Fabrication and characterization of rare earth hexaboride single-crystal materials

Oriented single crystals of LaB₆, CeB₆ and PrB₆ have been prepared by the arc float-zone refining technique, which is described. A critique of the various possible preparation techniques is given. Wet chemical methods were used to determine bulk stoichiometries of samples prepared from starting materials of various R/B compositions (where R = rare earth). It was found that LaB_6,09, a desired composition, could be grown in single-crystal form from LaB_6.2 starting material, the excess boron compensating for preferential boron vaporization. Starting materials with B/La < 6.2 produce a B/La gradient in two-pass zone-refined specimens. The level of metallic impurities in arc zone-refined LaB₆ is lower than that reported elsewhere for laser zone-refined LaB₆. Residual oxygen present in the zone-refined specimens was found to produce second-phase inclusions, which were studied in detail. The effect of contamination of LaB₆ surfaces by evaporated refractory metals has also been investigated.     

Co掺杂CeO2稀磁氧化物陶瓷和纳米颗粒的铁磁性能

分别采用固相烧结法及激光液相烧蚀(LAL)技术,成功制备出Co掺杂CeO_2稀磁氧化物陶瓷块体和纳米颗粒。XRD和SEM研究发现所制备的材料具有良好的结晶性和形貌。Co掺杂CeO_2稀磁氧化物陶瓷块体和纳米颗粒均为多晶立方结构,与纯立方相的CeO_2结构相同,说明Co掺杂未形成其他结构和杂相。磁性测量表明固相烧结法和激光烧蚀液相法制备的Co掺杂CeO_2样品均具有较高的室温铁磁性,且远高于文献中报道的结果。将陶瓷块材经激光烧蚀成纳米颗粒后,纳米颗粒的铁磁性与陶瓷块材保持一致。这说明激光烧蚀法制备的纳米材料可以很好地保持母材的特性,是一种很好的纳米颗粒制备方法。根据XRD和SEM研究结果,笔者认为Co掺杂CeO_2陶瓷块材及纳米颗粒的室温铁磁性是内禀性质;磁性产生的机理源于氧空位诱导的铁磁性耦合。     

Nb3Al superconducting tape prepared by CO2 laser beam irradiation

A15 Nb₃AI tape superconductors have been prepared by high power and continuous wave CO₂ laser irradiation onto moving NbAl composite tapes made by the powder method. The irradiated area was melted and then immediately resolidified, which resulted in a dendritic microstructure consisting of Nb dendrites embedded in a Nb₃AI matrix. Irradiated only specimens showed critical temperatures, T_c, >16.0 K and subsequent heat treatment at 750°C enhanced T_c to 18.6 K. For laser irradiated and then heat treated specimens, critical currents, I_c, were little decreased, even at >20 T, compared with specimens which were only heat treated. A maximum critical current density, J_c, for the reacted area of 4.8 × 10⁴ A cm⁻² was obtained at 23 T and 4.2 K.     

Dynamic fracture behaviour of Fe78Si9B13 metallic glass ribbon under laser shock loading

The fracture behaviour of Fe₇₈Si₉B₁₃ metallic glass under laser shock loading was investigated. Morphologies of the fracture surface and laser irradiated surface were characterized using scanning electron microscope. The results show that the fracture surface consists of sliding region and final fracture region with crack propagation. Liquid droplets and melted belts are scattered on the fracture surface as the notable features compared with fracture surface morphology under quasistatic loading, indicating the significant temperature increase in shear bands during dynamic loading. The primary and secondary shear bands are distributed on the specimen surface resulting from the simultaneous operation of multiple shear bands at high strain rates. Ripples with the characteristic spacing of about 1 µm are generated on the laser irradiated surface because of the interaction of laser pulse with solid surface.     

Effects of substrate temperature on Bi0.8La0.2FeO3 thin films prepared by pulsed laser deposition

Bi_0.8La_0.2FeO₃ thin films on Pt/TiO₂/SiO₂/Si substrates at various substrate temperatures from 500 °C to 750 °C are prepared by pulsed laser deposition, and their microstructures and ferroelectric/magnetic properties are carefully investigated using various techniques. It is observed that the crystallographic orientation and Fe-ion valence state depend significantly on the substrate temperature, which consequently influences considerably on the ferroelectric and magnetic properties of the thin films. A considerable improvement of the ferroelectric and magnetic properties of the thin films can be achieved by optimizing the substrate temperature for deposition.     

