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A single layer of La2Zr2O7 (LZO), deposited on textured Ni and Ni–1.7% Fe–3% W (Ni–W) tapes by a low-cost sol-gel process, is used as buffer layer for the growth of YBa2Cu3O7−δ (YBCO) coated conductors. It is shown for the first time that such single buffer layers can be used for the deposition of YBCO yielding critical current densities (Jc) that are comparable to those typically obtained using CeO2/YSZ/Y2O2 trilayers on identical substrates, i.e., in excess of 1 MA/cm2 at 77 K and self-field. The properties of the YBCO films and the dependence of Jc on thickness of the LZO layer are investigated.
The microstructure of a low-carbon steel after high current density electropulsing treatment was characterized by high-resolution transmission electron microscopy. It was found that nanostructured γ-Fe could be formed in the coarse-grained steel after the electropulsing treatment. The mechanism of the formation of a nanostructure was discussed. It was thought that change of the thermodynamic barrier during phase transformation under electropulsing was a factor that cannot be neglected. It was reasonable to anticipate that a new method might be developed to produce nanostructured materials directly from the conventional coarse-grained crystalline materials by applying high current density electropulsing.
Molecular dynamics (MD) were employed in atomic-level simulations of fundamental damage production processes due to multiple ion–solid collision events in cubic SiC. Isolated collision cascades produce single interstitials, vacancies, antisite defects, and small defect clusters. As the number of cascades (or equivalent dose) increases, the concentration of defects increases, and the collision cascades begin to overlap. The coalescence of defects and clusters with increasing dose is an important mechanism leading to amorphization in SiC and is consistent with the homogeneous amorphization process observed experimentally in SiC. The driving force for the crystalline– amorphous (c–a) transition is the accumulation of both interstitials and antisite defects. High-resolution transmission electron microscopy (HRTEM) images of the defect accumulation process and loss of long-range order in the MD simulation cell are consistent with experimental HRTEM images and disorder measurements. Thus, the MD simulations provide atomic-level insights into the interpretation of experimentally observed features associated with multiple ion–solid collision events in SiC.
In flip-chip packages, the effect of Ni metallization on the substrate side on interfacial reactions between solders and an Al/Ni(V)/Cu under-bump metallization (UBM) on the chip side was investigated during the reflow process. The Ni substrate metallization greatly accelerated interfacial reactions on the chip side and quickly degraded the thermal stability of the UBM due to a fast consumption of the Ni(V) layer. This phenomenon can be explained in terms of rapid Ni or Sn diffusion in the ternary (Cu,Ni)6Sn5 phase, which was formed in the solder adjacent to the Ni(V) layer and the enhanced dissolution of (Cu,Ni)6Sn5 into the molten solder. Without the Ni metallization on the substrate side, the Al/Ni(V)/Cu UBM remained very stable with both eutectic SnPb and Pb-free solders.
The wear behavior of bulk Zr41Ti14Cu12.5Ni10Be22.5 metallic glasses has been studiedusing sliding wear tests and scanning electron microscopy in both as-prepared and annealed samples. It was found that the wear resistance of differently processed samples increases in the following order: crystallized state; as-prepared state; relaxed state. The thermal stability of worn samples was also investigated by means of differential scanning calorimetry. Under the experiment conditions, no sliding wear-induced crystallization is observed in either as-prepared or relaxed samples indicating good thermal stability of the bulk metallic glasses.
Nanocrystalline KTiOPO4 powders were prepared through a chemical process. This process involved the hydrolysis of KOOCCH3 · 2H2O, Ti(OC4H9)4, and PO(OR)3 to produce a homogeneous solution. A gel was formed by the partial evaporation of this solution. After the gel was decomposed at 450 °C, white amorphous powder remained. On calcinating up to 550 °C, the amorphous powder began to transform into nanocrystalline KTiOPO4 powders with an average particle size of 30–50 nm. The KTiOPO4 powders were investigated through x-ray diffraction, infrared spectroscopy, and transmission electron microscopy studies.
The effect of BaO addition on the crystallization of Li2O–Al2O3–ZnO–SiO2–TiO2– ZrO2 (LAZSTZ) glass was investigated and discussed. The crystallization temperature of BaO-added LAZSTZ precursor glass (LBAZSTZ) was decreased due to the increase in the number of broken ionic bonds, while the transparency of the glass–ceramic was maintained. The lowered activation energy for crystallization in LBAZSTZ glass strongly supports the increase in the number broken bonds. During heating LBAZSTZ glass, phase separation did not occur most probably due to the competition between metal cations having a certain degree of ionic field strength. The formation of β-spodumene as a major phase in LBAZSTZ glass–ceramic was identified using x-ray diffraction.
