Ceramics
Ceramics
Ceramics include a variety of materials such as clay and clay products, cement, enamels, glasses, refractories etc. In short ceramics includes all articles which are essentially silicates. It can be concluded that ceramics is a branch of technology which deals with the manufacture of ceramics articles, their characteristic properties and the raw materials used.
CLASSIFICATION
1. Based on general classification.
2. Based on commercial classification.
3. Based on their field of use.
1) Based on general classification-In this type of classification, ceramics are classified according to their properties after firing. When a clay body is dried and fired, its structure is altered completely. Finally, a hard rigid material is formed which is sometimes called artificial stone.
2) Based on commercial classification-Ceramics is classification as rough ware and fine ware
(i)Rough wares: These have a porous nature and non uniform structure. It is made from coarse grained ceramics drought. Examples -refractories.
(ii)Fine wares: These ceramics have fine pores and uniform structure; example is porcelain and special ceramics.
3) Based on field of use: it is classified as follows:
i) Refractories: These are materials which retain their mechanical properties even at high temperature without undergoing deformation in its shape. These are used in lining industrial furnaces, ovens, etc.
ii) Structural ceramics: Ceramics articles used in building construction belongs to these categories. Example is buildings bricks such as common bricks, hollow tile, roof tile, etc.
iii) Facing material: Ceramics articles used for internal and external facing of buildings; for examples: facing bricks and oven tile.
iv) Special ceramics: These are ceramics with specific properties and used in radio industry, aviation instruments manufacture etc.
CHARACTERISTIC PROPERTIES OF CERAMICS
1)Chemical and physical properties : The close packing of crystal structure, high field strength of small cations of high charge and the presence of chemical components such as oxides and carbides gives high stability to ceramics. Most of the oxides present in ceramics are resistant to highly oxidizing and reducing atmosphere and also to fluctuations in temperature.
2) Mechanical properties: The mechanical strength of ceramics is controlled by important factors such as temperature, composition and microstructure. Ceramics are highly resistant to compression and are brittle solids.
3. Electrical and magnetic properties: Ceramics with oxides as their chemical components are generally bad conductors in their normal oxidation states. However, nonoxide ceramics can be used as semi conductors and ceramics can be used as semiconductors and ceramics containing transition metal ion show magnetic properties. In other words, magnetic ceramics contain molecules with odd number of electrons and incompletely filled 3d, 4f and 5f orbital’s. It responds to magnetic field. Unlike magnetic metals, magnetic ceramics are bad conductors of electricity.
MEHODS OF FABRICATION OF CERAMICS
Different methods of fabrication of ceramics ware include:
(a)Soft mud process: This is the classical fabrication of clay on the potter’s wheel. Jiggering is the soft mud process used in white ware manufacture to form plates and high voltage insulators. In this process, a column of de-aired clay is fed to the jigger machine and spread on the mould where the jiggering tool forms the profile surface. It is then dried to remove the ware from mould.
(b)Stiff mud process: In this process, stiff mud containing 12 t0 20% water is forced through a steel die of any desired shape in a pug mill. The column thus obtained can be cut in to desired length. This method is mainly used for manufacture of bricks, hollow tiles, electrical insulators etc.
(c)Dry pressing: In this method, the ceramic ware is shaped under pressure with a moisture content of 5-15%. Hydraulic or automatic pressing is actually employed. This method is used for shaping floor, high grade refractories, electrical insulators etc.
(d)Dry hot pressing: it is usually used for sintering of solid particles below their melting point. The die and the plunger used for compressing are made up to graphite and heating is done electrically. Shaping and firing take place together in this process. The process is used for preparing dense shaped pieces of refractory oxides and carbides.
(e)Slip casting: In this process, ceramics material suspension in water is poured in to a porous mould made up of plaster of paris which sucks the water from the contact area. Thus a hard layer of clay is formed on the mould surface. This process is continued until the entire inner part of the mould is filled. This method is known as solid casting.
DRYING OF CERAMIC WARE
The shape ceramic obtained by the above processes must be first dried carefully before firing. Direct firing of the shapes in the kiln results in cracking of the ware. The drying rate should be carefully controlled by regulating the humidity, flow rate of the air used for drying and its temperature. Excess shrinkage during leads to crack formation. Shrinkage can be reduced by controlling the particle size and the initial water content.
FIRING OF CERAMIC WARE
The dried ceramics ware is fired at temperature in the range of 700-2,000°C (depending on the composition and the properties desired) to impact hardness, durability and strength. Unglazed ceramic ware is usually fired only once whereas glazed ware is fired twice i.e. biscuits firing and glost firing, in special type of kiln such as (a) Continuous or counter flow kiln (e.g. up-draft kiln, down draft kiln or moving fire zone kiln and tunnel kiln or fixed zone kiln).
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