CERAMICS MATERIALS

1. CERAMICS MATERIALS • Applications of ceramics • refractories • sensors • capacitors • the magnetic strip on a credit card • space shuttle protection from high temperature • addition to paints • found in bone and teeth • spark plugs Ceramics exhibit good strength under compression and virtually no ductility under tension. Ceramics are inorganic and non-metallic materials that are commonly electrical and thermal insulators, brittle and composed of more than one element (e.g., two in Al2O3)

2. DEFINITIONS The word ceramic, derives its name from the Greek keramos, meaning "pottery", which in turn is derived from an older Sanskrit root, meaning "to burn". The Greeks used the term to mean "burnt stuff" or "burned earth". Thus the word was used to refer to a product obtained through the action of fire upon earthy materials Ceramics make up one of three large classes of solid materials. The other material classes include metals and polymers. The combination of two or more of these materials together to produce a new material whose properties would not be attainable by conventional means is called a composite. Examples of composites include steel reinforced concrete, steel belted tyres, glass or carbon fibre - reinforced plastics (so called fibre-glass resins) used for boats, tennis rackets, skis, and racing bikes.

3. CEMENTS ADVANCED CERAMICS GLASSES FIRECLAY GLASSES STRUCTURAL CLAY PRODUCTS SILICA GLASS CERAMICS WHITEWARES BASIC SPECIAL CERAMIC GROUPING

4. Ceramics can be classified based on chemical composition – oxides, carbides, nitrides, sulfides and fluorides. Or they can be grouped according to their major functions. Ceramics found in coatings – glazes, are ceramic coatings applied to glass objects and enamels are ceramic coatings applied to metallic objects. Alumina (Al2O3): Diamond (C): Silica (SiO2): Silicon carbide: Silicon nitride (Si3N4): Titanium oxide (TiO2): Zirconia (ZrO2):

7. Properties of Ceramics

8. Tensile strength = T; Flexural strength = F; Young’s modulus = Y; Compressive strength = C; Fracture toughness = Ft; Density = D.

9. Synthesis of ceramic powders Ball milling, blending, spray drying of powders using processing additives Shaping of powders into useful shapes (green ceramics) using pressing, slip casting, tape casting Consolidation into a dense, monoclinic object using sintering or firing Secondary processing (e.g. grinding, cutting, polishing, electroding, coating etc.) Final sintered ceramic product Synthesis and processing of ceramic powders

10. DIFFERENT TECHNIQUES FOR PROCESSING OF ADVANCED CERAMICS These techniques are used to convert properly processed powders into desirable shape to form the green ceramic (is a ceramic that has not yet been sintered. Slip casting; Compaction (uniaxial or isostatic); Tape casting; Extrusion; Injection molding; See Figure 15-2 on page 539 of Askeland and Phule

11. Characteristics of sintered ceramics Important in sintered ceramics – grain size, grain size distribution, and the level and type of porosity. Grains and grain boundaries: Ceramics with small grain size are stronger than coarse – grained ceramics. Finer grain size reduces stresses that develops at grain boundaries due to anisotropic expansion and contraction. Average grain size properly controlled produces magnetic, dielectric and optical properties. Porosity: Pores represent defects in polycrystalline ceramics and are usually detrimental to the mechanical properties of bulk ceramics. Pores may be either interconnected or closed. The apparent porosity measures the interconnected pores and determine permeability or ease with which gases or fluids seep through ceramic components. Apparent porosity = Ww – Wd / Ww – Ws x 100 (W = weight either after removal from water (s), dry (d) or suspended in water (s). True porosity = ρ - B / ρ x 100 (B = bulk density, ρ = true density or specific gravity of the ceramic material.

12. Example 15.1 on page 544: Silicon carbide particles are compacted and fired at a high temperature to produce a strong ceramic shape. The specific gravity of SiC is 3.2 g/cm3. The ceramic shape is weighed when dry (360 g), after soaking in water (385 g) and while suspended in water (224 g). Calculate the apparent porosity, the true porosity, and the fraction of the pore volume that is closed. Apparent porosity = Ww – Wd / Ww – Ws x 100 = 385 – 360 / 385 – 224 x 100 = 15.5% Need bulk density to calculate true porosity; Bulk density (B) = Wd / Ww - Ws = 360 / 385 – 224 = 2.24 g.cm-3 True porosity = ρ - B / ρ x 100 = 3.2 – 2.24 / 3.2 x 100 = 30% Closed pore percentage = true porosity – apparent porosity; = 30 – 15.5 = 14.5% Therefore, fraction closed pores = 14.5 / 30 = 0.483

13. Crystalline Undercooled liquid Glass liquid Density Tg Tm Inorganic Glasses Non-crystalline materials especially based on silica (others are based on fluorides, sulfides and alloys). Define glass: a metastable material that has hardened and become rigid without crystallizing. Below the glass temperature the rate of volume contraction on cooling is reduced and material considered glass not undercooled liquid. When silica crystallizes on cooling, abrupt change in the density is observed

