U.S. patent application number 13/955247 was filed with the patent office on 2013-11-28 for silicon carbide material, honeycomb structure, and electric heating type catalyst carrier.
This patent application is currently assigned to NGK Insulators, Ltd.. The applicant listed for this patent is NGK Insulators, Ltd.. Invention is credited to Yoshimasa KOBAYASHI, Takahiro TOMITA, Miyuki YABUKI.
Application Number | 20130316129 13/955247 |
Document ID | / |
Family ID | 46602692 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130316129 |
Kind Code |
A1 |
YABUKI; Miyuki ; et
al. |
November 28, 2013 |
SILICON CARBIDE MATERIAL, HONEYCOMB STRUCTURE, AND ELECTRIC HEATING
TYPE CATALYST CARRIER
Abstract
A silicon carbide material according to the present invention
includes a substrate containing, as a main component, silicon
carbide or containing, as main components, silicon carbide and
metallic silicon, and a film covering at least a portion of the
surface of the substrate. The film contains, as a main component, a
phase including at least four elements: lithium (Li), aluminum
(Al), silicon (Si), and oxygen (O). One example of such a silicon
carbide material includes a substrate having a structure in which
silicon carbide particles are bonded by metallic silicon, and a
lithium aluminosilicate film covering at least a portion of the
surface of the silicon carbide particles. Such a silicon carbide
material can be used for a DPF, an electric heating type catalytic
converter, or the like.
Inventors: |
YABUKI; Miyuki;
(Nagoya-City, JP) ; TOMITA; Takahiro;
(Nagoya-City, JP) ; KOBAYASHI; Yoshimasa;
(Nagoya-City, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGK Insulators, Ltd. |
Nagoya-City |
|
JP |
|
|
Assignee: |
NGK Insulators, Ltd.
Nagoya-City
JP
|
Family ID: |
46602692 |
Appl. No.: |
13/955247 |
Filed: |
July 31, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/JP2012/051940 |
Jan 30, 2012 |
|
|
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13955247 |
|
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Current U.S.
Class: |
428/116 ;
428/312.6; 428/334; 428/446; 502/439 |
Current CPC
Class: |
C04B 35/565 20130101;
B01J 35/04 20130101; C04B 2235/606 20130101; Y10T 428/249969
20150401; C04B 2111/00793 20130101; C04B 2235/72 20130101; F01N
3/2828 20130101; C04B 41/009 20130101; C04B 2235/5436 20130101;
B01J 23/04 20130101; C04B 35/6316 20130101; C04B 2235/3203
20130101; C04B 2235/3418 20130101; Y10T 428/263 20150115; C04B
41/5024 20130101; C04B 2235/3217 20130101; Y10T 428/24149 20150115;
C04B 2111/0081 20130101; C04B 35/565 20130101; C04B 41/4556
20130101; C04B 41/4535 20130101; C04B 41/85 20130101; C04B 41/009
20130101; C04B 41/455 20130101; C04B 38/0006 20130101; B01D 53/944
20130101; C04B 35/19 20130101; C04B 2235/428 20130101; C04B 41/5024
20130101 |
Class at
Publication: |
428/116 ;
428/446; 428/312.6; 428/334; 502/439 |
International
Class: |
F01N 3/28 20060101
F01N003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2011 |
JP |
2011-022498 |
Claims
1. A silicon carbide material, including: a substrate containing,
as a main component, silicon carbide or containing, as main
components, silicon carbide and metallic silicon, and a film
covering at least a portion of the surface of the substrate,
wherein the film contains, as a main component, a phase including
at least four elements: lithium (Li), aluminum (Al), silicon (Si),
and oxygen (O).
2. The silicon carbide material according to claim 1, wherein the
film does not contain potassium.
3. The silicon carbide material according to claim 1, wherein the
film contains a lithium aluminosilicate.
4. The silicon carbide material according to claim 3, wherein the
lithium aluminosilicate is spodumene.
5. The silicon carbide material according to claim 1, wherein the
substrate has a structure in which silicon carbide particles are
bonded by metallic silicon.
6. The silicon carbide material according to claim 1, wherein the
substrate has a porosity of 20% to 70%.
7. The silicon carbide material according to claim 1, wherein the
film has the maximum thickness of 2 to 102 .mu.m.
8. The silicon carbide material according to claim 1, wherein the
film has the film weight ratio of 4.1% to 58.3% by weight.
9. The silicon carbide material according to claim 1, wherein the
film has a composition including 1% to 37% by weight of lithium
oxide (Li.sub.2O), 3% to 49% by weight of aluminum oxide
(Al.sub.2O.sub.3), and 48% to 96% by weight of silicon oxide
(SiO.sub.2).
