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In a vacuum furnace, materials can behave differently than they do under atmospheric pressure. Silver brazing alloys typically consist of silver along with other metals such as copper, zinc, and sometimes cadmium. Each of these metals has its own vapor pressure, which is the pressure at which a material will start to evaporate.
When you heat these alloys in a vacuum furnace, the lack of atmospheric pressure means that metals with a high vapor pressure are more likely to evaporate if the temperature gets high enough. This is because vapor pressure increases with temperature and in a vacuum, there’s no atmospheric pressure to counteract this.
So, while silver and copper have relatively low vapor pressures and thus high boiling points, other components of a silver brazing alloy might not. If a silver brazing alloy contains zinc, for instance, the zinc may start to evaporate at high temperatures because it has a high vapor pressure.
The specific temperature at which a silver brazing alloy might start to evaporate in a vacuum furnace would depend on the exact composition of the alloy and the level of vacuum achieved. Manufacturers of vacuum furnaces and brazing materials can often provide specific guidance based on their products’ characteristics.
Without atmospheric pressure to keep the materials condensed, even metals like silver could potentially evaporate at temperatures lower than their standard boiling points if the vacuum is strong enough and the temperature is sufficiently high. However, the term “evaporate” might be somewhat misleading as it usually refers to liquids turning into gas, and in the context of metals in a vacuum, it would actually be sublimation (solid turning directly into gas) or the metals may simply vaporize.
Therefore, the answer is yes, silver braze can evaporate in a vacuum furnace, especially if the vacuum is strong and the temperature is high enough to exceed the vapor pressures of the constituent metals. However, silver itself is less likely to evaporate compared to other, more volatile components of the brazing alloy.