Analysis of the Growth Mechanism of Carbon Nanotubes by C2 Ingestion

Abstract

Cap structures capable of sustaining infinite growth by ingestion of C₂ fragments to give a carbon nanotube were screened systematically. For this purpose a total of 23,296 IPR cap structures having characteristic tubule vectors of (5～20, 0～10) were generated by the use of net algorithm. Eighteen recurring patterns were found which involve 50 cap structures having tubule vectors (6～11, 5).     

High-Resolution Electron Microscopic Study of the Growth Mechanism of Gigantic Tubular Super-Fullerenes

I report a new synthesis of gigantic tubular super-fullerenes : tubular carbon cages with shell structure showing maximum sizes of ∼ 10 μ m in length and ∼ 30 nm in the outermost diameter. For the innermost tubule a similar diameter to C₆₀ is found occasionally. Direct evidence of coaxial tubules capped on the ends is presented by high-resolution electron microscopy. I propose a new growth model: growth of the tubular super-fullerenes initiates at the outermost cage and proceeds towards the core. I would like to thank Oosuna, T. for obtaining TEM images and Shida, S. for preparing photographs.     

Production of Metallic Foams Having Open Porosity Using a Powder Metallurgy Approach

A technique has been recently developed to produce foamed metallic structures from dry powder blends containing a metallic powder, a polymeric binder, and a foaming agent. The blend is molded and heat-treated to foam and consolidate the material. The final properties may be tailored by varying the sintering temperature. Microstructure, chemical composition, and properties of nickel (Ni) foams sintered at different temperature are presented and discussed. The resulting material has an open cell microstructure with three levels of porosity. This structure leads to materials having low density (∼ 90% porosity) and high specific surface area. The specific surface area is reduced and the mechanical strength is increased when the sintering temperature increases.     

Design and in vitro evaluation of polymeric formulae of simvastatin for local bone induction

Simvastatin (SVS), a cholesterol-lowering drug, has been shown to stimulate bone formation. This study deals with the design and in vitro evaluation of local delivery systems for simvastatin. They are intended to treat bony defects resulting from periodontitis or to induce osteogenesis around the titanium implants. Granules and gels were formulated using bioerodible/biocompatible polymers, namely hydroxypropylmethyl cellulose (H), sodium carboxymethyl cellulose (C), and chitosan (Ch). The in vitro release profiles and kinetics were evaluated and the swelling and/or erosion was monitored. Differential scanning calorimetry (DSC) and infrared (IR) were used to detect any SVS/polymer interactions that may affect drug release. The results revealed variable extents of controlled drug release from the designed formulae depending on the polymer nature. About 50% cumulative SVS was released from both H granules and gel formulae within 24 h and ∼66% and ∼88% from C granules and gel, respectively. Ch formulae exhibited ∼50% release from granules and ∼30% from gel.     

Fabrication, Processing, and Characterization of Braided, Continuous SiC Fiber-Reinforced/CVI SiC Matrix Ceramic Composites

Three-dimensionally reinforced continuous-fiber ceramic matrix composites (CFCCs) were fabricated from preforms of braided SiC fiber (Nicalon(™)) tows that had been coated with 0.4 μm thick layer of pyrolytic graphite. A hybrid infiltration process of chemical vapor infiltration and polymer impregnation pyrolysis was used to form the matrix. The as-fabricated CFCCs were composed of ∼34 vol% fiber and ∼36 vol%matrix with ∼29 vol% residual porosity. Appropriate specimens were tested at 20° and 1000°C along the longitudinal braiding direction in uniaxial tension, compression and three and four-point flexure. Linear stress-strain responses to well-defined proportional limits (∼75 MPa) occurred for tension and flexure at both temperatures. Nonlinear stress-strain behaviour occurred beyond the proportional limit up to fracture at ultimate strengths in tension on the order of 175-200 MPa for 20°C and 100 MPa for 1000°C. SEM fractography revealed fiber pullout for the 20°C tests and evidence of brittle fracture due to environmental degradation for the 1000°C tests.     

