Preparation And Performance Optimization Of P-Type AgCuTe Based Thermoelectric Materials

AgCuTe is a new ternary“liquid-like”thermoelectric material with excellent thermoelectric properties in the medium to high temperature range（500～800 K）due to its ultra-low lattice thermal conductivity and highly symmetrical cubic crystal structure.However,the lower symmetry of the hexagonal crystal structure near room temperature results in a higher thermal conductivity.In addition,the higher carrier concentration results in a lower Seebeck coefficient.This seriously affects the application of AgCuTe-based thermoelectric materials at room temperature.Therefore,there has been much interest in improving the thermoelectric properties of AgCuTe-based thermoelectric materials at room temperature.In this thesis,a series of AgCuTe thermoelectric bulk materials were prepared by ball milling combined with discharge plasma sintering under the guidance of the“phononic liquid-electron crystal”concept,and the effects of heterogeneous atom doping and microstructure on their thermoelectric properties were analysed.The following results were obtained from the experimental studies.（1）In this experiment,AgCuTe-based thermoelectric materials were successfully prepared by mechanically alloying Ag,Cu and Te powders by a planetary ball mill combined with discharge plasma sintering.On this basis,the ratio of Ag₂Te to Cu₂Te is adjusted to optimise the thermoelectric properties by regulating the content of Ag and Cu.A high Seebeck coefficient value of about 171μV K^-1 was obtained for the Ag_0.95Cu_1.05Te sample at 623 K.Finally,a z T value of 0.91 was obtained for Ag_0.95Cu_1.05Te,which is an improvement of about 21%compared to AgCuTe（z T=0.75）.（2）The doping of Ag_0.95Cu_1.05Te with Ni in the cationic position was carried out.The doping of Ni at the cationic site was demonstrated by XRD tests to shift the main peak to a higher angle,indicating that Ni can be successfully doped at the cationic site.Also,the presence of a 3～10 nm nano-second phase（Ni Te）in the matrix was observed by TEM and EDS characterisation,as well as the presence of some dislocations and layers of dislocations,which facilitated the scattering of phonons.At 623 K,a reduced lattice thermal conductivity of 0.23 W m^-1 K^-1 was obtained for the Ag_0.95Cu_1.02Ni_0.03Te sample,with a maximum z T value of～1.23 and a z T_avg of 0.77.（3）On the basis of Ag_0.95Cu_1.05Te,doping with S elements at the Cu site was carried out.In order to solve the problem of unsatisfactory thermoelectric properties of AgCuTe based thermoelectric materials due to phase transformation.Samples doped with S elements in the AgCuTe base were successfully prepared and the mechanism of the change in thermoelectric properties was investigated.It was demonstrated that the doping of S elements at the Te site could inhibit the transition from the cubic to the hexagonal AgCuTe phase.The maximum power factor of about 1058μW m^-1 K^-2 was obtained for sample Ag_0.95Cu_1.05Te_0.94S_0.06 after coordinated optimisation.At 623 K,the lattice thermal conductivity of sample Ag_0.95Cu_1.05Te decreased from 0.33 W m^-1 K^-1 to 0.19 W m^-1 K^-1 for sample Ag_0.95Cu_1.05Te_0.94S_0.06 Ultimately,the z T value of sample Ag_0.95Cu_1.05Te_0.95S_0.05reached a maximum value of～1.32 with a z T_avg of 0.81.