In crystal growth, a Knudsen cell is an effusion evaporator source for relatively low partial pressure elementary sources (e.g. Ga, Al, Hg, As). Because it is easy to control the temperature of the evaporating material in Knudsen cells, they are commonly used in molecular-beam epitaxy.
Development
The Knudsen effusion cell was developed by
Martin Knudsen (1871–1949). A typical Knudsen cell contains a
crucible (made of
pyrolytic boron nitride,
quartz,
tungsten or
graphite), heating filaments (often made of metal
tantalum),
water cooling system,
, and an
orifice shutter.
Vapor pressure measurement
The Knudsen cell is used to measure the
of a solid with very low vapor pressure. Such a solid forms a vapor at low pressure by sublimation. The vapor slowly effuses through the pinhole, and the loss of mass is proportional to the vapor pressure and can be used to determine this pressure.
[Peter Atkins and Julio de Paula, Physical Chemistry (8th ed., W.H.Freeman 2006) p.756 ] The heat of sublimation can also be determined by measuring the vapor pressure as a function of temperature, using the Clausius–Clapeyron relation.
[Drago, R.S. Physical Methods in Chemistry (W.B.Saunders 1977) p.563 ]
