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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°C/min to a maximum temperature of 1200°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°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. [ABSTRACT FROM AUTHOR]

Milk quality is influenced by the nature of animal feeding, genetic potential, as well as the hygiene level in the process of obtaining it, which affects the presence of mechanical impurities and bacteria. The uncontrolled use of drugs is often accompanied by their appearance in milk, which affects not only the product quality, but also makes it unsuitable for further use and consumption. The aim of this work was to study the contamination of raw milk with bacteria and antibiotics. During 2019-2020, 425 milk samples were subjected to laboratory tests, of which 407 were tested for the presence of antibiotics. It was found that 58.12% of raw milk samples for microbiological examination are received from June to October. More than half of all samples containing microorganisms were found from May to August. At the same time, the highest microbial contamination was determined in July and August (65.4% and 54.0% of all examined samples, respectively), and the lowest one was determined from September to November (30.6%, 34.7% and 34.3%, respectively). Staphylococcus spp. 41.6%, the bacteria of the Escherichia coli group - 35.4% and Streptococcus spp. - 21.4% dominated among the microorganisms found in raw milk. It was revealed that 55.28% of raw milk samples delivered to the laboratory were contaminated with antibiotics. The most common antibiotic found in raw milk is amoxicillin (38.67% of all samples containing antibiotics), doxycycline was the second one by occurrence (20.89%), then erythromycin and azithromycin were presented (10.67% each). Levofloxacin was less common and it found in 8.44% of milk samples with antibiotics; in rare cases, the presence of streptomycin (6.22%) and trimethaprim (4.44%) was recorded. The most common use of antibiotics was during the summer. On the contrary, the cases of antibiotic detection in raw milk are rare during winter. [ABSTRACT FROM AUTHOR]

MgAl2O4 samples were microwave sintered to near‐full density in rapid processes with heating rates on the order of 100°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°C) and about 97% in 1 wt.% Y2O3‐doped samples (1700°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°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/cm³, similar to the values observed in dc field‐assisted flash sintering experiments. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

MgAl2O4 samples were microwave sintered to near‐full density in rapid processes with heating rates on the order of 100°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°C) and about 97% in 1 wt.% Y2O3‐doped samples (1700°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°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/cm³, similar to the values observed in dc field‐assisted flash sintering experiments. [ABSTRACT FROM AUTHOR]