Selective Microwave Heating Research Articles

Microwave hydrothermal in situ synthesis of highly dispersed metal-organic framework@SiC as a promising structured catalyst

Metal-organic framework (MOF) materials with loaded catalysts have recently garnered substantial interest in the catalysis field. However, the catalytic performance of MOF catalysts is hindered by particle aggregation. This study introduces a microwave-assisted anchoring strategy to improve the dispersion of UIO-66-NH2 on SiC surfaces. The dispersion can be effectively controlled by varying the SiC carrier content. Scanning electron microscopy and transmission electron microscopy analyses show that as the SiC content increases from 2% to 10%, MOFs exhibit a more dispersed distribution on the surface. This is exemplified in the esterification reaction between acetic acid and cyclohexene, where a higher MOF dispersion leads to a conversion rate boost from 70% to 75%. Numerical simulations reveal that microwave's selective heating property plays a crucial role in achieving this enhanced dispersion. This work contributes a systematic approach to enhance MOF dispersion and advances the development of structured catalysts based on MOFs.

Rapid and selective actuation of 3D-printed shape-memory composites via microwave heating

Three-dimensional (3D) printing allows the fabrication of complex shapes with high resolutions. However, the printed structures typically have fixed shapes and functions. Four-dimensional printing allows the shapes of 3D-printed structures to be transformed in response to external stimuli. Among the external stimuli, light has unique advantages for remote thermal actuation. However, light absorption in opaque structures occurs only near the sample surface; thus, actuation can be slow. Here, we propose and experimentally demonstrate the rapid and selective actuation of 3D-printed shape-memory polymer (SMP) composites using microwave heating. The SMP composite filaments are prepared using different amounts of graphite flakes. Microwave radiation can penetrate the entire printed structures and induce rapid heating. With sufficient graphite contents, the printed SMP composites are heated above their glass transition temperature within a few seconds. This leads to rapid thermal actuation of the 3D-printed SMP structures. Finally, dual-material 3D printing is demonstrated to induce selective microwave heating and control actuation motion. Our experiments and simulations indicate that microwave heating of SMP composites can be an effective method for the rapid and selective actuation of complex structures.

Chemical recycling of polymer composites induced by selective variable frequency microwave heating

AbstractThe application of variable frequency microwave (VFM) heating to achieve rapid thermal depolymerization of polymer composites is reported for the first time. The thermal and chemical influence of composite additives on polymer decomposition has been studied for a set of chemically recyclable polymers, including polyphthalaldehyde, polypropylene carbonate, two polyhydroxyalkanoates, and nylon 6. Carbon‐based additives, specifically nanocarbon particles, were used as effective microwave absorbers to controllably degrade the surrounding polymer matrix into valuable, monomeric compounds. Depolymerization and byproduct confirmation were quantified by gel permeation chromatography and nuclear magnetic resonance spectroscopy. Synergistic effects can be leveraged from pre‐treating composite samples to further reduce the total microwave energy required to depolymerize composite samples. This study establishes the use of VFM heating for chemical recycling of polymer composites that can be leveraged toward a plastic circular economy.

Ultrafast Strategy to Fabricate Sulfur Cathodes for High-Performance Lithium-Sulfur Batteries.

Based on the different dielectric properties of materials and the selective heating property of microwaves, the ultrafast (30 s) preparation of S-NiS2@SP@Bitu as a cathode material for lithium-sulfur batteries was achieved using bitumen, sulfur, Super P, and nickel naphthenate as raw materials for the first time, under microwave treatment. NiS2@SP@Bitu forms Li-N, Li-O, Li-S, and Ni-S bonds with polysulfide, which contributes to promoting the adsorption of polysulfide, reducing the precipitation and decomposition energy barrier of Li2S, and accelerating the catalytic conversion of polysulfide, as result of inhibiting the "shuttle effect" and improving the electrochemical performance. S-NiS2@SP@Bitu as the sulfur cathode material demonstrates outstanding rate performance (518.6 mAh g-1 at 4C), and stable cycling performance. The lithium-sulfur battery with a sulfur loading of 4.8 mg cm-2 shows an areal capacity of 4.6 mAh cm-2. Based on the advantages of microwave selective and rapid heating, this method creatively realized that the sulfur carrier material was prepared and sulfur was fixed in it at the same time. Therefore, this method would have implications for the preparation of sulfur cathode materials.

