U.S. patent application number 12/536626 was filed with the patent office on 2011-02-10 for sintering aid coated yag powders and agglomerates and methods for making.
Invention is credited to Ishwar D. Aggarwal, Shyam S. Bayya, Woohong Kim, Bryan Sadowski, Jasbinder S. Sanghera, Guillermo R. Villalobos.
Application Number | 20110034319 12/536626 |
Document ID | / |
Family ID | 43535264 |
Filed Date | 2011-02-10 |
United States Patent
Application |
20110034319 |
Kind Code |
A1 |
Villalobos; Guillermo R. ;
et al. |
February 10, 2011 |
Sintering Aid Coated YAG Powders and Agglomerates and Methods for
Making
Abstract
Particles including a YAG core and a coating of sintering aid
deposited thereon. The particles and agglomerates thereof maybe
formed as a powder. The coated YAG-containing particles are
well-suited to production of polycrystalline YAG-containing
ceramics. The coated YAG-containing particles may be fabricated
using a novel fabrication method which avoids the need for
formation of a homogeneous powder mixture of YAG and sintering aid.
In the method, a solution including a sintering aid or sintering
aid precursor is prepared and mixed with YAG-containing particles
to form a mixture. The mixture may be sprayed into a drying column
and dried to produce coated particles. Alternatively, the YAG
particles and sintering aid or sintering aid precursor solution may
be separately introduced to the drying column and dried to form
coated YAG-containing particles.
Inventors: |
Villalobos; Guillermo R.;
(Springfield, VA) ; Sanghera; Jasbinder S.;
(Ashburn, VA) ; Kim; Woohong; (Lorton, VA)
; Bayya; Shyam S.; (Ashburn, VA) ; Sadowski;
Bryan; (Arlington, VA) ; Aggarwal; Ishwar D.;
(Fairfax Station, VA) |
Correspondence
Address: |
NAVAL RESEARCH LABORATORY;ASSOCIATE COUNSEL (PATENTS)
CODE 1008.2, 4555 OVERLOOK AVENUE, S.W.
WASHINGTON
DC
20375-5320
US
|
Family ID: |
43535264 |
Appl. No.: |
12/536626 |
Filed: |
August 6, 2009 |
Current U.S.
Class: |
501/152 |
Current CPC
Class: |
C04B 2235/3224 20130101;
C04B 35/44 20130101; C04B 35/6281 20130101; C04B 35/62828 20130101;
C04B 35/62897 20130101; C04B 2235/3222 20130101; C04B 35/62886
20130101; C04B 2235/3225 20130101; C04B 35/62807 20130101; C04B
35/62655 20130101; C04B 2235/9653 20130101; C04B 35/645 20130101;
C09K 11/025 20130101; C09K 11/7774 20130101 |
Class at
Publication: |
501/152 |
International
Class: |
C04B 35/50 20060101
C04B035/50 |
Claims
1. A coated particle comprising: a core comprising YAG; and a
sufficient amount of a sintering aid coated on a surface of said
core, wherein the coating is sufficiently continuous to
substantially prevent a large number of sites where another core
may come into contact with the coated YAG-containing core.
2. The particle according to claim 1, wherein said core further
comprises a rare earth element dopant.
3. The particle according to claim 2, wherein said rare earth
element dopant is selected from the group consisting of Yb, Nd, Er
and combinations thereof.
4. The particle according to claim 2, wherein said core comprises
less than about 10% by weight of the dopant.
5. The particle according to claim 2, wherein said core comprises
from about 1% to about 2% by weight of the dopant.
6. The particle according to claim 1, wherein the sintering aid
comprises a material selected from the group consisting of: silicon
dioxide, fluoride salt, magnesium oxide and combinations
thereof.
7. The particle according to claim 6, wherein the fluoride salt is
selected from the group consisting of aluminum fluoride and lithium
fluoride.
8. The particle according to claim 6, wherein the sintering aid is
silica.
9. The particle according to claim 1, wherein said core has a
diameter of from about 10 nanometers to about 100 microns and
wherein said coating has a thickness of from about 1 nanometer to
about 500 microns.
