Cu_1.8S Composite,Cu₂S Doping Modulation And Their Thermoelectric Performance

As a new type of clean energy material,thermoelectric material can directly realize the conversion between thermal energy and electric power.It has great potential in the fields of waste heat recovery and refrigeration,and is expected to become an important method to improve energy efficiency and alleviate environmental pollution problems.The chalcogenide materials have been widely investigated as promising thermoelectric materials in recent years because of their low cost,low toxicity and large reserves.Cu2-xS as a typical sulfide system,has a series of compounds from CuzS rich in copper to CuS deficient in copper.However,the performance of Cu2-xS-based thermoelectric materials is still relatively low in the medium-temperature zone,which needs to be further enhanced to improve the thermoelectric conversion efficiency.In this paper,Cu1.8S and Cu2S were selected as the research objects,and their thermoelectric properties are improved through composite structure and doping strategies,which the compounds are prepared by mechanical alloying combined spark plasma sintering process.Cu1.aS has high electrical conductivity,but its low Seebeck coefficient and high thermal conductivity prevent it from obtaining high ZT value.In this study,the porous structure Cu1.sS induced by NH4Cl was prepared by mechanical alloying combined with spark plasma sintering.The scale of medium pores(0.1-2.0 ?m)were introduced in the Cu1.8S matrix,which effectively reduced the thermal conductivity.The point defects are also formed after adding NH4C1 to increase the carrier concentration,which is conducive to maintaining high electrical conductivity.Finally,the maximum ZT value reached 0.3,which is 15%higher than that of the pure Cu1.8 sS.The results show that NH4Cl composite in thermoelectric materials is a simple and rapid strategy to prepare porous bulk materials,which can reduce thermal conductivity and improve the thermoelectric properties of materials.Cu2S has intrinsic low thermal conductivity,but its electrical conductivity is very low.In this study,homologous element Se doping was adopted to reduce the band gap,change the band structure and improve the electrical conductivity.At the same time,the point defects formed by the replacement of Se for Te,resulting in the reduced thermal conductivity.The synergistically improvement of the electrical and thermal transport performance was realized in this system.The highest ZT value is 0.74 at 723 K.Furthermore,Te alloying is selected to fabricate series of Cu2S1-XTex compounds.With the increasing Te content,the phase changes from monoclinic Cu2S(x=0),monoclinic Cu2S1-xTex solid solution(0.02<x<0.06),hexagon Cu2S1-xTex solid solution(0.08<x<0.1),coexisting Cu2S and Cu2Te(0.3?x?0.7),to Cu2Te phase(x=1).)All Cu2S1-xTex(0?x?1)alloys haveimproved thermoelectric properties.The highest ZT value of 1.18 was obtained at 723 K.For the Cu-site of Cu2S,magnetic element Ni was doped to generate new hybrid energy level to improve the electrical conductivity by using the 3d hybridization of Ni.When more Ni content introduced,Cu2-xS phases formed in the matrix,which obey the effect of "phonon blocking/electron penetration" to improve the electrical conductivity.The highest ZT value is 1.10 at 773 K.Inspired by the appearance of Cu2-xS phases after excessive Ni doping,we dope the trivalent cation of A1 in the Cu2S.Cu2S,Cu1.97S,and Cu1.96S were co-existed after A1 doping,and the rod structure can be observed in the SEM,which is beneficial to the carrier transport.The thermal conductivity is reduced owing to the muti-phases boundaries scattering.The maximum ZT value reached 1.51 at 773 K.