ZnO/ZnS Heterostructure Enhances The Thermoelectric Properties Of ZnO

There is a serious energy crisis in world today.It is undeniable that fossil energy has an indelible contribution to the development of the world.But now fossil energy is drying up,and its serious pollution to the environment is also troublesome.Therefore,it is necessary to develop a non-polluting and convenient energy material to replace fossil energy.Thermoelectric materials have entered the“candidate area”.Nowadays,thermoelectric materials with high figure of merit(ZT)in the middle and high temperature region are basically prepared from toxic and harmful or expensive raw materials.Therefore,ZnO with high temperature stability and simple preparation have entered everyone's vision.However,there are some disadvantages in ZnO that as thermoelectric materials.The wide band gap leads to poor electrical performance,and the larger thermal conductivity also makes ZT value low.In this paper,ZnO and ZnO/ZnS heterostructure thermoelectric ceramics were prepared by solution method combined with microwave sintering technology in order to comprehensively improve their thermoelectric properties.(1)Firstly,ZnO powders were synthesized by hydrothermal method and precipitation method,respectively.The particle sizes of the synthesized powders were?145 nm and 45 nm,respectively.The powders were microwave sintered in air and argon atmosphere,respectively.The sintering conditions were 1000°C and 60 min.The relative densities of ZnO powders synthesized by hydrothermal method after sintering in air and argon were 85.8%and 75.6%,respectively.The relative densities of ZnO powders synthesized by precipitation method after sintering at two atmospheres were 83.3%and 75.2%,respectively.Due to the decrease of oxygen vacancy concentration caused by sintering in air,the electrical properties of ZnO samples synthesized by the two methods cannot be both tested.The ZT values of the samples synthesized by hydrothermal method and precipitation method at 900 K are0.2 and 0.17,respectively.(2)ZnO/ZnS heterostructures were obtained by substitution reaction between ZnO powder synthesized by hydrothermal method and thioacetamide(TAA).The relative amount of ZnS and ZnO was adjusted by changing the amount of TAA.When the ratio of sulfur and oxygen atoms was 5%,the thermal conductivity of the sintered sample was 1.67 Wm^-1K^-1at 900 K.Finally,the maximum ZT of the sintered sample with a sulfur-oxygen atomic ratio of 5%was 1.77 at 900 K.(3)In order to explore the effect of powder ZnO particle size on the properties of final samples,ZnO/ZnS heterostructures were prepared by the reaction of ZnO powder synthesized by precipitation method with TAA.Due to the small particle size and larger specific surface area of ZnO powders synthesized by precipitation method,the TAA concentration required for the formation of suitable ZnS layer is smaller,so the selected sulfur-oxygen atomic ratio is 0.1%-0.5%.At the same time,due to the smaller grain size after sintering,more grain boundaries are produced,resulting in more phonon scattering.At 900 K,the thermal conductivity of the sintered sample with sulfur-oxygen atomic ratio of 0.5%is 1.24 Wm^-1K^-1,and the ZT is 1.38.