U.S. patent application number 12/982054 was filed with the patent office on 2011-06-30 for anisotropic cte lsm for sofc cathode.
This patent application is currently assigned to Saint-Gobain Ceramics & Plastics, Inc.. Invention is credited to Hansong Huang, F. Michael Mahoney, Yeshwanth Narendar.
Application Number | 20110158880 12/982054 |
Document ID | / |
Family ID | 44187810 |
Filed Date | 2011-06-30 |
United States Patent
Application |
20110158880 |
Kind Code |
A1 |
Mahoney; F. Michael ; et
al. |
June 30, 2011 |
ANISOTROPIC CTE LSM FOR SOFC CATHODE
Abstract
An anisotropic coefficient of thermal expansion (CTE) cathode of
a solid oxide fuel cell (SOFC) is formed by placing a layer of
perovskite powder between two platens, and sintering the layer
while applying pressure to the platens, thereby forming the
anisotropic CTE cathode. The perovskite can be lanthanum strontium
manganite (LSM).
Inventors: |
Mahoney; F. Michael;
(Holliston, MA) ; Narendar; Yeshwanth; (Westford,
MA) ; Huang; Hansong; (Natick, MA) |
Assignee: |
Saint-Gobain Ceramics &
Plastics, Inc.
Worcester
MA
|
Family ID: |
44187810 |
Appl. No.: |
12/982054 |
Filed: |
December 30, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61335085 |
Dec 31, 2009 |
|
|
|
Current U.S.
Class: |
423/263 ;
264/618 |
Current CPC
Class: |
C04B 2235/3206 20130101;
C04B 2235/668 20130101; C04B 2235/656 20130101; C04B 2235/3229
20130101; H01M 4/9025 20130101; C04B 35/645 20130101; C04B 2235/662
20130101; Y02E 60/50 20130101; C04B 2235/661 20130101; C04B
2235/9607 20130101; C04B 35/016 20130101; H01M 4/88 20130101; H01M
4/8875 20130101; C04B 2235/3227 20130101; C04B 2235/3208
20130101 |
Class at
Publication: |
423/263 ;
264/618 |
International
Class: |
C01F 17/00 20060101
C01F017/00; C04B 35/36 20060101 C04B035/36 |
Claims
1. A cathode of a solid oxide fuel cell, comprising a sintered
layer having an anisotropic coefficient of thermal expansion (CTE)
defined by an out-of-plane CTE and an in-plane CTE with a
difference of at least 1.times.10.sup.-6.degree. C..sup.-1 between
the out-of-plane CTE and the in-plane CTE.
2. The cathode of claim 1, wherein the sintered layer is a
perovskite.
3. The cathode of claim 2, wherein the perovskite is lanthanum
strontium manganite (LSM).
4. The cathode of claim 3, wherein the LSM has an in-plane CTE in a
range of between about 10.6.times.10.sup.-6.degree. C..sup.-1 and
about 11.8.times.10.sup.-6.degree. C..sup.-1.
5. The cathode of claim 4, wherein the LSM has an in-plane CTE in a
range of between about 11.0.times.10.sup.-6.degree. C..sup.-1 and
about 11.4.times.10.sup.-6.degree. C..sup.-1.
6. The cathode of claim 3, wherein the out-of-plane CTE of the LSM
is about 13.0.times.10.sup.-6.degree. C..sup.-1 and the in-plane
CTE of the LSM is about 12.0.times.10.sup.-6.degree. C..sup.-1.
7. The cathode of claim 3, wherein the out-of-plane CTE of the LSM
is about 14.0.times.10.sup.-6.degree. C..sup.-1 and the in-plane
CTE of the LSM is about 10.6.times.10.sup.-6.degree. C..sup.-1.
8. The cathode of claim 1, wherein the cathode exhibits a
non-linear stress-strain curve at a temperature in a range of
between about 20.degree. C. and about 50.degree. C.
9. A method for forming a low coefficient of thermal expansion
(CTE) cathode of a solid oxide fuel cell, comprising the steps of:
a) placing a layer of perovskite powder between two platens; and b)
sintering the layer while applying pressure to the platens, thereby
forming the low CTE cathode.
