U.S. patent application number 10/601884 was filed with the patent office on 2004-12-30 for spinel and process for making same.
Invention is credited to Aggarwal, Ishwar D., Bayya, Shyam S., Sanghera, Jas S., Villalobos, Guillermo R..
Application Number | 20040266605 10/601884 |
Document ID | / |
Family ID | 33539465 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040266605 |
Kind Code |
A1 |
Villalobos, Guillermo R. ;
et al. |
December 30, 2004 |
Spinel and process for making same
Abstract
This invention pertains to product and process. The product is a
transparent product of a density in excess 99.5% comprising spinel
and having uniform mechanical properties. The process pertains to
fabrication of a transparent spinel product comprising the steps of
dissolving a sintering aid in water to form a neutral sintering aid
solution, adding a suitable additive to the sintering aid solution,
applying the sintering aid solution to spinel particles to form a
spinel dispersion, sub-dividing or atomizing the spinel dispersion
to form droplets comprising one or more spinel particles coated
with the final spinel solution, drying the droplets to form dried
coated particles comprising one or more spinel particles coated
with a dried layer of the sintering aid, and densifying the dried
coated particles to form a transparent spinel product having
uniform optical and mechanical properties in absence of grains of
exaggerated size.
Inventors: |
Villalobos, Guillermo R.;
(Springfield, VA) ; Sanghera, Jas S.; (Ashburn,
VA) ; Bayya, Shyam S.; (Ashburn, 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: |
33539465 |
Appl. No.: |
10/601884 |
Filed: |
June 24, 2003 |
Current U.S.
Class: |
501/108 ; 419/19;
419/23; 419/32; 419/35; 428/701; 501/118; 501/120 |
Current CPC
Class: |
C04B 2235/78 20130101;
C04B 2235/3206 20130101; C04B 2235/658 20130101; C04B 35/64
20130101; C04B 2235/3222 20130101; C04B 2235/6581 20130101; C04B
35/6264 20130101; C04B 2235/661 20130101; C04B 35/62655 20130101;
C04B 35/6303 20130101; C04B 35/628 20130101; C04B 2235/3201
20130101; C04B 2235/3203 20130101; C04B 2235/445 20130101; C04B
2235/444 20130101; C04B 2235/72 20130101; C04B 35/62828 20130101;
C04B 2235/724 20130101; C04B 2235/784 20130101; Y10S 264/91
20130101; C04B 35/443 20130101; C04B 35/62886 20130101 |
Class at
Publication: |
501/108 ;
501/118; 501/120; 419/019; 419/023; 419/032; 419/035; 428/701 |
International
Class: |
C04B 035/443; B22F
001/02; C04B 035/10 |
Claims
What is claimed
1. A product that is essentially devoid of a sintering aid
components comprising spinel that has porosity of less than 0.2%,
is transparent to light having wavelengths in the range of 0.4-5.5
microns, has uniform properties, is devoid of grains larger than
about 1 mm and is devoid of grains of exaggerated size.
2. The product of claim 1 wherein its spinel grains are less than
about 300% of the average-sized grain.
3. The product of claim 1 having transparency in excess of 50% and
its spinel grains are within about 300% of the size of an average
grain.
2. The product of claim 1 wherein its spinel grains are less than
about 300% of the average-sized grain.
4. The product of claim 3 having transparency of at least 60% at a
wavelength of 4 microns and the spinel is a hard crystalline solid
selected from the group consisting of oxides of magnesium and
aluminum.
5. The product of claim 2 having transparency of at least 60% at a
wavelength of 4.0 .mu.m and the spinel is a hard crystalline solid
MgAl.sub.2O.sub.4.
6. A process for preparing a transparent ceramic product comprising
the steps of: (a) dissolving a sintering aid in a suitable solvent
to form a sintering aid solution, (b) applying the sintering aid
solution to ceramic particles to form a ceramic dispersion, (c)
sub-dividing the ceramic dispersion to form droplets comprising at
least one ceramic particle coated with the sintering aid solution,
(d) drying the droplets to form dried coated particles comprising
at least one ceramic particle coated with a dried layer of the
sintering aid, and (e) densifying the dried coated particles to
form a transparent ceramic product having uniform optical and
mechanical properties and being devoid of grains larger than about
1 mm and grains of exaggerated size.
