Page 43 - Curriculum Visions Dynamic Book

grained powders. Most ceramic powders have grains smaller than a micron (a millionth of a metre). That is smaller than the size of many ordinary clays.
Even mixing is also extremely difficult to achieve by simple techniques such as stirring. As a result, special chemical methods are used. For example, the materials are dissolved in a solution and then precipitated again. This produces very fine particles that are very evenly mixed. Alternatively, a solution containing the ceramic can be freeze-dried (in a way not unlike freeze-drying coffee) by spraying it into a freezing environment.
The sprayed liquid immediately forms very tiny frozen crystals. The spray can also be directed into an oven. The liquid evaporates, and the dissolved materials settle out as a fine powder.
Making the powders into useful things
Once the powders have been made, they have to be turned into useful products. That can be achieved by pressing them into a mould or by turning them into a muddy liquid and casting them, just as is done to clay. In general, however, the moulding and casting are very precise. Often the powders are mixed with a resin (a plastic substance), which sets as soon
as the mixture has been extruded through a die.
In this way, for example, tiny honeycomb frames are produced in which the platinum of a catalytic converter can be sprayed. The walls of this ceramic frame are less than a tenth of a millimetre thick.
See Vol. 1: Plastics for more on plastics.
(Above and below) Blades for a turbine can be made from advanced ceramics.
43