10. The method of claim 9, wherein the perovskite is lanthanum
strontium manganite (LSM).
11. The method of claim 9, wherein the layer has a thickness in a
range of between about 0.5 mm and about 4 mm.
12. The method of claim 9, wherein the layer is sintered at a
temperature in a range of between about 1200.degree. C. and about
1400.degree. C. for a time period in a range of between about 15
minutes and about 3 hours.
13. The method of claim 12, wherein the layer is sintered at a
temperature in a range of between about 1270.degree. C. and about
1290.degree. C. for a time period in a range of between 15 minutes
and about one hour.
14. The method of claim 9, wherein the pressure applied to the
platens is in a range of between about 0.02 MPa and about 20
MPa.
15. The method of claim 14, wherein the pressure applied onto the
platens is in a range of between about 0.02 MPa and about 12 MPa.
Description
RELATED APPLICATION(S)
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/335,085, filed on Dec. 31, 2009. The entire
teachings of the above application are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] A fuel cell is a device that generates electricity by a
chemical reaction. Typically, in a fuel cell, an oxygen gas, such
as O.sub.2, is reduced to oxygen ions (O.sup.2-) at the cathode,
and a fuel gas, such as H.sub.2, is oxidized with the oxygen ions
to form water at the anode. Among various types of fuel cells,
solid oxide fuel cells (SOFCs) use hard ceramic compounds of metal
oxides (e.g., calcium or zirconium oxides) to form components of
the fuel cell, such as, for example, the anode, cathode,
electrolyte, and interconnect. Fuel cells are generally designed as
stacks, whereby subassemblies, each including a cathode, an anode
and a solid electrolyte between the cathode and the anode, are
assembled in series by locating an electrical interconnect between
the cathode of one subassembly and the anode of another.
[0003] One SOFC cell design consists of five layers. Two of these
five layers are relatively thick layers: anode bulk and cathode
bulk. A relatively thin layer of electrolyte and relatively thin
functional anode and cathode layers are sandwiched between the bulk
layers. Typically, the thin layer thickness is only about
1/100.sup.th of the bulk layer thickness. The most important
mechanism in stress generation in a SOFC co-fired cell is the step
of cooling down from the sintering temperature, typically
1300-1400.degree. C., to room temperature, due to the mismatch in
the coefficients of thermal expansion (CTE) between the materials
of the five layers. Where there is sufficient mismatch of CTE among
the layers, cooling, or any temperature change that is too rapid,
can cause fracture and consequent failure of the SOFC. Because of
the much larger thickness, the majority of the stress is generated
by the mismatch between the CTEs of the anode bulk layer and the
cathode bulk layer. Since most ceramics show linear elastic
stress-strain behavior up to failure in the temperature range that
a SOFC operates, for a fixed geometry design, there are only two
material properties that affect the thermal mismatch stress:
modulus and coefficient of thermal expansion of the materials.
[0004] In order to reduce the thermal mismatch stress, it is
desirable to have both the cathode and anode materials of a SOFC
have a CTE that is as close as possible to the CTE of the
electrolyte, which is typically made of yttria-stabilized zirconia
(YSZ). A typical CTE of YSZ generally is in a range of between
about 10.510.sup.-6.degree. C..sup.-1 and 1110.sup.-6.degree.
C..sup.-1, which is much lower than the CTE of most cathodes.
Specifically, while a typical anode material has a CTE of
11.310.sup.-6.degree. C..sup.-1, the most commonly used cathode
material for SOFCs is lanthanum strontium manganite (LSM),
La.sub.0.2Sr.sub.0.8MnO.sub.3 (LSM20/80), which has a CTE in a
range of between about 12.210.sup.-6.degree. C..sup.-1 and about
12.410.sup.-6.degree. C..sup.-1 (average CTE between room
temperature and 1200.degree. C.). See L. Kindermann, et al.,
Synthesis and properties of La--Sr--Mn--Fe--O perovskites,
Proceedings of the 3.sup.rd European solid oxide fuel cell forum,
1998, pp. 123. The difference between the CTEs of the LSM and YSZ
materials would generate a large thermal mismatch stress in the
SOFC. With a Sr content increase above 0.2, the CTE of the LSM
material would further increase. Id. On the other hand, it is often
undesirable to use an LSM with Sr content lower than 0.2, due to
diminished electrochemical performance.
