Cooling Stage

Material to be frozen can be fully immersed in a cold liquid. This might be a brine, in which case the material may have to be wrapped in a plastic bag to avoid contact with the liquid. The sodium chloride and glycol brines cannot be used cold enough to get complete freezing, so this may be a first pre-cooling stage before a final air blast. Alternatively, liquid nitrogen (- 196°C) or carbon dioxide (-78.5°C) can be sprayed onto the surface. 

Another very useful technique for cryo-crystaUographic studies is optical microscopy, in particular under polarized light. Cooling stages can be easily... 

Figure 2.2 An example of cure schedule—two steps with constant temperature and two different heating rates (dTJdt) for the curing stage one step with constant temperature and one heating rate for the postcure stage one step with one heating rate for the cooling stage...

A direct-contact gas cooler system operates as follows Approximately 35,000 lb/hr of bone-dry air is passed over hot trays. The air is heated from 150°F to 325°F as it passes over the trays. It exits from the unit with a due point of 105°F. The hot air is sent to a direct-contact cooler, where its temperature is reduced back to 150°F. During the cooling stage, the air is dehumidified with water that is heated frpm 75°F to 105°F. The unit is rated at 3.5 inches of water pressure drop (a) Determine the number of diffusion units needed for this operation and (b) Establish the required dimensions for the direct-contact cooling tower (Hint Use standard low-pressure-drop data from the literature. Some of the older literature give pressure drop data for simple fill. See Sherwood, T. K. and C. E. Reed [6]. 

Fig. 2. Deuterium (D) and free carrier concentration (n) profiles of a n-type GaAs Si bulk sample exposed to a rf deuterium plasma for 90 min. at 250°C (rf power density = 0.2W/cm2). The loss of free carriers occurring only in.the deuterated region suggests that hydrogen plays a major role in the free carrier concentration decrease. The deuterium concentration drop in the near surface region is attributed to a deuterium out-diffusion during the cooling stage of the sample with the plasma off. J. Chevallier et al., Materials Science Forum, 10-12, 591 (1986). Trans. Tech. Publications.

Sintering at about 1350 °C in air and the subsequent cooling stages have to be carefully controlled since this is when the barrier-layer characteristics are established. The conditions must be such as to allow a barrier layer of optimum thickness (0.1-1 /mi) to form, while the grains grow, ideally uniformly, to the optimum size, normally about 50/.an. 

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