Ceramics

Cement

Glass

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Pottery, porcelain, brick, and tile are all very similar in composition and method of manufacture, and with glass they provide the class of materials known as ceramics. These products are made from varying mixtures of clay, silica, and feldspar. The very first of them are usually prepared from natural clays which already contain silica and feldspar. These natural clays are made into a workable mud with water and then fired in special ovens. As the temperature rises, water is first driven from the mixture, then various decomposition products are formed and above 1250˚ C the feldspar fuses and binds the other materials present.

Porcelain is made of very fine white clay known as kaolin. Then very pure sands and feldspar are added.

Silicate materials include glass, porcelain, enamels and cement. Ordinary glass is a mixture of silicates in the form of super cooled liquid. It is made by melting a mixture of sodium carbonate, limestone and sand. Ordinary glass contains about 10% of sodium, 5% of calcium, and 1% of aluminium; the remainder is silicon and oxygen. It consists of an aluminosilicate tetrahedral framework, within which are embedded sodium ions and calcium ions.

Portland cement is an aluminosilicate powder which sets to a solid mass on treatment with water. Portland cement before treatment with water consists of a mixture of calcium silicates and calcium aluminate. When treated with water the calcium aluminate hydrolyzes and form calcium hydroxide and aluminium hydroxide. These substances react further with the calcium silicates and produce calcium aluminosilicates in the form of crystals.

Ceramics is an applied science dealing with the preparation and application of a great number of organic chemicals. Until recently, ceramic materials were primarily silicate-bearing compounds. Now they include oxides, graphite, carbides, borides, silicides, nitrides, and systems compounded of these groups. Typical ceramic products are pottery, porcelain, enamels, refractories, glass, insulators, cement, bricks and building materials, and abrasives. Clay is still the basis of most commercial ceramic products, despite recent developments of new types of ceramic materials.

Ceramic products are usually man-made, although a number of ceramic products exist in a natural state, e.g. diamond, graphite, mica and marble. Ceramic products can be classified by the method of preparation into two groups: sintered and fused.

Silica, SiO2, is the main ingredient of all ceramic products. Its most common form is quartz. Kaolin or clay minerals are compound of silica and other metal oxides, particularly aluminium and magnesium, and are used in pottery manufacture. Mica minerals also consist of complex silicates. Mica is used as an insulator in electrical and electronic equipment.

An extensive technology is now developing around newer ceramics. Oxides have great potential because of their resistance to high temperatures and their electrical (insulation) properties. Graphite, because of its high melting point and high temperature strength, is finding wide-spread use in rocket building.

Ferrites (mixed oxides of iron and other metals) have magnetic properties which are of interest for computer components. Titanates, sapphire, rube and garnets are findig application in advanced communication equipment and energy and power-conversion equipment.

Cermets (combination of metals and ceramics) are becoming important ceramic systems.

Дата добавления: 2015-01-03