Mixed powder

Calcination. Calcination involves a low (<1000° C) temperature soHd-state chemical reaction of the raw materials to form the desired final composition and stmcture such as perovskite for BaTiO and PZT. It can be carried out by placing the mixed powders in cmcibles in a batch or continuous kiln. A rotary kiln also can be used for this purpose to process continuously. A sufficiendy uniform temperature has to be provided for the mixed oxides, because the thermal conductivity of powdered materials is always low. 

Urea forms a 1 1 adduct with hydrogen peroxide. Urea peroxohydrate [124-43-6] CO(NH2)2 202, is made simply by mixing powdered urea and 35% hydrogen peroxide in the presence of stabili2ers, and crystaUi2ing the product by cooling or concentration. It is available in the form of crystals or tablets. The former contain about 35% H2O2, the latter about 34%. The solubihty in water is 510 g/L at 20°C. The solution decomposes above 55°C. 

Fan, Chen and Lai, Recent Developments in Solids Mixing, Powder Technol. 61,255-287 (1990). 

Small particles can be introduced into metals or ceramics in other ways. The most obvious is to mix a dispersoid (such as an oxide) into a powdered metal (aluminium and lead are both treated in this way), and then compact and sinter the mixed powders. 

The strong influence of morphology and mixing is well illustrated with the composite particle investigation. These particles were composed of a nickel shell coated on spherical aluminum particles by hydrogen reduction in aqueous metal salt solution. The overall ratio of material in a particle was about 80 wt% Ni and 20 wt% aluminum. With these particles, the ratio of reactants was approximately the same as in the mixed powders, but the morphology of the reactants is radically different. 

The shock-modified composite nickel-aluminide particles showed behavior in the DTA experiment qualitatively different from that of the mixed-powder system. The composite particles showed essentially the same behavior as the starting mixture. As shown in Fig. 8.5 no preinitiation event was observed, and temperatures for endothermic and exothermic events corresponded with the unshocked powder. The observations of a preinitiation event in the shock-modified mixed powders, the lack of such an event in the composite powders, and EDX (electron dispersive x-ray analysis) observations of substantial mixing of shock-modified powders as shown in Fig. 8.6 clearly show the first-order influence of mixing in shock-induced solid state chemistry. 

Patent pending). Some designs work well on mixing powders. There are over 30 different models of static mixers worldwide [38],... 

Mix 20g of mixed powder and 25 ml PVA solution to form the concentrated slurry support. A different composition of mixed powder and PVA solution was used. 

Continuous Model "C0NGAS". This model predicts performance of an ideal continuous wellstirred polyreactor. The model system consists of a continuous backmix reactor in which the total powder volume is held constant. There are four inlet streams 1) Makeup of pure propylene, 2) Catalyst feed, 3) Hydrogen feed, and 4) Recycle. The single effluent powder stream is directed through a perfect separator that removes all solids and polymer and then the gases are recycled to the reactor. The makeup propylene is assumed to disperse perfectly in the well-mixed powder. 

Chemical engineers could also work to devise processes to improve the flow characteristics of powders after they are formed. Such research would help control agglomeration of particles in subsequent processing steps as well as facilitate the production of compacted ceramic preforms. For example, gas-solid chemical reactions might be used to tailor the chemical composition of powders. As another example, better methods of componnding powders with binders might be achieved by processes that mix powders with suitable binders in a liquid and then spray dry the resulting suspension. 

For obvious reasons, the possibility of compressing mixed powders into tablets without an intermediate granulating step is an attractive one. For many years,... 

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. 

Consider as an illustration measurements of the carbon content of a mixed powder fed to a plant over a period of a month (Davis and Goldsmith, 1972). We have a total of 178 measurements, covering a range of 4.1% to 5.2%. This range was divided into groups such as 4.10-4.19 and so on. The results are displayed in Table 1. 

TABLE I Percentage of Carbon in a Mixed Powder (from Davis and Goldsmith, 1972)... 

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. 

These hydrogenations are rapid and quantitative. Conversely, the gas-solid hydrogenations of alkenes that were doped by platinum metal compounds on their recrystallization maybe incomplete as, for example, the hydrogenation of trans-cinnamic acid or N-vinylisatin [58,61], which should be milled for completion. Furthermore, the spillover technique by mixing powders of substrate and catalyst followed by application of hydrogen and several hours or days rest [69,70] appears inappropriate for quantitative conversions. Milling is, however, not applicable if volatile liquids have to be constantly pumped off [69] or if the products become liquid or sticky [70]. 

In an early experiment, metal/graphite composite anodes were made of bored graphite rods that were packed with pressed mixtures of metal oxide powder, graphite powder, and pitch (12,13). The packed graphite rods had to be heated to about 1600°C for several hours under vacuum in order to cure the pitch. Subsequently the preparation of composite anodes was simplified by simply packing with a mixed powder of metal oxide (or metal) and carbon. Several kinds of composite rods, in which metal particles are uniformly dispersed in graphite, are now commercially available (31). 

Turpin s Explosive, described by Colver (AddnlRef B) was patented in 1885 for filling HE shells. It vas prepd by mixing powdered PA with small amounts of collodion cotton, grease, or aqueous gum arabic and forming into blocks of the desired shape and size Note Accdg to Stettbachet (AddnlRef C, 15), Turpin disco verdd in 1885 that PA (Picric Acid) can easily be detonated by means of a MF cap and for this reason can serve as... 

The hydrolytic resistance of the products was investigated quantitatively in 0.4 N potassium hydroxide and 0.5 N sulfuric acid solutions by mixing powdered materials (40 mg) in a suitable solvent (20 mL) at 50 0.1°C for 24 h. 

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