Microwave circulator

Fig. 5.24. (a) Microwave circulator and (b) its schematic representation. (Adapted from Moulson Herbert, 1990.)... 

Microwave Ferrites. Microwave devices employing ferrites make use of the nonreciprocal propagation characteristics that are close to or at a gyromagnetic-resonance frequency at ca 1—100 GHz. The most important devices are isolators and circulators (see Microwave technology). 

Some power tubes can be operated without the need for a protective ferrite isolator. One example is the cooker magnetron (700 W) used in modern microwave ovens (57). At higher power levels, such as 25 kW, it is more common to employ a protective ferrite device, particularly in the form of a circulator (58), as shown in Figure 3. This results in a power loss equivalent to a few percentage points in system efficiency. The ferrite circulator prevents reflected power from returning to the power tube and instead directs it into an auxiHary dummy load. The pulling of tube frequency is thus minimised. 

Freeze drying has also been carried out at atmospheric pressure in fluid beds using circulating refrigerated gas. Vacuum-type vibrating conveyors, rotating multishelf dryers and vacuum pans can be used as can dielectric and microwave heating. 

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-... 

Although many spectrometer designs have been produced over the years, the vast majority of laboratory instruments are based on the simplified block diagram shown in Figure 1.4. Plane-polarized microwaves are generated by the klystron tube and the power level adjusted with the Attenuator. The Circulator... 

Current single-mode continuous-flow microwave reactors allow the processing of comparatively small volumes. Much larger volumes can be processed in continuous-flow reactors that are housed inside a multimode microwave system. In a 2001 publication, Shieh and coworkers described the methylation of phenols, indoles, and benzimidazoles with dimethyl carbonate under continuous-flow microwave conditions using a Milestone ETHOS-CFR reactor (see Fig. 3.11) [104]. In a typical procedure, a solution containing the substrate, dimethyl carbonate, 1,8-diazabicy-clo[5.4.0]undec-7-ene (DBU) base, tetrabutylammonium iodide (TBAI), and a solvent was circulated by a pump through the microwave reactor, which was preheated to 160 °C and 20 bar by microwave irradiation (Scheme 4.31). Under these condi-... 

Fig. 10.4 Schematic diagram of the microwave system for catalytic study 1. generator, 2. water load, 3. circulator, 4. switch,...

Fig. 14.5 A modified MW oven for microwave photochemistry experiments. A. magnetron, B. reaction mixture with the EDL and a magnetic stir bar, C. aluminum plate, D. magnetic stirrer, E. infrared pyrometer, F. circulating water in a glass tube, G. dummy load inside the oven cavity [88]. With permission from Elsevier Science.

Figure 15.1—Inductively coupled plasma torch. A radiofrequency current (between 27 and 50 MHz) that induces circulation of the electrons in the inert gas drives the torch. The argon serves as an auxiliary gas, a cooling gas and the nebulisation gas. In the upper right is shown an optic device used to collect emitted light in the longitudinal axis of the plasma. Lower down, plasma generated by microwave.

Replacing the field plates of Fig. 14.7 with the resonant microwave cavity shown in Fig. 15.4 allowed an increase in the circulating microwave power by Q, the quality factor, of the cavity.8 The cavity is a piece of WR-90 (X band) waveguide 20 cm long which is closed at both ends. The inside dimensions of the cavity are... 

Microwave heat-assisted enzyme-linked immunosorbent assay (ELISA) has been used for measuring anti-glomerular basement membrane (GBM) antibodies in kidney serum (Van Dorp et al., 1991) The presence of GBM antibodies is one of the characteristics of Goodpasture s syndrome. The application of microwave heating reduces the duration of incubation to assay circulating anti-GBM autoantibodies. 

Su, L., H. Zhang, Q. Wang, and H. Xie. 2007. On-line microwave digestion for the determination of total phosphorus in circulating water by flow injection spectrophotometry. Gongye Shuichuli 27 80-82. 

Earlier, the group of Laporterie reported on another prototype CF microwave reactor [80]. Solvent-free Friedel-Crafts reactions have been successfully carried out employing only catalytic amounts of the FeCl3 catalyst (Scheme 19). At a flow rate of 20-22 mL min 1 the corresponding substrates have been circulated in a molar scale (2 1 ratio) in the apparatus. Thus, 150-250 g products could be isolated. Excess substrates have been recovered by evaporation and recycled in the process. 

Unlike the case of the Faraday rotation isolator, even an elementary description of the operational principle of the circulator is not easy, involving as it does the dimensional resonance of the microwave field within the ferrite cylinder. In this context the word resonance does not signify gyromagnetic resonance but a standing-wave resonance determined by the dimensions of the... 

In addition to its use as a circulator, the device can be operated as an isolator, by terminating one of the ports with a matched load, or as a microwave power switch, by reversing the sense of the biasing field. 

---