Giant and Reversible Inverse Barocaloric Effects near Room Temperature in Ferromagnetic MnCoGeB0.03

Colossal Reversible Barocaloric Effects in a Plastic Crystal Mediated by Lattice Vibrations and Ion Diffusion

Solid-state methods for cooling and heating promise a sustainable alternative to current compression cycles of greenhouse gases and inefficient fuel-burning heaters. Barocaloric effects (BCE) driven by hydrostatic pressure (p) are especially encouraging in terms of large adiabatic temperature changes (|ΔT| ≈ 10 K) and isothermal entropy changes (|ΔS| ≈ 100 J K-1 kg-1). However, BCE typically require large pressure shifts due to irreversibility issues, and sizeable |ΔT| and |ΔS| seldom are realized in a same material. Here, the existence of colossal and reversible BCE in LiCB11H12 is demonstrated near its order-disorder phase transition at ≈380 K. Specifically, for Δp ≈ 0.23 (0.10) GPa, |ΔSrev| = 280 (200) J K-1 kg-1 and |ΔTrev| = 32 (10) K are measured, which individually rival with state-of-the-art BCE figures. Furthermore, pressure shifts of the order of 0.1 GPa yield huge reversible barocaloric strengths of ≈2 J K-1 kg-1 MPa-1. Molecular dynamics simulations are performed to quantify the role of lattice vibrations, molecular reorientations, and ion diffusion on the disclosed BCE. Interestingly, lattice vibrations are found to contribute the most to |ΔS| while the diffusion of lithium ions, despite adding up only slightly to the entropy change, is crucial in enabling the molecular order-disorder phase transition.

Low pressure reversibly driving colossal barocaloric effect in two-dimensional vdW alkylammonium halides

Plastic crystals as barocaloric materials exhibit the large entropy change rivalling freon, however, the limited pressure-sensitivity and large hysteresis of phase transition hinder the colossal barocaloric effect accomplished reversibly at low pressure. Here we report reversible colossal barocaloric effect at low pressure in two-dimensional van-der-Waals alkylammonium halides. Via introducing long carbon chains in ammonium halide plastic crystals, two-dimensional structure forms in (CH3-(CH2)n-1)2NH2X (X: halogen element) with weak interlayer van-der-Waals force, which dictates interlayer expansion as large as 13% and consequently volume change as much as 12% during phase transition. Such anisotropic expansion provides sufficient space for carbon chains to undergo dramatic conformation disordering, which induces colossal entropy change with large pressure-sensitivity and small hysteresis. The record reversible colossal barocaloric effect with entropy change ΔSr ~ 400 J kg-1 K-1 at 0.08 GPa and adiabatic temperature change ΔTr ~ 11 K at 0.1 GPa highlights the design of novel barocaloric materials by engineering the dimensionality of plastic crystals.

Ultrasensitive barocaloric material for room-temperature solid-state refrigeration

One of the greatest obstacles to the real application of solid-state refrigeration is the huge driving fields. Here, we report a giant barocaloric effect in inorganic NH₄I with reversible entropy changes of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {S}_{{P}_{0}\to P}^{{{\max }}}$$\end{document}ΔSP0→Pmax ∼71 J K⁻¹ kg⁻¹ around room temperature, associated with a structural phase transition. The phase transition temperature, T_t, varies dramatically with pressure at a rate of dT_t/dP ∼0.79 K MPa⁻¹, which leads to a very small saturation driving pressure of ΔP ∼40 MPa, an extremely large barocaloric strength of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left|\Delta {S}_{{P}_{0}\to P}^{{{\max }}}/\Delta P\right|$$\end{document}ΔSP0→Pmax/ΔP ∼1.78 J K⁻¹ kg⁻¹ MPa⁻¹, as well as a broad temperature span of ∼41 K under 80 MPa. Comprehensive characterizations of the crystal structures and atomic dynamics by neutron scattering reveal that a strong reorientation-vibration coupling is responsible for the large pressure sensitivity of T_t. This work is expected to advance the practical application of barocaloric refrigeration.

A small driving pressure is desirable for practical application of barocaloric materials. Here, the authors demonstrate a sensitive barocaloric effect in NH₄I due to strong reorientation-vibration coupling.

Giant barocaloric effects with a wide refrigeration temperature range in ethylene vinyl acetate copolymers

Solid-state cooling technology based on the caloric effects of phase-transition materials has been a research hotspot due to its environmental friendliness and high efficiency, but limited for practical applications due to its narrow working temperature region. Here, we report giant barocaloric effects based on pressure-driven liquid-solid phase transitions in elastic copolymers of ethylene and vinyl acetate. Giant adiabatic temperature changes of up to 29.6/-26.9 K are directly observed under rapid compressions/decompressions of 400 MPa near the liquid-solid phase transition points. Strikingly, since both the solid and the liquid sides can show giant barocaloric effects, a very broad refrigeration temperature region of more than 110 K is achieved in these copolymers. Furthermore, a cooling prototype is designed to demonstrate the potential applications of these liquid/elastic barocaloric materials. Our study sheds light on exploring liquid-solid phase transition materials for the next-generation refrigerators.

