Electrode for preventing noise electric wave and method thereof

This invention relates to a distributor rotor electrode for preventing noise electric wave which prevents the generation of noise electric wave, especially, the generation of noise electric wave for the radio which is loaded on automobiles and the like. The electrode according to the present invention is used as a rotor electrode of a distributor of automobile engines.

Description of the Related Art

In conventional distributor of an internal combustion of automobiles, a rotor electrode rotates to intermittently oppose a side-fixed electrode having a small clearance between them. The rotor electrode and the side-fixed electrode discharge between them so that they feed a number of ignition plugs. However, in this conventional feeding method, noise electric wave (ignition noise) is generated due to spark discharge between the rotor electrode and the side-fixed electrode. Since the noise electric wave has wide and high frequency band, it causes hindrance on radiocommunication such as TV or radio, electronic equipments loaded on automobiles and the like; for example, EFI (electronical controlled fuel injection apparatus), ESC (electronic skid control apparatus), EAT (electronic control automatic transmission).

As shown in Figure 15, the above spark discharge current comprises capacity discharge current and induction discharge current. The capacity discharge current is high-frequency current which flows for 10 micron seconds from the beginning of discharge at the initial discharge stage due to rapid build-up. The induction discharge current is low-frequency current (about 10 to 100mA) which continuously flows for 500 to 1500 micron seconds soon after the capacity discharge current flows.

Ignition energy supplied for the ignition plug is proportionated with the product of the induction discharge current and its discharge duration. Concerning the induction discharge current, since the absolute value level of the current value is low, it has little influence on the noise electric wave. Therefore, in order to effectively prevent the noise electric wave without decreasing the ignition energy, it is important that the starting voltage and the capacity discharge current are firmly decreased.

Conventionally, various measures for preventing noise electric wave have been taken. For example, a method for placing the resistor outside or inside of the plug, a method for introducing resistance to a part of high-voltage wiring, a method for establishing a condenser in order to prevent noise. However, in these methods, effects are not sufficient and reliability is deteriorated.

Japanese Patent Registration No. 858984 discloses that high electrical resistance substance is formed on the surface of the discharge electrode in order to prevent the generation of noise electric wave caused by discharge gap. However, in this method, only 5 to 6dB of noise can be decreased so that required performance cannot be achieved.

Japanese Unexamined Patent Publication No. 50735/1979 discloses the technique in which the discharge electrode which is one element of ignition distributor of internal combustion is performed by surface treatment so that the starting voltage and the capacity discharge current are decreased, thereby preventing noise electric wave. In this technique, mixed powder comprising CuO (cupric oxide) and Al₂O₃ (alumina) is thermal sprayed on the surface of the discharge electrode to form the layer for preventing noise electric wave. Thus, the layer for prevention of noise electric wave is formed on the surface of the discharge electrode which is faced to an opposite electrode. In the electrode for preventing noise electric wave, preliminary micro discharge is generated between CuO as oxide resistor and Al₂O₃ as oxide dielectric substance, so main discharge voltage generated between CuO and the opposite electrode is reduced, thereby decreasing the capacity discharge current. The effect of the preliminary micro discharge is called as Malter effect, and the method for preventing noise electric wave which makes use of Malter effect is recently noticed.

Japanese Examined Patent Publication No. 22472/1989 discloses one example of the electrode for preventing noise electric wave which makes use of Malter effect. This electrode comprises an electrode substrate and a resistive material layer coated on the surface of the electrode substrate which is faced to the opposite electrode. The resistive material layer is made of semi-conductive alumina-ceramics material. The resistive material layer is formed on the surface of the electrode substrate because titania (TiO₂) is added to oxide ceramics mainly comprising alumina (Al₂O₃), and reducing treatment is performed in reducing atmosphere. In the electrode for preventing noise electric wave, preliminary micro discharge is generated between titania having semi-conductivity (resistivity) and alumina as dielectric substance, so main discharge voltage generated between the electrode for preventing noise electric wave and the opposite electrode is reduced, thereby decreasing the capacity discharge current.

However, in the method for preventing noise electric wave which makes use of Malter effect, the effect for preventing noise electric wave is not sufficient so that more effect is required. As a result, a bonding wire or a H/T code for prevention of noise electric wave is required. Therefore, there are disadvantages in cost and assembling manhour.

