Coating cryopump surfaces with molecular sieve materials A Sands and S M Dick, International Research and Development Co Ltd, Newcastle upon Tyne

Several methods of coating cryopumping surfaces with molecular sieve materials have been developed and tested. Adhesion, life expectancy under thermal cycling, and gas load capacity have been investigated. Plasma sprayed coatings seem attractive. From these initial tests it is concluded that suitable techniques have been evolved for the production of cryopanels varying in complexity from plane surfaces to helical tubular coils.

I. In t roduct ion One of the most recent developments in the general field of pumping by cryogenic techniques has been the exploitation of the properties of the so-called molecular sieve materials. The simple tests described in this paper were performed as part of a development programme aimed at sim0lifying and improving methods of constructing non-standard cryopump elements and arrays.

The paiameters relevant to the application of coated elements to vacuum systems are defined and the ease with which devices may be constructed in the laboratory is demonstrated.

2. Cryopumping Cryogenic pumping in the high and ultrahigh vacuum ranges is effective ff the constituent gases can be condensed at the temperature of the cryopanel. Recent work by Gareis and Hagenback I showed that if pressures of the order of 10--; a torr are to be obtained by cryogenic techniques alone in systems where appreciable quantities of hydrogen, neon and helium may be present, then the principles of cryosorption and cryo- trapping offer practicable mechanisms fol the removal of these gases from the vacuum space and their retention at the cryo- surface. These mechanisms differ from condensation cryo- pumping in that physical adsorption processes are involved.

Metallic substrates coated with synthetic zeolite materials are incorporated in some commercially available cryopump arrays and the manufacturers state that they appear to be most effective in having near theoretical pumping speeds for hydrogen and helium when cooled to 20°K and 5°K respectively.

For a technique apparently offering such attractive prospects a thorough investigation into the possibility of developing more sophisticated devices with probable operating temperatures above 4.2°K is merited. With this in mind a review of the sieve materials with regard to their physical chemistry and the kinetics of the adsorption process is presented.

3. Molecular s ieve materials Molecular sieves are minutely porous crystals of natural or synthetic zeolites, the pores having molecular dimensions. Molecules small enough to enter the pores are adsorbed on the zeolite crystals while larger molecules cannot enter. This porosity is at t r ibutable to the crystal lattice of zeolite which is essentially tetrahedral with some comparatively large cavities

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where water molecules have been driven off after crystallization. The shape of these cavities is determined by the chemical composition of the particular zeolite and the walls of the cavities are composed of ions of silicon and other elements.

Molecules of different gases or vapours have characteristic sizes usually defined in terms of cross section and length, the critical diameter being the factor which decides whether or not the molecule will pass through an aperture to enable sorption to occur. The selection of sieve material is therefore dependent on the surface area, pore size, shape and position or distribution of sorbent, and also on the boiling point, molecular volume, polarization and for organic molecules the degree of unsatura- tion of the sorbate.

In a dynamic vacuum system where leaks and outgassing of components and differential thermal distribution exists, the equilibrium state of gas and sieve will only approximate to that predicted by the relevant isotherm.

Figure 1 (after Barrer2) illustrates the sorption of nitrogen,

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A Sands and S M Dick: Coating cryopump surfaces with molecular sieve materials

argon and oxygen on 5A sieve, that is, molecular sieve with a nominally 5/~ pore size, (this pore is not necessarily circular). At critical and well-defined points (--100°C and --150°C) there are peaks in the adsorption curves for argon and nitrogen respectively. At temperatures lower than these the adsorption rapidly tails off until at liquid temperature it is insignificant.

One of the causes of this behaviour is shrinkage of the crystal apertures to sizes which exclude the nitrogen and argon molecules (which have critical diameters of 3.0/~ and 3.8 A) but permit the continued adsorption of oxygen molecules with a critical diameter of 2.8/~.

Table 1. Specific surface of adsorbents (m2/gm)

Chabazite 750

Mordenite 440

Synthetic zeolites Types NaA and CaA 750-800 Types CaX 1030

Activated Carbon 800-1050

Silica Gel 500-600

Activated Alumina 230-380

Other types of porous glass 100-200

The capacity of sieve materials for various gases is dependent mainly on the specific surface. Table 1 illustrates the specific surfaces for the more common porous materials, synthetic zeolite being the obvious choice of sorbent followed by the activated carbons.

Specific surface for synthetic molecular sieve suitable for removing atmospheric gases from a vacuum system is typically 1000 m2/gm; this together with pore size serves to determine the suitability of the material for a particular application, which may be selective rather than total removal of gaseous or vapour species.

slurry with various sizes of molecular sieve particles and the slurry was applied to the element. The drying technique presented the major problem with this method as any attempt to accelerate drying, such as by exposure to ammonia vapour, resulted in crazing of the surface and reduction in adhesion (Figures 3b and 3c). However, some successful elements were produced by this method and subsequently withstood tens of cycles from 77°K to 373°K, this being the temperature obtain- able with a soldering iron element as a heater for regeneration (Figure 2). The thickest coatings so far have been produced using this method.

