1. Field of the Invention
The invention pertains to a method for the manufacture of stems for vacuum tubes making it possible, in particular, to improve the quality of the seal between metallic bushings and the glass of the stem. The invention also pertains to a stem obtained by the method of the invention.
2. Description of the Prior Art
The proper functioning and longevity of a vacuum tube, for example a cathode tube, are related to the preservation of the vacuum made in the tube and, consequently, are related to the quality of the imperviousness of the metallic bushings which electrically connect the inside of the tube to its outside through a glass stem.
The imperviousness of the bushing or bushings depends on the quality of the metal-glass bond, the bond being due to the dissolving, in the glass, of the metallic oxide formed on the surface of the metal or the conductor used to make the bushing.
Generally, the metallic bushing is made with a metallic wire (ferro-nickel lined with copper for example), and it is the dissolving of copper oxide in glass which is liquid or almost liquid that gives a precise wetting of the glass on the copper of the conductor. This dissolving or diffusion of metal oxide takes place all the more efficiently as the temperature of the glass is higher.
For series manufacture, the stems are usually made on an indexing table type transfer machine that has a number of molds set at the edge of the table. The rotation of the table makes the molds go past several different positions where they undergo an operation before going on to a following position. These operations are, for example, loading, pre-heating, molding, etc. The mold comprises a hollow mold (called the bottom mold) with a certain number of bores intended to receive conductors set in a circle. The bottom mold is loaded with the conductors and then loaded with cylindrical glass pieces one of which is placed inside the laying-out circle and the other outside it. The set is then heated, generally by means of burners, firstly in order to provoke the appropriate reaction at the surface of the conducting metal to make the said metal oxidize and, secondly, to carry the glass pieces, in a substantially uniform way, to a temperature known as a working temperature which is close to the melting temperature.
The metallic conductors are then embedded in molten glass by pressing the glass pieces between the bottom mold and a top mold. The result of this process, after cooling, is a pre-formed stem. This preliminary stem is then re-heated and re-molded to obtain a final part with a given geometry.
It is observed that a considerable number of stems made in this way have imperviousness defects at the metallic bushings, and the author of the present invention attributes these defects to a lack of homogeneity in the layer, a lack by which the metal oxide is formed around and on the embedded length of conducting metal. For, since the quality of the glass-metal bond is related to the dissolving in the liquid glass of the metallic oxide that has formed on the surface of the conductor, when the oxide dissolves incompletely, there remains a more or less crumbly oxide interface on the surface of the metal between the glass and the metal, and the crumbly oxide interface is capable of breaking under the effect of mechanical or thermal stresses. The solidity of the glass-metal bond then depends on the thickness of the oxide layer.