Martensitic Transformation and Barocaloric Effect in Co-V-Ga-Fe Paramagnetic Heusler Alloy

In the present study, polycrystalline Co50V34Ga16−xFex (1≤x≤2) quaternary Heusler alloys were fabricated by the arc-melting method. It was found that they undergo a paramagnetic martensitic transformation (MT) from the L21-type cubic austenitic structure to the D022 tetragonal martensitic structure. With the increase of the Fe concentration, the MT shifts towards a higher temperature range, which is strongly related to the variation of the valence electron concentration. Moreover, it was also found that the MT exhibited by every alloy is sensitive to the applied hydrostatic pressure due to a relatively high difference in volume between the two phases. By using the quasi-direct method based on caloric measurements, the barocaloric effect (BCE) associated with the hydrostatic pressure-driven MT was estimated in the studied alloys. The results demonstrated that the sample with x=1.5 performs an optimal BCE among these three alloys.

Colossal and reversible barocaloric effect in liquid-solid-transition materials n-alkanes

Emerging caloric cooling technology provides a green alternative to conventional vapor-compression technology which brings about serious environmental problems. However, the reported caloric materials are much inferior to their traditional counterparts in cooling capability. Here we report the barocaloric (BC) effect associated with the liquid-solid-transition (L-S-T) in n-alkanes. A low-pressure of ~50 MPa reversibly triggers an entropy change of ~700 J kg⁻¹ K⁻¹, comparable to those of the commercial refrigerants in vapor-based compression systems. The Raman study and theoretical calculations reveal that applying pressure to the liquid state suppresses the twisting and random thermal motions of molecular chains, resulting in a lower configurational entropy. When the pressure is strong enough to drive the L-S-T, the configurational entropy will be fully suppressed and induce the colossal BC effect. This work could open a new avenue for exploring the colossal BC effect by evoking L-S-T materials.

Barocaloric effect, previously reported in solid-solid phase transition materials, offers a green alternative to current cooling technology. Here the authors report colossal BCE in n-alkanes associated with liquid–solid transition.

Giant and Reversible Barocaloric Effect in Trinuclear Spin-Crossover Complex Fe3 (bntrz)6 (tcnset)6

A giant barocaloric effect (BCE) in a molecular material Fe₃ (bntrz)₆ (tcnset)₆ (FBT) is reported, where bntrz = 4-(benzyl)-1,2,4-triazole and tcnset = 1,1,3,3-tetracyano-2-thioethylepropenide. The crystal structure of FBT contains a trinuclear transition metal complex that undergoes an abrupt spin-state switching between the state in which all three Fe^II centers are in the high-spin (S = 2) electronic configuration and the state in which all of them are in the low-spin (S = 0) configuration. Despite the strongly cooperative nature of the spin transition, it proceeds with a negligible hysteresis and a large volumetric change, suggesting that FBT should be a good candidate for producing a large BCE. Powder X-ray diffraction and calorimetry reveal that the material is highly susceptible to applied pressure, as the transition temperature spans the range from 318 at ambient pressure to 383 K at 2.6 kbar. Despite the large shift in the spin-transition temperature, its nonhysteretic character is maintained under applied pressure. Such behavior leads to a remarkably large and reversible BCE, characterized by an isothermal entropy change of 120 J kg^-1 K^-1 and an adiabatic temperature change of 35 K, which are among the highest reversible values reported for any caloric material thus far.

On the instability of the giant direct magnetocaloric effect in CoMn0.915Fe0.085Ge at. % metamagnetic compounds

The giant magnetocaloric effect was quantified in CoMn_1-xFe_xGe (x = 0.085–0.12) nom. at. % polycrystals across the high temperature hexagonal (P6₃/mmc) to low temperature orthorhombic (Pnma) phase transition via differential scanning calorimetry (DSC) and multiple (thermo) magnetization measurements. It was found that increasing Fe content led to the decrease of both the martensitic transformation temperature and entropy change (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta S$$\end{document}ΔS) at the point of the phase transition. Moreover, first-time magnetocaloric measurements resulted in irreproducible entropy change versus temperature diagrams, which was attributed to the release of internal pressure in bulk samples that disintegrated into powder upon transformation. CoMn_1-xFe_xGe demonstrated larger magnetic field-induced entropy changes and giant magnetocaloric effect (MCE) compared to other CoMnGe alloys doped with Si, Sn, Ti, and Ga. However, the observed brittleness and apparent change in volume at the magnetic transition was posited to influence the material’s potential for regenerative applications.

Caloric materials for cooling and heating

Magnetically driven thermal changes in magnetocaloric materials have, for several decades, been exploited to pump heat near room temperature. By contrast, their electrocaloric and mechanocaloric counterparts have only been intensively studied and exploited for little more than a decade. These different caloric strands have recently been unified to yield a single field of research that could help combat climate change by generating better heat pumps for both cooling and heating. Here we outline the timeliness of the present activity and discuss recent advances in caloric measurements, materials, and prototypes.