Evaporation ideal vacuum

The simplest system in which useful products are obtained by thermochemical processing involves the evaporahon of an element or elements in vacuum in order to produce thin hlms on a selected substrate. This process is usually limited to the production of thin hlms because of the low rates of evaporation of the elements into a vacuum under conditions which can be controlled. These rates can be calculated by the application of the kinetic theory of ideal gases. 

In a highly idealized case representing the case of steady-state evaporation into a vacuum (pG = 0), Hsu and Graham (1976) use Eq. (2-92) to compute the rates of... 

The simplest recording medium is a bilayer structure. It is constructed by first evaporating a highly reflective aluminum layer onto a suitable disk substrate. Next, a thin film (15-50 nm thick) of a metal, such as tellurium, is vacuum deposited on top of the aluminum layer. The laser power required to form the mark is dependent on the thermal characteristics of the metal film. Tellurium, for example, has a low thermal diffusivity and a melting point of 452 °C which make it an attractive recording material. The thermal diffusivity of the substrate material should also be as low as possible, since a significant fraction of the heat generated in the metal layer can be conducted to the substrate. For this reason, low cost polymer substrates such as poly (methylmethacrylate) or poly (vinyl chloride) are ideal. 

A medium vacuum of about 10-12mmHg is ideal for a variety of tasks including rotary evaporation, distillations, and for serving double manifold-type inert gas lines. 

The equation assumes that the evaporation of the liquid takes place at or near the end of the capillary. However, it can be calcrrlated that the evaporation rate of water at 50°C from a 25-pm-lD tube is ca. 50 rrl/s. Therefore, the evaporation does not take place at or near the end of the capillary, but somewhere inside the capillary. The competition between evaporation rate F, and liqttid flow-rate F, is schematically depicted in Figure 4.2 [7]. The situation described above is marked (a) in Fignre 4.2. By increasing the inlet pressrrre it must be possible to go from the situation (a) to the ideal situation (b) or even to the situation (c). However, the resrrlting flow-rate will necessitate a larger pumping capacity of the vacuum system. The situation marked (c) does not resrrlt in stable ion sotuce pressures, because the evaporation surface area is not constant. 

PIPAAm gradually change from the surfaces to the inside of the film for both films cast on NTPSD and PTPSD. For the film cast on PTPSD in vacuum condition, more significant compositional difference was observed between the dish side and the air side than the film cast on NTPSD in vacuum. In combination with XPS and EDX mapping measurements on the films cast on NTPSD and PTPSD at ambient and vacuum conditions, respectively, it was revealed that both the oxidized hydrophilic surface and evaporation rate of water molecules contribute to the formation of an ideal gradient structure in the HA/PIPAAm blend system (Hexig et al, 2010). 

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