Page 46 - Curriculum Visions Dynamic Book

Alumina
The main ceramic material used in electronics is called alumina (aluminium oxide, Al2O3). It is a strong material with good insulating properties; it does not corrode, and it seals well to other materials. But it has its drawbacks. It is not good
at drawing away heat. This is a problem because chips warm up in use. If they are sealed in by alumina, there is no way for the heat to get out except through the alumina. If the heat cannot get out, the electronics overheat and break down.
To make a base for a chip, a thin film
of alumina is created by draping a paste
containing alumina powder onto a tape and
then heating the tape. The mixture is 99.5% alumina, the rest being magnesia (MgO) and silica (SiO2). The additives turn glassy when they are heated. They stick the alumina powder together and the components of the chip to the base. The tape is usually a film of about 1 to 2mm depending on what is needed.
Ceramic capacitors
Capacitors are tiny devices designed to store electricity in an electronic circuit. A capacitor consists of two metal plates separated by a material that does not conduct electricity. Ceramics do not conduct electricity and make ideal capacitors. Nine out of ten of all modern capacitors are ceramics.
Most ceramic capacitor-separating materials are made of barium titanate (BaTiO3). It is made by mixing and then firing powders of barium carbonate and titanium dioxide. They are first spread onto a tape and then punched out of the tape. After that they are fired.
To make the capacitor, two wafer-thin palladium or palladium-silver alloy electrodes are cemented to opposite surfaces of the ceramic. These metals have a melting point that is higher than the firing temperature of the ceramic (1,250°C). This means that the capacitor
(Above) Alumina powder.
Non-conducting ceramic between metal sheets
Metal sheet
(Above) A ceramic capacitor.
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