Powder formation

Table 7.3 Newer techniques for ceramic powder formation...

In view of the above, instead of avoiding powder formation regimes in discharges, careful powder management involving optimization of the ratio of radicals and silicon nanoparticles arriving on the substrate has been proposed [379]. 

A considerable decrease both in the deposition rate and in powder formation was found when the 13.56-MHz excitation was modulated with a square wave of 2 Hz [510, 511]. In the y -regime, Biebericher et al. [512] have observed a decrease in deposition rate from 1.0 nm/s in a continuous-wave (cw) 50-MHz SiH4-H2 plasma, to 0.2 nm/s in a similar (i.e. with the same average power of 10 W) plasma, modulated by a frequency of 100 kHz. 

Limiting currents measured for a deposition reaction may be excessively high due to surface roughness formation near the limiting current. Rough deposits in the case of copper deposition have been mentioned several times in previous sections, since this reaction is one commonly used in limiting-current measurements. However, many other metals form dendritic or powdery deposits under limiting-current conditions, for example, zinc (N lb) and silver. Processes of electrolytic metal powder formation have been reviewed by Ibl (12). 

The original method of polymers stabilization was invented by Gladyshev and coworkers [14-18]. They proposed to introduce in polymer a metal compound inert toward dioxygen. This compound is decomposed at elevated temperatures with production of a thin metal powder. Formates, oxalates, and carbonyls of metals were suggested as predecessors of an active metal powder. For example, ferrous oxalate decomposes at 600-630 K with the formation of pyrofore iron and ferrous oxide... 

Freeze-drying (lyophilization) if the product is to be marketed in a powdered format. 

Reduction oC3,5-Octadiyne with Activated Zinc Powder - Formation or Z-3-Octen-5- vne... 

Reduction of 1 -Trimethylsilyl-1 J-heptadivne with Activated Zinc Powder. Formation of l-Trimethylsrlyl-3-hepten-l-yne... 

Besides the already mentioned techniques, a low-temperature plasma has been adopted to enhance the reaction in CVC. Through the synthesis of AIN UFPs by an RF-plasma-enhanced CVC using trimethylaluminum [A1(CH3)3] and NH3 as reactants, the effect of experimental parameters on the rate of powder formation, particle size, and structure was examined (60). A high RF current was primarily connected to a high electron density, which activated the gas-phase reaction to promote the powder formation rate. The increase of both susceptor temperature and A1(CH3)3 concentration also increased the powder formation rate and enhanced the grain growth, where both mechanisms—coalescence by particle collision and vapor deposition on to particle surfaces—were believed to occur. 

The powder metallurgy process consists of three basics steps powder formation powder compaction and sintering. Each of the steps in powder metallurgy will be described in more detail. 

Powder Formation. Metallic powders can be formed by any number of techniques, including the reduction of corresponding oxides and salts, the thermal dissociation of metal compounds, electrolysis, atomization, gas-phase synthesis or decomposition, or mechanical attrition. The atomization method is the one most commonly used, because it can produce powders from alloys as well as from pure metals. In the atomization process, a molten metal is forced through an orifice and the stream is broken up with a jet of water or gas. The molten metal forms droplets to minimize the surface area, which solidify very rapidly. Currently, iron-nickel-molybdenum alloys, stainless steels, tool steels, nickel alloys, titanium alloys, and aluminum alloys, as well as many pure metals, are manufactured by atomization processes. 

The formation of a thin invisible film, such as a film of oxide, on the surface greatly affected the properties of the catalyst and brought about powder formation. 

The present study on Ti02 powder formation from Ti(0-iC3H7>4 in supercritical isopropanol has allowed the determination of reaction kinetic constants and activation energy in a temperature range from 531 to 568 K at 10 MPa. The proposed mechanism is based on a hydrolytic decomposition of the alkoxide initiated by water formed in alcohol dehydration catalysed by reactor walls. The derived reaction kinetic order is unity in accordance with experimental results. Such a mechanism also explains that special cares must be taken about the internal surface state of the reactor in order to obtain reproducible results. 

