Absorption Of Microwave Energy Research Articles

Size-effect in microwave processing of engineering materials - A review

The size of material units is especially critical in manufacturing processes where thermal energy interacts with the material. The microwave energy is widely used to process the materials in industries such as food processing, chemical, manufacturing etc. due to its unique heating characteristics. In microwave processing, energy is generated and absorbed inside the material during irradiation. The energy absorbed per unit volume of the material depends upon its size. The smaller size candidate materials have more effective surface area to absorb microwave energy than the bulk ones and usually yield lesser defects. This review paper summarizes the fundamentals of size-effect, microwave–materials interaction and input/output parameters in microwave material processing. Further, size-effect in microwave processing of different type of engineering materials (metal based, ceramic based and polymer based) have been discussed in terms of energy absorption and improvement in product attributes. The challenges in microwave processing of metal based materials have been identified and opportunities have been outlined in order to improve the properties vis-à-vis particle sizes during microwave processing.

Modelling of moving drying process and analysis of drying characteristics for germinated brown rice under continuous microwave drying

To understand the complex continuous microwave drying of germinated brown rice (GBR) and clarify the drying characteristics, the heat and mass transfer of GBR under continuous microwave drying was investigated numerically and experimentally. A three-dimensional model of coupled multi-physics fields involving the transmission of microwave field, heat transfer and mass (moisture) transfer was developed to characterise the drying process of GBR in a continuous microwave dryer. The implementation strategy based on discrete-combined approach was proposed to achieve the simulation of continuous microwave drying of moving materials with the mutual cooperation of the computer simulation software and independently developed program code. For the continuous microwave drying of GBR, the relatively uniform distribution of electric field strength applied to the grain layer depended on the reasonable arrangement pattern of magnetrons and suitable microwave power output. The movement of materials can effectively reduce excessive absorption of microwave energy by the grain layer, and achieve uniform distribution of temperature and moisture content and high drying uniformity assisted by the synergistic effects of microwave heating, moisture evaporation and ventilation convection. The developed model and simulation strategy may provide guidance for understanding and analysis of continuous microwave drying process of granular materials such as GBR.

Simple pyrolysis of alginate-based hydrogel cross-linked by bivalent ions into highly porous carbons for energy storage

Searching for a sustainable precursor that is capable of absorbing microwave energy is crucial for its rapid conversion into porous carbon via microwave heating. Here, alginate-based hydrogel beads cross-linked by bivalent ions (Cu2+, Zn2+, and Ca2+) are converted into porous carbons (SPCs) in 10 min by applying simple microwave-assisted pyrolysis. Water wrapped in hydrogel beads and bivalent ions serve as initial microwave absorbers to convert alginate into char which acts as a good microwave absorber in the following stages. Additionally, bivalent ions also act as a gelling agent to generate hydrogel beads, as a porogen to obtain a highly porous structure, and as a metal donor to form the SPC/Cu composite. The resultant SPC has a high specific surface area of 1336 m2 g−1, a large total pore volume of 0.56 cm3 g−1, interconnected macropores, as well as a high oxygen content of up to 13.2%. These attractive characteristics give SPC a remarkable rate capability of 72% at 50 A g−1. Interestingly, the interconnected macropores in SPC offer sufficient space for Cu growth via the redox reactions of Cu2+; thus, the SPC/Cu composite without acid wash shows a specific capacitance as high as 804 F g−1 at 0.5 A g−1.

Microwave-Enhanced Chemistry at Solid-Liquid Interfaces: Synthesis of All-Inorganic CsPbX3 Nanocrystals and Unveiling the Anion-Induced Evolution of Structural and Optical Properties.

