Solid-state reactions chemical decompositions

When the reactivity of a solid is controlled by the crystal structure, rather than by the chemical constituents of the crystal, the reaction is said to be topochemically controlled. The nature of products obtained in a decomposition reaction is frequently decided by topochemical factors, particularly when the reaction occurs within the solid without separation of a new phase (Thomas, 1974 Manohar, 1974). A topotactic reaction is a solid state reaction where the atomic arrangement in the reactant crystal remains largely unaffected during the course of the reaction, except for changes in dimension in one or more directions. Dehydration of Mo03-2H20 is a typical example of a topotactic reaction ... 

Photolysis of oxalate complexes show that there is a strong tendency to undergo photoredox decompositions, resulting in oxides of carbon. Two useful applications of this are (i) the system based on the redox photolysis of aqueous [Fe(C204)3]3- is widely used for chemical actinometry 100 and (ii) UV irradiation of (phos)2M(C204) complexes (M = Pd, Pt) result in loss of two molecules of carbon dioxide and production of the synthetically useful, coordinatively unsaturated M° complex (phos)2M.19 One reaction which, if generally applicable to dicarboxylate complexes, may have considerable impact upon the validity of physical measurements upon these systems is the rather unusual, room temperature, solid-state reaction (2).102... 

In solid phase reaction s3mthesis, there are three types of chemical reactions oxidation or reduction of a solid, thermal decomposition of a solid, and solid state reaction between two t3 s of solid. With liquid phase ssmthesis of ceramic powders, there are five different methods drying of a liquid, precipitation, sol-gel sjmthesis, hydrothermal S5m-thesis, and reactions of a liquid metal melt with a gas to give a solid ceramic. There are basically three operational principles for precipitation temperature change, evaporation, and chemical reaction. Sol—gel... 

Preparation of a peritectic compound requires a solid state (solid-solid) or solid-liquid reaetion. A solid state reaction requires transport of matter (diffusion) in the direetion of a chemical potential gradient (i.e., a chemical composition gradient). Thus solid state reactions are diffusion controlled, with diffusion fastest across grain boundaries. Within a crystal, diffusion is enhanced by defects. Clearly the preparation of a peritectic alloy should be carried out at the highest possible temperature just below the peritectic decomposition temperature. The speciality of powder metallurgy exploits the optimum conditions for solid state reactions. 

The theory of solid state reaction kinetics includes no consideration of surface properties other than the recognition that crystal faces are the most probable location for nucleation in many reactions. Dehydration studies have provided evidence that, in many such processes, all surfaces are modified soon after the onset of chemical change [20,21], This is ascribed to a surface reaction that is limited in extent and can continue only at local sites of special reactivity where the recrystallization required for nucleation is possible. In other decompositions there is evidence that the modified reactivity associated with surfaces may influence the overall reaction [11] and may also preserve the identity of crystals. This, incidentally, masks the occurrence of melting during decomposition [22],... 

The decomposition pressure of NiS2(cr) was measured as a function of temperature by means of a tensimetric method. The solid phase, NiS2(cr), was prepared by solid state reaction between NiS(cr) and sulphur. The composition of the product was checked by chemical analysis (no X-ray diffraction analysis was performed). 

As a pronounced contrast, in the field of high performance non-oxide ceramics, currently only binaries are in use. Thus, the manifold opportunities for creating new and ever more capable nitride or carbide ceramics offered by the use of multi-component systems seem to have remained essentially unexplored. The main chemical reason for this lagging behind of non-oxide ceramics are clearly the extremely low self-diffusion coefficients of silicon or boron in their nitrides or carbides [6, 7]. Although the experimental data available are rather limited, the numbers presented in Table 1 [8] suggest that the temperatures needed to complete a solid state reaction between SiC and Si3N4 in an acceptable length of time reach, or even exceed, the decomposition temperature of at least one of the reactants. 

There are three types of solid-state reaction (i) chemical decomposition, (ii) chemical reaction between solids, and (iii) chemical reduction. The first two reactions will be introduced, while the third one is commonly used for nonoxides... 

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