U.S. patent application number 10/362869 was filed with the patent office on 2004-01-22 for high temperature amorphous composition based on aluminum phosphate.
Invention is credited to Sambasivan, Sankar, Steiner, Kimberly A..
Application Number | 20040011245 10/362869 |
Document ID | / |
Family ID | 24585143 |
Filed Date | 2004-01-22 |
United States Patent
Application |
20040011245 |
Kind Code |
A1 |
Sambasivan, Sankar ; et
al. |
January 22, 2004 |
High temperature amorphous composition based on aluminum
phosphate
Abstract
A composition providing thermal, corrosion, and oxidation
protection at high temperatures is based on a synthetic aluminum
phosphate, in which the molar content of aluminum is greater than
phosphorus. The composition is annealed and is metastable at
temperatures up to 1400.degree. C.
Inventors: |
Sambasivan, Sankar;
(Chicago, IL) ; Steiner, Kimberly A.; (Chicago,
IL) |
Correspondence
Address: |
REINHART BOERNER VAN DEUREN S.C.
ATTN: LINDA GABRIEL, DOCKET COORDINATOR
1000 NORTH WATER STREET
SUITE 2100
MILWAUKEE
WI
53202
US
|
Family ID: |
24585143 |
Appl. No.: |
10/362869 |
Filed: |
July 15, 2003 |
PCT Filed: |
August 20, 2001 |
PCT NO: |
PCT/US01/41790 |
Current U.S.
Class: |
106/14.12 ;
423/305; 427/372.2; 427/427; 427/430.1 |
Current CPC
Class: |
C04B 2235/44 20130101;
C04B 35/63488 20130101; C08K 3/32 20130101; C04B 2235/402 20130101;
C09D 7/48 20180101; C08K 3/22 20130101; C08K 7/04 20130101; C04B
2235/721 20130101; C04B 2235/3227 20130101; C04B 2235/5228
20130101; C04B 41/4584 20130101; C23C 22/74 20130101; C04B 35/62873
20130101; C04B 2235/3232 20130101; C23C 30/00 20130101; C04B
2235/9607 20130101; C04B 2235/3224 20130101; C04B 2235/77 20130101;
C23C 4/134 20160101; C04B 35/62655 20130101; C04B 2235/3217
20130101; C04B 35/62881 20130101; C01B 25/36 20130101; C04B 33/36
20130101; C23C 2/04 20130101; C23C 4/11 20160101; C04B 41/009
20130101; C04B 33/30 20130101; C04B 35/6303 20130101; C04B
2235/3418 20130101; C04B 2235/5436 20130101; C04B 35/447 20130101;
C03C 10/00 20130101; C04B 35/62268 20130101; C03C 3/17 20130101;
C04B 33/14 20130101; C03C 17/02 20130101; C04B 41/4584 20130101;
C04B 41/4535 20130101; C04B 41/5048 20130101; C04B 41/4584
20130101; C04B 41/5092 20130101; C04B 41/009 20130101; C04B 14/4625
20130101; C04B 41/009 20130101; C04B 14/4656 20130101 |
Class at
Publication: |
106/14.12 ;
423/305; 427/372.2; 427/430.1; 427/427 |
International
Class: |
C04B 009/02 |
Claims
1. A high temperature stable composition comprising aluminum
phosphate wherein the ratio of aluminum to phosphorus is greater
than one-to-one, said composition characterized by containing at
least 50 percent by weight of an amorphous content, said
composition being metastable of temperatures from ambient up to
1400.degree. C.
2. The composition of claim 1 additionally comprising a substrate,
said composition being a coating on said substrate.
3. The composition of claim 1 wherein said composition is in the
form of a fiber.
4. The composition of claim 2 wherein said coating protects said
substrate from oxidation at elevated temperatures.
5. The composition of claim 2 wherein said coating protects said
substrate from corrosion at elevated temperatures.
6. The composition of claim 1 comprising an additional metal.
7. An aluminum phosphate composition, said composition comprising
aluminum phosphate wherein the amount of aluminum relative to
phosphorus in said composition exceeds five percent, said
composition being metastable at temperatures up to 1400.degree.
C.
8. A method for protecting a substrate from corrosion and oxidation
at elevated temperatures, said method comprising the steps of
applying a precursor solution to said substrate, said precursor
solution comprising phosphorus pentoxide and an aluminum salt,
wherein the ratio of aluminum to phosphorus is greater than one to
one, and thereafter drying annealing said solution on said
substrate.
9. A metastable material comprising an aluminum phosphate
composition having the formula Al.sub.1+xPO.sub.4+3x/2, wherein x
is about 0 to about 1.5, said composition having structural
components absorbing radiation in the infra red spectrum at about
795 cm.sup.-1 to about 850 cm.sup.-1, said components present at
temperatures of at least about 1000.degree. C.
10. The material of claim 9 wherein x is about 0.
11. The material of claim 10 wherein x is about 0.1 to about
1.0.
12. The material of claim 11 wherein said material is substantially
amorphous.
13. The material of claim 11 wherein said material is metastable at
temperatures at least about 1200.degree. C.
14. The material of claim 11 further including crystalline
particles.
15. The material of claim 14 wherein said crystalline particles are
ErPO.sub.4.
16. A method of using an aluminum phosphate composition to enhance
substrate oxidation resistance, said method comprising: providing
an aluminum phosphate composition, said composition having the
formula Al.sub.1+xPO.sub.4+3x/2, wherein x is about 0 to about 1.5;
and applying said composition to a substrate.
17. The method of claim 16 wherein said composition is annealed
prior to said application.
18. The method of claim 16 wherein said composition is annealed
after said application.
19. The method of claim 16 wherein said composition is dip-coated
onto a substrate.
20. The method of claim 16 wherein said composition is
plasma-sprayed on a substrate.
21. The method of claim 16 wherein said composition is
aerosol-sprayed onto a substrate.
22. The method of claim 16 wherein said composition is a slurry in
a solution of a compositional precursor.
23. A method of using the aluminum content of an aluminum phosphate
material to affect the metastability of said material, said method
providing an aluminum phosphate material using an aluminum salt
precursor compound, said material having an aluminum content
corresponding to the aluminum content of said precursor compound,
said precursor compound and said material aluminum content
sufficient to provide a material metastability.
24. The method of claim 23 wherein said material aluminum content
is stoichoimetric.
25. The method of claim 23 wherein said material aluminum content
is greater than stoichoimetric.
26. The method of claim 23 further including a second component
selected from the group consisting of silicon, lanthanum and
zirconium.
27. An aluminum phosphate product having Al--O--Al structural
moieties absorbing radiation in the infra red spectrum at about 795
cm.sup.-1 to about 850 cm.sup.-1, said product obtainable by mixing
an alcoholic solution of phosphorus pentoxide with a solution of an
aluminum salt, and heating the admixture.
28. The product of claim 27 wherein said product is substantially
amorphous.
29. The product of claim 27 further including crystalline
particles.
30. The product of claim 29 wherein said particles are crystalline
ErPO.sub.4 inclusions prepared by incorporating an erbium salt with
said aluminum salt solution.
31. The product of claim 27 further including metal oxide
particles, said particles selected from the group consisting of
Group IIIA and IIIB-VIB metal oxides, said particles in an amount
sufficient to modify the thermal expansion coefficient of said
product.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to synthetic inorganic compositions
which remain metastable and possess other desired properties at mid
and high temperature, for example, from 800.degree. C. to
1400.degree. C. and greater.
[0002] It is known to use metal oxide coatings for high temperature
protection of substrates or other surfaces. Up to the present time,
however, there are no known synthetic oxides which can remain
amorphous and metastable at temperatures up to 1400.degree. C. or
greater. Silica, for example, is known to devitrify/crystallize at
temperatures slightly greater than 850.degree. C. Other non-oxide
materials, such as silicon oxy-carbide and silicon oxy-nitride
rapidly oxidize and form crystalline phases at high temperatures in
air.
[0003] Aluminum phosphate is a well known inorganic material that
has found many uses in applications including catalysts,
refractories, composites, phosphate bonded ceramics, and many
others. Aluminum phosphate has a low density (d=2.56 g/cm.sup.3).
