Gas evaporation

Equation (12-47) assumes that all heat sources are at the same temperature and that the convection coefficients to the evaporating surface and to the unwetted portions of the tray are equal. When radiation occurs from a source at a different temperature, the radiation coefficient can be corrected to the same basis by multiplying by the ratio (t — f()/(U i ), where t, t, and t are the diying-gas, evaporating-surface, and radiator temperatures respectively. 

Heat exchanger Cocurrent, countercurrent Differential Gas Evaporation, dephlegmation... 

Mattox, D., Vacuum Deposition, Reactive Evaporation and Gas Evaporation, m ASM Handbook, Surface Engineering, 5 556-572, ASMPubl. (1994)... 

Persistency The persistency will depend upon munitions used and the weather, ffeavily splashed liquid persists 1-2 days under average weather condition. GA evaporates about 20x more slowly than water. GA in water can persist about 1 day at 20°C and about 6 days at 5°C. GA persists about twice as long in seawater. 

It is more convenient to use the enthalpy of the cold gas evaporating from the LHe in a continuous gas flow cryostat (Fig. 5.4). The flux and hence the temperature of the experiment can be regulated by a heater or a needle valve. 

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). 

Ohno T (2002) Morphology of composite nanoparticles of immiscible binary systems prepared by gas-evaporation technique and subsequent vapor condensation. J Nanoparticle Res 4 255-260... 

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. 

Fig. 8.1.4 Schematic illustration of the evaporation apparatus with electron beam heating, DP and MBP mean diffusion pump and mechanical booster pump, respectively. (Reprinted from J Cryst Growth, 56 S Iwama et a].. Ultrafine powders of TiN and AIN produced by a reactive gas evaporation technique with electron beam heating, pp 265-269. Copyright 1982, with permission from Elsevier Science )...

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. 

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

Gas evaporation using Ar for the preparation of various sort of metal fine powders was first reported by Kimoto et al. in 1963 (5). The production chamber of this method is basically the same as that of a vacuum sublimation chamber. A target material is heated in this chamber with several torr inert gas atmosphere. The nanometer-sized particles are easily formed in the chamber space. However, by this method, it is difficult to get genuine nanoparticles whose sizes are several nanometers. This is because of the radiation heating in a production chamber, resulting particle coalescence on the chamber wall or particle collector, as well as the direct particle contact in the deposited particle layer (powders). Therefore the size becomes several tens to hundreds of nanometers. Several ultrafme metallic powders are now commercially available, including Cu, Ag, Al, Ni, Co, Fe, and Au, with a size of several tens of nanometers. 

Fig. 9.4.7 Electron microscopic images of Mg small particles prepared by different methods and their size histograms. (A) Mg fine powders produced by a conventional gas-evaporation method with Ar at 4 kPa. (B) Mg fine particles produced by a matrix isolation method with Ar at 300 Pa in tetrahydrofuran. (C) Mg ultrafine particles produced by a matrix isolation method with He at 1.3 kPa in tetrahydrofuran. The scale bar for (C) is the same as for (B). Abscissa at top (n) is a rough estimate of the number of Mg atoms in a single particle whose diameter is represented by logarithm of diameter (nm) in the bottom scale. Ordinate (N ) is a normalized number of particles in a unit size width. (From Ref. 4.)...

Size and Yield as a Function of Flow Rate and Gas Pressure. The effect of gas pressure on the size of particles prepared by the gas evaporation method was noticed at an early period of the development of this method (17). It was reported that to obtain the same size, the pressure of helium had to be about 10 times as large as that of argon, and the size was larger under high-pressure gas. More quantitative work was done by Yatsuya et a). (18) on the pressure dependence of the size of the Al fine particle. In these studies, particles were sampled in the gas phase. 

Be. Figure 9.4.14 shows a TEM photograph of Be fine particles produced by a conventional gas evaporation technique. The size of the smallest particles is down to 5 nm. 

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