JPH05234665A - Ceramic exothermic body - Google Patents

Abstract

PURPOSE:To enhance durability and reliability in service at high temperatures for a long time by embedding an exothermic resistor which is made of non-organic conductive material, in a silicone nitriding sintered body where the molar ratio of silicone deoxide content to the content of rear earth elements converted to an oxide base lies in a specified range. CONSTITUTION:A ceramic exothermic body 1 is so constituted that an exothermic resistor 2 is embedded in a silicone nitriding sintered body 3 where the molar ratio of silicone dexide (SiO2) content to the content of rear earth elements converted to an oxide base is 1.0 to 2.5. The resistor 2 is composed of non-organic conductive material, such as W, Mo and the like, the negative electrode of the resistor 2 is formed into a fitting 6, and the positive electrode of the resistor is also formed into a terminal bar 13, and they are insulated to form a glow plug. In addition, the exothermic body 1 is so constituted that the resistor 2 is rested on a silicone nitriding compact in a semi-circular shape in cross section, and a separate silicone nitriding compact is piled up over the aforesaid compact thereafter, so that they are pressed so as to be sintered while being integrated. This configuration thereby allows deposition resistant strength in service at high temperatures for a long time to be maintained, and thereby allows durability and reliability to be increased without the occurrence of cracks and the deterioration of textures.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high temperature ceramic heating element used for glow plugs for accelerating the starting of diesel engines, ignition heaters for various combustion equipment and heating heaters for heating equipment.

[0002]

2. Description of the Related Art Conventionally, as a glow plug used for accelerating the starting of a diesel engine and various ignition and heating heaters, a heat resistant insulating powder is filled in a sheath made of a heat resistant metal, and nickel ( A sheathed heater in which a heating resistor made of a refractory metal wire mainly composed of Ni) -chromium (Cr) or the like is embedded, and various igniters utilizing high-voltage spark discharge have been used.

However, since the sheathed heater transfers the heat of the heat-generating resistor through the heat-resistant insulating powder filled in the sheath made of heat-resistant metal, it is difficult to rapidly raise the temperature in a short time and, in addition, it has abrasion resistance. In addition to the problem of poor durability, various ignition devices using the spark discharge also cause radio interference such as noise at the time of ignition, lack of reliability from the viewpoint of reliable ignition, in the case of unignition There were drawbacks such as safety issues.

Therefore, the temperature can be rapidly raised in a short time, no radio wave interference occurs, and the ignition is surely performed to ensure safety, and it can be used for a long time regardless of the atmosphere, and is wear-resistant. As a highly reliable and highly reliable heating element, a ceramic heating element in which a heating resistor made of an inorganic conductive material is embedded in a ceramic sintered body has been widely used.

Above all, a silicon nitride sintered body having a heat shock resistance and a high temperature strength remarkably superior to those of other ceramics is used as a substrate of a heater, and in general, tungsten (W) is used.
A heating resistor made of a high melting point metal such as molybdenum or molybdenum (Mo) or a compound thereof is embedded in a substrate, or a heating resistor paste mainly containing the high melting point metal or a compound thereof is pattern-printed on the substrate. Those that are integrated by firing are widely used.

However, since the grain boundary phase of the sintered body generally forms a glass material having a low melting point, the ceramic heating element using the silicon nitride sintered body as the base material of the heater is heated by electric heating. When the heating element exceeds 1000 ° C., the strength of the silicon nitride sintered body is deteriorated due to the softening of the grain boundary phase and the structure is deteriorated due to the ion migration of the grain boundary phase due to the applied voltage. Due to the difference in thermal expansion between the bodies, there are drawbacks such as cracks in the silicon nitride sintered body near the heating resistor and poor oxidation resistance.

Therefore, by crystallizing the grain boundary phase of the silicon nitride sintered body, the strength deterioration of the ceramic sintered body due to the softening of the grain boundary phase at high temperature is prevented, and the grain boundary phase of the sintered body is reduced by the applied voltage. Proposed ceramic heating element that prevents ion migration and prevents the silicon nitride sintered body near the heating resistor from cracking and the silicon nitride sintered body itself from causing structural deterioration. (JP-A-1-
See Japanese Patent No. 3133362).