10. A method for forming a coated particle comprising YAG, said
method comprising the steps of: preparing a solution comprising a
sintering aid or sintering aid precursor; mixing YAG-containing
particles with said solution to form a slurry; and drying said
mixture to form YAG-containing particles with a coating of
sintering aid on a surface thereof.
11. The method of claim 10, wherein said sintering aid precursor is
a silica precursor.
12. The method of claim 11, wherein said silica precursor is
selected from the group consisting of: tetraethoxysilane,
tetramethoxysilane, alkoxy silanes, silicate oxides and
combinations thereof.
13. The method of claim 10, wherein said drying step is carried out
at a temperature of from about 200.degree. C. to about 500.degree.
C.
14. The method of claim 10, wherein said drying step gradually
increases temperature from a first temperature to a higher
temperature.
15. The method of claim 14, wherein said YAG particles comprise a
dopant.
16. The method of claim 15, wherein said dopant is selected from
the group consisting of Yb, Nd, Er and combinations thereof.
17. The method of claim 10, wherein the YAG-containing particles
are mixed with the sintering aid solution to form a slurry and said
slurry is sprayed into a drying column for said drying step.
18. The method of claim 10, wherein the YAG-containing particles
are provided to said drying step separately from said sintering aid
solution.
19. The method of claim 10, wherein said sintering aid comprises a
material selected from the group consisting of: silicon dioxide,
fluoride salt, magnesium oxide and combinations thereof.
20. The method of claim 15, further comprising the step of heat
treating said coated YAG-containing particles to fabricate a
transparent, shaped polycrystalline YAG ceramic.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to the field of yttrium
aluminate garnet (YAG) particles. In particular the present
invention relates to yttrium aluminate garnet particles coated with
a sintering aid for use in fabricating ceramics.
[0003] 2. Description of the Related Technology
[0004] Hard, strong and thermally conductive YAG particles are
generally used to fabricate ceramics used in the semiconductor and
optics industry. Although single crystal YAG particles are widely
used to construct single crystal laser hosts, these crystals are
expensive to manufacture, too small in size and do not provide
sufficiently good beam quality to be used in high power lasers of
greater than about 5 kW. By contrast, polycrystalline materials
possess good beam quality and can be made in large sizes.
[0005] Processing YAG particles to form polycrystalline YAG
ceramics overcomes many of these limitations. Traditional
processing of polycrystalline YAG ceramics, however, leads to high
scattering and absorption losses that are distributed in localized
yet random regions. Polycrystalline YAG materials produced by this
method do not posses uniform optical loss and therefore have a poor
yield. Furthermore, traditional processing is expensive and
inadequate for producing large sized polycrystalline YAG ceramics
or materials having different shapes. Consequently, YAG materials
produced by this method are unsuitable for many applications,
including construction of high power lasers.
[0006] One of the primary problems with traditional processing is
the difficulty involved in sintering YAG-containing materials. In
general, sintering is the act of consolidating powder into a dense
shape. The powder being sintered cannot melt to a great extent,
although some melting of the secondary phase in the powder or
surface melting is allowed under this definition. If the material
melts, the process is referred to as fusion casting. Sintering,
either with pressure, i.e. hot pressing, or without pressure,
requires a vast amount of material transport to consolidate an
aggregate of loose powder particles into a dense shape. In the case
of porcelains and clay products, the secondary phases of these
materials melt and bind the primary solid particles together with a
glass phase; these types of systems were the first to be used due
to their ease of sintering. However, advanced ceramics do not have
these intrinsic sintering aids and thus sintering aids must
therefore be added to the materials.
[0007] Sintering aids work in a variety of fashions. They may
liquify at or somewhat below the primary material's densification
temperature thereby promoting liquid phase sintering. Certain
sintering aids exhibit higher solid-state diffusion coefficients
than the primary material's self-diffusion. The sintering aid may
conversely have a lower solid-state diffusion coefficient that
prevents exaggerated grain growth and promotes grain boundary
refinements and pinning. The sintering aid may also simply clean or
etch the primary material's surfaces thereby enhancing solid-state
diffusion.