When the conventional electrode for preventing noise electric wave disclosed in Japanese Unexamined Patent Publication No. 50735/1979 is applied for a rotor electrode of distributor, noise is generated in the radio loaded on automobiles. In this case, radio noise is terrible as compared with the case in which the rotor electrode without layer (thermal sprayed layer) for preventing noise electric wave is used.

Since the radio is easily influenced by electric wave and electric noise, the radio loaded on automobiles has PNL (Pulse Noise Limiter) function in order to control noise generation due to ignition noise. The PNL function is the function in which ignition noise in sound signal is absorbed by shutting the gate for a predetermined time (about 20 micron seconds) when the pulse noise above the predetermined level is input through antenna.

There are two kinds of rotor electrodes: one is the rotor electrode in which the layer (thermal sprayed layer) for preventing noise electric wave is formed on the surface of the rotor electrode faced to the opposite electrode by use of the normal thermal spraying method that thermal spraying is performed in the direction perpendicular to the surface, and the other is the rotor electrode without the layer. Figure 16 shows the difference of electric wave form between them at the time of induction discharge. Al₂O₃ + 60wt%CuO is used as thermal spraying material.

As shown in Figure 16, as compared with the rotor electrode without the layer, in the rotor electrode with the layer (thermal sprayed layer) for preventing noise electric wave, induction discharge in which the absolute level of the current value is high can be maintained for a long time. In accordance with this, PNL operating time becomes longer. There are interrelation between the PNL operating time and the level of the radio noise. Therefore, in the electrode with the layer for preventing noise electric wave which is formed by use of the normal thermal spraying method, the radio noise becomes deteriorated.

US-A-4 091 245 discloses a distributor rotor electrode comprising a substrate, an intermediate layer of nickel aluminide and a further layer of electrically high resistive material.

US-A-3 992 230 discloses a method of providing an electrode comprising a substrate and a layer of nickel aluminide with a surface layer of an electrically high resistive material such as CuO.

In the conventional electrode for preventing noise electric wave disclosed in Japanese Examined Patent Publication No. 22472/1989, there are drawbacks in the durability. When the conventional electrode had been used for a long time, electric noise (radiation field intensity) had been increased, and required efficiency level could not be obtained.

In order to study the cause of the above problems, inventors have observed the discharge generating situation. As a result, although the resistive material layer having high electric resistive value has a close distance from the opposite electrode, discharge is not generated at the resistive material layer. Only at the part of the electrode substrate having low electric resistive value which is near the opposite electrode, namely, at the portion of the electrode substrate which is near a boundary portion between the electrode substrate and the resistive material layer, discharge is generated. Inventors have examined the relationship between the discharge generating situation and noise electric generating situation, and they found that the discharge generating situation is closely related to the durability of the electrode for preventing noise electric wave. When discharge is generated at the portion of the electrode substrate which is near the boundary portion between the electrode substrate and the resistive material layer, the electrode substrate is fused by heat at the time of discharge since the electrode substrate comprises metal materials having lower fusing point than that of ceramics. Inventors have found that the temperature at the time of discharge reaches about 1300 to 1500°C sectionally. As a result, when the electrode had been used for a long time, a concave portion is formed at the portion of the electrode substrate which is near the boundary portion between the electrode substrate and the resistive material layer due to fused loss, and discharge is generated at the bottom of the concave portion. Then, discharge is hard to occur, or micro discharge is frequently occurred and relatively large induction discharge current continuously flows since the discharge passage becomes complicated. Therefore, noise electric is increased.

SUMMARY OF THE INVENTION

In view of the above disadvantages, an object of the present invention is to further prevent noise electric wave by means of improvement of electrode.

The distributor rotor electrode for preventing noise electric wave and for solving the above object according to claim 1 of the present invention comprises a substrate; a first layer on the surface of the substrate faced, during use of the electrode in a distributor, to an opposite electrode; and a second layer comprising metal oxide, being formed on the surface of the first layer faced to the opposite electrode, characterized in that said first layer comprises metal oxide and has a smaller resistivity than that of said second layer.

