JP2012057508A - Exhaust gas purification device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purification device which has electrodes excellent in heat resistance, oxidation resistance under a high temperature, which can mitigate stress generated between a carrier and thereof.SOLUTION: The exhaust gas purification device 1 includes the carrier 20 which supports a catalyst, and a pair of electrodes 30, 30 provided on an outer peripheral surface of the carrier 20. The electrode 30 includes: a first layer 31 formed on the outer peripheral surface of the carrier 20; metal foil 32 arranged on the first layer 31; and a second layer 33 formed to cover the metal foil 32. The first layer 31 contains Ni-Cr alloy or Co-Ni-Cr alloy, and silicon and silica, and graphite. The second layer 33 contains Ni-Cr alloy or Co-Ni-Cr alloy, and silicon and silica equal to or less than the amount of the silicon and silica contained in the first layer 31, and graphite equal to or less than the amount of the graphite contained in the first layer 31.

Description

  The present invention relates to an exhaust purification device, and more particularly to an exhaust purification device that purifies exhaust gas discharged from an engine.

  2. Description of the Related Art Conventionally, an automobile or the like is provided with an exhaust purification device on an exhaust path in order to purify exhaust gas discharged from an engine.

As the exhaust purification apparatus as described above, a catalyst such as platinum or palladium is supported, a honeycomb structure carrier made of ceramics such as SiC (silicon carbide), and the carrier is electrically connected to the outer peripheral surface thereof. An electrically heated catalyst (EHC) including a pair of electrodes provided to face each other is known (see, for example, Patent Document 1).
In EHC, a carrier is energized and heated between a pair of electrodes using a power source, whereby the catalyst supported on the carrier is heated to an activation temperature and discharged from the engine to pass through the carrier. Harmful substances such as (unburned hydrocarbon), CO (carbon monoxide), and NOx (nitrogen oxide) are purified by catalytic reaction.

Some EHC electrodes are formed into a film on the outer peripheral surface of the carrier by thermal spraying. Such an electrode is formed while accumulating internal stress in the process where the molten metal collides with the carrier to be sprayed and rapidly solidifies.
For this reason, when the volume of the electrode formed by thermal spraying increases, the accumulated internal stress exceeds the strength of the carrier, and the carrier may be destroyed.

In addition, when the electrode is formed on the outer peripheral surface of the carrier and subjected to a thermal cycle assuming that the EHC is disposed on an exhaust path of an automobile or the like, the difference in linear expansion coefficient between the carrier and the electrode is increased. As a result, cracks occur in the electrode and the carrier, and problems such as peeling of the electrode from the carrier occur.
Furthermore, as the oxidation of the electrode proceeds due to the thermal cycle, the electrical resistance of the electrode increases, and in the worst case, there is a problem that the carrier cannot be conducted and the carrier cannot be heated and energized.

Japanese Patent No. 3334897

  An object of the present invention is to provide an exhaust emission control device including an electrode that is excellent in heat resistance and oxidation resistance at high temperatures and can relieve stress generated between the carrier and the carrier.

  The exhaust emission control device of the present invention comprises a carrier on which a catalyst is supported, and a pair of electrodes provided on an outer peripheral surface of the carrier, and the catalyst is activated by heating the carrier through the pair of electrodes. The pair of electrodes includes a Ni—Cr alloy or a Co—Ni—Cr alloy, and includes graphite.

  In the exhaust emission control device of the present invention, it is preferable that the pair of electrodes further includes silicon and silica.

  In the exhaust emission control device of the present invention, the pair of electrodes are formed to cover the first layer formed on the outer peripheral surface of the carrier, the metal foil provided on the first layer, and the metal foil. And the first layer includes a Ni—Cr alloy or a Co—Ni—Cr alloy and includes graphite, and the second layer includes a Ni—Cr alloy or Co. It is preferable that the alloy contains a Ni—Cr alloy and not more than the amount of graphite contained in the first layer.

  In the exhaust emission control device of the present invention, it is preferable that the first layer and the second layer are formed by thermal spraying.

  In the exhaust emission control device of the present invention, it is preferable that the first layer further includes silicon and silica, and the second layer further includes silicon and silica equal to or less than the amount of silicon and silica included in the first layer. .

  In the exhaust emission control device of the present invention, the volume ratio of silicon and silica in the electrode is preferably 10 to 30 [vol%] in combination.

