K. Ishizaki

Applicability Study of Classical and Contemporary Models for Effective Complex Permittivity of Metal Powders

Abstract Microwave thermal processing of metal powders has recently been a topic of a substantial interest; however, experimental data on the physical properties of mixtures involving metal particles are often unavailable. In this paper, we perform a systematic analysis of classical and contemporary models of complex permittivity of mixtures and discuss the use of these models for determining effective permittivity of dielectric matrices with metal inclusions. Results from various mixture and core-shell mixture models are compared to experimental data for a titanium/stearic acid mixture and a boron nitride/graphite mixture (both obtained through the original measurements), and for a tungsten/Teflon mixture (from literature). We find that for certain experiments, the average error in determining the effective complex permittivity using Lichtenecker’s, Maxwell Garnett’s, Bruggeman’s, Buchelnikov’s, and Ignatenko’s models is about 10%. This suggests that, for multiphysics computer models describing the processing of metal powder in the full temperature range, input data on effective complex permittivity obtained from direct measurement has, up to now, no substitute.

Mechanism and kinetics of the reduction of magnetite to iron during heating in a microwave E-field maximum

The time-resolved x-ray diffraction method was applied in situ to investigate the carbothermal reduction of magnetite by carbon black in a 2.45 GHz microwave field. The kinetics of the phase transformation sequence is analyzed within the Kolmogorov–Johnson–Mehl–Avrami formalism. The first reduction stage is the solid-state transition of magnetite to wustite, followed by the partial conversion of nearly stoichiometric wustite to iron. The kinetics of the initial reduction of Fe3O4 to primary wustite is phase boundary controlled. Behind the reaction front, primary wustite rapidly evolves toward stoichiometric FeO. Cation vacancies and the dynamic clustering of structural defects influence the kinetics of the reduction of the nearly stoichiometric secondary wustite to porous iron.

Nanocrystallization of amorphous alloys using microwaves: in situ time-resolved synchrotron radiation studies

Important energy and time savings can be achieved with the thermal treatment of materials by replacing conventional heating methods with microwave heating. The nano- crystallization of Co-Fe-W-B amorphous alloy powders under microwave irradiation was followed for the first time by in situ time-resolved synchrotron radiation powder diffraction. It is shown that even a very short exposure to the electromagnetic field (single pulse microwave application) typically of the order of a few seconds is sufficient to obtain the bulk nano- crystalline state. A metastable high-temperature Co-W-B orthorhombic phase forms during the microwave heating, which gradually transforms to the tetragonal Co2B stable phase.

Rapid synthesis and densification of single-phase Al–Cu–Fe quasicrystals by spark plasma sintering or microwave heating

Quasicrystalline (QC) phases are often stable only within narrow composition domains. For this reason, the synthesis of larger amounts of single-phase quasicrystalline powders is difficult. Powder metallurgical approaches, based on mechanical milling followed by conventional heating, have been explored in the recent past. The manufacturing process for single-phase quasicrystals – either in the form of powders or as bulk parts – can be accelerated by orders of magnitude using rapid heating methods that involve pulsed electric currents and/or high-frequency electromagnetic fields. Prior knowledge of the phase transformation sequence and transformation kinetics, as revealed by in situ time-resolved synchrotron radiation experiments, is crucial in obtaining single-phase quasicrystals. We report on the simultaneous synthesis and densification of bulk single-phase Al–Cu–Fe QCs by spark plasma sintering (SPS) within minutes and on the ultrafast synthesis of single-phase Al–Cu–Fe quasicrystalline powders by microwave heating within seconds. The effect of electric current application in the rapid processing of pre-alloyed powders is discussed in relation to the faster diffusion and enhanced phase transformation kinetics.

Microwave energy absorption driven by dynamic structural and magnetization states in Fe85B15 metallic glass ribbons

The kinetics of the microwave crystallization of Fe85B15 metallic glasses was investigated in situ using the time-resolved x-ray diffraction method. It is shown that the recorded thermal profile during the microwave exposure of the ribbons bears a close relationship with the dynamic magnetization state during the decomposition of the amorphous phase into nanocrystalline α-Fe and Fe3B phases.