7. The process of claim 6 wherein said densifying step is carried
by applying minimal pressure of about 50 psi to the coated
particles while raising temperature to above the melting
temperature of the sintering aid and pressing the dried coated
particles at above about 5000 psi while increasing temperature to
above about 1500.degree. C.
8. The process of claim 6 wherein said ceramic particles are spinel
MgAl.sub.2O.sub.4, wherein said densifying step is accomplished in
a hot press by ramping temperature from ambient to above
1500.degree. C.
9. The process of claim 6 wherein said ceramic particles are spinel
MgAl.sub.2O.sub.4, and wherein said densifying step is accomplished
by ramping steps to an elevated temperature with intermittent
holding periods to allow the sintering aid to liquify and
escape.
10. The process of claim 9 wherein the ramping steps are about
20.degree. C./minute and the holding periods are between the
ramping steps and are about one half hour.
11. The process of claim 10 wherein there is an initial ramping
step to a temperature of less than 100.degree. C. above the melting
point of the sintering aid followed by a holding period to liquify
the sintering aid, an intermediate step to less than about
550.degree. C. above the melting point of the sintering aid,
followed by a holding period to allow vaporized sintering aid or
components thereof to escape, and a final ramping step to above
1500.degree. C., followed by a holding period to fully densify the
dried coated particles to a transparent spinel product.
12. The process of claim 8 wherein the spinel particles making the
spinel dispersion have particle size in the range of 500 nm to 100
.mu.m; wherein the solvent includes water and an additive selected
from the group consisting of ethanol, isopropanol, and mixtures
thereof; and the ratio of water to hydrocarbon to LiF sintering aid
to spinel particles is about 220 ml, about 780 ml, 0.2 grams, and
10 grams, respectively.
13. The process of claim 12 wherein pH of the final sintering aid
solution is about 7
14. A process for preparing a transparent MgAl.sub.2O.sub.4 spinel
product having maximum transparency in excess of about 60%
comprising the steps of: (a) dissolving LiF sintering aid in water
to form a sintering aid solution of about a neutral pH, (b) mixing
the sintering aid solution and a volatile, low surface
tension/viscosity additive selected from the group consisting of
ethanol, isopropanol, and mixtures thereof, (c) applying the
sintering aid solution to MgAl.sub.2O.sub.4 spinel particles to
form a spinel dispersion, (d) atomizing the spinel dispersion to
form droplets comprising at least one spinel particle coated with
the final spinel solution, (e) drying the droplets to form dried
coated particles comprising at least one spinel particle coated
with a dried layer of the sintering aid, and (f) densifying the
dried coated particles to form a transparent MgAl.sub.2O.sub.4
spinel product having uniform optical and mechanical properties in
absence of grains of exaggerated size.
15. The process of claim 14 wherein said densifying step is
accomplished in a hot press by ramping temperature from ambient to
above about 1500.degree. C.
16. The process of claim 15 wherein said densifying step is
accomplished by ramping steps to an elevated temperature with
intermittent holding periods to allow the sintering aid to liquify
and escape.
17. The process of claim 17 wherein the ramping steps are about
20.degree. C./minute and the holding periods are between the
ramping steps and are about one half hour.
18. The process of claim 17 wherein there is an initial ramping
step to a temperature of about 950.degree. C. followed by a holding
period to liquify the sintering aid, an intermediate ramping step
to about 1200.degree. C., followed by a holding period to allow
vaporized sintering aid to escape, and a final ramping step to
above 1500.degree. C., followed by a holding period to fully
densify the dried coated particles to a transparent spinel product.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention pertains to the field of sintered ceramics,
particularly magnesium aluminum spinel, and to a process for
preparing sintered ceramic articles from ceramic powders.
[0003] 2. Description of Related Art
[0004] Sintering is defined as the act of consolidating powder into
a dense shape. The powder being sintered must additionally not melt
to a great extent, some melting of secondary phases in the powder,
or surface melting is allowed under this definition. If the
material completely melts, the process is referred to as fusion
casting. Sintering, both presureless and with pressure, or hot
pressing, requires solid, liquid or gas material transport to
consolidate an aggregate of loose powder particles into a dense
shape. In the case of porcelains and clay products, secondary
phases do melt and "glue" the primary solid particles together with
a glassy 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 they must therefore, be
added. For small samples, the powdered sintering aids are mixed
with the powder to be sintered with a mortar and pestle. In larger
samples, mixing is accomplished by ball milling, attritor milling,
high shear wet milling, and variations or combinations of these
methods.
