U.S. patent application number 16/668441 was filed with the patent office on 2020-05-21 for support for electric heating type catalyst and exhaust gas purifying device.
This patent application is currently assigned to NGK INSULATORS, LTD.. The applicant listed for this patent is NGK INSULATORS, LTD.. Invention is credited to Takayuki Inoue, Naoki OKAMOTO.
Application Number | 20200157998 16/668441 |
Document ID | / |
Family ID | 70470619 |
Filed Date | 2020-05-21 |
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United States Patent
Application |
20200157998 |
Kind Code |
A1 |
OKAMOTO; Naoki ; et
al. |
May 21, 2020 |
SUPPORT FOR ELECTRIC HEATING TYPE CATALYST AND EXHAUST GAS
PURIFYING DEVICE
Abstract
A support for an electric heating type catalyst comprises: a
honeycomb structure having partition walls that define a plurality
of cells, each of cells extending from one end face to other end
face to form a fluid path for a fluid; and a pair of electrode
layers formed on a side surface of the honeycomb structure, the
pair of electrode layers being arranged so as to face each other
across a center of the honeycomb structure. The pair of electrode
layers is electrically connected to the honeycomb structure. The
honeycomb structure side of each of the electrode layers comprises
a portion having a thermal expansion coefficient lower than that of
the honeycomb structure.
Inventors: |
OKAMOTO; Naoki; (Nagoya-Shi,
JP) ; Inoue; Takayuki; (Nagoya-Shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGK INSULATORS, LTD. |
Nagoya-Shi |
|
JP |
|
|
Assignee: |
NGK INSULATORS, LTD.
Nagoya-Shi
JP
|
Family ID: |
70470619 |
Appl. No.: |
16/668441 |
Filed: |
October 30, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01N 13/1811 20130101;
F01N 13/1838 20130101; F01N 2330/30 20130101; F01N 2450/28
20130101; F01N 3/2013 20130101; F01N 3/2026 20130101; F01N 3/0222
20130101; F01N 3/2828 20130101 |
International
Class: |
F01N 13/18 20060101
F01N013/18; F01N 3/022 20060101 F01N003/022; F01N 3/20 20060101
F01N003/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2018 |
JP |
2018-215939 |
Claims
1. A support for an electric heating type catalyst, comprising: a
honeycomb structure having partition walls that define a plurality
of cells, each of cells extending from one end face to other end
face to form a fluid path for a fluid; and a pair of electrode
layers formed on a side surface of the honeycomb structure, the
pair of electrode layers being arranged so as to face each other
across a center of the honeycomb structure; wherein the pair of
electrode layers is electrically connected to the honeycomb
structure, and wherein the honeycomb structure side of each of the
electrode layers comprises a portion having a thermal expansion
coefficient lower than that of the honeycomb structure.
2. The support for the electric heating type catalyst according to
claim 1, wherein a difference between a thermal expansion
coefficient of the portion having the thermal expansion coefficient
lower than that of the honeycomb structure and the thermal
expansion coefficient of the honeycomb structure is 1.0 [ppm]/K or
more.
3. The support for the electric heating type catalyst according to
claim 1, wherein the thermal expansion coefficient of the portion
having the thermal expansion coefficient lower than that of the
honeycomb structure is lower than the thermal expansion coefficient
of each of the electrode layers.
4. The support for the electric heating type catalyst according to
claim 1, wherein the portion having the thermal expansion
coefficient lower than that of the honeycomb structure is an
intermediate layer having a thermal expansion coefficient lower
than that of the honeycomb structure.
5. The support for the electric heating type catalyst according to
claim 4, wherein the intermediate layer has a thickness of 3 to 400
.mu.m.
6. The support for the electric heating type catalyst according to
claim 4, wherein the intermediate layer has an area larger than
that of each of the electrode layers, and wherein, in a projection
plane on the side surface of the honeycomb structure, a projection
plane of each of the electrode layers is completely included in a
projection plane of the intermediate layer.
7. The support for the electric heating type catalyst according to
claim 1, wherein the honeycomb structure comprises a
silicon-silicon carbide composite material or silicon carbide as a
main component.
8. The support for the electric heating type catalyst according to
claim 1, wherein the portion having the thermal expansion
coefficient lower than that of the honeycomb structure comprises an
oxide ceramic, or a mixture of a metal or a metal compound and an
oxide ceramic.
9. The support for the electric heating type catalyst according to
claim 1, wherein each of the electrode layers comprises a mixture
of a metal or a metal compound and an oxide ceramic.
10. An exhaust gas purifying device, comprising: the support for
the electric heating type catalyst according to claim 1, the
support being disposed in an exhaust gas flow path through which an
exhaust gas from an engine is allowed to flow; and a cylindrical
metal member for housing the support for the electric heating type
catalyst.
Description
TECHNICAL FIELD
[0001] The present invention relates to a support for an electric
heating type catalyst and an exhaust gas purifying device. More
particularly, the present invention relates to a support for an
electric heating type catalyst including a honeycomb structure and
electrode layers, which can effectively suppress cracking in the
honeycomb structure, caused by thermal stress due to a difference
between a thermal expansion coefficient of the honeycomb structure
and a thermal expansion coefficient of the electrode layers; and to
an exhaust gas purifying device using the support for the electric
heating type catalyst.
