Performance Optimization Of N-type Bi₂Te₃ Based Thermoelectric Materials Doped With MgB₂ And B

Nowadays,the society has a huge energy consumption and a large requirement for waste heat utilization.Thermoelectric materials and thermoelectric devices have a great prospect in the application of waste heat recovery and power generation,which is one of the international research hotspots.Improving the comprehensive performance of thermoelectric materials is the basis of all their applications.In this dissertation,two series of samples(Bi₂Te_2.7Se_0.3)1-x(MgB2)x and(Bi₂Te_2.7Se_0.3)1-x Bx were prepared by means of high-temperature smelting and spark plasma sintering(SPS).Bi₂Te_2.7Se_0.3 nanostructured materials were synthesized by solution method,and a series of nanocomposited thermoelectric materials were prepared by MgB2 doping and combining the SPS technology.The effects of MgB2,B and solution synthesis on the thermoelectric and mechanical properties of n-type Bi2Te3-based thermoelectric materials were studied.Specific research results are listed as following:(1)The study on the series of samples(Bi₂Te_2.7Se_0.3)1-x(MgB2)x prepared by smelting method show following results:By doping a small amount of superconducting material MgB2,the thermoelectric and mechanical properties of n-type Bi₂Te_2.7Se_0.3 alloy prepared by one-step spark plasma sintering method can be enhanced simultaneously,among which Mg and B atoms play an important role in the optimization of carrier concentration and the enhancement of hardness.In addition to optimizing carrier concentration,MgB2 doping can also increase the carrier mobility and reduce the lattice and bipolar thermal conductivity,so that the ZT peak value of Bi₂Te_2.7Se_0.3(BTSMB)alloy doped with 0.5%MgB2 can reach 0.96 at 325 K,and the average ZT value within 300-500 K can reach 0.88.The ZT peak and average ZT values of our optimized BTSMB sample are higher than the Bi2Te3-based alloys prepared by zone melting(ZM)method.In addition,the hardness of the optimized BTSMB sample was nearly 70%higher than that of the commercial ZM ingot.(2)The study on the series of samples(Bi₂Te_2.7Se_0.3)1-x Bx prepared by smelting method show following results:The B doping can not only effectively reduce the carrier concentration,greatly improve the Seebeck coefficient,but also further enhance the mechanical properties.Compared with the n-type Bi₂Te_2.7Se_0.3 alloy,the thermoelectric performance of Bi₂Te_2.7Se_0.3(BTSB)alloy doped with 6%B is effectively improved at room temperature.The corresoponding peak ZT can reach 0.95 at 350 K,and the average ZT value can reach 0.88 at300-473 K.The average ZT value of doped 2-6%BTSB samples is above 0.85.Our results show that the doping B optimization strategy can provide excellent thermoelectric performance in a large temperature range and has great application potential.Furthermore,compared with commercial ZM samples,the hardness was improved by nearly 68%and the compressive resistance was improved by nearly 210%.(3)The study of MgB₂ doped series samples of Bi₂Te_2.7Se_0.3 nanocomposite prepared by solution method show following results:The MgB2 doping can further optimize the carrier concentration and obtain an obvious enhancemence of thermoelectric performance on the basis of obtaining low lattice thermal conductivity from nanocomposited thermoelectric material prepared by solution method.At room temperature,the 10%MgB2 doped sample can obtain the lowest lattice thermal conductivity(0.34 Wm^-1K^-1)as compared to the undoped Bi₂Te_2.7Se_0.3sample with lattice thermal conductivity of 0.70 Wm^-1K^-1,which is nearly 50%lower.In addition,the total thermal conductivity of the doped samples can be greatly reduced and the electrical properties can also be greatly improved.