It is chemically inert and stable at high temperatures, as well as
being chemically compatible with many metals and with most widely
used ceramic materials including silicon carbide, alumina, and
silica over a moderate range of temperatures.
[0004] Aluminum phosphate, however, is unsuitable for use as a high
temperature ceramic material because it undergoes polymorphic
transformations (quartz-type, tridymite and cristobalite) with
corresponding large molar volume changes. Thus, it would be
desirable to provide a synthetic form of aluminum phosphate which
is metastable and remains substantially amorphous at increasing
temperatures, or during heating and cooling cycles. Another
desirable property would be to provide an aluminum phosphate
composition having a low oxygen diffusivity at high temperatures or
in harsh environments, in order to provide oxidation protection and
corrosion resistance to substrates such as metals and ceramics.
SUMMARY OF THE INVENTION
[0005] The present invention relates generally to substantially
amorphous aluminum phosphate materials and/or compositions
exhibiting metastability and various other related properties under
high temperature conditions. In part, metastability can be
evidenced by retention of amorphous characteristics under oxidizing
conditions, such morphology and the extent thereof due at least in
part to the aluminum content of such materials/compositions, with
those having an overall stoichiometric excess of aluminum
exhibiting enhanced amorphous character and associated stability as
compared to their stoichiometric counterparts. Such properties and
related high-temperature attributes can be effected in the
preparation of such materials/compositions, primarily by admixture
of the aluminum precursor to the corresponding phosphate precursor
to initiate various structural and/or compositional modifications
which manifest themselves in the high temperature performance of
the resulting aluminum phosphate material/composition. In
particular, and as illustrated more clearly in the following
examples, aluminum can be identified upon addition to a phosphate
precursor and shown as contributing to the amorphous character and
associated metastability of the resulting aluminum phosphate
material/composition. Addition of a stoichiometric excess of
aluminum precursor enhances the resulting amorphous character and
metastability.
[0006] In part, the present invention is a metastable material
including an aluminum phosphate composition, such a composition as
can be represented having the formula Al.sub.1+xPO.sub.4+3x/2
wherein x is about 0 to about 1.5. The composition of such
materials can be characterized by structural/compositional
components absorbing radiation in the infra red spectrum at about
795 cm.sup.-1 to about 850 cm.sup.-1, and can be further
characterized by their presence at temperatures of at least about
1000.degree. C. Without regard to any particular material or
compositional phase, in various preferred embodiments of this
invention, as discussed more fully below, x is 0 or about 0. In
various other preferred embodiments, depending upon desired
performance properties of the metastable material and/or end use
application, x is about 0.1 to about 1.0. Generally, such materials
are substantially amorphous, the degree to which in part dependent
upon the value of x and the aluminum content of the entire
composition. Depending upon such content and morphology, such
materials are metastable at temperatures at least about
1200.degree. C. As illustrated below, in the following examples,
such materials can also include crystalline particles including but
not limited to CaWO.sub.4, Al.sub.2O.sub.3 and ErPO.sub.4, such
inclusions as can result from temperature treatment and/or
incorporation of suitable precursor components. Such inclusions can
be provided, as desired, to affect various material physical
characteristics and/or performance properties, including, but not
limited to, modification of the material thermal expansion
coefficient for a particular end use or composite fabrication.
[0007] In light of the above and in conjunction with the following
examples and detailed descriptions, the present invention can also
be a method of using the aluminum content of an aluminum phosphate
composition to affect and/or control the metastability thereof.
Such a method includes providing an aluminum phosphate composition
from or using an aluminum salt precursor compound. The resulting
aluminum phosphate composition has an aluminum content
corresponding to that of the precursor compound, such aluminum
content sufficient to provide a desired and/or predetermined
compositional metastability. As illustrated below, and as would be
understood to those skilled in the art, metastability of such a
material can be evidenced spectroscopically showing an amorphous
and/or non-crystalline aspect of the material. The material can
have a certain metastability with an aluminum content
stoichoimetric with respect to phosphate. Generally, the
metastability of such a material can be enhanced with a
stoichoimetric excess of aluminum. Aluminum content and resulting
stability can be effected upon preparation of the corresponding
precursor and admixture with a suitable phosphate precursor, as
illustrated elsewhere herein. Metastability and various other
optical, chemical and/or physical properties can benefit by
inclusion of other metal components including but not limited to
silicon, lanthanum and zirconium upon choice of and/or modification
of a suitable precursor.
[0008] Accordingly, the present invention also includes, in part, a
method for preparing a precursor solution for the formation of a
metal phosphate composition, preferably an aluminum phosphate
composition. Such a method includes preparation of a first solution
of a metal and/or aluminum salt; preparing a phosphorus component;
and admixing the solution and component. Typically, the phosphorus
component is an alcoholic solution of phosphorus pentoxide, but
other phosphorus components/phosphate precursors can be used with
comparable effect as described elsewhere herein. Likewise and
without limitation, the metal/aluminum component is provided in
alcoholic solution, with choice of solvent dependent upon
metal/aluminum solubility and compatibility with the corresponding
phosphorus/phosphate component. Among the various departures from
the prior art, this aspect of the present invention contemplates
use of a stoichiometric excess of the corresponding metal and/or
aluminum component in the preparation of such a precursor and use
thereof in the subsequent formation of the desired metal and/or
aluminum phosphate material/composition. As described more fully
elsewhere herein, preferred embodiments of this invention are
directed toward suitable aluminum salt components, precursors and
resulting materials/compositions, but various other metal
components can be incorporated into the precursor solution to
provide the resulting material/composition associated thermal,
optical, chemical and/or physical properties.
[0009] In part, the present invention also includes a method of
using an aluminum phosphate composition to enhance the oxidation
resistance of an associated substrate. The method includes (1)
providing an aluminum phosphate composition of this invention,
preferably one having the formula Al.sub.1+xPO.sub.4+3x/2, wherein
x is about 0 to about 1.5; and (2) applying the composition to a
suitable substrate. In various preferred embodiments, depending
upon end use application and/or fabrication technique, the
composition can be annealed either prior or subsequent to substrate
application. Regardless, as illustrated below, such a composition
can be dip-coated to provide a film on the substrate.
Alternatively, without limitation, the composition can be prepared
as a powder then either plasma--or aerosol--sprayed onto a
substrate.
[0010] In part, the present invention can also include an aluminum
phosphate product with aluminum-oxygen-aluminum structural
moieties, absorbing radiation in the infra red spectrum at about
795 cm.sup.-1 to about 850 cm.sup.-1. Such a product is obtainable
and/or can be produced by (1) mixing an alcoholic solution of
phosphorus pentoxide with a solution of an aluminum salt, the salt
either stoichiometric or in stoichiometric excess with respect to
the phosphorus precursor; and (2) heating the resulting admixture.
Such a product is substantially amorphous, but can also provide for
crystalline particulate inclusions, as discussed above. As a
representative embodiment, such particles are crystalline erbium
phosphate inclusions prepared by incorporating an erbium salt with
the aforementioned aluminum salt solution. Alternatively,
illustrating another aspect of this invention, the product can
include Group III A and/or Group III B-VI B metal oxide particles
in amounts sufficient to modify the thermal expansion coefficient
of the resulting product.
[0011] With regard to one or more aspects of the preceding
discussion, the present invention can include a new class of
phosphate compounds formulated to contain an excess amount of metal
species in the composition; that is, with reference to a preferred
embodiment, the aluminum atoms exceed the number of phosphorus
atoms found in stoichiometric aluminum phosphate. The excess can be
more than one percent and preferably greater than five percent.
[0012] Whether compositions of this invention are stoichiometric or
reflect an excess metal component methods for their preparation
include those disclosed in U.S. Pat. No. 6,036,762, the entirety of
which is incorporated herein by reference. In accordance therewith,
a precursor solution is formed from two liquid components. The
first component is a metal salt dissolved in alcohol. The second
component of phosphorus pentoxide is dissolved in alcohol. The two
components are then mixed together in the desired molar proportions
to provide a stable precursor solution, with the phosphate portion
at least partially esterfying to form a polymer-like structure
which uniformly entraps the metal ion.
[0013] The solution, as such, may be heated directly to remove the
alcohol portion and other species and provide a pure metal
phosphate. Preferably, however, the solution is applied as a
coating to a non-porous or porous substrate using any suitable
method, and the coated substrate is heated, typically to a
temperature of less than 600.degree. C., to obtain a uniform and
pure coating of the metal phosphate on the substrate.
