Study On Phase Transition And Thermal Properties Of NiAs-type Iron-based Sulfides

With the continuous increase in global energy demand and the growing concern over environmental pollution,the research and development of highly efficient,environmentally friendly,and low-cost thermo-functional materials have become a critical focus in the field of science and technology.Thermo-functional materials are those capable of performing the conversion,transmission,storage,and control of thermal energy.Due to their diverse structural forms and rich physical properties,these materials can be widely utilized in energy conversion,energy storage,and optoelectronic devices,among other fields.The FeS compound with a NiAs structure has a rich crystal structure and potential applications in fields such as energy conversion,transmission,and optoelectronic devices.However,research on the thermophysical properties of FeS system compounds,such as thermal expansion,phase transition latent heat,thermal stability,and thermal conductivity,is limited.This paper focuses on the FeS system compound and systematically investigates its thermophysical properties at the phase transition point through element doping and thermal treatment methods.The study reveals the promotion effect of the micro-porous structure on the apparent negative thermal expansion of Fe1-xCoxS(x≤0.15),resulting in the preparation of self-composite Fe1-xCoxS(x≤0.12)materials with low thermal expansion properties.Furthermore,novel latent heat storage materials with high energy density and thermal stability are obtained in the form of Fe1-xS(x≤0.125).This research provides a reference for the thermophysical mechanism and regulation strategics of transition metal sulfides and offers an effective candidate system for developing new thermophysical controllable materials.The main conclusions of this study are as follows:1.Hexagonal Fe1-xCoxS(x≤0.15)with adjustable negative thermal expansion(NTE)performance was reported,and the promotion effect of micro-porous structure on apparent NTE was revealed.By systematically studying the cooperative mechanism between micro-porous structure and lattice anisotropic thermal expansion,it was found that this synergy can significantly enhance the apparent NTE performance of Fe1-xCoxS(x≤0.15)samples.When the doping amount of cobalt element was x=0.10,the inherent volumetric NTE coefficient measured by the diffraction method was-29.86 ppmK-1,while the apparent volumetric NTE coefficient of the bulk sample measured by strain method was as high as-97.18 ppmK-1.2.Based on a simple low-temperature annealing method,the low coefficient of thermal expansion of Fe1-xCoxS(x ≤0.12)in the widened temperature zone was experimentally studied.Phase analysis shows that the annealing process promotes the emergence of a non-stoichiometric precipitate phase with a few cation vacancies,which corresponds to a lower operating temperature window than the original stoichiometric matrix material.Therefore,the negative thermal expansion temperature range of the precipitate phase overlaps with the positive thermal expansion temperature range of the matrix phase,forming a self-composite material and producing a wide temperature range low expansion effect.For example,the Fe0.94Co0.06S sample annealed exhibits a thermal expansion coefficient of only 1.9 ppmK-1 in the temperature range of 76 K-252 K.The mechanical modulus difference between the two phases of this self-composite material is very small,and it exhibits good stability in thermal cycling.3.The phase transition enthalpy and phase transition thermal stability of hexagonal FeS was studied,and its potential as a solid-state phase change thermal storage material in the intermediate temperature range was evaluated-Hexagonal FeS was synthesized,and phase separation occurred after annealing at 450 K for 3 hours,generating a non-stoichiometric precipitate phase Fe1-δS(δ≤0.125)with cation vacancies.Neutron diffraction results showed that the annealing process promoted the escape of iron elements,causing the precipitate phase Fe1-δS(δ≤0.125)and the matrix FeS to reach equilibrium,with a molar ratio of about 55:45.The FeS compound annealed has a large latent heat storage density(136 Jcm-3).