Cold plasma oxidation

You should be able to oxidize things that are difficult to oxidize using a rapidly cooled air plasma, and this should be feasible even without refractory electrodes and at atmospheric pressure.

The basic principle

If you convert a gas into a hot electric plasma between two electrodes, then cool the plasma rapidly to the gas’s original temperature, the cooled plasma will still be different from the original gas in several ways: its molecule energies will still be far from the Maxwell–Boltzmann distribution, as it will still have a significant number of very hot molecules and free electrons (this is called an “anisothermal plasma”); many of the molecules will still be ionized; many of the non-ionized molecules will have excited electrons; and, if it’s not a noble gas, many of the molecules will have been ripped apart, and some will have reformed in new conformations. In particular, if the source contained O₂, the resulting gas will also contain O and O₃, which are very strong oxidizers, and if it contained O₂ and N₂, it will also contain a variety of nitrogen oxides, some of which are also very strong oxidizers.

Wikipedia's nonthermal plasma article tells me that the oxygen atoms have a lifetime of about 14 μs.

Applications

Directing a stream of air, converted to this cool plasma, against a material that should have an excellent chance of oxidizing it further, if it's not already fully oxidized with oxygen or something stronger, without necessarily setting it on fire or even heating it much, and volatilizing some of the oxide, which could be useful for a variety of purposes:

• removing organic contaminants from oxide surfaces, such as calcite, clay, concrete, glass, sapphire, or viridian, including the glass, sapphire, and viridian passivating layers formed naturally on silicon, aluminum, chromium, and stainless steel surfaces in air;
• metal passivation, including selective thickening of passivation layers, including on iron;
• selective functionalization of organic surfaces;
• selective production of oxidation products on organic surfaces — in some cases, for example, this can be used to make a dark mark, making the device a printer;
• low-temperature cutting of materials whose oxides are volatile, including any organic material (other than teflon), graphite, diamond, zinc, and possibly even silicon carbide, molybdenum, iron, and steel (certainly at higher temperatures where the oxide will melt and can be blown off);
• acquiring trace amounts of volatilized oxides and nitrides for spectroscopic analysis;
• gasifying an oxidizable blowing agent from a matrix without heating it excessively, producing a foam, as in the traditional means of producing firebrick by burning organic material out of the pores of a matrix;
• sterilizing drinking water or solid objects, especially solid objects that would be damaged by high temperatures, although maybe you should see if an ordinary ozone generator without plasma jets would be adequate (I see that this is currently under investigation for food processing);
• selective oxidative polymerization of, for example, linseed oil, tung oil, or stand oil, for example for 3-D printing, for 2-D printing with the polymer as a binder, or for depositing resists to guide later etching — and polymerizing almost any energetically favorable monomer should be possible at the right temperature, perhaps including polymerizing aqueous silicic acid into silica gel, as plasma polymerization is already used to form films of polystyrene, polyethylene, and poly(methyl methacrylate), among others, though normally from low-pressure gas-phase monomers;
• curing linseed-oil or stand-oil finishes very quickly;
• very rapid selective polymerization of cyanoacrylate by selectively adding hydroxyl radicals, although maybe ordinary water vapor is a better way to do this;
• oxidizing hazardous organic chemicals such as mustard gas, PCBs, benz(a)pyrene, and possibly-unknown mixtures into tamer and simpler oxides that can be more easily disposed of;
• cutting composites made of oxide grains cemented together with a carbon or organic binder.

Possible applications of non-air plasma ingredients include surface-nitriding metals and selectively “salt-glazing” silicate ceramics with a part-sodium plasma, or superpassivating surfaces that need to withstand exposure to strong oxidants by using a part-fluorine plasma. If the sodium source is chlorine-free (for example, NaOH or NaNO₃), the chlorine and HCl emissions that plague salt-glazing will not occur.

Directing the plasma

If the stream is sufficiently cool, its rate of erosion of already-oxidized materials such as teflon, soda-lime glass, iron oxides, quartz, viridian, sapphire, zircon, or zirconia should be relatively low, so it might be feasible to use a nozzle made from these materials to direct it onto the workpiece without eroding the nozzle too rapidly.

Cooling the plasma

Rapidly cooling of the plasma that has just passed through the arc can be effected by misting water into the plasma. The water will flash to steam, some of which will itself ionize and dissociate, contributing further reactive oxygen species to the mix.

Alternatively, you might be able to use corona discharge to ionize enough of the air to be useful, without ever heating it to arcing temperatures — maybe like a garden-variety ozone generator with more concentrated output and maybe gold-plated or carbon points for longer life.

Liquid electrodes

Optionally, the electrodes themselves can be covered with a constantly replenished liquid electrolyte, such as water including a substantial mixture of NaOH or KOH, or molten NaNO₃ or KNO₃. This avoids the need for electrodes of refractory materials such a graphite, silicon carbide, tungsten, or hafnium. Some of the electrolyte will find its way into the arc, which may be preferable to the evaporation from solid refractory electrodes under some circumstances.

(Why don’t potters just use chlorine-free sources for salt-glazing in a kiln? NaOH and NaNO₃ convert to NaO at high temperatures, which doesn’t boil until 1950°, much higher than NaCl’s 1413°, so a pottery kiln at 1100°–1200° has a hard time volatilizing the sodium from NaOH or NaNO₃ without an admixture of the lower-boiling NaCl, while the arc should have no trouble. However, they do use NaCO₃ and NaHCO₃ for “soda firing”, sprayed into the kiln during firing, so I don’t know.)

Hazardous emissions

However, if made from air, the cooled plasma inevitably contains sufficient ROSes and nitrogen oxides to be hazardous; the resulting gas needs to be thoroughly reduced unless you’re doing this at a very small scale somewhere adequately ventilated. The standard approach to this problem is a platinum catalytic converter, but maybe bubbling the gas through consumable linseed oil or passing it over a consumable bed of sulfur, phosphorus, powdered lanthanoids, or really any easily oxidized chemical would be adequate.