Investigation On Barocaloric Refrigeration And Thermal Storage Performances Of New Plastic Crystal Materials

The refrigerants used in conventional gas compression refrigeration technology lead to a higher greenhouse effect,which is not conducive to the priority task of promoting green and low-carbon development planning in our country.To achieve the strategic goal of"carbon neutrality",it is crucial to develop new technologies for energy saving and emission reduction.In recent years,solid-state phase change refrigeration has been attracting attention as a new type of refrigeration technology because of its clean,efficient,and low-carbon properties.Among them,the barocaloric refrigeration technology based on plastic crystal materials is considered as one of the potential alternatives to conventional gas compression refrigeration technology due to its colossal isothermal entropy change during the phase transition process.Although some progress has been made in the research of barocaloric effects based on plastic crystals,there are still some problems.Firstly,there are fewer plastic crystal systems,and new types of plastic crystal systems need to be developed.Secondly,the current research mainly focuses on the performances of the barocaloric effects,while the intrinsic phase transition mechanisms of the barocaloric effects have not been sufficiently investigated.These problems restrict the applications of the barocaloric effect of plastic crystals in practice.Therefore,finding new plastic crystal materials with the high performance of barocaloric effects and revealing the phase transition mechanisms of the barocaloric materials are the key issues in the current field.In this paper,a series of new plastic crystal materials are investigated and the microscopic mechanisms of the barocaloric effects are deeply explored by neutron scattering as the main technical means.In particular,the inverse colossal barocaloric effect is found in NH₄SCN and applied to the field of controllable thermal storage.This research achievement is of great significance for improving energy utilization efficiency and reducing carbon emissions.The main content of this article is as follows:（1）Based on the study of the colossal barocaloric effect of neopentylglycol,its local structures and phase transition processes were further investigated.The colossal barocaloric effect of neopentylglycol was characterized by the high-pressure differential scanning calorimeter,and the critical pressure and saturation pressure were explored.The local atomic structures during the plastic crystal phase transition were studied using the pair distribution function analysis method.The significant phase transition asymmetry during the plastic crystal phase transition was discovered from heat flow curves and time-resolved X-ray diffraction spectra,where the time required for the phase transition during cooling was significantly less than that during heating.The phase transition asymmetry was not only affected by temperature ramping rates but also by pressure.As the pressure increased,the phase transition asymmetry decreased in neopentylglycol.（2）To explore new barocaloric materials,the barocaloric effects of 2-methyl-2nitro-1-propanol,formylferrocene,and 2-methyl-1,3-cyclohexanedione were investigated.2-methyl-2-nitro-1-propanol has a similar structure to neopentylglycol but has different molecular groups.In the study of the colossal barocaloric effect of 2-methyl2-nitro-l-propanol,the effects of different molecular groups on the performances of the barocaloric effect were compared.Under a driven pressure of 60 MPa,2-methyl-2nitro-1-propanol reaches a maximum isothermal entropy change of 360 J·kg^-1·K^-1.Formylferrocene belongs to organometallic plastic crystal materials and reaches the maximum isothermal entropy change of 186 J·kg^-1·K^-1 at 40 MPa.Through the study of formylferrocene,the research direction of barocaloric effects in organometallic plastic crystal materials has been opened up.2-methyl-1,3-cyclohexanedione undergoes a second-order phase transition at 244.6 K,with a thermal hysteresis of 0.7 K.The barocaloric effect of the second-order phase transition was characterized based on 2methyl-1,3-cyclohexanedione,achieving an almost completely reversible barocaloric effect.（3）The barocaloric thermal storage performances were investigated based on the inverse colossal barocaloric effect of the plastic crystal material NH₄SCN.The barocaloric thermal storage absorbs waste heat upon pressurization through the orderto-disorder plastic crystal phase transition of NH₄SCN,releasing about 43 J·g^-1 during depressurization,which is 11 times more than the input mechanical energy.The NH₄SCN can maintain the disordered phase stably under pressure to achieve longterm heat storage.We characterized the performances of the barocaloric thermal storage,which can reach a maximum temperature change of 12 K during depressurization.The stability of the barocaloric thermal storage performance was demonstrated through variable-pressure heat flow measurements under different temperatures.The inverse colossal barocaloric effect of NH₄SCN was analyzed.Under the saturation pressure of 80 MPa,the maximum isothermal entropy change was 128 J·kg^-1·K^-1,making it the first material with inverse colossal barocaloric effect.Finally,the application scenarios of barocaloric thermal storage were analyzed in detail,promoting its application in various fields.（4）The mechanism of the inverse colossal barocaloric effect of NH₄SCN was studied.Neutron scattering and synchrotron radiation were used to determine the crystal structures and lattice parameters of each phase of NH₄SCN,and revealed the connection between negative thermal expansion and the inverse barocaloric effect.The evolution process of the local structures during plastic phase transition was studied by combining the analysis method of the pair distribution function,confirming that the disordered state in plastic phase transition originates from the orientational disorder motion of NH₄⁺.The dynamic modes of NH₄⁺were analyzed using quasi-elastic neutron scattering.Raman scattering spectra at different temperatures and pressures explained the anharmonic dynamic process of the phase transition,and combined with computational simulation methods to explore the mechanism of the inverse colossal barocaloric effect:the hydrogen bonds between the monoclinic phase ions are weakened under applied pressure,causing NH₄⁺ from an ordered state bound by hydrogen bonds to an orientationally disordered state.