Vacuum science facts and inventions
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Want to know more about the foundations of vacuum technology? We gathered some facts and stories about the science of vacuum.
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The legendary French scientist Fernand Holweck was born in 1890 and made significant impact on a breathtaking range of activities which are still highly relevant today. He studied at the Ecole de Physique et Chimie and graduated in 1910 having been amongst such luminaries as Pierre Curie and Paul Langevin.

In 1912 he became an assistant to Marie Curie and played an intrinsic role in the development of the Curie Institute. He was an experimentalist par excellence and is credited with contributing more than anybody else to the systematization of the radioactive technique, which was created in the Curie Laboratory and spread thence throughout the world.

During the First World War he worked with Louis de Broglie on developing ultrasound techniques for the detection of submarines. In 1922 he received his Doctorate for his studies on soft x-rays, bridging the gap in understanding between the far ultraviolet region and x-rays: a classical study on the x-ray spectra of the elements of low atomic number. Amongst other contributions Holweck developed the gravimetric pendulum (for surveying), a demountable high power radio tube, he worked on thermoionic valves and a constructed the first successive acceleration X-ray tube cascade. He was also the first to develop the focusing of electrons and electron optics; in this respect he was at the forefront of the development of television.

In these latter activities his use of, and interest in improving, vacuum techniques was expanded. To this end he designed and built the Holweck molecular vacuum pump in 1920 achieving vacuum levels of 10-6 mbar, which contributed significantly towards vacuum-reliant research and industry. In the Holweck molecular pump the pumping action is produced by a rotor usually in the form of a smooth cylinder. The stator is provided with spiral guide grooves. The design of the construction can also be reversed, with the stator being smooth and the rotor having the guide grooves. The principle is one of the exploitation of molecular drag. Today there are still totally Holweck based pumps, but the Holweck principle is now used mainly in combination with bladed turbomolecular pumps whereby the Holweck stage facilitates exhaust to high backing pressures and gives high process gas throughout. These pumps play a crucial and integral part in the vacuum techniques used in contemporary microelectronics production.

Holweck continued to use this pump for his research into x-rays and radio-biological applications at the Pasteur Laboratory. In 1929 he independently confirmed, the quantized interpretation of the biological action of radiation on microorganisms and later on bacteria, fungi and viruses.

In 1938-1939, Holweck joined a group of French vacuum scientists from academia and industry to form the first national Vacuum Society aiming to promote vacuum sciences and techniques through education, which later became the ‘French society for vacuum engineers and Technicians’ .

During the German occupation of France in 1940 Holweck and his work were closely monitored. Although his personal safety was at high risk, he refused to leave Paris and joined the resistance. He was arrested by the Gestapo in December 1941 and died shortly after his arrest whilst under torture.

In 1945 the French and British Physical Societies, as a memorial to Fernand Holweck, initiated the Holweck medal. The award is presented alternatively by the Council of the Institute of Physics to a French physicist and by the Council of the French Society to a physicist based in the UK or Ireland. The selection is made from a list of three nominees submitted by the other Council.

Holweck’s legacy to vacuum and science in general is undisputable however it is maybe best to leave the final words to one of his obituarists writing in 1942 in Science (Vol. 96 No. 2493 p33) ‘He has paid with his life for his love for freedom and for his country. His example will inspire all scientists of the world in their fight for the cause of liberty and democracy’.

Martin Knudsen made a tremendous contribution to vacuum science and especially in the understanding of flow in different parts of the vacuum spectrum. He was born in the Fyn region of Denmark in 1871 and in 1896, after 6 years of study, gained a Masters degree in science majoring in (the relatively new established discipline of) physics. This was at the the University of Copenhagen during which time he worked as an assistnat to Christiansen who soon later was to guide Neils Bohr.

Knudsen was highly interested in kintec theory of gases and was the first to apply it to rarefied gases so as to become the ‘father’ of modern vacuum science. His supreme experimental skills allowed the verification of the prediction from the Maxwell-Boltzmann distribution of the flow of gases through an aperture. From this is the concept of a Knusden cell which is the basic element for molecular beam epitaxy.

His analysis of the thermal effects at surfaces led him to develop the Knusden gauge and introduce the thermal accommodation coefficients. He later looked at a viscosity gauge by analysing the movement of gas molecules between moving plates.

