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Ceramic MaterialsGuangzhao Mao

Homework:13.313.1613.2113.46

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Topics

? Applications? Atomic Structure and bonding? Crystal structure? Mechanical properties

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Applications of Ceramics? Construction (clays, bricks, tiles, windows, etc.).? Insulators such (electrical porcelain, alumina, etc.).? Ceramic chip capacitors in electronic circuits.? Semiconductors using sintered oxides such as iron oxide.? Wearing, grinding, and cutting tools.? High temperature applications (ovenware, engine components,

rocket components, etc.).? Optical lenses and optical fibers.

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Applications

AGT-100 Automotive Engine:• Gasifier turbine• Combustor• Power turbine• Turbine inlet guide vane• Exhaust diffuser• Regenerator

Thermal conduction module:• Integrated circuit chips• Multilayer ceramic substrate• Aluminum pistons for heat removal

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Atomic Structure and Bonding

? Ceramics: compounds between metallic and nonmetallic elements.

? Bonding type: ranging from purely ionic to purely covalent depending on electronegativities of the elements.

% ionic character = {1-exp[-0.25(XA-XB)2]} × 100

? Ceramic atomic compositions:? Simple ceramics (NaCl, MgO, ZnS, CaF, etc.).? Silicate ceramics (SiO4

4- and metal elements).? Carbonaceous ceramics (diamond, graphite, etc.).? Engineering ceramics (Si3N4, SiC, ZrO2, Al2O3,

BaTiO3-barium titanate, etc.)

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Silicate Ceramics

Basic unit:

Simple silicates:

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Silicate CeramicsLayered silicates:

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Silicate Ceramics – Networked Silicates

Crystalline silica: Silica glasses:

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More about Silica GlassesGlasses• Noncrystalline silicates containing other oxides.• Optically transparent (containers, windows, lenses, fibers, etc.)• Ease of fabrication.

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Thermal Properties in Glass Forming? Melting temperature ? Working temperature? Softening temperature? Annealing temperature? Glass transition temperature ? Strain temperature

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More about Carbonaceous Ceramics

diamond graphite

nanotubefullerene

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Crystal Structure of Simple Ceramic Compounds

• Criteria for formation of ionic crystals:• The crystal must be electrically neutral.• The coordination number (C.N.) is determined by rC (radius of cation)/rA(radius of anion).

• Cations and anions must be in contact.• Cations and anions must have maximum nearest neighbors.

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Example 13.1: Correlation between C.N. and rC/rA

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Cont’d

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General Rules between C.N. and rC/rA

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Example: Prediction of Crystal C.N.

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Crystal Structure of Ionic Crystals

Rock salt structure:• Example: NaCl.• Composition: AX.• C.N. = 6.• Anions pack in FCC.

Cesium chloride structure:• Example: CsCl.• Composition: AX.• C.N. = 8.• Anions pack in simple cubic.

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Crystal Structure in Ionic Crystals

Zinc Blende structure:• Example: ZnS.• Composition: AX.• C.N. = 4.• Anions pack in FCC.

Calcium Fluorite structure:• Example: CaF2.• Composition: AX2.• C.N. = 8 (A) and 4 (X).• Anions pack in simple cubic.

Perovskite structure:• Example: BaTiO3.• Composition: ABX3.• C.N. = 12 (A), 6 (B), 6 (X).• Anions pack in FCC.

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Review for Exam #3Chap. 6? Stress-strain curve

? Definitions? Modulus of elasticity? Yield/fracture strength? Ductility? Toughness? Resilience

? Hardness tests? Data statistics? Design and safety factors

Chap. 8? Fracture

? K, Kt, KC, KIC

? KIC, a, and ?? Griffith theory? Design Ex. 8.1

? Fatigue? S-N curve? Fatigue life prediction

? Creep? Creep curve/steady-state creep? Creep rate, T, and ?

? Difference among fracture, fatigue, and creep tests

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Summary of Crystal Structures

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Density Calculations? ?

AC

ACNV

AAn ? ???

'?

? ?C

3AA

3CC

C

SV

RnRn34

VV

APF??

??

? ? ? ?? ?C

3AA

3CC

C

SV

RnRn34

VV

IPF????

??

n’: the number of formula units per unit cell.? AC (? AA): the sum of the atomic weights of all

cations (anions) in the formula unit.VC: unit cell volume.NA: Avogadro’s number.VS: volume of spheres per unit cell.nC (nA): number of cations (anions) per unit cell.rC (rA): cation (anion) radius.

