Exploring new materials for caloric cooling
Caloric materials produce nominally reversible thermal changes when subjected to an applied driving field. These materials are widely considered a potential replacement to the environmentally harmful refrigerant fluids that are currently used in vapour compression systems. Barocaloric materials that are driven by changes in hydrostatic pressure are comparatively the most promising type of caloric material, as they display the largest thermal changes, are practically more cost-effective, and can withstand large cyclic changes in hydrostatic pressure without breaking down or decreasing performance. This work reports on a selection of superionic conductors and metal-organic frameworks that were chosen based on their promising thermodynamic properties for barocaloric studies. Furthermore, novel ways of driving caloric effects are explored by exploiting latent heat from first-order phase transitions in a smart polymer driven by stimuli unexplored in the field of calorics thus far. Despite displaying large thermally driven entropy changes (|∆S₀|) near superionic transitions, the superionic conductors studied here showed a tendency to exist in many stoichiometric compositions and metastable states, which hindered their barocaloric performance. Complex calorimetric data from synthesised Ag-Cu-S ternary materials were understood to be an effect of variation in the pressure-dependence of the transition in different stoichiometries of the individual Ag-Cu-S ternary phases present. Furthermore, both conventional and inverse barocaloric effects were produced within each individual sample, which resulted from two different phases of the ternary system. Similar phase-related complexity was demonstrated for Ag₂Se and Cu₂Se. The metal-organic frameworks studied display improved barocaloric performance. Large reversible isothermal entropy changes |∆S(it)| ∼ 14 J K¯¹ kg¯¹ and adiabatic temperature changes |∆T(ad)| ∼ 5 K due to changes in applied pressure |∆p| ∼ 1 kbar were evaluated in a nitrogenated sample of the zeolitic imidazolate framework ZIF-4(Zn) near its first-order phase transition at T₀ ∼ 185 K. These barocaloric effects could be driven over an exceptionally wide range of starting temperatures (∼75 K when driven by |∆p| ∼ 1 kbar) due to the large tunability of T₀ with pressure in this compound. While initial calculations yielded |∆S₀| ∼ 16 J K¯¹ kg¯¹, extended analysis correlating time-dependent calorimetric measurements with x-ray diffraction data and the Clausius-Clapeyron relation indicated that a certain fraction of the sample does not undergo the transition, which therefore largely dwarfed |∆S₀| due to mass normalisation. By correcting for this inactive mass, a maximum barocaloric response of |∆S(it)| ∼ 155 J K¯¹ kg¯¹ and |∆T(ad)| ∼ 50 K driven by |∆p| ∼ 1 kbar was evaluated. Furthermore, a novel type of caloric effect, tentatively termed barosorptiocaloric in this work, was investigated in MIL-53(Fe) near a sorption driven first-order phase transition, in which adsorption and desorption of water molecules in the porous MIL-53(Fe) framework was regulated by changes in hydrostatic pressure. This new driving mechanism delivered |∆S(it)| ∼ 10 J K¯¹ kg¯¹ and |∆T(ad)| ∼ 7 K near room temperature, with nominally no hysteresis. Two more types of novel caloric effects were investigated in the smart polymer, poly(N-isopropylacrylamide) [p(NIPAM)], and its acidic functional derivative, by using changes in solvent concentrations and pH to drive a first-order conformational phase transition. The caloric responses from these previously unexplored caloric driving fields, herein termed solvocaloric effects and pH-caloric effects respectively, were evaluated. Changes in pH only shift the transition temperature marginally, but changes in ethanol:water solvent ratios led to a large shift in transition temperature of ∼20 °C, leading to large solvocaloric changes in entropy of ∼16 J K¯¹ kg¯¹ near room temperature in p(NIPAM), with nominally no hysteresis. Direct measurements of adiabatic temperature change were performed to demonstrate this novel effect.
barocaloric, barosorptiocaloric, first-order transitions, metal-organic frameworks, pH-caloric, smart polymer, solvocaloric, superionic conductors
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