Caloric effects in liquid crystal-based soft materials

Abstract

With the increased environmental awareness, the search for environmentally friendlier heat-management techniques has been the topic of many scientific studies. The caloric materials with large caloric effects, such as the electrocaloric (EC) and elastocaloric (eC) effects, have increased interest due to their potential to realize new solid-state refrigeration devices. Recently, caloric properties of soft materials, such as liquid crystals (LCs) and liquid crystal elastomers (LCEs), are getting more in the focus of caloric materials investigations, stimulated by large caloric effects observed in these materials. Here, an overview of recent direct measurements of large caloric effects in smectic LC 14CB and main-chain LCEs is given. Specifically, high-resolution thermometric measurements revealed a large EC response in 14CB LC exceeding 8 K. Such a large effect was obtained at a relatively moderate electric field of 30 kV/cm compared to solid electrocaloric materials. We demonstrate that such a small field can induce the isotropic to smectic A phase transition in 14CB, releasing or absorbing relatively large latent heat that enhances the EC response. Furthermore, it is demonstrated that in main-chain LCEs, the character of the nematic to isotropic transition can be tuned from the supercritical towards the first-order regime by decreasing the crosslinkers' density. Such tuning results in a sharper phase transition and latent heat that enhance the elastocaloric response, exceeding 2 K and with the eC responsivity of 24 K/MPa, about three orders of magnitude larger than the average eC responsivity found in the best shape memory alloys. Significant caloric effects in soft liquid crystal-based materials, observed at much smaller fields than in solid caloric materials, demonstrate their ability to play an important role as new cooling elements, thermal diodes, and caloric-active regeneration material in new heat-management devices.

Criticality Controlling Mechanisms in Nematic Liquid Crystals

“…The formulation developed in this work can serve as a guide towards the development of various applications, e.g., in the area of anomalously sensitive detectors [59,60] or caloric devices [7,8], especially those based on the barocaloric effect [7,8]. In the proximity of critical points, soft materials exhibit a highly increased susceptibility to even weak external stimuli.…”

“…Furthermore, susceptibilities of certain systems are related to the piezoelectric com-pliance [61], which is related to the electro-mechanical response. Consequently, mastering the critical point conditions is interesting for diverse thermal (e.g., heat management) applications, actuators, and sensitive sensors [7,8]. In these effects, the adiabatic variation in an external (e.g., electric, magnetic, elastic) field enforces a large temperature change that can be maximized near critical points [4].…”

“…In studying P-driven changes, we assume that the dominant pressure-dependent free energy contribution is also introduced by the first term in Equation (8), where we set [17] T * ∼ T * 0 + α∆P, (10) where P = P 0 + ∆P, P 0 ∼ 10 5 N/m 2 stands for the atmospheric pressure, T * (P = P 0 ) = T * 0 , and α is constant. With this in mind, we obtain…”

“…At the CP, the system exhibits anomalous sensitivity to different stimuli [4]. Consequently, establishing CP conditions is of interest for various technological applications (e.g., sensors, caloric responses) [4,[7][8][9].…”

“…In order to gain a profound understanding of universal features (i.e., scaling behavior) and to effectively manipulate and master the CP behavior, it is essential to identify simple experimental systems with a well-developed theoretical background. It is important to stress that normally significant external fields [4,7,8] are needed to approach CP conditions. Furthermore, applications of external fields could induce unwanted effects such as resistive Joule heating or Eddy currents if electrical or magnetic fields are used for such purposes.…”