Hello everyone. In this lecture, we are going to talk about the n-type thermoelectric oxide. We are focusing on the strontium titanate based oxide. As you know, strontium titanate is a dielectric ceramic. So they are insulator. But electronic conduction behavior can be induced by doping with higher valence ions. Now, for example, lanthanum three plus ion doping at strontium two plus site, and niobium five plus the doping at titanium four plus site, we can induce the electrical conduction behavior in insulating strontium titanate oxide. As shown here, by doping of lanthanum or niobium, we can activate one electrons from the valence band to the conduction band of titanium 3d orbital. One important advantage of strontium titanate is large density of states effective mass based to value. So based on this equation, then we can obtain the large Seebeck coefficient value due to the high density of state effectiveness, and then, the electric current conduction behavior can be easily controlled by doping of large. So due to the large Seebeck coefficient, and high electrical conductivity, we can obtain the high power factor. So you know, the power factor is the Seebeck coefficient to scale times electrical conductivity, and also this material has high melting temperature, about 2080 degrees C. So this material has good stability, even at higher temperatures, and another important advantage of this material is low cost and non-toxic. The epitaxial thin film of niobium doped to strontium titanate can be easily fabricated by the pulsed laser deposition technique. As shown here, by using the zero, zero, one, face of lanthanum aluminate, single-crystalline substrate, we can fabricates the high quality epitaxial thin film of niobium doped to strontium titanate, and also as shown here, we can obtain the high density of states effective mass, which it can be resulted in large Seebeck coefficient, and also obtain the high hole mobility, which can be resulted in high electrical conductivity. So calculating the power factor, is about the 1.5 milliwatt per meter Kelvin scale. So this is the moderate, the power factor value. However, the thermal conductivity of strontium titanate is very high. So it reaches about the 4.5 at higher niobium concentration. So the required ZT is relatively low, but just to reach it in value about 0.37 at 1,000 Kelvin. Do you remember the maximum ZT value of a p-type thermoelectric oxide, such as sodium cobalt oxide, is over 0.9 at high temperature. So we should increase the ZT of n-type thermoelectric oxide, in order to make high efficient thermoelectric module. One important approach to enhance the power factor of strontium titanate, is to make the 2DEG-like superlattice thin film. So as shown here, so we can fabricate the superlattice thin film, which is composed of insulating strontium titanate and conducting niobium doped to strontium titanate, and then, due to the quantum confinement effect. In this like [inaudible] quantum confined structure, then we can obtain the enlarged density of states, and then they can be resulted in large Seebeck coefficient value as shown here. So this like 2DEG property can be used to increase the power factor of strontium titanate, and another offered to increase the ZT of strontium titanate is to reduce the lattice thermal conductivity of a material. One promising candidate material is layered perovskite. It is Ruddlesden-Popper phases. As shown here, this like layer, the perovskite, the structure of the materials, have high density interface between SrO, rock salt type layer and strontium titanate layer. So in the presence of this like a high density interface. So this material can provide us the very low lattice thermal conductivity. So the epitaxial thin film of niobium doped to [inaudible] can be easily fabricates by the pulsed layer deposition technique, and as shown here, you can confirm the epitaxial thin film gross of niobium doped to strontium oxide, to strontium titanium oxide. So by using [inaudible] analysis and AFM analysis. But as shown in AFM image, we can just obtain the polycrystalline epitaxial film of this material. Actually, the lattice thermal conductivity of this material, that shows the reduced value compared with the strontium titanate, due to the intensified [inaudible]. But unfortunately, as shown here, this material shows the very low Seebeck coefficient value compared to the perovskite, strontium titanate, mainly due to the decrease in density of state effectiveness. The Seebeck coefficient originated from the deformed the density of state structure, that by crystal field splitting. Then compared with the perovskite type strontium titanate with regular titanium oxide octahedra, so this like layer the perovskite structure material has irregular the titanium oxide octahedra. So the bottom of the conduction band is composed of a single degenerate, the titanium 3d_xy orbital. So this resulted in the small dose, and this provide us the reduced Seebeck coefficient value. So we should find the sum the appropriate defect engineering and the structure engineering techniques, so in order to enhance the ZT in thermoelectric oxide based on the nanostructuring, and defect engineering technologies. Thank you.
