JPH044715B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing 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 heating element for a glow plug. 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 metal heating elements, the operating temperature is
The temperature limit is about 1000 to 1100°C, and at higher temperatures, oxidation corrosion, melting, etc. occur, making it unusable. Silicon carbide (SiC) can be used up to 1,600°C, and molybdenum silicide (MoSi ₂ ) 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 1,300°C. As a result, it began to soften, causing problems in terms of high temperature strength and thermal shock resistance, and could not meet the required specifications for, for example, glow plugs for automobiles. SUMMARY OF THE INVENTION An object of the present invention is to utilize the extremely excellent heat resistance of molybdenum disilicide, eliminate the disadvantage of being susceptible to thermal shock, and provide a method for manufacturing a ceramic heater having a large temperature coefficient of resistance. The details of the present invention will be explained below based on experimental examples. Oxidation tests were conducted on various high-temperature materials shown in Table 1 in an electric furnace at 1000°C for 15 hours. The weight change rate and initial resistivity before testing of each material are shown in the same figure. The samples used were material powders that were sintered to a density of 90% or more of the theoretical density using hot pressing, and each sample was cut into the same shape.The rate of weight change was determined by washing the samples with acetone after the test and measuring the weight change. I asked. Note that a commercially available Ni-Cr material was used. From Table 1, it can be seen that SiC and MoSi ₂ have extremely good oxidation resistance, while other materials are considerably oxidized. However, SiC has a very high initial resistivity, making it extremely difficult to apply it to small heating elements such as glow plugs that are operated by a low-voltage power source. In Table 1, the reason why the weight change rate of ZrB ₂ is negative is recognized to be because the oxidized portion was peeled off during the acetone cleaning. From the above, MoSi ₂ is the most excellent 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 in the high temperature range of 1000°C or higher. In order to improve this drawback, the inventors invented a method for manufacturing a ceramic heater made of molybdenum disilicide and silicon nitride. However, there are manufacturing constraints in obtaining a mixed sintered body of MoSi ₂ and Si ₃ N ₄ , and in particular, it is important to note that the mixture of MoSi ₂ and Si ₃ N ₄ is strongly affected by the atmosphere during firing. discovered. Figure 2 explains the effect. Figure 2 shows that the starting materials listed in Figure 2 are mixed with a plurality of organic solvents, and a sheet formed by the doctor blade method is laminated.
1600℃×1Hr, 500Kg/in each atmosphere shown in the figure
Molybdenum disilicide (MoSi ₂ ) and molybdenum trisilicide (Mo ₅ Si ₃ ) produced after hot pressing at cm ²
This shows the ratio of However, this ratio is the ratio of the heights of the strongest peaks of MoSi ₂ and Mo ₅ Si ₃ in X-ray diffraction {MoSi ₂ (dA=2.02) Mo ₅ Si ₃ (dA=2.156)}, and is the true It's not a molar ratio. Alumina and magnesia spinel (MgAl ₂ O ₄ ) is added in small amounts as a sintering aid, and Mo metal
18.7 mol%, Si metal 37.5 mol%, Si ₃ N ₄ 43.8 mol%
After firing, the ratio is MoSi ₂ 30 mol%, Si ₃ N ₄ 70 mol%
corresponds to the ratio of From Figure 2, MoSi ₂ and Si ₃ N ₄
When hot pressing a mixture of
It turns out that Mo ₅ Si ₃ is generated, whereas in argon, most of it remains as MoSi ₂ .
In addition, when using 100% MoSi ₂ as the starting material,
Mo ₅ Si ₃ is hardly generated even in N ₂ .
Furthermore, it can be seen that the sintering aid has almost no effect on this phenomenon. Furthermore, when Si ₃ N ₄ is added to the starting materials Mo metal and Si metal, which are the raw materials for MoSi ₂ , Mo ₅ Si ₃ is generated in N ₂ and MoSi ₂ is generated in Ar.