The distributor rotor electrode for preventing noise electric wave and for solving the above object according to claim 2 of the present invention corresponds to the electrode according to claim 1, wherein said first layer and said second layer comprise resistive oxide material.

The distributor rotor electrode for preventing noise electric wave and for solving the above object according to claim 3 of the present invention corresponds to the electrode according to claim 1, wherein said first layer comprises oxide dielectric substance and resistive oxide material and said second layer comprises oxide dielectric substance.

However, there is the problem on the electrode for preventing noise electric wave according to claim 3. Namely, when the electrode has been used, pin hole is generated on the surface of the electrode, and oxide dielectric substance on the surface of the electrode is omitted. As a result, the effect for preventing noise electric wave cannot be maintained for a long time. Then, inventors have been further studied and completed the electrode which prevents pin hole and can maintain the effect for preventing noise electric wave for a long time.

The completed distributor rotor electrode for preventing noise electric wave and for solving the above object according to claim 4 corresponds to the electrode according to claim 1, wherein said first layer and said second layer oxide dielectric substance and resistive oxide material.

In the above electrodes for preventing noise electric wave according to claims 1 to 4, the method for forming the first layer and the second layer is not restricted, and various methods for forming the layer can be applied, for example, plasma spraying method, ion plating method, sputtering method and so on. However, when the second layer comprising metal oxide and having larger resistivity than that of the first layer is formed on the surface of the first layer comprising metal oxide, considering the cost, it is preferable that the second layer is formed on the surface of the first layer by performing oxidation treatment on the surface of the first layer.

(Effects)

In the electrode for preventing noise electric wave according to claims 1 to 4, the second layer which is located at outside has larger resistivity than that of the first layer.

When the discharge occurs, the flow of electron in the resistive oxide material continuously exists for a predetermined time. However, when electron is once emitted, the electronic supply performance at the top surface layer has an enormous influence on the current value. As a result, it is advantageous that the impedance of the second layer is high. In the electrode having the above construction according to claims 1 to 4, it is possible to prevent the generation of noise electric wave as compared with the conventional electrode having single layer comprising high electrical resistance substance.

In the electrode for preventing noise electric wave according to claim 3, discharge is generated from the resistive oxide material (having lower resistance than that of the oxide dielectric substance) in the first layer at the time of voltage apply, but the oxide dielectric substance exists in the second layer. Therefore, discharge is generated from the upper and lower surfaces of the electrode, and current flows along the surface of the first and second layers to be creeping discharge. When electron is moved along the surface of the first and second layers having high electric resistance, energy of the discharge is damped and the generation of the electric field and magnetic field which causes noise can be decreased. Furthermore, since the second layer comprising oxide dielectric substance is formed on the first layer, the outflow of electron charged in the resistive oxide material in the first layer can be prevented when the electron of the creeping discharge is moved from the electrode for preventing noise electric wave (cathode) to the opposite electrode (anode). When the outflow of electron charged in the resistive oxide material in the first layer occurs, discharge current value is increased and noise electric wave is increased.

In the electrode for preventing noise electric wave according to claim 3, when the second layer comprising oxide dielectric substance is too thick, impedance of whole electrode is increased and discharge voltage is increased. As a result, the effect for preventing noise electric wave is deteriorated. Therefore, it is preferable that the thickness of the second layer is not more than 0.1mm. In order to effectively demonstrate the effect of the above creeping discharge, the total thickness of the first layer and the second layer is in the range of 0.1 to 1.0mm. When the total thickness of the first layer and the second layer is not less than 1.0mm, impedance of whole electrode is increased. Furthermore, discharge is generated at the resistive oxide material in the first layer from which discharge is likely to be generated, and at the field which is nearest to the opposite electrode. Namely, discharge is generated from the boundary portion between the first layer and the second layer, thereby deteriorating the effect of creeping discharge.

The electrode of the present invention is exposed to the high density energy due to the discharge. In the electrode according to claim 3, when the oxide dielectric substance of the second layer is used as a barrier for saving the discharge electron, large amounts of energy is absorbed, and the electrode sectionally becomes high temperature. As a result, when the resistive oxide material having comparatively low fusing point such as CuO is used for the first layer, the resistive oxide material are fused by the heat. Furthermore, when the electrode rotates at high speed like a rotor, the oxide dielectric substance of the second layer is also scattered to generate pin hole.

