JPH11117001A - Composite powder with heat resistance and electric conductivity, and its use - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively provide a heat resistant and electrically conductive member, particularly an electrode, having superior electric conductivity even under a high temp. oxidizing atmosphere and also to inexpensively provide a composite body, particularly a heater and a diesel particulate filter(DPF), causing neither peeling off from ceramics nor cracking even if the composite body is subjected to repeated thermal hysteresis and excellent in thermal shock resistance. SOLUTION: The composite powder with heat resistance and electric conductivity has a composition containing, by weight, 40-80% silver powder, 4-40% metallic silicon powder, 2-25%, expressed in terms of metallic components, of heat resistant and electrically conductive powder of at least one kind selected from the group consisting of metals of nickel, cobalt, and chromium and compounds thereof, and 0.5-25% ceramic powder. A sintered compact is composed of a heat treated material of this composite powder. The electrode is constituted of this sintered compact. The composite body is composed of an integrated material of this sintered compact and a ceramic sintered compact. The heater is composed of this composite body, and the DPF has this heater.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant and conductive composite powder suitable for producing electrodes, heaters, and the like, and to its use.

[0002]

2. Description of the Related Art In an electric heater or the like, a power supply terminal is connected to both ends of a heat generating element to energize the heat generating element to generate heat. Therefore, when the heat generating element is made of ceramics, the power supply terminal is connected. It is indispensable to form electrodes for ceramics on the ceramics.

Conventionally, conductive materials such as aluminum, nickel, copper and their alloys have been used as electrodes of ceramic heating elements such as silicon carbide and molybdenum disilicide, but these are easily oxidized in a high-temperature oxidizing atmosphere. There is a problem that is. For example, when an electrode is formed from ceramics with these conductive materials, and the power supply terminal is pressed against the ceramic to conduct electricity to the ceramic, the electrode surface is oxidized and the contact resistance increases, causing problems such as local heat generation and conduction inhibition. . Therefore, conventionally, the electrode portion is separated from the heat generating portion by using a heat insulating material and air-cooled so that the operating temperature of the electrode portion is 400 ° C. or less. Had significant limitations.

[0004] One application of a composite material in which electrodes are formed on a ceramic heating element is an electrically heated diesel particulate filter (hereinafter, referred to as "DPF"). In the DPF, when combustible fine particles contained in the exhaust gas of the diesel engine are collected on the porous wall of the DPF, the pressure loss increases, and the collection efficiency decreases. Therefore, it is necessary to regenerate the DPF by periodically removing the combustible fine particles collected and deposited. In the regeneration, the DPF itself is used as a heating element and is heated by energization to incinerate and remove combustible fine particles.

[0005] Since the entire DPF, including the portion where the electrode is formed, is placed in the heat insulating can, not only is it exposed to high-temperature exhaust gas at the time of collecting combustible fine particles, but also the combustible fine particles at the time of regeneration are collected. The combustion raises the internal temperature of the DPF to 600 ° C. or higher. Therefore, as an electrode formed on the DPF,
It is required to have high durability in an oxidizing atmosphere at 600 ° C. or higher. Further, the electrode is required to have excellent thermal shock resistance because it receives a heat history associated with repeated collection and regeneration.

Conventionally, as a heat-resistant material having conductivity,
For example, heat-resistant steels represented by SUS-310, nickel-based heat-resistant alloys, cobalt-based heat-resistant alloys, M-Cr-Al-based alloys (M = Fe, Ni) (JP-A-54-74811), M- Cr-Al-Y alloy (M = Fe, Ni, C
o, Ni-Co) [JP-A-61-106763]
Etc. are known. These mainly form an oxide layer on the surface from the viewpoint of a structural material, function as a protective film for preventing the progress of oxidation, and improve scale peeling and reduction in high-temperature strength. However, these are oxidized at a temperature of about 600 ° C. and the surface layer is insulated, so that they cannot be used as DPF electrodes.