10. A honeycomb structure which includes the silicon carbide
material according to claim 1.
11. An electric heating type catalyst carrier which uses the
silicon carbide material according to claim 1.
12. The silicon carbide material according to claim 4, wherein the
substrate has a structure in which silicon carbide particles are
bonded by metallic silicon.
13. The silicon carbide material according to claim 4, wherein the
substrate has a porosity of 20% to 70%.
14. The silicon carbide material according to claim 4, wherein the
film has the maximum thickness of 2 to 102
15. The silicon carbide material according to claim 4, wherein the
film has the film weight ratio of 4.1% to 58.3% by weight.
16. The silicon carbide material according to claim 4, wherein the
film has a composition including 1% to 37% by weight of lithium
oxide (Li.sub.2O), 3% to 49% by weight of aluminum oxide
(Al.sub.2O.sub.3), and 48% to 96% by weight of silicon oxide
(SiO.sub.2).
17. A honeycomb structure which includes the silicon carbide
material according to claim 4.
18. An electric heating type catalyst carrier which uses the
silicon carbide material according to claim 4.
Description
TECHNICAL FIELD
[0001] The present invention relates to a silicon carbide material,
a honeycomb structure, and an electric heating type catalyst
carrier.
BACKGROUND ART
[0002] Exhaust gas emitted from a diesel engine contains
particulate matter (PM), and therefore, after the PM is collected
by a diesel particulate filter (DPF) installed in the exhaust path,
the exhaust gas is discharged into the atmosphere. On the other
hand, exhaust gas emitted from a gasoline engine contains
hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx),
and therefore, after these substances are converted into water,
carbon dioxide, and nitrogen by means of oxidation-reduction
reactions by a catalytic converter installed in the exhaust path,
the exhaust gas is discharged into the atmosphere. As such a DPF or
catalytic converter, a honeycomb filter including silicon carbide
(SiC) as a main component may be used in some cases. For example,
Patent Literature 1 discloses a honeycomb filter which contains
silicon carbide particles and metallic silicon, in which a film
covering the surface thereof has a phase including silicon oxide.
Furthermore, Patent Literature 2 discloses a honeycomb filter which
contains silicon carbide particles and metallic silicon, in which a
film covering the surface thereof has a mullite crystal phase.
Furthermore, Patent Literature 3 discloses a silicon carbide
heating element which has a film containing a potassium
aluminosilicate.
[0003] Patent Literature 1: Japanese Unexamined Patent [0004]
Application Publication No. 2002-154882
[0005] Patent Literature 2: WO2005/014171
[0006] Patent Literature 3: Japanese Unexamined Patent [0007]
Application Publication No. 2-186598
SUMMARY OF INVENTION
[0008] However, when the materials disclosed in Patent Literatures
1 to 3 are used for the DPF or catalytic converter described above,
problems may occur in terms of thermal shock resistance in some
cases. That is, in the material of Patent Literature 1, since the
thermal expansion coefficient of silicon oxide in the film is high,
the thermal expansion coefficient of the material is also high,
resulting in low thermal shock resistance, which is a problem. In
the material of Patent Literature 2, since the thermal expansion
coefficient of the mullite contained in the film is not so low,
thermal shock resistance is not satisfactory. In the material of
Patent Literature 3, since the thermal expansion coefficient of the
potassium aluminosilicate is high, thermal shock resistance is low,
which is a problem. Such problems are particularly marked in the
case of catalytic converters of a type in which the catalyst is
activated early by electric heating.
[0009] The present invention has been achieved in order to solve
such problems. It is a main object of the present invention to
provide a silicon carbide material having excellent thermal shock
resistance compared with the related art.
[0010] The present inventors have conducted a wide variety of
studies on the material of a film which covers at least a portion
of the surface of a substrate containing silicon carbide, and as a
result, have found that lithium aluminosilicates typified by
spodumene are excellent in terms of thermal shock resistance
compared with cristobalite, mullite, quartz, and potassium
aluminosilicates. Thereby, the present invention has been
completed.
[0011] That is, a first aspect of the present invention relates to
a silicon carbide material which includes a substrate containing,
as a main component, silicon carbide or containing, as main
components, silicon carbide and metallic silicon, and a film
covering at least a portion of the surface of the substrate. The
film contains, as a main component, a phase including at least four
elements: lithium (Li), aluminum (Al), silicon (Si), and oxygen
(O). The term "surface of the substrate" includes, in addition to
the outer surface of the substrate, the inner surface of pores in
the case where the substrate is porous. Furthermore, as long as the
"film" contains four elements: Li, Al, Si, and O, it may be a
compound, mixture, or composite, or may be a crystalline phase or
amorphous phase.
