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Ceramics and Composites

Ceramics

Ceramics cover a very wide range of materials from structural materials like concrete to technical ceramics like PZT – a piezoelectric.  Usually they are defined as solids with a mixture of metallic or semi-metallic and non-metallic elements (often, although not always, oxygen), that are quite hard, non-conducting and corrosion-resistant.

Techniques for identifying ceramics

It is effectively impossible to identify ceramics by eye. Optical microscopy will allows the examination of the microstructure to identify the method of processing, however, it does not allow the identification of different phases.

The most useful technique for finding the composition of a ceramic is energy dispersive x-ray spectroscopy (EDS).  Note that for non-conducting ceramics the surface of the sample must be covered with a metallic coating (often gold) to prevent charge build-up.

Here is an example EDS for PZT – a piezoelectric ceramic: Pb[ZrxTi1-x]O3, this data gives the formula to be: Pb0.7[Zr0.49Ti0.44]O3.  For the piezoelectric ceramic we would expect to have x ~ 0.52.

EDS data for PZT
Element

Weight%

Atomic%

O K 17.26 63.49
Ti K 7.58 9.32
Zr L 16.15 10.42
Pb M 59.01 16.77

Totals

100.00

100.00

Another appropriate method is X-ray diffraction. This allows you to detect the phase or phases present as well as measuring lattice parameter(s) in order to specify precise compositions.

Processing techniques for ceramics

Ceramics are mostly made by powder processing techniques, for example sintering .  It may be possible to identify the kind of processing from directionality or porosity in the sample.

Composites

Composites are often used in applications that require specific ‘conflicting’ properties such as a high strength and high toughness. The properties may be conflicting because having a high yield stress sometimes relies on trapping and tangling dislocations, but these reduce the ductility and toughness of the material.  Composites often consist of a matrix and fibres or particles that affect the properties (see the TLP on the Mechanics of Fibre-Reinforced Composites).

Usually for composites, once they have been identified as such, it is better to treat each part of the composite as a separate material, and then subsequently look at costs of manufacture and processing.

One important distinction to make is the structure of the two parts that make up the composite – i.e. is it a matrix with long, aligned fibres? Or a matrix with particles? etc

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