ELECTRICALLY HEATED CATALYTIC CONVERTER AND METHOD OF MANUFACTURING THE SAME

Abstract

Provided is an electrically heated catalytic converter including at least a conductive substrate and an electrode member that is fixed to the substrate, in which a protective film is formed on a surface of at least a portion of the electrode member. In the electrically heated catalytic converter, at least a portion of the protective film is formed of Al.sub.2O.sub.3, SiO.sub.2, a composite material of Al.sub.2O.sub.3 and SiO.sub.2, or a composite oxide including Al.sub.2O.sub.3, SiO.sub.2, or a composite material of Al.sub.2O.sub.3 and SiO.sub.2 as a major component, the protective film has an amorphous structure or a partially crystalline glass structure having a crystallization rate of 30 vol % or lower with respect to the entire portion of the protective film, and a thickness of the protective film is in a range of 100 nm to 2 μm.

Claims

1. An electrically heated catalytic converter comprising: a conductive substrate that includes a catalyst coating layer; an electrode member that is fixed to the substrate; and a protective film that is provided on a surface of at least a portion of the electrode member, wherein the protective film is formed of i) Al.sub.2O.sub.3, ii) SiO.sub.2, iii) a composite material of Al.sub.2O.sub.3 and SiO.sub.2, or iv) a composite oxide including Al.sub.2O.sub.3, SiO.sub.2, or a composite material of Al.sub.2O.sub.3 and SiO.sub.2 as a major component, the protective film has a configuration in which an entire portion is formed of an amorphous structure or in which a partially crystalline glass structure having a crystallization rate of 30 vol % or lower with respect to the entire portion of the protective film is provided, and a thickness of the protective film is in a range of 100 nm to 2 μm

2. The electrically heated catalytic converter according to claim 1, wherein the conductive substrate is a conductive ceramic including SiC as a major component.

3. The electrically heated catalytic converter according to claim 1, wherein the electrode member is a single member made of a metal or a ceramic or a composite member made of a metal and a ceramic, the electrode member is constituted with a surface electrode and a wiring fixing layer, and a porosity of the surface electrode and the wiring fixing layer is 5% or higher.

4. A method of manufacturing an electrically heated catalytic converter, the electrically heated catalytic converter including at least a conductive substrate that includes a catalyst coating layer and an electrode member that is fixed to the substrate, in which a protective film is formed on a surface of at least a portion of the electrode member, and the method comprising: preparing a sol-gel solution by adding any one of i) a nanomaterial of Al.sub.2O.sub.3, ii) a nanomaterial of SiO.sub.2, iii) a nanomaterial of a composite material of Al.sub.2O.sub.3 and SiO.sub.2, and iv) a nanomaterial of a composite oxide including Al.sub.2O.sub.3, SiO.sub.2, or a composite material of Al.sub.2O.sub.3 and SiO.sub.2 as a major component to a solvent; and manufacturing the electrically heated catalytic converter by applying the sol-gel solution to the surface of at least the portion of the electrode member, drying the sol-gel solution to form a coating film, and firing the coating film at a temperature of 500° C. or lower to form the protective film.

5. The method according to claim 4, wherein the conductive substrate is a conductive ceramic including SiC as a major component.

6. The method according to claim 4, wherein the electrode member is a single member made of a metal or a ceramic or a composite member made of a metal and a ceramic, and the electrode member is constituted with a surface electrode and a wiring fixing layer the electrode member is a porous member in which a porosity of the surface electrode and the wiring fixing layer is 5% or higher.

7. The method according to claim 4, wherein the sol-gel solution is applied to the surface of at least the portion of the electrode member and is dried to form the coating film, and the coating film is fired at a temperature of 100° C. to 200° C. to form the protective film.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

[0030] FIG. 1 is a schematic diagram showing an embodiment of an electrically heated catalytic converter (EHC) according to the disclosure;

[0031] FIG. 2 is a schematic diagram showing a specimen;

[0032] FIG. 3A is an SEM image showing a portion IIIa of FIG. 2;

[0033] FIG. 3B is an SEM image showing an enlarged view of a portion b of FIG. 3A;

[0034] FIG. 4 is a graph showing the results of an oxidative degradation test;

[0035] FIG. 5A is a graph showing an XRD profile of an Al.sub.2O.sub.3 film; and

[0036] FIG. 5B is a graph showing an XRD profile of a SiO.sub.2 film.

