Glass-to-metal seal

Tungsten and molybdenum, which have been used extensively for metal-to-glass seals, suffer from the disadvantages of being expensive... 

Metal-to-Glass Seals using Kovar Tubing... 

The filler tube is next attached, followed by the gas outlet tube. The metal-to-glass seals with the electrodes are put in last. 

One of the most satisfactory metal-to-glass seals involves an Fe-Ni-Co alloy called Kovar. The coefficient of expansion of this alloy is close to that of Pyrex, to which it can be directly bonded however, an even better match in expansion coefficients is obtained with a special borosilicate glass (Corning 7052, Kimble EN-1 or K-650). A variety of tubes is available commercially involving a Kovar tube sealed to a Pyrex tube through one or more intermediate glasses. Die properties of Kovar are discussed in Appendix IV under iron (Section IV.I.E). 

Use Coating nuclear fuel rods, corrosion-resistant alloys, photo flashbulbs (foil), pyrotechnics, metal-to-glass seals, special welding fluxes, getter in vacuum tubes, explosive primers, acid manufacturing plants, deoxidizer and scavenger in steel manufacturing, laboratory crucibles, spinnerettes. 

Teflon front ferrules in Swagelok fittings provide leak free glass to copper seals and eliminate the need for more expensive and fragile Kovar or other metal to glass seals. 

A simple sealed-tube sample holder is shown in Figure 6.2(g). Other sealed-tube sample and reference holders are shown in Figure 6.16 (59). In one case, (a), the thermocouples were sealed directly into the tubes, and this presented problems with metal-to-glass seals. Heat transfer from the sample to the sensing thermocouple is fairly low in most of the examples given. The system in (e) is much better than most of the others because the... 

Fig. 2. Some tantalum seals and apparatus. (A) Crimped and welded (B) capped and welded (C) apparatus for biphasic, isopiestic equilibrations (D) thermal analysis container (E) tantalum tube to be welded under vacuum [tube is brazed to metal-to-glass seal and closed with joint and stopcock at top) not shown)] (F) sublimation container with reservoir or condenser swaged or flared to fit (G) reaction container sealed in silica jacket under vacuum.

Use of the evacuated apparatus (style E) may improve some transport reaction rates. The tantalum portion is attached to a glass joint and stopcock by a brazed metal-to-glass seal. The flat part can be produced by a pair of vise blocks which are driven together by tightening bolts through them while the apparatus is attached to the vacuum line. The evacuated and closed container is then welded from the side at the flat portion and cut off above the weld. The same blocks may be utilized within the dry box instead of a vise. 

Medium high-pressure g.I.c. does require more sophisticated apparatus as is shown in the flow diagram of Figure 4. It must include a high-pressure injector, pressure-control values, special metal-to-glass seals, and a flow meter capable of operating at pressures of 1200 kPa. These will be discussed later in this section. 

The second and later version of the J-tube, which was used for mixtures was similar except that a demountable glass-to-metal-to-glass seal was incorporated in the sealed limb of J-tube. The glass-to-metal-to-glass seal was removed from the sealed capillary and the appropriate mixture placed in the sealed tip. The demountable coupling was reassembled and the degassing procedure was then identical to that used in the first version. 

Corrosion of Low Coefficient of Expansion Alloys in Metal-to-Glass Seals... 

Iron-nickel-cobalt alloys with low coefficients of thermal expansion are commonly used for metal-to-glass seals and for encapsulating optical elements for use in undersea applications. However, these alloys are subject to stress corrosion cracking in humid atmospheres in the presence of a tensile stress, which may be residual from wire drawing or machining or it may be applied during device processing in operations... 

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