[0014] A particular advantage of this approach is that the
precursor solution provides a substrate coating of even and uniform
thickness for substrate application. After initial heat treatment,
subsequent coatings may be applied to increase coating thickness.
This methodology is applicable to the formation of precursor
phosphate solutions containing more than one metal ion. The ability
to adjust the concentration of the composite solution over a wide
range is another distinct advantage, allowing for precise or
controlled amounts of metal phosphates to be formed.
[0015] Further and as directed more particularly to the present
invention, the aforementioned admixture/precursor solution can be
dried and then annealed, for example, at temperatures of up to
800.degree. C. or greater, in air. The annealing step is believed
to cause a transformation of the molecular structure, with the
final product being more than 50% amorphous in content, and with
the amorphous nature being sustained for long periods at
temperatures up to 1400.degree. C. or greater without oxidation.
Depending on the synthetic procedure and presence of other
components or additives, the composition may also contain small
crystalline inclusions which can impact other desirable properties,
such as toughness and optical activity. The composition exhibits
other desirable properties, such as very low oxygen diffusivity,
low thermal conductivity and high emissivity. Thus, a particularly
suitable application is to use the composition as a coating on a
substrate to minimize oxidation of the substrate at high
temperatures.
[0016] The initially formed organic solution can be converted into
any desired form. For example, the solution may be applied to a
metal, ceramic or other substrate, such as ceramic composites and
then annealed, or it may be converted into any desired shape, such
as fibers or filaments or in any other desired molded form, or may
be converted into a powder for application to substrates using a
suitable spray technique. Various other end use applications are
provided elsewhere, herein. Various materials/composites of this
invention are available under the Cerablak trademark from Applied
Thin Films, Inc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1. Uncoated and AlPO.sub.4 coated type 304 stainless
steel after 1000.degree. C. 100 h. The coated piece showed
remarkably little weight gain from oxidation (0.08-0.24%) compared
to the uncoated piece (4.5-8.6%).
[0018] FIG. 2. TEM micrographs of powders annealed to a)
1200.degree. C. 420 h b) 1300.degree. C. 100 h c) 1400.degree. C.
10 h d) electron diffraction 1200.degree. C. 100 h.
[0019] FIG. 3. TEM micrograph of a coating on a Nextel 720 fiber,
with an alumina overcoat, annealed 1200.degree. C. 2 h.
[0020] FIG. 4. XRD pattern of a stoichiometric material annealed to
a) 1100.degree. C. for 1 hour, b) 1100.degree. C. for 163 hours.
Note the splitting of the peak at 21.5, indicating the presence of
crystalline tridymite and cristobalite phases.
[0021] FIG. 5. a) XRD pattern of an aluminum phosphate material
(x=0.75) annealed at 1100.degree. C. for 1 hour, b) XRD pattern of
the same composition annealed to 1100.degree. C. for 163 hours.
Note the lack of differentiation of the tridymite peaks.
[0022] FIG. 6. Thermal expansion measurements for an aluminum
phosphate composition, in accordance with this invention.
[0023] FIG. 7. TEM micrograph and electron diffraction pattern of
AlPO.sub.4 coated Nextel 720 fiber annealed to 1200.degree. C. for
100 hours.
[0024] FIG. 8. TEM bright field image of AlPO.sub.4 nanocrystal
embedded in the amorphous matrix.
[0025] FIG. 9. TEM micrograph of Er-doped aluminum phosphate
annealed to 1000.degree. C. for 1 hour.
[0026] FIG. 10. a) Thermal conductivity of AlPO.sub.4 (lower line)
with YSZ, fused silica, mullite alumina and spinel. b) Thermal
conductivity of AlPO.sub.4 (lower line) and YSZ, a common thermal
barrier material.
[0027] FIG. 11. SEM micrograph of a cross-section of an AlPO.sub.4
fiber.
[0028] FIG. 12. X-ray diffraction patterns of a) powder annealed at
1100.degree. C., 1 hr and b) powder annealed at 1200.degree. C.,
500 hr, 10 atm, 15% steam (white).
[0029] FIG. 13. TEM micrograph of crushed white pellet of Example
31, showing nanocrystalline inclusions in an amorphous matrix and
electron diffraction pattern from different area of the same
sample.
[0030] FIG. 14. Raman spectra from AlPO.sub.4 annealed 1400.degree.
C., 1 hr; a) black area, and b) white area from the same
material/composition sample.
[0031] FIG. 15. .sup.31P NMR of phosphorus pentoxide dissolved in
ethanol. a) soon after dissolution b) after 24 hours reflux.
[0032] FIG. 16. Liquid .sup.31P NMR spectra of admixed precursor
solutions, showing the effect of aluminum addition. a) full
spectrum b) plot showing peaks reflecting the presence of
aluminum.
[0033] FIG. 17. X-ray diffraction patterns of the solutions shown
in FIG. 16 after anneal to 1100.degree. C., 160 hrs. a) full scale
b) zoom to show the differences in the peaks near 21.degree..
[0034] FIG. 18. FTIR of stoichiometric and non-stoichiometric
compositions annealed to 1100.degree. C. for 1 hr.
[0035] FIG. 19. Deconvolution of .sup.27Al NMR spectrum of
stoichiometric AlPO.sub.4 annealed to 1100.degree. C., 1 hr.
[0036] FIG. 20. Deconvolution of .sup.27Al MAS NMR of an excess
aluminum composition Al/P=2 (1-fold excess, x=1.0) after anneal at
1100.degree. C., 160 h.
DETAILED DESCRIPTION OF SEVERAL PREFERRED EMBODIMENTS
[0037] As discussed above, the present invention relates to a new
class of metastable high-temperature amorphous inorganic
compositions. A unique amorphous structure can be derived using a
simple, low-cost sol-gel precursor. The thermal stability of the
amorphous material is primarily controlled by metal content of the
corresponding precursor, aluminum in preferred embodiments. Several
compositions have been synthesized in amorphous form and shown to
be stable for hundreds of hours above 1200.degree. C. Most
crystalline materials of the prior art synthesized using sol-gel
routes undergo amorphous to crystalline transition below
1000.degree. C. In this case, however, and with aluminum phosphate
as a generic example, thermodynamic equilibration to stable
crystalline alumina and AlPO.sub.4 phases does not occur until
annealing above 1500.degree. C. Calorimetric measurements revealed
highly exothermic dissolution behavior suggesting the material to
be thermodynamically unstable or metastable. Extremely low oxygen
diffusivity in the amorphous material, which may be attributed to a
special "Al--O--P" cluster, appears to be dominating the sluggish
kinetics. Hermetically dense and adherent thin films (1000 .ANG. or
less) deposited by a simple dip coating process demonstrate
remarkable ability of the material to protect stainless steel from
oxidation at 1000.degree. C. (see, FIG. 1 and Example 2,
below).
[0038] When prepared as a film or coating, the material tends to
remain completely amorphous whereas powders derived therefrom
contain amorphous material with minor amounts, up to about 20-30%,
of nanocrystalline inclusions (varying in size from 5-60 nm) of
stoichiometric aluminum phosphate (FIGS. 2 and 3). Likewise, as
described elsewhere herein, the amorphous content and presence of
nanocrystalline inclusions can also be affected by the
stoichiometry of an aluminum precursor, with the use of a
stoichiometric excess thereof reducing the incidence of such
inclusions, increasing amorphous content and enhancing the
metastability of the entire material/composition. Several
properties characterizing such compositions of this invention are
provided in Table 1, below.