DETAILED DESCRIPTION OF EMBODIMENTS

[0037] Hereinafter, an embodiment of an electrically heated catalytic converter according to the present disclosure and a method of manufacturing the same will be described with reference to the drawings.

[0038] (Embodiment of Electrically Heated Catalytic Converter and Method of Manufacturing the Same)

[0039] FIG. 1 is a schematic diagram showing an embodiment of an electrically heated catalytic converter (EHC) according to the disclosure. An electrically heated catalytic converter 10 shown in the drawing is incorporated into an exhaust system for exhaust gas. Specifically, in the exhaust system, an engine (not shown), the electrically heated catalytic converter (EHC) 10, a three-way catalytic converter (not shown), a sub muffler (not shown), and a main muffler (not shown) are disposed in this order and are connected to each other through a system pipe. When the engine starts, a noble metal catalyst constituting the electrically heated catalytic converter 10 is heated to a predetermined temperature as soon as possible, exhaust gas flowing from the engine is purified by the noble metal catalyst, and a portion of the exhaust gas which is not purified by the electrically heated catalytic converter 10 is purified by the three way catalyst purifier positioned downstream of the electrically heated catalytic converter 10.

[0040] The electrically heated catalytic converter 10 is fixed through an external metal pipe (metal case; not shown) and a mat (holder; not shown) which is provided in a hollow portion of the external pipe, and is configured overall to include: a substrate having a honeycomb structure in which a catalyst coating layer (not shown) is provided on surfaces of cell walls 1a; a cable (not shown) through which a pair of electrode members 5 disposed on a surface of the substrate 1 are connected to each other; and an external circuit that is provided midway the cable and includes a power supply (not shown). Among the pair of electrode members 5, one electrode member is connected to a positive pole, and the other electrode member is connected to a negative pole. The substrate 1 is electrically heated by supplying a current thereto through the electrode members 5.

[0041] Each of the electrode members 5 includes: a surface electrode film 2 that is disposed on a surface of the substrate 1; a comb-shaped wiring 3 that is disposed on a surface of the surface electrode film 2; and wiring fixing layers 4 to which a plurality of first wirings 3a, which are included in the comb-shaped wiring 3 and extend in a circumferential direction of the substrate 1, are fixed.

[0042] The comb-shaped wiring 3 includes: a second wiring 3b that extends in a longitudinal direction of the substrate 1; and the plurality of first wirings 3a that are branched from the second wiring 3b and extend in the circumferential direction of the substrate 1. The first wirings 3a and the second wiring 3b are electrically connected to the surface electrode film 2.

[0043] In a case where the power supply is turned on during an engine start, a current is supplied to the pair of electrode members 5 positioned at the center of the substrate 1, a path extending along a diameter of a section of the substrate 1 and a path linearly extending in a section of the substrate 1 through the surface electrode film 2 are formed. In this way, due to a current diffusion function of the surface electrode film 2 constituting the electrode member 5, a current can be supplied to the entire portion of substrate 1 as uniformly as possible, and the diffusion and rectification of an equal amount of current can be realized.

[0044] In the substrate 1, an exhaust gas passage having a honeycomb structure which includes a plurality of cell walls 1a is formed, and a catalyst coating layer (not shown) is formed on the cell walls 1a. The catalyst coating layer is formed by causing a platinum group element such as palladium (Pd), rhodium (Rh), or platinum (Pt) or a platinum group element compound to be supported on an oxide such as alumina (Al.sub.2O.sub.3) or causing another noble metal or a compound thereof to be supported on alumina or a ceria-zirconia (CeO.sub.2—ZrO.sub.2) composite oxide, adjusting this noble metal catalyst with alumina sol or water to prepare a slurry, and applying the slurry to the skeleton of the substrate using an impregnation method, an ion exchange method, a sol-gel method, a wash coating method, or the like.