Multicriteria Optimal Control of Polypropylene Terepthalate Polymerization Reactor

Tetrabutoxytitanium (TBOT) is a proven catalyst for the esterification step of the polypropylene terepthalate (PPT) polymerization process. Previous studies show that the performance of TBOT is superior in terms of the enhanced degree of polymerization and less processing time to other competitive catalysts. But, interesting observation left was to investigate whether with other process objectives like by-product minimization and controlled growth of desired functional groups, any other catalyst offers better system performance or not. Present study carries out the exercise of searching other catalytic options along with TBOT for the process improvements and gives a detailed process analysis through different sets of optimized operations. A well-validated kinetic model for esterification step of PPT polymerization process and the advanced Real-Coded Nondominated Sorting Genetic Algorithm-II (Real-Coded NSGA-II) optimization routine have been used in this current effort. For process objectives like by-product minimization, TBOT though become a marginal winner, Sn- and Zn-based catalysts compete with each other. Zn-based catalyst is found probably the most suitable catalyst in terms of the overall process performance with excellent by-product minimization (∼10 times better), higher degree of polymerization (∼1.75 times better), and tight quality control (∼5 times better).     

Enhanced oral bioavailability of a poorly water soluble drug PNU-91325 by supersaturatable formulations

Supersaturatable cosolvent (S-cosolvent) and supersaturatable self-emulsifying drug delivery systems (S-SEDDS) are designed to incorporate water soluble cellulosic polymers such as hydroxypropyl methylcellulose (HPMC), which may inhibit or retard drug precipitation in vivo. A poorly soluble drug, PNU-91325, was used as a model drug in this study to illustrate this formulation approach. The comparative in vitro studies indicated that the presence of a small amount HPMC in the formulation was critical to achieve a stabilized supersaturated state of PNU-91325 upon mixing with water. An in vivo study was conducted in dogs for assessment of the oral bioavailability of four formulations of PNU-91325. A five-fold higher bioavailability (∼ 60%) was observed from a S-cosolvent formulation containing propylene glycol (PG) + 20 mg/g HPMC as compared to that (∼ 12%) of a neat polyethylene glycol (PEG) 400 formulation. The low bioavailability of the PEG 400 formulation is attributed to the uncontrolled precipitation of PNU-91325 upon dosing, a commonly observed phenomenon with the cosolvent approach. A S-SEDDS formulation composed of 30% w/w Cremophor (surfactant), 9% PEG 400, 5% DMA, 18% Pluronic L44, 20% HPMC, and other minor components showed an oral bioavailability of ∼ 76%, comparable to that of a neat tween formulation (bioavailability: ∼ 68%). The significant improvement of the oral bioavailability of the supersaturatable S-cosolvent and S-SEDDS formulations is attributed to a high free drug concentration in vivo as a result of the generation and stabilization of the supersaturated state due to the incorporation of polymeric precipitation inhibitor.     

Effect of Substituents on the Redox Potentials of C60 Derivatives

The electrochemical properties of four C₆₀ derivatives (compounds 1∼4) were investigated with cyclic voltammetry (CV), and compared with that of C₆₀. The reduction potentials of compound 1∼3 shifted to more negative positions than C₆₀, and their oxidation peaks appeared at the lower potentials. Significantly, the reduction potentials of compound 4 shifted to more positive positions than C₆₀, which is advantageous to form charge-transfer complexes with more donors. So the reduction properties in different solvents and in situ NIR absorption spectra of compound 4 were studied in more details. In addition, the AM1 molecular orbital calculation was performed on C₆₀ and compound 1∼4 for the explanation of the potential shifts. The reduction potentials of these compounds exhibited good linear relationships with the calculation LUMO or LUMO+1 energy levels.     

Metallic Oxide Catalyzed Growth of Carbon Nanotubes

Carbon nanotubes were created in the cathode deposit by DC arc-discharge evaporation of the graphite rods containing Y₂O₃, La₂O₃ or SC₂O₃ These metallic oxides have the catalytic action to promote the growth o nanotubes. The most remarkable catalytic effect was observed for the case of metallic oxide annexation of 2∼8wt % and arc current of 165∼196A.     