[0012] Second and third aspects of the present invention relate to
a honeycomb structure and an electric heating type catalyst, each
including the silicon carbide material according to the first
aspect of the present invention.
[0013] The silicon carbide material according to the first aspect
of the present invention has excellent thermal shock resistance
compared with the related art. The reason for this is believed to
be that, since the thermal expansion coefficient of the film is
lower than the thermal expansion coefficient of the substrate,
thermal expansion of the substrate can be suppressed by the film.
The honeycomb structure according to the second aspect of the
present invention and the electric heating type catalyst carrier
according to the third aspect of the present invention each use the
silicon carbide material according to the first aspect of the
present invention, and therefore they have excellent thermal shock
resistance.
BRIEF DESCRIPTION OF DRAWINGS
[0014] FIG. 1 shows a SEM photograph (reflected electron image) of
a substrate of Example 1.
[0015] FIG. 2 shows a SEM photograph (reflected electron image) of
a sample of Example 1.
DESCRIPTION OF EMBODIMENTS
[0016] A silicon carbide material according to the first aspect of
the present invention includes a substrate containing, as a main
component, silicon carbide or containing, as main components,
silicon carbide and metallic silicon, and a film covering at least
a portion of the surface of the substrate. The film contains, as a
main component, a phase including at least four elements: lithium
(Li), aluminum (Al), silicon (Si), and oxygen (O).
[0017] The substrate is not particularly limited as long as it
contains, as a main component, silicon carbide or contains, as main
components, silicon carbide and metallic silicon. In particular, a
substrate having a structure in which silicon carbide particles are
bonded by metallic silicon is preferable. The substrate may
contain, as a sintering aid, boron, carbon, or a metal oxide. In
particular, the substrate preferably contains B.sub.4C or an oxide
of an alkaline earth metal or rare-earth metal. Furthermore, a
porous body having a porosity of 20% to 70% may be used as the
substrate. In such a case, when the silicon carbide material is
used for a DPF, in order to prevent an increase in pressure loss
while maintaining the strength of the substrate, the porosity is
set preferably at 40% to 70%, and more preferably at 45% to 60%.
When the silicon carbide material is used for an electric heating
type catalyst carrier, in order to facilitate supporting of a
catalyst and electric heating, the porosity is set preferably at
20% to 50%, and more preferably 30% to 45%. The porosity can be
adjusted by the amount of a pore-forming material, the Si/SiC
ratio, the amount of a sintering aid, the firing atmosphere, and
the like. Examples of the shape of the substrate include a
plate-like shape, a tube-like shape, a lotus root-like shape, and a
honeycomb shape. In the case of the honeycomb shape, for example,
the thickness of the wall may be set at 50 to 500 .mu.m, and the
cell density may be set at 10 to 100 cells/cm.sup.2. The term
"substrate containing, as a main component, silicon carbide" refers
to a substrate containing 50% by weight or more of silicon carbide,
which may contain oxides, nitrides, and the like of silicon.
[0018] Preferably, the film does not substantially contain
potassium. This is because there is a possibility that elemental
potassium will corrode silicon carbide. The term "does not
substantially contain" includes the case where the content is zero
and also includes the case where a very small amount (for example,
an amount below the detection limit) is contained as an impurity.
Furthermore, preferably, the film contains a lithium
aluminosilicate. Examples of the lithium aluminosilicate include
spodumene, eucryptite, and quartz solid solutions. Among these,
spodumene is preferable. The reason for this is that spodumene has
the highest melting point among the lithium aluminosilicates and is
a thermally stable crystal phase.
[0019] Preferably, the film has a maximum thickness of 2 to 102
.mu.m. When the maximum thickness is in this range, satisfactory
thermal shock resistance can be obtained. As the thickness
increases, it is necessary to increase the concentration of a raw
material to be applied. Consequently, the thickness is not likely
to become uniform, application to the inside of pores is not likely
to occur, and the drying time of the material applied increases,
all of which are problems. Therefore, the maximum thickness is more
preferably 2 to 50 .mu.m, and still more preferably 2 to 20 .mu.m.