[0002] Conventionally, a member in which a catalyst is supported on
a honeycomb structure made of cordierite or silicon carbide is used
for treatment of harmful substances in exhaust gases discharged
from motor vehicle engines. Such a honeycomb structure generally
has a pillar shaped honeycomb structure that includes partition
walls defining a plurality of cells extending from one end face to
the other end face to form flow paths for an exhaust gas.
[0003] For the treatment of the exhaust gas with the catalyst
supported on the honeycomb structure, a temperature of the catalyst
is required for being increased to a predetermined temperature.
However, as the engine is started, the catalyst temperature is
lower, conventionally causing a problem that the exhaust gas is not
sufficiently purified. Therefore, a system called an electric
heating catalyst (EHC) has been developed. In the system,
electrodes are disposed on a honeycomb structure made of conductive
ceramics and the honeycomb structure itself generates heat by
electrical conduction, whereby the temperature of the catalyst
supported on the honeycomb structure is increased to an activation
temperature before or during starting of the engine.
[0004] Patent Document 1 proposes a honeycomb structure which is a
catalyst support and also functions as a heater by applying a
voltage, and which can suppress a bias of a temperature
distribution when a voltage is applied. More particularly, it
proposes that the bias of the temperature distribution generated
when the voltage is applied is suppressed by disposing a pair of
electrode portions (hereinafter also referred to as "electrode
layers") in the form of strip on a side surface of the pillar
shaped honeycomb structure in an extending direction of a cell of
the honeycomb structure, and disposing one electrode portion of the
pair of electrode portions on a side opposed to the other electrode
portion of the pair of electrode portions across a center of the
honeycomb structure, in a cross section orthogonal to the extending
direction of the cell.
[0005] The disposing of the electrode layers on the honeycomb
structure requires good bonding strength of a connected portion, a
decreased change ratio of electrical resistance at the connected
portion, and lower contact thermal resistance of the connected
portion. As its approach, Patent Document 2 discloses "a honeycomb
structure comprising: a honeycomb ceramic body having partition
walls and an outer peripheral wall positioned at the outermost
periphery, the partition walls defining a plurality of cells
extending from one end face to other end face to form flow paths
for a fluid, the honeycomb ceramic body containing metallic Si; and
a connected portion bonded to an outer peripheral surface of the
ceramic body, wherein the connected portion has: a diffusion layer
placed at the outer periphery side of the ceramic body, the
diffusion layer being mainly based on a metallic silicide; and a
metal layer formed on the diffusion layer, and wherein the metal
layer contains a metal component as a main component and has a
diffused compound having a thermal expansion coefficient of
5.0.times.10.sup.-6/.degree. C. or less".
CITATION LIST
Patent Literatures
[0006] Patent Document 1: WO 2013/146955
[0007] Patent Document 2: Japanese Patent Application Publication
No. 2014-51402 A
SUMMARY OF INVENTION
[0008] In the invention disclosed in Patent Document 2, the
diffusion layer mainly based on a metal silicide and the electrode
layer (metal layer) formed on the diffusion layer are disposed on
the outer peripheral surface side of the honeycomb structure.
However, the electrode layer generally has a higher thermal
expansion coefficient than that of the honeycomb structure.
Therefore, in the invention disclosed in Patent Document 2, the
relationship of the thermal expansion coefficient represents the
honeycomb structure<the diffusion layer<the electrode layer,
which causes a problem that a thermal stress is generated due to a
difference in thermal expansion of each member as the temperature
of the support for the electric heating type catalyst increases,
and cracks are generated in the honeycomb structure.
[0009] In order to suppress the thermal stress due to the
difference in the thermal expansion of each member, it is
considered that the thermal expansion coefficient of the electrode
layers is decreased to reduce the difference in the thermal
expansion. However, the materials of the electrode layers are
affected on performance, so that it is difficult to adjust the
thermal expansion coefficient. Therefore, there is a need for other
approaches to alleviate the thermal stress.
[0010] The present invention has been made in view of the above
problems. An object of the present invention is to provide a
support for an electric heating type catalyst having a honeycomb
structure and electrode layers, which can effectively suppress
cracking in the honeycomb structure caused by thermal stress due to
a difference between a thermal expansion coefficient of the
honeycomb structure and a thermal expansion coefficient of the
electrode layers; and an exhaust gas purifying device using the
support for the electric heating type catalyst.
[0011] As a result of intensive studies, the present inventors have
found that the above problems can be solved by the presence of a
portion having a certain thermal expansion coefficient on the
honeycomb structure side of each electrode layer. Thus, the present
invention is specified as follows:
[0012] (1)
[0013] A support for an electric heating type catalyst,
comprising:
[0014] a honeycomb structure having partition walls that define a
plurality of cells, each of cells extending from one end face to
other end face to form a fluid path for a fluid; and
[0015] a pair of electrode layers formed on a side surface of the
honeycomb structure, the pair of electrode layers being arranged so
as to face each other across a center of the honeycomb
structure;
[0016] wherein the pair of electrode layers is electrically
connected to the honeycomb structure, and
[0017] wherein the honeycomb structure side of each of the
electrode layers comprises a portion having a thermal expansion
coefficient lower than that of the honeycomb structure.
[0018] (2)
[0019] The support for the electric heating type catalyst according
to (1), wherein a difference between a thermal expansion
coefficient of the portion having the thermal expansion coefficient
lower than that of the honeycomb structure and the thermal
expansion coefficient of the honeycomb structure is 1.0 [ppm]/K or
more.
