(2022). Editor's message: sufficient details in a manuscript, originality, and similarity score. Journal of Microwave Power and Electromagnetic Energy: Vol. 56, No. 2, pp. 69-70.

AbstractIt seems possible that Solid-State Radio Frequency (RF) transistors will one day replace magnetrons in consumer microwave ovens. These new ovens have the potential to be smaller and more energy efficient. Because they are closed loop systems, they can precisely cook food in ways that consumers have not seen before. This review outlines the basics of Solid-State RF systems and the consumer ovens that are currently available. There are two potential approaches for this new technology: first, the oven could be like current appliances and serve as a reheating device. Second, the oven could take advantage of the new technology to be a high-end cooking appliance. The food industry would be best served by learning more about this technology change and preparing for their products to be cooked using this type of equipment. Due to the even heating and overall consistency of the technology, it is possible that there are benefits to both quality and food safety for this technology.

AbstractTo effectively separate the valuable minerals in ludwigite, this study investigated the mineralogy of ore, particle-size distribution and mineral liberation characteristics. Most of the magnetite was intergrown with ascharite and serpentine. Nearly half of the serpentine existed in a dense, massive body. Most of the ascharite was encased in magnetite, and a small amount was embedded in the serpentine phase. After the microwave radiation, some intergranular and through cracks were formed to improve mineral liberation during the grinding process. The mass fractions for the +75 μm range decreased from 43.16 wt% for untreated samples to 24.31 wt% for treated samples with a microwave power of 3 kW for 40 s. By contrast, the mass fractions of particles in the −75 μm range increased from 56.84 wt% for untreated samples to 75.69 wt% for treated samples. D50 decreased from 53.54 µm for the untreated sample to 21.58 µm for the treated sample. The liberation degrees after the treated samples clearly improved and increased by 26.64% for magnetite, 18.92% for ascharite and 14.10% for serpentine.

AbstractThis article deals with the investigations on mullite ceramics formation from Kyanite sample- an alumino-silicate mineral with microwave heating. In the present investigation kyanite was heated in microwave sintering furnace with calcined alumina, silicon carbide, zirconium dioxide and binder for mullite formation as well as for the preparation of zirconia toughened mullite product. The results of these studies reveal that how a small amount of zirconium dioxide helps in toughening the mullite formation for refractory and ceramics applications. The results shows that microwave heating method is one of the most novel and eco-friendly process for such toughened mullite formation, well within very short period of time. The observations with mullite formation and zirconia toughened mullite formation were observed with the structural data using X-ray diffraction data, field effect scanning electron microscope and energy dispersive spectroscopy. Future studies are still in process.

(2021). Editor message: farewell to our friend Bob Schiffmann. Journal of Microwave Power and Electromagnetic Energy: Vol. 55, No. 4, pp. 271-272.

AbstractThis article presents information about the design and purpose of microwave processing plants of thermal, non-thermal microwave modification and microwave processing plants with hybrid working chambers that have not yet been widely used. This article also presents and justifies advantages of electric microwave processing plants with hybrid chambers over electric microwave processing plants used for thermal treatment of dielectric materials and non-thermal treatment of polymer materials. These advantages give reason to talk about the expediency of widespread use of electric microwave processing plants with hybrid working chambers in industry and agriculture. This article formulates and justifies the problems of electric microwave processing plants with hybrid working chambers. Thus, high prices for microwave generators reduce the profitability of the plant. The research is being carried out within the framework of a grant as subsidies in science from the budget of the Republic of Bashkortostan for the state support of young scientists – candidates of sciences (the competition code is REC–Federal and State Management – 2022).

AbstractThe main aim of the present investigation is to develop and synthesis the precious nano-metals such as nano gold and its by product components such as nano copper, nano silver etc. The investigations were basically consisting of two level environmental friendly process i.e., recovering of the gold from discarded E-waste components and secondly with the proper reaction after boiling. The precious nano materials so obtained will be very much useful for sensors, organic photovoltaic materials, other electronic conductors, probes, drugs and medicines in bio-medical applications, catalysis and reagent in bio-chemical applications etc. The results with XRD, FESEM-EDX and UV analysis studies from present investigations clearly confirm about the presence of nano gold particles and its by products. All the investigations were performed with microwave heat treatment as an environmental friendly method.

AbstractThe common practice of applying reflection loss to material is incorrect as it can only be applied to film. Also the dominant theory of impedance matching should be replaced by wave cancellation theory. Here, we show that experimental data obtained from microwave absorption by the film of the titled composite can be successfully analyzed theoretically from our new theories. Formulae for the scattering parameters s11 and s21 of the film have been derived based on the wave addition model which clearly differentiates between microwave absorption mechanisms from interface and film. The experimental values, the patterns from s11 and s21, and the relationships between the patterns from the experimental values have been reproduced and interpreted both quantitatively and qualitatively from the new perspective. This specific new analysis for the verification of the new theory involving a large data set provides a clear indication that the conclusions obtained are not only valid for one unique system but are of general applicability. Thus, the new theory has been verified and can be used to reveal the mechanisms of microwave absorption by material and device. The successful interpretations are contrasted with the inadequacies of previous applied methods involving reflection loss and impedance matching.

