Microwave Heating Research Articles

Understanding Structural Changes in Recycled Aggregate Concrete under Thermal Stress

Objective: This study investigates the influence of high-temperature treatment on the deformation properties and structural deformation of recycled aggregate concrete (RAC) in response to potential fire hazards in the construction industry. Methods: Standard-cured 28-day RAC specimens were subjected to microwave heating at 300 °C and 600 °C, with subsequent uniaxial compression tests utilizing a WDW-2000 machine and a VIC 3D strain measurement system to analyze strain data through digital image correlation (DIC) technology. Results: After treatment at 300 °C, recycled aggregate concrete (RAC) demonstrated superior mechanical properties to fresh concrete aggregates. This enhancement may be attributed to the more robust siloxane bonds (Si-O-Si) in the recycled materials. Conversely, exposure to 600 °C intensified internal structural damage, notably lowering the material’s elastic modulus and peak stress. DIC analysis highlighted the correlation among temperature, volumetric strain, and crack development patterns, with more extensive cracking at 600 °C. Conclusions: Moderate-temperature treatment enhances RAC’s structure and deformation properties, while high-temperature treatment diminishes its performance. These findings provide valuable insights for assessing building safety post-fire and the application of RAC, emphasizing its suitability at moderate temperatures and risks at high temperatures.

Simple and low‐cost preparation of functionalised graphene by microwave expansion combined with ball milling grafting

AbstractThe preparation of functionalised graphene often involves consuming significant amounts of organic solvents, complicated steps, and expensive equipment. This study presented a simple, low‐cost, and efficient method for preparing well‐dispersed functionalised graphene. This method involved the microwave heating of commercial graphene precursors and ball milling of grafted expanded graphite, resulting in a short and straightforward preparation process without requiring large amounts of organic solvents. This process enabled the preparation of few‐layer graphene with a thickness of only 3.5 ± 0.5 nm. During this process, the majority of the surface oxygen‐containing groups were replaced by polyetheramine (D2000) at a grafting rate of up to 5.14%, which improved the interface adhesion strength between the graphene and the epoxy resin. The fabricated altered graphene notably enhanced the mechanical characteristics of the epoxy resin., that is, the toughening effect reached up to 171% with a graphene content of only 0.3 wt%, while the Young's modulus and tensile strength values increased by 54% and 39%, respectively. This process is cost‐effective, easy to operate, and highly efficient, making it suitable for the large‐scale production of well‐dispersed functionalised graphene.Highlights Pioneers mechanical chemical energy in graphene, a new materials science direction. First ball milling on microwave graphene, merging milling benefits with graphene. Ball milling cuts D2000 grafting time on graphene, boosting efficiency. Reduces organic solvent use, cutting costs and environmental effects. Ball milling lowers costs and impacts, aiding graphene material commercia‐lization.

Investigation of Mn–Co oxides loaded ceramic structured catalyst for microwave enhanced catalytic degradation of benzene

Compared with traditional heating, microwave heating has unique advantage and can enhance the chemical reaction. It is a promising technology for catalytic degradation of VOCs gas. Here we prepared a structured catalyst (Mn–Co/SiC) suitable for VOCs degradation under microwave. The catalyst shown excellent stability under microwave heating and microwave empowered the catalyst with good water resistance. Microwave heating exhibits high energy efficiency. Density functional theory is employed to investigate the microwave-enhanced catalytic degradation of VOCs. The theoretical calculation results indicate that the adsorption energy is more negative under microwave electric field, suggesting that microwave electric field is conducive to the activation of benzene. The electrons on the adsorption surface of the catalyst are redistributed to different degrees under the action of different microwave electric fields, thus affecting the activation of adsorbed molecules and chemical reaction process. The local density of states further reveals that electric fields may facilitate the electron transport.

Characterization of Plant-Based Meat Treated with Hot Air and Microwave Heating.