[0005] Therefore, there is a need to overcome or minimize the
above-mentioned problems.
SUMMARY OF THE INVENTION
[0006] The invention generally is directed to a cathode having an
anisotropic CTE and a method for forming the anisotropic CTE
cathode of a solid oxide fuel cell.
[0007] In one embodiment, a cathode of a solid oxide fuel cell
includes a sintered layer having an anisotropic coefficient of
thermal expansion (CTE) defined by an out-of-plane CTE and an
in-plane CTE with a difference of at least
1.times.10.sup.-6.degree. C..sup.-1 between the out-of-plane CTE
and the in-plane CTE. The sintered layer can be a perovskite. In
some embodiments, the perovskite can be lanthanum strontium
manganite (LSM). In these specific embodiments, the LSM can have an
in-plane CTE in a range of between about
10.6.times.10.sup.-6.degree. C..sup.-1 and about
11.8.times.10.sup.-6.degree. C..sup.-1, preferably in a range of
between about 11.0.times.10.sup.-6.degree. C..sup.-1 and about
11.4.times.10.sup.-6.degree. C..sup.-1. In some embodiments, the
out-of-plane CTE of the LSM can be about
13.0.times.10.sup.-6.degree. C..sup.-1 and the in-plane CTE of the
LSM is about 12.0.times.10.sup.-6.degree. C..sup.-1. In other
embodiments, the out-of-plane CTE of the LSM can be about
14.0.times.10.sup.-6.degree. C..sup.-1 and the in-plane CTE of the
LSM is about 10.6.times.10.sup.-6.degree. C..sup.-1. The
anisotropic CTE cathode can exhibit a non-linear stress-strain
curve at a temperature in a range of between about 20.degree. C.
and about 50.degree. C.
[0008] In another embodiment, a method for forming a low
coefficient of thermal expansion (CTE) cathode of a solid oxide
fuel cell includes the steps of placing a layer of perovskite
powder between two platens, and sintering the layer while applying
pressure to the platens, thereby forming the anisotropic CTE
cathode. The layer can have a thickness in a range of between about
0.5 mm and about 4 mm. In some embodiments, the layer can be
sintered at a temperature in a range of between about 1200.degree.
C. and about 1400.degree. C. for a time period in a range of
between about 15 minutes and about 3 hours. In certain embodiments,
the layer can be sintered at a temperature in a range of between
about 1270.degree. C. and about 1290.degree. C. for a time period
in a range of between 15 minutes and about one hour. The pressure
applied to the platens can be in a range of between about 0.02 MPa
and about 20 MPa. In some embodiments, the pressure applied onto
the platens is in a range of between about 0.02 MPa and about 12
MPa.
[0009] This invention has many advantages, including maintaining
low thermal mismatch stresses in the cell and stack of a SOFC. As
will be further described below, it has been found that compression
of LSM during heat treatment lowers the CTE of the LSM, thereby
minimizing or significantly reducing failure of SOFCs that employ
LSM during use.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic illustration of the application of the
methods of the invention.
[0011] FIG. 2 is a graph of tangent CTE as a function of
temperature for free-sintered and hot-pressed CREE LSM20.
[0012] FIG. 3 is a graph of tangent CTE pre- and post-annealing
compared to the CTE of free-sintered and hot-pressed LSM20.
[0013] FIG. 4 is a schematic illustration of test samples for the
CTE anisotropy tests.
[0014] FIG. 5 is a graph of the anisotropic CTE test results
[0015] FIG. 6 is a graph of CTE as a function of pressure.
[0016] FIGS. 7A-C are graphs of four-point bending load as a
function of displacement for cathode bulk materials.
[0017] FIG. 7D is a graph of stress as a function of strain for
cathode bulk materials.
