Vapor condensation methods

Ullrafine particles (UFPs) of metal and semiconductor nitrides have been synthesized by two major techniques one is the reactive gas condensation method, and the other is the chemical vapor condensation method. The former is modified from the so-called gas condensation method (or gas-evaporation method) (13), and a surrounding gas such as N2 or NII2 is used in the evaporation chamber instead of inert gases. Plasma generation has been widely adopted in order to enhance the nitridation in the particle formation process. The latter is based on the decomposition and the subsequent chemical reaction of metal chloride, carbonate, hydride, and organics used as raw materials in an appropriate reactive gas under an energetic environment formed mainly by thermal healing, radiofrequency (RF) plasma, and laser beam. Synthesis techniques are listed for every heal source for the reactive gas condensation method and for the chemical vapor condensation method in Tables 8.1.1 and 8.1.2, respectively. 

There are numerous approaches to the synthesis of highly dispersed metal oxides in addition to those discussed. These include some methods that have been less commonly used to prepare metal oxides, such as vapor condensation methods, spray pyrolysis and templated techniques. 

Insofar as mercury and cadmium are concerned, and lead to a lesser extent, no matter how the incinerators are operated, a significant fraction of these materials will be volatilized during incineration and enter the ecosystem via the airborne path, unless recovered from the flues by fly ash precipitation and vapor condensation, methods of questionable merit for large scale MSW operations, uie remainder of the cadmium and lead will end up in the incinerator ash and in the incinerator residues, but all the mercury may be expected to be volatilized. This means that unless the reduction of the toxic materials at the source can be practiced, the incinerator residues and flues will need to be processed to remove lead and cadmium for recycling or for safe disposal in some other manner. The most effective and also the most economical way to prevent mercury from entering the environment from batteries is to phase out the use of mercury in batteries to the ftillest extent possible, an effort already instituted by the battery manufacturers, and to maintain an effective collection system for the mercury batteries still in use. 

Avery and Ramsay (129) prepared Si02 (also ZrOa) particles about 4 nm in diameter as a voluminous powder by a vapor condensation method and compacted this in stages up to 100 tons in. . In the original Duffy powder it was not possible to determine any definite pores but at the point beyond where a coherent body was obtained, the approximate relation between pressure and the coordination number of silica particles was as follows ... 

Smith, M. W. Jordan, K. C. Park, C. Kim, J.-W. Lillehei, P. T. Crooks, R. and Harrison, J. S. Very long single- and few-walled boron nitride nanotubes via the pressurized vapor/condenser method. 

The chemical vapor condensation method was applied for the preparation of ferromagnetic nanoparticles with a core-sheU structure by the pyrolysis of iron pentacarbonyl ([Fe(CO)5]) [176, 177]. Among the factors which strongly affect the characteristics of finally formed particles is the decomposition temperature of the precursor at the tubular furnace. During decomposition of the precursor vapor in the heated furnace, nuclei are formed and grown to form the observable particles. A saturation vapor pressure ratio increases with an increase of the decomposition temperature. A higher saturation vapor pressure ratio results in the larger particle formation. The relationship between decomposition temperature and particle size... 

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

Metallization layers are generally deposited either by CVD or by physical vapor deposition methods such as evaporation (qv) or sputtering. In recent years sputter deposition has become the predominant technique for aluminum metallization. Energetic ions are used to bombard a target such as soHd aluminum to release atoms that subsequentiy condense on the desired substrate surface. The quaUty of the deposited layers depends on the cleanliness and efficiency of the vacuum systems used in the process. The mass deposited per unit area can be calculated using the cosine law of deposition ... 

Distillation is a method of separation that is based on the difference in composition between a Hquid mixture and the vapor formed from it. This composition difference arises from the dissimilar effective vapor pressures, or volatihties, of the components of the Hquid mixture. When such dissimilarity does not exist, as at an a2eotropic point, separation by simple distillation is not possible. Distillation as normally practiced involves condensation of the vaporized material, usually in multiple vaporization/condensation operations, and thus differs from evaporation (qv), which is usually appHed to separation of a Hquid from a soHd but which can be appHed to simple Hquid concentration operations. 

The vapor deposition method at variable substrate temperature provides additional insight into the structure and wetting properties of 8CB in its various phases. If the substrate temperature is between 41 and 33°C, fiat islands 32 A thick are formed if only a small amount of 8CB is condensed on the surface. The size of these fiat islands increases with deposition time while their height remains constant until a uniform layer is formed. If more 8CB is deposited, droplets form on top of the film. This is shown in the image of Figure 14. 

Elace. Hence, if particle buildup is to De sought, the scrubber should e preceded by an appropriate gas-conditioning section. On the other hand, particle collection by Stefan flow can be induced simply by scrubbing the hot, humid gas with sufficient cold water to bring tne gas below its initial dew point. Any practical method of inducing condensation on the dust particles will incidentally afford opportunities for the operation of the Stefan-flow mechanism. The hot gas stream must, of course, have a high initial moisture content, since the magnitude of the effects obtained is related to the quantity of water vapor condensed. 

Rapid solidification and devitrification of amorphous metals and metallic glasses Combustion-flame chemical vapor condensation processes (Kear) Induction-heating chemical vapor condensation processes DC and RF magnetron sputtering, inclusive of the method of thermalization Laser ablation methods Supercritical fluid processing... 

Condensation—The condensation method requires the formation of a vapor-air mixture that is supersaturated with respect to the vapor. Condensation from the supersaturated vapor may then... 

Nanomaterials are also prepared by chemical vapor deposition (CVD) or chemical vapor condensation (CVC). In these processes, a chemical precursor is converted to the gas phase and then it undergoes decomposition to generate the nanoparticles. These products are then subjected to transport in a carrier gas and collected on a cold substrate, from where they are scraped and collected. The CVC method may be used to produce a variety of powders and fibers of metals, compounds, or composites. The CVD method has been employed to synthesize several ceramic metals, intermetallics, and composite materials. 

Hot-vapor bypass pressure control. A more modern way of controlling a tower s pressure is shown in Fig. 13.6. This is the hot-vapor bypass method. When the control valve on the vapor bypass line opens, hot vapors flow directly into the reflux drum. These vapors are now bypassing the condenser. The hot vapors must condense in the reflux drum. This is because there are no vapors vented from the reflux drum. So, at equilibrium, the hot vapors must condense to a liquid on entering the reflux drum. They have no other place to go. 

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