Electrocaloric and elastocaloric effects in soft materials

Abstract

Materials with large caloric effect have the promise of realizing solid-state refrigeration which has potential to be more efficient and environmentally friendly compared with current cooling technologies. Recently, the focus of caloric effects investigations has shifted towards soft materials. An overview of recent direct measurements of the large electrocaloric effect (ECE) in a composite mixture of a liquid crystal and nanoparticles (NPs) and large elastocaloric (eC) effect in main-chain liquid crystal elastomers is given. In mixtures of 12CB liquid crystal with functionalized CdSSe NPs, an ECE exceeding 5 K was found in the vicinity of the isotropic to smectic A phase transition. It is shown that the NPs smear the isotropic to smectic coexistence range in which a large ECE is observed due to latent heat enhancement. NPs acting as traps for ions reduce the moving-ion density and consequently the Joule heating. Direct eC measurements indicate that the significant eC response can be found in main-chain liquid crystalline elastomers, but at a fraction of the stress field in contrast to other eC materials. Both soft materials could play a significant role as active cooling elements or parts of thermal diodes in development of new cooling devices. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

Torsional refrigeration by twisted, coiled, and supercoiled fibers

“…Using the above volume-average cooling on isometric twist release, the coefficient of performance (COP) for NR twist fridges was obtained. The COP can be described as the ratio of the cooling energy (the product of the volume-average cooling, the gravimetric heat capacity, and the mass of the fiber) to either the input mechanical energy or the net energy consumed during a mechanical cycle (called COPHC and COPFC, for a half-cycle and a full-cycle, respectively) (5). COPHC is useful when device simplicity or miniaturization is more important than increasing efficiency by recapturing part of the input mechanical energy.…”

“…An ordinary rubber band is an example, which was reported in 1805 to heat when stretched and to cool when stretch is released (1). Alternative materials for refrigeration are electrocaloric (2)(3)(4)(5)(6) and magnetocaloric (5,7), meaning they provide cooling because of changes in electric or magnetic fields, respectively. While these solid-state cooling mechanisms have been widely investigated for diverse materials, none yet meets the performance needs for wide scale applications.…”

Elastocaloric effect in liquid crystal elastomers from molecular simulations

“…3. Note, however, that in experiments [8] such a secondary peak has not been observed. Its presence in the simulation may be a mere consequence of local irregularities in our relatively small sample.…”

“…Experimentally, a measurable eC response has already been observed in main-chain LCE [8], but at a fraction of the stress field required in other eC materials, e.g., in NiTi-based shape-memory alloys [3,[9][10][11]. The amplitude of the observed eC effect was of the order of 0.1 K and the corresponding responsivity in the range of a few K/MPa.…”

“…In the so-called direct approach, ∆T is measured under adiabatic application or removal of external force F like in Ref. [8]. On the other hand, the indirect approach that we follow here starts by considering the total entropy of the sample, S, as a function of temperature T and external force F, and setting S = const.…”