Microwave dielectric measurements of ferroelectrics up to 800 K

A new device for high frequency dielectric measurements of ferroelectric ceramics at temperatures from 300 up to 800 K is presented. The apparatus involves a circular coaxial guide short-circuited at its end and extended by an air line for the measurement in temperature. The calculations of the real and imaginary parts of the complex permittivity were computed using a mode matching method. In fact, the main innovation of the present work is the technical adaptation in temperature. The device was preliminary tested with BaTiO3 ceramics and validated up to 450 K. The first new study concerns KNbO3 derived ceramics from 300 K to temperatures greater than Tc ∼ 670 K. A high frequency relaxation was shown and characterized. Such a new device will be very useful for the study of ferroelectrics with Curie temperature higher than 450 K; this temperature is the limit generally reached for the measurement of high permittivities at frequency up to 109 Hz.