Furthermore, preferably, the film weight ratio is 4.1% to 58.3% by
weight. When the film weight ratio is in this range, satisfactory
thermal shock resistance can be obtained. As the film weight ratio
decreases, it is possible to reduce the raw material cost by
decreasing the coating weight of the raw material. Therefore, the
film weight ratio is more preferably 4.1% to 50% by weight, and
still more preferably 4.1% to 30% by weight. The film weight ratio
is defined as a value obtained by dividing the difference between
the weight of the silicon carbide sintered body after formation of
the film and the weight of the silicon carbide sintered body before
formation of the film by the weight of the former and expressed in
percent. Furthermore, preferably, the film has a composition
including 1% to 37% by weight of lithium oxide (Li.sub.2O), 3% to
49% by weight of aluminum oxide (Al.sub.2O.sub.3) , and 48% to 96%
by weight of silicon oxide (SiO.sub.2). When these ranges are
satisfied, satisfactory thermal shock resistance can be obtained.
Furthermore, in order to increase the amount of the lithium
aluminosilicate so as to produce a more thermally stable substance,
the film preferably has a composition including 20% to 37% by
weight of Li.sub.2O, 3% to 49% by weight of Al.sub.2O.sub.3, and
48% to 96% by weight of SiO.sub.2, and more preferably has a
composition including 20% to 37% by weight of Li.sub.2O, 3% to 45%
by weight of Al.sub.2O.sub.3, and 48% to 96% by weight of
SiO.sub.2.
[0020] A method for producing a silicon carbide material according
to the first aspect of the present invention, for example, includes
a step (a) of applying alumina or an alumina precursor which is
converted into alumina by heat treatment and lithium oxide or a
lithium oxide precursor which is converted into lithium oxide by
heat treatment to the surface of a substrate containing silicon
carbide, and a step (b) of forming a film containing a lithium
aluminosilicate on at least a portion of the surface of the
substrate by performing heat treatment on the substrate obtained in
the step (a) in an oxygen-containing atmosphere at 900.degree. C.
to 1,300.degree. C. In the step (a), silicon oxide or a silicon
oxide precursor which is converted into silicon oxide by heat
treatment may be further applied.
[0021] The substrate is the same as described above. Examples of
the alumina precursor include, but are not limited to, alumina sol,
basic aluminum chloride, basic aluminum lactate, aluminum chloride,
aluminum nitrate, and aluminum sulfate. Among these, alumina sol
and basic alumina lactate sol are preferable from the standpoints
that the coating weight can be easily controlled, reactivity is
high, and the lithium aluminosilicate is easily formed. Examples of
the lithium oxide precursor include, but are not limited to,
lithium oxide sol, lithium hydroxide, lithium acetate, lithium
citrate, and lithium carbonate. Among these, lithium hydroxide is
preferable from the standpoints that reactivity is high, and the
lithium aluminosilicate is easily formed. Examples of the silicon
oxide precursor include, but are not limited to, colloidal silica
and ethyl silicate. Among these, colloidal silica is preferable
from the standpoints that the coating weight can be easily
controlled, reactivity is high, and the lithium aluminosilicate is
easily formed. The thickness and film weight ratio of the film
containing the lithium aluminosilicate can be changed arbitrarily,
for example, by adjusting the amounts of use of lithium oxide or a
precursor thereof, the aluminum oxide material or a precursor
thereof, and silicon oxide or a precursor thereof.
[0022] In the step (a), in applying alumina or an alumina precursor
to the surface of the substrate, it is preferable to use an aqueous
solution or aqueous slurry containing alumina or the alumina
precursor, and in applying lithium oxide or a lithium oxide
precursor, it is preferable to use an aqueous solution or aqueous
slurry containing lithium oxide or the lithium oxide precursor.
Furthermore, in the step (a), in the case where silicon oxide or a
silicon oxide precursor is further applied, it is preferable to use
an aqueous solution or aqueous slurry containing silicon oxide or
the silicon oxide precursor. In such a case, it is water vapor that
is mainly generated in the step (b), and it is not necessary to use
a device for eliminating toxic volatile components, thus enabling
simplification of the production process.
[0023] In the step (a), in applying alumina or an alumina
precursor, lithium oxide or a lithium oxide precursor, and silicon
oxide or a silicon oxide precursor to the surface of the substrate,
dipping, coating, spraying, or the like can be performed.
[0024] In the step (b), silica constituting the lithium
aluminosilicate contained in the film may contain silica obtained
by oxidation of the silicon component in the substrate by the heat
treatment. In such a manner, in the case where silicon oxide is
applied to the substrate, the amount of silicon oxide can be
decreased. Alternatively, silica constituting the lithium
aluminosilicate may be all supplied by silica obtained by oxidation
of the silicon component in the substrate by the heat treatment. In
such a manner, the step of applying silicon oxide to the substrate
can be omitted.
