U.S. patent application number 15/454549 was filed with the patent office on 2017-06-29 for transparent spinel article and tape cast methods for making.
The applicant listed for this patent is Corning Incorporated. Invention is credited to Michael Edward Badding, Weiguo Miao, Nathan Michael Zink.
Application Number | 20170183265 15/454549 |
Document ID | / |
Family ID | 53762313 |
Filed Date | 2017-06-29 |
United States Patent
Application |
20170183265 |
Kind Code |
A1 |
Badding; Michael Edward ; et
al. |
June 29, 2017 |
TRANSPARENT SPINEL ARTICLE AND TAPE CAST METHODS FOR MAKING
Abstract
A transparent, tape casted, spinel article, as defined herein.
Also disclosed is a method of method of making the tape casted,
transparent spinel, and laminates of the tape casted, transparent
spinel, as defined herein.
Inventors: |
Badding; Michael Edward;
(Campbell, NY) ; Miao; Weiguo; (Horseheads,
NY) ; Zink; Nathan Michael; (Painted Post,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Corning Incorporated |
Corning |
NY |
US |
|
|
Family ID: |
53762313 |
Appl. No.: |
15/454549 |
Filed: |
March 9, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14736935 |
Jun 11, 2015 |
9624136 |
|
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15454549 |
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62019649 |
Jul 1, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 35/638 20130101;
C04B 2235/612 20130101; C04B 35/443 20130101; C04B 2235/661
20130101; C04B 2235/725 20130101; C04B 2237/704 20130101; C04B
2235/3222 20130101; C04B 2235/6562 20130101; C04B 2235/9653
20130101; C04B 35/634 20130101; C04B 2235/726 20130101; C04B
2235/721 20130101; C04B 2235/77 20130101; C04B 2235/785 20130101;
C04B 2235/6025 20130101; C04B 2235/6585 20130101; C04B 2235/6565
20130101; C04B 2235/96 20130101; C04B 2235/606 20130101; C04B
2235/5454 20130101; C04B 2235/5409 20130101; C04B 2235/728
20130101; C04B 2235/663 20130101; C04B 2235/608 20130101; C04B
35/6455 20130101; C04B 2235/786 20130101; C04B 35/6261 20130101;
C04B 2235/6567 20130101; C04B 2237/343 20130101; C04B 2235/72
20130101; C04B 2235/3206 20130101; C04B 35/6264 20130101; C04B
2235/5445 20130101; C04B 35/62625 20130101 |
International
Class: |
C04B 35/443 20060101
C04B035/443; C04B 35/634 20060101 C04B035/634; C04B 35/645 20060101
C04B035/645; C04B 35/626 20060101 C04B035/626 |
Claims
1.-6. (canceled)
7. A method of making a tape casted, transparent spinel,
comprising: attrition milling, for from 10 min to 10 hrs to form a
slurry, a batch mixture comprising a spinel powder having a mean
particle size of from 75 to 500 nanometers, a binder, a dispersant,
a plasticizer, a defoaming agent, and an aqueous solvent; degasing
the resulting isolated slurry under vacuum; tape casting the
degased slurry to a wet thickness of from 20 to 2,000 micrometers;
controlled drying using an under-bed heater and heated flowing air
at from 20 to 100.degree. C. of the tape casted slurry to form a
green tape having a dry thickness of from 5 to 1,000 micrometers;
and firing the green tape for a sufficient time and temperature to
provide a sintered transparent spinel.
8. The method of claim 7 further comprising laminating the green
tape into a plurality of green tape layers and then forming to
provide a laminated transparent spinel.
9. The method of claim 7 wherein the spinel powder has a purity of
from 99.5% to 99.9 wt %, and the spinel powder has a low sulfur
content of from 0.01 wt % to 0.001 wt %.
10. The method of claim 7 wherein the spinel powder, prior to
introduction into the slurry, has a BET surface area of from 2 to
30 m.sup.2/g, and the microstructure of the green tape is
homogeneous.
11. The method of claim 7 wherein the spinel powder has a ceramic
solids loading in the slurry of from 5 to 60 vol %, and the green
tape has a ceramic solids loading of from 35 and 85 vol %.
12. The method of claim 7 wherein firing the green tape comprises:
sintering the green tape and accomplishing binder burn out at 1500
to 1600.degree. C. for 2 to 8 hrs to obtain a sintered transparent
spinel; hot isostatic pressing the sintered transparent spinel at
1500 to 1600.degree. C. for 4 to 12 hr, and a pressure of from 5 to
60 kpsi to reduce residual porosity in the spinel, wherein the
total porosity of the sintered transparent spinel after hot
isostatic pressing is less than about 500 ppm; and oxygen hot
isostatic pressing of the HIP sintered transparent spinel at 1000
to 1200.degree. C. for 2 to 8 hrs, and a pressure of from 0.2 to 30
kpsi, to reduce objectionable color centers in the resulting
oxygen, hot isostatic pressed, sintered transparent spinel.
13. The method of claim 7 wherein firing is accomplished free of a
sintering aid.
14. The method of claim 7 wherein the aqueous solvent is deionized
water, and the binder, dispersant, plasticizer, defoaming agent,
and aqueous solvent have a pH from 9 to 12.
15. The method of claim 7 wherein at least one step of the method
is accomplished in a particulate controlled environment.
16. The method of claim 7 wherein the green tape has a porosity of
from about 0.01 to about 25 vol %.
17. The method of claim 7 further comprising shape forming the
green tape into a desired shape.
18. The method of claim 7 further comprising filtering the degased
slurry to remove contaminants.
19. The method of claim 7 further comprising surface finishing the
sintered transparent spinel to a desired thickness and surface
texture.
20. The method of claim 8 wherein the sintered tape or the sintered
tape laminate is insensitive to slight variations in either the
amount of the binder in the tape cast slurry or the amount of the
porosity, if the green tape ceramic solids loading is greater than
45 volume percent.
Description
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn.119 of U.S. Provisional Application Ser. No.
62/019,649 filed on Jul. 1, 2014 the content of which is relied
upon and incorporated herein by reference in its entirety.
[0002] The entire disclosure of publications and patent documents
mentioned herein are incorporated by reference.
BACKGROUND
[0003] The present disclosure generally relates to a tape casting
method for making thin transparent spinel and laminate transparent
spinel.
SUMMARY
[0004] In embodiments, the present disclosure provides one or more
of:
[0005] a tape casting method of making a transparent spinel
sheet;
[0006] a tape casting method of making a transparent spinel sheet
based on an aqueous binder system;
[0007] a tape casting method which provides a uniform green
microstructure throughout the thickness of the casted tape;
[0008] a tape casting method of making a transparent spinel sheet
without or in the absence of a sintering aid;
[0009] a tape casting method of making a transparent spinel sheet
that provides a uniform green tape having a solids-loading of, for
example, from 35 to 85 vol %;
[0010] a tape casting method of making a transparent spinel sheet,
which method provides very thin tapes, for example, of from 10
micrometers to 1 millimeter via tape casting, suitable for
lamination to a desired thickness of several centimeters or more;
and
[0011] a tape casting method of making a transparent spinel sheet,
which method can be accomplished using either aqueous tape casting
or non-aqueous tape casting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] In embodiments of the disclosure:
[0013] FIGS. 1A to 1I show an exemplary flow chart for the
disclosed tape casting process accomplished on a laboratory
scale.
[0014] FIG. 2 shows experimental results from tapes made using
three powders having different properties, and the effect on volume
percent spinel in the tape.
[0015] FIG. 3 shows SEM images for the as-received powders (top),
and the prepared green tapes (bottom).
[0016] FIG. 4 shows TGA analysis for a binder system to determine a
superior binder burn-off schedule for the tape.
[0017] FIG. 5 shows long (triangles) and short(circles) sintering
curves used for binder removal and air sintering of spinel
parts.
[0018] FIG. 6 shows the HIP sintering cycle having conditions
defined herein.
[0019] FIG. 7 shows the results of the subsequent O.sub.2 HIP
processing, which removes color centers, and moves the
transmittance value close to the maximum.
[0020] FIG. 8 shows the measured transmittance curve for parts
having a thickness of about 325 micrometers that were prepared by
tape firing, lamination, and polishing.
[0021] FIG. 9 is a plot that shows the effect of lamination
pressure on solids loading in the green tape, which pressure
removes tape porosity through compression.
[0022] FIG. 10 is a Weibull probability plot of unpolished parts
(left side points) and polished parts (right side points) and
demonstrates that polishing increases the average part
strength.
DETAILED DESCRIPTION
[0023] Various embodiments of the disclosure will be described in
detail with reference to drawings, if any. Reference to various
embodiments does not limit the scope of the invention, which is
limited only by the scope of the claims attached hereto.
