U.S. patent application number 10/924313 was filed with the patent office on 2005-01-27 for chemical vapor deposition and powder formation using thermal spray.
Invention is credited to Hornis, Helmut G., Hunt, Andrew T..
Application Number | 20050019551 10/924313 |
Document ID | / |
Family ID | 21699191 |
Filed Date | 2005-01-27 |
United States Patent
Application |
20050019551 |
Kind Code |
A1 |
Hunt, Andrew T. ; et
al. |
January 27, 2005 |
Chemical vapor deposition and powder formation using thermal
spray
Abstract
A method for chemical vapor deposition using a very fine
atomization or vaporization of a reagent containing liquid or
liquid-like fluid near its supercritical temperature, where the
resulting atomized or vaporized solution is entered into a flame or
a plasma torch, and a powder is formed or a coating is deposited
onto a substrate. The combustion flame can be stable from 10 torr
to multiple atmospheres, and provides the energetic environment in
which the reagent contained within the fluid can be reacted to form
the desired powder or coating material on a substrate. The plasma
torch likewise produces the required energy environment, but,
unlike the flame, no oxidizer is needed so materials stable in only
very low oxygen partial pressures can be formed. Using either the
plasma torch or the combustion plasma, coatings can be deposited
and powders formed in the open atmosphere without the necessity of
a reaction chamber, but a chamber may be used for various reasons
including process separation from the environment and pressure
regulation.
Inventors: |
Hunt, Andrew T.; (Atlanta,
GA) ; Hornis, Helmut G.; (Atlanta, GA) |
Correspondence
Address: |
ALFRED H. MURATORI
MICROCOATING TECHNOLOGIES, INC.
5315 PEACHTREE INDUSTRIAL BLVD
ATLANTA
GA
30341-2107
US
|
Family ID: |
21699191 |
Appl. No.: |
10/924313 |
Filed: |
August 23, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10924313 |
Aug 23, 2004 |
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09921437 |
Aug 3, 2001 |
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6793975 |
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09921437 |
Aug 3, 2001 |
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09293867 |
Apr 16, 1999 |
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09293867 |
Apr 16, 1999 |
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08691853 |
Aug 2, 1996 |
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5997956 |
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60002084 |
Aug 4, 1995 |
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Current U.S.
Class: |
428/323 ;
427/421.1; 427/446; 428/328; 428/336 |
Current CPC
Class: |
Y10T 428/265 20150115;
C01B 13/34 20130101; Y02P 20/54 20151101; C23C 16/453 20130101;
Y02T 50/60 20130101; B22F 9/28 20130101; Y10T 428/25 20150115; B05D
1/08 20130101; B05D 2401/90 20130101; C23C 16/4486 20130101; Y10T
428/256 20150115 |
Class at
Publication: |
428/323 ;
428/336; 428/328; 427/421.1; 427/446 |
International
Class: |
B05D 001/08; B32B
005/16 |
Claims
We claim:
1) A chemical process in which a reactable spray composed of
droplets predominantly less than 10 microns in size are reacted in
a reaction process to form carbonaceous material.
2) The process of claim 1 wherein the droplets are predominantly
less than 2 microns in diameter.
3) The process of claim 1 wherein the reaction process is a
combustion reaction.
4) The process of claim 1 wherein the reactable spray contains a
cation precursor.
5) The process of claim 4 wherein the material formed is an
inorganic and carbonaceous material composite.
6) The process of claim 1 in which a liquid source for the droplets
contains liquefied or dissolved gas.
7) The process of claim 1 in which a liquid source of the said
droplets is heated sufficiently and released through a nozzle to
yield the desired formed size and distribution of carbonaceous
material.
8) The process of claim 1 in which the composite material formed is
a powder.
9) The process of claim 1 in which the composite material formed is
a coating or layer.
10) The process of claim 1 in which the material is formed at or
above ambient pressure.
11) The process of claim 1 in which the material is formed in
vacuum.
12) The process of claim 1 in which at least one additional
material is formed and becomes apart a composite with the
carbonaceous material.
13) The process of claim 1 in which the material formed is an
inorganic material.
14) The process of claim 1 in which the spray is a combustable
spray and the reaction occurs due to a flame.
15) The process of claim 14 in which the spray velocity is greater
than the flame speed and there is an ignition source to support the
combustion.
16) A nanophase composite, said composite comprising inorganic
material in part with a carbonaceous material.
17) The composite of claim 16 in which the composite is a powder
with a particle size of less than 1 micron.
18) The composite of claim 16 in which the composite is a powder
with particle size of less than 100 nm.
19) The composite of claim 16 in which the composite is a layer or
coating.
20) The composite of claim 16 in which the composite is a powder
with additional material bonded to the surface of the carbonaceous
material.
21) The composite of claim 20 in which the thus formed material is
electrochemically active.
22) The composite of claim 16 in which the inorganic material is a
metal.
23) The composite of claim 16 in which the formed material adds
electrical conductivity to a medium when the medium and the
composite are combined.
Description
RELATED CASES
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/921,437 filed Mar. 8, 2001, which is a
divisional of U.S. patent application Ser. No. 09/293,867 filed
Apr. 16, 1999, (now abandoned), which is a divisional of U.S.
patent application Ser. No. 08/691,853, filed Aug. 2, 1996, now
U.S. Pat. No. 5,997,956, which claims the benefit of U.S.
Provisional Application Ser. No. 60/002,084, filed Aug. 4, 1995,
the contents of all of which are hereby incorporated in their
entirety by this reference.
II. FIELD OF THE INVENTION
[0002] This invention relates to methods of powder formation and
thin film deposition from reagents contained in liquid or
liquid-like fluid solutions, whereby the fluid solution, near its
supercritical point temperature, is released into a region of lower
pressure causing a superior, very fine atomization or vaporization
of the solution. Gasses are entrained or fed into the dispersed
solution and rapidly flow into a flame or plasma torch. The
reagents react and form either: 1) powders which are collected; or
2) a coating from the vapor phase onto a substrate positioned in
the resulting gases and vapors. Release of the near supercritical
point temperature fluid causes dispersion and expansion resulting
in a very fine nebulization of the solution, which yields improved
powder and film quality, deposition rates and increases the number
of possible usable precursors.
III. BACKGROUND OF THE INVENTION
[0003] Chemical vapor processing has been used extensively for the
production of powders and coatings. Chemical vapor deposition
("CVD") is the term used when coatings onto a substrate are formed.
CVD production of coatings is widespread. Many of these coating are
only nanometers thick and smooth to less than 5% percent of coating
thickness. Reaction and agglomeration of the reacted vapor material
in the gas stream forms powders which can be commercially useful.
In fact, nanopowders are required in the formation of nanomaterials
which have different properties from those of bulk materials. These
materials' properties can be tailored by controlling the cluster
size of the nanopowder. Similarly, coatings of less than 50 nm can
have properties which are different from thicker films, and the
properties change further as the coating thins.
[0004] It is desirable to form such powders and coatings at low
production and capitalization costs and with simple production
processes. However, for many materials there is a very limited
selection of available precursors which can be vaporized and used
for traditional CVD. Being able to form coatings in the open
atmosphere tremendously eases substrate handling and flow through
the coating process. In addition to thin films low cost quality
thick coatings and bulk materials are also desirable.
[0005] Combustion chemical vapor deposition ("CCVD"), a recently
invented CVD technique, allows for open atmosphere deposition of
thin films. The CCVD process offers several advantages over other
thin-film technologies, including traditional CVD. The key
advantage of CCVD is its ability to deposit films in the open
atmosphere without any costly furnace, vacuum, or reaction chamber.
As a result, the initial system capitalization requirement can be
reduced up to 90% compared to a vacuum based system. Instead of a
specialized environment, which is required by other technologies, a
combustion flame provides the necessary environment for the
deposition of elemental constituents from solution, vapor, or gas
sources. The precursors are generally dissolved in a solvent that
also acts as the combustible fuel. Depositions can be performed at
atmospheric pressure and temperature within an exhaust hood,
outdoors, or within a chamber for control of the surrounding gasses
or pressure.
[0006] Since CCVD generally uses solutions, a significant advantage
of this technology is that it allows rapid and simple changes in
dopants and stoichiometries which eases deposition of complex
films. In contrast to conventional CVD, where the precursor vapor
pressure is a concern which dictates expensive high vapor pressure
precursors, the CCVD technique generally uses inexpensive, soluble
precursors. In addition, precursor vapor pressures do not play a
role in CCVD because the dissolution process provides the energy
for the creation of the necessary ionic constituents. In general,
the precursor materials used for traditional CVD depositions are
between 10 and 1000 times more expensive than those which can be
used in CCVD processing. By adjusting solution concentrations and
constituents, a wide range of stoichiometries can be deposited
quickly and easily. Additionally, the CCVD process allows both
chemical composition and physical structure of the deposited film
to be tailored to the requirements of the specific application.
[0007] Unlike CVD, the CCVD process is not confined to an
expensive, inflexible, low-pressure reaction chamber. Therefore,
the deposition flame, or bank of flames, can be moved across the
substrate to easily coat large and/or complex surface areas.
Because the CCVD process is not limited to specialized
environments, the user can continuously feed materials into the
coating area without disruption, thereby permitting batch
processing. Moreover, the user can limit deposition to specific
areas of a substrate by simply controlling the dwell time of the
flame(s) on those areas. Finally, the CCVD technology generally
uses halogen free chemical precursors having significantly reduced
negative environmental impact compared to conventional CVD,
resulting in more benign by-products.
[0008] Numerous materials have been deposited via CCVD technology
with the combustion of a premixed precursor solution as the sole
heat source. This inexpensive and flexible film deposition
technique permits broader use of thin film technology. The CCVD
process has much of the same flexibility as thermal spraying, yet
creates quality, conformal films like those associated with CVD.
Traditional CVD often requires months of effort to successfully
deposit a material. With CCVD processing, a desired phase can be
deposited in a few days and at a frction of the cost of traditional
CVD.
[0009] By providing these coating capabilities inexpensively, the
CCVD process can broaden the commercial opportunity for thin films,
including use in tribological, thermal protective, wear, space
environment protective, optic, electronic, structural and chemical
resistant applications. Thus, government and commercial users can
benefit from the advantages of thin films over thick films,
including their high adhesion to the substrate, controlled
microstructure, greater flexibility, reduced raw material
consumption and reduced effect on the operating characteristics
and/or dimensions of the coated system.
