JPS6028194A - Ceramic heater - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a conductive ceramic heater that has excellent oxidation resistance, good thermal shock resistance, high-temperature strength, and a large temperature coefficient of resistance, and can be used, for example, as a glow plug river heating element. .

Conventionally, heat-resistant alloys such as nickel-chromium alloys and iron-chromium-aluminum alloys have been used as heating elements in the case of metals, and silicon carbide, molybdenum silicide, etc. have been used in the case of ceramics.

However, in the case of a fully bent heating element, the operating temperature is 1000
The upper limit is about 1100°C, and at higher temperatures, oxidation corrosion, fusing, etc. occur, making it unusable. Silicon carbide (S i C) can be used up to 1,600°C, and molybdenum silicide (MoSi2) can be used up to about 1,800°C, but silicon carbide has an extremely high resistivity, so miniaturization is a problem, and molybdenum silicide can be used up to 1,800°C. It begins to soften at temperatures above 1300°C, causing problems in terms of high-temperature strength and thermal shock resistance, and has not been able to meet the required specifications for, for example, automobile glow plugs.

An object of the present invention is to provide a ceramic heater that utilizes the extremely excellent heat resistance of molybdenum disilicide, eliminates the disadvantage of being weak in heat (weight part), and has a large temperature coefficient of resistance.

The details of the present invention will be explained below based on experimental examples.

An oxidation test was conducted on various high-temperature materials shown in FIG. 5 in an electric furnace at 1000° C. for 115 hours.

The weight change rate and initial resistivity before testing of each material are shown in the same figure. The samples used were material (゛) powder that was sintered to a density of 90% or more of the theoretical density using a photopress, and each sample was cut into the same shape.The rate of weight change was determined by washing the sample with acetone after the test was completed. I measured it and found it.

Note that a commercially available Ni-Cr material was used.

From FIG. 5, it can be seen that SiC and MoSi2 have extremely good oxidation resistance, while other materials are considerably oxidized.

However, SiC has a very high initial resistivity, and it is extremely difficult to apply it to a small heating element such as a glow plug, which is operated by a low-voltage power source. In the table, the weight change rate of M o B 2 is negative because
It is recognized that this is due to the oxidized part peeling off during acetone cleaning.

From the above, M o Si 2 is the best heater material in terms of oxidation resistance and specific resistance, but it has the disadvantage of being significantly inferior in thermal shock resistance and softening at high temperatures of 1000°C or higher. . In order to improve this drawback, the inventors devised a heater made of molybdenum disilicide and silicon nitride. However, there are restrictions in manufacturing in order to obtain a mixed sintered body of M o S i2 and S i 3N4.
It has been discovered that the mixture of i 2 and S i 3N4 is strongly affected by the atmosphere during firing. Figure 2 explains the effect. Fig. 2 shows that the starting materials listed in Fig. 2 are mixed with a plurality of organic solvents, and a sheet formed by the doctor blade method is laminated, and the sheets are heated for 1600'cxlHr in each atmosphere shown in Fig. 2.
Molybdenum disilicide (MoSi2) and molybdenum trisilicide (M
o5Si3).

However, this ratio is determined by the X-ray diffraction M o S i 2 and M
.

The strongest peak with 5Si3 (Mo Si 2 (
dA72.02) Mo s S i 3 (dA=2
．． 156) It is the height ratio of l, not the true molar ratio. Alumina and magnesia spinel (MgAβ2o
4) is a small amount added as a sintering aid, and MO
Metal 18.7 mol%, Si metal 37.5%/l/%, 5
The ratio of f3N443.8 mol% is M o Si after firing.
230 mol%, corresponding to a ratio of 5f3N470 mol%. From Figure 2, M o S i 2 and Si 3 N
When the mixture of a is pot-pressed, most of the Mo
5Si3 is produced, whereas in argon it is found that most of the MoSi2 remains as it is. Furthermore, when MoSi2 is used as a starting material at 100%, almost no Mo5Si3 is generated even in N2. Furthermore, it can be seen that the sintering aid has almost no effect on this phenomenon.

In addition, Mo metal, which is the raw material for Mo Si 2, and St
Even when 3i3N4 is added as a gold metal starting material, Mo5Si3 is generated in N2, and M in Ar.
o Generate S i 2. From the above, M o S
It has been found that nitrogen makes a very large contribution to the reaction in which i 2 changes to Mo s S i 3, and that this reaction is significantly accelerated by the presence of silicon. Mo3i2
In the case of 1QQ%, since the above reaction hardly occurs in N2, the conversion of MO312 to M O5S i 3 in the presence of N2 +; t, Mo5i2 and S i 3 N 4
It is considered to be characteristic in a mixture of The reason why molybdenum silicide has excellent oxidation resistance is that when heated in an oxidizing atmosphere, a 5i02 film is formed on the surface layer, and this film prevents oxidation from progressing to the inside.

This oxide film is related to the Si content, and when the Si content is low, the oxidation resistance is low because less S t O2 is generated. Comparing MoSi2 and Mo5S i 3, the ratio of Si amount is higher in MoSi2 than in Mo5
It is considerably higher than Si3, and therefore has excellent oxidation resistance.

Therefore, if oxidation resistance is considered, M o 5
It is necessary to fabricate the heating element so as to eliminate the conversion of S i 3.

In FIG. 2, the nitrogen partial pressure is gradually increased and M.

The data showing the state of conversion of Si2 to M o 5 S i 3 is also included (1 to K). Conversion to M o 53 i 3 proceeds as the nitrogen partial pressure increases.

FIG. 1 shows the change in resistance after the heating element manufactured under the conditions shown in FIG. 2 was polished to the sample dimensions shown in FIG. ff11 and subjected to electrical heating. Each sample was heated to 1,400 degrees Celsius by applying electricity to a thin part with a diameter of 2 cranes.
Durable for 100 hours continuously.

