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The specific surface area of synthetic goethite depends on the preparation: the Fe(III):OH ratio, the rate of base titration of Fe salt, and the temperature and time of crystallization. The crystals also have different morphologies as determined by SEM or TEM. Carbon coating is used to improve the quality of SEM images of nonconducting specimens. We show here that needle-like goethite particles become substantially thicker in the course of standard carbon coating, and the length-to-width ratio obtained for carbon-coated particles is lower than that for the original goethite particles. The morphology of the goethite particles was also studied by tapping mode AFM.
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PMID:Morphology of synthetic goethite particles. 1497 1

Carbon nanotubes were synthesised within the pores of an alumina membrane. The membrane had 200 nm diameter pores and 60 microm thickness, and ethylene was used as carbon source. Membrane dissolution by HF results in a bundle of parallel open tubes, aligned without macroscopic defects. The external diameter of the tubes is uniform and there is no evidence of any amorphous carbon. Wall thickness control was obtained by varying the reaction time, length by the thickness of alumina membrane, and external tube diameter by the membrane pore size. Scanning (SEM) and transmission (TEM) electron microscopy, atomic force microscopy (AFM), X-ray diffraction, thermogravimetric analysis (TG) and surface area evaluation by nitrogen adsorption were used for the characterization of membrane and nanotubes.
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PMID:Hydrocarbon decomposition in alumina membrane: an effective way to produce carbon nanotubes bundles. 1557 Sep 60

Natural bentonite spent in the process of plant oil bleaching was used as an initial material for preparation of carbon-mineral adsorbents. The spent bleaching earth was treated using four procedures: T (thermal treatment); H (hydrothermal treatment); C (thermal treatment with addition of CCl4 vapor); M (modification of porous structure). Raw bentonite, RB (raw bleaching earth), and carbon materials prepared using plant oil were compared. The physicochemical characteristics of the adsorbents were determined using different methods: nitrogen adsorption/desorption, XRD, TEM, and MS-TPD. Carbon-mineral adsorbents contain from 5.23 to 19.92% C (w/w) and carbon adsorbents include from 84.2 to 91.18% C (w/w). Parallel processes of organic substance carbonization, porous structure modification, sublimation or evaporation of metal chlorides, and removal of hydrogen chloride take place during pyrolysis of waste mineral materials in the CCl4 atmosphere.
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PMID:Carbon-mineral adsorbents prepared by pyrolysis of waste materials in the presence of tetrachloromethane. 1575 82

We monitored exposure to various fibers among workers in eight plants operated by ConocoPhillips that produce green or calcined petroleum coke. Carbon/coke and other fibers, including calcium silicate, cellulose, gypsum, and iron silicate, were found in occupational samples. Carbon/coke fibers were found in bulk samples of calcined petroleum coke, the probable source of these fibers in occupational samples. Time-weighted average (TWA) total fiber concentrations were approximately lognormally distributed; 90% were < or = 0.1 f/ml. Although consistently low, TWA total fiber concentrations varied with plant, job (tasks), and type of coke. This was expected given the substantial differences in plant configuration, technology, and workplace practices among refineries and carbon plants. Carbon/coke fibers (identified and measured using transmission electron microscopy [TEM]) were found at all plants producing all types of calcined coke and not detected at any plant producing only green coke. Approximately 98% of all carbon/coke TWAs were < or = 0.1 f/ml. Analysis of task length average (TLA) data by various statistical techniques indicates that the average carbon/coke TLA is certainly < or = 0.05 f/ml and probably < 0.03 f/ml.
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PMID:Occupational exposure to carbon/coke fibers in plants that produce green or calcined petroleum coke and potential health effects: 2. Fiber concentrations. 1632 98