[0008]

However, the ceramic heating element based on the above-mentioned silicon nitride sintered body is a melilite phase represented by Si ₃ N ₄ .Y ₂ O ₃ having poor oxidation resistance and 4Y. Besides, a crystal phase of a YAM phase typified by ₂ O ₃ , SiO ₂ , Si ₃ N _{4 and the} like is generated, and besides, a glass material having a low melting point remains at the crystal grain boundaries even if it is a small amount.

Moreover, the glow plugs and ceramic heating elements as various ignition and heating heaters generally have a high temperature of 1000 to 1300 ° C. when ignited, and some of them are exposed to an ignited flame and exceed 1350 ° C. is there.

When continuous energization is carried out for a long time in such a situation, it is impossible to prevent the migration of the low melting glassy ions, and the crystals of the grain boundary phase are also oxidized to cause the ion migration. It causes a crack on the side surface of the exposed silicon nitride sintered body near the pipe-shaped metal fittings or causes deterioration of the structure, which shortens the life of the ceramic heating element rapidly and loses its function. There was a problem of lacking.

[0011]

SUMMARY OF THE INVENTION The present invention has been developed in view of the above-mentioned drawbacks, and an object thereof is to provide a ceramic heating element excellent in oxidation resistance and durability which can be continuously used at a high temperature for a long time. ..

[0012]

The ceramic heating element of the present invention is a ceramic heating element in which a heating resistor made of an inorganic conductive material is embedded in a silicon nitride sintered body containing a rare earth element and silicon oxide. The silicon nitride-based sintered body contains silicon oxide (S
The molar ratio of the content of iO ₂ ) is 1.0 to 2.5.

In the ceramic heating element of the present invention, the molar ratio of the content of silicon oxide (SiO ₂ ) to the content of the rare earth element in the silicon nitride sintered body converted to oxide, that is,
When the value of the content of SiO ₂ (mol%) / the content of the rare earth element converted to oxide (mol%) is less than 1.0, the melilite phase or the YAM phase is generated, and the crystal phase has an oxidation resistance. Since it is easily oxidized badly, the grain boundary phase is oxidized and volume-expanded due to ion migration caused by energization at high temperature for a long time, for example, several thousand cycles or less, and the exposed silicon nitride sintered body near the pipe-shaped metal fittings. On the anode side of the embedded heating resistor, the embedded heating resistor is apt to be oxidized, resulting in disconnection.

On the other hand, when the molar ratio exceeds 2.5, a low melting point glass represented by SiO ₂ glass is generated in the grain boundary phase in the crystal, and under the above-mentioned conditions of use, it takes about 1000 cycles. Due to the ion migration of the low melting point glass, the strength was deteriorated, and due to the difference in thermal expansion between the heating resistor and the silicon nitride sintered body, it was embedded on the exposed side surface of the silicon nitride sintered body near the pipe-shaped metal fitting. A crack is generated on the cathode side surface of the heating resistor.

Therefore, the molar ratio of the content of silicon oxide (SiO ₂ ) to the content of the rare earth element in the silicon nitride sintered body converted to oxide is 1.0 to 2.5, preferably 1.3.
Is specified in the range of ˜1.9.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The ceramic heating element of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows a ceramic heating element applied to a glow plug used to accelerate starting of a diesel engine according to an embodiment of the present invention.

In FIG. 1, reference numeral 1 is a ceramic heating element in which a heating resistor 2 is embedded in a silicon nitride sintered body 3, and the ceramic heating element 1 has a pipe-shaped metal fitting 4 having a step portion 8.
Is externally fitted and brazed so as to be connected to the lead portion 5 of the heat generating resistor 2 and led out as one electrode terminal. Further, at least a portion of the mounting metal fitting 6 at which the pipe-shaped metal fitting 4 and the mounting metal fitting 6 overlap each other. The pipe-shaped metal fitting 4 is provided so that there is a gap on the front end side of the mounting bracket 6 from the step seat 7 provided inside the front end.
The stepped portion 8 is electrically contacted and fixed to the stepped seat 7 provided inside the front end of the mounting member 6 through the conductive gasket 9.

