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Chapter 12-1 CHAPTER 12: STRUCTURE AND PROPERTIES OF CERAMICS How do ceramics differ from metals ? Keramikos ~ burnt stuff –Heat treatment is necessary.

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Presentation on theme: "Chapter 12-1 CHAPTER 12: STRUCTURE AND PROPERTIES OF CERAMICS How do ceramics differ from metals ? Keramikos ~ burnt stuff –Heat treatment is necessary."— Presentation transcript:

1 Chapter 12-1 CHAPTER 12: STRUCTURE AND PROPERTIES OF CERAMICS How do ceramics differ from metals ? Keramikos ~ burnt stuff –Heat treatment is necessary Usually a compound between a metal and a non-metal –Bonding displays a mixture of ionic and covalent character Generally hard and brittle, have high melting temperature –Why ? Generally thermally and electrically insulating Can be opaque, semi-transparent or transparent Traditional ceramics ~ based on clay (china, porcelain, bricks, tiles) and glasses Hi-tech ceramics => electronic, communication, computer hardware, aerospace industries

2 Chapter 12-2 Bonding: --Mostly ionic, some covalent. --% ionic character increases with difference in electronegativity. What is electronegativity ? Adapted from Fig. 2.7, Callister 6e. (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. Large vs small ionic bond character: CERAMIC BONDING

3 Chapter 12- Crystal Structure of Ionicly Bonded Ceramics Crystal structure is defined by 2 criterions 1.Magnitude of the electrical charge on each ion. Charge balance dictates chemical formula (Ca 2+ and F - form CaF 2 ). 2.Relative sizes of the cations and anions. Cations wants maximum possible number of anion nearest neighbors and vice-versa. Stable ceramic crystal structures require anions surrounding a cation to be all in contact with that cation. For a specific coordination number there is a critical or minimum cation/anion radius ratio r C /r A for which this contact can be maintained. Pure geometrical consideration…

4 Chapter 12-3 1.Charge Neutrality: --Net charge in the crystal structure should be zero. --General form: 2.Maximize the # of nearest oppositely charged neighbors --stable structures: Adapted from Fig. 12.1, Callister 6e. IONIC BONDING & CRYSTAL STRUCTURE

5 Chapter 12-4 Coordination # increases with Adapted from Table 12.2, Callister 6e. Adapted from Fig. 12.2, Callister 6e. Adapted from Fig. 12.3, Callister 6e. Adapted from Fig. 12.4, Callister 6e. COORDINATION # AND IONIC RADII

6 Chapter 12-5 On the basis of ionic radii, what crystal structure would you predict for FeO? Answer: based on this ratio, --coord # = 6 --structure = NaCl (rocksalt) Data from Table 12.3, Callister 6e. EX1: PREDICTING STRUCTURE OF FeO Two penetrating FCC units; other examples are MgO, MnS, LiF….

7 Chapter 12-6 Consider CaF 2 : Based on this ratio, coord # = 8 and structure = CsCl. Result: CsCl structure w/only half the cation sites occupied. Only half the cation sites are occupied since #Ca 2+ ions = 1/2 # F - ions. Adapted from Fig. 12.5, Callister 6e. EX2: A m X p STRUCTURES Empty

8 Chapter 12- EX3: ZnS - ZincBlende Structure Zn 2+ + S 2- What is the CN ? What should be the structure ?

9 Chapter 12- Ceramic Density Computations n’: number of formula units in unit cell (all ions that are included in the chemical formula of the compound = formula unit) ∑A C : sum of atomic weights of cations in the formula unit ∑A A : sum of atomic weights of anions in the formula unit V C : volume of the unit cell N A : Avogadro’s number, 6.023 X 10 23 (formula units)/mol

10 Chapter 12- EX4: NaCl density n’ = 4 in FCC lattice ∑A C = A Na = 22.99 g/mol ∑A A = A Cl = 35.45 g/mol V C = a 3 =[2 (r Na + r Cl )] 3 a

11 Chapter 12- Silicate Ceramics Composed mainly of silicon and oxygen, the two most abundant elements in earth’s crust (rocks, soils, clays and sand- SiO 2 silica) Basic building block: SiO 4 4- tetrahedron: –Si-O bonding is largely covalent, but overall SiO 4 block has charge of –4 –Various silicate structures – different ways to arrange SiO 4 4- blocks

12 Chapter 12- EX: Crystalline form of SiO 2 Three polymorphs of SiO 2 : Quartz, Crystobalite, Tridymite Not a very closed pack structure low density ~ 2.65 g/cm 3 3D networks of SiO 4 4- tetrahedra Each O atom is shared by an adjacent tetrahedron

