Evaporation preferential

The disadvantage of lasers with nanosecond-picosecond pulse duration for depth profiling is the predominantly thermal character of the ablation process [4.229]. For metals the irradiated spot is melted and much of the material is evaporated from the melt. The melting of the sample causes modification and mixing of different layers followed by changes of phase composition during material evaporation (preferential volatilization) and bulk re-solidification [4.230] this reduces the lateral and depth resolution of LA-based techniques. 

Evaporation from an alloy source largely avoids the problems of source geometry which may be encountered in simultaneous evaporation from separate pure metal sources. The success of the method depends on the relative vapor pressures of the component metals in the alloy source because one component metal with a substantially higher vapor pressure will evaporate preferentially. The alloy film will then be rich in the more volatile component compared with the composition of the source and may also be stratified into layers of different composition. Evaporation from alloy sources is discussed in detail by Dushman (43) and Holland (44) on the basis of Raoult s law, and it is possible to predict which metal pairs... 

The kinetic models all allow for evaporation-condensation to be a significant mechanism in surface reconstruction. In particular, as noted earlier, it was frequently suggested that a metal or metal oxide would evaporate preferentially from certain planes, leading to a surface (presumably equilibrium) consisting of planes with the lowest evaporation rates. Net weight loss was anticipated. Yet, no evidence of weight loss is available from the early literature. 

If a two-phase system in the composition range between/and h is heated, then at t liquids/and g are present, and vapor h appears. The system at t is invariant. Since the vapor is richer in butanol than the original overall composition, the butanol-rich layer evaporates preferentially, leaving liquid/and vapor h. As the temperature rises, the liquid is depleted in butanol finally only vapor remains. 

FIGURE 3.4 Evaporation of a mixture of acetone and cyclohexane at 25°C. If a mixture of acetone-cyclohexane (50 50) is allowed to evaporate, the vapour that comes off will have a composition of = 69 31, i.e. acetone will evaporate preferentially. This progressively changes the composition of the mixture until, at 74% acetone, the vapour that comes off is also 74% acetone. This constant evaporation mixture will then evaporate until dry. Similarly if one starts with a mixture of acetone-cyclohexane (90 10), the v our that comes off will have a composition of = 83 17, i.e. cyclohexane will evaporate preferentially until the azeotropic composition of 74% acetone is reached. The change of solubility parameters of the solvent will affect the stability, viscosity, etc. of a polymer solution. Azeotropes are usually described as constant boiUng mixtures, but their behaviour at room temperature is more important for conservators. There are fewer published details of the mixtures at room temperature. The effect of the dissolved polymer on the relative evaporation rates has yet to be elucidated. 

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