The stracture of the multilayered emulsions may be preserved during spraydrying, enabling the delivery of emulsions with multilayered interfaces in a powder format. Spray-dried tuna oil powders made from emulsions containing oil droplets with lecithin-chitosan membranes, with added com symp showed good oil retention and water dispersibility (Klinkesom et al. 2006). 

FIGURE 6.36 Characteristic etnalysis on typical ceramic powder formation reactions. Data plotted according to equation (6.135). Redrawn, with permission from Dirksen, S. Benjelloun, and Ring [120]. 

The formation of powders in luminous gas phase has significant implications in the processing of LCVD. In an attempt to obtain a uniform nanofilm on a substrate, the powder formation in gas phase ruins the product. On the other hand, the analysis of powder formation as a function of operational parameters provides important information pertinent to the growth mechanism and the deposition mechanism of LCVD, by which growing species deposit on the surface. 

The excessive formation of powders occurs only under limited conditions, although powder formation has been observed in reactors of different designs and types of discharge and with various monomers, particularly in a specific section of a reactor that is related to the flow pattern of gas. Therefore, powder formation provides an excellent opportunity for examining the basic principles of the polymer deposition mechanism. 

In 1972, Liepins and Sakaoku [7] reported that polymeric powders were formed nearly exclusively in the radio frequency reactors shown in Figures 8.12 and 8.13, in which an organic vapor was introduced into the glow discharge of a carrier gas. The monomers that formed powders nearly exclusively and the yield of powder formation are summarized in Table 8.1. Monomers that did not form powders exclusively (i.e., formed plasma polymer in the form of a film or a film with powders) are shown in Table 8.2. The significant points about these experiments are as follows ... 

Most powders consisted of a large portion of the soluble polymer (as high as 90%) in tetrahydrofuran, which indicates that the kinetic pathlength in powder formation is very short. 

On the other hand, the distribution of polymer depositions in an inductive radio frequency discharge reactor described in Chapter 20, in which monomer is introduced into luminous gas phase and does not go through an inductive coil zone just like in the cases of powder formation studies described above, indicates the following trends ... 

Combining these trends, we can postulate the mechanism of polymer powder formation as follows. When a relatively high concentration (pressure) of monomer vapor meets with the luminous gas phase of a carrier gas in a relatively small volume at sufficiently high pressure, the formation occurs quickly in a relatively small-volume element. Because a sufficient quantity of reactive species are created in the small-volume element, the polymer formation steps approach a critical level above which particles cannot stay in the gas phase without the reactive species diffusing out of the volume element. In other words, the kinetic pathlength is very short under such conditions, as proved by the fact, shown in Table 8.1, that most powders are soluble in solvent. 

Because powder formation can be characterized as the rapid formation of polymeric species in a localized gas phase, the quantity of particles or powders mixed in a coherent film that forms at a substrate surface should also be related to the rate of film formation. Thompson and Smolinsky [8] found a direct correlation between the particle density on the surface of a plasma-polymerized film and the growth rate of the film as depicted in Figure 8.14. 

The inclusion of particles in a film of plasma polymer was once considered by some investigators to be a characteristic problem due to the plasma polymerization mechanism, which hampers the practical use of plasma polymers in some applications. In contrast to this view, the formation of powder or the inclusion of particles in a film is related to the polymer deposition part of polymerization-deposition mechanisms. The inclusion or elimination of particles, therefore, could be accomplished by selection of the proper operational parameters and reactor design. The data of Tiepins and Sakaoku [7] are a typical demonstration that powders can be formed nearly exclusively if all conditions are selected to favor powder formation. An important point is that the monomers used in their study were those commonly used by other investigators for the study of film formation by plasma polymerization in other words, no special monomer is needed to form powders exclusively. 

Because powder formation depends on the polymer deposition portion of the polymerization-deposition mechanisms of LCVD, its dependence on operational parameters such as the flow rate and system pressure is not necessarily the same in... 

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