AbstractCereal production in Australia is severely impacted by stubble-borne pathogens which can survive multiple seasons within cereal residues (stubble). Microwave radiation may be able to reduce or eliminate the pathogens, but the energy requirements first need defining. Hence, the dielectric properties of wheat and barley stubble with different pathogen loads were investigated at 10%, 15%, 30% and 100% moisture content using an open-ended coaxial probe in a spectral band covering 915, 2450, and 5800 MHz. A significant increase in dielectric constant and loss factor was observed with increasing stubble moisture. The dielectric constant and loss factor were lower in the crown (basal) section of stubble compared with the stem (20 cm from base), due to differences in density. When stubble moisture was 100%, the loss factor of barley was higher than wheat. Infection of stubble by different pathogens did not affect the dielectric properties. Microwave heating could therefore be an effective method to heat cereal stubble to eradicate a range of cereal pathogens, especially at lower frequencies and high moisture content, at which the loss factor is high. This research serves as a starting point to define requirements for further development of effective microwave radiation treatments under field conditions.

AbstractThis study aims to analyze the patch antenna for moisture content measurement of made tea. The patch antenna-based system uses an air gap between the ground plane and an electrically conductive patch. The tea sample is contained in the air gap. The patch antenna design has a resonance frequency of 5 GHz. The resonance frequency and S11 parameters are measured for tea samples in the moisture range of 2–10%. The resonance frequency of the patch antenna was observed to decrease with an increase in the moisture content of tea. However, the return loss of patch antenna was observed to increase with an increase in the moisture content of made tea. Linear model parameters were calculated for the prediction of the moisture content of tea leaves.

AbstractIn this study, sour cherry nectar and apple juice were concentrated using thermal concentration (75, 80, and 85 °C), and intermittent microwave concentration (180, 300, and 450 W) under vacuum condition (at 250 mbar). Thirteen different mathematical models were used to describe the concentration kinetics of the samples. The Midilli model exhibited the best fit to all experimental data (R2 ≥ 0.9972; χ2 ≤ 1.6724; RMSE ≤ 1.2932). The concentration time was reduced with increasing microwave power (from 180 W to 450 W) and thermal vacuum concentration temperature (from 75 °C to 85 °C). In addition, the main quality parameters of the concentrates were comparatively investigated after reconstitution. Generally, major differences were not observed in the physicochemical properties of samples concentrated by thermal and microwave methods. Total phenolic content of the apple juice and sour cherry nectar samples were ranged between 209.87-216.67 mg/L and 571.00-588.57 mg/L, respectively. Fructose was the main sugar in both juices, followed by glucose and sucrose. The particle size of the juice samples increased with concentration processing.

AbstractNon-uniform heating is a significant challenge in radiofrequency (RF) heating of low moisture foods. Previous experiments showed that the immersion of fluids (air, deionized water, and soybean oil) changed the RF heating uniformity and rate of cornflour. However, the behind mechanism is not well understood. This study developed a finite-element-based model that incorporated quasi-static electromagnetics and Fourier’s heat transfer to understand the effect of the three immersion fluids on the RF heating performance of cornflour. The model was validated and showed good agreement with experimental thermal images. The simulation results showed that the immersion of soybean oil increased the average heating rate and improved the heating uniformity compared to immersions of air and deionized water. Less distortion of electric potential reduced the fringe effect of edge heating and thus improved the heating uniformity. The higher heating rate was attributed to more dissipated power within the cornflour sample and less surface heat loss from cornflour to the surrounding fluid than the immersion of air or water. The use of soybean oil as immersion fluid could be a promising strategy to be implemented with RF technology to improve heating performance of low moisture food products.

AbstractIncreasing heating performance and preservation of product quality are critical factors in the food industry. Although microwave-vacuum heating is applied in many food processes, there are many unanswered questions about its effects on sensitive foods. In this study, the tomato juice was concentrated in microwave-vacuum heating system and results compared with conventional and conventional-vacuum heating methods. The effect of pressure and microwave power was investigated on quality parameters, energy consumption, exergy performance, and processing kinetics of tomato juice concentration. The processing time in the microwave-vacuum method was 2.3 to 6.3-fold lower than conventional and conventional-vacuum methods. The exergy performance was obtained in the range of 7.16–13.29% for microwave-vacuum heating and 2.04–2.99% for conventional methods. Microwave vacuum heating could reduce energy consumption by 30–71%, compared with conventional-vacuum methods. The minimum decline in ascorbic acid and total phenol content of tomato juice concentrate was observed for microwave-vacuum evaporation. Microwave-vacuum heating was an excellent concentration technique that provided tomato paste with improved nutritional quality, increasing exergy efficiency, reducing processing time and energy consumption compared to conventional and conventional vacuum processes.

