JP2735725B2 - Ceramic heating element - Google Patents

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 a glow plug for accelerating the start of a diesel engine, an ignition heater for various kinds of combustion equipment, and a heating heater for a heating equipment.

[0002]

2. Description of the Related Art Conventionally, a heat-resistant metal sheath is filled with a heat-resistant insulating powder as a glow plug or a heater for various ignition and heating, which is conventionally used for accelerating the starting of a diesel engine. Seed heaters in which a heating resistor composed of a high melting point metal wire mainly composed of Ni) -chromium (Cr) or the like is embedded, and various ignition devices that use high-voltage spark discharge have been used.

[0003] However, since the sheathed heater transmits heat of the heating resistor through a heat-resistant insulating powder filled in a heat-resistant metal sheath, it is difficult to quickly raise the temperature in a short period of time, and furthermore, it has a high wear 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 reliability from the viewpoint of reliable ignition, There are drawbacks such as a problem with safety.

Therefore, it is possible to quickly raise the temperature for a short time, to prevent radio interference, and to ensure the safety by igniting reliably, to be able to be used for a long time regardless of the atmosphere and to have abrasion resistance. As a highly reliable and highly durable 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.

[0005] Among them, a silicon nitride sintered body, which is significantly superior in thermal shock resistance and high-temperature strength to other ceramics, is used as a base of a heater and is generally made of tungsten (W).
Or a heat-generating resistor made of a high-melting metal such as molybdenum (Mo) or a compound thereof is embedded in a substrate, or a heat-generating resistor paste mainly containing the high-melting metal or a compound thereof is pattern-printed on the substrate. What is integrated by sintering is widely used.

However, the ceramic heating element using the silicon nitride sintered body as a base of the heater has a grain boundary phase of the sintered body generally forming a glass material having a low melting point. When the temperature of the heating element exceeds 1000 ° C., the strength of the silicon nitride-based sintered body is degraded due to softening of the grain boundary phase, and the structural deterioration is caused by the ion transfer of the grain boundary phase due to the applied voltage. Due to the difference in thermal expansion of the body, cracks occur in the silicon nitride sintered body in the vicinity of the heating resistor, and there are defects such as poor oxidation resistance.

Therefore, by crystallizing the grain boundary phase of the silicon nitride-based sintered body, strength deterioration of the ceramic sintered body due to softening of the grain boundary phase at a high temperature is prevented, and the grain boundary phase due to an applied voltage is prevented. A ceramic heating element has been proposed that prevents ion migration and prevents the silicon nitride-based sintered body near the heating resistor from cracking or causing the silicon nitride-based sintered body itself to cause structural deterioration. (Japanese Unexamined Patent Publication No.
No. 313362).

[0008]

However, the ceramic heating element based on the silicon nitride-based sintered body has a melilite phase or 4Y which is poor in oxidation resistance and is typified by Si ₃ N ₄ .Y ₂ O _3. A crystal phase of a YAM phase typified by ₂ O ₃ · SiO ₂ · Si ₃ N ₄ or the like is generated, and in addition, a small amount of low-melting glassy material remains at a crystal grain boundary even if it is a small amount.

Moreover, the glow plug and the ceramic heating element as various heaters for ignition and heating generally have a high temperature of 1000 to 1300 ° C. at the time of ignition, and some of them exceed 1350 ° C. when exposed to the ignited flame. is there.

If continuous energization is performed for a long time in such a situation, the low-melting glassy ion transfer cannot be prevented, and the crystal of the grain boundary phase is also oxidized to prevent the ion transfer. Causes cracks on the exposed side surface of the silicon nitride sintered body near the pipe-shaped metal fittings and causes structural deterioration.The life of the ceramic heating element is shortened sharply and loses its function. There was a problem of lack.

[0011]

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

[0012]

A ceramic heating element according to 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 has a silicon oxide (S
molar ratio of the content of iO ₂₎ is characterized in that 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 based sintered material in terms of oxide, that is,
If the value of the content of SiO ₂ (mol%) / the content of the rare earth element in terms of oxide (mol%) is less than 1.0, the melilite phase or the YAM phase is formed, and the crystal phase has oxidation resistance. Because it is badly oxidized, ion migration occurs at high temperature for a long time, for example, when current is applied for several thousand cycles or less, and the grain boundary phase is oxidized and expanded in volume. Cracks occur on the side surface of the buried heating resistor and on the side surface on the anode side of the buried heating resistor, and especially the buried heating resistor is easily oxidized and disconnected.

On the other hand, if the molar ratio exceeds 2.5, a low-melting glass typified 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 glass, the strength deteriorated and the thermal expansion difference between the heating resistor and the silicon nitride-based sintered body caused the silicon nitride-based sintered body to be exposed and buried near the pipe-shaped metal fitting. Cracks occur on the side of the heating resistor on the cathode side.

Therefore, the molar ratio of the content of silicon oxide (SiO ₂ ) to the content of the rare earth element in the silicon nitride based sintered material in terms of oxide is 1.0 to 2.5, preferably 1.3.
To 1.9.