To prevent the above disadvantages, the oxide dielectric substance and the resistive oxide material have preferably high fusing point. Inventors have found that the temperature at the time of discharge reaches 1300 to 1500°C sectionally. Therefore, the oxide dielectric substance and the resistive oxide material have preferably the fusing point of not less than 1500°C. The oxide dielectric substances include Al₂O₃, ZrO₂, MgO, BeO and so on. The resistive oxide materials include TiO₂, CaO, MnO, ZnO, BaO, CeO₂, NiO, CoO, Fe₃O₄, Cr₂O₃, V₂O₃ and so on.

However, in the case of the above composition, the pin hole is sometimes generated. So, in the electrode for firmly preventing the pin hole according to claim 4, the second layer comprises both the oxide dielectric substance and the resistive oxide material. As a result, the resistivity value of the second layer is decreased as compared with the electrode according to claim 3. Furthermore, the performance of the barrier is deteriorated, and the absorbed energy can be decreased. Therefore, sectionally temperature rising of the electrode can be controlled, and it is possible to prevent the fusion of the resistor and the dielectric substance which causes the generation of the pin hole.

In the present invention, oxide is used for the resistor or the dielectric substance. When carbide or nitride is used for the resistor or the dielectric substance, the discharge in the atmosphere causes the deterioration of oxidation, and there are disadvantages in the durability of the performance for preventing noise electric wave.

In the electrodes according to claims 1 to 4, it is preferable that the thickness of the first layer is in the range of 0.1 to 1.0mm, and the total thickness of the first layer and the second layer is not more than 1.0mm. When the thickness of the first layer is less than 0.1mm, it is difficult to decrease the discharge voltage. When the total thickness of the first layer and the second layer is more than 1.0mm, it is easy to cause coming-off or loss of the layer. In case of the rotor electrode, there are bad influence on the performance of the ignition of engine. Furthermore, in the electrode according to claim 2, the effect of creeping discharge is deteriorated.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of its advantages will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings and detailed specification, all of which forms a part of the disclosure:

Figure 1 is a main cross-sectional view for showing the electrode for preventing noise electric wave according to the First Preferred Embodiment of the present invention.

Figure 2 is a whole cross-sectional view for showing the electrode for preventing noise electric wave according to the First Preferred Embodiment of the present invention.

Figure 3 is a bar graph for showing the level of noise electric wave of the electrode according to the First Preferred Embodiment of the present invention.

Figure 4 is a graph for showing the relationship between the level of noise electric wave and the ratio of the resistivity of the second layer to the resistivity of the first layer according to the First Preferred Embodiment of the present invention.

Figure 5 is a graph for showing the degree of the discharge voltage of the electrode according to the Second Preferred Embodiment of the present invention.

Figure 6 is a graph for showing the degree of the noise electric current of the electrode according to the Second Preferred Embodiment of the present invention.

Figure 7 is a graph for showing the degree of the noise electric field intensity of the electrode according to the Second Preferred Embodiment of the present invention.

Figure 8 is an enlarged photograph for showing the particle structure on the surface of the second layer of the electrode according to the Second Preferred Embodiment of the present invention after the electrode is used.

Figure 9 is a bar graph for showing the level of noise electric wave of the electrode according to the Third Preferred Embodiment of the present invention.

Figure 10 is a graph for showing the relationship between the level of noise electric wave and the thickness of the first layer according to the Third Preferred Embodiment of the present invention.

Figure 11 is a graph for showing the relationship between the level of noise electric wave and the thickness of the second layer according to the Third Preferred Embodiment of the present invention.

Figure 12 is a bar graph for showing the level of noise electric wave of the electrode according to the Fourth Preferred Embodiment of the present invention.

Figure 13 is a graph for showing the relationship between the level of noise electric wave and the amount of TiO₂ according to the Fourth Preferred Embodiment of the present invention.

Figure 14 is a graph for showing the relationship between the level of noise electric wave and the thickness of the first layer according to the Fourth Preferred Embodiment of the present invention.

Figure 15 is a graph for showing the result of examining the electric current profile model at the time of first discharge in the conventional electrode for preventing noise electric wave.