  In the exhaust emission control device of the present invention, the volume ratio of graphite in the electrode is preferably 40 to 60 [vol%].

  According to the present invention, it is possible to suppress the generation of cracks in the electrode and the oxidation of the electrode at a high temperature, and to stably heat the carrier by energization.

The figure which shows the exhaust gas purification apparatus which concerns on this invention. Sectional drawing which shows an electrode. The figure which shows the relationship between the thermal expansion coefficient of a thermal spray coating, and the volume ratio of the silicon | silicone and silica which are contained in a thermal spray coating. The figure which shows composite powder.

Below, with reference to FIG.1 and FIG.2, the exhaust gas purification apparatus 1 which is one Embodiment of the exhaust gas purification apparatus which concerns on this invention is demonstrated.
The exhaust purification device 1 is an electrically heated catalyst (EHC) that is provided on an exhaust path of an automobile or the like and purifies exhaust gas discharged from an engine.

  As shown in FIG. 1, the exhaust purification apparatus 1 includes a hollow case 10 that forms an exterior, a carrier 20 housed in the case 10, and a pair of electrodes 30 and 30 provided on the outer peripheral surface of the carrier 20. It comprises.

  The case 10 is a hollow member that forms the exterior of the exhaust purification device 1 and forms part of an exhaust pipe through which exhaust gas discharged from the engine flows.

  The carrier 20 is a porous member having a honeycomb structure made of conductive SiC (silicon carbide), and supports a catalyst such as platinum or palladium. The carrier 20 is formed in a cylindrical shape, and is disposed inside the case 10 so that exhaust gas discharged from the engine passes through the inside of the carrier 20 along the axial direction. In addition, the gap between the inner peripheral surface of the case 10 and the outer peripheral surface of the carrier 20 is sealed between the inner peripheral surface of the case 10 and the outer peripheral surface of the carrier 20 while preventing the displacement of the carrier 20. A holding member (not shown) is provided.

The electrodes 30 and 30 are a pair of electrodes that are formed independently of each other on the outer peripheral surface of the carrier 20 and are heated by passing a current through the carrier 20. The electrodes 30 and 30 are each provided in a predetermined range in the circumferential direction of the carrier 20 and are formed across both ends in the axial direction of the carrier 20 in a state of being opposed to each other.
A terminal (not shown) is electrically connected to the electrodes 30, 30, and power can be supplied from a power source such as a battery provided outside the case 10 by the terminal. It should be noted that the direction in which the current flows is not limited, regardless of which of the electrodes 30 and 30 is the plus or minus electrode.

  The electrode 30 includes a first layer 31 formed on the outer peripheral surface of the carrier 20, a plurality of metal foils 32, 32... Provided on the first layer 31, and a first so as to cover the metal foil 32. A plurality of second layers 33, 33... Formed on the layer 31.

  As shown in FIG. 2, the first layer 31 is a film formed on the outer peripheral surface of the carrier 20 by thermal spraying, and is electrically connected to the carrier 20. The first layer 31 is provided in a predetermined range in the circumferential direction of the carrier 20, and is formed across both ends in the axial direction of the carrier 20.

  The metal foil 32 is a thin plate made of a metal such as an Fe—Cr alloy. The metal foil 32 is provided on the first layer 31 and is electrically connected to the first layer 31. The metal foil 32 extends over the entire range in which the first layer 31 in the circumferential direction of the carrier 20 is formed, and a plurality of metal foils 32 are arranged at predetermined intervals along the axial direction of the carrier 20.

The second layer 33 is a film formed on the first layer 31 so as to cover the metal foil 32 by spraying in order to fix the metal foil 32, and is electrically connected to the first layer 31 and the metal foil 32. ing. A plurality of second layers 33 are provided for one metal foil 32, and are arranged at predetermined intervals along the extending direction of the metal foil 32 (the circumferential direction of the carrier 20).
In the metal foils 32 and 32 adjacent to each other, a plurality of second layers 33, 33... Provided on one metal foil 32 and a plurality of second layers 33 provided on the other metal foil 32. ... Are arranged with their phases shifted from each other.

  As described above, in the exhaust gas purification apparatus 1, the carrier 20 is energized and heated between the electrodes 30 and 30 to raise the temperature of the catalyst supported on the carrier 20 to the activation temperature, and is discharged from the engine and discharged from the engine. Toxic substances such as HC (unburned hydrocarbon), CO (carbon monoxide), and NOx (nitrogen oxide) in the exhaust gas passing through the catalyst are purified by catalytic reaction.

