Plastic Crystal Neopentyl Glycol/Multiwall Carbon Nanotubes Composites for Highly Efficient Barocaloric Refrigeration System

Plastic crystal neopentyl glycol(NPG)exhibits colossal barocaloric effect with high entropy changes.However,their application is restricted in several aspects,such as low thermal conductivity,substantial supercooling effect,and poor springback properties.In this work,multi-walled carbon nanotubes(MWCNTs)with ultra-high thermal conductivity and high mechanical strength were selected for performance enhancement of NPG.The optimal mixing ratio was determined to be NPG with 3 wt%MWCNTs composites,which showed a 6K reduction in supercooling without affecting the phase change enthalpy.Subsequently,comprehensive performance of the composites with optimal mixing ratio was compared with pure NPG At 40 MPa,390J·K^(-1)·kg^(-1)change in entropy and 9.9 K change in temperature were observed.Furthermore,the minimum driving pressure required to achieve reversible barocaloric effect was reduced by 19.2%.In addition,the thermal conductivity of the composite was increased by approximately 28%,significantly reducing the heat exchange time during a barocaloric refrigeration cycle.More importantly,ultra-high pressure release rate resulted in a73.7%reduction in the springback time of the composites,offering new opportunities for the recovery of expansion work.

Thermodynamic Analysis on the Performance of Barocaloric Refrigeration Systems Using Neopentyl Glycol as the Refrigerant

Barocaloric refrigeration is regarded as one of the next-generation alternative refrigeration technology due to its environmental friendliness.In recent years,many researchers have been devoted to finding materials with colossal barocaloric effects,while neglecting the research on barocaloric refrigeration devices and thermodynamic cycles.Neopentyl glycol is regarded as one of the potential refrigerants for barocaloric refrigeration due to its giant isothermal entropy changes and relatively low operating pressure.To evaluate the performance of the barocaloric system using Neopentyl glycol,for the first time,this study establishes a thermodynamic cycle based on the metastable temperature-entropy diagram.The performance of the proposed system is investigated from the aspects of irreversibility,operating temperature range,and operating pressure,and optimized with finite-rate heat transfer.The guidance for the optimal design of the system is given by revealing the effect of the irreversibility in two isobaric processes.The results show that a COP of 8.8 can be achieved at a temperature span of 10 K when the system fully uses the phase transition region of Neopentyl glycol,while a COP of 3 can be achieved at a temperature span of 10 K when the system operates at room temperature.Furthermore,this study also shows that the system performance can be further improved through the modification of Neopentyl glycol,and some future development guidance is provided.

Barocaloric Material with High Thermal Conductivity for Room-Temperature Refrigeration

Barocaloric refrigeration technology,one of the caloric-effect refrigeration technologies,is drawing more and more attention.Neopentyl glycol(NPG)was reported to have a giant barocaloric effect,making it a potential barocaloric material.However,the high solid-solid(S-S)phase transition temperature and low thermal conductivity hinder the application of NPG in barocaloric refrigeration.This work lowers the S-S phase transition temperature and improves the thermal conductivity of the NPG-based barocaloric material.An NPG/TMP(TMP:Trimethylolpropane)binary system with an S-S phase transition temperature of 283.15 K is prepared,in which the mass ratio of TMP is 20%.Graphene is then added to the binary system to enhance thermal conductivity,and the optimal mass ratio of graphene was determined to be 5%.The thermal conductivity of this composite is 0.4 W/(m·K),increased by 110%compared to the binary system.To predict the effect of enhanced thermal conductivity on the cold-extraction process of the barocaloric refrigeration cycle,a numerical model is developed.The results show that the cold-extraction time of the barocaloric refrigeration cycle utilizing the composite with 5%graphene as the refrigerant is shortened by 50%compared with that using the binary system.

Supergiant Barocaloric Effects in Acetoxy Silicone Rubber over a Wide Temperature Range:Great Potential for Solid-state Cooling

Solid-state cooling based on caloric effects is considered a viable alternative to replace the conventional vapor-compression refrigeration systems.Regarding barocaloric materials,recent results show that elastomers are promising candidates for cooling applications around room-temperature.In the present paper,we report supergiant barocaloric effects observed in acetoxy silicone rubber—a very popular,low-cost and environmentally friendly elastomer.Huge values of adiabatic temperature change and reversible isothermal entropy change were obtained upon moderate applied pressures and relatively low strains.These huge barocaloric changes are associated both to the polymer chain rearrangements induced by confined compression and to the first-order structural transition.The results are comparable to the best barocaloric materials reported so far,opening encouraging prospects for the application of elastomers in near future solid-state cooling devices.

