Vacuum fluids

Vacuum fluids

Vacuum received L fluids* 12 March Laurenson. 1980 ResearchfDevelopment, Edwards High Vacuum, Crawley, Sussex, UK The properties and appli...

656KB Sizes 11 Downloads 67 Views

Vacuum received L

fluids*

12 March

Laurenson.

1980

ResearchfDevelopment,

Edwards

High

Vacuum,

Crawley,

Sussex,

UK

The properties and applications of the various vacuum fluids and greases at present available discussed. The information is presented to aid in making the best choice of fluids for specific applications.

are

Introduction

The choice of fluid for vacuum pumpsrequiresconsideration of the condition to which the pumpsare to be subjected.Not only must the vacuum performancebe considered,but also the compatability of the fluid with the processor instrumentwhich is to be pumped.To discussthis, the paper is divided into three sections; (a) rotary pump fluids, (b) vapour pump fluids and (c) vacuum greases.There is obviously some inter-relation betweenthe sections,as a basic chemicalspeciemay be used in all three applications. In such a case,the properties of the chemical will be dealt with under their first use and unless qualified thesepropertiescan be assumedto apply to the other applicationsof the substance.

Rotary pump fluids

There are a number of criteria that can be applied to such a fluid-it has to be a lubricant, have a viscosity appropriate to the pump’s mechanical design, have a fairly low vapour pressure, be compatible with the process involved, and if possiblebe non-toxic. For many years the only fluids available for rotary pumps were purified mineral oils, which, for heavy duty applications, were fortified by additives to reduce, for example, corrosion, oxidation and foaming. Mineral oils can easily be tailored to satisfy requirementsof lubrication, viscosity and vapour pressure, but for someapplicationsthey are far from suitable; the pumping of oxygen or acidsare prime examples.As processes becamemore sophisticated,mineral oilsbecamelessacceptable, for example light componentsof the oil made by cracking at the rubbing surfacesin the pump had to be trapped out. Trapping is normally carried out usingeither liquid nitrogen cooled surfacesor a sorbentmaterial suchasartificial zeolite, charcoal or activated alumina. Liquid nitrogen traps have the disadvantage of requiring frequent replenishment, otherwise trapped oil will be re-releasedinto the vacuum. The author and his colleaguesfound activated alumina to be the most satisfactory of the sorbents1*2.If it is necessaryto regeneratethe sorbent by baking, this must not be done in situ otherwisea

l A paperin our educational series‘The Theory and Practiceof Vacuum Scienceand Technologyfor Schoolsand Colleges”,presentedat the VacuumTechnologyMeeting, heldat the Middlesex HospitalMedicalSchool,London,11July 1979.

Vacuum/volume30/number7.

Pergamon

Press

LtdlPrinted

in Great

Figure 1. Graphicalrepresentation of backstreaming attenuationby

liquid nitrogentrap and air purge. (Reproducedfrom Baker and Laurenson,Yuc~ntm, 16, 1966, 633.) proportion of the sorbedbackstreamingspecieswill be driven off into the vacuum, or at bestcondensedon the pipework and valve to the vacuum chamberand enter it on the next pumping cycle. An alternative to trapping is to provide a gas leak to ensurea fairly high pressurein backing and roughing lines to prevent backmigration. Figure I showsthe speedwith which backstreamingtakesplaceand the efficacy of both trapping and usinga controlled leak. It wasobserved3that when usedin rotary pumps,even very low vapour pressurefluids gave similar backstreamingratesto the more normally usedoils due to cracking of the oils. There

Britain

275

L Laurenson:

Vacuum fluids

was not, as shown in Table 1, an order difference in backstreamingrates betweenSantovac 5 and conventional mineral oils although the vapour pressuresof the parent fluids differed by a factor of nearly 105.The rangeof propertiesof oils suitable for rotary pumpsare given in Table 2. The most suitable oil for a specific pump would be recommendedby the pump manufacturer.For many yearspumpingof aggressivematerial, with the obvious exception of oxygen, was carried out using mineral oils, changing the oil as it becamedegraded, maybe after only a few hours. However, with changesin economic conditionsand the advent of processes usingevenmore aggressivematerials,alternative fluids had to be found. Among these, phosphateesterswere used to pump oxygen and lately two halogenatedmaterials have come to the fore for use with oxygen and for other aggressiveduties.

