124 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



it was machined from the package by cutting the copper extension to 

 length, leaving the second groove for use in brazing the seal to the re- 

 peater and removing the container tubing and the support members. 



The basic seal consists of the cup, central conductor and glass. The 

 cup (smaller cylindrical item in the upper lefthand corner of Fig. 4) 

 was machined from Kovar rod. The wall of the cup is tapered from a 

 thickness of 0.025 inch at the base to 0.002 inch at the lip. The last O.OOG 

 inch of the lip is further tapered from this 0.002 inch to a razor edge. 

 The internal surface is better than a 63-micro-inch turned finish and 

 was also liquid honed to give it a uniform matte finish. The central con- 

 ductor (slim piece in the upper right-hand corner of Fig. 4) was also 

 machined from Kovar rod. Both the cup and central conductor were 

 further processed by pickling, hypersonically cleaning in deionized water, 

 and decarburizing. The glass, a borosilicate type of optical quality, was 

 cut from heavy walled tubing. The glass tubing was hand polished, 

 lapped and etched to remove surface scratches, and to arrive at the spe- 

 cified weight. It was also fire polished and hypersonically cleaned to 

 remove all traces of surface imperfections and to assure maximum clean- 

 liness. 



In order to make the basic glass seal, the metal parts had to be oxidized 

 under precisely controlled conditions. For the oxidizing operation, a 

 suitable fixture was loaded with brazed shell-cup assemblies, central 

 conductor assemblies, and a Kovar disc, which had been prepared in 

 precisely the same manner as the cups and central conductors. The 

 disc was carefully weighed before and after oxidizing and the increase in 

 weight divided by the area involved yields the weight gain due to oxida- 

 tion for each run. Limits of 1.5 to 2.5 milligrams per square inch of oxide 

 were set. This operation was performed by placing the loaded, sealed 

 retort, through which passed a metered flow of dried air, into a furnace 

 for a specified time-temperature cycle. 



In the glassing operation the oxidized shell assembly, the carbon mold 

 and the central conductor were placed in a fixture and held in the proper 

 relationship. The carbon mold served to support the glass, while it was 

 being melted, in that section between the cup and central conductor 

 where the glass was normally unsupported. The prepared cut glass tubing 

 was loaded into the Kovar cup and the fixture was sealed into the retort. 

 During the glassing cycle, a constant flow of nitrogen passed through 

 the retort to provide an atmosphere Avhich minimized any reduction or 

 further oxidation of the already carefully oxidized parts. After the proper 

 purging period, the retort was placed in the furnace. In the furnace, the 

 glass melted and formed a bond with the oxidized Kovar of the cup and 



