Powdered reactants

Vapor—vapor reactions (14,16,17) are responsible for the majority of ceramic powders produced by vapor-phase synthesis. This process iavolves heating two or more vapor species which react to form the desired product powder. Reactant gases can be heated ia a resistance furnace, ia a glow discharge plasma at reduced pressure, or by a laser beam. Titania [13463-67-7] Ti02, siUca, siUcon carbide, and siUcon nitride, Si N, are among some of the technologically important ceramic powders produced by vapor—vapor reactions. 

Thermal solid-state reactions were carried out by keeping a mixture of powdered reactant and reagent at room temperature or elevated temperature, or by mixing with pestle and mortar. In some cases, the solid-state reactions proceed much more efficiently in a water suspension medium or in the presence of a small amount of solvent. Sometimes, a mixture of solid reactant and reagent turns to liquid as the reaction proceeds. All these reactions are called solid-state reactions in this chapter. Solid-state reactions were found to be useful in the study of reaction mechanisms, since it is easy to monitor the reaction by continuous measurement of IR spectra. 

This first industrial device has been designed by MES company [65] for drying. It could be used for solid state reactions with powder reactants. Consequently, the reactor cannot be a classical chemical vessel or a classical chemical reactor with stirrer and others associated technical devices but a container able to enclose a reactant powder layer. The geometrical shape of the microwave applicator is parallelepiped box and the reactants are supported by a dielectric conveyor belt with edges as described by the Fig. 1.18. 

An old, simple and still widely used method to perform a solid-state reaction is to mix together the powdered reactants, possibly press them into a pellet and then, generally under a controlled atmosphere, heat it in a furnace for prolonged periods. In a number of cases, especially if fine, well mixed, and compacted component powders are used, this treatment will be sufficient to obtain a complete reaction. In other cases more complex treatments will be necessary (for instance to pulverize the partially reacted pellet and to mix, compact and heat it again). 

FIGURE 6.25 In this preparation of rocket fuel for the space shutde, powdered reactants are mixed with a liquid polymer base and hardened inside the booster rocket shell. 

To provide a well-defined boundary between reactants, experiments have been conducted using ensembles of laminated Ni and Al metallic foils (Anselmi-Tamburini and Munir, 1989). These experiments were motivated by modeling efforts that used a foil geometry to understand interactions between powder reactants (Hardt and Phung, 1973 see Section IV,B). In the experimental technique, nickel (12.5 and 125 tm) and aluminum (12.5 fim) foils were laminated in specific stoichiometric amounts corresponding to the NiAl and NiAls compounds. These ensembles were then ignited in a chemical oven consisting of Ni-I-Al pow-... 

In both the electrothermographic and foil assembly methods, the rapid heating rates associated with combustion synthesis are reproduced. However, the powder reactant contact found in a compacted green mixture of particulate reactants is not adequately simulated. One way to overcome this is to investigate interactions of particles of one reactant placed on the surface of the coreactant in the form of a thin foil. The physical simulation corresponds to the reaction of a powder mixture where the particle size of one reactant is small while that of the coreactant is relatively large. Two methods have been used to initiate the interaction. 

The second approach considers the heterogeneous structure of the compact particulate reaction mixture. Since the combustion wave propagates through a mixture of powder reactants with varying particle sizes (O.l-KX) m), nonuniformity of the combustion front and variations in propagation velocity can be expected at the microscopic scale. 

Powder Reactants Dilution Heat Treatment ( C) Particle Size (nm) Ref. 

Distilling the acid in the presence of red phosphorus is rapid and satisfactory except for the foaming induced by the presence of the insoluble powdered reactant. Treatment with hypophosphorous acid is preferable. 

The reaction rate increases by using powdered reactants with large specific surface area (SSA) and high contact area between their particles the diffusion lengths are also reduced. The SSA of a nonporous solid increases as the size of the particle decreases assiuning homogeneous, nonporous, spherical particles, the relationship SSA-r-5 = 3 holds, where r stands for the radius of the particle and 5 for its density. 

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