Electrically conductive ceramic material

Abstract: An electrically conductive ceramic material containing at least Mo₅Si₃C is disclosed. Furthermore, a self-controlling type glow plug is disclosed wherein a resistor for controlling supply of electric current to a heating element is connected in series to the heating element. This self-controlling type glow plug comprises an electrically conductive ceramic material for the heating element containing at least Mo₅Si₃C and a material for the resistor having a larger resistance-­ temperature coefficient than that of the heating element.

Description: The invention relates to an electrically conductive ceramic material. Chem.Abstr. No. 104:134624f, Vol. 104 (1986), page 291 discloses in connection with solid-state interfacial reaction in molybdenum-carbide system ceramics at high temperature and pressure and its application to bonding technology a ceramic material having a Mo₅Si₃C layer. Proc. of the 10th Plansee-Seminar 1981, Vol.1, Ed: H.M. Ortner, Van Loo et al, pages 145-146 discloses also ceramic materials having a thin Mo₅Si₃C layer. This publication refers to phase relations and diffusion paths in the Mo-Si-C-System at 1200°C. US-A-4 634 837 discloses a sintered ceramic electric heater suitably applicable to a glow plug of a diesel engine, wherein the heater element is formed as a sintered body of a mixture of MoSi₂ powder having an average diameter not greater than 2µm and 35 to 75 Mol% of Si₃N₄ powder. Hitherto, ceramic heating elements have been prepared by combining and dispersing electrically conductive ceramics such as TiN and MoSi₂ and insulating type ceramics such as Si₃N₄ and sintering these combined ceramics. However, a sufficiently reduced resistance-temperature coefficient could not be obtained in a ceramic heating element of such a type as described above, because attempts to reduce the resistance-temperature coefficient meet inherent limitations due to usual influence of the specific temperature-resistance coefficient of electrically conductive ceramics such as TiN and MoSi₂. For example, in a known self-controlling type glow plug provided with the above-described type ceramic heating element and a resistor for controlling electric current connected in series to said element. Since the resistance-temperature coefficient of the element cannot be sufficiently reduced, the heating element itself acts as the resistor when the temperature of the heating element is raised, the amount of electric current to be supplied to the heating element cannot be controlled properly with the resistor for controlling electric current, and satisfactory temperature-rising characteristics cannot be obtained. An object of the present invention is to provide an electrically conductive ceramic material of an extremely reduced resistance-temperature coefficient, and as well to obtain a self-controlling type glow plug exhibiting excellent temperature-rising characteristics. The first part of the object is achieved by the features in claim 1. According to the present invention, a self-controlling type glow plug is provided wherein a resistor for controlling the supply of electric current to a heating element is connected in series to the heating element, the self-controlling type glow plug comprising an electrically conductive ceramic material at least containing Mo₅Si₃C, and a material for the resistor having a larger resistance-temperature coefficient than that of the heating element. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view illustrating a glow plug according to one embodiment of the present invention. Fig. 2 is a graph showing the electricity supplying characteristics of the glow plug of Fig. 1. Fig. 3 is a cross-sectional view illustrating another glow plug according to the present invention. Fig. 4 is a graph illustraing the temperature-rising characteristics of the glow plug of Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENTS Since Mo₅Si₃ proposed as the conventional electrically conductive ceramic material contains an excessive amount of Mo atoms, its resistance-temperature coefficient cannot be sufficiently reduced. Furthermore, since MoSi₂ contains an excessive amount of Si atoms in which the ratio of Mo atoms is smaller than in Mo₅Si₃, the resistance-temperature coefficient cannot be reduced sufficiently. The present inventors have found that Mo₅Si₃C can serve as a ceramic material exhibiting a sufficiently reduced resistance-temperature coefficient. The present invention employs Mo₅Si₃C as electrically conductive ceramic material wherein Mo₅Si₃C can be expressed more accurately by the formula Mo 5-x Si₃C 1-Y (0 ≦ X ≦ 2, 0 ≦ Y < 