Magnetic properties of carbon nanotubes produced by the arc-discharge method under different conditions

Physics of the Solid State - The diamagnetic susceptibility and EPR signals of multi-walled carbon nanotubes produced at different helium pressures and rates of feed of the graphite anode are...     

Protection against corrosion of iron alloys by aluminized coatings produced using two different processes

Aluminized coatings were produced on iron by means of two different processes: electron beam deposition under UHV of Al on iron samples previously covered with ⁵⁷Fe films, and hot-dipping of iron samples in molten aluminium. Aluminized samples were submitted to thermal treatments in order to promote interdiffusion at the Fe–Al interface and favour the formation of Fe–Al intermetallic compounds of composition suitable to protect the underlying iron from oxidation. Phase composition, structure and morphology of both as deposited and thermally treated coatings were characterized by means of X-ray diffraction, Mössbauer spectroscopy and metallographic techniques. Significant differences among the effects of the Fe–Al interdiffusion occurring for Al layers produced with the two processes are pointed out and discussed.     

Efficient solar energy conversion using TiO2 nanotubes produced by rapid breakdown anodization – a comparison

We report on dye‐sensitization of different TiO₂ nanotube layers, their photoelectrochemical response and their efficiency for solar energy conversion. The tubes compared in this study were either grown by controlled Ti anodization in fluoride containing electrolytes or by rapid breakdown anodization (RBA) of Ti in fluoride free electrolytes. After converting the different tube layers to anatase and sensitizing with Ru‐dye (N719), clearly layers consisting of RBA‐NTs show a significantly higher photoresponse and conversion efficiencies than tubes formed under self‐ordering conditions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Carbon nanotubes produced by tunneling of cobalt nanoparticles in carbon fibers

Carbon nanotubes (CNTs) have been produced by the tunneling of cobalt nanoparticles in carbon fibers that are derived from electrospun polyacrylonitrile (PAN) fibers. During annealing, the PAN fibers transform to a composite of cobalt nanodroplets and carbon fibers. Driven by the high chemical potential of wrinkled graphene platelets and amorphous carbon with respect to graphite, the cobalt nanodroplets are to tunnel in the carbon fibers. When cobalt nanodroplets have an elongated shape, carbon atoms dissolved in the droplets precipitate preferentially and completely at their lateral sides, producing perfect CNTs that form bulk structures.     

Synthesis and crystallinity of carbon nanotubes produced by a vapor-phase growth method

We have studied the effect of temperature on the growth and crystallinity of carbon nanotubes (CNTs), synthesized by a vapor-phase growth method using a catalytic reaction of iron pentacarbonyl (Fe(CO)₅) and acetylene (C₂H₂) gas. By increasing the growth temperature from 750 °C to 950 °C, both the growth rate and the diameter of the CNTs increase. Moreover, the crystallinity of the graphite sheets improves progressively with increasing growth temperature. Adjustment of the growth temperature gives potential for controlled growth of CNTs in a large-scale synthesis of CNTs. PACS 61.46.+w; 68.37.-d; 81.07.De     

Growth of carbon nanotubes by Fe-catalyzed chemical vapor processes on silicon-based substrates

In this paper, a site-selective catalytic chemical vapor deposition synthesis of carbon nanotubes on silicon-based substrates has been developed in order to get horizontally oriented nanotubes for field effect transistors and other electronic devices. Properly micro-fabricated silicon oxide and polysilicon structures have been used as substrates. Iron nanoparticles have been obtained both from a thin Fe film evaporated by e-gun and from iron nitrate solutions accurately dispersed on the substrates. Single-walled nanotubes with diameters as small as 1 nm, bridging polysilicon and silicon dioxide “pillars”, have been grown. The morphology and structure of CNTs have been characterized by SEM, AFM and Raman spectroscopy.     

Structure of Lennard–Jones nanowires encapsulated by carbon nanotubes

Molecular dynamics simulations have been performed to investigate the structures of Lennard–Jones(LJ) nanowires(NWs) encapsulated in carbon nanotubes(CNTs). We find that the structures of NWs in a small CNT only adopt multi-shell motifs, while the structures of NWs in a larger CNT tend to adopt various motifs. Among these structures, three of them have not been reported previously. The phase boundaries among these structures are obtained regarding filling fractions, as well as the interaction between NWs and CNTs.     