Table 1. Backstreaming ratesfor differentfluids in a two-stage 35 We/minute pump (ED35). (Reproduced from Fulker ef al, Vacuum, 19, 1969, 556)

Oil or fluid

Backstreaming rate (pgm cm-* min-‘)

Normal uninhibited mineral oil (Edwards 16 oil)

18

Strippedmineraloil

18

Mineral oil + 1% MoS, Mineraloil + 1% oleic acid

Observations

4.7 6.5

Oleicacid backstreaming evident

Strippedsiliconefluid (DC 704)

10

Polyphenylether (unstripped)

13

ApiezonC oil

15

Pumpseized after + h of

operation Difficult to start

pumpdueto high viscosity

Table 2. Range of properties for rotary pump oils

Vapour pressure (20°C) Molecular weight

Viscosity(20°C) Pour point (“C) Additives

Sulphurcontent

chemical attack and can be selectedto have vapour pressure and viscosity properties which allow close matching in these respectsto the mineral oils in mechanicalpumps.The result is the isolation of a cut with a viscosity of lessthan SAE 20 and a vapour pressureof lessthan 10m4mbar at 50°C (the operating temperatureof most rotary pumps)4.They are also immiscible with many commonsolvents,a property which can prove useful when the pumping of such substancesis required. If it is necessaryto dissolvethis material a highly fluorinated solvent such as trichloro-trifluoroethanet should be used. However the use of an inert fluid immiscible with most solvents and aggressivematerialsdoesnot automatically solve the problem of pumpingaggressivematerials,asthesewill still be presentat the pump dischargeoutlet; in the caseof perlluoro polyether, with a specificgravity of 1.9, they will be floating on the top of the oil reservoir. Where the quantity is not great it can be removed by keepinga clear exhaust which doesnot allow any condensateto flow back into the pump. Due also to the oil’s non-misciblenature, water and other solvents are not held in the fluid and generally gasballastingis not required. By virtue of its inert nature it has no detergent properties and if much particulate material is expected a dust filter betweenthe vessel and the rotary pump is recommended. However, to conclude,vacuum applicationsare still growing and the requirementsare becoming even more exacting, for example, X-ray spectrometerworkers are now calling for a sulphur- and fluorine-free oil. Thus continued work is required to extend the range of lubricants suitablefor rotary pump use and tailored for particular applications.

Lessthan 10e6 mbar 420-520 lo-30 SAE -9 to -18 Optional Less than 1.5 %

One is a fluoro chloro chain compoundand the other a fully fluorinated compound, a perfluoro polyether. The fluoro chloro chain materialsare inert to oxygen and many aggressive chemicalsbut suffer from a tendency to be hydrolysed to form hydrochloric acid. They alsohave vapour pressureand viscosity characteristicswhich cannot be exactly matched to those of mineraloils. The fully fluorinated materials(Fomblin*), on the other hand, do not hydrolyse, are somewhatmore resistantto

Vapour pumpfluids There is no universal fluid and to make a choice parameters suchas the ultimate vacuum needed,the critical backing pressure required and the processto be carried out have all to be considered.The criteria required for vapour pumpsare somewhat different from thosefor a rotary pump. Consideringthese it is unnecessaryfor a diffusion pump fluid to be a lubricant, although in rugged applications rust protection could be desirable.Viscosity is of no consequenceprovided it is sufficiently fluid to flow back to the boiler when cold. Its vapour pressuremust be fairly low to provide a good ultimate vacuum. It must be able to withstand being boiled under reducedpressure without significantdecompositionand ideally it shouldbe resistant to oxidation if exposed to air while at its operating temperature. It must be compatible with the processbeing evacuated,and in considerationof this its chemicalresistance and its behaviour under energetic particle bombardment (i.e. whether it forms polymer films and whether theseare conducting or insulating) is important. Finally it should be non-toxic and a non-pollutant. Historically mercury was the first fluid employed in this capacity, beingusedby Gaedein 1913and in an improved form of pump by Langmuir in 19166.This was followed by the use of mineraloil in 1928by Burch’. Thesetwo materialswere used almost exclusively until the late 1940swhen other fluids were developed. The shortcomings of the above fluids were as follows. Mercury has a vapour pressureof around lo-” mbar at room temperature;asa result, to obtain pressuresbelow this

* Tradename of MontedisonSpA (Vacuumgradeshavethe suffix Y VAC and are marketed by Edwards High Vacuum). 276

t Tradenames Arklone P, Algofrene 113, Arcton 113 and Freon 113.