Figure 16 is a graph for showing the result of comparison between the electric current profile model at the time of first discharge in the conventional electrode with the layer for preventing noise electric wave and the electric current profile model at the time of first discharge in the conventional electrode without the layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Having generally described the present invention, a further understanding can be obtained by reference to the specific preferred embodiment which is provided herein for purposes of illustration only and is not intended to limit the scope of the appended claims.

In the following preferred embodiments, the present invention is applied for a rotor electrode of distributor of automobiles. Figure 2 is a whole schematic diagram of this electrode. The distributor comprises a rotor 1 which is rotatable at high speed, a rotor electrode 2 which is installed on the rotor 1 and a side electrode 3 which is opposite to the tip of the rotor electrode 2 with the clearance therebetween.

First Preferred Embodiment

Embodiment 1

Figure 1 is a cross-sectional view for showing the rotor electrode 2 according to the Embodiment 1. The rotor electrode 2 comprises a substrate 20 made of brass, a substrate layer 21 formed on the surface of the substrate 20, a first layer 22 formed and coated on the surface of the substrate layer 21 and a second layer 23 formed and coated on the surface of the first layer 22.

The substrate layer 21 is formed in such a manner that the first layer 22 is firmly adhered to the substrate 20 by thermal spraying. The substrate layer 21 is made of Ni-5%Al alloy and has the thickness of 100 µm (microns). The substrate layer 21 is formed by plasma spraying method.

The first layer 22 is made of CuO as oxide resistor and has the thickness of 200 µm (microns). The resistivity value R₁ of the first layer 22 is in the range of 10³ to 10⁴ ohm centimeters.

The second layer 23 is made of BaO as resistive oxide material and has the thickness of 200 µm (microns). The resistivity value R₂ of the second layer 23 is in the range of 10⁹ to 10¹⁰ ohm centimeters. Therefore, R₂ is larger than R₁.

Both the first layer 22 and the second layer 23 are formed by plasma spraying method.

Comparative Example 1

An electrode according to Comparative Example 1 comprises only a substrate 20.

Comparative Example 2

An electrode according to Comparative Example 2 has the same construction as that of the Embodiment 1 except that the second layer 23 is not existed.

Comparative Example 3

An electrode according to Comparative Example 3 has the same construction as that of the Embodiment 1 except that the first layer 22 formed on the surface of the substrate layer 21 is made of BaO and has the thickness of 200 µm (microns), and that the second layer 23 is not existed.

Comparative Example 4

An electrode according to Comparative Example 4 has the same construction as that of the Embodiment 1 except that the first layer 22 formed on the surface of the substrate layer 21 is made of BaO and has the thickness of 200 µm (microns), and that the second layer 23 is made of CuO and has the thickness of 200 µm (microns). In this case, the resistivity value R₂ of the second layer 23 is smaller than the resistivity value R₁ of the first layer 22.

(Evaluation)

Figure 3 shows the result of measuring the level of noise electric wave at the time of discharge concerning each electrode. As seen from Figure 3, the electrode according to the Embodiment 1 shows the most excellent effect for preventing noise electric wave. As seen from Comparative Example 4, when the first layer 22 and the second layer 23 in the Embodiment 1 are exchanged with each other, there is no effect for preventing noise electric wave.

Figure 4 shows the change of the level of noise electric wave when the ratio of R₂ to R₁ (R₂ / R₁) is variously changed. As seen from Figure 4, when R₁ is larger than or equal to R₂, there is no effect for preventing noise electric wave. Furthermore, when R₂ is larger than R₁, there is remarkable effect for preventing noise electric wave.

Second Preferred Embodiment

Embodiment 2

An electrode according to the Embodiment 2 has the same construction as that of the Embodiment 1 except that the construction of the first layer 22 and the second layer 23 is different. The first layer 22 is made of the mixture comprising Al₂O₃ as oxide dielectric substance and CuO as resistive oxide material, and the weight ratio of Al₂O₃ to CuO is 4 : 6. The first layer 22 has the thickness of 400 µm (microns), and the resistivity value is in the range of 10⁴ to 10⁶ ohm centimeters.

The second layer 23 is made of only Al₂O₃ as oxide dielectric substance. The second layer 23 has the thickness of 50 µm (microns), and the resistivity value is 10¹⁴ ohm centimeters. The second layer 23 has the larger resistivity than that of the first layer 22.

