MICROWAVE DIELECTRIC MEASUREMENTS OF FERROELECTRICS UP TO 800 K

A new device for high frequency dielectric measurements of ferroelectr ic ceramics at temperatures from 300 up to 800 K is presented. The app aratus involves a circular coaxial guide short-circuited at its end an d extended by an air line for the measurement in temperature. The calc ulations of the real and imaginary parts of the complex permittivity w ere computed using a mode matching method. In fact, the main innovatio n 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 similar to 670 K. A high frequency re laxation was shown and characterized. Such a new device will be very u seful for the study of ferroelectrics with Curie temperature higher th an 450 K; this temperature is the limit generally reached for the meas urement of high permittivities at frequency up to 10(9) Hz. (C) 1997 E lsevier Science S.A.