[0008] Recently, sintering aids have been used to facilitate
processing of YAG particles by enhancing densification and
assisting sintering. In general, these sintering aids tend to be
solid inorganic particles, such as silica. Since the YAG particles
are also solid inorganic particles, the sintering aid and YAG
particles must be mixed homogeneously for the sintering aid to be
effective. To date this has typically been accomplished by some
form of mechanical mixing. For small samples, the mixing may
involve the use of a mortar and pestle. In larger samples ball
milling, attritor milling, high shear wet milling, and variations
or combinations of these methods may accomplish mixing. However,
due to the nature of this type of particle-particle interaction,
none of these mechanical mixing methods produce a homogenous
mixture wherein the silica adequately coats the YAG particles.
Furthermore, mechanical mixing has the added problem of
contaminating the YAG with the milling material. Inhomogeneity
results in areas having too much, too little or no sintering aid
and causes significant problems in the fabrication of transparent
ceramics, electronic ceramics, and refractory ceramics.
Consequently, the ceramic materials produced from these YAG
particles typically contain inhomogeneous regions as well as opaque
regions having low yield that must be drilled out and removed. This
can be expensive and lead to small sized ceramic products.
Production of uniform and highly transparent polycrystalline YAG
products therefore depends upon the uniform mixture of YAG
particles and sintering aids, which as discussed above, is
typically non-ideal when mechanical mixing methods are
employed.
[0009] The use of solutions and suspensions including YAG particles
and sintering aids for fabricating polycrystalline YAG products has
also been investigated. These investigations have demonstrated that
the production of a high yield and uniform ceramic depends on
uniformly coating the YAG particles with the sintering aids so that
the YAG particles do not substantially directly contact one
another. Merely creating a solution or suspension of YAG particles
and a sintering aid is insufficient to achieve this effect. Rather,
further processing is required of these solutions and suspensions
to produce a uniform coating.
[0010] For example, U.S. Patent application publication no.
2005/0281302 (Lee), disclose a transparent polycrystalline YAG
material that may be doped with rare earth elements for use in
lasing systems. The material was fabricated by dispersing YAG
powders in a solution of ethyl alcohol and TEOS or colloidal
silica, and the resultant suspension was dried under stirring and
subsequently calcinated. The process of stirring and calcinating a
YAG particle and silica precursor solution alone, however, does not
produce a uniform coating of silica on the YAG particles. Although
this process provides a better distribution of silica on the YAG
particles in comparison to mechanical mixing, there are still areas
on the YAG particles with either too much or not enough silica
sintering aid. Consequently, Lee does not teach a method for
producing a uniform coating of silica on YAG particles. In Lee's
method, adjacent YAG particles can be directly in contact with each
other.
[0011] Similarly, U.S. Patent application publication no.
2007/0182037 (Rabinovitch) discloses a transparent ceramic
constructed from YAG that is produced by suspending doped YAG
particles in deionized water and colloidal silica, agitating the
solution and subsequently filtering out and compacting the
resultant YAG particles. Rabinovitch employs a particulate silica
suspension but does not indicate that the silica particles were
deposited the YAG particles to establish a uniform coating.
Notably, the resulting silica coating is less uniform than that
taught by Lee. Rabinovitch, therefore, does not teach a method for
producing an adequate uniform coating of silica on YAG particles.
In Rabinovitch's method, adjacent YAG particles can be directly in
contact with each other.
[0012] U.S. Pat. No. 7,449,238 (Villalobos) discloses yttria
particles coated with LiF. According to Villalobos, a LiF salt is
dissolved in water and caused to precipitate on the particle as the
particle is dried. Unfortunately there are no commercially suitable
salts that produce SiO.sub.2 without calcination at temperatures
above 700.degree. C. and long residence times measured in hours.
The maximum temperatures of spray dryers are in the 500.degree. C.
range and the residence times are measured in seconds instead of
hours. Although a salt could be precipitated on the particles,
subsequent heat treatment at 700.degree. C. for 2 hours to form
SiO.sub.2 would produce hard agglomerates that do not sinter and
would also cause spalling of the coating. Therefore, Villalobos
does not provide a teaching for creating a substantially uniform
silica coated YAG particle. By contrast, applying a uniform coating
of silica to a YAG particle, involves a distinctly different
chemical process that requires binding the silica to the YAG
particle surface. Consequently, the coating method disclosed in
Villablobos would not be applicable to establishing a uniform
silica based coating. Additionally, Villalobos does not suggest
that its coating method would be compatible with YAG particles.