[0010] Ichinose, H., Shiwa, Y., and Nagano, M., Synthesis of
BaTiO.sub.3/LaNiO.sub.3 and PbTiO.sub.3/LaNiO.sub.3 Thin Films by
Spray Combustion Flame Technique, Jpn. J. Appl. Phys., Vol. 33, 1,
10 p. 5903-6 (1994) and Ichinose, H., Shiwa, Y., and Nagano, M.,
Deposition of LaMO.sub.3 (M.dbd.Ni, Co, Cr, Al)--Oriented Films by
Spray Combustion Flame Technique, Jpn. J. Appl. Phys., Vol. 33, 1,
10 p. 5907-10 (1994) used CCVD processing, which they termed spray
combustion flame technique, by ultrasonically atomizing a precursor
containing solution, and then feeding the resulting nebulized
solution suspended in argon carrier gas into a propane combustion
flame. However, these atomization techniques cannot reach the
highly desirable submicron capabilities which are important to
obtaining improved coating and powder formation.
[0011] U.S. Pat. No. 4,582,731 (the "'731 patent") discloses the
use of a supercritical fluid molecular spray for the deposition of
films. However, the '731 patent is for physical vapor deposition
(PVD), which differs from the independently recognized field of CVD
by having no chemical reagents and normally being operated at high
vacuum. Additionally, no flame or plasma torch is used in this
method, and only supercritical fluid solutions are considered.
Chemical reagents are beneficial because of there physical
properties, including higher solubility. The flame and plasma torch
enable coatings in the open atmosphere with no additional heat
source. The '731 deposition material, however, does not start from
a reagent, and thus will not react at supercritical conditions.
[0012] U.S. Pat. No. 4,970,093 (the "'093 patent") discloses the
use of supercritical fluids and CVD for the deposition of films.
Work related to the '083 patent is described in B. M. Hybertson, B.
N. Hansen, R. M. Barkley and R. E. Sievers, Supercritical Fluid
Transport-Chemical Deposition of Films, Chem. Mater., 4, 1992, p.
749-752 and Hybertson et al and B. N. Hansen, B. M. Hybertson, R.
M. Barkley and R. E. Sievers, Deposition of Palladium Films by a
Novel Supercritical Fluid Transport-Chemical Deposition Process
produce, Mat. Res. Bull., 26, 1991, p.1127-33. The '093 patent is
for traditional CVD without a flame or plasma torch and does not
consider open atmosphere capable techniques such as CCVD, which has
the associated advantages discussed above. Additionally, only
supercritical fluid solutions are considered; liquid solutions near
the supercritical point are not addressed. All of the precursors of
the '093 patent are carried in the supercritical solution which can
limit the usable precursors due to reactivity and solubility in
supercritical fluids.
[0013] B. M. Merkle, R. N. Kniseley, F. A. Smith and I. E.
Anderson, Superconducting YBaCuO Particulate produced by Total
Consumption Burner Process produce, Mat. Sci. Eng., A124, p. 31-38
(1990), J. McHale et al., Preparation of High-Tc Oxide Films via
Flaming Solvent Spray, J. Supercond. 5 (6), p. 511 (1992), and M.
Koguchi et al., Preparation of YBa.sub.2Cu.sub.3O.sub.x Thin Film
by Flame Pyrolysis, Jpn. J. Appl. Phys. 29 (1), p. L33 (1990)
describe the use of a flame to deposit films in what was termed a
"spray pyrolysis" technique. Both Merkle et al. and McHale et al.
deposited YBa.sub.2Cu.sub.3O.sub.x from a combusted sprayed
solution onto substrates, but the deposition conditions resulted in
low quality pyrolysis and particulate type coatings. Koguchi et al.
atomized a 0.03 mol/l aqueous solution and transported the
resulting mist into a H.sub.2--O.sub.2 flame and deposited a 10
.mu.m thick coating in 10 minutes on a yttria stabilized zirconia
(YSZ) substrate heated by the flame with much of the sprayed
material being lost in transport due to the method used. The
temperature, measured at the back of the substrate, reached a
maximum of 940.degree. C. However, the flame side of the substrate
is generally expected to be 100.degree. C. to 300.degree. C. higher
in temperature than the back which would be in the melting range of
YBa.sub.2Cu.sub.3O.sub.x. The resulting Koguchi et al. film had a
strong c-axis preferred orientation and, after a 850.degree. C.
oxygen anneal for eight hours, the film showed zero resistivity at
90.degree. K. Koguchi et al. termed their method "flame pyrolysis,"
and were probably depositing at temperatures near the melting point
of YBa.sub.2Cu.sub.3O.sub.x. The solution concentrations and
deposition rates were higher than those useful in CCVD processing.
Therefore, there exists a need for a coating method which achieves
excellent results at below the coating materials' melting point.
The present invention fulfills this need because the finer
atomization of the near supercritical fluid improves film quality
by enabling the formation of vapor deposited films at lower
deposition temperatures.
[0014] McHale et al. successfully produced 75 to 100 .mu.M thick
films of YBa.sub.2Cu.sub.3O.sub.x and
Bi.sub.1.7Pb.sub.0.3Ca.sub.2Sr.sub.2Cu.sub.3- O.sub.10 by
combusting a sprayed solution of nitrates dissolved in liquid
ammonia in a N.sub.2O gas stream, and by combusting nitrates
dissolved in either ethanol or ethylene glycol in an oxygen gas
stream. The results suggest the films were particulate and not
phase pure. The YBa.sub.2Cu.sub.3O.sub.x coatings had to be
annealed at 940.degree. C. for 24 hours and the
Bi.sub.1.7Pb.sub.0.3Ca.sub.2Sr.sub.2 Cu.sub.3O.sub.10 coatings heat
treated at 800.degree. C. for 10 hours and then at 860.degree. C.
for 10 hours to yield the desired material. Even after oxygen
annealing, zero resistivity could never be obtained at temperatures
above 76.degree. K. The solution concentrations used were not
reported, but the deposition rates were excessively high. In both
Koguchi's and McHale's methods, the reported solution and resulting
film stoichiometries were identical. Conversely, in CVD and in the
present invention, the solution stoichiometry may differ from the
desired film stoichiometry. Additionally the resulting droplet size
of sprayed solutions was excessively large and the vapor pressure
too low for effective vapor deposition.
[0015] A nebulized solution of precursors has been used with a
plasma torch in a process termed "spray inductively coupled plasma"
("spray-ICP" or "ICP"). See M. Kagawa, M. Kikuchi, R Ohno and T.
Nagae, J. Amer. Ceram. Soc., 64, 1981, C7. In spray-ICP, a reactant
containing solution is atomized into fine droplets of 1-2 mm in
diameter which are then carried into an ICP chamber. This can be
regarded as a plasma CVD process, different from flame pyrolysis.
See M. Suzuki, M. Kagawa, Y. Syono, T. Hirai and K. Watanabe, J.
Materials Sci., 26, 1991, p. 5929-5932. Thin films of the oxides of
Ce, La, Y, Pr, Nd, Sm, Cr, Ni, Ti, Zr, La--Sr--Cu, Sr--Ti, Zn--Cr,
La--Cr, and Bi--Pb--Sr--Ca--Cu have successfully been deposited
using this technique. See M. Suzuki, M. Kagawa, Y. Syono and T.
Hirai, Thin films of Chromium Oxide Compounds Formed by the
Spray-ICP Technique, J. Crystal Growth, 99, 1990, p. 611-615 and M.
Suzuki, M. Kagawa, Y. Syono and T. Hirai, Thin films of Rare-Earth
(Y, La, Ce. Pr, Nd, Sm) Oxides Formed by the Spray-ICP Technique,
J. Crystal Growth, 112, 1991, p.621-627. Holding the substrate at
an appropriate distance from the plasma was crucial to synthesizing
dense films. The range of desired deposition distances from the
plasma source was small due to the rapid temperature drop of the
gases. CVD type coatings were achieved using ultrasonically
atomized 0.5-1.0 M solutions of metal-nitrates in water which were
fed into the ICP at 6-20 ml/h using Ar flowing at 1.3-1.4 l/min.
Only oxides were deposited and liquid or liquid-like solutions near
the supercritical temperature were not used. The use of near
supercritical atomization with ICP was not considered in this broad
review of ICP nebulization techniques. See T. R. Smith and M. B.
Denton, Evaluation of Current Nebulizers and Nebulizer
Characterization Techniques, Appl. Spectroscopy, 44, 1990, p.
214.
[0016] Therefore, it is highly desirable to be able to form
nanopowders and coatings at low production and capitalization costs
and with simple production processes. It is also desirable to be
able to form coatings in the open atmosphere without any costly
furnace, vacuum, or reaction chamber. It is further highly
desirable to provide a coating process which provides for a high
adhesion to the substrate, controlled microstructure, flexibility,
reduced raw material consumption and reduced effect on the
operating characteristics and/or dimensions of the coated system
while being able to retain highly desirable submicron capabilities
which are important to obtaining improved coating and powder
formation. Moreover, it is highly desirable to provide a process
which uses solutions near their supercritical point, and,
therefore, achieves excellent results at below the coating
materials' melting point.
SUMMARY OF THE INVENTION
[0017] The present invention fulfills these needs and defines
plasma torch and CCVD produced vapor formed films, powders and
nanophase coatings from near supercritical liquids and
supercritical fluids. Preferably, a liquid or liquid-like solution
fluid containing chemical precursor(s) is formed. The solution
fluid is regulated to near or above the critical pressure and is
then heated to near the supercritical temperature just prior to
being released through a restriction or nozzle which results in a
gas entrained very finely atomized or vaporized solution fluid. The
solution fluid vapor is combusted to form a flame or is entered
into a flame or electric torch plasma, and the precursor(s) react
to the desired phase in the flame or plasma or on the subsrate
surface. Due to the high temperature of the plasma much of the
precursor will react prior to the substrate surface. A substrate is
positioned near or in the flame or electric plasma, and a coating
is deposited. Alternatively, the material formed can be collected
as a nanophase powder.
