Variable Frequency Microwave Research Articles

Resonance-driven microwave heating for improved methane conversion to hydrogen

Efficient hydrogen production through methane pyrolysis is hindered by high temperature required and frequent catalyst deactivation due to carbon deposition. This study proposes a novel use of variable-frequency microwave heating to enhance the efficiency and stability of methane pyrolysis by leveraging the resonance effects between microwave frequencies and dielectric properties of activated charcoal (AC). A solid-state variable-frequency microwave reactor was employed to modulate microwave frequencies to match the dielectric properties of AC, with a peak tanδ of 0.19 at 4600 MHz. Experimental results showed that at a resonance frequency of 4650 MHz and a microwave power of 100 W, the reaction temperature of AC reached 960 °C within 2 min, achieving a 100% methane conversion rate. However, carbon deposition reduced the conversion rate to 35.1% after 150 min. By cyclically adjusting the microwave frequencies between 4650 MHz and 4640 MHz, the catalyst was successfully reactivated, restoring the conversion rate to 100%. This frequency modulation method enabled continuous methane pyrolysis for 1300 min, highlighting the potential of variable-frequency microwave heating as a novel and efficient approach for a sustainable production of green hydrogen.

Beyond microwave susceptors: Exploring 5525 MHz frequency for efficient biomass pyrolysis

The transition from fossil fuels to renewable energy sources necessitates innovative approaches to biomass utilization. Despite the recognized benefits of microwave-assisted biomass pyrolysis, the reliance on costly microwave susceptors poses challenges. This study centers on the innovative utilization of a variable-frequency microwave approach, particularly at 5525 MHz, eliminating the need for microwave susceptors. Remarkably, at this frequency, corn stalk pyrolysis achieved a temperature of 589 °C with a rapid heating rate of 327.7 °C/min, showcasing a 69.5% conversion rate without susceptors. Furthermore, the study investigated the interplay between microwave frequency and alkali metal content, revealing a direct correlation between K+ and biomass conversion rates. Our findings highlight the transformative potential of 5525 MHz in biomass pyrolysis, offering a cost-effective and environmentally friendly alternative to microwave susceptors, with implications for sustainable energy and industrial applications.

Tailoring microwave frequencies for high-efficiency hydrogen production from biomass

Generating green hydrogen through biomass thermochemical conversion is pivotal for contributing to carbon neutrality. However, the efficiency of hydrogen production is hindered by the byproduct of tar formed during biomass depolymerization. This study employed a variable-frequency microwave ranging from 2430 to 6000 MHz to investigate the influence of microwave frequency on tar removal and hydrogen recovery over activated charcoal (AC). Experimental results indicated that microwave frequencies significantly impact the temperature distribution and heating rates of AC. Notably, at 4640 MHz the AC rapidly reached 1145 °C within seconds, with an instantaneous heating rate of 2022 °C/min. The ultra-fast heating rate crucially contributes to achieving 100 % efficiency in tar cracking, resulting in a hydrogen yield of 32.8 mmol/g. With a hydrogen recovery rate surpassing 94 %, the 4640 MHz frequency represents a 15.4-fold increase compared to the conventional 2450 MHz. This innovative approach holds significant promise for enhancing energy efficiency in the pursuit of carbon-neutral energy production.

Variable frequency microwave induced CO2 Boudouard reaction over biochar

The Boudouard reaction presents promising application prospects as a straightforward and efficient method for CO2 conversion. However, its advancement is hindered primarily by elevated activation energy and a diminished conversion rate. This study employed a microwave reactor with a variable frequency as the initial approach to catalyze the CO2 Boudouard reaction over biochar, with the primary objective of producing renewable CO. The study systematically investigated the influence of various variables, including the heating source, microwave frequency, microwave power, gas hourly space velocity (GHSV), and carrier gas, on the conversion of CO2 and the selectivity towards CO. The experimental findings indicate that under static conditions, with a fixed microwave frequency set at 2450 MHz and 100 W microwave power, the Boudouard reaction did not initiate. Conversely, a CO2 conversion rate of 8.8% was achieved when utilizing a microwave frequency of 4225 MHz. Under this unique frequency, further elevating the microwave power to 275 W leads to the complete conversion of CO2. Furthermore, a comparative analysis between microwave and electrical heating revealed that the CO production rate was 37.7 μmol kJ−1 for microwave heating, in stark contrast to the considerably lower rate of 0.2 μmol kJ−1 observed for electric heating. Following the reaction, the biochar retained its robust 3D skeleton structure and abundant pore configuration. Notably, the dielectric constant increased by a factor of 1.8 compared to its initial state, rendering it a promising microwave-absorbing material.Graphical

