K. I. Rybakov

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.

Preferred orientation of pores in ceramics under heating by a linearly polarized microwave field

A spherical pore in the ionic crystalline material subjected to a linearly polarized microwave radiation is shown to flatten along the electric field vector. The deformation of the pore occurs due to rectification of high-frequency flows of charged vacancies in the course of their nonlinear interaction with the electric field near the pore surface. The estimates show that the effect is most pronounced in the materials with a significant contribution of mobile vacancies into microwave absorption. Preferred orientation of pores has been observed experimentally in a zirconia ceramic sample sintered under heating by a linearly polarized microwave radiation.

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.

Stability of microwave heating of ceramic materials in a cylindrical cavity

Microwave heating of ceramic materials in a cylindrical wave field is analyzed by numerical simulation. The issues of temperature uniformity and stability inside the sample undergoing microwave heating are addressed. The heating regimes leading to development of global and localized thermal instabilities are illustrated. The role of the temperature dependence of the materials microwave dielectric properties in the formation of instabilities is demonstrated. The tuning and matching of a cylindrical cavity containing the sample is analyzed within a simple model for the microwave and millimetre-wave frequency ranges.

Fabrication of metal-ceramic functionally graded materials by microwave sintering

A model of microwave sintering of multilayer graded metal-ceramic structures is presented. Based on the sintering kinetic data for pure materials, numerical calculations of densification of multilayer compositions have been accomplished. Experimentally, the compaction conditions of the multilayer powder compositions and the microwave heating regimes that ensure obtaining integral multilayer structures Al2O3-Ni, Al2O3-NiAl and ZrO2-Mo with stepwise varying composition have been determined.

Microwave Sintering of Nanostructured Ceramic Materials

Experimental works devoted to studies of the sintering processes of ceramic and composite materials upon heating by microwave radiation have been reviewed. Since the above processes are presently considered to comprise one of the most promising methods of obtaining bulk articles with nanosized structure, special attention has been given to works in which the microwave heating application made it possible to obtain high-density materials while preserving an average grain size of about 100 nm. Also, a significant part of this review is related to studies of the microwave-field nonthermal effect on mass-transfer processes in a polycrystalline solid. The prospects of purposefully using the electromagnetic field effect to obtain nanostructured materials with predetermined properties have been discussed.

Microwave Joining of ZrO2 and Al2O3 Ceramics Via Nanostructured Interlayers

In recent years considerable interest has been drawn to the development of functionally graded materials, and in particular, graded thermal barrier coatings (TBC) [1,2]. One widely studied TBC system is ZrO2 — Al2O3 — metal, in which zirconia is responsible for high-temperature stability of the coating, and alumina prevents oxygen diffusion to the metal and thereby enhances its resistance to corrosion. One of the most promising methods of creating thermal barrier structures on the basis of the ZrO2 + Al2O3 ceramic composition is high-temperature diffusion joining of these materials. A major problem with the joining of dissimilar materials is high residual stresses in the contact zone and its vicinity. These stresses result from a mismatch of the coefficients of thermal expansion (CTEs) when the joint is cooled down after processing.

Flash Sintering of Oxide Ceramics under Microwave Heating

We report on the results of the analysis of the effect of flash sintering, which is observed upon heating compacted powder materials by high-intensity microwave radiation. Ceramic samples of Y2O3, MgAl2O4, and Yb: (LaO)2O3 were sintered to a density exceeding 98–99% of the theoretical value during 0.5–5 min without isothermal hold. The specific microwave power absorbed volumetrically in the samples was 20–400 W/cm3. Based on the analysis of the experimental data (microwave radiation power and heating and cooling rates) and of the microstructure of the obtained materials, we propose a mechanism of flash sintering based on the evolution of the thermal instability and softening (melting) of the grain boundaries. The proposed mechanism also explains the flash sintering effect observed when a dc or a low-frequency ac voltage is applied to the samples. The microwave heating makes it possible to implement flash sintering without using electrodes for supplying energy to the articles being sintered.

Microwave Heating of Metal Power Clusters

The results of simulating the rapid microwave heating of spherical clusters of metal particles to the melting point are reported. In the simulation, the cluster is subjected to a plane electromagnetic wave. The cluster size is comparable to the wavelength; the perturbations of the field inside the cluster are accounted for within an effective medium approximation. It is shown that the time of heating in vacuum to the melting point does not exceed 1 s when the electric field strength in the incident wave is about 2 kV/cm at a frequency of 24 GHz or 5 kV/cm at a frequency of 2.45 GHz. The obtained results demonstrate feasibility of using rapid microwave heating for the spheroidization of metal particles with an objective to produce high-quality powders for additive manufacturing technologies.