[0025] In the step (b), the oxygen-containing atmosphere may
contain water vapor. In this case, water vapor may be mixed from
the outside into the atmosphere. Furthermore, the heat treatment is
performed at 900.degree. C. to 1,300.degree. C. In this range, the
formation rate of the lithium aluminosilicate is high, and it is
possible to obtain a silicon-containing material having excellent
thermal shock resistance.
[0026] The silicon carbide material according to the first aspect
of the present invention can be used in a honeycomb structure. The
honeycomb structure can be used for a DPF or catalytic converter on
which, for example, a noble metal catalyst is carried. That is, one
utilization form of the honeycomb structure is a catalyst carrier.
Furthermore, in catalytic converters of a type in which the
catalyst is activated early by electric heating, high thermal shock
resistance is required, and therefore use of the silicon carbide
material according to the first aspect of the present invention is
particularly preferable.
EXAMPLES
Example 1
[0027] A porous substrate composed of silicon carbide particles and
metallic silicon with a porosity of 40% was prepared. The substrate
was produced as follows. Silicon carbide (SiC) powder and metallic
silicon (Si) powder as ceramic starting materials were mixed at a
weight ratio of 80:20, and hydroxypropyl methylcellulose serving as
a binder and water were added thereto to prepare a forming
material. The forming material was kneaded, and a cylindrical
puddle was formed by a vacuum pug mill. The binder content was 7%
by mass relative to the total of the silicon carbide (SiC) powder
and the metallic silicon (Si) powder. The water content was
appropriately adjusted so that the puddle had a hardness suitable
for extrusion, and was about 23% by mass relative to the total of
the silicon carbide (SiC) powder and the metallic silicon (Si)
powder. The average particle size of the silicon carbide powder was
33 .mu.m, and the average particle size of the metallic silicon
powder was 6 .mu.m. The average particle sizes of silicon carbide,
metallic silicon, and the pore-forming material are the values
measured by laser diffractometry.
[0028] The resulting puddle was formed, using an extruder, into a
honeycomb formed body. The resulting honeycomb formed body was
dried by high-frequency dielectric heating, and then dried at
120.degree. C. for 2 hours using a hot air drying machine. The
dried honeycomb formed body was degreased in the atmosphere at
550.degree. C. over 3 hours, and then fired in an Ar inert
atmosphere at about 1,450.degree. C. for 2 hours. Thereby, a porous
substrate in which SiC crystal grains were bonded by Si was
obtained. Note that the porosity was measured by a mercury
porosimeter. FIG. 1 shows a SEM photograph of this substrate.
[0029] The porous substrate was dipped in an acetic acid-stabilized
alumina sol having an alumina particle size of 10 to 100 nm and a
concentration of 10% by weight so that alumina was applied to the
surface of the substrate, and drying was performed in the air at
110.degree. C. Subsequently, the substrate was dipped in a 2 wt %
aqueous solution of lithium hydroxide so that lithium hydroxide was
applied to the surface of the substrate, and drying was performed
in the air at 110.degree. C. Subsequently, the substrate was
subjected to heat treatment in the air at 1,100.degree. C. for 3.5
hours. Thereby, a sample of Example 1 composed of a
silicon-containing material including a substrate containing
silicon carbide and metallic silicon and a film covering at least a
portion of the surface of the substrate was obtained. The film of
the sample contains a lithium aluminosilicate formed by reaction
among lithium oxide, alumina, and silica, but does not
substantially contain elemental potassium. The term "does not
substantially contain" includes the case where the content is zero
and also includes the case where a very small amount (for example,
an amount below the detection limit) is contained as an impurity.
It was confirmed by the peak appearing at 2.theta.=25.0.degree. to
26.3.degree. in X-ray diffraction (X-ray source Cu-K.alpha.) that
the film was composed of a lithium aluminosilicate. Among lithium
aluminosilicates, in each of spodumene, eucryptite, and a quartz
solid solution, a peak appears at this position. Furthermore, it
was confirmed by the peak appearing at 2.theta.=22.0.degree. to
23.0.degree. in X-ray diffraction (X-ray source Cu-K.alpha.) that a
spodumene crystal phase was contained. While a peak appears at this
position in spodumene, a peak does not appear at this position in
eucryptite or a quartz solid solution.
[0030] The film weight ratio (wt %), the composition ratio (weight
ratio) of the film, the maximum thickness (.mu.m) of the film, the
change in thermal expansion (%) of the sample, and the change in
strength (%) of the sample were calculated for the sample of
Example 1, and the results thereof are shown in Table 1. The
parameters were measured.
Film Weight Ratio (wt %)
[0031] The film weight ratio was calculated in accordance with the
following formula:
Film weight ratio=(weight of sample-weight of substrate)/weight of
sample.times.100
[0032] In the formula, the sample refers to a silicon carbide
material in which a film is formed on a substrate, and the
substrate refers to the silicon carbide material not having the
film.