Additionally, any examples set forth in this specification are not
limiting and merely set forth some of the many possible embodiments
of the claimed invention.
Definitions
[0024] "Volume percent solids loading," "vol % solids loading," or
like expressions refer to the inorganic solids in the casted tape.
Vol % solids loading only takes into account the inorganic
components (i.e., spinel). Typical vol % solids loading can be, for
example, from 45 to 65 vol %, from 50 to 65 vol %, from 55 to 65
vol %, from 60 to 65 vol %, including intermediate values and
ranges.
[0025] "Tape green density" refers to the combination of the spinel
powder (the inorganic component) and the binder system (the organic
component) in the tape in g/cm.sup.3. Green density is a
representation of the amount of porosity in the tape, which
considers both the organic and inorganic components. Typical tape
green density can be, for example, from 75 to 95% depending, for
example, on the starting powder and organic content. Table 8
provides a comparison of tape composition (vol %) and tape green
density (g/cm.sup.3) of disclosed example formulations.
[0026] "Transmittance" refers to the fraction of incident light at
a specified wavelength that passes through a sample.
[0027] "Transparency" refers to the property of the spinel that
permits light to pass through without being scattered.
[0028] "Include," "includes," or like terms means encompassing but
not limited to, that is, inclusive and not exclusive.
[0029] "About" modifying, for example, the quantity of an
ingredient in a composition, concentrations, volumes, process
temperature, process time, yields, flow rates, pressures,
viscosities, and like values, and ranges thereof, or a dimension of
a component, and like values, and ranges thereof, employed in
describing the embodiments of the disclosure, refers to variation
in the numerical quantity that can occur, for example: through
typical measuring and handling procedures used for preparing
materials, compositions, composites, concentrates, component parts,
articles of manufacture, or use formulations; through inadvertent
error in these procedures; through differences in the manufacture,
source, or purity of starting materials or ingredients used to
carry out the methods; and like considerations. The term "about"
also encompasses amounts that differ due to aging of a composition
or formulation with a particular initial concentration or mixture,
and amounts that differ due to mixing or processing a composition
or formulation with a particular initial concentration or
mixture.
[0030] "Optional" or "optionally" means that the subsequently
described event or circumstance can or cannot occur, and that the
description includes instances where the event or circumstance
occurs and instances where it does not.
[0031] The indefinite article "a" or "an" and its corresponding
definite article "the" as used herein means at least one, or one or
more, unless specified otherwise.
[0032] Abbreviations, which are well known to one of ordinary skill
in the art, may be used (e.g., "h" or "hrs" for hour or hours, "g"
or "gm" for gram(s), "mL" for milliliters, and "rt" for room
temperature, "nm" for nanometers, and like abbreviations).
[0033] Specific and preferred values disclosed for components,
ingredients, additives, dimensions, conditions, times, and like
aspects, and ranges thereof, are for illustration only; they do not
exclude other defined values or other values within defined ranges.
The composition and methods of the disclosure can include any value
or any combination of the values, specific values, more specific
values, and preferred values described herein, including explicit
or implicit intermediate values and ranges.
[0034] In embodiments, the present disclosure provides a tape
casting method for making a thin sheet of transparent spinel.
Spinel is a ceramic having excellent transmission properties in the
visible spectrum and in portions of the ultraviolet and infrared
wavelength regions. Transparent spinel has been widely researched
for over forty years. However, due to issues related to hydrolysis,
a commercially viable aqueous-based slurry process for spinel
manufacture has not been reported. Consequently, all of the past
forming work has focused on the dry pressing of spinel powder
using, for example, a uni-axial, a cold isostatic, or a hot
isostatic press. A major issue with dry pressing is the
non-uniformity of the green body, which results in inferior
attributes of the final product.
[0035] Dry pressing can also present limitations in the overall
thickness of parts that can be made. In general, thicknesses of 1
mm can be achieved. However, high aspect ratio parts need to be
much thicker than this, for example, 1 cm or more.
[0036] In embodiments, the present disclosure provides a range of
sheet thickness from thin sheets, for example, having a thickness
of from 10 to 15 micrometers, to thicker sheets, for example,
having a thickness of from 40 to 100 micrometers, which thin
sheets, intermediate thickness sheets, thick sheets, or
combinations thereof, can be laminated to achieve a desired
thickness of up to several centimeters or more.
[0037] Existing processes have also encountered considerable
difficulty in attaining both transparency and strength targets. In
the existing processes, parts must be fully densified, typically
through use of a lithium fluoride (LiF) sintering aid, to achieve
optical transparency. The sintering aid causes exaggerated grain
growth and can result in a typical grain size larger than 100
micrometers, post-sintering. This results in a dramatic decrease in
strength. The presently disclosed tape casting process yields
extremely high green densities, for example, 75 to 95% of the
theoretical tape density, which eliminates the need for sintering
aids. This allows grain sizes to remain smaller, e.g., less than 5
micrometers, resulting in far superior sintered strength compared
to spinel parts prepared with a sintering aid. The reduced grain
size results in higher sintered part strength. Parts that were
prepared with a sintering aid typically have a strength of, for
example, 100 to 200 MPa, corresponding to the grain size 100 to 200
micrometers. In contrast, disclosed parts prepared without the
sintering aid have a smaller grain size and have a sintered
strength of, for example, 300 to 500 MPa, corresponding to grain
size less than 10 micrometers.
[0038] One industry practice is to use hot pressing to form
transparent spinel. Due to part-size and throughput limits, this is
an expensive process. Accordingly, potential product applications
are constrained to cost-indifferent markets such as military and
defense systems, or niche products. Practical use in broader
markets such as consumer electronics is foreclosed. Another
advantage of the disclosed process is the significantly decreased
cost for transparent spinel products, creating significant
commercial potential.
[0039] In embodiments, the disclosure provides both aqueous- and
non-aqueous-based tape-casting methods and provide data
demonstrating the production of transparent spinel having highly
desirable properties.
[0040] In embodiments, the disclosure provides a transparent, tape
casted, spinel article, comprising:
[0041] at least one of:
[0042] a single layer thickness of 10 to 300 micrometers, or
[0043] a laminate comprising a plurality of laminated single
layers, the laminate having a thickness of 50 micrometers to 10
millimeters, or more;
[0044] the spinel article having:
[0045] a transparency of 80% to 87%, e.g., 84% to 87% based on 87%
maximum theoretical transparency; and
[0046] a sintered grain size of, for example, from 1 to 7
micrometers, from 2 to 7 micrometers, from 3 to 6 micrometers, from
4 to 5 micrometers, including intermediate values and ranges.
[0047] In contrast, the prior art mentions transparent tape casted
spinel using a sintering aid, such as lithium fluoride, having a
sintered grain size of from 100 to 200 micrometers, see for
example,
armorline.com/pdf/ArmorLine%20Corp-Transparent%20Spinel%20Brochure.pdf.
[0048] In embodiments, the spinel article comprises a spinel powder
having a narrow median particle size diameter of from 80 to 500
nanometers and a relatively low BET surface area (SA) of from 5 to
30 m.sup.2/g prior to firing.
[0049] In embodiments, the spinel comprises a spinel powder having
a narrow median particle size diameter of 100 to 300 nanometers and
relatively low BET surface area 6 to 15 m.sup.2/g prior to
firing.
[0050] In embodiments, the spinel article can have a Knoop hardness
number measured with a 200 g load of from 10 to 16 GPa, of from 11
to 15 GPa, of from 12 to 15 GPa, such as 14.1 GPa, including
intermediate values and ranges.
[0051] Sintered grain size measurements were obtained for three
transparent spinel samples that were prepared from S15CR spinel
particles using different hot isostatic pressing (HIPing)
conditions:
[0052] 4.6 microns at 1550.degree. C., 4 hr HIP;
[0053] 3.6 microns at 1500.degree. C., 4 hr HIP; and
[0054] 2.8 microns at 1475.degree. C., 4 hr HIP.
[0055] In embodiments, the spinel article has a purity of from
99.5% to 99.9 wt %, and has a low sulfur content of from 0.01 wt %
to 0.001 wt %, or less.
[0056] In embodiments, the disclosure provides a method of making a
tape casted transparent spinel, comprising:
[0057] attrition milling, for from 10 min to 10 hrs, preferably
from 30 mins to 6 hrs, and more preferably from 1 to 3 hours, to
form a slurry, a batch mixture comprising a spinel powder having a
mean particle size of from 75 to 500 nanometers, preferably from
100 to 500 micrometers, and more preferably from 200 to 400
micrometers, a binder, a dispersant, a plasticizer, a defoaming
agent, and an aqueous solvent;
[0058] degasing the resulting isolated slurry under vacuum;
[0059] tape casting the degased slurry to a wet thickness of from
20 to 2,000 micrometers, preferably from 50 to 1,000 micrometers,
and more preferably from 100 to 500 micrometers;
[0060] controlled drying of the tape casted slurry using an
under-bed heater and heated flowing air at from 20 to 100.degree.
