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The magnetic properties and the domain structure of anisotropic melt-spun SmCo6.5Zr0.5 alloys with C addition was investigated by means of x-ray diffraction (XRD), magnetic measurement, and magnetic force microscopy. The XRD analyses showed that the addition of a few percent of C led to a significant increase in the coercivity and simultaneously affected the characterization of crystalline texture of the ribbons. The easy magnetization c axis changed from parallel to the ribbon plane for SmCo6.5Zr0.5 ribbons to normal to the ribbon plane for SmCo6.5Zr0.5C0.25−0.75 ribbons. An optimal coercivity of 0.92 T was obtained for the SmCo6.5Zr0.5C0.5 ribbon spun at 5 m/s. The corresponding remanence measured normal or parallel to the ribbon plane was 7.1 kGs or 3.1 kGs, respectively. The domain structure was studied by magnetic force microscopey. A strip-shaped domain was observed on the surface of the SmCo6.5Zr0.5 ribbons and the walls lay straight and parallel. For C-doped ribbons, the domain walls formed a maze domain pattern of grains with c axis normal to the ribbon plane. Scanning electron micrographs showed that a dendrite structure was present in the SmCoZr ribbon surface, and C addition caused the above-mentioned dendrite to diminish.
Thin epitaxial TiC and VC films and superlattices have been deposited on MgO(001) by simultaneous sputtering of the metals and evaporation of C60. It was found that epitaxial growth conditions for TiC could be maintained down to a temperature of 100 °C, while the epitaxial growth of VC required 200 °C. Epitaxial VC films were completely relaxed at all growth temperatures, while a change from a relaxed to a strained growth behavior was observed for TiC films. The structural quality of the TiC films was better than for the VC films. A general observation was that a plasma-assisted deposition process yields films with a higher quality and allows epitaxial growth at lower temperatures than for a pure coevaporation process.
The effects of boron addition on the microstructure and afterglow properties of the long-phosphorescent SrAl2O4:Eu2+,Dy3+ (SAED), synthesized via a novel sol-gel route, were systematically investigated. Significant improvement on luminescence intensity and the lengthening of afterglow persistent time in boron-added SAED (BSAED) phases were observed, as compared to those without boron addition and commercial phosphors. Typical bluish-green emissions attributed to the doublet phosphorescence with wavelengths peaking at 412 and 501 nm for BSAED phase and 398 and 486 nm for the pristine SAED phase were observed. Afterglow with wavelengths peaking at 403 and 485 nm was observed for BSAED phase, whereas that with wavelengths peaking at 486.5 nm was found for the pristine SAED phase, as indicated by time-dependent afterglow decay profiles. Results from scanning electron microscopic morphological studies were used to investigate the modification of microstructure of the BSAED phases.
Polycrystalline aluminas sintered with 10 wt% additions of calcium oxide (CaO) and silica (SiO2) in varying molar ratios were fabricated via precipitation, calcination, and hot pressing. Alumina microstructures were analyzed by scanning electron microscopy in terms of their mean grain size, grain size distribution, and grain aspect ratios. High-resolution transmission electron microscopy (HRTEM) showed the presence of an amorphous intergranular glassy phase at two- and three-grain boundaries. The intergranular film width at two-grain boundaries, determined by HRTEM, appeared to vary with the [CaO]:[SiO2] ratio of the additive as did the chemical composition and local chemistry, determined by high-resolution analytical transmission electron microscopy and scanning transmission electron microscopy (using both energy dispersive x-ray and electron energy loss spectroscopy). The factors influencing the erosive wear rate are discussed including the chemistry and associated fracture energy of the intergranular glassy film. Wet erosive wear rates of the densified materials were determined and had a strong dependence on the [CaO]:[SiO2] ratio in the additive.
The thermodynamic stability of tetragonal (t-) ZrO2 nanocrystallites below the bulk stability temperature 1200 °C was studied through specially synthesized crystallites that exhibited an extremely slow coarsening rate. The nanocrystallites were mechanically transformed to the monoclinic (m-) structure, and, because the crystallite size was kept below approximately 20 nm, the t-structure was completely recovered solely by thermal treatments between 900 and 1100 °C. These results gave strong evidence to the notion that, for sufficiently small crystallite size, nanocrystalline t-ZrO2 is not just kinetically metastable but can be truly thermodynamically more stable than the mpolymorph in air below 1200 °C.
The microstructures and phase compositions of Cu–Ag–Ti active-metal brazing alloys have been studied by scanning electron microscopy and energy dispersive x-ray spectroscopy to evaluate alloy wetting on AlN and Cu brazing on AlN. Titanium is segregated from the original alloy, and a Ti-rich layer is formed between the brazing alloy and AlN substrate. The alloy components are able to penetrate into the grain boundary of AlN during wetting or brazing, and the interfacial reaction takes place along the grain and outer boundary of AlN. The bonding of brazing alloys to AlN substrate often induces cracks in the AlN side.