1TABLE I Selected Illustrative Properties Oxygen Diffusivity
.about.1 .times. 10.sup.-12 cm.sup.2/sec (chemical diffusivity @
1400.degree. C.) Thermal expansion 5 .times. 10.sup.-6 K.sup.-1
Thermal conductivity 1.0-1.5 W/mK (RT-1300.degree. C.) Dielectric
constant 3.3-6.35 (for x = 0.5-0.75)
[0039] The very low oxygen diffusivity allows for the use of
extremely thin amorphous protective coatings (50-100 nm) where
cracking due to thermal stresses is less of a concern. This unique
property can be exploited to provide protection for many metals and
ceramics used in high temperature applications. Formation of
nanocrystalline glass-ceramic composites may also provide the
opportunity to tailor physical, thermal, mechanical, and optical
properties for a number of applications. The materials/compositions
of this invention can be formed as a continuous film or as a powder
(that can be plasma sprayed) or in a near-net shaped consolidated
form. Some of the potential applications include oxidation and
corrosion protection (low oxygen diffusivity and chemical
durability), thermal protection for aerospace and space vehicles
(high emissivity, low thermal conductivity, and low oxygen
diffusivity), low observable thermally stable coatings (low
dielectric constant), protection against molten metal (non-wetting
character), interface coatings (non-wetting) and matrices (high
strength and ease of fabrication) for ceramic matrix composites
(CMCs).
[0040] A preferred method for making the composition of the present
invention is described in the aforementioned U.S. Pat. No.
6,036,762. An aluminum salt, such as aluminum nitrate having water
of hydration is dissolved in an organic solvent, preferably an
alcohol such as ethanol. A quantity of phosphorus pentoxide
(P.sub.2O.sub.5) is dissolved in a separate container in the same
solvent. The molar ratio of Al to P in the Al solution is greater
than a one-to-one ratio with phosphorus and is preferably at least
1% and most preferably at least 5% greater. The upper practical
limit of excess aluminum has not been determined, but compositions
containing ten times excess aluminum have been prepared, and a 1.5
to 3.5 excess molar ratio appears to be most promising in terms of
retaining the amorphous content at high temperatures.
[0041] More generally, as applicable to broader aspects of this
invention, this synthetic approach provides a metal phosphate
precursor solution from two separate liquid components using a
common organic solvent. While a variety of organic solvents may be
potentially useful, liquid alcohols are preferred, such as methanol
or ethanol, with ethanol being most preferred. Accordingly, a first
component of the precursor solution is prepared from a metal salt
dissolved in alcohol such as ethanol. A mixture of salts of
different metals may be employed. Nitrates, chlorides, acetates or
any salt of metal soluble in alcoholic media may be used. The
choice of metal salt and/or alcohol is limited only by associated
solubility considerations.
[0042] The salt of any metal may be employed in the first
component. For the preparation of coatings for use in high
temperature reactive environments, reference is made to U.S. Pat.
No. 5,665,463, incorporated herein by reference. The metal salt may
comprise a monazite having the general formula MPO.sub.4, where M
is selected from the larger trivalent rare earth elements or the
lanthanide series (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd and Th).
Xenotimes as described in the above patent can also be prepared.
Other di- and tri-valent metals such as aluminum are especially
suitable.
[0043] The second component of the precursor solution is phosphorus
pentoxide (P.sub.2O.sub.5) dissolved in the same solvent such as
ethanol. Without limitation, there is believed a controlled
reactivity between the alcohol and phosphorus pentoxide in which
phosphate esters are produced. The esterification process
continues, forming ester chains while the solution ages, and the
solution becomes sufficiently polymeric such that good film forming
properties are attained. The metal salt solution is preferably
added to the phosphorus pentoxide solution shortly after
preparation of the latter and before extensive esterification
occurs.
[0044] The precursor solution can be prepared at a variety of
concentrations, depending on the desired film microstructure. For
example, using lanthanum nitrate, a solution providing up to 160
grams per liter yield of lanthanum phosphate can be obtained. The
metal salt and the phosphate are provided in the mixture in either
stoichiometric proportions or with an excess of metal salt to yield
the desired metal phosphate.
[0045] As described more fully elsewhere, herein, the solution
comprising the two components is shelf stable and can be converted
to metal phosphate by heating. Since the solution has good wetting
and coating properties, however, the preferred method of employment
is as a coating on porous and non-porous substrates. For example,
lanthanum phosphate has substantial utility as a coating on ceramic
fibers, fabrics or in other structures used at high temperatures,
i.e., in excess of 1200.degree. C. The phosphate coating allows for
increased toughness for the composite as referred in U.S. Pat. No.
5,665,463. The solution may be applied as a coating on non-porous
materials such as metals and metal alloys.
[0046] Upon pyrolysis of the precursor coated substrate, much of
the solvent evaporates at a relatively low temperature, leaving a
continuous film of residual precursor material on the substrate.
Upon additional heating, all species except for metal and phosphate
are removed, leaving a coating of the metal phosphate. The
temperature to which heating is required may be evaluated by
differential thermal analysis. For the LaPO.sub.4 precursor,
heating to a temperature of 600.degree. C. for a brief period
assures total conversion. X-ray diffraction of the film obtained
confirmed the formation of a single phase lanthanum phosphate.
Scanning electron microscopy analysis of the film showed it to be
smooth, uniform, continuous and stoichiometric. The use of a
volatile solvent system allows the metal phosphate to form at
relatively low temperatures.
[0047] The precursor liquid can be coated onto a suitable
substrate, such as a metal or alloy or ceramic or mixed with
particles of ceramic material requiring oxidation and/or corrosion
protection. In addition, the liquid can be drawn into fibers,
placed in a mold, or used alone. Regardless, the liquid is
converted into solid, stable form by annealing or pyrolysis in air.
Typically, for aluminum compositions, this requires heating to
temperatures normally above 750.degree. C. for a period of time,
for example, for one hour, or at higher temperatures. Complete
annealing becomes evident when the composition assumes a black or
dark grey color.
[0048] With regard to at least the aluminum phosphate compositions
of this invention, it is believed that the decomposition behavior
of organic based precursor at least partially controls the
molecular events leading to a unique inorganic compound. The
material contains in excess of 50% of an amorphous compound and may
also contain nanocrystals. The material remains amorphous and
metastable when heated to temperatures from ambient and up to
1400.degree. C. or greater for extended period of time. It is
believed that increased storage time of the precursor solution
increases amorphous content.
[0049] Based on initial observations, it has been found that the
amorphous content of the annealed composition of the present
invention may be influenced by at least two factors, namely,
application and age of the precursor solution. As an example of the
first effect, coatings of solution applied on fibrous substrates
appear to be substantially completely amorphous even after
annealing at 1200.degree. C. for two hours. This has been initially
confirmed by TEM analysis of solution coated and annealed on
mullite-alumina fibers with an overcoat of alumina. On the other
hand, powders synthesized in alumina crucibles at 1000.degree. C.
for 30 minutes contain a significant fraction of AlPO.sub.4
crystallites. Aging of the precursor solution appears to have a
significant effect on the phosphorus environment in the precursor
as well as the amorphous content in the pyrolyzed product. Storage
of the solution in a refrigerator for a period of up to two years
or at room temperature for over one month tends to yield more pure
amorphous content.
[0050] Of the AlPO.sub.4 samples tested, the composition/material
had a low density in the order of 1.99 to 2.25 g/cm.sup.3, in
comparison with 3.96 g/cm.sup.3 for alumina. The composition the
chemical diffusivity was in the order of
1.times.10.sup.-12cm.sup.2/sec at 1400.degree. C. The material also
exhibits a high emissivity, potentially useful in thermal
protection systems, such as space applications. Thermal
conductivity has been measured at 1 to 1.5 W/m.k. The material is
inert in various harsh environments, and has a non-wetting
character to most materials, including molten aluminum and solid
oxides. Coatings as thin as 0.25 microns are capable of protecting
metallic and other surfaces.
[0051] Potential applications include thermal, corrosion and
oxidation protection for metals and metal/ceramic-based thermal
protection systems, high emissivity coatings, interface coatings
for silicon carbide and oxide based ceramic matrix systems,
environmental barrier coatings for metal and ceramic based systems,
fibers for composites and fiber lasers, corrosion protection in
molten metal processing, monolithic materials for thermal
insulation, catalyst supports, as well as many others. The material
may also possess a low dielectric constant, making it useful in
Radome applications.
