Microwave Heating Research Articles

Characteristics and optimization strategy of microwave thermal regeneration of spent activated carbon based on a multi-physical field model

Microwave thermal regeneration is an efficient and low-consumption way to recycle spent activated carbon. However, traditional experimental approaches are difficult to analyze the electromagnetic field distribution in the microwave cavity, heat and mass transfer process of adsorbates, and optimization of the microwave reactor. Therefore, in this work, a multi-phase porous media model coupled with electromagnetism, heat and mass transfer is established to study the characteristics of microwave thermal regeneration procedure and to propose an optimization strategy for the microwave reactor. The results showed that the absorption of microwave by the sample can be strengthened by adjusting the width to height ratio of the waveguide. The optimal waveguide size (width: length = 80 mm: 10 mm) increased the microwave utilization efficiency from 65.4 % to 98.5 %. In addition, the combination of rotary and intermittent microwave heating significantly optimized the temperature uniformity in both lateral and vertical directions and reduced the microwave heating time. The combined microwave heating time was reduced from the initial 136 s to 80 s, saving 41 % of energy consumption. The temperature difference of the sample was also reduced from the initial 536 K to 191 K, and the covariance COVT decreased from 0.52 to 0.24. The results provide a valuable technical guideline for the optimization of the microwave reactor in order to improve the regeneration efficiency and the quality of the regenerated carbon.

Influence of localized thermal effect of microwave heating on the carbothermic reduction process of ZnFe2O4

As an environmentally efficient method, microwave heating has been proposed to recover Zn from electric arc furnace dust (EAFD) recently. During the process, microwave irradiation has been shown to promote the reaction by lowering the reaction temperature and activation energy, in addition to providing efficient heat. However, the mechanism by which microwave heating promotes the zinc removal reaction of EAFD remains unclear. Thus, this study focused on ZnFe2O4, the primary component of EAFD, and developed an electromagnetic-thermal-chemical reaction coupling model to investigate the factors influencing the carbothermic reduction process of ZnFe2O4 and analyze the promoting mechanism of the microwave on the reaction. The main results indicate that the localized thermal effect, determined by the microwave distribution, exacerbates temperature differences and leads to non-uniform reaction behavior inside the sample. Increasing microwave power and graphite addition enhances the localized thermal effect, worsening field uniformity. In the simulation, the localized thermal effect of microwave heating reduced the activation energy of the zinc removal reaction to 252.97 kJ/mol, with an R2 reaction model. The assistance of the non-thermal effect in enhancing ion diffusion leads to a significant decrease in the activation energy observed in the actual microwave heating experiments.

Effect of microwave energy combined with hot air on the functional properties and antioxidant activity and pasting properties of Samh (Mesembryanthemum forsskalei Hochst) seeds

The study aimed to investigate the effect of microwave heating combined with hot air (70 °C) at different application times (0, 90 &180 s,) on the colour, digestible a soluble protein, functional and pasting properties, antioxidant capacity of Samh seeds (8, 12 & 16 % moisture content). The results indicated that microwave heating caused a significant change in Samh seed's colour and enhanced the protein solubility of the seeds, and the functional properties and viscosities of the Samh seeds. Moreover, the results showed that the moisture content of the Samh seeds and application time significantly impact the seeds' quality parameters. However, the partial least square (PLS) model validated that the microwave treatments of the samh seed with (16 %, 180 s) were treated with microwave energy combined with hot air circulation Accordingly, microwaves may offer the potential of being an effective emerging technology for improving the quality and functional characteristics of Samh seeds.