Composition Ratio (Weight Ratio) of Film
[0033] Chemical analysis was performed on the sample and the
substrate in accordance with JIS R 1616. By subtracting the
quantitative value of the substrate from the quantitative value of
the sample, the weights of three components (Li.sub.2O,
Al.sub.2O.sub.3, and SiO.sub.2) were obtained. The composition
ratio of the three components was calculated on the basis of the
weights.
Maximum Thickness (.mu.m) of Film
[0034] The thickness of the film was measured using a SEM image
(magnification 1,000) of a cross section of the sample. The maximum
value among the measured values was defined as the maximum
thickness. FIG. 2 shows a SEM image of the sample of Example 1.
Dark grey portions correspond to silicon carbide particles, and
light grey portions correspond to metallic silicon. In FIG. 2, it
was confirmed that the maximum thickness of a lithium
aluminosilicate film formed on the surfaces of the silicon carbide
particles and the metallic silicon was 5 .mu.m. The thickness was
measured in a field of view of 95.times.120 .mu.m (5 spots per
field of view).
Change in Thermal Expansion (%)
[0035] An average linear thermal expansion coefficient from room
temperature to 800.degree. C. was measured in accordance with JIS
R1618, and the resulting value was defined as the thermal expansion
coefficient. The change in thermal expansion was calculated in
accordance with the formula:
(value of sample-value of substrate)/value of substrate.times.100
(%)
Spodumene Crystal Phase
[0036] Powder X-ray diffraction (X-ray source Cu-K.alpha.) was
measured in accordance with JIS K0131, and substances were
identified. It was confirmed by the peak appearing at
2.theta.=22.0.degree. to 23.0.degree. that a spodumene crystal
phase was contained.
Examples 2 to 14
[0037] As in Example 1, by adjusting the Si/SiC ratio, the amount
of the sintering aid, the firing atmosphere, the amount of the
pore-forming material, and the like, substrates with a porosity
shown in Table 1 were produced. The aluminum source and the lithium
source were applied to the substrates while adjusting the coating
weights, and then heat treatment was performed at 900.degree. C. to
1,300.degree. C. Thereby, samples, each including a film covering
at least a portion of the surface of the substrate, were obtained.
The parameters were measured for the resulting samples as in
Example 1. The results thereof are also shown in Table 1.
Example 15
[0038] A dense substrate composed of silicon carbide particles was
prepared. This substrate was produced as follows. 100 Parts by
weight of silicon carbide powder with an average particle size of
0.6 .mu.m, 2.0 parts by weight of B.sub.4C powder with an average
particle size of 1.5 .mu.m, and 1.5 parts by weight of carbon
powder with an average particle size of 0.02 .mu.m were prepared.
The preparation was fed into a media-agitation type pulverizer, and
mixing and pulverization were performed for 20 hours to obtain a
slurry. Then, the slurry was dried using a spray dryer to produce
granulated powder with an average particle size of 50 pl. The
granulated powder was cast in a rubber mold, and hydrostatic
pressure forming was performed at a pressure of 2.5 ton/cm.sup.2.
The formed body was fired in an argon atmosphere of 1 atm at
2,100.degree. C. for one hour. Thereby, a dense SiC sintered body
was obtained. The porosity was measured in accordance with JIS R
1634. An aluminum source and a lithium source were applied to the
substrate, followed by heat treatment at 1,100.degree. C. Thereby,
a sample including a film covering at least a portion of the
surface of the substrate was obtained. The parameters were measured
for the resulting sample as in Example 1. The results thereof are
also shown in Table 1.
Example 16
[0039] 45 Parts by weight of coarse silicon carbide powder with an
average particle size of 100 .mu.m and 55 parts by weight of fine
silicon carbide powder with an average particle size of 2 .mu.m
were mixed, and 20 parts by weight of water and 2 parts by weight
of a binder were added to the mixture to obtain a slurry. The
slurry was poured into a gypsum mold and formed. The resulting
formed body was fired in argon at 1 atm and 2,200.degree. C. for
two hours. Thereby, a porous recrystallized silicon carbide
sintered body was obtained. The porosity was measured by a mercury
porosimeter. An aluminum source and a lithium source were applied
to the substrate, followed by heat treatment at 1,100.degree. C.
Thereby, a sample including a film covering at least a portion of
the surface of the substrate was obtained. The parameters were
measured for the resulting sample as in Example 1. The results
thereof are also shown in Table 1.
