Barocaloric response of plastic crystal 2-methyl-2-nitro-1-propanol across and far from the solid-solid phase transition

Abstract

Plastic crystals have emerged as benchmark barocaloric materials for potential solid-state cooling and heating applications due to huge isothermal entropy changes and adiabatic temperature changes driven by pressure. In this work we investigate the barocaloric response of the neopentane derivative 2-methyl-2-nitro-1-propanol (NO2C(CH3)2CH2OH) in a wide temperature range using X-ray diffraction, dilatometry and pressure-dependent differential thermal analysis. Near the ordered-to-plastic transition, we find colossal barocaloric effects of ≅400 J K-1 kg-1 and ≅5 K upon pressure changes of 100 MPa. Although reversible effects at the transition are obtained only from higher pressure changes due to hysteretic effects, we do obtain fully reversible BC effects from any pressure change in individual phases, that become giant at moderate pressures due to very large thermal expansion, especially in the plastic phase. From our measurements, we also determine the crystal structure of the low temperature phase and estimate the contribution of the configurational disorder and the volume change to the total transition entropy change.

Landau theory of barocaloric plastic crystals

“…Our model predicts that the onset of long-range orientational ordering leads to (i) a change ∆α in the coefficient of thermal expansion (equations ( 10) and ( 11)), (ii) a change in volume ∆V t (equations ( 10) and ( 12)), (iii) a change in entropy ∆S t (equations ( 15) and ( 16)), and (iv) a linear temperature-pressure phase boundary (equation ( 9)), which is in agreement with experiments [7][8][9][10][11][12][13][14][15][16][17][18][19]. We note that ∆α, ∆V t , and dT c /dP (equation ( 14)) depend on the strain-orientation coupling.…”

“…We note that ∆α, ∆V t , and dT c /dP (equation ( 14)) depend on the strain-orientation coupling. The main discrepancy between our model and experiments [7,9,10,12,13,16]), is that the predicted ∆V t and ∆S t are independent of pressure. This shortcoming stems from neglecting non-linear invariants beyond those we have considered in the strain-orientation coupling energy G ϵQ , which yield the change ∆A t independent of P (equation ( 13)).…”

“…For applied hydrostatic pressures in the range in which BC effects have been experimentally observed (0 − 0.60 GPa) [16], the transition temperature increases up to about 360 K. The orientational OP of the phase transition is a tetrahexacontapole (64-pole) [45], and while moments of lower order such as the quadrupole Q vanish due to molecular symmetry, we still use our model, as equation ( 7) is isomorphic to the free-energy density of fullerite [45]. We obtain model parameters by performing the following fits to experimental data [16]: T 0 c and dT c /dP are obtained by fitting equation ( 9) to the temperature-pressure phase boundary on heating; α, ∆α, and ∆V t are obtained by fitting equation (10) with V 0 = 1 to the observed volume at ambient pressure. The results are as follows: T 0 c = 263 K, dT c /dP = 165 K GPa −1 , α = 42 × 10 −6 K −1 , ∆α = 36 × 10 −6 K −1 , and ∆V t = −0.012.…”

“…These colossal BC effects are comparable to the pressure-driven thermal changes seen in hydrofluorocarbons/hydrocarbons used in vapor-compression refrigeration 4 . PCs exhibiting colossal BC effects include neopentane derivatives [7][8][9][10], adamantane [11], adamantane derivatives [12,13], carboranes [14], and alkali hydrido-closoborates [15]. In addition, orientationally disordered solids which are not as mechanically soft as PCs, can also display large [16,17] and colossal [18] BC effects.…”