Ceramics , Basics of material science , General studies , UPSC ESE

Ceramics 

• Keramikos - burnt stuff. 
• Inorganic materials. 
• These are generally compounds between metallic and nonmetallic elements.
• Atomic bonding is predominantly ionic but some covalent character may also be present.
  
  

There are two types of ceramics 

1. Traditional ceramics 

• These are mainly clay based products. 
• The basic constituents are 

1. Clay :it provides workability to the material ,constituting major body of material and hardens its before firing. 
2. Silica flint: it has high melting temperature and it is used as refractory material in traditional ceramics. 
3. Feldspar: potash feldspar has low melting temperature and it makes glass when ceramic mixture is fired. It bonds refractory component together. 

2. Engineering or technical ceramics 

• These are relatively pure compounds such as oxides, carbides, nitrides, etc. 
• The basic constituents are 

1. Alumina :it has very high melting temperature so it is used for refractory tubing and for making high-purity crucible. It has high electrical insulation resistance so it is also used in quality electrical application such as in spark plug insulator material. 
2. Silicon nitride : reaction bonding, Nitriding, Probabily Si3N4 has the best combination of engineering properties among all engineering ceramics. 
   
3. Silicon carbide: carbides are hard and refractory materials which are used for high temperature application but metal carbide such as TiC , ZrC do not resist of oxidation very well at elevated temperature however silicon carbide is exceptional among the carbides with outstanding resistance to oxidation at elevated temperature. At high temperature silicon carbide forms a skin of silicon oxide which protects the main body of material. Silicon carbide is used to make special refractories for very high temperature application. 
   

Zirconia (ZrO2)

• Pure zirconia is polymorphic and transforms from tetragonal to monoclinic structure at approx 1170 degree Celsius with an expansion in volume so it is subjected to cracking. 
• However zirconia is mixed with other oxides such as calcium oxide, magnesium oxide then its cubic structure can be stabilized which have especially high fracture and toughness strength leading to several new ceramic applications. 

Ceramic crystal structure (ionic packing theory) 

• 
  For those ceramics in which atomic bonding is predominantly ionic it is assumed that they are made up of electrically charged ions instead of atoms. These ions are assumed to be hard and spherical. 
• Packing of ions in ceramic crystal is governed by following two criteria. 

1. The magnitude of electrical charge on each of the component ion: The crystal should be electrically neutral that is all the cation positive charges should be balanced by equal number of anion negative charges. For example in CaF2 for stable crystal structure there should be twice F1- ions as compared to Ca2+ ions.
2. The relative sizes of cations and anions: 

• For a stable critical structure all the anions which are surrounding the cation should be in direct contact with that cation. This cation and anion contact is established for a specific value of rc/ra hence coordination number and rc/ra are related with each other. 
• This relationship between coordination number and rc/ra is determined from pure geometrical consideration and assuming hard spherical ions so these relationships are only approximate and there are exceptions also for example in some ceramic compounds with  rc/ra greater than 0.4 14 in which atomic bonding is highly covalent exhibit coordination number 4 instead of 6. 

Zinc blende structure or sphalerite structure 

• Example: ZnS , ZnTe, SiC
• Third and fifth group compound semiconductors like gallium ,arsenic ,etc. 

Note: 

• NaCl ---- rock salt type structure 
• Al2o 3 ---- corandum structure 
• Batio3---- pervoskite structure 
• In a cubic unit cell of zns all the corner and face position of the cube are occupied by sulphur atoms whereas zinc atoms fill in interior tetrahedral position. Each zn atom has 4 Sulphur atoms as nearest neighbours so the coordination number is 4 or tetrahedral coordination. 

Electrical properties of ceramics 

• High electrical insulation resistance. 
• High dielectric constant  
• High dielectric strength. 
• Low dielectric losses. 

Ceramics with Er<12 

• These are used in bulk as electric insulator. 
• For example alumina ,electrical porcelain , steatite, etc. 

Ceramics with Er>12

• These are used to design capacitors. 
• These are mainly mineral rutile (TiO3) and titanates such as batio3 ,etc. 

Mechanical properties of ceramics

• Corrosion resistance : high 
• Hardness :high 
• Strength: ceramics are stronger in compression than in tension. 
• Brittleness, ductility and malleability :ceramics are brittle materials with very low ductility and malleability. 
• Elasticity: ceramic have linear elastic behaviour. 
• 
  Toughness :low 
• Crack propagation: ceramics developed crack at the stress concentration points. These cracks can propagate under tensile load but not under compressive load. 
• Fracture: ceramics fail under brittle mode. 

Thermal properties of ceramics 

• High melting point. 
• High thermal insulation. 
• Low thermal conductivity and expansion . 
• Ceramics can withstand high thermal shock so they do not normally creep up to work cable high temperatures. 

Applications 

1. Refractories 

• Should have high melting point. 
• Should have high thermal insulation or heat reflection. 
• Should have capability to with stand high load at elevated temperatures. 
• Should not crumble or cracked easily. 
• Should not react with molten metal or slag. 
• Should have high degree of non porosity. 

Types 

• Fireclay refractories. 
• Acid refractories: silica-based. 
• Basic refractories: magnesia based  
• Neutral refractories: chromite/ chromite- magnesite based 
• Special refractories: these are made up of high purity oxides (such as Al2O3,SiO2,etc) carbides (such as ZrC, SiC,etc) with a high degree of non porosity.

Note: carbon and graphite are also very good refractory material and they are used for high temperature applications but they do not resist oxidation very well at temperature greater than 800 degree celcius.

2. Abrasives

• Abrasives ceramics are used to wear grind or cut away other materials which necessarily is softer , therefore the prime requirement for this group of material is hardness or wear resistance , in addition a high degree of toughness is essential to ensure that the abrasive particles do not easily fracture , furthermore high temperature may be produced from abrasives friction forces so some refractoriness is also desirable .
• Diamonds both materials and synthetic are utilised as abrasives , however they are relatively expensive . The more common ceramic abrasives include silicon carbide , tungsten carbide , aluminium oxide and silica sand.