[0013] Therefore, there is a need in to develop YAG particles
suitably coated with a sintering aid in order to enhance the
reproducibility of production of polycrystalline YAG products.
[0014] There is also a need for methods for making such coated YAG
particles that do not require the advance preparation of a
homogenous mixture of silica and YAG particles.
SUMMARY OF THE INVENTION
[0015] The present invention is directed to YAG particles coated
with a sintering aid. In a first aspect, the invention is directed
to a particle having a core including YAG and a sintering aid
coated on a surface thereof, wherein the coating is sufficiently
continuous to substantially prevent a large number of sites where
another core may come into contact with the coated YAG-containing
core.
[0016] The invention is also directed to a method for forming a
particle comprising the steps of: preparing a solution including
YAG particles and a sintering aid precursor in a solvent, mixing
the solution to form a slurry, spraying the slurry and drying the
sprayed slurry to form a coating of the sintering aid on the YAG
particles.
[0017] These and various other advantages and features of novelty
that characterize the invention are pointed out with particularity
in the claims annexed hereto and forming a part hereof. However,
for a better understanding of the invention, its advantages, and
the objects obtained by its use, reference should be made to the
drawings which form a further part hereof, and to the accompanying
descriptive matter, in which there is illustrated and described a
preferred embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWING
[0018] FIG. 1 is a schematic representation of a silica coated YAG
particle.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS)
[0019] For illustrative purposes, the principles of the present
invention are described by referencing various exemplary
embodiments. Although certain embodiments of the invention are
specifically described herein, one of ordinary skill in the art
will readily recognize that the same principles are equally
applicable to, and can be employed in other systems and methods.
Before explaining the disclosed embodiments of the present
invention in detail, it is to be understood that the invention is
not limited in its application to the details of any particular
embodiment shown. Additionally, the terminology used herein is for
the purpose of description and not of limitation. Furthermore,
although certain methods are described with reference to steps that
are presented herein in a certain order, in many instances, these
steps may be performed in any order as may be appreciated by one
skilled in the art; the novel method is therefore not limited to
the particular arrangement of steps disclosed herein.
[0020] It must be noted that as used herein and in the appended
claims, the singular forms "a", "an", and "the" include plural
references unless the context clearly dictates otherwise. Thus, for
example, reference to "a particle" may include a plurality of
particles and equivalents thereof known to those skilled in the
art, and so forth. As well, the terms "a" (or "an"), "one or more"
and "at least one" can be used interchangeably herein. It is also
to be noted that the terms "comprising", "including", "composed of"
and "having" can be used interchangeably.
[0021] For purposes of the present invention, the "coating" of one
or more sintering aids herein refers to applying a sufficient
amount of one or more sintering aids on a particle core in an
arrangement that substantially prevents direct contact of the
coated particle core with other coated particle cores. The coating
should be sufficiently continuous and/or have a sufficient
thickness that the coating minimizes the number of sites where
coated particle cores can come into contact with one another
without an intervening layer of a sintering aid. The coating should
be sufficient to prevent a large number of sites where a
YAG-containing core comes into contact with another core. While the
coating may be hermetic and/or continuous, this is not necessary to
achieve a useful coating. In an exemplary embodiment, the sintering
aid may be intermittently and strategically deposited on the
surface of a particle core so as to minimize the number sites where
particle cores directly contact one another. For example, in one
embodiment, although discontinuous, the sintering aid coating may
be substantially thick and/or the uncoated surface of the particle
core may be sufficiently small so as to prevent direct contact with
other particulate cores.
[0022] For purposes of the present application, "boiling" as used
herein includes heating a solvent to a temperature above the
boiling point as well as conditions that cause rapid and
significant evaporation of the solvent to prevent the formation of
a sintering aid coating of salt on the YAG particles.