[0005] Spinel is defined as a crystalline structure of the type
AB.sub.2O.sub.4 where A is a 2+ cation occupying tetrahedral
lattice site in an oxygen cubic close packed structure and B is a
3+ cation occupying octahedral lettice site. In a preferred
embodiment, spinel is MgAl.sub.2O.sub.4 consisting of an oxide of
magnesium and aluminum. Spinel powder can be prepared by wet
chemistry, solid state diffusion of oxides or calcination. Spinel
powder particles consist of crystallites which are less than 500 nm
in size that can also be agglomerated into larger sizes varying
from 500 nm to 100 .mu.m, more typically 1-50 .mu.m.
[0006] Spinel is important because it is strong and transparent
from visible to 5.5 .mu.m wavelength. Its mechanical properties are
several times greater than that of glass and make it a leading
candidate for use as a transparent armor and window material.
Commercially, it can be used as a stronger and thinner window for
many applications including lap top computers, cell phones,
automotive glassing and headlamps, aerospace windshields, and
industrial blast shields.
[0007] Dense, transparent spinel articles are not currently
available from a commercial source although there are companies
currently trying to develop a viable manufacturing process. Since
there is no viable manufacturing process, the cost of spinel
products is so high that even the military avoids its use.
[0008] Difficult to sinter materials, such as spinel, are typically
mixed with a sintering aid or a secondary material that aids in
densification. The sintering aids work in a variety of fashions.
The sintering aids may liquefy at or somewhat below the primary
material's densification temperature thereby promoting liquid phase
sintering. Certain sintering aid materials exhibit higher
solid-state diffusion coefficients than the primary material's
self-diffusion coefficient. The secondary material may conversely
have a lower solid-state diffusion coefficient that prevents
exaggerated grain growth and promotes grain boundary refinement and
pinning. The sintering aid may also simply clean or etch the
primary material's surfaces thereby enhancing solid-state
diffusion. These are broad examples of the mechanisms by which
sintering aids enhance densification. In actual practice, sintering
aids may not fit into just one of the categories outlined and the
same aid may have different functions in different material
systems, or have no effect in other systems.
[0009] Sintering aids tend to be solid inorganic particles at room
temperature. Sintering aid particles henceforth are defined as
comprising crystallites (.ltoreq.500 nm), crystals (>500 nm),
and agglomerates of crystallites and/or crystals. Since the
materials to be densified are generally also solid inorganic
particles, the two materials must be mixed homogeneously for the
sintering aid to be effective. This is accomplished by some form of
mechanical mixing. However, due to the nature of particle-particle
interactions, the mixture is far from homogeneous. Inhomogeneity in
the mixture results in areas that have too much sintering aid and
other areas that have little or no sintering aid. This is a major
problem in the fabrication of transparent ceramics, electronic
ceramics, and in high tech refractory ceramics.
[0010] The Sellers et al U.S. Pat. No. 3,768,990 discloses an
optical element having transparency in the visible and infrared
wave lengths that is made by heating at an elevated temperature a
composition having sub-micron particle size of magnesium oxide and
aluminum oxide having uniformly mixed therethrough 0.2-4% by weight
of powdered LiF. It is believed that optical and mechanical
properties of the Seller's optical element are negatively impacted
by the inhomogeneous presence of substantial amount of LiF. This
leads to microstructural regions that are highly porous and other
microstructural regions that exhibit exaggerated grain growth, all
of which lead to inferior optical and mechanical properties. This
has prevented the use of spinel in practical applications since the
Seller's patent issued in 1973. Furthermore, it is believed that
the atomic concentrations of lithium and fluorine will be greater
than about 1000 ppm and 100 ppm, respectively due to the fact that
LiF is well known to react with alumina, which Seller's uses as a
starting powder.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
[0011] It is an object of this invention to make spinel products or
products that contain spinel that are moderately priced.
[0012] It is another object of this invention to increase
transmission of spinel products from about 5% at 0.525 .mu.m, which
is relatively opaque, to at least 50% at 0.525 .mu.m, which is
transparent.