Continuous laser irradiation under ambient conditions: A simple way for the space-selective growth of gold nanoparticles inside a silica monolith

Thanks to the potential and various applications of metal-dielectric nanocomposites, their syntheses constitute an interesting subject in material research. In this work, we demonstrate the achievement of gold nanocrystals growth through a visible and continuous laser irradiation. The in situ and direct space-selective generation of metallic nanoparticles is localized under the surface within transparent silica monoliths. For that purpose, the porous silica monoliths are prepared using a sol-gel route and post-doped with gold precursors before the irradiation. The presence of Au nanoparticles inside the irradiated areas was evidenced using absorption spectroscopy, X-ray diffraction analysis and transmission electron microscopy. The comparison between the results obtained after a laser irradiation and by a simple heat-treatment reveals that the local precipitation of gold nanoparticles by continuous photo-irradiation occurs following a photo-thermal activated mechanism.     

Pr2/3Sr1/3MnO3薄膜外场诱导自旋输运特性

采用脉冲激光沉积法制备了稀土掺杂钙钛矿锰氧化物Pr2/3Sr1/3MnO3(PSMO)外延薄膜,研究了薄膜在磁场、激光和电流作用下的自旋输运特性.在低温铁磁金属相,激光作用使薄膜的电阻增大,而磁场和电流则诱导电阻减小;在高温顺磁绝缘态,外场诱导均使电阻减小.在铁磁金属相,外场诱导输运特性的变化可归结于外场对体系电子自旋系统的影响:磁场和电流加强材料中eg电子和t2g局域电子间的自旋平行,增强了双交换作用;激光作用可产生光致退磁效应,减弱双交换作用.在顺磁绝缘态,场致电阻降低源于外场致使小极化子的退局域化效应.     

Out‐of‐Plane Magnetic Patterning Based on Indentation‐Induced Nanocrystallization of a Metallic Glass

Periodic arrays of micrometer‐sized ferromagnetic structures with perpendicular magnetic anisotropy are prepared by nanoindentation at the surface of a Fe_67.7B₂₀Cr₁₂Nb_0.3 glassy ribbon initially showing in‐plane magnetic anisotropy. The indented regions exhibit enhanced coercivity and saturation magnetization with respect to the surrounding nondeformed matrix. These effects are due to a mechanically driven selective nanocrystallization of the metallic glass, induced by nanoindentation, even without the need for thermal annealing. In addition, while the amorphous matrix becomes paramagnetic above 325 K, the crystallized regions (consisting of α‐Fe) remain ferromagnetic upon heating to high temperatures. The local change in the magnetic anisotropy direction is ascribed to a certain degree of crystallographic texture, together with the inverse magnetostriction effect caused by the compressive indentation stresses.     

Growth of epitaxial γ-Al2O3 films on rigid single-crystal ceramic substrates and flexible,single-crystal-like metallic substrates by pulsed laser deposition

Epitaxial γ-Al₂O₃ thin films were grown on diverse substrates using pulsed laser deposition. The high quality of epitaxial growth and cubic structure of γ-Al₂O₃ films was confirmed by X-ray diffraction. SrTiO₃ and MgO single crystal substrates were used to optimize the growth conditions for epitaxial γ-Al₂O₃ film. Under the optimized conditions, epitaxial γ-Al₂O₃ thin films were grown on flexible, single-crystal-like, metallic templates. These included untextured Hastelloy substrates with a biaxially textured MgO layer deposited using ion-beam-assisted-deposition and biaxially textured Ni-W metallic tapes with epitaxially grown and a biaxially textured, MgO buffer layer. These biaxially textured, γ-Al₂O₃ films on flexible, single-crystal-like substrates are promising for subsequent epitaxial growth of various complex oxide films used for electrical, magnetic and electronic device applications.     

Characteristic and magnetic properties of Ni1–xZnxFe2O4 ferrites

Ferrites are magnetic ceramic materials which have additional metallic ion in ferrous oxide compounds. Ferrites are usually classified as soft or hard ferrites. In this study, characteristics and magnetic properties of magnetic materials having NiO_1–xZnO_xFe₂O₄ structure were investigated. Mechanical mixing of high purity NiO, ZnO and Fe₂O₃ powders were done to obtain homogenous NiO_1–xZnO_xFe₂O₄ powder mixture for x = 0.15, x = 0.50 and x = 0.85. These powder mixtures were pressed using hydraulic press machine and then subjected to sintering at same temperatures of 1000 °C for 1 hour. Obtained specimens were analyzed with scanning electron microscopy (SEM) imaging and energy dispersive X‐ray fluorescence (EDXRF) technique for the investigation of structural analysis; magnetic properties were determined using vibrating sample magnetometer (VSM). However, effects of composition, specimens and Zn% element in magnetic materials after energy dispersive X‐ray fluorescence on maximum magnetic moment (M_s) were analyzed using Taguchi orthogonal array design of experiments technique. The study indicates that Zn% element is the main process parameter that has the highest statistical influence on maximum magnetic moment. However, another parameter, composition, also has a significant effect on maximum magnetic moment. Then, Zn‐content was found to have a significant influence on the magnetic properties of the system.     

