The present invention relates to a ceramic heater employed as a glow plug in a diesel engine or the like, especially to a ceramic heater provided with a heater element having excellent oxidization resistance and thermal shock resistance, and high bending strength at a high temperature.
Conventionally, the heater element has been made of heat resistant alloy such as nickel-chrome alloy, iron-chrome-aluminum alloy or ceramic such as silicon carbide or molybdenum silicide.
However, the heater element made of the above described heat resistant alloy has a problem to be oxidized or melt at a temperature above 1100.degree. C.
The heater element made of silicon carbide can be used at a temperature under 1600.degree. C. but the specific resistance thereof is too high for making the heater element small.
The heater element made of molybdenum silicide is superior in oxidization resistance but inferior in bending strength at a high temperature and thermal shock resistance.
Therefore, the heater element made of the above described material cannot be used in an atmosphere of which temperature drastically changes in a high temperature range.
The heater, for example a glow plug which is used for preheating an air-fuel mixture at a starting time of a diesel engine, is especially required to exhibit a rapidly heating property that is to generate heat immediately upon receiving an electric current in order to improve the startability of an engine.
Most of the conventional glow plugs for diesel engines are of sheath type. In the glow plug of this type, a heating wire such as nickel-chromium alloy is wound like a coil and is disposed within a bottomed outer casing made of thermal corrosion resistant ally such as stainless steel, or inconel. Within the outer casing, an insulating material such as magnesium oxide is charged so as to surround the coil shaped heating wire.
In this glow plug, the heat of the heating wire is transmitted to the outer casing through the insulating material so that it takes a time to heat the surface of the outer casing enough to ignite the air-fuel mixture of a diesel engine. Therefore, the glow plug of this type is short of a rapidly heating property so that speedy startability of a diesel engine cannot be obtained.
In order to overcome this problem, it has been proposed to dispose a heater element made of heat resistant metal such as tungsten, nickel, platinum or molybdenum on an outer surface of the glow plug.
The glow plug of this type exhibits a rapidly heating property but various properties are lowered at about 1000.degree. C. due to the oxidization except for that comprising the heater element made of platinum.
Therefore, the glow plug of this type is not practical.
And platinum is too expensive to be applied to the mass production.
Furthermore, when the heater element is formed of a sintered body of ceramic such as silicon carbide or molybdenum silicide, the sintering operation has been conventionally performed under pressure by means of a hot press or hot isostatic press. Therefore, conventionally, it is difficult to produce a heater element made of a ceramic sintered body having a complex shape.
From the above circumstances, it has been required to produce a heater element made of a ceramic sintered body which has excellent oxidization resistance, and thermal shock resistance and high bending strength at a high temperature by atmospheric sintering (pressureless sintering).