Localized Microwave Heating Research Articles

Preliminary study on localized microwave sintering of lunar regolith

The pioneering objective of a permanent outpost on the lunar surface requires the development of technologies for in situ resource utilisation. Consolidation of lunar soil into products is imperative to avoid unaffordable transportation burdens. The paper addresses the feasibility of the Local Microwave Heating effect as a promising technology for sintering lunar regolith based on microwave radiation, with a simple solid-state generator requiring a relatively modest amount of energy. Experiments were conducted using terrestrial simulants with physical and chemical properties similar to those of lunar regolith. Laboratory equipment with a purpose-built microwave actuator was designed and manufactured with “Design for Embarking” in mind. Experiments have demonstrated the feasibility of the proposed technology with a reasonable degree of confidence. Solid artefacts were obtained with proper setting of microwave parameters and appropriate scanning strategy, proving the feasibility of regolith sintering with a localised microwave source A roadmap for future development of the technology has been established.

Sequential Solidification of Metal Powder by a Scanning Microwave Applicator.

This study examines the fundamental feasibility of sequential metal-powder solidification by localized microwave-heating (LMH) provided by a scanning, all-solid-state microwave applicator. This continuous process is considered for the additive manufacturing (AM) and 3D printing (3DP) applications of metal parts. In previous studies, we employed LMH for the incremental solidification of small batches of metal powder in a stepwise vertical manner. Here, we study a continuous lateral LMH process, layer by layer, in a fashion similar to laser scanning in powder beds, as performed in common laser-based AM systems. LMH solidification at scanning rates of ~1 mm3/s is obtained in bronze powder using ~0.25-kW microwave power. The effect is studied here by LMH scanning in one lateral dimension (~20-mm long) in layers, each of ~1-4 mm thickness and ~2-4 mm width (mechanically confined). Imperfect solid bars of ~20×4×5 mm3 are obtained with rough surfaces. Their joining in an L shape is also demonstrated. The experimental solidified products are tested, and their hardness and density properties are found to be comparable to laser-based AM products. The capabilities and limitations of the LMH scanning concept for metal-powder solidification are evaluated. The potential feasibility of a solid-state LMH-AM technology is discussed.

Fast microwave-assisted synthesis of iron–palladium catalysts supported on graphite for the direct synthesis of H2O2

While carbon supported noble metal catalysts have been extensively used in the direct synthesis of H2O2 (DSHP), unsatisfactory H2O2 yield remains a difficult problem to solve. In this work, a facile and efficient preparation of the FePd catalysts supported on graphite was realized by fast microwave-irradiated carbon nanotube (CNT) growth and metal reduction. For this purpose, ferrocene was used as a catalyst for microwave-assisted CNT growth as well as a carbon source, and carbon fibers and hexane were only used as carbon sources for CNT growth. The fast and localized microwave heating enabled the fast pyrolysis of ferrocene and the subsequent decomposition of other carbon sources, which efficiently regulated the Pd nanoparticle (NP) size during the microwave-irradiated reduction of Pd. The carbon sources significantly affected the geometric and electronic characteristics of Pd on carbon surface, and therefore, the catalytic activity for the DSHP reaction was affected. The additional use of hexane decreased the Pd NP size and increased the Pd2+/Pd0 ratio under microwave-assisted reduction. In the presence of FePd/GR-H catalyst, the H2O2 productivity and selectivity reached as high as 1492.3 mmol H2O2/g-Pd.h and 68.8%, respectively, in the absence of any corrosive promoters. The result clearly demonstrates that the effect of microwave irradiation for the catalyst preparation highly depended on carbon sources, and therefore, catalytic activity can be finely controlled by microwave irradiation condition.