The third and fourth methods attempted were variations on plasma spraying techniques. Plain plasma spraying is a process involving the introduction of material in granular form to a high temperature gas stream and its transfer to a substrate where adhesion occurs. The dwell time of the particles in the high temperature region is usually sufficient for total or par~tal melting to occur. Use of molecular sieve material in tla;s manner results in a coating with excellent adhesion but almost totally destroys the unique porous structure. Marginal improve- ments in porosity can be produced by varying the spraying parameters: stream temperature, gas flow, rate of particle introduction, and time of flight.

The final method, a modification of the above technique, involved the introduction of a secondary stream of molecular sieve particles immediately before the substrate. The resultant coating is a mixture of porous molecular sieve and material with no sieve properties acting as a binding agent. The typical appearance of such a coating is shown in Figure 3d.

Qualitative performance assessments for the four methods of coating were made by first flushing the chamber several times with dry gaseous nitrogen effluent from a storage dewar, finally allowing a pressure of 100 microns to build up. The chamber was then isolated from both pump and flushing line, and liquid nitrogen was introduced to the cryopump element.

The pressure in the test chamber (volume 1500 cc) was moni- tored by a Pirani gauge and recorded at suitable intervals for up to three minutes at which time no furthm reduction in pressure could be observed. The rate of introduction of liquid nitrogen was the same in each case.

4. Experimental coating of surfaces The object in performing the following tests was to establish the feasibility of coating a cryopanel more complex than commer- cially available elements. The test element was a stainless steel tube crimped and welded at one end and fitted with stainless steel Edwards coupling at the other, the overall length being 5~ in. (Figure 2). The Edwards coupling was used inside out with regard to the vacuum and utilised a Viton "O" ring at near liquid nitrogen temperatures.

The available literature indicated that various resins were being used to coat surfaces with sieve particles a. Initially a two- phase Viton coating process using materials with the trade names Adcora V and Adcora VT was chosen 4. One of these paints acts as an undercoat to give good adhesion, the other as the adhesive for the sieve particles. No trouble was experienced in coating tubes with various sizes of granules, Figure 3a being a typical example.

The second method, tried in an attempt to avoid organic adhesives, was to use an investment casting technique on both smooth and artificially roughened stainless steel surfaces. Silica sols (Pyramol A30 and F30) 5 were used to prepare a Figure 2.

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A Sands and S M Dick: Coating cryopump surfaces with molecular sieve materials

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A Sands and S M Dick: Coating cryopump surfaces with molecular sieve materials

S. Apparatus Figure 4 illustrates the equipment in which the assessments of the sieve properties after processing were made. An Edwards 1S/50 rotary pump and a 203B oil diffusion pump were used to evacuate the test chamber. This chamber was a modified phos- phorous pentoxide trap and proved ideal for the purpose, having a suitably placed window for observations to be made of any deterioration of the elements during thermal cycling. Initial conditioning and regeneration of the elements was accomplished using a 65 watt soldering iron element plasma- sprayed with a coating of alumina for electrical insulation.

6. Results Of the four types of coated elements tested, three produced identical base pressures, Figure 5. Similar, but not identical amounts of 13X grade molecular sieve were used in each case. Thus it appeared that the final pressure obtained was an equilibrium pressure for the sieve material under the given experimental conditions and independent of the coating method. To ensure this effect was not due to saturation of the material, further quantities of nitrogen were added and the same base pressures reached without regeneration of the element.

Saturation did occur, however, in the case of the plain plasma sprayed element.

7. Conclusions There are many methods of coating cryopump surfaces with molecular sieve materials and the main problems in their application to vacuum systems are adhesion under thermal extremes and contamination of the residual atmosphere in the case of organic or volatile adhesives.

The plasma spraying process when modified to obtain optimum retention of sieve properties seems to offer an attrac- tive solution since no contaminants are involved and the final surface exhibits good durability and adsorption efficiency.

Effort is currently directed to improving the plasma spraying process and proposed further work includes a study of the adsorption rate and a more detailed examination of the coating structure.

Acknowledgements The assistance of M Winch in conducting the plasma spraying and of R Browell in performing some of the tests is gratefully acknowledged.

References 1 p j Gare i s a n d G F H a g e n b a c k , Industrial and Engineering Chemistry, 57 (5), May 1965, 27-32. z R M Barre r , British Chemical Engineering, 4 (5), May 1959, 267-279. 3 R S Narc is i et al , Trans 9 A VS Nat Vac Syrup, 1962, 232. 4 Private communication with manufacturers. E and F Richardson Ltd, Buckingham. 5 Private communication with manufacturers. Joseph Crossfield & Sons Ltd, Warrington.

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