14. Silicate glasses are most widely used, fused silica made from pure silica has high melting point and the dimensional changes during heating and cooling are small. Oxides can be classified as glass formers (silica), intermediates (aluminium oxide) and modifiers (magnesium oxide) – cause glass to devitrify or crystallize as they break up the network structure. Modified silicate glasses; Modifiers break up the network of the silica. When Na2O is added Na+ ions enters the holes in the network while O2- ions becomes part of the network structure. The O:Si ratio becomes large and when it reaches 2.5 glass is difficult to form. Modifiers reduces melting points and viscosity of the silica making it possible to produce glass at lower temperatures. Glass formers – B2O3, SiO2, GeO2, P2O5, V2O3 Intermediates – TiO2, ZnO, PbO2, Al2O3, BeO Modifiers – Y2O3, MgO, CaO, PbO, Na2O

15. Glass manufacturing At high temperatures and with viscosity controlled so that glass can be shaped without breaking. Liquid range – sheet and plate glass produced when glass is in a molten state. Liquid tin used to form smooth surface on glass. Working range – shapes for containers or light bulbs can be formed by pressing, drawing or blowing glass into molds. Glass is heated in the working range so that is formable but not runny. Annealing range – annealed to reduce residual stresses during forming. Large glass castings are often annealed and slowly cooled to prevent cracking. Tempered glass – quenching the surface of plate glass with air causing the surface layers to cool and contract. Used in car and home windows shelving for refrigerators, ovens, furnitures. Laminated glass – consist of two annealed glass pieces with a polymer (polyvinylbutyral, PVB) in between used to make car wind shields.

17. Glass Composition – most glass are based on silica, modifiers such as Na2O (soda), CaO. Common commercial glass contains approximately 75% SiO2, 15%Na2O and 10% CaO = soda line glass. Borosilicate glass – contains 15% B2O3, used in lab glassware, glass ceramics and containers for high level radioactive waste. Calcium aluminosilicate glass or E-glass (20% Al2O3, 12%MgO and 3%B2O3) – used for general purpose fiber for composite materials such as fiber glass. Fused silica – gives best resistance to high temperature, thermal shock and chemical attack. Photochromic glass – darkened by the UV portion of sunlight used for sunglasses. Polychromatic glasses – sensitive to all light not just UV light.

18. Melting 1600 liquidus Forming 1250 -4 x 104 Growth 900 Softening point -107.6 Viscosity µ(poise) Temperature (°C) Nucleation 800 Annealing point -1013.4 650 Time Glass ceramics – are crystalline materials derived from amorphous glasses. Glass-ceramics have a substantial level of crystallinity (>70 – 99%). Formability and density of glass becomes important. Glass is crystallized using heterogeneous nucleation by such oxides such as TiO2 and ZrO2. In making glass ceramics – first step is to assure that no crystallization during cooling from the forming temperature. Heat treatment profile for glass-ceramic fabrication. Cooling must be rapid to avoid the start of crystallization. Isothermal and continuous cooling values for lunar glass. The rate of nucleation of precipitates is high at low temperatures, whereas rate of growth is high at higher temperatures

19. Nucleation of the crystalline phase is controlled in two ways; first the glass contains agents such as TiO2, that react with other oxides and form phases that provides the nucleation sites. Second heat treatment is designed to provide number of nuclei, the temperature should be relatively low in order to maximize the rate of nucleation. Processing and Applications of clay products: Clay products are traditional ceramics used for producing pipe, brick, cooking ware. Clay – kaolinite and water serve as the initial binder for the ceramic powders, which are typically silica. Feldspar, [(K,Na)2O.Al2O3.6SiO2 , is used as a flux (glass forming) agent during later heat treatment.

20. Glassy bond Liquid phase formed Clay Firing time Forming techniques for clay products; The powders, clay, flux and water are mixed and formed into shape. Dry or semi-dry mixtures are mechanically pressed into green shapes, isostatic pressing may be done, the powders are placed into a rubber mold and subjected to high pressures Drying and Firing of Clay products; During drying, excess moisture is removed and large dimensional changes occur. Temperature and humidity are controlled to provide uniform drying – minimizing stresses, distortion and cracking. Firing – produces rigidity and strength of the ceramic materials. During heating the clay dehydrates an vitrification or melting begins. Impurities and fluxing agent reacts with ceramic materials and clay producing low-melting point liquid phase at the grain surfaces. The liquid helps eliminate porosity and after cooling changes to a rigid glass. The glassy phase provides ceramic bond and shrinkage

21. Refractories – components of equipments used in production, refining and handling of metals and glasses, for constructing heat treating furnaces. They must survive high temperatures without being corroded or weakened. Refractory bricks contain 20 to 25% apparent porosity to provide improved thermal insulation. Acid refractories - Base refractories - Neutral refractories -

22. Compositions of typical refractories (weight percentage)

23. Other ceramic materials Cements: A chemical reaction converts a liquid resin to a solid that joins the particles. CO2 gas acts as a catalyst to dehydrate sodium silicate to produce glassy material. xNa2O.ySiO2.H2O + CO2 = glass Coatings: Thin films and Single crystals: Fibers: