Week 4_Session 1.pdf

Ceramics and Glasses Rojin Eghbali  Crystal for Metal Metal BCC FCC CPH 2 atoms/unit; Iron (RT) CP direction diagonal thru cell; Chromium...

Ceramics and Glasses Rojin Eghbali  Crystal for Metal Metal BCC FCC CPH 2 atoms/unit; Iron (RT) CP direction diagonal thru cell; Chromium APFBCC=0.68 Tungsten Copper, Nickel 4 atoms/unit; Aluminum CP direction runs diagonally across Gold each face of the cell. Titanium Zinc 6 atoms/unit; Magnesium CP direction runs diagonally across CP plane. APFCPH=0.74  Crystal structure of simple ceramics Ceramics usually are compounds consisting of two or more elements (having different atomic radius). They have covalent, ionic, or combined covalent/ionic bonding. The atoms are in a repeatable (ordered) arrangement.  Ceramics Crystal Structure Oxide structure Oxygen anions larger than metal cations (size effect) Close packed oxygen in a lattice (usually FCC) Cations fit into interstitial sites among oxygen ions  Factor that Determine Crystal Structure 1. Relative size of ions- Formation of stable structures: maximize the # of oppositely charged ion neighbors. 2. Maintenance of charge Neutrality: Net charge in ceramic should be zero Reflected in chemical formula AmXp  AX-Type Ceramic Crystal Structures AX-type ceramic crystals have either FCC or BCC unit cells. They have the same number of “A” atoms as “X” atoms in the unit cell. The “A” and “X” atoms are usually of different size. The smaller atom sits in interstitial locations in the unit cell while the larger atom sits in the regular atomic sites. （interstitial：The gaps between the regular atom locations） We will consider 3 most common AX-Type crystal structures: 1) The Sodium Chloride (Rock Salt) Structure. 2) The Zinc Blende Structure. 3) The Cesium Chloride Structure.  The NaCl “Rock Salt” Structure FCC The Cl- (larger ion) occupies the regular FCC sites. The Na+ (smaller ion) occupies the interstitial positions.  The ZnS “Zinc Blende” Structure FCC The S atoms occupy the regular FCC sites. The Zn atoms occupy interstitial positions.  The Cesium Chloride (CsCl) Structure BCC unit cell with the chlorine ions (Cl-) at the corners and a cesium ions (Cs+) in the body. Cl- Cs-  Equation for calculating density of a crystalline compound (i.e. an AX-type ceramic) Where: n’ is the number of formula units (molecules) within the unit cell ∑AA= the sum of the atomic weights of all the “A” atoms in the formula unit ∑AX= the sum of the atomic weights of all the “X” atoms in the formula unit V=volume of the unit cell N=Avogadro’s number  Calculate the Density of Salt (NaCl) The atomic weight of Na = 22.99 g/mol The atomic weight of Cl = 35.45 g/mol rNa+ = 0.102 nm, rCl- = 0.181 nm Note: Both Na+ and Cl- ions form FCC unit cells. Therefore, n’ = 4 Recall ∑AA= 22.99 g/mol ∑Ax=35.45 g/mol  Coordination number and Ionic Radii Coordination number: Number of nearest neighbouring atoms for a particular atom. To form a stable structure, how many anions can surround around a cation? ZnS (Zinc blende) NaCl CsCl  Example: Predicting the Crystal Structure of FeO On the basis of ionic radii, what crystal structure would you predict for FeO Cation Ionic radius (nm) Al3+ 0.053 𝑟𝑐𝑎𝑡𝑖𝑜𝑛 0.077 = = 𝟎. 𝟓𝟓𝟎 Fe2+ 0.077 𝑟𝑎𝑛𝑖𝑜𝑛 0.140 Fe3+ 0.069 Ca2+ 0.100 Coordination number-6 Crystal structure-NaCl Anion Ionic radius (nm) O2- 0.140 Cl- 0.181 F- 0.133  AX2 Crystal Structure Fluorite Calcium Fluorite(CaF2) Cations in the cubic site ZrO2, CeO2, etc. Antifluorite structure-position of cations and anion reversed  ABX3 Crystal Structure Perovskite Structure Complex oxide BaTiO3  Glass materials Quartz: Crystalline SiO2 Very strong (but brittle). Very high melting temperature. - Tmelt ≈ 1750oC Window glass: Non-crystalline SiO2 Very weak (but still brittle at room temp.) Very low melting temperature. -Tmelt ≈ 780oC Becomes very ductile (viscous) at high temperature.