Three-dimensional observations of the electric field distribution of variable frequency microwaves, and scaling-up organic syntheses

Variable Frequency Microwave (VFM) radiation provides a solution to the inhomogeneity of the electric field in the cavity, which has long led to a decline in the reliability of microwave chemical data and its industrial utilization. Herein, we report in-situ three-dimensional experimental measurements of the electric field’s uniform distribution of VFMs within a multimode cavity under high power conditions, and their subsequent comparison to Fixed Frequency Microwaves (FFM) that could only be assessed earlier through theoretical analysis. We also examine the consequences of changes in VFM irradiation conditions and elucidate the threshold at which VFM irradiation might prove beneficial in syntheses. With an ultimate focus on the use of VFM microwave radiation toward industrial applications, we carried out an effective synthesis of 4-methylbyphenyl (4-MBP) in the presence of palladium (the catalyst) supported on activated carbon particulates (Pd/AC), and revisited two principal objectives: (a) the effective suppression of discharge phenomena (formation of hot spots), and (b) synthesis scale-up using a 5-fold increase in sample quantity and a 7.5-fold larger reactor size (diameter) than otherwise used in earlier studies.

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.

Design of variable frequency dual transponder system for fixed point ranging

AbstractAiming at the problems of high system cost, complex ranging process, low ranging accuracy, and limited measurement distance in commonly used methods for ground fixed point distance measurement journals, a variable frequency microwave ranging system based on a dual transponder architecture is proposed in this letter. The system uses carrier dual transponder as an active forwarding architecture, utilizing the digital phase‐locked loop to obtain the high‐precision phase difference between two signals, and resolving the ambiguity through microwave frequency sweep within a certain range. The simulation results show that the system can quickly obtain the unambiguous distance between two fixed points, besides, driven by a high‐stability crystal oscillator, submillimetre accuracy can be achieved in km‐level ranging.

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.

Cover Image, Volume 139, Issue 45

The cover image by Prof. Satoshi Horikoshi shows how incorporating a microwave absorption heating element (MAHE) powder into an epoxy adhesive significantly shortened the microwave curing time upon using fixed frequency microwaves (FFM); however, the extent of hardening of the adhesive was highly heterogeneous. To resolve this problem, usage of variable frequency microwaves (VFM) enabled uniform curing of the adhesive, while maintaining the short curing times at 12 s. DOI: 10.1002/app.53010

Curing of an epoxy adhesive with fixed‐frequency microwaves in the presence of a microwave absorber (activated carbon) and with the variable frequency microwave method

AbstractThis study examines the curing characteristics of an epoxy resin adhesive mixed with activated carbon (AC) particulates as the microwave absorption heating element (MAHE) using a non‐fluctuating microwave irradiation device consisting of a semiconductor microwave generator and a single‐mode applicator. The advantages of the microwave curing method are the more rapid heating of the epoxy resin adhesive and the associated reduction in curing time. However, problems arose unique to microwave heating as the AC particulates formed aggregates as a result of the microwaves' electric field (E‐field), which also led to a temperature distribution greater than 100°C generated in the epoxy resin adhesive. This problem could not be resolved by commercial microwave chemical equipment. Resolution of the problem was achieved using a variable frequency microwave (VFM) irradiation method using a semiconductor generator; high‐speed electrical diffusion of the E‐field was performed to irradiate with an average electric field without fluctuations. This achieved a uniform curing of the adhesive containing the MAHE particulates using the VFM method, which also proved effective in preventing adhesive ignition accidents. E‐field was performed to irradiate with an average electric field without fluctuations. This achieved a uniform curing of the adhesive containing the MAHE particulates using the VFM method, which also proved effective in preventing adhesive ignition accidents.

Elucidation of the principle of microwave rubber vulcanization based on dielectric parameters and vulcanization of tire rubber by variable frequency microwave

AbstractThe microwave vulcanization of tire rubber was investigated by monitoring the dielectric properties of the rubber polymer and other components of the formulation (including carbon filler, antioxidant, and vulcanization agents) as functions of frequency and temperature. The effect of the vulcanization reaction on the dielectric properties during heating was also assessed. The physical interaction between the crosslinked structure and the carbon black significantly favors the responsiveness to microwaves during the vulcanization reaction. Based on these basic data, microwave irradiation conditions were determined, and model tire samples were microwave vulcanized in a PTFE mold. It has achieved a shorter time of heating and 65% energy saving, but on the other hand, it has become clear that the vulcanization produces very poor physical quality compared to vulcanization by electric furnace heating. To solve this problem, we performed microwave irradiation using variable frequency microwave (VFM; 5.85 ~ 6.65 GHz) using glass fiber on polyetherketoneketone (PEEK/GF) mold. In VFM, compared to conventional fixed frequency microwave (FFM), successfully synthesized high‐quality tire rubber because it showed 4.2, 1.1, 2.0, 1.2, and 1.8 times higher values in terms of cross‐linking density value, hardness value, tensile strength, elongation at break, and 200% modulus, respectively.

