Yehuda Meir

Localized Rapid Heating by Low-Power Solid-State Microwave Drill

This paper presents a theoretical and experimental study of a locally induced microwave-heating effect implemented by a low-power transistor-based microwave drill. A coupled thermal-electromagnetic model shows that the thermal-runaway instability can be excited also by relatively low microwave power, in the range ~ 10-100 W, hence by solid-state sources rather than magnetrons. Local melting then occurs in a millimeter scale within seconds in various materials, such as glass, ceramics, basalts, and plastics. The experimental device employs an LDMOS transistor in an oscillator scheme, feeding a miniature microwave-drill applicator. The experimental results verify the rapid heating effect, similarly to the theoretical model. These findings may lead to various material-processing applications of local microwave heating implemented by solid-state devices, including local melting (for surface treatments, chemical reactions, joining, etc.), delicate drilling (e.g., of bones in orthopedic operations), local evaporation, ignition, and plasma ejection (e.g., in microwave-induced breakdown spectroscopy (MIBS) for material identification).

Observations of Ball-Lightning-Like Plasmoids Ejected from Silicon by Localized Microwaves

This paper presents experimental characterization of plasmoids (fireballs) obtained by directing localized microwave power (<1 kW at 2.45 GHz) onto a silicon-based substrate in a microwave cavity. The plasmoid emerges up from the hotspot created in the solid substrate into the air within the microwave cavity. The experimental diagnostics employed for the fireball characterization in this study include measurements of microwave scattering, optical spectroscopy, small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Various characteristics of these plasmoids as dusty plasma are drawn by a theoretical analysis of the experimental observations. Aggregations of dust particles within the plasmoid are detected at nanometer and micrometer scales by both in-situ SAXS and ex-situ SEM measurements. The resemblance of these plasmoids to the natural ball-lightning (BL) phenomenon is discussed with regard to silicon nano-particle clustering and formation of slowly-oxidized silicon micro-spheres within the BL. Potential applications and practical derivatives of this study (e.g., direct conversion of solids to powders, material identification by breakdown spectroscopy (MIBS), thermite ignition, and combustion) are discussed.

Plasma column and nano-powder generation from solid titanium by localized microwaves in air

This paper studies the effect of a plasma column ejected from solid titanium by localized microwaves in an ambient air atmosphere. Nanoparticles of titanium dioxide (titania) are found to be directly synthesized in this plasma column maintained by the microwave energy in the cavity. The process is initiated by a hotspot induced by localized microwaves, which melts the titanium substrate locally. The molten hotspot emits ionized titanium vapors continuously into the stable plasma column, which may last for more than a minute duration. The characterization of the dusty plasma obtained is performed in-situ by small-angle X-ray scattering (SAXS), optical spectroscopy, and microwave reflection analyses. The deposited titania nanoparticles are structurally and morphologically analyzed by ex-situ optical and scanning-electron microscope observations, and also by X-ray diffraction. Using the Boltzmann plot method combined with the SAXS results, the electron temperature and density in the dusty plasma are estimated as ∼0.4 eV and ∼1019 m−3, respectively. The analysis of the plasma product reveals nanoparticles of titania in crystalline phases of anatase, brookite, and rutile. These are spatially arranged in various spherical, cubic, lamellar, and network forms. Several applications are considered for this process of titania nano-powder production.

Underwater microwave ignition of hydrophobic thermite powder enabled by the bubble-marble effect

Highly energetic thermite reactions could be useful for a variety of combustion and material-processing applications, but their usability is yet limited by their hard ignition conditions. Furthermore, in virtue of their zero-oxygen balance, exothermic thermite reactions may also occur underwater. However, this feature is also hard to utilize because of the hydrophobic properties of the thermite powder, and its tendency to agglomerate on the water surface rather than to sink into the water. The recently discovered bubble-marble (BM) effect enables the insertion and confinement of a thermite-powder batch into water by a magnetic field. Here, we present a phenomenon of underwater ignition of a thermite-BM by localized microwaves. The thermite combustion underwater is observed in-situ, and its microwave absorption and optical spectral emission are detected. The vapour pressure generated by the thermite reaction is measured and compared to theory. The combustion products are examined ex-situ by X-ray photo-electron spectroscopy which verifies the thermite reaction. Potential applications of this underwater combustion effect are considered, e.g., for detonation, wet welding, thermal drilling, material processing, thrust generation, and composite-material production, also for other oxygen-free environments.

Transistor-based miniature microwave-drill applicator

This paper explores the microwave self-focusing effect, and examines various new applications of transistor-based microwave-drills at <100-W microwave power range. The thermal-runaway instability is analyzed, and a drillability factor is defined heuristically for the minimal power needed to induce hotspots in specific materials. A practical transistor-based microwave-drill scheme using LDMOS-FET is introduced for delicate operations. Variants of this device are proposed for other applications, like local melting, ignition, indentation, plasma generation, and material identification by microwave induced breakdown spectroscopy (MIBS). Experimental results of glass processing, basalt melting and drilling, and thermite powder ignition are introduced as examples for future applications.

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

Purpose This 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/approach Following 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. Findings The powder properties are characterized, and a theoretical LMH model is used to simulate the LMH mechanism dominated here by magnetic eddy currents. Originality/value The 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.