Large Electrocaloric Effects in Lead-free BaTiO₃ Multilayer Capacitors

Electrocaloric (EC) materials, presenting large adiabatic temperature change or isothermal entropy change under the application (or removal) of electric fields, hold the potential to offer more efficient alternatives to caloric heat pumps for replacing hazardous gases used in traditional vapour-compression systems. Recently, large EC effects with 5.5 K have been reported in Pb(Sc_0.5Ta_0.5)O₃ multilayer capacitors (MLC) [1] thanks to its strong first-order phase transition, and then a 13 K of temperature span is reported in a prototype based on these MLC [2]. However, the toxicity of lead forces researchers to find eco-friendly materials exhibiting EC performances competitive with lead-based. Here, we study the EC effects in lead-free BaTiO₃ (BTO) MLC using an infrared camera. Unlike the commercial BTO MLC, we prepared our sample without scarifying the feature of first-order transition in BTO. The adiabatic temperature variation versus temperature profile shows two peaks, which matches with two first-order phase transitions of BTO in the same temperature range, as observed by Differential Scanning Calorimetry. We measured a temperature drop of ~ 0.9 K with a temperature span of 70 K under 170 kV cm^-1, starting at 30 °C near the tetragonal-to-orthorhombic transition. Under the same electric field, a larger temperature change of ~ 2.4 K was recorded above 126 °C, the Curie temperature of BTO. Our findings suggest that further optimized BTO MLC can offer a path for designing lead-free caloric cooling prototypes with promising results.<br/><br/>[1] Nair, Bhaskaran, et al. "Large electrocaloric effects in oxide multilayer capacitors over a wide temperature range." Nature 575.7783 (2019): 468-472.<br/>[2] Torelló, A., et al. "Giant temperature span in electrocaloric regenerator." Science 370.6512 (2020): 125-129.