C., preferably from 40 to 80.degree. C., and more preferably from
60 to 70.degree. C., of the tape casted slurry to form a green tape
having a dry thickness of from 5 to 1,000 micrometers, preferably
from 20 to 500 micrometers, and more preferably from 40 to 200
micrometers; and
[0061] firing the green tape for a sufficient time and temperature
to provide the sintered transparent spinel.
[0062] In embodiments, the method can further comprise laminating
the green tape into a plurality of green tape layers, that is two
or more layers or multiple layers, such as from 2 to about 28
layers, wherein the laminating can be accomplished by, for example,
compression methods.
[0063] In embodiments, the spinel powder, prior to introduction
into the slurry, has a BET surface area of from 2 to 30 m.sup.2/g,
more preferably from 5 to 20 m.sup.2/g, and even more preferably
from 6 to 15 m.sup.2/g, and most preferably 6 to 8 m.sup.2/g. In
embodiments, the surface area of one preferred spinel powder was 15
m.sup.2/g, and having a mean particle size distribution of from 50
to 1,000 nm, more preferably from 100 to 600 nm, and even more
preferably from 200 to 400 nm. A caveat regarding PSD measurement
methods is that they measure agglomerate size and are not
especially reliable for nano powders.
[0064] In embodiments, the spinel powder has a ceramic solids
loading in the slurry of, for example, from 5 to 60 vol %,
preferably from 10 and 40 vol %, and more preferably from15 to 25
vol %, and the green tape has a ceramic solids loading of, for
example, from 35 and 85 vol %, preferably from 45 and 75 vol %, and
more preferably from 55 and 75 vol %, which solids loading levels
permit sintering of the ceramic to high transparency. In
embodiments, a preferred green tape had a solids loading from 45 to
from 55 vol %.
[0065] In embodiments, firing the green tape can comprise, for
example:
[0066] sintering the green tape and accomplishing binder burn out
(BBO), for example, at 1500 to 1600.degree. C. for 2 to 8 hrs to
obtain a sintered transparent spinel;
[0067] hot isostatic pressing ("HIPing"), such as in an inert gas
atmosphere of argon, the sintered transparent spinel, for example,
at 1500 to 1600.degree. C. for 4 to 12 hr .degree. C., and a
pressure, for example, of from 5 to 60 kpsi, preferably from 10 to
40 kpsi, and more preferably from 20 to 30 kpsi. In embodiments, a
successful hot isostatic pressing was accomplished at about 29 kpsi
or about 200 MPa, to reduce residual porosity in the spinel,
wherein the total porosity of the sintered transparent spinel after
hot isostatic pressing is less than about 500 ppm, preferably less
than about 200 ppm, more preferably less than about 100 ppm, even
more preferably less than about 50 ppm, including intermediate
values and ranges; and
[0068] oxygen hot isostatic pressing ("O.sub.2 HIP") of the HIP
sintered transparent spinel, for example, at 1000 to 1200.degree.
C. for 2 to 8 hrs, and a pressure of from 0.2 to 30 kpsi,
preferably from 1 to 20 kpsi, and more preferably from 5 to 10 kpsi
including intermediate value of ranges.
[0069] In embodiments, a successful oxygen hot isostatic pressing
was accomplished at 8 to 10 kpsi, to reduce objectionable color
centers in the resulting oxygen, hot isostatic pressed, sintered
transparent spinel. Hot isostatic pressing (HIP) is a manufacturing
process used to reduce the porosity and to increase the density of
many ceramic materials. This improves the material's mechanical
properties and workability. The HIP process subjects a component to
both elevated temperature and isostatic gas pressure in a high
pressure containment vessel. The most widely used pressurizing gas
is argon. An inert gas can be used, so that the material does not
chemically react. A HIP chamber is heated, causing the pressure
inside the vessel to increase. Many systems use associated gas
pumping to achieve the necessary pressure level. Since pressure is
applied to the material from all directions the pressing is
"isostatic".
[0070] In embodiments, the firing can be accomplished in one or two
steps to provide the sintered tape cast transparent spinel. In a
one-step firing process the BBO and sintering is accomplished in
same furnace, then H.sub.2 atmosphere or HIP. In a two-step firing
process the BBO and sintering are accomplished in separate
furnaces, then H.sub.2 atmosphere or HIP.
[0071] In embodiments, the firing can be accomplished free of a
sintering aid.
[0072] In embodiments, the aqueous solvent can be deionized
water.
[0073] In embodiments, the at least one step of the method is
accomplished in a particulate controlled environment, e.g., a Class
100 clean room, a Class 1000 clean room, or like controlled
environment having low or no particulate contamination.
Additionally, the particulate controlled environment can be
chemically clean and biologically sterile.
[0074] In embodiments, the green tape has a porosity, for example,
of from about 0.01 to about 25 vol %, preferably from 1 and 20 vol
%, and more preferably from 2 to 10 vol %, including intermediate
value of ranges. In embodiments, a successful green tape had a
porosity, prior to lamination processing, of from 7 to 17 vol
%.
[0075] In embodiments, the method can further comprise shape
forming, i.e., green forming, the green tape into a desired shape
or object, e.g., other than a sheet, or a windable tape.
[0076] In embodiments, the method can further comprise filtering
the degased slurry to remove contaminants, which contaminants are
large particulate or large scale contaminants, such as milling
media, etc., or agglomerates that are larger than the primary
particles.
[0077] In embodiments, the method can further comprise surface
finishing the sintered transparent spinel to a desired thickness
and surface texture.
[0078] In embodiments, sintering a tape or a tape laminate was
insensitive to either the amount of the binder in the tape cast
slurry, or the porosity of the green tape, if the green tape
ceramic solids loading was greater than 45 volume percent, that is,
advantageously slight variations in binder content or in green tape
porosity did not significantly alter the quality of the sintered
tape or sintered laminate products.
[0079] In embodiments, the microstructure of the preferred green
tape is homogeneous when observed in an SEM image. For example,
excellent particle de-agglomeration is observed. The particles are
approximately evenly spaced having binder and porosity situated
between the particles, see for example, FIG. 3 (lower middle image
of S15CR green tape) having an excellent microstructure, and FIG. 3
(lower right image of S8CR green tape) having a comparatively poor
microstructure (S8 tape). The differences in homogeneity among the
tapes prepared from different sized particles is readily
apparent.
[0080] In embodiments, a tape cast transparent spinel article can
be prepared by the preparative methods disclosed herein.
[0081] In embodiments, a tape cast and laminated transparent spinel
article can be prepared by the preparative methods disclosed
herein.
[0082] In embodiments, the present disclosure provides a tape
casting method of making a transparent spinel sheet.
[0083] In embodiments, the disclosure provides a tape casting
method of making a transparent spinel sheet having an aqueous
binder system including a dispersant. The binder system including a
dispersant has an relatively high basic pH, such as from 8.5 to 13,
from 9 to 12, from 9.5 to 11, from 9.5 to 10.5, including
intermediate values and ranges, which basic pH can be achieved
with, for example, aqueous ammonia, and which binder system
prevents hydrolysis and gelation of the spinel powder during slurry
preparation and casting.
[0084] In embodiments, the disclosure provides a tape casting
method of making a transparent spinel sheet, which method provides
a uniform green microstructure throughout the thickness of the
casted tape. The uniform green microstructure can be accomplished
by, for example, using a starting spinel powder having a
well-defined specific surface area of, for example, from 2 to 30
m.sup.2/g, more preferably from 5 to 20 m.sup.2/g, and even more
preferably from 6 to 8 m.sup.2/g, and a very narrow particle size
distribution of, for example, from 50 to 1,000 nm, more preferably
from 100 to 600 nm, and even more preferably from 200 to 400 nm.
Though extensive experimentation, it was discovered that it was
possible to make the disclosed transparent ceramic, such as
transparent spinel, using a tape casting method if the starting
spinel powder had a surface area and particle size distribution
powder properties corresponding to those described above.
[0085] In embodiments, the disclosure provides a tape casting
method of making a transparent spinel sheet, having a uniform green
tape having a solids-loading of from 35 to 85 vol %, more
preferably of from 45 to 65 vol %, and even more preferably of from
55 to 65 vol %, which solids-loading permits sintering to
transparency. Without a sufficiently high green density as obtained
by the disclosed method it was not possible to achieve a
transparent spinel. Though extensive experimentation, it was also
discovered that it was not possible to make the disclosed
transparent ceramic, such as transparent spinel, using a tape
casting method if the green tape solids-loading and resulting green
density were other than those described above.
