Preparation And Properties Of Nise₂、cu_xs Based Thermoelectric Materials

Over the past few years,thermoelectric materials have redrawn considerable attentions among physics,chemistry,and materials researcher due to their capability of direct conversion between heat and electricity,which is today well recognized as viable renewable-energy sources.The sulfides TE materials are a serier of novel TE materials which have more advantages such as low costs,low toxicity,and environment friendliness compared to the traditional TE materials of Bi2Te3,and PbTe.We focused on the NiSe2+x and CuxS materials and improved their thermoelectric properties by preparing process adjusting,mircosturcuture controlling and component optimizing.The pure NiSe2 was synthesised by Mechanical alloying(MA)and Sparkle Plasma Sintering(SPS)method.The carrier concentration increased with volatilized Se.The electrical conductivity increased ascribed to the formation of second phase Ni3Se2 own high electrical conductivity.The bulk SPSed at 793 K showed the best electrical conductivity value 8716 Scm-1 at 323 K.The thermal conductivity remained 7～8 Wm-1K-1 for all sample.The maximum power factor and ZT at 323 K were abtained 101 μWm-1K-2 and 0.0045 for the bulk SPSed at 773 K,respectively.N-type NiSe2+x(x = 0,0.01,0.02,0.06)bulks were fabricated by applying MA and SPS technique.The excessively added Se initially enters into the Se vacancy,and then into the Se octahedral interstice,which all lead to the decrease carrier concentration.The largest Seebeck coefficient absolute value of 9.27 μVK-1 was attained in the NiSe2.06 at 323 K.The thermal conductivity value reached 3.43 Wm-1K-1 for the NiSe2.01 at 323 K owing to the finest grains and the lowest relative density,which is also the lowest value in the literature.The NiSe2 powder was synthesized by MA.The NiSe2 coating SiO2 precursor powder was prepared by sol-gel method.Then xSiO2/NiSe2 precursor powders were sintered at 773 K using SPS methods.The largest Seebeck coefficient absolute value of 9.27μVK-1 was attained and the minimum thermal conductivity value reached 4.26 Wm-1K-1 for the sample with x=2.5 at 323 K.As x=0.5,the maximumpower factor at 323 K was acquired 53.93 μWm-1K-2.The transport mechanism of the carrier and phonon via the interface and carrier screen effect in the core-shell structure material are revealed by building core-shell structure theory model and the calculation of the electronic band structure of NiSe2 and SiO2.Like this theory illustrates,the potential barrier scattering the low energy carrier to enhance thermoelectric properties effectively ascribed to the increased Seebeck and reduced the thermal conductivity.The calculated band structures for Cu2S and Cu1.96S from room temperature to 700 K were performed using the generalized gradient approximation(GGA)Perdew-Burke-Ernzerhof(PBE)exchange function with projector augmented-wave(PAW)potentials as implemented in VASP(Vienna Ab-initio Simulation Package).The band gap of Cu2S were 0.48 eV,0.37 eV and 0 eV in different temperature,respectively.The valence band around Femi energy of Cu2S was mainly formed by the combination of Cu 3d and S 3p orbital.Low temperature Cu1.96S phase exhibit indirect gap semiconductor and Femi energy passed through the valence band in the medium and high temperature phase.It speculated that there would be a high electrical conductivity for the Cu1.96S phase attribute to gathering more carriers around Femi energy.Low cost and non-toxic CuxS(x=1.92,1.94,1.98)bulks were synthesized by a simple route that combines MA and SPS technologies.As x decreases,Seebeck coefficient decreased while electrical conductivity increased proportionally to give a maximum electrical conductivity value of 538.7 Scm-1 at 673 K for the sample with x=1.92.The power factor had been improved remarkably from 177.5 to 902.3 μWm-1K-2.The CuxS bulks had intrinsically low lattice thermal conductivity(-0.5 Wm-1K-1)due to their superionic structures.The highest ZT value,1.23,was attained at 773 K as x=1.94.