[0023] For purposes of the present application, "aerosol," includes
the slurry material present both before and after any evaporation
of solvent.
[0024] For purposes of the present application, "spalling," refers
to when the outer portion of a liquid coating of solution first
dries to form a crust around the still liquid inner portion of the
coating. When solvent later evaporates from the inner portion, the
gaseous expansion optionally blows off the crust, creating an
incompletely coated particle.
[0025] The present invention is directed to novel coated yttrium
aluminate garnet (YAG) particles. As used herein, references to
"YAG particles" and "YAG cores" include both particles and cores
made of substantially pure YAG, as well as particles and cores
containing YAG and some amount of other materials such as dopants.
The YAG particles are coated with a sintering aid. The present
invention is also directed to methods for making the coated YAG
particles. As shown in the exemplary embodiment of FIG. 1, the
coated particle 100 of the present invention includes a core 1
composed of YAG and has a layer of one or more sintering aids 3
coated on the surface of core 1. Because the coating substantially
prevents direct contact between the YAG cores 1 of the particles
100, powders composed of particles 100 and agglomerates thereof may
be used to manufacture dense, highly transparent and uniform
polycrystalline YAG ceramic materials. It is envisioned that
particles 100 and ceramic products formed therefrom may have a wide
variety of applications in the fields of semiconductors and
optics.
[0026] Core 1 of coated particle 100 includes YAG. In one
embodiment, the YAG may be undoped. Alternatively, the YAG may be
doped with one or more dopants. Exemplary dopants may include rare
earth elements, such as Nb, Yb, Er or other rare earth elements and
combinations thereof. In an exemplary embodiment, the dopant may be
present in an amount up to about 10% by weight of particle 100,
preferably about 1% to about 2% by weight of particle 100.
[0027] Core 1 may have any suitable shape, size or configuration.
The cores may be, but are not limited to, approximately spherical
or elliptical shapes. In an exemplary embodiment, core 1 may have a
diameter in the range of about 10 nm to about 100 um, preferably
about 100 nm to about 30 um.
[0028] A coating of one or more sintering aids 3 is disposed about
a surface of core 1 to facilitate sintering, reduce porosity and
decrease exaggerated grain growth. Sintering aid 3 may include any
material suitable for facilitating sintering. In an exemplary
embodiment, the sintering aid may be silica, a lithium salt,
magnesium oxide or combination thereof. Exemplary sintering aids
may include aluminum fluoride and lithium fluoride. As shown in
FIG. 1, sintering aid 3 preferably coats the surface of core 1 so
as to minimize the possibility that particle core 1 directly
contacts another particle core 1, after coating is complete. In one
embodiment, the coating may be continuous and/or hermetic.
Alternatively, the coating may be intermittent while substantially
preventing contact between two coated YAG cores 1. For example, a
thick coating with small holes will successfully prevent two YAG
cores from contacting one another. The thickness of the coating of
sintering aid 3 may be uniform or may vary over the surface of core
1. Preferably, the sintering aid 3 is homogenously distributed over
the surface of core 1 on a nanometer scale so as to create a
substantially uniform coating thickness. In an exemplary
embodiment, the selected sintering aid is silica, and the silica
coating 3 may have a thickness of from about 1 nm to about 500 um,
more preferably about 10 nm to about 200 um.
[0029] Coated particles 100 and agglomerates thereof have a number
of advantages and may be used for a wide variety of applications.
Specifically, the combination of a YAG particle core 1 and a
coating of one or more sintering aids enable fabrication of high
yield ceramics with no or minimal light scattering sites. This
allows a reduction in the total amount of sintering aid used and
consequently reduces the amount of unwanted reaction byproducts
that are left in the material as scattering sites. The coating of
the sintering aid also substantially simplifies the densification
process during ceramic fabrication, enabling densification under
less harsh conditions than are traditionally used in order to
achieve highly dense and uniform shapes of hard to sinter
materials. Consequently, the coated YAG particles, agglomerates and
powders of the present invention may be used to reproducibly and
economically construct highly dense, transparent polycrystalline
YAG monoliths as well as ceramics that may be useful in the
semiconductor and optical field. Specifically, coated particles 100
may be particularly useful for constructing polycrystalline YAG
ceramics to be used in optical fibers and lasers.
