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Lectures on industrial applications of high vacuum technology
Vaouunz,
Vol. II
0.5% by dry weight of the product. Undoubtedly there are many substances which show better keeping qualities with increased dryness. On the other hand there is now evidence that it is possible that some biological materials can be spoilt by overdrying. Suspensions of bacteria fall into this category. Methods must be employed, therefore, which limit the final residual moisture content of the material. Hitherto this has been achieved biochemically by including a polysaccharide in the suspending medium, in which the organisms are dried. This method is somewhat haphazard and a more precise method of obtaining a predetermined residual moisture content has been evolved. In the suggested method the dried product is exposed to a system containing water vapour at a predetermined saturation pressure until its pressure equilibrates with the latter. Research work on these lines will be helped by a recently devised vacuum procedure of determining residual moisture content by means of equilibrium vapour pressure.
Glass-to-Metal Seals for High Vacuum Apparatus. Presented by : 0. Adams. Although much has been written and many theories proposed to explain the mechanism by which vitreous enamels adhere to iron, comparatively little has been published on the allied problem of the adherence of glass to metal. Glass-to-metal seals are usually made in air at high temperatures and thus the metal oxidises during the process unless precautions are taken to prevent this. Consequently glass is separated from the metal by a layer of oxide and most glass-to-metal joints have a characteristic colour due to the reaction of the oxide layer and the glass. This oxidation of the metal would seem to be necessary for the formation of a good seal between glass and metal since A. W. Hull and E. E. Burger1 have stated that molten glass will not ‘ wet ’ the surface of clean metal in an atmosphere of hydrogen, pure nitrogen or carbon dioxide. The glass did spread rapidly over the surface however if the metal was oxidised. However it has been possible to make alkali-free glasses adhere to tungsten by heating in an atmosphere of dry hydrogen. The resulting seals appear to be free from any oxide but it has not yet been possible to prove that this is so. J. H. Partridge2 has stated that the surface of iron
386
October. 1952
No. 4
became slightly oxidised when heated in the presence of molten enamel in carbon dioxide or nitrogen at low pressure and that a slight visible oxide film was sufficient to promote adhesion. Metal oxides are quite soluble in glass at sealmaking temperatures and thus adherence between metal oxide and glass is usually very good. Mechanical strength tests on seals have shown that fractures occur either in the glass near the joint or at the interface between metal and metal oxide, with oxide adhering to the glass. Fracture seldom occurs in the oxide layer itself unless the layer is too thick. This the nature of the metal oxide and its adherence to the parent metal are of great importance in glassto-metal seals. For example, the oxide layer on 50150 nickel iron alloy is mainly ferric oxide with a phase of the magnetite type containing nickel. During the manufacture of a glass-to-metal seal, however, the oxide appears to change and the only oxide detectable in the final seal is magnetite. There is a considerable expansion difference between magnetite and the original ahoy so that adherence between the two is poor. For this reason, it is usual for nickel iron to be plated with copper before being sealed into glass. The copper is then coated with a thin film of fused borax (as is usuai with copper-glass seals) to prevent excessive oxidation during seal manufacture. The greenish oxide layer on 26% chromium iron, however, appears to consist of a mixture of chromic and ferric oxides and the thermal expansion of such a mixture (between 8 and 10 x 10m6)per “C depending on the proportions) is not greatly different from that of the alloy itself or the glass which is sealed to it. Glass adheres firmly to alloys of this type. These are but two examples of one aspect of the problem of adhesion between glass and metal and much more information is required before the mechanism of this adhesion can be satisfactorily explained. L
Physics, 5. 1934. 384-405. * PARTRIDCL?, J. H., Glass-to-Metal Technology, Sheffield, 1949).