On the other hand, silver, palladium, platinum and the like, which are noble metals, exhibit good oxidation resistance at a use temperature lower than their melting points, but have a problem in wettability with ceramic materials.
Since the wettability with a porous ceramic material such as PF is extremely poor, the contact resistance at the interface between the electrode layer and the ceramic layer increases, resulting in abnormal heat generation and sparks at the interface during energization, resulting in poor conductivity. Will be enough. In addition, since these noble metal-based materials generally have a larger thermal expansion coefficient than ceramic materials, when used as an electrode of a ceramic heating element such as silicon carbide or molybdenum disilicide, the electrode layer is likely to peel off due to heat history. There was a problem. in addition,
Since these precious metal materials are expensive, using them industrially in large quantities poses a major problem in terms of cost.

The present inventors have proposed a noble metal-based composite material as a conductive material having excellent oxidation resistance and thermal shock resistance (Japanese Patent Application No. Hei 9-100525). Nevertheless, since expensive palladium powder is an essential component, the problems of productivity, mass productivity, and cost have not been solved.

[0009]

SUMMARY OF THE INVENTION In view of the above, an object of the present invention is to provide a heat-resistant and conductive material having good conductivity even in a high-temperature oxidizing atmosphere, in particular, an electrode at a low cost. Another object of the present invention is to provide an inexpensive composite, particularly a heater and a DPF, which is excellent in thermal shock resistance without peeling and cracking from ceramics even when subjected to repeated heat history. .

[0010]

That is, the present invention provides at least one kind of heat-resistant material selected from the group consisting of 40 to 80% by weight of silver powder, 4 to 40% by weight of metallic silicon powder, nickel, cobalt, and chromium metals and their compounds. A heat-resistant and conductive composite powder comprising 2 to 25% by weight of a conductive powder in terms of a metal component and 0.5 to 25% by weight of a ceramic powder. Further, the present invention provides a sintered body comprising a heat-treated product of the above composite powder, an electrode constituted by the sintered body, and a composite comprising an integrated body of the sintered body and a ceramic sintered body. It is. Furthermore, the present invention is a heater comprising the above-described composite, and a diesel particulate filter (DPF) provided with the heater.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail.

The constituent components of the composite powder of the present invention are a heat-resistant conductive powder comprising at least one selected from silver powder, metallic silicon powder, nickel, chromium, cobalt metals and their compounds, and ceramic powder. In addition to these components, a small amount of noble metal components such as Au and Pt, Mn, Fe, C
A transition metal component such as u may be included.

The ratio of each component of the composite powder of the present invention is as follows:
Silver powder 40 to 80% by weight, preferably 60 to 75% by weight, metallic silicon powder 4 to 40% by weight, preferably 10 to 30% by weight,
2 to 25% by weight, preferably 4 to 15% by weight, and 0.5 to 25% by weight of ceramic powder in terms of metal component
Preferably it is 2 to 15% by weight. The reasons for limiting to such a ratio are as follows.

The silver powder component is for imparting conductivity to the sintered body obtained by heat-treating the composite powder of the present invention. If the content is less than 40% by weight, the conductivity becomes insufficient. If it exceeds 80% by weight, the coefficient of thermal expansion of the sintered body increases, and the thermal shock resistance deteriorates.

The metallic silicon powder component functions as a melting point depressant or a fluidizing agent during melting of the silver alloy by eutectic with the silver component, and improves the wettability with the ceramic.
Therefore, when the sintered body obtained by heat-treating the composite powder of the present invention and the ceramic sintered body are integrated to form a composite, the joining property is improved. Further, when an electrode film is formed from the composite powder of the present invention, the structure becomes dense and oxidation resistance is improved. If the content of metallic silicon is less than 4% by weight, its sufficient effect cannot be obtained, and if it exceeds 40% by weight, the oxidation resistance is impaired by excessive metallic silicon.

The heat-resistant conductive powder component imparts conductivity and heat resistance to the sintered body obtained by heat-treating the composite powder of the present invention. A further important effect is to react with the metal silicon component to form metal silicide around the metal silicon particles, thereby suppressing oxidation of the metal silicon. If the proportion of the heat-resistant conductive powder component is less than 2% by weight in terms of the metal component, the above-mentioned effect cannot be sufficiently obtained, and if it exceeds 25% by weight, the brittleness increases, which is not preferable.

