This study examines microwave heating of cold-pressed aluminum nanoparticle compacts as a function of relative density relative to the theoretical maximum density (TMD). Spherical aluminum nanoparticles (18 nm average diameter) were compressed into cylindrical compacts at 20%, 30%, 40%, and 50% TMD and exposed to 2.45 GHz microwave radiation for five minutes under identical field conditions. Two-dimensional transient surface temperatures were measured in situ using infrared thermography, enabling spatially resolved thermal analysis during exposure. Increasing relative density from 20% to 50% TMD produced higher heating rates and elevated steady-state temperatures, indicating enhanced microwave–thermal energy conversion with reduced interparticle spacing. To interpret these trends, electromagnetic simulations were performed using ANSYS HFSS, modeling periodic arrays of core–shell aluminum nanoparticles with varying interparticle air-layer thickness. The simulations demonstrated increased power dissipation with decreasing air spacing, consistent with experimental observations. These trends were independently corroborated using a Bruggeman effective-medium model, which showed that the dielectric loss factor of the compacts increases monotonically with relative density, while normal-incidence surface reflectance also rises, representing a competing loss mechanism that is expected to dominate at higher densities. The combined experimental and numerical results establish relative density as a governing parameter in microwave heating of porous aluminum nanoparticle compacts and identify an optimal intermediate density range for efficient microwave coupling in metallic nanoparticle systems.

Membrane distillation (MD) has attracted significant research interest for desalinating hypersaline brine. However, the lack of robust hydrophobic membranes and lower energy efficiency requirements restrict its true potential. Designing and fabricating a hydrophobic membrane that enables surface heating at the mass transfer interface provides a potential route for efficient desalination with MD. This study aims to study a new class of surface-heated membranes that can be triggered by radiofrequency (RF) electromagnetic waves. We developed hydrophobic membranes that were prepared by CO2 laser ablation of a polyethersulfone (PES) membrane substrate. The proposed single-step laser modification converts the PES membrane surface to laser-induced graphene (LIG), which is hydrophobic and electroconductive, making it suitable for surface heating. The hydrophobic nature of the prepared PES-LIG membrane is confirmed from the surface water contact angle (143.7 degrees), and the surface heating potential is studied by investigating the thermal response of the membrane exposed to RF fields. The membrane surface average temperature can reach up to similar to 140 degrees C with optimized RF frequency and power. The PES-LIG membrane's mechanical and thermal properties are characterized to investigate its feasibility for MD application. In this work, vacuum MD (VMD) is studied by integrating with RF heating and a permeate flux of up to 13.5 L m(-2) h(-1) with >99% salt rejection is reported. Cyclic thermal and mechanical stability tests and long-term VMD tests show the stable performance of the PES-LIG membranes. This work demonstrates a novel MD strategy that can potentially address challenges impeding its commercialization.

A new radar architecture that features a compact size and only one radio frequency (RF) active device was designed, fabricated, and tested for small motion detection and human range tracking. Unlike conventional board-level radar systems, it contains a single antenna and does not use any low noise amplifier (LNA) or circulator in the RF front end. The I/Q amplitude and phase imbalance present on the system were measured to be ${< }1{\%}$ and 5.17°, respectively. Besides the local oscillator (LO), there is no other active device consuming dc power. The fabricated radar system has dimensions of $36\times72$ mm ( ${L} \times {W}$ ) and a power consumption of 290 mW in continuous operation. Moreover, a 24-GHz single-channel radar system was designed, fabricated, and tested using the proposed architecture. The dimensions of fabricated board are $24\times27$ mm ( ${L} \times {W}$ ) and total power consumption of 800 mW dissipated by the LO. The range tracking capabilities of the implemented radar systems were demonstrated by successfully tracking the range of a human target walking back and forth on a hallway. Moreover, the high sensitivity of the proposed quadrature 2-port monostatic Doppler radar was demonstrated by effectively measuring a sinusoidal movement with an amplitude of $2.5 \mu \text{m}$ with a 5.8-GHz carrier.

Graphene is an excellent choice for heating applications due to its high thermal conductivity and is considered an interesting candidate for application in flexible heaters. The major challenge, though, is the costly and chemical-intensive pathways to produce graphene on a large scale. Laser ablation of polymeric substrates is a relatively recent technique for a facile, single-step, chemical-free fabrication of graphene, referred to as laser-induced graphene (LIG). This work demonstrates the fabrication of patterned LIG-based flexible heaters and their response to radio frequency (RF) electromagnetic waves. Polymeric substrates were scribed with laser patterns in both raster and vector modes and subjected to RF electromagnetic fields to test their heating response. We confirmed different graphene morphologies of the lased patterns through various materials characterization methods. The maximum steady-state temperature observed for the LIG heater was approximately 500 °C. Unprecedented heating rates, as high as 502 °C/s, were observed when LIG heaters were exposed to RF fields at 200 MHz frequency and 4.6 W power. Mechanical and thermal stability tests for the best heater were also performed showing a stable thermal response for 1000 bending cycles and 20 cycles of the heating test for 8.5 h, respectively. Our work suggests that LIG heaters produced in vector mode lasing outperformed those lased in raster mode which can be attributed to the improved graphene quality for RF absorbance.

