Iron–cobalt catalysts synthesized by a reverse micelle impregnation method for controlled growth of carbon nanotubes

Fe–Co bimetallic metals supported on CaCO₃ support have been prepared using three different methods for the preparation of the catalysts; wet impregnation (IMP), deposition–precipitation (DP) and reverse micelle impregnation (RM). By careful choice and optimization of CVD reaction conditions and catalyst preparation procedures, we have tailored the nanofabrication of CNTs using 5 wt.% Fe–Co/CaCO₃. High yields of ‘clean’ MWNTs with near 100% selectivity and excellent reproducibility were obtained (1215% and 835% yields for the IMP and DP methods respectively). The RM technique enabled more precise control over the size of the CNTs and a narrower diameter distribution (3–5 nm i.d. and 10–15 nm o.d.) was obtained.     

Neurobiology of cerebral malaria and African sleeping sickness

Prolonged depolarization induces a slow inactivation process in some K+ channels. We have studied ionic and gating currents during long depolarizations in the mutant Shaker H4-Delta(6-46) K+ channel and in the nonconducting mutant (Shaker H4-Delta(6-46)-W434F). These channels lack the amino terminus that confers the fast (N-type) inactivation (Hoshi, T., W.N. Zagotta, and R.W. Aldrich. 1991. Neuron. 7:547-556). Channels were expressed in oocytes and currents were measured with the cut-open-oocyte and patch-clamp techniques. In both clones, the curves describing the voltage dependence of the charge movement were shifted toward more negative potentials when the holding potential was maintained at depolarized potentials. The evidences that this new voltage dependence of the charge movement in the depolarized condition is associated with the process of slow inactivation are the following: (a) the installation of both the slow inactivation of the ionic current and the inactivation of the charge in response to a sustained 1-min depolarization to 0 mV followed the same time course; and (b) the recovery from inactivation of both ionic and gating currents (induced by repolarizations to -90 mV after a 1-min inactivating pulse at 0 mV) also followed a similar time course. Although prolonged depolarizations induce inactivation of the majority of the channels, a small fraction remains non-slow inactivated. The voltage dependence of this fraction of channels remained unaltered, suggesting that their activation pathway was unmodified by prolonged depolarization. The data could be fitted to a sequential model for Shaker K+ channels (Bezanilla, F., E. Perozo, and E. Stefani. 1994. Biophys. J. 66:1011-1021), with the addition of a series of parallel nonconducting (inactivated) states that become populated during prolonged depolarization. The data suggest that prolonged depolarization modifies the conformation of the voltage sensor and that this change can be associated with the process of slow inactivation.     

Physico-chemical characteristics of some Lesotho’s clays and their assessment for suitability in ceramics production

The physico-chemical properties of three clays (reddish, yellow and gray) collected at Ha Motloheloa quarry in Lesotho were determined. The main components in all the clays were SiO₂, Al₂O₃, Fe₂O₃ and K₂O in the form of mica, CaO, MgO, Na₂O, TiO₂ and P₂O₅ with less than 6% loss of ignition (LOI). The clays showed a wide variation in particle sizes with clay fractions from 15.81% to 27.38%, silt fractions from 41.95% to 52.32% and sand fractions from 30.6% to 31.9%. The surface area of the clays increased with a decrease in particle size. All the clays showed water content below 9% and shrinkage value below 6%. The clays had moderate plasticity indexes between 20 and 25 and less than 9% water content with 17.32%–20.72% water required to produce sufficient workability. Their densities were less than 3?g/cm^?3 and had low green to dry shrinkage (4.19%–5.35%). For all firing temperatures used (950°C–1100°C), the flexural strength varied from 2.3 to 17.6?MPa, while the water content varied from 6.1% to 12.2%. The reddish clay can be used for making perforated/solid bricks and roofing tiles, while yellow and gray clays can be used only for solid bricks.     