Knudsen is probably best known and remembered as giving his name to the Knudsen number Kn = λ/d where λ is the mean free path of a gas molecule in the system and d is a characteristic dimension (usually the pipe or chamber diameter or transverse section length).

The continuum or viscous flow regime ii where Kn < 0.01 and molecule-molecule dominate gas behaviour which behaves as a fluid. In molecular or Knudsen flow where Kn > 1 (or for some authors >0.5 or > 3) molecule-surface collisions dominate and the interaction of a gas molecule with for example a chamber wall is crucial to understanding this flow regime. The transitional flow regime is where 1 > Kn > 0.01 this is a particularly difficult regime to analyse.

For a box of length l the number of molecule-surface/molecule-molecule collisions is 3 λ/l. hence the range 1 < Kn < 10 is known as an ‘almost-free’ passage of molecules down a tube. An application of this fact is particularly important in calculations of molecular transmission probabilities.

Knudsen’s analysis of the behavior of the molecules on a surface was also seminal. A molecule impinging on a surface accommodates to the surface and after a residence time (which can vary in a huge range). After leaving the surface (a process known as desorption) the molecule has no memory of the direction it took (or speed it had) to travel to the surface. Since molecule-wall collisions dominate in molecular flow this action at the molecule-wall interface dictates the molecular flow behavior.

This situation is that described by the Knudsen Cosine Law which states that the relative probability W of molecules leaving a surface into a solid angle dω forming an angle θ; with the normal to the surface is proportional to cos? i.e. W = (d?/p)cos? or the flux per unit solid angle is where J(0) is the flux (per unit solid angle) normal to the surface (θ = 0) which is the most probable direction. On average molecules depart at an angle normal to the surface.

Knudsen cosine distribution The polar diagram represents the locus of the flux (number density) of molecules emitted from a flat (average) surface element. The magnitude of each vector is proportional to cosine θ.

Interestingly this understanding if specific nature of gas desorption corrected Wolfgang Gaede’s erroneous propositions.

Knudsen had also a keen interest in hydrography and developed methods of defining properties of seawater (he was editor of Hydrological Tables in 1901), however it is his great contributions to vacuum science makes him a true Hero of Vacuum.

References:

An early classic paper is from 1910 (with Willard Fisher): The Molecular and the Frictional Flow of Gases in Tubes:Physical Review (Series I, Volume 31, p586 (1910).

His ideas on Kinetic Theory are summarized in his book, The Kinetic Theory of Gases (London, 1934).

Walter Steckelmacher’s article: Knudsen flow 75 years on (Reports on Progress in Physics ,Volume 49, p1083 -1986) is an excellent summary.

MKarl Manne Georg Siegbahn was a Swedish physicist who won the Nobel Prize for Physics in 1924 "for his discoveries and research in the field of X-ray spectroscopy.” Remarkably his son (Kai Manne Börje), in 1981 also won the Nobel Prize for Physics "for his contribution to the development of high-resolution electron spectroscopy".

Siegbahn senior’s very early work focused on problems of electricity and magnetism. He worked at Lund University with Rydberg and on whose death he became Professor in 1920. Siegbahn moved in 1923 to a Physics chair at the University of Uppsala and he was later (1937) to become Research Professor of Experimental Physics at the Royal Swedish Academy of Sciences. In the same year he became the first Director of the newly formed Physics Department of the Nobel Institute of the Academy came into being.

From 1912 onwards Seigbahn focused his studies onto X-ray spectroscopy. He was to develop novel techniques and practices (e.g. X-ray tubes and gratings) which enabled increasing radiation intensity and increased accuracy of measurements. In 1916he discovered the 3rd (M series) group of spectral lines. Seigbahn’s later work at the Institute was to oversee the development of a cyclotron for nuclear physics research.

Siegbahn utilized vacuum for his experiments and his search for higher vacuum levels led to his development of the Siegbahn Pump. This was a drag type mechanism which different from the Gadae and Holweck pumps in that a disc rotates inside a housing with spiral grooves. Patents were sought a few years after the pump was first built in 1926. It is not know whether Siegbahn was aware of the patent on the Holweck drag pump From 1926-1940 units were built in the university machine shop and Leybold held a licence for production up to 1931.

