Fast synthesis and improved electrical stability in n-type Ag_(2)Te thermoelectric materials

Cu-and Ag-based superionic conductors are promising thermoelectric materials due to their good electrical properties and intrinsically low thermal conductivity. However, the poor electrical and thermal stability restrict their application. In this work, n-type pure phase Ag_(2) Te compound is synthesized by simply grinding elemental powders at room temperature and compacted by spark plasma sintering. It is found that, because of the migration of Ag+after the phase transition around 425 K, submicron pores are formed inside the samples during the electrical performance measurement, resulting in poor electrical stability and repeatability of Ag_(2) Te samples. However, Pb-doped Ag_(2-x)Pb_(x)Te(x = 0–0.05) specimens exhibit improved electrical stability by the precipitation of the secondary phase Pb Te in the Ag_(2) Te matrix, which is confirmed via cyclic electrical property measurement and microstructure characterization.A maximum z T = 0.72 is obtained at 570 K for x = 0.03 mainly due to the increased power factor.

Mid-temperature thermoelectric performance of zone-melted Sb_(2)(Te,Se)_(3) alloys near phase transition boundary

(Bi,Sb)_(2)(Te,Se)_(3) alloys are widely used commercial thermoelectric(TE)materials for solid-state refrigeration around room temperature.The composition-induced structural phase transition could be realized by varying the compositions in these alloys,which may largely alter the electronic structure and phonon dispersion.Among them,the Se-alloyed Sb_(2)Te_(3) accompanied with structural transition is seldom reported.Herein,the interrelations of Se-alloying induced changes in structural phase transition,band structure and TE properties of p-type zone-melted Sb_(2)Te_(3-x)Se_(x)(x=1.5-2.4)alloys near phase transition boundary are systematically investigated.The results demonstrate that Sb_(2)Te_(3-x)Se_(x) shows a structural transition from a rhombohedral phase to mixed structure at x=2.0.The carrier concentration and bandgap at room temperature of Sb_(2)Te_(3)-xSex(x=1.5-2.4)constantly decrease with increasing Se contents x.The zT peak of the Sb_(2)TeSe_(2) matrix is improved and shifted to higher temperature by optimizing carrier concentration via Ag doping.A maximum zT of~0.4 is obtained at 680 K in Sb_(1.97)Ag_(0.03)TeSe_(2) alloy,about 100% enhancement compared with the undoped sample.

Low contact resistivity and long-term thermal stability of Nb0.8Ti0.2FeSb/Ti thermoelectric junction

Although half-Heusler compounds are quite promising for thermoelectric power generation,there is only limited research on the interfacial structure between metal electrode and half-Heusler compounds for device applications.This work reports on the characteristics of Nb0.8Ti0.2Fe Sb/Ti junction and its evolution behavior during 973 K.The Nb0.8Ti0.2Fe Sb/Ti interface consists of one Ti0.9Fe0.1 layer and one Fe-poor layer.There is an Ohmic contact and a low contact resistivity(0.15μΩcm^-2)in this junction,on account of the matching of working functions between Nb0.8Ti0.2Fe Sb and Ti0.9Fe0.1 interlayer.The high doping of Ti high carrier concentration in Nb Fe Sb matrix leads to a high carrier concentration,which results in inducing a large tunneling current at this interface.After aging treatment at 973 K,the Fe-poor layer and the Ti0.9 Fe0.1 layer continues to expand,resulting in the increase of the thickness of the interfacial layer and the contact resistivity.The interfacial electrical is only 1.9μcm^-2 after 25 days’aging.The thickness of the interface layer has a good linear relation with the square root of aging time,which firmly indicates that the growth of the layer is determined by mutual diffusion of Fe and Ti atoms across the interface.The low contract resistivity and long-time thermal stability demonstrate the great potential of Nb0.8Ti0.2Fe Sb/Ti thermoelectric junction in high efficiency half-Heusler TE devices.