  Below, with reference to FIG.3 and FIG.4, the 1st layer 31 and the 2nd layer 33 which comprise the electrode 30 are demonstrated in detail.

  The first layer 31 is mainly composed of a Ni—Cr alloy or a Co—Ni—Cr alloy, and includes a predetermined amount of Si (silicon) and Si oxide (silica), and further includes a predetermined amount of graphite.

Here, the thermal expansion coefficient of the Ni—Cr alloy or the Co—Ni—Cr alloy is about 13 [× 10 ⁻⁶ / ° C.]. On the other hand, the thermal expansion coefficient of Si (silicon) is about 2.8 to 7.3 [× 10 ⁻⁶ / ° C.], and the thermal expansion coefficient of Si oxide (silica) is about 0.5. 3 [× 10 ⁻⁶ / ° C.], and the thermal expansion coefficient of graphite is about 0.6 to 4.3 [× 10 ⁻⁶ / ° C.].
Therefore, Si (silicon) having a thermal expansion coefficient smaller than that of Ni-Cr alloy or Co-Ni-Cr alloy, and Si oxide (silica) having a smaller thermal expansion coefficient, and By including graphite in the first layer 31, the coefficient of thermal expansion of the first layer 31 can be lowered.
Thereby, the coefficient of thermal expansion of the first layer 31 can be predicted by a composite law and controlled (reduced) to a desired value.

The first layer 31 contains Si (silicon), Si oxide (silica), and graphite at a predetermined volume ratio so that the thermal expansion coefficient thereof is close to the thermal expansion coefficient of the carrier 20. .
It is preferable that Si (silicon) and Si oxide (silica) contained in the first layer 31 are 10 to 30 [vol%] in combination with the volume ratio in the first layer 31.

As shown in FIG. 3, when the volume ratio of Si (silicon) and Si oxide (silica) in the first layer 31 is less than 10 [vol%], the thermal expansion coefficient of the first layer 31 increases. The difference from the coefficient of thermal expansion of the carrier 20 becomes large.
FIG. 3 shows the thermal expansion coefficient [× 10 ⁻⁶ ] of the thermal spray coating when Ni-50Cr, which is an Ni—Cr alloy, is used and a thermal spray coating with a graphite content of 25 [wt%] is produced. / ° C.] is a diagram showing the relationship (predicted value) between the volume fraction [vol%] of Si (silicon) and Si oxide (silica) contained in the sprayed coating.

On the other hand, when the volume ratio of Si (silicon) and Si oxide (silica) in the first layer 31 exceeds 30 [vol%], the electrical resistance of the first layer 31 increases.
As described above, in the first layer 31, the thermal expansion coefficient and the electrical resistance are contradictory in accordance with the contents of Si (silicon) and Si oxide (silica). The volume ratio of (silica) is preferably 10 to 30 [vol%] in total.
In addition, since the thermal expansion coefficient of graphite is smaller than the thermal expansion coefficient of the metal (Ni—Cr alloy or Co—Ni—Cr alloy) contained in the first layer 31, the volume of graphite in the first layer 31 is small. By making it contain in the 1st layer 31 so that it may become 40-60 [vol%] by a rate, it contributes to the fall of the thermal expansion coefficient of the 1st layer 31 together with the fall effect | action of the Young's modulus mentioned later. From these points, it is possible not to contain Si (silicon) and Si oxide (silica) in the first layer 31 but to contain only a predetermined amount of graphite in the first layer 31.

Moreover, it is possible to lower the apparent Young's modulus of the first layer 31 by adding graphite having an extremely small strength compared to the strength of Ni—Cr alloy or Co—Ni—Cr alloy.
Thereby, the buffer function can be given to the first layer 31, and the stress generated based on the difference between the thermal expansion coefficient of the first layer 31 and the thermal expansion coefficient of the carrier 20 can be relaxed.

The first layer 31 contains graphite at a predetermined volume ratio so as to have a Young's modulus that can relieve stress generated based on the difference between the coefficient of thermal expansion and the coefficient of thermal expansion of the carrier 20.
The graphite contained in the first layer 31 is preferably 40 to 60 [vol%] in volume ratio in the first layer 31.