[0020] (3)
[0021] The support for the electric heating type catalyst according
to (1) or (2), wherein the thermal expansion coefficient of the
portion having the thermal expansion coefficient lower than that of
the honeycomb structure is lower than the thermal expansion
coefficient of each of the electrode layers.
[0022] (4)
[0023] The support for the electric heating type catalyst according
to any one of (1) to (3), wherein the portion having the thermal
expansion coefficient lower than that of the honeycomb structure is
an intermediate layer having a thermal expansion coefficient lower
than that of the honeycomb structure.
[0024] (5)
[0025] The support for the electric heating type catalyst according
to (4), wherein the intermediate layer has a thickness of 3 to 400
.mu.m.
[0026] (6)
[0027] The support for the electric heating type catalyst according
to (4) or (5), wherein the intermediate layer has an area larger
than that of each of the electrode layers, and wherein, in a
projection plane on the side surface of the honeycomb structure, a
projection plane of each of the electrode layers is completely
included in a projection plane of the intermediate layer.
[0028] (7)
[0029] The support for the electric heating type catalyst according
to any one of (1) to (6), wherein the honeycomb structure comprises
a silicon-silicon carbide composite material or silicon carbide as
a main component.
[0030] (8)
[0031] The support for the electric heating type catalyst according
to any one of (1) to (7), wherein the portion having the thermal
expansion coefficient lower than that of the honeycomb structure
comprises an oxide ceramic, or a mixture of a metal or a metal
compound and an oxide ceramic.
[0032] (9)
[0033] The support for the electric heating type catalyst according
to any one of (1) to (8), wherein each of the electrode layers
comprises a mixture of a metal or a metal compound and an oxide
ceramic.
[0034] (10)
[0035] An exhaust gas purifying device, comprising:
[0036] the support for the electric heating type catalyst according
to any one of (1) to (9), the support being disposed in an exhaust
gas flow path through which an exhaust gas from an engine is
allowed to flow; and
[0037] a cylindrical metal member for housing the support for the
electric heating type catalyst.
[0038] According to the present invention, it is possible to
provide a support for an electric heating type catalyst having a
honeycomb structure and electrode layers, which can effectively
suppress cracking in the honeycomb structure caused by thermal
stress due to a difference between a thermal expansion coefficient
of the honeycomb structure and a thermal expansion coefficient of
the electrode layers; and an exhaust gas purifying device using the
support for the electric heating type catalyst.
BRIEF DESCRIPTION OF DRAWINGS
[0039] FIG. 1 is a view showing an example of a honeycomb structure
according to the present invention.
[0040] FIG. 2 is a view showing a structure of a support for an
electric heating type catalyst according to an embodiment of the
present invention.
[0041] FIG. 3 is a projection plane on a side surface of a
honeycomb structure according to an embodiment of the present
invention.
[0042] FIG. 4 is a view showing a structure of a support for an
electric heating type catalyst produced according to an embodiment
of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] Hereinafter, embodiments of a support for an electrically
heating type catalyst according to the present invention will be
described with reference to the drawings. However, the present
invention is not limited to the embodiments, and various changes,
modifications, and improvements may be added without departing from
the scope of the present invention, based on knowledge of those
skilled in the art.
[0044] (1. Honeycomb Structure)
[0045] FIG. 1 is a view showing an example of a honeycomb structure
in the present invention. For example, the honeycomb structure 10
includes: porous partition walls 11 that defines a plurality of
cells 12, the cells 12 forming flow paths for a fluid, the cells
extending from an inflow bottom face that is an end face on an
inflow side of the fluid to an outflow bottom face that is an end
face on an outflow side of the fluid; and an outer peripheral wall
located at the outermost periphery. The number, arrangement, shape
and the like of the cells 12, as well as the thickness of each
partition wall 11, and the like, are not limited and may be
optionally designed as required.
[0046] A material of the honeycomb structure 10 is not particularly
limited as long as it has conductivity, and metals, ceramics and
the like may be used. In particular, from the viewpoint of
compatibility of heat resistance and conductivity, preferably, the
material of the honeycomb structure 10 is mainly based on a
silicon-silicon carbide composite material or silicon carbide, and
more preferably, it is a silicon-silicon carbide composite material
or silicon carbide. Tantalum silicide (TaSi.sub.2) and chromium
silicide (CrSi.sub.2) may also be added to lower the electric
resistivity of the honeycomb structure. The phrase "the honeycomb
structure 10 is mainly based on a silicon-silicon carbide
composite" means that the honeycomb structure 10 contains 90% by
mass or more of the silicon-silicon carbide composite material
(total mass) based on the entire honeycomb structure. Here, for the
silicon-silicon carbide composite material, it contains silicon
carbide particles as an aggregate and silicon as a bonding material
for bonding the silicon carbide particles, and a plurality of
silicon carbide particles are bonded by silicon so as to form pores
between the silicon carbide particles. The phrase "the honeycomb
structure 10 is mainly based on silicon carbide" means that the
honeycomb structure 10 contains 90% by mass or more of silicon
carbide (total mass) based on the entire honeycomb structure.
[0047] The electric resistivity of the honeycomb structure 10 may
be set as needed depending on voltage to be applied, including, but
not particularly limited to, for example 0.001 to 200 .OMEGA.cm,
for example. For a higher voltage of 64 V or more, it may be 2 to
200 .OMEGA.cm, and typically 5 to 100 .OMEGA.cm. Further, for a
lower voltage of less than 64 V, it may be 0.001 to 2 .OMEGA.cm,
and typically 0.001 to 1 .OMEGA.cm, and more typically 0.01 to 1
.OMEGA.cm.
