An Investigation of Surface Polarization in Single and Polycrystalline NaCl at Elevated Temperatures
Date of Award
3-1974
Document Type
Dissertation - Restricted
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering
First Advisor
Martin Seitz
Second Advisor
Robert N. Blumenthal
Third Advisor
Robert F. Brebrick
Abstract
Electrical polarization phenomena in materials have been extensively investigated since the time of MaxwelI. Bulk polarization mechanisms responsible for the dielectric behavior of solids at low temperatures and high frequencies are theoretically welI-defined processes. In addition to these, other types of polarization processes associated with a material's surfaces may also occur in ionicly bonded compounds in response to electrical excitations In the audio frequency range and below and at sufficiently high temperatures, usually above one half the melting temperature of the compound. This general class of polarization phenomena, referred to as surface polarization, is less clearly defined and may result in an apparent component of polarization much in excess of that due to bulk electronic, ionic, and dipole orientation phenomena. Several theories have been developed which are purported to explain how this effect is obtained. Various types of space charge, Faradaic, and heterogeneous polarization models have been employed to account for surface polarization effects occurring in binary compounds. Unfortunately some controversy exists regarding which of these surface polarization theories is the more correct explanation. In this study the dielectric behavior of single crystal NaCl was investigated at low frequencies and at elevated temperatures to determine which of these surface polarization theories was more credible. Similar studies were also performed on polycrystalline NaCl to determine how the polycrystalline nature of the material affected surface polarization processes .
Sodium chloride was employed as a test material to limit the number of variables in the data analysis and to exemplify the limiting case behavior expected of other more complex compounds in their ionic conduction domains. Since NaCl is isotropic, exhibits stoichiometric disorder and has approximately equal anion and cation vacancy defect mobilities its properties closely approximate many of the conditions assumed in the derivation of the space charge and the Faradaic surface polarization theories. The dielectric behavior of NaCl under the conditions of low frequency and elevated temperatures is therefore expected to correspond to that predicted by a particular surface polarization theory.
The dielectric behavior of pure single and polycrystalIine NaCl was measured as a function of the frequency of the applied voltage, temperature, and specimen thickness and in terms of the equivalent parallel capacitance and resistance of the specimens . The dielectric properties were not dependent on the magnitude of the a.c. voltage although the condition, eVapplied << kT, was not strictly fulfiIled. Various types of materials were employed as electrodes to determine the role of the electrodes in surface polarization phenomena. The data was interpreted in terms of a lumped parameter equivalent circuit derived from space charge and Faradaic polarization considerations. The object of this investigation was to determine how well the behavior of real material was approximated by the various surface polarization theories The dielectric behavior of single and polycrystalline materials was compared to investigate the influence of grain boundaries on the surface polarization phenomena. This study also served to test the effectiveness of using dielectric measurements for determining the defect parameters of an ionic substance.
A considerable number of determinations of the intrinsic defect parameters of NaCl, i.e. the average change in enthalpy and entropy required to produce a Schottky defect pair, have been previously reported in, the literature. These determinations usually resulted from an analysis of the temperature variations of the conductivity of pure and doped NaCl. Nevertheless some uncertainty remains regarding the values of these parameters. Under the interpretations imposed in this study and due to the conditions fulfiIled by the dielectric behavior of NaCl it was possible to determine approximate values for the enthalpy and entropy of formation for a Schottky defect pair. These numbers are in good agreement with other recent determinations and tend to reinforce the validity of the interpretations applied.