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The use of perfluoroalkyl polyether fluids in vacuum pumps
The use of perfluoroalkyl polyether fluids in vacuum pumps received 4 August 1971 M A Baker, L Holland and L Laurenson,
Crawley, Sussex, England
Central Research Laboratory, EdwardsHigh VacuumInternational, Manor Royal,
A fluorocarbon fluid ("Fomblinl') * has been investigated as a lubricant for a rotary mechanical pump. Although the backstreaming rate was only marginally (30 per cent) less than that of a mineral oil the emitted vapour and fragmented species (CF=, CF3, C3F O were resistant to the formation of carbonaceous and polymerized films (as occurs with hydrocarbons or silicone vapours) when exposed to electron bombardment. Resistance to polymerization and carbon deposition may arise from the low adsorption time and high bond strength of CF-radicals. The fluorocarbon fluid can also be evaporated by electron bombardment without leaving a decomposition residue. A rotary pump was operated at 110°C for 500 hr without signs of wear so that vapour with a saturated vp at this temperature may be pumped without gas-ballasting. The fluid shows marked resistance to reaction with exhausted gases (eg fluorinated species) and can afford corrosion protection to the pump. In conclusion experiments show that an all fluorocarbon system is possible with both diffusion and rotary pumps charged with the fluid, such an arrangement would be of value for electron beam systems if fluorocarbon radicals could be tolerated. However, the high cost of the fluid will currently restrict its use.
1. Introduction In January 1969 the writers presented a paper at the "Symposium on Lubrication In Hostile Environments" in which they described their studies of the sources of backstreaming from mineral oils used in rotary mechanical vacuum pumps. It was shown from friction experiments using different lubricants and abraders that fluid degradation occurred when the sliding coefficient of friction was sufficiently high to cause thermal hot spots on the rubbing surfaces. These experiments confirmed the belief that the volatiles emitted from rotary pumps charged with mineral oils come from decomposition as well as evaporation of light fractions in the original oil. In a subsequent discussion at the conference with D r Grazzini of Montecatini Edison SpA it was learnt that his company had developed a new type of synthetic lubricant. D r Grazzini queried whether the new lubricant, perfluoroalkylpolyether* with a high thermal stability, might give lower backstreaming rates when used as a mechanical vacuum pump lubricant. A few weeks after the conference a sample of the lubricant was supplied by Montecatini Edison for mechanical vacuum pump tests in the writers' laboratory.']"
2. Backstreaming measurements Backstreaming rate. Backstreaming tests were carried out with an Edwards ED35 two-stage vacuum pump (35 I/min). The backstreaming rates were measured directly above the vacuum inlet to the pump using a quartz crystal microbalance 1 cooled with liquid nitrogen. Rates were measured with the pump charged with a normal mineral oil (Edwards 16 oil) and then after *Trade name "Fomblin" of Montecatini, Edison S.p.A. can be obtained for vacuum uses from Edwards Vacuum Components. t"A relevant Research Note R.N. 47 was deposited with the National Reference Library of Scienceand Invention, in December 1969.