[0048] Each partition wall 11 of the honeycomb structure 10
preferably has a porosity of 35 to 60%, and more preferably 35 to
45%. The porosity of 35% or more can further suppress deformation
during firing, which is preferable. The porosity of 60% or less
maintains sufficient strength of the honeycomb structure. The
porosity is a value measured by a mercury porosimeter.
[0049] Each partition wall 11 of the honeycomb structure 10
preferably has an average pore diameter of 2 to 15 .mu.m, and more
preferably 4 to 8 .mu.m. The average pore diameter of 2 .mu.m or
more can prevent excessively high electric resistivity. The average
pore diameter of 15 .mu.m or less can prevent excessively low
electric resistivity. The average pore diameter is a value measured
by a mercury porosimeter.
[0050] The shape of each cell 12 in a cross section of each cell
orthogonal to a flow path direction is not limited, but it may
preferably be a square, a hexagon, an octagon, or a combination
thereof. Among these, the square and hexagonal shapes are
preferable. Such a cell shape leads to a decreased pressure loss
when an exhaust gas flows through the honeycomb structure 10, and
improved purification performance of the catalyst.
[0051] The outer shape of the honeycomb structure 10 is not
particularly limited as long as it presents a pillar shape, and it
may be, for example, a shape such as a pillar shape with circular
bottoms (cylindrical shape), a pillar shape with oval shaped
bottoms, and a pillar shape with polygonal (square, pentagonal,
hexagonal, heptagonal, octagonal, and the like) bottoms, and the
like. Further, for the size of the honeycomb structure 10, the
honeycomb structure preferably has an area of bottom surfaces of
2000 to 20000 mm.sup.2, and more preferably 4000 to 10000 mm.sup.2,
in terms of increasing heat resistance (preventing cracks generated
in a circumferential direction of the outer peripheral side wall).
Further, an axial length of the honeycomb structure 10 is
preferably 50 to 200 mm, and more preferably 75 to 150 mm, in terms
of increasing the heat resistance (preventing cracks generated in a
direction parallel to a central axis direction on the outer
peripheral side wall).
[0052] Further, the honeycomb structure 10 can be used as a
catalyst support by supporting a catalyst on the honeycomb
structure 10.
[0053] Production of the honeycomb structure can be carried out in
accordance with a method for making a honeycomb structure in a
known method for producing a honeycomb structure. For example,
first, a forming material is prepared by adding metallic silicon
powder (metallic silicon), a binder, a surfactant(s), a pore
former, water, and the like to silicon carbide powder (silicon
carbide). It is preferable that a mass of metallic silicon powder
is 10 to 40% by mass relative to the total of mass of silicon
carbide powder and mass of metallic silicon powder. The average
particle diameter of the silicon carbide particles in the silicon
carbide powder is preferably 3 to 50 .mu.m, and more preferably 3
to 40 .mu.m. The average particle diameter of the metallic silicon
particles in the metallic silicon powder is preferably 2 to 35
.mu.m. The average particle diameter of each of the silicon carbide
particles and the metallic silicon particles refers to an
arithmetic average diameter on volume basis when frequency
distribution of the particle size is measured by the laser
diffraction method. The silicon carbide particles are fine
particles of silicon carbide forming the silicon carbide powder,
and the metallic silicon particles are fine particles of metallic
silicon forming the metallic silicon powder. It should be noted
that this is formulation for forming raw materials in the case
where the material of the honeycomb structure is the
silicon-silicon carbide composite material. In the case where the
material of the honeycomb structure is silicon carbide, no metallic
silicon is added.
[0054] Examples of the binder include methyl cellulose,
hydroxypropyl methyl cellulose, hydroxypropoxyl cellulose,
hydroxyethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol
and the like. Among these, it is preferable to use methyl cellulose
in combination with hydroxypropoxyl cellulose. The content of the
binder is preferably 2.0 to 10.0 parts by mass when the total mass
of the silicon carbide powder and the metallic silicon powder is
100 parts by mass.
[0055] The content of water is preferably 20 to 60 parts by mass
when the total mass of the silicon carbide powder and the metallic
silicon powder is 100 parts by mass.
[0056] The surfactant that can be used includes ethylene glycol,
dextrin, fatty acid soaps, polyalcohol and the like. These may be
used alone or in combination of two or more. The content of the
surfactant is preferably 0.1 to 2.0 parts by mass when the total
mass of the silicon carbide powder and the metallic silicon powder
is 100 parts by mass.
[0057] The pore former is not particularly limited as long as the
pore former itself forms pores after firing, including, for
example, graphite, starch, foamed resins, water absorbing resins,
silica gel and the like. The content of the pore former is
preferably 0.5 to 10.0 parts by mass when the total mass of the
silicon carbide powder and the metallic silicon powder is 100 parts
by mass. An average particle diameter of the pore former is
preferably 10 to 30 .mu.m. The average particle diameter of 10
.mu.m or more can allow sufficient formation of pores. The average
particle diameter of 30 .mu.m or less can allow prevention of a die
from being clogged with the pore former during forming. The average
particle diameter of the pore former refers to an arithmetic
average diameter on volume basis when frequency distribution of the
particle size is measured by the laser diffraction method. When the
pore former is the water absorbing resin, the average particle
diameter of the pore former is an average particle diameter after
water absorption.
