ZnO samples with an addition of 0, 0.035, 0.1, and 0.35 mol.% Bi2O3 were microwave sintered at heating rates 10 and 50 degrees C/min to a maximum temperature of 1200 degrees C with zero hold time. The densification curves obtained by optical dilatometry have been studied in their dependence on the dopant concentration and the heating rate. Direct volumetric absorption of microwave radiation resulted in a 50-60 degrees C shift of the densification curves toward low temperatures compared to susceptor-assisted heating. An analysis of the effect of the volumetrically absorbed microwave power on the formation of grain-boundary phases that facilitate densification is presented.

Ultra-rapid microwave sintering of ceramics has been recently demonstrated by the authors. In the experiments with oxide ceramic samples carried out in a 24 GHz gyrotron system for microwave processing of materials, full density was achieved in the sintering processes with a duration of the high-temperature stage of one to several minutes and zero hold at the maximum temperature. The implementation of the ultra-rapid microwave sintering processes was made possible due to fast and efficient control over the temperature of the materials and the supplied microwave power. The absorbed microwave power density was typically in the range of 10–100 W/cm^3, which is within the same order of magnitude as the power of Joule heat in the DC electric field–assisted flash sintering processes. At this power level, a thermal instability is triggered by the volumetric heating, which results in a drastic enhancement of mass transport. In addition, possibility of ultra-rapid microwave sintering of powder metals has been demonstrated within a model accounting for the effective electromagnetic properties and resonant absorption effects.

MgAl2O4 samples were microwave sintered to near-full density in rapid processes with heating rates on the order of 100 degrees C/min and zero isothermal hold. The experiments were carried out using a gyrotron system for microwave processing of materials operating at a frequency of 24 GHz with a maximum power of 6 kW. In the regimes with a preset heating rate sustained by the automatically regulated microwave power, the maximum achieved density was about 95% of the theoretical value in pristine MgAl2O4 samples (maximum sintering temperature 1650 degrees C) and about 97% in 1 wt.% Y2O3-doped samples (1700 degrees C). In the regimes with a fixed microwave power (about 3.5 kW), translucent spinel samples with a relative density above 99% were obtained at 1700 degrees C. The duration of the high-temperature stage of sintering was 1.5-10 minutes. The suggested mechanism responsible for the enhanced densification involves development of a thermal instability and formation of transient liquid phases at grain boundaries. The estimated specific absorbed power in the samples during the high-temperature stage of ultra-rapid microwave sintering was 27-80 W/cm(3), similar to the values observed in dc field-assisted flash sintering experiments.

A 45-GHz gyrotron-based microwave oscillator setup has been purposefully developed for powering of a superconducting electron cyclotron resonance (ECR) ion source. The setup provides a smoothly regulated output power in the 0.1–20-kW range (up to 26 kW in manual mode) with an efficiency of up to 50% and collector depression in both continuous wave (CW) and pulse modes. Pulse mode specifications: duration 5–200 ms, leading edge less than ${2}~\mu \text{s}$ , trailing edge less than ${1}~\mu \text{s}$ , repetition rate 1–10 Hz, and triggered operation using an external reference oscillator. The set of high-voltage power supplies (HVPSs) is equipped with a fast electronic protective system to ensure trouble-free operation of the gyrotron. The functional structure of the setup corresponds to the general design method for feedback-controlled microwave sources. The feedback loop consists of a power meter (or a detector unit, to the user’s choice), a control system, and an anode HVPS, the regulated voltage of which controls the output power of the gyrotron. The feedback characteristic time is less than 1 s. Feedback control is provided for both CW and pulse modes. The setup includes a unique quasi-optical line that transmits the microwave beam through a 2-m air gap, as well as a synthesized mode converter from the Gaussian beam to the TE01 or TE11 mode (to the user’s choice) installed on a 300-kV platform. During the tests, the developed setup proved to be a reliable and user-friendly tool applicable for long-term powering of the fourth-generation ECR ion sources.

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.

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.