Carbon nanotubes were used to induce the formation of spherical vaterite crystals and stabilize the metastable crystals in the biomimetic mineralization of CaCO3 for the first time. It was found that carboxyl-functionalized multiwalled/single-walled carbon nanotubes (MWNT-COOH/SWNT-COOH) can favor the formation of spherical vaterite crystals and stabilize the crystals. In the presence of CNT-COOH, CaCO3 vaterite crystals with diameters of ca. 1-7 microm coated and embedded with the carbon nanotubes (CNTs) were obtained in 30 min by adding Na2CO3 aqueous solution to the aqueous solution of CaCl2. The spherical vaterite crystals covered by the carboxylic CNTs can exist stably in water for a week. Carboxylic-polymer-functionalized CNTs can also facilitate the formation of spherical vaterite crystals, whereas the formed crystals completely transformed into thermodynamically stable calcite crystals in water within 10 h. "Offline" TEM observations of the mineralization process of CaCO3 in the presence of CNT-COOH or pristine CNTs revealed the stability mechanism of vaterite crystals with carboxylic CNTs. The crystals nucleate at the carboxyl groups of CNT-COOH, grow around the CNTs, and finally form spherical vaterite crystals embedded and covered by the CNTs. The strong interaction between CNT-COOH and crystals together with the strong mechanical strength of CNTs stabilizes the formed vaterite crystals and makes them difficult to dissolve in water. These findings announce that nanomaterials could strongly influence the mineralization of biomineralization matters, which may help us prepare novel biomaterials and bionanomaterials.
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PMID:Efficiently stabilized spherical vaterite CaCO3 crystals by carbon nanotubes in biomimetic mineralization. 1735 86

We present the synthesis and characterization of Fe-doped carbon aerogels (CAs) and demonstrate the ability to grow carbon nanotubes directly on monoliths of these materials to afford novel carbon aerogel-carbon nanotube composites. Preparation of the Fe-doped CAs begins with the sol-gel polymerization of the potassium salt of 2,4-dihydroxybenzoic acid with formaldehyde, affording K+-doped gels that can then be converted to Fe2+- or Fe3+-doped gels through an ion exchange process, dried with supercritical CO2, and subsequently carbonized under an inert atmosphere. Analysis of the Fe-doped CAs by TEM, XRD, and XPS revealed that the doped iron species are reduced during carbonization to form metallic iron and iron carbide nanoparticles. The sizes and chemical composition of the reduced Fe species were related to pyrolysis temperature as well as the type of iron salt used in the ion exchange process. Raman spectroscopy and XRD analysis further reveal that, despite the presence of the Fe species, the CA framework is not significantly graphitized during pyrolysis. The Fe-doped CAs were subsequently placed in a thermal CVD reactor and exposed to a mixture of CH4 (1000 sccm), H2 (500 sccm), and C2H4 (20 sccm) at temperatures ranging from 600 to 800 degrees C for 10 min, resulting in direct growth of carbon nanotubes on the aerogel monoliths. Carbon nanotubes grown by this method appear to be multiwalled (approximately 25 nm in diameter and up to 4 microm long) and grow through a tip-growth mechanism that pushes catalytic iron particles out of the aerogel framework. The highest yield of CNTs was grown on Fe-doped CAs pyrolyzed at 800 degrees C treated at CVD temperatures of 700 degrees C.
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PMID:Iron-doped carbon aerogels: novel porous substrates for direct growth of carbon nanotubes. 1738 Nov 46

Carbon films 250 division by 500 nm in thickness deposited on Si wafers from mass-selected flow of accelerated C60 ions with energies of 5.0 +/- 0.1 keV at temperatures of 300 K and 673 K are characterized by TEM and nanoindentation. On the TEM images of the films deposited at 673 K, nanocrystalline graphite with the typical grain size of -6 nm is observed. The films deposited at 300 K are transparent in visible light. TEM study of these films has revealed structural elements with lattice spacing close to that of diamond and the grain size of about 4 nm. Nanohardness and elastic modulus of the films prepared at a substrate temperature of 300 K were 23.1 +/- 0.2 GPa and 200 +/- 1 GPa, respectively. Possible mechanisms of the carbon films structure formation are suggested in the framework of a hydrodynamic shock wave model.
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PMID:Nanocrystalline diamond thin films deposited from C60 monoenergetic fullerene ion beam. 1745 Sep

