Hpht Synthesis And Properties Of High Performance Cosb₃-based Thermoelectric Materials

Thermoelectric material (TE) is a kind of function materials that can convertelectricity and heat directly. Thermoelectricity is becoming increasingly important inthe field of cooling, heating, generating power, and recovering waste heat, which arebased on thermoelectric materials could convert heat to electricity, transferred throughor pumped by the charge carriers in the thermoelectric materials, and rejected at theheat sink. The efficiency of TE devices is mainly limited by the currently available TEmaterials, a lot of studies shows that the performance of TE materials needs to beenhanced significantly to complete with the conventional cooling andenergy-conversion system.The basic criteria for the selection of possibly efficient materials include a large unitcell, complex crystal structure, heavy constituent atoms, highly covalent atomic bonds,large effective mass of the charge carriers, high carrier mobility (~103cm2/Vs), narrowband-gap energy (~10kBT), and low electronegativity differences between theconstituent atoms. The use of TE device is limited by their low efficiencies. Theefficiency of advanced thermoelectric technology is governed by the materialsdimensionless thermoelectric figure of merit ZT=α2σT/κ, which is defined by thefollowing transport parameters: absolute temperature T, thermopower S, electricalconductivity σ, and thermal conductivity κ.CoSb3-based skutterudites have been studied extensively for their promisingthermoelectric properties. CoSb3skutterudite crystallize in a body-centered-cubic structure with space group Im3and have interstitial voids at the2a positions(12-coordinated) in the lattice. Each M atom is octahedrally surrounded by X atomsforming a MX6octahedron. Binary CoSb3is a semiconductor with small band gap,high carrier mobility, and modest thermopower. However, it also possesses highthermal conductivity values that limit its thermoelectric performance. So, in order tolower the thermal conductivity and higher thermopower, the works of fabricatingdoped CoSb3and filled CoSb3were reported. For doped CoSb3, the mechanism ofphonon scattering by the introduced defects could reduce the lattice thermalconductivity. For filled CoSb3, the foreign species (filled atoms) fill the empty voids,which could significantly depress the lattice thermal conductivity due to the rattling ofthe filled atoms.The group of J.V. Badding and Sergey V Ovsyannikov found that the relationshipof pressure and thermoelectric properties. The power factor of PbTe based materialscould be improved one hundred times nearly. But, these high results can not be keptafter unload pressure. While the high temperature combined with high pressure couldresolve this defect for high pressure solely.Recently, a lot of reported results show that the thermoelectric properties of manymaterials can be optimized extraordinarily under high pressure. However, regretfully,the high performance under high pressure returns generally to the primary state oncethe pressure is unloaded. In order to keep the samples' high performance under highpressure, HPHT method was used to prepare thermoelectric materials. The method ofHPHT has a lot of advantages, such as, the ability to tune rapidly and cleanly,restraining the disorder phase separation and other complicating factor during thepreparation of materials. So in this study, we focus our attention on the TE materialswith high power factor and low thermal conductivity. The conclusions are listed below:1. Skutterudite compounds PbxCo4Sb11.5Te0.