[0016]

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

In FIG. 1, reference numeral 1 denotes a ceramic heating element in which a heating resistor 2 is embedded in a silicon nitride sintered body 3, and a pipe-shaped metal fitting 4 having a step 8 is provided on the ceramic heating element 1.
Is externally fitted and brazed so as to be connected to the lead portion 5 of the heating resistor 2, and is led out as one electrode terminal. Further, at least at the portion where the pipe-shaped metal fitting 4 and the mounting metal 6 overlap, at least the mounting metal 6 The pipe-shaped metal fitting 4 is provided so as to have a gap on the tip side of the mounting fitting 6 from the step 7 provided inside the tip.
The stepped portion 8 is electrically connected and fixed to the stepped seat 7 provided inside the front end of the mounting bracket 6 via a 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 fitting 4
A terminal rod 13 having a flange inserted through an insulator 12 is in contact with an end surface of an electrode extraction portion 11 connected to the lead portion 10 of the mounting member 6, and a rear end peripheral edge of the mounting bracket 6 is fixed by caulking at an end surface of the insulator 12. Then, the pipe-shaped fitting 4 and the mounting fitting 6 are pressure-bonded without brazing to form a negative electrode, and the electrode extraction portion 11 and the terminal rod 13 brazed to the rear end of the ceramic heating element 1.
Similarly, a positive electrode is formed by pressure bonding, and a negative electrode of the mounting bracket 6 and a positive electrode of the terminal rod 13 are insulated by fixing an insulating washer 14 such as bakelite to the terminal rod 13 with a nut 15. A glow plug is configured.

The ceramic heating element 1 has a heating resistor 2 placed on a silicon nitride molding 16 having a semicircular bar shape as shown in FIG. The silicon moldings 17 are stacked and fired under pressure to be integrated.

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

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 as an unavoidable impurity to be contained, ie, silicon oxide (SiO ₂ ) is 3%.
Weight percent or less, a silicon nitride (Si ₃ N ₄ ) powder having a crystal conversion rate of 97%, a rare earth element oxide and alumina (Al ₂ O ₃ ) as a sintering aid, and silicon nitride Raw material powders having various amounts of silicon oxide (SiO ₂ ) for adjusting the amount of oxygen in the sintered body are wet-mixed by a ball mill for 24 hours.

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

Next, a heating resistor 2 composed of a coiled tungsten wire having a substantially U-shape and tungsten wires constituting the lead portions 5 and 10 connected to the coiled tungsten wire is formed on the plane of the molded body 16. Was mounted, and another silicon nitride-based molded body 17 of the same shape was stacked and fired under pressure so as to sandwich the heating resistor 2.

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

On the other hand, an electrode extraction portion 11 made of a wire is similarly joined to the lead portion 10 exposed at the end of the sintered body by using a silver solder, and then inserted into the tip end of the mounting bracket 6. Metal fitting 6, pipe-shaped metal fitting 4, and ceramic heating element 1
The electrode take-out part 11 and the terminal rod 13 which were soldered to the rear end were press-bonded to form positive and negative electrodes, respectively, to produce a glow plug for evaluation.

At the same time, the silicon nitride sintered body obtained by sintering only the silicon nitride molded body under pressure under the same conditions is used as a sample for oxygen content analysis and bending strength evaluation. The total oxygen content of the sintered body was measured, and the oxygen content as inevitable impurities contained in the silicon nitride (Si ₃ N ₄ ) powder, the oxide of the rare earth element added as a sintering aid and alumina (Al ₂ O) ₃ ) The amount of oxygen calculated from the amount was subtracted, and the amount of oxygen was calculated assuming that the remaining amount of oxygen reacts 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 flexural strength at room temperature and 1400 ° C. was measured by using the evaluation sample according to the JIS three-point bending strength test method. Further, using the evaluation sample, X
The magnitude of the peak intensity was determined by the use of a line diffractometer together with the presence or absence of the crystal phase of the YAM phase.

On the other hand, an endurance test was conducted in which a glow plug for evaluation was energized from a DC power supply, rapidly heated to a temperature of 1400 ° C., and then stopped after being energized and blown with compressed air for forced cooling. After 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 2
Sample Nos. 1, 10, 17, and 24 having a molar ratio of ₂ (mol%) / rare earth element oxide (mol%) of less than 1.0 show the exposed side surface of the silicon nitride sintered body near the pipe-shaped fitting. In addition, cracks occur on the side surface on the anode side of the embedded heating resistor, and the heating resistor itself is disconnected.

Samples Nos. 9, 16, 23, and 30 in which the molar ratio of SiO ₂ (mol%) / rare earth element oxide (mol%) both exceeded 2.5 were obtained at a high temperature of the silicon nitride sintered body. Cracks are formed on the side surface of the exposed silicon nitride sintered body near the pipe-shaped fitting and on the cathode side of the buried heating resistor.

On the other hand, the silicon nitride sintered body of the ceramic heating element of the present invention maintained high bending strength, and no change was observed in the current 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 in terms of oxide is 1.0 to 2.5. Since the heating resistor made of an inorganic conductive material is embedded in the silicon nitride-based sintered body, while maintaining high bending strength even at high temperatures, the heating resistor can be mounted on the silicon nitride-based sintered body on either the positive or negative electrode side of the heating resistor. In addition, it is possible to provide a ceramic heating element having excellent durability and reliability which can be used repeatedly for a long time at a high temperature without generation of cracks, deterioration of the structure, and excellent oxidation resistance. .

[Brief description of the 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 for accelerating the start 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]

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

 ────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-252973 (JP, A) JP-A-5-36470 (JP, A) JP-A-5-1817 (JP, A) 259781 (JP, A) JP-A-4-370690 (JP, A) JP-A-4-124467 (JP, A) JP-A-63-88777 (JP, A) JP-A-2-20293 (JP, U)

Claims (1)

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