Hisayuki Suematsu

Magnetic properties of nanosize NiFe2O4 particles synthesized by pulsed wire discharge

Nanosize particles of nickel ferrite, NiFe2O4, have been successfully synthesized by pulsed wire discharge (PWD). In PWD, a simple circuit consisting of a capacitor and a gap switch drives the discharge. The wires of Ni and Fe were simultaneously discharged in a chamber filled with oxygen. The particles floating in the ambient gas were collected by pumping the gas through a membrane filter, and subjected to further analysis. The specific surface area of the particles were measured by the Brunauer-Emmet-Teller (BET) method. X-Ray diffraction showed the formation of NiFe2O4 and the inclusion of NiO. The NiO inclusion was estimated to be 18 vol.%. Magnetization hysteresis was measured for the particles synthesized at 600 torr. X-Ray diffraction and BET measurements reveal that particle size increases with increase in oxygen pressure. The saturation magnetization was found to be 33 emu/g for the particle with 45 nm in the size.

Materials modification using intense ion beams

Pulsed intense ion beams have been developed for applications including surface modification and alloying, and thin-film and nanopowder synthesis. Rapid thermal processing with ions is quite promising for large-scale commercial use, due to the high specific ion energy deposition (joules per cubic centimeter) without reflection, and to the relative efficiency and low cost of the pulsed power ion-beam drivers compared to other high-kinetic energy alternatives. We discuss in this paper the basis for the use of ions in materials processing and the methods of beam formation and impingement on material to be treated, and give examples of recent and ongoing work in materials processing.

Preparation of Cr(Nx,Oy) thin films by pulsed laser deposition

Abstract Chromium oxynitride [Cr(Nx,Oy)] thin films have been prepared by pulsed laser deposition (PLD). The thin films were prepared by depositing chromium metal vapor in a nitrogen ambient gas with residual oxygen. By composition analysis using X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy, it was found that the thin films contained approximately ∼46 at.% oxygen. In the Fourier transform infrared spectra, the peak originating from the CrN bond was found, and no peak originating from the CrO bond of the Cr2O3 phase was observed. X-ray diffraction results indicated that the thin film was in a B1 (NaCl) structure. From these results, it has been concluded that the Cr(Nx,Oy) thin film, which structurally resembles CrN where nitrogen atoms were partially substituted by oxygen, has been successfully produced. It was also found that the CrN phase maintains a B1 structure, even if 46 at.% oxygen is dissolved into the structure. By decreasing nitrogen pressure, the preferential orientation of the thin films changes from [200] to [111].

High-resolution electron microscopy of a SiC/SiC joint brazed by a Ag-Cu-Ti alloy

A high-resolution electron microscope observation (HREM) was performed on the joined portion of a brazed polycrystalline or single crystal SiC to itself with (Ag-28wt% Cu) + 2wt% Ti alloy foil. The brazing was done under vacuum at temperatures of 800° C to 950° C with a holding period of up to 30 min. Reaction products formed at the joined interface were found to be mainly TiC. In the specimen brazed at 800° C with the holding time of 0 min, reaction product TiC formed itself into small crystallites with a diameter of less than 20 nm, and an amorphous like layer was found between SiC and TiC. On the other hand, TiC was formed as a layer along the joined interface for the specimen brazed at 950° C for the holding time of 30 min. Lattice matching of SiC to TiC crystals appeared to be good so the high bonding strength of the joint was attributed to the formation of this epitaxial interface between SiC and TiC.

Enhancement of energy deposition in pulsed wire discharge for synthesis of nanosized powders

Nanosized particles, less than 100 nm in diameter, have been successfully synthesized by pulsed wire discharge (PWD). The powders prepared by PWD contained submicron-sized particles, in the range of 0.1 /spl mu/m-1 /spl mu/m in diameter. The existence of submicron-sized particles is attributed to liquid droplets due to lower energy deposition in the wire than vaporization energy of the whole wire. The effect of the energy deposition on particle size distribution was investigated with the copper powders prepared in the atmospheric gas of nitrogen. The energies deposited in the wire were estimated by measuring the currents and voltages for various discharge conditions. Under the conditions of high atmospheric pressure and fast-current rise, the energy deposition was significantly enhanced. The energy deposition mainly affects the quantity of submicron-sized particles that originates from unvaporized liquid droplets rather than the average particle size of nanosized powders.

