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Wednesday, 24 April 2013

Polyvinyl alcohol

Polyvinyl alcohol commonly known as PVA is an important material in materials science research and textile industry. It is used as a facilitator for speedy mixing of ceramic oxides. The hydrolysis of polyvinyl acetate in the presence of acids or alkalies yields polyvinyl alcohol. There is always a chance of retaining around 2 mole percentage of acetyl groups in the end product. Polyvinyl alcohol is easily soluble in water. When it is reacted with iodine, gives an intense blue colour. It can be used in the production of emulsions since it is a good protective colloid for aqueous emulsions. The other industries which use polyvinyl alcohol are suspension systems industry and adhesive industry. In textile industry, it is used only after the treatment with a concentrated aqueous solution of sodium sulphate containing calculated amounts of sulphuric acid and formaldehyde. Superlative water absorption property makes PVA fibers more attractive. Some other properties of PVA fibers are superior dimensional stability, quick washing and drying property, resistance to abrasion and outstanding tenacity.

Wednesday, 17 April 2013

Hall Effect

Hall Effect can be used effectively in material science in characterization of materials. When a current carrying conductor is kept in a transverse magnetic field, an electric field in a direction perpendicular to both the current and applied magnetic field is generated. This is Hall Effect and the developed field is known as Hall field. To evaluate many physical parameters of metals and semiconductors this property is used. Experimentally it is possible to determine Hall coefficient which is defined as the Hall field generated in the material per unit magnetic field applied when the current density is unity. In the case of semiconductors, Hall coefficient is positive for p-type and negative for n-type semiconductors. Thus Hall Effect is an important tool to identify the type of semiconductor. The carrier concentration, mobility, magnetic field etc. can be measured using Hall Effect based devices. Gauss meter makes use of this principle.