[0018] FIG. 8 is a graph of an XRD pattern of in-plane and
out-of-plane surfaces of a hot-pressed sample.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The foregoing will be apparent from the following more
particular description of example embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating embodiments of the present invention. The
teachings of all patents, published applications and references
cited herein are incorporated by reference in their entirety.
[0020] The invention generally is directed to a cathode having an
anisotropic CTE and a method for forming the anisotropic CTE
cathode of a solid oxide fuel cell. A SOFC stack can include the
anisotropic CTE cathode, wherein the anisotropic CTE cathode can be
made separately before stack fabrication or processed
simultaneously with other layers of the SOFC stack. As used herein,
the term solid oxide fuel cell includes a solid oxide electrolyser,
that is, a solid oxide fuel cell operated in a regenerative mode
for the electrolysis of water.
[0021] In one embodiment, illustrated in FIG. 1, a method for
forming an anisotropic coefficient of thermal expansion (CTE)
cathode of a solid oxide fuel cell includes the steps of placing
layer 10 of perovskite powder between two platens 20, and sintering
the layer while applying pressure to platens 20, thereby forming
the anisotropic CTE cathode. The perovskite can be lanthanum
strontium manganite (LSM). Other perovskites and their CTE's are
listed in Table 1.
[0022] Layer 10 can have a thickness in a range of between about
0.5 mm and about 4 mm. Layer 10 can be green-formed from the
perovskite powder by, for example, die pressing, tape
casting/lamination, extrusion, and gel casting.
TABLE-US-00001 TABLE 1 Examples of perovskites and their CTE's
Typical Material CTE value family Example (.times.10.sup.-6
C.sup.-1) Lanthanum (La.sub.0.8Ca.sub.0.2)MnO.sub.3 11.1 Calcium
(150.degree. C.- Manganite 900.degree. C.) (LCM) Cerium
(La.sub.0.69Ca.sub.0.2Ce.sub.0.11).sub.0.99(Mn.sub.0.91Mg.sub.0.09)-
O.sub.3 11.2 Magnesium (150.degree. C.- doped 900.degree. C.)
Lanthanum Calcium Manganite (LCCMM) Lathanum
(La.sub.0.8Sr.sub.0.2)Fe O.sub.3-.delta. 10.0-11.8 Strontium
(20.degree. C.- Ferrite 1000.degree. C.) Cobalt doped
(La.sub.0.6Sr.sub.0.4)(Co.sub.0.2Fe.sub.0.8)O.sub.3-.delta. 15
Lanthanum (300.degree. C.- Strontium 700.degree. C.) Ferrite
[0023] In some embodiments, layer 10 can be sintered at a
temperature in a range of between about 1200.degree. C. and about
1400.degree. C., preferably between about 1270.degree. C. and about
1290.degree. C., for a time period in a range of between about 15
minutes and about 3 hours, preferably in a range of between about
15 minutes and about one hour. The pressure applied to platens 20
can be in a range of between about 0.02 MPa and about 20 MPa,
preferably in a range of between about 0.02 MPa and about 12 MPa,
with a constant pressure being applied to platens 20 during the
sintering time period.
[0024] The resulting low CTE cathode can have a CTE in a range of
between about 10.6.times.10.sup.-6.degree. C..sup.-1 and about
11.8.times.10.sup.-6.degree. C..sup.-1, preferably in a range of
between about 11.0.times.10.sup.-6.degree. C..sup.-1 and about
11.4.times.10.sup.-6.degree. C..sup.-1. The CTEs listed herein are
linear secant CTEs between room temperature and about 900.degree.
C., to avoid complications from continued sintering above about
1000.degree. C.
[0025] The CTE can be anisotropic. As illustrated in FIG. 4, the
CTE can be defined by an out-of-plane CTE and an in-plane CTE. The
out-of-plane CTE of the low CTE cathode can be about
14.0.times.10.sup.-6.degree. C..sup.-1 and the in-plane CTE of the
low CTE cathode can be about 10.6.times.10.sup.-6.degree.