Reduced graphene oxide catalytically enhances the rate of cyanate ester curing under variable frequency microwave heating

AbstractThe curing of Lonza Primaset PT‐30 novolac cyanate ester resin and EPON 826 bisphenol‐A diglycidyl ether were investigated using convective thermal heating and variable frequency microwave (VFM) heating. The addition of 1 part per hundred reduced graphene oxide (r‐GO) to PT‐30 novolac cyanate ester increased the VFM cure rate compared to thermal heating. Curing it at 160°C for 240 min with VFM heating resulted in a 55% degree of cure compared to a 26% degree of cure with thermal heating. This observed VFM rate enhancement is due to selective microwave heating of the r‐GO particles in the resin resulting in increased r‐GO catalytic activity toward cyanate ester curing. It is both a thermal and catalytic effect, the latter of which is absent when r‐GO is added to a bisphenol‐A diglycidyl ether resin with o‐phenylenediamine hardener. Impurities present in the PT‐30 matrix do not appear to contribute to its overall cure kinetics, nor do they participate in the observed VFM rate enhancement.

Low-cost, high-density Mn–Co spinel coatings for stainless steel interconnect via efficient microwave heating

The high preparation cost and poor density of spinel coatings for stainless steel interconnect have always been puzzling conundrums for the solid oxide fuel cell stack developers. Herein, the microwave heating technology for fabricating Mn–Co spinel coating is developed to address these issues. We designed a device spontaneously providing reducing atmospheres and realized the low-cost, high-efficiency reduction of Mn–Co spinel via microwave heating. Dense coatings (MnO + Co) are formed on the steel surface at ∼1100 °C. The excellent density is related to the melting of Co particles induced by microwave selective heating. At initial, microwave heating is dominated by dipolar polarization, interfacial polarization, and natural resonance. After forming metallic Co, conductive loss and the lens effect further accelerate the reduction process. High-quality spinel coatings are formed during the subsequent oxidation at 800 °C, exhibiting an extremely low area-specific resistance increase rate.

Purification and characterization of amorphous boron powder by microwave heating combined with acid leaching

Mg removal is the key to the purification of amorphous boron powder, and the specific surface area of boron powder has a great influence on its activity. Therefore, it is important to improve the purity and activity of boron powder. In this study, the shells of boron powder particles are cracked using the microwave selective heating manner, which results in cracks and exposes impurities in boron particles. This is convenient for impurities to dissolve by acid. In addition, the specific surface area of boron powder particles is increased by 5.11 m 2 /g. The optimal experimental results show that the magnesium content in amorphous boron powder is reduced to 0.85%. It was deduced that the activity of boron powder was improved. The initial oxidation temperature of boron powder decreases by 64 °C, and the termination oxidation temperature decreases by 16 °C. It opens the way for unconventional modification of the boron powder. • A shell of boron powder is cracked and impurities expose by microwave heating. • Mg is decreased to 0.85% and purification of boron powder reaches 95.87%. • Specific surface area of boron powder are increased by microwave heating. • The activity of boron powder was improved after microwave heating.

Microwave selective heating ultrafast construction of coral-like TiO2-MXene /graphene hybrid architectures for high-performance lithium-ion battery

Graphene and MXenes, both as an emerging two-dimensional (2D) material, have become the predominant choice of electrochemical energy electrode materials owing to large surface-area-to-volume ratio and high electrical conductivity. However, severe restacking of such 2D ultrathin nanostructures largely lowers active surfaces and thus constrains their applications. Herein, we develop a microwave selective heating strategy that is highly efficient for ultrafast and scalable fabrication of novel coral-like porous MXene/graphene composite with high-quality graphene and well-dispersed TiO2 nanocrystals. The MXene not only works as a source material but also serves as an excellent microwave absorbent, which leads to that reduction of graphene oxides can be completed within only 10 s under microwave irradiation at a much lower applied power of 200 W than the reported value of 1000 W. Especially the coral-like TiO2-MXene/graphene material with large porosity delivers much higher capacity (356 mAh g−1 at 50 mA g−1) and much better rate capability than those for pure graphene, MXene and other MXene/graphene composites. That is attributed to the composites avoids restacking, assures structural stability, and provides high specific surface area and excellent electrons/ions conductivity. This work provides a new route to large-scale and low-cost production of high-performance electrode materials for lithium-ion battery.