Microwave-ignited DC-plasma ejection from basalt: Powder-generation and lightning-like effects

A phenomenon of dusty plasma ejected from basalt is presented, in the form of a stable, long-lived fire-pillar, solely sustained by a direct-current (DC) supply. The plasma is emitted to the air atmosphere from a molten hotspot, initially created in the basalt rock by localized microwave-heating (LMH). After an LMH-to-DC transition, the plasma column is solely sustained as a fire-pillar of ∼2 cm diameter, ∼4-cm height using a DC supply of ∼0.2 kV, ∼5 A, steadily lasting for periods longer than 3 min. The dusty-plasma products, accumulated in a powder form, consist of micro- and nanoparticles of basalt's components, including oxides of Si, Na, Fe, P, K, Mg, and Ti. The experimental conditions for the DC sustainability of basalt's dusty-plasma column are analyzed by various theoretical estimates. A possible relevance of these laboratory observations is attributed to atmospheric fire-pillar-like effects, associated, for instance, with volcanic-, ball-lighting, and lunar dusty-plasma phenomena. A potential application, utilizing this effect of DC-sustained dusty-plasma columns, is proposed for the direct extraction of mineral powders from rocks.

Designing Local Microwave Heating of Metal Nanoparticles/Metal Oxide Substrate Composites

Designing Local Microwave Heating of Metal Nanoparticles/Metal Oxide Substrate Composites

Concrete Cutting by a Solid-State, Localized Microwave-Heating Applicator

This article presents experimental results of concrete cutting by a localized microwave-heating (LMH) applicator, fed by a ~0.25-kW solid-state microwave generator. The LMH applicator is a modified microwave-drill, operating in a stepwise pecking mode. Compared to the previously-presented, magnetron-based microwave-drill for concrete, the new device operates at a lower power; it is more compact, and it enables arbitrary cutting contours. In each pecking step in this mode, a small amount of concrete is softened by LMH, and mechanically removed. These sequential pecks are accumulated to form a long deep groove along the desired cutting contour. The lower microwave power applied in the pecking mode enables a narrower LMH applicator, which reduces the total amount of concrete removed in the cutting process (the kerf), and hence improves the overall efficiency. The solid-state LMH pecking module can be remotely controlled, and it is applicable to other masonry as well. Its silent and clean operation (with no vibration or dust effusion) classifies it as an environmental-friendly technology. The article introduces the LMH pecking concept for concrete cutting, and presents its experimental results. Concrete removal rates of ~0.35 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /min in ~0.5-cm pecking steps (accumulated to ~6-cm-long, ~6-cm-deep grooves) were obtained by applying a ~0.25-kW microwave power, in agreement with the LMH theory. The feasibility of a faster concrete cutting in practice by advanced LMH-based devices, also in array schemes, is discussed.

Local melting and cutting of iron bulks by a synergic microwave–DC thermal skin effect

Microwaves are widely utilized in heating processes, in domestic, industrial, scientific, and medical applications. However, metal bulks (unlike metal powders or thin sheets) are commonly considered as hardly susceptible to microwave heating (due to their micrometric skin-depth, which prevents the microwave penetration into the bulk). Here, we show that adding a relatively small direct current (DC) may catalyze a localized microwave-heating (LMH) effect in the iron bulk, up to its local melting (and even further to ablation and dusty-plasma ejection). The combined DC-LMH effect is demonstrated here by cutting 8-mmØ iron rebars (made of carbon steel, with no susceptors added), which is not feasible by sole microwave or DC in these conditions. The synergic microwave and DC effect is attributed here to a combined thermal skin evolution, which jointly forms a hotspot in a mutually intensified thermal-runaway instability and deepens the microwave penetration into the iron bulk. This interpretation of the experimental findings is supported here by a simplified theoretical model of the combined microwave–DC interaction in iron, which demonstrates the thermal skin layer evolution and confirms the feasibility of a hotspot formation. The synergic DC-LMH effect may advance potential applications of microwaves in thermal processing of metals, such as melting, cutting, joining, sintering, casting, and 3D printing.

Temperature Homogeneity under Selective and Localized Microwave Heating in Structured Flow Reactors

Selective heating of different phases of multiphase systems via microwaves can result in energy savings and suppression of side reactions. However, materials properties and operating conditions tha...