Both the first layer 22 and the second layer 23 are formed by plasma spraying method which is the same method as that in the Embodiment 1.

Comparative Example 1

An electrode according to Comparative Example 1 comprises only a substrate 20.

Comparative Example 5

An electrode according to Comparative Example 5 has the same construction as that of the Embodiment 2 except that the second layer 23 is not existed.

Comparative Example 6

An electrode according to Comparative Example 6 has the same construction as that of the Embodiment 1 except that an insulation layer is formed on the surface of the substrate layer 21 in order to propagate electric power by use of creeping discharge. The insulation layer is made of Al₂O₃ and has the thickness of 400 µm (microns).

(Evaluation)

Concerning each electrode, the discharge voltage, noise electric current and noise electric field intensity were measured. The result is shown in Figures 5, 6 and 7.

As seen from these figures, both the discharge voltage and noise electric current of the Embodiment 2 are controlled to be low. As a result, noise electric field intensity is remarkably decreased. As compared with Comparative Examples 5 and 6, the electrode of the Embodiment 2 has about 2.5 to 3 times effect for decreasing noise electric wave.

Third Preferred Embodiment

Embodiment 3

When the electrode of Embodiment 2 is used, as shown in Figure 8, a number of pin holes (circle and black portions) are generated on the surface of the second layer 23, and the effect for preventing noise electric wave is gradually decreased. Therefore, an electrode according to the Embodiment 3 has the same construction as that of the Embodiment 1 except that the first layer 22 is made of electromelting grinding material such as Al₂O₃-13%TiO₂ (in case of not more than 44%TiO₂, it exists as Al₂TiO₅ and Al₂O₃) and has the thickness of 20 µm (microns), and that the second layer is made of Al₂O₃ and has the thickness of 50 µm (microns).

The electromelting grinding material comprising Al₂O₃-13%TiO₂ is now put on the market, and it is excellent in its uniformity of dispersion and the cost. When the electromelting grinding material is used as the first layer, it is possible to manufacture the electrode for preventing noise electric wave having excellent performance inexpensively.

Embodiment 4

An electrode according to the Embodiment 4 has the same construction as that of the Embodiment 3 except that the thickness of the first layer 22 is 70 µm (microns).

Embodiment 5

An electrode according to the Embodiment 5 has the same construction as that of the Embodiment 3 except that the thickness of the first layer 22 is 100 µm (microns).

Embodiment 6

An electrode according to the Embodiment 6 has the same construction as that of the Embodiment 3 except that the thickness of the first layer 22 is 200 µm (microns).

Embodiment 7

An electrode according to the Embodiment 7 has the same construction as that of the Embodiment 3 except that the thickness of the first layer 22 is 800 µm (microns).

Embodiment 8

An electrode according to the Embodiment 8 has the same construction as that of the Embodiment 3 except that the thickness of the first layer 22 is 400 µm (microns), and that the thickness of the second layer 23 is 20 µm (microns).

Embodiment 9

An electrode according to the Embodiment 9 has the same construction as that of the Embodiment 3 except that the thickness of the first layer 22 is 400 µm (microns).

Embodiment 10

An electrode according to the Embodiment 10 has the same construction as that of the Embodiment 3 except that the thickness of the first layer 22 is 400 µm (microns), and that the thickness of the second layer 23 is 100 µm (microns).

Embodiment 11

An electrode according to the Embodiment 11 has the same construction as that of the Embodiment 3 except that the thickness of the first layer 22 is 400 µm (microns), and that the thickness of the second layer 23 is 200 µm (microns).

Embodiment 12

An electrode according to the Embodiment 12 has the same construction as that of the Embodiment 3 except that the thickness of the first layer 22 is 400 µm (microns), and that the thickness of the second layer 23 is 400 µm (microns).

Comparative Example 1

An electrode according to Comparative Example 1 comprises only a substrate 20.

Embodiment 13

An electrode according to the Embodiment 13 has the same construction as that of the Embodiment 3 except that the first layer 22 is made of the mixture comprising Al₂O₃ and CuO (the weight ratio of Al₂O₃ to CuO being 4 to 6) and has the thickness of 400 µm (microns), and that the thickness of the second layer 23 is 100 µm (microns).

