Samples of alumina - 3 % yttria-stabilized zirconia (YSZ) composites were sintered in rapid processing regimes using 24 GHz microwave heating at rates of up to 200 degrees C/min and zero hold time. The final relative density was 96-99 % for the samples containing 1.5 and 7.5 wt % YSZ and 98-99 % for the samples containing 13 wt % YSZ. The microwave sintering kinetics were compared for the processes carried out by direct and susceptor-assisted microwave heating. Under direct microwave heating, the effect of an intense microwave electromagnetic field with an estimated absorbed power density of up to 130 W/cm(3) resulted in a shift of the shrinkage curves by about 100( degrees )C towards lower temperatures compared to the case of susceptor-assisted heating. The grain size of the samples sintered by direct microwave heating decreased with an increasing heating rate. The mechanical properties were slightly higher for the materials sintered under susceptor-assisted microwave heating. The samples containing 13 wt % YSZ exhibited a microhardness of about 20 GPa and a fracture toughness of about 7 MPa m (1/2) .

Using the method of sintering by microwave heating at a rate of 10–100°C/min to a temperature of 1250 °C without isothermal hold, we obtain ceramic samples with the composition Ba0.75Sr0.25TiO3. The experiments are performed on a gyrotron complex for the high-temperature microwave treatment of materials, which is operated at a frequency of 24 GHz with a maximum power of 5 kW. The features of the formation of a solid solution in the sintered samples are analyzed. The dependence of the intensity of recrystallization growth of grains on the microwave-heating rate during the sintering is demonstrated. The permittivity of the samples obtained by microwave sintering with heating rates of 10 and 30 °C/min amount to 3600–4400 in the frequency range 25Hz–3MHz at room temperature.

Rapid microwave sintering processes with heating rates of up to 300 degrees C/min and zero isothermal hold have been implemented using a 5 kW 24 GHz gyrotron system for high-temperature microwave processing of materials. ZnO-based varistor ceramics, BaTiO3/SrTiO3 dielectric ceramics and Gd:CeO2 ceramics for solid electrolyte applications have been sintered to densities of up to 96% of the theoretical value. Using in situ optical dilatometry, correlation between the development of thermal instability under intense volumetric microwave heating and the early onset of densification has been revealed. The influence of the absorbed microwave power on densification and grain growth has been studied by comparing direct and susceptor-assisted microwave heating processes. The possibility of tailoring the microstructure and functional properties of the obtained materials by choosing optimal regimes of rapid microwave sintering is discussed.

Gd:CeO2 ceramic samples with a density of up to 96% of the theoretical value have been obtained by rapid 24 GHz microwave sintering with heating rates of up to 300 degrees C/min and zero hold time. It has been found that a higher absorbed microwave power density lowers the densification onset temperature by up to 150 degrees C. The grain size in the final materials decreased with an increase in the heating rate at a lower power density (a few W/cm(3)) but increased at a higher power density (similar to 40 W/cm(3)). The ionic conductivity of the sintered materials depended on the microwave heating rate; samples sintered at 30 - 100 degrees C/min had an ionic conductivity of approximately 0.5 S/m at a temperature of 560 degrees C.

24 GHz microwave sintering of BaTiO3 ceramics has been studied under rapid heating at a rate of up to 300 degrees C/min to a maximum temperature of 1150-1300 degrees C with no isothermal hold. Under direct volumetric microwave heating the temperature difference measured between the center and periphery of the samples was 200-300 degrees C, and the estimated volumetrically absorbed power density was about 40 W/cm(3). The use of a SiC susceptor made it possible to reduce the required microwave power and equalize the temperature distribution, but in this case densification started at a higher temperature. Evidence of recrystallization-induced microstructure transformation propagating outward from the core of the sample is presented. The possiblity of implementing control over the microstructure and functional properties by choosing the parameters of the microwave sintering regime is discussed.