Assessment of different methods for the removal of biofilms and lichens on gravestones of the English Cemetery in Florence

The control of biodeterioration encompasses the operations undertaken to eliminate the biological growth and, possibly, to delay a new colonization. The current attitude is generally oriented toward its planned removal whenever it causes an objective damage and/or structural impairments to the substratum. The English Cemetery, located in the centre of Florence, offers interesting features for a research focused on the removal of biofilms and lichens growing on stone surfaces of some tombs. The study compared the efficacy of two methods based on physical approach (mechanical cleaning with a brush and microwave heating) with a chemical approach using biocide ROCIMA™ 103 to remove biofilms and lichens from each tombstone. The research, focusing on methodologies with low impact for the environment, tested the efficacy of an innovative portable system that produces localized microwave heating. Its great advantage lays on lack of the potential risks associated with the irreversible application of microbicides. The assessment of the treatments' efficacy was carried out monitoring the chlorophyll a fluorescence's parameters, informative on the vitality and stress responses of photosynthetic organisms. The long-term monitoring of the recolonization after the treatments was performed for five years. The mechanical cleaning eliminated the superficial layer of biofilms and lichens but not the cells within the stones. The biocide was efficient in killing the biological growth; almost no recolonization was observed after about five years. The innovative microwave treatment was effective on biofilms and lichens, eliminating also cells present in the bulk of the substrata, but recolonization was observed after 15 months. This suggests that, dopo treatment aggiungere virgola the microwave treatment should be performed more frequently than biocide treatments yet guaranteeing lower impact on the environment.

SCIENTIFIC BASIS OF SILVER NANOPARTICLES COATINGS FORMATION ON SURFACE OF POLYPROPYLENE ULTRATHIN FIBERS

New principles of silver nanoparticles coatings formation on the ultrathin fibers surface using electromagnetic radiation of the deci- and nanometer ranges for bactericidal modification of liquid media were proposed. Coating production involves the formation of silver nanoparticles as a result of photoreduction from an aqueous solution of AgNO3 with a concentration of 0.001M at the presence of ultrathin polypropylene fibers with their subsequent stabilization on the surface of the fibers with microwave radiation of the fiber material are initiated, which prevents agglomeration and stabilize silver nanoparticles on their surface. Ultrathin polypropylene fibers obtained as a result of pneumatic spraying of the melt are represented by an average diameter dav = 1.5-6 μm and a fairly wide lognormal distribution. The possibility of a two-step formation of coatings on the polymer fibers surface has been demonstrated using the method of numerical simulation of the self-assembly and self-organization of silver nanoparticles, taking into account the dissipation of their energy into the polymeric matrix through the exciton and phonon channels. It has been established that as a result of local microwave heating of silver nanoparticles, processes of the formation of supramolecular structures of the fiber material are initiated, which prevents agglomeration and stabilizes silver nanoparticles on their surface. The coatings obtained were characterized by transmission electron microscopy and spectroscopy. For example, silver nanoparticles formed and attached to the surface of fibers as a result of a ten-minute exposure to ultraviolet radiation have a size of 50 ± 18 nm. Studies of bactericidal activity showed that ultrathin polypropylene fibrous materials with coatings based on silver nanoparticles can effectively perform antibacterial treatment of natural waters with a high level of microbiological contamination. After 10 cycles of water purification, the amount of silver in the samples, according to atomic emission spectroscopy, remains unchanged.

Incremental solidification (toward 3D-printing) of metal powders by transistor-based microwave applicator

Incremental solidification (IS) of small batches of metal powders is a fundamental process in various additive-manufacturing (AM) and 3-D printing (3DP) operations. Here we show the feasibility of an IS process implemented by a compact, all-solid-state microwave applicator. In this compact scheme, the localized microwave-heating (LMH) process is conducted in a nitrogen shielding environment, and hence the microwave power is reduced to only ∼0.1 kW (compared to ∼1 kW in a previously reported magnetron-based IS process). This shielding prevents the plasma excitation (which intercepted most of the microwave power in the previous experiments in air atmosphere), and hence significantly improves the efficacy of the LMH-IS process. This paper introduces the compact LMH-AM concept, and its novel implementation by a transistor-based microwave system. Experimental and theoretical results of the LMH-AM process applied to bronze- and iron-based powders are presented. The powder is provided to the LMH interaction region in small batches, on demand. A contactless magnetic confinement is demonstrated for ferromagnetic powder batches. The experimental products obtained are tested, and their mechanical properties are evaluated. The potential applicability of the solid-state LMH technology to full AM and 3DP processes is discussed.