careful cleaning and recharging with the fluorocarbon lubricant. ~ The measured backstreaming rates were as follows: Mineral oil (pump temp 60°C) = 15/~g cm -~ min -1. Fluorocarbon lubricant (pump temp. 70°C)~10 pg cm -~ min -1. Gas analysis. In addition to the backstreaming tests the vacuum atmosphere above the working pump charged with the fluorocarbon lubricant was examined with a mass spectrometer. A 60 ° magnetic sector field instrument was used with a mass discrimination up to 350 amu. To permit reduction of the rotary pump gas pressure to the optimum value for the mass spectrometer samples of the gas above the rotary pump were fed to the ion source of the mass spectrometer via a heated capillary tube coupled to the vacuum inlet of the rotary pump. When the pump was vented to the mass spectrometer dominant peaks appeared in the mass spectra with mass to charge ratios of 50: (CF=) 69 (CF3) and 93 (C8F3); these radicals being characteristic of the breakdown of fluorocarbon compounds. Breakdown could have occurred in the ion source of the mass spectrometer and by fluid degradation at hot spots generated by friction at blade tips and other sliding surfaces in the pump. As the backstreaming rate of the rotary pump was only reduced by 30 per cent when replacing the mineral oil by the fluorocarbon fluid it was concluded that decomposition of the latter still occurred, ie assuming that evaporation of volatile fractions was not mainly responsible for the backstreaming. The fluorocarbons fluid used had an average molecular weight of 6500 amu whereas mineral oils used as rotary pump lubricaras typically have an average weight of 480 amu. However - - C H ~ - units in a hydrocarbon chain have a mass of 14 amu compared with 50 ainu for - - C F ~ - - units so that as a rough approximation fluorocarbon molecules comparable in size to mineral oils would have 3~, times their mass. Although the original purpose of using the fluorocarbon in a
Vacuum/volume 21/number 10. PergamonPress LtdlPrinted in Great Britain
479
M A Baker, L Holland and L Laurenson: The use of perfluoroalkyl polyether fluids in vacuum pumps rotary pump was to reduce the backstreaming rate and this had only been marginally achieved there were many operational reasons why the fluid could be preferred to mineral oils. However, the fluid is at present expensive so its immediate uses are likely to be limited and we have discussed some examples and related tests below. 3. Resistance to polymerisation by electron bombardment Both the parent fluorocarbon and its fragmented species possessed a stable C - F bond and it was believed that this would suppress the production of polymer or carbonaceous coatings on surfaces which are formed readily when hydrocarbon or siloxane molecules are present in electron beam accelerator systems. Also, most fluorocarbons have a low affinity for surfaces due to the strong shielding effect of the fluorine ion. Thus fragmented species were likely to remain in the gas phase and adsorption, which is necessary for irradiation cross-linking of hydrocarbons or silicone gas molecules, would be negligible. These assumptions were tested as follows: Electron bombardment/fluid evaporation. A tray containing 6 cc of the fluorocarbon fluid was exposed to electron bombardment (1 keV 0.15 mA/cm ~) and after 55 rain 3 cc of the fluid had evaporated from a fluid area of 6 cm~; polymer formation was not observed in the liquid which had remained colourless. The high rate of evaporation arose from a combination of electron bombardment and radiant heating from the simple electron source. Also there were no signs of polymer growth on the vacuum vessel walls from the evaporated fluorocarbon molecules although electrons were bombarding these surfaces. When a silicone fluid was treated under identical conditions a polymer skin was formed on the liquid surface within 15 min. The tests demonstrated the remarkable resistance of the fluorocarbon fluid to polymerization or formation of non-volatile residues when exposed to electron bombardment which is in contrast to silicone and hydrocarbon fluids. Further experiments of directly electron bombarding the fluorocarbon fluid were made but this time a comparison was made with silicone (MS704), polyphenylether (Santovac 5) and Apiezon (C) fluids placed on the same anode target. An average current density of - 100/~A/cm ~ was used for a few minutes. Shown in Figure 1 are the residues remaining after the four fluids were electron bombarded. The samples were bombarded simultaneously and it is possible that some vapour interaction could have occurred. However, it is believed that any such effects would be small and could be neglected. The
fluids with the exception of the fluorocarbon all gave light brown polymer residues after electron bombardment. The brown colour of the silicone polymer was due to the relatively high energy of the electron bombardment (l keV). Low energy bombardment (200 eV) of silicone produces a white transluscent polymer. The fluorocarbon evaporated without leaving a polymer deposit and the fluid residue remaining was still completely liquid and transparent without optical absorption. In the centre of the bombarded area a brown polymer film had formed by interaction of electrons with organic vapours. As the fluorocarbon evaporated the liquid at its edges formed, under surface tension, droplets which then completely vaporized. The outline of these evaporated droplets, as clean patches, can be observed in the figure because a brown polymer from hydrocarbon vapours had been deposited in the regions between the evaporating liquid droplets. The experiment was repeated using the fluorocarbon alone and neither the liquid residue nor the completely evaporated zones showed any sign of degraded material or polymer formation. Thus if electron bombardment causes degradation the components formed must evaporate with the parent molecules. In a third experiment a diffusion pump was charged with a fluorocarbon fluid and used without a cold trap. The ion gauge was operated with an emission current of l0 mA and the glass envelope observed for formation of a polymer coating as would occur with an untrapped pump using conventional fluids. No deposit was observed after 5 hr although the pressure was about 5 X 1 0 -7 torr. These results indicate that if fluorocarbon molecules and their radicals can be tolerated in a vacuum system then they have the great advantage of resisting the formation of insulating or semi-conductor films as occurs with silicones and hydrocarbons such as mineral oils and polyphenyl ethers.