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Example 13.3: Density Calculation

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Cont’dSelect the unit length for a calculation.

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Example: Density Calculation

Calculate the density of zinc blende (ZnS). Assume the structure to consist of ions and that the ionic radius of Zn2+ = 0.060 nm and that of S2- = 0.174 nm.

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Example: Density Calculation

Calculate the density of UO2 which has the calcium fluorite structure. (Ionic radii: U4+ = 0.105 nm and O2- = 0.132 nm.

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Example: Ionic Packing Factor (IPF) Calculation

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Defects in Ceramic Crystals

? Frenkel defects.? Schottky defects.? Nonstoichiometric defects.? Impurities.

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Example13.32 If CuO is exposed to reducing atmospheres at elevated temperatures, some of the Cu2+ ions will become Cu+. (a) Under these conditions, name one crystalline defect that you would expect to form in order to maintain charge neutrality. (b) How many Cu+ ions are required for the creation of each defect? (c) How would you express the chemical formula for this nonstoichiometric material?

(a) For a Cu2+O2- compound in which a small fraction of the Cu2+ ions exist as Cu+,

for each Cu+ formed there is one less positive charge introduced (or one more negative charge). In order to maintain charge neutrality, we must either add an additional positive

charge or subtract a negative charge. This may be accomplished be either creating Cu2+

interstitials or O2- vacancies.

(b) There will be two Cu+ ions required for each of these defects.

(c) The chemical formula for this nonstoichiometric material is Cu1+xO or CuO1-x, where x is

some small fraction.

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Example 13.4If electroneutrality is to be preserved, what point defects are possible in NaCl when a calcium ion (2+) substitutes a sodium ion (1+)? How many of these defects exist for every calcium ion?

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Mechanical Properties of Ceramics

? General: hard, brittle, low impact resistance, high T performance.? Modulus of elasticity: 11 (extruded graphite) to 1200 (diamond)

GPa.

? Tensile strength: 14 (extrude graphite) to 1500 (sintered zirconia) MPa.

? Mechanisms for deformation: ? Covalent ceramics: little deformation due to separation of electro-pair

bonds under stress.? Ionic ceramics: capable of plastic deformation due to slip planes.? Noncrystalline ceramics: viscous flow.

? Factors affecting strength: pores, flaws, and grain size.

? ?20 P9.0P9.11EE ??? ? ?nPexp0fs ???? P: pore volume fraction

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Flexural Property Measurement Ceramics can be too brittle for tensile tests!

? ??

??

?/1

212 ??

???

???

F

T

ts

fs

VV

? ??

??? /1

212 ??

???

???

F

T

ts

fs

VV

3

3

4bdmL

EF ?

? : shape parameterVT: sample volume in tension testVF: sample volume in flexural testm: initial slope of load deflection curve

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More on Deformation

Viscous FlowSlip

More common

Less common

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Mechanical Properties of Ceramics? Fracture toughness: small, ranging from 0.2 (concrete) to 12 (zirconia)

MPa-m1/2.? Transformation toughening of zirconia.

? Fatigue failure: rare due to absence to plasticity. ? Hardness: high for abrasive ceramics (HK > 1000).

? Creep: only at high temperature. Transformation toughening

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ExampleA reaction-bonded silicon nitride ceramic has a strength of 300 MPa and a fracture toughness of 3.6 MPa-m1/2. What is the largest-size internal crack that this material can support without fracture? Assume Y = 1.

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Example – 13.39The fracture strength of glass may be increased by etching away a thin surface layer. It is believed that the etching away alter surface crack geometry (i.e. reduce crack length and increase the tip radius). Compute the ratio of the original and etched crack tip radii for an eightfold increase in fracture strength if two-thirds of the crack length is removed.

?t = original crack tip radius, and

?t' ?= etched crack tip radius ? f' ?= ? f

a' = a3

?o' ?= 8?o

? f = 2?o???

???a

?t

1/2 = ? f' ?= 2?o' ??

????a'

?t'?1/2

? t'

?t =

???

???? o'

?o 2

??

??a'

a =

???

???8?o

? o 2

??

??a/3

a = 21.3

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Summary

? Common ceramic materials.? % ionic character.? Packing of cations and anions:

? C.N. vs. radii ratio? Unit cell structures for simple ceramics.? Density and IPF calculations.? Defects.

? Mechanical properties:? Unique properties of ceramics.? Application of prior knowledge.