US 2604229 A
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July 22, 1952 E. scHwARz cLosURE SEAL FOR VACUUM vassms Filed March 14, 1949 gaining!! :l
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Patented July 22, 1952 CLOSURE SEAL FOR VACUUM VESSELS Ernst Schwarz, London, England, assigner to Hilger & Watts, Limited, London, England, a company of Great Britain Application March 14, 1949, Serial No. 81,381 In Great Britain March 25', 1948 (Cl. 22B-81) Claims.
A problem frequently arising in the physical laboratory and in industry is that ci' fixing a window of some special material to a vessel which has to be evacuated to a certain degree. Examples of such vessels are the envelopes of X-ray tubes and discharge lamps and apparatus for producing and measuring infra-red rays. The main part of the vessel may be of metal or glass, but the window must be of some material capable of transmitting the particular radiation concerned. In the case of 1 infra-red transmitting apparatus especially* the vmost suitable material known at present for the window is an alkali halide such as rock salt or potassium bromide.
The invention relates to a vacuum seal suitable for any window material including such fragile materials as those mentioned. In the method known hitherto the window was sealed by a cement of low vapour pressure such asl that known as W-wax. This cement softens at about 50 C.- and is fairly liquid at 80 C. The vessel and the window were therefore heated up to about 80 C., the cement was applied, and the parts so iixed together were cooled down slowly to room temperature. j
The disadvantage of this method is that the vessel cannot be degassed satisfactorily while the vacuum is maintained inside, since the temperatur'e to which the apparatus can be safely heated is limited by the softening point of the cement, viz., 50 C. for W-wax. On the other hand, the degassing of glass, i. e., the removal of the absorbed layer of water and gas from the surface of the glass, requires a temperature not less than 150 C. and preferably higher. The temperature limitation imposed by the properties of W-wax results in the vacuum inside the vessel deteriorating with time. I
It is possible to iind cements which have a low vapour pressure and a higher softening temperature, such as silverlchloride and some high polymers. The use of such cements involves another diiiiculty however. To apply the cement it is necessary to heat at least the parts near the seal to a temperature above the softening point of the cement. A high enough temperature has to be applied after assembly for degassing. At the temperatures necessary for these two processes difliculties arise from the difference in co-efcient of expansion of the materials of the vessel and the Window. The strain set up by differential contraction on cooling usually causes the weaker member of the assembly to break.
For instance, lead glass has a coeflicient of expansion of 9.5 6 per C'. and potassium bromide one of L1x10-5. If then a window of potassium bromide is cemented direct to an opening in a lead glass vessel and is subjected in the fixing or in subsequent degassing to a temperature above 100 C., the bromide crystal invariably breaks, however slowly cooling takes place` According to the present invention the window is supported on the peripheryof an aperture in the vessel in such a manner as to withstand the mechanical stresses due to the internal vacuum and to permit the window to expand and contract independently of the vessel, and ther vacuum seal is effected by a foil of. yieldableV metal cemented to the outside of the window and tothe parts of the vessel immediately surrounding. the window. The cement is placed in the form of two closed curves one within the other, and the part of the foil between the curves is supported against external atmospheric pressure by the edgesv of the window andthe adjacent parts of the vessel.
To facilitate relative movement between the window and the vessel during temperature changes it is advantageous, especially with glass vessels, to place a metal washer between the window and the vessel.
The foil should bethick enough to provide an l effective vacuum seal and to withstand the stresses due to pressure differences, but it should not'be so thick .as-to hinder the relativel movement of the window and vessel during temperature changes. Thepreferred metal for the foil in most cases are silver and aluminum. ATin is alsosuitable though not quite so much so as silver. Sometimes the vessel is required to contain a liquid;
if so, the foil should be of a metal not attacked by' the liquid, which may for instance involve the use of gold or platinum. Y
The invention is illustrated by two forms lof construction shown in the accompanying drawing, in which I Figure 1Y is a face view partly broken away of a seal according to the invention applied to a window on the side of a glass tube;
Figure 2 is a transverse section through the same, taken on the line II-II of Figure 1,
Figure 3 is a longitudinal section through the same taken on the line III-III of Figure 1, and
Figure 4 is an axial section through a part of a glass tube having a window on the end with a seal according to the invention.
The representations in the drawing are not to scale, since it has been necessary for the sake of clarity greatly to exaggerate some of the thicknesses and distances. Some dimensions will be given in the text where advisable for the guidance of constructors.
