Search

Ultrathin films of polystyrene were deposited on the surfaces of carbon nanofibers using a plasma polymerization treatment. A small percent by weight of these surface-coated nanofibers were incorporated into polystyrene to form a polymer nanocomposite. The plasma coating greatly enhanced the dispersion of the nanofibers in the polymer matrix. High-resolution transmission-electron-microscopy (HRTEM) images revealed an extremely thin film of the polymer layer (∼3 nm) at the interface between the nanofiber and matrix. Tensile test results showed considerably increased strength in the coated nanofiber composite while an adverse effect was observed in the uncoated composites; the former exhibited shear yielding due to enhanced interfacial bonding while the latter fractured in a brittle fashion.

Ultrathin films of pyrrole were deposited on the surfaces of carbon nanotubes using a plasma polymerization treatment. High-resolution electron transmission microscopy images revealed that an extremely thin film of the polymer layer (2∼7 nm) was uniformly deposited on the outer and inner surfaces of the nanotubes. The nanotubes of all sizes exhibited equally uniform ultrathin films, indicating well-dispersed nanotubes in the fluidized bed reactor during the plasma treatment. In particular, the inner wall of the nanotube was also coated with a uniform ultrathin film of only ∼1–3 nm. Time-of-flight secondary ion mass spectroscopy experiments confirmed the highly branched and cross-linked polymer thin films on the carbon nanotubes. The plasma deposition mechanism is discussed in this letter.

Surface modification of nanoparticles will present great challenges due to their extremely small dimensions, high surface areas, and high surface energies. In this research, we demonstrate the uniform deposition of ultrathin polymer films of 2 nm on the surfaces of alumina nanoparticles. The deposited film can also be tailored to multilayers. Time-of-flight secondary ion mass spectroscopy was used to confirm the pyrrole thin film on the nanoparticle surfaces. Using such a nanocoating, it is possible to alter the intrinsic properties of materials that cannot be achieved by conventional methods and materials.

Domain-orientation dependence of levitation force has been determined for single-domain YBa2Cu3Ox. The single-domain material is obtained from a seeded melt growth process. The levitation force has been found to reach a maximum as the c axis of the domain is parallel to the direction of the force. The levitation force decreases in a cosine law fashion as the angle θ (the angle between the direction of the force and the c axis) increases from 0° to 60°. A maximum anisotropy of levitation force of 2.29 has been found. A physical model is proposed to explain the observed orientation dependence.

A method was developed to melt join two YBa2Cu3Ox domains together. After melt joining, the scanning electron microscopy analysis did not reveal any interfaces near the original domain contact surfaces. Both samples with equal geometry and orientation before and after the melt‐joining process exhibited the same magnetic hysteresis, indicating that the melt‐joined sample was strongly coupled. Also discussed is a possible domain growth mechanism during the melt‐joining process.

A high‐resolution electron microscopic (HREM) study of the interface structure between diamond film and its silicon substrate has been carried out. The HREM images reveal that there is an amorphous layer between diamond film and its substrate for a sample grown by the hot filament chemical vapor deposition. β‐SiC crystallites are embedded in this amorphous layer. The HREM images of cross‐sectional specimens reveal that the diamond crystallites can nucleate directly on either the intermediate amorphous layer, the β‐SiC crystallites, or at some scratches of the Si substrate.

Photoluminescence (PL) of Fe3O4 nanoparticle was observed from the visible to near-infrared (NIR) range by laser irradiation at 407 nm. PL spectra of ∼10 nm diameter Fe3O4 nanoparticles organized in different spatial configuration, showed characteristic emissions with a major peak near 560 nm, and two weak peaks near 690 nm and 840 nm. Different band gap energies were determined for these Fe3O4 nanoparticle samples corresponding to, respectively, the electron band structures of the octahedral site (2.2 eV) and the tetrahedral site (0.9 eV). Photothermal effect of Fe3O4 nanoparticles was found to be associated with the photoluminescence emissions in the NIR range. Also discussed is the mechanism responsible for the photothermal effect of Fe3O4 nanoparticles in medical therapy.