Plant-based meat is growing globally due to health, environmental, and animal welfare concerns, though there is a need for quality improvements. This study assessed how different ratios of wheat gluten (WG) to soy protein isolate (SPI) and various baking methods-hot air (HA), microwave (MW), and a combination of both (HA-MW)-affect the physicochemical properties of plant-based meat. Increasing the SPI from 0% to 40% significantly enhanced lightness, hardness, chewiness, water-holding capacity, moisture content, and lysine (an essential amino acid) (p ≤ 0.05). Hardness and chewiness ranged from 4.23 ± 1.19 N to 25.90 ± 2.90 N and 3.44 ± 0.94 N to 18.71 ± 1.85 N, respectively. Baking methods did not affect amino acid profiles. Compared to HA baking, MW and HA-MW baking increased lysine content (561.58-1132.50 mg/100 g and 544.85-1088.50 mg/100 g, respectively) while reducing fat and carbohydrates. These findings suggest that a 40% SPI and 60% WG ratio with microwave baking (360 W for 1 min) optimizes plant-based meat, offering benefits to both consumers and the food industry in terms of health and sustainability.

Multi-physical field analysis based on a multi-material mode stirrer and interlayer for microwave processing fluid

Microwave technology is widely recognized for its efficiency in the chemical, food and energy industries, and has attracted great attention due to its ability to induce both thermal and non-thermal effects. However, the use of microwave reactors often leads to thermal runaway phenomenon. To address this challenge, a multi-material mode stirrer has been integrated with a microwave reactor structured with an interlayer, and the materials of mode stirrer and interlayer inner wall have been changed to address issues related to temperature uniformity. The combined effects of multi-material mode stirrer and different material inner walls on the microwave heating characteristics have been studied by using the evaluated coefficient of temperature variation (COV) and the mean fluids temperature. It is discovered that: (1) the effect of mode stirrer on heating characteristics depends on its material and the coefficient of temperature variation of Aluminum-Alumina mode stirrer is reduced by 44 %; (2) under the action of mode stirrer, the corundum inner wall can improve the heating efficiency, but the glass inner wall has a poor effect on it; (3) changing the structure materials of the microwave reaction kettle has little effect on the heating efficiency and the combination of Polyethylene-Aluminum mode stirrer and silicon carbide inner wall can improve the heating uniformity.

Flexible and efficient direct-insertion microwave heating with a dielectric wedge

Traditional microwave heating requires the heated object to be placed in a metal cavity, which limits its flexibility. Moreover, the microwave heating efficiency is affected by the shape, size, and permittivity of the load. This paper proposes a direct-insertion microwave heating device based on a dielectric wedge. It offers flexible and efficient heating for different loads. Firstly, a dielectric wedge with a gradient refractive index was designed, which can convert electromagnetic waves into surface waves propagating along the dielectric. Secondly, efficient heating of materials with permittivity ranging from 10 to 80, regardless of their shape, is achieved by inserting the dielectric wedge into the load. Microwave heating efficiencies above 90 % were consistently observed in heating tests with beverages, mashed potatoes, and minced fish of varying shapes. Finally, A high-power continuous flow heating scheme was introduced, utilizing the device as a heating module and scaling up by adding more modules to boost microwave power. Multi-physics simulations confirmed the device's efficiency and versatility in microwave continuous flow heating, with minimal energy interference between microwave sources.

Buoyancy-driven nano-suspension subject to interstitial solid/nanofluid heat transfer coefficient: Role of local thermal non-equilibrium (LTNE)