[0013] It is another object of this invention to eliminate or
reduce the Hot Isostatic Pressing (HIP) procedure in the
fabrication of the spinel products.
[0014] Another object of this invention is the more uniform
distribution of the sintering aid on the spinel particles.
[0015] Another object of this invention is to increase
densification of sintered spinel products beyond 99.8%.
[0016] Another object of this invention is the more complete and
more uniform covering on surfaces of the spinel particles by a
sintering aid which is achieved by dissolving the sintering aid in
a solvent to create a sintering aid solution and then adding the
spinel particles in the sintering aid solution to form a
dispersion, also referred to as a ceramic dispersion.
[0017] These and other objects of this invention can be achieved by
a transparent spinel product that is fabricated by sintering and
densifying spinel powder in absence of the HIP procedure by
dissolving a sintering aid in a solvent to form a sintering aid
solution, adding spinel particles in the sintering aid solution to
form a dispersion, maintaining the dispersion in a state which
discourages precipitation of the solid sintering aid, spray-drying
the dispersion to form spinel particles coated with a sintering
aid, and densifying the coated spinel particles to form a
transparent product.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic illustration of the process of coating
spinel particles with LiF sintering aid and formation of the final
densified MgAl.sub.2O.sub.4 product;
[0019] FIG. 2 is a schematic illustration of a preferred
spray-drying system;
[0020] FIG. 3 is a graph of percent (%) Transmission versus
Wavelength of densified spinel products wherein the curve marked
#1, representing prior art, is mechanically mixed spinel particles
with 0.5% by weight LiF sintering aid as opposed to spray-dried
spinel particles with 0.5% by weight (curve #2) and 2.0% (curve #3)
LiF sintering aid, which represent the invention herein. Curve #4
represents theoretical transmission.
[0021] FIG. 4 is a representation of three discs made from coated
spinel particles and correspond to curves #1, #2 and #3 in FIG. 3
showing pictorially opaque disc #1, which represents prior art, and
transparent discs #2 and #3, which represent this invention.
[0022] FIG. 5 is a table showing relative parameters of spinel
(MgAl.sub.2O.sub.4) and glass;
DETAILED DESCRIPTION OF THE INVENTION
[0023] This invention pertains to a sintered and transparent spinel
product and to a process for making it which is characterized by
spraying a dispersion consisting of spinel particles in a sintering
aid solution to form spinel particles coated with the sintering
aid. The sintering aid coating on the spinel particles need not be
continuous, although it should be sufficient to prevent a large
number of sites where the particles contact each other without an
intervening layer of a sintering aid.
[0024] The sintered and densified spinel product of this invention
is novel and unobvious when compared to prior art. Preparation of a
spinel product by prior art procedure yields a product that is not
transparent but opaque afrer hot pressing and requires extended
HIPing to render the product transparent, however, with consequent
loss of optical and mechanical properties. To explain more fully,
the prior art product can be made transparent by extending the
HIPing procedure beyond the customary period of on the order of a
day, however, prolonging the hot processing and/or the HIPing
procedure introduces exaggerated grain growth which renders the
resulting product non-uniform in terms of optical and mechanical
properties such as elastic modulus, flexure strength and fracture
toughness, properties which are paramount for spinel character. The
novel and unobvious product, made in absence of the HIP procedure,
as described herein, is transparent over the wavelength range of
about 0.3-5.5 .mu.m, has uniform optical and mechanical properties
and its grains are smaller than about 1 mm, typically smaller than
about 500 .mu.m. The spinel product of this invention does not have
the exaggerated grain growth or is essentially devoid of grains of
exaggerated size. There are no grains larger than about 1 mm.
Exaggerated grain growth typically leads to grains that are greater
than 3 times larger to several orders of magnitude larger than an
average sized grain. Transparency of the spinel product of this
invention, made in the manner disclosed herein and in absence of
the HIP procedure, is above about 50% and up to about 90%.