AbstractThis study investigates the modeling of the concentration of hibiscus extracts and the effect of the concentration process on some quality properties of hibiscus extracts. Absolute pressure was applied as 250 mbar in all concentration processes. Thermal vacuum concentration process was applied at 75 °C and 80 °C, thermosonic vacuum concentration process was applied at 75 °C and 0.02 W/mL AED, and microwave vacuum concentration treatment was applied at 180 and 300 W. The shortest processing time was achieved with the microwave vacuum concentration process at 300 W. Among the 13 models used, Midilli was the most successful model (R 2 ≥ 0.9951, χ2 ≤ 1.0185 and RMSE ≤ 1.0092)) in describing the concentration data. No significant change was determined in the physicochemical properties of the concentrated samples. However, a significant increase was determined in the turbidity values of the concentrated hibiscus samples, while a significant decrease was noted in the anthocyanin content and composition and antioxidant activity values of the concentrated samples compared to the initial sample.

AbstractThe dielectric properties and electromagnetic wave absorbing performance of granular polysilicon have important influences on microwave smelting efficiency. In this study, the dielectric parameters (dielectric constant, dielectric loss factor and loss tangent) of granular polysilicon were measured from room temperature (25 °C) to 800 °C at 2450 MHz by using the cylindrical cavity perturbation method, and the reflection loss of granular polysilicon was calculated according to the theory of electromagnetic wave transmission line, and then the effects of temperature on microwave absorbing performance was analyzed. The results indicated that the dielectric constant and dielectric factor loss of granular polysilicon change little before 300 °C, but when the temperature is higher than 300 °C, the dielectric constant and dielectric loss factor increase gradually. The microwave absorbing performance of granular polysilicon fluctuates with the increase of granules thickness, there are multiple absorption peaks, and the position and intensity of absorption peaks change with the increasing temperature. This study provides theoretical guidance for the optimization of smelting process parameters of granular polysilicon and the design of microwave smelting equipment.

AbstractIn mathematical models of microwave heating with infinite-dimensional characteristics, it is difficult to use traditional numerical methods to improve computational efficiency. In this work, we propose a fast and accurate method to calculate the temperature distribution of materials under microwave heating. First, we analysed the relationship between the choice of model order and the solution accuracy by downscaling the infinite-dimensional heat conduction partial differential equation (PDE) model into a finite-dimensional ordinary differential equation (ODE) model. Additionally, the effect of different boundary conditions on the global temperature distribution was analysed. Second, the equilibrium conversion matrix was calculated using the singular value decomposition (SVD) truncation method under homogeneous boundary conditions. Using this matrix, a lower-dimensional microwave heating ODE model was further obtained. Finally, the numerical simulation results showed that the root mean square error (RMSE) was only 0.07 and the maximum relative error was only −0.85%. The computation time of the equilibrium conversion matrix was 2.12 ∼ 3.00 ms, and the model calculation time was reduced by 97.78%. We compared the calculated temperature rise curves with those obtained using the conventional COMSOL model. The SVD truncation method achieved an efficient and accurate solution for the microwave heating model.

Published in Journal of Microwave Power and Electromagnetic Energy (Vol. 56, No. 3, 2022)

AbstractIn present studies, dielectric properties of eggshell powder were determined at microwave frequencies 2.45 and 5.8 GHz by varying temperature and moisture content in the range 15–75 °C and 0.5–8% (wet-basis) respectively, using cavity perturbation technique. Values of dielectric constant (ε′) and loss factor (ε″) increase with temperature and moisture content at both of these frequencies but ε′ is higher at 2.45 GHz than 5.8 GHz whereas ε″ shows opposite trend. The calculated penetration depths of microwaves are more at 2.45 GHz than 5.8 GHz and it decreases with increase in temperature as well as moisture content. The determined data of ε′ and ε″ were used to generate the third order regression equations with high correlation coefficients (R2 > 0.95). These findings are useful for modelling and designing the microwave applicator for heat treatment to eggshell powder. Additionally, X-ray diffraction spectra was also recorded.

AbstractIn this study, a self-sensing smart structure was developed with a novel approach. Electromagnetic wave based self-sensing structure designed by sending waves to the reinforced concrete structure through the reinforcement by using a coaxial cable. EM measurements were based on estimating the reflected EM waves through the reinforced concrete. Correlations between reflected EM wave, damage in reinforced concrete and strength development in concrete was estimated. According to test results, it was seen that with increasing crack width of reinforced concrete by applying monotonic load, reflection of EM wave through the reinforced concrete decreased. It was also seen that with strength development of the concrete, reflection of EM through the reinforced concrete increased. In the light of the findings of this study it can be said that, damage assessment and monitoring strength development of the reinforced concrete is possible in real time with applying the proposed novel technique.

(2022). Editor’s message: the solid-state applications era. Journal of Microwave Power and Electromagnetic Energy: Vol. 56, No. 1, pp. 1-2.