Microwave application

Gadolinium yttrium garnets are used in microwave applications and gadolinium compounds are used as phosphors in color television sets. 

Table 3.23 Vessel types for microwave applications Vessel type Description...

Table 3.23 gives an overview of the vessel types in use for microwave applications. It is especially important to distinguish between open vessel (as used in Sox wave ) and closed vessel (pressurised) microwave heating systems (as in MAE). Both open-vessel and closed-vessel microwave systems use direct absorption of microwave radiation through essentially microwave transparent vessel materials (Teflon, PC). 

Numerous microwave applications have been published on decomposition, fusion, dry and wet mineralisation, ashing and extraction. Knapp et al. [67] have reported decomposition efficiencies of over 96% for PE, PVC, PS and PB, using PMD. Boron in polyolefins was determined after high-pressure microwave digestion followed by ICP-MS [80]. 

After some considerations relating to microwave technology, we will examine microwave ovens and reactor background. The limits of domestic ovens and temperature measurements will be analyzed, as well as design principles of microwave applicators. Next, a brief overview of laboratory, experimental and industrial equipment will be given. 

First, laboratory and experimental reactors will be described. The vessel containing reactants or their supports are made of convenient dielectric materials (cylindrical or egg-shaped reactor). Original microwave reactors will be described. The first one is a metallic cylindrical reactor which is also the microwave applicator. It allow to reaches high pressures. The other one is a egg-shaped microwave reactor leading to high focusing level of microwave power. 

For several years M. Delmotte et al. have designed a microwave reactor for high pressure chemistry [63]. The microwave applicator and reactor are identical in order to accommodate the mechanical constraints induced by high pressure within liquids. This is the main interest of this device. The metallic cylindrical pipe is simultaneously a waveguide and the reactor. The device is described by Fig. 1.15. 

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. 

This industrial equipment has been designed by MES company [67]. The Thermostar system is constituted by cylindrical vessel associated with parallelepiped applicator. Circular pipes are very classical geometrical shape for industrial reactors. The Thermostar device is constituted of parallelepiped microwave applicators crossing by a dielectric pipe. Two variants of this device have been designed in relation to reactants state (liquid or solid). 

The second variant is designed for solid state reactants to the exclusion of liquid or gas. This powder variant of Thermostar is described by the Fig. 1.19 (right). The microwave applicator is the same as for the device for liquids heating but the reactant transport is ensured by a metallic screw set within the dielectric pipe. This specific traveling metallic screw crosses all the microwave applicators. The coexistence of this metallic screw with the electric field is ensured by the fact that the major electric field direction is parallel to the major direction and perpendicular to the local curving of the screw. A typical industrial unit for solid or liquid reactants is powered with microwave generators units of 2 or 6 kW for a total microwave power close to 20 or 60 kW. 

Fig. 10.5 Schematic diagram of the microwave applicator 1. reactor, 2. wave guide, 3. fritted silica disc, 4. thermal insulation, 5. catalyst, 6,...

N.T. Link, Two-Dimensional Electron Gas FETs Microwave Applications M. Abe et al., Ultra-High-Speed HEMT Integrated Circuits... 

Folate inhibitors, bacterial resistance mechanisms, 3 32t Folate metabolism, 23 502 Folcysteine, 13 431, 52 Folded guide microwave applicator, 16 522 Folding-carton inks, 14 321 Folding carton packaging, converting, 25 21-22... 

Single-line processing units, 21 847 Single-loop feedback controller, 20 695, 696 Single-mode cavity microwave applicators, 16 521-522... 

Slow-wave microwave applicator structures, 16 522 SL/RN process, 24 516, 518 Sludge... 

S. Stone-Elander, N. Elander, Microwave applications in radiolabelling with shortlived positron-emitting radionuclides, J. Label. Compds Radiopharm. 45 (2002) 715-746. 

The remote nature ofthe interaction between microwaves and the sample means that when the power is switched off the sample rapidly cools. Consequently, it is possible to use the microwave applicator in a more flexible fashion than a normal heating furnace. This becomes an important consideration in the economics of ceramics processing, since the ability to use the furnace flexibly for several processes combined with the shorter reaction times can lead to a more economic use of plant than the conventional thermal processing. 

The Cannizzaro reaction, that is, the base-catalysed disproportionation of a carbonyl compound to an alcohol and a carboxylic acid, has gained some importance as an economically viable alternative to the reduction with borohydrides. However, the reaction is restricted to carbonyl compounds without any a-hydrogen, which do not undergo competing aldol reactions. Thus, mainly aromatic aldehydes are used for this kind of transformation. The protocols developed for microwave applications typically involve solvent-free conditions using alumina as the solid support. Under these conditions, a significant acceleration of the reaction was achieved. 

Perovskites AB)/3C2/303 (A = Ba, Sr, B = Zn, Mg, Co, Ni C = Nb, Ta) are promising compounds for microwave applications. It is important to synthesize these complex oxides as pure perovskite phases because the slightest admixture of a second phase hinders drastically the dielectric properties of ceramics, which sinter only at very high temperatures (1400 to 1500°Q. The precursor chemistry resembles greatly that of BaTi03 formation by alkoxide or alkoxide-hydroxide routes. Below we summarize the 3 approaches to the synthesis of these perovskites by the sol-gel method ... 

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