JPS632916B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a ceramic conductor for a heater. [Background of the invention] As a heating element that can be used at high temperatures of 2000℃ or higher,
Mo, W, C, etc. are known, but they cannot be used in an oxidizing atmosphere. Heating elements that can be used in oxidizing atmospheres include silicon carbide (SiC), zirconium oxide (ZrO ₂ ), and LaCrO ₃ , but all of them have relatively high electrical resistivity, and the electrical resistance decreases as the temperature rises. Therefore, it is easy to cause thermal runaway and temperature control is difficult. It also has low mechanical strength. [Object of the Invention] The present invention has been made with the aim of solving the above-mentioned problems in conventional heating elements, and provides a ceramic conductor for heaters that has a low electrical resistivity, a positive temperature coefficient of resistance, and high mechanical strength. It provides: [Summary of the Invention] That is, the characteristics of the present invention are a, a, and a, which have good conductivity but are difficult to sinter and have poor oxidation resistance.
Fine particles of nitrides and carbides of group a transition elements,
To obtain a ceramic conductor for heaters that has both characteristics by dispersing and mixing it in silicon nitride (Si ₃ N ₄ ), which is an insulator but has high mechanical strength and good oxidation resistance, and sintering it densely. It is in. a used as a conductive material in the present invention,
Va, carbides and nitrides of Group A transition elements are refractory and highly hard, and also exhibit metallic conductivity, with a low resistivity of approximately 10 ^-5 Ωcm or less and a positive temperature coefficient of resistance, making them suitable for use in heaters. Suitable as a ceramic conductor. The reason why ceramic conductors for heaters are limited to nitrides and carbides is that oxides do not have electrical conductivity, and borides do not have good sinterability when combined with Si ₃ N ₄ . In order to uniformly disperse the nitride and carbide raw material powders in Si ₃ N ₄ and improve conductivity, it is preferable that the powders be as fine as possible, and preferably have an average particle size of 5 μm or less.
The reason for limiting the mixing ratio of the conductive compound to 30 to 70 parts by volume is that when conductive particles are dispersed in an insulator, the electrical resistivity decreases rapidly at around 20 parts by volume, and the reproducibility of the resistivity value is limited. This is because the amount of the conductive particles is extremely poor, and if the amount exceeds 70 parts by volume, the insulating particles, which are the matrix, will be unable to firmly hold the conductive particles, resulting in a decrease in mechanical strength. The reason why Si ₃ N ₄ was chosen as the matrix phase is that it has high mechanical strength, low coefficient of thermal expansion, and excellent heat resistance. Moreover, when combined with the above compounds, it forms an oxide film that is stable at high temperatures.
This is more advantageous since it serves to prevent the internal compound particles from being oxidized. Si ₃ N ₄ is difficult to sinter, so it is used as a sintering aid.
It is necessary to add 1 to 10 parts by weight of MgO, Y ₂ O ₃ , Al ₂ O ₃ , AlN, etc. per 100 parts by weight of Si ₃ N ₄ . The amount of the sintering aid is limited because if it is less than 1 part by weight, it will not be effective as a sintering aid, and if it exceeds 10 parts by weight, the grain boundary phase will increase and the high temperature strength will decrease. The manufacturing method involves preparing a predetermined amount of Si ₃ N ₄ , a conductive compound, and a sintering aid, mixing it using a sieve machine or a pole mill, and then adding a small amount of a molding binder such as polyvinyl alcohol (PVA). After granulation, it is molded into a predetermined shape. This is sintered in a nitrogen atmosphere at a temperature of 1,600 to 1,850°C by a hot press method or an atmospheric pressure sintering method. 1600
Dense sintering does not occur below 1850°C, and when it is higher than 1850°C
This is because the decomposition of Si ₃ N ₄ becomes intense and densification is inhibited. The above-mentioned sintered ceramics must have an electrical resistivity of 10 ^-2 Ωcm or less at room temperature and a positive temperature coefficient of resistance. [Examples of the Invention] Next, the present invention will be explained in detail with reference to Examples. Experimental example 1 9 parts by weight of Y ₂ O ₃ and 4 parts by weight as sintering aids
Si ₃ N ₄ powder with an average particle size of 0.7 μm added with Al ₂ O ₃ ,
30 parts by volume of various A, A, A group nitrides and carbides were prepared, 10% of a 5% PVA solution was added, and the mixture was mixed in a stirrer. Next, the mixed powder was filled into a mold and molded at a pressure of 1 t/cm ² . This was placed in a graphite die, and the temperature was 1750°C and the pressure was 1 atm nitrogen.
Hot pressing was carried out under the conditions of 300Kg/cm for ² hours and 1 hour.
The properties of the obtained sintered body are shown in the table. Relative densities of 97 to 98% or higher have been obtained for most compounds. Also, the resistivity varies from 10 ¹ to 10 ^-4 Ωcm depending on the compound.
Although a wide range of values were obtained, the temperature coefficient of resistance was all positive. Bending strength is 350~540MN/ ^m2
It was recognized that the mechanical strength was also high. Oxidation resistance is particularly good with TiN and TiC additions, 1100
After being left in the air for 92 hours at ℃, the oxidation weight gain was 2.8 and 3.3 mg/cm ² , respectively.

【table】

〔Effect of the invention〕

According to the present invention, any resistivity of 10 ^-2 Ωcm or less can be easily obtained. In addition, since the temperature coefficient of resistance is positive, thermal runaway does not occur during heating with electricity, the mechanical strength is sufficient, and the durability with electricity is excellent. Therefore, the ceramic conductor for heaters of the present invention can provide highly reliable heaters for glow plugs, gas igniters, and various heating elements.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the relationship between the compound mixture ratio and electrical resistivity in the Si ₃ N ₄ -TiN system and Si ₃ N ₄ -TiC system, and Figure 2 is the relationship between the compound mixture ratio and the temperature coefficient of resistance of the above ceramics. FIG. 3 is an external view of a prototype heater. 1...Silicon nitride-based conductive ceramics, 2...
...Aluminum nitride, 3...Lead wire.

Claims (1)

[Scope of Claims] 1 (a) 30 to 70 parts by volume of silicon nitride, (b) 30 to 70 parts by volume of carbides and/or nitrides of Group A, Group A, and/or Group A transition elements,
and (c) a sintered body containing 1 to 10 parts by weight of magnesium oxide, yttrium oxide, aluminum oxide and/or aluminum nitride per 100 parts by weight of the mixtures (a) and (b), the sintered body being kept at room temperature. A ceramic conductor for a heater, characterized in that it has an electrical resistivity of 10 ^-2 Ωcm or less and a positive resistor temperature coefficient.