crystal growth Crystal growth is a major stage of a crystallization, crystallization process, and consists of the addition of new atoms, ions, or polymer strings into the characteristic arrangement of the crystalline lattice. The growth typically follows an ini ...

, a Knudsen cell is an

effusion In physics and chemistry, effusion is the process in which a gas escapes from a container through a hole of diameter considerably smaller than the mean free path of the molecules. Such a hole is often described as a ''pinhole'' and the escape ...

evaporator An evaporator is a type of heat exchanger device that facilitates evaporation by utilizing conductive and convective heat transfer, which provides the necessary thermal energy for phase transition from liquid to vapour. Within evaporators, a ci ...

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 Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors. MBE is used to make diodes and MOSFETs (MOS field-effect transis ...

Development

The Knudsen effusion cell was developed by Martin Knudsen (1871–1949). A typical Knudsen cell contains a

crucible A crucible is a container in which metals or other substances may be melted or subjected to very high temperatures. Although crucibles have historically tended to be made out of clay, they can be made from any material that withstands temperat ...

(made of pyrolytic boron nitride,

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...

graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...

), heating filaments (often made of metal

tantalum Tantalum is a chemical element; it has Symbol (chemistry), symbol Ta and atomic number 73. It is named after Tantalus, a figure in Greek mythology. Tantalum is a very hard, ductility, ductile, lustre (mineralogy), lustrous, blue-gray transition ...

water cooling file:KKP Auslauf.jpg, Cooling tower and water discharge of a nuclear power plant Water cooling is a method of heat removal from components and industrial equipment. Evaporative cooling using water is often more efficient than air cooling. Water i ...

system, heat shields, and an orifice shutter.

Vapor pressure measurement

The Knudsen cell is used to measure the

vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...

s 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

References

Crystallography Semiconductor growth {{crystallography-stub