[0025] In the preparation of the sintered spinel product of this
invention, the sintering aid, such as LiF or any other suitable
sintering aid such as NaCl, NaF, LiCl, etc., is dissolved in a
suitable solvent, typically water, to form a sintering aid
solution. The sintering aid can be in any condition, however, it is
typically particulate with particle sizes in the range of 500 nm-10
.mu.m. This, of course, is not important because the sintering aid
is eventually dissolved to form a sintering aid solution. To
enhance spraying of the sintering aid solution, ethanol or
isopropanol or another suitable diluent is admixed with the aqueous
sintering aid solution to form a modified sintering solution, which
henceforth is referred as the sintering aid solution. Aqueous
solution of LiF by itself is not readily sprayable using an
ultrasonic atomizer in absence of a low surface tension/viscosity
liquid component, which is typically a suitable additive, such as
an alcohol. The overriding consideration in adding another
component to water is to enhance solubility of the sintering aid
and the sprayability of the modified sintering aid solution. If the
sintering aid is other than LiF, other component(s) known to a
person skilled in the art may be used. A typical sintering aid
solution is prepared by admixing 0.2 g LiF sintering aid, 220 ml
water, and 780 ml ethanol or another like component. Typically, the
ratio of the components is on this order of magnitude. The
sintering aid solution should not contain too much sintering aid,
such as about in excess of about 10% by weight.
[0026] Coating of the spinel particles can be effected in any
desired manner in order to deposit a uniform layer of the sintering
aid on the particles. The sintering aid coating can be applied onto
the spinel particles in any suitable manner such as in a fluidized
bed, by a wet chemistry technique, by CVD, plasma enhanced CVD,
laser assisted deposition, by sputtering, by an evaporation
technique, and the like. The coating need not be continuous, but
should be sufficient to prevent a large number of sites where the
particles contact each other without an intervening layer of a
sintering aid material. Spraying of the final sintering aid
solution can also be used to deposit at least a monolayer of the
sintering aid on the spinel particles. Another way of accomplishing
this objective is simply to immerse the spinel particles in the
final sintering aid solution until the spinel particles acquire a
coating of the sintering aid of sufficient thickness and sufficient
uniformity.
[0027] The sintering aid solution is maintained in a state where it
is on the spinel particles and the sintering aid is in solution and
not precipitated on the spinel particles. This may require
adjustment of temperature, pH, and/or another parameter(s) to
discourage precipitation of the sintering aid on the particle
surface. If the sintering aid is LiF, precipitation thereof can be
discouraged or prevented by maintaining a neutral pH of about 7 of
the sintering aid solution.
[0028] The spinel particles are typically in the range of 500 nm to
100 .mu.m and amount of the sintering aid on the spinel particles
is typically 0.05-10% on weight basis, more typically 0.1-2% by
weight of the spinel particles. FIG. 1 is illustrative of the
coating process and shows spinel particles 12 coated with sintering
aid 14, sintered to produce product 16. Spinel particles must be
insoluble in mixtures of water and the additives used.
[0029] The dispersion resulting when spinel particles are mixed
with the sintering aid solution is delivered to an atomizer where
the dispersion is sprayed, causing sub-division into droplets which
are transported into a drying zone where vaporizable matter is
removed from the droplets and the solid coating is formed thereon.
The droplet size can be up to 1000 .mu.m but is typically up to 500
.mu.m, more typically 5 nm to 250 .mu.m, and especially 50 nm to 50
.mu.m. Any suitable atomizer can be used, including mechanical,
piezoelectric (ultrasonic) and electrostatic, as long as droplets
containing the desired number of spinel particles are formed and
the preponderance of resulting coated droplets are completely or
hermetically sealed or coated.
[0030] Whatever atomizer is used, chemistry of the dispersion
should be such as to prevent premature precipitation of the coating
on the spinel particles, and the droplets issuing from the atomizer
should contain at least one of the spinel particles per droplet.
Typically, an ultrasonic atomizer is used at a variable frequency
since size of a droplet can be controlled by varying atomizer
frequency. For instance, at atomizer frequency of 20 kHz, droplets
of about 90 .mu.m can be formed; at frequency of 40 kHz, droplets
of about 45 .mu.m can be formed; and at 80 kHz, droplets of about
20 .mu.m can be formed. As is apparent, the inverse relationship
between atomizer frequency and the droplet size can be used to
control the droplet size.
