先进复合材料结课论文.docx

先进复合材料结课论文题目：The research progress of oxide thermoelectric materials 姓名： 彭帮强 学号： 201512006002 日期： 2016年6月25日 Abstract Oxides thermoelectric materials have virtues such as high temperature resistance, anti-oxidation, environment friendly, and the preparation technology is not complicated, do not need to vacuum,which is considered to be one of the most promising thermoelectric materials. Analyzed several typical oxide material crystal structure and thermal performance, and mainly of high temperature oxide thermoelectric materials were reviewed the research progress of preparation, modification methods and application.Key words: oxide thermoelectric material,thermoelectric property一、The realistic significance research of oxide thermoelectric materialsWith the rapid development of national economy, energy shortage and environment pollution problem increasingly prominent, thermoelectric materials due to the energy recycling and the special function of environmental protection, has become the hotspot in the field of material research. In fact, thermoelectric materials have been widely used in navigation, remote space over a long distance detection, waste heat power generation, thermoelectric refrigeration, space exploration, infrared stealth, etc. But, at present the technology is relatively mature, the performance good thermoelectric materials for metal semiconductor alloy, they have high thermoelectric conversion efficiency, but at high temperature is not stable, easy oxidation, and most of the heavy metal containing harmful to human body. By contrast, the oxide thermoelectric materials with excellent structural stability and chemical stability, can under the high temperature oxidation resistance, long service life, safe and non-toxic, simple preparation, high temperature is considered to be a condition has potential application prospects of the new thermoelectric conversion material.二、Main kinds of oxides thermoelectric materials1、Na-Co-O thermoelectric materialsCobalt oxide with layered structure, the general formula for AxBO2 (0.5 x 1) NaCo2O4 was Na + Co+2 and triangular lattice structure layer along the c axis arranged alternately stacked layer structure, the height of 2 d features. In layer (Na +), 50% 70% of the Na + are arranged without rules, make its have low thermal conductivity, and can be used as electric storage layer to provide electronic, stable crystal structure. When the doping of Na + mass fraction is 50% (x = 1) when the thermoelectric performance best substitutes NaCo2O4 represent such oxide materials. And Co+2 layer by the side of CoO6 octahedral connection, is responsible for electrical transport materials. In this kind of structure, can take advantage of the eight sides in the larger gap, instead of through the fill certain elements or elements, induce the lattice distortion, increasing the phonon scattering effect, improve the thermoelectric performance of materials. At the same time, in this kind of layer structure, different interval not only has different crystal symmetry, and with different chemical properties and electronic structure, which makes a single material has the versatility, result in cobalt oxide has high Seebeck coefficient and lower phonon thermal conductivity。2. Ca-Co-O thermoelectric materialsAt present, about the Ca-Co-O thermoelectric material research mainly involves the Ca3Co4O9, Ca2Co2O5, and three types of Ca3Co2O6 layered structure materials, including research more for Ca3Co4O9。Research shows that when the room temperature of layered cobalt base oxide Ca3Co4O9 with NaCo2O4 quite thermoelectric performance, and its temperature higher than 1000 k still can maintain stable performance in air or oxygen, therefore is a kind of extremely potential new thermoelectric materials in high temperature. It has high temperature resistance, stable performance, long service life, etc. The advantages of low temperature thermoelectric materials can not reach. But when the temperature more than 1193 k, the Ca3Co4O9 is likely to break down and make the thermoelectric performance.Ca3Co4O9 has similar to NaCo2O4 layered structure, by the insulation Ca2CoO3 ,CoO2 and conductive layer along the c axis arranged alternately. In Ca2CoO3 layer, Ca-O and Co-O are combined, in the form of ionic bond and CoO2 layer to the octahedral structure, Co is located in the center of octahedral atom, O atom array point in octahedron. When Co coordination number is 6, the valence + 3 and + 4, it is at the halfway point of the adjacent atom of oxygen, which may produce a shift in the center of the octahedron, so that the Co - O keys have different length. Band structure of this kind of structure change makes the material change, to improve seebeck coefficient.3.Perovskite oxides thermoelectric materialsPerovskite structure oxides thermoelectric materials were various, general formula for the ABO3, an ion has A larger ionic radius, with 12 oxygen atom ligand, B ion has A smaller ionic radius, coordination and six oxygen atoms. These compounds have excellent ferroelectric, piezoelectric, dielectric and pyroelectric performance, is widely used in the manufacture of random access memory, piezoelectric sensor, actuator, pyroelectric sensor array and condenser, etc. But because of the seebeck coefficient of these compounds is not high, so the thermoelectric merit ZT is not high, the thermoelectric performance needs to be improved. Research suggests that by doping methods for this kind of oxide modified materials, can change the band structure, improve the seebeck coefficient, while the electrical conductivity of some materials may reduce, but on the whole, the thermoelectric performance significantly improved.