EXAMPLES OF THE INVENTION
Example 1
[0052] To make 850 mL of 75.46 g/L a precursor solution to
synthesize the amorphous aluminum phosphate material with a 1.75:1
Al:P ratio (0.375 molar excess Al.sub.2O.sub.3), 408.94 g
Al(NO.sub.3).sub.39H.sub.2O was dissolved in 382 ml ethanol to make
500 ml of solution. In a separate container in an inert atmosphere,
25.23 g P.sub.2O.sub.5 was dissolved in 300 ml ethanol. After the
P.sub.2O.sub.5 is dissolved, the two solutions were mixed together
and allowed to stir for several minutes. After the solution was
thoroughly mixed, it was placed in a large container in an oven at
150.degree. C. for one or more hours. After the resulting powder is
completely dried, it was annealed in air to 1100.degree. C. for one
hour to form amorphous aluminum phosphate powder with 0.75 moles
excess aluminum per mole aluminum phosphate.
Example 2
[0053] To form an oxidation resistant amorphous aluminum phosphate
coating on a rectangular coupon of 304 stainless steel, the piece
was dipped in the precursor solution of Example 1, diluted to a
certain concentration and removed. The sample was dried in flowing
air to remove the solvent. The sample was dried more thoroughly in
an oven at 65.degree. C. The piece was annealed in air to
1000.degree. C. (at a ramp rate of 10C./minute) for 100 hours and
cooled to room temperature at 10C/minute, along with an uncoated
piece of 304 stainless steel of the same size and shape. The weight
of each uncoated piece was measured prior to anneal. The weight was
measured again after coating and anneal. The amorphous aluminum
phosphate coated piece showed remarkably less weight gain. The
weight gain data is given in the table below.
2TABLE I Weight gain of uncoated, and coated (AlPO.sub.4, with 75%
excess Al) stainless steel coupons annealed to 1000.degree. C. in
air. The weight gain is related to the weight of the annealed,
uncoated coupon. Original Weight after Weight % Weight Sample
weight (g) anneal (g) gained (g) gained Amorphous 20.3727 20.4207
0.048 0.24% aluminum phosphate (incl. coating) Uncoated 20.6303
22.4123 1.782 8.64%
Example 3
[0054] To form an amorphous aluminum phosphate coating on a solid
substrate by plasma spray, amorphous aluminum phosphate powder made
in Example 1 is milled to a small and uniform size (around 20
microns) in a ball mill. The powder is then deposited using the
small particle plasma spray process (see, U.S. Pat. No. 5,744,777
incorporated herein by reference in its entirety).
Example 4
[0055] Bulk amorphous aluminum phosphate is formed by
electroconsolidation (U.S. Pat. No. 5,348,694). Finely ground
amorphous aluminum phosphate powder was mixed with a binder (1 wt %
PEG 8000 and 2 wt % PEG 20M) and then pressed into a pellet. This
pellet was pre-sintered at 1200.degree. C. for five hours. The
pellet was then electroconsolidated at 1300.degree. C. for 30
minutes. The final pellet had a density of 1.99 g/cm.sup.3.
Example 5
[0056] Amorphous aluminum phosphate fibers were made from viscous
polymer formed from the precursor solution of Example 1. The
AlPO.sub.4 solution was dried at 50-65.degree. C. until 40-30% of
the weight is retained. The residue had a mainly clear, glassy
appearance with a high viscosity. Green fibers were pulled with a
needle, inserted into the viscous residue and quickly removed. The
fibers were dried immediately in flowing air at 650.degree. F. The
green fibers were then annealed to at least 900.degree. C. to form
amorphous aluminum phosphate fibers.
Example 6
[0057] Rare earth and other metal ions can be incorporated into the
amorphous aluminum phosphate structure. An erbium doped precursor
solution with 0.75 moles excess metal (aluminum and erbium) of
which 5 mol % is erbium was synthesized in a manner similar to the
amorphous aluminum phosphate solution of Example 1. 31.2 g Al
(NO.sub.3).sub.3 9H.sub.2O was dissolved in 75 ml ethanol. In an
inert atmosphere glove box in a separate container, 1.94 g
Er(NO.sub.3).sub.3 5H.sub.2O was dissolved in 20 ml ethanol. The
erbium nitrate solution was added to the aluminum nitrate solution
and left to stir for several minutes. In a separate container in an
inert atmosphere glove box, 3.55 g P.sub.2O.sub.5 was dissolved in
40 mL ethanol. After the P.sub.2O.sub.5 was dissolved, the aluminum
nitrate and erbium solution was added and left to stir for several
minutes. The solution was then dried at 150.degree. C. for about an
hour and annealed to 1000.degree. C. for one hour. X-ray
diffraction of this material annealed to 1000.degree. C. for one
hour confirms the amorphous structure, with no erbium phosphate
crystalline.
Example 7
[0058] FIG. 4 is a typical x-ray diffraction pattern (XRD) pattern
obtained from stoichiometric aluminum phosphate synthesized from an
ethanolic precursor solution containing nominal equimolar amounts
of aluminum nitrate nonahydrate and phosphorus pentoxide. The
solution was dried and the powder obtained was calcined to
1100.degree. C. in air for one hour and is "jet" black in color. It
is immediately evident from the pattern that the material is not
fully crystalline and may contain a significant amount of
crystalline disorder or amorphous content. Closer examination of
the broad peaks reveals the presence of disordered tridymite and
cristobalite forms of AlPO.sub.4. Further annealing of this
material for longer times in air (1100.degree. C., 163 hours) does
induce significant crystallization as seen in FIG. 4 where the
tridymite peaks are much better defined, and the cristobalite peak
is separate from the main tridymite peak.
[0059] In contrast, FIG. 5 shows the XRD pattern of aluminum
phosphate synthesized with excess aluminum (x=0.75, 75% molar
excess) in the precursor solution. The striking difference between
the patterns in FIGS. 4 and 5 is immediately apparent: the
diffraction pattern for the material with excess aluminum retains
broad, low-intensity peaks indicative of a large degree of
non-crystalline amorphous structure and enhanced metastability.
[0060] Without restriction to any one theory or mode of operation,
the precursor design, along with excess aluminum, is believed a
factor in the preparation of the present compositions. The
poly-esterification of P.sub.2O.sub.5 by ethanol and hydrolysis
controls the chemistry of clusters in liquid during which time a
sequence of molecular events occur yielding unique spatial
coordinations between P, Al, O, and --OH which is preserved through
gelation and calcination. Synthesis of AlPO.sub.4 with excess
aluminum significantly enhances the thermal stability of the
resulting material/composition.
Example 8
[0061] The addition of excess aluminum to the precursor solution
results in the presence of a substantial number of coordinations
other than regular tetrahedral coordinations, including but not
limited to distorted octahedrally coordinated aluminum, in the
pyrolyzed product. Crystalline aluminum phosphate of the prior art
consists of tetrahedrally coordinated aluminum and phosphorus, but
.sup.27Al MAS NMR of the aluminum phosphate materials/compositions
of this invention shows the presence of both 4- and 6-fold aluminum
(see, also Examples 35a and 35b, and FIGS. 19 and 20, below)
consistent with the metastability exhibited therewith.
Example 9
[0062] Thermal expansion of an electroconsolidated aluminum
phosphate pellet was measured by dilatometry from room temperature
to 1100.degree. C. (FIG. 6). The thermal expansion coefficient is
considerably lower than steel which has a thermal expansion
coefficient around 13.times.10.sup.-6/K. But, as evident from
stainless steel coating experiments, very thin coatings of such
materials are able to withstand the thermal expansion mismatch and
remain adherent and crack free even after heating to 1000.degree.
C. and returning to room temperature.
Example 10
[0063] TEM analysis of a 50 nm coating on Nextel 720
alumina/mullite fiber annealed to 1200.degree. C. for 100 hours
shows that the aluminum phosphate composition of this invention has
remained completely amorphous (FIG. 7). No nanocrystalline
inclusions are evident.
Example 11
[0064] Nickel-based superalloys are frequently used in high
temperature applications such as turbine blades. However, oxidation
at high use temperatures is still a problem. Ni-based superalloy
pieces were coated with an AlPO.sub.4 material of this invention to
substantially reduce the kinetics of alumina scale growth and
spallation, demonstrating this invention dramatically reduces
high-temperature oxidation.
