JPS58128687A - 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 This invention relates to a conductive ceramic heating element that has excellent oxidation resistance, good spalling characteristics and high-temperature strength, and can be used as a heating element for electric furnaces, glow plugs, etc. It is.

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 KFi operating temperature is 100
0 to 1100℃ is the limit for papermaking, and higher temperatures cause oxidation corrosion, fusing, etc., making it unusable.In the case of silicon carbide (StO), the temperature is 1600℃, and molybdenum silicide (m
os5) can be used up to about 1800°C, but silicon carbide has extremely high resistivity, so there is a delay in miniaturization, and molybdenum silicide begins to soften at temperatures above 1300°C, resulting in high-temperature strength, S+N-IJ There are problems with the performance of
For example, it was not possible to meet the required specifications for glow plugs in automobiles.

It is an object of the present invention to provide a ceramic heating element that utilizes the extremely excellent oxidation resistance of molybdenum silicide, eliminates the drawback of softening at high temperatures, lowers the coefficient of thermal expansion, and improves spalling characteristics. It is. To achieve this objective, the present invention combines molybdenum silicide, which is a conductive material with excellent oxidation resistance, with silicon nitride, which is a 11C insulating material with a small thermal W# film coefficient and has an excellent heat resistance.
515M< ) is mixed and sintered to provide a ceramic heating element. More specifically, the present invention provides a ceramic heating element formed by sintering a mixture of silicon nitride and molybdenum silicide in an amount of 70% by weight or less (hereinafter simply referred to as %).

The purpose of the present invention is to provide a ceramic heating element which has improved characteristics as described above and which can be sintered under normal pressure. *os), mixture of aluminum oxide and magnesium oxide (Mg0), spinel of aluminum oxide and magnesium oxide (Mg*ts
A ceramic heating element is provided by sintering a mixture containing at least IIIll of o 4 ) with the balance being molybdenum silicide.

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

Chemical tests were conducted on each of the high-temperature materials shown in Table 1 in an electric furnace at tl-1000°C for 15 hours.

The weight change rate and initial resistivity before testing of each material are shown in the same table. The samples used were material powders sintered by hot pressing to a theoretical density of 90 or higher, each cut into the same shape.The rate of weight change was determined by washing the samples with acetone after the test and changing the temperature by 171 degrees. It was determined by measurement. Note that a commercially available Ni-0r material was used.

From Table 1, it can be seen that S10 and Mo51g have extremely good oxidation resistance, and the other materials are considerably oxidized.

However, SiO has a very high initial resistivity, and if a heating element is formed using it, the size of the heating element increases, making it impractical. In the table, although the weight change rate of MoB1 is i-minus, it is recognized that this is due to the oxidized portion being peeled off during the acetone cleaning.

Table 2 shows the physical properties of sintered crystals obtained by mixing powders of Mo11, Si, and N4 in the proportions of 1 and 2 and sintering by hot pressing. As in Table 1, the weight change rate is 1 in air.
This is the rate of weight change when an oxidation test was conducted at 000'C for 15 hours, and the bending strength is the load when the sample was broken or deformed in a 3-point bending test at 1300°C. The bending strength test was carried out using a sample cut into a 40×3×4 size at a crosshead speed of α5/min.

The coefficient of thermal expansion is the average coefficient of thermal expansion from room temperature to 800'C. The sample is pressure 1501I910ISQ degree 1500~17
The material was sintered in a hot press at 0°C to a degree of sintering (bulk density/theoretical density) of 90% or more.

As shown in Table 2, the weight change due to oxidation is the same for all samples as in the case of Mo5txjl, 81, M,
It can be seen that there is almost no amount due to contamination. The specific resistance of 811N is completely insulating when it is alone (Mo81i=O), but when Mo5it is mixed in, it rapidly decreases, and when Mo
81g It decreases to a practical level at 30% or more. The high-temperature bending strength increases rapidly as the mixing ratio of 81°M4 increases, and the thermal expansion coefficient becomes 81. As the proportion of M4 increases, it decreases.81. N, approaches that of itself.