AC-conductance and capacitance measurements for ethanol vapor detection using carbon nanotube-polyvinyl alcohol composite based devices

We report the preparation of inexpensive ethanol sensor devices using multiwalled carbon nanotube-polyvinyl alcohol composite films deposited onto interdigitated electrodes patterned on phenolite substrates. We investigate the frequency dependent response of the device conductance and capacitance showing that higher sensitivity is obtained at higher frequency if the conductance is used as sensing parameter. In the case of capacitance measurements, higher sensitivity is obtained at low frequency. Ethanol detection at a concentration of 300 ppm in air is demonstrated. More than 80% of the sensor conductance and capacitance variation response occurs in less than 20 s.     

Composites of polyvinyl alcohol and carbon (coils, undoped and nitrogen doped multiwalled carbon nanotubes) as ethanol, methanol and toluene vapor sensors

We investigate the chemical sensing behavior of composites prepared with polyvinyl alcohol and carbon materials (undoped multiwalled carbon nanotubes, nitrogen-doped multiwalled carbon nanotubes and carbon nanocoils). We determine the sensitivity of thin films of these composites for ethanol, methanol and toluene vapor, comparing their conductance and capacitance responses. The composite that exhibits highest sensitivity depends on specific vapor, vapor concentration and measured electrical response, showing that the interactivity of the carbon structure with chemical species depend on structural specificities of the carbon structure and doping.     

A facile procedure to shorten multiwalled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) with > 95% purity were synthesized over a Fe-Co/CaCO3 catalyst using chemical vapour deposition (CVD). Both the CNT yield and the outer diameters increased with time on line in the presence of acetylene. More significantly, the tubes were reduced in length and became stub-like with time. TEM analysis revealed that the CNTs commenced shortening after 2 h of reaction time. Reagent residues (e.g., Ca, CaO, OH/COOH groups and Fe-Co oxides) were found not to influence the CNT bond breaking reaction. CNT growth over Fe-Co supported on silica or CaCO3-Ca3(PO4)2 gave similar results. Further, MWCNTs produced by a floating catalyst method, carbon helices produced from Fe-Co-In/A2O3, and N doped CNTs also revealed tube shortening as a function of reaction time under a flow of acetylene. It is thus apparent that MWCNTs can readily be shortened by the facile procedure of depositing carbon from excess C2H2 on the outer walls of CNTs.     

GaN nanostructures-poly(vinyl alcohol) composite based hydrostatic pressure sensor device

The wide band-gap semiconductor material gallium nitride was synthesized using a one step microwave-assisted solution phase technique. The synthesized GaN nanocrystals showed an intense ultraviolet-blue emission typical of GaN materials. Hydrostatic pressure sensors were fabricated using a GaN/polyvinyl alcohol (PVA) composite film deposited onto an interdigitated electrode and studied by measuring the change in alternating current conductance of the devices at varied applied pressures. Three different GaN concentrations of 29, 50 and 67% were used. A very high sensitivity in the range 100–200 kPa was observed for these devices. The composite devices demonstrated both response and recovery times of less than 16 s.     

Development of antifouling polyamide thin‐film composite membranes modified with amino‐cyclodextrins and diethylamino‐cyclodextrins for water treatment

The fouling behavior of polyamide thin‐film composite (TFC) membranes modified with amino‐ and diethylamino‐cyclodextrins (CDs) through an in situ interfacial polymerization process is reported. Modified polyamide TFC membranes exhibited improved hydrophilicity, water permeability, and fouling resistance as compared to the unmodified TFC membranes, while restricting the passage of NaCl salt (98.46 ± 0.5%). The increase in hydrophilicity was attributed to the secondary and tertiary hydroxyl groups of the CDs, which were not aminated. The membranes modified with amino‐CDs had increased surface roughness while the membranes modified with diethylamino‐CDs had smoother surfaces. However, despite the surface roughness of the membranes modified with amino‐CDs, low fouling was observed due to the highly hydrophilic surfaces, which superseded the roughness. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40109.     

Carbon Nanotube Docking Stations: A New Concept in Catalysis

The mobility of surface-bound metallic nanoparticles (NPs) on catalyst supports results in agglomeration leading to a subsequent decrease in effectiveness of the catalytic behavior of the metal NPs over time. We report here the synthesis and characterization of a carbon nanotube (CNT) catalyst support system enhanced with ‘docking stations’ along the exterior which limit the surface mobility of ultra small iron catalyst particles on CNT surfaces during Fisher–Tropsch synthesis.