Comparative Example 1
[0040] A porous substrate produced in Example 1 was subjected to
heat treatment in the air at 1,100.degree. C. for 3.5 hours.
Thereby, a sample including a silica film covering at least a
portion of the surface of the substrate was obtained. The
parameters were measured for the resulting sample as in Example 1.
The results thereof are also shown in Table 1. It was confirmed
that the maximum thickness of the silica film formed on the surface
of the silicon carbide particles was 2 .mu.m.
Comparative Example 2
[0041] A porous substrate produced in Example 1 was dipped in the
alumina sol used in Example 1 such that alumina was applied to the
surface of the substrate, and then drying was performed in the air
at 110.degree. C. Subsequently, heat treatment was performed in the
air at 1,300.degree. C. for 3.5 hours. Thereby, a sample including
an aluminosilicate-containing film covering at least a portion of
the surface of the substrate was obtained. The parameters were
measured for the resulting sample as in Example 1. The results
thereof are also shown in Table 1.
Comparative Example 3
[0042] A porous substrate produced in Example 1 was dipped in the
aqueous solution of lithium hydroxide used in Example 1 such that
lithium hydroxide was applied to the surface of the substrate, and
then drying was performed in the air at 110.degree. C.
Subsequently, heat treatment was performed in the air at
1,100.degree. C. for 3.5 hours. Thereby, a sample including a
lithium silicate-containing film covering at least a portion of the
surface of the substrate was obtained. The parameters were measured
for the resulting sample as in Example 1. The results thereof are
also shown in Table 1.
Comparative Example 4
[0043] A porous substrate produced in Example 12 was subjected to
heat treatment in the air at 1,300.degree. C. for 3.5 hours.
Thereby, a sample including a silica film covering a very small
portion of the surface of the substrate was obtained. The
parameters were measured for the resulting sample as in Example 1.
The results thereof are also shown in Table 1.
Comparative Example 5
[0044] A porous substrate produced in Example 1 was dipped in the
alumina sol used in Example 1 such that alumina was applied to the
surface of the substrate, and then drying was performed in the air
at 110.degree. C. Subsequently, the substrate was dipped in a 2 wt
% aqueous solution of potassium chloride such that potassium
chloride was applied to the surface of the substrate, and then
drying was performed in the air at 110.degree. C. Subsequently,
heat treatment was performed in the air at 1,300.degree. C. for 3.5
hours. Thereby, a sample including a potassium
aluminosilicate-containing film covering at least a portion of the
surface of the substrate was obtained. The parameters were measured
for the resulting sample as in Example 1. The results thereof are
also shown in Table 1.
[0045] Among the samples of the examples and comparative examples,
using the samples of Examples 1, 7, 10, and 14 and Comparative
Examples 1, 2, 3, and 5, an electric furnace spalling test (rapid
cooling test) was carried out. As a result, no cracking occurred in
all of the samples of the examples, and cracking occurred in all of
the samples of the comparative examples. The electric furnace
spalling test is a test, in which, specifically, a sample is heated
in an electric furnace at 550.degree. C..times.2 h, the temperature
is made uniform, then the sample is recovered to room temperature,
and thermal shock resistance is evaluated depending on the presence
or absence of occurrence of cracks in the sample.
[0046] As shown in Table 1, the change in thermal expansion in each
of Examples 1 to 16 is negative, while the change in thermal
expansion in each of Comparative Examples 1 to 5 is positive. A
change in thermal expansion being negative means that the film has
a lower thermal expansion coefficient than that of the substrate.
In each of Examples 1 to 16, thermal expansion of the substrate can
be suppressed by the film having a low thermal expansion
coefficient compared with each of Comparative Examples 1 to 5, and
as a result, thermal shock resistance is improved. Furthermore, as
the absolute negative value of the change in thermal expansion
increases, the thermal shock resistance increases. That is, the
change in thermal expansion can be an index for evaluating the
thermal shock resistance. The reason for the fact that the change
in thermal expansion in each of Examples 1 to 16 is negative is
believed to be that the film contains, as a main component, a phase
having a low thermal expansion coefficient, specifically, a phase
including four elements: lithium (Li), aluminum (Al), silicon (Si),
and oxygen (O) (in particular, a phase including a lithium
aluminosilicate).
[0047] Furthermore, when comparison is made between Example 4 and
Example 9, Example 9 has a larger negative absolute value of the
change in thermal expansion. Consequently, Example 9 has a larger
effect of suppressing thermal expansion of the substrate, and the
thermal shock resistance is further improved. The reason for this
is assumed to be that the lithium aluminosilicate constituting the
film of Example 9 is spodumene having a low thermal expansion
coefficient, while the lithium aluminosilicate constituting the
film of Example 4 is a crystal phase having a thermal expansion
coefficient not as low as that of spodumene.