[0018] The method of the present invention provides for very fine
atomization, nebulization, vaporization or gasification by using
solution fluids near or above the critical pressure and near the
critical temperature. The dissolved chemical precursor(s) need not
have high vapor pressure, but high vapor pressure precursors can
work well or better than lower vapor pressure precursors. By
heating the solution fluid just prior to or at the end of the
nozzle or restriction tube (atomizing device), the available time
for precursor chemical reaction or dissolution prior to atomization
is minimized. This method can be used to deposit coatings from
various metalorganics and inorganic precursors. The fluid solution
solvent can be selected from any liquid or supercritical fluid in
which the precursor(s) can form a solution The liquid or fluid
solvent by itself can consist of a mixture of different
compounds.
[0019] A reduction in the supercritical temperature of the reagent
containing fluid demonstrated superior coatings. Many of these
fluids are not stable as liquids at STP, and must be combined in a
pressure cylinder or at a low temperature. To ease the formation of
a liquid or fluid solution which can only exist at pressures
greater than ambient, the chemical precursor(s) are optionally
first dissolved in primary solvent that is stable at ambient
pressure. This solution is placed in a pressure capable container,
and then the secondary (or main) liquid or fluid (into which the
primary solution is miscible) is added. The main liquid or fluid
has a lower supercritical temperature, and results in a lowering of
the maximum temperature needed for the desired degree of
nebulization. By forming a high concentration primary solution,
much of the resultant lower concentration solution is composed of
secondary and possible additional solution compounds. Generally,
the higher the ratio of a given compound in a given solution, the
more the solution properties behave like that compound. These
additional liquids and fluids are chosen to aid in the very fine
atomization, vaporization or gasification of the chemical
precursor(s) containing solution. Choosing a final solution mixture
with low supercritical temperature additionally minimizes the
occurrence of chemical precursors reacting inside the atomization
apparatus, as well as lowering or eliminating the need to heat the
solution at the release area. In some instances the solution may be
cooled prior to the release area so that solubility and fluid
stability are maintained. One skilled in the art of supercritical
fluid solutions could determine various possible solution mixtures
without undue experimentation. Optionally, a pressure vessel with a
glass window, or with optical fibers and a monitor, allows visual
determination of miscibility and solute-solvent compatibility.
Conversely, if in-line filters become clogged or precipitant is
found remaining in the main container, an incompatibility under
those conditions may have occurred.
[0020] The resulting powder size produced by the methods and
apparatuses of the present invention can be decreased, and
therefore, improved by: 1) decreasing the concentration of the
initial solution; 2) decreasing the time in the hot gasses; 3)
decreasing the size of the droplets formed; and/or 4) increasing
the vapor pressure of the reagent used. Each of the variables has
other considerations. For instance, economically, the concentration
of the initial solution should be maximized to increase the
formation rate, and lower vapor pressure reagents should be used to
avoid the higher costs of many higher vapor pressure reagents.
Decreasing the time in the hot gasses is countered by the required
minimum time of formation of the desired phase. Decreasing the size
of the droplets formed can entail increased fluid temperature which
is countered by possible fluid reaction and dissolution effects.
Similarly, coating formation has parallel effects and
relationships.
[0021] Another advantage is that release of fluids near or above
their supercritical point results in a rapid expansion forming a
high speed gas-vapor stream. High velocity gas streams effectively
reduce the gas diffusion boundary layer in front of the deposition
surface which, in turn, improves film quality and deposition
efficiency. When the stream velocities are above the flame
velocity, a pilot light or other ignition means must be used to
form a steady state flame. In some instances two or more pilots may
be needed to ensure complete combustion. With the plasma torch, no
pilot lights are needed, and high velocities can be easily achieved
by following operational conditions known by one of ordinary skill
in the art.
[0022] The solute containing fluid need not be the fuel for the
combustion. Noncombustible fluids like water or CO.sub.2, or
difficult to combust fluids like ammonia, can be used to dissolve
the precursors or can serve as the secondary solution compound.
These are then expanded into a flame or plasma torch which provides
the environment for the precursors to react. The depositions can be
performed above, below or at ambient pressure. Plasma torches work
well at reduced pressures. Flames can be stable down to 10 torr,
and operate well at high pressures. Cool flames of even less than
500.degree. C. can be formed at lower pressures. While both can
operate in the open atmosphere, it can be advantageous to practice
the methods of the invention in a reaction chamber under a
controlled atmosphere to keep airborne impurities from being
entrained into the resulting coating. Many electrical and optical
coating applications require that no such impurities be present in
the coating. These applications normally require thin films, but
thicker films for thermal barrier, corrosion and wear applications
can also be deposited.
[0023] Further bulk material can be grown, including single
crystals, by extending the deposition time even further. The faster
epitaxial deposition rates provided by higher deposition
temperatures, due to higher diffusion rates, can be necessary for
the deposition of single crystal thick films or bulk material.
[0024] Additional advantages of the invention will be set forth in
part in the description which follows, and in part will be obvious
from the description, or may be learned by practice of the
invention. The advantages of the invention will be realized and
attained by means of the elements and combinations particularly
pointed out in the appended claims. It is to be understood that
both the foregoing general description and the following detailed
description are exemplary and explanatory only and are not
restrictive of the invention, as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 shows a schematic diagram of the apparatus of the
invention.
[0026] FIG. 2 shows a schematic diagram of an apparatus for the
deposition of films and powders using near supercritical and
supercritical atomization.
[0027] FIG. 3 shows a detailed schematic view of the atomizer used
in the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The present invention may be understood more readily by
reference to the following detailed description of preferred
embodiments of the invention and the Figures.
[0029] Before the present methods and apparatuses are disclosed and
described, it is to be understood that the terminology used herein
is for the purpose of describing particular embodiments only and is
not intended to be limiting. It must be noted that, as used in the
specification and the appended claims, the singular forms "a," "an"
and "the" include plural referents unless the context clearly
dictates otherwise.
[0030] Throughout this application, where publications are
referenced, the disclosures of these publications in their
entireties are hereby incorporated by reference into this
application in order to more fully describe the state of the art to
which this invention pertains.
[0031] The present invention provides a method for coating a
substrate with a selected material. The method comprises, at a
first selected temperature and a first selected pressure,
dissolving into a suitable carrier to thereby form a transport
solution one or more reagents capable of reacting (where, for a
single precursor reagent, the precipitation of the reagent from the
solution is herein considered a "reaction") to form the selected
material. At some time prior to the actual deposition, a substrate
is positioned in a region having a second selected pressure. The
second selected pressure can be ambient pressure and is generally
above 20 torr. The tansport solution is then pressurize to a third
selected pressure above the second selected pressure using a
pressure regulating means. One of skill in the art would recognize
that there are many suitable pressure regulating means, including,
but not limited to compressors, etc. Next, the pressurized,
transport solution is directed to a fluid conduit having an input
end and an opposed output end having a temperature regulating means
positioned thereon for regulating the temperature of the solution
at the output end. The output end of the conduit further comprises
an outlet port oriented to direct the fluid in the conduit into the
region and in the direction of the substrate. The outlet port can
be of a shape similar to a nozzle or restrictor as used in other
spraying and CVD applications. Thereafter, the solution is heated
using the temperature regulating means to a second selected
temperature within 50.degree. C. above or below the critical
temperature, T.sub.c, of the solution while maintaining the third
selected pressure above the second selected pressure and above the
corresponding liquidus or critical pressure, P.sub.c, of the
solution at the second selected temperature using the pressure
regulating means. Then, the pressurized, heated solution is
directed through the outlet port of the conduit into the region to
produce a nebulized solution spray in the direction of the
substrate. As the solution is directed into the region, one or more
selected gases are admixed into the nebulized solution spray to
form a reactable spray and, thereafter, this reactable spray is
exposed to an energy source at a selected energization point. The
energy source provides sufficient energy to react the reactable
spray (which contains the one or more reagents of the transport
solutions) thereby forming the material and coating the substrate
therewith.
[0032] In a further embodiment of this method, the energy source
comprises a flame source and the selected energization point
comprises an ignition point. In an alternate embodiment, the energy
source comprises a plasma torch.
[0033] In a further embodiment of the method, the second selected
pressure of the region is ambient pressure.
[0034] In yet another embodiment, the nebulized solution spray is a
vapor or an aerosol having a maximum droplet size of less than 2
.mu.m.
[0035] In a further embodiment, the second selected pressure of the
region is reduced to produce a combustion flame having a
temperature of less than 1000.degree. C.
[0036] In yet another embodiment, the carrier is propane and the
transport solution comprises at least 50% by volume propane. In a
further embodiment, the trnsport solution further includes butanol,
methanol, isopropanol, toluene, or a combination thereof. In yet
another embodiment, the carrier is selected such that the transport
solution is substantially precipitate free at standard temperature
and pressure for a period of time sufficient to carry out the
method.
[0037] In an alternate embodiment of the method, a pressurized
container is used and before, during or after the pressuring step,
a standard temperature and pressure gas is also contacted with the
trnsport solution at a selected pressure sufficient to form a
liquid or supercritical fluid (depending upon the temperature). In
a preferred embodiment, the transport solution containing the
standard temperature and pressure gas is substantially precipitate
free at the selected pressure for a period of time sufficient to
carry out the method. In yet another embodiment, the reagent
concentration of the transport solution is between 0.0005 M and
0.05 M.
[0038] In a further embodiment, the outlet end of the conduit
further comprises a fluid introduction port and, prior to directing
the pressurized, heated solution through the outlet port of the
conduit, fluid is added to the pressurized, heated solution through
the fluid introduction port. Such introduction forms a combined
solution having a reduced supercritical temperature.
[0039] In yet another embodiment, each of the one or more reagents
has a vapor pressure of no less than about 25% of the vapor
pressure of the carrier.
[0040] In a further embodiment, the outlet end of the conduit
comprises tubing having an internal diameter of 2 to 1000 .mu.m,
more preferably 10 to 250 .mu.m. In a more preferable embodiment,
the outlet end of the conduit comprises tubing having an internal
diameter of 25 to 125 .mu.m. In yet a further preferable
embodiment, the outlet end of the conduit comprises tubing having
an internal diameter of 50 to 100 .mu.m.
