Ceramic Materials to Replace Silicon, Because Ceramic Are Tougher, More Refractory and More Inert E.G

Ceramics are inorganic, non -metallic and crystalline materials that are typically produced using clays and other minerals from the earth or CERAMIC chemically processed powders MATERIALS Ceramics are crystalline and are compounds formed between metall ic MATERIALS and non -metallic elements such as aluminium and oxygen (alumina -

Al 2O3 ), silicon and nitrogen (silicon nitride - Si 3N4) and silicon and carbon (silicon carbide -SiC).

Glasses are non -metallic, inorganic but amorphous . They are often considered as belonging to ceramics.

Characteristics of Ceramics Characteristics of Ceramics Structure of Ceramics

Ceramics Metals Polymers Ceramics exhibit ionic, covalent bonding or a combination of the two

(like in Al 2O3) Low density High density Very low density High T m Medium to high T m Low Tm Type of bonding strongly influences the crystal structure of cer amics High elastic modulus Medium to high elastic Low elastic modulus Brittle modulus Brittle modulus Ductile and brittle lCeramics crystallise in two main groups: Non -reactive Ductile Goff electrical and Reactive (corrode) thermal insulators Good electrical and 1. Ceramics with simple crystal structure (e.g; SiC, MgO) High hardness and thermal conductors wear resistance 2. Ceramics with complex crystal structures based on silicate SiO 4 (known as silicates) Ceramic Bonding • Bonding: Ionic bonding : metallic ions + nonmetallic ions -- Mostly ionic, some covalent. Cations Anions -- % ionic character increases with difference in electronegativity. Stable structure • Large vs small ionic bond character:

Coordination Number: RC/R A CaF 2: large SiC: small

RC/R A = 0.155

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. 6

Ceramic crystal structure considerations

Charge Neutrality The bulk ceramic must remain electrically neutral

For example, the compound MgO 2 does not exist Mg +2 & O -2: net charge / molecule = 1(+2) + 2( -2) = -2 must MgO

Coordination Number (CN) : The number of atomic or ionic nearest neighbors. Depends on atomic size ratio

CN increases as the RC/R A increases CN determines the possible crystal structure, Thus, CN determines the physical properties MgO and FeO Examples of AX type structure

MgO and FeO also have the NaCl structure Cs +

2- O rO = 0.140 nm Cl - 2+ Mg rMg = 0.072 nm

Rock Salt Structure

= Na +

Adapted from Fig. = Cl - 12.2, Callister 7e. Each oxygen has 6 neighboring Mg 2+

AX Crystal Structures AX Crystal Structures

AX–Type Crystal Structures include NaCl, CsCl, and zinc blende Zinc Blende structure

Cesium Chloride structure:

Ex: ZnO, ZnS, SiC

r + .0 170 Cs = = .0 939 r − .0 181 Cl

+ - Each Cs has 8 neighboring Cl Adapted from Fig. 12.4, Callister 7e. Adapted from Fig. 12.3, Callister 7e.

11 12 ABX Crystal Structures AX 2 Crystal Structures 3

Fluoride structure • Perovskite

• Calcium Fluoride (CaF ) 2 Ex: complex oxide BaTiO • cations in cubic sites 3

• UO 2, ThO 2, ZrO 2, CeO 2

Adapted from Fig. 12.6, Callister 7e.

Adapted from Fig. 12.5, Callister 7e.

13 14

Mechanical Properties

We know that ceramics are more brittle than metals. ceramics Why? • Consider method of deformation slippage along slip planes Glasses Refractories Cements Eng. Ceramics in ionic solids this slippage is very difficult Clay Products Abrasives too much energy needed to move one anion past another anion Glasses Glass - ceramics Our focus is HERE !!!

15 Engineering ceramics are generally Bioceramics classified into the following:

Cutting tools Structural ceramics, Industrial wear parts, bioceramics, cutting tools, engine components Electrical and Electronic ceramics, Capacitors, insulators, substrates, IC packages, piezoelectrics, magnets, superconductors Engine parts Ceramic coatings, Industrial wear parts, cutting tools, engine components

Chemical processing & environmental Coating ceramics Filters, membranes, catalysts

Silicates Silicate Ceramics Si -O Tetrahedron O 4- Combine SiO 4 tetrahedra by having them share Most common elements on Si corners, edges, or faces earth are Si & O

The strong Si-O bond leads to a strong, high melting material Adapted from Fig. (1710ºC) 12.12, Callister 7e. Mg 2SiO 4 Ca 2MgSi 2O7

Cations such as Ca 2+ , Mg 2+ , & Al 3+ act to neutralize & provide ionic bonding 20 Si -O Tetrahedron O Silicate Ceramics O

Two most common silicate ceramics are: 2. Silica Glasses Si Silica and silica glasses If the tetrahedra are randomly arranged, a non - crystalline structure, known as Glass is formed.

