Materials Science Forum Vols. 561-565

Abstract: Free surface motion of a liquid metal submerged in an alternating magnetic field has been examined. A copper vessel filled with a liquid gallium is set in a coil for the imposition of the alternating magnetic field. The alternating magnetic field penetrates into a liquid gallium only from an upper free surface because thickness of the copper vessel is larger than the electromagnetic skin layer of copper. Time variation of displacement of the standing wave loop excited on the free surface is detected by a laser level sensor. The standing wave was suppressed not only by intensification of the magnetic field magnitude but also increase of the magnetic field frequency.

Abstract: This work presents an investigation of the spray forming and downstream processing of Al alloys that are difficult to produce in bulk by conventional solidification processing: Al-Fe-Cr-Ti alloys for intermediate temperature applications and Al-Si-Li alloys for high stiffness, low density applications in fast moving machinery. For the Al-Fe-Cr-Ti alloys, spray forming is being investigated to allow the scale-up of alloy compositions previously explored only as ribbons or powders in traditional rapid solidification routes. For Al-Si-Li alloys, spray forming is used to provide globular primary AlLiSi in a fully divorced AlLiSi/α-Al eutectic structure. For both alloys, the as spray formed and downstream processed microstructure of 20kg billets has been investigated by scanning electron microscopy, electron probe microanalysis, and X-ray diffractometry. Preliminary mechanical properties have also been investigated.

Abstract: The lithium ions doped red-emitting phosphors of (Ca,Eu)WO4 and (Ca,Eu)MoO4 were synthesized by solid-state reaction method, and their luminescent properties were investigated. The XRD patterns show that the phosphors are isostructural and share a tetragonal scheelite structure, even some lithium ions are doped. The excitation spectra consist of broad charge transfer bands in the short-wave UV region of W6+→O2- and Eu3+→O2- with three sharp lines around 395, 465 and 535 nm of the Eu3+ 4f excitation transitions. The characteristic emissions of WO4 2- and MoO4 2- are quenched absolutely and red light emission of Eu3+ exhibits predominate peak around 615 nm due to the electric dipole energy transition of 5D0→7F2. Excited with 395 nm, the evident enhanced emission of the phosphors were observed when Li+ doped in the host, which implies that the addition of some Li+ is beneficial for the energy transfer from WO4 2- or MoO4 2- to Eu3+.

Abstract: Using nitrate precursors, a novel microwave-assisted progress for the preparation of nanocrystalline pure YFeO3 phase has been developed. Synthesis of perovskite YFeO3 has been achieved with carbon to enhance microwave absorption, polyvinyl alcohol as inhibitor and urea as homogeneous precipitator. The precursor led to formation of phase pure YFeO3 nanocrystalline (52-64 nm) after calcination. The obtained YFeO3 samples were characterized by XRD analysis, N2 adsorption, FT-IR, SEM, TEM, UV-vis/DRS. The synthesis is a fast, simple and inexpensive process and such nanosized YFeO3 has a potential application in photocatalysis under visible light irradiation due to its structure of perovskite-type and photo-absorption property.

Abstract: The single phase of a cubic NaZn13-type La1-zNdz(Fe0.88Si0.12)13 is obtained in the region z ≤ 0.2. The field-induced first-order transition from the paramagnetic to ferromagnetic state, that is, the itinerant-electron metamagnetic (IEM) transition is kept after the substitution of Nd. In addition, a discontinuous change of magnetization at the Cure temperature becomes larger with increasing z. As a result, the isothermal magnetic entropy change and the relative cooling power in the magnetic field change from 0 to 5 T increase to –27 J/kg K and 518 J/kg, respectively, by the partial substitution of z = 0.2.

Abstract: The carbon-coated Fe(C), Co(C) and Ni(C) nanocapsules were prepared by a modified arc-discharge method in methane atmosphere. The nanocapsule powder, 50 % by weight, was mixed uniformly with paraffin wax to form measurement samples. Their electromagnetic (EM) wave reflection loss of the composite samples was calculated using the relative complex permeability and permittivity measured in microwave frequency range of 2-18 GHz. It is indicated that Fe(C), Co(C) and Ni(C) nanocapsules show the broadband characteristics with strong absorption from 2 to 18 GHz for a coating thickness of about 2-3 mm. The excellent EM wave absorption properties are mainly attributed to the proper electromagnetic match in microstructure, strong natural resonance as well as multi-polarization mechanisms, etc. As an inspiration, we found this kind of nanocapsules with a dielectric shell and a ferromagnetic core are very promising for new EM wave absorption materials.

Abstract: Electron beam induced-deposition (EBID) is a promising technique for fabricating nanometer-sized structures in a position- and size-controlled manner. The resolution of EBID is now reaching down to subnanometers. However, the deposits obtained by EBID contain a large amount of carbon. Thus, carbon reduction techniques are needed. In this study, nanostructures, such as nanowires, were fabricated by EBID using an iron pentacarbonyl precursor. Several techniques to reduce carbon were applied, including post-deposition heat-treatments and the modification of precursor. It was found that the post-deposition heat-treatment in air resulted in a formation of Fe2O3, and that carbon-free Fe3O4 was formed by mixing a small amount of water vapor in the iron pentacarbonyl precursor.

Abstract: The influence of Mn content on the structure and magnetic permeability of Nd3Fe68-xMnxCo18B11 (x = 0, 1, 2) amorphous soft magnetic alloys was studied. The ribbons were prepared by melt-spining. The permittivity–frequency and permeability–frequency properties were determined in the microwave frequency regime of 2-18 GHz by vector network analysis. XRD spectra showed that only α-Fe diffraction peak was observed in the as-spun alloy. It is found that the acquired complex permittivity and permeability values match the microwave frequency when the 1at% Mn content doped. The results suggest a new design of microwave absorbers based on electromagnetic wave absorbing materials.

Abstract: The magnetic field strength, Hs, at which rearrangement of martensite variants initiates has been investigated in Ni2MnGa ferromagnetic shape memory alloy by magnetization measurements in the [001]P direction ("P" stands for the parent phase). We have also calculated Hs from the magnetocrystalline anisotropy constant Ku, spontaneous magnetization Ms, twinning shear s and twinning stress τreq by considering the condition for the rearrangement of martensite variants reported previously [Int. J. Appl. Electromagnetics and Mechanics, 23 (2006) 45]. The calculated value of Hs is in good agreement with the experimental value for all the examined temperatures. The agreement confirms the applicability of the reported condition.