GaN Solid State Power Amplifiers for Microwave Power Transfer and Microwave Heating

GaN Solid State Power Amplifiers for Microwave Power Transfer and Microwave Heating

Impact of thermal processing methods on physicochemical properties and flavor-related compounds of Pearl gentian grouper with waterless preservation

This study investigated the impact of various treatments (boiling, steaming, microwave heating, and baking) on the physicochemical properties and flavor profile of pearl gentian grouper after undergoing waterless preservation. The grouper meat treated with Csteaming (non-preservation steaming treatment) and WLsteaming (waterless preservation steaming treatment) exhibited the highest levels of springiness (P < 0.05). Additionally, Csteaming and WLsteaming significantly reduced the hardness and cooking loss (P < 0.05) and demonstrated the highest umami values. The TBARS value of the baking groups exhibited a significantly lower level compared to that of the other heat treatment groups (P < 0.05). The primary volatile compounds in the heated grouper samples were nitrogen oxide, sulfides, and long-chain alkanes. The volatile flavor compounds of heated grouper samples were determined by headspace solid-phase microextraction gas chromatography-mass spectrometry, and a total of 35 volatile compounds were identified, including 8 aldehydes, 8 alcohols, 1 acid, 5 nitrogen-containing compounds, 6 hydrocarbons, and 7 ketones. The boiling treatments exhibited the highest levels of 5′-IMP and lactic acid. Meanwhile, steaming treatment emerged as a preferred method for maintaining the umami taste. Additionally, the waterless preservation technique effectively enhanced the content of succinic acid in pearl gentian grouper.

Energy and exergy performances of low-density polyethylene plastic particles assisted by microwave heating.

Plastic waste can exist naturally for hundreds of thousands of years and harm humans, animals, and the environment. In this study, the energy and exergy performances (absorbed energy, energy efficiency, absorbed exergy, and exergy efficiency) of LDPE (low-density polyethylene) plastic particles assisted by microwave heating based on the experimental data as affected by microwave power, feeding load, and chamber volume were evaluated and analyzed. The results showed that as the microwave power raised from 500 to 900 W, the feeding load changed from 10 to 30g, and the chamber volume decreased from 200 to 100ml, (a) the absorbed energy at the heating time of 60min increased from 19.73kJ, 5.84kJ, and 22.71kJ to 37.69kJ; (b) the energy efficiency for the whole heating process increased from 1.10%, 0.32%, and 1.26% to 2.09%; (c) the absorbed exergy at the heating time of 60min increased from 0.308, 0.091, and 0.091 to 0.724kJ; and (d) the exergy efficiency for the whole heating process increased from 0.017, 0.005, and 0.023 to 0.040%, respectively.

Improving the stability of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ/Crofer 22 H glass-sealed joints in humid oxidizing environments with a Mn-Co spinel coating

Sealing the BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) electrolyte to stainless steel interconnect is necessary for proton ceramic fuel cell stack packaging. However, oxidation of the interconnect in the glass-sealed joint presents a significant challenge, especially in humid and oxidizing environments. In this work, a Mn-Co spinel coating was fabricated on Crofer 22 H using microwave heating to extend its service life. The coated Crofer 22 H was successfully sealed to BZCYYb by a SiO2-TiO2-K2O-Na2O-Al2O3-ZnO glass sealant, achieving a sufficient joining. The joint is defect-free and dense, with a shear strength of 20 MPa. Detailed analysis using transmission electron microscopy revealed the underlying sealing mechanism. The Mn-Co spinel coating not only enhances the corrosion resistance of Crofer 22 H but also significantly improves the service life of the joint without compromising strength. The joint remains robust and defect-free after aging in a humid oxidizing atmosphere at 600 °C for 300 h.

Preparation of Carbon Quantum Dot with Copper Nanoparticles Using Mugwort Leaves as Carbon Source to Construct FRET Fluorescent Sensor for Melamine Detection

AbstractCarbon quantum dots (CQDs) have gained extensive attention due to their excellent properties and wide applications. In this study, we selected the common Chinese herb mugwort as the carbon source and employed the microwave heating method to synthesize CQDs with a quantum yield of 13.54 %. These CQDs exhibited blue fluorescence under UV light irradiation. Furthermore, we constructed a fluorescence resonance energy transfer (FRET) mechanism by combining CQDs with copper nanoparticles (CuNPs) for the detection of melamine. The detection limit for melamine was determined to be 0.83 nmol/L, with a linear range of 1–100 nmol/L. The results demonstrated the potential of this sensing system for the sensitive and selective detection of melamine.

