JPS6156187B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of manufacturing silicon carbide heating elements with improved strength and low resistance. Conventionally, silicon carbide heating elements have been used as heating elements for conventional electric furnaces. Recently, it is also used as a heating element for various industrial equipment, and this application requires high strength, low electrical resistance, small negative temperature coefficient of resistance, and high oxidation resistance at high temperatures, but recently developed Introducing a few methods for sintering SiC that have been developed. By sintering a mixture of these materials in Ar at 2050°C for 60 minutes, a sintered body with a theoretical density of over 95% can be obtained. However, the specific resistance was several Ω-cm or more, and the high temperature resistance was extremely low, making it unsuitable as a heating element. If this is sintered from the beginning at 2200℃ or lower in nitrogen, the resistivity will be 1.0Ω-cm or less, but it will only be sintered to about 80% of the theoretical density and its strength will be weak. Although it can be sintered to about 90% of its theoretical density, it has a large resistivity of 10 to 10 ⁶ Ω-cm and is unstable. (2) JP-A-52-1 proposed by Carborundum
No. 110499 "Fuel igniter made of a new silicon carbide composition and method for producing the composition" is a method of sintering SiC powder containing 95% or more to 2.5 g/cc by hot pressing,
This is a method of doping N, P, As, Sb, Bi in solid phase or gas phase to make the resistivity at room temperature 0.66 Ω-cm, but the resistivity at 1350℃ is 0.12 Ω-cm and the temperature coefficient of resistance is 0.66 Ω-cm. Unfortunately, it was unsuitable as a power-saving heating element. (3) JP-A-53-121810 ``High Density Thermal Shock Resistant Sintered Silicon Carbide'' proposed by the same Carborundum company contains additives BN, BP, 0.3 to 3.0% by weight to SiC.
A pressureless sintered body proposed by General Electric Company is made by adding AiB <sub>2</sub> and 150 to 500% by weight of C to this additive and heating it to 1900 to 2500°C to achieve a density of 85% or more of the theoretical density. However, a sintered body having a density of 95% or more of the theoretical density has a problem in that its oxidation resistance is slightly inferior to that of a system containing B and B <sub>4</sub> C. Therefore, the present applicant previously proposed in Japanese Patent Application No. 54-72464 ``Method for manufacturing silicon carbide heating element'' an average particle size of 1.0.
Boron or a boron compound in an amount equivalent to 0.3 to 3.0% by weight of boron and carbon or a carbonaceous compound in an amount equivalent to 0.1 to 3.0% by weight of carbon are added to SiC powder with a size of less than μ, and the mixture is molded and vacuum molded. Primary sintering to 70-95% of the theoretical density in an inert atmosphere other than nitrogen, followed by sintering at a temperature of 1,600-2,200°C in a nitrogen gas atmosphere to a resistance of 80% or more of the theoretical density or less than 1.0Ω-cm. A method for manufacturing a silicon carbide heating element is provided, which comprises secondary firing of resintering the silicon carbide heating element. The material obtained by this method has high strength and low resistance because of its high density, and its high temperature resistance does not become extremely low. However, by changing the nitrogen atmosphere during the subsequent secondary firing to a pressurized nitrogen atmosphere, it was possible to further increase the effect of nitrogen addition and reduce the resistance value. As a result, the gist of the present invention is that the average particle size is 1.0μ.
Boron or a boron compound in an amount equivalent to 0.3 to 3.0% by weight of boron and carbon or a carbonaceous compound in an amount equivalent to 0.1 to 6.0% by weight of carbon are added to the following SiC powder, mixed and molded in a vacuum. Or silicon carbide with a specific resistance of 80% or more of the theoretical density and 1.0Ω-cm or less at a temperature of 1500 to 2300°C in a pressurized nitrogen gas atmosphere after primary sintering to 70 to 95% of the theoretical density in an inert atmosphere. This provides a method for producing a silicon carbide heating element, which comprises secondary firing of resintering the heating element, and the pressurized atmosphere of nitrogen is preferably 1.5 atm or more and 200 atm or less, preferably 1.5 atm or more and 100 atm or less. is particularly preferred, and the reason why sintering is divided into primary firing, which is sintering in a vacuum or an inert atmosphere, and secondary firing, which is sintering in pressurized nitrogen, is because sintering is performed in pressurized nitrogen from the beginning. This is because although it is possible to obtain a sintered body with a resistivity controlled within the desired value, the density does not reach 80% or more of the theoretical density, and therefore the breaking strength does not reach the required value. Also, the sintered density of the primary firing is 70 to 95 of the theoretical density.
% because if it is less than 70%, even if secondary firing is performed, it will not reach the final theoretical density of 80%, and the upper limit is set to 95% because if it is above 95%, the This is because closed pores are formed in the compact and open pores that penetrate to the center are not formed, and the influence of nitrogen does not reach the center during secondary firing in nitrogen. It is also a starting material
One of the reasons for setting the SiC grain size to 1.0μ or less is that a sintered body with the required density cannot be obtained if it is larger than that, and the other reason is to increase the specific surface area of the primary sintered body. This is to ensure sufficient nitrogen doping. The boron compound to be added is preferably elemental boron, boron carbide or other boron compounds, which improve the sinterability of SiC, but the lower limit is set at 0.3%.
Below 0.3%, the effect of improving sinterability is small, and the upper limit was set at 3.0% because above 3.0%, the boron compound
This is because it promotes the grain growth of SiC, which deteriorates the sinterability, and because the boron element is an electrically positive doping agent, the electrical properties cannot be improved. On the other hand, carbon has the effect of deoxidizing the oxidation on the surface of SiC particles, improving sinterability, and suppressing grain growth, but the lower limit was set at 0.1% because these effects cannot be expected below 0.1%. The upper limit was set at 6.0% because adding more than 6.0% will result in free carbon.
This is because it harms the physical properties of the sintered body, especially its breaking strength. In addition, the second stage of sintering in nitrogen has the effect of reducing the resistivity, but the reason why this sintering temperature was set at 1500 to 2300°C is because below 1500°C, the effect of reducing the resistivity due to the intrusion of nitrogen atoms is poor. This is because, at temperatures above 2300°C, not only is there a large amount of volatilization, N ₂ and SiC react, scale is formed on the surface of the sintered body, and the specific resistance is rather large, ranging from 10 to 10 ⁶ Ω-cm. The state is shown in FIG. In the figure, the horizontal axis shows the secondary firing temperature, the vertical axis shows the specific resistance of the sintered body, the dotted line shows the case when the N ₂ atmosphere pressure is 1 atm, and the solid line shows the case when the N 2 atmosphere pressure is 80 atm. According to this, when sintering in N ₂ at 80 atmospheres, for example, the resistance decreases overall, and therefore the specific resistance can be made 1.0 Ω-cm or less at 1500 to 2300°C. The reason why the secondary firing atmosphere is a pressurized atmosphere is that, as shown in Figure 2, by increasing the _N2 gas pressure,
This is because specific resistance can be significantly lowered.
According to this, as the pressure of the _N2 gas atmosphere is increased above 1 atm (atmospheric pressure), the specific resistance decreases, but the efficiency of this effect decreases as the pressure increases; Even if the amount is increased, the effect will not be very noticeable, and the problem will be that the equipment cost will increase significantly. For the same reason, 1.5 atmospheres or less is most preferable. in this way
The reason why resistivity can be lowered by pressurizing the _N2 gas atmosphere is not clear, but pressurization makes the solid solution of N atoms more effective, and the increase in free electrons due to the substitution of C atoms and N atoms becomes more pronounced. This is thought to be due to the purpose of The reason why the density is set to 80% or more of the theoretical density is because the breaking strength is too low if it is less than 80%, and the reason why the specific resistance is set to 1.0Ω-cm or less is that if it is more than 1.0Ω-cm, it is a power-saving heating element. This is because it is difficult to design as A more specific explanation will be given below with reference to Examples. Example 1 β-SiC powder with an average particle size of 0.3 μ and 0.5 μ
% of boron carbide powder is dispersed in an acetone solution containing 6% of phenolic resin per SiC powder, wet mixed, dried, passed through a sieve, pressed, and heated at 800℃ in vacuum.
The mixture was calcined to prepare a molded body. This molded body was sintered at 1800 to 2050°C for 60 minutes in an Ar flow to perform primary firing to obtain a primary sintered body. After measuring the density of the obtained primary sintered body, 8
Surface ground to x25 x 4mm, digital tester (1
The room temperature resistivity was measured using the four-point needle method (when the resistance was above Ωcm) or the four-point needle method (when the resistance was below 1Ωcm). FIG. 3 shows the relationship between primary firing temperature and specific resistance. Next, the primary sintered body was reheated in N ₂ at the temperature and pressure shown in Table 1 for 60 minutes to perform secondary firing and obtain a secondary sintered body. The density and room temperature specific resistance of the obtained secondary sintered body were measured in the same manner as in the case of the primary sintered body, and the bending strength by three-point bending was also measured. The results are shown in Table 1.