L Laurenson:

Vacuum

fluids

a low-temperature trap is essential. The presence of mercury in systems where metals are being processed, such as electronic circuitry and metal coating, can be catastrophic. The chief disadvantage of mineral oils is their relatively poor thermal stability and oxidation resistance. However, possibly due to their low cost, they are still in use to this day. Apart from the limitations of mercury and mineral oils, the arrival of specific and more demanding applications required the development of other fluids. To simplify the discussion of such fluids they will be subdivided into two groups, those suitable for vapour booster pumps and those for the better known diffusion pumps. A full discussion of the use and design of the two types of pump has been given by Colwell* and it is sufficient to say here that the booster pump works in a higher pressure region (1Oo-1O-4 mbar) than the diffusion pump (10s3 to less than lo-lo mbar) and thus the two are somewhat different in characteristics. Booster pump fluids. In addition to the criteria already listed, for booster pumps cost becomes a significant factor, as a large booster can take 55 1 of fluid. As booster pumps operate over a higher pressure range than diffusion pumps the fluids are more volatile. Over the years, a number of fluids have been used. Among the early fluids were glycerine and butyl phthalate. Although both of these performed satisfactorily as pumping fluids they had drawbacks. Glycerine is hygroscopic and thus provides little corrosion protection. As a result when pumps so charged are left open to the atmosphere the interiors become rusty. Butyl phthalate is readily hydrolized when pumping water vapour to become phthalic acid, which forms cjstals in the pump. After glycerine and butyl pthalate chlorinated biphenyls such as Aroclort and Klophenes were used for many years. These provided extremely satisfactory pumping fluids but were found to be environmental pollutants which available evidence indicates are toxic to mammals. As a result their use was discontinued in many applications, including vacuum pumps. The inadvisability of using chlorinated biphenyls has required the selection of a suitable substitute and it is true to say that a fluid equal to the biphenyls in performance in every way has not yet been found. There are three choices. 1. Mineral oil. A sufficiently light volatile fraction can be produced for use in a booster pump and its oxidation resistance can be greatly enhanced by anti-oxidant additives of the same vapour pressure. Even so such a pump fluid falls short of the chlorinated biphenyls on three counts: (a) It gives a lower mass throughput of gas in the 1Oo-1O-2 mbar pressure range, a region where, in applications such as vacuum furnaces, a maximum throughput is required. (b) Although much improved by the additive it is still far from a rugged fluid, coke forming on the jet assemblies after relatively few exposures to air. (c) It has the fairly low auto-ignition temperature of 305°C in air, which means that if a sealed pump so charged is

$.Tradename of DuPont. 5 Tradename of Bayer.

switched on at atmospheric pressure and then exposed to air when hot, auto-ignition of the fluid could result. A suitable mineral oil for booster pump use is marketed under the name of Apiezon AP201. 2. Silicone fluids. At present the fluids readily available are those developed for conventional diffusion pump use and as such are not sufficiently volatile for optimum booster pump use. This results in even poorer throughput performance than with mineral oils in the pressure range 1Oo-1O-2 mbar. However, against this they are very oxidation resistant and have a low fire risk. Their auto-ignition temperature is around 500°C. Both DC702 and DC704 have been used for this purpose but the DC702 being more volatile gives the better pumping performance. 3. Esters. Conventional esters have relatively poor resistance. to hydrolysis and oxidation. However, hindered esters with suitable additives can be made to withstand repeated exposure to air without significant deterioration. An ester with a suitable vapour pressure curve has been developed (Edwards 200) which gives a performance comparable to that of mineral oil but without decomposing to carbon and with a much higher auto-ignition temperature (370°C against 305°C for a suitable mineral oil). An additional advantage of this fluid is that the charge in the pump does not disappear as rapidly as does a mineral charge. The reason for charge depletion in a booster pump is not fully understood. It is obviously a function of the fluid and has been suggested that it is either the degradation of some fluids to gaseous components which are pumped away, or the formation of fine droplets which aid the fluid’s escape from the pump. Thus depending on requirements the choice of fluid can be made. If cost is the main consideration then a mineral oil is the best choice. If a fluid with high chemical and oxidation resistance and of high physiological compatability is required then a silicone should be chosen and the reduced throughput tolerated. (In respect of physiological compatibility it has been known for pumps used in the food industry to be charged with glycerine.) The ester provides a good compromise between the two, giving an identical pumping performance to the mineral oil, but with an auto-ignition temperature which lies between the silicones and mineral oils and good oxidation resistance. Diffusion pump fluids. Apart from mercury, diffusion pump fluids can be divided into five groups in common use in diffusion pumps today. These are: (i) mineral oils, (ii) silicones, (iii) polyphenyl ethers, (iv) perfluoro polyethers and (v) other miscellaneous man-made fluids. Some comments on each of these will be made. Normally diffusion_pumps are so made that a fluid from any of these groups can be used provided all traces of previous fluids are removed. This is particularly important if a volatile fluid is being replaced by a more phlegmatic one, as the boiler temperature could decompose oil residues of the more volatile type, resulting in bad pumping performance. Pumps made for use with mercury are not suitable for other fluids and vice-versa. This is due to the fact that no provision for fractionation is made in mercury pumps and jet clearances are different. However, a limited performance can sometimes be obtained from a mercury pump charged with organic or silicone fluids. 277

