Hello, everyone. In this lecture, we are going to talk about another important functionality of ceramics. It is thermoelectrics. Actually, we already discussed about the thermoelectric effect. If you remember, thermoelectric effect is the direct conversion phenomenon between electrical energy and thermal energy. It is temperature difference. So as shown here, if we apply the heat energy at one end of the material, the carriers can be activated from valence band to the conduction band, and then they diffuse from the hot side to the cold side. So this like a thermal effect, can be used to generate electricity, to measure temperature, or to cool and to heat object. The thermoelectric effect include two important effect. So let's make this a unicouple thermoelectric module. It is composed of one p-type semiconducting thermoelectric material and the n-type semiconducting thermoelectric material, and yellow colored electrode. So if we apply the electric current to this unicouple thermoelectric module, the electron diffuse to the opposite direction to the electrical current, and holes diffused along the same direction to the electrical current. These electrons and holes should have the thermal energy. The top side of the thermoelectric module should be cooled. This is the Peltier effect, which is the basic principle of thermoelectric cooling and heating technology. On the other hand, if we heat energy to the top side of this unicouple thermoelectric module, so at the hot side of the p-type thermoelectric material and n-type thermoelectric material, holes and electrons should be generated. They can be diffused from the hot side to the cold side, and then generate electricity. This is the Seebeck effect, which is the basic principle for thermoelectric power generation technology. The system absorption of some electric is dependent on the performance of the material, which is defined by ZT, and we already discussed about the things ZT. It's given by Seebeck coefficient square, the electrical conductivity times absolute temperature, over thermal conductivity. So we already discussed about the two trade-off relationship among these three transport parameters. One is trade-off between the thermal conductivity and electrical conductivity, and the other is the trade-off between Seebeck coefficient and electrical conductivity according to the carrier concentration. An important critical parameters, the electronic transport parameters, Seebeck coefficient is thermal power and thermoelectric power is magnitude of an induced thermoelectric voltage in response to a temperature difference across that material. So SI unit of the Seebeck coefficient is micro-watt per Kelvin. As shown here, this is the multi-relationship about the Seebeck coefficient. The Seebeck coefficient can be determined by the density of state material. So due to this relationship with the electronic structure dependent on the thermoelectric performance. So a shown here, a lot of intermetallic compound and alloyed based the thermoelectric material can show the high ZT value. But as shown here, the thermoelectric ceramics, the ZT value is relatively lower than those in alloys and intermetallic compound. So oxides are poor thermoelectric materials. Due to the large electronegativity difference among the constituent element, this results in the strong localization charge-carrier scattering by optical phonons. So we can only obtain the raw electrical conductivity in oxides. Also, the large bonding energy and small mass of oxygen should be resulted in high velocity of sound. This is the origin for the high lattice thermal conductivity of oxide. But in high operating temperature, over 900 Kelvin, the lower ZT of oxide can be compensated by the high temperature because the ZT is proportional to the operating temperature and also, the oxides are good chemical stability material even at higher temperatures. Another, the advantage of oxide thermoelectric material is low cost. So widely researched p-type thermoelectric oxide are layered cobaltite, such as A. A is the lithium, sodium, calcium, strontium, intercalated cobalt oxide and calcium cobalt oxide. So they have largest Seebeck coefficients due to low spin state of cobalt ion. Due to the variation in cobalt balance, we can easily change the oxygen content. This like the layered cobaltite to contain cobalt dioxide plane. So it provides us a path for p-type conduction. We can also find so many interfaces between layers. So this can provide the important scattering center for reduced pull on thermal conductivity. So cobalt oxide has CO2 minus plane that's separated by Ca2CoO3 plus layer. In sodium intercalated, the cobalt oxide, CoO2 minus layers are separated by a layer of sodium ions. So this layer, the cobaltite can be easily fabricated by using reactive solid-phase epitaxy, R-SPE technology. So by using R-SPE, we can fabricate the single crystalline films of layered oxide. As shown here, the first step to fabricate the single crystal films over layered cobaltite is the preparation of epitaxial film of cobalt oxide onto the 0001, Alpha alumina substrate, and then capping of YSZ, and then just heating at 700 degree C with sodium source, strontium source, calcium source. We can fabricate the lithium cobalt oxide, stratum cobalt oxide, and and calcium cobalt oxide films by using R-SPE technology. As shown here, we can easily fabricate the high-quality epitaxial films of sodium cobalt oxide and calcium cobalt oxide with very high crystal quality. Then let's think about the thermoelectric properties of layered cobaltite. So power factor; the power factor is the Seebeck coefficient to scale times electrical conductivity. It's about the 10 milliwatt per meter Kelvin square in the wide range of temperature from 100 to 1,000 Kelvin. The thermal conductivity is about three watt per meter Kelvin and ideally, independent of temperature. So the acquired ZT is about 0.3 at 1,000 Kelvin. So the ZT of the inter-metallic and alloy compound is over 1.0. This like the moderate ZT value, is comes from the moderator power factor value and high thermal conductivity of layered cobaltite. In bulk material, if we use some defect engineering technologies, we can obtain the enhanced ZT about 0.5 at 1,000 Kelvin as shown here. How about the thermoelectric properties of sodium cobalt oxide? Due to the high power factor compared with the calcium cobalt oxide and reduce the lattice thermal conductivity, which can be realized in sodium cobalt oxide. As shown here, we can obtain a very high ZT value, about 0.9 at 900 Kelvin. But the ZTs of layered cobaltite are still relatively lower than that of alloys and intermetallic based thermoelectric material. So we should find a new strategy to enhance the ZT of the p-type and n-type thermoelectric ceramics. Thank you.