Synthesis and Characterization of Ferromagnetic Nickel Nanoparticles

Nickel nanoparticles on mass production scale have been prepared using a modified polyol process with Ni(CH₃COO)₂?4H₂O, NaOH, 1,2 propandiol and hydrazinium hydroxide (N₂H₄?H₂O). A?mixture of face centered cubic (fcc) metallic nickel nanoparticles with 12 nm diameter were obtained. We have experimentally studied the structure of nanoparticle by X-ray and Scanning Electron Microscopy (SEM, EDS). The magnetic properties of the prepared Ni film have been studied by Ferromagnetic Resonance (FMR) technique, Vibrating Sample Magnetometer (VSM) and Vector Network Analyzer (VNA). The effective g-factor and magnetic anisotropy constant were determined as g _eff=2.25 and K _eff=85?Oe, respectively.     

Fabrication of multi-filamentary Y123 superconductor

The precursor Y₁Ba₂Cu₃O_x (Y123) filaments were prepared by solution spinning through aqueous poly(vinyl alcohol) solution containing mixed Y, Ba and Cu acetates. The as-drawn filaments were heated to remove volatile components and to generate a superconducting phase. The filamentary superconductors were passed through Ag paste and sintered. By controlling the heating condition, the composite with excellent connection between the filaments and metallic Ag matrix was obtained. The overall transport critical current density (J_e) of more than 2000 A/cm² at 77 K and 0 T was achieved for the composite with partially melted Y123 filaments. Although the J_e value decreased by applying a low magnetic field less than 0.1 T, the superconductivity of the composite maintained at 10 T at 77 K.     

Phase-precipitation studies of Fe-B-Si metallic glasses using Mössbauer spectroscopy

Phase-precipitation studies have been performed on samples of the metallic glasses Fe₇₉B₁₆Si₅ and Fe₇₈B₁₃Si₉, heated in the range 300–475 °C for various times (1–16 h) using ⁵⁷Fe Mössbauer transmission spectroscopy and X-ray diffraction methods. These measurements have helped in identifying the temperature ranges and annealing durations in which the amorphous structure of these metallic glasses is retained. The results revealed that the thermal stability increases as boron is replaced by silicon in the Fe-B-Si metallic glasses and that these alloys remain amorphous below 450 °C. The various phases precipitated above this temperature were identified as -Fe, -(Fe, Si), Fe₃B, and Fe₂B. The direction of magnetization in the two metallic glasses appears to change upon annealing.     

Effect of laser irradiation on superconducting properties of bulk MgB<Subscript>2</Subscript> sintered at different temperatures

The surfaces of MgB₂ bulk samples produced by a pellet/closed tube method at two different sintering temperatures of 650 and 850?°C, after hot pressing at 200?°C, were irradiated with the same irradiation dose by using an Nd:YVO₄ laser in order to study the possible potentiality of laser irradiation to improve pinning performances and critical current density of MgB₂ superconductor. The measurements showed that the magnetic field dependence of the critical current density values of irradiated sample sintered at 650?°C slightly increased with a narrowing in superconducting transition region as compared to the reference sample sintered at same temperature. However, irradiated sample sintered at 850?°C showed a decrease in pinning performance and similar critical temperature values as compared to the corresponding reference sample. From these results it can be said that the same laser irradiation dose affects superconducting properties of bulk MgB₂ in different ways depending on sintering temperature of the superconductor.     

Effects of ultrasonic processing on phase transition of flame-synthesized anatase TiO2 nanoparticles

The effect of ultrasonic processing on the phase transformation of flame-synthesized anatase TiO₂ nanoparticles heated to the rutile phase was investigated. TiO₂ nanoparticles of various sizes were prepared using a coflow hydrogen diffusion flame and an ultrasonic processor. Smaller nanoparticles having a similar portion of anatase phase using the ultrasonic processor were produced. On the basis of scanning electron microscopy images and specific surface areas, we observed that smaller nanoparticles tended to be sintered more easily than larger nanoparticles. From X-ray diffraction analysis, we demonstrated that when heated, TiO₂ nanoparticles synthesized using the ultrasonic processor at 60% of its maximum amplitude were transformed from the anatase phase to the rutile phase more easily than those formed without or with the ultrasonic processor operated at 20% of its maximum amplitude.     

