JPH044717B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a ceramic heater suitable for use in, for example, a preheating plug for a diesel engine. As a conventionally known heater of this type, there is a
As shown in Japanese Patent No. 84144, there is one that uses a test tube-shaped ceramic heating element. Such conventional devices have a problem in that they cannot effectively utilize the heat from the inner surface of the heating element. Therefore, the present inventors have already filed an application for a ceramic heater in which the ceramic heating element is made into a U-shape in cross section, and the inside of the slit in the U-shaped portion is opened to effectively utilize the heat inside the slit. There is. However, since it is a U-shaped heating element,
The length of the U-shaped portion on the inner surface of the slit of the heating element is inevitably shorter than the length of the U-shaped portion on the outer surface, resulting in a smaller path resistance. Therefore, electric power is concentrated on the U-shaped portion on the inner surface, and a large temperature difference is generated between the U-shaped portion on the outer surface and the U-shaped portion on the outer surface. As a result, in order to raise the surface temperature of the outer surface to a predetermined temperature, it is necessary to increase the temperature of the inner surface, but there is a problem in that more power than necessary is required. In view of the above-mentioned points, the present invention provides a ceramic heater in which the temperature difference between the inner and outer surfaces of a U-shaped heating element is minimized. According to the present invention, this objective is to set x and y in the range of 0.73≦x/y≦1.0, where x is the length of the U-shaped portion on the inner surface of the heating element, and y is the length of the U-shaped portion on the outer surface of the heating element. This is achieved by setting. Hereinafter, the present invention will be explained in detail using specific examples. First, to describe the overall structure of the ceramic heater, in Figures 1 and 2 a and b, 1
2 is a heating element, 2 is an electric insulator, and 3 is a U-shaped heating element. The electric insulator 2 is composed of a mixed composition of, for example, 70% by mole of Al ₂ O ₃ and 30% of Si ₃ N ₄ . The heating element 3 is made of, for example, MoSi ₂ 70 mol% and Si ₃ N ₄ 30
It is composed of a mixed composition with mol%. The electric insulator 2 and the heating element 3 are integrally joined, and the joining parts are the two ends of the U-shaped part of the heating element 3 and the inner surface of the U-shaped part. Reference numeral 4 denotes two thin plate-like leads made of heat-resistant metal such as Mo and W. These leads 4 are buried inside the electrical insulator 2 and are connected to the heat generating element 3.
The two ends are connected inside. Reference numeral 5 denotes a sleeve made of heat-resistant metal, and this sleeve 5 is brazed to the outer periphery of the electrical insulator 2 of the element 1 via a metallized layer (not shown). Reference numeral 6 denotes a mounting housing made of heat-resistant metal, which is connected to the sleeve 5 by brazing. One of the leads 4 is connected to the housing 6 through a sleeve 5, and the other lead 4 is connected to a center electrode 9 through a metal holding pin 7 and a metal cap 8. Note that 10 represents an electrically insulating bush and 11 represents a mounting nut. The heating element 1 described above is manufactured as follows. In other words, as shown in FIG.
Insert between sheets. Then, as shown by the arrows in the figure, hot pressing is performed at low temperature to adhere each sheet. After bonding, it is fired under high temperature and pressure to create a dense fired body. The operation of the ceramic heater configured as described above will be described. When the center electrode 9 is connected to a power source, for example, the positive electrode of an on-board battery, and the housing 6 is connected to the negative electrode of the battery, a current flows and the heating element 3 generates heat due to Joule heat. As a result, fuel injected from an injector (not shown) is ignited. Next, specific experimental results of the present invention will be explained. First, each dimension of the heating element described above is set to a to f as shown in Fig. 4, and the length of the U-shaped part on the inner surface of the heating element in Fig. 2a is x, and the length of the U-shaped part on the outer surface is set as x. When set to y, the part A in Figure 5 is set to 1000.
We investigated the temperature at the bottom when kept at ℃. The results of this experiment are shown in Table 1. In each experiment No., the resistance value was 0.13Ω, and the applied voltage was
The voltage was 9.3V, and a commercially available thermospot sensor was used to measure the temperature in parts A and B above. Set the spot of the luminous flux (1 mmφ) projected from inside the device at the center of each of the above A and B parts, and apply voltage until the heating element becomes red hot and the temperature indication by the thermo spot sensor becomes stable. Table 1 shows the average value of the temperature measurements taken 1 minute after the sample was left to stand and stabilized as the temperature of each part. In Table 1, No. 2 has a standard U-shaped heating element, and the other No. 3 to No. 20 have a heating element with a standard U-shape.