C..sup.-1. Without wishing to be bound by any particular theory, it
is suggested that the pressure induced change in CTE can be caused
by the creation of anisotropy. Single crystalline LSM has different
thermal expansion coefficients along its three axes. See Neumeier,
J. J. et al., 1999, Thermal expansion of single-crystalline
La.sub.0.83Sr.sub.0.17MnO.sub.3: the importance of
temperature-induced strain for electrical resistivity, Physical
review B, vol. 58, No. 3, 1701-1705, for the CTE of
single-crystalline La.sub.0.83Sr.sub.0.17MnO.sub.3 along its a, b,
and c axes, but only below room temperature (RT). The literature
results show that the single crystalline LSM has anisotropic CTE.
The temperature dependent crystal lattice parameters can be found
in the literature. Gaviko, et al., 2005, X-ray studies of the
(La,Sr)MnO.sub.3 perovskite manganite structure, Physics of the
solid state, Vol 47, No. 7, 1299-1305; Iberl, A, et al., 1991,
High-temperature x-ray diffraction measurements of phase
transitions and thermal expansion in(La,Sr)(Mn,Co)O.sub.3-cathode
materials, Proceedings of the 2.sup.nd international symposium on
solid oxide fuel cells, 527-535. The macroscopic CTE is the average
of random distribution of the anisotropic single-crystalline LSM
that appears isotropic macroscopically. It is possible that
pressure applied at high temperature disturbs the statistically
random orientation of crystalline LSM, thus generating
anisotropy.
[0026] The cathode can exhibit a non-linear stress-strain curve at
a temperature in a range of between about 20.degree. C. and about
50.degree. C.
EXEMPLIFICATION
[0027] All samples were sintered at about 1280.degree. C. for about
30 minutes in air unless otherwise noted. For the hot-pressed
(HPed) samples whose results are shown in FIG. 6 and Table 7 below,
a constant pressure in a range of between about 0.02 MPa and about
3 MPa was maintained throughout the sintering step. Table 2 below
lists the CTEs measured from room temperature (RT) to 900.degree.
C. of several LSM materials (in ppm, i.e 10.sup.-6.degree.
C..sup.-1) from Praxair (Praxair, Woodinville, Wash.)
La.sub.0.2Sr.sub.0.8MnO.sub.3 (LSM20), Praxair LSM20 with 0%, 2%,
and 5% A-site deficiency, LSM20 produced in-house (CREE LSM20)
either free-sintered or bisque-fired, and NexTech (NexTech
Materials, Lewis Center, Ohio) LSM20. Unless noted otherwise, the
samples listed in Table 2 were produced by free-sintering the
green-formed corresponding LSM powder in air.
TABLE-US-00002 TABLE 2 CTE (RT-900.degree. C.) of free-sintered
LSM20 Materials CTE (ppm) Materials CTE (ppm) CREE LSM20 11.9-12
Praxair LSM20 12-12.2 (CB0616 free-sintered) 2% A-site CREE LSM20
12-12.2 Praxair LSM20 12.1 (CB0704 free-sintered) 5% A-site CREE
LSM20 12.1-12.2 Praxair LSM20 12 (CB0704 bisque-fired) (0% A-site)
Nextech LSM20 11.9-12.1
[0028] To demonstrate the effect of hot-pressing the LSM samples,
three sets of samples were made from the same batch of CREE LSM20.
Set A was free-sintered in a box oven with a spacer covering the
top and bottom surfaces. Set B was free-sintered in a hot-press
oven with a sapphire spacer covering the top and bottom surfaces,
and Set C was hot-pressed. Each set of samples consisted of at
least 4 samples and two rounds of repeated tests. The CTE results
were:
[0029] Set A (free-sintered in box oven, covered by
spacer)--12-12.2 ppm
[0030] Set B (free-sintered in hot-press oven, covered by
spacer)--11.9-12.2 ppm
[0031] Set C (hot-pressed)--11-11.2 ppm
A comparison of differential CTEs between hot-pressed and free
sintered CREE LSM20 powder is shown in FIG. 2. As shown in FIG. 2,
the hot-pressed LSM20's tangent CTE is lower than that of
free-sintered LSM20 over the entire temperature range.
[0032] Additional results for low CTE LSM were obtained from a
series of pre-sintered (bisque-fired) cathode bulk samples. Three
sets of identical 1 inch (25 mm) disks were first free-sintered.