[0058] Then, the resulting forming raw materials are kneaded to
form a green body, and the green body is then extruded to prepare a
honeycomb structure. In extrusion molding, a die having a desired
overall shape, cell shape, partition wall thickness, cell density
and the like can be used. Preferably, the resulting honeycomb
structure is then dried. When the length in the central axis
direction of the honeycomb structure is not the desired length,
both the bottom faces of the honeycomb structure can be cut to the
desired length.
[0059] (2. Electrode Layer)
[0060] As shown in FIG. 2, the support for the electric heating
type catalyst includes a pair of electrode layers 13a, 13b on the
outer peripheral wall of the honeycomb structure 10. Each of the
electrode layers 13a, 13b is formed into a strip shape extending in
the extending direction of the cell 12 of the honeycomb structure
10. Each of the electrode layers 13a, 13b is electrically connected
to the honeycomb structure 10. In a cross section of the honeycomb
structure 10 orthogonal to the extending direction of the cell 12,
the pair of electrode layers 13a, 13b are arranged so as to face
each other across a center O of the honeycomb structure 10. Such a
configuration allows suppression of any bias of a current flowing
in the honeycomb structure 10 and suppression of any bias of a
temperature distribution in the honeycomb structure 10 when a
voltage is applied.
[0061] The electrode layers 13a, 13b are formed of a material
having conductivity. The electrode layers 13a, 13b are preferably
made of an oxide ceramic or a mixture of a metal or a metal
compound and an oxide ceramic. The metal may be either a single
metal or an alloy, including, for example, silicon, aluminum, iron,
stainless steel, titanium, tungsten, Ni--Cr alloys and the like.
Examples of the metal compound other than the oxide ceramic include
metal oxides, metal nitrides, metal carbides, metal silicides,
metal borides, composite oxides, and the like, and for example,
FeSi.sub.2, CrSi.sub.2, alumina, silica, titanium oxide and the
like can be preferably used. Each of the metal and the metal
compound may be used alone or in combination of two or more
types.
[0062] Specific examples of the oxide ceramic include glass,
cordierite, and mullite.
[0063] The glass may further contain an oxide consisting of at
least one component selected from the group consisting of B, Mg,
Al, Si, P, Ti and Zr. The further containing of at least one
selected from the above group is preferable in order to allow
further improvement of the strength of each electrode layer.
[0064] Further, although not shown, the support for the electric
heating type catalyst further includes a pair of electrode portions
which are electrically connected to the electrode layers 13a, 13b,
respectively. According to the arrangement, when a voltage is
applied through the electrode layers 13a, 13b, the metal electrode
portions can be energized to cause the honeycomb structure 10 to
generate heat by Joule heat. Therefore, the honeycomb structure 10
can be suitably used as a heater. The applied voltage is preferably
12 to 900 V, and more preferably 64 to 600 V. However, the applied
voltage may be changed as needed.
[0065] For the electrode portions, a metal, a ceramic or the like
may be used. Examples of a metal include, but not limited to,
representatively, silver, copper, nickel, gold, palladium, silicon,
and the like, in terms of ease of availability. It is also possible
to use carbon. Non-limiting examples of ceramics include ceramics
containing at least one of Si, Cr, B, Fe, Co, Ni, Ti and Ta, and
illustratively, silicon carbide, chromium silicide, boron carbide,
chromium boride, and tantalum silicide. Composite materials may be
formed by combining the metals with the ceramics.
[0066] (3. Portion Having Thermal Expansion Coefficient Lower than
that of Honeycomb Structure)
[0067] As shown in FIG. 2, the support for the electric heating
type catalyst according to the present embodiment includes portions
14a, 14b each having a thermal expansion coefficient lower than
that of the honeycomb structure 10 on the honeycomb structure side
of each of the electrode layers 13a, 13b (hereinafter abbreviated
as "portions 14a, 14b"). Here, it is important that the portions
14a, 14b have a lower thermal expansion coefficient than that of
the honeycomb structure 10.
[0068] As described above, the electrode layers generally have a
higher thermal expansion coefficient than that of the honeycomb
structure. Therefore, there has been a problem that a thermal
stress is generated due to a difference in thermal expansion of
each member as the temperature of the support for the electric
heating type catalyst increases, thereby generating cracks in the
honeycomb structure. So, the portions 14a, 14b having the thermal
expansion coefficient lower than that of the honeycomb structure 10
are provided on the honeycomb structure sides of the electrode
layers 13a, 13b. Since the thermal expansion coefficient of the
portions 14a, 14b is lower than that of the honeycomb structure 10,
the thermal expansion of the portions 14a, 14b is less than that of
the honeycomb structure 10 even if the temperature of the support
for the electric heating type catalyst is increased, whereby the
thermal expansion of parts near the electrode layers 13a, 13b in
the honeycomb structure 10 is suppressed, and cracks can be
prevented.
[0069] The above effects can be obtained if the thermal expansion
coefficient of the portions 14a, 14b is lower than that of the
honeycomb structure 10. However, in order to obtain more remarkable
effects, a difference between the thermal expansion coefficient of
each of the portions 14a, 14b and the honeycomb structure 10 is
preferably 1.0 [ppm]/K or more. The difference of 1.0 [ppm]/K or
more can more strongly suppress the thermal expansion of the
honeycomb structure 10, so that the cracks are further prevented.
From this viewpoint, the difference between the thermal expansion
coefficient of each of the portions 14a, 14b and the thermal
expansion coefficient of the honeycomb structure 10 is more
preferably 1.3 [ppm]/K or more, and more preferably 1.5 [ppm]/K or
more.
