Noboru Yoshikawa

Heating of metallic powders by microwaves: Experiment and theory

It is known that bulk metallic samples reflect microwaves while powdered samples can absorb such radiation and be heated efficiently. In the present work we studied heating mechanisms of metallic powders in a multimode 2.45 GHz microwave applicator. The present paper shows direct evidence of penetration of a layer of metallic powder by microwave radiation and provides theoretical explanation of such behavior.The most effectively heated powder is Fe because both eddy current loss (in alternating H-field) and magnetic reversal loss (in alternating E-field) mechanisms act in case of such metal. Diamagnetic metals Sn and Cu are heated better than paramagnetic Ti while Au is also only slightly heated. Cu- and Ni-based metallic glassy powders are also moderately heated. Weak heating of Au powder (which is a noble metal) can be explained by the absence on the particles of the oxide layer (shell), which allows eddy currents flowing through larger area compared to other metals, and reflection mechanism works much ...

Low-temperature growth of ferroelectric lead zirconate titanate thin films using the magnetic field of low power 2.45 GHz microwave irradiation

Pb(Zr(x)Ti(1-x))O(3) (PZT) thin films were coated on Pt/Ti/SiO(2)/Si substrates by the sol-gel method and then crystallized by using the magnetic field of 2.45 GHz microwave irradiation. The elevated temperature generated by microwave irradiation used to obtain the perovskite phase was only 450 degrees C, which is significantly lower than that of conventional thermal processing. The PZT films crystallized by microwave irradiation showed similar ferroelectric properties to those of the films crystallized by conventional thermal processing at 600 degrees C. It is clear that single-mode 2.45 GHz microwave irradiation in the microwave magnetic field is effective for obtaining perovskite PZT thin films at low temperatures. (c) 2008 American Institute of Physics.

Fundamentals and applications of microwave heating of metals.

Abstract As the fundamentals of microwave (MW) interaction with metals, boundary conditions of electromagnetic (EM) field on metal surface are discussed, which consider the EM field in the metal surface layer in terms of surface impedance. Experimental report on heating behavior of separated electric (E-) and magnetic (H-) fields of metal particles and films are shown. Temperature peak formation at the first heating curves was observed in both cases, which are discussed considering the microstructural alteration by MW heating. In the last half section, various reports on MW heating of metal are reviewed. They were classified into the major application for sintering and materials fabrication. And also, its usage as a heating aid of glasses and soils, topics on metal hydride and catalytic metal particles are included.

Microwave heating origination and rapid crystallization of PZT thin films in separated H field

In the separated microwave H and E fields, the heating behaviour of a SiO2/Si substrate was first investigated. Subsequently, the heating behaviour was further compared with the samples deposited with Pt/Ti electrodes and PZT films, respectively. It was found that PZT thin films were much more efficiently heated and crystallized in the H field than in the E field; moreover, the heating of the samples was derived mainly from the contribution of Pt/Ti and Si, which is distinct from the previous assumption about microwave dielectric heating of the E field. This unexpected foundation indicates that susceptors or high power are not necessary for microwave annealing of thin films, and furthermore, this method provides a new path for annealing any kind of thin film deposited on semiconducting or conductive substrates.

Growth rates and microstructures of reacted layers between molten Al–Fe alloy and SiO2

Abstract Fabrication of composite materials consisting of Al2O3 and Al–Fe alloy was attempted by means of an in-situ reaction between SiO2 and molten Al–Fe alloys. In this study, SiO2 rods were immersed into the molten alloy at various conditions. Growth rate of the reacted layer was measured and its microstructure was observed by using various techniques. The growth rate decreased as an increase of Fe content in the molten alloys, and microstructures of the reaction layer became fine, comparing with that of Al/Al2O3 (reaction product between pure Al and SiO2). Concentration gradients of Fe and Si existed in the reacted layer, namely, Si and Fe contents were higher at the reaction interface than in the edge. Si was segregated in the metal area having higher Fe content. Reasons for the decreases in growth rate were discussed considering the observed microstructures. Judging from the growth rate dependence on the Fe content, it is impossible to fabricate FeAl (intermetallic compound)/Al2O3 composite materials in-situ by simple immersion of SiO2 into the molten Al–Fe alloy. The alternative two-stage immersion process was proposed.

Mechanical properties of Al/Al2O3 composites fabricated by reaction between SiO2 and molten Al, Al-Cu

