Seals, graded

This is made by the General Electric Co., and is used for medical ampoules. It loses very little alkali to aqueous solutions. The linear expansion coefficient is 7-3 x 10 from 20 to 350°C. This is intermediate between the soft and hard glasses, and M.6. is sometimes used in graded seals. 

Graded seal, 101 Graded seal glasses, 25 Grinding glass, 40,45,82,141 Grinding wheel, 45... 

Fig. 3.20. Common conductivity probe for vacuum work. A couplings to shielded cables from conductivity meter 5 a B. 19 or B.24 socket fitting onto cone on observation/reaction vessel C graded-seal soda glass to borosilicate glass D shielded leads spot-welded to thick Pt wires E sealed through the soda glass probe and held together by the lead glass bead F.

Fig. 3.25. The Pask-Nuyken device for measuring simultaneously the conductivity and the UV spectrum of a reaction mixture. A mixing chamber, B conductivity cell with jacket, C graded-seal borosilicate glass-soda glass, D jacketed quartz cell, E copper leads to platinum electrodes Pt, F graded-seal borosilicate glass-quartz.

Fig. 3.26. The Holdcroft-Plesch observation vessel to find the conductivity and the UV spectrum of a solution (a) plan, (b) elevation. A mixing vessel also serving as conductivity cell, and burettes, Q, Q, Q PTFE taps, D Pt wire electrodes fastened together by a lead-glass bead, E terminals from screened cables leading to D, F graded-seal soda glass-Pyrex, G cone for connection to vacuum and supply lines, 1 cm and 0.1 cm quartz cells.

Rogier, R., Pernot, F., (1991), Glass-ceramic metal composites for making graded seals in prosthetic devices , J. Mat. Sci. Med., 2, 153-161. 

Graded seals can be used to join not only different borosilicate glasses, but different types of glasses (and even metals) of different thermal coefficients of expansion. Obviously, it is most efficient and desirable to select glasses and metals with close thermal coefficients of expansion, because it reduces the amount of stress with which the glassblower must deal. 

W. Silica window attached to horosilicate glass envelope with a graded seal... 

The thermal expansion also depends on the composition and varies (from 3.3 x 10 per °C for ordinary borosilicate glass (widely used for laboratory glassware) to 7.2 x 10 per °C for speeial types used in graded seals. The annealing temperature, depending on composition, is from 510°C to 600°C. 

Should it be necessary to join two glasses together whose expansion coefficients differ by more than 1.5 x 10 per °C, a number of intermediate glasses must be used. Such a series of joins is known as a graded seal and is used to seal metals to otherwise unsuitable glasses, or to join silica tubes to borosilicate or soda glass tubes. 

Unless graded seals are required in reasonably large numbers, or for unusual purposes, it is more economical to buy them ready-made, rather than to maintain a stock of the intermediate glasses. These glasses have limited application and are expensive. 

Table 4 gives graded seal glasses and is taken from the makers catalogues. 

---