[0030] The invention is further directed to novel methods for
making coated YAG-containing particles 100. Unlike the prior art,
it is not necessary to prepare a homogenous mixture of dry
particulate sintering aid and YAG particles in order to implement
the present invention. Rather, a coating of sintering aid 3 on
YAG-containing core 1 may be achieved by depositing a sintering aid
3 on a surface of core 1 using a novel spray and column-based
drying process. Unlike prior art references that teach randomly
precipitating a sintering aid precursor solution, such as TEOS. the
present method uses a spray drying technique that forces the
sintering aid precursor solution to precipitate in a spray droplet
form, uniformly deposit on and contain YAG-containing cores 1. This
process produces a substantially uniform coating of sintering aid 3
on the YAG-containing cores 1. The resultant coated particles 100
further have a lower melting point than that of untreated YAG
particles, thereby enabling high liquid phase transport when
sintering and fabricating YAG based materials.
[0031] The method of the present invention involves preparing a
sintering aid and YAG-containing particle slurry and subsequently
coating the YAG-containing particles with the sintering aid. The
resultant coated particles 100 and agglomerates thereof may then be
shaped to form a ceramic product.
[0032] In one embodiment, doped or undoped YAG particles may be
added to and mixed with a precursor solution for a sintering aid to
form a slurry. Exemplary precursor solutions may include
tetraethoxysilane (TEOS), tetramethoxysilane, alkoxy silanes,
silicate oxides and combinations thereof. In a similar manner,
various magnesium alkoxides can be used to form MgO coatings such
as magnesium-s-butoxide, magnesium ethoxide and combinations
thereof. Exemplary precursor solutions for forming fluoride salt
sintering aids may also be formed by simply dissolving fluoride
salts in water or another liquid. The fluoride salt can be made to
precipitate by boiling off the liquid. In this invention, the
sintering aid is dissolved in the slurry prior to spray drying. The
use of undissolved sintering aids in the slurry is not
preferred.
[0033] The slurry may further include a solvent selected based on
the sintering aid precursor solution. Exemplary solvents may
include ethanol, methanol, isopropyl or combinations thereof. At
this point, the sintering aid should be completely dissolved in the
slurry and should not precipitate until the coating, i.e. spraying,
step. The YAG particles, however, should not dissolve in the
solvent. Premature contact between a precipitated sintering aid and
YAG particle may produce inhomogeneous regions that cause
scattering in ceramics fabricated from these particles. In an
exemplary embodiment, the slurry may be diluted with a compound to
optimize the concentration of the solution. Exemplary diluents may
include ethyl alcohol, other compatible alcohols, additional water
or combinations thereof.
[0034] The sintering aid, e.g. silica may subsequently be coated on
the YAG particles by spraying and heat processing the slurry. In
one embodiment, the YAG particles, sintering aid precursor and
solvent are mixed together to form a slurry that is sprayed into a
heated drying chamber having any suitable shape, such as a drying
column. The slurry may enter the column as an aerosol under thermal
conditions that avoid boiling the solvent. In an exemplary
embodiment, the slurry may be sprayed into the column at a rate of
about 15 ml/min. Sintering aid 3 does not precipitate until the
aerosol moves through the spray drying column. In an exemplary
embodiment, the aerosol enters the column at temperature of about
room temperature to about 180.degree. C., preferably about room
temperature to about 150.degree. C. Slightly elevated temperatures
may alternatively used. Additionally, an ultrasonic spray head may
optionally be used to spray the slurry into the drying column.
[0035] In an alternative embodiment, mixing and spraying may occur
simultaneously when the YAG particles and the sintering aid
precursor solution are separately sprayed into the drying column to
accomplish mixing in the drying column itself. Other ingredients
may optionally be added during the mixing and spraying process. In
one non-limiting example, various components of the slurry are
separately sprayed into the drying column. To enhance sprayability,
the YAG particles may be optionally mixed with a liquid that
immediately evaporates upon entering the column or otherwise does
not significantly affect the coating process.
