"The transport of heat in dielectric solids is attributed entirely to lattice waves whereas in metals there is in addition a transfer by the flow of valence electrons. The thermal conductivity of a non-electrical-conductor is limited by a number of scattering processes giving rise to thermal resistances. Scattering results from impurity atoms and from vacancies and dislocations within the crystal structure and from the interaction of the lattice vibrations amongst themselves. Scattering also occurs at the boundaries of the crystal. The thermal resistance of an ideal crystal at low temperatures is due almost entirely to boundary scattering and at high temperatures is due primarily to anharmonic coupling. With increasing temperature the former decreases and the latter increases and consequently the thermal conductivity passes through a maximum.
In 1914, Debye showed from theoretical considerations that the conductivity is approximately proportional to 1/T for temperatures above the region of maximum conductivity. A more recent and rigorous treatment of the problem by Peierls, using quantum mechanics, resulted in the same temperature dependence. In 1941, Pomeranchuk, published a paper showing the necessity of including an additional term in the interaction potential. His calculations showed a mixed type of conductivity proportional to 1/T and 1/[T3/2].
Although a considerable amount of experimental work has been done in the past, most of it has dealt with temperatures below the boiling point of water. Consequently the measurements of thermal conductivities above this temperature have not been sufficiently extensive for a detailed comparison between theory and experiment to be made. The wide discrepancy in the results of early experimenters is believed to be due largely to the defects and impurities of the various samples used. Thus it seems that if a comparison between the experimental and theoretical values is to be made, the conductivities of a single crystal must be determined over a rather large temperature range. The ultimate goal of this project is to investigate the temperature dependence of the conductivity of various alkali halides up to their melting points.
Equipment was available at the beginning of this investigation that had been previously operated by Weisbrod and Brown. The apparatus was designed to measure absolute conductivities rather than relative ones. Near perfect cylindrical crystals of sodium chloride whose length and diameter were approximately 1.2 and 1.5 centimeters respectively were available. The most apparent deficiency of the equipment was that of background (explained elsewhere). The error in the calculated conductivities resulting from this effect increased with increasing temperature and amounted to as much as 8% at 300°C as was shown by Brown. The purpose of this investigation was to improve the operating performance of the equipment and to reproduce and extend the work done by Weisbrod and Brown"--Introduction, pages 1-3.
Bessey, William H.
M.S. in Physics
Missouri School of Mines and Metallurgy
v, 28 pages
© 1956 Paul E. Ohlsen, All rights reserved.
Thesis - Open Access
Salt -- Analysis
Materials at high temperatures
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Ohlsen, Paul E., "Thermal conductivity of sodium chloride within the temperature range 375°K to 637°K" (1956). Masters Theses. 2573.