We demonstrate how microwaves could enhance the chemistry at interfaces of heterogeneous reactions involved in the microwave-solvothermal (MW-ST) synthesis of all-inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (PNCs) within 6 min, unlike a conventional hot-injection method that requires 3 h. The enhanced MW-ST reaction rate was quantitatively analyzed by the Eyring equation, and it has been observed that the decreased activation free energy (ΔG⧧) and increased activation entropy (ΔS⧧) are caused by changes in the relative energies of reactants at their solid-liquid interfaces, leading to the formation of "hot spots", where microwave energy absorption is at its maximum. This rapid and homogeneous microwave heating could facilitate the self-assembly of uniformly distributed CsPbX3 nanocubes with precise control over the stoichiometric ratio, as confirmed by high-resolution transmission electron microscopy and energy-dispersive X-ray analyses. X-ray diffraction and Raman results indicate that lattice contraction and expansion in CsPbBr3-yXy have occurred because of an increase in the metal-halide bond length upon moving down the groups Cl → Br → I, as further ascertained by the Rietveld refinement studies. These anion-induced structural variations accordingly affected the electronic properties of MW-ST-synthesized CsPbX3 PNCs, which is apparent from the shifts in their conduction-band (CB) and valence-band (VB) positions. Consequently, the optical properties were also altered, resulting in a color-tuned emission from blue to red, with excellent photoluminescence quantum yields (up to 92%) and narrow emission line widths, as is evident from UV-vis and photoluminescence spectroscopy. The MW-ST-synthesized CsPbX3 PNCs were used as color-conversion layers for the fabrication of light-emitting diodes (LEDs) with commercial 456 nm UV-LED chips.

Catalyst‐Free Synthesis of Few‐Layer Graphdiyne Using a Microwave‐Induced Temperature Gradient at a Solid/Liquid Interface

AbstractGraphdiyne (GDY), a 2D carbon allotrope, is predicted to possess high carrier mobility and an intrinsic bandgap. However, the controlled synthesis of mono‐ or few‐layer GDY with good crystallinity remains challenging because of the instability of the monomers. Herein, a rapid and catalyst‐free synthetic method is presented for few‐layer GDY involving the use of a solid/liquid interface and a microwave‐induced temperature gradient. Sodium chloride, which can absorb microwave energy, is used as the solid substrate in a nonabsorbing solvent. A temperature gradient is formed at the solid/liquid interface under microwave irradiation, facilitating the cross‐coupling reaction of monomers at the solid surface and stabilizing the monomers in the bulk solution. Few‐layer GDY with an average thickness of less than 2 nm, a field‐effect mobility of 50.1 cm2 V−1 s−1, and p‐type characteristics is successfully obtained. This wet chemical approach may be extended to the synthesis of other few‐layered 2D materials with improved quality.

Surface morphology, magnetic resonant and antistatic properties of 07S11-KVfabric coated with stainless steel

A study of the mixed 07С11-KВ fabric (produced by Mogotex) with a coating of steel 12X18H10T, obtained by the method of pulsed cathode arc deposition in a vacuum of 3.5×10–3 Pa, was conducted. It is shown that optical microscopy has a number of advantages in studying the surface morphology of such objects as compared to scanning electron microscopy. The most contrast image is formed using dark-field illumination. When coating is applied, a droplet phase is formed, the droplet sizes vary from 2 to 10 microns. By the method of electron paramagnetic resonance, it has been established that the spectrum of coated fabric has an asymmetrical spectral line with a width of 101mT,which indicates a high concentration of magnetic resonance centers and a significant resonant absorption of microwave energy. Resonant absorption at low magnetic fields is determined by clusters of iron, nickel, chromium, titanium, etc., with weak nonresonant absorption. The specific surface resistance of the fabric (side 1 / side 2) is 3.3×105 ohm and 5.6×105 ohm,respectively.

Bio-fuels generation and the heat conversion mechanisms in different microwave pyrolysis modes of sludge

In order to explore the different characteristics of bio-fuels generated by microwave pyrolysis of sludge in the temperature and power control modes, the differences in the distribution, composition, lower heating value and energy utilization efficiency have been analyzed. The heat conversion mechanism was preliminarily explored via changes in the dielectric constants of the sludge. The results showed that: First, there were differences between the two modes during the pyrolysis process. This was the main reason that the lower heating value of the products in the temperature control mode was higher than it in the power control mode. For instance, the lower heating values of bio-gas and bio-oil generated at 800 °C were 16.04 MJ/kg and 35.96 MJ/kg, respectively. Second, the energy utilization efficiency of the temperature control mode was higher than that of the power control mode. This was because the required microwave energy input in the temperature control mode (40.32–65.52 MJ) was lower than that in the power control mode (72.80–88.00 MJ). This can be proven by the integral calculation results of output power and reaction time. Third, during the process of microwave pyrolysis, the ability of sludge to absorb microwave energy and convert it into heat (tanθ) first decreased and then increased due to the water evaporation and subsequent formation of pyrolysis residues. Therefore, choosing a suitable pyrolysis mode (temperature control mode) and condition (800 °C) is crucial to improving the efficiencies of resource recovery and energy utilization.