Two of them were then hot-pressed under different temperatures and
pressures, listed in Table 3 below. Their CTEs, also listed in
Table 3, were measured afterwards. As shown in Table 3,
hot-pressing lowered the CTE of the LSM material.
TABLE-US-00003 TABLE 3 Bisque-fired LSM20 CTE Bisque Hot-press CTE
Sample condition condition (RT-900.degree. C. ppm) 1 1350.degree.
C. N/A 12.1-12.2 2 1350.degree. C. 1320.degree. C./7 MPa 11.3-11.5
3 1350.degree. C. 1280.degree. C./10 MPa 11.1-11.4
[0033] An additional sample made from Praxair LSM20 and calcined
before hot-pressing also showed a low CTE after hot-pressing.
Thermal Stability Results
[0034] Hot-pressed CREE LSM20 was subjected to thermal cycle tests
in which the samples were exposed to 900.degree. C. for an
accumulated time period of about 16 hours. The CTE before the
cyclic test (during the initial heat-up) and after the test (during
final cycle cool-down) were calculated. The two CTE values were the
same for all three types of samples tested (11-11.2 ppm for
hot-pressed CREE LSM20, 11.9-12.2 ppm for free-sintered Praxair
LSM20 and free-sintered CREE LSM20). Therefore, the relatively
short-term stability of the CTE is demonstrated at 900.degree.
C.
Annealing Results
[0035] A set of hot-pressed CREE LSM20 were annealed in a
dilatometer in air at 900.degree. C. (4 hrs), 1200.degree. C. (4
hrs), 1280.degree. C. (6 hrs), and 1380.degree. C. (2 hrs)
respectively. The CTEs before and after annealing were measured.
Pre-annealing CTE was measured during the heat-up phase of
annealing. Post-annealing CTE was measured both during cool-down
phase of annealing and in at least one subsequent separate CTE
measurement run. The results are shown in Table 4 below.
TABLE-US-00004 TABLE 4 CTE results for annealed hot-pressed CREE
LSM20 Anneal Pre-annealing CTE Post-annealing CTE Sample condition
(ppm) (ppm) 1 900.degree. C./4 hrs 11.2 11.2 2 1200.degree. C./4
hrs 11.3 11.4 3 1280.degree. C./6 hrs 11.3 11.9 4 1380.degree. C./2
hrs 11.3 12.1
[0036] The CTE remained unchanged after annealing at and below
1200.degree. C., but increased back to (or almost to) the CTE of
free-sintered LSM20 after annealing above 1200.degree. C. The trend
can also be seen from the differential CTE plot shown in FIG.
3.
[0037] The annealing study was also done with the pre-sintered
(bisque-fired) samples. The results are listed in Table 5
below.
TABLE-US-00005 TABLE 5 CTE results for pre-sintered (bisque-fired)
LSM20 Anneal Pre-anneal Post-anneal Sample Bisque-fire condition
condition CTE (ppm) CTE (ppm) 1 Bisque-fire 1350.degree. C.
1280.degree. C. 12.1 11.9 6 hrs 2 Bisque-fire 1350.degree. C. ->
1280.degree. C. 11.3 12 Hot-press 1320.degree. C. + 6 hrs 7 MPa 3
Bisque fire 1350.degree. C. -> 1280.degree. C. 11.4 12 Hot-press
1280.degree. C. + 6 hrs 10 MPa
[0038] As shown by the results in Table 5, the low CTE of the LSM
obtained after hot-pressing reverses back to the CTE of
free-sintered LSM after annealing at or above 1280.degree. C.
Without wishing to be bound by any particular theory, it is
possible that the CTE reversal might be activated by the mobility
of the crystalline structure, similarly to the continued sintering
observed in hot-pressed LSM20 above 1100.degree. C.
Anisotropy Results
[0039] The CTE of hot-pressed LSM20 was tested in two orthogonal
directions, shown in FIG. 4. Due to the limitation of hot-pressed
disk thickness (4 mm), the test samples were much shorter than the
typical dilatometer test sample length (25 mm). The dimensions of
the anisotropy test samples were 4 mm.times.4 mm.times.4 mm.