[0070] It is preferable that the thermal expansion coefficient of
each of the portions 14a, 14b is lower than the thermal expansion
coefficient of each of the electrode layers 13a, 13b. This can
provide advantages that the portions 14a, 14b more reliably ensure
the above role, as well as the thermal expansion coefficient of
each of the electrode layers 13a, 13b can be more freely set, and a
range for selecting the materials is expanded.
[0071] In the drawing, each of the portions 14a, 14b forms a
continuous layer (see FIGS. 2 and 3). However, each of the portions
14a, 14b does not necessarily have to form a layer alone, and can
be arranged in any form as long as the portions 14a, 14b can
achieve their functions. That is, the portions each having the
thermal expansion coefficient lower than that of the honeycomb
structure only need to be present on the honeycomb structure side
of the electrode layers, and each portion may form a part of the
electrode layer, or may be included in a separate layer, or may
form a layer itself that is different from the electrode layer. For
example, each of the portions 14a, 14b may be included in one layer
(which may be each of the electrode layers 13a, 13b or may be a
layer provided separately from each of the electrode layers 13a,
13b) in a state where each of the portions 14a, 14b is continuous
to a certain extent. It should be noted that each of the portions
14a, 14b necessarily does not have a constant thermal expansion
coefficient, and may vary continuously or discontinuously as long
as it has the thermal expansion coefficient lower than that of the
honeycomb structure 10. In the embodiment where each of the
portions 14a, 4b is included in a part of each of the electrode
layers 13a, 13b, it is assumed that there is a portion having a
thermal expansion coefficient higher than that of the honeycomb
structure 10 on the radially outer side of each of the portions 14,
14b in each of the electrode layers 13a, 13b.
[0072] It is, of course, preferable that each of the portions 14a,
14b forms a layer in view of more reliably obtaining the effect of
the present invention, industrial convenience and the like (see
FIG. 3). In this case, since each of the portions 14a, 14b is
located between the honeycomb structure 10 and each of the
electrode layers 13a and 13b, these portions are called
intermediate layers. FIG. 3 is a view of a projection plane of the
side surface of the honeycomb structure 10. In the drawing, the
portions 14a, 14b and the electrode layers 13a, 13b are all
rectangular, and the electrode layers 13a, 13b are arranged at the
center positions of the portions 14a, 14b, respectively. However,
their shapes and positions are not limited thereto. As with the
above description, each intermediate layer only needs to have a
thermal expansion coefficient lower than that of the honeycomb
structure 10. That is, the thermal expansion coefficient of each
intermediate layer does not need to be constant. For example, a
case where the thermal expansion coefficient continuously or
discontinuously increases (or decreases) from the honeycomb
structure 10 side toward each of the electrode layers 13a, 13b is
also included in the scope of the present invention.
[0073] Further, when the portions 14a, 14b form the intermediate
layers, each of them preferably has a thickness in a range of 3 to
400 .mu.m. The thickness of each intermediate layer of 3 .mu.m or
more can provide a more remarkable effect. On the other hand, the
thickness of each intermediate layer of 400 .mu.m or less can
suppress the influence on a current flowing through the honeycomb
structure 10, so that the influence on the original function of the
support for the electric heating type catalyst can be minimized.
From the above viewpoint, the thickness of each intermediate layer
is more preferably 5 to 100 .mu.m.
[0074] Further, when the portions 14a, 14b form the intermediate
layers, an area of each intermediate layer is larger than that of
each of the electrode layers 13a, 13b in terms of more reliably
suppressing thermal stress due to the difference in thermal
expansion between the honeycomb structure 10 and each of the
electrode layers 13a, 3b, so that a projection plane of each of the
electrode layers 13a, 13b is completely included in a projection
plane of each intermediate layer in a projection plane on the side
surface of the honeycomb structure 10 (see FIG. 3). That is, since
each of the portions 14a, 14b always lies between the honeycomb
structure 10 and each of the electrode layers 13a, 13b, any
generation of thermal stress in the local portion can be
suppressed. It should be noted that pinholes may be generated in
each intermediate layer, through which the components of the
electrode layers 13a, 13b may penetrate, due to the setting of the
thickness of each intermediate layer and production restrictions,
but the above effect is not inhibited because their influences are
smaller.
[0075] The problem of the present invention can be solved as long
as the thermal expansion coefficient of each of the portions 14a,
14b is lower than the thermal expansion coefficient of the
honeycomb structure 10. However, in terms of maintaining the
thermal resistance at a lower level and maintaining the change rate
of electrical resistance of the connected portion and contact
thermal resistance of the connected portion at lower levels, each
of the portions 14a, 14b is preferably made of an oxide ceramic or
a mixture of a metal or a metal compound and an oxide ceramic, as
with the electrode layers 13a, 13b as described above.
[0076] The metal may be either a single metal or an alloy,
including, for example, silicon, aluminum, iron, stainless steel,
titanium, tungsten, Ni--Cr alloy and the like. Examples of the
metal compound other than the oxide ceramic include oxides, metal
nitrides, metal carbides, metal silicides, metal borides, and
complex oxides. For example, FeSi.sub.2 and CrSi.sub.2 can be
preferably used. Each of the metal and the metal compound may be
used alone or in combination of two or more types. Specific
examples of the oxide ceramic include glass, cordierite, and
mullite.