[0031] The higher the temperature in the drying zone, the shorter
residence time is required of the droplets in the zone to have the
vaporizable matter removed from and the coating formed thereon. The
temperature in the zone should be high enough to drive-off volatile
matter from the particles but not so high as to impart thermal
damage to the particles or the coating. Furthermore, temperature in
the zone should be high enough to drive-off volatiles from the
droplets in a reasonable or desired time, which can be adjusted by
changing temperature in the zone, with higher temperature in the
zone reducing residence time of the droplets to form dry, coated
particles. Typically, depending on many factors, temperature in the
drying zone should be in excess of about 100.degree. C. and below
600.degree. C., more typically 200.degree. C.-500.degree. C.; and
speed of the droplets through the drying zones in the droplet
direction is typically 0.1-1000 cm/sec, more typically 50-500
cm/sec. When moving in the zone, the droplets are entrained in hot
air or in an inert gas or a reactive gas. Residence time in the
zone is instantaneous to a fraction of a minute, typically 0.1-10
seconds.
[0032] FIG. 2 illustrates a preferred spray drying system 210
composed of a pair of 5-foot long silica tubes of 1/8 of an inch in
wall thickness joined end to end to form a continuous vertically
disposed conduit about 10 feet long. The spinel dispersion is taken
to an atomizer and introduced into the drying system through the
top. Three heat or drying zones 212, 214, 216, are arranged around
the conduit. The first heating zone 212 is disposed around the
upper portion of the conduit but about 3/4 of a foot below the top
of the conduit where it maintains a temperature of 150.degree. C.
within the conduit; the second heating zone 214 is disposed around
the lower portion of the conduit and spaced 5 feet below the bottom
of the first heating zone where it maintains a temperature of
350.degree. C. within the conduit; and the third heating zone 216
is disposed around the bottom portion of the conduit and spaced
half a foot below the bottom portion of the second heating zone
where it maintains a temperature of 430.degree. C. Below the third
heating zone 216 is a 2-foot long section 218 that is a
continuation of the conduit and below section 218 is a half-foot
long conical section 220 which terminates in an opening 2 inches in
diameter. If temperature in the sections is not sufficiently high,
LiF sintering aid will remain wet and the droplets will stick
together, however, if the droplets are dried too fast, the
deposited LiF will spall off the spinel droplets. If the given
temperature profile disclosed herein is adhered to, satisfactory
drying in a reasonable time is attained.
[0033] Below conical section 220 is cyclone separator 222 wherein
the coated particles (dried droplets) are separated from the gas
stream and left in collection bin 224. Suction hose 226 transports
the gaseous components to exhaust. Coated particles from the
collection bin are taken to densification.
[0034] The spray-dried coated particles, which can contain at least
one spinel particle, are hot-pressed in an inert atmosphere at
about 1550.degree. C. and 5000 psi for about 2 hours to obtain a
transparent monolithic sintered spinel product of less than 0.2%
porosity. When in the hot press, minimal pressure of about 50 psi
is initially applied until the coated spinel powder starts to
densify at about 1100.degree. C. and thereafter, pressure is raised
to about 5000 psi when a temperature of about 1450.degree. C. is
reached. In a preferred embodiment, the heating schedule in this
densifying procedure, which is conducted under initial vacuum of
about 10.sup.-4 Torr, includes about a 20.degree. C./minute ramp
from ambient temperature to 950.degree. C., about a half hour hold
to allow the sintering aid LiF to melt and clean/etch the spinel
particle surfaces, another 20.degree. C./minute ramp to
1200.degree. C., another half hour hold to allow vaporized
sintering aid to leave the hot press, still another 20.degree.
C./minute ramp to about 1550.degree. C., a 2-hour hold to fully
densify the spinel powder into a transparent shape, and turning off
the heating elements and the hydraulic pump to allow pressure to
bleed-off and temperature of the product in the hot press to cool
to about room temperature under a vacuum of less than 10.sup.-3
Torr. Melting point of LiF is 850.degree. C. and its boiling is
about 2000.degree. C. but starts vaporizing above its melting
point. The heating and pressure schedule can be modified to
accommodate other sintering aids.
[0035] In densifying the coated particles, it is important to keep
in mind the necessity of removing the sintering aid, such as LiF,
at the lowest possible temperature below about 1000.degree. C. to a
level below about 500 ppm lithium from the sintering aid LiF, more
typically below about 100 ppm lithium, and below about 50 ppm
fluorine from the sintering aid LiF, more typically below about 10
ppm fluorine, hereafter, referred to as essentially devoid of the
sintering aid components, in this case lithium and fluorine. It is
necessary to abide by these limits since at higher temperatures,
reaction between the sintering aid and spinel is made more likely
with the imminent production of impurities, such as lithium
aluminate, lithium aluminum oxyfluoride and lithium aluminum
fluoride which may negatively affect properties, such as the
optical and mechanical properties.