[0041] In another embodiment, the temperature regulating means
comprises means for resistively heating the conduit by applying
thereto an electric current of a selected voltage from an electric
current source. In a preferred embodiment, the voltage is less than
115 Volts. In yet another preferred embodiment, the means for
resistively heating the conduit comprises a contact positioned
within 4 mm of the outlet port.
[0042] Moreover, the present invention also provides the above
method wherein the carrier and one or more reagents are selected
such that the second selected temperature is ambient
temperature.
[0043] The above method may be practiced wherein the material that
coats the substrate comprises a metal. Alternatively, the material
that coats the substrate comprises one or more metal oxides. In a
further alternative embodiment, the material that coats the
substrate comprises a carbonate, a sulfate, or a phosphate. In yet
a further embodiment, the material that coats the substrate
comprises at least 90% silica.
[0044] In a further embodiment, the reactable spray comprises a
combustible spray having a combustible spray velocity and wherein
the combustible spray velocity is greater than the flame speed of
the flame source at the ignition point and further comprising one
or more ignition assistance means for igniting the combustible
spray. In a preferred embodiment, each of the one or more ignition
assistance means comprises a pilot light In yet another embodiment,
the combustible spray velocity is greater than mach one.
[0045] In a further embodiment, the ignition point or flame front
is maintained within 2 cm of the outlet port.
[0046] The present invention also provides a method where, during
the exposing step, cooling the substrate using a substrate cooling
means. In a preferred embodiment, the substrate cooling means
comprises a means for directing water onto the substrate. However,
one of ordinary skill in the art would recognize that many other
suitable cooling means could be used.
[0047] In a further embodiment, the material that coats the
substrate comprises a carbonaceous material. In another embodiment,
the material that coats the substrate comprises diamond. In an
alternate embodiment, the material that coats the substrate
comprises (1) diamond and (2) a metal oxide or a metal. In a
further embodiment, the material that coats the substrate has a
thickness of less than 100 nm. In yet another embodiment, the
material that coats the substrate comprises a graded composition.
In another embodiment, the material that coats the substrate
comprises an amorphous material. In a further embodiment, the
material that coats the substrate comprises a nitride, carbide,
boride, metal or other non-oxygen containing material.
[0048] The present invention also provides a method further
comprising flowing a selected sheath gas around the reactable spray
thereby decreasing entrained impurities and maintaining a favorable
deposition environment.
[0049] In a preferred embodiment, the second selected pressure is
above 20 torr.
[0050] In addition to the above methods, the present invention also
provides an apparatus for coating a substrate with a selected
material. Referring now to FIG. 1, the apparatus 100 comprises a
pressure regulating means 110, such as a pump, for pressurizing to
a first selected pressure a transport solution T (also called
"precursor solution") in a transport solution reservoir 112,
wherein the transport solution T comprises a suitable carrier
having dissolved therein one or more reagents capable of reacting
to form the selected material and wherein the means for
pressurizing 110 is capable of maintaining the first selected
pressure above the corresponding liquidus (if the temperature is
below T.sub.c) or critical pressure, P.sub.c, of the transport
solution T at the temperature of the transport solution T, a fluid
conduit 120 having an input end 122 in fluid connection with the
transport solution reservoir 112 and an opposed output end 124
having an outlet port 126 oriented to direct the fluid in the
conduit 120 into a region 130 of a second selected pressure below
the first selected pressure and in the direction of the substrate
140, wherein the outlet port 126 further comprises means 128 (see
FIGS. 2 and 3, atomizer 4) for nebulizing a solution to form a
nebulized solution spray N, a temperature regulating means 150
positioned in thermal connection with the output end 124 of the
fluid conduit 120 for regulating the temperature of the solution at
the output end 124 within 50.degree. C. above or below the
supercritical temperature, T.sub.c, of the solution, a gas supply
means 160 for admixing one or more gases (e.g., oxygen) (not shown)
into the nebulized solution spray N to form a reactable spray, an
energy source 170 at a selected energization point 172 for reacting
the reactable spray whereby the energy source 170 provides
sufficient energy to react the reactable spray in the region 130 of
the second selected pressure thereby coating the substrate 140.
[0051] In a further embodiment of the apparatus, the energy source
170 comprises a flame source and the selected energization point
172 comprises an ignition point. In an alternate embodiment, the
energy source 170 comprises a plasma torch. In yet another
embodiment, the outlet port 126 further comprises a pressure
restriction (see FIG. 3, restrictor 7).
[0052] In a further embodiment of the apparatus, the second
selected pressure of the region is ambient pressure.
[0053] In yet another embodiment, the nebulized solution spray N is
a vapor or an aerosol having a maximum droplet size of less than 2
.mu.m.
[0054] In a further embodiment, the second selected pressure of the
region is reduced to produce a combustion flame having a
temperature of less than 1000.degree. C.
[0055] In yet another embodiment, the carrier is propane and the
transport solution comprises at least 50% by volume propane. In a
further embodiment, the transport solution further includes
butanol, methanol, isopropanol, toluene, or a combination thereof.
In yet another embodiment, the carrier is selected such that the
transport solution is substantially precipitate free at standard
temperature and pressure for a period of time sufficient to carry
out the method.
[0056] In an alternate embodiment of the apparatus, a pressurized
container (not shown) is provided and a standard temperature and
pressure gas is also contacted with the transport solution at a
selected pressure sufficient to form a liquid or supercritical
fluid. In a preferred embodiment, the transport solution contaning
the standard temperature and pressure gas is substantially
precipitate free at the selected pressure for a period of time
sufficient to carry out the method. In yet another embodiment, the
reagent concentration of the transport solution is between 0.0005 M
and 0.05 M.
[0057] In a further embodiment, the outlet end 124 of the conduit
120 further comprises a fluid introduction port (see FIG. 2, feed
lines 17 or 19) and, prior to directing the pressurized, heated
solution through the outlet port 126 of the conduit 120, fluid is
added to the pressurized, heated solution through the fluid
introduction port. Such introduction forms a combined solution
having a reduced supercritical temperature.
[0058] In yet another embodiment, each of the one or more reagents
has a vapor pressure of no less than about 25% of the vapor
pressure of the carrier.
[0059] In a further embodiment, the outlet end of the conduit
comprises tubing having an internal diameter of 2 to 1000 .mu.m,
more preferably 10 to 250 .mu.m. In a more preferable embodiment,
the outlet end of the conduit comprises tubing having an internal
diameter of 25 to 125 .mu.m. In yet a further preferable
embodiment, the outlet end of the conduit comprises tubing having
an internal diameter of 50 to 100 .mu.m.
[0060] In another embodiment, the temperature regulating means 150
comprises means for resistively heating the conduit by applying
thereto an electric current of a selected voltage from an electric
current source. In a preferred embodiment, the voltage is less than
115 Volts. In yet another preferred embodiment, the means for
resistively heating the conduit comprises a contact 152 positioned
within 4 mm of the outlet port 126.
[0061] Moreover, the present invention also provides the above
apparatus wherein the carrier and one or more reagents are selected
such that the second selected temperature is ambient
temperature.
[0062] The above apparatus may be used wherein the material that
coats the substrate 140 comprises metal. Alternatively, the
material that coats the substrate 140 comprises one or more metal
oxides. In a further alternative embodiment, the material that
coats the substrate 140 comprises a carbonate, a sulfate, or a
phosphate. In yet a further embodiment, the material that coats the
substrate 140 comprises at least 90% silica.
[0063] In a further embodiment, the reactable spray comprises a
combustible spray having a combustible spray velocity and wherein
the combustible spray velocity is greater than the flame speed of
the flame source at the ignition point 172 and further comprising
one or more ignition assistance means 180 for igniting the
combustible spray. In a preferred embodiment, each of the one or
more ignition assistance means 180 comprises a pilot light. In yet
another embodiment, the combustible spray velocity is greater than
mach one.
[0064] In a further embodiment, the ignition point 172 or flame
front is maintained within 2 cm of the outlet port.
[0065] The present invention also provides a substrate cooling
means 190 for cooling the substrate 140. In a preferred embodiment,
the substrate cooling means 190 comprises a means for directing
water onto the substrate 140. However, one of ordinary skill in the
art would recognize that many other suitable cooling means could be
used.
[0066] In a further embodiment, the material that coats the
substrate 140 comprises a carbonaceous material. In another
embodiment, the material that coats the substrate 140 comprises
diamond. In an alternate embodiment, the material that coats the
substrate 140 comprises (1) diamond and (2) a metal oxide or a
metal. In a further embodiment, the material that coats the
substrate 140 has a thickness of less than 100 .mu.m. In yet
another embodiment, the material that coats the substrate 140
comprises a graded composition. In another embodiment, the material
that coats the substrate 140 comprises an amorphous material. In a
further embodiment, the material that coats the substrate 140
comprises a nitride, carbide, boride, metal or other non-oxygen
containing material.
[0067] The present invention also provides an apparatus further
comprising a means (see FIGS. 2 and 3, feed line 17 or 19) for
flowing a selected sheath gas around the reactable spray thereby
decreasing entrained impurities and maintaining a favorable
deposition environment
[0068] In a preferred embodiment, the second selected pressure is
above 20 torr.
[0069] In addition, the present invention also provides a method
for creating a powdered material in a region. This powder forming
method comprises, at a first selected temperature and a first
selected pressure, dissolving into a suitable carrier to thereby
form a transport solution one or more reagents capable of reacting
to form the powdered material in the region, wherein the region has
a second selected pressure lower than the first selected pressure.
The powder forming method then involves pressurizing the transport
solution to a third selected pressure above the second selected
pressure using a pressure regulating means. Thereafter, the
pressurized, transport solution is directed to a fluid conduit
having an input end and an opposed output end having a temperature
regulating means positioned thereon for regulating the temperature
of the solution at the output end, wherein the output end further
comprises an outlet port oriented to direct the fluid in the
conduit into the region. The solution is then heated using the
temperature regulating means to a second selected temperature
within 50 degree C. above or below the critical temperature,
T.sub.c, of the solution while maintaining the third selected
pressure above the second selected pressure and above the
corresponding liquidus or critical pressure, P.sub.c, of the
solution at the second selected temperature using the pressure
regulating means. The pressurized, heated solution is then directed
through the outlet port of the conduit into the region to produce a
nebulized solution spray. One or more selected gases are admixed
into the nebulized solution spray to form a reactable spray.