Silica glasses is a dense form of amorphous Silica 1. Silica (SiO ) Silica (SiO 2) silica - Charge imbalance corrected with + If the tetrahedra are arranged in a “counter cations” such as Na regular and ordered manner, a -Borosilicate glass is the pyrex glass crystalline structure is formed. Silica used in labs have 3 different types: quartz, -better temperature stability & less crystobalite and tridymite brittle than sodium glass

Amorphous Silica

Other oxides may also be incorporated into a • Silica gels - amorphous SiO 2 glassy SiO 2 network in different ways: Si 4+ and O 2- not in well-ordered lattice 1. Network formers: form glassy structures (B O ) Charge balanced by H + (to form 2 3 OH -) at “dangling” bonds 2. Network modifiers: added to terminate (break 2 very high surface area > 200 m /g up) the network (CaO, Na 2O). These are added to silica glass to lower its viscosity (so SiO 2 is quite stable, therefore unreactive that forming is easier) makes good catalyst support 3. Network intermediates: these oxides cannot form glass network but join into the silica network and substitute for Si. Adapted from Fig. 12.11, Callister 7e.

23 Carbon Forms Carbon Forms

• Carbon black – amorphous – • layer structure – aromatic layers surface area ca. 1000 m 2/g • Diamond tetrahedral carbon hard – no good slip planes brittle – can cut it Adapted from Fig. large diamonds – jewellery 12.17, Callister 7e. small diamonds often man made - used for cutting tools and polishing Adapted from Fig. 12.15, Callister 7e. weak van der Waal’s forces between layers diamond films hard surface coat – tools, planes slide easily, good lubricant medical devices, etc. 25 26

Carbon Forms Defects in Ceramic Structures • Frenkel Defect • Fullerenes or carbon nanotubes -a cation is out of place. wrap the graphite sheet by curving into ball or tube • Shottky Defect -- a paired set of cation and anion vacancies. Buckminister fullerenes Shottky Like a soccer ball C 60 - also C 70 + others Defect: Adapted from Fig. 12.21, Callister 7e. (Fig. 12.21 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.) Frenkel Defect

−Q / kT Adapted from Figs. • Equilibrium concentration of defects D 12.18 & 12.19, ~ e Callister 7e.

27 28 Mechanical Properties of Ceramics Mechanical Properties of Ceramics

Ceramics have inferior mechanical properties compared to metals, and this Ceramics have excellent compressive strength (used in cement and has limited their applications concrete in foundations for structures and equipment)

The main limitation is that ceramics fail in “brittle ” manner with little or no The principles source of fracture in ceramics is surface cracks, porosity, plastic deformation. inclusions and large grains produced during processing.

Fracture strength of ceramics are significantly lower than predi cted by Testing ceramics using the usual tensile testing is not possible , so a theory because of the presence of very small cracks in the mater ial (stress transverse bending test is used and a modulus of rupture (MOR) is concentrators). determined.

Lack of ductility in ceramics is due to their strong ionic and c ovalent bonds. Strength of ceramics can only be described by statistical method s and it is dependent on specimen size.

Transverse rupture Compressive Elastic strength strength modulus Hardness Poisson’s Density Material Symbol (MPa) (MPa) (GPa) (HK) ratio ( n) (kg/m 3)

Aluminum Al 2O3 140–240 1000–2900 310–410 2000–3000 0.26 4000–4500 oxide Cubic boron CBN 725 7000 850 4000–5000 — 3480 nitride Diamond — 1400 7000 830–1000 7000–8000 — 3500

Silica, fused SiO 2 — 1300 70 550 0.25 — Silicon SiC 100–750 700–3500 240–480 2100–3000 0.14 3100 carbide

Silicon Si 3 N4 480–600 — 300–310 2000–2500 0.24 3300 nitride Titanium TiC 1400–1900 3100–3850 310–410 1800–3200 — 5500–5800 carbide Flexural 3F L Tungsten WC 1030–2600 4100–5900 520–700 1800–2400 — 10,000–15,000 f carbide 2 Rectangular cross section strength, σfs = 2 Partially PSZ 620 — 200 1100 0.30 5800 bd stabilized zirconia 3 Ff L The properties vary widely depending on the condition of the material (crack size) = 3 Circular cross section πR Factors Affecting Strength of Ceramics Failure of ceramics occurs mainly from structural defects; surface cracks, porosity, inclusions and large grains during processing. Toughening Mechanisms of Ceramics

Porosity in ceramics acts as stress Fracture strength or toughness of ceramics can be improved only by concentrators: crack forms and propagates mechanisms that influence the crack propagation (ceramics always leading to failure. contain cracks). Once cracks start to propagate, they will continue to grow until fracture occurs. There are various methods used to improve the toughness of Porosity also decrease the cross -sectional ceramics: area over which a load in applied: lower the stress a material can support.