Se&,
(The
Society
of
Glass
Vacuum Technology in the Valve and Cathode Ray Tube Industry. Presented by : R. 0. Jenkins. The subject of vacuum technology in the valve and cathode ray tube industry is covered in three lectures. The first lecture introduces the subject and then deals
Octobor, 1952
Vac,uttm,
with the properties of the various materials used in valves and cathode ray tubes and their pre-treatment. In particular the properties of pure tungsten, thoriated tungsten and oxide coated cathodes are discussed and how these properties affect the vacuum techniques that can be used during processing valves. Vacuum and hydrogen cooking of components and getters of the dispersed and non-dispersed types are described. The vacuum properties of hard and soft glasses and treatments usually employed are also detailed. The second lecture covers the evacuation of transmitting valves and cathode ray tubes. Suitable vacuum systems are described and processing schedules for transmitting valves from the high
Personal Data ADAMS, O., Research Laboratories, The General Electric Co. Ltd., Wembley, Middlesex. BECKETT, L. G., Research Laboratories, W. Edwards & Co. (London) Ltd., Manor Royal, Crawley, Sussex. HOLLAND, L., Research Laboratories, W. Edwards & Co. (London) Ltd., Manor Royal, Crawley, Sussex. JENKIN$~ R. O., Research Laboratories, The General Electric Co. Ltd., Wembley, Middlesex.
Vol. I I No. 4
power type tungsten cathode down to low power oxide cathode types are discussed, and the general principles on which they are based. Treatment of valves with various gas fillings is also covered. The processing of various sizes of cathode ray tubes is dealt with similarly. In the third lecture, vacuum systems for bench pumping of receiving valves are described, together with typical evacuation schedules. The extension of this to mass-production systems and vacuum schedules is finally outlined. Editors Note. T h e s u m m a r i e s o f t h e p a p e r s h a v e b e e n compiled by the lecturers themselves and have been r e c e i v e d b e t w e e n J u l y a n d D e c e m b e r 1953 .
of Authors
LECK, J. H., Dept. Electrical Engineering, University of Liverpool, Brownlow Street, Liverpool, 3M~TSON, G., Research Branch, Post Office Engineering Dept., Dollis Hill, London, N.W.I. RIDDIFORD,L., Physics Dept., The University, Edgbaston, Birmingham 15. ZEHDEN, W., Hilger Div., Hilger & Watts Ltd., 98, St. Pancras Way, Camden Road, London, N.W.r.
a87
Vol. II
0.5% by dry weight of the product. Undoubtedly there are many substances which show better keeping qualities with increased dryness. On the other hand there is now evidence that it is possible that some biological materials can be spoilt by overdrying. Suspensions of bacteria fall into this category. Methods must be employed, therefore, which limit the final residual moisture content of the material. Hitherto this has been achieved biochemically by including a polysaccharide in the suspending medium, in which the organisms are dried. This method is somewhat haphazard and a more precise method of obtaining a predetermined residual moisture content has been evolved. In the suggested method the dried product is exposed to a system containing water vapour at a predetermined saturation pressure until its pressure equilibrates with the latter. Research work on these lines will be helped by a recently devised vacuum procedure of determining residual moisture content by means of equilibrium vapour pressure.
Glass-to-Metal Seals for High Vacuum Apparatus. Presented by : 0. Adams. Although much has been written and many theories proposed to explain the mechanism by which vitreous enamels adhere to iron, comparatively little has been published on the allied problem of the adherence of glass to metal. Glass-to-metal seals are usually made in air at high temperatures and thus the metal oxidises during the process unless precautions are taken to prevent this. Consequently glass is separated from the metal by a layer of oxide and most glass-to-metal joints have a characteristic colour due to the reaction of the oxide layer and the glass. This oxidation of the metal would seem to be necessary for the formation of a good seal between glass and metal since A. W. Hull and E. E. Burger1 have stated that molten glass will not ‘ wet ’ the surface of clean metal in an atmosphere of hydrogen, pure nitrogen or carbon dioxide. The glass did spread rapidly over the surface however if the metal was oxidised. However it has been possible to make alkali-free glasses adhere to tungsten by heating in an atmosphere of dry hydrogen. The resulting seals appear to be free from any oxide but it has not yet been possible to prove that this is so. J. H. Partridge2 has stated that the surface of iron
