Thermal Conduction Measurements of Materials using Microwave Energy
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Thermal Conduction Measurements of Materials using Microwave Energy R. E. Giedd, G. R. Giedd, Center for Scientific Research, Southwest Missouri State University, 901 South National Ave, Springfield, Mo. 65802. ABSTRACT We have developed a new technique to measure the thermal conductivity of materials using microwave energy. A thermal wave is induced in a material of unknown thermal conductivity using a pulse of microwave energy. This energy is incident on one side of the material. The corresponding temperature rise of the opposite side of the material is measured. The thermal diffusivity of the material can then be determined in the same way as "laser flash"[1]. Some of the advantages of the microwave system are the relatively low cost of the magnetron compared to the high energy laser, easily variable pulse length, and accurate measurement of the reflected energy. The microwave system consists of a 2.45 GHz magnetron that is pulse modulated to energies as high as 10 J. A typical pulse lasts for 1 ms with rise and fall times of 10ps. This is achieved by a high voltage source (5 - 8 kV at 1 - 2 A), switched by a high power, rf transmitting tube connected in the filament circuit of the magnetron. INTRODUCTION The problem of binding one material phase to another is of profound practical and theoretical significance. It is difficult, however, to characterize these interfacial regions since they are only a small percentage, by volume, of the bulk material. In composites these interfacial regions can provide a "weak link" where cracks and slippages occur and therefore an understanding of their characteristics is vital to the overall performance of the material. This new technique allows the investigator to examine these interfacial regions using microwave energy in non-destructive, dynamic thermal dissipation experiments. Thermal conduction in materials is a sensitive function of the above mentioned defects, since thermal phonons have very short wavelengths near room temperature[2]. These thermal phonons "probe" the structure of the materials since they scatter from disruptions in long range order. Thermal conduction depends upon two independent quantities which describe different aspects of the material in question. First, the specific heat per unit volume; second, the thermal diffusivity. The dependence is described by: r = pcf
(1)
where p is the mass density, c is the specific heat, and ci is the thermal diffusivity[2,3]. The specific heat is a measure of the excitable thermal modes in a material. If a material has a low heat capacity it will be a relatively poor thermal conductor (if diffusivity is held constant) since it will not absorb energy as completely as a material with a high heat capacity. However most materials at room temperature and above have similar heat capacities. The orders of magnitude difference between the thermal conductivity of copper and polystyrene, around room temperature, does not lie in the differences in heat capacity (indeed polystyrene has a higher heat capacity than copper) but in th
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