generate. From the above, it was confirmed that the contribution of nitrogen to the reaction in which MoSi ₂ changes to Mo ₅ Si ₃ is very large, and that this reaction is significantly accelerated by the presence of silicon. In the case of 100% MoSi ₂ , the above reaction hardly occurs even in N ₂ , so _the
The conversion to Mo ₅ Si ₃ in the presence of N ₂ is considered to be characteristic in the mixture of MoSi ₂ and Si ₃ N ₄ . The reason why molybdenum silicide has excellent oxidation resistance is that when heated in an oxidizing atmosphere, a SiO ₂ 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 those with a low Si content have low oxidation resistance because less SiO ₂ is generated. _{MoSi2 and Mo5Si3}
When compared with _MoSi2 , the ratio of Si amount is _higher than that of Mo5
It is considerably higher than Si ₃ and therefore has excellent oxidation resistance. Therefore, considering oxidation resistance,
It is necessary to manufacture the heating element so as to eliminate the conversion of Mo ₅ Si ₃ . Figure 2 shows that by gradually increasing the nitrogen partial pressure,
Data showing the conversion of MoSi ₂ to Mo ₅ Si ₃ is also included (I to K). Conversion to Mo ₅ Si ₃ progresses as the nitrogen partial pressure increases. Figure 1 shows the heating element manufactured under the conditions shown in Figure 2.
This figure shows the change in resistance after the sample was polished to the dimensions shown in the figure and subjected to electrical heating. Each sample was heated to a temperature of 1400°C in a thin part with a diameter of 2 mm by applying electricity, and was continuously used for 100 hours. Figure 1 shows the ratio between the resistance value after durability and the initial resistance, and if it is 1.0, there is no change in resistance. In the case of Mo ₅ Si ₃ (Sample B), the resistance value increases compared to MoSi ₂ (Samples A and G). Furthermore, even when the nitrogen partial pressure is changed, an increase in resistance is observed starting from around sample K, which was manufactured under the condition of N ₂ partial pressure of 0.3 atm. Table 2 shows the rate of increase in resistance of the samples used in FIG. 1 with respect to temperature. 1000 of each sample
The ratio between the resistance value at °C and the resistance value at room temperature is shown. Mo ₅
As the ratio of Si ₃ increases, the rate of increase in resistance decreases. In a system that detects the resistance value of the heating element and feeds it back to control the temperature of the heating element, the higher the resistance temperature coefficient, the better; the control limit is (1000℃).
R at room temperature/R at room temperature) is about 2.3. From the above, it has been found that a heating element made of a mixed sintered body of MoSi ₂ and Si ₃ N ₄ needs to be manufactured in a non-oxidizing atmosphere with a nitrogen partial pressure of 0.3 atmospheres or less. It has been confirmed that helium, which is a non-oxidizing atmosphere that does not contain nitrogen, and argon are effective in preventing the decomposition of MoSi ₂ even in a vacuum atmosphere. In this example, MgAl ₂ O ₄ was used as a sintering aid, but
Alternatively, alumina (Al ₂ O ₃ ) or a mixture of alumina and magnesia (MgO) may be used. In addition, in this example, for example, _N2 partial pressure is 0.2 atm,
Although sintering was performed in an atmosphere with a partial pressure of Ar of 0.8 atm and a total pressure of 1 atm, it is also possible to sinter under a high pressure such as under a partial pressure of N ₂ of 0.2 atm and a partial pressure of Ar of 9.8 atm, a total of 10 atm. However, the _N2 partial pressure is still below 0.3 atmospheres in that case. Table 3 shows MoSi ₂ and Si ₃ N ₄ mixed in various proportions,
The characteristic values are shown for a sheet made by adding an organic solvent and hot pressed in argon gas at 1600°C for 1 hour and 500kg/cm ² . The bending strength is 1300℃