The ongoing COVID-19 pandemic has increased the use of single-use medical fabrics such as surgical masks, respirators, and other personal protective equipment (PPE), which have faced worldwide supply chain shortages. Reusable PPE is desirable in light of such shortages; however, the use of reusable PPE is largely restricted by the difficulty of rapid sterilization. In this work, we demonstrate successful bacterial and viral inactivation through remote and rapid radio frequency (RF) heating of conductive textiles. The RF heating behavior of conductive polymer-coated fabrics was measured for several different fabrics and coating compositions. Next, to determine the robustness and repeatability of this heating response, we investigated the textile's RF heating response after multiple detergent washes. Finally, we show a rapid reduction of bacteria and virus by RF heating our conductive fabric. 99.9% of methicillin-resistant Staphylococcus aureus (MRSA) was removed from our conductive fabrics after only 10 min of RF heating; human cytomegalovirus (HCMV) was completely sterilized after 5 min of RF heating. These results demonstrate that RF heating conductive polymer-coated fabrics offer new opportunities for applications of conductive textiles in the medical and/or electronic fields.

Mitigation of multipactor in waveguides is of importance, and strategies have included the addition of external fields, materials engineering, or surface modifications. Here, geometry modifications of rectangular waveguide surfaces and the application of an axial magnetic field are investigated for suppressing multipactor growth. A Monte Carlo approach has been used to simulate electron dynamics. The empirical secondary electrons yield is modeled based on a modified Vaughan approach. The electric fields driving electron transport were derived from separate electromagnetic calculations to adequately include field perturbations due to the presence of surface patterns in the rectangular waveguide structure. Combinations of grooves and a DC magnetic field are shown to effectively mitigate multipactor growth at field strengths up to ∼105 V/m. Finding optimal combinations for an arbitrary field and operating frequency requires further work.

The conversion of polyacrylonitrile (PAN) to carbonaceous structures is a complex but common processing step in the aerospace industry. Here, we show that this thermal stabilization process can occur based on the volumetric heating of PAN using radio frequency (RF) fields, in the 1-200 MHz range. Unlike many other polymers, neat PAN films show a surprisingly rapid RF heating response (5 degrees C/s at 103 MHz and 30 W power), without the need for any RF susceptor fillers. The RF response drops as the polymer oxidizes during heating and as the cyanide bond disappears to form a cyclic bond. Strangely, PAN nanofiber mats did not respond to RF fields. The measured AC conductivity and dielectric constant of fiber mats are lower (2.4 X 10(-4) S/m and 3.66, respectively, at 103 MHz) compared to 1.4 X 10(-2) S/m and 18.8 for PAN films. The dielectric properties are correlated with the bulk heating responses and stem from differences in morphology.

This article presents a new method for dielectric characterization as a function of temperature and frequency for nanomaterial-loaded polymers, while they are heated with radio frequency (RF) energy. Specifically, the dielectric constant and electrical conductivity of polymers with conductive nanomaterials are measured in the RF frequency range (up to a few hundreds of MHz), while the material is undergoing a curing reaction using a high-power RF energy source. The nanomaterials act as susceptors for RF energy to facilitate heating to the desired temperatures for processing and curing polymers. Measurement techniques were developed to enable simultaneous in situ RF heating and characterization of the polymer. Since some polymers used in this work are liquids at room temperature, such samples are placed in an alumina crucible located at the end of a microstrip-type applicator. Temperature is monitored using an FLIR thermal camera. Multiphysics simulations are used to help develop the appropriate applicator for RF heating and dielectric measurements. An appropriate calibration procedure along with a circuit model to enable extraction of the dielectric constant and electrical conductivity from the measured reflection coefficient is introduced.

The massive deployment of wireless sensors is a fundamental piece in the growing internet of things (IoT) industry. Therefore, it is imperative to use already existing hardware to realize new sensing functions with very few or no hardware added. As wireless power transfer (WPT) and near field communication (NFC) become standard features in smart phones, this article investigates beverage freshness sensing based on the WPT/NFC technology compatible with smart phones. A circuit model for the beverage-coil interaction was developed and the performance of features from different nature (e.g., magnitude, amplitude, phase) for classification was analyzed and tested. Accuracies up to 96.7% were achieved using supervised machine learning for milk freshness classification, when 5 different types of milk were used and up to 100% when just 2% fat milk was used for classification. Additionally, the radio frequency bandwidth needed for classification was reduced to 10 MHz using singular value decomposition (SVD) and boxplot analysis without affecting the classification accuracy for two different methods of feature extraction.