When the volume ratio of graphite in the first layer 31 is less than 40 [vol%], the apparent Young's modulus of the first layer 31 is increased, and the thermal expansion coefficient of the first layer 31 and the thermal expansion coefficient of the carrier 20 are increased. The stress generated based on the difference between the two cannot be sufficiently relaxed.
On the other hand, when the volume fraction of graphite in the first layer 31 exceeds 60 [vol%], the first layer 31 becomes brittle and the first layer 31 may be peeled off from the carrier 20.
Therefore, in the first layer 31, the volume ratio of graphite is preferably 40 to 60 [vol%].
Although the apparent Young's modulus of the first layer 31 can be reduced by dispersing a resin such as polyester having low strength in the first layer 31, the coefficient of thermal expansion is 0.6-4. Compared to graphite, which is relatively small at 3 [× 10 ⁻⁶ / ° C.] and high in electrical conductivity, a resin such as polyester is not suitable for the present invention because it has a high coefficient of thermal expansion and is an insulator.

  As described above, Si (silicon), Si oxide (silica), and graphite are included in the first layer 31 to reduce the difference between the thermal expansion coefficient of the first layer 31 and the thermal expansion coefficient of the carrier 20. At the same time, by causing the first layer 31 to contain graphite so that the first layer 31 has a buffer function, the stress generated based on the difference between the thermal expansion coefficient of the first layer 31 and the thermal expansion coefficient of the carrier 20 is reduced. It is possible to relax and suppress the occurrence of cracks in the first layer 31.

  The first layer 31 can be produced, for example, by the following procedure.

First, a powder of Ni—Cr alloy or Co—Ni—Cr alloy (hereinafter simply referred to as “base metal”), Si (silicon) powder, and graphite powder are prepared.
In addition, it is preferable that the content rate of Cr (chromium) in a base metal is 20-50 [wt%] so that the 1st layer 31 has sufficient heat resistance and oxidation resistance.

Next, base metal and Si (silicon) are compounded with graphite to produce a composite powder thereof.
Specifically, as shown in FIG. 4, base metal and Si (silicon) particles are attached (clad) around graphite particles, and then sintered to produce a composite powder.
At this time, Si (silicon) and Si oxide (silica) are included at a predetermined volume ratio in the first layer 31 to be finally produced, and graphite is included at a predetermined volume ratio. The amount of Si (silicon) and graphite is adjusted so that has a desired coefficient of thermal expansion and Young's modulus.
As a method of combining the base metal and Si (silicon) with graphite, an alloying method, a mechanical alloying method in which the surface of the base metal and Si (silicon) particles are mechanically coated on the surface of the graphite particles, and It is also possible to apply a spray drying method or the like. However, a method of simply mixing the base metal, Si (silicon), and graphite is also conceivable, but this method is excluded because the uniform and stable first layer 31 cannot be produced due to a difference in specific gravity between the base metal, Si, and graphite.

Finally, the composite powder is sprayed onto the outer peripheral surface of the carrier 20. Thereby, the first layer 31 is formed on the outer peripheral surface of the carrier 20.
In the thermal spraying of the composite powder, plasma spraying is applied so that a part of Si (silicon) is oxidized during the thermal spraying and a Si oxide (silica) layer is formed on the first layer 31. It is preferable. For example, when HVOF spraying is applied, the particle speed of the composite powder at the time of spraying is high and the oxidation of Si (silicon) is small, so that a sufficient layer of Si oxide (silica) is formed in the first layer 31 However, when gas spraying is applied, there is much oxidation of Si (silicon), and an unnecessary layer of Si oxide (silica) is formed on the first layer 31.

Thus, the first layer 31 can be produced by the above procedure.
In addition, the volume ratio of Si (silicon) and Si oxide (silica) and the volume ratio of graphite in the first layer 31 have predetermined values, respectively, and the first layer has a desired coefficient of thermal expansion and Young's modulus. The method is not limited as long as the first layer 31 can be manufactured in a uniform and stable manner. For example, instead of using the base metal and Si (silicon), the Ni—Cr—Si alloy is used, and the Ni—Cr—Si alloy powder is combined with the graphite powder to form the first layer 31. It is also possible to produce it.
Further, instead of oxidizing Si (silicon) to produce Si oxide (silica), Si oxide (silica) may be compounded with the base metal and graphite at the stage of producing the composite powder. In this case, since it is not necessary to oxidize Si (silicon) to generate Si oxide (silica), the type of thermal spraying is not limited.