A new technique of surface modification by diffusion coating for AZ91D alloy was developed. A 1.0–2.0-mm alloy layer, which has hardness four to five times higher than the substrate metal, was formed after the treatment. Consequent solution treatment and aging could further improve the hardness of the alloy layer. Microstructure and chemical composition were investigated using optical microscope and electron probe.
Mixed-phase diamond/β–SiC composite films with compositional gradient were prepared by microwave plasma-assisted chemical vapor deposition using a gas mixture of hydrogen, methane and tetramethylsilane (TMS). Single-crystalline silicon wafers, pretreated with diamond nanoparticles before deposition, were used as substrates. The film characterization by scanning electron microscopy, electron probe microanalysis, and energy-dispersive x-ray analysis shows that the contents of diamond and silicon carbide in the films vary with TMS flow rate. Diamond/β–SiC composite films with compositional gradients are achievable by varying the TMS flow rate during the film growth process.
KnbO3 is a ferroelectric material with a Curie temperature (TC) at 415°C, thus giving it the potential to be a material for high-temperature positive temperature coefficient of resistivity (PTCR) applications. In this study, we investigated the PTCR effect in donor-doped KnbO3 ceramics containing 0, 0.1, 0.2, and 0.3 mol% PbO. The donor-doped KnbO3 ceramics exhibited a PTCR anomaly with a relatively low room-temperature resistivity. The temperature of the tetragonal-to-cubic phase transition (TC) of the KnbO3 decreased with the amount of added PbO, while the orthorhombic-to-tetragonal phase transition (TOT) remained unchanged.
We demonstrated rapid prototyping of templates for replica molding using a conventional laser printer. A polymer, polydimethylsiloxane, was cast directly on the transparency templates to make the replicas. The templates and replicas were characterized by scanning electron microscopy, profilometry, and optical microscopy. Four patterns, including an Electronic Industries Association resolution test pattern, were printed on transparencies at 600 dots per inch on a HP LaserJet 4M printer (Hewlett-Packard, Palo Alto, CA). Optimal precision and clarity occurred between intensity settings of 50–100. Mean pattern height/depth ranged from 8–13 μm, and width was as small as a few tenths of a millimeter. Mean surface roughness of the template patterns ranged from 1 to 4 μm on the top surface and from 5 to 10 nm on the bare transparency surface. This method provides access to microfabricated patterns for the broader research community without the need for sophisticated micromachining facilities.
Nd2−xCexCuO4 (x = 0 to 0.15) thick films were grown directly on LaAlO3 substrates and surface-oxidized Ni tapes by fast liquid-phase processing methods. The films had a smooth surface and a very good biaxial texture, with the full width at halfmaximum equal to 0.8° and 5° on LaAlO3 substrates and surface-oxidized Ni tapes, respectively. Films of thickness of 5–15 μmm were grown at rates in excess of 2 μm/min. Nd2−xCexCuO4 has a good lattice and thermal-expansion match to rare-earth Ba2Cu3O7−δ (REBCO), minimum reaction with the high-temperature CuO:BaO solutions, and is nonpoisoning to superconductivity. It is an ideal buffer for liquidphase expitaxy processing of REBCO thick films.
We examined the densification of 12CaO · 7Al2O3 (C12A7), which has a microporous lattice framework with a cavity of approximately 0.4 nm. Fully densified translucent ceramics are obtained when hydrated C12A7 powders prepared as precursors are sintered in a dry oxygen atmosphere at 1300 °C. The average transmittance between 400–800 nm for 1-mm-thick samples is improved up to approximately 70% with a 48-h increase in the sintering time. Dissociation of water molecules at the grain–pore interface into grains which entraps hydroxide ions in the crystallographic cage, and the release of the hydroxide ions into the atmosphere during the sintering procedure are considered to play crucial roles in the pore-annihilation processes.
Si film has been grown on a wurtzite gallium nitride layer on sapphire by low-pressure chemical vapor deposition. Uniform nitrogen incorporation was found in the Si film at the concentration of 5%, indicating an incorporation-limited process through interstitial diffusion from GaN layer to Si layer. The nitrogen occupied the substitutional sites in the Si film, leading this Si layer to be n-type doping with the carrier concentration of 1.42 × 1018/cm3 and the hall mobility of 158 cm2/(V s). This is consistent with other calculated and experimental results, which suggest that only 5% nitrogen can occupy the substitutional sites in the nitrogen-doped Si materials.