[Table] From the results in Table 1, by secondary sintering the primary sintered body sintered to 70-95% of the theoretical density in a pressurized nitrogen gas atmosphere, the density is 80% or more of the theoretical density. It was found that a sintered body having a specific resistance of 1 Ωcm or less can be obtained. Table 2 shows the characteristics of the conventional product. As can be seen from Table 1, the present invention provides a method for producing a silicon carbide heating element that has a specific resistance of 1.00 Ω-cm or less, a small negative temperature coefficient, and a large transverse rupture strength, and is of high industrial value. It has a wide range of uses as an igniter, igniter, and other heating elements for internal combustion engines. Example 2 Tests were conducted in the same manner as in Example 1 except that the SiC raw material was SiC consisting of 5% α-SiC and 95% β-SiC, and the results were as shown in Table 3.

【table】

[Table] As a result, a small amount of α-SiC was added to β-SiC.
It was found that even when SiC was used, the performance was not much different from when only β-SiC was used.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the firing temperature and resistivity during secondary firing, and Figure 2 shows the relationship between nitrogen gas pressure and resistivity when the primary sintering was performed at 1950°C and the secondary sintering was performed in nitrogen. It is. FIG. 3 is a diagram showing the relationship between primary firing temperature and specific resistance.

Claims (1)

[Claims] 1. SiC powder with an average particle size of 1.0μ or less and 0.3 to 3.0μ
Add boron or a boron compound in an amount equivalent to 0.1 to 6.0 weight % of carbon, and mix and mold in vacuum or in an inert atmosphere to a theoretical density of 70%. Primary firing to sinter to ~95% and then ~1500 in a pressurized nitrogen gas atmosphere
Specific resistance 1.0Ω over 80% of theoretical density at a temperature of 2300℃
A method for producing a silicon carbide heating element comprising secondary firing of resintering to form a silicon carbide heating element of -cm or less. 2. A method for manufacturing a silicon carbide heating element according to claim 1, wherein the atmospheric pressure during secondary firing is more than 1 atmosphere (atmospheric pressure) and less than 200 atmospheres. 3. A method for manufacturing a silicon carbide heating element according to claim 1, wherein the atmospheric pressure during secondary firing is 1.5 atm or more and 100 atm or less.