Preparation and Characterization of (1 —x) Pb(Mg1/3Nb2/3) O3 —x PbTiO3 Electrocaloric Ceramics

Non-contact and direct electrocaloric effect measurement for high-throughput material screening

A non-contact and direct electrocaloric effect (ECE) measurement system was developed for rapid ECE measurement. The ECE of ferroelectric materials was measured directly using two measurement methods, namely, the constant heating rate method (CH method) and one-dimensional temperature gradient method (1D method), with the measurement system. The CH method continuously measures the ECE while gently heating the sample, and it can evaluate the performance of the sample more quickly than conventional methods that measure the ECE at steady state. The 1D method directly measures the ECE using a 1D temperature distribution applied to the sample. This method can measure the temperature dependence of the ECE faster than the CH method. The measurement system achieved a high signal-to-noise ratio, and the temperature dependence of the ECE measured by the two methods was consistent. The proposed measurement system and methods enable promising candidate materials for electrocaloric cooling systems to be screened more rapidly than when using conventional approaches.

Pyroelectric energy conversion: optimization principles

In the framework of microgenerators, we present in this paper the key points for energy harvesting from temperature using ferroelectric materials. Thermoelectric devices profit from temperature spatial gradients, whereas ferroelectric materials require temporal fluctuation of temperature, thus leading to different applications targets. Ferroelectric materials may harvest perfectly the available thermal energy whatever the materials properties (limited by Carnot conversion efficiency) whereas thermoelectric material's efficiency is limited by materials properties (ZT figure of merit). However, it is shown that the necessary electric fields for Carnot cycles are far beyond the breakdown limit of bulk ferroelectric materials. Thin films may be an excellent solution for rising up to ultra-high electric fields and outstanding efficiency. Different thermodynamic cycles are presented in the paper: principles, advantages, and drawbacks. Using the Carnot cycle, the harvested energy would be independent of materials properties. However, using more realistic cycles, the energy conversion effectiveness remains dependent on the materials properties as discussed in the paper. A particular coupling factor is defined to quantify and check the effectiveness of pyroelectric energy harvesting. It is defined similarly to an electromechanical coupling factor as k2=p2theta0/(epsilontheta33cE), where p, theta0, epsilontheta33, cE are pyroelectric coefficient, maximum working temperature, dielectric permittivity, and specific heat, respectively. The importance of the electrothermal coupling factor is shown and discussed as an energy harvesting figure of merit. It gives the effectiveness of all techniques of energy harvesting (except the Carnot cycle). It is finally shown that we could reach very high efficiency using 1110.75Pb(Mg1/3Nb2/3)-0.25PbTiO3 single crystals and synchronized switch harvesting on inductor (almost 50% of Carnot efficiency). Finally, practical implementation key points of pyroelectric energy harvesting are presented showing that the different thermodynamic cycles are feasible and potentially effective, even compared to thermoelectric devices.