Example 12
[0065] Aluminum phosphate powders synthesized as described herein
can contain nanocrystalline inclusions embedded in the amorphous
matrix, in contrast to completely amorphous material obtained as a
thin film. TEM examination of annealed powders showed two distinct
types of material. After 1 hour anneal at 1100.degree. C., about
20-30% of the powder sample contained isolated aluminum phosphate
crystallites. Most of the sample, however, contained an
amorphous/glassy matrix with nm-sized crystalline inclusions that
ranged from 5-30 nm (FIG. 8) and well dispersed. TEM studies of
powder annealed to 1300.degree. C. for 100 hours shows that the
overall fraction of nanocrystals in the material is essentially the
same, with grain size increasing slightly (25-60 nm).
Example 13
[0066] Similar results were obtained with ErPO.sub.4
nanocrystallites in a matrix. 5 mol % Er-doped powders were
prepared as described elsewhere, herein. TEM analysis of Er-doped
material annealed to 1000.degree. C. for 1 hour shows an increase
in nanocrystalline fraction over the undoped material (FIG. 9). EDS
confirms that Er is present in these nanocrystals. XRD analysis of
Er-doped material annealed to 1100.degree. C. for 1 hour shows
definite ErPO.sub.4 peaks.
Example 14
[0067] The compositions of this invention have a low thermal
conductivity (1.0-1.5 W/mK)--lower than yttria-stabilized zirconia,
a commonly used thermal barrier coating material (FIG. 10). Such
materials therefore show potential as providing both an
environmental and thermal barrier simultaneously, and as can be
achieved in application of in one coating. Thermal barrier coatings
are frequently plasma sprayed, which involved partial melting of
the powder. An AlPO.sub.4 powder has been plasma sprayed onto steel
and cast iron, and the XRD pattern does not indicate any changes in
the structure.
Example 15
[0068] An AlPO.sub.4 composition of this example is prepared as
smooth, dense amorphous ceramic fibers, having high strength and
high creep resistance in the absence of grain boundaries where
flaws form easily. A high strength, creep resistant fiber stable to
high temperatures would have great structural potential. (See, FIG.
11 and several other examples, below.)
Example 16
[0069] The aluminum phosphate materials/compositions of this
invention are non-wetting and non-bonding. With thermal stabilities
up to 1400.degree. C., they may provide a high-temperature
substitute for Teflon.RTM.-like non-stick coatings in applications
ranging from cookware to engine components.
Example 17
[0070] A slurry of fine particles in solution can be applied by
aerosol spraying onto a substrate. Accordingly, a slurry of
AlPO.sub.4 powder (average particle size=16 microns) is mixed into
an AlPO.sub.4 solution in a ratio of 5 g powder/100 mL solution.
This slurry is aerosol sprayed onto a heated stainless steel
coupon. The result is a coating of AlPO.sub.4 particles embedded in
an AlPO.sub.4 coating. The coating adheres well to the surface of
the steel.
Example 18
[0071] A coating of a composition/material of this invention is
obtained by chemical vapor deposition (CVD). CVD coatings can be
deposited at low temperature, thereby creating an amorphous
coating. CVD also allows for good stoichiometry control.
Accordingly, aluminum acetylacetonate and trimethyl phosphate are
dissolved in toluene. This solution is placed in a liquid-delivery
assisted CVD system. This liquid precursor will allow for careful
mixing and stoichiometric control. The solution is transferred into
a flash evaporator, where it is vaporized. This vapor enters into
the reactor, and reacts and deposits as a solid on the
substrate.
Example 19
[0072] Another route for the preparation of compositions of this
invention can be through a reaction of a solid with a liquid
phosphorus source (phosphoric acid, phosphorus pentoxide solutions,
etc.). The solid may contain aluminum, which will promote the
formation of amorphous aluminum phosphate. Accordingly, a solid
containing a small amount of aluminum is dipped in phosphoric acid.
When this solid is heated above 800.degree. C., the phosphorus on
the surface reacts with the small amount of aluminum to form
amorphous aluminum phosphate.
Example 20
[0073] Composite coatings of this invention can be deposited on a
substrate. Solid particles are added to an AlPO.sub.4 solution to
form a slurry. This composite coating can be deposited on a
substrate by dip coating, applying with a brush, aerosol spraying,
etc. When this coating is fully formed under a heat lamp or in a
furnace, a composite coating, containing particles embedded in an
AlPO.sub.4 coating is produced. The particles can be of any
composition.
Example 21
[0074] Glasses are susceptible to corrosion in a variety of
atmospheres, from distilled water to humid air. Sodium silicate
glass is a common glass and is very susceptible to corrosion
whether the glass is immersed in liquid, being rained on, or simply
being stored in a humid warehouse. Glass containers are susceptible
to corrosion from the liquid they are containing. Water and acidic
and basic media promote corrosion of glass. Glass windows are
subject to corrosion when it rains. Glass that is being stored is
subject to localized pitting as atmospheric humidity deposits as
drops on the surface.
[0075] Through an ion exchange reaction with hydrogen ions, the
sodium ions dissolve into the surrounding water. The hydroxyl
content in the water dissolves the silica as well, but this process
is much slower.
[0076] Several methods of combating glass corrosion are used. Some
common methods of increasing durability of commercially produced
glasses include: the addition of other components to the melt and
forming a protective coating. CaO, Al.sub.2O.sub.3 and MgO are
commonly added to the sodium silicate melt to retard leaching of
the sodium. Surface coatings are provided by treatment with
SO.sub.2 gas to form sodium sulfate and annealing the glass in a
trace fluorine atmosphere.
[0077] The compositions/materials of this invention provide a
transparent coating on glass. The coating goes down very smoothly,
and the coating is only readily visible in areas where it has been
disturbed (such as where it was held during dip-coating). Such
coatings could be used to increase chemical durability without
decreasing transparency. This invention could provide a protective
layer, which limits the transport of hydrogen or hydroxyls to the
glass surface, and the transport of the corrosion products out of
the glass.
Example 22
[0078] Aqueous solutions of this invention are desired as a
non-flammable, non-toxic alternative to their alcoholic
counterparts. Aqueous solution does not require special hazardous
labeling during shipping, does not require the large amount of
ventilation in the workplace, and is more attractive to
manufacturers accustomed to working with aqueous systems and
processes. Dried AlPO.sub.4 gel was produced by heating solution at
100C. in a convection oven. This dried gel is white and fluffy.
This gel was dissolved into deionized water. The gel goes into
solution easily, forming a viscous, yellowish solution. When
annealed to 1100.degree. C. for 1 hr. the XRD pattern shows the
typical aluminum phosphate diffraction pattern. When annealed to
1000.degree. C. for 1/2 hr, the XRD pattern showed an amorphous
hump, that is typical of an aged composition/material. The powder
is jet black and glassy in appearance.
[0079] An aqueous solution can be made much more concentrated: up
to 25% by weight AlPO.sub.4 versus 10-15 wt % in ethanol. This
solution was coated onto a glass slide by standard dip coating. A
concern with aqueous solutions is that the film formation
characteristics are different and preparation of a continuous,
smooth film may be more difficult than through use of an alcoholic
solution.
Example 23
[0080] With reference to the preceding discussion regarding
precursor solutions, a viscous, clear liquid can be prepared from
which fibers can be pulled by inserting and retracting a needle.
The fiber precursor is made by concentrating precursor solutions in
a rotary evaporator to approximately 30 wt % concentration. The
fiber precursor can be difficult to prepare at the risk of becoming
too concentrated. The fiber precursor is unstable on its own. After
concentration, a clear liquid is left. This liquid is stable for 10
minutes--hours, but will eventually spontaneously decompose in a
strong exothermic reaction. The resulting fiber precursor can be
used, but there is generally a lot of foam. However, if the
solution is put in a water bath immediately after removing it from
the rotary evaporator, it keeps the decomposition from proceeding
so violently, and a clear, slightly yellow liquid results.
Accordingly, 100 mL of 9.1 wt % solution is condensed to 40 mL in a
rotary evaporator. The temperature was 60.degree. C., and the
pressure was varied to keep the ethanol evaporating. After the
solution was concentrated, it was poured into a jar and kept in a
water bath to sit. After 15 minutes, the decomposition started, and
clear, viscous, yellow liquid remained.
