S. V. Egorov

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.

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.

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.