[0086] In embodiments, the disclosure provides a tape casting
method of making a transparent spinel sheet, which method can make
very thin tapes of, for example, from 10 micrometers to 1
millimeter via tape casting, and the very thin tapes can then be
laminated to a desired thickness of several centimeters or more.
Although not wishing to be limited by theory, the thickness of the
transparent laminate available using the disclosed method is
limited only by the size of the available lamination equipment.
[0087] In embodiments, the disclosure provides a tape casting
method of making a transparent spinel sheet, which method can be
accomplished using either aqueous tape casting (e.g., water alone
or in combination with miscible solvents such as alcohols) or
non-aqueous tape casting if desired (e.g., with solvents such as
ethanol, toluene, or MEK, and a PVB or carbonate binder
system).
[0088] The disclosed composition, articles, and methods are
advantaged by providing, for example, at least one of the
following:
[0089] A tape casting method that forms high density green tapes
that allows sintering of the transparent spinel without the use of
a sintering aid. The absence of a sintering aid limits grain growth
during the sintering process. This results in a final grain size of
about 1 to 5 micrometers, compared to grain sizes of greater than
100 micrometers when using a LiF sintering aid and a hot pressing
method. The smaller grain size can provide significantly increased
strength to the resulting spinel when compared to other
commercially available spinels (e.g., Surmet or Armorline).
[0090] A tape casting and lamination method that is scalable to a
large scale and low cost compared to other commercially available
processes. For example, the disclosed tape casting method can
produce a transparent sheet material having properties similar to
synthetic sapphire crystal, at a fraction of the cost (e.g., 50% or
less).
[0091] A tape casting and lamination method that has flexibility to
make parts of different thicknesses.
[0092] A tape casting and lamination method that can make large,
thin, flat sheets for consumer electronics applications.
[0093] A tape casting and lamination method that uses existing
process equipment.
[0094] Referring to the Figures, FIGS. 1A to 1I show a flow chart
for the disclosed tape casting process.
[0095] FIG. 2 shows experimental results from tapes made using
three powders having different properties, and the effect on volume
percent spinel in the tape. Each powder has a different surface
area (SA) (30, 15, or 10.5 m.sup.2/g). The difference in green tape
solids loading is indicated by the three data points. The decrease
in SA of the powder allows for an increase in solids loading of the
tape. A higher solids loading allows for sintering to transparency.
Specific details of the BET surface area (SA) properties of the
three different powders tested are contained in Table 1. Preferred
tapes were made using least amounts of binder possible and which
amounts did not produce tape cracking. The lower surface area (SA)
powders having a relatively larger average particle size were able
to use less binder and achieve a higher tape density without
cracking.
TABLE-US-00001 TABLE 1 Spinel powder properties. Powder Name S30CR
S15CR S8CR BET SA (m.sup.2/g) 30.4 15.4 10.5
[0096] Table 2 summarizes the elemental analysis of the three
selected spinel powder samples and lists their impurity levels.
TABLE-US-00002 TABLE 2 Spinel powder elemental analysis (impurities
in ppm). Impurity S30CR S15CR S8CR Na 40 34 8.8 K 100 Not measured
19 Fe 2 7 5.6 Si 33 26 43 Ca 12 6 6 S <80 <80 <80
[0097] FIG. 3 shows SEM images for the as-received powders (top),
and the prepared green tapes (bottom). Both sets of images have a
two (2) micron scale bar. Of the powders tested the S15CR powder
had superior results. The S30CR powder has an extremely high SA,
and the tape needed a large amount of binder, such as about 15 to
20 vol % in the batch slurry and greater than about 50 vol % in the
dried casted tape, to prevent cracking of the tape during drying.
Although not limited by theory, each solid particle should be
surrounded by organic binder material to prevent cracking. With the
high binder content, the maximum solids loading in the tape was
only was 36 to 37 vol %, which is too low to sinter transparent
spinel. The S8CR has a higher SA, but had a non-uniform particle
size distribution (PSD), as can been seen in the SEM images in top
of FIG. 3. The PSD is bimodal, having large, strongly agglomerated
particles of up to 1 micrometer. This PSD does not allow for
uniform sintering. The S15CR powder provided tapes that were
superior to the tapes prepared from the S30CR or S8CR powders. The
S15CR powder has a narrow PSD and relatively low SA, which
characteristics allow for excellent tape formation and sintering.
Again, although not limited by theory, the available results
suggest that an even more preferable powder would be similar to
S15CR with the exception of having a slightly larger particle size,
e.g., about 300 micrometers, and a smaller SA of, for example,
about 7 m.sup.2/g.
[0098] FIG. 4 shows a TGA analysis for a binder system to determine
a superior binder burn-off phenomena for the material. Notable
temperatures were identified at 180, 350, and 600.degree. C., where
mass loss identified binder removal. Sintering curves based on this
analysis are shown in FIG. 5.
[0099] FIG. 5 shows long (triangles; slow) and short (circles;
fast) sintering curves used for binder removal and sintering of
spinel parts. Notable temperature holds for binder removal and to
minimize part warp/camber were at 180, 350, and 600.degree. C. The
sample was then sintered to 1550.degree. C. to densify the part for
Argon HIPing. The long sintering profile can be used for thicker or
larger parts to assure that binder removal is complete. For thin,
small parts the short sintering cycle has proven to be sufficient.
Table 3 and 4, respectively, list the slow and fast binder burn out
(BBO) schedules, and sintering schedules in greater detail. Longer
dwell times can be used at notable temperatures as needed depending
on part size. Alternatively, slower ramp rates can also be used for
BBO (binder burn-off). The BBO is not limited to only this heating
schedule.
TABLE-US-00003 TABLE 3 TGA based slow binder burnout (BBO) and
sintering details. Ramp End Step Total Start Temp Rate Dwell Temp
Time Time Step (.degree. C.) (.degree. C./hr) (hr) (.degree. C.)
(hrs) (hrs) 1 25 120 0 180 1.3 1.3 2 180 0 4 180 4.0 5.3 3 180 120
0 350 1.4 6.7 4 350 0 4 350 4.0 10.7 5 350 120 0 600 2.1 12.8 6 600
0 10 600 10.0 22.8 7 600 120 0 1550 7.9 30.7 8 1550 0 4 1550 4.0
34.7 9 1550 480 0 25 3.2 37.9
TABLE-US-00004 TABLE 4 TGA based fast binder burnout (BBO) and
sintering details. Ramp End Step Total Start Temp Rate Dwell Temp
Time Time Step (.degree. C.) (.degree. C./hr) (hr) (.degree. C.)
(hrs) (hrs) 1 25 120 0 180 1.3 1.3 2 180 0 2 180 2.0 3.3 3 180 120
0 350 1.4 4.7 4 350 0 2 350 2.0 6.7 5 350 120 0 600 2.1 8.8 6 600 0
4 600 4.0 12.8 7 600 120 0 1550 7.9 20.7 8 1550 0 4 1550 4.0 24.7 9
1550 480 0 25 3.2 27.9
[0100] After the initial air sintering process, the parts are argon
HIPed to sinter to transparency. An alternative route to this
process step includes increasing the green density of the parts,
using a binder burn out process, and then hydrogen sintering, which
is less costly than HIPing. Hydrogen sintering is a known process
for other transparent or translucent oxide materials such as
alumina for the lighting industry, and YAG for other advanced
ceramic applications.
[0101] FIG. 6 shows the HIPing sintering cycle and Table 5 provides
a listing of conditions. After argon HIPing (Ar HIP), parts were
oxygen HIPed (O.sub.2 HIP) to removed oxygen vacancies, which
vacancies can cause a darkening of the part.
TABLE-US-00005 TABLE 5 HIPing sintering cycle. Ramp Start Start
Rate End End Step Total Temp Pressure (.degree. C./ Dwell Temp
Pressure Time Time Step (.degree. C.) (psi) hr) (hr) (.degree. C.)
(psi) (hrs) (hrs) 1 25 1500 858 0 1450 28500 1.7 1.7 2 1450 28500 0
0.08 1450 29000 0.1 1.7 3 1450 29000 600 0 1550 29000 0.2 1.9 4
1550 29000 0 8 1550 29000 8.0 9.9 5 1550 29000 8100 0 200 29000 0.2
10.1 5 200 29000 2160 0 20 1500 0.1 10.2
[0102] After sintering, the sintered parts were polished as
desired. The majority of the defects observed in the parts,
although relatively minor, are due to surface contamination during
the process. These defects can be substantially decreased, for
example, by about 75% or more, when a proper cleanroom process
environment is selected.
