Hot and Cold Stages

Hot and cold stages can easily be attached to optical, electron, and atomic force microscopes [220, 221]. Cold stages are very common in TEMs, where they may reduce specimen contamination by stopping surface diffusion. In biology, cold stages are used to keep hydrated samples frozen and stable for observation in the vacuum of the TEM or SEM (e.g., [222]), 

A major appUcation of dynamic microscopy is the study of pol)nner structure and its development as a function of temperature in a hot stage. This type of work is rare in electron microscopes. Heating a polymer sample in an electron microscope requires caution, as not only will heating increase the rate of radiation damage, but also the polymer may outgas or degrade and evaporate, contaminating the microscope vacuum system and associated x-ray detectors. 

Spencer, Fundamentals in Light Microscopy (Cambridge University Press, Cambridge, 1982). 

Rawlins, Light Microscopy (BIOS Scientific Publishers, Oxford, 1992). 

Hartley, Hartley s Microscopy (Senecio Publishing, Charlbury, 1981). 

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There are two types of stages for dynamic microscopy (a) hot and cold stages and (b) tensile stages [1], Hot stages are most commonly used for the dynamic microscopy of polymers [1,43]. Thermal analysis in the OM is complementary to other thermal analysis methods, such as differential thermal analysis (DTA) [1], Direct observation of the structural changes of a polymer as a function of temperature can determine the nature of phase changes and thermal decomposition [1], It also measures the transformation temperatures. 

Aqueous heterophase polymerizations may be carried out in microwave ovens because the polar nature of the continuous phase allows for efficient microwave coupling. This dielectric heating is extremely fast as the reaction mixture can be warmed up within about 12 seconds from room temperature to >90 °C. Comparable with radiation induced polymerization pulsed thermal polymerizations (FTP) with alternating hot and cold stages as illustrated in Figure 4 give rise to... 

Illustration of the pulsed thermal polymerization (FTP) procedure (left) with cycles of alternating hot and cold stages and the temperature profiles during polymerizations with pulsed and permanent heating (right). 

Mechanical Instability. The obtainable resolution depends not only on the lens characteristics but also to a large extent on the mechanical stability of the micro.scope. which should be installed on a vibration-free heavy concrete block. In many cases the mechanical stability is actually the resolution-determining factor. The specimen holder should moreover be creep free, which is a problem for hot and cold stages. High-resolution work is therefore performed almost exclusively at room temperature. 

Attachment of a hot or cold stage to the ordinary microscope stage allows the specimen to be observed while the temperature is changed slowly, rapidly, or held constant somewhere other than ambient. This technique is used to determine melting and freezing points, but is especially useful for the study of polymorphs, the determination of eutectics, and the preparation of phase diagrams. 

Other instruments have been described that have application in the electronics field. Special metallurgical hot and cold st es are being produced, and stages capable of large motions with sub-pm accuracy and reproducibility will become common. 

Fig. 26. The production of heavy water is based upon the behavior of deuterium in a mixture of water and hydrogen sulfide. When liquid H2O and gaseous H2S are thoroughly mixed, the deuterium atoms exchange freely between die gas and file liquid. At high temperatures, file deuterium atoms tend to migrate toward file gas, while they concentrate in file liquid at lower temperatures. In the first and second stages of production, file towers of a heavy water plant are operated with the top section cold and file lower section hot. Hydrogen sulfide gas is circulated from bottom to top and water is circulated from top to bottom through the tower. In the cold section, the deuterium atoms move toward file water and are carried downward, while in file hot section, they move toward the gas and are carried upward. The result is that, both gas and liquid are enriched in deuterium at the middle of the tower. A series of perforated trays are used to promote mixing between the gas and water in the towers. A portion of the HjS gas, enriched in deuterium, is removed from file tower at the juncture of file hot and cold sections and is fed to a similar tower for the second stage of enrichment...

The entire rectifying section can be pressurized, and the heat can be transferred between any desired stages of the rectifying and stripping sections. This is called secondary reflux and vaporization (SRV) distillation. It reduces the consumption of both hot and cold utilities, and the sizes of the reboiler and condenser. However, capital cost is increased by additional intermediate heat exchangers moreover, since the process is more thermodynamically reversible, it requires more stages to achieve the desired separation. 

The first stage of form, Aristotle believed, was found in the four elements of Earth, Air, Fire, and Water. The elements, while distinguished from each other, are also related by four qualities. These qualities are dry, moist, hot, and cold. Each element possesses two qualities, of which one predominates, and each element is linked to two other elements by the quality... 

The degree of separation obtainable in thermal diffusion (the difference in composition between hot and cold walls) is much less than in other diffusion processes, so that use of a column to multiply the composition difference is practically essential. The stage type of thermal diffusion has been used only to measure the thermal diffusion coefficient and is never used for practical separations. In some thermal diffusion columns, htu s are as low as 1.5 cm, and as many as 800 stages of separation have been obtained from a sin e column. Even with such a great increase in separation, it is often necessary to use a tapered cascade of thermal diffusion columns for isotopic mixtures, to minimize hold-up of partially enriched isotopes and to reduce equilibrium time. 

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