| Energy and Environment are two important issues of the human society, they arenot only independent, but also dependent each other. In the development of the humansociety, it is the sign for the First Industrial Revolution that the steam engine is insteadof the hand labor. This revolution immensely promotes the development of the humansociety to make our industry, economy, science, etc step to a new era. However, thebad side is the serious environmental pollution following the industrial revolution. Ithas surpassed the ability of the natural septic systems because of overly exploitingfossil fuel, immoderate burning and unscrupulous exhaust emission. As a result, thenature environment has been badly polluted, leading to threaten human health andabundant species extinctions.Human has begun to find and develop the new green energy and the materials ofenergy conversion, when the energy crisis and environmental deterioration have beenthe inevitable issues of the human society. Currently, the wind power, thehydro-energy and the solar energy have been gradually applied in our daily life. Thescientist found and kept studying a new green energy conversion materials‘Thermoelectric Materials’ for effectively using the wasteful energy in the productionand living. Thermoelectric materials are a kind of function materials which candirectly convert heat energy into electrical energy and vice versa. It had attractedattention by the scientific community based on its advantages such as the stableproperties, non-pollution, small volume, long service life, etc. The development ofthermoelectric materials can effectively enhance the efficiency of the energyutilization. For example, the waste heat in industry and body temperature can be usedto produce the electric power. Therefore, it is one of the most important subjects in thescientific community that is to actively research the high performance and variousthermoelectric materials. Now, the thermoelectric materials are not used in daily life due to the lowconversion efficiency. To judge a thermoelectric material, we usually adopt thenon-dimensional thermoelectric figure of merit (ZT), which is defined as ZT=TσS2/κ,where T is the absolute temperature, S is the Seebeck coefficient, σ is the electricalresistivity and κ is the thermal conductivity. The increased ZT will directly result inthe enhancement in conversion efficiency. Therefore, the important target is to studyand achieve high ZT of thermoelectric materials in the thermoelectric field. However,the basic parameters of the ZT are not independent, limiting the significant increase ofZT values. The ZT≥1has been regarded as a benchmark for many thermoelectricmaterials for a long time. The Bi2Te3-based materials due to the optimal ZT～1beingnear room temperature become one of the best important thermoelectric materials, andgive rise to a lot of attentions from the researchers.Bi2Te3-based materials has an R-3M space group, the unit cell is therhombohedral layer structure. Each crystal layer consists of the same atom, Bi and Teatoms are arranged following the c axis as Te1-Bi-Te2-Bi-Te1, which are stacked basedon five atom layers. The traditional preparation methods for Bi2Te3-based materialsare hydrothermal synthesis, high-energy ball milling, molecular beam epitaxy, etc.Currently, the development of the nanotechnology significantly promotes thethermoelectric field. The synthesis and adjustment of the Bi2Te3-based nanostructureseffectively improve the figure of merit. Meantime, the new experimental techniquealso significantly develops the Bi2Te3-based materials such as the SPS and the meltspinning. The long synthetic period, complicated process, reaction difficulty, etc. limitthe development of large-scale production. However, high-pressure technique exhibitsits unique advantages in long developing process such as fast reaction, the broadeningof research space, the discovery of new materials, etc.In this paper, we researched the synthesis and property modulation of theBi2Te3-based materials based on the pressure by the high-pressure andhigh-temperature (HPHT) technique. It mainly researched the effects of pressure andelement composition on the crystal structure, electron structure, band structure andmicrostructure of the Bi2Te3-based materials, modulating the carrier effective mass, carrier concentration, carrier mobility, Fermi level of the Bi2Te3-based materials torealize the breakthrough in thermoelectric performance. The main research resultshave been shown as below:1. We had studied the thermoelectric properties of the Bi2Te3bulk materialssynthesized under different pressures. The pressure can significantly change thetextural feature of reaction products, effectively modulating carrier and phononscattering to improve the thermoelectric properties of Bi2Te3alloys. As a result, themaximum ZT of n-type Bi2Te3has been achieved to be0.97.2. We studied the effects of high pressure and Sb substitution on thermoelectricproperties of Bi2Te3-based materials produced by HPHT. The results indicated that thepressure can effectively optimize the thermoelectric properties, which are caused bySb substitution, improving further the high thermoelectric performance ofBi2Te3-based materials. In addition, we had concluded an optimal carrierconcentration range, which was beneficial to the improvement of thermoelectricproperties.3. The BiSbTe3bulk materials had been synthesized under different pressures byHPHT, and the thermoelectric properties of BiSbTe3bulk materials had beeninvestigated carefully. The results indicated that high pressure can effectively adjustthe electrical transport properties. Meantime, high pressure can also introduce the newvibration modes to realize the reduction in thermal conductivity of reaction products.As a result, the p-type BiSbTe3bulk materials achieved a maximum ZT of1.4.4. Bi0.5Sb1.5Te3bulk materials had been synthesized by HPHT. We mainlyresearched the thermoelectric properties of Bi0.5Sb1.5Te3bulk materials synthesizedunder different pressures. The results indicated that high pressure can produce thestrain inside the crystals, and induced the structure modulation and abundant latticedefects, promoting the phonon scattering to decrease the thermal conductivity. Inaddition, high pressure can effectively improve the thermal stability of samples, whichis beneficial to the practical application of thermoelectric materials. The maximum ZTof the synthesized p-type Bi0.5Sb1.5Te3is1.3, and the average ZT is1.1in themeasured temperature range. 5. We conducted the Se substitution experiment of Bi2Te3-based materials underthe condition of high pressure. The results indicated that pressure role can effectivelymodulate the electrical and thermal transport properties, achieving high σ/κ. As aresult, the maximum ZT is1.03from the synthesized n-type Bi2Te2.73Se0.27bulkmaterials by HPHT.In summary, the research results of the different Bi2Te3-based materials indicatethat high pressure exhibits the irreplaceable advantages during the synthesis processof Bi2Te3-based materials. The pressure effect can significantly change the texturalfeature and crystal structure, induce the reduction of thermal conductivity and the newmaterial phases, and expand the research field of thermoelectric materials. The HPHTmethod can achieve the fast preparation of high thermoelectric performance ofBi2Te3-based materials, and avoid the disadvantages of traditional methods during theimprovement of thermoelectric performance, simultaneously. Therefore, HPHTprovides an effective technique and a production basis for the further developmentand research of thermoelectric materials. |
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