Hello everyone. In this lecture, continue to previous lecture that we are going to talk about ionic and mixed conduction. The total electrical conductivity of materials can be expressed by the summation of electronic contribution by holes and electrons and ionic contribution by anion, cation and proton. So total electrical conductivity of a material sigma is given by hole and electron conductivity plus ionic conductivity. Basically the electrical conductivity of a material is based on this equation eun, e, is charge of carrier, u is mobility of carrier and n is concentration of carrier. So the oxide ion conduction can be also expressed by this equation. Ionic conductivity is related with the concentration and mobility, and needs for high oxide ion conductivity are high concentration. It is related with the density of oxygen vacancies and high mobility, which is related with the large free volume. So oxide ion conductivity is related with the oxygen vacancies. So these oxide material which can be formed oxygen vacancy are promising candidate material for oxide ion conductors, for example, the fluorite structure zirconium, cerium, bismuth oxide, bismuth vanadium oxide can be on the promising candidates for oxide ion conductors. Also perovskite structure, lanthanum aluminate, lanthanum gallate, lanthanum mandate, lanthanum indate and LaYO3 can also be used as oxide ion conductors. Another example can be found in perovskite related structure oxide and apatite structure oxide. Firstly, lets think about the oxygen vacancy formation in fluorite structure, the AO2 oxide material. The representative material is zirconium, ZrO2, and we can generate the oxygen vacancy by doping of yttrium with three plus charge at zirconium with four plus charges site. So if we doped yttrium at zirconium site, the charge state of yttrium is always three plus and the charge state of zirconium is always four plus. So yttrium substituted at zirconium site has minus one charge. Then in order to make charge neutrality, the oxygen vacancy with two positive charges should be formed. Then let's think about the oxygen vacancy formation in perovskite structured lanthanum gallium oxide. The left side figure shows the crystal structure of lanthanum gallium oxide. We already discussed about the several times of this material, and we can generate oxygen vacancies by doping at lanthanum site and also by doping at gallium site. So for example, if we doped barium at lanthanum site, the charge state of barium is always two plus and the charge state of lanthanum is always three plus. So barium substituted at lanthanum site has minus one charge, and in order to make charge neutrality, one over two oxygen vacancy with two plus charges should we formed. Then also if we doped magnesium at gallium site, the charge state of magnesium is always two plus and charge state of gallium is always three plus. So magnesium substituted at gallium site has minus one charge, and in order to make charge neutrality, one over two oxygen vacancy with two plus charge should to be formed. Brownmillerite structured material such as Ba2In2O5 has intrinsic oxygen vacancies. As shown here, the oxygen vacancies are ordered along 110 direction in brownmillerite structured material and has the alternating layers of InO6 octahedra, and InO4 tetrahedra. So this crystal structure can be understood by perovskite structure with one over six of oxygen missing in ordered row. Another important ionic conduction is proton conduction. The proton conductivity also related with the concentration and mobility. So needs for high proton conductivity are high concentration of protonic defect and high mobility. In order to induce the proton conduction, the first step is formation of oxygen vacancy, and then incorporation of water vapor is required. Actually, water vapor can be decomposed into one hydroxyl ion and one proton, and hydroxyl ion can be incorporated into oxygen vacancy and remained proton can be bonded to another oxygen, so this make two protonic defect. Then let's think about the enthalpy of a platonic defect formation reaction in A, B, or C perovskite structured material. So as shown here, you can find the correlation between hydration enthalpy, delta H and difference of electronegativities of A-site cation and B-site cation in perovskite structured oxide. You can calculate the water incooperation energy of this material from lattice energy. As shown here, the water in cooperation energy of lanthanum gallium oxide has large plus value. So lanthanum gallium oxide constitutes the pure oxide ion conduction behavior. Whereas, lanthanum yttrium oxide or lanthanum scandium oxide, that they have the large negative value of water incooperation energy about minus 1.36 electron volt. So they can show pure protonic conduction behavior and lanthanum indium oxide, it has moderate minus value of water incooperation energy. So this material can show mixed conduction of oxide ion and proton. Then how can we clarify the proton conduction in material? As shown here, you can find increased conductivity by proton conduction under wet nitrogen atmosphere. But below 600 degree C, the electrical conductivity in wet nitrogen is much higher than that in dry nitrogen. That this means that the generation of a protonic defect and then proton conduction can be occurred. Then proton conduction can also be clarified by DTA and TGA analysis. As shown here, you can find two endothermic reaction in DTA analysis. So one endothermic reaction about 120 degree C is later released but desorption of absorbed water on the surface of the sample and another endothermic picks, that which can be found near the 340-410 degree C is related with the dehydration of the incorporate water. This is protonic defect. So as shown here, proton conductivity is rather larger compared with oxide ion conductivity at lower temperature. But at higher temperature, proton conductivity should be decreased due to the dehydration of platonic defect. So now we use the oxide ion conductor as an electrolyte material for solid oxide fuel cell. So in order to realize the proton conductor basis solid oxide pure cell, we should develop the high temperature proton conductor. Thank you.