4. Physical and chemical properties The fluorocarbon lubricant possesses many other properties of importance in vacuum work. These are: a high chemical resistance to most common acids, alkalies and corrosive gases, a low vapour pressure, non-miscibility with most common solvents and complete safety when exposed to neat oxygen. The foregoing are of value in chemical engineering and low pressure processing where gas sealed pumps must be used for safety purposes. Our tests have shown that fluorinated active gases can be
(
Figure 1. Residues resulting from the electron bombardment of (= 100 #A/cm2) of fluorocarbon (Fomblin YR) mineral oil (Apiezon C) silicone (M.S. 704) and polypheny ]ether (Santovac 5). 480
M A Baker, L Holland and L Laurenson : The use of perfluoroalkyl polyether fluids in vacuum pumps successfully exhausted by a mechanical vacuum pump lubricated with the fluorocarbon fluid without any reaction or degradation of the fluid. There is also evidence that the fluorocarbon lubricant affords protection to the exposed surfaces of a mechanical pump which is exhausting a corrosive gas. Operating temperature and vapour exhaustion. The high thermal stability of the fluorocarbon lubricant permits the operation of mechanical pumps at elevated temperature with efficient lubrication and absence of thermal degradation of the lubricant ie apart from that arising from friction effects. Thus after completion of the backstreaming tests described above the pump containing the fluorocarbon lubricant was operated for a further 500 hr at about 110°C, which was about 40°C higher than its normal operating temperature. When the pump was stripped down it showed no evidence of lubrication failure or excessive wear. The temperature limit of 110°C in the test was imposed by the mechanical tolerances of the pump but it should be possible to use the fluid as a vacuum pump lubricant to at least 200°C. * Exhausting condensable vapeurs. Operating a rotary pump at an elevated temperature can permit the exhaustion of a vapour without condensation occurring in the pump providing its temperature is sufficient for the corresponding saturated vapour pressure to exceed the expulsion pressure, which allowing for atmospheric pressure and exhaust valve loading is about I000 torr. Operating with a pump at an elevated temperature can be a more efficient method of handling a condensable vapour than by gas-ballast flow into the compression stage of the pump. Gas-ballasting can impair the ultimate pressure and an inert ballast gas can be essential when exhausting reactive vapours. Of course fluid evaporation can be enhanced at high temperature and require the addition of a cold trap. More experiments are needed using fluids whose viscosity and vapour pressure are adapted to high temperature operation. Also, * Thermal degradation of the fluorocarbon fluid occurs rapidly above 360°C and the degradation temperature is lowered to ~--320°Cif oxygen and some metals or their oxides are present.
experiments using heated fluid for specific chemical engineering purposes are still to be made.
5. Discussion and conclusions Although our work on the fluorinated lubricant in vacuum mechanical pumps is still incomplete it is sufficient to indicate that a f l u i d is now available which is greatly superior in its physical and chemical properties to the commonly used mineral oil. F o r the present its high cost will greatly limit its application but we are beginning to use the fluid in laboratory forepumps employed for evacuating the target vessels of ion beam systems and related plant. Experiments are also commencing on its application in sputtering systems where film contamination can arise from hydrocarbon degradation. The use of such fluids in diffusion pumps, as mentioned above, comes to mind with the possible achievement of a vacuum combination operating fully on fluorocarbon fluids. Tests on such a system have commenced and a further report will be made of its use on systems involving surface bombardment by energetic particles. Finally, it has proved extremely difficult because of operational and constructional costs to design routine vacuum pumping systems free from the use of organic or silicone fluids. Providing the fluorocarbons can be reduced in production cost and their radicals are not a source of chemical reaction then their resistance to polymerization and decomposition residues could be of great importance in vacuum systems for physical electronic instruments such as electron microscopes. However, it will be necessary to investigate whether fluorocarbon gases react with specimen materials, particularly in the presence of bombardment by energetic beams.
References 1 L Laurenson, L Holland and M A Baker, Paper 11. Degradation of lubricated fluids in a vacuum. Symp on Lubrication in Hostile Environments. London 15-16 January (1969). The Institution of Mechanical Engineers. 2 M Baker and L Laurenson, Vacuum, 16, 1966, 633-637.
