Hello, everyone. Welcome to this lecture. In the previous modules, we talked about the fundamentals on ceramics including their compositions, crystal structures, and classifications. Then we discussed about the two important materials technology. They are nano-structuring and defect engineering. Then I provided the several material since as the technology to fabricate the ceramics of particles, [inaudible] and thin films. So based on these fundamentals, in this module, we are going to talk about the physical properties and applications of ceramics. As shown here, the physical properties of the materials include; electrical properties, such as electrical conductivity, ionic conductivity, dielectric constant, and permittivity; and thermal properties such as thermal conductivity, heat capacity, and thermal expansion coefficient; and mechanical properties such as strength, hardness, toughness, ductility, brittleness, and creep and slip; and optical properties such as transmittance, reflection, absorption, color, polarization, photoluminescence, and diffraction; and also include the magnetic properties such as ferromagnetic, paramagnetic, diamagnetic, magnetocaloric, and magnetostrictive. Let's think about the ionic and mixed conduction behavior in ceramics. This, like electrical conduction property, is important in fuel cell. A fuel cell is a device that generate electricity by a chemical reaction. We need two electrode: anode and cathode, and an electrolyte. Reactions that produce electricity take place at the electrode. An electrolyte carries ions, such as oxide ion and proton from one electrode to the other. Hydrogen is the basic fuel, but fuel cells also require oxygen. Hydrogen atoms enter a fuel cell at the anode, and then chemical reaction strips them of their electrons, and generated electrons provide the current through wires. Generated electricity with very pollution is the main advantage of fuel cell. Byproduct is just water vapor. The types of fuel cells is defined by the electrolyte material. For SOFC, solid oxide fuel cell, the electrolyte material is solid oxide. For MCFC, the molten carbonate fuel cell, the electrolyte material is molten carbonate. For PAFC, the phosphoric acid fuel cell, the electrolyte material is phosphoric acid. For PEMFC, the proton exchange membrane fuel cell, the electrolyte material is proton exchange membrane. For AFC, alkaline fuel cell, the electrolyte material is alkaline. Among these various fuel cells, the SOFC is a ceramic-based fuel cells. Let's think about the operation principle of SOFC. If we use the hydrogen as a fuel; at the anode, hydrogen and oxide ion makes two water vapor and two electrons as shown here. Electrons flow around the external circuit and then meet oxygen at cathode, then make two oxide ions. Oxide ions can be transferred through the electrolyte. The product water as a steam, available for steam, reformation of a fuel. If we use the carbon monoxide as a fuel; at the anode, due to the reaction between two carbon monoxide and two oxide ions to make two carbon dioxide and two electrons. Electrons flow round the external circuit and then they meet with the oxygen at the cathode, it makes two oxide ions. Then oxide ion can be transferred through the electrolyte. This is the operating principle of SOFC. The normal operating temperature of SOFC is above 800 degrees C. We can find four, the key element, key components, in solid oxide fuel cell, including electrolyte, interconnector, anode, and cathode. Let's think about the require the properties for SOFC component. The electrolyte should have high ionic conductivity and low electronic conductivity. Interconnector should have low ionic conductivity and high electronic conductivity. Anode and cathode should have high mixed conductivity of ions and electrons. The electrolytes and interconnector should have high stability under oxidation and reduction atmosphere. The anode should have high stability under reduction atmosphere. Cathode should have high stability under oxidation atmosphere. One important requirement from the viewpoint of a chemical reactivity, there is no chemical reaction with other components. Electrolyte and interconnector should be dense. Anode and cathode should be porous. These four components should have similar thermal expansion coefficient with other component. Then electrical conductivity of a material can be measured by using d.c.4-probe method. As shown here, if we apply the current through the outer two probes and then if we measure the voltage drop between two inner two probes, based on the Ohm's law, we can obtain the resistance value. Then by using the dimension of sample, then we can calculate the conductivity of sample. But just like a conductivity which can be measured by the d.c.4-probe method, is the summation of the bulk grain conductivity and grain-boundary conductivity. Grain-boundary conductivity is variable with heating condition. An important thing is the grain-boundary conductivity is not serious for thin film and single crystal. So we should separate the bulk grain conductivity from total conductivity by using a.c. method. As shown here, the a.c. method is based on the measurement of impedance, and capacitance of a material is given by this equation: one over two Pi times f, frequency; times r, resistance. You know, the capacitance for grain is about picofarad order, so they can be found at high frequency region. Also, the capacitance for grain-boundary is about nanofarad order, so if they can be found at relatively lower frequency region. So as shown here, you can find two separated semicircles after measuring of a.c. method. You can separate the grain contribution and grain-boundary contribution to the total electronic conduction. Then let's think about the separation of oxide ion conductivity. As you know, the total electric conductivity of the materials contains the electronic contribution by holes and electrons and also includes the ionic contribution by oxide ion and proton. As you know, the electronic transport property is variable with oxygen partial pressure. But ionic contribution is constant at a constant temperature. So oxide ion conductivity can be separated from the total conductivity with oxygen partial pressure dependent conductivity measurement. So electronic conduction is strongly correlated with the oxygen vacancies. We already discussed about this matter in [inaudible]. Anyway, in the presence of oxygen vacancy, if we increase the oxygen partial pressure, the oxygens can be incorporated into oxygen vacancies and then make oxygen at oxygen site with charge neutrality and the two plus charge in oxygen vacancy makes two holes. In this case, the electronic conductivity by holes is proportional to oxygen partial pressure to the one over four. Then at higher oxygen partial pressure, the oxygens can be incorporated into inter-cell site, and this makes inter-cell oxygen with two minus charges and then in order to make a charge neutrality, two holes should we formed. In this case, electronic conductivity by hole is proportional to oxygen partial pressure to the one over six. Then if we reduce the oxygen partial pressure, the oxygen can be diffused out from the lattice, so inter-cell oxygen can be decomposed into oxygen plus inter-cell vacancy and then make two electrons. In this case, electronic conductivity by electron is proportional to oxygen partial pressure to the minus one over four. Then at lower oxygen partial pressure, the oxygen at oxygen site can be diffused out from the lattice then make one over two oxygen and oxygen vacancy with two plus charge. In order to make charge neutrality, two electrons should be formed. In this case, electronic conduction by electron is proportional to oxygen partial pressure to the minus one over six. So total electrical conductivity is given by this equation: oxide ion conductivity plus hole conductivity plus electron conductivity. Hole conductivity and electronic conductivity is dependent on the oxygen partial pressure. So this equation can be rewritten by oxide ion conductivity plus Sigma_0 times oxygen partial pressure to the a plus Sigma_0 dash times oxygen partial pressure to the P. So you can find these four different features in electrical conduction behavior of a material. From this measurement, you can separate the oxide ion conductivity from total electrical conductivity. Thank you.