Example 24
[0081] Intended fiber applications include a) structural ceramic
fibers used in ceramic matrix composites, metal matrix
composites--currently, SiC and a variety of oxide fibers are being
developed, b) fiber-optic amplifiers, and c) fiber lasers. Fibers
have been hand-drawn from fiber precursor (for scale-up, the
precursor will be fed into a spinerette to produce continuous
single or multi-filaments (typically 10 microns in diameter. The
fibers are drawn by putting a thin rod into the precursor, then
quickly withdrawing it. The resulting fibers are smooth and dense.
The diameter is not uniform, but due only to the hand drawing
process. The fibers show stability up to 10 hours at 1200.degree.
C., but after 100 hours at 1200.degree. C., dramatic phosphorus
loss is seen. An attractive advantage of such fibers will be the
use of nanocrystalline inclusions within the amorphous matrix to
improve its strength, toughness, creep resistance, and thermal
expansion properties. Accordingly, a small metal spatula is
immersed slightly in the precursor from the preceding example. The
spatula is withdrawn at a steady rate, and the fiber is caught on a
piece of stainless steel mesh. The mesh is bent into a C shape, so
the fiber was touching the steel at only 2 points. The fibers were
put in a furnace and annealed in air at 900.degree. C. for 30
minutes.
Example 25
[0082] Films can be produced by dip coating onto a variety of
substrates, steel being the most common. The sample is air dried,
and then heated with an infrared lamp to cure the coating. The
coating cures much more quickly than in the furnace, 30 sec to 3+
minutes depending on the substrate. This eliminates the step of
putting the sample in the furnace and reduces substrate temperature
and heating times. Species removed from the precursor state in
order of their volatility are ethanol and other hydrocarbons (below
100.degree. C.), nitrates (typically above 500.degree. C.), and
hydroxyls (at least above 1000.degree. C. in case of powders). For
very thin films (below 500 angstroms), the temperature limits may
be much lower. It is worth noting that the exothermic peak in the
DTA around 225.degree. C. suggests formation of the amorphous
phosphate phase. Accordingly, a piece of stainless steel is
half-dip coated in precursor solution. The piece is heated with the
IR lamp for 2 minutes. The resulting piece shows the bottom half
well-coated and the top half still appears as it did previously. In
contrast, when stainless steel is half-dipped and annealed in the
furnace, the bottom half shows a good compositional coating, but
the whole piece is slightly discolored from oxidation.
Example 26
[0083] The compositions of this invention have been spin coated
onto silicon and steel in a standard spin coater. Aluminum
phosphate prepared as described herein has also been coated in a
3-dimensional process onto steel by immersing the piece and
removing it, then spinning the whole piece (e.g., use of a drill
press). The coatings seem to be more uniform and have fewer cracks
than standard dip coated pieces. Accordingly, a piece of stainless
steel is immersed completely in a 6.6 g/L aluminum phosphate
solution. The piece is withdrawn, and immediately spun (rpm to be
determined, but less than 540). The piece is cured with the IR
lamp. The piece is cured slowly by slowly bringing the IR lamp
closer to the piece, over a period of 5 minutes.
Example 27
[0084] Because P.sub.2O.sub.5 is very hygroscopic, preparation is
best carried out inside an dry glove box. To test the possibility
of working in the open atmosphere, the P.sub.2O.sub.5 was weighed
outside the glove box and left to sit overnight. Overall, the
P.sub.2O.sub.5 picked up 3.8 g of water from an original 19 g
P.sub.2O.sub.5. This syrupy P.sub.2O.sub.5 was dissolved in
ethanol, and an aluminum nitrate solution was added. The XRD
pattern shows the desired aluminum phosphate composition after
1100.degree. C. 1 h anneal, proving it can be synthesized in
ambient atmospheres without using controlled environment, thus
reducing the need for expensive atm control. Accordingly, 19.57 g
P.sub.2O.sub.5 is weighed out and left to sit in the laboratory and
was left for 22 hours, taking up water to provide a syrupy
consistency, instead of powder as it is when it is dry. This was
dissolved in ethanol and added to aluminum nitrate solution. The
XRD shows the resulting aluminum phosphate after 1100.degree. C., 1
hr anneal.
Example 28
[0085] Composition of this invention can be applied to glass by a
dip coating process. The resulting coating is very smooth and
transparent. Where the coating is continuous, it is featureless
under the optical microscope, and is only noticeable when held up
to the light. Coatings on glass are needed for corrosion
protection, as a glass-strengthening aid (heal surface flaws), and
for altering optical properties. Accordingly, a glass microscope
slide is dipped in a 17.6 g/L solution. The piece is blow dried
with cold air until dry. A low power IR lamp provides gentle heat.
After it is dry, the high power IR lamp is turned on and the piece
is heated for 4 minutes.
Example 29
[0086] Dip coated silicon can be annealed 1200.degree. C., for
extended periods of time. Coatings on silicon may be useful in the
semiconductor industry as a low dielectric stable coating
(dielectric constants lower than 2.9 is desired); a typical
aluminum phosphate powder of this invention with dielectric
constant as low as 3.3 has been made; further optimization may even
lower it further to meet the 2.9 criteria, providing an inexpensive
way to make these coatings and achieved such results. Accordingly,
a piece of silicon was dip coated and annealed at 1200.degree. C.
for 180 hours. There is evidence of some coating degradation, as
there is no phosphorus evident in the TEM cross-section. Similar
techniques can be used to coat molybdenum substrates.
Example 30
[0087] Solution of the present invention can be spray dried. The
resultant particles had a mean diameter of 11.5 microns and were
generally between 5-25 microns. Powders annealed at 1100.degree.
C., 1 hr retained the characteristic spectral patterns.
Example 31
[0088] It has been determined that the Raman peaks at 1350 and 1600
cm.sup.-1 are related to elemental carbon. It was also determined
that the peak near 1350 cm.sup.-1 in some FTIR spectra showed was
the result of atmospheric contamination, not P.dbd.O. The presence
of nanoinclusions of carbon is believed responsible for the black
color of the powders of this invention. Nanocrystalline carbon
(with a grain size as small as 15 .ANG.) shows peaks at 1350 and
1600 cm.sup.-1 in the Raman spectrum. Carbon has a weak IR
spectrum, which explains why there are no carbon peaks in the
FTIR.
[0089] XPS analysis was performed on annealed powders. Both
as-annealed specimens and crushed powders (to expose fresh
surfaces) were analyzed by Physical Electronics Corporation (MN,
USA). The as-annealed specimens showed carbon content of less than
0.1% whereas the crushed powders did show carbon presence near
1.6%. However, the report cast doubt on the 1.6% for the crushed
powder. The skepticism was based on powder dispersion in the
chamber, and may be the result of some surface contamination not
removed during extensive sputtering to remove 1500 .ANG. of the
surface from the crushed powder (500 521 was removed from the
as-annealed powder to remove surface contamination that was
reported to be typical for any material exposed to air). This
assessment was also supported by TEM and SEM analysis with low Z
detectors, although the detection limits with energy dispersive
spectra (EDS) are generally above at least 1 wt %. In addition, no
graphite inclusions have been observed within the amorphous matrix.
It is indeed possible that the size of these inclusions are below 5
nm and are randomly distributed or that it is present in a glassy
form mixed in with the amorphous oxide matrix.
[0090] Both Raman spectroscopy and CHNS
(Carbon-Hydrogen-Nitrogen-Sulfur) analysis have confirmed the
presence of carbon in the aluminum phosphate materials of this
invention. The amount of carbon present is indicated by the color
of the powder. Black powder contains more carbon than lighter
powder. Even powders described as "gray" or "light" are not truly
gray, they are a mixture of black and white domains that appear
gray when crushed.
[0091] A pellet of the black composition/powder was sent to Oak
Ridge National Lab for testing in the "Kaiser rig." The pellet was
annealed at 1200.degree. C. for 500 hours in a total pressure of 10
atmospheres, with 15% steam. The pellet lost approximately 5 wt %
during the experiment, but otherwise appeared intact. The pellet
was almost completely white. The surface was removed to eliminate
effects of any surface contaminants and the pellet was x-rayed. The
X-ray diffraction pattern was similar to the original powder (FIG.
12). The XRD pattern did not indicate significant crystallization.
When observed under the optical microscope, the pellet had a few
isolated black grains, but was over 98% white.
[0092] TEM analysis of the crushed pellet indicates that there are
nanocrystalline inclusions embedded in an amorphous matrix (FIG.