High temperature strength of silicon nitride ceramics with ytterbium silicon oxynitride

Silicon nitride ceramics with ytterbium silicon oxynitride (Yb{sub 4}Si{sub 2}O{sub 7}N{sub 2}) as secondary phase were fabricated by hot-pressing the powder mixtures, including 50.0 to 97.0 mol{percent} of silicon nitride with a mixture of Yb{sub 2}O{sub 3} and SiO{sub 2} (Yb{sub 2}O{sub 3}/SiO{sub 2}=4). Sinterability of the materials with Yb{sub 2}O{sub 3} was higher than that with Y{sub 2}O{sub 3} in same composition of raw powder mixtures. High density materials was obtained under the condition of 50.0 to 89.1 mol{percent} of silicon nitride in raw powder mixtures. Mechanical properties of silicon nitride containing 97.6 mol{percent} of Si{sub 3}N{sub 4} and 2.4 mol{percent} of Yb{sub 4}Si{sub 2}O{sub 7}N{sub 2} were measured. Fracture toughness measured by indentation technique was 5.9 MPam{sup 1/2}. Bending strength at room temperature and at 1500{degree}C was 977 MPa and 484 MPa, respectively. The silicon nitride grains were consisted of highly elongated rod-like grains and thin needle-like grains. The Yb{sub 4}Si{sub 2}O{sub 7}N{sub 2} grains were crystallized at multigrain junctions and bonded close to Si{sub 3}N{sub 4} grains. High strength at high temperature is supposed to be based on the presence of crystalline Yb{sub 4}Si{sub 2}O{sub 7}N{sub 2} having high melting point. {copyright} {ital 1997 Materials Research Society.}

Enhancement of Nitridation in Synthesis of Aluminum Nitride Nanosize Powders by Pulsed Wire Discharge

High-purity aluminum nitride (AlN) nanosize powders were synthesized by pulsed wire discharge (PWD) of aluminum wires in nitrogen gas mixed with ammonia. The AlN content in the powders, which was determined by X-ray diffraction analysis, was increased by increasing the discharge energy. The highest AlN content of 98 wt% was obtained at the discharge energy of 360 J, and the average particle size was 30 nm. The increase in the AlN content at higher discharge energy can be explained by the energy deposited in the wire during the ohmic heating process. The number of micrometer-sized Al particles, which might be generated from liquid droplets, decreased by increasing the amount of energy deposited in the wire. The production rate of AlN powder was evaluated to be 46 g/kWh, which was 15 times higher than that obtained by the conventional arc discharge method.

Oxidation behavior of Cr-Al-N-O thin films prepared by pulsed laser deposition

Abstract Chromium aluminum oxynitride (Cr-Al-N-O) films have been successfully prepared by pulsed laser deposition (PLD). Experiments were carried out by changing the surface area ratio of the target [ S R = S AlN /( S Cr 2 N + S AlN )] under a pressure of 1×10 −5 torr. The composition of the film prepared at fluence of F =5 J/cm 2 and S R =75% was determined to be Cr 0.11 Al 0.39 N 0.25 O 0.25 by Rutherford backscattering spectroscopy (RBS). The oxidation of the Cr-Al-N-O film was observed above 900 °C. Additionally, the film heat-treated at 1100 °C consisted mainly of B1 (NaCl) structure. From the result of grazing angle X-ray diffractometry (GXRD), the oxidation resistant of Cr-Al-N-O film was found to be improved due to the fact that Cr 2 O 3 and α-Al 2 O 3 is formed near the film surface.

Strongly overdoped states and irreversibility-field characteristics of the Hg-1223 and Cu-1223:P superconducting cuprates

Essentially single-phase Hg-1223 and Cu-1223:P samples in wide hole-doping ranges were successfully synthesized by a high-pressure (5 GPa) technique utilizing internal and/or external oxidizing agents and by subsequent post-annealings performed in a thermobalance. The hole-doping level of the as-synthesized samples was found to be in the overdoped side for both of the phases by thermoelectric power measurements. values as low as 107 K and 67 K were obtained for the overdoped Hg-1223 and Cu-1223:P samples, respectively. The values of and were calculated from the measured M versus H data employing, respectively, the Bean model and two different criteria: (at ) and (as ). It was found that the heavier the doped hole concentration was, the more improved were the versus characteristics for both the Hg-1223 and the Cu-1223:P phases. This tendency was the same for versus characteristics calculated using either criterion. The improvement with increasing hole-doping level was especially profound in the case of Cu-1223:P: the versus characteristics of a Cu-1223:P sample with K, probably being close to the optimally doped state, were as poor as that reported for Bi-2212, while an overdoped Cu-1223:P sample with K was superior to an overdoped Hg-1223 sample with K. The mechanisms as well as the usefulness of the observed phenomena are discussed.

Photoluminescence properties of crystallized strontium aluminate thin films prepared by ion-beam evaporation

Abstract Photoluminescent SrAl 2 O 4 :Eu,Dy thin films were successfully prepared by a pulsed ion-beam evaporation method without substrate heating or sample annealing. The crystallinity of the films was observed by X-ray diffraction and large-area high-resolution electron microscopy. From the above results and photoluminescence spectra (PL), it was concluded that the intensity at the peak in PL spectra is related to the volume fraction of a crystallized SrAl 2 O 4 :Eu,Dy phase in the thin films.