Investigation On Thermal Functional Properties Of Fe-Doped Nickel Sulfides With Hexagonal Structure

From electricity generated by burning fossil fuels,to waste heat recovery and utilization,to building heating,and to thermal management of power battery and microelectronics,heat generation and transmission are inseparable.Statistically,roughly 90%of the world's energy use involves generation or manipulation of heat.Therefore,the precise control of heat transfer is of great important to elevate energy efficiency,emission reduction and sustainable development.In addition,for the storage and utilization of waste heat generated in industrial production and solar energy in renewable energy,it can not only solve the problem of time or regional imbalance of energy supply,but also significantly improve the energy utilization efficiency.Hexagonal sulfide Ni1-xFexS are well-known compound because of its first-order phase transition.At transition temperature,the antiferromagnetic(AFM)nonmetal(or semiconductor)state transforms to paramagnetic(PM)metal state,which is concomitant with the changes of lattice,electron and spin.This multi-degree-offreedom coupling makes makes it have rich functional properties and physical properties,such as metal-nonmetal transition,barocaloric effect and large latent heat.In this paper,Ni1-xFexS(x ? 0.2)were systematically studied with high contrast thermal conductivity,and combining theoretical calculations its physical mechanism is clarified.Moreover,the thermal rectification of the thermal diode composed of Ni0.85Fe0.15S and Al2O3 was investigated.Additionally,Ni1-xFexS with high figure of merit(FOM)can as a new type solid-state thermal energy storage material,which is discussed in the field of heat storage.The main contents of this paper are as follows:1.Hexagonal sulfide Ni1-xFexS were investigated with high contrast thermal conductivity.Here,we report a sharp change of thermal conductivity(??/?0)up to 200%within a temperature interval of?40 K in hexagonal sulfides Ni1-xFexS bonded with silver.Such ??/?0 values exceed those reported in other thermal regulation materials.Based on density functional theory,the results shows that the electronic thermal conductivity of high temperature paramagnetic metal phase is much higher than that of low temperature antiferromagnetic phase.The phonon spectrum analysis shows that when the phase changes from antiferromagnetic phase to paramagnetic phase,the frequency of optical phonon mode increases and the frequency of acoustic mode decreases,and their contributions to thermal conductivity offset each other.Therefore,the change of phonon thermal conductivity before and after phase transition is small.Moreover,the addition of silver overcomes the inherent brittleness of Ni1-xFexS and thus improves machinability and thermal cycling stability,which is beneficial to the practical applications.Our work presents a new promising material for thermal management applications.2.The thermal rectification of the thermal diode constructed by Ni1-xFexS and Al2O3 was investigated.A high rectification factor ? of 1.51 was achieved with a relatively small temperature bias of 97 K near room temperature when the cold terminal temperature is set at 250 K.At the same time,it is found that its thermal rectification effect is strongly dependent on the cold terminal temperature.The excellent rectification effect is attributed to the high contrast change in thermal conductivity of Ni0.85Fe0.15S.In addition,we demonstrated that the maximum thermal rectification ratio is strongly dependent on the cold terminal temperature of the thermal diode,the length ratio between the two segments,and the steepness of first-order phase transition of Ni0.85Fe0.15S.3.The Ni1-xFexS with the large latent heat and high thermal conductivity were studied,which can be as a new type solid-state thermal energy storage material.The maximum latent heat can reach 109.23 Jcm-3 when x=0.125.Meanwhile,these materials have high thermal conductivity(about 13 WK-1m-1)and high density(about 5.5 gcm-3),which much higher than that of conventional phase change heat storage materials.Combining these advantages,its means that the figure of merit(FOM)of Ni1xFexS is much higher than that of conventional phase change heat storage materials,which means that as a phase change heat storage material,it has a faster heat exchange efficiency and higher volumetric heat storage density.