Investigation And Optimization Of Transport Properties Of Chalcogenide-based Thermoelectrics

(Ag,Cu)₂X?X=S,Se,Te?compounds have attracted wide spread attention because of their controllable preparation processes and non-toxic constituent elements.In general,this kind of material exhibits a low-symmetry structure at low temperature,and transforms into“electron-crystal,phonon-liquid”cubic structure with excellent thermoelectric performance at high temperature.However,the poor low-temperature performance and unavoidable phase transition are not conducive to further commercial applications.Therefore,in this study,the thermoelectric properties of Cu₂Te and Ag Cu Te are enhanced greatly,especially at low temperature,and the phase transitions are suppressed,which are of great significance for thermoelectric applications.In addition,after evaluating the repeatability and stability,the thermoelectric conversion efficiency based on the studied materials is tested,and a new thermoelectric hybrid battery system has been proposed to utilize the battery waste heat.The main conclusions are summarized below:?1?The thermoelectric performance of Cu₂Te is optimized by?Ag,S?co-alloyed.Specially,the formation energy of Cu vacancy is increased by Ag alloying,resulting in a decreased cation vacancy concentration and carrier concentration,thereby the Seebeck coefficient and power factor are optimized in Ag_xCu_2-xTe.For Cu₂Te,the carrier concentration is 4.53×10²⁰ cm^-3,and the power factors are 2.7?W cm^-1 K^-2 and 5.4?W cm^-1 K^-2 at room temperature and 773 K,respectively.While the carrier concentration decreases to 2.05×10²⁰ cm^-3 for Ag_0.7Cu_1.3Te,along with the power factors of 4.4?W cm^-1 K^-2 and 10.1?W cm^-1 K^-2 at corresponding temperatures.In addition,the carrier concentration is further decreased to 8.95×10¹⁹ cm^-3 for Ag_0.7Cu_1.3Te_0.92S_0.08,resulting in a decreased carrier thermal conductivity.Combined with the intrinsic low lattice thermal conductivity,the thermal conductivity of Ag_0.7Cu_1.3Te_0.94S_0.06 is lower than 0.6 W m^-1K^-1,and the highest ZT of 1.4 is obtained at 773 K.?2?A stable cubic phase can be introduced at room temperature by doping I into Ag Cu Te,thereby successfully avoiding the phase transition within the studied temperature range.In addition,iodine doping can effectively reduce the hole concentration,from 1.26×10²⁰ cm^-3 in Ag Cu Te to 5.14×10¹⁹ cm^-3 in Ag Cu Te_0.875I_0.125,resulting in an increased Seebeck coefficient and decreased carrier thermal conductivity.Compared with Ag Cu Te,the room-temperature Seebeck coefficient increased by 122%,and the carrier thermal conductivity decreased by 81%for Ag Cu Te_0.9I_0.1.Therefore,the thermoelectric performance is enhanced immensely.For Ag Cu Te_0.9I_0.1,the ZTs increase to 0.3 and 0.9 at 300 K and 463 K,respectively,and the average ZT reaches up to 0.64between 300 K and 463 K,which are much higher than that of other similar materials.Besides,good repeatability and stability can also be proved by repetitive experiments.?3?By replacing a portion of Te atoms with smaller size Se atoms,the interstitial space can be increased and the cubic phase can be stabilized at room temperature,leading to an optimized low-temperature thermoelectric performance.The highest power factor of 6.8?W cm^-1 K^-2 is realized at 353 K in Ag Cu Te_0.9Se_0.1,and a competitive average power factor of 6.2?W cm^-1 K^-2 can also be obtained between 300 K and 673 K.In addition,the carrier thermal conductivity and lattice thermal conductivity are reduced simultaneously due to the decreased electrical conductivity and introduced defects,therefore the room-temperature ZT of 0.3 is obtained in Ag Cu Te_0.9Se_0.1.Besides,the Seebeck coefficient and power factor are further improved by regulating the cation vacancy and carrier concentration.Among them,the power factor of 11.4?W cm^-1 K^-2 at room temperature,and the highest power factor of 13.8?W cm^-1 K^-2 can be realized in?Ag Cu?_0.995Te_0.9Se_0.1.Hence,the room-temperature ZT reaches up to 0.72,the peak value of 1.1 can be realized at 353 K,and the average ZT of 0.96 is obtained between 300 K to673 K.?4??Ag Cu?_0.995Te_0.9Se_0.1 shows good thermal stability and repeatability after reliability testing.Based on this material,a single leg of power generation was prepared,and the output power together with thermoelectric conversion efficiency were tested.Experimental results show that the output power is strongly dependent on the current and temperature difference,which increases with increasing hot side temperature as well as temperature difference.When the hot side temperature is 723 K and the applied current is 1.2 A,the output power reaches the maximum value of 46.7 m W.When the hot side temperature is 723 K,cold side temperature is 322 K,the current density is 6.8 A cm^-2,the thermoelectricconversion efficiency reaches up to 11%.Considering that thermoelectric materials have the ability to realize the conversion between heat and electricity,thermoelectric hybrid battery system is proposed to realize the green closed-loop working mechanism of battery waste heat-power generation-charging-cooling,therefore the power density and safety of batteries can be optimized simultaneously.Meanwhile,the suitable n-type mateiral is matched according to the studied p-type material,and ANSYS simulation is carried out after optimizing the geometric parameters.When the hot side temperature is 723 K,the output power is about 52 m W,and the efficiency reaches up to 7.8%.