Enhanced densification and phase transformations during rapid microwave sintering of alumina – yttria-stabilized zirconia ceramics

Alumina – 7.5wt. % yttria-stabilized zirconia (YSZ) ceramic composites were sintered using 24GHz microwave heating at rates of 10 – 200°C/min with zero isothermal hold. The starting powders were nanophase η-Al2O3 and YSZ prepared by a laser evaporation method. The final densities of the sintered samples were up to 97.5% of the theoretical value. The samples exhibited rapid densification until transformation to the α-Al2O3 phase. The temperature of the densification rate peak (and hence of the phase transformation) decreased consistently with increasing microwave electromagnetic field intensity (varied by using different susceptor materials). The densification peak temperature difference between microwave and conventional sintering experiments exceeded 200 °C.

Green synthesis of carbon dots nanoparticles from edible swiftlet nest and evaluation of their antioxidant activity

Edible swiftlet nest (ESN) confers various health benefits to humans, including immune system enhancement, anti-inflammatory properties, bone strengthening, and antioxidative effect. ESN is notably rich in protein and minerals, with its proteins serving as non-enzymatic antioxidants capable of binding free radicals. However, the antioxidant capacity of ESN is comparatively lower than that of other free radical scavengers, such as carbon dots nanoparticles (CNPs). CNPs features charged ligands on their surface that act as electron donors for free radical binding. The presence of the carbon chain forming proteins in ESN suggests their potential as the primary source for CNPs formation. The enhancement of hydroxyl groups and delocalized electrons is imperative for enhancing the antioxidant activity of ESN. This study aims to enhance the antioxidant activity of ESN by converting it into CNPs nanoparticles. The results demonstrated the effective synthesis of CNPs from the ESN solution using microwave methods. This was evidenced by XRD patterns indicating CNPs formation, and an average particle size of 4.86 nm as indicated by TEM analysis. The optimal microwave heating duration of 30 min yielded CNPs with a prominent emission spectrum peak at 425 nm and significantly high intensity. Absorbance data revealed the presence of C=C bonds, consistent with aromatic CNPs bonds observed in FTIR studies. CNPs possessed hydroxyl and carboxyl linkages, suggesting their potential as antioxidants. The percent inhibition results indicated that CNPs exhibited a substantial percentage (62.5%) at a concentration of 50 mg ml−1. The free radical scavenging activity of the CNPs significantly elevated compared to ESN.

Microwave-assisted synthesis of silver nanoparticles using extract of unbaked cilembu sweet potato

Abstract Silver nanoparticle (AgNP) is one of attractive nanomaterials for biomedical applications, such as electroactive scaffold, antimicrobial treatment, anticancer therapy, and wound healing. Previously, AgNP was successfully synthesized using extract of Cilembu sweet potato (CSP) only if the extract was baked in oven at 120 °C for 60 minutes, which is an energy-intensive and time-consuming process. In this study, AgNPs was fabricated using unbaked extract of CSP with employing fast microwave heating instead of baking process. Microwave irradiation time were varied (0, 30, 60 and 90 seconds) to know the influence of microwave irradiation time on particles size and morphology of the obtained AgNPs. The obtained samples were evaluated using UV-Visible spectroscopy, dynamic light scattering and transmission electron microscopy to know the surface plasmon resonance characteristic, average particles size and morphologies of the obtained AgNPs respectively.

Impacts of microwaves on the pectin extraction from apple pomace: Technological properties in structuring of hydrogels

The valorization of by-products eliminates the risks of economic barriers and reduces the overall amount of pollution and the carbon footprint of the products. In this context, conventional heating extraction (CE) and microwave-assisted extraction (MAE) were compared to extract pectin from apple pomace. The effects of temperature on MAE were evaluated based on the resulting properties obtained in subsequent pectin hydrogels. A pectin yield of 10.6 ± 0.3% was obtained by MAE with 5 min extraction, while CE showed a yield of 9.5 ± 0.3% after 2 h. The pectin extracted by both methods had a low methoxyl content, indicating a low degree of esterification. The galacturonic acid content was greater than 60% for most samples. Zeta potential analysis indicated the ability of the pectin to form stable gels. According to the X-ray diffractograms, increasing the extraction temperature led to a decrease in the crystallinity of the pectin structure. Rheological tests showed increased storage modulus as the temperature increased by MAE from 80 to 100 °C for the pectin-based hydrogel. The water holding capacity was greater than 50% for all hydrogels. Furthermore, an analysis of electrical consumption and energy costs highlighted the economic advantage of using microwave heating technology to extract pectin.