Generally, fabrication costs of composite materials are high because several processes are involved, such as embedding ®bres or particles within the matrix materials followed by various heat treatments. In situ composites have recently attracted attention [1], as their fabrication processes are expected to be effective for reducing the fabrication costs and the desired micro=macro structures of the composite material are obtained during the fabrication process. In the case of Al=Al2O3 composites, a process of direct oxidation of molten Al has been developed as the DIMOX process [2], and fundamental studies have been reported on their reaction kinetics [3, 4] and the microstructure [4, 5]. On the other hand, it has been reported that the Al=Al2O3 composite materials can also be obtained [6±8] by means of a substitutional reaction between SiO2 (or mullite [9]) and molten Al, which have similar microstructure to that obtained by direct oxidation. However, the reported studies have mainly dealt with the relationships between the process conditions and the microstructure [6, 7]. The detailed relationships between the process conditions, microstructures and the properties are still not clari®ed. In this study, Al=Al2O3 composite materials were fabricated between SiO2 and molten Al at different reaction temperatures and their microstructures and mechanical properties are investigated. Moreover, modi®cation of the hardness was attempted, by utilizing molten Al-Cu for the reactant, followed by heat treatment. The Al sections utilized for the reactions was cut from an ingot of 99.99% purity, etched with 0.1 N NaOH solution for 1.8 ks then cleaned with puri®ed water. Commercial fused silica rods having diameters of 5 mm (Toshiba Ceramics Inc.) were dipped into molten Al in air. After suf®cient time for completion of the reaction (investigated for the reaction kinetics [10], in advance) the specimens were cut and the microstructures were observed by optical microscope and scanning electron microscope (SEM). Compositional analysis was carried out with energy dispersive X-ray analysis (EDX), attached to the SEM, and with wavelength dispersive X-ray analysis (WDX). The Vickers hardness was measured by applying a load of 5 kg. Compression tests were conducted for the specimen rods having the following dimensions: length 7 mm, diameter 5 mm. Compressive fracture strength (of ) was also measured. Optical micrographs of the specimen microstructures are shown in Fig. 1. The bright regions in the photographs correspond to Al and the dark regions to Al2O3. Microstructures of the specimens fabricated at temperatures higher than 1373 K were coarse, compared with those fabricated at 1073 K. Microstructures coarsened as the reaction temperature increased.

Microwave-induced heating of a single glassy phase and a two-phase material consisting of a metallic glass and Fe powder

Metallic glasses exhibit low viscosity in a temperature range between the glass transition and the crystallization temperature, a feature that allows successful sintering of glassy powders. Microwave heating, being volumetric, has significant advantages over conventional heating in materials processing, such as substantial energy savings, high heating rates and process cleanliness. In the present study, we investigate the stability of Cu50Zr45Al5 glassy powders and the formation of a bulk two-component metallic glassy-crystal sample by microwave heating in a single-mode cavity (915 MHz) in an alternating magnetic field.

Ferromagnetic resonance heating of Fe and Fe3O4 by 5.8 GHz microwave irradiation

Compressed Fe3O4 powder and Fe sheet were pre-heated to various temperatures higher than 300 °C in a microwave magnetic field (hrf). Then an external static magnetic field (Hext) was imposed perpendicularly to hrf and the temperature increased (ΔT: temperature difference from the pre-heated temperature, ΔTmax: ~200 °C (Fe), ~50 °C(Fe3O4)). It had a peak at almost the same Hext during ascent and descent of Hext because of ferromagnetic resonance (FMR). In fact, ΔT became smaller as the preheating temperature was raised to approach the Curie temperature, and Hext corresponding to the temperature peak shifted higher. This report provides FMR heating data under various conditions with quantitative discussion and proposes application of a new heat treatment to materials.

Microwave heating of soda-lime glass by addition of iron powder

Experimental studies were conducted to investigate the microwave (MW) heating behavior of soda-lime glass beads with added iron powder. These studies were intended to obtain fundamental knowledge for vitrification solidification and for the fabrication of metal-reinforced glass-matrix composites. The glass beads (0.2 mm diameter) did not heat very well by themselves at temperatures greater than 200 °C within 600 s in a multimode applicator at a power of 0.67 W. The addition of iron powder (average 70 μm, volume fraction 18%) made it possible to heat the glass beads above 700 °C within 60 s. At lower fractions of 3–11 vol%, however, a sudden temperature rise [thermal runaway (TRW)] occurred after the incubation time period. A single-mode MW applicator was used for clarifying the electric (E)-field and magnetic (H)-field contributions to the heating of each material and their mixtures. The results of this study demonstrated that the H-field contributed to the heating of the iron and then triggered the heating of the glass. The E-field component is necessary for heating the glass to a temperature higher than 800 °C. The factors determining the threshold values of the volume fraction causing TRW are discussed.

Tetragonal to monoclinic transformation in Y-TZP joined with metallic materials

Joints between Y-TZP (Yttria containing Tetragonal Zirconia Polycrystal) and metallic materials (type SUS304 stainless steel and Mo) were fabricated, using brazing alloy (Ag-Cu-Ti) sheet. Y-TZP disks having different Y2O3 contents, grain size were prepared for changing their transformability from tetragonal to monoclinic phase. Y-TZP disks with various thickness were joined with metal disks with constant thickness in order to change the thermal stress states. Transformation in Y-TZP was investigated by changing cooling rates from the joining temperature.Transformed fraction was larger under presence of tensile thermal stress (σrr). The transformed fraction decreased when cooled at a faster rate, which was related with time-dependent characteristics of the transformation in Y-TZP. A large fraction of transformation was detected in the coarse grained Y-TZP joined with Mo, although no transformation was detected in the unjoined state when cooled at the same rate. Transformation of Y-TZP joined with metallic materials was discussed, considering the effects of residual stress and the time dependent features of the transformation.