[0045] Exhaust gas flowing down (X direction) from the upstream side of the exhaust system for exhaust gas is purified by the activity of the noble metal catalyst while flowing through the exhaust gas passage including the plurality of cell walls 1a, and the purified exhaust gas flows from the electrically heated catalytic converter 10 to the downstream side of the exhaust system.

[0046] First, the substrate 1, and the surface electrode film 2 and the wiring fixing layers 4 which constitute the electrode member 5 are formed of a metal material or a ceramic material. The substrate 1 can be formed of, for example, SiC, a composite material of SiC and Si, or a composite material of SiC and MoSi.sub.2. The surface electrode film 2 and the wiring fixing layers 4 which constitute the electrode member 5 can be formed of a thermal spraying material such as Ni—Cr, CrB—Si, MoSi.sub.2—Si, or TiB.sub.2—Si.

[0047] A protective film 6 is formed on a surface of the electrode member 5. The surface which the protective film 6 is formed includes an area corresponding to a portion where the electrode member 5 is in contact with the substrate 1. The protective film 6 is formed of i) Al.sub.2O.sub.3, ii) SiO.sub.2, iii) a composite material of Al.sub.2O.sub.3 and SiO.sub.2, or iv) a composite oxide including Al.sub.2O.sub.3, SiO.sub.2, or a composite material of Al.sub.2O.sub.3 and SiO.sub.2 as a major component. The protective film 6 has a configuration in which the entire portion is formed of an amorphous structure or in which a partially crystalline glass structure having a crystallization rate of 30 vol % or lower with respect to the entire portion of the protective film is provided. A thickness of the protective film is in a range of 100 nm to 2 μm.

[0048] The protective film 6 has a configuration in which the entire portion is formed of an amorphous structure or in which a partially crystalline glass structure having a crystallization rate of 30 vol % or lower with respect to the entire portion of the protective film is provided. That is, the protective film 6 has no crystal structure or a small amount of crystal structure. Therefore, the Young's modulus or thermal expansion coefficient of the protective film 6 can be suppressed to be low, and thus the protective film 6 has cracking resistance performance.

[0049] Further, by setting the thickness of the protective film 6 to be in a range of 100 nm to 2 μm, thermal stress is not likely to be generated, and thus the protective film 6 is not likely to crack. It is preferable that the thickness of the protective film 6 is equal to or less than 1 μm to prevent the protective film 6 from cracking.

[0050] Next, the method of manufacturing the electrically heated catalytic converter 10 will be described focusing on a method of forming the protective film 6.

[0051] First, a sol-gel solution is prepared by adding any one of i) a nanomaterial of Al.sub.2O.sub.3, ii) a nanomaterial of SiO.sub.2, iii) a nanomaterial of a composite material of Al.sub.2O.sub.3 and SiO.sub.2, and iv) a nanomaterial of a composite oxide including Al.sub.2O.sub.3, SiO.sub.2, or a composite material of Al.sub.2O.sub.3 and SiO.sub.2 as a major component to a solvent including water and alcohol (first step).

[0052] Next, the sol-gel solution is applied to a surface of the electrode member 5 and is dried to form a coating film through a chemical reaction such as hydrolysis or condensation polymerization. Next, the coating film is fired at a temperature of 500° C. or lower to remove the solvent remaining therein and to promote densification, thereby the forming the protective film 6. As a result, the electrically heated catalytic converter 10 is manufactured (second step).

[0053] In this way, by using the sol-gel solution which is prepared by adding any one of i) a nanomaterial of Al.sub.2O.sub.3, ii) a nanomaterial of SiO.sub.2, iii) a nanomaterial of a composite material of Al.sub.2O.sub.3 and SiO.sub.2, and iv) a nanomaterial of a composite oxide including Al.sub.2O.sub.3, SiO.sub.2, or a composite material of Al.sub.2O.sub.3 and SiO.sub.2 as a major component to a solvent, the firing temperature during the formation of the protective film 6 can be made to be 500° C. or lower.

