if a chamber were to be completely filled with oxygen gas only (let's say atmospheric presssure in a 1L vacuum chamber) and a piece of metal, maybe iron or maybe steel wool, is heated so it oxidizes, would it be able to achieve a vacuum level higher than a vacuum pumps such as this one (https://www.vevor.com.au/vacuum-pump-c_11109/single-stage-3-cfm-refrigeration-air-conditioning-vacuum-pump-r410a-gauges-p_010497372373), assuming the amount of iron in the chamber is 3 times the stoichiometrically needed amount to completely react with all the oxygen? I'm curious because I've read that to get a high vacuum level, we need cryopumps or diffusion pumps and i'm curious if something like this can reach a comparable vacuum level. I get a feeling it won't but i'm still curious.


1 Answer 1


(The device concept you're describing is called a getter.)

The thermodynamic aspect is addressed in Ellingham diagrams:

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You place a straightedge on point "O" (for oxygen) and on the relevant material line at the temperature of interest. The straightedge intersects the corresponding equilibrium oxygen partial pressure $p_{\text{O}_2}$. At 1000°C, for example, pure iron oxidation is expected to be spontaneous down to an oxygen partial pressure of around $10^{-14}$ to $10^{-16}$ atm.

Uh oh, the numbers actually increase with increasing temperature. That's because oxidation (except oxidation of carbon to form carbon monoxide) removes high-entropy gas, and Nature increasingly rejects any entropy destruction with increasing temperature, even if it's paid for with enthalpic heating of the rest of the universe.

So a higher temperature actually gives worse results—thermodynamically. Kinetically, a higher temperature increases the reaction rate, and you presumably don't wish to wait years for the chamber atmosphere to be depleted. You can expect the reaction rate to increase exponentially with increasing temperature $T$ as $\propto \exp\left(-\frac{\Delta G}{RT}\right)$, where $\Delta G$ is the relevant activation energy and $R$ is the gas constant.

Note that the reaction rate will also fall as the oxide thickness grows, also in a material-dependent way. In a practical example, bulk aluminum is not generally preferred as an oxygen getter because the well-bonded (see the high oxide melting temperature in the diagram) aluminum oxide is an effective barrier to further oxygen transport. However, aluminum powder may provide sufficient total surface area even as the barrier thickens.


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