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A review of theories of ferroelectricity and of experimental data on ferroelectric ceramics leads to the conclusion that an analytic theory of such ceramics on microscopic basis does not seem feasible. A thermodynamic continuum theory is developed here, which is based on the postulated isomorphism, relevant to small signals only, between a polarized ceramic and one under bias fields near the limit of its nonpolar state; this bears analogy to Mueller's "lassical theory of Rochelle salt. The results of the theory include an elastic relation, which supplements existing experimental data, and a piezoelectric one, which relates to retained polarization and which agrees with measurements.

In transducer applications of ferroelectric ceramics it is standard practice to rely for the electromechanical response on retained polarization. This is practicable only for moderate driving amplitudes and sufficiently low temperature. At higher driving fields dielectric losses increase inordinately and lead to eventual depolarization and loss of response. This can be remedied by application of a comparatively modest dc bias. The quantitative loss behavior of various ceramic bodies over extended ranges of temperature, field amplitude, and for various values of aiding bias is investigated. Results are also presented on various other effects obtained with bias operation, such as increase of electromechanical coupling and shifts of thermodynamic transition points. Work on other associated phenomena—electrostriction, secular relaxation, etc.—will be reported in part II.

The effect of a maintained aiding direct current bias field on the performance of prepolarized barium titanate ceramics has been investigated with respect to both loss factor and electromechanical response over the temperature range from 25° to 120°C as a function of bias field strength. In addition the effect on the temperature variation of the reversible dielectric constant was studied. The dielectric losses under high ac fields are considerably reduced by the application of bias, and the magnitude of this effect shows a pronounced temperature dependence. Aiding bias also proves beneficial by substantially increasing the electromechanical coupling, and it raises the Curie point by up to about 4°C. Dielectric loss factors were determined semiquantitatively by bridge measurements at moderate ac fields and by means of hysteresis loop areas at higher fields. Both the electrostrictive coefficient u31 relating transversal strain to the square of the total charge density and the linear piezoelectric coefficient g31 relating the increment of strain to the increment of charge density in prepolarized ceramics have been measured directly. From these results, approximate values of retained polarization referring to various physical conditions have been determined; they range to about 11 microcoulombs per square centimeter.

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