Interface/Dipole Polarized Antibiotics-Loaded Fe3O4/PB Nanoparticles for Non-Invasive Therapy of Osteomyelitis Under Medical Microwave Irradiation.

Due to their poor light penetration, photothermal therapy and photodynamic therapy are ineffective in treating deep tissue infections, such as osteomyelitis caused by Staphylococcus aureus (S. aureus). Here, a microwave (MW)-responsive magnetic targeting composite system consisting of ferric oxide (Fe3O4)/Prussian blue (PB) nanoparticles, gentamicin (Gent), and biodegradable poly(lactic-co-glycolic acid) (PLGA) is reported. The PLGA/Fe3O4/PB/Gent complex is used in combination with MW thermal therapy (MTT), MW dynamic therapy (MDT), and chemotherapy (CT) to treat acute osteomyelitis infected with S. aureus-infected. The powerful antibacterial effect of the PLGA/Fe3O4/PB/Gent is determined by the synergistic effects of heat and reactive oxygen species (ROS) generation by the Fe3O4/PB nanoparticles under MW irradiation and the effective release of Gent at the infection site via magnetic targeting. The antibacterial mechanism of the PLGA/Fe3O4/PB/Gent under MW irradiation is analyzed using bacterial transcriptome RNA sequencing. The MW heat and ROS reduce the activity of the protein transporters on the bacterial membrane, along with the transport of various ions and the acceleration of phosphate metabolism, which can lead to increased permeability of the bacterial membrane, damage the ribosome and DNA, and accompany the internal protein efflux of the bacteria, thus effectively killing the bacteria.

Dynamics of cyanogenic glycosides in apple and plum fruits, products, and byproducts: A concise review.

Apples (Malus domestica) and plums (Prunus domestica) are important fruit crops belonging to the Rosaceae family. The edible parts of fruits and seeds contain phytochemicals; however, the seeds are rich in cyanogenic glycosides (CNGs), which release toxic hydrogen cyanide (HCN) upon the loss of plant cell integrity. This review aims to explore the quantitative and qualitative CNG profiles in apples and plums, focusing on their distribution in different parts of the fruit, changes during fruit development, and environmental impacts on their biosynthesis. It also discusses the intricate dynamics of CNGs in processed fruits and waste-derived products and the effects of the processing methods on CNG content. There is considerable variation in the CNG content of fruit crops, as well as in its distribution in fruit parts other than seeds and shifts during fruit maturation. Although several studies have attempted to explain this variability by the influence of cultivars and exogenous factors, there is insufficient evidence to draw reliable conclusions. Furthermore, due to the lack of studies, the dynamics of CNGs during the storage of fresh or preserved fruit remains unaddressed. In the context of reusing plum stones from waste to produce distillates, it is recommended to monitor the HCN content in the products during storage, as it can increase significantly over time. Processing methods influence CNG levels, with strategies like seed separation, soaking, and microwave heating showing promise in reducing cyanide (CN-) presence. The insights from this review will provide direction for future detailed research.

Upgrading Biomass Wastes to Graphene Quantum Dots with White-Light-Emitting Features in the Solid State

The emergence of bio-based carbonaceous materials for various applications has attracted significant attention during the last few years. Here, we report a rapid, efficient, and reproducible microwave-assisted synthesis of graphene quantum dots (GQDs) with identical features irrespective of the nature of biomass waste investigated. The synthesized GQDs were fully characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, transmission electron microscopy, and dynamic light scattering. The nanoparticles displayed narrow sizes of 1–2 nm and high solubility in polar solvents such as water and ethanol. The protocol described herein is advantageous in comparison to dealing with the synthesis of GQDs from biomass waste previously reported since our protocol is faster owing to the use of microwave heating and the avoidance of dialysis for the purification step. Furthermore, in solution, the water-soluble particles showed excitation-dependent photoluminescence ranging from blue to orange emission wavelengths. Interestingly, thin films displayed white-light emission under 325 nm UV-light excitation, while aggregation-induced quenching was usually observed, opening the way for their potential use as a phosphor in white-light-emitting diodes.