A laser optoacoustic method of inducing high-energy states and the investigation of phase transitions in metals at high pressures

Results are given of investigations of high-energy states and phase transitions, using lead as an example, under the effect of nanosecond laser pulse in a wide range of intensities. The metal surface being irradiated is mechanically confined by a transparent dielectric plate in order to realize conditions of highly effective generation of pressure while maintaining local thermodynamic equilibrium. The dynamics of thermodynamic state of metal are analyzed by the shape and amplitude of pressure pulse propagating from the surface being heated and by the variation of reflectivity of the metal surface being irradiated.The measurements of pressure pulse make possible the recording of phase transitions in lead: melting on the sample surfaces being irradiated at a pressure up to P _max ≈ 0.1 GPa. It is demonstrated that the pressure amplitude prior to melting is proportional to the intensity of laser radiation, and that upon passing the melting threshold — to the absorbed energy of laser pulse. The instant of time, at which the “deformation” of the leading edge of pressure pulse begins, corresponds to the beginning of melting. When high-energy states are realized in the thin surface layer of metal, the density decreases significantly; this results in a significant decrease in both reflectivity and electrical conductivity.     

Trivalent Ce2O3 and CeO2−x intermediate oxides induced by laser irradiation of CeO2 powders

A strong non-stoichiometry of pure fcc CeO₂ was induced by laser irradiation. The increase of laser power and/or energy density had a saturable effect on particle size growth. The possibility of CeO₂ reduction to A-Ce₂O₃ by laser irradiation was demonstrated. Particles of stable Ce₇O₁₂ phase were observed in all specimens irradiated at low laser-power densities. An epitaxial relationship between triclinic Ce₁₁O₂₀ and cubic Ce₁₂O₂₂ phases was found. The controversial C-Ce₂O₃ phase was detected at the limits of a bcc particle. An unknown bcc phase of acicular morphology, strongly related to C-Ce₂O₃, was also registered. The dose dependence of CeO₂ structural modifications obtained by laser irradiation as a function of laser energy density variation could be explained by a simple defect aggregation model implying lattice defects (oxygen vacancies and Ce³⁺ ions).     

Fabrication of Fe-based bulk metallic glass by selective laser melting: A parameter study

Soft magnetic Fe-based bulk metallic glass cylindrical specimens with a diameter of 2 mm and height of 6 mm have been successfully fabricated by selective laser melting (SLM) and the effect of scan speed v and laser power P on the microstructure, thermal stability and soft magnetic properties has been investigated. The results indicate that low v and high P lead to the formation of SLM samples with high relative densities, which can reach values of about 99.7%. This can be ascribed to the optimal energy transfer during processing at low v and high P. Structural and calorimetric studies prove that the SLM samples are fully amorphous. In addition, magnetic measurements reveal that the amorphous structure of the SLM material is identical to the parent atomized powders. Although additional work is required to remove the residual porosity and to avoid the formation of cracks during processing, the present results confirm that additive manufacturing by SLM represents an alternative processing route for the preparation of bulk metallic glass components with designed geometry having excellent magnetic softness.     

Reactive sputtering of titanium and properties of titanium suboxide films for photochemical applications

The effect of oxygen partial pressure on the deposition rate, crystalline structure and optical absorption of thin film titanium suboxides prepared by the reactive sputtering of a titanium metal target is given. A wide variety of films ranging from metallic through semiconducting to dielectric specimens were deposited in a reproducible manner simply by controlling the oxygen content in the sputtering plasma atmosphere. In addition, polycrystalline stable semiconducting TiO₂ electrodes were deposited onto heated glass substrates. The spectral response was investigated; a main absorption edge of about 410 nm was obtained. The design of a special substrate table with a unique mask changer that allowed for the fabrication of different geometrical film patterns is also given.     

XAFS and CEMS study of dilute magneto-optical semiconductor, Fe doped TiO2 films

TiO₂ films doped with 6% Fe were prepared by pulsed laser deposition (PLD) under different oxygen pressures, and characterized by X-ray absorption fine spectra (XAFS) and conversion electron Mössbauer spectra (CEMS). The edge energy and spectrum profiles of Fe- and Ti K X-ray absorption showed only Fe³⁺ and Ti⁴⁺ states for rutile TiO₂ films prepared under 10^− 1 Torr, the metallic Fe and Ti⁴⁺ for rutile TiO₂ films prepared in 10^− 6 Torr, and the metallic Fe and the average valance of less than “4+” for Ti in Ti_nO_2n−x films prepared by the PLD under 10^− 8 Torr. The metallic Fe clusters are also found in the TEM images of Ti_nO_2n−x film. Magnetic property of Fe doped TiO₂ films prepared by PLD at high vacuum (10^− 6 and 10^− 8 Torr) is considered to originate mainly from the magnetic metal iron clusters.