Poly(lactic acid) composites containing a mixture of calcium carbonates (vaterite, aragonite, and calcite) were prepared by a carbonation process in methanol. Soaking of the composites for 3 h in simulated body fluid (SBF) at 37 °C resulted in the deposition of bonelike apatite particles on the composite surface. After soaking the composites, vaterite phase in the composites was forward to dissolve rapidly, resulting in increase the supersaturation of the apatite in SBF. 13C cross-polarization magic angle spinning nuclear magnetic resonance (13C CP/MAS-NMR) spectra of the composites suggested the formation of a bond between Ca2+ ion and the COO- group, which induces the apatite nucleation. These results may elucidate the mechanism of means to reduce the induction period for apatite formation.
Boron nitride nanotubes (BNNTs) filled with zirconium oxide (ZrO2) nanorods were synthesized by the improved solid-gas multiphase reaction method. The structure of ZrO2 nanorods was monoclinic single crystal or multi-twin crystal. The diameters of ZrO2 nanorods varied from 20 to 40 nm. The inner diameters of BNNTs were similar to those of corresponding ZrO2 nanorods. The BNNTs exhibited open end, closed end, or an end connected with a short tube grown from the tip of the ZrO2 nanorod. No preferred orientation was observed for the growth of ZrO2 nanorods.
Smooth polycrystalline diamond films were deposited onto silicon substrates using a newly developed time-modulated chemical vapor deposition (TMCVD) process. The distinctive feature of the TMCVD process involves pulsing the hydrocarbon gas, methane, at different flow rates for varying durations into the vacuum reactor during the chemical vapor deposition (CVD) process. Generally, CVD diamond films display nonuniformity in the crystal sizes and surface roughness along the film growth profile. The TMCVD method was specifically developed to (i) deposit smooth films, (ii) control film microstructure and morphology, and (iii) improve film reliability. We show that the TMCVD process produces diamond films with improved surface smoothness as compared to films of similar thickness produced by conventional CVD method under similar conditions. Surprisingly perhaps, the TMCVD method gave growth rates much higher than the conventional CVD method without reducing the film quality as revealed by the SEM micrographs and micro-Raman spectra.
Superhard and elastic carbon nitride films with hardness and elastic recovery of 47 GPa and 87.5%, respectively, were synthesized by using a double-bend filtered cathodic vacuum arc combined with radio-frequency nitrogen ion beam source. The bombardment of energetic nitrogen atom onto the growing film surface results in the high atomic ratio of N/C (0.4), which contributes to the high sp2 content and the formation of a five-membered ring structure in the carbon nitride film at room temperature. The buckling of the five-membered ring basal planes may facilitate cross-linking between the planes through sp3 coordinated carbon atoms. A rigid three-dimensional network is formed, which contributes to the high hardness and elastic recovery of the deposited films.
It is well known that plastic deformation induced by conventional forming methodssuch as rolling, drawing or extrusion can significantly increase the strength of metalsHowever, this increase is usually accompanied by a loss of ductility. For example, Fig.1 shows that with increasing plastic deformation, the yield strength of Cu and Almonotonically increases while their elongation to failure (ductility) decreases. Thesame trend is also true for other metals and alloys. Here we report an extraordinarycombination of high strength and high ductility produced in metals subject to severeplastic deformation (SPD). We believe that this unusual mechanical behavior is causedby the unique nanostructures generated by SPD processing. The combination ofultrafine grain size and high-density dislocations appears to enable deformation by newmechanisms. This work demonstrates the possibility of tailoring the microstructures ofmetals and alloys by SPD to obtain both high strength and high ductility. Materialswith such desirable mechanical properties are very attractive for advanced structuralapplications.
The eutectic 99.3Sn–0.7Cu solder (wt%, Sn–0.7Cu) is the most promising lead-free replacement for the eutectic Sn–Pb solder in wave-soldering applications. In this study, the effect of a small perturbation in the Cu concentration on the reaction between the Sn–0.7Cu solder and Ni was investigated. Specifically, four Sn–xCu solders (x = 0.2, 0.4, 0.7, and 1) were reacted with Ni at 250 °C. A slight variation in Cu concentration produced completely different reaction products. When the Cu concentration was low (x = 0.2), the reaction product was (Ni1–xCux)3Sn4. At high Cu concentrations (x = 0.7 and 1), the reaction product was (Cu1–yNiy)6Sn5. When the Cu concentration was in-between (x = 0.4), both (Ni1–xCux)3Sn4 and (Cu1–yNiy)6Sn5, formed. The above findings were rationalized using the Cu–Ni–Sn isotherm. The results of this study imply that the Cu concentration must be strictly controlled in industrial production to produce the desired intermetallic at the interface.