In the case where a ceramic powder component is subjected to heat treatment of the composite powder of the present invention to form a sintered body, particularly an electrode film, uniform distribution of the heat-resistant conductive powder during melting is eliminated. Is a component necessary for producing a sintered body and reducing the thermal expansion coefficient of the sintered body to improve the thermal shock resistance. If the proportion of the ceramic powder is less than 0.5% by weight, the effect is not sufficient, and if it exceeds 25% by weight, conductivity and oxidation resistance are adversely affected.

Regarding the type of ceramic powder, silicon carbide, silicon nitride, molybdenum disilicide, alumina, silica,
It can be used without distinction of zirconia, etc., for example, when integrated with a ceramic sintered body such as an electrode part of a heater, when the same type of ceramic powder as the main component of the ceramic sintered body, for example, DPF It is more preferable to use silicon carbide or molybdenum disilicide in terms of thermal shock resistance.

Regarding the particle size of each component of the present invention, silver powder has an average particle size of 10 μm or less, particularly 5 μm or less, metal silicon powder has an average particle size of 10 μm or less, and heat-resistant conductive powder has a maximum particle size of 50 μm.
Those having an average particle size of 10 μm or less are preferred. As the ceramic powder, an irregular-shaped ceramic powder having few projections is suitable, and the average particle size is 10 μm or less, particularly 5 μm or less.
μm or less is preferred.

The composite powder of the present invention may be used as a sintered body, an electrode, a composite, a heater and a D, which will be described below.
In addition to PF, it can be used as a brazing material for joining ceramics to metal or ceramics, a powder for metallizing, and the like.

The sintered body of the present invention is a heat-treated product of the composite powder, and is characterized by containing a metal silicide. The heat treatment temperature is preferably from 900 to 1400C, particularly preferably from 1000C to 1200C. If the heating temperature is lower than 900 ° C., the formation of metal silicide becomes insufficient and the oxidation resistance does not improve.
Since the melting of the metallic silicon becomes severe, the thermal shock resistance deteriorates.

The sintered body of the present invention can be used as an electrode or a circuit, and can be integrated with a ceramic sintered body to form a composite, and used as a plate or DPF.

The ceramic sintered body constituting the composite of the present invention can be selected from various shapes and types depending on the purpose and application. However, those having generally heat resistance are preferred, and alumina, zirconia, etc. Non-oxide ceramics such as oxide ceramics, silicon carbide, silicon nitride and molybdenum disilicide can be used. Among them, silicon carbide and molybdenum disilicide are suitable as a heater (ceramic heating element) because of their excellent electrical characteristics and heat resistance.

The method of integrating the sintered body of the present invention with the ceramic sintered body is not particularly limited. However, when the sintered body of the present invention is used as an electrode of a ceramic heating element (heater), The simplest method is to directly apply the paste-shaped composite powder of the present invention to a ceramic sintered body and heat it. The paste is prepared by mixing the composite powder of the present invention with a medium such as α-terpineol, ethyl carbitol, carbitol acetate, butyl cellosolov, and an organic binder such as polyisobutyl methacrylate, polyvinyl alcohol, methyl cellulose, and carboxymethyl cellulose. . The content of the composite powder in the paste is preferably 20 to 60% by weight, particularly preferably 30 to 60% by weight. After paste application, 100 ~ 200 ℃ 1 ~
After drying for 10 hours, under a vacuum atmosphere, an inert gas atmosphere or a reducing gas atmosphere, at a temperature of 900 to 1400
Heat treated at ℃.

When the composite of the present invention is used for DPF, the ceramic sintered body is preferably a honeycomb structure composed of porous walls mainly composed of silicon carbide or molybdenum disilicide. The thickness of the electrode to be formed is preferably about 10 to 500 μm. When the film thickness is less than 10 μm, the effects of heat resistance and conductivity become insufficient, and problems such as an increase in resistance and local heat generation during energization occur. On the other hand, when the thickness exceeds 500 μm, the thermal stress generated by the heat cycle of heating and cooling cannot be alleviated, and peeling from the ceramic sintered body or cracking of the ceramic sintered body occurs.

The shape of the honeycomb structure of the DPF is as follows.
For example, the length in the axial direction is 20 to 500 mm, the thickness of the porous wall is 0.1 to 1.0 mm, and the cell pitch of the through holes is 1.14 to
3.59 mm, through-hole cell density of 5 per square inch
0 to 500 pieces.