Carbon nanotubes (CNTs) were synthesized by low-pressure chemical vapour deposition (LPCVD) using N2:C2H2:H2 gas mixtures on nanocrystalline Ni95Ti5 film. This nanocrystalline film was deposited on silicon substrate using vapour condensation method. The growth temperature and growth time was kept at 800 degrees C and 30 mins, respectively and the pressure was maintained at 10 Torr. The growth mechanism of CNTs was investigated using FESEM, TEM, HRTEM, and Raman Spectroscopy. From FESEM image of Ni95Ti5 nanocrystalline film, it is clear that the particle size varies from 5-10 nm. EDX analysis suggests that Ni95Ti5 alloy contains Ni and Ti both. It is clear from TEM images that CNTs are multiwalled with the diameter varying from 10-30 nm and length of several micrometers. HRTEM image shows that the structure of these multi-walled nanotube (MWNTs) is bamboo-shaped and the catalyst exists at the tip of MWNTs. Fourier Transform Raman Spectroscopy confirmed that graphitic structure of as-prepared CNTs. Field emission measurements reveal that the carbon nanotubes grown for 30 mins showed a turn-on field of 7.2 V/microm, when the current density achieves 10 microA/cm2. The field enhancement factor was calculated to be 708.50 for carbon nanotubes grown for 30 mins.
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PMID:Synthesis of carbon nanotubes using Ni95Ti5 nanocrystalline film as a catalyst. 1765 54

A series of developments have been made in synthesizing Carbon Nanotubes (CNTs) by Catalytic Vapour Deposition (CVD) methods since its discovery as a possible route to the large scale and high quality production of CNTs. In this study, CNTs were synthesized continuously in a swirled floating catalytic chemical vapour deposition reactor using acetylene as carbon source, ferrocene as catalyst, with argon and hydrogen as carrier gases within the temperature range of 900-1050 degrees C. The effects of pyrolysis temperature, acetylene flow rate, hydrogen flow rate, and ratio of flow of acetylene to hydrogen on the rate of production of CNTs were investigated. The CNTs produced were purified with dilute nitric acid and the nature and quality of the CNTs were analysed by TEM, Raman spectrometer, EDX, and TGA. Results obtained revealed that a mixture of single and multi wall carbon nanotubes were produced continuously with a maximum yield rate of 0.31 g/min at 1000 degrees C and a flow ratio of acetylene to hydrogen of one to five.
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PMID:Synthesis of carbon nanotubes by swirled floating catalyst chemical vapour deposition method. 1801 55

The straightforward, efficient, solventless, RAPET (reactions under autogenic pressure at elevated temperature) approach was explored for the fabrication of core-shell nanomaterials. Carbon-encapsulated SnS and SnSe nanorods were synthesized by a one-step thermal decomposition of tetramethyltin in the presence of either S or Se powder in a closed reactor at 700 degrees C for 40 min, under their autogenic pressure in an inert atmosphere. The powder X-ray diffraction measurements provided structural evidence for the formation of pure orthorhombic phases of SnS or SnSe particles. The Raman spectroscopy measurements ensured that the nature of the coated carbon was semigraphitic. The scanning electron micrographs verified the 1D morphology of the formed SnS and SnSe chalcogenides, and their stoichiometry was confirmed by EDAX measurements. The HR-TEM micrographs distinguished between core and shell morphologies. The nitrogen gas adsorption on the surface of core-shell nanostructures was determined by BET surface area analysis. The plausible mechanism for the creation of chalcogenide cores (SnS or SnSe) with a carbon shell was elucidated.
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PMID:Core-shell nanorods of SnS-C and SnSe-C: synthesis and characterization. 1836 19


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