(5x=0.1,0.2,0.3,0.5)was synthesizedsuccessfully by high temperature and high pressure (HTHP) method and characterizedwith X-ray diffractometry and thermoelectric properties measurements. The electrical resistivity, Seebeck coefficient and Power factor were all depending on syntheticpressure and the Pb content of the Skutterudite compounds were performed at roomtemperature. As our expected, Seebeck coefficient and electrical resistivity allincreased with the increasing of synthetic pressure. These results indicated that HTHPtechnique may be helpful to prepare thermoelectric materials with enhancedthermoelectric properties.2. Skutterudite compounds PbxBayCo4Sb11.5Te0.5(x=0.03,0.05,0.23; y=0.14,0.25,0.27) with bcc crystal structure have been prepared by HPHT method. The studyexplored a chemical method that filling Pb and Ba atoms into the voids of CoSb3tooptimize the thermoelectric figure of merit ZT in the system of PbyBaxCo4Sb11.5Te0.5.The structure of PbxBayCo4Sb11.5Te0.5skutterudites was evaluated by means of x-raydiffraction. Seebeck coefficient, electrical resistivity and power factor were performedbetween room temperature and about720K. Thermal conductivity was performedbetween room temperature and about720K. Among all the samples,Pb0.03Ba0.27Co4Sb11.5Te0.5showed the highest power factor of31.64μwcm-1K-2at663K.Pb0.05Ba0.25Co4Sb11.5Te0.5showed the lowest thermal conductivity of2.73Wm-1K-1at663K, and the maximum ZT value reached0.63at673K due to its lower thermalconductivity. We have studied the impact of synthetic pressure on the thermoelectricproperties of Pb0.05Ba0.25Co4Sb11.5Te0.5. The absolute Seebeck coefficients ofPb0.05Ba0.25Co4Sb11.5Te0.5increased with the increase of synthetic pressure, and thethermal conductivity decreased with the increase of synthetic pressure. Adimensionless thermoelectric figure of merit of1.0at720K was achievable for n-typea0.25Pb0.05Co4Sb12Te0.5synthesized at2.3GPa finally.3. Skutterudite compounds InxCo4Sb12(0.1≤x≤0.5) with bcc crystal structure havebeen prepared by HPHT method. The structure of InxCo4Sb12skutterudites wasevaluated by means of x-ray diffraction, and transport properties measured on thesynthesized samples have been compared with each other. Seebeck coefficient,electrical resistivity, power factor, and thermal conductivity were performed between room temperature and about665K. Among all the samples, In0.5Co4Sb12showed thehighest power factor of31.3μWcm-1K-2at616K and showed the lowest thermalconductivity of2.193Wm-1K-1at568K. At last, the maximum ZT value ofIn0.5Co4Sb12reached0.88at665K due to its lower thermal conductivity and higherpower factor.4,Skutterudite compounds InxM0.2Co4Sb12(M=Ba and Pb) with bcc crystalstructure have been prepared by HPHTmethod. Through this method, the processingtime can be reduced from a few days to half an hour. The structure of InxM0.2Co4Sb12skutterudites was evaluated by means of x-ray diffraction, and Seebeck coefficient,electrical resistivity, power factor and thermal conductivity of InxM0.2Co4Sb12were allperformed in the temperature range of314.9-673K. The transport properties measuredon the synthesized samples have beencompared with each other. Remarkably thecombination of In and Ba was much more effective in reducing lattice thermalconductivity κLthan In and Pb. The figure of merit, ZT, of samples all increased withthe increasing temperature. Among all the samples, In0.5Ba0.2Co4Sb12showed thehighest power factor of30.3μw/cmK2at643.5K, lowest lattice thermal conductivity1.128Wm-1K-1at643.5K and the maximum ZT value0.80at673K.5. In-filled and Ge-doped skutterudites In0.5Co4Sb12-xGex(0.5≤x≤1) with bcccrystal structure have been prepared by HPHT method. The structure ofIn0.5Co4Sb12-xGexskutterudites was evaluated by means of x-ray diffraction. Seebeckcoefficient, electrical resistivity, power factor and thermal conductivity wereperformed between room temperature and about663K. We found that the absoluteSeebeck coefficients of In0.5Co4Sb11Ge1increased with the increase of syntheticpressure, and the thermal conductivity decreased with the increase of syntheticpressure. Among the different Ge content samples which were prepared at3GPa,In0.5Co4Sb11.5Ge0.5showed the highest power factor of21.98μWcm-1K-2at615K andthe maximum figure of merit (ZT) value of0.58at663K.In conclusion, the results indicated that high pressure combined with alloying could modulate the thermoelectric properties of CoSb3system effectively. Theseresults indicated that HPHT is very useful for further investigation into differentcompositions of the skutterudite family of compounds for potential thermoelectricapplications. In the condition of HPHT, the double-filled will be an effective way toreduce the thermal conductivity of CoSb3.