Glow-plug having ceramic base matrix and conducting element dispersed therein

#The present invention concerns ignition glow-plugs in which the basic matrix phase of both the conducting and insulating elements is made of a same ceramic, electrical conductivity of the conducting elements being provided by particles of one or more comminuted conductive materials dispersed in said matrix phase. The ignition glow-plugs of this invention are usable as fast response ignition plugs in high-compression thermal engines, e.g. Diesel engines. The invention also deals with a method for fabricating ceramic ignition glow-plugs.

#To start high-compression engines under cold conditions, one uses electrical ignition glow-plugs which must reach the operational temperature (1000.degree. C. or more) before the starter motor is switched on. Now, the time required to preheat glow-plugs may last, depending on the outside temperature, from a few seconds to several tens of seconds because the heating element of the plug has a substantial degree of thermal inertia; hence one has sought to reduce the delay as much as possible by using very large heating currents as well as automated systems for controlling this current when the desired temperature is attained, thereby avoiding premature deterioration of the plug. When a glow-plug normally operates under the foregoing conditions, it is subject to high stress and thermal shocks which threaten to prematurely end its operating life.

#Moreover, when the motor is in normal operation, the fuel combustion effects in the cylinders followed by the rapid cooling due to the outflow of exhaust gases will also contribute, together with the heat developed by the glow-plug, to generate thermal oscillations which may result in cracking and premature failure of the plug components, especially if the thermal expansion factors of the insulating and conducting components are markedly different from one another.

#The invention is illustrated by embodiments of glow-plugs represented in the annexed drawing.

#FIG. 1 is a schematic cross-sectional view of a glow-plug according to the invention.

#FIG. 2 is a radial cross-section along line II--II of FIG. 1.

#The glow-plug represented schematically in FIG. 1 consists essentially of a heating substrate or body comprising a conductor element 1 and an insulator element 2, both elements being made of a base ceramic matrix of a same nature, e.g. of alumina. The conductor element is made of a cermet of alumina and chromium powder of particle grade 1-5 .mu.m incorporated in the ceramic in a volume proportion of 20-40%. The heating body is provided with a connection wire 3 and it is securely sealed in a tubular casing or socket 4 which also contains threaded portion 5 and an axial threaded rod 6 tightened by an annular gasket 7 of insulating material; the wire 3 is welded to the rod 6 which is also provided, externally to the casing 4, with an insulating washer 8, a nut 9 and a lock-nut 10.

#To manufacture the plug, the element 1 of electroconductive cermet is first made by extrusion of a cermet paste as a soft rod which is bent 180.degree. and inserted into a green alumina matrix forming the insulator 2; then the whole cermet-ceramic composite is heated according to usual ceramic making conditions to effect co-sintering of both elements 1 and 2. The sintered heating body is then inserted into casing 4 and fastened therein by usual sealing means (crimping), such that the external surface of element 1 be in positive electrical contact with the inside surface of socket 4. Then the remaining elements of the glow-plug are installed and assembled according to conventional practice.

#Naturally, the ceramic of the insulator element 2 of this embodiment can also include, in dispersed form, a thermally conductive additive which imparts thereto enhanced thermal conductivity and reduces the thermal expansion differences between the conductor 1 and insulator 2 elements; this additive can be a proportion of chromium powder, the particles of which are provided with an insulating layer of chromium oxide.