High frequency impedance spectroscopy analysis of doped PZT ceramics

An impedance spectroscopy method was employed for the determination of radiofrequency (RF) and microwaves (MW) dielectric characteristics of co-doped dense PZT ceramics. The particular behavior of the cylindrical ceramic samples, i.e. the “self-resonances” were observed and analyzed. In order to have a simple and clear explanation of the RF and MW sample resonances, a dielectric resonant cavity model was developed and used. The PZT samples were considered as dielectric resonant cylindrical cavities characterized by the resonance frequency fr1 for the fundamental TM110 mode and fr2 for the TM210 resonance mode, which were observed experimentally in the frequency range of 100 MHz – 1 GHz. The simplified theoretical approach, the particular case of the frequencies shifts for different positions of the samples inside the measurement head of the Agilent E4991A RF Impedance/Material Analyzer and the comparison between the calculated and measured data, for both low and high frequencies, are presented. The specified material analyzer is designed to work properly in certain permittivity frequency domains, for samples having the dimensions closed to the measuring head’s electrodes diameter [1]. Getting out from these requirements, as for higher then 500 relative permittivity materials and for frequencies higher then 100 MHz, induces big errors on the directly measured values of the permittivity and permeability. Evaluating the resonance frequencies of high permittivity ceramic samples considered as dielectric resonant cavities is very useful in determination of the “true” value of the effective complex permittivity of a given material, in measurements as Impedance Spectroscopy, Frequency Domain Reflectomery FDR and Time Domain Reflectomery TDR. The paper also presents a detailed model for calculating the needed corrections, starting with the high frequencies data obtained by using the specified Impedance/Material Analyzer and the low frequencies data obtained by using the SOLATRON 1260A system, the condition of continuity at 1 MHz and the high frequencies resonances of the doped PZT samples.