Powder mixing Diffusion

In plasticized PVC, Hquid plasticizers first fill the voids or pores in the PVC grains fairly rapidly during powder mixing. If a large amount of plasticizer is added, the excess plasticizer beyond the capacity of the pores initially remains on the surface of the grains, making the powder somewhat wet and sticky. Continued heating increases the diffusion rate of plasticizer into the PVC mass where the excess Hquid is eventually absorbed and the powder dries. 

The reduction diffusion process has also been used for the production of powders of the magnetic neodymium-iron-boron alloy (Nd15Fe77B8). The reaction involves use of a powder mix of neodymium oxide, iron, ferroboron and calcium. The reaction is conducted by heating the powder charge mixture at 1200 °C for 4 h under vacuum. Neodymium-iron-boron alloys are much more prone to oxidation than samarium-cobalt alloys and a proprietary leaching procedure is used for the separation of the alloy and calcium oxide. 

The electron microscopic analysis revealed that the hexaaluminate powder prepared by hydrolysis of the alkoxides crystallized in plate-like microcrystals.The size of the powder is 30 nm in thickness and 200 nm in diameter. The plate-like crystal was also observed for the sample derived from powder mixture however, the sample contains granular-shaped crystals and amorphous regions. The plate-like shape is characteristic of the hexaaluminate microcrystals due to their hexagonal layered structure. The granular crystals and amorphous regions observed in the calcined powder mixture are the unreacted particle or the intermediate phases. The composition of each microcrystal is not uniform in the sample from the powder mixture. This means that the bulk diffusion significantly limits the phase formation of hexaaluminate in the case of powder mixing. 

The cost of Gas Diffusion Media (MPL + GDL) [13] is quoted to be aroimd 10- 15 per m, and is likely to be primarily determined by processing costs. Both carbon/graphite paper as woven stmctures are being used as substrate, which is made hydrophobic by, for example, a PTFE coating. The microporous layer mostly consists of carbon powder mixed with a PTFE emulsion, which is cured by a heat treatment. Clear directions for cost reduction have not been found. As the gas diffusion media play a critical role in the performance of the PEMFC, especially determining the maximum power output makes it worthwhile to focus on the GDM performance, rather than on the cost per m. ... 

Calciothermic reduction of samarium oxide, in the presence of cobalt powder, yields samarium-cobalt alloys in the powder form. The process is popularly known as reduction diffusion. Samarium oxide, mixed with cobalt powder and calcium hydride powder or calcium particles, is heated at 1200 °C under 1 atm hydrogen pressure to produce the alloys. Cobalt oxide sometimes partly replaces the cobalt metal in the charge for alloy preparation. This presents no difficulty because calcium can easily reduce cobalt oxide. A pelletized mixture of oxides of samarium and cobalt, cobalt and calcium, with the components taken in stoichiometric quantities, is heated at 1100-1200 °C in vacuum for 2 to 3 h. This process is called coreduction. In reduction diffusion as well as in coreduction, the metals samarium and/or cobalt form by reduction rather quickly but they need time to form the alloy by diffusion, which warrants holding the charge at the reaction temperature for 4 to 5 h. The yield of alloy in these processes ranges from 97 to 99%. Reduction diffusion is the method by which most of the 500 to 600 t of the magnetic samarium-cobalt alloy (SmCOs) are produced every year. 

Stable, conductive electrodes would also be a problem. Preliminary experiments, were carried out in a cell, using simulated flue gas nearly identical to that shown in Fig. 24. In these tests, the membranes were hot-pressed from mixed powders of electrolyte (ternary eutectic of [Na, Li, K]2 S04) with LiA102 as matrix. The electrodes were constructed of cold-pressed Li20-9Cr203, partially sintered to give a highly-porous gas-diffusion structure. The tests were encouraging up to 50% of the S02 was removed from the simulated flue gas with the application of current. Simultaneously, a stream of concentrated S03 and Oz was evolved at the anode. 

Reactants and reagents can be conveniently loaded into the dry zeolite by adsorption. This can be accomplished by intimately mixing the solid or liquid reactant and the powdered zeolite, by absorption from the gas phase, or by diffusion in a solvent slurry containing the zeolite and dissolved reactant. The choice of solvent for the slurry method is critical. It must be volatile enough to be removable at a pressure and temperature that does not result in evacuation of the reactant or its decomposition. In addition, the reactant must have a greater affinity for the interior of the zeolite than for the slurry solvent itself. The lack of affinity for the interior of the zeolite is an acute problem for non-polar hydrocarbons that lack binding sites for the intrazeolitic cations. The use of fluorocarbons such as perfluorohexane as slurry solvents takes advantage of the fluorophobicity of many hydrocarbons and has alleviated this problem to some extent.29... 

By mixing an absorbing granulate or powder with the product to be dried, the distances the diffusion can become very small, or the water molecules may move by surface diffusion. In both cases, the problem is the same First to find an acceptable drying agent (absorber) and then to separate it quantitatively from the dried product. 

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

A similar RP-HPLC method has been employed for the determination of the effect of reduced glutathion (GLT) and hydroxypropyL/Tcyclodcxtrin (HP-/LCD) on the stability of the colour pigments of paprika. Samples were prepared by mixing 5 per cent GLT (w/w) and 5 per cent HP-/TCD (w/w) with paprika powder and stored at room temperature (22 1°C) in diffuse light in Petri dishes covered by glass plates. Samples without additives served as a control, lg of paprika was taken after 14, 28, 42 and 56 days of storage... 

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