Chapter 12: Structures & Properties of Ceramics

Transcription

1 Chapter 12: Structures & Properties of Ceramics ISSUES TO ADDRESS... Structures of ceramic materials: How do they differ from those of metals? Point defects: How are they different from those in metals? Impurities: How are they accommodated in the lattice and how do they affect properties? Mechanical Properties: What special provisions/tests are made for ceramic materials? Chapter 12-1

2 Ceramic Bonding Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. Large vs small ionic bond character: CaF 2 : large SiC: small Adapted from Fig. 2.7, Callister 7e. (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 12-2

3 Ceramic Crystal Structures Oxide structures oxygen anions much larger than metal cations close packed oxygen in a lattice (usually FCC) cations in the holes of the oxygen lattice Chapter 12-3

4 Which sites will cations occupy? 1. Size of sites Site Selection does the cation fit in the site 2. Stoichiometry if all of one type of site is full the remainder have to go into other types of sites. 3. Bond Hybridization Chapter 12-4

5 Ionic Bonding & Structure 1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors unstable Charge Neutrality: --Net charge in the structure should be zero stable stable CaF2: Ca 2+ cation + Adapted from Fig. 12.1, Callister 7e. F - anions F - --General form: AmXp m, p determined by charge neutrality Chapter 12-5

6 Coordination # and Ionic Radii Coordination # increases with Issue: How many anions can you arrange around a cation? r cation r anion r cation r anion < Coord # 2 linear ZnS (zincblende) Adapted from Fig. 12.4, Callister 7e Adapted from Table 12.2, Callister 7e. triangular T D O H cubic NaCl (sodium chloride) Adapted from Fig. 12.2, Callister 7e. CsCl (cesium chloride) Adapted from Fig. 12.3, Callister 7e. Chapter 12-6

7 Cation Site Size Determine minimum r cation /r anion for O H site (C.N. = 6) 2r anion 2r cation = 2a a = 2r anion 2r anion 2r cation = 2 2r anion r anion r cation = 2r anion r cation = ( 2 1)r anion rcation = r anion Chapter 12-7

8 2. Stoichiometry Site Selection II If all of one type of site is full the remainder have to go into other types of sites. Ex: FCC unit cell has 4 O H and 8 T D sites. If for a specific ceramic each unit cell has 6 cations and the cations prefer O H sites 4 in O H 2 in T D Chapter 12-8

9 Site Selection III 3. Bond Hybridization significant covalent bonding the hybrid orbitals can have impact if significant covalent bond character present For example in SiC X Si = 1.8 and X C = % ionic character = 100 {1- exp[-0.25( X X ) ]} 11. 5% ca. 89% covalent bonding both Si and C prefer sp 3 hybridization Therefore in SiC get T D sites Si C = Chapter 12-9

10 Example: Predicting 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, Callister 7e. Answer: rcation = r anion = based on this ratio, --coord # = 6 --structure = NaCl Chapter 12-10

11 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 prefer O H sites Adapted from Fig. 12.2, Callister 7e. Chapter 12-11

12 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 O H sites Adapted from Fig. 12.2, Callister 7e. So each oxygen has 6 neighboring Mg 2+ Chapter 12-12

13 AX Crystal Structures AX Type Crystal Structures include NaCl, CsCl, and zinc blende Cesium Chloride structure: r Cl Cs = = r cubic sites preferred So each Cs + has 8 neighboring Cl - Adapted from Fig. 12.3, Callister 7e. Chapter 12-13

14 AX Crystal Structures Zinc Blende structure r Zn r O 2 2 = = O H?? Size arguments predict Zn 2+ in O H sites, In observed structure Zn 2+ in T D sites Why is Zn 2+ in T D sites? bonding hybridization of zinc favors T D sites Adapted from Fig. 12.4, Callister 7e. Ex: ZnO, ZnS, SiC So each Zn 2+ has 4 neighboring O 2- Chapter 12-14

15 AX 2 Crystal Structures Fluorite structure Calcium Fluorite (CaF 2 ) cations in cubic sites UO 2, ThO 2, ZrO 2, CeO 2 antifluorite structure cations and anions reversed Adapted from Fig. 12.5, Callister 7e. Chapter 12-15

16 Perovskite ABX 3 Crystal Structures Ex: complex oxide BaTiO 3 Adapted from Fig. 12.6, Callister 7e. Chapter 12-16

17 Mechanical Properties We know that ceramics are more brittle than metals. Why? Consider method of deformation slippage along slip planes in ionic solids this slippage is very difficult too much energy needed to move one anion past another anion Chapter 12-17

18 Ceramic Density Computation Number of formula units/unit cell = n ( A V C C A N A A ) Volume of unit cell Chapter 12-18

19 Silicate Ceramics Most common elements on earth are Si & O Si 4+ O 2- Adapted from Figs , Callister 7e. crystobalite SiO 2 (silica) structures are quartz, crystobalite, & tridymite The strong Si-O bond leads to a strong, high melting material (1710ºC) Chapter 12-19