1). The reason why X is 2 or less lies in that the two atoms of the five Mo atoms can easily give rise to lattice defects. Furthermore, the reason why Y is smaller than 1 also lies in that C atom can easily give rise to lattice defects. Since the normal case in which no lattice defects are generated can be realized with a composition expressed by Mo₅Si₃C, the "Mo₅Si₃C" will be used hereinafter for the above-described formula. A sufficiently reduced resistance-temperature coefficient can be obtained with Mo₅Si₃C since the content of Mo atoms in Mo₅Si₃C is smaller than that in Mo₅Si₃, that is, the content of Mo atoms is not excessive, and the content of Si atoms in Mo₅Si₃C is smaller than that in MoSi₂, that is, the content of the Si atoms is not excessive. Furthermore, a self-controlling type glow plug exhibiting an excellent temperature-rising characteristics can be obtained by using this ceramic material as the heating element. That is, the resistance can be made sufficiently larger than the resistance of the resistor at the initial stage of the electricity supply to the glow plug. On the other hand, the resistance of the heating element can be made smaller than the resistance of the resistor at the time when the electricity supply to the same is controlled, causing the heating rate of the heating element to be increased and an excessive rise in the temperature of this heating element to be prevented. Example 1 A method of preparing Mo₅Si₃C will now be described. As shown in Table 1, 0.5 to 3.0 wt% of carbon black was added to Mo₅Si₃ powder having a mean particle size of 1 µm, and then they were mixed and pulverized in a ball mill. The thus-obtained material was burned in a hot press at 1700°C for 60 minutes under a pressure of 300 kg/cm² in an atmosphere of Ar at 1 atm. Thus, a ceramic body was prepared. The obtained ceramic bodies with the compositions of Mo₅Si₃C of samples 1 to 6 were heated up to 900°C under vacuum. Then, the volume resistivity of each of the samples was obtained by a 4-terminal method, and the change rate with respect to the volume resistivity at room temperature was obtained. The results are shown in Table 1. In addition, the results of some samples obtained by burning MoSi₂, TiN, and Mo₅Si₃ under the same conditions are also shown in Table 1 as comparative examples. As is seen from Table 1, as the quantity of the carbon black added to Mo₅Si₃ increases, a quantity of Mo₅Si₃C having the completely different structure from Mo₅Si₃ increases in addition to Mo₅Si₃ in the burned ceramic. As a result, a further smaller resistance-temperature coefficient than the smallest resistance-temperature coefficient ever realized by the Mo₅Si₃ conventionally used was obtained. Furthermore, when the carbon black was added in a quantity exceeding 2.0 wt%, Mo₅Si₃ was substantially converted to Mo₅Si₃C, causing a satisfactory stable resistance-temperature coefficient. The thus obtained stable resistance-temperature coefficient of the electrically conductive ceramic material had a sufficiently reduced value which was substantially 1/5 to 1/20 of the resistance-temperature coefficient obtained by the electrically conductive ceramic materials according to the Comparative Examples 1 to 3. Although Mo₅Si₃C was obtained by adding carbon black to the starting material Mo₅Si₃ in the embodiment above, the present invention should not be limited to this embodiment. For example, a similar effect can be obtained from a material obtained by mixing Mo fine particles, Si fine particles, and carbon fine particles at a predetermined proportion. Example 2 Next, a ceramic heater was manufactured by using the electrically conductive ceramic Mo₅Si₃C. In order to realize the atomic ratio of Mo:Si:C = 5:3:1, Mo₅Si₃ having a mean particle size of 1 µm and carbon black were mixed, and the thus mixed powder and Si₃N₄ having a mean particle size of 10 µm or 1 µm were mixed in order to realize Si₃N₄:Mo₅Si₃C = 20 to 80:80 to 20. Then, the material to which each 5 wt% of Y₂O₃ and Al₂O₃ were added as a burning aid was wet-mixed for 12 hours in a ball mill. As a solvent, ethanol was used. The thus mixed powder was burned in a hot-press at 1700°C for 60 minutes under a pressure of 300 kg/cm² and in an atmosphere of Ar at 1 atm. The thus obtained samples 7 to 17 were subjected to measurements of resistivity and change rate of resistance as shown in Example 1. The results are shown in Table 2. In addition, the bending strength σ of these samples is also shown. Furthermore, the characteristics of the ceramic heaters formed by Si₃N₄ and MoSi₂ are respectively shown as Comparative Examples 4 to 5. As is seen from Table 2, a sufficiently