Usually taken as being equal to around 9.8 m s-2. The value varies across the world due to a number of factors such as the centripetal acceleration due to the Earth’s rotation, the deviation for the Earth’s shape from a perfect sphere and variation in the density and depth of the Earth’s crust. The actual value varies between 9.78 m s-2 at the equator to 9.83 m s-2 at the poles.
An alternative to cast and crush for the production of catalytic powders. The molten material is released through a nozzle and the stream is atomised by a jets of gas passing around the outside of the nozzle. The result is a fine solidified powder that can have its characteristics controlled by changing the gas flow rate and the nozzle shape.
Gas atomised powder
An advanced internal combustion engine. Air is drawn in to the front of the engine, in to the compressor stages. There liquid fuel is introduced and ignited in the now hot air. The hot combustion products are allowed to expand through one or more turbine stages and then out of the engine. The turbine stages then drive the compressor stages.
Different designs exist such as:
Turbojet engine.
Turbofan engine.
Turboprop engine.
Turboshaft engine.
Schematics of various gas turbine engines (images courtesy of wikipedia commons).
The part of a casting tree that leads into the individual components.
Winner of the 2007 Nobel Prize for Chemistry. The prize was awarded in recognition for his work on catalysis, in particular his working out of the precise mechanism of the Haber Process.
The temperature below which rubbers and other polymers change to a brittle state. This change in a solid booster O-ring seal was responsible for the Challenger Space Shuttle disaster in 1986.
An area within a metal where the atoms are arranged in a regular manner. Essentially a grain is one small crystal within a polycrystalline solid. Grain size can vary from a few nanometres up to several millimetres across. 10 to 100 microns is a typical size.
Etched aluminium ingot showing large, individual grains.
The jumbled, chaotic area between grains. The irregular nature of the grain boundary is one source of creep in metals but it is a barrier to dislocation mobility.
Representation of the grain boundary between three separate grains.
(n.b. all the atoms are the same species and the colour is only used to separate the individual grains)
A technique used to improve the yield stress of a metal. The surface area of a grain is proportional to the square of its linear size whilst the number of dislocations is proportional to the third power. By reducing the grain size (up to a critical point) the number of dislocations that ca pile up at the grain boundary is reduced. This leads to a greater stress being required to force the dislocations through the grain boundary in order to create more dislocations on the other side. This results in a greater yield stress. In many alloys ‘grain refiners’ are introduced that provide nucleation sites for the cooling melt. This results in many small grains being produced, thereby producing a stronger alloy.
The speed of an aircraft measured relative to the ground over which it is flying rather than the air through which it is moving. Since the air itself is likely to be moving air and ground speeds are rarely the same.