[0054] Regarding the firing temperature of 500° C. or lower, it is preferable that the firing temperature is in a range of 100° C. to 200° C., which is lower than 500° C., because firing can be performed at a slow heating rate such that the protective film 6 can be formed as uniformly as possible.

[0055] Using this manufacturing method, the electrically heated catalytic converter 10 which includes the protective film 6 having an oxidation resistance temperature of 1000° C. or higher can be manufactured at as a low atmosphere temperature as possible which is 500° C. or lower.

[0056] (Oxidative Degradation Test and Results Thereof)

[0057] The present inventors performed an experiment for verifying a relationship between oxidative degradation and whether or not the protective film was formed. Specifically, as shown in FIG. 2, an electrode terminal D (thickness: 50 μm or less) formed of a Ni—Cr thermal spraying material was formed on a surface of a porous substrate K formed of SiC/Si to prepare a specimen TP. A porosity of the wiring fixing layer of the specimen TP was obtained based on the image analysis of the sectional image of the specimen TP. The obtained porosity was 14% or higher. One of the following two protective films was formed on a surface of the electrode terminal D.

[0058] In Example 1, a protective film formed of SiO.sub.2 (amorphous silica) was formed. Regarding a method of forming the protective film, perhydropolysilazane (PHPS) as a precursor of an inorganic polymer was applied to a surface of the electrode terminal using a dipping method in a reduced-pressure atmosphere, was dried in air at 400° C. for 1 hour, and was fired in a N.sub.2 gas atmosphere at 500° C. for 1 hour. The thickness of the protective film can be controlled by adjusting a diluted concentration of PHPS and firing conditions.

[0059] On the other hand, in Example 2, a protective film formed of Al.sub.2O.sub.3 (amorphous alumina) was formed. Regarding a method of forming the protective film, an Al.sub.2O.sub.3 sol was applied to a surface of the electrode terminal using a dipping method in a reduced-pressure atmosphere and was fired in air at 200° C. for 2 hours. The thickness of the protective film can be controlled by adjusting a diluted concentration of the Al.sub.2O.sub.3 sol and firing conditions. As a comparative example to Examples 1 and 2, a specimen including no protective film was prepared.

[0060] Regarding a test method, each of the specimens was treated in air (aging treatment) at 1000° C. for 24 hours to undergo oxidative degradation, and then the volume resistivity thereof was measured.

[0061] The specimen TP shown in FIG. 2 is coated with the protective film formed of Al.sub.2O.sub.3 (amorphous alumina) according to Example 2. FIG. 3A is an SEM image showing a portion Ma of the specimen TP covered with the protective film formed of Al.sub.2O.sub.3, and FIG. 3B is an SEM image showing an enlarged view of a portion b of FIG. 3A. It can be seen from FIG. 3B that surfaces of primary particles were coated with the Al.sub.2O.sub.3 film.

[0062] FIG. 4 shows the results of the oxidative degradation test. It can be seen from FIG. 4 that, in Comparative Example, an increase in the volume resistivity caused by the oxidative degradation was significantly large compared to the initial state.

[0063] On the other hand, it can be seen that, in Examples 1 and 2, an increase in the volume resistivity caused by the oxidative degradation was extremely small compared to the initial state.

[0064] It can be seen from the experiment that, by forming the protective film formed of amorphous Al.sub.2O.sub.3 or amorphous SiO.sub.2 on the surface of the electrode terminal, a change in the volume resistivity is suppressed and the oxidation resistance is improved.

[0065] FIG. 5A shows an XRD profile of the Al.sub.2O.sub.3 film, and FIG. 5B shows an XRD profile of the SiO.sub.2 film.

[0066] In FIGS. 5A and 5B, a peak derived from crystallinity is not shown, and thus it can be seen that the films have an amorphous structure.

[0067] Hereinabove, the embodiments of the disclosure have been described with reference to the drawings, but specific configurations thereof are not particularly limited to the above-described embodiments. Within a range not departing from the scope of the disclosure, design changes and the like can be made and are embraced in the disclosure.