1. Field of the Invention
The present invention is directed to a device for assembling two elements together, the elements being constituted of materials that have substantially different coefficients of thermal expansion, and, in use, are subjected to substantial temperature variations, in the order of several hundred degrees centigrade, causing differential expansions that result in stresses which can adversely affect either or both of the elements.
More specifically, the present invention is directed to a plate assembly composed of a refractory material mounted on a metallic wall, and, in particular, to a plate assembly, known as a "hot bed" in a divergent plasma nozzle. However, the present invention is not limited to such a particular embodiment, but finds a more general application to any assembly of plates in which the plates are subjected to substantial temperature variations. For example, the assembly of plates, planar or otherwise, can comprise a coating for a wall, which is either internal, such as the wall of a conduit or an enclosure, or which is external, and whose respective materials, such as a decorative plate and a corresponding receiving wall, are subject to substantial temperature variations.
2. Discussion of Background Information
The hot bed of a plasma nozzle is a substantially planar plate formed of a refractory material, such as a silicon foam, located in the continuity of a internal wall of the divergent nozzle, and is adapted to form a localized screen to the cooling system of the internal wall 21, enabling the level of available heat flow at the exit of the nozzle to be substantially increased.
Such a plate is usually fixed by adhesion onto a metallic sole which is in contact, by its opposing face, with the cooled structure of the nozzle with which it is affixed.
The main problem in this type of assembly is the substantial differential expansion between the plate composed of the refractory material and the metallic sole, with the plate being capable of being raised to temperatures reaching or exceeding 1700.degree. C., such phenomena occurring for prolonged durations, such as 30 minutes.
This is the reason why, for a hot bed approximately 50 centimeters long, it is normal to note an elongation differential between the bed and sole in the order of 2 to 3 mm, which obviously poses problems of retention, both for the refractory material of the bed and for the adhesive material of the bed on the metallic sole.
Such a problem is also encountered in other fields of application. One such application, for example, involves refractory coatings for conduits adapted to transport gas flows that are raised to temperatures as high as 2000.degree. C. to 2500.degree. C.
French Patent FR-2,577,471, by the same assignee, has proposed a type of coating for solving the problem of differential expansion. Specifically, in this document, the coating is composed of a refractory structure formed of several superposed layers having different thermal characteristics enabling differential expansions to be absorbed. However, this technology is complex and gives rise to substantially thick, multi-layered structures that are particularly inappropriate for incorporation of a hot bed in a plasma nozzle structure.