[0077] The glass may further, contain an oxide consisting of at
least one component selected from the group consisting of B, Mg,
Al, Si, P, Ti and Zr. The further containing of at least one
selected from the above group is preferable in order to allow
further improvement of the strength of each electrode layer.
[0078] Each intermediate layer and each electrode layer can be
produced by the following method, for example. For a method for
forming each intermediate layer, metal powder (metal powder such as
metal silicide, stainless steel) and glass powder are mixed
together to prepare a ceramic raw material. To the ceramic raw
material are added a binder and a surfactant, and further added
water to prepare an intermediate layer paste. The intermediate
layer paste is applied onto the honeycomb structure using screen
printing or the like to form a coated film. The coated film is
dried and then fired together with the honeycomb structure under
vacuum conditions to form each intermediate layer. As a material
for each electrode layer, metal powder (metal powder such as metal
silicide and stainless steel) and glass powder are then mixed to
form a ceramic raw material. To the ceramic raw material are added
a binder and a surfactant, and further added water to prepare an
electrode layer paste. The electrode layer paste is applied onto
the honeycomb structure on which each intermediate layer has been
formed, by screen printing or the like to form a coated film. The
coated film is dried, and the honeycomb structure on which the
coated film has been formed is then fired under vacuum conditions
to form each electrode layer.
[0079] In the above description, each intermediate layer and each
electrode layer are formed by carrying out the firing every time
the respective layers are formed, in such a manner that the coated
film for the intermediate layer is fired to form each intermediate
layer, and the coated film for the electrode layer is fired to form
each electrode layer. However, the firing for forming each
intermediate layer may not be performed. That is, each intermediate
layer and each electrode layer can be formed at the same time by
forming the coated film for the electrode layer on the coated film
for the intermediate layer and firing this. In this case, when the
same material is used for the intermediate layer and the electrode
layer, the materials of the intermediate layer and the electrode
layer are intermingled during firing, so that there is no interface
between the intermediate layer and the electrode layer and the
electrode layer and the intermediate layer are integrated. In this
case, there is a portion having a thermal expansion coefficient
lower than that of the honeycomb structure on the honeycomb
structure side of each electrode layer.
[0080] The support for the electric heating type catalyst according
to the present invention can be used in an exhaust gas purifying
device, That is, another aspect of the present invention is an
exhaust gas purifying device, comprising: the support for the
electric heating type catalyst according to the present invention,
the support being disposed in an exhaust gas flow path through
which an exhaust gas from an engine is allowed to flow; and a
cylindrical metal member for housing the support for the electric
heating type catalyst. As will be understood from the above
descriptions, in such an exhaust gas purifying device, any cracking
in the honeycomb structure is suppressed, so that higher thermal
shock resistance can be expected.
EXAMPLES
[0081] Hereinafter, Examples is illustrated for better
understanding of the present invention and its advantages, but the
present invention is not limited to these Examples.
[0082] (1. Production of Honeycomb Dried Body)
[0083] Silicon carbide (SiC) powder and metallic silicon (Si)
powder were mixed in a mass ratio of 60:40 to prepare a ceramic raw
material. To the ceramic raw material were added hydroxypropyl
methyl cellulose as a binder, a water absorbing resin as a pore
former, and water to form a forming raw material. The forming raw
material was then kneaded by means of a vacuum green body kneader
to prepare a circular pillar shaped green body. The content of the
binder was 7 parts by mass when the total of the silicon carbide
powder (SiC) and the metallic silicon (Si) powder was 100 parts by
mass. The content of the pore former was 3 parts by mass when the
total of the silicon carbide powder (SiC) and the metallic silicon
(Si) powder was 100 parts by mass. The content of water was 42
parts by mass when the total of the silicon carbide powder (SiC)
and the metallic silicon (Si) powder was 100 parts by mass. The
average particle diameter of the silicon carbide particles in the
silicon carbide powder was 20 .mu.m, and the average particle
diameter of the metallic silicon particles in the metallic silicon
powder was 6 .mu.m. The average particle diameter of the pore
former was 20 .mu.m. The average particle diameter of each of the
silicon carbide particles, the metallic silicon particles and the
pore former refers to an arithmetic mean diameter on volume basis,
when measuring frequency distribution of a particle size by the
laser diffraction method. In addition, a Young's modulus and a
porosity of each honeycomb structure to be finally obtained are
shown in Table 1.
[0084] The resulting pillar shaped green body was formed using an
extruder to obtain a pillar shaped honeycomb formed body in which
each cell had a square cross-sectional shape. The resulting
honeycomb formed body was subjected to high-frequency dielectric
heating and drying and then dried at 120.degree. C. for 2 hours
using a hot air drier, and a predetermined amount of both end faces
were cut to prepare a honeycomb dried body.
[0085] (2. Formation of Intermediate Layer)
[0086] Each intermediate layer was provided as a portion having a
thermal expansion coefficient lower than that of the honeycomb
structure. The conditions are as follows:
[0087] (1) Metal powder (powder of a metal such as metal silicide
and stainless steel) and glass powder were mixed at each volume
ratio as shown in Table 1 to prepare a ceramic raw material. The
average particle diameter was 10 .mu.m for the metal powder and 2
.mu.m for the glass powder. The average particle diameter refers to
an arithmetic average diameter on volume basis when frequency
distribution of the particle size is measured by a laser
diffraction method.
[0088] (2) To the above ceramic raw material were added 1% by mass
of a binder, 1% by mass of a surfactant, and 30% by mass of water
to form a paste.
