Novel mechanocaloric materials for solid-state cooling applications

Abstract

Current refrigeration technologies based on compression cycles of greenhouse gases are environmentally threatening and cannot be scaled down to on-chip dimensions. Solid-state cooling is an environmentally friendly and highly scalable technology that may solve most of the problems associated with current refrigerant methods. Solid-state cooling consists of applying external fields (magnetic, electric, and mechanical) on caloric materials, which react thermally as a result of induced phase transformations. From an energy efficiency point of view, mechanocaloric compounds, in which the phase transitions of interest are driven by mechanical stresses, probably represent the most encouraging type of caloric materials. Conventional mechanocaloric materials like shape-memory alloys already display good cooling performances however in most cases they also present critical mechanical fatigue and hysteresis problems that limit their applicability. Finding new mechanocaloric materials and mechanisms able to overcome those problems while simultaneously rendering large temperature shifts, is necessary to further advance the field of solid-state cooling. In this article, we review novel families of mechanocaloric materials that in recent years have been shown to be specially promising in the aspects that conventional mechanocaloric materials are not, and which exhibit unconventional but significant caloric effects. We put an emphasis on elastocaloric materials, in which the targeted cooling spans are obtained through uniaxial stresses, since from an applied perspective these appear to be the most accomplished. Two different types of mechanocaloric materials emerge as particularly hopeful from our analysis, (1) compounds that exhibit field-induced order-disorder phase transitions involving either ions or molecules (polymers, fast-ion conductors, and plastic crystals), and (2) multiferroics in which the structural parameters are strongly coupled with polar and/or magnetic degrees of freedom (magnetic alloys and oxide perovskites).Keywords: solid-state cooling, caloric materials, field-induced transformations, entropy potential positive impact in world's sustainability caused by even modest improvements in energy efficiency is enormous. Meanwhile, for microchips to perform optimally the heat generated by electric currents needs to be removed; efficient micro-sized coolers operating near room temperature, therefore, are pressingly needed for successful engineering of faster and more compact electronic devices.Solid-state cooling is an environmentally friendly, highly energy-efficient, and highly scalable technology that may solve most of the problems associated with current refrigerant methods. Solid-state cooling relies on caloric materials and the application of external fields. In caloric materials reversible temperature shifts are achieved through the application/removal of electric, magnetic, or mechanical fields that render electrocaloric, magnetocaloric, or mechanocaloric effects, respectively. These caloric effects are ori...

Near-room-temperature reversible giant barocaloric effects in [(CH3)4N]Mn[N3]3 hybrid perovskite

“…27 This type of order-disorder phase transitions have been identified as mechanisms for potential barocaloric effects. 29 Variable-temperature powder X-ray diffraction data (Fig. 1b) revealed a large increase in specific volume of |DV 0 | B 9.40 AE 0.03 Â 10 À6 m 3 kg À1 on heating across the transition, which represents a relative change of B1.5%.…”

Colossal Barocaloric Effects in The Complex Hydride Li2B12H12

“…Therefore, we proceeded to estimate the barocaloric isothermal entropy, ∆S, and adiabatic temperature, ∆T , shifts induced by pressures of 0 ≤ P ≤ 0.4 GPa in bulk LBH. For this end, and in view of the first-order nature of the α ↔ β phase transformation [18], we employed the Clausius-Clapeyron method [2,3], which involves the knowledge of the α-β coexistence line slope (Fig. 2), ∆V (Fig.…”

“…This ionic diffusion behaviour is highly anomalous since hydrostatic compression typically hinders ionic transport [9][10][11]. On the other hand, the reorientational motion of the (BH) −2 12 icosahedra behaves quite normally, that is, decreases under pressure [3,4]. For instance, at T = 525 K and zero pressure we estimate λ B12H12 = 1.4 • 10 8 s −1 whereas at P = 0.4 GPa and same temperature we obtain 0.7•10 8 s −1 (Fig.…”

“…Solid-state cooling is an environmentally friendly, energy efficient, and highly scalable technology that can solve most of the problems associated with conventional refrigeration methods based on compression cycles of greenhouse gases (i.e., environmental harm and lack of downsize scaling). Upon application of magnetic, electric or stress fields good caloric materials undergo noticeable temperature changes (|∆T | ∼ 1-10 K) as a result of induced phase transformations that involve large entropy variations (|∆S| ∼ 10-100 JK −1 kg −1 ) [1][2][3]. Solidstate cooling capitalizes on such caloric effects to engineer refrigeration cycles.…”

“…Solidstate cooling capitalizes on such caloric effects to engineer refrigeration cycles. From a performance point of view, that is, largest |∆T | and |∆S| (although these are not the only parameters defining cooling efficiency [4]), barocaloric effects driven by small hydrostatic pressure shifts appear to be the most promising [1][2][3].…”