Thomson and collisional regimes of in-phase coherent microwave scattering off gaseous microplasmas

The total number of electrons in a classical microplasma can be non-intrusively measured through elastic in-phase coherent microwave scattering (CMS). Here, we establish a theoretical basis for the CMS diagnostic technique with an emphasis on Thomson and collisional scattering in short, thin unmagnetized plasma media. Experimental validation of the diagnostic is subsequently performed via linearly polarized, variable frequency (10.5–12 GHz) microwave scattering off laser induced 1–760 Torr air-based microplasmas (287.5 nm O2 resonant photoionization by ~ 5 ns, < 3 mJ pulses) with diverse ionization and collisional features. Namely, conducted studies include a verification of short-dipole-like radiation behavior, plasma volume imaging via ICCD photography, and measurements of relative phases, total scattering cross-sections, and total number of electrons N_{e} in the generated plasma filaments following absolute calibration using a dielectric scattering sample. Findings of the paper suggest an ideality of CMS in the Thomson “free-electron” regime—where a detailed knowledge of plasma and collisional properties (which are often difficult to accurately characterize due to the potential influence of inhomogeneities, local temperatures and densities, present species, and so on) is unnecessary to extract N_{e} from the scattered signal. The Thomson scattering regime of microwaves is further experimentally verified via measurements of the relative phase between the incident electric field and electron displacement.

Variable Frequency Microwaves For Warming of Vitrified Organs

Variable Frequency Microwaves For Warming of Vitrified Organs

Low-Temperature Processing of Electronic Materials Using Uniform Microwave Fields

This article presents the recent literature on the unique and surprising capability of microwave energy to effect chemical reactions at lower temperatures. These lower processing temperatures provide improved thermomechanical properties of organic and inorganic material combinations used in microelectronic assemblies. Variable-frequency microwave (VFM) technology eliminates arcing and produces the large and highly uniform fields necessary for use in high-volume manufacturing. Wafer coatings of polyimide thermoplastics can be fully polymerized 100-200°C lower with VFM than with thermal ovens, which lowers stress and warpage. VFM curing of thermoset adhesives for underfill, bonding, and package assembly can be done well below the usually inviolate “ultimate” glass transition temperature without vitrification and in less time. The unique kinetics and thermodynamics of microwave energy produce high chemical reactivity even after the high-viscosity thermoset gelation point. VFM-cured thermosets are highly uniform with less stress and warpage even on large substrates and panels. Field uniformity and penetration depth produce better adhesion and higher fracture strength. The lower process temperatures inherently produce more stable and reliable final products and allow for less costly low-temperature build materials and temperature-sensitive device designs. The anneal and activation process temperatures of implanted ions in silicon can also be reduced by half with VFM, which results in nearly zero dopant diffusion and high-quality SPE regrowth and low numbers of defect sites.

Application of Variable Frequency Microwaves in Microwave-Assisted Chemistry: Relevance and Suppression of Arc Discharges on Conductive Catalysts

The application and advantages of variable frequency microwaves (VFM; range, 5.85–6.65 GHz) are reported for the first time in microwave chemistry, particularly when carrying out reactions catalyzed by metallic conductive catalysts so as to avoid the formation of arc discharges, and especially when using a strong microwave absorber such as activated carbon (AC) particulates as supports of metal-based catalysts. Two model reactions performed in low boiling point nonpolar solvents are described wherein arc discharges easily occur under the more conventional fixed frequency microwave (FFM) approach: (i) the synthesis of 4-methylbiphenyl (4MBP) by the Suzuki-Miyaura cross-coupling process catalyzed by Pd particles supported on AC particulates (Pd/AC), and (ii) the synthesis of toluene via the dehydrogenation of methylcyclohexane (MCH) catalyzed by Pt particles dispersed on AC particulates (Pt/AC). Contrary to the usage of fixed frequency microwaves (5.85 GHz and 6.65 GHz), the use of VFM microwaves increased the chemical yields of 4MBP {49% versus 5–8% after 60 min} and toluene {89% versus 24% after 10 min} by suppressing the formation of discharges that otherwise occur on the catalyst/AC surface with FFM microwaves. Consequently, relative to the latter approach, the VFM technology is significantly advantageous, especially in reactions with solid conductive catalysts, not least of which are the reduction in power consumption, thus energy savings, and the prevention of potential mishaps.