[0036] After spraying, the aerosol moves through the column or, if
the column is vertically disposed, falls down the column. As the
aerosol moves, the thermal conditions in the column are such as to
evaporate the solvent, so that the sintering aid reaches a
saturation point and subsequently precipitates from the slurry and
deposits on the surface of the YAG particles to form a coating. The
thermal conditions and spraying rate may be selected to
substantially avoid spalling. In general, higher temperatures or
rapid increases in temperature may cause spalling. If the slurry
spray droplets are dried too fast, the deposited silica will fall
off the particles. If the temperature is not sufficiently hot, the
silica will remain wet, and the particles will stick together
rather than precipitating to form a coating on the YAG particles.
In one embodiment, the temperature may be set to about 350.degree.
C. In another exemplary embodiment, the drying column may have a
temperature gradient, wherein the temperature of the column
increases from about 200.degree. C. to about 500.degree. C. as the
aerosol travels through the column. This may or may not be a linear
change in temperature. Other gradients or multiple gradients may
also be used. In an alternative embodiment, the column may have
three temperature gradients of about 190.degree. C. to about
300.degree. C., 375.degree. C. to about 415.degree. C. and
390.degree. C. to about 430.degree. C. In an exemplary embodiment,
the particle speed through the column may be about 0.3 to about 30
msec with a residence time of about 0.1 to about 10 sec.
[0037] After spraying, the resulting coated YAG particles may be
collected in a cyclone separator. If necessary, the coated
particles may also be heat treated after fabrication in different
environments, such as air or oxygen, to reoxidize all or portions
of the materials in the coated particles.
[0038] Optionally, the resultant YAG particles coated with a
sintering aid may be subsequently formed and shaped into a ceramic
material. The process of shaping the coated YAG particles may or
may not involve application of pressure and/or heat. Exemplary
shaping methods may involve hot pressing, pressing or packing the
particles 100 or agglomerates. In one embodiment, the sintering aid
coated particles may be sintered at about 1450.degree. C. for about
2 hours Exemplary sintering temperatures may be in the range of
about 1300.degree. C. to about 1900.degree. C. with soaking times
of about 30 minutes to about 24 hours and ramp rates of about
1.degree. C./min to about 50.degree. C./min. Optionally, the
sintering temperature may be held at one or more temperatures to
facilitate the removal of volatile species and sintering aid
related products. In addition to shaping, the application of heat
may also function to remove any unwanted impurities, such as
carbon, hydrocarbons, alcohols and other volatiles species. In an
exemplary embodiment, the resultant shaped ceramic material may be
a high yield, transparent, dense polycrystalline YAG material that
is at least about 98% above bulk transmission that enables
transmission of light in the visible and infrared wavelength
regions.
EXAMPLES
Example 1
[0039] In a first example, SiO.sub.2 was coated on undoped YAG
particles.
[0040] The coated particles were produced by first preparing a
tetraethoxysilane (TEOS) stock solution that was made by mixing
about 0.08 ml TEOS, about 30 ml ethanol, about 0.2 ml water and
about 0.62 ml HCL. One gram of YAG particles was then mixed with
about 2.1 ml of the TEOS stock solution and about 600 ml of ethanol
to produce a slurry.
[0041] The slurry was then sprayed into a drying column at a
temperature of about 350.degree. C. and at a rate of about 15
m/min. The drying column was constructed as a vertical glass tube
about 3 m long having a diameter of about 15 cm with an ultrasonic
atomizer positioned at the top and a cyclone separator, including a
fan to create a downward suction, positioned at the bottom of the
tube. The slurry was pumped into the atomizer using a piston
pump.
[0042] Three furnaces were positioned along the length of the
sprayer. The first furnace produced a temperature region having
temperatures ranging from about 190.degree. C. to about 300.degree.