Values for the thermal conductivity κ and the thermal diffusivity D of four oxide single crystals were obtained. Near room temperature, the values for κ (W cm−1 K−1) and D (cm2 s−1) are as follows: LaAlO3, κ = 0.115, D = 0.0446; NdGaO3, κ = 0.068, D = 0.0197 for one structural orientation, and κ = 0.059, D = 0.0195 for an orthogonal orientation; (LaAlO3)0.3–SrAl0.5Ta0.5O3, κ = 0.051, D = 0.0133; and, ScAlMgO4, κ = 0.062, D = 0.0229. The relative standard uncertainties in these values are ±10% (1 σ). These values allowed us to calculate the specific heat of the materials. The thermal conductivity was measured by a dc heated bar method, and the thermal diffusivity was measured by a modification of Ångström's method.
Large-scale SiO2 nanowires were synthesized by using a simple but an effective approach at low temperature. Scanning electron microscopy, transmission electron microscopy, and x-ray photoelectron spectroscopy were employed to characterize the samples. The results indicated that SiO2 nanowires with a uniform diameter of about 20 nm and a length up to 10 μm have been synthesized. Photoluminescence measurement showed that the SiO2 nanowires emitted blue light at 2.8 and 3.0 eV. The possible growth process of the SiO2 nanowires is discussed. Using this method, large panels of SiO2 nanowires can be made under conditions that are suitable for device fabrication.
Nuclear resonance reaction analysis has been applied for the first time to measure the development of the hydrogen depth profile in the early stages of hydration of tricalcium silicate using the 1H(15N,αγ)12C reaction. The surface layer had an H concentration and thickness consistent with a few unit cells (1.1 nm) of tobermorite-like material. The inner regions exhibited diffusion-controlled growth with time until the hydrogen concentration approaches that of the surface layer at 4.25 ± 0.07 h. This event marked the end of the induction period and the onset of the rapid hydration reaction period.
Electron backscatter diffraction (EBSD) has been applied to characterize Pb(Mg1/3Nb2/3)O3–35 mol%PbTiO3 single crystals grown by the seeded polycrystal conversion method. Macroscopically triangular crystal growth fronts were shown to each be associated with discrete crystals that originated from slightly misoriented segments of an initially cracked single-crystal seed plate. Various types of crystal imperfections, including voids, second-phase regions, and polycrystalline matrix grains trapped within the grown region, were readily identified and distinguished from one another using EBSD. Further, it was shown that trapped matrix grains in the grown region had consistently small misorientations with respect to the grown single crystal and this may be qualitatively explained by a simple boundary energetics argument. The significance of the trapped grains is discussed.
The results of an in situ transmission electron microscopy study of the formation of Co-silicides on patterned (001) Si substrates are discussed. It is shown that the results of the in situ heating experiments agreed very well with the data based on standard rapid thermal annealing experiments. Fast heating rates resulted in better definition of the silicide lines. Also, better lines were obtained for samples that received already a low-temperature ex situ anneal. A Ti cap layer gave rise to a higher degree of epitaxy in the CoSi2 silicide.
The structure and magnetic properties of Nd8.4Fe86Mo1.1B4.5 nanostructured magnets prepared by mechanical alloying (MA), compared with those by mechanical milling (MM), were studied. The intrinsic coercivity μ0Hc, the reduced remanence Jr/Js, and the maximum magnetic energy product (BH)max for Nd8.4Fe86Mo1.1B4.5 magnets were prepared by MM were notably higher than the values of the corresponding MA-prepared samples. The average grain sizes of both α–Fe and Nd2Fe14B in the MM-prepared samples were measurably smaller than corresponding values of the MA-prepared samples. A more homogeneous distribution of α–Fe grains in the MM-prepared samples than in the MA-prepared samples was obtained.
The creep deformation in fine-grained polycrystalline Al2O3 is highly suppressed by the addition of 0.1 mol% LuO1.5. The transient creep behavior in Lu-doped Al2O3 was examined at the testing temperature of 1250–1350 °C, and the data were analyzed in terms of the effect of stress and temperature on the extent of transient time and strain. The experimental data on the transient creep in Lu-doped Al2O3 showed good agreement with the prediction from a time function of the transient and the steady-state creep associated with grain boundary sliding as well as an undoped one. The difference in the transient creep between Lu-doped and undoped Al2O3 can also be explained by the retardation of grain boundary diffusion due to the Lu3+ ions segregation in the grain boundaries.
The statistical behavior of ceramic and metal laminates was studied, and in particular the relationship between the failure distribution of the laminate and that of the ceramic layers that it comprises. An important result of this investigation is the ability to predict the misfit stresses due to thermal processing of the laminates. A finite different scheme that was capable of calculating the local stresses at each constituent and of considering the thermal residual stresses was developed. The laminated system was constructed from 380-μm-thick alumina alternating with nickel foil 25- or 50-μm thick.