BackgroundBecause of the prominent temperature discrepancy between fluid and solid in porous material, local thermal equilibrium (LTE) is not suitable in case of high-conductivity foams and electronic equipment. In view of above situation, Darcy-Brinkman-Forchheimer model subject to local thermal non-equilibrium (LTNE) is implemented. LTNE technique finds real world applications include groundwater pollution, geothermal extraction, microwave heating, industrial separation process, and transpiration cooling featuring with porous structure. The present study aims at the investigation of the entropy and hydrothermal characteristics of buoyancy-driven TiO2-H2O nanofluid inside a cross-shaped domain embodying two hot and cold rings influenced by LTNE. MethodsFinite element method (FEM) has been considered to solve the dimensionless form of governing equations. Significant findingsAmplification of interstitial solid/nanofluid heat transfer coefficient accounts for the intensification of streamlines, velocities, and diminution of isothermal lines in both nanofluid and solid matrix phases under the influence of LTNE. Strengthening of medium porosity whittles down entropy due to thermal effects in both nanofluid and solid phases, and that ameliorates entropy due to fluid friction and porous medium irreversibilities. Local and average Nusselt numbers in nanofluid phase reduce by 29.31 %, 20.72 %, 17.16 %, and 14.78 % while that in solid phase decays by 13.18 %, 7.63 %, 4.9 %, and 2.8 % for rise in εfrom 0.1 to 0.3, 0.3 to 0.5, 0.5 to 0.7, and 0.7 to 0.9, respectively. Introduction of Darcy-Brinkman-Forchheimer model subject to LTNE yielded better results in hydrothermal behavior of TiO2-H2O nanofluid inside a cross-shaped domain emplacing two hot and cold rings than earlier published results.

Rapid 24 GHz microwave sintering of alumina – yttria-stabilized zirconia ceramic composites

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 °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/cm3 resulted in a shift of the shrinkage curves by about 100 °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 m1/2.

Rapid microwave fabrication of highly luminescent nitrogen and phosphorous co-doped carbon quantum dots for the determination of glutathione in pharmaceutical supplements

A facile and sensitive nano-probe for spectrofluorometric glutathione (GSH) determination has been developed based on dual-doped nitrogen and phosphorus carbon quantum dots (NP@CQDs). The NP@CQDs were prepared, through a rapid, one-step microwave heating method in only 180 s with a high quantum yield (0.63) using ethylenediaminetetraacetic acid as carbon and nitrogen precursor and diammonium hydrogen phosphate as nitrogen and phosphorus dopant. The synthesised NP@CQDs had a maximum bluish fluorescence emission at 420 nm, after excitation at 360 nm. The luminescence of the prepared NP@CQDs was quenched via a static quenching mechanism by ferrous ions, while ferric ions did not affect the emission of NP@CQDs. GSH was mixed with ferric ions before adding NP@CQDs to exploit this observation. The attenuation in emission observed after adding NP@CQDs to GSH/ferric ions was directly related to the GSH concentration, as GSH worked to reduce the ferric ions present and produce ferrous ions. The proposed fluorometric method was validated in compliance with the International Council on Harmonization (ICH) guidelines. The proposed fluorescent nano-probe showed good linearity for GSH determination over a range of 0.5–10 µg/mL with % recoveries ranging between 99.50 and 101.86 %, and RSD less than 2 %. NP@CQDs were implemented for GSH determination in pharmaceutical supplementary capsules with respectable accuracy and precision. This approach is simple, reliable, accurate, specific, and it also does not require expensive chemicals or sophisticated equipment. This work opens up other uses of NP@CQDs in pharmaceutical and environmental analysis.

Bael (Aegle marmelos) beverage pasteurization by non-isothermal heating with continuous microwave to achieve microbial safety

The efficacy of conventional thermal (batch) and continuous microwave (MW) heating to pasteurize bael beverage was examined by inactivating Escherichia coli, Listeria monocytogenes, Salmonella Typhimurium, Saccharomyces cerevisiae, their cocktail, and native microbial groups. Continuous-MW was operated at 2 kW power and 45–80 °C final temperature. For 25–90 °C beverage temperature, the fluid followed a laminar flow. Data were appropriately modelled using first-order kinetics. MW-heating effectively inactivated all microorganisms. S. cerevisiae was most resistant towards thermal inactivation under single (D70 °C = 0.03 min) and cocktail scenario (D80 °C = 0.014 min) and towards MW under cocktail scenario (DMW_67.5 °C = 0.23 min). The resistance towards temperature rise was highest by L. monocytogenes. Based on lower zMW-values, the thermal sensitivity of microbes improved under MW heating. Morphological alteration in microbial cells of S. cerevisiae &L. monocytogens reveals that the lethality of MW inactivation is mainly based on its thermal effects. Industrial relevanceBael fruit is a nutritious tropical fruit whose beverage is relished by consumers for its refreshing flavour and numerous health benefits. The food sector is presently concentrating on creating innovative, minimally processed goods that are ready-to-eat or handy, shelf-stable, and of higher quality. As an alternative to conventional thermal processing, several novel and sustainable processing methods are now being investigated. To determine if microwave processing is a suitable substitute for thermal processing, the inactivation of artificially inoculated microorganisms, their cocktail, and indigenous microorganisms in the bael beverage were examined in this work. Kinetic study, beverage rheology and changes in cell morphology were also studied. The study's findings indicate that continuous microwave technology can be a more suitable method of pasteurizing fruit beverages than conventional heating due to its >7 log reduction in a shorter time. Except for a few differences, the inactivation mechanisms of thermal and microwave are comparable.

