Carbon nanotube/epoxy resin composite: Correlation between state of nanotube dispersion and Zener tunneling parameters

Carbon nanotube/epoxy resin composites with different levels of nanotube dispersion in the polymer matrix were prepared. The effects of nanotube dispersion on tunneling conductive behavior of such composites were investigated. The composites with homogeneous nanotube dispersion were found to exhibit larger static electrical conductivity and smaller percolation threshold than those with poorer nanotube dispersion. In addition, uniformly dispersed nanotubes induced strong Zener effect under the application of an electric field. The static conductivity and Zener tunneling parameters were shown to be good indicators for the state of nanotube dispersion.     

AES and SIMS investigations of the oxide films on Fe-40Cr-Ru alloys

Auger and SIMS depth profiling have been used to investigate the effect of ruthenium addition on the oxidation behaviour of the Fe-40Cr-Ru alloys oxidized in air at 500 °C. Auger results revealed that the oxide films formed on the Fe-40Cr-Ru alloys consisted of a thin iron oxide external layer and a chromium-rich internal layer. Ruthenium was not found in these oxide layers. Secondary ion mass spectrometry measurements also indicated a thin Fe⁺-rich film as an outer layer with a predominantly chromium oxide in the internal layer of the film. Furthermore, ruthenium was incorporated in the entire oxide film and promoted the formation of a thin compact film.     

Oxidation of Fe---25Cr---4Al---Pd and Fe---25Cr---4Al---Ru alloys

Fe---25Cr---4Al alloys containing Pd and Ru additions were oxidized in air at 1090°C. Scanning electron microscopy, dispersive x-ray analyses, and x-ray mapping were used to characterize the morphologies of the scales formed on these alloys. The results showed that Pd tended to segregate at the oxide-alloy interface as discrete and platelike particles during elevated temperature oxidation of the Pd-containing Fe---Cr---Al alloys. These Pd particles appeared to improve the adherence of the alumina scale. However, no improvement of the scale adherence of Ru-containing Fe---Cr---Al alloys was observed as outward diffusion of Ru cations to the external alumina scale led to the formation of RuO₂ within this scale.     

Granulocyte-macrophage colony stimulating factor suppresses lipopolysaccharide-induced osteoclast-like cell formation in mouse bone marrow cultures

Lipopolysaccharide (LPS) is a potent bone resorbing factor. We investigated the effect of LPS on osteoclast formation in three types of cultures. LPS inhibited osteoclast formation induced by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], in a dose-dependent manner, in cultures of whole bone marrow cells without dexamethasone. LPS increased the amount of granulocyte-macrophage colony stimulating factor (GM-CSF) in the culture supernatant, and anti-GM-CSF antiserum almost abolished the inhibition of osteoclast formation by LPS, thereby indicating that GM-CSF generated by treatment with LPS may be responsible for the inhibition of osteoclast formation. In cultures with dexamethasone, the amount of GM-CSF was decreased to one-third of that with 1,25(OH)2D3 alone and was not changed by treatment with LPS. In this culture system, LPS enhanced osteoclast formation. In the coculture system of nonadherent bone marrow cells and a stromal cell line in the presence of 1,25(OH)2D3 and dexamethasone, where no detectable GM-CSF was present in the supernatant, LPS markedly enhanced osteoclast formation, whereas exogenously added GM-CSF (100 pg/ml) almost completely inhibited osteoclast formation. LPS stimulated pit formation on dentin slices by the osteoclast-like cells formed by in vitro culture system.     

SIMS / XPS studies of the passive film on nickel

Passive films were formed on nickel in borate buffer solution (pH 8.4) at various anodic potentials. XPS and SIMS techniques were used to determine the passivating species present on the surface film. It is shown that the film formed at low passivating potentials consists of Ni (OH)₂ and NiO, and the film formed preceding oxygen evolution consists mainly of Ni₂O₃.     

Novel polypropylene biocomposites reinforced with carbon nanotubes and hydroxyapatite nanorods for bone replacements

Multi-walled carbon nanotubes (MWNTs) of 0.1 and 0.3 wt.% and hydoxyapatite nanorods (nHAs) of 8–20 wt.% were incorporated into polypropylene (PP) to form biocomposites using melt-compounding and injection molding techniques. The structural, mechanical, thermal and in vitro cell responses of the PP/MWNT–nHA hybrids were investigated. Tensile and impact tests demonstrated that the MWNT additions are beneficial in enhancing the stiffness, tensile strength and impact toughness of the PP/nHA nanocomposites. According to thermal analysis, the nHA and MWNT fillers were found to be very effective to improve dimensional and thermal stability of PP. The results of osteoblast cell cultivation and dimethyl thiazolyl diphenyl tetrazolium (MTT) tests showed that the PP/MWNT–nHA nanocomposites are biocompatible. Such novel PP/MWNT–nHA hybrids are considered to be potential biomaterials for making orthopedic bone implants.     