386
October. 1952
No. 4
became slightly oxidised when heated in the presence of molten enamel in carbon dioxide or nitrogen at low pressure and that a slight visible oxide film was sufficient to promote adhesion. Metal oxides are quite soluble in glass at sealmaking temperatures and thus adherence between metal oxide and glass is usually very good. Mechanical strength tests on seals have shown that fractures occur either in the glass near the joint or at the interface between metal and metal oxide, with oxide adhering to the glass. Fracture seldom occurs in the oxide layer itself unless the layer is too thick. This the nature of the metal oxide and its adherence to the parent metal are of great importance in glassto-metal seals. For example, the oxide layer on 50150 nickel iron alloy is mainly ferric oxide with a phase of the magnetite type containing nickel. During the manufacture of a glass-to-metal seal, however, the oxide appears to change and the only oxide detectable in the final seal is magnetite. There is a considerable expansion difference between magnetite and the original ahoy so that adherence between the two is poor. For this reason, it is usual for nickel iron to be plated with copper before being sealed into glass. The copper is then coated with a thin film of fused borax (as is usuai with copper-glass seals) to prevent excessive oxidation during seal manufacture. The greenish oxide layer on 26% chromium iron, however, appears to consist of a mixture of chromic and ferric oxides and the thermal expansion of such a mixture (between 8 and 10 x 10m6)per “C depending on the proportions) is not greatly different from that of the alloy itself or the glass which is sealed to it. Glass adheres firmly to alloys of this type. These are but two examples of one aspect of the problem of adhesion between glass and metal and much more information is required before the mechanism of this adhesion can be satisfactorily explained. L
Physics, 5. 1934. 384-405. * PARTRIDCL?, J. H., Glass-to-Metal Technology, Sheffield, 1949).
Se&,
(The
Society
of
Glass
Vacuum Technology in the Valve and Cathode Ray Tube Industry. Presented by : R. 0. Jenkins. The subject of vacuum technology in the valve and cathode ray tube industry is covered in three lectures. The first lecture introduces the subject and then deals
Octobor, 1952
Vac,uttm,
with the properties of the various materials used in valves and cathode ray tubes and their pre-treatment. In particular the properties of pure tungsten, thoriated tungsten and oxide coated cathodes are discussed and how these properties affect the vacuum techniques that can be used during processing valves. Vacuum and hydrogen cooking of components and getters of the dispersed and non-dispersed types are described. The vacuum properties of hard and soft glasses and treatments usually employed are also detailed. The second lecture covers the evacuation of transmitting valves and cathode ray tubes. Suitable vacuum systems are described and processing schedules for transmitting valves from the high
Personal Data ADAMS, O., Research Laboratories, The General Electric Co. Ltd., Wembley, Middlesex. BECKETT, L. G., Research Laboratories, W. Edwards & Co. (London) Ltd., Manor Royal, Crawley, Sussex. HOLLAND, L., Research Laboratories, W. Edwards & Co. (London) Ltd., Manor Royal, Crawley, Sussex. JENKIN$~ R. O., Research Laboratories, The General Electric Co. Ltd., Wembley, Middlesex.
Vol. I I No. 4
power type tungsten cathode down to low power oxide cathode types are discussed, and the general principles on which they are based. Treatment of valves with various gas fillings is also covered. The processing of various sizes of cathode ray tubes is dealt with similarly. In the third lecture, vacuum systems for bench pumping of receiving valves are described, together with typical evacuation schedules. The extension of this to mass-production systems and vacuum schedules is finally outlined. Editors Note. T h e s u m m a r i e s o f t h e p a p e r s h a v e b e e n compiled by the lecturers themselves and have been r e c e i v e d b e t w e e n J u l y a n d D e c e m b e r 1953 .
of Authors
LECK, J. H., Dept. Electrical Engineering, University of Liverpool, Brownlow Street, Liverpool, 3M~TSON, G., Research Branch, Post Office Engineering Dept., Dollis Hill, London, N.W.I. RIDDIFORD,L., Physics Dept., The University, Edgbaston, Birmingham 15. ZEHDEN, W., Hilger Div., Hilger & Watts Ltd., 98, St. Pancras Way, Camden Road, London, N.W.r.
a87