Chapter 12: Structures & Properties of Ceramics

Transcription

1 Chapter 12: Structures & Properties of Ceramics ISSUES TO ADDRESS... How do the crystal structures of ceramic materials differ from those for metals? How do point defects in ceramics differ from those defects found in metals? How are impurities accommodated in the ceramic lattice? In what ways are ceramic phase diagrams different from phase diagrams for metals? How are the mechanical properties of ceramics measured, and how do they differ from those for metals? Chapter 12-1 Ceramic materials Compounds between metallic and monmetallic elements keramikos ; burnt stuff desirable properties are achieved through a high-temp. heat treatment process called firing( 燒成 ) Traditional ceramics : clay china, porcelain, bricks, tiles, glass and high-temp. ceramics A new generation of ceramics: much broader ex. Electronic, computer, communication, and aerospace Chapter 12-2

2 Atomic Bonding in Ceramics Bonding: -- Can be ionic and/or covalent in character. -- % ionic character increases with difference in electronegativity of atoms. Degree of ionic character may be large or small: CaF 2 : large SiC: small Adapted from Fig. 2.7, (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.) Chapter 12-3 Ceramic Crystal Structures Oxide structures oxygen anions larger than metal cations close packed oxygen in a lattice (usually FCC) cations fit into interstitial sites among oxygen ions Chapter 12-4

3 Factors that Determine Crystal Structure 1. Relative sizes of ions Formation of stable structures: --maximize the # of oppositely charged ion neighbors unstable stable stable 2. Maintenance of Charge Neutrality : --Net charge in ceramic CaF2: Ca 2+ cation should be zero. --Reflected in chemical formula: AmXp + Adapted from Fig. 12.1, m, p values to achieve charge neutrality F - anions F - Chapter 12-5 r cation r anion < Coordination # and Ionic Radii r cation Coordination # increases with r anion To form a stable structure, how many anions can surround around a cation? Coord # 2 linear ZnS (zinc blende) Adapted from Fig. 12.4, Adapted from Table 12.2, triangular tetrahedral octahedral cubic NaCl (sodium chloride) Adapted from Fig. 12.2, CsCl (cesium chloride) Adapted from Fig. 12.3, Chapter 12-6

4 Computation of Minimum Cation-Anion Radius Ratio Determine minimum r cation /r anion for an octahedral site (C.N. = 6) 2 anion cation r 2r 2a a 2r anion 2r anion 2r cation 2 2r anion r anion r cation 2r anion r cation ( 2 1)r anion r r cation anion Chapter 12-7 Bond Hybridization Bond Hybridization is possible when there is significant covalent bonding hybrid electron orbitals form For example for SiC X Si = 1.8 and X C = 2.5 % ionic character 100 {1- exp[-0.25(x Si X C ) 2 ]} 11.5% ~ 89% covalent bonding Both Si and C prefer sp 3 hybridization Therefore, for SiC, Si atoms occupy tetrahedral sites Chapter 12-8

5 Example Problem: Predicting the Crystal Structure of FeO On the basis of ionic radii, what crystal structure would you predict for FeO? Cation Al 3+ Fe 2+ Fe 3+ Ca 2+ Anion O 2- Cl - F - Ionic radius (nm) Data from Table 12.3, Answer: rcation ranion based on this ratio, -- coord # = 6 because < < crystal structure is NaCl Chapter 12-9 Rock Salt Structure Same concepts can be applied to ionic solids in general. Example: NaCl (rock salt) structure r Na = nm r Cl = nm r Na /r Cl = cations (Na + ) prefer octahedral sites Adapted from Fig. 12.2, Chapter 12-10