[0103] The transmittance measurements were performed on: parts
after Ar HIPing (with color centers in); and parts after Ar HIPing
and then O.sub.2 HIPing, to remove the color centers. FIG. 7 shows
the results of the subsequent O.sub.2 HIP, which removes many color
centers, and moves the transmittance value (% T) to close to the
theoretical maximum.
[0104] FIG. 8 shows the measured transmittance curve for the
laminated, polished parts having a thickness of about 325
micrometers. Transmittance (% T) reaches the theoretical maximum
value of 87% limited by refractive index. The deviation of actual
from theoretical transmittance is within experimental error.
[0105] FIG. 9 is a plot that shows the effect of lamination
pressure on solids loading in the green tape. Batch composition of
Example 1 was used. The volume percent of spinel (diamonds; left
scale), organics (squares; left scale), and porosity (triangles;
right scale) is plotted as cast tape (a single layer; lamination
pressure=0), and five laminated samples each sample having a
thickness of twenty eight (28) laminate layers formed at different
lamination pressures of from 3,000 to 30,000 psi. A decrease in
porosity is observed as the lamination pressure increases over
3,000 to 30,000 psi. This results in an increase in green tape
solids loading from about 47% in the as-cast tape to about 54% in
the sample laminated at 30,000 psi. Increased spinel solids loading
in the green state allows for: improved sintering; lower
temperatures; and optionally hydrogen sintering. All tapes were
laminated at 70.degree. C., held at 1000 psi for 15 minutes, then
pressed to the target pressure of from 3,000 to 30,000 psi, and
held for an additional 15 minutes. The results listed in Table 6
are graphically illustrated in FIG. 9.
TABLE-US-00006 TABLE 6 Lamination Spinel Organic Pressure Solids
Solids Porosity (psi) (wt %) (wt %) (vol %) 0 46.8% 37.8% 15.5%
3000 48.2% 39.0% 12.8% 6000 49.3% 39.8% 10.9% 12000 51.2% 41.3%
7.5% 24000 53.1% 42.9% 4.0% 30000 54.0% 43.6% 2.4%
Example Batch Composition Formulation for Tape Cast Slurry.
Spinel Powder.
[0106] WB4101 is a proprietary acrylic binder having additives in
the solution. DF002 is a non-silicone de-foaming agent. DS001 is a
polymeric dispersant. PL005 is a high pH plasticizer. These
components were formulated for aqueous ceramic tape casting by
Polymer Innovations, Inc., of Vista, Calif.
[0107] The components in a tape cast slurry formulation of a S15CR
spinel particle batch composition are listed in Table 7A. The
quantities of each component are significant for forming the
disclosed tape, which tape doesn't crack, has a high green density,
and can be laminated in one or more layers, that is, a plurality of
layers, with itself or with other materials. This slurry has a
lower viscosity due to higher water content compared to, for
example, the slurry of Table 7B, and the tape cracked when trying
to cast thicker tapes having a dried thickness of greater than
about 50 micrometers.
[0108] The components in another tape cast slurry formulation of a
S15CR spinel particle batch composition are listed in Table 7B.
This formulation has improvements compared to the formulation of
Table 7A. The Table 7B slurry has lower water content, and higher
powder content. This leads to reduced drying stresses during the
tape casting process and allows for thicker tapes having minimal
cracking, up to about 100 micrometers in thickness.
[0109] Table 7C lists the quantities of each component for
preparing a tape cast slurry of an S15CR spinel particle batch
composition.
TABLE-US-00007 TABLE 7A Batch composition for tape cast slurry.
Volume Weight Component Density Percent Percent Name Supplier
H.sub.2O 1.00 60.25% 42.82% water -- NH.sub.4OH 1.00 3.32% 2.36%
ammonium -- hydroxide WB4101 1.03 18.28% 13.38% binder Polymer
Innovations PL005 1.03 1.08% 0.79% plasticizer Polymer Innovations
DF002 1.20 0.19% 0.16% defoamer Polymer Innovations DS001 1.03
1.72% 1.26% dispersant Polymer Innovations MgAl.sub.2O.sub.4 3.64
15.16% 39.23% S15CR Baikowski particles
TABLE-US-00008 TABLE 7B Batch composition for tape cast slurry.
Volume Weight Component Density Percent Percent Name Supplier
H.sub.2O 1.00 52.38% 36.28% water -- NH.sub.4OH 1.00 3.17% 2.20%
ammonium -- hydroxide WB4101 1.03 25.10% 17.91% binder Polymer
Innovations PL005 1.03 0.80% 0.57% plasticizer Polymer Innovations
DF002 1.20 0.22% 0.18% defoamer Polymer Innovations DS001 1.03
1.85% 1.32% dispersant Polymer Innovations MgAl.sub.2O.sub.4 3.64
16.48% 41.54% S15CR Baikowski particles
TABLE-US-00009 TABLE 7C Batch composition for tape cast slurry.
Volume Component Density Percent Name Supplier H.sub.2O 1.00 45 to
60% water -- NH.sub.4OH 1.00 0 to 5% ammonium -- hydroxide WB4101
1.03 15 to 30% binder Polymer Innovations PL005 1.03 0 to 5%
plasticizer Polymer Innovations DF002 1.20 0 to 5% defoamer Polymer
Innovations DS001 1.03 0 to 5% dispersant Polymer Innovations
MgAl.sub.2O.sub.4 3.64 10 to 30% S15CR Baikowski spinel powder
General Overview of the Tape Casting Process
[0110] A conventional tape casting process and apparatus are
disclosed and illustrated in "Principles of Ceramic Processing" by
James S. Reed, 1995, 2.sup.nd Ed., ISBN-13: 978-0471597216.
[0111] The following description introduces the disclosed method of
making and identifies differences from conventional tape casting
processes.
[0112] A lab scale tape casting process representative of the
disclosed method of making in shown in FIGS. 1A to 1I and as
discussed further below.
[0113] Batching (FIG. 1A): spinel powder was mixed with a
water-based tape casting system including a binder, a dispersant, a
plasticizer, and a defoaming agent.
[0114] Milling (FIG. 1B): The batched material was milled and mixed
in a mill by, for example: ball milling; high shear mixing;
attrition milling; vibratory milling; roller milling; and like
methods.
[0115] Degasing (FIG. 1C): After milling was completed, the milling
media was strained from the slurry, and the slurry was
de-aired/degased using a vacuum. This removes entrapped air from
the milled product that would otherwise end up as bubbles within
the mix.
[0116] Filtration: The slurry was optionally filtered to remove any
large scale contamination from the mixture that would otherwise
give adverse optical properties in the sintered material. Filtering
can be accomplished with, for example, 50 micrometers, 25
micrometers, 10 micrometers, or 1 micrometer filters made of, for
example, nylon, fiber, or other suitable materials.
[0117] Tape Casting (FIG. 1D): The slurry was then tape cast under
a doctor blade at a desired thickness to form a wet, thin sheet of
ceramic slurry. The wet tape was dried under controlled conditions
to form a thin ceramic/polymer composite tape, referred to as a
tape in the "green state" or alternatively "green tape", which can
be formed to the desired shape.
[0118] Blanking (FIG. 1E): Blank (i.e., punch cut) the desired part
geometry from a roll of tape from the tape casting process.
Blanking forms the near net shape. Next, the final part can be
formed in the green state, or formed in a post sintering process,
for example, with various finishing methods, such as cutting,
polishing, and like finish operations.
[0119] Stacking (FIG. 1F): Blanked layers were stacked to achieve
desired part thickness after sintering. Interleaf material can be
inserted between stacked parts as needed to laminate multiple parts
at the same time under the same lamination process conditions.
[0120] Lamination (FIG. 1G): Desired, multiple layers of the
ceramic tape can be stacked and laminated using uniaxial or
isostatic pressing to create a thicker tape. This is an optional
step and is only necessary if the desired tape thickness cannot be
achieved in the as-cast state.
[0121] Green Forming: The green tape is optionally formed to the
desired shape using any suitable ceramic forming techniques, for
example: laser cutting; hot knife cutting; punching; stamping;
pressing; and like methods, or combinations thereof. Alternatively
or additionally, the tape can be fired and then formed to shape in
the sintered state using, for example, laser cutting or
slicing.
[0122] Firing/Sintering (FIG. 1H): The tape can be fired in a one
or two-step process. The one step process removes binder and
sinters the tape in a single firing. In the two step process the
binder is removed in one furnace and then the part is sintered to
final density in a second furnace. Final firing can be achieved
using a hydrogen furnace, vacuum furnace, hot isostatic press
(HIP), N.sub.2 or Ar furnace, or an air furnace. Typical firing
temperatures can be, for example, of from 1400 to 1800.degree.
C.
