gas-phase synthesis with vapour condensation otherwise evaporation-condensation method (rus. синтез, газофазный с конденсацией паров otherwise метод испарения и конденсации) — a method to produce nanopowders of metals, alloys or compounds by condensation of their vapors at controlled temperature in an inert low pressure atmosphere.

Description

In contrast to vacuum evaporation, atoms vapourised in an inert low-density atmosphere rapidly lose their kinetic energy due to collisions with gas atoms and form clusters. Metal is evaporated from the crucible or injected into the heating and evaporation zone in the form of wire, injected metal powder, or in a jet of a liquid. Metal sputtering with argon ion beam is also used. Energy is supplied by direct heating, passing electric current through a wire, electric discharge in plasma, induction heating by high and ultrahigh frequency currents, laser radiation, electron beam heating.

Condensation of a vapour mixture with a temperature of 5000-10000 K may occur when it enters a chamber with a large cross-section and volume filled with cold inert gas; in this case cooling is due to expansion of the gas mixture and its contact with a cold inert atmosphere. There are types of process equipment where the condensation chamber coaxially receives two streams: vapour-gas mixture is fed along the chamber's axis, and an annular jet of cold inert gas flows along the periphery. Turbulent mixing makes the temperature of the metal vapour drop down and supersaturation increase which results in rapid condensation.

favourable conditions for the condensation of metal vapours are formed by adiabatic expansion in a Laval nozzle when rapid expansion causes a high temperature gradient and vapors are condensed almost instantaneously.

Independent problem is the collection of resulting condensation nanopowder because its particles are so small that they are in constant Brownian motion and remain suspended in the moving gas not being settled by gravity. Powder particles are collected by means of special filters and centrifugal sedimentation; in some cases, liquid film capture is used.

The main regularities in the formation of nanoparticles by evaporation and condensation are as follows.

- Formation of nanoparticles occurs when vapours cool down in the condensation zone which is larger as the gas pressure is lower. The inner boundary of the condensation zone is located near the evaporator, and as the gas pressure goes down its outer edge can move beyond the reaction vessel. In the process of condensation, convective gas flows play an important role.

- As the gas pressure grows up to several hundred Pa, the average particle size at first increases rapidly and then slowly approaches the limiting value at pressures over 2500 Pa.

- Under the same gas pressure, transition from helium to xenon i.e. from the less dense inert gas to the more dense one is accompanied by an increase in particle size by several times.

Under the same evaporation and condensation conditions, metals with a higher melting point form smaller particles. By adjusting the composition of the gas phase which contains, in addition to the inert gas, two elements or more, small particles of different shapes varying in degrees of crystallinity can be grown.

Illustrations

Production of nanopowders from metals using the gas-phase synthesis technique.

Production of nanopowders from metals using the gas-phase synthesis technique.


Author

  • Gusev Alexander I.

Sources

  1. Gusev A. I. Nanomaterials, Nanostructures, and Nanotechnologies (in Russian) // Fizmatlit, Moscow (2007) - 416 pp.
  2. Gusev A. I., Rempel A. A. Nanocrystalline Materials. — Cambridge: Cambridge International Science Publishing, 2004. — 351 p.

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