Magnetic hysteresis data were taken from 4.2 to 35 K on Bi2Sr2CaCu2Ox samples that were hot isostatically pressed at 105 MPa in an inert atmosphere at 825 °C. One set of samples was pressed for only 15 min while the other was pressed for 120 min. The samples pressed for 15 min contained a high density of dislocations and planar faults, while the samples pressed for 120 min contained fewer dislocations and faults, with most dislocations present within subgrain boundaries. The samples with the complex dislocation/planar fault structures exhibited substantially larger hysteresis loops, suggesting enhanced flux pinning.

Magnetic relaxation data have been taken on zone‐melted and sintered YBa2Cu3Ox samples over wide temperature and magnetic field ranges. We have found that magnetic relaxation varies greatly in the samples with different microstructures and crystallographic orientations. Flux‐creep rates have been found to be reduced in the zone‐melted samples owing to large amounts of intragranular crystal defects such as edge dislocations and stacking faults. An important flux‐pinning behavior related to the pinning potential distributions has been observed in the samples. The relationship between microstructural orientation and pinning potential distributions in YBa2Cu3Ox is discussed.

A uniform grain‐oriented microstructure was developed in 50‐mm‐long bar samples of bulk YBa2Cu3Ox prepared by a partial melt growth process. In sharp contrast to previous reports, the textured products contained only small amounts of second phases. The samples were dense and nearly free of porosity. Transport measurements showed that the textured samples could carry a critical current density greater than 4.4×104 A/cm2 at 77 K and 1.8 T. The possible effects of processing parameters on the superconducting properties of the tested materials are discussed.

Quantum dots (QDs) and Fe3O4 nanoparticles were conjugated onto the surfaces of carbon nanotubes (CNTs) for medical diagnosis and treatment. The nanoassembly was designed to meet the specific needs in cancer in vivo imaging and simultaneous treatment. The key functionalities needed for clinical applications were integrated, including CNT surface functionalization for attachment of biological molecules in targeting, drug storage capabilities, fluorescent emissions near the infrared range, and magnetic hyperthermia. CNT-QD-Fe3O4 developed exhibited a strong fluorescence near the infrared region for noninvasive optical in vivo imaging. Magnetization measurements showed nearly reversible hysteresis curves from CNT-QD-Fe3O4 nanoassembly. Fe3O4 conjugated CNT was found to experience hyperthermia heating under alternating electromagnetic field.

The effects of plasma surface modification on interfacial behaviors in carbon nanotube (CNT) reinforced alumina (Al2O3) nanocomposites were studied. A unique plasma polymerization method was used to modify the surfaces of CNTs and Al2O3 nanoparticles. The CNT-Al2O3 nanocomposites were processed by both ambient pressure and hot-press sintering. The electron microscopy results showed ultrathin polymer coating on the surfaces of CNTs and Al2O3 nanoparticles. A distinctive stress-strain curve difference related to the structural interfaces and plasma coating was observed from the nanocomposites. The mechanical performance and thermal stability of CNT-Al2O3 nanocomposites were found to be significantly enhanced by the plasma-polymerized coating.

Hydroxylapatite (HA) nanoparticles were functionalized by depositing rare-earth-doped Y2O3 nanoparticles on the surface, and the structural evolutions of both HA and Y2O3 phases at different annealing temperatures were investigated by x-ray diffraction and transmission electron microscopy. Laser spectroscopy indicated that the surface functionalized HA nanoparticles exhibited strong visible emissions. No visible emissions were observed from rare-earth-doped Y2O3 without any substrate, suggesting a doping-induced environmental change of optically active rare-earth elements in the functionalized HA nanoparticles. The luminescent hydroxylapatite nanoparticles may find important applications as a biodegradable substrate for biomarking and drug delivery.

