A. G. Eremeev

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.

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.

On the Mechanism of Microwave Flash Sintering of Ceramics

The results of a study of ultra-rapid (flash) sintering of oxide ceramic materials under microwave heating with high absorbed power per unit volume of material (10–500 W/cm3) are presented. Ceramic samples of various compositions—Al2O3; Y2O3; MgAl2O4; and Yb(LaO)2O3—were sintered using a 24 GHz gyrotron system to a density above 0.98–0.99 of the theoretical value in 0.5–5 min without isothermal hold. An analysis of the experimental data (microwave power; heating and cooling rates) along with microstructure characterization provided an insight into the mechanism of flash sintering. Flash sintering occurs when the processing conditions—including the temperature of the sample; the properties of thermal insulation; and the intensity of microwave radiation—facilitate the development of thermal runaway due to an Arrhenius-type dependency of the material’s effective conductivity on temperature. The proper control over the thermal runaway effect is provided by fast regulation of the microwave power. The elevated concentration of defects and impurities in the boundary regions of the grains leads to localized preferential absorption of microwave radiation and results in grain boundary softening/pre-melting. The rapid densification of the granular medium with a reduced viscosity of the grain boundary phase occurs via rotation and sliding of the grains which accommodate their shape due to fast diffusion mass transport through the (quasi-)liquid phase. The same mechanism based on a thermal runaway under volumetric heating can be relevant for the effect of flash sintering of various oxide ceramics under a dc/ac voltage applied to the sample.

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.

CW Ka-Band Kilowatt-Level Helical-Waveguide Gyro-TWT

Experimental results and design details are presented on an amplifier setup based on a helical-waveguide gyrotron traveling-wave tube delivering continuous-wave kilowatt-level output power with an instantaneous frequency bandwidth of about 3%.

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.

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.

Prospective gyro-devices for technological applications

Two kinds of gyro-devices are considered: gyrotrons for material processing and tunable microwave devices for Electron Cyclotron Ion Sources. The possibility of the significant output efficiency enhancement due to energy recovery for the gyrotrons operating at cyclotron harmonics was demonstrated. The second-harmonic 24 GHz gyrotron for technology application with output efficiency of 60% was developed and tested at IAP RAS. Record efficiency was obtained by optimizing the profile of the magnetic field and using the energy recovery for the spent electron beam. Different varieties of gyro-devices providing broadband radiation (multi-frequency or fastswept in time) with CW or average power of order of 10–15 kW and the center frequency within the range of 24–60 GHz are also 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.

Effect of Microwave Heating on the Mass Transfer, Phase Formation, and Microstructural Transformations in the Y2O3–Al2O3 Diffusion Couple

The phase composition, phase growth kinetics, and structures of diffusion zones formed under microwave heating (24 GHz) (MWH) and conventional heating (CH) in two-layer Al2O3–Y2O3 samples are studied by optical and scanning electron microscopy and electron microprobe analysis. Diffusion annealing was conducted at 1700°C for 5 h in vacuum, the heating rate being 10°C/min in all experiments. The diffusion couples included alumina layers, such as coarse-grained polycore or sintered Al2O3–5 vol.% ZrO2 layers, and yttria layers, such as sintered coarse-grained samples or fine Y2O3 powder layers on the Al2O3 surface. It is shown that the phases formed during reactive diffusion do not uniquely correspond to the phase diagram, but depend on the initial structure of contacting layers and the type of heating. This is attributed to the contribution of kinetic factors to the competitive phase growth, particularly to the structural sensitiveness of diffusion coefficients whereby the diffusive phases grow and the stresses appearing when new phases form. It is found that MWH influences the competitive phase growth in the Al2O3–Y2O3 system, which involves both the change in the phase composition of the diffusion zone compared to that formed under CH and the acceleration of phase growth. The maximum effect of the phase growth acceleration under MWH is observed for the YAG phase, which is 30 times as fast as that under CH. It is suggested that the structure of grain boundaries changes and, accordingly, their permeability increases under MWH. The accelerated GB diffusion under MWH promotes the YAG phase growth in both oxides as a result of opposite diffusion flows of Al and Y ions along GBs. Under TH the YAG phase is formed only in Y2O3 oxide because of the unipolar diffusion of Al3+ ions to Y2O3. The validity of the proposed mechanism is confirmed by numerical evaluations.