Evaporation

Evaporation in gases is a method that evaporates metals, alloys, or ceremics so that the atoms bump each other and also atoms of inert gases, or they react with active gases. Condensation in the cold gases results in the formation of nanoparticles (or ultrafine particles as defined in some literature many years ago) and nanocapsules. Heating by a resistance heater or a high frequency induction furnace is the simplest method in laboratories for evaporating the metals [29-41]. The resistance heater can be made of graphite, tungsten wire, or other kinds of metal wires. In the earliest experiments, metals were put in a tungsten basket inside a vacuum chamber and then evaporated in argon atmosphere of 1-50 Torr [30]. The metal smoke deposited on the water-cooled inner wall of the chamber. Figure 1 shows iron and copper smoke [35]. The nanoparticles were collected by a TEM mesh grid for observation by a TEM. The crystallographic properties of various nanoparticles, such as Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, Cd, Sn, Au, Pb, and Bi, were investigated systematically by TEM observations. It is a common fact that for all metals, the size of the particles is reduced with decreasing atmospheric pressure [30-35]. It is interesting to note that usually a shell of oxidations (commonly in amorphous) of several nanometers forms on the surface of the nanoparticles. The different metal nanoparticles have different crystallographic structures. For instance, Al, Bi, Sn, and Pb particles are balls, Mg particles are hexagons, and Cr particles are body-centered cubic (bcc) structures. Magnetic particles, like Fe and Co, can align as a necklace or a chain [30-35] due to the existence of stable magnetic moments and magne-tostatic forces. Dipolar interaction plays an important role on such alignment of magnetic nanoparticles, which may become dominant compared to the van der Waals attraction and steric repulsion that usually lead to a close-packed arrangement [85, 86]. The magnetic domains in the particles were observed by means of Lorentz electron microscopy [87-90].

The character (i.e., the shapes and colors) of the metal smokes was found to depend on the gas pressure/density

Figure 1. Iron and copper smokes. The smoke looks white for all metals and no color is observed. A beautiful color, however, characteristic of each metal, is seen in the neighborhood of the heater. (a) shows iron smoke in 50 Torr of helium, which has a typical candle-flame shape with inner, intermediate, and outer zones. The bright intermediate zone implies that the smoke is denser there than in the dark inner zone. In the case of iron, red light is emitted from the neighborhood of the heater. (b) shows copper smoke in 30 Torr of argon, which is thinner, because 30 Torr of argon is denser than 50 Torr of helium. The blue color is seen near the evaporation source Adapted with permission from [35], R. Uyeda, in "Morphology of Crystals" (I. Sunagawa, Ed.), Ch. 6, p. 367. Terra Scientific, Tokyo, 1987. © 1987, Terra Scientific.

Figure 1. Iron and copper smokes. The smoke looks white for all metals and no color is observed. A beautiful color, however, characteristic of each metal, is seen in the neighborhood of the heater. (a) shows iron smoke in 50 Torr of helium, which has a typical candle-flame shape with inner, intermediate, and outer zones. The bright intermediate zone implies that the smoke is denser there than in the dark inner zone. In the case of iron, red light is emitted from the neighborhood of the heater. (b) shows copper smoke in 30 Torr of argon, which is thinner, because 30 Torr of argon is denser than 50 Torr of helium. The blue color is seen near the evaporation source Adapted with permission from [35], R. Uyeda, in "Morphology of Crystals" (I. Sunagawa, Ed.), Ch. 6, p. 367. Terra Scientific, Tokyo, 1987. © 1987, Terra Scientific.

and the temperature/velocity of evaporation [35, 39, 42, 43]. The shape of the metal smoke can be candle-fire-, ring-, tulip-, and butterfly-like. The color of the smoke can be purple (for Al, Ag), green (for Au), red (for Be, Co, Fe, Ni), blue (for Cu, Nb, W), orange (for Ti), green plus orange (for Cr, Fe), yellow plus green (for Mo), etc. [35, 43, 46, 49, 50]. Granqvist and Buhrman obtained nanoparticles with an average size of about 10 nm by evaporating Al, Mg, Zn, Sn, Cr, Fe, Co, Ni, and Ca in argon atmosphere of 0.5-4 Torr by applying a facility with a graphite heater [91-96]. Increasing one of several factors (velocity of evaporation, temperature of the evaporating materials, evaporating pressure of the materials, press of inert gases) would increase the size of the nanoparticles/nanocapsules. Evaporation by a resistance heater is usually suitable for producing a small amount of the particles in laboratories, while heating by a high frequency induction furnace could be applied for producing a large amount of materials in the industrial scale. The advantages of the latter method are the stability of evaporation temperature, the homogenous melting alloys, the high energy power supply, etc. For details of the crystal habits and crystal structures of nanoparticles, the reader is referred to Uyeda's book [35] and other original contributions [30, 36-53, 97-101].

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