Carbon brushes used for the collection of current from rotating electrical machines are made from mixtures of natural and by-product carbons by bonding with a secondary coke formed by the thermal decomposition of a temporary hydrocarbon binder. When heat treated to a temperature of some 1200K-1400K the composition of the body is essentially the element carbon but with the constituents in various degrees of atomic structural order. On further heat treatment to about 3000K some carbons will change dramatically in their response to applied mechanical forces,and this is accompanied by improved resolution in their electron and X-ray spectra. Use is made of this behaviour for composite carbons to be used as electrical brushes as after the high temperature heat treatment they are stronger than natural graphite but have satisfactory self lubricating properties to run against a slip ring or commutator and are good conductors of electricity for current collection. The choice of ingredients and method of manufacture have a profound influence on the performance in practice and one particular aspect of this is the ability of a carbon brush to remain in good electrical contact with its counterface in the prescence of mechanical vibration. This report describes work carried out on two series of electrographitic carbons where in 1) the matrix composition and structure have been kept constant and the volume fraction porosity varied and 2) the porosity was kept constant and a range of petroleum cokes known to reach different states of structural order after heat treatment were used as matrices. These materials have been examined by microscopic and diffraction techniques to achieve a method of characterising their state of strucure numerically to relate with their static mechanical properties and damping capacity ability. It has been found that the mechanical damping capacity of these materials is very strongly amplitude dependant, and is a function of both the atomic order of the carbon ingredients and of the volume fraction porosity present.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:636023 |
| Date | January 1981 |
| Creators | Bale, E. S. |
| Publisher | Swansea University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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