【table】

[Table] As shown in Figure 2a, the heating element has a wedge-shaped inner surface. As understood from this Table 1,
When the value of x/y is 0.73 or more, the temperature difference between the A part and the B part (see FIG. 5) of the heating element is very small. Note that the value of x/y may be 1.0, but if it exceeds 1.0, this means that the path of x has many irregularities, but in this case, the current flows along the shortest path, so there is no effect. It matches the case of 1.0, but is difficult to manufacture. In Table 1 above, select the heater with structure No. 2 (element resistance: 0.13Ω) and the heater with structure No. 3 (element resistance: 0.13Ω). The same voltage (9.3V) was applied to both, and the relationship between heating element temperature and energization time was observed. The results are shown in FIG. The method for measuring the temperature of the heating element is the same as the temperature measuring method in parts A and B of Table 1. As can be seen from FIG. 7, in the No. 3 heater, the temperature difference between the A part and the B part is much smaller than that in the No. 2 heater. Also, as understood from Figure 7, No.
Heater No. 3 also has improved heating speed. In this way, in the present invention, by setting x and y in the heating element in the range of 0.73≦x/y≦1.0, the temperature difference between the inner and outer surfaces of the heating element is extremely small, and the temperature is almost uniform. temperature and can also improve rapid heating properties. Next, the ceramic material constituting the heating element will be explained. Such ceramic materials include:
When this heating element is used for a diesel engine, it is necessary that the temperature coefficient of resistance is high. MoSi ₂ is optimal for meeting this requirement, but it has some drawbacks in high-temperature strength and thermal shock resistance. It has been found that to solve this problem, it is sufficient to add Si ₃ N ₄ . According to the inventor's experiments, _MoSi2 is 30 to 90
The mole% of Si ₃ N ₄ is preferably 70 to 10 mole%. _Si3N
When the content of the ceramic heater exceeds 70 mol%, the resistivity increases, while when it falls below 10 mol%, the effect of adding Si ₃ N ₄ disappears. Experimental data on various properties depending on the mixing ratio of MoSi ₂ and Si ₃ N ₄ are as follows. Oxidation resistance test: 1000℃ x 15hr, high strength in air: sample 40 x 3 x 4mm, loading rate 0.5mm/min 1300℃, 3-point bending test in air, load when sample breaks or significantly deforms shows. Thermal expansion coefficient: average thermal expansion coefficient from room temperature to 800℃

【table】

[Table] The present invention is not limited to the embodiments described above, and various modifications can be made as described below. (1) As shown in FIG. 6, the heating element may have a shape in which a recess G is provided at the bottom of a U-shaped portion and a corner portion A of the outer periphery is removed. With this shape, the temperature difference between the inner and outer surfaces of the heating element can be further reduced. (2) The material of the heating element is not limited to MoSi ₂ -Si ₃ N ₄ but may be any ceramic material that generates heat when energized, such as SiC or TiC. The temperature difference due to the values of x and y in the heating element is not due to the material,
Due to its U-shape, there is no need to limit it to a specific ceramic material. (3) Although an electrical insulator is located on the inside surface of the U-shaped part of the U-shaped heating element (inside the slit), it is of course possible to leave this inside surface open. (4) The lead may be a wire instead of a thin plate. (5) The present invention can be applied to various uses such as a fuel ignition heater for a space heater in addition to a preheating plug for a diesel engine.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing one embodiment of the present invention, and FIG.
The figure shows the heating element in Figure 1, and Figure 2 a.
is a front view, FIG. 2b is a side view, FIG. 3 is a perspective view for explaining the fabrication of the element in FIG. 2, and FIGS. 4 and 5 are heating element elements for explaining experimental examples in the present invention. FIG. 6 is a front view showing another embodiment of the present invention, and FIG. 7 is a characteristic diagram for explaining the present invention. 2... Electric insulator, 3... Heating element.

Claims (1)

[Claims] 1 A ceramic heater having a U-shaped heating element in cross section made of a ceramic material that generates heat when energized, where the length of the U-shaped portion on the inner surface of the U-shaped heating element is x, and the length of the U-shaped portion on the outer surface of the U-shaped heating element is x. A ceramic heater characterized in that x and y are set in the range of 0.73≦x/y≦1.0, where y is the length of the part.