Process intensification using microwave heated multiphase reactors

The quest for transition to clean energy sources and intensified chemical processes has led to growing interest in microwave-heated reactors. Microwave generated from renewable electricity is a clean energy source and can intensify chemical processes with its unique heating characteristics, especially selective heating. Selective microwave heating creates a temperature differential between phases in a multiphase reactor, allowing a new paradigm in process intensification. Numerous studies have demonstrated the potential of microwaves to achieve process intensification. Nevertheless, detailed explanations for the observations are lacking primarily due to the unavailability of accurate temperature measurements. This perspective article discusses the significant challenges hindering the development and deployment of microwave multiphase reactors and the opportunities for further research. Structured reactors, such as monoliths, are emphasized as a canonical setup for experiments and modeling.

Investigating the role of solvent type and microwave selective heating on the extraction of phenolic compounds from cacao (Theobroma cacao L.) pod husk

Cacao pod husk (CPH) is a primary waste in the cacao industry that contains favourable natural antioxidants based on phenolic compounds. This study reported an investigation of the effect of extraction parameters to maximise the bioactive yields of CPH extract. The preliminary extraction was focused on high total phenolic content and continued to maximise the total monomeric anthocyanin and antioxidant activity that have the potential to be applied as food additives. The solvent selection and particle size were the key parameters for extraction to reach the maximum phenolic yield (100.4 ± 0.5 mg GAE/g dw). It was confirmed that 50% (v/v) aqueous ethanol was the most appropriate solvent, either based on experimental results or Hansen Solubility parameter prediction. At the same time, the comparison of microwave and conventional heating suggested that Microwave-assisted Extraction was the best method to get high phenolic content due to its selective heating effects. The results showed that the maximum bioactive yields were 0.37 ± 0.0 mg Cy3GE/g dw of anthocyanin and 3.36 ± 0.02 mg TE/g dw of antioxidants obtained under 50 °C and 5 min extraction time. Gallic acid, catechin, epicatechin, coumaric acid and quercetin were identified in CPH extract using High-Performance Liquid Chromatography.

Microwave-accelerated regeneration of a non-aqueous slurry for energy-efficient carbon sequestration

We have developed a slurry of basic immobilized amine sorbent (BIAS) and polydimethylsiloxane (PDMS) oil for a CO2 adsorption/desorption process aided by the Microwave-Accelerated Regeneration of a Slurry (MARS). Selective microwave heating of CO2-adsorbed BIAS can accelerate the CO2 desorption flux by 10 times and consume much less energy for sorbent regeneration, compared to thermal heating. Furthermore, the microwave regeneration temperatures of the slurry can be lowered to 65–85 °C. These results indicate that the microwave-accelerated swing method is highly energy-efficient, which can substantially decrease the energy penalty for CO2 capture and downscale the capture process, especially the regeneration equipment. The microwave-assisted non-aqueous capture technology without water and steam use can decrease the cooling duty by 18–48%, compared to the aqueous amine-based capture technology and promote decarbonization in a more sustainable manner.

Microwave absorption properties of spent green phosphor and enhanced extraction of rare earths

The efficient extraction of rare earth elements from spent phosphor was achieved by microwave alkali fusion -leaching. Effect of temperature on the dielectric properties was studied. The addition of alkali greatly enhanced the microwave heating ability of the phosphor with loss tangent value increasing from 0.031 to 0.084 at 800 °C, which was ascribed to the strong susceptibility of molten sodium hydroxide and roasted products to microwave interaction. Effects of process parameters of roasting and acid leaching were investigated. When the phosphor was treated at 800 °C for 30 min with mass ratio of NaOH/phosphor of 2:1 followed by acid leaching of 4 mol/L HCl with the liquid-solid ratio of 10:1 at 60 ºC for 120 min, the leaching efficiencies of Ce and Tb were 97.86% and 95.75%, respectively. Microwave roasting showed the advantages of lower temperature, shorter time and higher leaching ratio of rare earths compared with conventional roasting. Enhanced thermal motion of sodium hydroxide, improved kinetic conditions and changes of crystal structures resulting from microwave selective heating may be responsible for microwave-enhanced rare earths extraction. Kinetic studies showed that both the alkali roasting and leaching process of rare earths were controlled by product layer diffusion.

Microwave-assisted reduction and sintering to construct hybrid networks of reduced graphene oxide and MXene for electromagnetic interference shielding

Graphene is a prospective electromagnetic interference (EMI) shielding material in light of remarkable electrical conductivity. Nevertheless, the preparation of graphene through reduction of graphene oxide (GO) is an energy/time consuming process. We utilized Ti3C2Tx as an efficient initiator to achieve the thermal reduction of GO under microwave irradiation. Simultaneously, the continuous networks of rGO/Ti3C2Tx were constructed in sintered composite by locally melting and welding polysulfone under appropriate irradiation conditions. The composite with a 1:4 ratio of GO to Ti3C2Tx treated by irradiation of 10 s possessed an EMI SE of 52.81 dB with a thickness of 2 mm, while the shielding effectiveness of reflection (SER) only reached 4.8 dB. Besides, the intensity ratio of D to G-band (ID/IG) for composite treated by appropriate microwave dropped from 1.20 to 0.74. Therefore, selective microwave heating might be a clean and low-consumption strategy for large-scale production of rGO and manufacture of high-performance composites with EMI shielding property.