1. Transformation toughening Strength of ceramics is thus determined by many factors: 2. Microcrack induced toughening 1. Chemical composition 2. Microstructure - In dense ceramics materials, no large pores, the flaw is 3. Crack deflection related to grain size. Finer grain size ceramics, smaller flaws size at the boundaries, hence stronger than large grain size. 4. Crack bridging 3. Surface condition 4. Temperature and environment (failure at RT, usually due to larg e flaws).

1. Transformation Toughening: e.g. Partially Stabilised Zirconia (PSZ) • This transformation is accompanied by a volume expansion, causin g a compressive stress locally and in turn a squeezing effect on the crack and enhancing the fracture toughness also significantly extends the reliability Zirconia (ZrO 2) exists on 3 different crystal structures: and lifetime of products made with stabilized zirconia. Matrix is cubic ZrO 2- MgO Melt Cubic Tetrag onal Monoclinic Precipitate is tetragonal ZrO -MgO 2680 oC 2370 oC 1150 oC 2 Precipitate around Transformation toughening is achieved by stabilising the tetrago nal Transformation toughening is achieved by stabilising the tetrago nal crack is monoclinic structure at room temperature by adding other oxides such as: Mg O, CaO, ZrO 2-MgO and Y 2O3 to zirconia.

If cubic ZrO 2 is stabilised, so it retains cubic structure at RT called fully stabilised zirconia.

If tetragonal ZrO 2 is stabilised, it called as PSZ. o Mixture of ZrO 2-9 mol %MgO is sintered at 1800 C, then rapidly cooled to RT become metastable cubic structure. The materials is reheated at 1400 oC for sufficient time, a fine metastable precipitate with tetrag onal structure known as PSZ formed.

As the crack propagates, it creates a local stress field that in duces transformation of the tetragonal structure to the monolithic (or monoclinic) structure in that region. 2. Micro -crack Induced Toughening: Single crystals of the cubic phase of Microcracks are purposely introduced by internal stresses during zirconia are commonly used as diamond processing of the ceramics tend to blunt the tip of the propagat ing crack and simulant in jewelery. thus reduce the stress concentration at the crack tip. This micro -crack will interfere the crack tip propagation.

3. Crack Deflection and Crack Bridging The cubic phase of zirconia also has a very low thermal conductivity, which has led to its use as a thermal barrier coating or TBC This is achieved by reinforcing the ceramics: produce ceramic ba sed in jet and diesel engines to allow operation composites (CMC) at higher temperatures.

Stabilized zirconia is used in oxygen sensors and fuel cell membranes because it has the ability to allow oxygen ions to move freely through the crystal structure at high temperatures.

Advanced ceramics

• MEMS – mechanical devices that integrated with large number of electrical elements on a substrate of Silicon – e.g. for microsensors Ceramic as abrasive materials • Current research on ceramic materials to replace silicon, because ceramic are tougher, more refractory and more inert e.g. silicon carbonitrides (silicon carbide-silicon • The high hardness of some ceramic materials makes them nitrides alloys) useful as abrasive materials for cutting, grinding, polishing

e.g. Al 2O3 and SiC, diamonds Heat Treatment of Glasses • Glasses can be rendered more fracture resistance by introducing compressive Properties of Glasses stresses on the glass surface. This is followed by glass temperi ng

1. Glass Annealing Glasses posses properties not found in other engineering materia ls. Used to reduce internal residual stresses, which weaken the glas s and may lead to fracture. The glass is heated to the annealing temperature, then slowly co oled to RT Combination of transparency, ability to transfer light, hardness at room temperature, a sufficient strength and corrosion resistance to m ost 2. Glass Tempering environments. These make glasses important for many applications : vehicle 2. Glass Tempering glazing, lamps, electronic industry, laboratory apparatus. Used to strengthen glass by inducing compressive stresses at the surface.

Tempering is achieved by heating the glass to a temperature > T g, then rapidly cooled to room temperature. Deformation of glass varies with temperature: The surface of the glass cools first and contracts; later the ce ntre cools and attempts to contract but is prevented from doing so by the rigid and stro ng surface. At high temperatures: viscous flow This produces high tensile stresses in the centre but compressiv e stresses at the At low temperatures: elastic and brittle surface. At intermediate temperatures: visco -elastic This tempering treatment increases the strength of the glass bec ause applied tensile stresses must surpass the compressive stresses on surface before fracture occurs. Tempered glass has higher impact resistance than annealed glass and about 4x stronger than annealed glass.