Rapid microwave sintering of different oxide ceramics with heating rates up to 300 °C/min and zero hold time has been implemented using a 24 GHz gyrotron-based system for high-temperature processing of materials. The design of the system, principle of operation, and process control are described. Particular attention is given to the design of thermal insulation assemblies and the implementation of temperature measurement in an environment with intense electromagnetic fields. A description of an optical system for dilatometry and temperature measurement is presented. The interrelation between the automatically regulated output power of the gyrotron and the microwave power absorbed volumetrically in the sample is analyzed on the basis of energy balance considerations. The analysis is illustrated by considering examples of rapid sintering processes with ZnO-based and BaTiO3 ceramic samples making use of direct and susceptor-assisted microwave heating. It is demonstrated that an increase in the volumetrically absorbed power leads to the development of a controlled thermal instability, which results in a lower temperature of the densification onset.

Using hydroxyapatite as an example, first results of research on a new additive method of manufacturing ceramic products are presented. The method consists of repeated sequential application of suspension layers with high content of powder material and their rapid 24 GHz microwave sintering. The stability of aqueous suspensions of hydroxyapatite powder with submicron particle size depending on the pH of the dispersion medium and dispersants was studied. Suspensions with a high value of the solid load mass and a fluidity sufficient to ensure the continuity of layers applied by the doctor blade method were obtained. By the method of layer-by-layer microwave sintering with a heating rate of up to 30°C/min and a maximum temperature of up to 1330°C, ceramic samples with a closed system of micron-sized pores and a density of up to 92% of the theoretical value were obtained.

Samples of ZnO + Bi2O3 + Sb2O3 varistor ceramics were microwave sintered using gyrotron systems operating at a frequency of 24 GHz. The microwave power was automatically regulated to implement heating at a constant heating rate of 10-130 degrees C/min up to a temperature of 1100-1300 degrees C with no isothermal hold. The final sintered density of the samples was 95-96 % of the theoretical value. Manifestations of the thermal instability associated with the liquid phase formation were observed at a temperature of about 600 degrees C. The estimated volumetrically absorbed power density at the onset of instability was >= 20 W/cm(3), and the temperature difference measured between the center and periphery of the samples reached 200 degrees C. Correlation has been revealed between the thermal instability occurrence and the shift of densification curves towards lower temperatures. A mechanism underlying enhanced densification in electromagnetic field-assisted sintering processes is suggested.

We demonstrate the possibility to use focused beams of millimeter-wave radiation to heat ceramic materials locally with the purpose of manufacturing sintered products on the basis of such materials by additive methods. The results of experiments on layer-by-layer sintering of ceramic hydroxiapatite samples heated by a gyrotron facility operating at a frequency of 24 GHz are presented.

Hydroxyapatite powder compacts have been sintered to a relative density of about 95% using rapid 24 GHz microwave heating at rates 10-100 degrees C/min to 1300 degrees C with zero hold time. An optical system based on an infrared camera has been developed to measure shrinkage and temperature distributions over the surface of the samples. Ultra-rapid localized consolidation of hydroxyapatite powder, with application prospects in additive manufacturing, has been achieved using heating by a focused beam of 263 GHz millimeter-wave radiation. (C) 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the III All-Russian Conference (with International Participation) "Hot Topics of Solid State Chemistry: From New Ideas to New Materials".

Compacted alumina samples with an addition of 1.0 and 2.5 wt % carbon nanotubes (CNT) have been microwave sintered at heating rates 50 and 100 degrees C/min to a maximum temperature of 1550-1600 degrees C with zero hold time. The densification kinetics has been studied using optical dilatometry. No noticeable influence of CNT on the development of thermal instability during rapid microwave sintering of alumina has been detected. The relative densities of the samples containing 1.0 and 2.5 wt % CNT sintered at a maximum temperature of 1550 degrees C and zero hold time were 93.8 and 87.5%, respectively. Microwave sintering with a repeated development of thermal instability has resulted in an increase in the final density to 95.0%.