不同靶材超热电子和非线性过程特性的实验研究

在“星光Ⅱ”上进行几种特殊靶材盘靶和特殊结构腔靶产生超热电子和非线性过程 (受激Raman散射 (SRS)、双等离子体衰变 (TPD)、受激布里渊散射 (SBS)特征实验研究。该实验表明 :低ZCH泡沫或CH膜对抑制超热电子预热和非线性过程的产生是不利的 ,它们不作腔的第一打击面。     

Photoluminescence of epitaxial InAs produced by different growth methods

InAs and its alloys are promising materials for the fabrication of mid infrared optoelectronic devices for use in gas sensors and many other applications. Photoluminescence (PL) spectroscopy has been a key technique in appraising the quality of this material, but a systematic comparative study of the photoluminescence of InAs grown by different methods has never been published. In this paper we present a comparison of the photoluminescence spectra from InAs layers grown by LPE, MBE, MOCVD and from substrate materials of various carrier concentrations. Excitonic recombination is observed in the MBE and MOCVD material, however the brightest emission is obtained from the LPE material which did not show excitonic recombination.     

Atomic force microscopy investigation of electrochemically produced carbon nanotubes

Carbon nanostructures have been synthesized in NaCl-MgCl₂ and in NaCl-CaCl₂ salt melts and the extracted material was investigated by tapping-mode atomic force microscopy (TM-AFM) and scanning electron microscopy. Some interesting new nanostructures were found and investigated as torus-shaped carbon structures with a ring diameter of 300-400 nm and 10-15 nm height. These tori are closely related to the wrapped SWNT rings described recently. They are probably formed during the electrolysis. A chain-like structure was also revealed.     

Release potential of single-wall carbon nanotubes produced by super-growth method during manufacturing and handling

We investigated the release potential of single-wall carbon nanotubes (CNTs) produced by the super-growth method during their manufacturing and handling processes at a research facility. We generally sampled air at points both outside and inside of protective enclosures such as a glove box and fume hood. Sampling the air outside of the enclosures was intended to evaluate the actual exposure of workers to CNTs, while sampling the air inside the enclosures was performed to quantify the release of CNTs to the air in order to estimate the potential exposure of workers without protection. The results revealed that airborne CNTs were generated when (1) CNTs were separated from the substrates using a spatula and placed in a container in a glove box; (2) an air gun was used to clean the air filters (containing dust that included CNTs) of a vacuum cleaner; (3) a vacuum cleaner was used to collect CNTs (emission with exhaust air from the cleaner); (4) the container of CNTs was opened; and (5) CNTs in the bin of the cleaner were transferred to a container. In these processes, airborne CNTs were only found inside the enclosures, except for a small amount of CNTs released from the glove box when it was opened. Electron microscopic observations of aerosol particles found CNT clusters, which were fragments of CNT forests, with sizes ranging from submicrometers to tens of micrometers.     

A comparison of different powder compaction processes adopted for synthesis of lead-free piezoelectric ceramics

Barium zirconium titanate, Ba(Zr_0.15Ti_0.85)O₃ nano-crystalline powders were synthesized using highenergy ball milling. The calcined powders were compacted adopting two different approachesviz. the conventional uniaxial pressing and cold-isostatic pressing (CIP) and the compactswere sintered at 1350 °C. Asingle phase perovskite structure was observed in both cases. BZT ceramics compacted usingCIP technique exhibited enhanced dielectric and ferroelectric properties compared toceramics compacted by uniaxial pressing. The polarization current peaks have been used inthis paper as an experimental evidence to prove the existence of ferroelectricity in theBZT ceramics under study. The peak polarization current was found to be ~700% higher in case of coldiso-statically compacted ceramics. Similarly electric field induces strain showed amaximum strain (S_max) of 0.08% at an electric fieldof 28 kV/cm. The dielectric and ferroelectric properties observed are comparable to singlecrystals of the same material.     

Investigation of relaxation processes in nanocrystalline cobalt produced by severe plastic deformation

It has been shown that the dependence of the width of X-ray diffraction lines of plastically deformed cobalt on the annealing temperature is described by the exponential function. Characteristic temperature regions corresponding to the processes of recovery and recrystallization have been established. It has been shown that the values of the activation energy of recrystallization determined from the experimental data are comparable with the activation energy of the grain-boundary diffusion in metals. The activation energy for the recovery region is considerably lower than the activation energy of migration of nonequilibrium grain boundaries in nanocrystalline metals. The X-ray diffraction data have been confirmed by the investigations of the microstructure and microhardness.     

A comparison of the titanomagnetites produced by several volcanoes in Iceland

Samples of titanomagnetite extracted from the ejecta of volcanoes in Iceland have been separated according to the intensity of their magnetisation and analysed with Mossbauer spectroscopy and electron microprobe analysis. It is shown that the magnetic peaks in the Mossbauer spectra vary widely between different source volcanoes and magnetic fractions, and thus that the Mossbauer spectrum of the titanomagnetite could be a useful fingerprint of the tephra.     