Colossal barocaloric effects discovered in plastic crystals

Partially supported by the National Natural Science Foundation of China,Profs.Li Bing(李昺),Zhang ZhiDong(张志东),and Ren WeiJun(任卫军)from the Institute of Metal Research,Chinese Academy of Sciences reported colossal barocaloric effects in plastic crystals and indicated a new direction for emergent solid-state refrigeration technologies.This work was conducted in cooperation with researchers from Japan,United States,and Australia,and published in Nature(2019,567:506).

Giant mechanocaloric materials for solid-state cooling

This article reviews the research progress of measurement techniques and materials on the mechanocaloric effect over the past few decades.Mechanocaloric materials can be divided into elastocaloric and barocaloric materials depending on the applied uniaxial stress or hydrostatic pressure.Elastocaloric materials include non-magnetic shape memory alloys,polymers,and rare-earth compounds.Barocaloric materials include magnetic shape memory alloys,ferroelectric ceramics,superionic conductors,and oxyfluorides.The mechanocaloric effects of these classes of materials are systematically compared in terms of the isothermal entropy change and adiabatic temperature change.In addition to the thermal effects,other characteristics closely related to the application of mechanocaloric materials are also summarized.Finally,perspectives for further development of mechanocaloric materials in the solid-state cooling area are discussed.

High-energy x-ray diffraction study on phase transition asymmetry of plastic crystal neopentylglycol

As a prototype material of colossal barocaloric effects, neopentylglycol is investigated by combining high-precision differential scanning calorimetric measurement and high-energy x-ray diffraction measurement. The diffraction data at constant temperatures indicate a first-order phase transition with thermal hysteresis as well as the phase transition asymmetry,specifically, the phase transition is completed faster at cooling than at heating. The analysis of resulting pair distribution function confirms the intermolecular disorder in the high-temperature phase. The phase transition asymmetry is quantitatively characterized by time-resolved x-ray diffraction, which is in agreement with the thermal measurement. Also, such an asymmetry is observed to be suppressed at high pressures.

A hot future for cool materials

The ewidespreadneed to pump heat necessitates simprovements that will increase energy efficiency and,more generally,reduce eenvironmental impact.As discussed at the recent Calorics 2022 Conference,heat-pump devices based on caloric materials offer an intriguing alternative to gas combustion and vapor compression.

Microstructure and giant baro-caloric effect induced by low pressure in Heusler Co_(51)Fe_(1)V_(33)Ga_(15) alloy undergoing martensitic transformation

Solid-state refrigeration based on the magneto-or mechano-caloric effect,including elasto-and barocaloric in ferroic phase transition materials is promising to replace the current vapor compression refrigeration in consideration of environmental-friendliness and energy-saving.However,both high driven field and small thermal changes in all of these caloric materials hinder the development of solid-state refrigeration.Here we report a giant baro-caloric effect near room temperature induced by a low hydrostatic pressure in Co-based Co_(51)Fe_(1) V_(33)Ga_(15) Heusler alloy.The maximum adiabatic temperature change under the applied pressure change ofΔp=0.1-100 MPa can be as high asΔ_(Tad)^(Max)=7.7 K(Δ_(Tad)^(Max)/Δpreaches up to~7.7 K kbar-1),surpassing theΔ_(Tad)^(Max)/Δpvalue reported hitherto in baro-caloric alloys.In addition,the microstructure is also studied by using the electron microscopes.Along with the austenite and martensite,the submicron V-rich particles are precipitated in this alloy,which are believed to account for enhancing mechanical properties.

Waste Tire Rubber-based Refrigerants for Solid-state Cooling Devices

Management of discarded tires is a compelling environmental issue worldwide.Although there are several approaches developed to recycle waste tire rubbers,their application in solid-state cooling is still unexplored.Considering the high barocaloric potential verified for elastomers,the use of waste tire rubber(WTR)as a refrigerant in solid-state cooling devices is very promising.Herein,we investigated the barocaloric effects in WTR and polymer blends made of vulcanized natural rubber(VNR)and WTR,to evaluate its feasibility for solid-state cooling technologies.The adiabatic temperature changes and the isothermal entropy changes reach giant values,as well as the performance parameters,being comparable or even better than most barocaloric materials in literature.Moreover,pure WTR and WTR-based samples also present a faster thermal exchange than VNR,consisting of an additional advantage of using these discarded materials.Thus,the present findings evidence the encouraging perspectives of employing waste rubbers in solid-state cooling based on barocaloric effects,contributing to both the recycling of polymers and the sustainable energy technology field.