Microwave-assisted KOH activation from lignin into hierarchically porous carbon with super high specific surface area by utilizing the dual roles of inorganic salts: Microwave absorber and porogen

Searching an appropriate biomass that is capable of absorbing microwave energy is significant to realize its rapid microwave-assisted conversion into porous carbons (PCs). Herein, the lignin derived from waste pulping black liquor via crude extraction is directly converted into high-performance PC for a very short duration of 10–30 min by microwave heating. The inherent inorganic salts in crude lignin with a high content of 17.3% is innovatively used as initial microwave absorber to achieve rapid conversion from lignin into bio-char, and further as porogen combined with KOH to synergistically obtain well-developed porous texture. The resultant PC possesses a super high specific surface area of 3065 m 2 g −1 , hierarchical pore size distribution with abundant micropores (0.73 cm 3 g −1 ) and a remarkable meso-/macropores ratio of 64.4%, highly disordered and amorphous structure, as well as oxygen-enriched structure with a surface oxygen content up to 16.5%. These attractive characteristics contribute to excellent electrochemical properties of the resultant supercapacitor electrode which exhibits a high specific capacitance of 325 F g −1 at 0.5 A g −1 in 6 mol L −1 KOH electrolyte, still maintaining a high value of 258 F g −1 (79.4%) at 50 A g −1 . The prepared supercapacitor can remain 81.9% energy density when the power density was increased by 20 folds, suggesting its excellent energy-power synergetic outputting property. These attractive results pave a new way to utilize such a biomass waste, especially with high inorganic salts content, into value-added electrode material for advanced energy storage application. • Conversion from waste lignin into porous carbon only required 10–30 min. • Inorganic salts act as microwave absorber and also porogen. • Super high specific surface areas surpassing 3000 m 2 g −1 were achieved. • A remarkably high rate capability of 79.4% was achieved at 50 A g −1 .

Dielectric properties of hard rock minerals and implications for microwave-assisted rock fracturing

Understanding microwave-assisted breakage and fracturing of hard rocks necessitates the characterisation of the dielectric properties of basic rock-forming minerals. In the literature, there is a significant discrepancy in the loss factors (i.e., the ability of minerals in absorbing microwave energy) of the gangue minerals due to the fact that different unsuitable and inconsistent measurement techniques are used. In this paper, the dielectric properties (i.e., the dielectric constant $$\varepsilon^{\prime}$$ and loss factor $$\varepsilon^{\prime\prime}$$) of 14 high-grade minerals, including 11 gangue minerals and 3 ore minerals that form most of the commonly found igneous and metamorphic hard rocks, were obtained using a customised resonant cavity and the effective medium theory at 2.45 GHz and room temperature. These minerals could be classified into three categories (i.e., low, medium and high loss) based on the magnitude of their loss factors. For the low-loss and medium-loss gangue minerals, their loss factors hardly correlate to either one or a combination of the metal elements or the crystal system. The dielectric properties of minerals were then used to explain the behaviour difference of granite and basalt when exposed to microwave radiation.

Dielectric properties and microwave heating behavior of neutral leaching residues from zinc metallurgy in the microwave field

AbstractThe method of microwave strengthens roasting neutral leaching slag contained germanium is put forward. The dielectric properties and temperature rising behavior of neutral leaching slag in the microwave field are analyzed via experimental and theoretical analysis. The experiment used the dielectric constant test device, the microwave roasting device and the temperature measuring device. The neutral leaching slag can absorb microwave energy and transform into heat energy. The specific surface area of the calcined slag containing the neutral leaching slag is increased by microwave roasting, and the microwave energy can reduce the particle size of the mineral particles to some extent. At the same time, cracks can be seen on the surface of the calcined sand after microwave roasting. The surface of neutral leaching slag roasted by microwave reformed cracks which help to open reaction channels making the oxygen to inside takes part in oxidizing reaction significantly improving germanium oxide reaction conditions, besides increasing the leaching process of the reaction area and increasing the leaching rate. The research on the dielectric constants and temperature characteristics of neutral leaching residue can provide the theoretical and experimental basis for microwave strengthen roasting.