[0036] Although the HIP procedure is typically omitted in this
novel and unobvious process, and thus the cost of resulting product
is reduced by 1/3 to 1/2, it may be desirable to subject the
product of this invention to the HIP procedure of short duration to
positively alter transparency, porosity or some other parameter(s)
of the product. The HIP procedure for a sintering aid, like LiF, is
carried out in an inert atmosphere at temperature above
1500.degree. C., such as about 1750.degree. C., and at a pressure
above 20,000 psi, such as about 30,000 psi, to obtain a transparent
product of porosity on the order of less than 0.2%. Duration of the
HIP procedure in the past has been 1 day, however, with the process
of this invention, HIP duration may be as short as about 1 hour and
is typically 1-4 hours, when used. Generally, density of spinel
products should be greater than 99.5% to have transparency since
such products with density of less than about 99.5% are milky white
and opaque.
[0037] The novel process disclosed herein is unobvious since it
yields a transparent product after hot pressing in absence of the
HIP procedure; reduces hot pressing conditions, provides for better
reproducibility; increases yield; provides for smaller average
grain size; reduces porosity; and minimizes exaggerated grain
growth. Generally, the smaller and the more uniform grain size, the
better the optical properties, up to a point. Once the particles
become too small, such smaller than about 0.05 .mu.m, strength, and
possibly other parameters are negatively impacted.
[0038] FIG. 3 is a graph which shows variation of transmission at
different wavelengths of light with the manner of preparing
sintered and densified spinel experimental disks which were 1" in
diameter and 1 mm in thickness. The disks were polished, first with
SiC and then with diamond before use. The curves in FIG. 3 are
identified as #1, #2, #3 and #4. Curve #1 was made by testing disks
prepared by mechanical mixing 0.5% by weight of the sintering aid
LiF with spinel particles, and is not representative of the
invention herein: curve #2 was made by testing disks prepared by
spraying 0.5% by weight of the sintering aid LiF pursuant to the
invention herein; curve #3 was made by testing disks prepared by
spraying 2% by weight of the sintering aid LiF pursuant to the
invention herein; and curve #4 represents theoretical or a solid
monolith of spinel MgAl.sub.2O.sub.4 where porosity was essentially
zero. Data for the theoretical curve #4 was obtained from
literature. Spinel powder particles that were used to prepare disks
#1, #2 and #3 correspond to curves #1, #2 and #3 of FIG. 3, were in
the range of 500 nm-10 .mu.m. In preparing disk #1, the spinel
particles and the particulate LiF sintering aid were mixed in a
mortar and pestle for about 5 minutes whereas for disks #2 and #3,
a sintering aid (LiF) solution was prepared, as described above,
and used to spray-dry a LiF coating on the spinel particles. For
disks #1, #2 and #3, densification in a hot press was carried out
at initial vacuum of 10.sup.-4 Torr pursuant to the following
schedule: 20.degree. C./minute ramp from ambient to 950.degree. C.;
30-minute hold period; another 20.degree. C./minute ramp from
950.degree. C. to 1200.degree. C.; another 30-minute hold period;
another 20.degree. C./minute ramp from 1200.degree. C. to
1550.degree. C.; 2-hour hold period; and an extended cool down and
pressure bleed-off period. Transparency or opacity of the disks
corresponding to the curves #1, #2 and #3 is illustrated in FIG. 4
where disk #1 is shown as being opaque, disk #2 is shown as being
partially transparent, and disk #3 is shown as being transparent.
From FIG. 3, it is apparent that in the visible region of 0.4-0.7
.mu.m, only disk #3 reaches transparency in the area of about 55%.
It should be noted that, based on the data for curve #4 in FIG. 3,
transmission drops from about 80% to about nil at a wavelength of
about 0.3 .mu.m.
