CERAMIC STRUCTURE S

CRYSTAL STRUCTURES AX-Type A m X p -Type A m B n X p -Type

Summary of Some Common Crystal Structures Structure Name Structure Type Anion Packing Coordination No.Examples CationAnion Rock salt (NaCl) AXFCC66NaCl, MgO, FeO Cesium Chloride AXSimple Cubic88CsCl Zinc blende (sphalerite) AXFCC44ZnS, SiC FlouriteAXSimple Cubic84CaF 2, UO 2, ThO 2 PerovskiteABX 3 FCC12(A) 6(B) 6BaTiO 3, SrZrO 3, SrSnO 3 SpinelAB 2 X 4 FCC4(A) 6(B) 4MgAl 2 O 4, FeAl 2 O 4

SILICATE CERAMICS materials composed primarily of silicon and oxygen e.g: bulk of soils rocks clays and sand Each atom of silicon is bonded to four oxygen atoms, which silicon atom is positioned at the center (SiO4 ) Various silicate structures arise from the different ways in which the SiO4 units can be combined into 1D,2D or 3D arrangements

TYPES OF SILICATE CERAMICS Characterized by the crystal structures of these materials in terms of unit cells NONCRYSTALLINE SILICA GLASSES CRYSTALLINE SILICA LAYERED SILICATES

CARBON DiamondGraphiteFullerenes Carbon nanotubes - Hardness - High thermal conductivity - High-temperature chemical stability - good lubricative properties - Electrically insulative - Conductive / Semiconductive - extremely strong & stiff - Conductive / Semiconductive

IMPERFECTIONS IN CERAMICS  With regards to atomic point defects,interstitials and vacancies for each anion and cation types are possible.  Inasmuch as electrical charges are associated with atomic points defects in ceramic materials,defects sometimes occur in pairs;for instance are Frenkel defect and Schottky defect,in order to maintain charge neutrality.  A stoichiometric ceramic is one in which the ratio of cations to anions is exactly the same as predicted by the chemical formula.  Nonstoichiometric materials are possible in cases where one of the ions may exist in more than one ionic stage.  Addition of impurity atoms may result in the formation of substitutional or interstitial solid solutions.

DIFFUSION IN IONIC MATERIALS Normally occurs by a vacancy mechanism Localized charge neutrality is maintained by the coupled diffusive motion of a charged vacancy and some other charged entity

CERAMIC PHASE DIAGRAM The general characteristic of ceramic phase diagram are similar to those for metallic systems. There are four phase diagram of the important systems in ceramic:  Al 2 O 3 -Cr 2 O 3 system  MgO-Al 2 O 3 system  ZrO 2 -CaO system  SiO 2 -Al 2 O 3 system

Example of Ceramic Phase Diagram Figure 1.a : MgO-Al 2 O 3 phase diagram Figure 1.b : ZrO 2 -CaO phase diagram

MECHANICA L PROPERTIE S OF CERAMICS

Brittle Fracture of Ceramics Ceramic materials – microcracks – Very difficult to control – Result in amplification of applied tensile stress and account for relatively low fracture strengths (flexural strengths) Variation in fracture strength for specimens of a specific material results in as much as the size of a crack-initiating flaw (vary form specimen to other specimen) Stress amplification doesn’t occur with compressive load (ceramic are stronger in compression) Fractographic analysis : – The fracture surface of a ceramic material may reveal the location and source of the crack-producing flaw

STRESS-STRAIN BEHAVIOUR  The stress-strain behaviours and fracture strengths of ceramic materials are determined using transverse bending tests.  Flexural strengths are measured from three-point bending test s may be determined for rectangular and circular cross-sections using respectively these two equation below:  The elastic stress-strain behaviour for ceramic materials using these flexure tests is similar to the tensile test results for metals.  = F L R³

Figure 2:A three-point loading scheme for measuring the stress-strain behaviour and flexural strength of brittle ceramics

Mechanisms of Plastic Deformation Crystalline Ceramics Dislocation motion The britleness of these materials is explained by the limited number of operable slip systems Noncrystalline Ceramics Viscous flow A material’s resistance to deform is expressed as vicosity (units:Pa.s) Room temperature: the viscosities of these materials are extremely high

MISCELLANEOUS MECHANICAL CONSIDERATION INFLUENCE OF POROSITY HARDNESS CREEP

APPLICATIO N OF CERAMICS

Ceramic Materials Glasses Glass- Ceramics Clay Products Structural Clay Products Whitewares Refractories Fireclay Silica Basic Special AbrasivesCements Advanced Ceramics

GLASSES Glasses Glasses o Familiar : noncrystalline silicates (contain other oxides) :- CaO, Na 2 O, K 2 O, Al 2 O 3 (influence glass properties) o Typical soda-lime glass :  70 wt% SiO 2  Balance mainly : Na 2 O (soda) and CaO (lime) o Two prime assets of these materials:  Optical transparency  Ease of fabrication

GLASSES Glass-Ceramics Glass-Ceramics o Initially fabricated as glasses o After heat treatment : crystallized to form fine- grained polycrystalline materials o Properties :  Improved mechanical strength  Lower coefficients of thermal expansion

CLAY PRODUCTS Reason for its popularity Inexpensive ingredients as its is found naturally in great abundance When mixed clay and water with proper proportions,its become very amenable to shaping Have two broad classification Structural clay productsWhitewares eg:building bricks, tiles and sewer pipes Become white after high temperature firing eg:porcelain,pottery, tableware,and plumbing fixtures

REFRACTORY CERAMICS  have the capacity to withstand high temperature without melting or decompositing and remains unreactive and inert when exposed to severe environments.  have ability to provide thermal insulation.  Typical applications including: - Furnace linings for metal refining - Glass manufacturing - Metallurgical heat treatment - Power generation  The performance of a refractory ceramic depends on its composition.  Porosity is one of the microstructural variable that must be control and the optimum porosity depends on the conditions of service.

REFRACTORY CERAMICS FIRECLAY REFRACTORIES BASIC REFRACTORIES SILICA REFRACTORIES SPECIAL REFRACTORIES Prime ingredient is silica Primary ingredient are highly-purity fireclays, alumina and silica mixtures Rich in periclase, or magnesia and also contain calcium, chromium and iron compound Other ceramic materials that are used for rather specialised refractory applications

ABRASIVES Used to: – Cut, grind and polish other softer materials Must be hard and though Be able to withstand high temperatures that arise from frictional forces Common abrasive materials : – Diamond, silicon carbide, tungsten carbide, corundum and silica sand

CEMENTS Characteristic feature : o When mixed with water (chemical reaction at ambient temperature)  They will form a paste that sets and hardens  Capable of assuming just about any desired shape Portland cement o Produced by calcination process : heating a mixture of clay and lime- bearing minerals in a rotary kiln o Result of ‘clinker’ product is ground into a very fine powder and it is added to a small amount of gypsum (CaSO4-2H2O)

ADVANCED CERAMICS MICROELECTROMECHANICAL SYSTEMS (MEMS) OPTICAL FIBERS CERAMIC BALL BEARINGS miniature “smart” system consisting of a multitude of mechanical devices that are integrated with large number of electrical elements on a substrate of silicon. made of extremely high – purity silica,which must be free of even minute levels of contaminants and other defects that absorbs, scatter and attenuate a light beam. consist of balls and races that are in contact with and rub against one another when in use.