Microwave solid-state reaction

Komatsu K (2005) The Mechanochemical Solid-State Reaction of Fullerenes. 254 185-206 Kremsner JM, Stadler A, Kappe CO (2006) The Scale-Up of Microwave-Assisted Organic Synthesis. 266 233-278... 

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. 

We are all familiar with use of microwave radiation in cooking food where it increases the speed of reaction. Recently this method has been used to synthesize solid state materials such as mixed oxides. The first solid state reaction experiments were performed in modified domestic ovens, and these are still used, but more specialised (and expensive ) ovens have also been developed to give more control over the conditions. We shall briefly consider how microwaves heat solids and liquids because this gives us insight into which reactions will be good candidates for this method. 

To use microwave heating in solid state synthesis, at least one component of the reaction mixture must absorb microwave radiation. The speed of the reaction process is then increased by both increasing the rate of the solid state reaction and by increasing the rate of diffusion, which, as we mentioned earlier, is often the rate-limiting step. 

Contamination of the sample being formed in a solid-state reaction may occur from the walls of the containment vessel, but is reduced under microwave conditions because of the lower temperatures at the interfaces between the sample and the crucible. 

Chemicals and the containment materials for chemical reaction do not interact equally with the commonly used microwave frequencies for dielectric heating and consequently selective heating may be achieved. Specifically, it is possible to cool the outside of the vessel with a coolant that is transparent to microwaves (solid C02 or liquid N2) and thereby have cold walls that still allowthe microwave energy to penetrate and heat the reactants, which are microwave active, in the vessel. Also for solid-state reactions contamination from the crucible walls may be minimised. 

These differential heating effects may also be used in solid-state reactions involving metal powders that are made into a fluidised bed by a counter stream of gas. The metal particles interact very strongly with the microwaves and rapidly heat, whereas the gases are transparent to microwaves therefore, the reaction is induced by a very selective interaction between the metal particles and microwaves. 

Whittaker, A.G. and MingosD.M.P., Microwave assisted solid state reactions involving metal powders,/. Chem. 

For the development of a sustainable chemistry based on clean technologies, the best solvent would be no solvent at all. For this reason, considerable efforts have recently been made to design reactions that proceed under solvent-free conditions, using modern techniques such as reactions on solid mineral supports (alumina, silica, clays), solid-state reactions without any solvent, support, or catalyst between neat reactants, solid-liquid phase-transfer catalysed and microwave-activated reactions, as well as gas-phase reactions [37-42]. However, not all organic reactions can be carried out in the absence of a solvent some organic reactions even proceed explosively in the solid state Therefore, solvents will still be useful in mediating and moderating chemical reactions and this book on solvent effects will certainly not become superfluous in the foreseeable future. 

Kniep R, Simon P (2007) Fluorapatite-Gelatine-Nanocomposites Self-Organized Morphogenesis, Real Structure and Relations to Natural Hard Materials. 270 73-125 Koenig BW (2007) Residual Dipolar Couplings Report on the Active Conformation of Rhodopsin-Bound Protein Fragments. 272 187-216 Komatsu K (2005) The Mechanochemical Solid-State Reaction of Fullerenes. 254 185-206 Kremsner JM, Stadler A, Kappe CO (2006) The Scale-Up of Microwave-Assisted Organic Synthesis. 266 233-278... 

Whittaker, A. G. and Mingos, D. M. P. (1993). Microwave-Assisted Solid-State Reactions Involving Metal Powders and Gases Journal of the Chemical Society-Dalton Transactions 2541-2543. 

K. They combined three experiments on two different morphologies (microwave absorption and magnetic susceptibility x of powders, and dc resistivity of films) to assert superconductivity imder unusually difficult, poorly controlled chemical conchtions. As the powder is prepared by a solid-state reaction, only the initial composition is known, so no claim could be made about the... 

Keywords Mineral solid supports, Phase transfer catalysis. Solid state reactions. Reactivity, Selectivity, Microwave activation. 

Solid-state reactions by microwave heating 11 -P-13 Solvent effects 18-P-07... 

There are distinct advantages of these solvent-free procedures in instances in which catalytic amounts of reagents or supported agents are used, because they enable reduction or elimination of solvents, thus preventing pollution at source . Although not delineated completely, reaction rate enhancements achieved by use of these methods may be ascribed to nonthermal effects. Rationalization of micro-wave effects and mechanistic considerations are discussed in detail elsewhere in this book [25, 244], There has been an increase in the number of publications [23c, 244, 245] and patents [246-256], and increasing interest in the pharmaceutical industry [257-259], with special emphasis on combinatorial chemistry and even polymerization reactions [260-263], and environmental chemistry [264]. The development of newer microwave systems for solid-state reaction [265], and introduction of the concepts of process intensification [266], may help realization of the full potential of microwave-enhanced chemical syntheses under solvent-free conditions. 

Thus, although it is still not clear whether specific microwave effects exist, the indisputable observation that a variety of reactions can be carried out under microwave conditions is of significant environmental and commercial value. Reaction times are drastically reduced, resulting in significant economies in electricity and heating costs. Solvent handling problems are also eliminated in microwave-assisted solid-state reactions and organic reactions carried out under dry conditions. 

Ganesh I, Srinivas B, Johnson R, Saha BP, Mahajan YR (2004) Microwave assisted solid state reaction synthesis of MgAl204 spinel powders. J Eur Ceram Soc 24 201-207... 

The formation of YAG through solid-state reaction is controlled by the diffusion of Al -" [156]. Such diffusion of Al -" was enhanced by the microwave field, so that the reaction temperature of YAG and its sintering temperature were decreased, as evidenced by the pores entrapped in grains, as shown in Fig. 7.44b. Similarly, the formation of the liquid phase was also promoted. In other words, the content of sintering aids could be reduced when using microwave sintering, which however needs further clarification. 

It was found that the increase in polymerization rate was not due to an increase in polymerization temperature, but the effect was consistent with direct heating of the condensate leading to enhanced diffusion rates (Table 11) [89]. In the case of microwave PET polymerization, it was noticed that an increase in polymerization rate for hydroxyl and carboxyl functional groups was not monotonous it was higher for hydroxyl end groups. In general, the increase in the solid-state reaction rate due to microwaves was about equivalent to an increase in reaction temperature of 10-15 °C. 

Beta-SiC powder can be produced in a microwave ovoi (2540 MHz) by simple solid-state reaction between silicon and charcoal powder at temperature lower than 1000"C.1"1... 

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