TABLE-US-00001 TABLE 1 Film Substrate Max- Property Sinter- Spodu-
imum Film Change in Si:SiC ing Aid Poros- Composition Ratio Lithium
mene Thick- Weight Thermal (Weight (Weight ity Li.sub.2O
Al.sub.2O.sub.3 SiO.sub.2 K.sub.2O Alumino- Crystal ness Ratio
Expansion Type Ratio) Ratio) (%) (wt %) (wt %) (wt %) (wt %)
silicate Phase (.mu.m) (wt %) (%) Example 1 Si--SiC 20:80 -- 40 3
21 76 -- Contained Contained 5 9.6 -2.3 Example 2 Si--SiC 20:80 --
40 2 19 79 -- Contained Contained 11 12.5 -4.7 Example 3 Si--SiC
20:80 40 5 41 54 Contained Contained 12 13.7 -5.3 Example 4 Si--SiC
20:80 -- 40 1 3 96 -- Contained Not Con- 3 6.0 -1.9 tained Example
5 Si--SiC 20:80 -- 40 37 6 57 -- Contained Contained 2 4.8 -2.2
Example 6 Si--SiC 20:80 -- 40 4 41 55 -- Contained Contained 18
19.6 -5.5 Example 7 Si--SiC 20:80 -- 40 2 11 87 -- Contained
Contained 102 58.3 -13.6 Example 8 Si--SiC 20:80 -- 40 2 3 95 --
Contained Contained 89 48.1 -11.2 Example 9 Si--SiC 30:70 -- 38 3
49 48 -- Contained Contained 3 5.3 -4.6 Example 10 Si--SiC 30:70
SrO: 1 31 1 19 80 Contained Not Con- 2 4.1 -1.1 tained Example 11
.sup. 1 Si--SiC 50:50 -- 20 5 37 58 -- Contained Contained 6 16.8
-2.3 Example 12 .sup. 1 Si--SiC 60:40 -- 0 1 5 94 -- Contained
Contained 18 8.6 -0.1 Example 13 Si--SiC 10:90 -- 48 3 20 77 --
Contained Contained 5 7.3 -4.4 Example 14 .sup. 2 Si--SiC 20:80
SrO: 30 70 10 12 78 -- Contained Contained 3 19.7 -6.8 CaO: 9
Example 15 SiC 0:100 -- 0 3 35 62 -- Contained Contained 48 25.8
-0.5 Example 16 SiC 0:100 39 5 21 74 Contained Contained 3 6.2 -1.8
Comparative Si--SiC 20:80 -- 40 -- -- 100 -- Not Con- Not Con- 2
5.8 9.1 Example 1 tained tained Comparative Si--SiC 20:80 -- 40 --
52 48 -- Not Con- Not Con- 7 11.8 6.8 Example 2 tained tained
Comparative Si--SiC 20:80 -- 40 4 -- 96 -- Not Con- Not Con- 3 6.9
9.0 Example 3 tained tained Comparative .sup. 1 Si--SiC 60:40 -- 0
-- -- 100 -- Not Con- Not Con- 3 0.2 0.9 Example 4 tained tained
Comparative Si--SiC 20:80 -- 40 -- 23 55 22 Not Con- Not Con- 3
12.8 11.4 Example 5 tained tained .sup. 1 In Examples 11, 12 and
Comparative Example 4, the formed body was fired in decompression
and an Ar atmosphere. .sup. 2 In Example 14, 20 parts of fly ash
balloon and 15 parts of starch are added as pore-forming materials
relative to 100 parts of weight of Sr--SiC.
[0048] As is evident from Table 1, in the samples of Examples 1 to
16, the thermal expansion coefficient is low compared with the
samples of Comparative Examples 1 to 4, and thus excellent thermal
shock resistance is exhibited.
[0049] This application claims the benefit of Japanese Patent
Application No. 2011-022498, filed Feb. 4, 2011, which is hereby
incorporated by reference herein in its entirety.
INDUSTRIAL APPLICABILITY
[0050] The silicon carbide material according to the first aspect
of the present invention can be used in a honeycomb structure. The
honeycomb structure can be used for a DPF or catalytic converter on
which, for example, a noble metal catalyst is carried. That is, one
utilization form of the honeycomb structure is a catalyst carrier.
Furthermore, in catalytic converters of a type in which the
catalyst is activated early by electric heating, high thermal shock
resistance is required, and therefore use of the silicon carbide
material according to the first aspect of the present invention is
particularly preferable.
* * * * *