Basic Technology

Ceramics, Types and Properties

Olawunmi Omosanya - Fri, 25th January, 2019 @ 02:09:28 AM

What are Ceramics?

Ceramics are not metals, neither are they plastics. They are all around us and include things like tile, bricks, spark plugs, diamond and glass. Ceramics as a topic and a product is indeed very broad and this had made it's defination somewhat complex. Infact, often times, It is defined as what it's not. Defining ceramics by it's characteristics can be a little complicated because not all of them share the same feature. For instance ceramics like glass and mugs tend to be hard and also are bad conductor of electricity. Graphite on the other hand is a ceramic (because it is inorganic and metallic), but it's brittle (soft) and also a good conductor of electricity. That's why you would see a lot of textbook define it as an inorganic and non-metallic material. They are also refractory by nature and require a very high amount of heat and subsquent cooling to be formed.

Note: inorganic imply that they they do not have their origin from any living organism while refratory means that they are capable of withstanding a variety of abuses like very high temperatures and chemical attacks.

Normally, ceramics are generally made by taking clay from the ground and mixing it with water. It is then shaped into any form and fired inside an oven (called kiln) at a very high temperature. They may then be decorated with a waterproof, paint-like substances called glazes.

Ceramics may be classified into two. These are:

(A) Traditional Ceramics

They are made completely form clay. We can put them into:

  1. Earthenware: These are clay fired at fairly low temperatures of about 900°C – 1,150°C. The result is the formation of a partially porous and rough object. As a fix for it's porosity, the resulting product is covered with a waterproof, paint-like substance or powered glass suspended in water (called glaze). It is then fired a second time. Delft and Majolica are examples of earthenware.
  2. Stoneware: They are clay fired at a temperature of about 1,250°C until they become vitrified, i.e glass-like. Stoneware is non-porous and if glaze was to be used, then it's only for decorative purposes. The products of stoneware are a strong and durable material which may be used as cooking pots, jugs and plates.
  3. Porcelain: They are made from kaolin clay, glass, granite, feldspar and water. Once made into desired shape, they are fired in a kiln to between 1,200°C – 1,450°C. It may be decorated with glazes by additional firing. By characteristics, they are very hard, translucent white ceramic. Procelain is sometimes referred to as fine china because it had it's origin from China. They are used as vaces, jugs and plates.

 

(B) Advanced Ceramics

They are based on metallic oxides or non-oxides (or combinations of both) and are not generally clay based. The main oxides used in advanced ceramics are aluminium oxide (Al2O3), Magnesium oxide (MgO) and zirconium oxide (ZrO2). The non-oxides are mainly carbides, borides and silicides, e.g, boron carbide (B4C) and silicon carbide (SiC). Regarding how they are produced, firstly, these oxides are thoroughly grounded into very fine powders and then shaped into a green body fired at a very high-temperature (1,600–1,800°C) in an oxygen-free atmosphere (and allowing the tiny grains of each ceramic components to join together, a process called sintering). The end result is a very hard, durable and corrosion-resistant product. Advanced ceramics are used in metal production and electronics.

 

Properties of Ceramics

Ceramics may have constrasting physical and chemical properties, but in general,

  1. They have high melting points and are heat resistant.
  2. They can be very hard with great strength but a few are brittle, e.g graphite.
  3. They are durable (last for a long time).
  4. Most are electrical insulators (cannot conduct electricity) but graphite is an electrical conductor.
  5. They are chemically unreactive or inert.
  6. They are not easily oxidized.
  7. Most are non-magnetic.


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