Localized microwave-heating (LMH) of basalt \u2013 Lava, dusty-plasma, and ball-lightning ejection by a \u201cminiature volcano\u201d

This paper presents various phenomena obtained by localized microwave-heating (LMH) of basalt, including effects of inner core melting, lava eruption and flow (from the molten core outside), plasma ejection from basalt (in forms of fire-column and ball-lightning), and effusion of dust (deposited as powder by the plasma). The experiments are conducted by irradiating a basalt stone (~30-cm3 volume, either naturally shaped or cut to a cubic brick) in a microwave cavity, fed by an adaptively-matched magnetron (~1 kW at 2.45 GHz). Effects of LMH and thermal-runaway instability in basalt are observed and compared to theory. Various in- and ex-situ diagnostics are used in order to characterize the dusty-plasma observed and its nanoparticle products. The resemblance of the experimental phenomena obtained in small laboratory scale to gigantic volcanic phenomena in nature is noticed, and its potential relevance to further volcanic studies is discussed. In particular, we show that LMH could be instrumental for laboratory demonstrations and simulations of miniature-volcano effects, such as lava flows, formation of volcanic glass (obsidian), eruption of dusty-plasma and volcanic ash, and ejection of ball lightning. These findings might be significant as well for various applications, such as drilling and mining, microwave-induced breakdown spectroscopy (MIBS), mineral extraction, and powder production directly from basalts.

Technology of local microwave heating

Experimental studies of the possibilities of local microwave heating technology practical implementation were carried out. The correspondence of the existing theoretical models of the process to what is happening in practice is shown. The results of measuring the temperature distribution over the sample when exposed to a radiating microwave coaxial probe are given. The directions for further improvement of the operating modes and the main components of the microwave modifier are determined.

Thermoelectric Graphene Nano-Constrictions as Detectors of Microwave Signals

Microwave power detectors were realized using nano-constrictions fabricated from commercially-available monolayer graphene, synthesized by means of chemical vapor deposition. The graphene nano-constrictions (GNCs) were fabricated so as to shunt the inner discs to the concentric outer annuli of micrometer-scale Corbino disc test structures. The test structure was fabricated on a layer of electrically-insulating silicon dioxide situated on an electrically-conductive, doped silicon substrate. The substrate was used as a gate for controlling the Fermi level in the nano-constriction. Probe station measurements were conducted on a detector with nano-constriction dimensions of 500 nanometers in width and 5 micrometers in length. The annulus of this device was electrically grounded and held in thermal equilibrium with chassis of the probe station. Measurements obtained with a vector network analyzer provided the reflection coefficient of the detector. Using a test signal frequency of 433.92 MHz, microwave power detection characterizations, which accounted for signal reflection, revealed a peak detection sensitivity of 60.25 mV/mW. The gate voltage dependence of the power detection sensitivity was found to be strongly correlated with that of the Mott formula used for calculating the Seebeck coefficient of graphene. This observation suggests that localized microwave heating of the inner disc, relative to the temperature of the outer annulus, establishes a thermal gradient across the nano-constriction and results in a thermoelectric response that is proportional to the power level of the incident microwave signal.

Incremental solidification (toward 3D-printing) of magnetically-confined metal-powder by localized microwave heating

PurposeThis paper aims to present an experimental and theoretical study oriented to investigate the potential use of localized microwave-heating (LMH) in 3D-printing and additive-manufacturing (AM) processes.Design/methodology/approachFollowing a previous study by the authors, a magnetic confinement technique is developed here as a non-contact support for the incremental solidification by LMH of small metal-powder batches. This approach, which saves the need for a mechanic support in contact with the powder-batch during the microwave heating, may significantly simplify the LMH–AM process.FindingsThe powder properties are characterized, and a theoretical LMH model is used to simulate the LMH mechanism dominated here by magnetic eddy currents.Originality/valueThe experimental products are analyzed, and their hardness, porosity and oxidation are evaluated. Practical considerations and further improvements of the non-contact LMH–AM process are discussed.

