Electrothermal effect in nematic liquid crystal

Liquid crystal based active electrocaloric regenerator

The active electrocaloric (EC) regenerator exploiting electric conversion into thermal energy has recently become important for developing a new generation of heat-management devices. We analyze an active EC regenerator numerically. We establish a temperature span across the regenerator Δ T by commuting a liquid crystalline (LC) unit between regions with and without an external electric field E. In modelling, we use Landau-de Gennes mesoscopic approach, focusing on the temperature regime where isotropic (paranematic) and nematic phase order compete. We determined conditions enabling a large enough value of Δ T suitable for potential applications. In particular, (i) the vicinity of the paranematic-nematic (P-N) phase transition, (ii) large enough latent heat of the transition, (iii) strong enough applied external field (exceeding the critical field E _c at which the P-N transition becomes gradual), and (iv) relatively short contact times between LC unit and heat sink and heat source reservoirs are advantageous. Our analysis reveals that Δ T ≫ 1 K could be achieved using appropriate LC material.

Field-induced orientational switching produces vertically aligned Ti3C2Tx MXene nanosheets

Controlling the orientation of two-dimensional materials is essential to optimize or tune their functional properties. In particular, aligning MXene, a two-dimensional carbide and/or nitride material, has recently received much attention due to its high conductivity and high-density surface functional group properties that can easily vary based on its arranged directions. However, erecting 2D materials vertically can be challenging, given their thinness of few nanometres. Here, vertical alignment of Ti₃C₂T_x MXene sheets is achieved by applying an in-plane electric field, which is directly observed using polarised optical microscopy and scanning electron microscopy. The electric field-induced vertical alignment parallel to the applied alternating-current field is demonstrated to be reversible in the absence of a field, back to a random orientation distribution. Interdigitated electrodes with uniaxially aligned MXene nanosheets are demonstrated. These can be further modulated to achieve various patterns using diversified electrode substrates. Anisotropic electrical conductivity is also observed in the uniaxially aligned MXene nanosheet film, which is quite different from the randomly oriented ones. The proposed orientation-controlling technique demonstrates potential for many applications including sensors, membranes, polarisers, and general energy applications.

In this work, authors demonstrate reversible vertical alignment of Ti₃C₂T_x MXene sheets induced by an applied in-plane electric field. Further modulation of the field can achieve programmed patterns onto various electrode substrates.

Ferroelectric Smectic Liquid Crystals as Electrocaloric Materials

The 1980s saw the development of ferroelectric chiral smectic C (SmC*) liquid crystals (FLCs) with a clear focus on their application in fast electro-optic devices. However, as the only known fluid ferroelectric materials, they also have potential in other applications, one of which is in heat-exchange devices based on the electrocaloric effect. In particular, ferroelectric liquid crystals can be both the electrocaloric material and the heat exchanging fluid in an electrocaloric device, significantly simplifying some of the design constraints associated with solid dielectrics. In this paper, we consider the electrocaloric potential of three SmC* ferroelectric liquid crystal systems, two of which are pure materials that exhibit ferroelectric, antiferroelectric, and intermediate phases and one that was developed as a room-temperature SmC* material for electro-optic applications. We report the field-induced temperature changes of these selected materials, measured indirectly using the Maxwell method. The maximum induced temperature change determined, 0.37 K, is currently record-breaking for an FLC and is sufficiently large to make these materials interesting candidates for the development for electrocaloric applications. Using the electrocaloric temperature change normalised as a function of electric field strength, as a function of merit, the performances of FLCs are compared with ferroelectric ceramics and polymers.

Giant electrocaloric response in smectic liquid crystals with direct smectic-isotropic transition

Electrocaloric materials have become a viable technology for solid state heat management applications. We demonstrate both theoretically and experimentally that liquid crystals (LCs) can be exploited as efficient electrocaloric materials. Numerical and experimental investigations determine the conditions under which the strongest electrocaloric effect (ECE) responses are expected in LCs. Specifically, we show that a large ECE can be expected at the isotropic-nematic and in particular at the isotropic-smectic A phase transition. In our theoretical study, LC ordering is modelled using a Landau – de Gennes - Ginzburg mesoscopic approach. The simulation results are in qualitative agreement with our high precision electrocaloric measurements conducted on 8CB and 12CB liquid crystals. In the latter, we obtained ΔT_EC ~ 6.5 K, corresponding to the largest response measured so far in LCs. The fluid property of LC electrocaloric heat cooling elements could lead to the development of devices with a higher coefficient of performance and thus better cooling power yield per mass of the ECE-based device.

Electrocaloric and elastocaloric effects in soft materials

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'.

Kerr effect at high electric field in the isotropic phase of mesogenic materials

The well-known Kerr effect in isotropic fluids consists in the appearance of uniaxial orientational order and birefringence that grows as the square of the applied electric field. We predict and observe that at a high electric field, the Kerr effect displays features caused by the nonlinear dependence of dielectric permittivity on the field-induced orientational order parameter. Namely, the field-induced birefringence grows faster than the square of the electric field and the dynamics of birefringence growth slows down as the field increases. As a function of temperature, the field-induced birefringence is inversely proportional to the departure from an asymptotic critical temperature, but this temperature is no longer a constant (corresponding to the lower limit of the supercooled isotropic phase) and increases proportionally to the square of the electric field.

Polar and nonpolar orderings in the electrically induced isotropic-nematic phase transition

We study the dynamics of the isotropic-nematic phase transition caused by an applied electric field at the time scales of dielectric relaxation. In the classic Landau-Khalatnikov theory of the phase transition dynamics, the nematic (nonpolar) order parameter is an instantaneous function of the applied field. We demonstrate that, when the field is changing faster than the time of dielectric relaxation, the induced polar order dynamics influences the dynamics of the nonpolar order parameter. We develop a model based on the Langevin equation to describe the simultaneous dynamics of both polar and nonpolar order parameters; the model is supported by experiment.

Time resolved experimental analysis of the electric field induced biaxial order reconstruction in nematics

We present the time resolved experimental characterization of the biaxial switching between two topologically distinct textures of a nematic liquid crystal cell submitted to a strong electric field. This fast electro-optical effect is governed by the electric induced order reconstruction in the nematic bulk, which is an actual challenge for a complete theoretical description of the nematodynamics. The electric measurements across the cell are suitable to this purpose since they well discriminate among the dielectric, the ionic, and the order reconstruction contributions. A phenomenological model describes the experimental data, allowing the measurements of the order reconstruction characteristic time.

Electro-optic response of surface-induced nematic order above the nematic-isotropic phase transition temperature

The optical retardation of a liquid crystal above the nematic-isotropic phase transition temperature T(NI) and subjected to planar alignment conditions at the substrate was investigated in the presence of an electric field applied normal to the substrates. The response was found to exhibit "S"-shaped behavior with electric field, and was larger near T(NI) than well above T(NI). The results were examined in the context of a model that permits both biaxiality and a field-induced tilt of the molecular director. The results suggest that the primary effect of the electric field is to induce biaxiality, and in consequence suppress the order induced by the surface. No clear indication of a Freedericksz-like transition, either experimental or theoretical, was observed.