三、The oxide thermoelectric materials preparation methodsAt present，the preparation methods of oxide thermoelectric materials generally includes the following：(1) high temperature solid phase method. High temperature solid phase method generally used for reactions such as metal oxides, carbonates, or oxalic acid salt precursors, starting material been fully mixing, calcined reaction, synthesis temperature usually takes up to 1000 1200 . Mix between precursors in the method for solid and white mixed directly, so often exist shortcomings, such as mixing uneven, inadequate in the reaction process, the reactants to form product molecules of migration distance is long, often leads to lower the synthesis temperature is higher, the purity, poor uniformity. Therefore in the process of preparation, can use a ball mill grinding of reactivity to replace manual grinding, make fully mixing between the reactants, can overcome the above shortcomings to some extent. High temperature solid phase method is generally used in synthetic polycrystalline or grain size larger preferable solid materials, sintering, but due to the purity of product is low, particle size distribution is not uniform, so this method is suitable for the demand for materials such as purity requirement is too high and the larger the preparation of new materials.(2) Melt growth method. Traditional thermoelectric materials preparation by melt growth method, the method according to different process can be divided into three kinds: bridgman method, czochralski method, zone melting method. General preparation powder by mixing melting, alloy of components is the single crystal growth of the reentry after homogenization, made with orientation of single crystal materials. This method is the biggest advantage is that the operation is simple, but the biggest drawback is made of the sample is easy to form micro cracks, reduces the mechanical properties of thermoelectric materials. At the same time, this method is directly to the alloy elements according to the stoichiometric ratio of mixed sintering so this method is no advantage in improve the thermoelectric performance of materials.(3) Sol-gel method. Sol-gel method usually with metal salts and organic salts as precursors, with citric acid, tartaric acid and ethylenediamine tetraacetic acid, stearic acid, etc. With a strong organic acid as ligand. The advantages of this method lies in component mixing uniformity is good, easy to implement uniform doping, quantitative phase and a low temperature, easy to obtain dispersivity is good, high purity of the powder. This method in the film, the preparation of ceramic materials and the oxide materials occupies the important position, and through the control of the oxygen in the process of heat treatment atmosphere, still can produce oxygen measurement than different composite oxide materials. A shortcoming of this method in the raw materials used are usually more expensive, the gel contains a large number of pores, dry and heat treatment process can produce certain pollutants and shrinkage cavity, preparation period is longer, cost is high, its large-scale application is limited by certain.(4) Hot water and solvent thermal bonding diagnosis. Hydrothermal synthesis method refers to material in high temperature and high pressure sealing system of aqueous solution (or vapor fluid etc.) in the synthesis, separation and post-processing and then through get the required materials. Solvent thermal principle and hydrothermal synthesis, synthesis in organic solvent such as benzene or alcohol, etc.) instead of water, chemical reaction was achieved in sealing system. Hydrothermal solvent thermal characteristics of influence factors is more, such as temperature, pressure, time, concentration, ph value, material type, proportion, filling and packing order and the performance of the reaction kettle, etc. All have an impact on response. Its advantage is that dont need high temperature sintering, the synthesis temperature is low, the product directly as the crystalline state; When under the constant temperature and constant pressure reaction, easy to generate high crystallinity, particle size is easy to control, perfect crystal shape more rules. Using solvent hot method, therefore, synthetic materials for crystallization degree, particle size and morphology of the material control, in the preparation of ultrafine, no reunion or less gathering material, as well as the growth of single crystal, spherical, core-shell materials perovskite materials.