Temperature field distribution analysis for cargo oil on microwave heating process

This paper uses microwave technology to study the heating process of cargo oil in tanker side cabin under the static environment. The research is aimed to improve the heat transfer efficiency of cargo oil and solve the problems of uneven temperature distribution and energy consumption during the heating process of traditional crude-oil in oil tank. Based on the theory of microwave heating and heat transfer, the finite element simulation software is used to couple the microwave and temperature field during the cargo oil is heated to simulate the changing characteristics of the temperature field in static environment, and then verify the numerical results of the existing experiments. The simulation results show that the numerical results of this model agree well with the experimental data, and the error is in the range of 6.21-13.84%. Therefore, the accuracy of the numerical method can be verified. During the heating process, the absorption of microwave energy has a positive correlation with the electric field intensity distribution. The microwave field shows a ?strong point? distribution characteristic. Heat transfer is accompanied by natural-convection and heat conduction, and the oil temperature distribution is more balanced under the combined effect, which shows that the microwave heating of oil tankers have certain feasibility. The results of this study can provide a theoretical basis for studying the heat transfer law of oil tanker cargo steam and microwave combined heating process.

Carbonized cellulose nanofibers as dielectric heat sources for microwave annealing 3D printed PLA composite

Filament fused fabrication (FFF) is an extrusion-based 3D printing technology for manufacturing thermoplastic components. One major obstacle facing 3D printed thermoplastic material is the reduced crystallinity resulting from a fast quench when material exiting the 3D printer hot nozzle solidifies quickly at the low-temperature platform, leading to weak mechanical performance. Here, we report an accelerated annealing strategy with the assistance of microwave heating, aiming to enhance crystallinity and mechanical performance of FFF 3D printed polylactic acid (PLA) composite. We selected naturally abundant cellulose fibers as precursors for producing carbonized cellulose nanofibers (CCNFs), and compounded CCNFs with PLA to produce bi-component filament for 3D printing final composite. After being irradiated with microwave, the embedded CCNFs in composite selectively absorbed microwave energy and generated heat. Subsequently, the localized heat transferred to the adjacent PLA regions, triggering amorphous PLA chains to repack and convert to new crystallites. In this work, annealing conditions, including heating method (i.e., oven annealing vs. microwave annealing), time (0–120 min), and temperature (80 vs. 120 °C), were systematically studied to understand the relevant effects on the resulting parameters including composite crystallinity and tensile strength. Microwave annealing method was also compared with conventional oven annealing method and results shows that microwave annealing significantly reduced the required annealing time to reach the maximum crystallinity and tensile strength. Notably, microwave annealing performed below cold crystallization temperature was exceptionally suitable to develop an optimized crystallinity and tensile strength for 3D printed PLA composite.

Effect of dielectric properties on heat transfer characteristics of rubber materials via microwave heating

The effects of dielectric properties on temperature distribution, heat flux density distribution, heating rate, heating efficiency and heating uniformity of rubber sheets via microwave heating were studied. Experimental and numerical simulation studies were carried out, the reliability of the simulation method was verified. The results demonstrated that the ability of rubber to absorb microwave energy and convert it into heat is significantly affected by dielectric constant and dielectric loss tangent. Different dielectric properties led to different electric field strength distributions in rubber sheets during microwave heating process, resulting in different electrical energy densities in the rubber sheets, eventually, different amounts of electric field energy can be converted into heat, resulting in different temperature distributions. The temperature distributions of rubber sheets are uneven. The temperature difference of rubber sheet increases with increasing heating time, but the growth rate of temperature difference decreases with increasing heating time. The higher the dielectric constant or loss tangent of rubber, the higher the microwave heating efficiency and temperature rise rate, but the more uneven the temperature distribution. The longer the heating time, the higher the average body temperature of rubber sheet, but the growth rate of the average body temperature decreases with increasing heating time. The heat flux density of rubber sheet was also significantly influenced by the dielectric properties, the maximum temperature gradient value is in the central area of the rubber sheet rather than in the hot spot area.

