Glow plug with improved composite sintered silicon nitride ceramic heater

A method of making an improved ceramic heater for a glow plug, comprising the steps of:

• i) preparing a first mixture containing silicon nitride, at least one sintering additive selected from the group consisting of yttrium oxide, aluminium oxide and aluminium nitride, and 33-50 vol. % electrically conductive ceramic component selected from the group consisting of titanium nitride, titanium carbonitride and molybdenum disilicide, and thereafter casting a first ceramic tube, having a closed end and an internal diameter, made of said first mixture;
• ii) preparing a second mixture containing silicon nitride, at least one sintering additive selected from the group consisting of yttrium oxide, aluminium oxide and aluminium nitride, and 46-75 vol. % electrically conductive ceramic component selected from the group consisting of titanium nitride, titanium carbonitride and molybdenum disilicide, and thereafter casting a ceramic rod having a diameter which is less than said internal diameter of said ceramic tube cast in step i), made of said second mixture;
• iii) attaching the cast rod so obtained within said first cast tube in the proximity of the closed end, thereby forming an annular space around said rod and obtaining a unitary casting of an electrically resistive ceramic member;
• iv) preparing a third mixture containing silicon nitride, at least one sintering additive selected from the group consisting of yttrium oxide, aluminium oxide and aluminium nitride, and 0.5-28 vol. % electrically conductive ceramic component selected from the group consisting of titanium nitride, titanium carbonitride and molybdenum disilicide, and thereafter casting a second ceramic tube having a diameter and a wall thickness substantially corresponding to said annular space formed in step iii), thereby obtaining a tubular insulating member, subsequently inserting said tubular insulating member into said annular space, thereby obtaining a unitary casting of a ceramic heater;
• v) sintering the unitary casting so obtained, thereby obtaining a sintered unitary ceramic heater having a core composition, as defined in step ii), a insulating member sintered to said core having a composition as defined in step iv) and an outer tube sintered to said tubular insulating member and to one end of said core, having a composition as defined in step i).

An improved glow plug for a diesel engine comprising, a metallic housing adapted to support a ceramic heater and having means to provide electrical contact to said heater, and being connectible to a power source, and a unitary ceramic heater comprising,

• a rod composed of silicon nitride, at least one sintering additive selected from the group consisting of yttrium oxide, aluminium oxide and aluminium nitride, and 46-75 vol. % electrically conductive ceramic component selected from the group consisting of titanium nitride, titanium carbonitride and molybdenum disilicide,
• a tubular insulating member surrounding said rod and being bonded to said rod, composed of silicon nitride, at least one sintering additive selected from the group consisting of yttrium oxide, aluminium oxide and aluminium nitride, and 0.5-28 vol. % electrically conductive ceramic component selected from the group consisting of titanium nitride, titanium carbonitride and molybdenum disilicide, and
• an outer tube having a closed end, the inner surface of said outer tube being bonded to said tubular insulating member and to one end of said rod, composed of silicon nitride, at least one sintering additive selected from the group consisting of yttrium oxide, aluminium oxide and aluminium nitride, and 33-50 vol. % electrically conductive ceramic component selected from the group consisting of titanium nitride, titanium carbonitride and molybdenum disilicide.