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From Vijay, M.; Ramachandran, K.; Ananthapadmanabhan, P. V.; Nalini, B.; Pillai, B. C.; Bondioli, F.; Manivannan, A.; Narendhirakannan, R. T. Current Applied Physics 2013, 13 (3), 510–516. DOI 10.1016/j.cap.2012.09.014:

Since large quantity of energy is consumed to vaporize the solvent in liquid precursors, a novel method that involves thermal plasma in-flight oxidation of $\ce{TiH2}$ has been developed to synthesize nanocrystalline $\ce{TiO2}$."

What does in-flight mean in this context? Obviously it doesn't have anything to do with actual flying, but everywhere I look it says that it is used for something that occurrs or is provided during an aircraft flight.

The article is about photocatalytic inactivation of bacteria by reactive plasma nanocrystalline $\ce{TiO2}$ powder. It suggests a new method for the physical preparation of $\ce{TiO2}$ for a novel reactor.

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Original paper where this method (and, I guess, the term itself) has been introduced was written in Japanese in 1982 [1]. In 1997 there was a great overview published in English by Toyonobu Yoshida, author of the original paper, with a chapter (2.3) devoted to this particular method:

Processes used to manufacture materials by injecting a solid, liquid, or gas into a thermal plasma to use its high enthalpy and reactivity are referred to as in-flight plasma processes (IFP processes).

Due to the nature of the method which utilizes plasma jet with reactants generated via various torch-types, there is actually a flow of chemicals reacting while they "fly", of course, in the plasma area.

enter image description here

Various plasma torches provide various times-of-flight, temperatures and velocities. For example, the direct current jet torch has the following characteristics:

An argon plasma of about $\pu{80 kW}$ that has been generated in air reportedly produces a temperature of about $\pu{11600 K}$ and a velocity of about $\pu{410 m/s}$... The reaction time for reactants injected near the anode at high temperatures is about $\pu{400 \mu s}$ and the gaseous raw materials are mainly used for the synthesis of fine powders... The reaction time of the reactants is estimated to be $\pu{1 ms}$.

Bibliography

  1. Yoshida, T.; Akashi, K. Tetsu-to-Hagane 1982, 68 (10), 1498–1502. DOI 10.2355/tetsutohagane1955.68.10_1498.
  2. Ultra-fine particles: exploratory science and technology; Hayashi, C., Uyeda, R., Tasaki, A., Eds.; Materials science and process technology series; Noyes Publications: Westwood, N.J, 1997. ISBN 978-0-8155-1404-6.
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Further down in the paper, the following image and accompanying text are present:

enter image description here

Fig. 1. Schematic of plasma reactor (1 – Plasma torch, 2 – Reactor, 3 – Filter and 4 – Exhaust).

As the powder particles enter the plasma jet, they get heated up and $\ce{TiH2}$ dissociates into $\ce{Ti}$ particles and hydrogen gas. Both $\ce{Ti}$ particles and hydrogen gas react with oxygen to form $\ce{TiO2}$ and water vapor respectively; the latter escapes along with the exhaust gas stream. In addition to the particle heating by the plasma, the energy released by the exothermic reaction between Ti particles and oxygen helps to dissociate the formed $\ce{TiO2}$ particles into Ti vapor and oxygen gas. The Ti vapor reacts with oxygen at the tail of the plasma jet forming nanoclusters of $\ce{TiO2}$ particles.

So what happens is that a stream of $\ce{TiH2}$ particles are sent into the reactor on a stream of gas (in other words, flying into the reactor). The oxidation occurs while the particles are in-flight, not before they hit the plasma or after they're collected, presumably on the filter.

So in-flight here means the same thing it does in other contexts, but the flight is the flight of $\ce{TiH2}$ particles on a stream of gas/plasma.

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