Half of the world’s CO2 emissions can be attributed to heating and cooling. This is primarily due to heating with natural gas and cooling with compression of greenhouse gases, which are neither environmentally friendly nor energy efficient. There is great interest in developing energy-efficient solid-state heat pumps that can replace these environmentally damaging technologies. Caloric materials are at the core of novel solid-state heat-pump technologies. During this talk I will describe our work on electrocaloric effects driven by electric field, and barocaloric effects driven by hydrostatic pressure, on ferroelectric materials for low-carbon heating and cooling applications.

Waste heat recovery with the use of pyroelectric materials has attracted increasingly attention because it can convert low-grade thermal energy into electricity. However, the low output of the pyroelectric harvester significantly limits its application due to the low heat transfer of pyroelectric materials. Herein, we utilized a first-principles approach to simulate phonon scattering and phonon group velocity, and analyzed how these elements affect the heat transfer of pyroelectric materials with high thermal conductivity fillers. Additionally, we employed the Multiphysics software COMSOL to simulate the thermal and electrical fields of the pyroelectric materials with fillers. These simulations resulted in improved thermal conductivity, pyroelectric properties of the pyroelectric material, and the corresponding output performance of the energy harvester. Experimental results further contribute to the advancement of heat transfer control in pyroelectric materials and their potential applications.

The heat capacity of a ferroelectric crystal in a wide temperature range can be represented as the sum of the anomalous and background specific heats. Anomalous is called the part of the specific heat, which depends on the phase transition parameter and its derivatives. The background heat capacity of a crystal is the sum of its lattice heat capacity and other contributions that do not depend on the derivatives of the phase transition parameter, for example, the contribution to the heat capacity of thermal expansion. Calculation of the background heat capacity is a necessary stage in the interpretation of the experimental data on the temperature dependence of the heat capacity for ferroelectric crystals.
Since the background specific heat of a crystal is substantially determined by its lattice heat capacity, the determination of the background specific heat from the experimental temperature dependence of the heat capacity of the crystal makes it possible to obtain independent information on its phonon spectrum. Conversely, the existing data on the phonon spectrum of the crystal make it easier to determination its background specific heat.
Calculation of the background specific heat is quite facilitated in the presence of a sufficiently detailed temperature dependence of the heat capacity over a wide temperature interval including the ferroelectric phase transition. Obtaining such the dependences requires special thorough when planning and making a calorimetric experiment.
We made a comparative review of various methods for determining the background specific heat of ferroelectric crystals. The experimental and computational approaches are analyzed using examples of such model ferroelectrics as potassium dihydrophosphate and barium titanate. Also detailed are the results relating to prospective for applications crystals of lanthanum borogermanate and solid solutions based on lead titanate magnoniobate.
The specificity of ferroelectric crystals is that, as a rule, in their phonon spectrum the most active are one or several polar optical modes being discussed. This circumstance can be used both in constructing the lattice specific heat from data on the phonon spectrum of the ferroelectric crystal, and in constructing the interpolation schemes necessary for interpretation the temperature dependencies of the heat capacity of ferroelectrics.

The electrocaloric (EC) effect, defined as the reversible adiabatic temperature change or the isothermal entropy change of a dielectric material under the application of an electric field, is promising for the development of alternative cooling devices. EC materials should possess large temperature or entropy changes and low hysteresis in their intended working temperature range. Multilayer ceramic (MLC) components are beneficial because of their high refrigerant volume, their adjustable thickness to systems frequency needs, and their high dielectric strength. We develop lead-free ferroelectric bulk materials based on modified (Ba,Sr)(Sn,Ti)O3 (BSSnT) which show a large EC temperature change over a broad temperature range around room temperature. However, the fabrication of BSSnT MLC components for high EC effects remains challenging due to the occurring abnormal grain growth resulting in electrical conductivity paths between the inner electrodes. In the present work, we study the influence of grain growth inhibitors on the grain size of BSSnT. Additions of MgO, Y2O3 and MnCO3 significantly decrease the mean grain size of BSSnT from 40 µm to 0.4 µm, 0.8 µm and 0.4 µm, respectively. Ca-modification exhibit a gentler effect on grain growth suppression. Depending on sintering temperature and dwell time mean grain size will be 1.5 to 7.3 µm for these materials. For all compositions dielectric, ferroelectric and EC properties are characterized in detail and discussed with regard to grain size. Our investigations show that EC effect is significantly influenced by the microstructure of BSSnT. With increasing grain size from 1.5 to 7.3 µm, the maximum electrocaloric temperature change |ΔT_EC | of Ca-modified BSSnT increases from 0.15 K at 35 °C to 0.37 K at 20 °C at low electric fields of 2 V µm-1. Further optimization of the sintering process of modified BSSnT MLC components to increase the EC temperature change by keeping high dielectric strength is subject of our ongoing research.

This work is supported by the Fraunhofer lighthouse project “ElKaWe – Electrocaloric heat pumps”.

Keywords: lead-free, electrocaloric, sintering inhibitors, decreasing grain size.