Gas-phase precipitation

In some cases, the reaction may take place before the substrate is reached while still in the gas phase (gas-phase precipitation) as will be reviewed later. As can be expected, the mathematical modeling of these phenomena can be complicated. 

In the A sector (lower right), the deposition is controlled by surface-reaction kinetics as the rate-limiting step. In the B sector (upper left), the deposition is controlled by the mass-transport process and the growth rate is related linearly to the partial pressure of the silicon reactant in the carrier gas. Transition from one rate-control regime to the other is not sharp, but involves a transition zone where both are significant. The presence of a maximum in the curves in Area B would indicate the onset of gas-phase precipitation, where the substrate has become starved and the deposition rate decreased. 

There are, however, three important factors that control the nature and properties of the deposit to some degree which must be reviewed at this time epitaxy, gas-phase precipitation, and thermal expansion. 

As mentioned previously, a CVD reaction may occur in the gas phase instead of at the substrate surface if the supersaturation of the reactive gases and the temperature are sufficiently high. This is generally detrimental because gas-phase precipitated particles, in the form of soot, become incorporated in the deposit, causing nonuniformity in the structure, surface roughness, and poor adhesion. In some cases, gas-phase precipitation is used purposely, such as in the production of extremely fine powders (see Ch. 19). 

The alkali metals, sodium and potassium, shown in Table 3.1 are not generally used as reductant because their reductive power is so high that it tends to cause premature and detrimental gas-phase precipitation and, as mentioned above, high temperature is necessary to volatilize these metals. 

T orr. Reaction (1) is a simplified version of a reaction that involves the formation of subfluorides such as ReF4. Grain refinement is obtained by the addition of a small amount of H2O in the gas stream.The hydrogen reduction of the chloride is not normally used since it produces gas phase precipitation which is almost impossible to avoid. 

Roman, O. V., Kirilyuk, L., and Chernousova, S., Gas-Phase Precipitation of Tungsten Carbide Coatings,Metall, 6 53-56(1987)... 

A recent competitor to CVD in the planarization of silicon dioxide is the sol-gel process, where tetraethylorthosilicate is used to form spin-on-glass (SOG) films (see Appendix). This technique produces films with good dielectric properties and resistance to cracking. Gas-phase precipitation, which sometimes is a problem with CVD, is eliminated. 

The examples of this section illustrate the general approach to equilibrium problems. Notice that these examples include gas-phase, precipitation, and acid-base chemishy. We use a variety of equilibrium examples to emphasize that the general strategy for working with equilibria is always the same, no matter what type of equilibrium is involved. In Chapters T7 and 18 we apply these ideas in more detail to important types of equilibria. 

Porous aluminum oxide can be used as a template for the production of nanowires and nanotubes. For example, metals can be deposited on the pore walls by the following procedures deposition from the gas phase, precipitation from solution by electrochemical reduction or with chemical reducing agents, or by pyrolysis of substances that have previously been introduced into the pores. Wires are obtained when the pore diameters are 25 nm, and tubes from larger pores the walls of the tubes can be as thin as 3 nm. For example, nanowires and nanotubes of nickel, cobalt, copper or silver can be made by electrochemical deposition. Finally, the aluminum oxide template can be removed by dissolution with a base. 

A summary of the environmental half-lives of OC pesticides in Lake Ontario air and precipitation is provided in Table 6 [52,55,56]. The first-order half-lives for individual OC pesticide concentrations fall within narrow ranges. For example, half-lives of dieldrin in air, precipitation, and water ranged between 2.1 and 4.8 years. Similar decreases have been observed in dissolved concentrations throughout the Great Lakes [17]. Comparable half-lives in gas phase, precipitation, and dissolved phase suggested that the entire Great Lakes system is at or near equilibrium, and that the rate at which the equilibrium is achieved between the various compartments is relatively rapid, likely on the order of months [56]. 

Table 6 Comparisons of environmental half-lives (in years) of organochlorine pesticides in gas phase, precipitation, and dissolved phase in Lake Ontario, during 1992-2000 ...

A gas phase precipitation can only be carried out in this way when care is taken to suppress nucleation in the gas phase. This means that rapid homogeneous gas phase reactions should be avoided. Several ways to achieve ttiis are conceivable ... 

Metals can be precipitated from the Hquid or gas phase. For example, nickel ammonium carbonate gives nickel powder when subjected to hydrogen in an autoclave. Copper, cobalt, molybdenum, and titanium powders can also be formed by precipitation. 

Another method of manufacturing SiC by the decomposition of a gas mixture containing silane, propane, and hydrogen, and hydrogen chloride has been described (80). With such a mixture, it was possible to work at a relatively lower (1200°C) temperature and it was claimed that compact, homogeneous P SiC crystals were obtained. In a variation of this gas-phase synthesis theme, SiC has been produced from the reaction of SiCl and methane (81). SiC precipitates from 1000 to 3000°C. 

Polymers that form from the liqmd phase may remain dissolved in the remaining monomer or solvent, or they may precipitate. Sometimes beads are formed and remain in suspension sometimes emulsions form. In some processes solid polymers precipitate from a fluidized gas phase. 

At higher pressure (i.e., atmospheric), the reactant gas must be diluted with an non-reactive gas, such as hydrogen or argon, to prevent vapor-phase precipitation, while generally no dilution is necessary at low pressure. However, atmospheric-pressure reactors are usually simpler and cheaper, and, with proper control of the deposition parameters, satisfactory deposits may be obtained. 

If the temperature and supersaturation are sufficiently high in a CVD reaction, the product is primarily powder precipitated from the gas phase (see Ch. 2). Such powders have few impurities provided that the CVD precursors are carefully purified. Their small diameter and great uniformity are important factors in the production of high quality hot-pressed or sintered ceramic bodies with good mechanical and electrical properties. In addition, the sintering temperatures required for CVD powders are lower than those for conventional powders. 

Inside newiy formed caverns, calcium carbonate precipitates from water dripping from the ceilings of the chambers. This happens when water that is saturated with carbon dioxide and calcium hydrogen carbonate comes into contact with air. Some of the dissolved CO2 escapes into the gas phase. This shifts the two equilibria to the left, and solid calcium carbonate precipitates ... 

The appearance of CO2 in the gas phase before precipitation of metal implies the production of a soluble Pd(0) complex such as [Pd(CO)2Bt2] . This receives support from the observation that three times as much CO is consumed as CO2 liberated before deposition of Pd. The following reactions may be involved... 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

Gas-Phase Grafting Liquid-Phase Grafting Deposition-Precipitation... 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

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