四、Modification of oxide thermoelectric materialsGood oxides thermoelectric materials should possess high seebeck coefficient, high conductivity and low thermal conductivity, therefore, the researchers in order to improve the thermoelectric performance tried a variety of ways. At present, the high temperature modification of oxide thermoelectric materials mainly include the following three aspects。（1）the optimization of material thermal parameters. In general, the material of the seebeck coefficient, electrical conductivity and thermal conductivity is related with the carrier concentration, and relate to each other. Material seebeck coefficient decreases with the increase of carrier concentration, while electrical conductivity and thermal conductivity increases with the increase of carrier concentration, so how to improve the seebeck coefficient and electrical conductivity and lower thermal conductivity has become a key issue. At present, the more effective way is doped with metal ions in the crystalline structure of the original, under the condition of the optimization of carrier concentration increase phonon scattering, lower thermal conductivity, in order to improve its quality factor. (2) the preparation of nano structure materials. With the deepening of the nano materials research, people found in the thermoelectric performance of materials at the nanoscale tend to have significantly improved, the modified brings hope for thermoelectric materials. Some low (material quantum well can the seebeck coefficient, electrical conductivity and thermal conductivity of relatively independent control, thus in does not reduce the conductivity at the same time increase the seebeck coefficient; At the same time, low wiener m materials leading to high density of phonon selective grain boundary scattering, reduce the thermal conductivity of materials seem; In addition, the doping can also inhibit the role of electron to further improve the material carrier mobility, on the whole the realization of the aim of improve ZT values. Of course, compared with bulk materials, preparation and properties test of low-dimensional materials in difficult, it remains to be further improved. (3) preparation of superlattice materials. Relative to the current application of the solid solution alloys and intermetallic compounds, most of the easy preparation of oxide thermoelectric materials into superlattice, manifests the good thermoelectric performance。五、The application of oxide thermoelectric materialsTemperature is an important application of thermoelectric technology, can be used in solar power, geothermal power, gas power generation and other fields. Oxides thermoelectric materials in the application of thermal power generation mainly concentrated in the metal alloy not applicable on the high temperature heat source; Because of the special chemical composition and temperature sensitivity, oxides thermoelectric materials has important application value in terms of the sensor. For example, the oxide thermoelectric materials can be used for the preparation of high performance hydrogen sensor. The principle of it is in the part of the thermoelectric material surface coated hydrogen sensitive catalytic layer, when hydrogen existing in the ambient air, catalytic layer adsorption of hydrogen molecules react with oxygen in the air after send out quantity of heat, thereby having a temperature difference on both ends of thermoelectric materials. Due to material seebeck effect, the temperature difference is converted to electric potential, and the connection of alarm hydrogen leakage alarm signal could be sent。ConclusionsOxides thermoelectric materials has important application value in the high temperature heat source of power generation and temperature sensor. At present, their preparation methods, modification and application has a more in-depth study. But due to the thermoelectric performance of oxide thermoelectric materials is not high, from the point of development trend, future research will mainly focus on the following points: (1) to explore the new synthetic process and the preparation method. The synthesis methods of these oxides thermoelectric materials at present, a lot of, but can on the chemical composition and microstructure of thermoelectric material parameters for effective control methods is still rare, this aspect will be worth exploring. (2) to further explore ways to material modification. The material on the whole thermal efficiency is still inadequate, in many ways and application requirements and gap, this is also the key factors that limit the application of the. So, explore effective control material of thermoelectric parameters, to improve the efficiency of the modified approach will be one of the important research content. Predictably, with the deepening of the theoretical resea