Abstract
Sapphire-to-niobium and -tantalum metal seals were formed by using a crystalline oxide solder, and the interfaces between the materials were examined by optical microscopy and electron probe microanalysis. The results are summarized as follows:
(1) Vacuum tight seals were formed by the technique.
(2) The oxide solder composed of crystals of CaO, Al2O3, MgO and B2O3 turned into glass-ceramic composites by sealing operation. The solder of niobium metal seal showed optical microscopic appearances different from those of tantalum, but their major crystalline constituent phases were similarly 3CaO⋅Al2O3, CaO⋅Al2O3 and MgO⋅Al2O3 for niobium and tantalum metal seal.
(3). Diffusion of Nb into the solder was different from that of Ta. Ta concentration was relatively higher than Nb concentration up to 10μm from the interface. This is reflected by dissimilar microscopic appearances in the vicinity of interfaces.
The presumption that this difference could be explained by difference in oxidation between niobium and tantalum metal during sealing was confirmed by thermochemical calculations.
(4) B2O3 in the solder was very effective in improving adhesion and yield.
(5) At the solder-sapphire interface, a transition layer was formed by the diffusion of sapphire into the melted solder.
(6) At the solder-metal (niobium or tantalum metal) interface, the metal surface was oxidized by oxygen which was presumed to have originated from the ambient atmosphere and other sources. These metal oxides thus formed diffused into the melted solder to form transition layers.
(7) The transition layers at the interfaces are believed to be useful in reducing sealing stress and in improving adhesion.