Finally, the reactable spray is exposed to an energy source at a
selected energization point whereby the energy source provides
sufficient energy to react the reactable spray thereby forming the
powdered material in the region.
[0070] In a further embodiment of the method, the energy source
comprises a flame source and the selected energization point
comprises an ignition point. In an alternate embodiment, the energy
source comprises a plasma torch.
[0071] In a further embodiment, the powder-forming method may be
used to coat a particular substrate. For this method, the above
method is modified such that the method further comprises admixing
a selected substrate material with the transport solution.
[0072] In a further preferred embodiment of the powder forming
method, the transport solution concentration is between 0.005 M and
5 M.
[0073] In another embodiment, the present invention provides an
apparatus for creating a powdered material, comprising a pressure
regulating means for pressurizing to a first selected pressure a
transport solution in a transport solution reservoir, wherein the
transport solution comprises a suitable carrier having dissolved
therein one or more reagents capable of reacting to form the
selected material and wherein the means for pressurizing is capable
of maintaining the first selected pressure above the corresponding
liquidus or critical pressure, P.sub.c, of the transport solution
at the temperature of the transport solution, a fluid conduit
having an input end in fluid connection with the transport solution
reservoir and an opposed output end having an outlet port oriented
to direct the fluid in the conduit in the direction of a region of
a second selected pressure lower than the first selected pressure,
wherein the outlet port further comprises means for nebulizing a
solution to form a nebulized solution spray, a temperature
regulating means positioned on the output end of the fluid conduit
for regulating the temperature of the solution at the output end, a
gas supply means for admixing one or more gases into the nebulized
solution spray to form a reactable spray, and an energy source at a
selected energization point for reacting the reactable spray
whereby the energy source provides sufficient energy to react the
reactable spray in the region of the second selected pressure
thereby forming the powdered material. The apparatus is similar to
the coating apparatus, except that no substrate is positioned in
the region and, instead, the powder is collected.
[0074] In addition, the present invention provides a coating on a
substrate produced by the process described above. In addition, the
present invention provides a powder produced by the process
described above.
[0075] A flame requires that some oxidant be present. Thus,
materials which are not stable in the presence of oxidants cannot
be formed using a flame. See C. H. P. Lupis, "Chemical
Thermodynamics of Materials", Elsevier Science Publishing, 1993.
Instead, these materials are deposited using a plasma torch. When
using a plasma torch, a sheath gas, plasma enclosure tube, and/or
reaction chamber is necessary to maintain an air and oxidant free
environment. Because of the instability in the presence of
oxidants, the fluid and gasses used should not contain oxidizer
elements. Additionally, the use of hydrocarbon compounds can result
in the deposition of extensive carbon. While some carbon is needed
for the formation of carbides, too much carbon could result in high
C coatings, and possible elemental carbon. In such cases the
solution concentration should be increased, or a low carbon or
carbon free containing fluid should be used. One skilled in the art
of combustion could determine, with only routine experimentation,
the adjustments which would minimize the deposition or
incorporation of elemental carbon. This deposition is also called
"fouling" or "carbon buildup" in burner and engine applications.
For the deposition of nitrides, a plasma torch should be used, and
ammonia is one of the suitable solvents. Metal filns can be
deposited from chemical systems with no or low concentrations of
anions from thermodynamically stable phases. In a brief example, if
Ti metal was desired, then the concentration of C, N, and B would
preferably be low, and almost no Cl, F or O containing compounds
would be present. When using the plasma torch, it is easier to
deposit oxygen stable materials because the sheath gas or enclosure
tube need not be used. However, the cost of the flame deposited and
formed materials is less, and thus is preferred for materials
thermodynamically stable in environments containing sufficient
oxygen to maintain a flame.
[0076] It may be desired to form a coating completely or partially
composed of elemental carbon, in which case all elemental oxygen
and reactive oxygen containing compounds must be kept separated
from the carbon film as long as the film temperature is such that
the oxidation of carbon is thermodynamically favored over the
presence of elemental carbon. If trace amounts of oxygen are
present one can make the reaction atmosphere more reducing by
supplying elemental hydrogen. Diamond and diamond like carbon
coatings can be deposited from an oxygen containing flame, and the
necessary deposition conditions are known to one skilled in the
art. The addition of dopants or a second phase to the carbon
coating can be accomplished by adding a reagent to the flame or
plasma by the use of this present invention.
[0077] To simplify the operation, it is helpful to pump the
precursor/solvent solution to the atomizing device at room
temperature. Heating of the solution should occur as a final step
just prior to release of the solution into the lower pressure
region. Such late stage heating minimizes reactions and
immiscibilities which occur at higher temperatures. Keeping the
solution below the supercritical temperature until atomization
maintains the dissolved amounts of precursor in the region of
normal solubility and reduces the potential of developing
significant solvent-precursor concentration gradients in the
solution. These solubility gradients are a result of the
sensitivity of the solution strength of a supercritical solvent
with pressure. Small pressure gradients (as they can develop along
the precursor-solvent system delivery) can lead to significant
changes in solubility as has been observed. For instance, the
solubility of acridine in carbon dioxide at 308.degree. K can be
increased 1000 times by increasing the pressure from 75 atm to 85
atm. See V. Krukonis, "Supercritical Fluid Nucleation of Difficult
to Comnminute Solids", Presented at AIChE Meeting, San Francisco,
Nov. 25-30, 1984. Such solubility changes are potentially
detrimental because they may cause the precursor to be driven out
of solution and precipitate or react prematurely, clogging lines
and filters.
[0078] The rapid drop in pressure and the high velocity at the
nozzle cause the solution to expand and atomize. For solute
concentrations in the normal solubility range, preferred for
operation of the near supercritical atomization system of the
present invention, the precursors are effectively still in solution
after being injected into the low pressure region. The term
"effectively in solution" must be understood in conjunction with
processes taking place when a solution with solute concentrations
above the normal solvent strength is injected into the low pressure
region. In this case, the sudden pressure drop causes high
supersaturation ratios responsible for catastrophic solute
nucleation conditions. If the catastrophic nucleation rapidly
depletes the solvent from all dissolved precursor, the
proliferation of small precursor particles is enhanced. See D. W.
Matson, J. L. Fulton, R. C. Petersen and R. D. Smith, "Rapid
Expansion of Supercritical Fluid Solutions: Solute Formation of
Powders, Thin Films, and Fibers", Ind. Eng. Chem. Res., 26, 2298
(1987); H. Anderson, T. T. Kodas and D. M. Smith, "Vapor Phase
Processing of Powders: Plasma Synthesis and Aerosol Decomposition",
Am. Ceram. Soc. Bull., 68, 996 (1989); C. J Chang and A. D
Randolph, "Precipitation of Microsize Organic Particles from
Supercritical Fluids", AIChE Journal, 35, 1876 (1989); T. T. Kodas,
"Generation of Complex Metal Oxides by aerosol Processes:
Superconducting Ceramic Particles and Films", Adv. Mater., 6, 180
(1989); E. Matijevic, "Fine Particles: Science ad Technology", MRS
Bulletin, 14, 18 (1989); E. Matijevic, "Fine Particles Part II:
Formation Mechanisms and Applications", MRS Bulletin, 15, 16
(1990); R. S. Mohamed, D. S. Haverson, P. G. Debenedetti and R. K.
Prud'homme, "Solid Formation After Expansion of Supercritical
Mixtures," in Supercritical Fluid Science and Technology, edited by
K. P. Johnston and J. M. L. Penniger, p. 355, American Chemical
Society, Washington, D.C. (1989); R. S. Mohamed, P. G. Debenedetti
and R. K. Prudhomme, "Effects of Process Conditions on Crystals
Obtained from Supercritical Mixtures", AIChE J., 35, 325 (1989); J.
W. Tom and P. G. Debenedetti, "Formation of Bioerodible Polymeric
Microspheres and Microparticles by Rapid Expansion of Supercritical
Solutions", Biotechnol. Prog., 7, 403 (1991). Particles are
undesirable for the formation of thin coatings, but can be
beneficial during the formation of powders.
[0079] Thus the heated atomizer of the present invention provides
the further superior advantages, compared to an unheated device
that operates on rapid expansion of a solvent at exclusively above
the supercritical temperature, that (1) the temperature allows for
a well controlled degree of atomization of the precursor-solvent
mixture and (2) catastrophic nucleation of the precursors can be
omitted while still enjoying the benefits of supercritical
atomization. Supersonic velocities can be created forming a mach
disk which additionally benefits atomization.
[0080] By adjusting the heat input into the atomizing device, the
liquid solution can be vaporized to various degrees. With no heat
input to the atomizing device, liquid solutions of higher
supercritical temperature liquids, that are liquids at STP, can
exit in the form of a liquid stream which is clearly far from a
supercritical condition. This results in a poorly formed flame and,
possibly, undesirable liquid contact with the substrate. Decreasing
the temperature differential of the liquid solution to its
supercritical temperature at the nozzle causes the liquid solution
to break up into droplets forming a mist which is released from the
atomizing device. The droplets vaporize, and thus become invisible,
after a short distance. As the supercritical temperature at the
atomizing device is approached, the liquid solution droplets
decrease in size, and the distance to solution vaporization is
decreased. Using this atomizer the vapor droplet size was
determined using an aerosol vaporization tester and the obtained
droplet size was below the 1.8.mu. detection limit of the
instrument.
[0081] Further increasing the heat input results in a state of no
mist at the tip, or complete vaporization. Without wishing to be
bound by theory, this behavior of the solution can be attributed to
the combined supercritical properties of the reagents and solvents.
Solutions of precursors in lower supercritical temperature
solvents, that are gasses at STP, behave similarly, but the
emerging solution from the tip (also referred to as the "nozzle" or
"restrictor") does not form a liquid stream, even without heat
input. The amount of heat needed to obtain optimal vaporization of
the solution depends mostly on the heat capacity of the solution
and the differential between the supercritical temperature of the
solvent and the ambient temperature around the nozzle.