6 MgO and FeO MgO and FeO also have the NaCl structure O 2- Mg 2+ r O = nm r Mg = nm r Mg /r O = cations prefer octahedral sites Adapted from Fig. 12.2, So each Mg 2+ (or Fe 2+ ) has 6 neighbor oxygen atoms Chapter AX Crystal Structures AX Type Crystal Structures include NaCl, CsCl, and zinc blende Cesium Chloride structure: r r Cs Cl Adapted from Fig. 12.3, Since < < 1.0, cubic sites preferred So each Cs + has 8 neighbor Cl - Chapter 12-12

7 AX 2 Crystal Structures Fluorite structure Calcium Fluorite (CaF 2 ) Cations in cubic sites r Na /r Cl = 0.8, CN = 8 Half the center cube positions are occupied by Ca 2+ ions One unit cell consists of eight cubes Adapted from Fig. 12.5, UO 2, ThO 2, ZrO 2, CeO 2 Chapter ABX 3 Crystal Structures Perovskite structure Ex: complex oxide BaTiO 3 Adapted from Fig. 12.6, 1X1=1, 1/8 X 8=1 ½ X 6=3 Chapter 12-14

8 Density Computations for Ceramics Number of formula units/unit cell n ( A V C AA ) C N A Volume of unit cell Avogadro s number A C = sum of atomic weights of all cations in formula unit A A = sum of atomic weights of all anions in formula unit Chapter Density Computations for Ceramics n 4 V C 3 a ( 2r 2r ) Na Cl 3 n ( AC AA ) V N C [(2x (0.102X 10 A g / cm 7 4( ) ) 2(0.181X ] (6.022X )] Chapter 12-16

9 Silicate Ceramics Silicates are materials composed primarily of silicon and oxygen, the two most abundant element in the earth s cluster; ex. Soil, rocks, clays and sand Basic unit : SiO 4-4 tetrahedron various silicate structures arise from the various arrangement of an SiO 4-4 tetrahedron Si 4+ O 2- Adapted from Figs , Callister & Rethwisch 8e The most simple silicate materials is silica (silicon dioxide, SiO 2 ) - Silica polymorphic forms are quartz, cristobalite, & tridymite - The structures are open; atoms are not closely packed together low density ex. Density of quartz at RT = 2.65g/cm 3 - The strong Si-O bonds lead to a high melting temp. (1710ºC) for this material Chapter Silicate Ceramics Silica polymorphic forms : dependence of structure on temp. Cristobalite Chapter 12-18

10 Silica glasses Silicate Ceramics - if a silica melt is cooled quickly, its liquid structure will be reserved and it will turn into amorphous silica glass; having a high degree of atomic randomness - There is no well defined melting point for silica glass which slowly turns into a very viscous liquid upon heating. - It is often said that silica glass is an extremely viscous liquid, just like ordinary window glass, but both glasses are considered as regular solids. - Other oxides (eg. B 2 O 3 and GeO 2 ) may also form glassy structures ; network formers Chapter Glass Structure Basic Unit: 4- Si04 tetrahedron Si 4+ O 2- Quartz is crystalline SiO2: Glass is noncrystalline (amorphous) Fused silica is SiO 2 to which no impurities have been added Other common glasses contain impurity ions such as Na +, Ca 2+, Al 3+, and B 3+ network modifier ; lowers the mp and viscosity and makes it easier to form at lower temp. Na + Si 4+ O 2- (soda glass) Adapted from Fig , Chapter 12-20

11 Bonding of adjacent SiO 4 4- accomplished by the sharing of common corners, edges, or faces Silicates Mg 2 SiO 4 Ca 2 MgSi 2 O 7 Presence of cations such as Ca 2+, Mg 2+, & Al maintain charge neutrality, and 2. ionically bond SiO 4 4- to one another Chapter Layered Silicates Layered silicates (e.g., clays, mica, talc) SiO 4 tetrahedra connected together to form 2-D plane A net negative charge is associated with each (Si 2 O 5 ) 2- unit Negative charge balanced by adjacent plane rich in positively charged cations Adapted from Fig , Callister & Rethwisch 8e. Chapter 12-22