Fabricating bimodal microcellular structure in polystyrene/carbon nanotube/glass‐fiber hybrid nanocomposite foam by microwave‐assisted heating: A proof‐of‐concept study

AbstractIn the current study, polystyrene/carbon nanotubes/glass fiber (PS/CNT/GF) hybrid foam with a bimodal cellular‐structure has been fabricated via microwave heating as a novel energy source. Microwave‐assisted (MA) samples not only demonstrated the lowest density as compared to traditional methods but also represented superior compressive mechanical properties. Since the bimodal morphology has not been seen so far, the role of selective microwave heating has been thoroughly investigated from the microstructural point of view. Accordingly, the amount of receiving microwave radiations to heat up the system is controllable via the CNTs (as nucleating agents and absorbers) as well as radiation time. Moreover, 1–15 wt% fibers were incorporated to enhance mechanical performance, which led to turning uniform cellular structure to the bimodal pattern, and its cell morphological was studied thereafter. Electrical conductivity and dielectric permittivity properties have not been deprived of bimodality benefits which have been rigorously proven.

Accelerated thermal reaction kinetics by indirect microwave heating of a microwave-transparent substrate.

Macroscopically homogeneous mixtures of p-nitroanisole (pNA) and mesitylene (MES) can be selectively heated using microwave (MW) energy. The pNA solutes agglomerate into distinct phase domains on the attoliter-scale (1 aL = 10-18 L), and these agglomerates can be MW-heated selectively to temperatures that far exceed the boiling point of the surrounding MES solvent. Here, a 1 : 20 mixture of pNA : MES is used as a mixed solvent for aryl Claisen rearrangement of allyl naphthyl ether (ANE). ANE itself does not heat effectively in the MW, but selective MW heating of pNA allows for transfer of thermal energy to ANE to accelerate rearrangement kinetics above what would be expected based on Arrhenius kinetics and the measured bulk solution temperature. This focused study builds on prior work and highlights 1 : 20 pNA : MES as a mixed solvent system to consider for strategically exploiting MW-specific thermal effects.

Microwave selective heating of electric arc furnace dust constituents toward sustainable recycling: Contribution of electric and magnetic fields

Microwave heating of waste electric arc furnace dust (EAFD) for the purpose of its remediation has been studied extensively. However, these studies considered EAFD as a bulk material without addressing either the relative response of its individual constituents or the specific interactions with either the electric or magnetic fields of an electromagnetic wave. In this work, we present a study of the relative contribution of the electromagnetic wave components to the heating of EAFD constituents using separated electric and magnetic microwave fields. ZnO, ZnFe2O4, Fe3O4, and graphite were found to be the main contributors to the heating of EAFD based on their dielectric properties and heating profiles. ZnO and ZnFe2O4 heated only in the electric field yielding temperatures of 846 and 720 °C after heating for 30 and 40 s, respectively at a power input of 118 ± 12 W. Fe3O4 and graphite, in contrast, heated in both electric and magnetic fields owing to their respective magnetic and conductive nature. It is suggested that the selective magnetic heating of Fe3O4 has significant implications for the selective extraction of zinc and lead through thermal treatment of EAFD with halogenated plastics such as polyvinyl chloride (PVC).

Thermally Assisted Grinding of Cassiterite Associated with Pollimetallic Ore: A Comparison between Microwave and Conventional Furnaces

We investigated the influence of microwave and conventional heating pretreatments on the grinding of cassiterite associated with pollimetallic ore. The minerals that exhibited a stronger microwave absorption ability crushed first, which is the main difference between the microwave and the traditional heating pretreatments. The distribution of Fe, Pb, Zn, and Sn increased in the fine size range (−0.425 mm). The Fe and Pb grades in the size ranges of −3.2 + 2 mm and −2 + 1 mm after the microwave pretreatment (6 kW, 1 min) were lower than those of the traditional heating (12 kW, 400 °C, 20 min), indicating that the microwave selective heating was beneficial for pyrite and jamesonite. The grade and distribution of Sn decreased significantly in the size ranges of −3.2 + 2 mm and −2 + 1 mm and increased in the size ranges of −0.425 + 0.15 mm and −0.15 + 0.074 mm. Microwave heating treatment promoted the grinding of sulfide ore and reduced the cassiterite overgrinding.