[0082] It is desirable to maintain the pressure and temperature of
the system (before vaporization) above the boiling and the
supercritical point of the solution. If the pressure falls below
the liquidus or critical pressure, coincident with the temperature
above the boiling point, vaporization of the solvents will occur in
the tube prior to the tip. This leaves the solutes which can build
up and clog the atomizing device. Similarly the pressure is
preferably sufficiently high in the supercritical region so that
the fluid is more liquid-like. Liquid-like supercritical fluids are
better solvents than more gas-like supercritical fluids, further
reducing the probability of solutes clogging the atomizing device.
If the precursor-to-precursor interaction is higher than the
strength between solvent and precursor, the solvent-precursor bonds
can be broken and effectively drive the precursor out of solution.
Precursor molecules then form clusters that adhere to the atomizing
device and clog the restrictor. The problem can be solved, in most
cases, by shifting the vaporization point from the inside of the
tip to the end of the tip, which is accomplished by reducing the
heat input into the atomizing device. Another solution is to use a
solvent which forms stronger bonds with the precursor so a more
stable solution is formed. A small amount of mist at the tip
usually results in the best quality thin films. Nano- or
micro-spheres of the material will form if the temperature of the
solution it too high or too low. These spheres are detrimental if
dense coatings are desired.
[0083] If the no-mist condition is reached, the deposition is being
performed above the critical temperature. The heat of the flame and
mixing with external gasses keeps STP liquid solvents from
condensing and forming droplets. In the no-mist instance,
atomization and intermixing is very good but flow stability is
reduced, resulting in a flame that can jump from side to side with
respect to the direction of the tip. With such a flame behavior,
depositions remain possible, but it can be difficult to deposit
films requiring stringent thickness uniformity. Additionally, it is
necessary to maintain the temperature of the solution, prior to
release, below the temperature where either the solute precipitates
or reacts and precipitates. When using a solvent mixture it may be
possible during heating to cross the line for spinoidal
immiscibility. This causes the formation of two separate phases,
with the possibility of concentration differences in the two phases
due to different solubilities of the solutes. This may influence
the formation of precursor and product spheres at high atomization
temperatures. All of these factors demonstrate the preferability of
minimizing the solution's exposure to heating, if necessary, until
the tip so that possible unwanted equilibrium condition states of
matter do not have sufficient time to transpire. The structure of
the films deposited can thus be precisely controlled.
[0084] Due to this control, a number of film microstructures are
possible. By increasing solution concentration it is possible to
increase the deposition rate and the following microstructural
changes result with increasing solution concentration; dense to
porous, specular to dull, smooth to rough, columnar to hillocks,
and thin to thick. Graded and multilayered coatings can also be
produced. See Example VI. Multilayers can be formed by supplying
different precursor containing solutions to an individual flame.
Sequential multiple deposition flames may be used to increase
throughput for production applications. Some additional factors
controlling deposition parameters include; substrate surface
temperature which controls surface diffusion and nucleation;
pressure which controls boundary layer thickness and thus
deposition rate, solution composition and mix gasses varies the
material being deposited and thus the coatings growth habit, Flame
and plasma energy level effects where the reaction occurs and vapor
stability, and the distance to the substrate effects the time from
nebulization to reaction to deposition which can lead to particle
formation or increased diffusion time for larger clusters.
Additionally, electric and magnetic fields affect the growth habits
of some materials, or increase deposition efficiency. One of
ordinary skill in the art would recognize that such electric and
magnetic fields will affect the growth habits of some vapor
deposited materials, as well as vary the particular deposition rate
and efficiency.
[0085] Because the required energy input into the solution heating
atomizer varies for different
precursor/primary-solvent/secondary-solvent solutions, it is
preferred to deposit multilayer thin films from solutions with
constant primary to secondary solvent ratios. In so doing, it is
not necessary to change the energy input to the atomizer when
switching from one solution to another solution. The resulting
simplification of the setup produces increased performance and
reliability while reducing costs. Alternatively, the substrate can
be passed by flames containing different reagents to build the
desired multilayer.
[0086] A major difference between the use of flame fluids and
plasma fluids is that, often, the flame solution concentrations
must be related to the desired flame energy level. High solution
concentrations may lead to porous coatings or tube clogging. More
dilute solutions, higher vapor pressure precursors, higher
deposition temperatures and/or higher mobility and diffusion
deposition compounds require less solution fluid heating to achieve
dense coatings. Low vapor pressure precursors can form high quality
coatings, but the atomization and deposition parameters have less
variability than is possible for high vapor pressure reagents.
However, very low solution concentration yields unacceptably low
deposition rates.
[0087] When the solution provides the fuel for combustion,
concentrations up to 0.1 molar result in dense coatings depending
on the material. Most materials have preferred concentrations of up
to 0.01 molar. Materials with lower diffusion and mobility need
solution concentrations of less than 0.002. Solution concentrations
of less than 0.0001 molar result in very slow deposition rates for
most materials. Plasma torch and flame depositions with added
combustible materials can have higher concentrations, even
exceeding 1 M, but for the preferable vapor formation of the
precursors, high concentrations are less desirable unless the
precursor(s) have high vapor pressures. Low vapor pressure
precursor solution concentrations are preferably less than 0.002
molar.
[0088] Without wishing to be bound by theory, it is helpful to
understand that the principle of the deposition technique of the
present invention involves the finding that CVD its not limited to
reactions at the surface. See Hunt, A. T., "Combustion Chemical
Vapor Deposition, a Novel Thin Film Deposition Technique", Ph.D.
Thesis Georgia Inst. of Tech, Atlanta, Ga., (1993); Hunt, A. T.,
"Presubstrate Reaction CVD, and a Definition for Vapor", presented
at the 13th Int. Conf. on CVD, Los Angles, Calif. (1996), the
contents of which are hereby incorporated by this reference.
Reactions can occur predominately in the gas stream, but the
resulting material which is deposited must be subcritical in size
to yield a coating with vapor deposited microstructures. These
observations demonstrate that a vapor is composed of individual
atoms, molecules or nanoclusters which can be absorbed onto a
substrate and readily diffused into lower energy sites or
configurations. Thus the maximum cluster size must decrease with
lower substrate temperatures as does the critical nucleus size. It
is known by one of ordinary skill in the art that reagent clusters
are left after vaporization of the solvents, and the cluster size
is related to the reagent vapor pressure, initial droplet size and
the solution concentration. Therefore, atomization of low vapor
pressure reagents, which therefore do not vaporize in the flame,
must be very fine.
[0089] Preferred liquid solvents are low cost solvents include, but
are not limited to, ethanol, methanol, water, isopropanol and
toluene. Water solutions must be fed into a preexisting flame,
while the combustible solvents can themselves be used to form the
flame. It is preferable, but not required, to form the bulk of the
flame using the solution rather than feeding the solution into a
flame. Lower reagent concentration results this way, which eases
the formation of subcritical nucleus sized materials.
[0090] One preferred solvent and secondary solution fluid which is
propane, which is a gas at STP. However, it must be noted that many
other solvent systems are operable. See, e.g., CRC Handbook of
Chemistry and Physics. CRC Press, Boca Raton, Fla. Propane is
preferred because of its low cost, its commercial availability, and
its safety. Many low cost organometalics can be used in a
predominately propane solution. To ease handling, the initial
precursors can be dissolved in methanol, isopropanol, toluene or
other solvents compatible with propane. This initial solution is
then placed into a container into which liquid propane is added.
Propane is a liquid at above only about 100 psi at room
temperatures. The resulting solution has a much lower supercritical
point than the initial solution which eases atomization by lowering
the required energy input into the atomizer. Additionally, the
primary solvent acts to increase the polar solubility of the
propane, thus allowing higher solution concentrations for many
reagents than would otherwise be achieved by propane alone. As a
general rule, the polarity of the primary solvent should increase
with increasing polarity of the solute (precursor). Isopropanol can
thus aid in the solubility of a polar solute better than toluene.
In some cases the primary solvent acts as a shield between the
secondary solvent and a ligand on the solute. One example is the
dissolution of platinum-acetylacetonate
[Pt(CH.sub.3COCHCOCH..sub.3).sub.2] in propane, where the weight
ratios between precursor/primary solvent and primary
solvent/secondary solvent can be higher than those required in
other systems.
[0091] Ammonia has been considered and tested as a secondary
solvent for the deposition of coatings and powders. While ammonia
is an inexpensive solvent that is compatible with nitrite based
precursors, it is not easily usable with other secondary solvents
and problems stem from the general aggressiveness of pure ammonia.
The atomization properties of ammonia were tested without the
addition of a precursor and the used pressure vessel was
significantly attacked after the experiment even when an inert
Type-316 stainless steel vessel was used. In contrast to
hydrocarbon based solvents, ammonia also renders Buna-N and Viton
gaskets useless after only a few minutes. Even with a suitable
gasket material this is a problem since the desired coatings or
powders usually must not contain traces of iron washed from the
pressure vessel wall.
[0092] Other gas-like secondary solvents that were tested and can
be used include ethane, ethylene, ethane/ethylene mixture,
propane/ethylene mixture, and propane/ethane mixture. Ethane and
ethylene secondary solvents were used to coat sapphire single
crystal substrates with YSZ and YSZ-Alumina thin films resulting in
good quality dense filns. Platinum thin films were deposited from a
supercritical mixture of ethane and a platinum metalorganic. LCS
and PLZT thin films were deposited from supercritical ethane
mixtures. For these depositions, an unheated nozzle was used such
that the precursor solution underwent rapid expansion atomization
only. A large ID ("internal diameter") nozzle (compared to the
orifice) with a small orifice was used. Since the orifice is only
about 0.1 mm long (in the direction of the flowing precursor
solution), the pressure gradient across the restriction
approximates a discontinues transition. This sudden pressure drop
allows for an adiabatic expansion of the solvent. The processes
that take place in such a rapidly expanding non equilibrium systems
have been studied for various supercritical systems. See, e.g., C.
R. Yonker, S. L. Frey, D. R. Kalkwarf and R. D. Smith,
"Characterization of Supercritical Fluid Solvents Using
Solvatochromatic Shifts," J. Phys. Chem., 90, 3022 (1986); P. G.