  Similar to the first layer 31, the second layer 33 is mainly composed of a Ni—Cr alloy or a Co—Ni—Cr alloy, contains a predetermined amount of Si (silicon) and Si oxide (silica), and further includes graphite. Is included in a predetermined amount. However, the amount of Si (silicon) and Si oxide (silica) contained in the second layer 33 is equal to or less than the amount of Si (silicon) and Si oxide (silica) contained in the first layer 31. The amount of graphite contained in the layer 33 is equal to or less than the amount of graphite contained in the first layer 31.

As described above, the amount of Si (silicon) and Si oxide (silica) contained in the second layer 33 is equal to or less than the amount of Si (silicon) and Si oxide (silica) contained in the first layer 31. Therefore, the volume ratio of Si (silicon) and Si oxide (silica) in the second layer 33 is equal to or less than the volume ratio of Si (silicon) and Si oxide (silica) in the first layer 31. Therefore, the thermal expansion coefficient of the second layer 33 is a value equal to or higher than the thermal expansion coefficient of the first layer 31. However, the thermal expansion coefficient of the second layer 33 is set to be smaller than the thermal expansion coefficient of the metal foil 32.
Thereby, the difference of each thermal expansion coefficient of the 1st layer 31, the metal foil 32, and the 2nd layer 33 can be made small, and the stress which arises based on the difference of those thermal expansion coefficients can be relieved.
In addition, Si (silicon) and Si oxide (silica) contained in the second layer 33 are 10 to 30 [vol%] in combination with the volume ratio in the second layer 33 in the same manner as the first layer 31. It is preferable that

Further, as described above, since the amount of graphite contained in the second layer 33 is equal to or less than the amount of graphite contained in the first layer 31, the volume ratio of graphite in the second layer 33 is equal to that in the first layer 31. It is below the volume fraction of graphite. Accordingly, the Young's modulus of the second layer 33 is equal to or greater than the Young's modulus of the first layer 31.
Thereby, the metal foil 32 is fixed favorably by the second layer 33, and peeling of the metal foil 32 can be suppressed.
In addition, it is preferable that the graphite contained in the second layer 33 is 40 to 60 [vol%] in the volume ratio in the second layer 33 as in the first layer 31.

As described above, the difference between the thermal expansion coefficient of the second layer 33 and the thermal expansion coefficient of the first layer 31 and the difference between the thermal expansion coefficient of the second layer 33 and the thermal expansion coefficient of the metal foil 32 are reduced. By providing the second layer 33 with a Young's modulus that can fix the metal foil 32, the difference between the thermal expansion coefficient of the second layer 33 and the thermal expansion coefficient of the first layer 31, and the heat of the second layer 33. The stress generated based on the difference between the expansion coefficient and the thermal expansion coefficient of the metal foil 32 can be relieved, and peeling of the metal foil 32 can be suppressed.
Note that the method for producing the second layer 33 is substantially the same as that of the first layer 31, and therefore will be omitted.

  In general, in an exhaust emission control device, an oxide film of Cr (chromium) and Al (aluminum) is formed on a film constituting an electrode to protect the film from oxidation in a thermal cycle. However, when the coating is formed on the outer peripheral surface of the carrier by thermal spraying, the oxide coating becomes amorphous due to rapid solidification, and the function as a protective film is reduced.

Therefore, in the present invention, Si (silicon) having higher oxide forming ability than Cr (chromium) is introduced instead of Al (aluminum) to form a Si oxide (silica) layer in the sprayed coating.
The thermal expansion coefficient of Si (silicon) and Si oxide (silica) is relatively small. In particular, the thermal expansion coefficient of Si oxide (silica) is extremely small compared to the thermal expansion coefficient of the metal constituting the thermal spray coating. Therefore, by including Si (silicon) and Si oxide (silica) in the thermal spray coating, the thermal expansion coefficient of the thermal spray coating can be predicted by a composite law and controlled (reduced) to a desired value. .

  Furthermore, by including low-strength graphite in the thermal spray coating, the apparent Young's modulus of the thermal spray coating can be reduced, and the thermal expansion coefficient of the thermal spray coating can also be reduced.

