Electrocaloric Cooling Power and Long Term Stability of Barium Zirconate Titanate

Interest on the electrocaloric effect grew rapidly over the past decade. In this time, the electrocaloric temperature change was directly and indirectly determined in a lot of ferroelectric materials. To compare those materials with respect to electrocaloric applications not solely the electrocaloric but also the thermophysical performance characteristics need to be considered. Here, a material related cooling power is derived on basis of a Newtonian cooling model of a thin plate, which includes electrocaloric as well as thermophysical properties. From the material related cooling power a caloric figure of merit is derived which is used to compare materials of the Ba(ZrxTi1-x)O3 system. The electrocaloric temperature change, specific heat capacity and thermal conductivity of Ba(ZrxTi1-x)O3 are provided. The depicted compositions have different paraelectric to ferroelectric phase transition behavior, ranging from first order to second order character, diffusive phase transition and relaxor-like behavior. The largest caloric figure of merit is found for Ba(Zr0.13Ti0.87)O3 with a second order paraelectric to ferroelectric phase transition. The caloric figure of merit is further used to compare the electrocaloric effect with the magnetocaloric and mechanocaloric effect. It is found that multilayer structures of the best lead containing electrocaloric materials can compete with representative materials of the magnetocaloric effect. Whereas, NiTi, a representative of the mechanocaloric effect, exhibits a five times larger performance than the best magnetocaloric or electrocaloric materials. Phenomenological calculations are used to elaborate on the effect of critical end points, tricritical point and triple point on the electrocaloric behavior. The electric field – temperature phase diagram of BT is provided. The contribution of the latent heat, at the electric field induced first order phase transition, to the electrocaloric temperature change is subtracted and by this it is demonstrated that the largest electrocaloric responsivity is at the liquid – vapor type of critical end point. The phase diagram and electrocaloric temperature changes for Ba(ZrxTi1-x)O3 are calculated. A complete composition – temperature phase diagram with the position of a tricritical point and of a triple point are calculated. By considering the line of critical end points, an electric field – composition – temperature phase diagram is constructed. It is demonstrated that the triple point has a positive effect in the enhancement of the electrocaloric properties, whereas the tricritical point has no effect. The long term stability of the electrocaloric temperature change and the effect of oxygen vacancy migration is demonstrated. The movement of oxygen vacancies under strong electric fields, leads to a change in the defect chemistry and hence, to increased leakage current and Joule heating. It is demonstrated that the main conduction mechanism after 106 electrocaloric cycles changes from ionic to electronic conductivity. By changing the polarity of the electric field after every 105 cycles the oxygen vacancies can be redistributed and a large cycle number of 106 without decreasing ECE is obtained