Embodiment 14

An electrode according to the Embodiment 14 has the same construction as that of the Embodiment 3 except that the first layer 22 is made of the mixture comprising Al₂O₃ and CuO (the weight ratio of Al₂O₃ to CuO being 4 to 6) and has the thickness of 400 µm (microns), and that the thickness of the second layer 23 is 200 µm (microns).

(Evaluation)

Concerning each electrode, the decreasing amount of the level of noise electric wave (decreasing amount of noise) having 180MHz was measured at the initial stage and at 24 hours later. Furthermore, it was observed that the pin hole was generated or not after the electrode was used. The result is shown in Table 1 and Figure 9. Figure 10 shows the relationship between the thickness of the first layer 22 and the level of noise electric wave having 180MHz, and Figure 11 shows the relationship between the thickness of the second layer 23 and the level of noise electric wave having 180MHz. The decreasing amount of noise is calculated on the basis of the initial performance of the electrode of Comparative Example 1.

The electrode according to the Embodiment 3 shows the low level of noise electric wave at the initial stage and at 24 hours later. On the contrary, the electrode according to the Embodiments 2 and 13 shows low level of noise electric wave at the initial stage, but noise electric wave becomes increasing at 24 hours later. This is caused by the generation of pin hole. When the second layer 23 is made of only Al₂O₃ as oxide dielectric substance, pin hole is generated under the condition that CuO having comparatively low fusing point is included in the first layer 22.

In the Embodiment 14, no pin hole is generated and noise electric wave shows the same level at the initial stage and at 24 hours later. However, the level of noise electric wave is high since the thickness of the second layer 23 is thick. As seen from Figures 10 and 11, there is an appropriate thickness for preventing noise electric wave. The thickness of the first layer 22 is preferably not less than 0.lmm, more preferably, not less than 0.2mm. The thickness of the second layer 23 is preferably not more than 0.lmm, more preferably, not more than 0.05mm.

Fourth Preferred Embodiment

Embodiment 15

An electrode according to the Embodiment 15 has the same construction as that of the Embodiment 2 except that the second layer 23 is made of electromelting grinding material (Al₂O₃-2.3%TiO₂) as semi-conductive alumina and has the thickness of 50 µm (microns).

Embodiment 16

An electrode according to the Embodiment 16 has the same construction as that of the Embodiment 15 except that the amount of TiO₂ in the second layer 23 is 5%.

Embodiment 17

An electrode according to the Embodiment 17 has the same construction as that of the Embodiment 15 except that the thickness of the first layer 22 is 20 µm (microns), and that the amount of TiO₂ in the second layer 23 is 13%.

Embodiment 18

An electrode according to the Embodiment 18 has the same construction as that of the Embodiment 15 except that the thickness of the first layer 22 is 70 µm (microns), and that the amount of TiO₂ in the second layer 23 is 13%.

Embodiment 19

An electrode according to the Embodiment 19 has the same construction as that of the Embodiment 15 except that the thickness of the first layer 22 is 100 µm (microns), and that the amount of TiO₂ in the second layer 23 is 13%.

Embodiment 20

An electrode according to the Embodiment 20 has the same construction as that of the Embodiment 15 except that the amount of TiO₂ in the second layer 23 is 13%.

Embodiment 21

An electrode according to the Embodiment 21 has the same construction as that of the Embodiment 15 except that the thickness of the first layer 22 is 800 µm (microns), and that the amount of TiO₂ in the second layer 23 is 13%.

Embodiment 22

An electrode according to the Embodiment 22 has the same construction as that of the Embodiment 15 except that the amount of TiO₂ in the second layer 23 is 30%.

Embodiment 23

An electrode according to the Embodiment 23 has the same construction as that of the Embodiment 15 except that the amount of TiO₂ in the second layer 23 is 44%.

Comparative Example 7

An electrode according to Comparative Example 7 has the same construction as that of the Embodiment 15 except that the second layer is made of 99%TiO₂.

Comparative Example 1

An electrode according to Comparative Example 1 comprises only a substrate 20.