Gravitational Effects on the Gas Tungsten Arc Welds of 304 Stainless Steels

Gravitational effects on gas tungsten arc welding (GTAW) of 304 stainless steels were studied to determine the behavior of the weld pool geometry, and its impact on microstructure and solute distribution. This was accomplished through both a KC-135 flight experiment and a ground-based experiment. Gravitational level variation from low gravity (LG) (∼1.2 g) to high gravity (HG) (∼1.8 g) caused a 10% increase in weld width and a 10% decrease in depth, while maintaining the overall weld pool volume. Regardless of the gravitational level, the ferrite content and the solutes (Cr and Ni) distribution in the duplex microstructure remained constant within the measurement scatter.     

Fabrication of a depletion mode GaAs MOSFET using Al2O3 as a gate insulator through the selective wet oxidation of AlAs

A 1 μm thick undoped GaAs buffer layer, a 1500 Å thick n-type GaAs layer, an undoped 500 Å thick AlAs layer and a 50 Å thick GaAs cap layer were consecutively grown by molecular beam epitaxy (MBE) on a [100] oriented semi-insulating GaAs substrate. The AlAs layer was oxidized in a N₂ bubbled H₂O vapor ambient at 400°C for 3 h and fully converted to Al₂O₃ for use as a gate insulator. The I–V characteristics, having a maximum drain current of 10.6 mA, a current cut-off voltage of −4.5 V and a maximum transconductance value of 11.25 mS/mm, indicate that the selective wet thermal oxidation of AlAs/GaAs was successful in producing a depletion mode GaAs MOSFET.     

Hot Isostatic Pressing Modifications of Pore Size Distribution in Plasma Sprayed Coatings

Plasma sprayed coatings contain relatively large amount of pores. This is primarily due to the nature of deposition by the liquid droplets upon impact. This paper reports the modifications made in the pore size distribution of plasma sprayed yttria stabilized zirconia (YSZ) and Ti-6Al-4V/hydroxyapatite (HA) composite coatings following hot isostatic pressing (HIP). The pore size distribution was measured by a mercury intrusion porosimeter (MIP). The results indicated that the YSZ coatings which were HIPed for 1 hour and 3 hours in the temperature range 1000^° to 1200^°C and ∼185 MPa showed a small decrease in the average porosity (∼2.5%) for the 1 hour samples. However, the hardness increased ∼39%, and there was a corresponding increase in the coating density. This was due to reduction of the average pore size in the HIPed coatings. Thus, in the YSZ coatings, the pores responded to HIP by a general breakdown of large pores to smaller ones and effectively forming many 'new' interparticle contacts. Whilst the overall porosity was reduced marginally, the increase in physical property like hardness was significant because of the increase in interparticle and inter-lamellae contacts following HIP treatment. In the Ti-6A1-4V/HA composite coatings, the reduction of pores is most significant amongst the small pores. The porosity of the as sprayed 20 wt% HA composite was ∼19%. This value was reduced to 17% for the sample HIPed at 1,000%C for 1 hour. Although the reduction was relatively minor, the interesting aspect was the drastic reduction of small pores less than 0.3 μm. The average pore diameter was observed to increase from 0.1676 μm in the as sprayed coating to 0.787 μm in the sample HIPed at 1,000^°C, as a result of the elimination of the micro-pores. Physical properties such as microhardness, Young's modulus and density increased substantially. This is believed to be aided mainly by the plastic deformation of the ductile Ti-6A1-4V phase during HIP. Thus modification of the pore size distribution or even average pore size can elicit substantial improvement in the properties in two different material coating, albeit the difference in the manner the modification occur.     