Referring iirst to Figures 1, 2 and 3, a cylindrical glass tube I having an outside diameter of 13 mm. has a portion ground away at the side to a plane surface parallel with the axis. The length of the recess so formed is 30 mm. and the Width between the intersections of the section plane with the outside surface of the tube is 10 mm. The corresponding width on the inside surface is 7 mm.
A steel washer 2 is placed on the aperture. This washer is 0.2-0.5 mm. thick and rectangular in plan measuring 29 by 9 mm. outside, the width of the washer being 1.5 mm. all round. A rectangular window 3 measuring 20 by 8 mm. and 1 mm. thick is placed on the washer 2. The seal is completed by placing on the assembly a rectangular piece of silver foil with rectangular aperture therein measuring 26 by 6 mm. The part of the foil 1 to 1.5 mm. wide overlapping the marginal part of the outer face of the window .3'iscemented thereto by silver chloride or like cement, while the marginal part of the foil is similarly cemented to the outer cylindrical surface of the tube I to a width of 1 to 2 mm.
'I'he washer 2 forms a supportfor the window 3.against atmospheric pressure and permits free expansion and contraction in relation to the tube I, the friction being less than if the window were placed direct on the ground surface of the glass tube I. Moreover, the end portions of the Washer 2 form bridges to support the window and prevent it from bending under heat and pressure to anarcuate transverse section. The foil 4 is mechanically supported on the tube I and window. 3 and yields suiiiciently to permit expansion and contraction of the window with substantially no. constraint.
Figure 4 shows a seal devised on the same principle but applied to the circular end of a cylindrical glass tube. The parts have corresponding functions to those of the parts shown in Figures 1, 2 and 3 but diier therefrom in shape and dimensions. The tube 5 is thickened towards vthe end at 6 by decreasing the internal diameter and a cylindrical recess'l is ground therein of a diameter to receive the window 8 with small clearance and of depth to bring the outer face of the Window iiush with theend of the tube 5 when a circular metal Washer 9 is interposed between the shoulder 'I in the tube and the window. The sealing foil I0 is a fiat ring, being sealed to the marginal part of the outer face of the window 8 and to the end face of the tube 5.
1. A seal for a window of a vacuum vesse1 in which the window is supported on the periphery of an aperture in the vessel, the dimensions of said window being such as to provide a clearance gap between its edges and the adjacent edges of said aperture, to permit the window to expand and contract independently of the vessel, the
said seal being in the form of a flexible metal foil overlying the outer face of the window near the periphery thereof and the outer face of the parts of the vesse1 immediately surrounding the window, said metal foil being cemented to respective adjacent outer faces and yieldably bridging said gap.
2. A seal as defined in claim 1 comprising also a metal washer approximately matching the contour of the window and interposed between the Window and the periphery of the aperture in the vessel.
3. A seal as defined in claim 2 in which the periphery of the aperture in the vessel does not provide uninterrupted support for the window and the metal washer is made rigid enough to prevent the window from bending at the unsupported parts under external pressure.
4. A seal for a window of a vacuum vesse1 in which the window is supported on the periphery of'an aperture in the vessel, the dimensions of said window being such as to provide a clearance gap between its edges and the adjacent edges of said aperture, to permit the window to expand and contract independently of the vessel, the said seal being in the form of a flexible silver foil overlying the outer face of the window near the periphery thereof and the outer face of the parts of the vessel immediatelyv surrounding the window, said foil being cemented to respective adjacent outer faces and yieldably bridging said gap.
5. A seal for a window of a vacuum vessel in which the Window is supported on the periphery of an aperture in the vesse1, the dimensions of said window being such as to provide a clearance gap between its edges and the adjacent edges 0I said aperture, to permit the Window to expand and contract independently of the vessel, the said seal being in the form of a iiexible aluminum foil overlying the outer face of the window near the periphery thereof and the outer face of the parts of the vessel immediately surrounding the window, said foil being cemented to respective adjacent outer faces and yieldably bridging said gap.
REFERENCES CITED i The following references are of record in the le of this patent:
UNITED STATES PATENTS Number Name Date 261,263 Seymour July 18, 1882 2,172,864 Calva et al Sept. 12, 1939 2,178,826 Bowie Nov. 7, 1939 2,202,470 Schedel May 28, 1940 2,373,720 Stupakofl' Apr. 17, 1945 2,394,984 Claussen Feb. 19, 1946 2,396,320 Gaudenzo et al Mar. 12, 1946