Osmium vapor pressure, high temperature

For this purpose it is necessaiy to give sufficient contrast to a thin film of the frozen sample, for example, by use of osmium tetroxide. Then the sample can be viewed directly in the TEM (at — 196°C). The adjustment of the temperature to — 196°C produces a very low vapor pressure, especially of water, so that the examination of the probe is possible by preservation of the microstructure despite the high vacuum. A disadvantage of cryo-TEM is the classification of vesicles according to their size. Due to the fluid property of the vesicle dispersion prior to freezing, the thickness of the sample film varies from the center to the outside. Hence the smaller vesicles stay in the center, where the film is thin, while the larger ones remain at the outside margin in the thicker part of the film. In this outer part, the vesicles evade... 

For metals a sublimation temperature is chosen that corresponds to a vapor pressure of 10 Torr. This vapor pressure is not sufficient for all cases. Platinum and boron, e.g., have a vapor pressure of 10" Torr at 2100°C, yet platinum evaporates four times faster than boron at this temperature. The same difference in evaporation rate between metals and nonmetals is observed for osmium and carbon. Both have a vapor pressure of 10" Torr at 2650°C but their evaporation rates are as different as in the case of platinum and boron. Apparently the activation energy for evaporation of nonmetals is higher than for metals. Ease of evaporation or a high vapor pressure does not guarantee fast deposition rates even for metals. Although magnesium and zinc are volatile, they are difficult to deposit because they do not condense easily as their closed outer electron shell confers helium-like properties on their gaseous atoms. 

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