Inert-gas evaporation technique

Synthesis of nano-structured alloys by the inert gas evaporation technique A precursor material, either a single metal or a compound, is evaporated at low temperature, producing atom clusters through homogeneous condensation via collisions with gas atoms in the proximity of a cold collection surface. To avoid cluster coalescence, the clusters are removed from the deposition region by natural gas convection or forced gas flow. A similar technique is sputtering (ejection of atoms or clusters by an accelerated focused beam of an inert gas, see 6.9.3). 

Inert Gas Evaporation Technique. Inert gas evaporation technique is a familiar method in an aerosol production of ultrafine particles as already mentioned in Section... 

As mentioned in the preceding section, to stabilize a particle, metallic particles must be produced in an inert gas atmosphere and then trapped in appropriate liquids to finely disperse them. Unless liquid is used, particles tend to coalesce, forming a larger particle or aggregate, the size of which exceeds several tens of nanometers as a powdered sample. First we introduce the normal gas evaporation technique to show the principle of aerosol method. Then several modifications are described to get dispersed metallic systems as a colloid. 

The various methods of preparation employed to prepare nanoscale clusters include evaporation in inert-gas atmosphere, laser pyrolysis, sputtering techniques, mechanical grinding, plasma techniques and chemical methods (Hadjipanyas Siegel, 1994). In Table 3.5, we list typical materials prepared by inert-gas evaporation, sputtering and chemical methods. Nanoparticles of oxide materials can be prepared by the oxidation of fine metal particles, by spray techniques, by precipitation methods (involving the adjustment of reaction conditions, pH etc) or by the sol-gel method. Nanomaterials based on carbon nanotubes (see Chapter 1) have been prepared. For example, nanorods of metal carbides can be made by the reaction of volatile oxides or halides with the nanotubes (Dai et al., 1995). 

To achieve this, and to facilitate the control of the operating parameters, some authors recommended using a technique based on the flash vaporization of liquid substrates in an inert gas. This technique also increases the range of operating pressure, thus allowing the realization of reactions under reduced pressure. Figure 11.2 shows a solid/gas setup in which liquid substrates are injected in a flash evaporator and mixed with an inert gas to produce the required gas mixture. 

Physical vapor deposition (PVD) techniques such as evaporation or sputtering may also be used to synthesize 0-D nanoparticles. One technique, known as inert gas evaporation (Figure 6.36f), consists of evaporation of a precursor material within a cooling inert gas at low pressures ca. 100 Pa). Vaporization may be accomplished via resistive heating, ion bombardment (sputtering), or laser irradiation. As one... 

Ullrafine particles (UFPs) of metal and semiconductor nitrides have been synthesized by two major techniques one is the reactive gas condensation method, and the other is the chemical vapor condensation method. The former is modified from the so-called gas condensation method (or gas-evaporation method) (13), and a surrounding gas such as N2 or NII2 is used in the evaporation chamber instead of inert gases. Plasma generation has been widely adopted in order to enhance the nitridation in the particle formation process. The latter is based on the decomposition and the subsequent chemical reaction of metal chloride, carbonate, hydride, and organics used as raw materials in an appropriate reactive gas under an energetic environment formed mainly by thermal healing, radiofrequency (RF) plasma, and laser beam. Synthesis techniques are listed for every heal source for the reactive gas condensation method and for the chemical vapor condensation method in Tables 8.1.1 and 8.1.2, respectively. 

The most widely used vacuum deposition techniques are evaporation and sputtering, often employed for smaller substrates. In the evaporation process, heating the metal by an electron beam or by direct resistance produces the vapours. The system is operated at a very high vacuum (between 10-5 and 10 6 Torr) to allow a free path for the evaporant to reach the substrate. The rate of metal deposition by evaporation processes varies from 100 to 250,000 A min h These processes can be operated on a batch or a continuous scale. On the other hand, in the case of the sputtering technique, the reaction chamber is first evacuated to a pressure of about 10-5 Torr and then back-filled with an inert gas up to a pressure of 100 mTorr. A strong electric field in the chamber renders ionisation of the inert gas. These inert gas ions... 

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