Anisotropy of thermal conductivity of ferro-filled polyvinylchloride

Abstract

UDC 541.64:532.72 The contribution of the constant external magnetic field to the anisotropy of thermal conductivity of polyvinyl- chloride (PVC) containing powders of highly dispersed ferromagnetics (iron and magnetite) as a filler has been determined. The relationship between the thermal conductivity coefficient of such PVCs, the direction of the heat-flux vector, and the normal to the isotherm of the composite has been established. It has been shown that for the systems "PVC + 2.0 vol. % Fe" and "PVC + 2.0 vol. % Fe3O4" formed on exposure to an ex- ternal magnetic field, the value of the principal axes of the thermal-conductivity coefficient depends on the type of filler. A comparison of the calculated and experimental values of the sought quantities has been made. Introduction. Today's science and technology strongly require that heterogeneous systems based on flexible- chain polymers that contain highly dispersed ferrite (spinel) (1) or metal (2) powders as a filler be obtained, investi- gated, and used. This is a relatively new class of polymeric composite materials with physical properties diametrically opposite to the properties of other composite polymers. Thus, polymers that are diamagnetic substances (1) with a low thermal-conductivity coefficient (2) are isotropic under normal conditions (3). At the same time, using a constant ex- ternal magnetic field, one can create anisotropy of macroscopic magnetization (4) in a polymeric composite material containing ingredients in the form of ferromagnetic metals or semimetals due to the spin paramagnetism (nuclear or electronic). Structural changes produced in the polymeric composite material by the action of a constant external mag- netic field ensure new conditions for kinetic processes in them (5). The process of change in the heat conduction of such materials relative to the heat-flux vector and to the normal to the isotherm remains to be investigated; this proc- ess is important in the production and use of polymeric materials together with ferrites and ferromagnetics, particularly in spintronic devices. With account taken of this fact and in connection with the necessity of creating differential heat insulators, heat-transfer agents, and energy converters based on polymeric composite materials, the present work seeks to obtain polymeric composite materials based on flexible-chain polymers containing highly dispersed particles of fer- romagnetic semimetal (magnetite) or ferromagnetic (iron), to investigate these materials with a constant external mag- netic field, and to determine the possibility of controlling the anisotropy of their thermal conductivity. Formulation of the Problem and Mathematical Model. It has been established that oriented amorphous and crystalline polymers and expanded rubbers as well as heterogeneous systems based on them possess thermal-conductiv- ity anisotropy (3). Therefore, we cannot apply, to such materials, the generally recognized proposition that the vector of the heat flux q calculated as the quantity of heat transferred through a unit surface of a material in unit time is perpen- dicular, at any point of this material, to the isotherm passing through this point (2). In (3), it has been shown that in an anisotropic medium, q is not coincident with the normal n to the isotherm. Let dT ⁄ dn be the rate of change in the temperature along the normal to the isotherm passing through the point P, and dT ⁄ dm be the rate of change in the temperature along the direction of q at the same point. The direction cosines of this vector have the form q x ⁄ q, q y ⁄ q, and q z ⁄ q, whereas the quantity dT ⁄ dm is equal to dT dm = q x q dT