reduced resistance-temperature coefficient of 5.0 x 10⁻⁴ deg⁻¹ or less was realized in the electrically conductive ceramic material with respect to the case where MoSi₂ was used by dispersing Mo₅Si₃C serving as the electrically conductive ceramic material in Si₃N₄ serving as the insulating ceramic material and by simultaneously burning the thus dispersed materials. Furthermore, ceramics for a heater having a desired resistivity with the thus realized low resistance-temperature coefficient maintained was obtained. Although Si₃N₄ was used as the insulating ceramic material, the present invention should not be limited to the example above. All materials having an insulating property may be used in the present invention. Therefore, for example, Al₂O₃, AlN, and ZrO₂ can realize the similar effects. Example 3 Fig. 1 is a cross-sectional view illustrating a glow plug in which the electrically conductive ceramic material according to the present invention is employed. As shown in Fig. 1, this glow plug has such a structure that heater 1 of a U-shaped cross section is formed within the end of support member 2 having a circular cross-sectional shape. Furthermore, the support member 2 is provided with electrically conductive wires 3a and 3b of tungsten embedded therein, of which the ends are connected to the heater 1. Metallic pipe 4 is attached around the support member 2, and the end of metallic housing 5 in a cylinder form is connected to the metallic pipe 4. The other end of the electrically conductive wire 3a extends to the base portion of the support member 2 and is connected to metallic cap 6 installed around this base portion, so that the electrically conductive wire 3a is connected to a power source (omitted from illustration) via this metallic cap 6 and nickel wire 7. On the other hand, the other end of the electrically conductive wire 3b is connected to a metallic sleeve. The support member 2 and the metallic pipe 4 were coupled by applying nickel plating to the surface thereof and performing blazing of the plated surface. The pipe 4 and the housing 5 were coupled by blazing. The composition of the material for the heater 1 consisted of 70 wt% of Si₃N₄ having a mean particle size of 10 µm and 30 wt% of Mo₅Si₃C having a mean particle size of 1 µm. The composition of the material for the support member 2 consisted of 72 wt% of Si₃N₄ having a mean particle size of 1 µm and 28 wt% of MoSi₂ having a mean particle size of 1 µm. As a burning aid, Y₂O₃ and Al₂O₃ were added in each amount of 5%. In this example, the composition of the material for the support member 2 was arranged in such a manner that the thermal expansion coefficient of the material of the support member 2 and that of the heater 1 became the same. By virtue of employment of an Si₃N₄ powder having a mean particle size of 10 µm for the material of the heater 1, Mo₅Si₃C particles surrounded the Si₃N₄ particles, so that the Mo₅Si₃C particles were provided in a continuous manner as electrically conductive structure. Furthermore, by virtue of employment of Si₃N₄ particles having a mean particle size of 1 µm as the material for the support member 2, the electrically conductive MoSi₂ particles were surrounded by the insulating Si₃N₄ particles, so that the former were separated from one another. As a result, an electrically insulated structure was formed. The reason why Si₃N₄ having a mean particle size of 10 µm was used as the material of the heater 1 lies in that reduction in coefficient of thermal expansion and improvement in bending strength are intended by reducing, as described in Example 2, the quantity of Mo₅Si₃C which needs to be provided for the purpose of obtaining the desired resistivity. The reason why the MoSi₂ particles are dispersed in the material for the support member 2 lies in that the thermal stress which will be generated between the material for the heater 1 and that for this support member 2 is intended to be reduced by making its coefficient of thermal expansion equal to that of the material of the heater 1. Fig. 2 is a graph which illustrates an electric supplying characteristics of this ceramic heater. As is seen from Fig. 2, since the heater 1 containing Mo₅Si₃C was used, excellent characteristics for a constant electricity consumption type glow plug could be obtained having a satisfactory reduced resistance-temperature coefficient realized in the material for the heater 1. The structure of the glow plug is not limited to that employed in this example. Any ceramic heaters containing at least electrically conductive ceramic material Mo₅Si₃C may be employed. Although the ceramic heater can be obtained by mixing the electrically