I. V. Plotnikov

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.

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.

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.

Electromagnetic induction heating for cold wall rapid thermal processing

Electromagnetic Induction Heating is a novel heating technique by which shallow implanted dopants can be activated with minimal movement of the as implanted profile. Silicon wafers can be directly heated, without the use of a susceptor, in the RF (13.56 MHz), microwave (2.45 GHz), or millimeter wave (30.5 GHz) regimes. Heating rates of at least 125/spl deg/C/sec have been achieved and are shown along with temperature vs. power plots in the different frequency ranges. SIMS profiles of as implanted and EMIH annealed boron implants show that considerable activation can be achieved with no diffusion of the junction depth. Post-anneal junctions with sheet resistances that meet the 100 nm technology node and that, with optimization, may meet the 70 nm technology node have been realized. It is suspected that the high frequency electric fields present during annealing provide an additional driving force toward dopant activation, allowing for higher activation at lower temperatures.

24-28 GHz Gyrotron-based Sources for Technological Applications

A design of gyrotron-based systems and examples of their applications are presented. The field of application of the millimeter-wave (mm-wave) energy extends considerably in the last years. Several models of the 24 -28 GHz gyrotron-based systems ranging from 3 to 15 kW in continuous wave power have been recently developed by the Institute of Applied Physics. The design of these systems is presented. Examples of their use for materials processing, production of ion beams, and mm-wave plasma assisted chemical vapor deposition are given.

Development of gyrotron-based technological systems at Gycom/IAP

The line of gyro-device systems produced jointly by Gycom and IAP for millimetre-wave processing of materials has been extended by two new setups. The general description and major technical specifications of a 5 kW 24.1 GHz gyrotron-based system and a dual-frequency system exploiting a 15 kW 28 GHz gyrotron and a 2.5 kW 24.1 GHz gyro-backward-wave oscillator (BWO) frequency tuneable in the range of /spl plusmn/0.5 GHz are presented.

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.

Efficiency enhancement of gyrotron based setups for materials processing

Second harmonic 24–28 GHz CW gyrotrons and series of gyrotron based setups for material processing and Electron Cyclotron Resonance Ion Sources (ECRIS) has been developed at IAP RAS. The possibility of significant output efficiency enhancement due to energy recovery for gyrotrons operating at cyclotron harmonic is demonstrated. The efficiency of 60% at 6 kW output power level at second harmonic 24 GHz gyrotron has been obtained by magnetic field profile optimization and single-stage energy recovery of the spent electron beam. It is shown, that the energy to sintering process can be significantly reduced (1.5–2 times) by simultaneous control of gyrotron voltage and solenoid current. Different varieties of broadband frequency gyro devices — multi-frequency or fast-swept in time operation — are under investigation for ECRIS. The double-frequency CW 10 kW second harmonic gyrotron with 2% fast frequency and a frequency-tunable gyro-BWO operating at center frequency of 60 GHz with CW output power about 10 kW are presented.