Debenedetti, Homogeneous Nucleation in Supercritical Fluids", AIChE
J., 36, 1289 (1990); J. W. Tom and P. G. Debenedetti, "Particle
formation with Supercritical Fluids--A Review", J. Aerosol. Sci.,
22, 555 (1991).
[0093] Other tested solvents and solvent mixtures resulted in
similar quality, but were more complex to work with since their
boiling points are significantly lower, which required cooling of
the solution. The ease of handling makes propane the preferred
solvent but the other supercritical solvents are considered
alternatives to propane in cases where propane cannot be used, such
as when a precursor that is soluble in propane cannot be found.
Other fluids can be used to further reduce the supercritical
temperature if desired.
[0094] The propane fuel/solvent system was used to perform
deposition efficiency and deposition rate studies. Deposition
efficiencies of 17% were demonstrated for the deposition of fully
dense SiO.sub.2. Deposition rates of 1.mu.m/min were obtained for
dense SiO.sub.2.
[0095] During developmental experiments, it was established that
one heating method is the application of an electric current
between the nozzle end, where the precursor solution is injected
into the low pressure region, and the back of the restriction tube.
This directly heated restrictive tube method allows for fast
changes in atomization due to a short response time. The location
of most intense heating can be shifted toward the tip by increasing
the connection resistance between the tip and the electrical lead
connected to the tip. Thin walled restriction tubes possess a
larger resistance than thick walled tubes and decrease the response
time. Other heating methods can be applied and several have been
investigated, including but not limited to, remote resistive
heating, pilot flame heating, inductive heating and laser heating.
One of ordinary skill in the art could readily determine other
suitable heating means for regulating the temperature at the outlet
port of the atomizer.
[0096] Remote resistive heating uses a non-conducting restriction
tube that is located inside an electrically heated tube. The
non-conducting tube will fit tightly into the conductive tube.
Application of an electric current to the conductive type heats
that tube and energy is transferred into the inner, non-conductive
restriction tube. This method requires larger heating currents
compared to the directly-heated restrictive tube method and shows
longer response times, which can be advantages under certain
conditions since the increased response time results in a high
degree of thermal stability. On the other hand, pilot flame and
laser heating use the energy of the pilot flame or laser light,
respectively, to heat the restriction tube. This can be done in a
directly heated setup where the tip of the restriction tube is
subjected to the pilot flame or laser light or in an indirectly
heated configuration where the larger outer tube is heated. Because
the amount of energy that needs to be transferred into the solution
is quite large, the heated tube will, preferably, have a thicker
wall than in the case of direct electrical heating or remote
electrical heating. Subjecting an outer tube to the pilot flame or
laser light allows the use of a thin walled restriction tube.
[0097] Referring now to FIGS. 2 and 3, an apparatus 200 for the
deposition of films and powders using supercritical atomization is
shown. The apparatus 200 consists of a fixed or variable speed pump
1 that pumps the reagent transport solution 2 (also called
"precursor solution") from the solution container 3 into the
atomizer (also referred to as the "nebulizer" or "vaporizer") 4.
FIG. 3 is an inset view showing a more detailed schematic view of
the atomizer 4. The precursor solution 2 is pumped from the
precursor solution container 3 through lines 5 and filters 6 and
into the atomizer 4. The precursor solution 2 is then pumped into a
constant or variable temperature controlled restrictor 7. Heating
can be accomplished in many ways including, but not limited to,
resistive electrical heating, laser heating, inductive heating, or
flame heating. For resistive electrical heating, either AC or DC
current can be used. One of the electrical connections 8 to the
restrictor 7 is preferably placed very close to the tip of the
restrictor 7. In the case of heating by a DC source, this
connection 8 or pole can be either positive or negative. The other
pole 9 can be connected at any other point along the restrictor 7,
inside or outside the housing 10. For special applications such as
coating the inside of tubes, where a small total atomizer size is
advantageous, it is preferable to either connect to the restrictor
7 at the back of the housing 10 or to connect inside the housing
10. Gas connections at the back of the housing 10 are shown in an
on-line arrangement but can be placed in any other arrangement that
does not interfere with the function of the apparatus 200.
[0098] The thin gas A supply line 11, {fraction (1/16)}" ID in most
cases, carries a combustible gas mix to a small outlet 12 where it
can serve as a stable pilot flame, preferably within 2.5 cm of the
restrictor 7, for the combustion of the precursor solutions
supplied via the restrictor 7. Gas A supply is monitored by a flow
controller 13, controlling the flow of the individual gas A mix
components, 14 and 15. The gas A fuel component 14 is mixed with
the oxidizing component 15 in a mixing "T" 16 close to or inside
the atomizer 4. This late mixing is preferably for safety reasons
because it reduces potential flash-back. Distributions channels
inside the housing 10 connect the gas supply lines 11 to the gas A
feed 17. Gas B supply lines 18 are used to deliver gas B from the
supply 19 such that good mixing with the nebulized solutions spray
can be accomplished. In most cases a high velocity gas stream is
utilized. A number of gas B supply holes 20 (four for most cases,
more or less holes can be used depending on the particular
application) is placed around the restrictor 7 supplying gas B such
that the desired flow patter is obtained. The flow properties of
the gas B stream are influenced by such factors as gas B pressure
in the gas B storage container 21, flow rate as determined by the
flow controller 13, line diameters 5, and number of supply holes
20. Alternatively, gas B can be fed through a larger tube coaxial
to and surrounding the restrictor 7. Once the precursor solution 2
has been pumped into the precursor supply 22 its temperature is
controlled by the current flow (in the case of electrical heating)
through the restrictor 7 as determined by the power supply 23. This
heating current can then be adjusted such that the proper amount of
atomization (nebulization, vaporization) can occur. The stable
pilot flame is then capable of igniting the nebulized reactive
spray and depositing a powder or film on a substrate 24.
[0099] Many different coatings have been deposited using the
methods and apparatuses of the present invention. While propane was
used in most cases as the super critical secondary solvent (i.e. a
small amount of high precursor concentration primary solvent was
mixed with a large amount of secondary solvent), others solvents
have been used. Other possible secondary solvents include, but are
not limited to, ethylene, ethane, and ammonia. The following is a
list of materials deposited to date using the apparatus of FIG. 2
and only represents a few of the many possible materials which can
be deposited using the present invention. This list is not intended
to limit the scope of the present invention as set forth in the
claims:
[0100] METALS: Ag, Au, Cu, Ni, Pt and Rh;
[0101] OXIDES: Al.sub.2O.sub.3, 3Al..sub.2O.sub.3--2SiO.sub.2,
BaCeO.sub.3, BaTiO.sub.3, BST, Cr.sub.2O.sub.3 Cu.sub.2O, DLC,
In.sub.2O.sub.3, (K)--SiO.sub.2, LaPO.sub.4, LSC, LSM, MgO,
MnO--Pt, NiO, PbSO.sub.4, PdO, PLZT, PZT, RuO.sub.2, SiO.sub.2,
SnO.sub.2, SrLaAlO.sub.4 SrTiO.sub.3, TiO.sub.2,
YBa.sub.2Cu.sub.3O.sub.x, YIG, YSZ, YSZ-Alurina (ZTA),
Y.sub.2O.sub.3 and ZrO.sub.2;
[0102] OTHERS: BaCO.sub.3, LaPO.sub.4 and PbSO.sub.4
[0103] One of ordinary skill in the art would recognize that almost
any substrate can be coated by the method and apparatus of the
present invention. A substrate can be coated if it can withstand
the temperature and conditions of the resulting hot gases produced
during the process. Substrates can be cooled using a means for
cooling (described elsewhere herein), such as a water jet, but at
low substrate surface temperatures, dense or crystalline coatings
of many materials are not possible because of the associated low
diffusion rates. In addition, substrate stability in the hot gases
can be further accounted for by using a low temperature, low
pressure flame, either with or without additional substrate
cooling.
[0104] YSZ and Pt have been deposited as powders. Other materials
can be deposited as a film or powder with this technology. Thus, it
must be emphasized that the present invention is broad in
applicability and that the materials and substrates encompassed by
the present invention are not limited to the above-listed
materials.
[0105] With this in mind, examples using the preferred embodiments
of the above-described methods and apparatuses are set forth
hereinbelow. Other features of the invention will become apparent
from the following examples, which are for illustrative purposes
only and are not intended as a limitation upon the present
invention.
EXAMPLE I
[0106] To illustrate the coating deposition capability of the
process of the present invention, simple oxide coatings were formed
on a metal substrate. SiO.sub.2 was deposited onto water cooled
aluminum foil from a solution of tetraethoxysilane
[Si(OC.sub.2H.sub.5).sub.4] dissolved in isopropanol to 2.1 wt %
Si, additional isopropanol (3.2 ml) and propane (51 ml) were added
for an overall silicon concentration of 0.06 M. The gas temperature
for deposition was 1190.degree. C. The needle used to nebulize the
precursor, as seen in FIG. 3, was 304 stainless steel with OD=0.012
inches and ID=0.004 inches. The resistance over the electrical flow
length of the needle was about 1.6 W. Small pilot flames formed
from combusted ethane and oxygen were used throughout the
deposition to maintain the flame. The solution was pumped to the
needle at 3 ml/min and nebulized by controlling the amount of
current through the needle. In this example, the current was 2.65
A. The solution pressure from pumping during a deposition can vary
from one run to another and is not important as long as a minimum
pressure is maintained to ensure proper fluid properties. In this
case, the resulting pressure was 500 psi. Oxygen flowed around the
outside of the solution flame was measured through the flow meter
at a pressure of 30 psi and a flow rate of 4750 ml/min.
[0107] The aluminum foil substrate was cooled during the deposition
by a 80 psi generated air-water mist directed onto the substrate
side opposite to the side upon which deposition occurred. The
deposition rate for a coating applied at 1190.degree. C. for 48
seconds was approximately 1.mu.m/min and the deposition efficiency
was 16.6% (calculated by dividing the weight gain by the total
available precursor material as reacted silica). The amorphous
coating was dense and adherent on the substrate. Although the
surface of the coating was not as smooth as that achieved at lower
deposition rates, good thin film interference colors were observed.
No substrate oxidation occurred.
