JPH06196252A - Ceramic heating element - Google Patents

Abstract

PURPOSE:To provide a ceramic heating element having the improved metal fatigue resistance, capable of being continuously operated for a long period at a high temperature, and excellent in durability and reliability. CONSTITUTION:A ceramic heaving element 1 is buried with a heating resistor 5 in a silicon nitride sintered body 2, and at least the heating resistor 5 at the highest heating section 6 is constituted of a heating resistance wire 7 made of an inorganic conducting material and densely wound into a coil shape. The outer shape of the heating resistance wire 7 is formed into an elliptic shape having the ratio between the short diameter 8 and the long diameter 9, i.e., oblateness, of 50% or above, and the hardness of a silicon nitride sintered body 3 inside the elliptic coil shape is made equal to or above the hardness of a silicon nitride sintered body 4 surrounding the outer periphery of the heating resistance line 7.

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 as a glow plug for starting assistance of a diesel engine, a heater for ignition of various combustion equipment and a heater for various heating equipment.

[0002]

2. Description of the Related Art Conventionally, as a glow plug and various heaters for ignition and heating used for accelerating the starting of a diesel engine, a heat resistant insulating powder is filled in a sheath made of a heat resistant metal, and nickel (Ni) is contained in the heat resistant insulating powder. ) -Chromium (Cr) or the like, a sheathed heater in which a heating resistor made of a high melting point metal wire is embedded, and various ignition devices 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.

Among them, as shown in FIG. 3, a silicon nitride sintered body 18 having a thermal shock resistance and a high temperature strength remarkably superior to those of other ceramics is used as a substrate of a heater, and generally tungsten (W) or molybdenum ( A heating resistor wire 19 made of a high melting point metal such as Mo) or a compound thereof is wound into a coil to form a heating resistor, which is embedded in the base and integrated as a heater at about 1000 ° C. It is widely used including the glow plug (see Japanese Patent Publication No. 63-27835).

[0006]

However, in the ceramic heating element 20, when the coil-shaped heating resistance wire bent in a substantially U shape is embedded in the base of the silicon nitride sintered body 18 to be integrated. As for the heating resistance wire 19 which is the maximum U-shaped maximum heat generating portion in which the tip portion is closely wound, the heating resistance wire 19 in which the raw material powder of the silicon nitride sintered material 18 is wound is closely packed. Since it is difficult to be filled, the pressure at the time of pressure firing is directly applied to the heating resistance wire 19 serving as the highest heat generating portion, and unnecessary strain remains on the heating resistance wire 19.

When the ceramic heating element 20 is used as a high temperature heater, its temperature is generally 1 when ignited.
The temperature is about 000 to 1300 ° C., but it is exposed to an ignited flame and reaches a high temperature of 1350 ° C. or higher. Due to long-term operation, the heating resistance wire 19 has a residual stress and nitriding inside the heating resistance wire 19. Since the silicon-based sintered body 21 is not sufficiently densified, non-uniform stress due to a difference in thermal expansion is further applied, cracks are generated in the heating resistance wire 19 due to metal fatigue, and an electric resistance value is increased. There is a drawback that the coil-shaped heat generating resistance wire 19 is broken and the durability is extremely deteriorated.

[0008]

SUMMARY OF THE INVENTION The present invention was developed in view of the above-mentioned drawbacks, and an object thereof is to provide a ceramic heating element having improved metal fatigue resistance and capable of continuous operation at high temperature for a long time and having excellent durability. To provide.

[0009]

In the ceramic heating element of the present invention, the outer shape of the heating resistance wire wound in a coil form the heating resistance wire made of an inorganic conductive material embedded in a silicon nitride sintered body. A ceramic heating element having an elliptic coil shape having a flatness of 50% or more is wound into an elliptical coil shape formed in the axial direction of the heating resistance wire of at least the portion corresponding to the highest heating portion of the ceramic heating element. In addition, the hardness of the silicon nitride sintered body filling the inner side is equal to or higher than the hardness of the silicon nitride sintered body surrounding the outer circumference of the elliptic coil-shaped heating resistance wire.

[0010]

According to the ceramic heating element of the present invention, the hardness of the silicon nitride sintered body inside the elliptical coil formed by the heating resistance wire of at least the highest heating portion of the ceramic heating element is the outer circumference of the heating resistance wire. Since the hardness is equal to or higher than that of the silicon nitride sintered body surrounding the heating resistance wire, at least the concentration of stress on the heating resistance wire of the highest heating part is relieved, and the pressing force during pressure firing is dispersed to generate the heating resistance wire. Acts to suppress the deformation of.

[0011]

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 is a sectional view showing an embodiment in which the ceramic heating element of the present invention is applied to a glow plug used for starting assistance of a diesel engine.
FIG. 2 is an enlarged cross-sectional view of the glow plug of FIG. 1 taken along the line AA.

In FIGS. 1 and 2, reference numeral 1 is a ceramic heating element in which a heating resistor 5 is embedded in a silicon nitride sintered body 2, and the heating resistor 5 of the highest heating portion 6 of the ceramic heating element 1 is shown.
Is composed of a heating resistance wire 7 made of an inorganic conductive material that is densely wound in a coil shape, and the outer shape of the heating resistance wire 7 is an elliptic coil having a ratio of a minor axis 8 to a major axis 9, that is, an oblateness of 50% or more. The hardness of the silicon nitride sintered body 3 having a shape and having an elliptical coil shape formed by the heating resistance wire 7 of the highest heat generating portion 6 is the same as that of the silicon nitride sintered body 4 surrounding the outer circumference of the heating resistance wire 7. It is equal to or higher than the hardness of.

