Glass-to-metal Seal - Dumet Wire Seal

Dumet Wire Seal

In 1911 the Dumet-wire seal was invented which is still the common practice to seal copper leads through soda-lime or lead glass. If copper is properly oxidised before it is wetted by molten glass a vacuum tight seal of good mechanical strength can be obtained. After copper is oxidized, it is often dipped in a borax solution, as borating the copper helps prevents over-oxidation when reintroduced to a flame. Simple copper wire is not usable because its coefficient of thermal expansion is much higher than that of the glass. Thus, on cooling a strong tensile force acts on the glass-to-metal interface and it breaks. Glass and glass-to-metal interfaces are especially sensitive to tensile stress. Dumet-wire is a copper clad wire (about 25% of the weight of the wire is copper) with a core of nickel-iron alloy 42, an alloy with a composition of about 42% nickel. The core has a low coefficient of thermal expansion, allowing for a wire with a coefficient of radial thermal expansion which is slightly lower than the linear coefficient of thermal expansion of the glass, so that the glass-to-metal interface is under a low compression stress. It is not possible to adjust the axial thermal expansion of the wire as well. Because of the much higher mechanical strength of the nickel-iron core compared to the copper, the axial thermal expansion of the Dumet-wire is about the same as of the core. Thus, a shear stress builds up which is limited to a safe value by the low tensile strength of the copper. This is also the reason why Dumet is only useful for wire diameters lower than about 0.5 mm. In a typical Dumet seal through the base of a vacuum tube a short piece of Dumet-wire is butt welded to a nickel wire at one end and a copper wire at the other end. When the base is pressed of lead glass the Dumet-wire and a short part of the nickel and the copper wire are enclosed in the glass. Then the nickel wire and the glass around the Dumet-wire are heated by a gas flame and the glass seals to the Dumet-wire. The nickel and copper do not seal vacuum tight to the glass but are mechanically supported. The butt welding also avoids problems with gas-leakages at the interface between the core wire and the copper.