Thermomechanically micronized sugar beet pulp: Elucidating the interaction of soluble elements and insoluble fibrous particles for enhancing emulsification

The micronization of plant particles usually transfers substantial insoluble fraction into soluble elements, resulting in a soluble-insoluble binary system and the content of soluble elements can not be neglected. The present work aims to investigate the interaction between soluble elements and insoluble particles obtained from sugar beet pulp by microwave heating with ultrasonication, and their contribution to the synergistic emulsification performance. The soluble elements (∼40%) (sugar beet pectic substance, SBPS) and insoluble fibrous particles (IFPs) were separated from micronized sugar beet pulp (MSBP). Quartz crystal microbalance with dissipation analysis revealed that SBPS could irreversibly adsorb onto IFPs with an adsorbed mass of 32.2 μg/mg Fiber. By adopting sequential extraction and specific enzymatic hydrolysis for the modification of IFPs and SBPS, it was found that their non-saccharide fraction played an important role in the interaction. Larger molecular weight subfractions with abundant protein and ferulic acid content were the main adsorbable fraction in SBPS, and the interaction was driven by hydrophobic interaction. The soluble-insoluble binary system promoted the interfacial adsorption of both IFPs (from 65.6% to 82.2%) and SBPS (from 9.6% to 19.6%), forming a hybrid interfacial coverage (with SBPS-coated IFPs and SBPS) and endowing emulsion with better stability. These findings provided a guideline for understanding the emulsification mechanism of plant particles.

Effect of silicon carbide powder on asphalt material properties and microwave-induced self-healing

Silicon carbide (SiC) powder, an excellent microwave-absorbing material, can serve as a filler in asphalt mixtures. This study compares the particle size distribution, phase composition, pore structure, specific surface area, and electromagnetic properties of SiC powder and limestone powder. Asphalt mastic and mixtures were prepared with SiC powder replacing mineral powder at ratios ranging from 0 % to 60 %. The study examines how differences in particle size gradation, mineral composition, pore structure, and specific surface area contribute to deformation resistance and improve mechanical properties through micro-level interlocking and cementation. Electromagnetic parameter variations affect microwave heating performance, with uniform distribution of absorbing fillers ensuring heating homogenization. SiC powder also enhances the self-healing index, reaching 74.4 %, and maintaining a maximum healing index of 68.1 % after four damage-healing cycles. These findings support the practical application of SiC powder in asphalt concrete, enhancing maintenance efficiency and advancing microwave heating for self-healing and hot recycling.

Comparative study of microstructure and mechanical properties of Mg/B4C composites: Influence of sintering method and temperature