[0027]

The present invention will be described more specifically with reference to examples and comparative examples.

Examples 1 to 14, Comparative Examples 1 to 8 Silver powder (purity 99% or more, particle diameter 10 μm), metallic silicon powder (purity 98% or more, particle diameter 10 μm), nickel powder as heat-resistant conductive powder (Purity 99% or more, particle size 5 μm below),
Nickel oxide powder (purity 99% or more, particle diameter 5 μm), chromium oxide powder (purity 99% or more, particle diameter 5 μm), cobalt oxide powder (purity 99% or more, particle diameter 5 μm), and ceramic powder, Silicon carbide powder (purity of 99% or more, particle size of 5 μm or less) or molybdenum disilicide (purity of 99% or more, particle size of 5 μm or less) was blended in the ratio shown in Table 1. After 100 parts by weight of this compound was dry-mixed for 1 hour by a ball mill, 3 parts by weight of polyisobutyl methacrylate and 50 parts by weight of α-terpineol were added and uniformly mixed to prepare a paste.

A silicon carbide (SiC) -based or molybdenum disilicide (MoSi ₂ ) -based ceramics sintered body having a specific resistance shown in Table 1 is used. The above paste was added to the entire surface of the honeycomb structure having a diameter of 43 mm, a through-hole cell pitch of 2.54 mm, and a through-hole cell density of 100 / square inch.
After coating at a rate of 02 g / cm ² , the coating was dried at 150 ° C. for 1 hour, and then dried in a vacuum atmosphere at a temperature shown in Table 2 for 10 hours.
Heat treatment was performed for a minute, and baking was performed to form an electrode film on the honeycomb.

The thickness of the formed electrode film was measured using a micrometer, and the constituent components were analyzed by an X-ray diffraction method. Table 2 shows the results.

Next, the honeycomb structure was subjected to the following oxidation resistance test and thermal shock resistance test. Table 2 shows the results. (1) Oxidation Resistance The room temperature resistivity before the heat treatment and the room temperature resistivity after the heat treatment in air at 700 ° C. for 200 hours were measured. Since the specific resistance was measured by providing contacts on the electrode film at intervals of 80 mm, the measured value includes the contact resistance due to surface oxidation.

(2) Thermal shock resistance Immediately after being put in a furnace at 700 ° C. from the room temperature in the air and kept for 10 minutes, it is immediately cooled at room temperature for 10 minutes. This operation was defined as one cycle, and 200 cycles were performed while observing the sample appearance.
The test was performed up to the cycle, and the number of cycles at which the sample was peeled or cracked was measured.

[0033]

[Table 1]

[0034]

[Table 2]

Tables 1 and 2 show that the honeycomb structure having the electrode film formed of the composite powder of the present invention has excellent oxidation resistance and thermal shock resistance.

[0036]

According to the present invention, a sintered body which exhibits excellent conductivity even in a high-temperature oxidizing atmosphere, and which does not peel or crack from ceramics even when subjected to repeated thermal histories, is produced. Is provided. Moreover, since the composite powder of the present invention contains metal silicon powder and ceramic powder as constituents, the amount of expensive noble metal used is reduced, and the economic efficiency is improved.

Since the sintered body of the present invention has the above characteristics, it becomes an electrode having excellent heat resistance and conductivity, and the sintered body (for example, an electrode) and the ceramic sintered body of the present invention are integrated. The composite has improved thermal shock resistance, such as a heater,
This is suitable for producing a DPF. .

Claims (6)

[Claims] 1. Silver powder 40 to 80% by weight, metallic silicon powder 4 to
40% by weight, 2 to 25% by weight of at least one heat-resistant conductive powder selected from nickel, cobalt and chromium metals and their compounds in terms of metal component, and 0.5 to 25% by weight of ceramic powder. A heat-resistant and conductive composite powder characterized by comprising: 2. A sintered body comprising the heat-treated composite powder according to claim 1 and containing a metal silicide. 3. An electrode comprising the sintered body according to claim 2. 4. A composite comprising an integrated product of the sintered body according to claim 2 and a ceramic sintered body. 5. A heater comprising the composite according to claim 4. 6. A diesel particulate filter comprising the heater according to claim 5.