On the other hand, at the rear end of the ceramic heating element 1,
The other heating resistor 2 brazed at the same time as the pipe-shaped metal fitting 4
The terminal rod 13 having a flange inserted in the insulator 12 is brought into contact with the end surface of the electrode lead-out portion 11 connected to the lead portion 10, and the rear end peripheral edge of the mounting bracket 6 is caulked and fixed by the end surface of the insulator 12. Then, the pipe-shaped metal fitting 4 and the mounting metal fitting 6 are pressure-bonded without brazing to form a negative electrode, and the electrode lead-out portion 11 and the terminal rod 13 which are brazed to the rear end portion of the ceramic heating element 1 are formed.
Similarly, pressure bonding is performed to form a positive electrode, and by fixing an insulating washer 14 such as Bakelite to the terminal rod 13 with a nut 15, the negative electrode of the mounting bracket 6 and the positive electrode of the terminal rod 13 are insulated. A glow plug is constructed.

Further, in the ceramic heating element 1, after the heating resistor 2 is placed on the silicon nitride molding 16 formed into a rod shape having a semicircular cross section as shown in FIG. 2, another nitriding is performed on the upper surface thereof. The silicon-based compact 17 is overlaid, pressure-fired and integrated.

In the ceramic heating element of the present invention,
Examples of the heating resistor made of an inorganic conductive material include high melting point metals such as tungsten (W), molybdenum (Mo), and rhenium (Re), as well as tungsten carbide (W).
C), titanium nitride (TiN), molybdenum silicide (MoSi ₂ ), zirconium boride (ZrB ₂ ), etc. 4a, 5a, 6a carbide or nitride wire or thin layer What was formed is used suitably.

In evaluating the ceramic heating element of the present invention, first, the specific surface area is 12 m ² / g and the amount of oxygen contained as inevitable impurities, that is, silicon oxide (SiO ₂ ) is 3.
A silicon nitride (Si ₃ N ₄ ) powder having a crystallinity ratio of 97% by weight or less, a rare earth element oxide and alumina (Al ₂ O ₃ ) as a sintering aid, and a silicon nitride material. Raw material powders having various amounts of silicon oxide (SiO ₂ ) for adjusting the amount of oxygen in the sintered body are wet mixed in a ball mill for 24 hours.

The slurry thus obtained is spray-dried and granulated, and rod-shaped silicon nitride compacts 16 and 17 each having a semicircular cross section are produced by a press molding method.

Next, on the plane of the molded body 16, a heating resistor 2 comprising a substantially U-shaped coiled tungsten wire and a tungsten wire forming the lead portions 5 and 10 connected to the coiled tungsten wire. Was placed, and another silicon nitride compact 17 having the same shape was stacked so as to sandwich the heat generating resistor 2 and was fired under pressure.

The side surface of the thus obtained sintered body is polished to expose a part of the lead portion 5, and a metal coating film of nickel (Ni) or the like is formed on at least the exposed portion by a metallizing method or a plating method. After that, it is inserted into the pipe-shaped metal fitting 4 and joined with silver brazing in a reducing gas atmosphere.

On the other hand, the lead-out portion 10 exposed at the end of the sintered body is similarly joined with an electrode lead-out portion 11 made of a wire by silver brazing, and then inserted inside the tip of the mounting metal fitting 6 to attach the same. Metal fitting 6, the pipe-shaped metal fitting 4, and ceramic heating element 1
A glow plug for evaluation was produced by pressure-bonding the electrode lead-out portion 11 and the terminal rod 13 brazed to the rear end portion thereof to form positive and negative electrodes.

