Evaporation techniques, high vacuum

In a second TEM technique, a thin layer of gold was evaporated under high vacuum onto a microtomed thin film of the blend containing the maleated elastomer. TEM in Figure 3, shows small islands of gold (of order 5 nm) on the surface of the specimen which are more densely packed on the crystalline polyamide than on the amorphous rubber. The presence of subinclusions within the rubber domain is clearly revealed by the gold decoration. 

Molecular beam epitaxy (MBE) is a radically different growth process which utilizes a very high vacuum growth chamber and sources which are evaporated from controlled ovens (15,16). This technique is well suited to growing thin multilayer stmctures as a result of very low growth rates and the abihty to abmpdy switch source materials in the reactor chamber. The former has impeded the use of MBE for the growth of high volume LEDs. 

Dryers. Drying, another type of evaporation technique, is suited for waste streams of very high soHds content. Several common types of dryers are vacuum rotary dryers, dmm dryers, tray and compartment dryers, and pneumatic conveying dryers. 

Some elements, such as the rare eartlrs and the refractory metals, have a high afflnity for oxygen, so vaporization of tlrese elements in a irormaT vacuum of about 10 " Pa, would lead to the formation of at least a surface layer of oxide on a deposited flhrr. The evaporation of these elements therefore requires the use of ultra-high vacuum techniques, which can produce a pressure of 10 Pa. 

The advantages of this technique are that they are applicable to probes in ultra high vacuum, where any impurities are excluded, and also to samples at atmospheric pressure. The diameter of the evaporated cavity can be varied between 10 jam and 1 mm, and the quantity of the evaporated material may be less than 10" g. Solids in any desired form, crystalline or powered, are accessible to analysis. 

In ESI MS, a dissolved sample is sprayed through a capillary in an electric field which is situated in front of the vacuum inlet of the mass spectrometer [2]. Thus, in contrast to most other ionization techniques performed in high vacuum, the ionizahon process takes place at the atmospheric pressure. After leaving the capillary, the solvent forms a so-called Taylor-cone, which further forms a filament and finally, the spray of small droplets (Figure 14.2). These droplets carry charges on the surface this is frequently supported by the acidification of the solvent. The droplets shrink is caused by the evaporation of the solvent. This leads to an increase of the charge-per-surface ratio, finally... 

Technological developments in high vacuum evaporation techniques have been responsible for the rapid growth of the domestic citrus industry. These techniques were developed and refined, for the most part, during World War II and they made possible the manufacture and production of many perishable foods and medicines. Most notable for the domestic citrus industry was the development of frozen concentrated citrus juices which was made possible by the development of these high vacuum evaporators... 

The most widely used vacuum deposition techniques are evaporation and sputtering, often employed for smaller substrates. In the evaporation process, heating the metal by an electron beam or by direct resistance produces the vapours. The system is operated at a very high vacuum (between 10-5 and 10 6 Torr) to allow a free path for the evaporant to reach the substrate. The rate of metal deposition by evaporation processes varies from 100 to 250,000 A min h These processes can be operated on a batch or a continuous scale. On the other hand, in the case of the sputtering technique, the reaction chamber is first evacuated to a pressure of about 10-5 Torr and then back-filled with an inert gas up to a pressure of 100 mTorr. A strong electric field in the chamber renders ionisation of the inert gas. These inert gas ions... 

---