Effects Of Second-Phase Dispersion On The Thermoelectric Properties Of Bi₂Te₃-Based Alloys And Their Zone-Melting Preparation

In the course of rapid economic development,the problems of energy and the environment are becoming apparent.If this problem can be solved,the goal of sustainable development can be achieved.On the one hand,fossil fuels are being consumed in large quantities and reserves are nearly depleted,but there is still a huge and growing demand for energy.On the other hand,due to the excessive exploitation of fossil fuels and the wanton discharge of waste products,the environment has suffered serious damage,even irreversible damage.People are gradually realizing that social development cannot be achieved at the expense of the ecological environment,and clean energy is the general trend of future development.Thermoelectric materials use the movement of internal carriers to achieve the mutual conversion between thermal and electrical energy.For example,they can convert waste heat from production and life into electrical energy,which improve the overall utilization of materials;they can also solve the problem of relying solely on fossil fuels for electricity.Among them,Bi₂Te₃-based alloys,as one of the traditional thermoelectric materials,have been improving their performance with the progress of science and technology,and they are the most widely used thermoelectric materials in commercial applications,which have strong research value.This paper mainly discussed two n-type materials Bi₂Te₃ and Bi₂Te_2.7Se_0.3,which were synthesized by hydrothermal method and formed by hot pressing.It was proposed to enhance the ZT values of the original materials by introducing different second phases.Firstly,zero-dimensional carbon-based nanomaterials,CDs,were chosen as the second phase.In Bi₂Te₃+m wt%CDs（m=0,0.05,0.10,0.15）and Bi₂Te_2.7Se_0.3+n wt%CDs（n=0,0.05,0.10,0.15）,the thermal conductivity was effectively reduced while the power factor was improved.Finally the optimum thermoelectric properties were achieved at 432 K,and the specific result was that m=0.10 corresponds to the ZT value of 0.58,n=0.10 corresponds to the ZT value of 0.84.The effect of La₂O₃ particles as a second phase on the Bi₂Te₃-based alloy was next investigated.The results for this series of experiments were slightly different from those of CDs composites,with a greater reduction in thermal conductivity and a partial reduction in power factor.Combining these two changes,the performance of both Bi₂Te₃+s wt%La₂O₃（s=0,0.5,1,1.5）and Bi₂Te_2.7Se_0.3+t wt%La₂O₃（t=0,0.5,1,1.5）was improved.At 455 K,ZT values of about 0.61 were achieved for the s=0.5 sample;a maximum value of 0.94was reached for the t=0.5 sample.Finally,the method of preparing n-type Bi₂Te_2.7Se_0.3crystalline rods using the zone melting method was also attempted in this paper.At 359K,the ZT values reached 1.16,and the thermoelectric properties were higher than current commercial products.Comparing the thermoelectric properties in the vertical and parallel zone melting directions,it was found that the resistivity and thermal conductivity were significantly different along the different directions tested and little variation in the Seebeck coefficient simultaneously.It confirmed the anisotropy of the thermoelectric properties.