Influence of a Weak Magnetic Field on Photoconductivity Spectrum of C60 Single Crystal

Comparative research of the excitation photoconductivity spectra (quantum light energy 2-5 eV) of C₆₀ single crystal in and out of magnetic field at the temperature T = 250-350 K has been carried. The spectral evolution at this temperature range is described. It is shown that the spectra changes abrupt at temperature T₁ ∼ 260 K and T₂ ∼ 315 K. An increase in the photoconductivity up to 15% was observed in the magnetic field (B = 0.4 T) within the photon energy range 2.5-4.5 eV. Local photoconductivity peak's appearances in the magnetic field have been proven that the charge transfer excitons take part in a photoconductivity.     

Formation of metal–TiN/TiC nanocomposite powders by mechanical alloying and their consolidation

Fe–TiN, Ni–TiN, and SUS316–TiC nanocomposite powders were prepared by ball-milling Fe–Ti, Ni–Ti, and SUS316–TiC powder mixtures in a nitrogen or argon gas atmosphere. Fe–63vol.% TiN and Ni–44–64vol.% TiN milled powders were dynamically compacted by use of a propellant gun to produce bulk materials of nanocrystalline structure and having grain sizes between about 5 and 400 nm. SUS316–2.8–5.6vol.% TiC milled powders were consolidated by hot isostatic pressing (HIP) to produce bulk materials having grain sizes between about 100 and 400 nm. The possibility of using fine-dispersed TiN/TiC particles to pin grain boundaries and thereby maintain ultra-fine grained structures of grain sizes down to the nanocrystalline scale has been discussed.     

The Dislocation Structure of a Single-Crystal γ + γ′ Two-Phase Alloy After Tensile Deformation

The dislocation structures of an industrial single-crystal γ + γ′ two-phase alloy DD3 after tensile deformation from room temperature to 1273K were studied by transmission electron microscopy. The strength of this alloy decreased with an increase in the temperature, and showed a strength peak at 1033K. At room temperature, the dislocations shearing the γ′ particles were found to be 1/3<112> partial dislocations on the dodecahedral slip system <112>{111}. Some dislocation pairs on the cubic <110>{100} system that blocked the glide of dislocations were found at a medium temperature of 873K. As a result, dislocation bands were formed. Shearing of γ′ particles by 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} was also found at this temperature. At the peak temperature of 1033K, because of the strong interaction between dislocations on the {111} and {100} planes, the extent of dislocation bands with high dislocation densities was extensive. The 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} also existed. When the temperature reached the high temperature of 1133K, the range of dislocation bands was limited. The γ′ particles were sheared by <110> dislocation pairs on the octagonal <110>{111} system and the cubic <110>{100} system. At 1273K, the regular hexagonal dislocation networks were formed in the γ matrix and at the γ/γ′ interface. The Burgers vectors of the network were found to be b₁ = 1/2[110], b₂ = 1/2[1–10], b₃ = [100], and the last one was formed by the reaction of b₁ + b₂ → b₃. Dislocations shearing the γ′ particles were found to be <110> dislocation pairs on the octagonal system <110>{111} and cubic slip system <110>{100} at 1273K.     

Photoluminescence Spectra of Fullerene Films

Photoluminescence spectra of fullerene films were obtained in the range of 0.8∼1.8 eV by using a semiconductor pin Ge-detector and a photomultiplier. The effects of air intercalation on the optical properties of the fullerene film are discussed.     

A Novel Technique for Forming Self-Assembled Nanotube Structures

A suspension containing multi-wall, hollow structured carbon nanotubes were suspended in polyethylene glycol and electrosprayed. The results present an operational map identifying a parametric window in which stable cone-jet mode exists. At an electric field strength of ∼0.26 kV/mm and corresponding flow rate of ∼1×10^-8 m³s^-1 the finest droplets were generated. These droplets once deposited were analysed using optical and transmission electron microscopy and are compared with those deposits created through a control method. The electrosprayed deposits were further examined using electron diffraction and dispersive X-ray spectroscopy that forms the discussion in this article.     