Microwave-driven heterogeneous catalysis for activation of dinitrogen to ammonia under atmospheric pressure

This paper presents an innovative approach to producing energy-dense, carbon–neutral liquid ammonia as a means for carrying energy. This approach synergistically integrates microwave reaction chemistry with novel heterogeneous catalysis that decouples dinitrogen activation from high-temperature and high-pressure reactions, altering reaction pathways and increasing ammonia formation rate. Results presented here demonstrate that ammonia synthesis can be conducted at 280 ℃ and ambient pressure to achieve ~1 mmol ammonia/g cat/h over supported ruthenium catalyst systems utilizing microwave irradiation. It is further shown that adding promoter ions such as potassium, cerium, and barium significantly improves the ammonia production rate over undoped ruthenium-based catalysts. This effect could be attributed to enhanced dielectric loss processes that lead to stronger microwave absorption by the catalyst. Measurement of the equilibrium constant under microwave conditions showed a higher ammonia yield than under thermal equilibrium conditions for both the iron- and ruthenium-based catalysts. Finally, this study also illustrates the advantages of using a variable-frequency microwave reactor for ambient-pressure ammonia synthesis. Mechanistically, investigators believed that the oscillating electric fields of the radiation can couple with adsorbed nitrogen on the surface and accelerate its dissociation. Since dinitrogen dissociation on the surface is rate limiting, this effectively accelerates the reaction. Overall, the study provides an in-depth analysis of the parameters affecting the use of microwaves in catalyzed ammonia synthesis. This process is fundamentally different from the commercial Haber–Bosch process and provides an alternative method of ammonia synthesis for certain applications, as it is tolerant to intermittent supplies of renewable energy, therefore effectively operating at variable rates of production.

Simple, controllable fabrication and electromagnetic wave absorption properties of hollow Ni nanosphere

Electromagnetic wave (EW) absorption material with light weighted, low addition, high efficiency and wide variable frequency band is a highly concerned topic which needs to be explored. In this study, hollow Ni nanospheres with fascinating EW absorption properties and light-weighted superiority were successfully synthesized using a simple and rapid strategy with controllable structures and compositions. The shell thickness and average diameter of hollow Ni nanosphere were about 360 nm and 30 nm respectively. Moreover, the EW absorption of hollow Ni nanosphere was investigated. The reflection loss of hollow Ni nanospheres was up to − 43.6 dB, and the bandwidth achieved 3.6 GHz at 11 GHz with the thickness of only 2.1 mm. Furthermore, the wide variable frequency microwave absorption was successfully realized by the controllable structure and compositions, and the absorption peak shifted from 2.78 to 12.59 GHz and 14.1–17.66 GHz, covering 83% of the measured frequency range. In summary, it can be demonstrated that hollow Ni sphere is an excellent choice in the field of EW absorbing materials.

Time-resolved study of variable frequency microwave processing of silver nanoparticles printed onto plastic substrates

This paper demonstrates variable frequency microwave processing of silver nanoparticles inkjet-printed onto polyethylene terephthalate (PET) substrate. Printed patterns were in situ monitored with the use of a camera and specially developed two-color infrared pyrometer. It was demonstrated that direct microwave heating of metal inks is very challenging to control due to the progressive evaporation of polar solvents, and almost always leads to non-uniform heating and significant sample deformation. To overcome this difficulty, a PET-based substrate with a solvent-absorbing layer was used. Samples were analyzed at various heating periods and microwave power levels. The collected results reveal a sharp increase in the temperature at the very first moment of the microwave irradiation caused by concomitant interaction with electric and magnetic fields. Depending on the processing conditions, the sintered samples achieved relatively low electrical resistances at the very beginning of the irradiation periods. The lowest measured resistivity was about 30 cm after only 5 s of irradiation. Microstructural analysis of the film revealed inter-particle neck formation and no significant grain growth. The observed reduction in the electrical resistivity upon microwave irradiation was mainly ascribed to the neck formation and its progressive growth.

