JPH029269Y2 - - Google Patents

Description

[Detailed explanation of the idea]

(Field of Industrial Application) The present invention relates to a ceramic heater such as a glow plug for an internal combustion engine, particularly a glow plug suitable for a diesel engine. Currently, glow plugs are used in diesel engines as starting parts at low temperatures, and there is a demand for small, quick-heating glow plugs to improve the startability of diesel engines. Most conventional glow plugs are sheathed type, in which a heating wire made of Ni-Cr alloy is wound into a coil, and this is placed inside an outer cylinder with one end closed and made of stainless steel or a heat-corrosion-resistant alloy such as Inconel. Furthermore, the area around the heating element is filled with an insulating material such as magnesium oxide. Therefore, in this type, heat conduction between the heating wire and the outer cylinder occurs through the insulating material, so it takes time for the surface of the outer cylinder to become red-hot to ignite the air-fuel mixture, and the preheating time is long. There is a problem with becoming. Therefore, it was proposed that the outer surface of the glow plug itself be made of a heating element.
Cases have been reported in which heat-resistant metal heating elements such as Ni, platinum (Pt), and molybdenum (Mo) are used, but with the exception of platinum, all of these metals deteriorate due to oxidation at around 1000°C, making them impractical. Furthermore, since platinum is also expensive, there are problems in its application to mass-produced products. In view of the above circumstances, the inventors conducted various experimental studies to find materials suitable for use as heating elements in ceramic heaters, and found that molybdenum silicide MoSi ₂ has sufficient high-temperature oxidation resistance as a heating element in ceramic heaters. It was confirmed that the material satisfies the above requirements and also has excellent rapid heating properties. Therefore, the present invention provides a ceramic heater having a molybdenum silicide heating element on its outer surface. The present inventors conducted oxidation resistance tests on molybdenum silicide and various other high melting point materials in order to determine their suitability as heating elements exposed on the outer surface of ceramic heaters. The test pieces cut out to a common size were left in the air at 1000℃ for 15 hours.
Oxidation resistance was investigated by weight change. Results first
Shown in the table.

[Table] As shown in Table 1, SiC and MoSi ₂ have extremely low weight changes and excellent oxidation resistance.
It is. Compared to these, heat-resistant metals have inferior oxidation resistance, and when used as the outer surface heating element of a ceramic heater, they change over time and are not practical. Although Pt has good oxidation resistance test results,
Due to its high price, it is not suitable for mass production. SiC has good oxidation test results, but has a high resistance of 200Ω-cm.
It cannot be used with small heating elements such as ceramic heaters whose input power is as low as 12 to 24V. On the other hand, MoSi ₂ has a low resistance value of 4.2×10 ^-5 Ω-cm, and a required resistance value of 0.1 to 1.5 Ω can be sufficiently achieved with a small heating element such as a ceramic heater. MoSi ₂ also has good oxidation resistance and can be used satisfactorily as the outer surface heating element of ceramic heaters. As a heating element for a ceramic heater, it is desirable that the temperature coefficient of resistance is large. When the temperature coefficient of resistance is large, a large current flows in the initial stage of energization, and as the temperature of the heating element rises, the resistance increases, limiting the current value and preventing overheating. Figure 1 shows the relationship between the temperature and energization time of heating elements with different temperature coefficients of resistance, and those with a large temperature coefficient of resistance (line a) are able to initially pass a large current compared to those with a small temperature coefficient of resistance (line b). and rapid overheating is possible. Next, Table 2 shows the temperature coefficient of resistance of the main high melting point materials.

[Table] As shown in Table 2, MoSi ₂ has a large temperature coefficient of resistance, making it possible to increase the initial current value and rapidly heat it compared to conventional Ni-Cr heating elements. In this way, MoSi ₂ has excellent oxidation resistance,
MoSi ₂
was recognized as the only heating element suitable for practical use as a heating element for external surface heating type ceramic heaters. Furthermore, a mixture (sintered body) in which MoSi ₂ is mixed with at least one of Si ₃ N ₄ , SiC, and A ₂ O ₃ has improved strength compared to MoSi ₂ alone, and is effective as a ceramic heater heating element. Table 3 shows the properties of these mixtures and comparative materials. The test conditions are as follows. Oxidation resistance test: 1000℃ x 15hr high temperature strength in air; sample
40 x 3 x 3 mm, loading rate 0.5 mm/min, 1300°C, 3-point bending test in air, showing the load when the sample was destroyed or significantly deformed. Coefficient of Thermal Expansion: Average coefficient of thermal expansion from room temperature to 800<0>C. All samples were prepared with 70% by weight _MoSi2 and 30% by weight of other mixtures ( _Si3N4 , _SiC , _{A2O3 )} .
All mixtures have good oxidation resistance and high temperature strength.
Larger than MoSi ₂ alone. The specific resistance at room temperature increases,
The coefficient of thermal expansion decreases in the case of mixtures with Si ₂ N ₄ and SiC.