Electrical conductivity of polyvinylidene fluoride nanocomposites with carbon nanotubes and nanofibers

Polyvinylidene fluoride nanocomposites with low loading levels of pristine multiwalled carbon nanotubes, carboxyl functionalized multiwalled carbon nanotubes and vapor grown carbon nanofibers were prepared by a versatile coagulation method. The alternating current electrical conductivity of these composites in the frequency range of 40-12 MHz was investigated. The alternating current conductivity of percolating nanocomposites followed a universal dynamic response. Therefore, both the direct current plateau and frequency dependent regime were observed. The percolation threshold of three composite systems was determined to be 1.0, 0.98, and 1.46 vol.%, respectively. Moreover, the percolative nanocomposites exhibited nonlinear current-voltage responses, demonstrating the presence of tunneling conduction.     

Direct current conductivity of carbon nanofiber-based conductive polymer composites: effects of temperature and electric field

Polymer composites based on high density polyethylene (HDPE) and carbon nanofiber (CNF) were fabricated by melt compounding. The dependences of electrical conductivity of HDPE-CNF composites on filler concentration, temperature, and applied electric field were investigated. The results showed that the conductivity of the HDPE-CNF composites follows the scaling law of percolation theory. Increasing temperature caused a sharp increase in the resistivity of HDPE-CNF composites near the melting temperature of HDPE, yielding a positive temperature coefficient (PTC) effect of resistance. The potential mechanisms involved in the PTC effect of such composites were analyzed. An investigation of the effect of electric field on the conductivity of HDPE-CNF composites revealed the presence of tunneling conduction. The tunneling conductivity increased with increasing filler content because of high tunneling frequency, and decreased with rising temperature as a result of gap widening between conducting CNF fillers.     

Mechanical, thermal and bioactive behaviors of polyamide 6/hydroxyapatite nanocomposites

Polyamide-6 nanocomposites filled with different hydroxyapatite nanorod contents were injection molded. The thermal and tensile properties as well as bioactivity of such nanocomposties were investigated. The results showed that the thermal stabilities of polyamide-6 improve considerably by adding hydroxyapatite nanorods. Tensile measurements demonstrated that nanorods reinforce polyamide-6 effectively but reduce its tensile elongation and impact strength. Cell cultivation and viability tests showed that mouse osteoblasts adhere and proliferate readily on the nanocomposites containing high filler contents. Therefore, polyamide-6/hydroxyapatite nanocomposites show potential application in orthopedics for bone tissue replacements.     

Amplified on-chip fluorescence detection of DNA hybridization by surface-initiated enzymatic polymerization

We describe the incorporation of multiple fluorophores into a single stranded DNA (ssDNA) chain using terminal deoxynucleotidyl transferase (TdT), a template-independent DNA polymerase that catalyzes the sequential addition of deoxynucleotides (dNTPs) at the 3'-OH group of an oligonucleotide primer; we term this methodology surface initiated enzymatic polymerization (SIEP) of DNA. We found that long (>1 Kb) ssDNA homopolymer can be grown by SIEP, and that the length of the ssDNA product is determined by the monomer to oligonucleotide initiator ratio. We observed efficient initiation (≥50%) and narrow polydispersity of the extended product when fluorescently labeled nucleotides are incorporated. TdT's ability to incorporate fluorescent dNTPs into a ssDNA chain was characterized by examining the effect of the molar ratios of fluorescent dNTP to natural dNTP on the degree of fluorophore incorporation and the length of the polymerized DNA strand. These experiments allowed us to optimize the polymerization conditions to incorporate up to ~50 fluorescent Cy3-labeled dNTPs per kilobase into a ssDNA chain. With the goal of using TdT as an on-chip labeling method, we also quantified TdT mediated signal amplification on the surface by immobilizing ssDNA oligonucleotide initiators on a glass surface followed by SIEP of DNA. The incorporation of multiple fluorophores into the extended DNA chain by SIEP translated to a ~45 fold signal amplification compared to the incorporation of a single fluorophore. SIEP was then employed to detect hybridization of DNA, by the posthybridization, on-chip polymerization of fluorescently labeled ssDNA that was grown from the 3'-OH of target strands that hybridized to DNA probes that were printed on a surface. A dose-response curve for detection of DNA hybridization by SIEP was generated, with a ~1 pM limit of detection and a linear dynamic range of 2 logs.