20 Amorphous Silica Silica gels - amorphous SiO 2 Si 4+ and O 2- not in well-ordered lattice Charge balanced by H + (to form OH - ) at dangling bonds very high surface area > 200 m 2 /g SiO 2 is quite stable, therefore unreactive makes good catalyst support Adapted from Fig , Callister 7e. Chapter 12-20

21 Silica Glass Dense form of amorphous silica Charge imbalance corrected with counter cations such as Na + Borosilicate glass is the pyrex glass used in labs better temperature stability & less brittle than sodium glass Chapter 12-21

22 Silicates Combine SiO 4 4- tetrahedra by having them share corners, edges, or faces Mg 2 SiO 4 Ca 2 MgSi 2 O 7 Cations such as Ca 2+, Mg 2+, & Al 3+ act to neutralize & provide ionic bonding Adapted from Fig , Callister 7e. Chapter 12-22

23 Layered Silicates Layered silicates (clay silicates) SiO 4 tetrahedra connected together to form 2-D plane (Si 2 O 5 ) 2- So need cations to balance charge = Adapted from Fig , Callister 7e. Chapter 12-23

24 Layered Silicates Kaolinite clay alternates (Si 2 O 5 ) 2- layer with Al 2 (OH) 4 2+ layer Adapted from Fig , Callister 7e. Note: these sheets loosely bound by van der Waal s forces Chapter 12-24

25 Layered Silicates Can change the counterions this changes layer spacing the layers also allow absorption of water Micas KAl 3 Si 3 O 10 (OH) 2 Bentonite used to seal wells packaged dry swells 2-3 fold in H 2 O pump in to seal up well so no polluted ground water seeps in to contaminate the water supply. Chapter 12-25

26 Carbon Forms Carbon black amorphous surface area ca m 2 /g Diamond tetrahedral carbon hard no good slip planes brittle can cut it large diamonds jewelry small diamonds often man made - used for cutting tools and polishing diamond films hard surface coat tools, medical devices, etc. Adapted from Fig , Callister 7e. Chapter 12-26

27 Carbon Forms - Graphite layer structure aromatic layers Adapted from Fig , Callister 7e. weak van der Waal s forces between layers planes slide easily, good lubricant Chapter 12-27

28 Carbon Forms Fullerenes and Nanotubes Fullerenes or carbon nanotubes wrap the graphite sheet by curving into ball or tube Buckminister fullerenes Like a soccer ball C 60 - also C 70 + others Adapted from Figs & 12.19, Callister 7e. Chapter 12-28

29 Defects in Ceramic Structures Frenkel Defect --a cation is out of place. Shottky Defect --a paired set of cation and anion vacancies. Shottky Defect: Frenkel Defect Adapted from Fig , Callister 7e. (Fig is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.) Equilibrium concentration of defects ~ e Q D / kt Chapter 12-29

30 Impurities must also satisfy charge balance = Electroneutrality Ex: NaCl Na + Cl - Impurities Substitutional cation impurity Ca 2+ Na + cation vacancy Na + initial geometry Ca 2+ Ca 2+ impurity resulting geometry Substitutional anion impurity O 2- anion vacancy Cl - Cl - initial geometry O 2- impurity resulting geometry Chapter 12-30

31 Ceramic Phase Diagrams MgO-Al 2 O 3 diagram: Adapted from Fig , Callister 7e. Chapter 12-31

32 Measuring Elastic Modulus Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials. cross section b rect. d R circ. F L/2 L/2 Determine elastic modulus according to: F x slope = F d d linear-elastic behavior E = F d L 3 4bd 3 = F d rect. cross section Adapted from Fig , Callister 7e. d = midpoint deflection L 3 12pR 4 circ. cross section Chapter 12-32

33 Measuring Strength 3-point bend test to measure room T strength. cross section b rect. d R circ. F L/2 L/2 location of max tension Adapted from Fig , Callister 7e. d = midpoint deflection Flexural strength: F f s fs = F x dfs 1.5F f L = F f L bd 2 pr 3 rect. d Material Typ. values: Si nitride Si carbide Al oxide glass (soda) Data from Table 12.5, Callister 7e. s fs (MPa) E(GPa) Chapter 12-33

34 Measuring Elevated T Response Elevated Temperature Tensile Test (T > 0.4 Tm). creep test s s e x. slope = e ss = steady-state creep rate time Chapter 12-34

35 Summary Ceramic materials have covalent & ionic bonding. Structures are based on: -- charge neutrality -- maximizing # of nearest oppositely charged neighbors. Structures may be predicted based on: -- ratio of the cation and anion radii. Defects -- must preserve charge neutrality -- have a concentration that varies exponentially w/t. Room T mechanical response is elastic, but fracture is brittle, with negligible deformation. Elevated T creep properties are generally superior to those of metals (and polymers). Chapter 12-35

36 ANNOUNCEMENTS Reading: Core Problems: Self-help Problems: Chapter 12-36