C. with the temperature increasing in the downstream direction. The
second furnace produced a temperature region having temperatures
ranging from about 375.degree. C. to about 415.degree. C., with the
temperature increasing in the downstream direction, and a third
furnace produced a temperature region having temperatures ranging
from about 390.degree. C. to about 430.degree. C., with the
temperature increasing in the downstream direction. As the solvents
evaporate while falling through the drying column, the silica
reached saturation, precipitated and deposited on the YAG particles
to form a coating. In general, the slurry aerosol moved through the
drying tube at a rate of from about 0.3 m/sec to about 30 m/sec
with a residence time of about 0.1 sec to about 10 sec. The
resultant coated particles were collected in a cyclone
separator.
[0043] X-ray diffraction of the coated powder showed the presence
of YAG and a broad amorphous signal resulting from silica. SEM
analysis of the coated particles showed the presence of a uniform
coating, while EDS tests verified the presence of Y, Al, Si and
O.
[0044] The silica coating enabled the subsequent hot pressing of
the YAG particles in an inert atmosphere to form a shaped
transparent ceramic that was above 98% bulk transmission in the
visible and infrared wavelength region. The ceramic was produced by
placing the coated YAG particles in a grafoil-lined graphite hot
press die. The die was placed in an argon/vacuum atmosphere hot
press. Minimal pressure was applied to the die until densification
was initiated. The powder was then heated gradually at a rate of
from about 20.degree. C./min to about 1500.degree. C./min until it
formed a fully densified and transparent shaped ceramic. The
heating elements were then turned off to allow natural cooling of
the hot press, and the hydraulic motor was turned off to allow the
pressure to bleed off.
Example 2
[0045] In this example, SiO.sub.2 was coated on YAG particles doped
with 1% by weight Nd, based on the total weight of the combination
of YAG and Nd (as Nd.sub.2O.sub.3).
[0046] The particles were produced by first preparing a
tetraethoxysilane (TEOS) stock solution that was made by mixing
about 0.08 ml TEOS, about 30 ml ethanol, about 0.2 ml water and
about 0.62 ml HCL. The TEOS stock solution was further diluted with
ethyl alcohol. One gram of YAG particles doped with 1% by weight of
Nd (as Nd.sub.2O.sub.3) was then mixed with about 2.1 ml of the
TEOS stock solution and about 600 ml of ethanol to produce a
slurry. The slurry was then processed in accordance with the same
procedure as that disclosed in Example 1.
Example 3
[0047] In this example, SiO.sub.2 was coated on YAG particles doped
with 1% by weight Yb, based on the total weight of the combination
of YAG and Yb (as Y.sub.2O.sub.3).
[0048] The particles were produced by first preparing a
tetraethoxysilane (TEOS) stock solution that was made by mixing
about 0.08 ml TEOS, about 30 ml ethanol, about 0.2 ml water and
about 0.62 ml HCL. The TEOS stock solution was further diluted with
ethyl alcohol. One gram of YAG particles doped with 1% by weight of
Yb particles was then mixed with about 2.1 ml of the TEOS stock
solution and about 600 ml of ethanol to produce a slurry. The
slurry was then process in accordance with the same procedure as
that disclosed in Example 1.
Example 4
[0049] In a second example, SiO.sub.2 was coated on YAG particles
doped with 8% by weight Nd, based on the total weight of the
combination of YAG and Nd (as Nd.sub.2O.sub.3).
[0050] The particles were produced by first preparing a
tetraethoxysilane (TEOS) stock solution that was made by mixing
about 0.08 ml TEOS, about 30 ml ethanol, about 0.2 ml water and
about 0.62 ml HCL. The TEOS stock solution was further diluted with
ethyl alcohol. One gram of YAG particles doped with 8% by weight of
Nd (as Nd.sub.2O.sub.3) was then mixed with about 2.1 ml of the
TEOS stock solution and about 600 ml of ethanol to produce a
slurry. The slurry was then processed in accordance with the same
procedure as that disclosed in Example 1.
[0051] It is to be understood, however, that even though numerous
characteristics and advantages of the present invention have been
set forth in the foregoing description, together with details of
the structure and function of the invention, the disclosure is
illustrative only, and changes may be made in detail, especially in
matters of shape, size and arrangement of parts within the
principles of the invention to the full extent indicated by the
broad general meaning of the terms in which the appended claims are
expressed.
* * * * *