We report here on a series of experiments in which relatively low levels of in-plane bending strain were applied to oxidizing silicon substrates. These were found to result in significant decreases in oxide thickness in the ultrathin oxide regime. Both tensile and compressive bending resulted in roughly the same degree of thickness retardation, although compressive bending typically led to somewhat thinner oxides than did tensile bending. An examination of the experimental data indicate that the principal effect seems to occur in the very early stages of oxidation, with only minor effects on subsequent oxide growth. We hypothesize that the observed oxide thickness retardation is related to straining of the underlying silicon lattice at the oxidation front.
Some of the most interesting compounds from both technological and a scientific viewpoint can be found within the B–C–N composition triangle. Despite all the attention that some phases on the vertices and sides of the triangle have attracted, few works have focused inside the triangle itself. A laser-assisted chemical vapor deposition system was used to deposit B–C–N phases over fused quartz substrates. Two sets of gaseous precursors were used, namely B2H6 + NH3 + C2H4 and B2H6 + (CH3)2NH. The coatings were characterized regarding chemical composition, structure, and morphology. Hardness measurements were also carried out with a depth-sensing indentation instrument. It was found that depending on the gas phase, different regions of the BCN solid composition triangle are accessed. Coatings ranging from pure h-BN to pure B4C were obtained, as well as mixtures of these with BxCyNz compounds.
Morphological evolution of cobalt germanide epilayers, CoxGey, was investigated in situ by scanning tunneling microscopy and spectroscopy and reflection high-energy electron diffraction, as a function of deposition method and, hence, the phase content of the epilayer. During reactive deposition epitaxy, in which Co atoms were evaporated onto a flat pseudomorphic Ge/Si(001) wetting layer at 773 K, the first phase formed was cobalt digermanide, CoGe2, in the form of elongated pyramidal islands. Each of these three-dimensional islands has locally exerted an additional strain on the Ge wetting layer already strained at the Ge/Si(001) interface, lifting the layer metastability and causing, in turn, the formation of three-dimensional Ge pyramids underneath every CoGe2 island. Solid-phase epitaxy of Co onto the same Ge/Si(001) epilayer resulted in the formation of more Co-rich germanide islands. Coupling of strain from these germanides to the epitaxial Ge/Si(001) strain has also facilitated a two-dimensional-to-three-dimensional transition of the Ge layer, however, with the germanide islands located at the Ge pyramid troughs, rather than crests. The difference in the relative location of germanide and germanium islands in these two cases is explained by accommodation of the large lattice-constant germanides at the more relaxed regions of the Ge pyramid crests and the smaller lattice-constant at the compressed Ge pyramid troughs.
A ceramic/metal laminated system has lately been proposed by the authors. It is capable of maintaining high mechanical strength and structural integrity after high-temperature thermal shock. In this investigation, a multilayered, multimaterial system with strong interface, subjected to thermal shock loading, was analyzed. The analysis was based on a 1-D finite difference scheme and considers the thermal residual stresses. Using a failure criterion based on crack initiation, the number of broken layers due to thermal shock and the residual mechanical strength at room temperature was determined. A comparison with experimental results of three different lay-ups was made, demonstrating the ability of the program to predict the experimental results. The program was thus shown to be a significant tool for designing multimaterial multilayered systems for thermal shock applications.
A composite film of titanium dioxide (TiO2) nanoparticles and hydrolyzed styrene–maleic anhydride alternating copolymer (HSMA) was obtained on a substrate when a TiCl4 solution was heated at 80 °C with a spin-cast thin HSMA film present in the solution. The composite film was characterized with x-ray photoelectron spectroscopy and transmission electron microscopy. Results revealed that TiO2 nanoparticles discretely dispersed on the polymer layer, and they were dominantly rutile phase, of a spherical shape and 18–20 nm in diameter. In contrast, mainly amorphous TiO2 powders were obtained from the identical TiCl4 solution by drying the solution with the absence of the HSMA film. The TiO2 nanoparticles deposited on the polymer layer were regarded to contain polymer chains, and a multilayered core–shell model was suggested for the formation of these composite nanoparticles. It is regarded that the core of a composite particle consisted of an anatase-phase TiO2 colloidal nanoparticle, while the shell layer was made of rutile-phase TiO2/polymer multilayers; the composite particles formed by a layer-by-layer self-assembly of TiO2 and polymer layers analogous to biomineralization, where the polymer promoted the crystallization of rutile-phase TiO2 when TiO2 deposited from solution.
Electrical transport properties, electrical conductivity, and thermoelectric power of a single-crystalline Mn0.45Zn0.43Fe2.12O4 were measured as functions of temperature in the range of 25 to 1000 °C. According to the small polaron hopping model, the values of the activation energy for small polaron hopping (EH) were obtained from the conductivity data in three different temperature regions: 0.032 eV for T < TC, 0.12 eV for TC < T < 600 °C, and 0.25 eV for 600 °C < T < 1000 °C. The behavior of conductivity and thermoelectric power data above TC is discussed in connection with cation redistribution.