Preparation And Thermoelectric Properties Of Zn₄Sb₃ Based Compounds

In recent years,due to the shortage of fossil energy and the environmental problems caused by burning fossil energy,people need to find new renewable energy sources to replace or reduce the amount of fossil energy.Scientists have discovered a new type of green energy material,thermoelectric material,which can directly convert electrical energy and heat energy,which has aroused people's extensive attention.Thermoelectric conversion technology has the advantages of small size,no vibration,no noise,no pollution and high stability.It has good applications in medical,industrial,automotive and consumer electronics.Among them,the Zn4Sb3 based compound is the most excellent thermoelectric material with excellent performance in the middle temperature range and abundant reserves and low price.However,the low temperature phase transition,microstructure and how to improve its thermoelectric properties of this series of compounds are still worthy of further study.In this thesis,Zn₄Sb₃ based compounds were used as the research object.The dense Zn₄Sb₃ based compounds were synthesized by melt-quenching-annealing and plasma activated sintering.The defect structure,phase composition and low temperature phase transition and the influence law of thermoelectric performance of Zn4Sb3 based compounds were studied systematically.The main contents and conclusions are as follows:?1?Zn solid solution Zn_4+xSb₃ compounds were synthesized by the above synthesis method.Zn has a certain solid solubility in Zn₄Sb₃ compound,and the solid solution interval is-0.17<x<0.15;it can be effectively adjusted by adjusting the relative content of Zn.The low temperature phase transition of Zn_4+xSb₃ compounds is related to the Zn content.As the Zn content increases,the?'phase changes to?phase,so the increase of Zn solid solution will cause?'phase.The ratio of?is increased,while?is more disordered than?',so Zn solid solution increases structural disorder.It can increase the Zn4Sb3 disorder by controlling Zn content,enhance the scattering of phonons by the lattice,and reduce lattice thermal conductivity;?2?Simultaneous Zn solid solution will appear the second phase of nano-Zn and many nano-porous structures,enhance the interface to phonon scattering,and further reduce the lattice thermal conductivity.The thermal conductivity of all samples decreased with increasing temperature,then began to increase after 573 K.The carrier thermal conductivity?C increased with increasing Zn content,and the thermal conductivity of lattice thermal conductivity increased with Zn content.Increasing and decreasing,the total thermal conductivity decreases as the Zn content increases.The lowest lattice thermal conductivity of Zn_4.21Sb₃ sample at 673 K is 0.22 Wm^-1K^-1.In addition,Zn solid solution can optimize the carrier concentration.As the Zn content increases,the carrier concentration increases and the conductivity increases.The rate and the power factor were increased.The maximum power factor of the Zn_4.21Sb₃sample at 673 K was 1.35×10-3 Wm^-1K^-2.The final ZT value of the Zn_4.21Sb₃ sample was obtained.At 673 K,ZT_max=1.35,which was 35%higher than the intrinsic.?3?In-doped(Zn_1-xInx)_3.83Sb₃ and(Zn_1-xIn_x)_4.15Sb₃ compounds were synthesized by the above synthesis method.The In doping limit is different.The In doping limit is x<0.07 at the Zn-depleted limit,and the In-doping limit at the Zn-rich limit is x<0.03.The doping limit of In when Zn-depleted is significantly greater than that at Zn-rich.It can be inferred that the In atom will enter the Zn position when In doping,because when the Zn content is low,the Zn lattice position has more vacancies,and the In doping amount is large;when the Zn content is high,the Zn lattice point The space of the position is lacking,and the amount of In doping is small.By drawing the ternary phase diagram of Zn-In-Sb,it is found that when the stoichiometric ratio deviates from the purple region in the ternary phase diagram,other miscellaneous phases appear,and the range of the purple region provides theoretical guidance for studying the phase relationship of In doped Zn₄Sb₃;?4?By observing the change of thermoelectric properties of In doped samples when Zn is deficient,it is found that the conductivity decreases first and then increases with the increase of temperature.The Seebeck coefficient is opposite,the power factor always increases,and the thermal conductivity first decreases and then increases.The ZT value increases with increasing temperature.In addition,with the amount of In doping increases,the ZT value also increases.The sintered(Zn_0.94In_0.06)_3.83Sb₃compound obtains the maximum ZTmax of 1.10 at 673K.The sample with the least amount of In doping in this group increased by 12.2%.By observing the change of thermoelectric properties of In doped samples when Zn is rich,it is found that electrical transport characteristics and thermal transport characteristics and ZT are consistent with the Zn-depleted sample with temperature,and the ZT value increases with the amount of In doping.However,it can be found that the thermal conductivity of the entire range of 323-673 K of the In-doped sample is higher than that of the Zn-doped In-doped sample.Since the thermal conductivity of In-doped samples when Zn-rich is significantly lower than that of In-doped samples when Zn-deficient,the In-doped samples when Zn-rich have better thermoelectric properties,and the(Zn_0.96In_0.04)_4.15Sb₃compound sintered bulks at 673 K.,the maximum ZT_max was 1.43,which was 52.1%higher than that of the sample(Zn_0.99In_0.01)_4.15Sb₃,and the maximum ZT value of the In-doped sample when Zn-rich was increased by 30%compared with the Zn-depleted sample.