Microwave-assisted solubilization of microalgae in high-temperature ethylene glycol

Four types of microalgae (Chlorella vulgaris, Nannochloropsis oculata, Fistsulifera solaris (JPCC DA0580), and Phaeodactylum tricornutum Strain NRIA-065) were liquefied through noncatalytic solvothermal solubilization under microwave (MW) irradiation using ethylene glycol (EG) as a reaction media. C. vulgaris and N. occulata exhibited higher solubilization rates and bio-crude oil yields in a temperature range at 300 °C while diatoms (Fistsulifera. Solaris and Phaeodactylum tricornutum) were recalcitrant to MW treatment owing to its high ash content. Bio-crude oil yield increased depending on the reaction temperature and reaction time. In particular, nearly 38% bio-crude oil was obtained under a reaction at 300 °C for 40–60 min from C. vulgaris. Higher heating values of bio-crude oil of 39 and 28 MJ kg−1 were attained in the chloroform and ethyl acetate layers in EG soluble fraction, respectively. Dielectric measurement of the reaction system indicated the effectiveness of ethylene glycol as a medium for absorbing MW energy. EG was efficient reaction medium for rapid heating up by MWs to facilitate solubilization of microalgae at low pressure.

Direct Microwave Conversion from Lignin to Micro/Meso/Macroporous Carbon for High‐Performance Symmetric Supercapacitors

AbstractEnzymatic hydrolysis lignin (EHL) is a common biomass that is difficult to use for the synthesis of porous carbon (PC) via direct microwave heating because it absorbs very little microwave energy. In this study, a novel method was developed to synthesize high‐performance PC directly from EHL using only 8 min of microwave heating. The resulting PC exhibits a high specific surface area of 2482 m2 g−1 and a hierarchical pore distribution with a high mesopore content of 35.6 % and oxygen content of 17.1 %. Nitrogen (as a protecting gas) passes through water molecules provided by steam to absorb microwave energy and acts as a physical porogen to form mesopores. Using solid KOH allows one to bypass the time‐consuming drying process and generate sufficient micropores. Additionally, fast self‐pyrolysis and a high heating rate lead to the generation of both mesopores and macropores in a PC framework. Furthermore, multiple active centers can be formed during microwave heating and subsequently stabilized by oxidation once the PC is exposed to an air atmosphere, resulting in the attachment of oxygen functional groups to the PC surface. Based on these desirable physicochemical characteristics, the PC electrodes fabricated in this study exhibit a high specific capacitance of 338 F g−1 at 1 A g−1 and high capacitance retention of 86 % at 10 A g−1. The final PC‐based supercapacitor delivers a maximum energy density of 17.1 Wh kg−1 in a Na2SO4 electrolyte and 8.6 Wh kg−1 in a KOH electrolyte.

Microwave-Initiated Synthesis of Metal Chalcogenides/Graphene-Catalyst for Enhanced Hydrogen Evolution Reaction

In present days, new types of renewable energy resources are being developed to resolve the world energy crisis. Nanomaterials, such as carbon nanotubes, metal oxides, metal chalcogenides (MCs) and conducting polymers with superior mechanical, thermal and electrical properties, lead to broad applications in composite materials, smart structures, chemical sensors, energy storage and nano-electronic devices. However, the high cost and difficulty in producing large scale, high quality nanomaterials remain challenges. The present work successfully demonstrates the production of MCs on graphene substrate through ultra-fast (60 seconds), facile and energy-efficient microwave-initiated approach. Graphene plays a vital role during the synthesis process by absorbing microwave energy and converting it to heat energy, which facilitates the precursors to react vigorously. Moreover, high specific surface area and conductivity of graphene delivers a favorable conductive network for the uniform growth of MCs, along with rapid charge transfer kinetics. The electrochemical characterizations reveal that the as-produced nanocomposites can be used for hydrogen evolution reaction (HER) to generate useful hydrogen fuel. As an example, the molybdenum disulfide on graphene (MoS2/graphene) composite exhibits superior electrocatalytic activity for the HER in acidic medium, with a low onset potential of only 100 mV and a Tafel slope of 43.3 mV per decade, suggesting the Volmer-Heyrovsky mechanism of hydrogen evolution. Beyond excellent catalytic activity, MoS2/graphene reveals an unusual ability to enhance the accessible active sites during the HER proceeds. This leads to a long cycling stability with a very high cathodic current density, providing the opportunity of practical application for scalable processing. Therefore, this single-step microwave approach can be universally employed to produce other useful MCs (e.g., MoSe2, WS2, WSe2 etc.), that will catalyze substantial development in more widespread uses of MC-based nanocomposites for successful energy applications.