481
Crawley, Sussex, England
Central Research Laboratory, EdwardsHigh VacuumInternational, Manor Royal,
A fluorocarbon fluid ("Fomblinl') * has been investigated as a lubricant for a rotary mechanical pump. Although the backstreaming rate was only marginally (30 per cent) less than that of a mineral oil the emitted vapour and fragmented species (CF=, CF3, C3F O were resistant to the formation of carbonaceous and polymerized films (as occurs with hydrocarbons or silicone vapours) when exposed to electron bombardment. Resistance to polymerization and carbon deposition may arise from the low adsorption time and high bond strength of CF-radicals. The fluorocarbon fluid can also be evaporated by electron bombardment without leaving a decomposition residue. A rotary pump was operated at 110°C for 500 hr without signs of wear so that vapour with a saturated vp at this temperature may be pumped without gas-ballasting. The fluid shows marked resistance to reaction with exhausted gases (eg fluorinated species) and can afford corrosion protection to the pump. In conclusion experiments show that an all fluorocarbon system is possible with both diffusion and rotary pumps charged with the fluid, such an arrangement would be of value for electron beam systems if fluorocarbon radicals could be tolerated. However, the high cost of the fluid will currently restrict its use.
1. Introduction In January 1969 the writers presented a paper at the "Symposium on Lubrication In Hostile Environments" in which they described their studies of the sources of backstreaming from mineral oils used in rotary mechanical vacuum pumps. It was shown from friction experiments using different lubricants and abraders that fluid degradation occurred when the sliding coefficient of friction was sufficiently high to cause thermal hot spots on the rubbing surfaces. These experiments confirmed the belief that the volatiles emitted from rotary pumps charged with mineral oils come from decomposition as well as evaporation of light fractions in the original oil. In a subsequent discussion at the conference with D r Grazzini of Montecatini Edison SpA it was learnt that his company had developed a new type of synthetic lubricant. D r Grazzini queried whether the new lubricant, perfluoroalkylpolyether* with a high thermal stability, might give lower backstreaming rates when used as a mechanical vacuum pump lubricant. A few weeks after the conference a sample of the lubricant was supplied by Montecatini Edison for mechanical vacuum pump tests in the writers' laboratory.']"
2. Backstreaming measurements Backstreaming rate. Backstreaming tests were carried out with an Edwards ED35 two-stage vacuum pump (35 I/min). The backstreaming rates were measured directly above the vacuum inlet to the pump using a quartz crystal microbalance 1 cooled with liquid nitrogen. Rates were measured with the pump charged with a normal mineral oil (Edwards 16 oil) and then after *Trade name "Fomblin" of Montecatini, Edison S.p.A. can be obtained for vacuum uses from Edwards Vacuum Components. t"A relevant Research Note R.N. 47 was deposited with the National Reference Library of Scienceand Invention, in December 1969.
careful cleaning and recharging with the fluorocarbon lubricant. ~ The measured backstreaming rates were as follows: Mineral oil (pump temp 60°C) = 15/~g cm -~ min -1. Fluorocarbon lubricant (pump temp. 70°C)~10 pg cm -~ min -1. Gas analysis. In addition to the backstreaming tests the vacuum atmosphere above the working pump charged with the fluorocarbon lubricant was examined with a mass spectrometer. A 60 ° magnetic sector field instrument was used with a mass discrimination up to 350 amu. To permit reduction of the rotary pump gas pressure to the optimum value for the mass spectrometer samples of the gas above the rotary pump were fed to the ion source of the mass spectrometer via a heated capillary tube coupled to the vacuum inlet of the rotary pump. When the pump was vented to the mass spectrometer dominant peaks appeared in the mass spectra with mass to charge ratios of 50: (CF=) 69 (CF3) and 93 (C8F3); these radicals being characteristic of the breakdown of fluorocarbon compounds. Breakdown could have occurred in the ion source of the mass spectrometer and by fluid degradation at hot spots generated by friction at blade tips and other sliding surfaces in the pump. As the backstreaming rate of the rotary pump was only reduced by 30 per cent when replacing the mineral oil by the fluorocarbon fluid it was concluded that decomposition of the latter still occurred, ie assuming that evaporation of volatile fractions was not mainly responsible for the backstreaming. The fluorocarbons fluid used had an average molecular weight of 6500 amu whereas mineral oils used as rotary pump lubricaras typically have an average weight of 480 amu. However - - C H ~ - units in a hydrocarbon chain have a mass of 14 amu compared with 50 ainu for - - C F ~ - - units so that as a rough approximation fluorocarbon molecules comparable in size to mineral oils would have 3~, times their mass. Although the original purpose of using the fluorocarbon in a