The Effects of Different Cooking Systems on Changes in the Bioactive Compounds, Polyphenol Profiles, Biogenic Elements, and Protein Contents of Cauliflower Florets

In the current study, we examined the effects of boiling cauliflower in a pressure cooker, conventional boiling, conventional heating, and microwave heating on the chemical components, total phenol, flavonoids, antioxidant capacity (DPPH test), phenolic compounds, and mineral contents of cauliflower florets to reveal the differences between these cooking methods. Cauliflower is generally consumed either boiled or cooked in dry heat. In this study, different boiling and dry heat cooking methods were tried to reveal the changes in phytochemical composition and protein and mineral contents of cauliflower florets. Depending on the cooking methods of the cauliflower florets, the total phenolic and flavonoid contents of the cauliflower florets were determined to be between 273.72 (conventional heating) and 731.01 mg GAE/100 g (microwave heating) and 142.02 (conventional heating) and 797.10 mg/100 g (conventional boiling), respectively. The antioxidant capacity results of cauliflowers were found to be between 8.30 (conventional heating) and 33.69 mmol/kg (fresh). Statistically significant differences were detected in the moisture, total phenol, total flavonoid, and antioxidant activity values of cauliflower depending on the cooking techniques applied (p &lt; 0.05). The gallic acid and 3,4-dihydroxybenzoic acid values of fresh and cooked cauliflowers were identified to be between 10.93 (microwave heating) and 194.79 mg/100 g (boiling in pressure cooker) and 17.58 (conventional heating) and 145.80 mg/100 g (boiling in pressure cooker), respectively. In general, the lowest amounts of phenolic compounds were defined in cauliflower samples boiled with a conventional heating system, followed by cauliflower samples cooked with the microwave heating method. Considering the component amounts as a result of cooking, the highest phenolic component amounts were specified in the cauliflower sample cooked by boiling in a pressure cooker. The protein quantities of fresh and cooked cauliflowers were determined to be between 16.11 (fresh) and 19.79% (microwave heating). The K and S contents of fresh cauliflowers and cauliflowers cooked with different blanching methods were specified to be between 19,647.62 (conventional boiling) and 35,130.01 mg/kg (conventional heating) and 3196.54 (boiling in pressure cooker) and 5105.65 mg/kg (microwave heating), respectively. The K, Mg, S, Fe, Cu, Mn, and Zn results of cauliflowers cooked in an oven and microwave were higher than those cooked using the control and boiling methods.

Microwave-assisted organic acids and green hydrogen production during mixed culture fermentation.

The integration of anaerobic digestion into bio-based industries can create synergies that help render anaerobic digestion self-sustaining. Two-stage digesters with separate acidification stages allow for the production of green hydrogen and short-chain fatty acids, which are promising industrial products. Heat shocks can be used to foster the production of these products, the practical applicability of this treatment is often not addressed sufficiently, and the presented work therefore aims to close this gap. Batch experiments were conducted in 5L double-walled tank reactors incubated at 37°C. Short microwave heat shocks of 25min duration and exposure times of 5-10min at 80°C were performed and compared to oven heat shocks. Pairwise experimental group differences for gas production and chemical parameters were determined using ANOVA and post-hoc tests. High-throughput 16SrRNA gene amplicon sequencing was performed to analyse taxonomic profiles. After heat-shocking the entire seed sludge, the highest hydrogen productivity was observed at a substrate load of 50g/l with 1.09molH2/molhexose. With 1.01molH2/molhexose, microwave-assisted treatment was not significantly different from oven-based treatments. This study emphasised the better repeatability of heat shocks with microwave-assisted experiments, revealing low variation coefficients averaging 29%. The pre-treatment with microwaves results in a high predictability and a stronger microbial community shift to Clostridia compared to the treatment with the oven. The pre-treatment of heat shocks supported the formation of butyric acid up to 10.8g/l on average, with a peak of 24.01g/l at a butyric/acetic acid ratio of 2.0. The results support the suitability of using heat shock for the entire seed sludge rather than just a small inoculum, making the process more relevant for industrial applications. The performed microwave-based treatment has proven to be a promising alternative to oven-based treatments, which ultimately may facilitate their implementation into industrial systems. This approach becomes economically sustainable with high-temperature heat pumps with a coefficient of performance (COP) of 4.3.

