Yu. V. Bykov

High-temperature microwave processing of materials

This article reviews the physical aspects of a cross-disciplinary science and technology field: the microwave processing of materials. High-temperature microwave processing has a clear industrial perspective in such areas as the production of advanced ceramics, the deposition of thermal barrier coatings, the remediation of hazardous wastes etc. This review starts with the relevant fundamental notions regarding the absorption of electromagnetic waves, heat transfer and the electrodynamics of single- and multimode microwave cavities. Useful formulae, estimates, and interrelations between process variables are presented. This is followed by a review of process examples illustrating the specific features of microwave processing: reduction in energy consumption and process duration, rapid and controllable heating, peculiar temperature distribution, and selectivity of energy deposition. Much attention is given to the advantages of higher-frequency millimetre-wave processing, which include the enhanced absorption in many materials of industrial interest, improved uniformity of electromagnetic energy and temperature, and the possibility of surface treatment. The phenomenon of microwave process rate enhancement is addressed in connection with the problem of the non-thermal microwave effect on mass transport in solids. Both experimental and theoretical approaches to the identification of the mechanism responsible for this effect are illustrated. Finally, the physical and technical factors influencing microwave technology scaleup and transfer to industry are discussed.

Microwave heating of conductive powder materials

In recent years, a considerable interest has been drawn to microwave heating of powder metals and other electrically conductive materials. In this paper a consistent formulation describing the absorption of microwaves in electrically conductive materials under different microwave heating conditions is developed. A special case when conductive powder particles are surrounded by insulating oxide layers is investigated in detail using the effective-medium approximation. The conditions giving rise to skin effect governed, volumetric, and localized microwave heating are analyzed. Experimental observations of different microwave heating regimes in silicon, iron, and copper powder compacts are in general agreement with the theoretical model.

Evidence for microwave enhanced mass transport in the annealing of nanoporous alumina membranes

Results of a comparative study of pore evolution in nanostructured alumina membranes under annealing in a gyrotron microwave system and in conventional furnace are described. Microwave heating has resulted in an enhanced mass transport leading to reduction in the surface porosity of the membranes. Evolution patterns for the shape of individual pores are discussed and compared for microwave and conventional annealing. The notably different behavior of the pores suggests that microwave radiation provides an additional driving force for mass transport. The experimentally observed enhancement of mass transport appears to be stronger than predicted by the earlier proposed models.

Sintering of nanostructural titanium oxide using millimeter-wave radiation

Abstract A comparative study of sintering of different nanocrystalline TiO2 powder materials has been made. Two types of powders (produced by gas condensation and wire explosion methods) and two techniques of compacting were used for the fabrication of the green samples. Microwave and conventional sintering have been compared too. Grains grow dramatically with increasing of the density of samples. No significant dependence of the grain size on the methods of fabrication of powders, technique of compacting or method of processing has been found. Despite the larger amount of the anatase, the microhardness of microwave sintered samples is about 10% higher than that of the conventionally sintered samples.

Effect of microwave heating on phase transformations in nanostructured alumina

Microwave influence on phase transformations in nanostructured alumina has been investigated in a comparative study. It has been found that microwave heating results in a lower phase transformation temperature, not affecting other features of the phase transformation process such as grain size and the effect of dopant addition. For the first time, the dependence of the effect on microwave intensity has been characterized quantitatively. Unexpectedly, this dependence turns out to be non-monotonic, and the phase transformation rate reaches its maximum when moderate-intensity microwaves are used for heating.

Experimental investigation of a gyrotron with whispering-gallery modes

w One way of increasing the power of a gyrotron [i] consists in a matched increase in the diameters of the cathode and the resonator, making it possible to generate intense electron streams and to convert their energy effectively into electromagnetic radiation. As the relative (in the wavelength scale) transverse dimensions of the resonator increase, however, the natural-frequency spectrum becomes compressed so that to insure stable single-mode operation it is necessary to employ ever more precise methods for electronic and electrodynamic mode selection [2].

Edge effect in microwave heating of conductive plates

It has been observed that the microwave annealing of doped silicon wafers in the multimode cavity is accompanied by a specific temperature rise in the near-edge region of the wafer. Experimental investigation and theoretical analysis suggest that the effect is not a result of the microwave irradiation non-uniformity but occurs due to the diffraction of electromagnetic waves on the edge of a thin conducting plate. The level of local overheating depends on the polarization and propagation direction of the incident electromagnetic wave. It is most pronounced in the case when the wave vector is parallel to the plate surface but perpendicular to the plate edge. A method of the plate screening has been suggested to suppress the edge effect and improve the temperature uniformity over the plate during heating. The efficiency of the method has been confirmed by a FDTD numerical simulation of the microwave field near an edge of the plate irradiated isotropically in the multimode cavity.

28 GHz 10 kW gyrotron system for electron cyclotron resonance ion source

The 28 GHz gyrotron system (GS) has been purposely designed to meet present-day demand for a microwave source for powering an electron cyclotron resonance ion source capable of producing both high total current and high average ion charge. A gyrotron oscillator of GS operates in both continuous wave and pulse regimes with the output microwave power smoothly regulated in the (0.1–10) kW range. As a whole the functional structure of GS follows the general method of design for feedback controlled microwave sources. The fast feedback loop consists of microwave power detector unit, a purposely developed PC control system, and a high voltage power supply, the regulated voltage from which controls the gyrotron output power. The feedback characteristic time is less than 1 s. The accuracy of a pre-set microwave power maintenance in continuous wave operation of GS is better than 100 W at an output power of 10 kW, and about 50 W at a power of 1 kW. In pulse operation the pulse length can be varied in the range of (1...

Sintering of nanophase oxide ceramics by using millimeter-wave radiation

Abstract Sintering of nanocrystalline powder materials by microwave heating has been studied. Two types of materials have been investigated: compacts of Al 2 O 3  ZrO 2 (Y 2 O 3 ) (average grain size 15 nm), prepared by the sol gel method, and TiO 2 compacts (∼100 nm) produced by the blasted-wire method. The kinetics of Al 2 O 3  ZrO 2 compact densification and grain growth under microwave and conventional heating are intercompared With microwave heating densification proceeds in two stages, the first stage is characterized by a decrease in the apparent activation energy in comparison with conventional heating. It has been found that TiO 2 — ceramics with densities ∼99% of T.D. can be obtained by fast microwave heating. However, because of fast sintering, the samples exhibit incomplete anatase-rutile phase transformation and rather low values of microhardness.

Microwave source based on the 24 GHz 3 kW gyrotron with permanent magnet

A 24 GHz 3 kW CW gyrotron (n=2) with permanent magnet and microwave source based on the gyrotron are developed. The source incorporating a set of power supplies, transmission line, and PC-based RF power control extends a line of gyrotron systems designed as a versatile and user friendly tool for research in microwave energy applications.