Vacuum/volume 21/number 10. PergamonPress LtdlPrinted in Great Britain
479
M A Baker, L Holland and L Laurenson: The use of perfluoroalkyl polyether fluids in vacuum pumps rotary pump was to reduce the backstreaming rate and this had only been marginally achieved there were many operational reasons why the fluid could be preferred to mineral oils. However, the fluid is at present expensive so its immediate uses are likely to be limited and we have discussed some examples and related tests below. 3. Resistance to polymerisation by electron bombardment Both the parent fluorocarbon and its fragmented species possessed a stable C - F bond and it was believed that this would suppress the production of polymer or carbonaceous coatings on surfaces which are formed readily when hydrocarbon or siloxane molecules are present in electron beam accelerator systems. Also, most fluorocarbons have a low affinity for surfaces due to the strong shielding effect of the fluorine ion. Thus fragmented species were likely to remain in the gas phase and adsorption, which is necessary for irradiation cross-linking of hydrocarbons or silicone gas molecules, would be negligible. These assumptions were tested as follows: Electron bombardment/fluid evaporation. A tray containing 6 cc of the fluorocarbon fluid was exposed to electron bombardment (1 keV 0.15 mA/cm ~) and after 55 rain 3 cc of the fluid had evaporated from a fluid area of 6 cm~; polymer formation was not observed in the liquid which had remained colourless. The high rate of evaporation arose from a combination of electron bombardment and radiant heating from the simple electron source. Also there were no signs of polymer growth on the vacuum vessel walls from the evaporated fluorocarbon molecules although electrons were bombarding these surfaces. When a silicone fluid was treated under identical conditions a polymer skin was formed on the liquid surface within 15 min. The tests demonstrated the remarkable resistance of the fluorocarbon fluid to polymerization or formation of non-volatile residues when exposed to electron bombardment which is in contrast to silicone and hydrocarbon fluids. Further experiments of directly electron bombarding the fluorocarbon fluid were made but this time a comparison was made with silicone (MS704), polyphenylether (Santovac 5) and Apiezon (C) fluids placed on the same anode target. An average current density of - 100/~A/cm ~ was used for a few minutes. Shown in Figure 1 are the residues remaining after the four fluids were electron bombarded. The samples were bombarded simultaneously and it is possible that some vapour interaction could have occurred. However, it is believed that any such effects would be small and could be neglected. The
fluids with the exception of the fluorocarbon all gave light brown polymer residues after electron bombardment. The brown colour of the silicone polymer was due to the relatively high energy of the electron bombardment (l keV). Low energy bombardment (200 eV) of silicone produces a white transluscent polymer. The fluorocarbon evaporated without leaving a polymer deposit and the fluid residue remaining was still completely liquid and transparent without optical absorption. In the centre of the bombarded area a brown polymer film had formed by interaction of electrons with organic vapours. As the fluorocarbon evaporated the liquid at its edges formed, under surface tension, droplets which then completely vaporized. The outline of these evaporated droplets, as clean patches, can be observed in the figure because a brown polymer from hydrocarbon vapours had been deposited in the regions between the evaporating liquid droplets. The experiment was repeated using the fluorocarbon alone and neither the liquid residue nor the completely evaporated zones showed any sign of degraded material or polymer formation. Thus if electron bombardment causes degradation the components formed must evaporate with the parent molecules. In a third experiment a diffusion pump was charged with a fluorocarbon fluid and used without a cold trap. The ion gauge was operated with an emission current of l0 mA and the glass envelope observed for formation of a polymer coating as would occur with an untrapped pump using conventional fluids. No deposit was observed after 5 hr although the pressure was about 5 X 1 0 -7 torr. These results indicate that if fluorocarbon molecules and their radicals can be tolerated in a vacuum system then they have the great advantage of resisting the formation of insulating or semi-conductor films as occurs with silicones and hydrocarbons such as mineral oils and polyphenyl ethers.