Atmospheric plasma polymerization of perfluorohexane was investigated in this letter. A large quantity of single-crystalline polytetrafluoroethylene (PTFE)-like nanotubes were formed on a simultaneously deposited film at room temperature without any catalysts or templates. The outer diameter of the nanotubes varied from 60 to 1200 nm with a maximum aspect ratio up to 100:1. Transmission electron microscopy and x-ray diffraction results indicated a single crystal close-packed hexagonal (cph) structure in the nanotubes. Polarization optical micrographs of the nanotubes showed their thermal stability comparable to PTFE. It is suggested that the plasma filament played a key role in the rapid formation of the nanotubes. This atmospheric plasma discharge synthesis can serve as a common method for nanofabrication of many other single-crystalline polymer systems.

The thin film of carbonfluorine was deposited on the surfaces of aligned carbon nanotubes using a plasma polymerization treatment. High-resolution transmission electron microscopy images revealed that a thin film of the polymer layer (20nm) was uniformly deposited on the surfaces of the aligned carbon nanotubes. Time-of-flight secondary ion mass spectroscopy and Fourier transform infrared experiments identified the carbonfluorine thin films on the carbon nanotubes. The plasma deposition mechanism is discussed.

Flexible sintered composite YBa2Cu3O7−δ tapes on a silver substrate have been fabricated by the tape casting technique followed by appropriate sintering. The sintered composite tape can be bent into an arc of a 27‐mm‐diam circle without breaking. This represents approximately 2% strain, as compared with a typical YBa2Cu3O7−δ superconductor material without silver, which fractures at a strain of ∼0.1–0.2%. The silver addition does not adversely affect the superconducting properties of the composite.

Alloys of Nb73Al12Si14.5B0.5 were rapidly quenched using the melt spinning technique to form fine amorphous ribbons. These were then annealed into A‐15 tapes with a variety of grain sizes. The smaller the grain was the more flexible the tape and the higher the critical current density Jc. The best results were obtained for a tape with a grain size of 15 nm; it could be bent to a diameter of 1 mm without breaking and it had a Jc of 3.3×1010 A/m2 at a magnetic field of 20 T.

Large amounts of lattice defects have been observed in the crystallized metal‐oxide glasses in the Bi‐Sr‐Ca‐Cu‐O system prepared by the rapid solidification method. The density of such defects has been found to greatly increase as the calcium and copper content of the starting material increases. These lattice defects are formed as a result of supersaturation of calcium and copper in the sample matrix. The intragrain critical current density has been found to increase 30 times in magnitude as the density of the lattice defects is increased in the sample matrix. The corresponding flux pinning mechanisms are discussed.

A melt‐annealing method has been developed to deposit highly textured superconducting thick films on different substrates in the Bi‐Sr‐Ca‐Cu‐O system. As opposed to previous techniques, this method uses powder made from an extremely dense glass sample to yield very uniform and dense thick films. The x‐ray diffraction and scanning electron microscopy results indicate that the superconducting phase is strongly oriented along the c axis perpendicular to the substrate and the films are chemically uniform. Electrical transport measurements consistently show a Tc near 80 K for all samples.

The transport crystal current density Jc has been measured in Bi2Sr2CaCu2Ox single crystals. With a voltage criterion of 1.0 μV/cm, current density measurements yield Jc values ranging from 300 to 1000 A/cm2 at 77 K and zero field, depending upon crystal quality. The crystals are grown by using a flux method and have a typical size of approximately 10×4×1.5 mm. X‐ray diffraction, scanning electron microscopy, electrical resistivity, and magnetization shielding data are reported.

The kinetic process between the 60 and 90 K superconducting phase transition has been explored to study the oxygen ordering mechanisms. An abrupt change in Tc from 60 to 90 K is observed as the sample’s oxygen content is rapidly varied from around 6.6 to 6.9. No intermediate Tc values are obtained, which suggests an oxygen ordering instability, leading to a phase transition between the 60 and 90 K superconducting phases.

Kinetics of the 110 K superconducting phase formation has been studied in the Bi‐Sr‐Ca‐Cu‐O system with and without the addition of small amounts of lead. The kinetics of the phase transformation has been greatly affected by lead doping. The time required for the maximum formation of the 110 K phase is substantially reduced by the addition of lead. This phenomenon has been shown to be associated with the lead enhancing the nucleation and growth process of the 110 K phase and the diffusivity of calcium and copper.