12 Layered Silicates (cont.) Kaolinite clay alternates (Si 2 O 5 ) 2- layer with Al 2 (OH) 2+ 4 layer Adapted from Fig , Note: Adjacent sheets of this type are loosely bound to one another by van der Waal s forces. Chapter Polymorphic Forms of Carbon Diamond tetrahedral bonding of carbon hardest material known very high thermal conductivity; 900~2,320W/mK, cf. Silver ~429 large single crystals gem stones small crystals used to grind/cut other materials diamond thin films hard surface coatings used for cutting tools, medical devices, etc. Adapted from Fig , Chapter 12-24

13 Polymorphic Forms of Carbon (cont) Graphite layered structure parallel hexagonal arrays of carbon atoms Adapted from Fig , Callister & Rethwisch 8e. weak van der Waal s forces between layers planes slide easily over one another -- good lubricant Chapter Polymorphic Forms of Carbon (cont) Fullerenes ; the most symmetrical large molecule Spherical cluster of 60 carbon atoms, C 60, like a soccer ball; The 1996 Nobel Prize for Chemistry has been won by Harold W. Kroto, Robert F. Curl and Richard E. Smalley for their discovery of Fullerene in 1985 contains double bonds, but not aromatic In theory, an infinite number of fullerenes based on pentagonal and hexagonal rings can exist,; ex. C70, C76 and C84 Chapter 12-26

14 A scanning Polymorphic Forms of Carbon (cont) Carbon nanotubes (CNT) sheet of graphite(graphene) rolled into a tube diameter ~1nm, length ~ few nm to microns ends capped with fullerene hemispheres discovered by Iijima who was studying arc-evaporation synthesis of fullerenes in 1991 extraordinary properties - Young s Modulus ; E > 1,250GPa, cf. steel ~200GPa, C-fiber ~425GPa - Tensile strength ; 11-63Gpa, cf. steel ~2GPa, C-fiber ~3.5-6GPa - Thermal conductivity; ~3,320W/mK - Current capacity ; ~ 1GAmps/cm 2, cf. Cu wire ~1MAmps/cm 2 - Thermal stability ; ~750 o C, cf. metal wires ~600-1,000 o C Chapter Polymorphic Forms of Carbon (cont) Carbon nanotubes (CNT) Zigzag : semi-conductivity Applications antenna, composite, writing, field emission, transistor, yarn, microscopy, storage Chiral Armchair : metallic conductivity Chapter 12-28

15 Polymorphic Forms of Carbon (cont) Graphene : Graphite + -ene One single layer of graphite; Andre Geim and Konstantin Novoselov won the Nobel Prize in Physics in 2010 for groundbreaking experiments regarding the graphene.(2004, Science) Graphene is aromatic, but different from that in benzene; The six ring carbon atoms have two resonance bonds and four single bonds which covalently bind to six other carbon atoms (not six hydrogens with single bonds as in benzene) Chapter Polymorphic Forms of Carbon (cont) Graphene One single layer of graphite, but offers some impressive properties that exceed those of graphite - Electrical conductivity; best of any known metal,~ 10 8 S/m, cf. Silver 63.0X Thermal conductivity; 4,840~5,300W/mK, cf. Diamond~900-2,320 - Mechanical properties ; E ~1,100GPa (100x stronger than steel) - light(0.77mg/m 2 ) - nearly transparent - Thinnest(~1/3nm) - Flexible Applications Transparent, conducting and flexible electrodes, Nano-transistor Chapter 12-30

16 SUMMARY Interatomic bonding in ceramics is ionic and/or covalent. Ceramic crystal structures are based on: -- maintaining charge neutrality -- cation-anion radii ratios. Silicate ceramics: Silica, Silica glasses and Silicate Polymorphic forms of carbon: -- Graphite, Diamond, Fullerene, CNT, and Graphene Chapter Problems Concept Check : 12.1 Example problem : 12.1~12.3 Questions and Problems : , 12.7, , 12.23, Chapter 12-32