Hello, everyone. In this lecture, we are going to talk about the newly discovered ceramic material. It is electride. So as you know, the chemical bonding of materials is related with the minimum in energy. For example, hydrogen molecule has the more stable state than each hydrogen atoms so by making the bonding to overlapping electron orbitals. So this is like bonding characteristics such as ionic and covalent bonding, metallic bonding, and Van der Waals bonding, is strongly correlated with their physical and chemical properties such as electrical conductivity, electron transfer, the melting temperature, spin moment, stiffness and bending of materials. We already discussed about the ionic crystals, such as sodium chloride and magnesium oxide. They have the one cationic metal ions and one anions such as chlorine and oxygen. So they have ionic bonding characteristics. The electride are ionic crystals in which electrons serve as anions, as shown here. So anionic electrons are loosely bound to the positively charged lattice frameworks of sodium ion. Physical and chemical properties of electride can be determined by anionic electrons. They can show regular array of quantum dots. So we can obtain the low work function and high emission and catalytic properties. Sometimes, we can obtain unexpected electronic and magnetic properties in the presence of the loosely bounded, the confined electrons in electride. So the first form of electride is solvated electrons. Then in 1983, the first form of organic electride has been reported. The first inorganic form of electride, it is the calcium aluminate that has been reported in 2003. The active anions can be incorporated into nanospaces in crystal, such as nanocages structured material, and also channel structures such as apatite. They can be used as catalytic agent and anionic storage materials. The calcium aluminate, the composition is 12 calcium oxide and seven alumina C12A7 is a nanoporous crystal material. Actually, this material is one of the cement component. As shown here, the structure of C12A7 is constructed of only one type of cage with the inner free space of 0.5 nanometer. Cages are linked three dimensionally and sharing the framework with each other. The each unit cell contains 12 cages and can be denoted as the calcium 24, aluminum 28, oxygen 64 with four plus charged state. So in order to make the charge neutrality, we need two oxygen ions with two minus charge. So positive charge is compensated by oxygen two minus ions which are randomly incorporated into one over six of the cages. Oxygen two minus ions are not belongs to the frameworks so it can be understood as an extra-framework oxide ion. So we can obtain the fast ionic conductor behavior due to the extra-framework oxide ions which can migrate the three-dimensional linked cages at higher temperatures. The extra-framework oxide ion can be partially or completely replaced by other anions such as OH minus, O minus, F minus, Cl minus, H minus and Au minus. The oxygen radicals such as oxygen two minus, and the oxygen one minus, and the hydrogen minus ions can be incorporated in the cage by annealing at oxygen and hydrogen gas atmosphere. The cage in calcium aluminate can form on additional conduction band as shown here. It is cage conduction band, CCB to which located one or two electron volt below the bottom of framework conduction band, FCB to which is composed of calcium 5s orbitals. In this material, the electrons with low concentration induce a large lattice deformation due to the Coulomb attractive force between the entrapped electron and the two calcium two plus ions on the cage wall. So this results in electron localization. So due to this electron localization, the conduction can be occurred via the hopping of the electron from a localized deformed cage, an isolated quantum dot state to the cage conduction band so by polaron hopping. We can change the electronic transport behavior of calcium aluminate by reduction process. So for example, reduction process enable the extra-framework oxide ion to be replaced with electrons. So for example, by using titanium treatment at 700 to 1,100 degrees C because titanium forms stable non-stoichiometric oxide over a wide chemical composition range that acts between one to two in TiO two minus x, we can control the charging state of calcium aluminate. So as shown here, we can make the insulating state, the semiconducting state, and also metallic states of calcium aluminate by controlling the concentration of electrons. So calcium aluminate electride can be easily fabricated by melt-solidification process. By using the starting material of calcium aluminate with the oxygen ions, it can be decomposited into C3A and CA. Also, by using the another, the heat treatment under reducing atmosphere, we can obtain the electride form of calcium aluminate as shown here. Also, we can fabricate the single crystal form of calcium aluminate electride as shown here by using seed material with yellow color. It is calcium aluminate with the oxygen ion and by using some floating zone method, we can fabricate the electride form of calcium aluminate with the black color one as shown here. So in order to make the single crystal of electride material, we can use the crystal growth the technology to which is related with the stoichiometric melt. So one of the important processing technique is zone melting method that technique with partial melting of a long ingot and then traversing the molten zone along the ingot with or without a seed crystal as shown here. Thank you.