[0123] Polishing (FIG. 1I): After firing, the parts can be ground,
lapped, and polished to the final desired thickness and surface
finish. For example, parts can be rough ground to achieve
coplanarity of the top and bottom surface using, for example,
silicon carbide or diamond, papers, or slurries. Yttria and alumina
are also commonly used polishing agents that could be substituted
as polishing agents. After achieving coplanarity, the surfaces can
be polished using subsequently finer diamond slurries or tapes
(e.g., 9, 6, 3, and 1 micrometer are typical sizes), and the final
polishing can use a 1 micrometer diamond slurry. Finer polishes may
be selected for optical applications. The polishing procedure may
take from 1 to 24 hours, more typically 2 to 12 hours, and most
typically 4 to 8 hours, including intermediate values and
ranges.
[0124] Referring to FIG. 10, a Weibull probability plot of
unpolished parts (left side points) and polished parts (right side
points) shows that polishing can increase the average ring-on-ring
(ROR) part strength from 137 to 373 MPa, or provide about a 3 fold
(2.72 fold by calculation) increase, or about a 172% increase in
average part strength.
Significant Aspects of the Process
[0125] Milling (FIG. 1B): The milling process is a significant step
which must provide fully deagglomerated particles and create a
uniform, that is, a well dispersed, slurry. An attrition mill
(aka.: stirred ball mill), from Union Process, is a preferred mill
for achieving deagglomeration, i.e., breaking up agglomerates or
nano-agglomerates of the spinel powder, which deagglomeration is
difficult to efficiently and economically achieve with alternative
methods. The attrition mill has benefits over other milling
processes and equipment due to the high energy in-put to the
materials during the milling process. This allows a batch to be
milled to smaller particle sizes in a shorter period of time
compared to other techniques, for example, 1 to 3 hours versus 50
to 100 hrs with ball milling.
[0126] One attrition mill used had a total volume of 750 mL and a
working volume (working capacity) of 250 mL. The tank was loaded
with 130 mL of slurry, and 740 grams of 1 mm 99.9% pure
Al.sub.2O.sub.3 media (i.e., grinding media) from Union process.
The tank was water cooled to 15.degree. C. during the milling
process to avoid overheating, and reduce evaporation of solvent(s).
The slurry was initially milled for 5 minutes at 500 rpm to break
down large agglomerates, then the speed was increased to 1300 rpm
and milled for 1 hour. At the end of milling the tank was slowed to
170 rpm and a defoaming agent was added to remove entrapped air.
The slurry was then poured through a 80 to 120 mesh screen to
remove the milling media from the slurry before deairing.
[0127] Deairing (Degasing) Process (FIG. 1C): After straining the
milling media from the slurry, the slurry was deaired. Deairing was
accomplished with a desiccator chamber and then a Mazerustar vacuum
planetary mixer. The slurry was loaded into a desiccator chamber
and de-aired for up to 10 minutes. After the initial deairing, the
slurry was loaded into the planetary mixer and operated under
vacuum for 5 minutes. An alternative deairing procedure that can
eliminate the Mazerustar mixer is to use a higher vacuum in the
desiccator chamber.
[0128] Binder System: The organic binder composition is significant
to the disclosed superior spinel tape casting process. A binder
system from Polymer Innovations was used that includes an acrylic
based binder that is soluble in high pH water, for example, from
about pH 9 to 12. This binder system allows a stable spinel slurry
system, which does not flocculate and gel before tape casting.
[0129] Tape Casting: Initially tape casting was performed in a
standard lab environment. Samples prepared in the lab had high
amounts of contamination which drastically reduced the optical
quality of the material. The casting portion of the process was
then moved to a class 1000 clean room. This eliminated greater than
75% of the observed defects in the material. Accomplishing the
milling, deairing, and lamination steps in a clean room can reduce
the contamination levels even further. Samples were tape cast on a
silicone coated Mylar.RTM. film, which was approximately 50 to 100
micrometers thick. The silicone coating provides easy release of
the tape material after drying. Other suitable films for tape can
be, for example, Teflon.RTM., glass, a metal belt, and like
alternative materials. The slurry was passed under a doctor blade
which had a gap of about 4 to 20 mils (i.e., 100 to 500
micrometers), typically a 4 mil (100 micrometers) blade height was
used, to form a thin sheet of ceramic tape. After drying the
thickness of the tape was 40 to 60 micrometers thick, and after
sintering the thickness was about 20 to 40 micrometers thick. The
casting blade was moved across the Mylar.RTM. at, for example, a
speed of 10 mm/sec. The speed can be varied as needed to increase
process speed, and modify the thickness of the tape.
[0130] Lamination Process: The roll of green tape, which can be,
for example, from the size of a sheet of paper, to several meters
wide and several hundred meters long, was blanked (punched/cut)
into a desired rough shape, for example, 1, 2, or 3 inch squares,
and 1 or 3 inch diameter circles. The orientation of the tape was
marked so the casting direction, and top and bottom surface of the
tape is known for later orientation. If the morphology of the
ceramic particles is anisotropic they may preferentially align in
the casting direction causing differential shrinkage in the x-y
direction resulting in part camber upon sintering. Additionally, it
is possible for the polymeric chains of the binder system to
preferentially align in the casting direction also contributing to
non-uniform shrinkage. The top and bottom surface of the tape may
contain different amounts or concentrations of binder and porosity
due to drying kinetics. This can result in non-uniform shrinkage in
the z-direction (out-of-plane), another possible source of part
camber. With the casting direction known, the tape blanks were
stacked. To mitigate the effects of preferential particle
orientation during casting, the tapes are rotated, 90 degrees to
one another per layer (additionally, no rotation, or 180 degree
rotation can be used). The tapes are typically stacked on top of
one another without flipping. The bottom surface of the first layer
is placed on the top surface of the next layer, and the sequence
repeated for the desired number of layers. Due to drying kinetics,
the bottom surface of the tape will typically contain a higher
concentration of binder, while the top surface is more porous. By
placing the layers atop of one another the high binder surface is
compressed to the porous surface during the lamination process. It
is possible to laminate two high binder surfaces together, however
if two porous surfaces are laminated typically delamination was
observed, unless the binder concentration in the tape is high
enough to fill in the pores even at the top surface of the tape. A
certain amount of porosity in the green tape is necessary to allow
compression during the lamination step. Typically, a 5 to 10%
porosity was targeted, but lower or higher porosities were also
satisfactory. Using the desired rotation and stacking technique,
the tapes are placed on top of one another and stacked to the
desired number of layers. For example, from 4 to 28 layers of a
green tape having a thickness of about 40 micrometers each, gave
green tape laminates having a thickness of from 160 to 1120
micrometers. However, stacks having several hundred layers are
possible if a thicker part is desired.
[0131] In various lamination examples, from 4 to 28 layers of tape
were stacked, top-to-bottom, and orienting each added tape layer by
rotating 90 degrees. The stack of tape(s) was placed between two
pieces of silicone coated Mylar.RTM. ("bookends") to allow for
release after the lamination process. A piece of pressure
indicating paper can be placed on top of the Mylar.RTM. to
visualize the pressure distribution after the process was
completed. The pressure paper changes from white to red, and the
darker red color, the higher the applied pressure.
[0132] The stack having the "bookends" was placed between two metal
plates, vacuum sealed in a bag, and isostatically laminated
(alternatively a uniaxial press can be used). Typical pressures
used were 3,000 to 5,000 psi, at from 60 to 80.degree. C. However,
pressures from 1,000 to 10,000 psi, and 60 to 100.degree. C., can
be used, and more preferably 4,000 to 5,000 psi, at 70.degree. C.
The stack sample was placed in the 70.degree. C. preheated
laminator and pre-heated for 15 minutes with no or low pressure
(e.g., 150 psi). The sample was then ramped to the desired pressure
(e.g., 3,000 psi) and held for 15 mins. After the cycle was
complete the pressure was released and the samples were removed
from the chamber. The samples were allowed to cool to room
temperature and removed from the lamination plates and Mylar
carrier film. The sample "part" was then moved to the
de-bind/sinter step, or the part can be formed in the green state
using a punching or cutting method.
[0133] Firing Processes: After lamination, the green bodies went
through a binder burnout (BBO) and a sintering process, as shown in
FIG. 1H. After sintering, the fired spinel body has over 94%
density, indicating that substantially all the pores were closed.
To eliminate the any residual porosity, hot isostatic pressing
(HIP) was used. The HIP schedules for slow and fast BBO are listed
in Tables 3 and 4, respectively. A high power graphite furnace was
used for the HIPing ("Ar HIP") process to achieve the high
temperatures (e.g., 1,500.degree. C. or above). After argon HIPing,
the parts were dark, for example, showing color centers. Although
not limited by theory, the color centers are believed to be related
to oxygen vacancies within the parts, which act as light absorption
centers. To increase the transmittance, the color centers are
preferably removed from the parts. An "O.sub.2 HIP" process was
used to remove the color centers. During the "O.sub.2 HIP" process,
the parts were HIPed in a mixture of 80 vol % Ar and 20 vol %
O.sub.2 atmosphere, at about 1,000 psi or above (e.g., 10kpsi or
5,500 psi) and 600.degree. C. above (e.g., 1,100.degree. C.) for
several hours. Due to the high pressure of O.sub.2, the color
centers are eliminated by removal of the oxygen vacancies.