[0089] (3) The paste was applied on the above honeycomb dried body
(a substrate) using screen printing, such that the paste was
applied to a region where both sides of each electrode layer to be
provided is extended by +3 mm in the circumferential direction, and
both end faces of the paste in the axial direction were matched to
both end faces of the substrate in the substrate of length
direction (a cell extending direction) (see FIG. 4).
[0090] (4) After applying the paste, the paste was dried at
120.degree. C. for 30 minutes by a hot air dryer, and then fired
together with the substrate under vacuum conditions at 1100.degree.
C. for 30 minutes.
[0091] The compositions and characteristics of the intermediate
layer in each Example and Comparative Example are shown in Table
1.
[0092] (3. Formation of Electrode Layer)
[0093] Each electrode layer was provided on each intermediate
layer. The conditions for forming each electrode layer are as
follows:
[0094] (1) Stainless steel powder (SUS430) and glass powder were
mixed at a volume ratio of stainless powder ratio of 40% and glass
powder ratio of 60% to prepare a ceramic raw material. The average
particle diameter was 10 .mu.m for the stainless powder and 2 .mu.m
for the glass powder. The average particle diameter refers to an
arithmetic average diameter based on volume basis when frequency
distribution of the particle size is measured by a laser
diffraction method.
[0095] (2) To the above ceramic raw material were added 1% by mass
of a binder, 1% by mass of a surfactant, and 30% by mass of water
to form a paste.
[0096] (3) The paste was applied on the substrate provided with
each intermediate layer using screen printing, such that the paste
was applied over a width provided by an area of 99.degree. in the
circumferential direction, and both end faces of the paste in the
axial direction were matched to both end faces of the substrate in
the substrate L dimension direction (a cell extending direction)
(see FIG. 4).
[0097] (4) After applying the paste, the paste was dried at
120.degree. C. for 30 minutes by a hot air dryer, and then fired
together with the substrate under vacuum conditions at 1100.degree.
C. for 30 minutes to form each electrode layer on each intermediate
layer.
[0098] In each support for the electric heating type catalyst
obtained by the above procedure, a pair of electrode layers is
formed on the side surface of the honeycomb structure. In each of
Examples and Comparative Examples, the thermal expansion
coefficient of each electrode layer was 6.3 [ppm]/K.
[0099] Further, in each of Examples and Comparative Examples, the
thermal expansion coefficient of each honeycomb structure was 4.3
[ppm]/K.
[0100] Each support for the electric heating type catalyst obtained
by the above procedure was subjected to a heating/cooling cycle
test. The heating/cooling cycle test was carried out by placing
each support for electric heating type catalyst in a rapid
heating/cooling furnace, and increasing and decreasing the
temperature in one minute for heating and in one minute for cooling
over 50 cycles such that the increasing and decreasing of the
temperature of each honeycomb structure was repeated in the range
of 50.degree. C. to 950.degree. C. For each of Examples and
Comparative Example, 20 samples for the same support for the
electric heating type catalyst were prepared according to the above
procedure, and the number of samples where cracks were generated
per the 20 samples, was counted. The results are shown in Table
1.
TABLE-US-00001 TABLE 1 Honeycomb Structure Thermal Expansion
Thermal Expansion Coefficient - Compressive Coefficient
Intermediate Layer Stress Composition of of Intermediate Thermal
Expansion Young's of Honeycomb Intermediate Layer Layer Coefficient
Modulus Porosity Structure Number of (vol %) ([ppm/K) ([ppm/K]
(GPa) (%) (MPa) Cracks Comparative Non -- -- -- -- 19.1 20/20
Example 1 Comparative SUS304 20 5.2 -0.9 170 0 23.2 20/20 Example 2
Glass 80 Example 1 SUS430 20 4.0 0.3 170 0 14.5 14/20 Glass 80
Example 2 FeSi.sub.2 10 3.1 1.2 170 0 8.9 5/20 Glass 90 Example 3
SUS430 10 3.0 1.3 170 0 8.2 3/20 Glass 90 Example 4 SUS304 6 3.0
1.3 170 0 8.2 4/20 Glass 94 Example 5 CrSi.sub.2 10 2.9 1.4 170 0
7.5 4/20 Glass 80 Example 6 Glass 100 2.0 2.3 170 0 2.7 0/20
Example 7 SUS430 10 3.0 1.3 153 10 9.2 5/20 Glass 90 Example 8
SUS430 10 3.0 1.3 136 20 10.2 8/20
[0101] As shown in Table 1, it is understood that Examples of the
present invention effectively suppress cracks as compared with
Comparative Example. In particular, it can be seen that when the
difference between the thermal expansion coefficient of the
intermediate layer and the thermal expansion coefficient of the
honeycomb structure is 1.0 [ppm]/K or more, the generation of
cracks is further significantly reduced.
[0102] On the other hand, in Comparative Examples 1 and 2 that did
not include the portion having the thermal expansion coefficient
lower than that of the honeycomb structure, cracks were generated
for all the samples. Particularly, in Comparative Example 2, the
intermediate layers were formed, but the coefficient of thermal
expansion was beyond the range defined by the present invention, so
that any desired function could not be achieved.
DESCRIPTION OF REFERENCE NUMERALS
[0103] 10 . . . honeycomb structure [0104] 11 . . . partition wall
[0105] 12 . . . cell [0106] 13a, 13b . . . electrode layer [0107]
14a, 14b . . . portion having thermal expansion coefficient lower
than that of honeycomb structure
* * * * *