Large Electrocaloric Effect from Electrical Field Induced Orientational Order-Disorder Transition in Nematic Liquid Crys
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Large Electrocaloric Effect from Electrical Field Induced Orientational Order-Disorder Transition in Nematic Liquid Crystals Possessing Large Dielectric Anisotropy Xiao-Shi Qian1, S. G. Lu2, Xinyu Li1, Haiming Gu1, L-C Chien3, Q. M. Zhang1,2 1 Department of Electrical Engineering and 2Materials Research Institute The Pennsylvania State University, University Park, PA 16802, USA 3 Liquid Crystal Institute and Department of Chemical Physics, Kent State University, Kent, OH 44242, USA ABSTRACT Large electrocaloric (EC) effects in ferroelectric polymers and in ferroelectric ceramics have attracted great attention for new refrigeration development which is more environmental friendly and more efficient and thus could be an alternative to the existing vapor-compression refrigerators which consume large energy and release large amount of green house gas. However in the past, all EC effects investigations have been focused on solid state dielectrics. It is interesting to ask whether a large EC effect can also be realized in dielectric fluids. A dielectric fluid with large EC effect could lead to new design of cooling devices with simpler structures than these based on solid state EC materials, for example, they can be utilized as both the refrigerant and heat exchange fluid. Here we present that a large EC effect can be realized in the liquid crystal (LC) 5CB near it’s nematic-isotropic (N-I) phase transition. The LC 5CB possesses a large dielectric anisotropy which can induce large polarization change from the isotropic phase to the nematic phase near the N-I transition. An isothermal entropy change of more than 23 Jkg-1K-1 was observed near 39 oC that is just above the N-I transition. INTRODUCTION The electrocaloric (EC) effect is a reversible temperature and/or entropy change of an insulating polar material under applying and removing of an electric field E [1-5]. Dielectrics with a large EC effect are attractive for developing alternatives for conventional mechanical vapor compression cycle (MVCC) based air-conditioning and refrigeration which use strong greenhouse gases as the refrigerant, e.g., operating air-conditioning only with electrical power is of great benefit to electrical vehicles considering an extra-burden of the mechanical compressor required for MVCC. The recent findings of giant EC effects in ferroelectric polymers and in ferroelectric ceramics have attracted great interest for designing new cooling devices which are environmental friendly, compact, and more efficient [3-18]. In all refrigeration and air-conditioning, two key components are required. First, it is required for a refrigerant whose entropy depends on some properties other than temperature, e.g., pressure or magnetic field. Second, entropy needs to be transported from one temperature level to another temperature level in a reversible and cyclic manner in order to get high efficiency. In EC materials, insulating dielectric materials, whose entropy can be changed by external electric fields, play a role as a refrigerant. In order to transport
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