Embodiment 2

An electrode according to the Embodiment 2 has the same construction as that of the Embodiment 1 except that the construction of the first layer 22 and the second layer 23 is different. The first layer 22 is made of the mixture comprising Al₂O₃ as oxide dielectric substance and CuO as oxide resistor, and the weight ratio of Al₂O₃ to CuO is 4 : 6. The first layer 22 has the thickness of 400 µm (microns), and the direct current resistance value is in the range of 10⁴ to 10⁶ ohm centimeters.

The second layer 23 is made of only Al₂O₃ as oxide dielectric substance. The second layer 23 has the thickness of 50 µm (microns), and the direct current resistance value is 10¹⁴ ohm centimeters. The direct current resistance value is measured instead of the resistivity, but the second layer 23 has the larger resistivity than that of the first layer 22.

Embodiment 13

An electrode according to the Embodiment 13 has the same construction as that of the Embodiment 2 except that the thickness of the second layer 23 is 100 µm (microns).

Embodiment 14

An electrode according to the Embodiment 14 has the same construction as that of the Embodiment 2 except that the thickness of the second layer 23 is 200 µm (microns).

Embodiment 25

An electrode according to the Embodiment 25 has the same construction as that of the Embodiment 2 except that the thickness of the second layer 23 is 20 µm (microns).

(Evaluation)

Concerning each electrode, the decreasing amount of the level of noise electric wave (decreasing amount of noise) having 180MHz was measured at the initial stage and at 24 hours later. Furthermore, it was observed that the pin hole was generated or not after the electrode was used. The result is shown in Table 2 and Figure 12. Figure 13 shows the relationship between the added amount of TiO₂ to the second layer 23 and the level of noise electric wave having 180MHz, and Figure 14 shows the relationship between the thickness of the first layer 22 and the level of noise electric wave having 180MHz. The decreasing amount of noise is calculated on the basis of the initial performance of the electrode of Comparative Example 1.

The electrode according to the Embodiment 20 shows the low level of noise electric wave at the initial stage and at 24 hours later. On the contrary, the electrode according to the Embodiments 2, 13 and 25 shows low level of noise electric wave at the initial stage, but noise electric wave becomes increasing at 24 hours later. This is caused by the generation of pin hole. Although the first layer 22 includes CuO having comparatively low fusing point, pin hole is hardly generated when the second layer 23 is made of Al₂O₃ as oxide dielectric substance and TiO₂ as resistive oxide material.

In the Embodiment 14, no pin hole is generated and noise electric wave shows the same level at the initial stage and at 24 hours later. However, the level of noise electric wave is high since the thickness of the second layer 23 is thick. As seen from Figures 13 and 14, there is an appropriate thickness of the first layer 22 and an appropriate added amount of TiO₂ for preventing noise electric wave. The added amount of TiO₂ is preferably in the range of 5 to 44%, more preferably, in the range of 5 to 22%. Furthermore, it is preferable that the thickness of the first layer is not less than 0.lmm, more preferably, not less than 0.4mm.

In each electrode for preventing noise electric wave according to claims 1 to 4, it is possible to prevent noise electric wave for a long time. As a result, other step for preventing noise electric wave such as a bonding wire is not required, so it is possible to decrease the cost and the manhour. Furthermore, since each electrode has the same noise level as that of a ceramic rotor electrode which is expensive, it is possible to use each electrode as a substitution for the ceramic rotor electrode. Therefore, it is possible to lower the cost remarkably.

The electrodes for preventing noise electric wave according to claims 1 to 4 comprises the first layer comprising resistive oxide material and the second layer comprising resistive oxide material, and the resistivity of the second layer located at outside is larger than that of the first layer. Therefore, it is possible to effectively prevent the generation of noise electric wave as compared with the conventional electrode which has a single layer comprising high electrical resistance substance.

The electrode for preventing noise electric wave according to claim 3 shows further effect for preventing noise electric wave due to the effect of creeping discharge and the effect for preventing outflow of electron caused by the second layer which is functioned as the insulating layer.

The electrode for preventing noise electric wave according to claim 4 has the same construction as that of the electrode according to claim 3 except that the second layer comprises both the resistive oxide material and the oxide dielectric substance. As a result, the resistivity of the second layer is decreased, and the performance of the barrier at the time of discharge is decreased. Therefore, it is possible to prevent the generation of pin hole, and to improve the durability.