The Effect Of Self-doping On The Thermoelectric Properties Of AgSbX₂(X=Te,Se) Based Materials

With the continuous development of science and technology in the world today,the demand for energy is increasing year by year.The consumption of energy has gradually reduced the reserves of traditional fossil energy;on the other hand,the environmental pollution problems caused by the consumption of petroleum energy have become more and more serious.Thermoelectric materials can effectively reduce environmental pollution and provide sustainable energy,which has attracted widespread attention.The low-temperature thermoelectric material AgSbX₂(X=Te,Se)has a low intrinsic thermal conductivity,but its electrical performance is poor,so the thermoelectric figure is low.In this paper,the thermoelectric material AgSbX₂(X=Te,Se)was selected as the research object,the rapid preparation method of AgSbTe₂ and AgSbSe₂ materials and the effect of Te or Se self-doping on the adjustment of carrier concentration and thermoelectric properties of the material were studied.Moreover,the doping of Mn element was used to optimize the thermoelectric performance of AgSbX₂ material.Three series of AgSbTe_2+x,AgSbSe_2+x,and AgSb_1-xMn_xSe_2/2.01 samples were prepared through high-frequency melting,high-temperature melting,spark plasma sintering(SPS)processes.The phase constituent,microstructure and thermoelectric properties were studied and the main results were list as fellow:(1)The AgSbTe_2+x(x=0,0.01,0.02,0.03,0.05)samples were prepared by high-frequency melting and high-energy ball milling methods.The experimental results showed that the self-doping of excessive Te elements could effectively suppress the formation of second phase Ag₂Te.But the composition of the samples prepared by the ball milling method was not uniform,and there were some Ag-rich areas,leading to unsuccess of electrical performance improvement of the samples.For high-frequency smelting samples,the self-doping of Te increased the carrier concentrations of the samples and reduced the resistivities of the samples.At 548 K,the resistivity decreased from 74??m for the sample with x=0 to 35??m for x=0.05.At the same time,the excessive lattice distortion introduced by Te and the enhancement of the phonon scattering reduced the lattice thermal conductivities.Finally,the sample with x=0.05 obtained the highest ZT value of 1.69 at 548 K,which was 24%higher than the undoped sample,and higher than the maximum ZT value obtained by the traditional high-temperature melting method.(2)On the basis of excessive Te doping can significantly improve the thermoelectric properties of AgSbTe₂,the excessive self-doping of the Se element was carried out on the homogenous compound AgSbSe₂.AgSbSe_2+x(x=0,0.0025,0.005,0.0075,0.01,0.03)series samples were prepared by high temperature melting and SPS method.Experiments showed that excessive self-doping of Se element decreased the lattice constants of the samples,the sample reacheed saturation and some Se element precipitated after the Se doping amount more than0.0075.Se self-doping increased the carrier concentrations of the samples,and the resistivities were greatly reduced.At room temperature,the resistivity decreased from 610??m for the sample with x=0 to 140??m for x=0.01.At the same time,the Seebeck coefficient gradually decreased.In this system,the sample with x=0.01 reacheed the maximum power factor of 640?Wm^-1K^-2 at 673 K,which was 115%higher than the maximum power factor of 298?Wm^-1K^-2 for the undoped sample.The thermal conductivity for the samples with Se self-doping gradually increased,and finally the sample with x=0.01 reached the maximum ZT value of0.96 at 673 K,which was 96%higher than the sample with x=0,and was comparable to the ZT values in literatures.At the same time,the average ZT value increased from 0.32 for the sample with x=0 to 0.59 for x=0.01,increasing by 84%.(3)For the AgSb_1-xMn_xSe_2/2.01(x=0,0.01,0.03,0.05,0.07)samples with Mn doping the lattice constants of the sample were reduced,the carrier concentrations were enhanced effectively,and the resistivities were greatly reduced.At room temperature,the resistivity decreased from 610??m for the sample with x=0 to 57??m for x=0.07.At the same time,the Seebeck coefficient gradually decreased.At 673 K,the power factor of the sample with x=0.07 reacheed the maximum value of 705?Wm^-1K^-2,much higher than the maximum power factor of 305?Wm^-1K^-2 for the undoped Mn sample.The doping of Mn increased the thermal conductivity slightly,the maximum ZT value of 0.96 at 673 K was obtained in the sample with x=0.07,which was 96%higher than the undoped sample.The average ZT value increased from0.32 for x=0 to 0.63 for x=0.07.In the Mn-doped AgSbSe_2.01 samples,the carrier concentration continued to increase on the basis of AgSbSe_2.01.The resistivity decreased,and the power factor increased.At 623 K,the power factor of the sample with x=0.07 reacheed710?Wm^-1K^-2,which was 11%higher than the undoped AgSbSe_2.01 sample.The thermal conductivities after Mn doping were also increased to some degree.Finally the sample with x=0.07 reaches the maximum ZT value of 1.04 was obtained at 673 K for the sample with x=0.07,similar to the undoped AgSbSe_2.01 sample.Above all,AgSbTe_2+x samples were by successfully prepared high-frequency melting method,which enriched the preparation method of AgSbTe₂ thermoelectric materials.Self-doping of Te or Se elements for AgSbTe₂ and AgSbSe₂ systems can significantly improve their thermoelectric performance.On this basis,we have studied the AgSbSe₂ thermoelectric materials doped with Mn element,and achieved good thermoelectric performance.