L Laurenson:

Vacuum

fluids

(a)

345

331

317

303

289

275

2bl

247

233

219

I95

181

167

153

139

I25

,,I

97

83

69

(b)

y.

J

A 469

405

353

1

_

374

MI

9.

446

1 329

313

Ltl 297

,I.-

4

370

352

J 229

259

325

310

294

278

rL11.a

.

199

1

260

233

223

184

168

Figure 2. (a) Apiezon C-pressure = 1.7 x’ 10m6 mbar, source temperature = 150°C. (b) Silicone DC704-pressure source temperature = 150°C. (c) Santovac G-pressure = 5 .c lo-’ mbar, source temperature = 170-C.

1. Mineral oils. These are mineral oils which have been carefully distilled to give them suitable vapour pressure characteristics. They contain no anti-oxidants and as a result are prone to oxidation if exposed to air while hot. Mass spectrometric analysis of such oils [Figure 2(a)] show groups of peaks at 14 amu (CH,) intervals which start at the low masses and extend through the range’. However, the oil is so readily broken down by the ion source that the parent peaks arc not discernable. The salient properties of mineral oils are summarized in Table 3 (column I). The attractive aspects are in italics. One of the best known ranges of these oils is the Apiezon range (fluids A, B and C), although near-equivalent oils are offered by a number of vacuum companies. 2. Silicone fluids. The need for fast cycling valveless pumping systems led to the development of silicone fluids suitable for diffusion pump use. Such fluids are robust, being thermally and oxidatively stable with a high resistance to chemical attack. Chemically they are methyl phenyl siloxanes. In the well-known DOW Corning range of DC702, DC704 and DC705, DC704 and DC705 have specific chemical structures, being tetramethyl tetraphenyl trisiloxane and trimethyl pentaphenyl trisiloxane 278

= 1.7 x lo-’

mbar,

respectively, whereas DC702 is made up of various methyl phenyl siloxane chains. The mass spectrum from DC704 [shown in Figure 2(b)] is complex9 but the parent peak can just be seen. When making a choice of pumping fluids silicones can be regarded as one of the first to be considered, as they provide a range of ultimate pressures and critical backing pressures, and are resistant both to thermal degradation and chemical attack. Against this they form insulating polymeric layers when irradiated by electrons. This is obviously a great disadvantage where physical electronic systems are being pumped. Their properties are also summarized in Table 3. 3. Polyphenyl ether. Originally developed as a lubricant for use in space it was introduced as a vapour pump fluid by Hickman in 1961 to and is now marketed as Santovac 511 and Convalex IO:. Although these are processed for vacuum use by different manufacturers the starting material is chemically identical. The fluid is a blend of five-ring meta and meta-para

11Tradename of Monsanto. 7 Tradename of CVC.

L Laurenson:

Vacuum fluids

Table 3. Comparative properties of diffusion pump fluids Fluid

Mineral oils

Silicones

Polyphenyl ether

Perfluoro polyether

Miscellaneous

Ultimate vacuum** at 20°C (mbar) Critical backing** pressure (mbar) Energetic particle bombardment

5 x 1o-5-1o-**

10-~-10-9*

Less than 10m9

3 x 10-6

10-6-S x IO-P*

0.65-0.45’

1.2-0.6*

0.6

1.0

0.65-0.45’

Generally insulating polymers formed

Cotrdacrittg formed

Generally polymers

cottdactittg formed

polymers

No polymers

formed

(except with hydrogen ions)

Thermal stability

Pot-X

Fb-y good

E.WCllCtll

Oxidation resistance Chemical resistance

Poor to fair Poor

Excellent Very good

Relatively low

with a few exceptions

High Mass spectrometry, electron microscopes, ultra-high vacuum

High Electron microscopes, Ion implanters. chemical vapour deposition. pumping aggressive chemicals