Improved Performance and Reduced Cost through Advanced Electrode Drying Technology

The commercial success of lithium ion batteries hinges on significant improvements in both cost and energy density. The electrode coating operation presents the highest leverage opportunity to address both challenges. Navitas ASG with Lambda Technologies has recently demonstrated and scaled up a patent-pending advanced drying process (ADP) technology that breaks through the limitations on electrode thickness and drying rate that are inherent in the present coating technology. This technology can improve electrode loading by at least 50% and accelerate coating rates by >2X. The technology is drop-in compatible with capital equipment in operation at battery OEM plants. The technology will enable cathodes that can match emerging high capacity silicon alloy anode materials. The expected impact of this technology will be >20% improvement in the cell level Wh/kg and 8% reduction in the system level EV battery cost. Microwave drying introduces key advantages over radiant and convective cooling for electrode drying (figure 1). Microwaves efficiently transfer energy through selective absorption by polar solvent (e.g. NMP) or water molecules. Microwave energy is also uniformly deposited through the bulk of the coated electrode. Electrode drying, however, introduces two barriers to use of microwave energy: avoiding arcing with the presence of metal foil current collectors, and supporting roll-to-roll coating without leakage of the microwave field through the coating web entry and exit ports. Navitas worked with a microwave processing company Lambda Technologies to resolve these barriers through the use of variable frequency microwaves (VFM). VFM-dried electrodes were validated by Navitas through accelerated life testing and failure analysis of prototype cells. Navitas worked closely with Lambda to design a VFM module able to support continuous web drying operation – a key challenge. Navitas installed a VFM module onto our Toyo WI pilot coating line and performed pilot scale electrode coating validation for several established commercial anode and cathode formulations to demonstrate accelerated drying with the absence of any deleterious effects on the electrode. The work by Navitas also involved the production of state of the art commercial baseline (i.e. non-VFM dried) benchmark electrodes. The VFM process showed significant advantages, with drying acceleration of >2X for NMP and >5X for water borne slurries. Thorough evaluation of adhesion, cohesion, conductivity, binder distribution etc. has shown the VFM electrode physical properties are as good as or better than the baseline (identical electrode coating passed through coater at lower speed with VFM module toggled off). The Navitas pilot coating trials validated potential cost savings and provided data for presentation to prospective commercial partners to establish confidence that existing coating lines can be retrofitted with VFM. In the course of the validation runs the Navitas team observed that VFM electrodes showed significantly less binder segregation than conventionally dried electrodes. This is attributed to the fact that VFM deposits the drying energy more uniformly in the bulk of the electrode. Uniform energy distribution enables transport of the solvent to the surface as vapor phase rather than as a liquid that drags dissolved or suspended binder to the surface where radiant/convective drying energy transfer predominates. Our observation of reduced binder segregation led to the realization that the critical-to-quality features that limit the ability to coat cathodes at loadings >5mAh/cm2 in high volume production are greatly improved in the VFM process. These limitations include poor adhesion to the current collector (binder deprivation), cracking due to residual stress (binder deprivation), and defects or performance limitations associated with excess binder at surface. Data will be presented comparing VFM electrodes to commercial state of the art electrodes for electrode loading ranging from 3 to 8 mAh/cm2 Figure 1

Comparison of Conventional and Variable Frequency Microwave Curing of SU8 Photoresist: Effects on the Dielectric, Thermal, and Morphological Properties

Variable Frequency Microwave (VFM) is known to be a rapid, volumetric, and selective form of heating which has shown potential as an alternative technique for the processing of negative-tone SU8 photoresist in the Micro-Electro-Mechanical System (MEMS) industry. A comparison of thermal properties of the films cured during the softbake and post exposure bake process using different techniques, i.e. conventional thermal curing, VFM curing, and a combination of both (referred to as hybrid, HY), was investigated using a Differential Scanning Calorimeter (DSC) and Thermogravimetric Analysis (TGA). A significant increase on the degree of cure (between 13-23%) was observed using the VFM over the hybrid and hotplate curing which means that SU8 curing at lower temperatures or rapid curing is possible. The increase in cure rates can be attributed to a combination of heat transfer and the unique capability of microwave to couple with the sample (selective heating). The improvement in curing at the same processing temperatures has important implications for processing thick films. It was found that regardless of curing methods, crosslink densities increased as the baking temperature increased, resulting in lower dielectric properties. Despite higher crosslinking contents, VFM cured samples decomposed at 2-4°C lower temperatures. In addition to better thermal properties, VFM offered satisfactory microstructure at lower curing temperatures; however, high processing temperatures could result in film cracking.