Search for the Microwave Nonthermal Effect in Microwave Chemistry: Synthesis of the Heptyl Butanoate Ester with Microwave Selective Heating of a Sulfonated Activated Carbon Catalyst

The heptyl butanoate ester was synthesized from butanoic acid and heptanol in a heterogeneous medium in the presence of sulfonated activated carbon (AC-SO3H) catalyst particles subjected to microwave irradiation, which led to higher conversion yields (greater product yields) than conventional heating with an oil bath. The advantage of the microwaves appeared only when the moisture content in the butanoic acid batch(es) was high, suggesting that, unlike conventional heating, the reverse reaction caused by the moisture content and/or by the byproduct water was suppressed by the microwaves. This contrasted with the results that were found when carrying out the reaction in a homogeneous medium in the presence of the 2,4,6-trimethylpyridinium-p-toluene sulfonate (TMP-PTS) catalyst, as product yields were not improved by microwave heating relative to conventional heating. The removal of moisture/water content in the reaction solution was more pronounced when the reactor was cooled, as the reaction yields were enhanced via selective heating of the heterogeneous catalyst. A coupled electromagnetic field/heat transfer analysis gave credence to the selective heating of the AC-SO3H catalyst, which was further enhanced by cooling the reactor. It was deduced that unforeseen impurities and local high-temperature fields generated on the surface of small fine catalyst particles may have had an effect on the microwave chemistry such that the associated phenomena could be mistaken as originating from a nonthermal effect of the microwaves. Accordingly, it is highly recommended that impurities and selective heating be taken into consideration when examining and concluding the occurrence of a microwave nonthermal effect.

Nonradiative subdiffraction near-field patterns using metagratings

We present a synthesis scheme to mold periodic nonradiative field patterns in transmission using the recent concept of metagratings (MGs). To this end, we utilize our previously developed analytical model to analyze the interaction of an incoming plane wave with these sparse periodic arrangements of polarizable particles (meta-atoms). As the model reliably predicts coupling to all scattered Floquet–Bloch modes, both propagating and evanescent, desired reactive near-field profiles with deep subwavelength features can be generated. This approach forms an appealing alternative to previously proposed near-field plates based on metasurfaces, where abstract homogenization introduces uncertainties regarding utilization of highly evanescent spectrum, and meta-atom realization incurs full-wave optimization. In contrast, the outlined MG-based methodology, verified via full-wave simulations, directly yields fabrication-ready printed-circuit-board configurations, enabling versatile control of reactive near fields with no interfering radiative components, with potential uses in sensing, selective microwave heating, and wireless power transfer.

Comparison of microwave and conventional heating for CO2 desorption from zeolite 13X

This study investigates microwave irradiation as an alternative to conventional heating for temperature swing adsorption processes. The performance of microwave and conventional heating during sorbent regeneration was evaluated by measuring CO2 desorption from zeolite 13X at different temperatures. Experimentally, a fixed bed of zeolite 13X was saturated by a 150 sccm flow of 15 % CO2 at room temperature followed by sorbent regeneration under nitrogen at 55, 100, or 150 °C by applying either microwave irradiation or conventional heating. Microwaves reduced sorbent regeneration times by at least half compared to regeneration by conventional heating. Under conventional regeneration, desorption curves were resolved into two peaks representing physisorbed CO2 (mass diffusion limited) at low temperature and bicoordinated CO2 (thermally limited) with increasing temperature. Under microwave regeneration, only one desorption peak was observed suggesting that CO2 desorption was limited by mass diffusion through the porous zeolite 13X structure, rather than by temperature. Depending on microwave power, apparent activation energy of the microwave-assisted regeneration was 15.8–18.1 kJ/mol, compared to 41.5 kJ/mol for conventional regeneration. The reduction in apparent activation energy is mainly attributed to selective microwave heating of CO2 adsorption sites (Na+ sites) resulting in greater steady state temperatures of Na+ relative to framework atoms, suggesting greater heating efficiency due to microwaves compared to conventional heat transfer. Due to rapid cycling and efficient heat transfer to CO2 sites on zeolite 13X, microwave regeneration is found to increase adsorption/desorption cycling productivity and potentially reduce the energy penalty of temperature swing capture.