Comparative cost Uses

LOW

Electrical, electronics and accelerator applications

but co-polymerized by alkali metal Low to moderate Process equipment, vacuum deposition plant and tv tube processes

Very good

Dccomposrs alotte? Excellent Excellenr

lo gas

Generally polymers

condacring formed

Good Good to very good Good to fair dependent on nature of fluid LOW

Vacuum deposition plants, process equipment and accelerator applications

* Dependent on grade used. ** As obtained in Edwards diffstaks and diffusion pumps. t Decomnosition temnerature is normallv greater than 300°C. Gases formed are toxic. NB. Italics indicate best features of the t?uyd.

isomers of polyphenyl ether. Four-ring compounds have too high a vapour pressure, and six-ring compounds are rather too viscous and would require a very high temperature to provide a satisfactory vapour stream; at normal water cooling temperature they do not flow back to the boiler satisfactorily. Polyphenyl ether has found great use in producing ultra high vacuum due to its very low vapour pressure. Values of less than IO-” mbar at 20°C have been determined independently by the author and other workers.” For physical electronics applications its low vapour pressure, plus the fact that any polymer formed by the interaction of electrons with polyphenyl ether forms a conducting film, makes it an attractive fluid. Again, its properties are listed in Table 3. One of the chief drawbacks of polyphenyl ether is its high initial cost. Due to its low vapour pressure and resistance to breakdown under electron bombardment, the parent peaks are by far the major component of the fluid’s mass spectrum. This also makes it an excellent fluid for use in the evacuation of mass spectrometers. Figure I(c) shows the mass spectrum obtained when polyphenyl ether is fed into the ion source of the mass spectrometer. 4. Perfiuoro polyethers (Fomblin). As the name suggests these are fully fluorinated compounds having only carbon, fluorine and oxygen in their structure. They were developed chemically in the 1960s by Montedison SpA and first proposed for vacuum in 1971r2. They present a different philosophy to vacuum pumping in that energetic particle bombardment does not generally result in polymer formation, only gas. The only exception known to the author is when the bombarding particles are hydrogen ions. Thus if this pumping fluid is not harmful to the vacuum application then there is no need to exclude it from the vacuum. The fluids have the added advantage of being inert to many of the most aggressive chemicals, among them oxygen, halogens and mineral acids4.

The chief use of perfluoro polyethers is in places where polymer growth of any kind is undesirable (an example being in the conventional electron microscope), and in conditions where highly aggressive chemicals are present, such as ion implantation and low pressure CVD. The major properties are listed in Table 3. It is worth noting that in general lower pumping speeds are obtained using these fluids as opposed to other fluid types mentioned. On average the air pumping speeds are 83 o/0 and the hydrogen speeds 66% of those obtained using other fluids. The reason for this is not understood but is believed to be a function either of the high molecular weight or the molecular spread in the cuts used. It should be remembered that this material is a fluorinated compound and as such will, like polytetrafluoroethylene, break up into aggressive compounds if overheated (around 300”C**)r3. The toxicity of the material in its unbroken state is however very low. 5. Miscellaneous fluids. Apart from the foregoing materials a number of other fluids have been found to be suitable as diffusion pump fluids. These have included esters, ethers, sebacates, phthalates and napthalenes. Generally these offer no advantage over the main range of fluids already outlined. HOWever, some are considerably more oxidation resistant than the straight hydrocarbon oils; both hindered esters and napthalene based materials are particularly robust. In addition to the booster fluid ester already discussed, diffusion pump ester fluids are marketed. An example is AP301, which is in fact, at present, the only non-mineral based material in the Apiezon range. Its vacuum performance is similar to the mineral oil Apiezon B but with a very much improved thermal ** The exact temperature is dependent on the conditions. Itt uacuo or in completely inert conditions this could be as high as 350°C but in the presence of certain metals such as titanium, decomposition can start at a temperature below 300°C. 279

L Laurenson:

Vacuum

fluids

I. Apiezon

M.

IO.Smg

2. Apiezon

H.

144mg/~036mm

3. Apiezon

Ii.

139.3mq

f348mm.

4. ApiezonlOO

238mc~

]595mm

5. Apiezon

237mq

/592mm.

12.7mg

/032mm

M.

6. ~iezcolO0.

