X-ray diffraction analysis of the surface acoustic wave propagation in langatate crystal

X-ray diffraction on a langatate crystal (La₃Ga_5.5Ta_0.5O₁₄, LGT) modulated by a Λ=12 μm Rayleigh surface acoustic wave (SAW) was studied in a double axis X-ray diffractometer scheme at the BESSY synchrotron radiation source. SAW propagation in the crystal causes sinusoidal modulation of the crystal lattice and the appearance of diffraction satellites on the rocking curves, with their number, angular positions, and intensities depending on the wavelength and amplitude of acoustic vibrations of the crystal lattice. Strong absorption of X-ray radiation in LGT enables the observation of the diffraction spectra extinction at certain SAW amplitudes. X-ray diffraction spectra analysis makes it possible to determine SAW amplitudes and wavelengths, to measure the power flow angles, and investigate the diffraction divergence in acoustic beam in LGT.     

Effects of carbon fiber surface treatment on the tribological properties of 2D woven carbon fabric/polyimide composites

Polyacrylonitrile (PAN)-based carbon fabric (CF) was modified with strong HNO₃ oxidation and then introduced into polyimide (PI) composites. The friction and wear properties of the carbon fabric reinforced polyimide composites (CFRP), sliding against GCr15 stainless steel rings, were investigated on an M-2000 model ring-on-block test rig under dry sliding. Experimental results revealed that the carbon fiber surface treatment largely reduced the friction and wear of the CFRP. Compared with the untreated ones, the surface-modified CF can enhance the tribological properties of CFRP efficiently due to the improved adhesion between the CF and the PI matrix. Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) study of the carbon fiber surface showed that the fiber surface became rougher and the oxygen concentration increased greatly after surface treatment, which improved the adhesion between the fiber and the PI matrix and improved the friction-reduction and anti-wear properties of the CFRP. An erratum to this article can be found at     

Finite element simulation for estimating the mechanical properties of multi-walled carbon nanotubes

A finite element simulation technique for estimating the mechanical properties of multi-walled carbon nanotubes is developed. In the present modeling concept, individual carbon nanotube is simulated as a frame-like structure and the primary bonds between two nearest-neighboring atoms are treated as beam elements, the beam element properties are determined via the concept of energy equivalence between molecular dynamics and structural mechanics. As to the simulation of the interlayer van der Waals force which has intrinsic nonlinearity and complicated applying region, a simplifying method is proposed that the interlayer pressure caused by van der Waals force instead of the force itself is to be considered, and we make use of the linear part of the interlayer pressure near the equilibrium condition to avoid the nonlinearity in problem, then linear spring elements whose stiffness is determined by equivalent force concept can be utilized to simulate the interlayer van der Waals force such that significant modeling and computing effort is saved in performing the finite element analysis. Numerical examples for estimating the mechanical properties of nanotubes, such as axial and radial Young’s modulus, shear modulus, natural frequency, buckling load, etc., are presented to illustrate the accuracy of this simulation technique. By comparing to the results found in the literature and the possible analytical solutions, it shows that the obtained mechanical properties of nanotubes by the present method agree well with their comparable results. In addition, the relations between these mechanical properties and the nanotube size are also discussed.     

Measurement of the optical Stark effect in semiconducting carbon nanotubes

A strong optical Stark effect has been observed in (6,5) semiconducting single-walled carbon nanotubes by femtosecond pump-probe spectroscopy. The response is characterized by an instantaneous blueshift of the excitonic resonance upon application of pump radiation at photon energy well below the band gap. The large Stark effect is attributed to the enhanced Coulomb interactions present in these one-dimensional materials.     

Synthesis of multi-wall carbon nanotubes by copper vapor laser

Synthesis of multi-wall carbon nanotubes in a 1473 K furnace using a copper vapor laser (CVL) is reported. The operating parameters of this laser, i.e. a high fluence at the focal point and an extremely high frequency of 10 kHz, distinguished it from common laser sources in the synthesis of CNTs. Therefore, the unexpected experimental findings, the formation of MWNTs instead of the generally reported SWNTs, would be verified by these two notable parameters. Electron microscopy beside Raman spectroscopy illustrates the presence of multi-wall carbon nanotubes in the resulting product.     

Fast deuteron production from antiproton annihilation at rest on carbon and uranium targets

Deuteron yields from 20–350 MeV kinetic energy in antiproton annihilation at rest in¹²C and²³⁸U targets are compared with a coalescence model calculation. Agreement of the data with the model up to approximately 80 MeV is good. However, from 80 to 350 MeV the model increasingly underestimates the yield, by as much as on order of magnitude. These results, along with previously reported failures to explain proton spectra with first order rescattering of pions, suggest that other mechanisms are manifest in relatively rare, high energy light nucleus emission, e.g.B > 0 annihilations or possible six quark interactions.Work supported in part by the U.S. Air Force Office of Scientific Research and the National Science Foundation     

Thermal stability of catalytically grown multi-walled carbon nanotubes observed in transmission electron microscopy

The thermal stability of multi-walled carbon nanotubes (MWCNTs) was assessed in situ by transmission electron microscopy. Upon heating, Ni catalysts in MWCNTs containing bamboo structures shrank from the tail due to evaporation, leading to additional bamboo formation and tube elongation at 800°C, while the MWCNTs with FeSi catalysts remained intact up to 1050°C except for better crystallinity. The physisorbed carbon and/or hydrocarbon on surfaces and super-saturated carbon in the Ni catalysts should be responsible for the phenomena.     