4. Physical and chemical properties The fluorocarbon lubricant possesses many other properties of importance in vacuum work. These are: a high chemical resistance to most common acids, alkalies and corrosive gases, a low vapour pressure, non-miscibility with most common solvents and complete safety when exposed to neat oxygen. The foregoing are of value in chemical engineering and low pressure processing where gas sealed pumps must be used for safety purposes. Our tests have shown that fluorinated active gases can be
(
Figure 1. Residues resulting from the electron bombardment of (= 100 #A/cm2) of fluorocarbon (Fomblin YR) mineral oil (Apiezon C) silicone (M.S. 704) and polypheny ]ether (Santovac 5). 480
M A Baker, L Holland and L Laurenson : The use of perfluoroalkyl polyether fluids in vacuum pumps successfully exhausted by a mechanical vacuum pump lubricated with the fluorocarbon fluid without any reaction or degradation of the fluid. There is also evidence that the fluorocarbon lubricant affords protection to the exposed surfaces of a mechanical pump which is exhausting a corrosive gas. Operating temperature and vapour exhaustion. The high thermal stability of the fluorocarbon lubricant permits the operation of mechanical pumps at elevated temperature with efficient lubrication and absence of thermal degradation of the lubricant ie apart from that arising from friction effects. Thus after completion of the backstreaming tests described above the pump containing the fluorocarbon lubricant was operated for a further 500 hr at about 110°C, which was about 40°C higher than its normal operating temperature. When the pump was stripped down it showed no evidence of lubrication failure or excessive wear. The temperature limit of 110°C in the test was imposed by the mechanical tolerances of the pump but it should be possible to use the fluid as a vacuum pump lubricant to at least 200°C. * Exhausting condensable vapeurs. Operating a rotary pump at an elevated temperature can permit the exhaustion of a vapour without condensation occurring in the pump providing its temperature is sufficient for the corresponding saturated vapour pressure to exceed the expulsion pressure, which allowing for atmospheric pressure and exhaust valve loading is about I000 torr. Operating with a pump at an elevated temperature can be a more efficient method of handling a condensable vapour than by gas-ballast flow into the compression stage of the pump. Gas-ballasting can impair the ultimate pressure and an inert ballast gas can be essential when exhausting reactive vapours. Of course fluid evaporation can be enhanced at high temperature and require the addition of a cold trap. More experiments are needed using fluids whose viscosity and vapour pressure are adapted to high temperature operation. Also, * Thermal degradation of the fluorocarbon fluid occurs rapidly above 360°C and the degradation temperature is lowered to ~--320°Cif oxygen and some metals or their oxides are present.
experiments using heated fluid for specific chemical engineering purposes are still to be made.
5. Discussion and conclusions Although our work on the fluorinated lubricant in vacuum mechanical pumps is still incomplete it is sufficient to indicate that a f l u i d is now available which is greatly superior in its physical and chemical properties to the commonly used mineral oil. F o r the present its high cost will greatly limit its application but we are beginning to use the fluid in laboratory forepumps employed for evacuating the target vessels of ion beam systems and related plant. Experiments are also commencing on its application in sputtering systems where film contamination can arise from hydrocarbon degradation. The use of such fluids in diffusion pumps, as mentioned above, comes to mind with the possible achievement of a vacuum combination operating fully on fluorocarbon fluids. Tests on such a system have commenced and a further report will be made of its use on systems involving surface bombardment by energetic particles. Finally, it has proved extremely difficult because of operational and constructional costs to design routine vacuum pumping systems free from the use of organic or silicone fluids. Providing the fluorocarbons can be reduced in production cost and their radicals are not a source of chemical reaction then their resistance to polymerization and decomposition residues could be of great importance in vacuum systems for physical electronic instruments such as electron microscopes. However, it will be necessary to investigate whether fluorocarbon gases react with specimen materials, particularly in the presence of bombardment by energetic beams.
References 1 L Laurenson, L Holland and M A Baker, Paper 11. Degradation of lubricated fluids in a vacuum. Symp on Lubrication in Hostile Environments. London 15-16 January (1969). The Institution of Mechanical Engineers. 2 M Baker and L Laurenson, Vacuum, 16, 1966, 633-637.
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