At the same time, a silicon nitride sintered body obtained by pressure-calcining only the silicon nitride molded body under the same conditions was used as a sample for oxygen content analysis and bending strength evaluation, and a silicon nitride material was analyzed by an oxygen / nitrogen analyzer. The total amount of oxygen in the sintered body was measured, and the amount of oxygen as an unavoidable impurity contained in the silicon nitride (Si ₃ N ₄ ) powder and the oxide of a rare earth element added as a sintering aid and alumina (Al ₂ O 4 ₃ ) The amount of oxygen calculated from the amount was subtracted, and the amount of residual oxygen was calculated as reacting with silicon (Si) to form silica (SiO ₂ ).

On the other hand, the rare earth element was quantified by a wavelength dispersive X-ray microanalyzer, and the content of the rare earth element in terms of oxide was calculated.

Further, the bending strength at room temperature and 1400 ° C. was measured using the above-mentioned evaluation sample based on the JIS three-point bending strength test method. In addition, using the evaluation sample, X
The magnitude of the peak intensity was identified by the presence or absence of the crystal phase of the YAM phase by a line diffractometer.

On the other hand, an endurance test in which one cycle consists of energizing a glow plug for evaluation from a DC power source to rapidly heat it to a temperature of 1400 ° C., then stopping energization and blowing compressed air to forcibly cool it is one cycle. After carrying out 10,000 cycles, the resistance value before and after the durability test was measured to calculate the resistance change rate of the heating resistor.

Further, the presence or absence of cracks in the ceramic heating element was inspected by a fluorescent flaw detection method, and the surface condition of the ceramic heating element was observed with a microscope. The above results are shown in Tables 1 and 2.

[0031]

[Table 1]

[0032]

[Table 2]

As is clear from Tables 1 and 2, SiO
Sample Nos. 1, 10, 17, and 24 having a molar ratio of ₂ (mol%) / rare earth element oxide (mol%) of less than 1.0 are the exposed side surfaces of the silicon nitride sintered body near the pipe-shaped metal fittings. A crack is generated on the side surface of the embedded heating resistor on the side of the anode, and the heating resistor itself is disconnected.

Further, sample numbers 9, 16, 23 and 30 in which both the SiO ₂ (mol%) / rare earth element oxide (mol%) molar ratio exceeded 2.5 were measured at a high temperature of the silicon nitride sintered body. Has a low bending strength, and cracks are formed on the exposed side surface of the silicon nitride sintered body near the pipe-shaped metal fitting and on the side surface on the cathode side of the embedded heating resistor.

On the other hand, all of the silicon nitride sintered bodies of the ceramic heating element of the present invention maintained a high bending strength, and no change was observed in the electrical durability test.

[0036]

As described above, in the ceramic heating element of the present invention, the molar ratio of the content of silicon oxide (SiO ₂ ) to the content of the rare earth element converted to oxide is 1.0 to 2.5. Since the heating resistor made of an inorganic conductive material is embedded in the silicon nitride sintered body, the silicon nitride sintered body on either the positive or negative electrode side of the heating resistor can be maintained while maintaining high bending strength even at high temperatures. Also, it is possible to provide a ceramic heating element which is free from cracks and deterioration of the structure, has excellent oxidation resistance, and can be repeatedly used especially at high temperature for a long time and which has excellent durability and reliability. ..

[Brief description of drawings]

FIG. 1 is a partially broken cross-sectional view showing an embodiment in which a ceramic heating element according to the present invention is applied to a glow plug used to accelerate starting of a diesel engine.

FIG. 2 is a perspective view for explaining a manufacturing process of the ceramic heating element according to the present invention.

[Explanation of symbols]

 1 Ceramic heating element 2 Heating resistor 3 Silicon nitride sintered body

Claims (1)

[Claims] 1. A ceramic heating element in which a heating resistor made of an inorganic conductive material is embedded in a silicon nitride sintered body containing a rare earth element and silicon oxide, wherein the silicon nitride sintered body contains a rare earth element. A ceramic heating element, characterized in that the molar ratio of the content of silicon oxide (SiO ₂ ) to the content converted into oxide is 1.0 to 2.5.