Uniform thermal treatment by dielectric hysteresis of composites

The production and shaping of composite materials generally require the temperature of the matrix's elementary reactants to be raised in line with their own phase changes and with the chemical transformations corresponding to the various desired characteristics (mechanical, optical, electrical, etc.). The chosen temperature rise can be achieved either by heat transfer from a hot source located outside the objects made of the materials, or from a heat source created within the objects themselves.

The nature of composite materials will lead to a differentiation in the mode of temperature rise. In the case of metallic materials, temperature rise is mainly achieved by heat diffusion. Non-metallic materials, on the other hand, cannot be effectively raised by simple heat transfer, unless the temperature of the external heat source is raised or the duration of operations is increased. Non-metallic materials, such as the majority of composite materials with low thermal conductivity, are generally also electrical insulators, and the creation of internal sources due to the Joule effect is impossible. The phenomenon particularly well-suited to this situation is dielectric hysteresis, which is simply the manifestation of the delay in electrical structuring or polarization, relative to the electric field. This cannot be achieved by a closed electrical circuit, since the materials involved are electrically insulating. The adjective "dielectric" should be kept in its etymological sense, as a simple negation, i.e. as an electrical insulator.

The delay in dielectric polarization is characterized by a time constant generally in the nanosecond range and, consequently, can only be observed when a time-varying electric field is created at a frequency in the gigahertz range. The present dossier supplements the [AM 3 046] dossier, which mainly concerns the intrinsic characteristics of the reactive constituents. It develops the spatial effects of heat diffusion and the propagation of electric field carrier waves within the objects to be produced.

The spatial temperature uniformity achieved under these conditions was long taken for granted, simply because dielectric hysteresis is an internal phenomenon of the material. In addition to this uniformity, there were other supposed advantages, such as the speed of transformations, their selectivity, and the reduction of mechanical stress due to the absence of thermal gradients. However, to achieve spatially controlled...