This present work compares the effect of heating methods including microwave and spark plasma sintering (SPS) process on the microstructure and mechanical properties of Mg–B4C. 5 wt%B4C mixed with Mg powders through high-energy mixer mill for 10 min at ethanol media. After mixing, the green bodies were prepared by uniaxially pressing the mixture. The microwave heating was done at sintering temperatures of 600, 650 and 700 °C without soaking time in graphite bed. While, in the case of SPS, the mixture of the powders was directly inserted into a graphite mold and sintering process was performed at 450 °C with initial and final pressures of 10 and 30 MPa at vacuum condition of 17 Pa, sequentially. The corresponding XRD patterns revealed Mg and B4C crystalline phase alongside the reaction product of MgB2 and also MgO byproduct. The FESEM investigations depict uniform distribution of B4C reinforcements at the Mg matrix. Moreover, increasing microwave sintering temperature led to decreasing pores at the microstructure of the prepared sample. The microstructure of SPSed sample revealed almost full densification compared to that of the microwave processed samples at all the sintering temperatures applied. Also, The XPS results show the presence of Mg, O, C and B elements on the surface of prepared composite, and HRTEM with HAADF-STEM images were obtained to study the interface of matrix and reinforcement. The highest bending strength and Vickers hardness of 116 ± 6 Hv and 231 ± 11 MPa were obtained for SPSed sample as well as for the highest relative density. Although the microwave sample was sintered at 700 °C at low pressure, sintering method demonstrated considerable mechanical properties in comparison to SPSed specimen, indicating a successful preparation method as a low-cost, energy- and time-saving process.

Optimization of temperature uniformity for multi microwave source alternating heating based on superpermutation planning enhanced by Harris Hawks optimization

For the optimization of material temperature distribution during multi-source microwave heating processes, we propose utilizing the Harris Hawks Optimization Algorithm to enhance the alternating heating strategy among multiple waveguides with superpermutations. Firstly, we innovatively introduce the concept of superpermutations to assign character weights, which subsequently correlates the alternating sequences of different waveguides. Besides, we employ Hamiltonian path planning to determine the optimal order of waveguide alternation. Then, we apply the Harris Hawks Optimization Algorithm and improve the algorithm based on microwave heating evaluation criteria to optimize the strategy for waveguide alternation and enhance search performance. At last, experimental validation involves heating SiC materials in a microwave reactor to verify the credibility of physical experiments against the simulation model used. This analysis includes assessing temperature uniformity and energy conversion efficiency during the heating process, thereby demonstrating the effectiveness of the proposed heating strategy. The experimental results indicate that temperature uniformity improved by 37.75% to 80.25%, and energy conversion efficiency increased by 7.72%.

Study on recycling carbon fibers from carbon fiber reinforced polymer waste by microwave molten salt pyrolysis

The extensive application of carbon fiber reinforced polymers (CFRP) has led to a sharp rise in the waste generated from CFRP, which poses a significant issue in sustainability that needs to be addressed. This work presents an efficient treatment method to recycle CFRP waste. Under the combined action of microwave heating and ZnCl2/NaCl molten salt, the epoxy resin matrix was rapidly degraded at 300 ℃ in 20 min. Then the pyrolytic carbon produced on the surface of carbon fibers from this process was removed by oxidation procedure in an air atmosphere. The results showed that the surface morphology and microstructure of the recycled carbon fibers (RCF) had no obvious changes, and the tensile strength and tensile modulus of RCF remained at 87.8 % and 94.6 % of the virgin carbon fibers, respectively. The degradation of epoxy resin by microwave molten salt pyrolysis increased the output of H2, which reached 65.35 % of total pyrolysis gases. Furthermore, the epoxy resin can be transformed directionally. The main components of the pyrolysis oils were phenol and p-isopropyl phenol, which had a high utilization value. Therefore, this study designed a new and efficient process to recycle carbon fibers from CFRP waste, which has important application prospect.

Recovery of NMC-lithium battery black mass by microwave heating processes

The exceptional microwave absorption capabilities of carbon-based materials make them highly effective for facilitating various chemical reactions, such as synthesis, reduction, exfoliation, doping, and decoration. In this study, we applied microwave irradiation to the thermochemical treatment of spent nickel manganese cobalt (NMC622) lithium-ion batteries, obtained as a mix of the cathode and the anodic graphite. This approach significantly accelerated the chemical reactions, leveraging carbon's robust ability to absorb microwave energy. As a proof-of-concept, we demonstrated that within just 5 min, the carbothermic reduction reactions in the black mass facilitated the formation of water-soluble Li2O, and enhanced the solubility of Co, Mn, and Ni in a weak acid. More than 90% of Li and 87% of Mn are recovered. This method, which bypasses the need for cathodic and anodic separation, presents a promising new approach for recovering strategic metals from spent lithium-ion batteries.