EXAMPLE II
[0108] In addition to coatings formed on metal substrates, such as
the oxide deposited on aluminum in Example I, coatings have also
been formed on plastic substrates. Platinum was deposited onto
Teflon at a gas temperature of 200 to 260.degree. C. from a 0.005M
solution of platinum-acetylacetonate
[Pt(CH.sub.3COCHCOCH.sub.3)..sub.2], toluene and methanol. The
deposition apparatus used was similar to that used for Example I,
except two separate pilot lights were used and the oxygen was
supplied via a coaxial tube surrounding the reagent solution. The
solution flow rate was 2 ml/min with a pressure of 1500 psi and a
needle current of approximately 3.3 A. The oxygen flowed at a
pressure of 20 psi and a rate of 4750 ml/min. The resulting
adherent film was smooth, dense and uniform. X-ray diffraction
("XRD") confirmed the formation of platinum with a (111) preferred
growth direction.
[0109] This example also illustrates that the coatings produced by
the process of the present invention are not exclusively oxides.
Platinum was deposited as a pure element.
EXAMPLE III
[0110] The coatings developed by the present invention are not
limited to formation on planar substrates. Films have been
deposited on ceramic fiber tows using the apparatus of the present
invention. LaPO.sub.4 was deposited onto an alumina fiber tow from
a solution of triethylphosphate [C.sub.2H..sub.5O.sub.3PO.sub.4]
dissolved in toluene to 1.7 wt % P, lanthanum 2-ethylhexanoate
dissolved in toluene to 1 wt % La, additional toluene (16 ml) and
propane (273 ml). The resulting solution had concentrations of
0.0010 M P and 0.0013M La. The solution flowed at a rate of 3
ml/min with a pressure of 410 psi during the deposition and was
nebulized with a needle current of 2.36 A. The flow rate of oxygen
to the solution flame was 4750 ml/min at a pressure of 30 psi.
[0111] The 400 fibers in the tow were coated at the same time. Each
fiber was approximately 12 mm in diameter. The tow was slowly moved
through the deposition zone of the flame two times. Only two passes
through the flame (where the tow was rotated 180 degrees about its
long axis for the second pass relative to the first pass) were
needed to produce a uniform coating over individual fibers in the
tow. The dense, columnar coatings produced at a 900.degree. C. gas
temperature ranged from 300 to 500 nm in thickness for more than
50% of the fibers in the tow. No excessive fiber degradation
resulted from the flame exposure. XRD confirmed that the coating
formed on the alumina fibers was monzanite, LAPO.sub.4.
[0112] This example also illustrates that the oxide coatings
produced by the CCVD process are not limited to binary oxides. The
LaPO.sub.4 included in this example was formed from a solution
containing two cation-containing precursors added in a ratio to
obtain a specific film stoichiometry. EDX compositional analysis
showed that the atomic percent of each cation deposited was as
desired at both 900 and 1000.degree. C.
EXAMPLE IV
[0113] Example III illustrated the ability of the present invention
to deposit coatings composed of more than one cation. Coatings from
a solution with up to five different cation-providing precursors
have also been deposited using an apparatus similar to that
described in Example I. Nickel-, aluminum- and strontium-doped
LaCrO.sub.3 coatings were produced from a solution containing
lanthanum nitrate [La(NO.sub.3).sub.3] dissolved in ethanol to
32.077 wt % La, chromium nitrate [Cr(NO.sub.3).sub.3] dissolved in
ethanol to 13 wt % Cr, strontium nitrate [Sr(NO.sub.3).sub.2]
dissolved in ethanol to 2 wt % Sr, nickel nitrate
[Ni(NO.sub.3).sub.2] dissolved in ethanol to 2 wt % Ni, aluminum
nitrate [Al(NO.sub.3).sub.3 dissolved in ethanol to 0.1 wt % Al,
ethanol (12 ml), isopropanol (25 ml) and water (5 ml). The
resultant solution had concentrations of 0.045M La, 0.040M Cr,
0.005M Sr, 0.005M Ni and 0.005M Al. The solution flowed at 2 ml/min
with a pressure of 5200 psi, while oxygen flowed to the solution
flame at a rate of 1600 ml/min and a pressure of 35 psi. The
coatings were deposited at 1150 to 1250.degree. C. for 16 minutes
onto fused silica substrates. EDX analysis of a coating revealed
cation ratios of approximately 1:26:5:5:63 for Al, Cr, Ni, Sr and
La, respectively. The successful coating deposition from precursors
such as the low vapor pressure nitrates illustrates that the
present invention is not limited to deposition from metal organic
reagents.
EXAMPLE V
[0114] The ability to coat different substrates by the process of
the present invention was illustrated in the previous examples.
Structural relationships of the deposited film with a substrate
have also been demonstrated using a similar process as described in
Example I. PLZT (Pb, La, Zr, Ti) was deposited onto single crystal
(100) MgO from a solution containing Pb 2-ethylhexanoate
[Pb(OOCCH(C.sub.2H.sub.5)C.sub.4H.sub.9).s- ub.2] dissolved in
toluene to 4 wt % Pb, tris (2,2,6,6-tetramethyl-3,5-hep-
tanedionato) lanthanum [La(C.sub.11H.sub.sub.19O.sub.2).sub.3]
dissolved in toluene to 0.28 wt % La, Zr 2-ethylhexanoate
[Zr(OOCCH(C.sub.2Hs)C.sub- .4H.sub.9).sub.4] dissolved in toluene
to 6 wt % Zr, Ti (IV) i-propoxide Ti[OCH(CH.sub.3).sub.2].sub.4
dissolved in toluene to 0.82 wt % Ti and all combined in toluene
(95.8 ml). The resultant solution had concentrations of 0.0023M La,
0.0012M Zr, 0.0010M Pb and 0.0003M La. The solution flowed at a
rate of 1.5 ml/min with a pressure of 2400 psi during the
deposition, while oxygen flowed at a rate of 1600 ml/min and a
pressure of 30 psi.
[0115] An XRD pole figure pattern of the (101) peak for PLZT
deposited onto MgO at 700.degree. C. for 16 minutes showed a high
degree of epitaxy of the PLZT on the substrate. There were no
intensities higher than 50 except at the four 44 degree Psi
locations which were each at 90.degree. Phi to each other. Other
than 3 minor peaks which were 0.005% of the maximum, there were no
additional peaks higher than 0.002% of the maximum. No additional
epitaxial peaks were present with the PLZT.
EXAMPLE VI
[0116] Multilayer coatings have also been produced by the process
of the invention using the apparatus described in Example I. A
twenty-two layer coating of alternating A15YSZ-16YSZ was produced
from two different solutions. The YSZ was deposited from a solution
of 0.51 g of Zr 2-ethylhexanoate
[Zr(OOCCH(C.sub.2H.sub.5)C.sub.4H.sub.9).sub.4] dissolved in
toluene to 6 wt % Zr and 0.80 g of Y 2-ethylhexanoate
[Y(OOCCH(C.sub.2H.sub.5)C.sub.4H.sub.9).sub.3] dissolved in toluene
to 0.69 wt % Y all combined in 150 ml of toluene. The resulting
concentrations were 0.0022 M Zr and 0.0004 M Y. The YSZ-Al solution
consisted of the two precursors listed above at 0.38 g and 0.18 g
respectively and 0.08 g of aluminum acetylacetonate
[Al(CH.sub.3COCHCOCH.sub.3).sub.3] dissolved in toluene to 0.1 wt %
Al and combined in 150 ml of toluene.
[0117] The coating was produced from a single flame, where the
solution fed to the flame was alternated every 3 minutes with a
line-cleansing flow of toluene between each solution for one
minute. The solution flow rate was kept constant at 3 ml/min for
both solutions and the current through the needle was consistently
3.3 A. The solution pressure was 1700 to 2000 psi. The oxygen fed
to the flame flowed at a rate of 4750 ml/min and a pressure of 20
psi. The coating deposition took 90 minutes and was performed at
gas temperature of 1100 to 1150.degree. C. The resultant multilayer
coating was smooth, dense and 1 mm thick with no observed cracking.
Each layer was approximately 40 nm thick.
EXAMPLE VII
[0118] The CCVD process also allows the production of powder. The
ease in which powder can be created was demonstrated by the
formation of YSZ powder on the first attempt to deposit that
material as a powder. The solution used consisted of Zr
2-ethylhexanoate dissolved in toluene to 6 wt %, Y 2-ethylhexanoate
dissolved in toluene to 0.69 wt %, toluene (6.9 ml) and propane
(136.7 ml). The concentrations of the zirconium and yttrium were
0.005M and 0.0003M, respectively. The current through the needle
during the deposition was 2.66 A. The temperature at which the
powder deposition occurred was 700.degree. C. and the time of
deposition was 32 minutes. The powder was deposited onto an
aluminum foil container that was filled with ice water. The water
helped to cool the deposition surface to a temperature much cooler
than that of the flame. The surface was warm enough, though, that
there was no formation of excessive moisture condensation on the
deposition area.
[0119] Once the deposition was complete, some of the powder that
collected on the container was scraped off the foil and analyzed by
transmission electron microscopy (TEM). Individual formed powder
grains could be differentiated from the large clusters of substrate
grains that resulted from the scraping process. EDX was used to
confirm the presence of Zr and Y. The powder grain size ranged from
approximately 2 to 10 nm with most of the grains being 4 to 6 nm.
In addition, the electron diffiaction patterns obtained from the
film were in the form of ring patterns, indicating that the powder
was crystalline. The rings of the patterns were smooth and
continuous as expected due to the small grain size. The d-spacings
for the material were calculated from the rings, and the values
matched the d-spacings expected for YSZ. However, because of the
limited number of rings available for indexing and the similarity
between d-spacing values for different types of zirconia with or
without yttria, the specific structure (hexagonal, tetragonal,
etc.) of the powder could not be determined from electron
diffraction. X-ray diffraction of the plane powder yielded similar
results, although a close match was made with an yttrium stabilized
zirconium oxide.
[0120] It will be apparent to those skilled in the art that various
modifications and variations can be made in the present invention
without departing from the scope or spirit of the invention. Other
embodiments of the invention will be apparent to those skilled in
the art from consideration of the specification and practice of the
invention disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with the true scope
and spirit of the invention being indicated by the following
claims.
* * * * *