The coexistence of electrocaloric and magnetocaloric effects in Pb(Fe1/2Nb1/2)O3 ceramics
published: May 23, 2017, recorded: April 2017, views: 933
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Solid-state refrigeration technology represents a promising alternative for the replacement of low energy efficient and ecologically harmful conventional vapor-compression refrigeration systems. Most current activity in cooling research is looking at one of the caloric effects – electrocaloric (EC), magnetocaloric (MC) or mechanocaloric – where the material’s entropy changes under the application of external stimuli – electric, magnetic or mechanical (stress).1 In bulk ceramic materials the caloric effect is currently not large enough for commercial use. One idea how to overcome this problem is to prepare a multicaloric material where two or more single caloric effects coexist in one material in which the application of both stimuli can enhance the total, multicaloric effect. Even more, different caloric modes can be applied in different temperature regions extending the operating temperature range of the cooling device. The coexistence of the MC and EC effects had been theoretically proposed five years ago2 followed by experimental conformations.3,4 It is very challenging to prepare efficient multicaloric materials and the search for them is not finished yet. In this work we experimentally prove that the Pb(Fe1/2Nb1/2)O3 (PFN) exhibits both MC and EC effects and is therefore the multicaloric material.
For the preparation of the PFN the homogenized, stoichiometric powder mixture was mechanochemically activated in a high-energy planetary mill for 30 h at 300 rpm and milled in an attrition mill for 4 h at 800 rpm. The powder compacts were isostatically pressed and sintered in an oxygen atmosphere at 1273 K for 2 h. This method yielded PFN ceramics with a theoretical density of 96 % and uniform microstructure with average grain size of 2.3 μm. The dielectric permittivity and dielectric losses at room temperature and 10 kHz were measured with a HP 4284A precision LCR Meter and were 3580 and 0.038, respectively. For the indirect EC measurements, the polarization vs. electric field hysteresis loops were measured by an Aixacct TF analyser 2000. At room temperature the EC temperature change was 0.81 K at 80 kV/cm. The maximum EC temperature change of 1.29 K was obtained at 80 kV/cm and 373 K. The magnetization vs. temperature at different magnetic fields was measured using a Superconducting Quantum Interference Device. The maximum MC temperature change of 0.16 K was obtained at 50 kOe and 2 K.
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