Characterization of free falling drops inside a microwave cavity

Microwave Internal Gelation (MIG) is a chemical process proposed for the production of nuclear particle fuel. The reaction is triggered by a temperature increase by the means of microwave heating. Due to the short residence time of a solution droplet in the cavity a detailed knowledge of the interaction between microwaves and chemical solution (shaped in small drops) is required. This paper describes a new procedure that enables the measurement of dielectric properties of aqueous droplets that freely fall through a microwave cavity. These measurements provide the information to determine the optimal values of the parameters (such as frequency and power) that dictate the heating of a material under microwaves.

Microwave-induced electromigration in multicomponent metallic alloys

The crystallization of amorphous FeCoCuZrAlSiB alloy ribbons during microwave heating was investigated in situ using time-resolved X-ray powder diffraction. The formation of the nanocrystalline α-(Fe,Co)(SiAl) phase during the primary crystallization stage is followed by the crystallization of the residual glassy matrix. Scanning electron microscopy analysis after microwave exposure reveals the formation of nanosized hillocks evenly distributed over the ribbon surfaces. Local chemical composition analysis by energy-dispersive spectroscopy shows that the surface clusters are enriched in Cu and Al. The occurrence of this typical electromigration effect imposes a strong restriction on the duration of the exposure of metallic ribbons to microwave fields and reinforces the need for prior characterization in particular by in situ time-resolved techniques.

Developing an in situ EXAFS experiment of microwave-induced gelation

Internal gelation is an advanced route to produce small spheres of metal from a nitrate solution. In this work, microwave heating is used to trigger the gelation of the solution. X-ray absorption spectroscopy is a powerful tool to follow the gelation advancement and verify the completeness of a reaction, but in order to use it on falling nitrate droplets, the study is split into two experiments. First, a fluorescence measurement has been carried out on a trickle of small falling cerium nitrate droplets generated at high frequency to verify that capturing a discontinuous signal would not generate too much noise. Then, a measurement of silver nitrate undergoing gelation in a microwave cavity has been performed in transmission mode, which proved that a fast recording method is suitable for following the evolution of chemicals during such a reaction. The combination of both analyses confirms that it is possible to study microwave gelation of falling droplets using X-ray absorption spectroscopy at the SuperXAS beamline.Graphical Abstract

Real-time material's response to high power microwave irradiation revealed by in-situ synchrotron radiation methods

The complexity of microwave heating stems to a large extent from the intrinsic complexity of the materials exposed to microwave irradiation. For simple ideal homogeneous material, its size, shape and orientation with respect to the electromagnetic field influence the microwave absorption properties at the macroscopic scale. Furthermore, a number of material parameters, namely the complex permittivity and permeability, electrical conductivity, density etc. additionally determine the efficiency of the energy transfer from the microwave field to the sample. These material parameters are inter-correlated and may change significantly as function of frequency and temperature. For an homogeneous solid, the material parameters are defined by scalar values, i.e. having the same value throughout the specimen volume.

Dynamic High-Temperature Monitoring of Microwave Energy Absorption and Heating of Materials with Ultrafast In Situ Synchrotron X-Ray Tomographic Microscopy and Powder Diffraction Techniques

In-situ time- and temperature-resolved synchrotron radiation techniques like ultrafast synchrotron X-ray tomographic microscopy and powder diffraction are unique tools for the dynamic characterization of materials processing by microwave heating. The absorption of microwave energy is typically a very fast process, with heating rates of the order of hundreds of degrees per second being no exception. The microwave energy absorption efficiency changes significantly with increasing temperature. Another unique feature of microwave heating is the intrinsic dielectric and/or magnetic selectivity, which often translates into the preferential deposition of microwave field energy only into specific specimen regions. For inhomogeneous materials in particular, complex patterns for the dynamic electromagnetic and temperature field distributions can be expected thus making the use of 3D monitoring methods not only meaningful but also necessary. We report on our recent progress with the in-situ characterization of microwave heating of metallic, ceramic and composite materials at very high heating rates. Experimental methods with subsecond temporal resolution, in particular high-temperature time-resolved X-ray scattering and time-resolved X-ray microtomography using synchrotron radiation, are discussed. Examples include microwave-assisted structural phase transitions and sintering in Al-alloys, foaming of construction materials, microwave processing of ceramics and composites.