The first pumps were relatively small, 220 mm diameter, with and ultimate of 1e-5 mbar and fore-pressure of 0.1 mbar. Its pumping speed was only 2 l/s. After further development a pump with 30 l/s speed was produced in 1943. Siegbahn was later to describe a hybrid Seigbhan-Gaede mechanism of speed 48 l/s.

A large pump (disc diameter 540mm) was built for the cyclotron at the Nobel Institute which had 3 spiral grooves (in parallel) and a pumping speed of 73 l/s.

Generally the Holweck mechanism is more widely employed in drag pumps or drag stages of a turbomolecular pumps. Since the Siegbahn pump is a series of discs rather than cylinders it gives a more compact pump. In this case although the Holweck mechanism is more efficient the Siegbahn has more stages and this gives increased performance.

Reference: http://nobelprize.org/nobel_prizes/physics/laureates/1924/siegbahn-bio.html

Born in 1878, in what is now in the German port of Bremerhaven, Wolfgang Gaede made unique contributions to the theoretical and practical applications of vacuum technology during the era of accelerating industrialization which took place in the latter part of the 19th and first half of the 20th Centuries.

Wolfgang Gaede graduated from the University of Freiburg in physics in 1901, after previously having studied medicine. He developed the first of his vacuum pumps in response to the needs of his assistantship studies when only a Sprengel pump was available. His rotating mercury (high vacuum) pump was patented in 1905, a year after he was approached by Alfred Schmidt of Leybold to produce the pump. Leybold had lost out to Pfeiffer who had a license to manufacture a Geryk pump, but Gaede’s mechanism fitted the gap in Leybold’s vacuum line of business and was the start of a long (and in no small measure lucrative) partnership. Indeed his royalties partly funded his private laboratory where with additional funding from Leybold he developed products exclusively for Leybold to produce and market. Evidence suggests that Gaede never developed a pump at Leybold’s demand and glorified in his free-hand. In 1915 Gaede invented the high vacuum mercury vapor diffusion pump allowing hitherto un-paralleled high vacuum pressure.

Gaede had a range of interests outside vacuum with patents including wireless and refrigerators and received a full professorship in 1919 at the Institute of Technology, Karlsruhe. He was an acknowledged peer of luminary vacuum scientists of the age. He did however have a mistaken belief about the nature of desorption (antagonistic to Knuden’s cosinusoidal law), but despite this, he recognized the potential of molecular drag (friction) to develop the Gaede molecular pump (19). The drag process is the principle of molecular drag pumping mechanisms in contemporary technology. In the 1930’s Gaede further developed large capacity rotary vane pumps and the principle of gas ballast; an elegant application of thermodynamics and kinetic theory. Gas ballast is a controlled flow of gas into the chamber of a rotary before maximum compression is achieved. This allows discharge of vapour without condensation thus allowing vapour pumping without the stalling consequences and damaging effects to the pump resulting from condensation.

In 1934 Gaede became a victim of the Nazi government’s Gestapo ‘witch-hunts’ of universities and was forced to retire prematurely from his post. He later located to laboratories in Munich. Leybold paid his expenses and compensation as Gaede continued to hold the license but without receiving any royalties. Allied bombing destroyed his laboratory buildings in 1944 and Gaede died in 1945 .

References:

H. Henning Vakuum in Forschung und Praxis Volume 13, Issue 3 , Pages 180 – 186 (2001)

Vacuum Science and Technology: Pioneers of the 20th Century P. A. Redhead Springer (1994) p43 ISBN 1563962489

Born of Italian descent in Berlin in 1880, Marcello Pirani was destined to make a major input to vacuum technology at a very early age. He completed his studies in Mathematics and Physics and then postgraduate research in 1904, thereafter joining the Siemens & Halske (Gluhampenwerk) incandescent lamp factory. He was mainly concerned with sources of light but also the manufacture of tantalum lamps, the manufacture of which required a higher vacuum than carbon filament lamps.