Technologies to decontaminate aflatoxins in foods: a review

SummaryAflatoxins are toxic secondary metabolites produced by Aspergillus spp., found in staple food commodities. Aflatoxins are carcinogenic, teratogenic, and mutagenic and pose a serious threat to the health of humans. The identification and quantification of aflatoxins in foods is a major challenge to guarantee food safety. Therefore, developing feasible, sensitive, and robust methods for decontamination is paramount, with short processing time and negligible impact on quality. This review evaluates recent novel technologies for aflatoxins decontamination by physical methods (microwave heating, Gama and electron beam irradiation, pulse light and ultraviolet), chemical methods (ozone, natural plant extracts, and organic acids), and biological methods (atoxigenic Aspergillus strains, Trichoderma spp., and bacteria and yeast). The study highlights on the cutting‐edge technologies of smart packaging and artificial intelligence (AI). To achieve more efficiency and adaptability to different food matrices in aflatoxins decontamination, the study suggests integrating multiple strategies. The study also recommends integrating Partnership for Delivery (P4D) to share the responsibility to increase the chance for success and control aflatoxins in foods.

Numerical optimization of drying of white button mushroom (Agaricus bisporus) employing microwave and fluidized bed drying for preparing value added product

The microwave-assisted fluidized bed two-stage novel, energy-efficient drying method was used for Agaricus bisporus drying which can be further utilised for mushroom protein powder, and soup. The effect of microwave heating time (MHT), fluidized bed drying temperature (FBDT), and slice thickness (ST) on rehydration ratio, total colour difference (TCD), and overall acceptability of white button mushroom (WBM) powder was investigated. The optimized values, i.e., 1 min (MHT), 60 °C (FBDT), and 2 mm (ST), were obtained during optimization using design expert software (ver. 13.0.1). The result revealed that MHT, FBDT, and ST affect the responses significantly. TCD was found to be increased while overall acceptability decreased as MHT and FBDT increased. Rehydration is an important criterion for using dry products in curry or soup since they may absorb water. The optimized WBM (O-WBM) powder was used to prepare the soup mix. Out of three formulations (F1, F2, and F3), F3 had a 9 % O-WBM powder score with a maximum mean value for colour, taste, flavor, consistency, and overall acceptability, followed by F2 with a 7 % O-WBM powder. The soup’s composition includes functional ingredients such as garlic, shallots, and milk powder, making it healthier.

Simulation investigation on heating characteristics of catalysts under microwave irradiation for decomposition of waste plastic

Microwave heating is a promising technique for heterogeneous catalytic reactions in plastic decomposition. The microwave-insensitive plastic material requires microwave-absorbing catalysts to facilitate catalytic-assisted decomposition and synergistic heating. However, the heating characteristics of catalyst particles within the microwave system is still unclear. In this study, the effects of particle size, particle arrangement direction, and particle shape on the microwave heating behavior of particles was investigated, and the model was experimentally validated and analyzed using infrared temperature data. The simulation results indicated that the heating rate increased as the particle size enlarged, with an average heating rate of 5.56 °C/s for the particle with a radius of 5 mm in comparison to 4.29 °C/s for that of 1 mm. Additionally, when particles were aligned parallel to the applied electric field, the electric field was intensely focused at the interparticle area, with a maximum electric field strength difference of 2.2 × 104 V/m in the samples. In contrast, the horizontal placement resulted in reduced electric field intensity (4.7 × 103 V/m) and lower temperatures (62 °C) near the areas adjacent to the particles compared to the maximum values in the particles. With respect to particle shape, cylindrical particles possessing larger aspect ratios exhibited superior heating performance due to the extended span of intraparticle microwave transmission aligned with the electric field direction but also resulted in increased thermal field distribution inhomogeneity. The research offers theoretical guidance to prevent catalyst sintering and promote microwave-assisted catalytic plastic decomposition.

Preparation of Magnetite-silica Composite by Non-firing Solidification Method and Its Microwave Heating Properties

Preparation of Magnetite-silica Composite by Non-firing Solidification Method and Its Microwave Heating Properties