[0039] The coating thickness on the particles can be varied, inter
alia, by adjusting dilutions of the coating solution and/or by
adjusting frequency of the atomizer; if an ultrasonic atomizer is
used. For purposes herein, it has been found that coating thickness
in the range of 1-1000 nm, more typically 2-200 nm is suitable.
Uniformity of coating thickness was confirmed by scanning electron
microscopy.
[0040] FIGS. 5 shows superior properties of the magnesium aluminum
spinel product compared to glass.
[0041] Having described the invention, the following examples are
given as particular embodiments thereof and to demonstrate the
practice and advantages thereof. It is undersood that the examples
are given by way of illustration and are not intended to limit the
specification or the claims in any manner.
EXAMPLE 1
[0042] This example details the use of lithium fluoride (LiF)
sintering aid as a coating on magnesium aluminate
(MgAl.sub.2O.sub.4) spinel particles. The use of LiF coating allows
the hot pressing of the coated spinel particles in an inert
atmosphere into a sintered transparent shape that has 70%
transmission, as is detailed in Ex. 2.
[0043] The sintering aid in this example was 0.2 grams of LiF
powder with a particle size in the range of 500 nm-10 .mu.m. The
LiF was initially dissolved in 220 ml of deionized water by mixing
for about a quarter of one hour followed by addition of 780 ml of
ethanol with mixing for about another quarter of one hour.
[0044] Ten grams of the spinel powder was mixed with the LiF
sintering aid solution to form a dispersion that was delivered to
an ultrasonic atomizer at a rate of 30 ml/minute using a metering
pump and sprayed. The LiF sintering aid solution was at a neutral
pH of about 7 which prevented LiF dissolved in the solution from
precipitating on the spinel particles in the dispersion. The
atomizer was operated at a frequency of 40 kHz and produced a fine
stream of 45-micron droplets, containing 1-3 spinel particles, that
were passed through a drying system illustrated in FIG. 2 at a rate
of about 9 cm/second before being collected in a cyclone separator
to form dried coated particles.
[0045] The droplets from the atomizer were introduced into the
dryer system where the first drying zone was 1 foot from the top
and temperature therein was 150.degree. C. The second drying zone
was 4 feet from below the first and was also 1 foot in length, as
were all the others, but its temperature was 350.degree. C. The
third drying zone was 1/2 foot below the second and its temperature
was 430.degree. C. Below the third drying zone, there was a 2-foot
straight section followed by a three quarter of a foot conical
section. The dried coated particles issuing from the conical
section through a 2-inch opening were directed to a cyclone
separator where the dried coated particles were separated and kept
in a bin and the gaseous components were removed through the
suction hose. The coated particles from the bin were later removed
to be densified. The coated particles were characterized by X-ray
diffraction, scanning electron microscopy, and x-ray fluorescence.
The coated particles were characterized as containing spinel
(MgAl.sub.2O.sub.4) particles with a LiF coating.
EXAMPLE 2
[0046] This example provides details as to densification of the
dried coated spinel particles prepared in the manner described in
Ex. 1, above. The product had transmission of 70% and was prepared
in a manner that did not include the expensive HIP procedure.
[0047] The coated particles in powder form and prepared as
described in Ex. 1, above, were placed in grafoil-lined graphite
hot press die and the die was placed in an inert argon atmosphere
(or a vacuum of 10.sup.-4 Torr). Minimal pressure was applied until
the powder started to densify at about 1100.degree. C. and pressure
was stepped up to about 5000 psi when the temperature of about
1450.degree. C. was attained. The heating schedule included a
20.degree. C./minute ramp from ambient to 950.degree. C., a
30-minute hold to allow LiF to melt and clean/etch the spinel
particle surfaces, a 20.degree. C./minute ramp from 950.degree. C.
to 1200.degree. C., a 30-minute hold to allow vaporized LiF, and
probably other components, to escape the hot press die, a
20.degree. C./minute ramp to 1550.degree. C., and a 2-hour hold to
fully densify the spinel powder into a transparent shape. The
heating elements and the hydraulic pump were then turned off to
allow natural cooling of the hot press and allow the pressure to
bleed-off, which took about 3 hours.
[0048] While presently preferred embodiments have been shown of the
novel and unobvious sintered spinel products and their preparation,
persons skilled in this art will readily appreciate that various
additional changes and modifications can be made without departing
from the spirit of the invention as defined and differentiated by
the following claims.
* * * * *