A new copper based hermetic sealing material: A comparison with AgCu28GeCo0.3
For vacuum brazing applications Hermetic Sealing Materials (HSM) with low partial pressure are widely used to join copper or stainless steel parts. For joining copper AgCu28 is a widespread material, which is well characterized and investigated. For stainless steel, AgCu28Ge2Co0.3 is commonly used if the same brazing temperature as AgCu28 is required. Umicore Technical Materials has developed a new copper-based HSM. The tensile strength of the stainless steel joints on stainless steel is comparable to AgCu28Ge2Co0.3 and the wettability is significantly improved. In this article, the physical properties of the alloy and its brazed joints will be presented in comparison to AgCu28GeCo0.3. However, today the silver content of HSMs is more in focus than in the past decades, because it has the biggest impact on the material prices. To accommodate this trend the total silver content of the new HSM is reduced by almost 44 %.
New hermetic sealing material for vacuum brazing of stainless steels
For vacuum brazing applications such as in vacuum interrupter industry Hermetic Sealing Materials (HSM) with low partial pressure are widely used. AgCu28 dominates the hermetic sealing market, as it has a very good wetting behavior on copper and metallized ceramics. Within recent decades wetting on stainless steel has become more and more important. However, today the silver content of HSMs is more in focus than in the past decades, because it has the biggest impact on the material prices. Umicore Technical Materials has developed a new copper based HSM, CuAg40Ga10. The wettability on stainless steel is significantly improved compared to AgCu28 and the total silver content is reduced by almost 44%. In this article the physical properties of the alloy and its brazed joints will be presented compared to AgCu28.
Oxygen mass transfer at liquid-metal-vapour interfaces under a low total pressure
AbstractThe problem of oxygen exchange at the interface between a gas and a liquid metal is treated for systems under a “vacuum” (Knudsen regime, pressures lower than 1 Pa), where, due to the large mean free path of gas molecules in a vacuum, transport processes in the gas phase have no influence on the total interphase mass exchange, which is controlled by interface phenomena and by oxygen partition equilibrium inside the liquid. Owing to the double contribution of molecular O2 and volatile oxides to the oxygen flux from the surface, non-equilibrium steady-state conditions can be established, in which no variations in the composition of the two phases occur with time, as the result of opposite oxygen exchanges. The total oxygen and metal evaporation rates are evaluated as a function of the overall thermodynamic driving forces, and an account of the transport kinetics is given by using appropriate coefficients. A steady-state saturation degree sr, is defined which relates the oxygen activity in the liquid metal to the O2 pressure imposed and to the vapour pressures of volatile oxides. When metals able to form volatile oxides are considered, pressures of molecular O2 higher than those defined under equilibrium conditions have to be imposed in the experimental set-up in order to obtain a certain saturation degree, as a consequence of the condensation of the oxide vapours on the reactor walls. Effective oxidation parameters are determined, which define the conditions leading the liquid to a definite steady-state composition under a “vacuum” when it is out of equilibrium. The effective value of the oxygen pressure which corresponds to the complete oxygen saturation of the metal, 
$$P_{O_{2,} s}^E $$
, is evaluated at different temperatures for the systems Si-O and Al-O. The results are represented as curves of 
$$P_{O_{2,} s}^E $$
 against T, which separate different oxidation regimes; these results agree well with the experimental data found in the literature.