CHEMISTY: CERAMICS

CHEMISTY: CERAMICS

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Overview

• A ceramic is an inorganic, non-metallic solid prepared by the action of heating and subsequent cooling
• The earliest ceramic materials were pottery made from clay
• Ceramics are resistant to chemical erosion and high temperatures (up to 1600C)
  

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PROPERTIES OF CERAMICS

• Mechanical properties
  • Ceramic materials are usually formed by ionic or covalent bonds
  • These materials tend to not be elastic and fracture easily
  • Ceramics are also porous
  • In general ceramics have poor toughness and have low tensile strength
• Electrical properties
  • Some ceramics are semiconductors
  • Semiconducting ceramics are made using zinc oxide
  • Under extremely low temperatures, some ceramics exhibit superconductivity
  • Most ceramics exhibit piezoelectricity i.e. the conversion of mechanical stress to electrical signals. This effect is commonly used in quartz watches
• Optical properties
  • Ceramics (esp. those based on aluminium oxide) can be made translucent
  • This has immediate applications in sodium-vapour lamps and dental restorations
  • Ceramics can be made transparent with applications in laser technology

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TYPES OF CERAMICS

1. Structural ceramics such as bricks, pipes, floor, roof tiles etc
2. Refractory ceramics such as kiln lining, steel and glass making crucibles
3. Whitewares such as tableware, wall tiles, pottery, sanitary products
4. Technical ceramics such as jet engine turbine blades, ballistic protection etc

MANUFACTURE OF CERAMICS

1. Milling
   1. Process by which materials are reduced in size
   2. Involves breaking of cemented material or pulverization
   3. Techniques used include ball mill, roll crusher, jaw crusher, wet attrition mills
2. Batching
   1. Is the process of weighing the oxides according to recipes and preparing them for further processing
3. Mixing
   1. Involves mixing the various components in the appropriate proportions
   2. Uses ribbon mixers, Mueller mixers and pug mills
4. Forming
   1. This is the process of the making the mixed materials into desired shapes such as toilet bowls, spark plugs etc
   2. Forming techniques include extrusion, pressing and slip casting
5. Drying
   1. Controlled heat is applied to dry the materials and obtain rigid shape
6. Firing
   1. Dried parts are processed through a controlled heating process and oxides are chemically changed to cause sintering and bonding

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BIO-CERAMICS

• Bacteria, plants and animals exhibit a tendency to form crystalline materials composed of silicon
• These bioceramics show exceptional physical properties such as strength, fracture resistance etc
• Bio-ceramics are usually made of proteins such as keratin, elastin, chitin and collagen
• The mother-of-pearl portion of marine shells exhibit the strongest mechanical strength and fracture toughness of any non-metallic substance known

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APPLICATIONS OF CERAMICS

Application	Ceramic components	Notes
Armoured vests	Alumina, boron carbide	Protects against high-calibre rifle fire
Dental implants, synthetic bone	Artificial hydroxyapatite (natural mineral of bone)
Ball bearings	Silicon nitride	Harder, more resistant to heat than metal bearings
Earthenware	Kaolin, boll, flint	Opaque Used to make cups, saucers etc
Chinaware	Leached granite (to remove quartz and mica)	Translucent Resists scratching
Porcelain	Kaolin, feldspar, quartz	White, semi-opaque Highly resistant to scratching Stronger than glass
Stoneware	Kaolin, feldspar, quartz	Similar to porcelain but from poor grade raw materials Hard, infusible
Space shuttles	Extremely pure Silica	Used on the outer surface of shuttles to withstand heating during atmospheric re-entry Space shuttle Colombia burnt up on re-entry due to damage to ceramic tiles

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JOHAR