Recent advances in tunable materials development

D. Suvorov, M. Spreitzer
Advanced Materials Department, Jožef Stefan Institute, Ljubljana, Slovenia

Voltage-tunable materials play a major role in advanced communication systems, as phase shifters, tunable filters, tunable capacitors, etc. Their implementation requires a distinctive dependence of the dielectric constant on the DC-bias field, i.e., high tunability, as well as low dielectric losses, a low dielectric constant and temperature-stable performance. A great many investigations in the field have focused on paraelectric modifications near the ferroelectric transitions of the displacive type, in particular on (Ba1-xSrx)TiO3. However, the tunability of ferroelectrics increases with the dielectric constant and therefore the above requirements are contradictory for pure compounds. Therefore, various (Ba1-xSrx)TiO3-based composites have been investigated, which partially overcome some of the drawbacks.
We focused our investigation of tunable materials on ferroelectrics of the relaxor type, in particular on Na0.5Bi0.5TiO3. The temperatures of the Na0.5Bi0.5TiO3 phase transitions and thus the intensities of the dielectric relaxations were modified by the addition of various incipient-type ferroelectric compounds. Improved figures of merit were obtained for samples with increased additive concentrations, which are macroscopically cubic, have paraelectric polar order and show low-temperature dielectric relaxations. The enhanced characteristics of the samples are also expressed by the exceptionally high nonlinear coefficients β, which might be related to the dynamics of the nanosized polar regions present in the samples and their composite nature. In the contribution the voltage-tunable characteristics of the samples will be correlated with their microstructure and stoichiometry as well.