  In this way, the thermal expansion coefficient of the thermal spray coating is controlled (reduced) to reduce the difference between the thermal expansion coefficient of the carrier on which the thermal spray coating is formed and to reduce the apparent Young's modulus of the thermal spray coating. Thus, the stress generated in the thermal spray coating can be relaxed and the occurrence of cracks in the thermal spray coating can be suppressed.

  Below, based on the Example which concerns on this invention, the property of the electrode provided in the exhaust gas purification apparatus which concerns on this invention is demonstrated.

First, the 1st layer which comprises an electrode was produced in the following procedures.
As the base metal of the first layer, Ni-50Cr having a relatively low coefficient of thermal expansion and advantageous in cost was selected.

A Ni-50Cr powder having a particle size of about 1 to 8 [μm] and a Si (silicon) powder were prepared, and a graphite powder having a particle size of about 20 to 75 [μm] was prepared.
Ni-50Cr powder and Si (silicon) powder were sintered into graphite powder after being clad to prepare a composite powder.

Plasma spraying of the composite powder (primary gas: Ar 60 [l / min], secondary gas: H ₂ 4 [l / min]) on a carrier (thermal expansion coefficient: about 4) made of SiC (silicon carbide), (Current: 500 [A], spraying distance: 150 [mm]), the first layer was formed on the outer peripheral surface of the carrier.
The thermal expansion coefficient of the first layer was about 10 [× 10 ⁻⁶ / ° C.], and the Young's modulus was about 50 [Gpa].

Next, the 2nd layer which comprises an electrode was produced.
A second layer was produced in the same procedure as the first layer except that the second layer was partially formed so as to fix the metal foil disposed on the first layer. Thus, an electrode including the first layer, the metal foil, and the second layer was provided on the outer peripheral surface of the carrier.
However, the coefficient of thermal expansion of the second layer is the coefficient of thermal expansion of the first layer (about 10 [× 10 ⁻⁶ / ° C.]) and the coefficient of thermal expansion of the metal foil made of Fe—Cr alloy (about 11 [× 10 ^{− 6} / ° C.]), the amount of Si (silicon) contained in the second layer was adjusted.
Also, the Young's modulus of the second layer is made higher than that of the first layer (50 to 75 [GPa]), and the graphite contained in the second layer so that the second layer can fix the metal foil well. The amount of was adjusted.

  Finally, a thermal cycle test (100 ° C. to 900 ° C. 1000 cycles) was performed on the electrodes provided on the outer peripheral surface of the carrier.

As a result of the thermal cycle test, no crack was observed in the electrode, and almost no increase in the electrical resistance of the electrode was observed.
Thus, it has been clarified that the electrode provided in the exhaust purification apparatus according to the present invention is excellent in heat resistance and oxidation resistance at high temperatures, and can relieve stress generated in the electrode.

DESCRIPTION OF SYMBOLS 1 Exhaust gas purification apparatus 10 Case 20 Carrier 30 Electrode 31 1st layer 32 Metal foil 33 2nd layer

Claims (7)

A carrier on which the catalyst is supported;
A pair of electrodes provided on the outer peripheral surface of the carrier,
An exhaust gas purification apparatus that heats the carrier through the pair of electrodes and heats the catalyst to an activation temperature,
The pair of electrodes includes an Ni—Cr alloy or a Co—Ni—Cr alloy, and an exhaust purification device including graphite.   The exhaust purification device according to claim 1, wherein the pair of electrodes further includes silicon and silica. The pair of electrodes includes:
A first layer formed on the outer peripheral surface of the carrier;
A metal foil provided on the first layer;
A second layer formed so as to cover the metal foil,
The first layer includes a Ni-Cr alloy or a Co-Ni-Cr alloy and includes graphite,
2. The exhaust emission control device according to claim 1, wherein the second layer includes a Ni—Cr alloy or a Co—Ni—Cr alloy and includes graphite equal to or less than the amount of graphite included in the first layer.   The exhaust emission control device according to claim 3, wherein the first layer and the second layer are formed by thermal spraying. The first layer further includes silicon and silica;
5. The exhaust emission control device according to claim 3, wherein the second layer further includes silicon and silica equal to or less than silicon and silica contained in the first layer.   The exhaust gas purification apparatus according to claim 2 or 5, wherein a volume ratio of silicon and silica in the electrode is 10 to 30 [vol%] in total.   The exhaust gas purification apparatus according to any one of claims 1 to 6, wherein a volume ratio of graphite in the electrode is 40 to 60 [vol%].

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