[0134] Table 8 lists tape cast compositions that were prepared from
different spinel powder starting materials based on particle
properties and different binders ratios, and having a correlation
to green tape density.
[0135] Tapes were successfully prepared using the S15CR powder and
having a range of powder and binder ratios. Higher binder content
tape resulted in a lower cracking tendency, however these tapes
were limited to low green tape thicknesses to prevent cracking.
Samples prepared with the S15CR powder needed a top weight during
firing to hold the tape flat and minimize warping. All compositions
produced good sintered part transparency independent of tape
composition. Tapes made having a 50:50 or 55:45 spinel:organic
ratio are preferred as they provided the best tradeoff between tape
quality, transparency, and flatness.
[0136] In contrast, tapes prepared using the S30CR powder were only
castable at very high binder contents and had limited tape quality,
transparency, and flatness. The ability to process the S30CR based
tapes was poor due to the low thicknesses required to prevent
cracking. Often tape tearing would occur during handling due to the
extremely thin layer thickness. Translucent sintered tapes could be
obtained, but the limitations due to high surface area and small
particle size make the S30CR powder less suitable for a tape
casting process.
[0137] The S8CR powder enabled better tape quality (i.e.,
thickness) and sintered part flatness at the expense of
transparency. Due to the hard agglomerates and bimodal particle
size distribution of the S8CR starting powder, it was not possible
to create transparent parts after sintering. However, the improved
green density of the tape allowed for production of thicker tapes
without cracking and flat sintering without top weights on the
samples.
[0138] All parts prepared during early experimental work showed
some level of defects attributable to the proof-of-concept lab
scale process, which introduced defects to the laminated
layers.
[0139] Table 9 lists observed properties for the sintered tape cast
part properties of the compositions listed in Table 8.
TABLE-US-00010 TABLE 8 Tape cast compositions. Theoretical Tape
Spinel Composition Actual Tape Composition Green Density Powder
(vol %) (vol %) (g/cm.sup.3) ID Spinel Organics Spinel Organics
Porosity Theo. Actual % Theo S30CR 48 52 37 40 23 2.30 1.78 77%
S30CR 50 50 36 36 28 2.34 1.71 73% S30CR 55 45 N/A N/A N/A 2.47 N/A
N/A S30CR 59 41 N/A N/A N/A 2.55 N/A N/A S15CR 50 50 44 44 12 2.33
2.04 88% S15CR 55 45 46 38 16 2.46 2.08 85% S15CR 59 41 47 33 20
2.57 2.05 80% S15CR 64 36 51 30 19 2.68 2.17 81% S15CR 67 33 47 22
31 2.80 1.92 69% S8CR 56 44 51 41 8 2.48 2.28 92% S8CR 60 40 51 35
14 2.59 2.22 86% S8CR 65 35 51 28 21 2.71 2.15 79%
TABLE-US-00011 TABLE 9 Sintered tape cast part properties. Observed
Part Properties Powder Tape Quality.sup.1 Transparency.sup.2
Flatness.sup.3 Defects.sup.4 S30CR - o o o S30CR - o o o S30CR N/A
N/A N/A N/A S30CR N/A N/A N/A N/A S15CR + + o o S15CR o + o o S15CR
o + o o S15CR - + o o S15CR - + o o S8CR + - + o S8CR + - + o S8CR
o - + o .sup.1Tape Quality N/A = Tape cracked and not usable;
unable to determine property. - = Only extremely thin (i.e., less
than 40 microns) tapes were castable, which avoided tape cracking;
a small amount of tape cracking may be present. o = Able to make
acceptable tapes having 40 to 60 micron thickness without cracking.
+ = Able to make greater than 60 micron thick tapes without
cracking. .sup.2Transparency N/A = Tape cracked and not usable;
unable to determine property. - = Parts are opaque after sintering.
o = Parts are translucent after sintering. + = Parts are
transparent after sintering. .sup.3Flatness N/A = Tape cracked and
not usable; unable to determine property. - = Parts have warping
after sintering. o = Parts can be sintered flat with a weight on
top. + = Parts are flat without a weight. .sup.4Defects N/A = Tape
cracked and not usable; unable to determine property. - = Large
amount of defects due to raw material contamination and
agglomeration of particles. o = Some small defects present due to
processing contamination. + = Parts were free of defects.
[0140] The particle size values were all derived from BET surface
area analysis. It is difficult to measure individual nano scale
particles. A formula was used to calculate average particle size
for individual particles:
d=6.times.10.sup.3/(.rho..times.S.sub.BET)
where d is the diameter or average particle size (in nm), .rho. is
the density of spinel (3.58 g/cm.sup.3), and S.sub.BET is the BET
measured surface area (in m.sup.2/g).
[0141] Table 10 lists the calculated average particle size
diameters (d) obtained from the BET surface area analysis of
commercial powders S30, S15, and S8. Although not limited by theory
the "TARGET S" particle properties are expected, based on
predictive modeling, to provide tape casted parts having superior
properties to the actual tape casted parts disclosed herein (see
also prophetic Example 2).
TABLE-US-00012 TABLE 10 Spinel powder particle properties. Spinel
Powder Density Sample BET(m.sup.2/g) (g/cm.sup.3) d(nm) S30 30 3.58
56 S15 15 3.58 112 S8 10 3.58 168 TARGET S 6 3.58 279
Non-Aqueous Tape Casting
[0142] A non-aqueous tape casting process related to the disclosed
aqueous process to make transparent spinel was also demonstrated.
Spinel tapes can be made with an ethanol-based solvent system and
polyvinyl butyral binder. Commercial spinel powder, Baikowski
S30CR, was attrition milled for 1 hr, using 1 mm diameter 3YSZ
milling media, to break up agglomerates. The composition of the
mill batch (MB) is listed in Table 11.
TABLE-US-00013 TABLE 11 Non-aqueous Mill batch (MB) composition.
wt. in wt. fraction Ingredient MB in MB Ethanol 115.65 0.671018
Butanol 27.9 0.16188 Propylene 6.3 0.036554 Glycol Dyspersbyk- 22.5
0.130548 118
[0143] Dyspersbyk-118 is a commercial dispersant from BYK-Chemie.
Polyvinylbutyral binder (Butvar B98), dibutylpthalate plasticizer,
and extra solvent were mixed in the mill batch to make a casting
slip with component fractions shown in Table 12.
TABLE-US-00014 TABLE 12 Non-aqueous tape casting. Material
Properties Slurry Properties Tape (calculated) Actual Tape Wt %
Volume Volume Weight Vol % in Wt % in Weight Vol % in Wt % in
Component Solids Density Percent (cm.sup.3) Wt % (g) Tape Tape (g)
Tape Tape Ethanol 0% 0.785 61.22% 19.08 43.55% 14.977 -- -- 14.98
-- -- Butanol 0% 0.810 14.31% 4.46 10.51% 3.613 -- -- 3.61 -- --
Propylene 0% 1.030 2.54% 0.79 2.37% 0.816 10.4% 5.2% 0.82 -- --
Glycol D-118.sup.1. 100% 1.050 5.24% 1.63 4.98% 1.714 21.4% 10.8%
1.71 23.9% 11.44% DBT.sup.2. 100% 1.050 1.22% 0.38 1.16% 0.400 5.0%
2.5% 0.40 5.6% 2.67% PVB-98.sup.3. 100% 1.110 5.78% 1.80 5.82%
2.000 23.6% 12.7% 2.00 26.4% 13.35% MgAl.sub.2O.sub.4 100% 3.600
9.69% 3.02 31.61% 10.872 39.6% 68.8% 10.87 44.2% 72.55% Total
100.0% 31.17 100.0% 34.39 100.0% 100.0% 34.39 100% 100%
.sup.1"D-118" is Dyspersbyk-118. .sup.2"DBT" is dibutylpthalate.
.sup.3"PVB-98" is Butvar B98 polyvinylbutyral binder.
[0144] This slip was cast using a 14 mil doctor blade on a
Teflon.RTM.-coated Mylar.RTM. carrier film. After drying the tape
released well, was cut to shape, and sintered at 1,500.degree. C.
for 4 hours. The fired layer was approximately 30 micrometers thick
and transparent.