Next, a cylindrical metal fitting 10 is externally fitted to the ceramic heating element 1 and led out as one electrode terminal, and the cylindrical metal fitting 10 and the mounting metal fitting 11 are brazed to form a negative electrode.
An electrode lead-out fitting 12 is provided at the rear end of the ceramic heating element 1.
Is brazed to the other electrode terminal and connected to the terminal rod 14 through the lead wire 13 to form a positive electrode. The terminal rod 14 is inserted with an insulating washer 15 such as Bakelite and fixed with a nut 16. A glow plug 17 in which the negative electrode of the mounting member 11 and the positive electrode of the terminal rod 14 are insulated is configured.

The coil-shaped heat generating resistance wire 7 made of an inorganic conductive material which constitutes the heat generating resistor 5 is made of a refractory metal such as tungsten (W), molybdenum (Mo), rhenium (Re) or an alloy thereof. , Tungsten carbide (WC), titanium nitride (TiN), molybdenum silicide (MoSi ₂ ) and zirconium boride (Zr
Wires made of carbides or nitrides of Group 4a, Group 5a, Group 6a such as B ₂ ) are suitable.

In evaluating the ceramic heating element of the present invention, first, a mixture of a raw material powder containing silicon nitride (Si ₃ N ₄ ) as a main component and an oxide of a Group 3a element as a sintering aid is added. A rod-shaped silicon nitride molded body is produced by a press molding method using the granulated material.

Next, a heating resistor consisting of a coil-shaped heating resistance wire formed by winding a tungsten wire having a diameter of 0.20 mm and a tungsten wire forming a lead wire portion connected to the heating resistance wire is nitrided. Oxides of Group 3a elements containing silicon as a main component, alumina (Al ₂ O ₃ ), molybdenum (Mo)
After being dipped in a slurry consisting of a sintering aid such as a silicide of Titanium (Ti) or the like and dried, it is placed on the molded body and the heating resistor to which the composition is attached is sandwiched. Another silicon nitride molded body having the same shape as described above was overlaid and fired under pressure.

After polishing the side surface of the thus obtained sintered body to expose a part of the lead wire, and after forming a metal film of nickel (Ni) or the like on at least the exposed portion by a metallizing method, a plating method, or the like. , A tubular metal fitting is fitted on the outside, and is joined with silver brazing in a reducing gas atmosphere to form a negative electrode, while the lead wire exposed at the end of the sintered body is an electrode composed of a wire or a cap-shaped metal fitting. Similarly, the metal fittings were joined by silver brazing and connected as a positive electrode, and a positive and negative electrode was led out to produce a ceramic heating element for evaluation having a diameter of about 3.4 mm.

Next, a part of the ceramic heating element for evaluation is used to cut the highest heat generating portion in which the heat generating resistance wire is tightly wound, and a short diameter of an elliptic coil shape formed by the heat generating resistance wire is formed from the cross section. Measure the major axis with a magnifying projector, calculate the flatness from the ratio of the minor axis to the major axis, and use a Vickers hardness tester to measure the inside and outside of the elliptic coil-shaped heating resistance wire to the silicon nitride sintered body. At least 5 points were measured and the average value was used as each hardness.

On the other hand, another ceramic heating element for evaluation was used, and a direct current of 14 V was energized for 1 minute to rapidly raise the temperature to 1450 ° C., then the energization was stopped and compressed air was blown for 1 minute for forced cooling. The high load endurance test in which the above process was one cycle was continued until the wire was broken and no heat was generated. The above results are shown in Table 1.

[0020]

[Table 1]

As a result, in the ceramic heating element of the present invention in which the hardness of the silicon nitride sintered body inside the elliptic coil-shaped heating resistance wire is equal to or higher than the hardness of the silicon nitride sintered body on the outer periphery, 18000 cycles. Withstands the above durability tests, and 8
It was confirmed that the time required to reach the temperature of 00 ° C. was within 2 seconds, which was also very excellent in the rapid temperature rising characteristics.

The ceramic heating element of the present invention is not limited to the above-mentioned embodiment, and for example, in order to prevent unnecessary strain from remaining in the heating resistance wire, at least the portion corresponding to the maximum heating portion. In the elliptic coil shape formed by the heating resistance wire wound in the above-mentioned coil, a wire pin or a pre-sintered body made of ceramics or the like may be attached, or they may be used in combination with the sludge. .

[0023]

As described above, according to the ceramic heating element of the present invention, the hardness of the silicon nitride-based sintered body inside the elliptic coil formed by the heating resistance wire of at least the highest heating portion of the ceramic heating element. Since the hardness is equal to or higher than the hardness of the silicon nitride sintered body surrounding the heat generating resistance wire, at least the heat generating resistance wire of the highest heat generating portion is not broken in a short time at all, and the metal fatigue resistance is improved. It is possible to provide a durable and reliable ceramic heating element capable of continuous operation at a high temperature for a long time.

[Brief description of drawings]

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

FIG. 2 is an enlarged cross-sectional view taken along line AA of the glow plug of FIG.

FIG. 3 is an enlarged cross-sectional view showing a main part of a conventional ceramic heating element.

[Explanation of symbols]

 1 Ceramic Heating Element 2, 3, 4 Silicon Nitride Sintered Material 5 Heating Resistor 6 Highest Heating Section 7 Heating Resistance Wire

Claims (1)

[Claims] 1. A ceramic heating element having an elliptic coil shape in which a heating resistance wire forming a heating resistor made of an inorganic conductive material embedded in a silicon nitride sintered body has an oblateness of 50% or more. The hardness of the silicon nitride sintered body filling the inside of the elliptical coil-shaped heat generating resistance wire of the heat generating portion is equal to or higher than the hardness of the silicon nitride sintered body on the outer periphery of the heat generating resistance wire. Heating element.