Hello, everyone. In this lecture, we are going to talk about the relationship between defective structures and electrical transporting properties of material. As shown here, you can find a lot of electronic transport properties of material, including Seebeck coefficient, electrical conductivity, mobility and effective mass. As shown here, you can find that this electronic transport properties are strongly correlated with the defective structures. Already we talked about the relationship between defect structure and thermal transport properties of a material. So in this lecture, we are going to talk about the relationship between defective structure and electronic and thermal transport properties of materials focusing on thermoelectrics. So because thermoelectric contains the electronic transport and thermal transport properties of materials. So as you know, thermoelectric effect is direct convergent phenomenon of temperature differences to electric voltage and vice versa. As shown here, if we applied the heat at one end of the material, the carriers can be activated from valence band to the conduction band and they diffuse from the hot side to the colder side. This thermoelectric effect can be used to generate electricity, to measure temperature, to cool objects, and to heat them. You can find the two important effect in thermoelectrics. The first is peltier effect, which is the basic principle for thermoelectric cooling and heating and second is Seebeck effect, which is the basic principle for thermoelectric power generation. Let's make this like a thermoelectric module, which consists of P-type semiconducting thermoelectric material and N-type semiconducting thermoelectric material and yellow colored electrode. If we apply the electric current to this thermoelectric module the electrons diffuse along the opposite direction to the current and holes in p-type semiconducting material diffuse along the current direction. So because the holes and electrons should have heat energy, the top side of the module should be cooled, this is thermoelectricity. Then if we apply the heat at the top side of a thermoelectric module in n-type thermoelectric material, the electrons can be activated and in p-type semiconducting material, holes can be generated. Activated electrons and holes can be diffused from the hot side to the cold side, and then makes the electrical energy, this is thermoelectric power generation. The efficiency of thermoelectric cooling and thermoelectric power generation is dependent on the operating conditions, such as the temperature difference, and also dependent on the performance of thermoelectric material. As shown here thermoelectric performance of material that it is the combination actions between thermal and electrical energy. The ZT is given by this equation. In this equation, SC is the Seebeck coefficient, Sigma is the electrical conductivity, and Kappa is the thermal conductivity at a given absolute temperature T. So in order to increase the ZT of thermoelectric material, we should increase the Seebeck coefficient and also increase the electrical conductivity and reduce the thermal conductivity. However, there are two important trade-off relationship among these three parameters. The first trade-off is between the thermal conductivity and electrical conductivity. Second trade-off relationship is between the Seebeck coefficient and electrical conductivity according to carrier concentration. So based on the defect engineering technique. So we should find the way to overcome these two trade-off relationship. For the second trade-off relation, we should find a way to enhance the Seebeck coefficient while maintaining electrical conductivity of the material. One possible way to increase the Seebeck coefficient while maintaining electrical conductivity is density-of-state engineering. As shown here the Seebeck coefficient is proportional to the density of states effective mass m_star and m_star is proportional to the density-of-state. So by using the density-of-state engineering technique, we can enhance the Seebeck coefficient. So density-of-state engineering techniques, includes the carrier filtering, band flattening, band convergence, and resonant state, and these density-of-state engineering techniques strongly correlate with defects structure. The first, the carrier filtering effect can be realized by the introduction of three dimensional inclusions. The Seebeck coefficient can be defined by this inclusion, the first term of this inclusion is related with the density-of-state, and second term of this inclusion is related with the variation in carrier relaxation time tau. In warmer semiconducting material, the second term can be negligible. But if we introduce the three dimensional nano-inclusions, into the thermoelectric materials. As shown here, we can induce the band bending, by the matching of some rebel between the conducting nano-inclusion and thermal electric metrics. Due to the dislike band bending, we can induce the variation in carrier relaxation time. So the second term of this equation should be positive. So resultantly we can obtain the enlarged Seebeck coefficient value by the carrier filtering effect. The band convergence is another important density-of-state engineering technique, which can be realized by the formation of zero dimensional point defect. As shown here, you can find that they separated the band, which is colored green, compared with the violet color, the band structure and by the generation of zero dimensional point defect. We can convert this to different value as shown here and this results in the increase of carrier pocket number. So we can obtain the enlarged Seebeck coefficient due to this band convergence. We already discussed about the resonant state formation by the generation of a general demand point defect. So this like red color, dopant has bound state. We cannot obtain any change in density-of-state. However, the doping element, red colored doping element have this approach bound state that they can induce the ledger interstate formation. So density-of-state material can be increased as shown here due to the regional and state of formation, and if we use the formula of tuning technology, we can obtain the enlarged Seebeck coefficient value due to the enlarged density-of-states by enlarged state formation. Also the engineered band structure can be also obtained by atomic disordered structure. In exile dependents, you can find left-sided shoulder structure, the witchy that suggests that the generation of atomic disorder. You can also find the different two regions, the m and p with different uprightness in TEM images and in this like high relation TEM images, you can clearly find that the distinct difference in chemical ordering of two regions, and then this like of the generation of atomic disorders. The triggers the proton scattering by point defect and also triggers the density-of-state engineering as shown here. Slightly, this region has large bandgap, but highly disordered can provide us the small bandgap. So this is like, band fluctuation can be resulted in enlarged density-of-states, and then, finally, we can obtain the enhanced Seebeck coefficient value by deformation of atomic disorders. So in this lecture, we talked about the several important relationship between the physical properties, focusing on the thermoelectric material, which is related with the defect structure. Then, in the next module, we are going to talk about the synthesis technique of ceramics, which can be used as a fundamental technology to realize the nanostructured and defect engineered ceramic materials. Thank you.