[0040] Four sets of tests were done, with 32 expansion curves
recorded, 16 curves for each orientation. The test samples were cut
from the usual long dilatometer test bar (25 mm long). The
materials included free-sintered, hot-pressed, bisque-fired, and
bisque-fired/hot-pressed CREE LSM20. Measurements (expansion
curves) are shown in FIG. 5. Turning to FIG. 5, the measurements in
the out-of-plane direction (along the pressure direction) are
identified as HP Side 1, and the measurements in the in-plane
direction (perpendicular to the pressure direction) are identified
as HP Side 2. The measurements of free-sintered samples are
identified as Free Sinter Side-1&2. The small sample length in
this test led to large variations in the results. Although not
conclusive, the plot suggests that anisotropy existed in the
hot-pressed samples with the out-of-plane direction CTE of about 13
ppm, and the in-plane direction CTE of about 11 ppm. The
free-sintered samples had a CTE of about 12 ppm.
[0041] In another example, a thicker (23 mm) sample was made with a
sintering temperature of about 1320.degree. C. and a time of about
30 min under a pressure of about 3 MPa, so that longer (21 mm) CTE
bars could be made for measurements along both in-plane and
out-of-plane directions. Table 6 shows the CTE values of 3 in-plane
and out-of-plane samples, respectively, showing the anisotropy
behavior of the samples. The CTE values along the in-plane
direction listed in Table 6 are higher than those reported
elsewhere. However, the HPed sample used for measurements in Table
6 was much thicker than the regular ones discussed above (23 mm vs.
.ltoreq.4 mm). Without wishing to be bound by any particular
theory, it is reasonable to suggest that within the thick sample
the transition of pressure and the correspondent mass diffusion can
be different from those within a thin sample. This difference might
cause different CTE results, but the anisotropic behavior remained
true for both types of samples regardless of thickness.
TABLE-US-00006 TABLE 6 CTE values (ppm) of in-plane and
out-of-plane directions measured on a 23 mm thick HPed sample
Measurement #1 #2 #3 In-plane direction 11.9 11.6 12.0 Out-of-plane
direction 13.0 12.6 13.0
[0042] X-ray diffraction (XRD) characterization was applied on the
in-plane and out-of-plane surfaces of HPed samples. As shown in
FIG. 8, crystal anisotropy can be differentiated by XRD
characterization on the in-plane and out-of-plane surfaces. The
number of peaks and position of those peaks are identical for both
orientations. However, there are five pairs of peaks showing
apparent relative intensity change (Table 7). The index of those
"intensity increasing" peaks all have a big number at the
c-direction of the crystal lattice index, which suggests the
"in-plane" surface preferably aligned with the c-plane of the
crystal structure, that is, the in-plane surface shows a preferred
crystallographic orientation along the c-plane of the LSM crystal
structure. Although this may not be the only reason for the CTE
anisotropy seen above, it is expected that this crystal anisotropy
can be a significant contributor.
TABLE-US-00007 TABLE 7 Relative intensity change of XRD peaks
comparing "in-plane" surface against "out-of-plane" surface for
HPed LSM samples Peaks whose intensity Peaks whose intensity
relatively increase relatively decrease 2.theta. (degree) Index
2.theta. (degree) Index 40.48 00(0)6 40.05 20(-2)2 52.79 11(-2)6
52.44 12(-3)2 58.50 01(-1)8 57.85 30(-3)0 58.01 21(-3)4 68.50
20(-2)8 67.90 22(-4)0 77.84 12(-3)8 77.42 13(-4)4
Hot-Pressing Results with Variable Pressure
[0043] The anisotropy results described above strongly indicate
that pressure is the cause of low CTE. Hence, a series of samples
of CREE LSM20 were hot-pressed, but with different peak pressure: 1
MPa, 0.1 MPa, dead load (.about.0.02 MPa), and no pressure (but
with sapphire spacer covering). For each pressure level, at least
two samples were tested and each test consisted of at least two
repeated tests done on different weeks. Also tested was a Praxair
fine powder LSM20 that was hot-pressed at 3 MPa. The CTE results
are listed in Table 8. Also listed in Table 8 is the diameter
expansion of the sample disk after hot-press--to indicate whether
deformation has been caused by the pressure. The sample should
shrink in diameter during sintering, but if enough pressure was
applied to cause deformation (pressure induced mass diffusion), the
diameter of the disk could expand.