On the possibility of using detonation ceramic coatings as microwave energy absorbers

The developed absorbing microwave energy coatings of TiO2 and Al2O3/TiO2 deposited by the detonation method were investigated in this work. Thermal cyclic tests of the obtained samples of different layer thickness on copper substrates were carried out, the phase composition of the obtained coatings was investigated, the effect of high-temperature heating in hydrogen on the phase composition of the coating and its stability was determined. The gas emission was studied on evacuated and degassed experimental model.

Airborne disinfection using microwave-based technology: Energy efficient and distinct inactivation mechanism compared with waterborne disinfection

Microwave has been extensively applied to inactivate microorganisms in liquids, food, and surfaces. However, energy efficiency is a limiting factor for the environmental application. The utilization pathway and energy efficiency of the microwave in different media have not been investigated. In this study, the inactivation performance, energy utilization, and bactericidal mechanisms for microwave-irradiated airborne and waterborne Escherichia coli were compared. A Beer-Lambert law-based model was also developed and validated to compare the inactivation performance in different phases. Microwave had greater inactivation effect on airborne bacteria than waterborne bacteria. The inactivation rate constant for airborne E. coli (0.29 s−1) was nearly 20 times higher than that of waterborne species (0.014 s−1). Most of the absorbed microwave energy (92.3%) was converted to increase water temperature instead of inactivating the waterborne bacteria, because the microwave photons were easily absorbed by water molecules. By contrast, 45.4% of the absorbed energy could disinfect the airborne bacteria. Finally, the required energies for 1-log inactivation were calculated as 2.3 J and 116.9 J per log-inactivation for airborne and waterborne E. coli, respectively. The airborne and waterborne E. coli samples showed distinct microwave inactivation mechanisms. Waterborne E. coli disinfection was primarily due to thermal effect, while the non-thermal effect was the major mechanism for airborne E. coli inactivation.

Molecular dynamics simulation investigation of the microwave heating supercritical water

Nonequilibrium NVE molecular dynamics (MD) simulations were performed to investigate the heating process of supercritical water (SCW) by the microwaves (the field intensities are from 3 × 105V/m to 3 × 109V/m with the frequencies of 915 MHz and 2.45 GHz, respectively). The energy distribution, microscopic molecular structure, diffusivity and hydrogen bond dynamics were analyzed. Microwaves field dependence on the rotational and translational coupling mechanisms of water molecules have been detected. The microwave energy has been converted to the kinetic and intermolecular energies of water. There are about 40% absorbed microwave energy is stored as the intermolecular energy, and the proportion decline at higher field intensity. The diffusion of supercritical water is non-linearly fluctuation due to the coupling of rotation and translation motions. And this work may provide a theoretical basis to improve the efficiency of industrial application with microwave and SCW.

New approach to achieving smaller bubbles with various microwave irradiation modes

Previous studies have measured the sizes of bubbles in suspension during microwave irradiation to clarify the mechanism of bubble formation. Those results indicate that the particle number density and the dielectric constants of the particles and solvent are the most important factors determining bubble size during irradiation. The aim of this study is to determine whether bubble nucleation can be controlled by changing the microwave mode between continuous irradiation and the two-stage irradiation proposed in this study. The first irradiation of higher power rapidly accelerates bubble nucleation, whereupon the bubbles grow more slowly during the second irradiation of lower power. Because the absorbed microwave energy is distributed to the liquid–air interface of each highly suspended small bubble produced by the first irradiation, the absorbed energy per bubble decreases during the second irradiation. Finally, smaller bubbles are achieved at the target temperature. Because the bubble number density and size can be controlled by the rapid thermal response that is characteristic of microwaves, this method could be useful for preventing superheating and bumping during nano-particle synthesis.