100

I50

200

TimrIMinsJ

250

300

Figure 3. Vapour emission of high vacuum Apiezon greases. (Reproduced from Laurenson,

x Stop-code ‘3 High

stability and oxidation resistance.Napthalene-basedsynthetic fluids are now becoming available. The author has recently assessed oneof theseproducts.It gavea goodall-round vacuum performancewith an ultimate pressureabove a water-cooled baffle in the 10Wgmbar range and it has good chemicalresistance to many aggressivesubstances,such as halogens,acids and alkalis. Its good vacuum performance, oxidation and chemicalresistancemake it a good choice for many vacuum applications,particularly where siliconescannot be used.This fluid is now commerciallyavailable asEdwardsL9.

The properties of vacuum greaseshave been discussedfully elsewherer4and only a brief summaryof the salient features

vacuum 27, 1977, 431.)

qrewz

wcl~n

Figure 4. Vapour emission of Edwards silicone greases. (Reproduced from Laurenson,

280

/.026mm

gWO%?

Vacuum 27, 1977,431.)

will be given. Greasescan be classifiedin one or other of two ways, either according to their chemical structure-namely mineral, silicone or perfiuoro polyether-based-or by their manufacturingtechnique,which is either by moleculardistillation to produce a homogeneoussolid greaseor by the ‘filling’ of a liquid with a finely-divided powder until a gel is achieved. The first classificationallows one to decidewhether or not a greaseis compatible with the processin question. Obviously it would be unwise to use a mineral greasewhere aggressive chemicalsabound. Greaseshave showndifferencesin vacuumperformancewhich appear to depend on the method of manufacture; vapour emissionfrom molecularly distilled greaseis dependentonly on the surfacearea of greaseexposedto the vacuum, whereasfor filled greasesthe vapour emissionis dependenton the massof

L Laurenson:

Vacuum fluids

grease present. This can be explained as follows: in the molecularly distilled case the vapour emission is only due to the

available for vacuum applications giving the advantages and disadvantagesof the various materials. It is hoped that it will

surfaceof grease‘subliming’at a rate dependenton the vapour pressureof the material (this is shown in Figure 3), but for the filled grease the vapour is escaping from a surface which consistsof fluid and solid, and asa fluid moleculeescapesfrom the surfaceit can be rapidly replacedby one migrating from the bulk. An exampleof the behaviour of a filled greasecan be seen in Figure 4. Thus the vapour emissionwill continue to risewith increasedmassuntil it reachessomevalue dependenton either the saturatedvapour pressureof the liquid above the surfaceor the rate at which the liquid in the bulk can migrate to the surface. If a low enoughvapour pressurefluid is usedin formulating the greasethen the equilibrium condition will occur at such a low vapour emissionthat the massdependenceeffect is not observedexperimentally. In this respectdry mix compounding of low vapour pressure cuts of perfluoro polyether with polytetrafluoroethylene telomershasproducedgreaseswhich do not show a mass dependencewithin the limits of the present measurements.

help new vacuum workers

Conclusions

This paper hasbeenwritten asa review of the fluids and greases

to select the fluids most suitable for

their purpose.

References ’ M A Baker and L Laurenson, Vocrr~~n, 16, 1956. 633. ’ M J Fulker, Vocrnr~~, 18, 1968, 445. ’ M J Fulker. M A Baker and L Laurenson. Vucr,rr~~. 19. 1969. 555. 4 L Laurenson, J Newton and N T M Dennis, Vacur~p;, 2d, 1979; 4331 ’ D Latham. B D Power and N T M Dennis, Vucurrm, 2, 1952, 33. 6 B D Power, High Vncrrr~n PuI?@u~ Eq/lip~~e/~/, p 44. Chapman and Hall, London (1966). ’ C R Burch, Notrrre, 122, 1928, 729. ’ B H Colwell, C’ncr~o~, 10 be published. ’ J S Cleaver and P N Fiveash. Vncurnt~, 20, 1970, 49. ” K C D Hickman, Trans. 2nd lnt Cong Vacuum Technol, p 307. Pergamon Press, 1961. ‘I G Rettinghaus and W K Huber, J I’ucrn~?~ Sci T~~httol, 9, 1972, 416.

I2 M A Baker, L Holland and L Laurenson, Vncr~r~~, 21. 1971, 479. I3 G Caporiccio, R A Steenrod, Jnr and L Laurenson, J Vacuum Sci Tecltnol.

15,

” L Laurenson,

1978,

775.

I/ncrnrn~, 27, 1977, 431.

281