A particular problem was in the use of glass McLeod gauges for vacuum measurement. They presented problems in being both manually operated and particularly sensitive to breakage; spilling poisonous mercury when doing so. Pirani considered this problem and as a result in 1906 he published his paper entitled the ‘Directly Indicating Vacuum Gauge’ which became known as the ‘Pirani gauge’: the first automatically reading gauge.

The Pirani gauge was designed to exploit to measure low pressures by utilizing the variation of heat loss from a wire with the pressure of the surrounding. A heated metal filament (typically platinum in modern gauges) loses heat to the gas from collisions of gas molecules with the wire. The heat loss is dependent on the number of collisions made with the wire and hence the pressure/density of the gas. As the vacuum level increases the number of molecules present will fall proportionately. This has a reduced cooling effect for the wire.

The electrical resistance of a wire varies with its temperature. The Pirani gauge operates in one of three modes: constant voltage, constant current or constant resistance (i.e. temperature). The Wheatstone bridge circuit is usually used where the Pirani gauge filament is one arm of a four-armed bridge. The readings of the gauge have to be corrected or calibrated for different gases (which have different thermal conductivities). Compared to the McLeod gauge the Pirani Gauge has the advantage of being automatic. Modern day gauges can measure from 100/10 to 10-4 mbar with an extension to higher pressure by exploiting the pressure dependence of convection losses.

Pirani worked further on optical measurements of high temperatures and then joined Osram in 1919 as head of the scientific-technical bureau. There he researched widely on topics ranging form the sorption of gases by tantalum to the transition from incandescent to gas-discharge lamps. During his time in industry he held several positions at the Technical University and Technishe Hochschule, both in Berlin.

From 1936 Pirani worked in the UK on activities as varied as high temperature resistant materials to the utilization of fine coal dust. He returned to Germany in 1953 consulting for Osram before dying at the age of 88 years in the city of his birth.

Karl Manne Georg Siegbahn was a Swedish physicist who won the Nobel Prize for Physics in 1924 "for his discoveries and research in the field of X-ray spectroscopy.” Remarkably his son (Kai Manne Börje), in 1981 also won the Nobel Prize for Physics "for his contribution to the development of high-resolution electron spectroscopy".

Siegbahn senior’s very early work focused on problems of electricity and magnetism. He worked at Lund University with Rydberg and on whose death he became Professor in 1920. Siegbahn moved in 1923 to a Physics chair at the University of Uppsala and he was later (1937) to become Research Professor of Experimental Physics at the Royal Swedish Academy of Sciences. In the same year he became the first Director of the newly formed Physics Department of the Nobel Institute of the Academy came into being.

From 1912 onwards Seigbahn focused his studies onto X-ray spectroscopy. He was to develop novel techniques and practices (e.g. X-ray tubes and gratings) which enabled increasing radiation intensity and increased accuracy of measurements. In 1916he discovered the 3rd (M series) group of spectral lines. Seigbahn’s later work at the Institute was to oversee the development of a cyclotron for nuclear physics research.

Siegbahn utilized vacuum for his experiments and his search for higher vacuum levels led to his development of the Siegbahn Pump. This was a drag type mechanism which different from the Gadae and Holweck pumps in that a disc rotates inside a housing with spiral grooves. Patents were sought a few years after the pump was first built in 1926. It is not know whether Siegbahn was aware of the patent on the Holweck drag pump From 1926-1940 units were built in the university machine shop and Leybold held a licence for production up to 1931.

The first pumps were relatively small, 220 mm diameter, with and ultimate of 1e-5 mbar and fore-pressure of 0.1 mbar. Its pumping speed was only 2 l/s. After further development a pump with 30 l/s speed was produced in 1943. Siegbahn was later to describe a hybrid Seigbhan-Gaede mechanism of speed 48 l/s.

A large pump (disc diameter 540mm) was built for the cyclotron at the Nobel Institute which had 3 spiral grooves (in parallel) and a pumping speed of 73 l/s.

Generally the Holweck mechanism is more widely employed in drag pumps or drag stages of a turbomolecular pumps. Since the Siegbahn pump is a series of discs rather than cylinders it gives a more compact pump. In this case although the Holweck mechanism is more efficient the Siegbahn has more stages and this gives increased performance.

Reference: http://nobelprize.org/nobel_prizes/physics/laureates/1924/siegbahn-bio.html