[Table] Examples of the present invention shown in the drawings will be described below. Figure 2 shows a heat generating section having molybdenum silicide or a heat generating element mainly composed of molybdenum silicide.
The figure shows an example of a glow plug equipped with the heat generating part. As shown in FIG. 2, the heat generating portion 1 is made of sintered plates 11a and 11b of molybdenum silicide, sintered plates 12a and 12b of an electrically insulating ceramic material such as silicon nitride, and a heat-resistant metal such as tungsten. The silicon nitride sintered plates 12a and 12b constituted by the metal wires 13a and 13b have stepped portions and are thicker on the proximal end side. Moreover, a groove 121b is formed in the vertical direction on the surface of one of the sintered plates 12b. Bent portions 131a, 131b in the right angle direction are formed at the tips of the metal wires 13a, 13b. In order to manufacture the heat generating part 1, the metal wire 13
a, 13b in the groove 131b of the silicon nitride sintered plate 13b.
The sintered plate 12a is placed on top of the sintered plate 12a. At that time, the bent portion 131a of the metal wires 13a, 13b,
131b are passed through holes 122a and 122b provided at the tips of the sintered plates 12a and 12b, respectively, and the tips are bent. The molybdenum silicide sintered plates 11a, 11b are stacked on the upper and lower tip sides of the silicon nitride sintered plates 12a, 12b stacked in this way, and the sintered plates 11a, 11b are pressed and fired (hot pressed) in the sandwiching direction. 12a, 12b, and 11b are joined together and integrated. In the glow plug shown in FIG. 3 in which the heat generating part 1 thus obtained is assembled, the mounting part 2 mainly consists of a metal housing 21 configured to be attached to an engine head, and an insulating material 22 inside the metal housing 21.
The positive terminal 23 is fixed via the positive terminal 23. The base end of the heat generating part 1 is inserted into the tip opening of the housing 21, and the molybdenum silicide sintered plates 11a and 11b, which are heat generating elements, are inserted into the housing 2 through the metal cover 24.
1 and the opening are fixed to form a body ground. At the base end of the heat generating part 1 inserted into the housing 21, a metal cap 25 is joined to the end faces of the sintered nitrided sintered plates 12a, 12b so as to be in contact with the metal wires 13a, 13b. It is electrically connected to 23 by a stainless steel wire. However, in the plug having the above structure, the current flows from the positive terminal 23 to the stainless steel wire 26 and the metal cap 2.
5. It reaches the tips of the molybdenum silicide sintered plates 11a, 11b through the metal wires 13a, 13b, and is connected to the body ground from the cover 24 to the housing 21 via the sintered plates 11a, 11b. In the example of the present invention configured as described above, when the resistance of the heating element is 0.1Ω, the time taken for the outer surface to reach 800°C with 12V applied was measured.
It showed an extremely small value of 1.4 seconds. Furthermore, after a durability test in which the outer surface of the heating element was heated to 1000°C and continuous current was applied for 200 hours, the resistance value did not change at all, the oxidation resistance was good, and continuous heat generation was sufficiently possible. In addition, as a manufacturing method for the heat generating part, powder or green sheet is used as the heat generating material and the insulating material, and the heat generating material, the insulating material, the metal wire, the insulating material, and the heat generating material are sequentially laminated in a mold of a predetermined shape, and then hot pressed. Pressure firing may be used. As mentioned above, when molybdenum silicide is used as a heating element, it has better oxidation resistance and durability than the characteristics of molybdenum silicide, and has low resistance, so the ceramic heater can be made smaller, and the resistance temperature Since the coefficient is large, it has excellent rapid heating properties. In the ceramic heater of the present invention, the heat generating part is a hot press sintered heat generating part, so the heat generating element and insulating material that make up the heat generating part are sintered to almost the theoretical density, and the heat generating part itself has a dense structure. Therefore, it is possible to prevent oxygen from entering the inside of the heat generating part. As a result, it is possible to suppress oxidation of the heat generating part, and in addition, it is possible to suppress oxidation of the lead member inside the heat generating part, which is an excellent effect. Therefore, the ceramic heater of the present invention has excellent oxidation resistance.

[Brief explanation of the drawing]

Figure 1 shows heating elements with different temperature coefficients of resistance and the relationship between temperature and energization time. Figures 2 and 3 show an embodiment of the present invention. Figure 2 shows the heat generated by a glow plug. FIG. 3 is a longitudinal sectional view of the glow plug. 1...Heating part, 11a, 11b...Heating element, 12
a, 12b... central member, 13a, 13b, 26
...Metal wire, 2...Mounting part, 21...Housing.

Claims (1)

[Claims for Utility Model Registration] (1) A mounting part that is attached to the engine body, and a hot press sintered heat generating part that is held in the mounting part and ignites fuel, and the heat generating part is made of electrically insulating ceramic. It is composed of a central member and a heating element made of molybdenum silicide provided on the outer surface of the central member, and a lead member is disposed inside the central member, and the lead is placed in contact with the heating element. A ceramic heater in which the heating element is connected to a current supply means through a member. (2) The heating element is composed of molybdenum silicide, silicon nitride,
The ceramic heater according to claim 1, which is constituted by a sintered body of a mixture to which at least one of silicon carbide and alumina is added.