FIG. 1 shows the ratio between the resistance value after durability and the initial resistance, and if it is 1.0, it means that there was no change in resistance. In the case of M o 5 S i 3 (sample B), MOS
The resistance value increases compared to i2 (sample A, G). Furthermore, even when the nitrogen partial pressure is changed, an increase in resistance is observed starting from the sample produced under the condition of N2 partial pressure of 0.3 atm.

FIG. 3 shows the rate of increase in resistance of the sample used in FIG. 1 with respect to temperature. The ratio of the resistance value of each sample at 1000°C and the resistance value at room temperature is shown.

As the ratio of M05Si3 increases, the rate of increase in resistance decreases. In a system that detects the resistance value of a heating element and feeds it back to control the temperature of the heating element, the larger the resistance temperature coefficient is, the better, and the control limit is (R at 1000° C./R at room temperature) about 2.3.

From the above, M o S i 2. and S i 3N4
It has been found that the heating element made of a mixed sintered body with nitrogen must be manufactured in a non-oxidizing atmosphere with a nitrogen partial pressure of 0.3 atm or less. It should be noted that MOSi2 can be
It has been confirmed that it is effective in preventing the decomposition of In this example, MgAl2O3 was used as a sintering aid, but
In addition, alumina (Aj!203) or a mixture of alumina and magnesia (MgO) may be used. In addition, in this example, firing was performed in an atmosphere of 1 atm with a N2 partial pressure of 0.2 atm and an Ar partial pressure of 0.8 atm. Firing may be performed under high pressure such as atmospheric pressure. However, the N2 partial pressure is also 0.3 atm or less in that case.

Figure 4 shows a sheet made by mixing M o Si 2 and Si3N4 in various proportions and adding an organic solvent to it in an argon gas atmosphere at 1600°C X LHr and 500 kg/c.
The characteristic values of the product hot-pressed under the conditions of 1 are shown. The bending strength is the load when a three-point bending sample is broken or deformed at 1300°C, and the bending strength test is 40X3X4.
Crosshead speed Q, 5 mm
/min. Thermal expansion coefficient is room temperature ~ 80
It is the average coefficient of thermal expansion at 0°C. From Figure 4, M o S
It was confirmed that the addition of S i 3N4 to i 2 reduces the coefficient of thermal expansion and increases the strength. Thermal shock resistance becomes better as the coefficient of thermal expansion becomes smaller and as the breaking strength, that is, the bending strength becomes larger.

Therefore, the heating element of the present invention maintains the excellent oxidation resistance of M OS i 2, and overcomes the disadvantage of being weak against thermal shock, which is the biggest drawback of heating elements composed of M OS i 2 alone. This is a heating element that solved the problem. The resistance value of the heating element when used as a car heater such as a glow plug is in the range of 1 x 10-3 to 1 x 10-1 Ωcm at room temperature, and from Figure 4, the amount of MoSi2 is 5 to 50 mol. % range. Regarding the addition of silicon nitride, if the purpose is simply to lower the coefficient of thermal expansion, it is possible to add cordierite in addition to silicon nitride, but silicon nitride is a structural material with the highest strength among ceramics. Therefore, since it is most effective for improving strength, and the optimal firing condition for MOSi2 is around 1600°C, it is not possible to add a large amount of a substance with a low melting point. As mentioned in this example, even if fired in a non-oxidizing atmosphere that does not contain nitrogen, a small amount of M o +33 i 3 is generated, but the majority is in the state of M o S i 2, The heating element of the present invention has a resistance value at 1000° C. that is 2.3 times the resistance value at room temperature. Further, in addition to Mo5i7°S i 3N4, a sintering aid may be added, but it may be added as long as it does not impede the effect of adding silicon nitride. Also, MoS
i2. Even if Si metal is added in excess in addition to S i 3N4, there is no problem as long as the amount of excess Si metal is on the order of several percent, since the oxidation resistance performance will not be deteriorated so much. In the examples, Si metal and Mo metal were used as raw materials for forming MoSi2, but other materials such as MoSi
If there is a raw material that forms 2, it may be used as the starting raw material. Note that the temperature described in the present invention is a value measured with a pyrometer and is a value given as emissivity (ε)−1,00. The mixed sintered body of M o Si 2 and S i 3 N 4 can only be produced by firing in a non-oxidizing atmosphere with a nitrogen partial pressure of 0.3 atm or less, and in the presence of nitrogen above this condition. In this case, a large amount of M o 5 S i 3 is produced, which deteriorates the oxidation resistance and lowers the temperature coefficient of resistance.

In the case of 100% M a S i 2, there is no such reaction, so this reaction is a combination of M o S i 2 and S i
It is particularly noticeable in a mixture with 3N4. By manufacturing under the above conditions, 1000°C
The resistance value at is two to three times or more than the resistance value at room temperature.

As described above, according to the present invention, it is possible to utilize the extremely excellent heat resistance of molybdenum disilicide, the extremely excellent thermal shock resistance of silicon nitride, and its large temperature coefficient of resistance makes it suitable for use in glow plugs. Ideal as a heater for automobiles.

[Brief explanation of drawings]

1 to 5 are characteristic diagrams for explaining the present invention. Representative Patent Attorney Takashi Okabe

Claims (1)

[Claims] Composed of a sintered body of molybdenum disilicide and silicon nitride,
The electrical resistance value at 000°C is 2.3 times or more the resistance value at room temperature, and the proportion of molybdenum disilicide in the two-component system of molybdenum disilicide and silicon nitride is 5 to 5.
Ceramic heater with 50 mol%.