Diffusion pump: Difference between revisions

[[Image:M6 Diffusion Pump.jpg|right|250 px|thumb|Six inch oil diffusion pump.]]

'''Diffusion pumps''' use a high speed jet of vapor to direct gas [[molecule]]s in the pump throat down into the bottom of the pump and out the exhaust. They were the first type of high [[vacuum pump]]s operating in the regime of [[free molecular flow]], where the movement of the gas molecules can be better understood as [[diffusion]] than by conventional [[fluid dynamics]]. Invented in 1915 by [[Wolfgang Gaede]], he named it a ''diffusion pump'' since his design was based on the finding that gas cannot diffuse against the vapor stream, but will be carried with it to the exhaust.<ref>{{cite journal | author = D. G. Avery and R. Witty | title = Diffusion pumps: a critical discussion of existing theories| journal = [[Proc. Phys. Soc.]] | volume = 59 | year = 1947 | pages = 1016–1030 | doi =  10.1088/0959-5309/59/6/313 | issue = 6|bibcode = 1947PPS....59.1016A }}</ref> However, the principle of operation might be more precisely described as '''gas-jet pump''', since diffusion also plays a role in other types of high vacuum pumps. In modern textbooks, the diffusion pump is categorized as a [[vacuum pump#Momentum transfer|momentum transfer]] pump.

The diffusion pump is widely used in both industrial and research applications. Most modern diffusion pumps use [[silicone oil]] or [[polyphenyl ether]]s as the working fluid.

In the late 19th century, most vacuums were created using a [[Sprengel pump]], which had the advantage of being very simple to operate, and capable of achieving quite good vacuum given enough time. Compared to later pumps, however, the pumping speed was very slow and the [[vapor pressure]] of the liquid mercury limited the ultimate vacuum.

Following his invention of the [[Holweck pump|molecular pump]], [[Wolfgang Gaede]] invented the diffusion pump in 1915,<ref>{{cite journal | year = 1915 | journal = [[Annalen der Physik]] | volume = 46 | page = 357 | author = Gaede, W. | doi = 10.1002/andp.19153510304 | title = Die Diffusion der Gase durch Quecksilberdampf bei niederen Drucken und die Diffusionsluftpumpe | issue = 3|bibcode = 1915AnP...351..357G | url = https://zenodo.org/record/1447291 }}</ref> and originally used [[Mercury (element)|elemental mercury]] as the working fluid. After its invention, the design was quickly commercialized by [[Leybold GmbH|Leybold]].<ref>{{cite web | last=Sella | first=Andrea | title=Classic Kit: Gaede's diffusion pump | website=Chemistry World | date=2009-04-28 | url=https://www.chemistryworld.com/opinion/classic-kit-gaedes-diffusion-pump/3004912.article | access-date=2019-08-03}}</ref> It was then improved by [[Irving Langmuir]]<ref>{{cite journal|title=The Condensation Pump: An Improved Form of High Vacuum Pump|first=Irving|last=Langmuir|journal=General Electric Review|year=1916|volume=19|pages=1060–1071|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015075043383&view=1up&seq=1104}}</ref> and W. Crawford. [[Cecil Reginald Burch]] discovered the possibility of using [[silicone oil]] in 1928.<ref>{{cite journal | author = C. R. Burch | title = Oils, greases and high vacua | journal = [[Nature (journal)|Nature]] | volume = 122 | year = 1928 | page = 729 | doi = 10.1038/122729c0 | issue = 3080|bibcode = 1928Natur.122..729B | s2cid = 4126707 | doi-access = free }}</ref>

 

===Oil types===

[[Image:Langmuir mercury diffusion pump.jpg|thumb|Early Langmuir mercury diffusion pump ''(vertical column)'' and its backing pump ''(in background)'', about 1920.  The diffusion pump was widely used in manufacturing [[vacuum tube]]s, the key technology which dominated the radio and electronics industry for 50 years.]]