TABLE-US-00008 TABLE 8 CTEs of hot-pressed LSM20 at different
pressures Peak Disk Diameter CTE # Samples pressure Expansion by
(75-> (Test (MPa) HP 900.degree. C. ppm) Repeats) CREE LSM-(J) 0
(spacer) -1.2% 11.9-12.2 4 (1) CREE LSM-(E) 0.02 (dead 0.5%
11.1-11.3 3 (2) load) CREE LSM-(F) 0.1 1% 11.1-11.2 4 (1) CREE
LSM-(G) 1 3% 10.9-11.1 3 (2) Praxair LSM20 3 11.2-11.4 4 (2)
[0044] As demonstrated by the results shown in Table 8 and FIG. 6,
the hot-press pressure indeed has caused the low CTE of LSM20 after
hot-press. FIG. 6 shows the CTE values for each individual test
sample. It can be seen that even 0.02 MPa pressure was enough to
alter the material. From the disk diameter expansion results, it
can be seen that even 0.02 MPa pressure is large enough to induce
pressure induced deformation (mass diffusion). This observation
correlates with the hypothesis above on the creation of anisotropy
in the crystalline structure by the forced alignment of crystalline
orientation. It could also correlate with the observation above on
the temperature at which (above 1200.degree. C.) the CTE can be
reversed after annealing.
[0045] To underscore the effect of even relatively low pressure,
all 5 batches of tests results, for which samples were cut from the
same disk for each pressure level, for CR LSM-(E) (0.02 MPa) and CR
LSM-(G) (0.1 MPa) are given in Table 9 below. The samples were
deliberately alternated as to which slot in the dilatometer was
used for the test. The tests were conducted over a time span of two
months.
TABLE-US-00009 TABLE 9 Comparison between CTEs from pressures of
0.02 MPa and 0.1 MPa 1 2 3 4 5 CREE LSM-(E) 11.3 11.3 11.35 11.1
11.2 0.02 MPa CREE LSM-(G) 11 11 11.1 11.3 10.8 0.1 MPa
[0046] Non-Linear Stress-Strain Results
[0047] LSM20 powder (CREE LSM20 or Praxair LSM20) was hot-pressed
or free sintered into 2 inch disks. Bars were then cut from disks
with dimensions of 50 mm.times.4 mm.times.4 mm. The bars were then
subjected to four-point bending tests to measure the load vs.
displacement curves. The results are shown in FIGS. 7A-D. CB0513
was hot-pressed Praxair LSM20 (calcined powder) (FIG. 7A), CB0616
was free sintered CREE LSM20 (FIG. 7B), and CB0704 was hot-pressed
CREE SLM20 (FIG. 7C). CB0616 (free-sintered CREE LSM20) showed a
linear elastic to failure load-displacement curve under four point
bending, which is the expected behavior of ceramics materials at
room temperature under small loading. FIGS. 7A-C show the
load-displacement curves for each individual sample of the
respective composition. Both CB0513 (hot-pressed Praxair LSM20) and
CB0704 (hot-pressed CREE LSM20) clearly showed non-linear load vs.
displacement curves that are very much like plasticity yielding,
i.e., softening, a behavior that resembles the ferroelasticity
reported for some perovskite ceramics.
[0048] To verify the effect of hot-press sample preparation, two
sets of samples were identically processed with the only difference
being that one set was hot-pressed and one set was free-sintered.
The powder batch, green forming, and sintering time and temperature
were all the same. The result is shown in FIG. 7D. The free
sintered sample was shown as linear and the hot-pressed sample had
clear non-linear behavior that matched well with the CB0704
samples.
Incorporation by Reference
[0049] The teachings of all references identified above are
incorporated herein by reference in their entirety.
EQUIVALENTS
[0050] While this invention has been particularly shown and
described with references to example embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *