Metallic Vaporization in Vacuum
Date: Fall 2012
Lately I have been wondering about the following: Will liquid metal (say, steel) boil in space at just above its melting point, the way water will? Assume it was just melted by a powerful laser. I understand that the pressure of the atmosphere around a liquid affects its boiling point, and that in the vacuum of space, most liquids like water will boil as soon as they become liquid. However, I'm wondering if you know whether that would hold true of molten metal as well, or if there is some factor that would prevent this which I'm not seeing.
Back when I was a graduate student, it was routine to coat samples with gold or platinum by heating the metal in vacuum and allowing some of the vaporized metal to settle on the target sample. We did not even have to heat the metal to its melting point. The solid sublimed, going directly from solid to vapor at the temperature and pressure conditions we were using. So, yes, I imagine that if the atmospheric pressure is reduced enough, and the temperature is high enough, metal can either evaporate or sublime.
However, we should distinguish between evaporation/sublimation and "boiling point". By definition, boiling point is the temperature at which the rate of escape from the liquid phase is exactly matched by the rate of return to the liquid from the vapor phase. The vapor and liquid are in equilibrium. The temperature of boiling is controlled by two factors, the vapor pressure and the enthalpy of vaporization (the energy required to break the interatomic or intermolecular forces holding the atoms or molecules in place at the liquid phase). Since enthalpy is an intrinsic factor - dependent only on the type of substance, then the external factor is the only thing that we can control as far as lowering the boiling point. Lower the vapor pressure enough and the boiling point becomes reachable at lower temperatures (just like water boils at below 100deg C on a mountain top as opposed to at sea level). So, in the vacuum of space, we can expect the boiling point to be lower ... but it may not be so low as to be at room temperature or ambient temperature - the liquid may not spontaneously boil. So, as you mentioned, it may still require being heated in order to reach its boiling point. But as mentioned previously, a liquid does not have to be at its boiling point in order to begin evaporating. So we may find that molten metal may actually start to vaporize even though the temperature is not high enough to have the metal at its boiling point.
Greg (Roberto Gregorius)
Water, and liquids in general will undergo a drop in their boiling points
as atmospheric pressure is reduced. In the case of water, exposure to
a vacuum will result in initial boiling which carries off heat. Thus the
water will start cooling as a result of the heat lost by boiling, and when
its temperature reaches 0 degrees Celsius, the remaining water will
freeze solid and no further boiling will result.
Other liquids with lower freezing points, make it easier to see that all
that is happening is that reducing the air pressure results in a gradual
lowering of a liquid's boiling point. Usually, there will be a reduced
temperature, where (under a vacuum), the boiling will stop.
In your question about the effect of vacuum on the boiling point of iron
(i.e. steel), your comment that a powerful laser is used to supply the
required heat, is irrelevant. It makes no difference what supplies the
heat needed.... heat is heat!
Iron's melting point is 1535 degrees C, whereas its boiling point (at
normal atmospheric pressure) is 2750 degrees C.... far over 1000
degrees higher than its melting point. Because its boiling point is so
much higher than its melting point, a crucible of molten iron or steel,
when subjected to vacuum, will not boil. Its boiling point will be
reduced from the normal 2750 C value, to be sure, but not so much
that liquid iron, at or near its melting point, will start to boil.
This, of course, is rather good news for the steel industry, since
vacuum melting of steel in an electric arc furnace is a very common
way to produce steel with very low levels of impurities. If the liquid
steel started boiling under vacuum, this process would be useless.
It is quite interesting question and actually there was a similar question before. Please look at this link for the reference.
Simply, if you consider general sublimation process, you can see the direct transformation from the solid to gas phase without passing through an intermediate (liquid) phase. As most solids have some amount of vapor pressure at a certain temperature, they can sublime depending on their phase diagram. Unfortunately most metals have extremely low vapor pressure in general, which is why you don't see evaporation process of metals.
However, there are lots of practical applications of metal evaporation such as electrode deposition of the electric devices. For this purpose, there is another factor to consider; even metal atoms can evaporate, they need long mean free path to reach the surface of substrates because they cannot travel to the substrates but will be blocked with atmosphere gas molecules without high vacuum condition. At 1E-4 Pa, an 0.4 nm particle has a mean free path of 60 m, which is enough for most evaporation processes.
Weonkyu from Los Alamos
Matt, this is an interesting question because alloys (such as steel)
will act very differently than pure elemental metals (e.g. iron).
In the case of iron, at very low pressures, you will have vapor phase
above around 700C (it does not melt -- but there is enough
mobility/energy in the atoms for them to transition from solid to gas
without a liquid phase). Or, if you are less patient, you can just heat
it up to the boiling point, which increases the vapor pressure (vapor
pressure is a measure of the amount of atoms going into the vapor
phase). Scientists have measured iron's vapor pressure (specifically,
how much iron becomes a vapor at a given temperature and pressure) and
published those results.
In the case of steel, you have a situation that is more like
distillation. With many components, and assuming you heat the material
to a liquid phase, you will have a distillation-like situation where
the most volatile (volatile means higher vapor pressure, or more
likely to move into the vapor phase) molecules turn into gas first,
and as they are depleted, the next most volatile atoms will come
off... eventually you will be left with a liquid that is substantially
comprised of the least volatile liquid. If you look up the boiling
points of the components of an alloy, you'll find the general order in
which they will evaporate.
So far, I have only discussed the phase change aspect of boiling.
Another aspect of boiling is the rolling bubbles that you see in
boiling water. Technically, a liquid at its boiling point (which is a
function of pressure) is considered "boiling" even if it is not rolling
and bubbling. This is a function of how much energy is being moved
into the liquid (and therefore how much vapor is being created) and
the viscosity. Thicker liquids will tend to hold bubbles more (larger,
slower bubbling), while thinner liquids have fewer, smaller bubbles,
but roll more rapidly -- and the warmer the liquid, the less viscous
it will tend to be.
Finally, I want to mention your suggested energy-injection method,
lasers. With a point energy source, if you have that powerful of a
laser, you are probably going to be vaporizing the metal at the point
of the laser hitting the material -- heat transfer won't be fast
enough to get a uniform-temperature liquid (in other words, it will not
act like a pot of boiling water). However, for the sake of argument,
we can just assume that you are heating the metal uniformly and not
worry about how.
Hope this helps,
You need to be careful about the definition of “boiling point” and “vapor pressure”. The common definition of “boiling point” is the temperature at which the “vapor pressure” equals the applied pressure of the atmosphere. This is 100 C. for water at 1 atmosphere applied pressure. As the altitude increases the applied pressure of the atmosphere decreases and water “boils” at a lower temperature.
In contrast, the vapor pressure is the pressure exerted by a substance at a given temperature with no applied external (air) pressure. Water at 25 C. has a vapor pressure of 25 mm Hg (This is just a trick of the numbers and their units – no science is involved).
If you place a container of water in space and control its temperature at 25 C., its vapor pressure is still 25 mm of Hg. That water will disappear shortly because the water escapes even at 25 mm of Hg. It does not have “to boil”.
So your statement… “that in the vacuum of space, most liquids like water will boil as soon as they become liquid.” is not correct. A lump of ice will also evaporate in space, even at a temperature below its melting point. But the colder its temperature the lower its vapor pressure and the lower the vapor pressure the slower is the evaporation.
If you take a metal that has a low vapor pressure at the temperature it will not evaporate. Even mercury, a liquid at “room temperature” evaporates very slowly whether it is on the Earth or in outer space. It all depends on the vapor pressure. Your confusion of boiling point and vapor pressure is very common, so do not feel bad.
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