[0145] Other non-aqueous systems may be applied to casting spinel
tapes. For example, polypropylene carbonate (PPC) binders dissolve
in carbonate solvents such as dimethyl carbonate or diethyl
carbonate and can provide clean removal of the binder at low
temperature with no residual carbon. Eliminating residual carbon
may be especially important for achieving less than 0.1% porosity
in the fired tape, which low porosity is called for to achieve high
transparency. In addition, PPC binders decompose by pyrolysis and
may be cleanly removed under inert atmosphere. This may provide an
additional advantage in further reducing porosity at low
temperature, thus reducing the level of porosity which needs to be
removed through traditional sintering. Acrylic binders, which
"unzip" or depolymerize rather than combust are advantageous for
making ultra-low porosity fired tape.
EXAMPLES
[0146] The following Examples demonstrate making, use, and analysis
of the disclosed spinel articles in accordance with the above
general procedures.
Example 1
Method of Making a Tape Casted, Laminated, Transparent Spinel--S15
Powder; High Transparency.
[0147] An excellent method of making a transparent spinel part was
performed using the S15CR powder supplied by Baikowski. This
resulted in good tape quality and a high transparency part. The
binder system was prepared in a 250 mL Nalgene bottle by combining
126.49 grams of deionized water, 7.66 grams of 30% aqueous ammonium
hydroxide (Fisher Scientific), 62.43 grams of WB4101 binder
(Polymer Innovations), 1.99 grams of PL005 plasticizer (Polymer
Innovations), 0.64 grams DF002 defoamer (Polymer Innovations), and
4.60 grams DS001 dispersant (Polymer Innovations) (see batch
compositions in Tables 7A, 7B, and 7C). The bottle was closed and
shaken to sufficiently mix the binder solution ingredients. A
01-HDDM Union Process attrition mill equipped with a 1,400 cc
Tefzel coated milling chamber, Lub-R plastic agitator disks, and
99.9% pure 1 mm Al.sub.2O.sub.3 milling media was used to prepare
the slurry. The milling media and binder solution were added to the
mill, which was then turned on to a speed of 500 rpm. With the mill
running, 144.84 grams of S15CR spinel powder (Baikowski) was added.
After adding the powder, the milling speed was increased to 1,300
rpm, and the slurry was allowed to mill for 60 minutes. After
milling, the slurry was separated from the milling media by
straining through an 80 mesh nylon screen. The slurry was degased
using a desiccator chamber and Mazerustar planetary
mixer/deaerator. After degasing the slurry was loaded into a 60 mL
syringe, a 11 micrometers nylon filter was attached to the end of
the syringe for filtration during the casting process. Using a draw
down machine, the slurry was cast under a casting blade with a 100
micrometers gap onto a silicone coated Mylar carrier film at a
speed of 10 mm/sec. The tape was allowed to dry under ambient
conditions (about 70.degree. C., 35% RH), to a final thickness of
about 50 micrometers. The tape was cut into 1.times.1 inch square
pieces, which were then stacked 28 layers thick. The stacked layers
were loaded between two metal plates, and vacuum sealed into an air
tight bag for lamination. The parts were loaded into an isostatic
laminator which had been preheated to 70.degree. C., and pressed at
1,000 psi for 15 minutes, then 4,000 psi for 15 minutes. The sample
was removed from the laminator and prepared for sintering. The
sample was loaded onto a setter with a high purity alumina weight
placed on top to maintain flatness. The sample was then fired in a
standard box furnace in air; heated to 180.degree. C. at a rate of
120.degree. C./hr and held for 2 hrs, heated 350.degree. C. at a
rate of 120.degree. C./hr and held for 2 hrs, heated to 600.degree.
C. at a rate of 120.degree. C./hr and held for 4 hrs, heated to
1550.degree. C. at a rate of 120.degree. C./hr and held for 4 hrs,
then allowed to cool at a rate of 480.degree. C./hr. The sample was
then transferred to a HIP for subsequent sintering to remove the
final porosity and achieve transparency. The sample was heated to
1,550.degree. C. under 29,000 psi of argon and held at temperature
for 8 hours. After argon HIPing the sample was sintered in an
Ar/O.sub.2 mixture to remove oxygen vacancies and restore
transparency. The sample was HIPed in a mixture of 80 vol % Ar and
20 vol % O.sub.2 atmosphere, at 5,500 psi for 4 hrs and then
allowed to cool. After finishing the sintering process, the sample
was polished using a 1 micrometer diamond film for 4 hrs on each
side to remove the surface layer and achieve the final desired
transparency.
Example 2
Prophetic
Method of Making a Tape Casted Transparent Spinel
[0148] Example 1 is repeated with the exception that a different
spinel starting powder is selected, such as a prophetic TARGET S
powder (see Table 10) having an estimated BET surface area of 6
m.sup.2/g, a density of 3.58 g/cm.sup.3, and an estimated particle
diameter of 279 nm. The tape casted product is high in
transparency. A laminate having from 4 to about 28 layer of the
tape casted product is produced by the above mentioned lamination
process.
Comparative Example 2
Unsuccessful Method of Making a Tape Casted Transparent
Spinel--S30CR Powder; No Transparency; Cracks.
[0149] Example 1 was repeated with the exception that a different
spinel starting powder S30CR was used instead of S15CR, and slight
changes were made in binder content and reduced water content to
counter crack formation tendencies. The binder system was prepared
in a 125 ml Nalgene bottle using 105.35 grams of deionized water,
5.58 grams of 30% ammonium hydroxide (Fisher Scientific), 39.86
grams of WB4101 binder (Polymer Innovations), 1.22 grams of PL005
plasticizer (Polymer Innovations), 1.21 grams DF002 defoamer
(Polymer Innovations), and 2.95 grams DS001 dispersant (Polymer
Innovations), and 92.12 grams S30CR Spinel powder (Baikowski). The
batch was prepared in half the amount used for Example 1. The
sample was milled, strained, degased, filtered, cast, laminated,
and sintered as in Example 1. The composition is similar to that
used for the S15 powder in Example 1. The batch for this
Comparative Example 2 is listed in Table 13. An increase in binder
content, and a decrease in water content was used in an attempt to
reduce drying stresses observed in initial casts, which stresses
cause cracking. It was determined that high binder contents were
needed to prevent tape cracking. The high binder contents decreased
the spinel solids loading in the tape to a point where sintering to
transparency was not possible.
TABLE-US-00015 TABLE 13 Comparative Tape Cast Spinel. Volume Weight
Component Density Percent Percent Name Supplier H.sub.2O 1.00
58.53% 42.43% water -- NH.sub.4OH 1.00 3.10% 2.25% ammonium --
hydroxide WB4101 1.03 21.50% 16.05% binder Polymer Innovations
PL005 1.03 0.66% 0.49% plasticizer Polymer Innovations DF002 1.20
0.56% 0.49% defoamer Polymer Innovations DS001 1.03 1.59% 1.19%
dispersant Polymer Innovations MgAl.sub.2O.sub.4 3.64 14.06% 37.10%
S30CR Baikowski partiles
Comparative Example 3
Unsuccessful Method of Making a Tape Casted Transparent Spinel--S8
Powder; No Transparency; Low Binder Content.
[0150] Example 1 was repeated with exception that the spinel powder
selected was the S8 powder. A similar batch composition to that
used for the S15 powder in Example 1 was used and is listed in
Table 14. The resulting tape cast had no transparency. The binder
system was prepared in a 125 mL Nalgene bottle using 78.00 g of
deionized water, 5.58 g of 30% aqueous ammonium hydroxide (Fisher
Scientific), 26.00 grams of WB4101 Binder (Polymer Innovations),
1.17 grams of PL005 plasticizer (Polymer Innovations), 0.39 grams
DF002 defoamer (Polymer Innovations), and 2.98 grams DS001
dispersant (Polymer Innovations), and 92.89 grams S30CR Spinel
powder (Baikowski). The batch was prepared in one half the amount
used for Example 1. The sample was milled, strained, degased,
filtered, cast, laminated, and sintered as mentioned in Example
1.
TABLE-US-00016 TABLE 14 Comparative Tape Cast Spinel. Volume Weight
Component Density Percent Percent Name Supplier H.sub.2O 1.00
56.24% 37.68% water -- NH.sub.4OH 1.00 4.03% 2.70% ammonium --
hydroxide WB4101 1.03 18.20% 12.56% binder Polymer Innovations
PL005 1.03 0.82% 0.57% plasticizer Polymer Innovations DF002 1.20
0.23% 0.19% defoamer Polymer Innovations DS001 1.03 2.09% 1.44%
dispersant Polymer Innovations MgAl.sub.2O.sub.4 3.64 18.40% 44.87%
S8CR Baikowski particles
[0151] The disclosure has been described with reference to various
specific embodiments and techniques. However, many variations and
modifications are possible while remaining within the scope of the
disclosure.
* * * * *