| Layered high-performance Mg3Bi2-based thermoelectric materials which made from nontoxic and earth-abundant elements have been considered as the most promising substitute for the commercial n-type Bi2Te3 alloys.Based on references,grain boundary effects in polycrystalline Mg3Bi2 materials were proved to diminish their room-temperature mobility(usually below 500 K),which results in poor electrical transport performance.By comparison,single crystals with the least grain boundaries are expected to have more excellent performance.The study of anisotropic phonon and electron transport behaviors in layered structure play an important guiding role in further optimization and development of two-dimensional materials,however,few experimental studies are reported on the anisotropy of Mg3Bi2-based materials so far due to the difficulties of growing large-sized single crystals.Therefore,not only the study of anisotropy but also conversion efficiency can be well promoted by growing large-sized and high-quality single crystals.For the first time,a series of large-sized and compositioncontrollable n-type Mg3Bi2-based single crystals were directly grown by applying an In-situ Loading Bridgman method(ILBG).The crystal quality,thermoelectric properties,anisotropy,and stability related were systematically examined.The effectiveness of ILBG was further demonstrated by utilizing it to fast-grow uniform Mg3Bi1.49Sb0.5Te0.01 polycrystalline rods with high mechanic strength,while the outstanding thermoelectric cooling ability was confirmed by constructing a single-leg thermoelectric device.And finite-element analysis was also used to simulate the output performance of the device.Detailed outlines and conclusions of this dissertation are summarized as follows:1.An In-situ Loading Bridgman method was developed to grow single crystals of Mgcontaining alloys and for the first time,n-type Mg3Bi1.49Sb0.5Te0.01 bulk single crystals with the diameter size in centimeter scale were successfully grown.Since Mg is highly volatile and reactive,which makes the Mg-containing alloys corrosive to most crucibles and related crystal growth is a big challenge.By designing a special set-up,which allows to load the metal precursor into graphite crucible at high temperature,a series of Mg3Bi2-based single crystals can be successfully grown.Graphite crucibles with a conical tip have been proven to be most beneficial for eliminating crystal nucleus,and finally single crystals of a compositioncontrollable quaternary compound Mg3Bi1.49Sb0.5Te0.01 were prepared.The backscattering Laue XRD diffraction patterns and single crystal X-ray diffraction indicated that the crystals bear notable single-crystalline character.The single crystal exhibits intrinsic n-type transport with an excellent mobility of 220 cm2·V-1·s-1 and a low thermal conductivity of 1.2 W·m-1·K-1 observed at room temperature,resulting in decent ZT value of～0.8.This work provides a new approach for the single crystal growth of Mg-containing alloys,and with the single crystalline materials prepared reliably and conveniently,in-depth investigations on the intrinsic thermoelectric properties of Mg3Bi2-based materials can be realized.2.Crystals were grown with flat interfaces corresponding to the optimal processing parameters,therein high-quality and large-sized single crystals were obtained,especially along the c-axis direction.High-quality bulk single crystal growth was carried out on native n-type Mg3Bi1.5-xSb0.5Tex(x=0,0.01)and Mg3Bi2,the thermoelectric properties of each component along the ab-plane were measured and compared.We also revealed the influence of Bi/Sb alloy on the thermoelectric properties of the single crystal and the significant effect of Te doping on the carrier concentration and mobility of Mg3Bi1.5Sb0.5.The anisotropy of in-and out-of plane for Mg3Bi1.49Sb0.5Te0.01 single crystals were carefully investigated where the explanation of its mechanism was conducted in detail.Owing to the higher mobility(266 vs 189 cm2·V-1·s-1),the electrical conductivity along ab-plane is about 25%higher than that along c-axis at room temperature.Besides,Seebeck coefficient and lattice thermal conductivity are insensitive to the crystal orientation.With the improvement of crystal quality,Mg3Bi1.49Sb0.5Te0.01 single crystal achieved a remarkable average ZTavg of 1.26 over the temperature range from 300 to 523 K along the ab-plane,and a compelling theoretical conversion efficiency of 14.6%by a temperature difference of 450 K was also expected.These values rank as the top among the reported literatures.Further stability tests suggested that Mg3Bi2-based single crystals were somehow sensitive to moisture,and effective anti-moisture strategies will be helpful to improve the chemical stability and lifetime of both Mg3Bi2-based materials and related modules.Finiteelement analysis was also used to simulate the output performance of the device.Through a comprehensive evaluation of properties mentioned above,the results presented here will benefit the in-depth understanding of the novel Mg3Bi2-based thermoelectric system and suggest new ideas on design of the state-of-art thermoelectric materials with high performance.3.By extending above method to prepare the polycrystalline materials of Mg3Bi2-xSbx,highly uniform polycrystalline rods can be obtained by using a fast growth rate of 7 mm·h-1.The polycrystalline rods exhibit good thermoelectric performance as well as excellent mechanic strength,which are already suitable for device manufacturing.Compared with traditional preparation methods,this growth technique does not require multiple movements and exposures during the preparation process,effectively avoiding sample contamination and quality loss.A carrier concentration of 7.8 ×1019cm-3 observed for the polycrystalline sample is slightly higher than that of the single crystal at room temperature,which can be understood with more defects formed in the polycrystalline material and thus results in a lower mobility of 90 cm2·V-1·s-1.The polycrystalline rods exhibit high ZT values around 1.0 near room temperature,demonstrated by a single-leg device,a large temperature difference of 10.5 K was achieved at the supplied electric current of 0.6 A,much superior to the conventional room temperature stateof-the-art thermoelectric materials at the same current. |
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