Investigations concerning polarization in barium titanate ceramics

Considerable information may be obtained concerning polarization in barium titanate ceramics by a careful analysis of hysteresis loops. Mathematically it is shown that both symmetrical and unsymmetrical loops, the latter being characteristic of polarized specimens, can be represented best by a Fourier series. It was found that direct voltage and even harmonic components are present in the unsymmetrical loops. The per-cent second harmonic and the electromechanical coupling coefficient both rise with increasing depolarizing voltage and reach a maximum value at approximately 15,000 volts per cm (centimeter). The measurement of the second harmonic is possible even when the activity is so small that measurement of the coupling coefficient becomes very inaccurate or impossible. The addition of calcium titanate in small percentages markedly raises the coercive force but lowers the spontaneous and remanent polarization in the range from 0 to 80 C (degrees centigrade). However, it does not have marked effect between the latter temperature and the Curie point. It was found from studies of the discharge currents on depolarizing specimens by sudden immersion in hot oil that the polarization observed from the discharge curves is essentially the same as the remanent polarization determined from oscillograms of saturated hysteresis loops.