conductive ceramic material comprising Mo₅Si₃C and the insulating ceramic material, the electrically conductive ceramic material according to the present invention can be used for many other-purposes, for example, it may be used in a resistor employed as a multilayered substrate or in wiring on substrates. Example 4 The self-controlling type glow plug according to the present invention is schematically shown in Fig. 3. To the components in Fig. 2 are given the same reference numerals as shown in Fig. 1. Reference numeral 10 represents a ceramic element. The ceramic element 10 is structured in such a manner that heater 1 serving as the heating portion and electrodes 3a and 3b are formed therein, as shown in Eaxmple 3. The electrode 3a disposed within this ceramic element 10 is electrically connected to housing 5 via sleeve 4. On the other hand, the electrode 3b is electrically connected to second resistor 15 via cap 6 and lead wire 7. The other end of the resistor 15 is connected to electrode 8. This resistor 15 is made of a material with a larger resistance-temperature coefficient than that of the heater 1, for example, it is made of Fe or Ni. Then, the operation of this self-controlling type glow plug will be described with reference to Fig. 4. When the self-controlling type glow plug according to this example is heated by applying an electricity thereto, the temperature of the heater 1 is rapidly raised at the initial stage of the electricity supply since the resistance of the heater 1 is sufficiently larger than that of the resistor 15. However, as is seen from Fig. 4, the resistance of the resistor 15 becomes, in 40 seconds after the start of electricity supply, sufficiently larger than that of the heater 1, and an excessive supply of electricity to the heater 1 is prevented and the overheat of the heater 1 can be effectively prevented. Furthermore, the R.T. coefficient (ratio of a resistance at 900°C and that at 20°C) of the heating portion of the self-controlling type glow plug according to this example can be improved to be substantially 1.20, as compared with substantially 1.69 achieved by a glow plug in which a metallic heating element (tungsten-rhenium) is embedded therein. Therefore, a self-controlling type glow plug exhibiting an excellent temperature-rising characteristics is obtained, whereby time taken for the heating portion to rise to 900°C is significantly shorted from conventional value of 4.0 seconds to 3.0 seconds. According to the present invention, a ceramic material exhibiting a sufficiently reduced resistance-temperature coefficient can be obtained by using Mo₅Si₃C as the electrically conductive ceramic material. Furthermore, a self-controlling type glow plug structured by a heating portion and a control portion made of a material with a larger resistance-temperature coefficient than that of the material for the heating portion can be made a self-controlling type glow plug exhibiting an excellent temperature-rising characteristics since the heating portion thereof is made of an electrically conductive ceramic material containing Mo₅Si₃C of which the resistance-temperature coefficient is sufficiently reduced.

Claims: An electrically conductive ceramic material comprising a dispersion of Mo₅Si₃C in an insulating ceramic. An electrically conductive ceramic material according to claim 1, prepared by burning a mixture of Mo₅Si₃C of a particle size of 1µm with a powder of an insulating ceramic of a particle size of 10µm. An electrically conductive ceramic material according to claim 1 or 2, wherein the insulating ceramic is selected from Si₃N₄, Al₂O₃, AlN and ZrO₂. An electrically conductive ceramic material according to claim 3, wherein the content of Mo₅Si₃C is 20%-80% by weight based on the total weight of Mo₅Si₃C and Si₃N₄, when Si₃N₄ is used as the insulating ceramic. An electrically conductive ceramic material according to any one of claims 1-5 which has a resistance-temperature coefficient of 5 x 10⁻⁴ deg⁻¹ or less. A self-controlling type glow plug wherein a resistor for controlling supply of electric current to a heating element is connected in series to said heating element, said self-controlling type glow plug comprising an electrically conductive ceramic material for said heating element containing the ceramic material of claim 1 and a material for said resistor having a larger resistance-temperature coefficient than that of said heating element.

Inventors: ITO NOBUEI, NUNOGAKI NAOCHIKA, TOYAMA TETUO, ITO, NOBUEI, NUNOGAKI, NAOCHIKA, TOYAMA, TETUO
Applicants: NIPPON DENSO CO, NIPPONDENSO CO., LTD.
Application year: 1989
Main classification: H01B1
IPC codes: C04B35, F23Q7