Here we report radio frequency (RF) heating patterns that may be generalized across a wide range of nanomaterial-loaded materials. We used experiments and simulation to show that the heating rates are non-monotonically related with the conductivity of the materials. A major finding is that the maximum heating rate occurs at an optimum DC surface conductivity that is the same for thin films made using carbon nanotubes, carbon nanofibers, and laser-induced graphene. We also determine that this maximum heating is closely associated with the percolation threshold in a given structure. We show similar patterns for nano-filled thick thermoplastic parts as well. These findings can be used to optimize RF heating by tuning the bulk dielectric properties of the nanomaterial structures. Optimization of RF heating would lead to enhanced efficiency in RF-based material processing techniques being developed for automotive, aerospace, and additive manufacturing industries. (c) 2021 Elsevier Ltd. All rights reserved.

A complete system of wireless power transfer using a circularly polarized retrodirective array is presented. A dual frequency, active retrodirective array is proposed for a transmitter system. The antenna array uses circularly polarized microstrip patch antenna subarrays with sequential rotation and surface wave suppression. The designed antenna element eliminates undesired coupling between array elements due to surface waves present in conventional microstrip antenna arrays in order to improve array performance. A sequential rotation technique was implemented to improve impedance matching and circular polarization bandwidths. The proposed retrodirective array was designed to operate at about 2.4 GHz for the interrogating signal and about 5.8 GHz for the retransmitted signal. The beam scanning inherent in retrodirective arrays ensures a uniform power level available to the receiving devices, regardless of their location within the angular sector over which retrodirectivity is achieved. A rectenna was designed as a receiver in order to have a complete system the wireless power transfer. A zero bias Schottky diode with high detection sensitivity was used as the rectifying device. The shorting pins used in the antennas to suppress surface waves also act as return paths for the DC current, eliminating the need for an RF chock in the rectifier circuit. The design procedure, simulation results, and experimental measurements are presented.

Here, a novel integration is proposed for radio frequency (1-300 MHz) responsive nanomaterials with conventional catalytic materials to realize a new class of heterogeneous catalysts that undergo uniform volumetric and localized heating to drive chemical transformations at the modular scale. Approximately 80% of chemical manufacturing involves heterogeneous catalytic reactions, which currently require heating via steam utilities or fired furnaces, and thus contribute to global greenhouse gas emissions while also limiting distributed chemicals production. This approach uses an electric route to produce chemicals where radio frequency (RF) electromagnetic fields and their interaction with carbon/ceramic nanomaterials are utilized to selectively heat the catalyst composition. A proof-of-concept is demonstrated using the commonly studied methanol steam reforming reaction on a platinum catalyst. In this study, two RF susceptors are used: carbon nanotubes and silicon carbide fibers. The conversion rate of methanol using RF heating is comparable to oven heating at varying temperature and catalyst combinations. This is a potential improvement over conventional catalytic reactors in that it enables small, safe, sustainable, on-site, and on-demand production of chemicals in the absence of traditional manufacturing infrastructure.

In article number 2000095, Micah J. Green, Benjamin A. Wilhite and co-workers propose a new approach for direct use of sustainable electric energy (solar, wind, nuclear) to drive chemical reactions using modular, intensified, and distributed processes, i.e., in the absence of traditional infrastructure. The reported radio-frequency responsive novel materials are integrated with conventional catalysts to drive heterogeneous catalytic reactors. This method allows for uniform volumetric and localized heating at competitive efficiencies.

Here we report the effect of metallic (m-) and semiconducting (s-) properties of single-walled carbon nanotubes (SWCNTs) on the response of SWCNT films to radio frequency (RF) heating. We separated high-purity m- and s-SWCNTs from an initial SWCNTs mixture and prepared thin films using vacuum filtration method. The areal density of the films is 9.6 g cm(-2), and the DC conductivities are in the range of 7800-49000 S m(-1). We show rapid and non-contact Joule heating of films using a fringing-field RF applicator, and we observe maximum heating rates in the frequency range of 60-70 MHz. We determine that the more conductive m-SWCNT films reflect RF fields and heat at a maximum rate of 1.51 degrees C s(-1) compared to maximum heating rate of 25.6 degrees C s(-1) for s-SWCNT films. However, m-SWCNTs heat up faster than s-SWCNTs when dispersed in a dielectric medium. Our results confirm the non-monotonic relationship between RF heating rate and conductivity for CNT-based materials such that conductivity is required for heating but high values are correlated with reflections. Our findings also suggest that RF heating could be a possible metric for evaluating film purity because impurities in the films affect the conductivity and thus RF heating rate. We anticipate that RF heating may occur in SWCNT-based electronics and affect their performance.