In this way, Mo81m and 81. Although the sintered body of the N4 mixture has a specific resistance that is higher than that of Mo81m alone, it shows a practical resistivity.

The addition range of N is wide, and 81. High level of 1 due to the contamination of N.
It exhibits excellent effects such as increased strength, decreased coefficient of thermal expansion, and improved spreading properties. The amount of Si and N in the mixture should be less than 70%; if it exceeds 70%, an interlayer will appear in practice due to an increase in specific resistance. Also,
Even if the amount of 81sN4 added is small, it increases the bending strength and lowers the thermal expansion coefficient than Mo 81m alone.
Although it improves the ring properties, it is desirable to add Filo% or more to significantly improve these properties.

Next, although the sintered body of the mixture of Mo81B, sl, and M4 exhibits excellent properties as described above, it cannot be sintered unless it is sintered under pressure such as hot press or hot isostatic press. Incidentally, in experiments conducted by the inventors, when the above mixture was fired under normal pressure, the degree of sintering was at most 70%. As a result of further experimental research, the inventors added kl to the above mixture.

0” SA l @ mixture of Os and MgO, 'IKA
It was confirmed that a mixture containing at least 1 scoop of 1 and 0 can achieve a high degree of sintering even when fired under normal pressure.

The figure shows the experimental results. 1 to d are pressureless sintering,
・～h is a hot press, and the mixture composition is a and ・ both 50% Mo55 + 50% Si, N, , b and f are 45 dominant uos1=+50 *Si3N4+
5*AjlOn, 0 and g 45% Mo51g
+5 0% Si3N4+ 54 (MgO+personl.

0, ), d and h are 45% MO3il+5(lf8
1゜N4+5〦MgA! , 04. Normal pressure firing conditions were N.

The hot pressing conditions were 1,600° C. under 1 atm pressure of 150° C. for 2 hours, and 1,600° C. under 150/9/clll pressure for 1 hour.

As is known from
It exhibits an extremely good degree of sintering compared to that of solid steel. In addition, in the case of hot pressing, the degree of sintering increases when the 4-E additive is added. Table 3 shows the weight change rate and bending strength due to oxidation when MgAl*Omk was added.

Table 3 As in the case of Table 2, good results were obtained.

Note that the amount of the alumina-based additive added may be 1096 or less, and if it exceeds that amount, properties such as high temperature strength will deteriorate. Even if the amount of alumina-based additives added is small, it shows a certain tendency to sinter, but it is desirable to add three or more.

As explained above, the heating element of the present invention is a sintered body made by mixing a predetermined proportion of silicon nitride with molybdenum silicide and sintering the mixture, and has excellent oxidation resistance and high strength. It has a small coefficient of thermal expansion and good spalling properties. Furthermore, a ceramic heating element is produced by adding aluminum oxide, a mixture of aluminum oxide and magnesium oxide, and at least one camphor of these spinels to molybdenum silicide and silicon nitride in a predetermined proportion, and sintering the mixture. It has excellent properties and exhibits an extremely high degree of sintering even when pressureless sintering is used. With pressureless sintering, it is possible to produce products with irregular shapes, and the cost is low.

[Brief explanation of the drawing]

The figure shows the degree of sintering of products of the present invention and comparative products by pressureless sintering and real press. \;character/

Claims (1)

[Claims] (1) Conductive ceramic heating element made by sintering a mixture of 70% by weight or less of silicon nitride and the balance of molybdenum silicide (z) qoyitt*% or less of silicon nitride, 10% by weight or less of aluminum oxide, aluminum oxide A conductive ceramic heating element made by sintering a mixture of aluminum oxide and magnesium oxide, at least one portion of spinel of aluminum oxide and magnesium oxide, and the balance molybdenum silicide.