Colossal barocaloric effects near room temperature in plastic crystals of neopentylglycol

Abstract

There is currently great interest in replacing the harmful volatile hydrofluorocarbon fluids used in refrigeration and air-conditioning with solid materials that display magnetocaloric, electrocaloric or mechanocaloric effects. However, the field-driven thermal changes in all of these caloric materials fall short with respect to their fluid counterparts. Here we show that plastic crystals of neopentylglycol (CH₃)₂C(CH₂OH)₂ display extremely large pressure-driven thermal changes near room temperature due to molecular reconfiguration, that these changes outperform those observed in any type of caloric material, and that these changes are comparable with those exploited commercially in hydrofluorocarbons. Our discovery of colossal barocaloric effects in a plastic crystal should bring barocaloric materials to the forefront of research and development in order to achieve safe environmentally friendly cooling without compromising performance.

Fig. 1

Thermally driven phase transition in NPG at atmospheric pressure. a Measurements of dQ/|dT| after baseline subtraction, on heating (red) and cooling (blue) across the first-order cubic-monoclinic phase transition, revealing a large latent heat. The insets represent simplified plan views of the globular (CH₃)₂C(CH₂OH)₂ molecules (C = dark green spheres, H = grey spheres and O = light green spheres), which are configurationally ordered in the monoclinic ordered-crystal (OC) phase (left inset), and configurationally disordered in the cubic plastic-crystal (PC) phase (right inset). We assume only one molecule per unit cell for ease of representation. b Specific heat C_p either side of the transition on heating (red) and cooling (blue). c Entropy S′(T) = S(T)−S(250 $\mathrm{K}$), evaluated via $S\prime \left(T\right)=S\left(T\right)-S\left(250\mathrm{K}\right)={\int }_{250\mathrm{K}}^T\left(C_p+∣\mathrm{d}Q∕\mathrm{d}T\prime ∣\right)∕T\prime \mathrm{d}T\prime$, revealing a large entropy change |ΔS₀| for the transition. d Specific volume V(T) on heating, revealing a large volume change |ΔV₀| for the transition. Symbols represent experimental data, lines are guides to the eye

Fig. 2

Pressure-driven phase transition in NPG. a, b Measurements of dQ/|dT| on heating and cooling across the first-order PC-OC transition for different values of increasing pressure p, after baseline subtraction. c, d Transition temperature and entropy change |ΔS₀(p)| on heating (red symbols) and cooling (blue symbols), derived from the calorimetric data of a, b and equivalent data at other pressures (shown in Supplementary Fig. 2). Black lines in c are linear fits. Red and blue lines in c, d are guides to the eye. e Volume change for the transition |ΔV₀(p)|: solid symbols obtained from the dilatometric data (DD) in Supplementary Fig. 3a; open circle obtained from the x-ray diffraction data in Fig. 1c, open square obtained from the x-ray diffraction data in Supplementary Fig. 3b; orange line obtained from c, d via the Clausius–Clapeyron (CC) equation

Fig. 3

Colossal barocaloric effects in NPG near room temperature. a, b Entropy S’(T,p) with respect to the absolute entropy at 250 K and p ~ 0, on a heating and b cooling through the first-order PC-OC phase transition. c Isothermal entropy change ΔS for 0 → p deduced from b, and for p → 0 deduced from a. d Adiabatic temperature change ΔT versus starting temperature T_s, for 0 → p deduced from b. e Adiabatic temperature change ΔT versus finishing temperature T_f for p → 0 deduced from a

Fig. 4

Barocaloric performance near room temperature. a For NPG, we show the peak isothermal entropy change |ΔS_peak| for pressure changes of magnitude |p|, on applying pressure (blue symbols) and removing pressure (red symbols). For comparison, the green envelope represents state-of-the-art barocaloric materials (Table 1) that operate near room temperature, and the orange symbol represents the standard commercial fluid refrigerant R134a for which operating pressures are ~0.001 GPa. For NPG alone, we show the variation with |p| of b refrigerant capacity RC = |ΔS_peak| × [FWHM of ΔS(T)] and c peak values of the adiabatic temperature change |ΔT_peak|, on applying pressure (blue symbols) and removing pressure (red symbols) near room temperature