Electron-beam heating

Fig. 1. Vacuum evaporation process with use of electron beam heating where A represents the material to be deposited. The flux profile ( ) is at a...

Materials having a higher vapor pressure at low temperatures ate typically vaporized from resistively heated sources such as those shown in Figure 5a. Refractory materials requite a high temperature to be vaporized. A focused high energy electron-beam heating is necessary for vaporization (Fig. 5b). 

The co-condensation at low temperature of a metal vapor (commonly produced by resistance or electron-beam heating of metals) with a vapor of weakly stabilizing organic ligands (such as -pentane, toluene, tetrahydrofu-ran, acetone, or acetonitrile), using commercially available reactors, affords solid matrices, where reactions between the ligand molecules and metal atoms can take place (Scheme 1(A) Figure 1) [5]. 

The results indicate that a supersaturation of vacancies, c/co — 10, at the catalyst s surface is required to nucleate CS planes in M0O3 catalysts. CS planes are formed by the elimination of anion vacancies in supersaturation (where the supersaturation is defined relative to the background concentration of anion vacancies in equilibrium with CS planes, as described earlier) (Gai 1981, Gai et al 1982). The driving force for the nucleation of the CS fault is the difference between the chemical stress due to the supersaturation of anion vacancies of the faulted (defective) structure and the force required to create the fault. The estimate of Co is consistent with the equilibrium concentration of anion vacancies found in electron beam heating studies of M0O3 in vacuum (Bursill 1969). 

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 )...

Electron-beam heating, in metal vapor synthesis, 1, 232 Electron-beam vaporization, in metal vapor synthesis, 1, 224 Electron correlation, and computational chemistry, 1, 642 Electron counting, in transition metal complexes bridging ligands, 1, 14 examples, 1, 9 ligand electrons, 1, 5/... 

The results of EDS analysis obtained at two different points marked in the Fig. 1 are presented in the Table 1. The first point was selected in the crystalline region and the second one in the amorphous fraction. It is clear that Ar molecules are mainly concentrated in the crystalline region while DCB molecules containing Cl atoms are located preferably in amorphous phase. It is necessary to note that the absolute quantity of Ar obtained by EDS analysis and shown in Table 1 should be considered as a low limit estimation. Thermal desorption of Ar atoms as a result of electron beam heating leads to the reduction of measurement value during analyzing procedure. 

Tc 600°C), at which crystallization occurs. The released latent heat raises the local temperature further, and thermal conduction maintains self-sustaining explosive transformation until the crystallization front has propagated down the thermal gradient to a point below Tc. Pulsed electron beam heating experiments indicate that a-Si melts at 1170°K (Baeri et al., 1980). 

Similar structures are observed by Dravid et al. in the remains of a carbon arc discharge.[Dr93] This indicates that the special conditions of electron-beam heating are not necessary to produce multi-shelled onions. Dravid et al. also point out that some of these onions contain unterminated graphene sheets. 

While all vapor techniques depend on a condensation step, the direct vaporization techniques begin with the desired composition and evaporate or sublimate the material. This is straightforward and allows a diversity of compositions to be used, but excessively high temperatures are demanded to vaporize refractory ceramics. Various heating methods used, including dc arcs, dc plasmas, rf plasmas, and electron beam heating. These techniques are not popular for large-scale production or routine laboratory powder preparation. 

For evaporative or sublimation process, correct selection of evaporation method, the evaporation source, and the evaporation temperatnre is required to surmount the attractive intermolecular forces existing within the starting material. The parameters depend primarily on the materials nsed and the film purity required. Indirect resistance heating, flash evaporation, and electron beam heating techniques are used for this purpose. 

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