13). Electron diffraction patterns show diffuse amorphous rings
superimposed on spot patterns, which is typical for the
compositions of this invention.
[0093] Raman spectra were taken. The microRaman used has a spatial
resolution of approximately 3-5 microns, so spectra could be taken
of black and white domains in the same sample. Black areas
consistently showed peaks near 1350 and 1600 cm.sup.-1. The
intensity of these peaks scaled with each other from sample to
sample. White areas showed low intensity peaks which lined up with
crystalline berlinite, and did not show the peaks at 1350 and 1600
cm.sup.-1 (FIG. 14).
[0094] Further analysis is pending, but the results of this example
are encouraging for use of this invention in steam environments
including its use as an environmental barrier coating in SiC-based
composites used in coal combustion applications where low oxygen
diffusivity combined with resistance to high temperature steam are
critical needs. In addition, use of such coatings for steam-laden
atmospheres for mid-high T applications (such as petrochemical
processing) are also relevant.
[0095] To illustrate the results of this example and just one
application of the present invention, there is a need make
coal-fired power generation more energy efficient by increasing the
combustion temperature. Greater efficiency of power generation will
help alleviate increasing demand for power and reduce both solid
and gas phase hazardous waste products. As is demonstrated in the
state of California recently, power demand is increasing
dramatically. California has been faced with rolling blackouts
which have cost millions of dollars for businesses located there.
Certainly, more efficient power generation plants are necessary to
offset the increasing demand with limited environmental impact.
[0096] Currently, the temperatures in the boiler are
550-650.degree. C. Commonly used alloys do not have the required
properties for use at 700.degree. C. and above. The specifications
required for next-generation Ultra-Supercritical Boilers are high
creep rupture strength at 750.degree. C., and high corrosion
resistance, with the loss of no more than 1 mm in cross-section
after 100,000 hours of service. Austenitic stainless steels are
desirable as a replacement material because they are inexpensive
and can maintain necessary strength at high temperatures. However,
these alloys encounter problems both with high temperature
oxidation and sulfidation and corrosion by coal ash. There is also
a problem with coal ash erosion of steel parts. However, the coal
ash quickly coats the parts and, in effect, forms a protective
layer on the substrate.
[0097] Over the past decades, extensive R&D has been conducted
on protection of metals and alloys in coal combustion environments.
Many new alloys have been developed, along with coatings to slow
the rate of degradation. Researchers have explored the corrosion
resistance of commercial stainless steels, modified stainless
steels, nickel-base alloys and others. Ferritic stainless steels
corrode by the formation of FeSO.sub.4. They have found that nickel
and cobalt containing alloys corrode easily because of the ease of
formation of NiSO.sub.4 and CoSO.sub.4. Both of these sulfates form
a low-melting eutectic with Na.sub.2SO.sub.4, increasing the
corrosion of the alloy. High chronium content alloys (greater than
25%) show improved corrosion resistance, because a chromia scale
grows from oxidation. However, these alloys are subject to
corrosion as well, which is most severe at intermediate
temperatures where the chromia grows slowly. The sulfur present in
the combustion gas forms CrS.sub.2 which degrades the quality of
the oxide scale, further reducing its effectiveness as a protective
coating. In response to these problems groups have added other
alloying elements to the steels, such as tantalum and niobium which
have increased corrosion resistance. Aluminum containing alloys and
intermetallics (Fe.sub.3Al) have been applied in hope of developing
an aluminum oxide scale which shows superior oxidation and
corrosion protection. However, these high-tech alloys and most
coatings are prohibitively expensive for widespread use.
[0098] A suitable system which attains the necessary specifications
at a reasonable cost for use for general power plant use has
heretofore not been found. An ideal solution to this problem would
be an inexpensive coating that would serve as an oxidation and
corrosion barrier for a common austentic steel. The present
invention provides an inexpensive material that can be applied very
easily, and would be a low-cost solution to the problem. If the
temperature of the plant can be increased, the efficiency will also
increase, leading to benefits of increased output power for a given
amount of coal, which will give lower-cost energy, and
environmental benefits of having to bum less coal. This not only
saves money in processing power, but also reduces clean-up costs,
which can be substantial. The compositions are inexpensive and easy
to apply: spraying a solution onto the exterior of a heat exchanger
tube or inside a boiler, for example.
Example 32a
[0099] .sup.31P NMR spectra of a precursor solution for a preferred
aluminum composition shows that the aluminum nitrate is interacting
with the phosphorus pentoxide solution to form one or more unique
complexes. For basis of comparison, the .sup.31P NMR spectrum of
phosphorus pentoxide in ethanol is presented. FIG. 15a shows the
spectrum of P.sub.2O.sub.5 dissolved in ethanol, and was taken
shortly after dissolution. FIG. 15b shows the same solution after
24 hours of reflux.
Example 32b
[0100] The addition of aluminum nitrate
Al(NO.sub.3).sub.3.circle-solid.9H- .sub.2O to the phosphorus
precursor solution of Example 32a changes the .sup.31P spectrum
significantly. FIGS. 16a-b show three spectra from three admixed
precursor solutions: the bottom curve is from C-1 (stoichiometric
Al), and the other two are for increasing aluminum addition (C-1.5,
50% excess Al; and C-2, 100% excess Al). The differences between
these spectra and the spectra shown in FIG. 1 are readily
apparent.
[0101] A set of new peaks appears between [-15 and -24 ppm]. None
of these peaks are observed in the P.sub.2O.sub.5+ ethanol
precursor spectra. These peaks are believed due a complexation of
the aluminum with the phosphorus species. A pattern is observed
upon increase of Al content from stoichiometric to a 2-fold
excess.
Example 33
[0102] The solutions of Example 32b were annealed to high
temperature (1100.degree. C.) for long times (160 hrs.). The
stoichiometric composition becomes somewhat crystalline over time,
while those materials of this invention with excess aluminum
provide XRD patterns showing substantial amorphous
character--denoting enhanced metastability. (See, FIGS. 17a-b.)
Example 34
[0103] FTIR spectra of annealed materials/compositions of this
invention show several unique features. For short anneal times (1
hr) both stoichiometric (x=0) and non-stoichiometric (x=0.25, 0.5
and 0.75) compositions show similar features. The spectra are shown
in FIG. 18. The spectra show predominately Al--O--P bonds, but show
some features attributed to Al--O--Al and P--O--P. The Al--O--Al
seems to be present in increasing amounts with increasing aluminum
content. Stoichiometric compositions contain a very small amount of
Al--O--Al. Stoichiometric compositions show a fairly strong P--O--P
feature, which is smaller in Al:P=1.25, and is very small (or
nonexistent, it is hard to be certain) in Al:P=1.5 and does not
show up in Al:P=1.75.
Example 35a
[0104] Stoichiometric compositions show distortion in the aluminum
coordination after 1100.degree. C., 1 hr anneal. FIG. 19 shows a
deconvolution of this spectrum. There are four curves in the
deconvolution spectrum which add to the complete spectrum. The
sharp peak near 39 ppm indicates Al in regular tetrahedral
coordination. The other peaks indicate aluminum in distorted
coordination and are listed in Table II.
3TABLE II Deconvolution of.sup.27 Al NMR spectrum of stoichiometric
AlPO.sub.4. tetrahedral peak octahedral peak position position
relative area 38.163 100 33.222 45.52 10.749 11.11 -16.356
14.84
Example 35b
[0105] Deconvolution of the .sup.27Al MAS NMR spectrum for a
non-stoichiometric composition (x=1.0) shows there are distorted
4-fold aluminum species, along with more regular 4-fold aluminum
(FIG. 20.) The peak near 40 ppm is tetrahedral aluminum, and the
peak fit highlighted in green shows regular coordination, while the
peak highlighted in red shows aluminum in distorted octahedral
coordination. The regular 4-fold aluminum is believed present in
the nanocrystals while the distorted 4-fold and 6-fold aluminum is
present in the amorphous matrix. Table III shows the relative peak
positions and areas attributed to tetrahedral and octahedral
aluminum. Table III.
4TABLE III octahedral peak position tetrahedral peak position
relative area -9.37 15.667 7.027 26.06 38.847 100 40.206 43.13
62.638 11.9
* * * * *