Generators, microwave

A classical setup for microwave conductivity measurements is based on the utilization of the waveguides. A simple installation consists of a microwave generator (typically a gun diode) which, when the Ka-band is used, can be operated in the frequency region of 28-40 Gc/s this is protected by an isolator against back-reflections from the rest of the microwave circuit. The microwave power is conducted by an attenuator across a circulator into the microwave conductor branch at the end of which the electrochemical cell is mounted. The microwave power reflected from the electrochemical sample is conducted via the circulator into the microwave detector. It typically consists of a diode that acts as an antenna, receiving the electrical alternating field, rectifying it, and con-... 

This method emplosrs a molten flux which dissolves the material and re-deposits it upon a seleeted substrate. That is, the molten flux acts as a transport medium. The temperature of the flux can be varied to suit the material and to promote high solubility of the solute material in the molten solvent. One example is YIG", yttrium iron garnet, i.e.- Y3FesOi2 -This material is used in the Electronics Industry as single crystals for microwave generating devices. It can be grown via the molten flux method. 

In a typical MIP-MS instrument, the ICP portion is replaced with one of a variety of microwave discharge sources, usually a fairly standardised (modified) Beenakker cavity connected to a microwave generator. The analytical MIP at intermediate power (<500 W) is a small and quiet plasma source compared with the ICP. The mass spectrometer needs no major modifications for it to be interfaced with the MIP. With MIP used as a spectroscopic radiation source, typically consisting of a capillary (1mm i.d.), a power of 30-50 W and a gas flow below 1 L min 1, multi-element determinations are possible. By applying electrodeposition on graphite electrodes, ultratrace element determinations are within reach, e.g. pg amounts of Hg. 

Figure 1.7 Schematic drawing of a microwave-generating klystron tube.

Fig. 1.18 General view of the Pulsar system. Each applicator is connected to one or two microwave generators set in both side parts of the equipment (courtesy of MES company).

This equipment could be used for chemical reactions based on a strong solid-gas interaction with gas adsorbed on powder such as limited air oxidation or with gas release (water, ammonia) such as esterification. The oversized applicator structure permits the design of dielectric pipe to manage such matter transfers. This equipment can be also used for many reactions on solid supports. A typical unit is powered with microwave generators units of 2 or 6 kW for a total microwave power close to 20 or 60 kW. 

An alternative approach for the preparation of supported metal catalysts is based on the use of a microwave-generated plasma [27]. Several new materials prepared by this method are unlikely to be obtained by other methods. It is accepted that use of a microwave plasma results in a unique mechanism, because of the generation of a nonthermodynamic equilibrium in discharges during catalytic reactions. This can lead to significant changes in the activity and selectivity of the catalyst. 

Figure 15.8—Coupling of a gas chromatograph with an atomic emission spectrophotometer. Effluents from the capillary column are injected into the plasma and decomposed into their elements. Each chromatogram corresponds to the compound containing the element of interest. For a given retention time, indication as to the elements included in a compound can be obtained. The plasma in this example is generated by heating the carrier gas (He) with a microwave generator confined in a cavity at the exit of the column. A diode array detector system can be used for simultaneous detection of many elements (chromatograms courtesy of a Hewlett Packard document).

Coleman, C.M., MacElroy, J.M.D., Gallagher, J.F. and O Shea, D.F., Microwave parallel library generation comparison of a conventional- and microwave-generated substituted 4(5)-sulfanyl-lH-imidazole library, /. Comb. Chem., 2002, 4, 87-93. 

Polyatomic molecules have more complex microwave spectra, but the basic principle is the same any molecule with a dipole moment can absorb microwave radiation. This means, for example, that the only important absorber of microwaves in the air is water (as scientists discovered while developing radar systems during World War II). In fact, microwave spectroscopy became a major field of research after that war, because military requirements had dramatically improved the available technology for microwave generation and detection. A more prosaic use of microwave absorption of water is the microwave oven it works by exciting water rotations, and the tumbling then heats all other components of food. 

The basic components of a microwave system include a microwave generator (magnetron), a waveguide for transmission, a resonant cavity, and a power supply. For safety and other reasons, domestic microwave ovens are not suitable for laboratory use. There are two types of laboratory microwave units. One uses closed extraction vessels under elevated pressure the other uses open vessels under atmospheric pressure. Table 3.12 lists the features of some commercial MAE systems. 

Scheme 17 Microwave-generated symmetric HIV-1 protease inhibitors...

The largest microwave reactor for organic synthetic applications so far is a pilot plant scale prototype installed at Sairem in France, developed and designed in collaboration with BioEurope and De Dietrich. This custom-built 1 m3 reactor (Fig. 15) with a powerful 6 kW microwave generator is used for the production of Laurydone [49]. Running in a batch-type recycling process, the equipment accomplished a 40% power reduction compared to the... 

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