Superconductivity of Organic Materials: Beyond the Fullerenes

We first briefly review the application of the BCS mechanism with electron-phonon interaction to the superconductivity of the A₃C₆₀ compounds. We then explore an extension of this family of compounds by using aromatic cryptands that we consider as “pseudo fullerene molecules”. Preliminary measurements of low field microwave absorption are consistent with superconducting behavior and a Tc ∼ 50K.     

Some Pharmaceutical Properties of 2,3,6-partially Methylated-β- cyclodextrin and its Solubilizing and Stabilizing Abilities

The pharmaceutical properties of 2,3,6-partially methylated-β-cyclodextrin(PMCD) were investigated. The aqueous solubility of PMCD was much higher than that of the parent βCyD, and it exhibited endothermic dissolution in contrast to that of the conventional heptakis-(2,6-di-O-methyl)-β-cyclodextrin(DMCD). The acid-catalyzed hydrolysis rate of PMCD was faster than those of the parent β-CyD and DMCD. The hemolytic activity (human erythrocytes) of PMCD was similar to that of DMCD. PMCD was a more effective solubilizer for poorly water-soluble drugs than the parent β-CyD; however PMCD is not as effective as DMCD. The stabilizing effect of PMCD on chemically unstable drugs was higher than that of the parent β-CyD.

For 2,3,6 partially methylated-β-CyD(PMCD), in which the hydroxyl groups of cyclodextrin are substituted by a methyl group, the methylation ratios are as follows: 58-62% at the 2-position, 48∼52% at the 3-position, and 98∼100% at the 6-position¹⁾. This novel cyclodextrin derivative is synthesized by MERCIAN CORPORATION²⁾. The aqueous solubilities of conventional heptakis-(2,6-di-O-methyl)-β-cyclodextrin(DMCD) and heptakis-(2,3,6-tri-O-methyl)-β-cyclodextrin(TMCD) usually decreased with increasing temperature; however, PMCD exhibited endothermic dissolution in a manner similar to that of the parent β-cyclodextrin (PCyD). PMCD has received considerable attention in the pharmaceutical field; therefore, in this study some of the physicochemical properties of PMCD, such as surface activity, hemolytic activity and chemical stability in acid medium were investigated. In addition, the solubilizing and stabilizing abilities of PMCD for pooly water-soluble drugs were compared with those of β-CyDand DMCD.     

Release Kinetics of Acyclovir from a Suspension of Acyclovir Incorporated in a Cubic Phase Delivery System

Acyclovir is a widely used agent in the treatment of herpes virus infections of the skin, but owing to its poor physicochemical properties in terms of bioavailability and suboptimal formulations, the treatment is far from optimal. The liquid crystalline cubic phase system has been reported to act as a bioadhesive drug delivery system. In the present study, acyclovir was suspended in a cubic phase of glycerol monooleate (GMO) and water 65%:35% w/w, and the phase behavior and release kinetics were examined. X-ray diffraction and differential scanning calorimetry (DSC) measurements demonstrated that the cubic phase containing 1%-10% (w/w) acyclovir retains its phase condition in the temperature range investigated (20°C-70°C). Acyclovir can be incorporated in high amounts (∼40% w/w) without causing phase transition, as is shown in polarized light. This is probably because of its low solubility (∼0.1% w/w) in the cubic phase. The release characteristics of acyclovir incorporated as a suspension (1%-5% w/w) into a cubic phase were investigated using Franz diffusion cells. Acyclovir was quantified by high-performance liquid chromatography (HPLC). The drug was readily released from the system, and the release increased with the initial drug load concentration. About 25%-50% was released after 24 h. The release is dependent on the square root of time, and the kinetics can be described by the Higuchi theory. The rate-limiting step in the release process is most likely diffusion. The suggested theory is further supported by identical release data obtained for micronized and nonmicronized acyclovir. The fluxes for 1% and 5% w/w were 380 and 900 μg/h^1/2, respectively. Comparison of the release rates of acyclovir delivered from a cubic phase and from the commercial product, Zovir® cream, showed the rate to be six times faster from the cubic phase. The results indicate that the cubic phase is a promising drug delivery system for acyclovir.