V.E. Semenov

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.

Microwave ponderomotive forces in solid-state ionic plasmas*

Numerous observations have been reported in the literature of enhanced mass transport and solid-state reaction rates during microwave heating of a variety of ceramic, glass, and polymer materials. An explanation for these controversial observations has eluded researchers for over a decade. This paper describes a series of recent experimental and theoretical investigations that provide an explanation for these intriguing observations in terms of ponderomotive forces acting on mobile ionic species. The ponderomotive phenomenon, like its conventional-plasma analog, can be described in the continuum model limit by combining the continuity, Poisson’s, and transport equations. However, the solid-state plasma version typically manifests as a result of gradients in mobile charge mobility (e.g., near physical surfaces or interfaces), whereas the conventional plasma ponderomotive transport is typically a consequence of gradients in the radiation field intensity. Both cases can be captured in a single, general, mathematical articulation developed in terms of the mobile particle fluxes.

Multipactor in rectangular waveguides

Multipactor inside a rectangular waveguide is studied using both an analytical approach and numerical simulations. Particular attention is given to an analysis of the role of such effects as the velocity spread of secondary emitted electrons and the action of the rf magnetic field on the electron motion. Conventional resonance theory is shown to give correct predictions for the multipactor threshold in cases where the height of the waveguide is very small and first order resonance multipactor dominates. In cases of higher order resonances, an accurate prediction of the multipactor threshold requires that the spread of the normal component of the electron emission velocity is taken into account. Furthermore, the spread of the tangential component of the electron emission velocity and the action of the rf magnetic field are shown to be very important when the waveguide height exceeds a certain critical value, which depends on the waveguide width. A new theory is developed for predicting the multipactor thre...

A dielectric mixing law for porous ceramics based on fractal boundaries

The effect of porosity on the complex dielectric permittivity of microwave sintered zinc oxide at room temperature and 2.45 GHz is reported. The predictions of conventional Maxwell–Garnet theory and the effective medium approximation are in poor agreement with the experimental results. Various methods are employed to investigate the system in an effort to come up with new mixing laws, including combinations of these two analytic theories and finite difference electromagnetic simulations of representative microstructures. A model that assumes the existence of dielectrically inactive, fractal‐geometry boundaries between ceramic grains provides an excellent description of the results with no free parameters. It gives physical insight into the experimentally observed mixing law.

Multipactor in a coaxial transmission line. I. Analytical study

Theoretical considerations of the electron multipactor have mostly been restricted to the simplified case of plane-parallel geometry with a uniform field. However, a nonuniform field may not only affect the results quantitatively, but may also lead to qualitatively new effects. In the present work, the effects of a nonuniform radiofrequency field on the properties of multipactor initiation are analyzed in the case of a cylindrical coaxial transmission line. A useful approximate solution of the nonlinear differential equation of motion for the electrons is derived and simple analytical estimates are developed to show that in this system the multipactor mechanism becomes qualitatively different for sufficiently small inner coaxial radii. When the inner radius is of the order of the outer one, the multipactor properties are very similar to those of its counterpart in plane-parallel geometry. However, when the inner radius is less than a certain threshold value, single-surface multipactor becomes possible.

Adiabatic frequency up-conversion of a powerful electromagnetic pulse producing gas ionization

The theory of strong frequency upconversion of the powerful ionizing electromagnetic radiation in gases is presented based on the modified nonlinear geometrical optics approximation. The permanent spectrum upshift versus propagation path, exceeding considerably the initial frequency, is demonstrated without strong wave dissipation for the cases of impact and field-induced ionization in the high-intensity field range. Reflectionless propagation into the supercritical plasma and broad-band tuning of the laser radiation are emphasized as highly promising physical applications of the phenomenon described. >

Hybrid resonant modes of two-sided multipactor and transition to the polyphase regime

A plane parallel model of multipactor is studied in detail using both an analytical approach and numerical simulations. The analytical analysis is carried out within a widely exploited approximation, which assumes a fixed value for the initial velocity of secondary electrons. It is shown that the commonly accepted picture of the multipactor zones is not quite complete and should be modified by taking into account the existence of hybrid resonance modes and the important consequences of a secondary emission yield that significantly exceeds unity. Numerical simulations demonstrate that the chart of the multipactor zones is also very sensitive to a spread of the initial velocity of the electrons. In particular, the full effect of initial electron velocities cannot accurately be modeled by using a single fixed value only.

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.

Investigations of time delays in microwave breakdown initiation

A detailed theoretical as well as experimental investigation has been made of the statistical properties of rf corona breakdown thresholds, relevant to situations where no artificial electron seeding is used and the electron breakdown avalanche is initiated from the weak naturally occurring electron seeding, primarily due to cosmic radiation. Good qualitative as well as quantitative understanding has been obtained concerning the statistical properties of the breakdown initiation process and its consequences for the observed breakdown threshold. Comparisons between theoretical predictions and experimental results show a good agreement.