Gas evaporation method

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. 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. 

An Fe UFP and a precipitated Fe catalysts were used in the present work. The Fe UFP designated as Fe UFP 200 A was supplied by Vacuum Metallurgical Co., Ltd. (Chiba Prefecture, Japan) and was prepared by the gas evaporation method (refs. 4,5). Transmission electron microscopic (TEM) observation showed that the UFP was sintered in the course of preparation and was in the form of "necklace" as... 

A gas evaporation method in which a carbon rod vertically touches the top surface of a metal block in a vacuum chamber produces ultraflne particles of various carbides (1). As the electric current is increased until the carbon rod changes from red heat to white heat in a helium or argon atmosphere, the metal block melts and molten metal climbs up along the surface of the carbon rod. Spherical or trigonal smoked TiC with a diameter of about 50 nm can be most easily obtained (1). 

Y Ando, R Ueda. Preparation of ultrafine particles of refractory metal carbides by a gas-evaporation method. J Cryst Growth 52 178, 1981. 

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