13 Chapter 12- Window Glass Still SiO 4 4- tetrahedra are the basic building block. Most common window glasses are produced by adding other oxides (e.g. CaO, Na 2 O, B 2 O 3, etc) whose cations are incorporated within SiO 4 network. These cations break the tetrahedral network and glasses melt at lower temperature than pure amorphous SiO 2. A lower melting point makes it easy to form glass to make, for instance, bottles. Some other oxides (TiO 2, Al 2 O 3 ) substitute for silicon and become part of the network

14 Chapter 12- Carbon/Diamond/Fullerenes/ Nanotubes Read => p399-403

15 Chapter 12- Shottky Defect: Frenkel Defect Frenkel Defect -- a cation is out of place. Shottky Defect -- a paired set of cation and anion vacancies. Equilibrium concentration of defects Adapted from Fig. 13.20, Callister 5e. (Fig. 13.20 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.) See Fig. 12.21, Callister 6e. DEFECTS IN CERAMIC STRUCTURES ~exp  Q D /kT A c Point defects in ionic crystals are charged. The Coulombic forces that are generated due to defects are very large and any charge imbalance has a strong tendency to balance itself, electroneutrality. To maintain charge neutrality several point defects can be created at the same time: Anion interstitials are unlikely, why ? Charge neutrality of the crystal is maintained

16 Chapter 12-8 Impurities must also satisfy charge balance Ex: NaCl Substitutional cation impurity Substitutional anion impurity IMPURITIES

17 Chapter 12- Stoichiometry A state for ionic solids where there is an exact ratio of anions to cations defined by the chemical formula unit. –NaCl => anion to cation ratio is exactly 1:1 –Ca 2 F => 1:2, otherwise it is called nonstoichiometry –FeO => wüstite, Fe 2+ or Fe 3+ may exist depending on temperature and O partial pressure. For any Fe 3+, there has to be an extra vacancy so that the charge neutrality is preserved… But then, Fe 1-x O for x < 1…

18 Chapter 12- Impurities in Ceramics Impurity atoms can exist as either substitutional or as interstitial solid solutions in ceramics –Substitutional ions substitute for ions of like type (anion to anion, cation to cation) –Interstitial ions are small compared to host structure – formation of anion interstitials is unlikely (why?) –Solubility is higher if ion radii and charges match closely –Incorporation of ion with different charge state requires compensation by point defects to preserve charge neutrality

19 Chapter 12- Ceramic Phase Diagrams Al 2 O 3 -Cr 2 O 3 system; often they share a common element in their formula, in many cases it is OXYGEN. –Solubility is achieved by Al 3+ substituting Cr 3+ –Binary Isomorphous system

20 Chapter 12- Ceramic Phase Diagrams Al 2 O 3 -SiO 2 system

21 Chapter 12-9 Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials. Determine elastic modulus according to: Adapted from Fig. 12.29, Callister 6e. MEASURING ELASTIC MODULUS

22 Chapter 12- Mechanical Properties of Ceramics Ceramics are very brittle. (Fracture Toughness) –For brittle materials fracture stress concentrators are very important. (Chapter 8: measured fracture strengths are significantly smaller than theoretical predictions for perfect materials due to the stress risers) –Fracture strength of ceramic may be greatly enhanced by creating compressive stresses in the surface region (similar to shot peening, case hardening in metals, chapter 8) Compressive strength is typically ten times the tensile strength. This makes ceramics good structural materials under compression (e.g., cement, bricks in building apartments, stone blocks in the pyramids). Generally, tensile test is not used –Hard to machine, grippers may break the piece, fail after 0.1% strain. –Size is important due impact of # of cracks on strength, why ?

23 Chapter 12-10 3-point bend test to measure room T strength. Flexural strength: Typ. values: Si nitride Si carbide Al oxide glass (soda) 700-1000 550-860 275-550 69 300 430 390 69 Adapted from Fig. 12.29, Callister 6e. Data from Table 12.5, Callister 6e. MEASURING STRENGTH

24 Chapter 12-11 Elevated Temperature Tensile Test (T > 0.4 T melt ). Generally,... MEASURING ELEVATED T RESPONSE

25 Chapter 12-12 Ceramic materials have mostly covalent & some 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 brittle, with negligible ductility. Elevated T creep properties are generally superior to those of metals (and polymers). SUMMARY

26 Chapter 12- Reading: Chapter 12 Core Problems: Self-help Problems: 0 ANNOUNCEMENTS

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