Silicon carbide (SiC) fibers are widely used as a reinforcement in ceramic matrix composites due to their high mechanical strength and superior thermal resistance. Here, we investigate the rapid radio frequency (RF) heating response of two types of SiC fibers (Hi-Nicalon and Sylramic) in the 1-200 MHz frequency range. Hi-Nicalon fibers exhibit a surprisingly rapid RF heating response of 240 °C/s in the perpendicular orientation, and this property could be exploited for oven-free and noncontact processing of composites with SiC fibers. The presence of excess carbon on the surface of Hi-Nicalon fibers is most likely responsible for the RF heating response and significantly higher temperatures in the parallel as compared to perpendicular alignment of fibers to the electric field. The RF heating response of Hi-Nicalon SiC fibers was utilized to heat preceramic polymers (polycarbosilanes) infiltrated in SiC fibers and cure them to ceramic matrix composites (CMCs) using RF applicators. A noncontact RF heating setup to pyrolyze the precursor polymers under inert conditions and make SiC/SiC composites is also developed.

Here the authors demonstrate an oven‐free and mold‐free heating route to convert preceramic polymers to silicon carbide using carbon nanomaterials as susceptors. Silicon carbide is prized for its high thermal stability and low density and could be produced via slow oven heating of polycarbosilane (PCS). The authors show that addition of multiwalled carbon nanotubes (MWCNT) as susceptors to polycarbosilane results in rapid and volumetric heating upon exposure to microwaves and radio frequency. The authors assess microwave heating of polycarbosilane‐MWCNT composites; this process is capable of reaching pyrolysis temperatures, and the resulting crystal structure is cubic (β‐SiC). Dielectric properties of these composites in the radio frequency range is measured. The authors cure these composites using RF, and thermogravimetric data shows that the extent of cure for these samples is around 95%. The applicability of this study for 3D printing silicon carbides by successive iterations of layer deposition and rapid RF curing is demonstrated. The fly measurements of dielectric values of the 3D printing ink at different temperature while curing it is performed. The authors have also shown that these volumetric heating methods can rapidly cure polycarbosilane fibers to make silicon carbide fibers without melting them before crosslinking.

This paper presents radio frequency (RF) methods for processing preceramic polymers and their dielectric characterization throughout the curing process. Nanomaterial fillers are dispersed in the polymers to act as susceptors and make the material more responsive to RF energy. Conventional methods to produce ceramics such as silicon carbide (SiC) rely on sintering at very high temperatures using conventional heating. Potential benefits of volumetric heating using RF energy over slow conventional heating methods include rapid and more uniform curing. Polycarbosilanes are preceramic polymers that can be used to produce SiC, however, they have low conductivity making them challenging to heat with RF energy. Adding nanomaterial fillers such as multiwalled carbon nanotubes (MWCNTs) to the polymer changes its dielectric properties so that they can absorb RF energy efficiently and, as a result, heat rapidly [1]-[3].

This article presents a microwave imaging method based on the time-reversed finite-difference time-domain method (FDTD-TR) that enhances immunity to undesired interference and jamming signals. The method uses random noise as the source waveform that illuminates the imaging area. Signals scattered by objects in the imaging area are collected on a planar grid, followed by processing through a correlation operation, and then used in the FDTD-TR algorithm to construct images on any desired plane in the computational domain. The method capability in suppressing interference noise is demonstrated through successful reconstruction of images for a test case consisting of a dielectric object behind a wall. The method is compared to conventional FDTD-TR methods that fail the same image reconstruction test.

Here, we give the first-ever report of radio frequency (RF) electromagnetic heating of polymer nanocomposite materials via direct-contact and capacitively coupled electric field applicators. Notably, RF heating allows nanocomposite materials to be resistively heated with electric fields. We highlight our novel RF heating technique for multiwalled carbon nanotube (MWCNT) thermoplastic composites and measure their broadband dielectric properties. We also demonstrate three different electric field applicator configurations and discuss their practical use in an industrial setting. We demonstrate the use of RF heating to cure an automotive-grade epoxy loaded with MWCNTs. Our results show that lap shear joints cured faster with the RF method compared with control samples cured in an oven because of the heat-transfer advantages of directly heating the epoxy composite. Finally, we implement our RF curing technique to assemble an automotive structure by locally curing an epoxy adhesive applied to a truck chassis.