A Silent Microwave Drill for Deep Holes in Concrete

This paper presents a mechanically assisted microwave drill (MWD) capable of drilling 26-cm-deep 12-mm-diameter holes in concrete. This record significantly extends the inherent $\lambda $ /4-depth capability (~1.5 cm at 2.45 GHz) of the basic MWD scheme. Compared with conventional mechanical drills, this MWD is characterized by a relatively silent and vibration-free operation, but its drilling speed is yet slower than 1 cm/min. This paper reviews the fundamental MWD mechanism (utilizing localized microwave heating and thermal-runaway instability), and extends it for deeper holes by also using the coaxial applicator as a slowly rotating hollow reamer to remove the debris. The MWD prototype is introduced, including its adaptive impedance matching and remote-operation features, and its experimental results are presented. Theoretical and practical MWD aspects are discussed, and potential developments are indicated (e.g., for faster drilling and iron-rebar cutting). The present MWD performance can be useful for specific applications which critically require silent drilling operations in concrete.

Ultrafast and Highly Localized Microwave Heating in Carbon Nanotube Multilayer Thin Films

Carbon nanotubes have emerged as highly effective materials for microwave absorbing applications due to their high electrical conductivity, large aspect ratio, and high temperature stability against oxidation and corrosion. Ultrathin films are fabricated through layer‐by‐layer deposition of carbon nanotubes and poly(diallyldimethyl ammonium chloride) from aqueous suspensions under ambient conditions. The number of immersion cycles controls the assembled thickness of these coatings, which provides an effective means of precisely tailoring their electrical properties and thermal response to microwaves. Films of thickness below 200 nm are capable of heating to high temperatures when irradiated with low microwave power, reaching over 130 °C in 30 s, at less than 10 W forward power, and rapidly cooling when the power is removed. In contrast to metal thin films, the multilayer carbon nanotube films are active microwave absorbers over a wide range of thickness, making them interesting for applications such as electromagnetic interference shielding and microwave welding.

Localized microwave-heating intensification—A 1-D model and potential applications

This paper reviews the localized microwave-heating (LMH) phenomenon and its various applications in a paradigmatic approach. A simplified 1-D microwave-heating model is derived for a temperature-dependent dielectric medium in a cavity. This semi-analytical model shows the evolution of high-order spatial modes, and the concentration therein of the dissipated power. The LMH effect is associated with the localized hotspot formation due to the thermal-runaway instability. LMH intensification in solids and powders enables various applications using the microwave-drill technique, as reviewed in this paper. These include for instance local heating, up to >103K, also by LDMOS transistors; ignition of thermite powders in air atmosphere and underwater; generation of plasma columns and spheres from molten hotspots in solids (e.g. silicon, titanium), and production of nano-powders by dusty plasma. The potential for 3D-printing and additive manufacturing (AM), recently demonstrated by local solidification of metal powders in a stepwise manner by LMH, is discussed.

Proppant immobilization facilitated by carbon nanotube mediated microwave treatment of polymer-proppant structures

The application of multi-walled carbon nanotubes (MWCNTs) to facilitate the aggregation of ceramic particles (proppants) by embedding in the coating through localized microwave heating is reported. Thermogravimetric analysis (TGA), scanning electron microscopy (SEM) along with energy dispersive X-ray analysis (EDX) and atomic force microscopy (AFM) were used to characterize a series of covalently functionalized proppants with carboxylic acids (lysine and fumaric acid). The bi-functional acids allow for a controlled reaction with a coating (resin and polymer). The addition of carbon nanotubes results in the melting of the coating under exposure to microwave irradiation (20s @ 100W) joining the particles. The significant reduction in reaction time (compared to the thermal treatments of 2–24h) is due to the presence of the MWCNTs. The microwave heating of the MWCNTs can thus be used to create interaction and adhesion between functionalized particles subject to a specific stimulus.

In situ microwave-enhanced electrochemical reactions at stainless steel: Nano-iron for aqueous pollutant degradation

Iron nanoparticle deposition and stripping are observed from aqueous Fe2+ solution at pH3 on stainless electrodes in the presence of focused microwave activation. The effects of Fe2+ concentration and microwave power are evaluated. It is shown that the resulting iron nanoparticle deposit (i) gives well-defined anodic stripping responses, (ii) is readily released into the solution phase, and (iii) is highly reactive towards chlorinated hydrocarbons such as trichloroacetate. The combined effects of increased mass transport and localized microwave heating improve pollutant degradation treatments.