This is the load at which a three-point bending sample breaks or deforms.The bending strength test was carried out using a sample cut into a size of 40 x 3 x 4 mm at a crosshead speed of 0.5 mm/min. The coefficient of thermal expansion is the average coefficient of thermal expansion from room temperature to 800°C. From Table 3, it was confirmed that by adding Si ₃ N ₄ to MoSi ₂ , the coefficient of thermal expansion decreased and the strength increased. 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 obtained in this example maintains the excellent oxidation resistance of _MoSi2 , and
This heating element successfully solves the biggest drawback of heating elements composed of MoSi ₂ alone, which is that they are susceptible to thermal shock. When used as a car heater such as a glow plug, the resistance value of the heating element is 1 at room temperature.
It is in the range of ×10 ^-3 to 1 × 10 ^-1 Ωcm, and from Table 3
The amount of _MoSi2 will range from 5 to 50 mol%. In addition,
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, it is the most effective for improving strength, and
Since the optimal firing condition for MoSi ₂ is around 1600°C, it is not possible to add a large amount of a substance with a low melting point. Furthermore, as mentioned in this example,
Although a small amount of Mo ₅ Si ₃ is generated even if fired in a non-oxidizing atmosphere that does not contain nitrogen, most of it is in the state of MoSi ₂ , and the resistance value at 1000°C is the resistance value at room temperature. The heating element of this example was one that was 2.3 times or more the value. Further, a sintering aid may be added in addition to MoSi ₂ and Si ₃ N ₄ , but the addition may be made as long as it does not impede the effect of adding silicon nitride. In addition, when adding excessive Si metal in addition to MoSi ₂ and Si ₃ N ₄ , as long as the amount of excess Si metal is only a few percent, the oxidation resistance performance will not deteriorate significantly.
I don't mind. In the examples, Si metal and Mo metal were used as raw materials for forming MoSi ₂ , but if there are other raw materials for forming MoSi ₂ , they may be used as starting materials. Note that the temperature described in the present invention is a value measured with a pyrometer and is a value assuming emissivity (ε)=1.00. Furthermore, MoSi ₂ and Si ₃
A mixed sintered body of _N4 can only be produced by firing in a non-oxidizing atmosphere that does not contain nitrogen or a non-oxidizing atmosphere with a nitrogen partial pressure of 0.3 atm or less; in the presence of nitrogen above this condition, Mo. ₅ Generates a large amount of Si ₃ , which deteriorates oxidation resistance and lowers the temperature coefficient of resistance. Since there is no such reaction in the case of 100% MoSi ₂ , this reaction is particularly noticeable in a mixture of MoSi ₂ and Si ₃ N ₄ . By manufacturing under the above conditions, the resistance value at 1000°C is 2.3 times or more the resistance value at room temperature.

【table】

【table】

[Table] As described above, the present invention provides a ceramic heater that can utilize the extremely excellent heat resistance of molybdenum disilicide, the extremely excellent thermal shock resistance of silicon nitride, and has a large temperature coefficient of resistance. It has the excellent effect of being able to

[Brief explanation of drawings]

Figure 1 shows each heating element A~ produced under the conditions shown in Figure 2.
Characteristic diagram showing the resistance change ratio after polishing K to the dimensions shown in Fig. 1 and conducting an electric heating durability test, Fig. 2
The figure is a diagram for explaining the manufacturing conditions of each of the heating element samples A to K shown in FIG. 1.

Claims (1)

[Claims] 1. A material containing at least molybdenum disilicide and silicon nitride, and in which molybdenum disilicide accounts for 5 to 50 mol% in the binary system of molybdenum disilicide and silicon nitride, is made into a non-oxidized material that does not contain nitrogen. 1. A method for manufacturing a ceramic heater, characterized in that firing is performed in an atmosphere or in a non-oxidizing atmosphere containing nitrogen at a nitrogen partial pressure of 0.3 atmospheres or less.