Synthesis And Thermoelectric Properties Of N-type PbS Compounds

Thermoelectric (TE) materials, which can realize direct conversion between heat and electricity based on the Seebeck or Peltier effect, have shown great significance in many energy conversion fields; such as solar energy conversion, waste heat recoveries, TE refrigeration and so on. Recently, many significant progresses have been achieved in the research of PbTe-based compounds which has been playing a dominant role in the thermoelectric power generation application at middle temperature range. However, Te is a scarce element in the crust of the earth. Hence, the Te price is likely to rise sharply if Te-based thermoelectric materials reach mass markets. A broad search for more inexpensive alternatives is therefore warranted. PbS could potentially be such a good alternative because it has many attractive features. For example, PbS shares the same highly symmetric NaCl-type cubic structure and electrical band structure with PbTe, and it is composed of abundant elements. Previous works in the literature performed decades ago indicated that PbS was not an attractive thermoelectric material and little research has been reported on the thermoelectric properties of bulk PbS.In this study, we focus on the inexpensive n-type PbS compound. The compounds were synthesized by melting combined with spark plasma sintering. The influence of Ga, In or Sb doping and Se or Te alloying on the phase compositions, microstructures, carrier concentration and TE properties were systematically studied. The main content and results are listed as follows.The Group ⅢA elements Ga or In doping n-type PbS compounds were prepared. Our result reveals that Ga-doping optimizes the carrier concentrations and electrical transport properties. The1%Ga-doped PbS sample shows a highest ZT of0.76at750K, which is about100%improvement over the un-doped sample. Ga shows limited donor abilitiy in PbS while In-doping gives a great rise on the carrier concentrations which is up to2×1020cm-3.This leads to a marked increase in the electrical conductivity and the highest ZT is obtained in higher temperature, and0.3%In-doped PbS sample shows a highest ZT of0.78at825K.The PbS compounds with additional Sb were prepared. The results show that Sb occupy interstitial positions which has little influence on the lattice thermal conductivity. The execess Sb only acts as donors and optimizes the carrier concentrations. PbS-1%Sb sample shows a highest ZT of0.8at825K. Overall, the carrier concentrations of n-type PbS compounds were adjusted in a wide range by different doping, which have great impacts on the electrical transport properties. While the carrier concentration locates between4X1019cm-3and9X1019cm-3, the samples display high power factor. The PbS-1%Sb sample with a carrier concentration of5.57X1019cm-3shows a highest power factor of1.4mWK-2m-1at825K, which indicates the optimizing of carrier concentrations is an effective way to enhance thermoelectric performance.On the basis of optimized carrier concentrations, PbS1-xSex and PbS1-xTex solid solutions were prepared. Se completely enters the sub-lattice of S and form solid solution in entire range. The Se-alloying introduces many coherent or semi-coherent PbSe nano-inclusions into the PbS matrix, which effectively reduce the lattice thermal conductivity. Furthermore, the coherent or semi-coherent nano-inclusions show a good lattice alignment with the matrix and it has little influence on the electrons’ transport. This leads to a marked increase in the ZT values, and the PbS0.5Se0.5-1%Sb sample shows a highest ZT of0.91at825K. Te has very low alloying limit in PbS (lower than3%). So it introduces many incoherent PbTe nano-inclusions which reduce lattice thermal conductivity more significantly due to the great lattice mismatch. Although they have notable negative impact on the electrons’ transport, the PbS0.5Te0.5-1%Sb sample shows a highest ZT of1.0at825K, which is about200%higher than undoped PbS compound.