Structure and spectroscopic properties of organoclays doped by multiwall carbon nanotubes

A method to modify a montmorillonite (MMT) clay mineral (CM) surface by surfactant (SA) cations with simultaneous doping by multiwall carbon nanotubes (MWNT) has been proposed. The structure and spectroscopic properties of composites based on MMT from two deposits (Cherkassy and Pyzhevsk, Ukraine) that differ in the inorganic impurity contents and cation-exchange capacities (CEC) have been investigated. Cetyltrimethylammonium bromide (CTAB) was used as the SA. According to x-ray diffraction analysis, CTA⁺ cations intercalated into MMT interplanar spaces expand them significantly whereas MWNTs do not affect the MMT galleries due to the much larger sizes of the former. Studies of the composite materials by IR spectroscopy revealed the mutual influence of the components appearing as the ordering of near-surface layers in the aluminosilicate framework and a change in the modifier methylene chain conformation at the interphase boundary. The majority of CTAB (~90%) is shown to be located inside the MMT galleries, the packing arrangement of which depends on the CEC value and affects the interplanar distances in MMT. The alkyl chains of the CTA⁺ cations on the outer surface of the MMT plates are sorbed by nanotubes, thus providing contact between the organoclay and MWNT surfaces.     

Structure of carbinoid nanotubes and carbinofullerenes

Molecular mechanics methods have been used to calculate the geometrically optimized structure of carbinoid layers, carbinoid nanotubes, and carbinofullerenes consisting of carbine chains linked by atoms in sp ² and/or sp ³ hybridization states. Energy characteristics of carbinoid nanostructures have been calculated by semi-empirical quantum-mechanical methods. A structural classification of framework carbinoid nanostructures has been proposed. The dependence of specific binding energies of carbinoid nanostructures on the ratio sp ²/sp and their geometrical sizes has been determined.     

Structure and properties of BeO nanotubes

The structure of a new non-carbon (beryllium oxide BeO) nanotube consisting of a rolled-up graphene sheet is proposed, and its physical properties are described. Ab initio calculations of the binding energy, the electronic band structure, the density of states, the dependence of the strain energy of the nanotube on the nanotube diameter D, and the Young’s modulus Y for BeO nanotubes of different diameters are performed in the framework of the density functional theory (DFT). From a comparison of the binding energies calculated for BeO nanotubes and crystalline BeO with a wurtzite structure, it is inferred that BeO nanotubes can be synthesized by a plasma-chemical reaction or through chemical vapor deposition. It is established that BeO nanotubes are polar dielectrics with a band gap of ～5.0 eV and a stiffness comparable to that of the carbon nanotubes (the Young’s modulus of the BeO nanotubes Y _BeO is approximately equal to 0.7Y _C, where Y _C is the Young’s modulus of the carbon nanotubes). It is shown that, for a nanotube diameter D > 1 nm, the (n, n) armchair nanotubes are energetically more favorable than the (n, 0) zigzag nanotubes.     

Structure and physical properties of iron nanotubes obtained by template synthesis

Physics of the Solid State - Iron nanotubes with an aspect ratio of approximately 100 are synthesized by electrochemical deposition using polyethylene terephthalate templates. The structural and...     

Nitrogen-doped carbon nanotubes with tunable structure and high yield produced by ultrasonic spray pyrolysis

Nitrogen-doped carbon nanotubes (CN_x) were prepared by ultrasonic spray pyrolysis from mixtures of imidazole and acetonitrile. Imidazole, as an additive, was used to control the structure and nitrogen doping in CN_x by adjusting its concentration in the mixtures. Scanning electron microscopy observation showed that the addition of imidazole increased the nanotube growth rate and yield, while decreased the nanotube diameter. Transmission electron microscopy study indicated that the addition of imidazole promoted the formation of a dense bamboo-like structure in CN_x. X-ray photoelectron spectroscopy analysis demonstrated that the nitrogen content varied from 3.2 to 5.2 at.% in CN_x obtained with different imidazole concentrations. Raman spectra study showed that the intensity ratio of D to G bands gradually increased, while that of 2D to G bands decreased, due to increasing imidazole concentration. The yield of CN_x made from mixtures of imidazole and acetonitrile can reach 192 mg in 24 min, which is 15 times that of CN_x prepared from only acetonitrile. The aligned CN_x, with controlled nitrogen doping, tunable structure and high yield, may find applications in developing non-noble catalysts and novel catalyst supports for fuel cells.     

Optical properties of laser plasma produced by pulses of different duration

The results of experiments on the interaction of laser radiation pulses (at intensities of up to 1015 W cm−2) of different durations (t = 3ps; 300ps; 30ns) with targets in a vacuum chamber are presented. An investigation was made of the optical characteristics of laser plasma in vacuum ultraviolet and X-ray ranges. The intensity of the laser-produced plasma radiation in the range 1–50nm was estimated to be 1010–1012W cm−2. This revised version was published online in November 2006 with corrections to the Cover Date.