Crystallization process of LTA zeolite from alkoxides assisted by microwave heating

Microwave heating (MH) is a promising way for zeolite synthesis in terms of rapid and uniform reaction. However, detail process of the synthesis reaction and mechanism of its promotion by microwave are still under investigation. In the present study, LTA (Linde Type-A) zeolite was hydrothermally synthesized by 300 W of MH from hydrolyzed Al and Si alkoxides. The formation process of LTA was elucidated by nuclear magnetic resonance spectroscopy (29Si NMR), small angle X-ray scattering (SAXS), X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS) and scanning electron microscopy (SEM). 29Si NMR and SAXS analyses suggest that linear polycondensates of aluminosilicate in the precursor solution was immediately transformed into densely packed structure by MH. TEM observations reveals that crystalline phase with several nanometers was already formed in amorphous aluminosilicate nanoparticles prior to the MH while XRD shows that the amorphous phase had been transformed to LTA phase until 400 min of MH. It is basically consistent with the previous studies among zeolite synthesis assisted by both of conventional heating and MH. The aluminosilicate/LTA particle size was measured by DLS and SEM image as well as the yield was calculated from weight of the reaction products. The particle gradually grew up to 242 nm in diameter until 36 h while the yield sharply increased to nearly 100 % during 6.7 h and 12 h. In other words, all aluminosilicate in the solution once had polycondensed to form insoluble particle until 12 h followed by particle growth by dissolution-reprecipitation or agglomeration until 36 h.

Highly effective grafting of poly(l-lactide) onto cellulose by microwave heating in a CO2 switchable solvent for the fabrication of biodegradable cigarette smoke adsorbent material

In this study, a CO2 switching solvent was used to dissolve cellulose, and then poly(l-lactide) (PLLA) side chain was grafted onto cellulose backbone by ring-opening polymerization of lactide under microwave heating to realize the rapid and efficient synthesis of cellulose-graft-poly(l-lactide) (C-g-PLLA). The efficiency of this method is significantly greater than that of the traditional oil bath heating system. The structure and properties of C-g-PLLA were characterized by Fourier transform infrared, Proton nuclear magnetic resonance, X-ray diffraction, Thermogravimetric analysis, and Differential scanning calorimetry. More importantly, C-g-PLLA can be rapidly degraded in both compost and lake water. It also showed good adsorption capacity for nicotine comparable to the commercial cigarette filters made from cellulose acetate. Overall, a rapid and efficient method for preparing C-g-PLLA was proposed, and the feasibility of its application in degradable cigarette smoke adsorption materials was demonstrated.

Radio frequency-only thermal processing of skimmed milk powder: Case study on the influence of RF heating profile on quality and Salmonella Typhimurium inactivation.

Radio frequency (RF) is a dielectric heating technology that allows rapid and volumetric heating of milk powder, outperforming the heating uniformity of conventional powder heating methods. Typically, RF milk powder processing consists of a fast RF heating phase, followed by an oven heating phase in temperatures around 90 °C. This methodology can result in milk powder quality deterioration due to non-uniform temperature distributions and local overheating. Radio frequency-only processes with a more gradual heating rate are alternative solutions to minimise the impact on milk powder quality. This study investigated the effect of the heating rate on the microbial inactivation of Salmonella Typhimurium inoculated in skimmed milk powder, as well as the effect of each process on two quality characteristics, colour and solubility. Overall, a slower heating profile resulted in sufficient inactivation rates of Salmonella in skimmed milk powder, while still providing a high-quality end product. A 4-log reduction was achieved by treating the skimmed milk powder up to 95 °C using a slower, longer heating rate. No statistically significant changes were observed in the solubility of skimmed milk powder and only the harshest treatment to 95 °C led to a slight increase in the yellowness of the skimmed milk powder colour.