We describe a series of gyrotron facilities developed at the Institute of Applied Physics of the Russian Academy of Sciences for studying physical processes during interaction of millimeter-wave electromagnetic radiation and matter. This paper presents the universal principle of designing such systems on the basis of a facility having an output radiation power of 5 kW at a frequency of 24 GHz. The main components of the facility and their technical parameters are described. Design of high-efficiency radiation sources and radiation transmission lines for various research applications is a sophisticated radiophysical problem, and the need for long-term stable operation with automatic adjustment of the parameters of the generation regime requires unique engineering solutions. Application of multimode electrodynamic devices in the radiation transmission line allows one to treat materials with significantly different dielectric properties, in particular, heat them up to temperatures of about (and exceeding) 2000°C. The vacuum-tight working chamber of the facility is a high-Q untuned cavity resonator having a volume of about 0.1 m3, in which microwave heating of items with characteristic dimensions of more than 10 cm can be performed. The automatic control system of the facility ensures its reliable and long-term failure-free operation.

Additive manufacturing of ceramic articles making use of concentrated energy flows attracts the research interest worldwide. While the application of laser beams faces serious problems associated with high temperature of sintering and low thermal conductivity of ceramics, layer-by-layer sintering by focused millimeter-wave radiation appears to be a promising method of additive manufacturing. This paper describes the studies of fast millimeter-wave sintering of yttria-stabilized zirconia and hydroxyapatite ceramics. Coefficients of the millimeter-wave absorption have been determined in broad frequency and temperature ranges. Rapid sintering of compacted ceramics samples was accomplished using volumetric microwave heating in a work chamber of a 24 GHz / 5 kW gyrotron system. In addition, using a 263 GHz / 1 kW cwgyrotron millimeter-wave source and a purposely designed electrodynamic focusing structure, radiation intensities of up to 20 kW/cm 2 could be achieved, which was sufficient for fast localized heating of ceramic layers to the solidification temperature. The results of a study of the microstructure and mechanical properties of the sintered ceramics are presented.

Gyrotron systems operated at frequencies of 24 to 30 GHz with an output power of 3 to 15 kW have been used at the Institute of Applied Physics of the Russian Academy of Sciences for more than 20 years for the studies of high-temperature processes in polycrystalline dielectric materials under intense electromagnetic irradiation. The research has mostly been focused on the study of the physically specific features of diffusive mass transfer in solids and on the possible use of these features for applications. A distinguishing feature of the studied processes is a significant enhancement of their rates compared to similar processes performed with the use of conventional heating methods. Examples of enhanced sintering of a broad range of ceramic materials, including optical and laser ceramics and composition-graded metal–ceramic products are considered. The principles of the developed method of ultrafast sintering of oxide ceramics with rates exceeding those typical of the conventional methods by two or three orders of magnitude are described. The development of this method has resulted from a purposeful use of the functional capabilities of the gyrotron systems and the engineering solutions implemented therein.

The results of microwave sintering of powder materials in the regimes with high heating rates and zero hold time at maximum temperature are reported. Microwave processing of compacted samples based on Al2O3, Y2O3, MgAl2O4, and Yb:(LaY)(2)O-3 ceramics was carried out using a 6 kW/24 GHz gyrotron system. The volumetric absorption of intense microwave radiation resulted in a very rapid densification with the duration of the high-temperature stage of sintering on the order of one to several minutes. In the rapid microwave heating regimes the effective high-frequency conductivity of the materials increased sharply at a certain threshold temperature as a result of the overheating instability, also known as thermal runaway. This suggests that the ultra-rapid sintering occurs via grain-boundary softening and formation of transient liquid phases. The indications of the presence of such phases were observed in the microstructure of the sintered samples. The absorbed microwave power density required for the transition to the ultra-rapid sintering is on the order of 10 ... 100 W/cm(3) for a broad class of the materials. The obtained results suggest that ultra-rapid microwave sintering proceeds via essentially the same mechanism as the so-called flash sintering that occurs in the presence of dc or low-frequency ac electric field.

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.

The report presents review of technological gyrotron systems developed and made by Institute of Applied Physics of the Russian Academy of Sciences (lAP RAS) jointly with GYCOM Ltd. (Nizhny Novgorod, Russia). Typical microwave power of these systems is several tens kW with efficiency up to 0.4 (0.65 in the case of depressed collector) at the frequency range 24-45 GHz. The output power of gyrotrons can be smoothly regulated from about 5% to 100% of the full power. All CW gyrotrons can operate also in pulse regime with the same or slightly higher output power.