Fabrication of completely filled carbon nanotubes with copper nanowires in a confined space

Carbon nanotubes (CNTs) filled completely with polycrystalline Cu nanowires were synthesized by laser vaporization of Cu and graphite under high-pressure Ar gas atmosphere. Depending on the Ar gas pressure (0.1–0.9 MPa) and the Cu content (1–40 at.%) in graphite targets for laser vaporization, various products with different morphologies were observed by scanning and transmission electron microscopy. The ratios of the Cu-filled CNTs and carbon nanocapsules particularly increased as Ar gas pressure was increased. The maximum ∼60% fraction of Cu-filled CNTs with outer diameter of 10–50 nm and length of 0.3–3 μm was achieved at 0.9 MPa from graphite containing 20 at.% Cu. Most of the encapsulated Cu-nanowires were surrounded by single, double, or triple graphitic layers. Although the yield of the Cu-filled CNTs was also dependent on the Cu content in the graphite targets, no unfilled CNTs were produced even for low Cu content. The growth of Cu-filled CNTs is explained by the formation of molten Cu–C composite particles with an unusually C-rich composition in a space confined by high-pressure Ar gas, followed by precipitating Cu and C from the particles and subjecting them to phase separation.     

Experimental studies of pulsed signal propagation from the shelf to deep sea

Results of an experimental study of low-frequency broadband pulsed signal propagation in a waveguide that includes the shelf zone, the continental slope, and the deep sea region are presented. Using phase-manipulated signals with central frequencies of 366 and 600 Hz, pulsed characteristics are measured at six points along the propagation track, the maximal distance from the source being 368 km. It is experimentally demonstrated that, in the presence of a negative sound velocity gradient in the near bottom layer on the shelf with a small bottom slope, the choice of the source position at the shelf bottom near the shoreline provides the formation of a continuous illumination zone in the deep sea near the USC axis and a stable pulsed characteristic with two main sound energy arrivals. The propagation velocity of the pulse that is last to arrive is identical (within the measurement error) to the velocity of sound on the USC axis at the point of reception. Possibilities for practical application of the results obtained from the experiment are discussed.     

Pressure variation of Luttinger liquid parameters in single wall carbon nanotubes networks

We measure electrical transport on networks of single wall nanotube of different origin as a function of temperature T, voltage V and pressure P. We observe Luttinger liquid (LL) behavior, a conductance ∝T^α and a dynamic conductance ∝V^α. We observe a sample dependent P variation of the α parameters, interpreted as Fermi level changes due to pressure induced charge transfer. We show how, through standard four-leads and crossed configuration methods, it is possible to determine α_bulk and α_end, respectively. We study and discuss the pressure and doping level dependences of the number of channels N, the LL parameter g and the intra-rope tube-tube coupling constant U within a phenomenological model.     

First-principles study for transport properties of defective carbon nanotubes with oxygen adsorption

Oxygen gas usually presents in carbon nanotube (CNT) based devices and can affect their transport properties. Here, we perform simulations for O₂ adsorption on a (5, 5) CNT with a double vacancy. We first use first-principles plane-wave calculation to optimize the structures and then use single-particle Green function method to study their transport properties. It is found that an O₂ can be either physisorbed or chemisorbed on the defective CNT. The physisorption has only minor effects on the transport while the chemisorption can improve it and the resulting conductance is affected by the orientation of the O₂ bonding.     

Propagation of terawatt laser pulses in the air

One of the problems when increasing the intensity of a femtosecond laser pulse is the propagation of the beam. As the intensity increases nonlinear effects begin to play a significant role. When arriving to the terawatt domain, nonlinear effects and filamentation give rise to a new phenomenology in the propagation. The aim of this paper is to analyze new possibilities to control the beam shape to Taylor the interaction of the beam with the target at large distances.     

Hyperspherical approach to description of electron correlation in negative ion of exciton

The binding energy of excitonium negative ion for the ground^1,3S-states in bulk semiconductors GaAs and ZnO in the hyperspherical coordinate method was found. Angular and radial correlations between electrons in gerade and ungerade states were taken into account by channel functions, that are the eigenfunctions of Hamiltonian on the surface of the sphere in the three-dimensional configuration space. Energy values were calculated using the adiabatic and Born-Oppenheimer approximations. The obtained energy values are in agreement with those obtained using variational method.     

Dye-sensitized solar cells based on TiO2 nanotube/porous layer mixed morphology

Titania porous layer has been fabricated on titania nanotubes for dye sensitized solar cells and the photovoltaic performance of solar cells with mixed morphology has been investigated. The porous layer results in a similar improvement in the short circuit current density to conventional TiCl₄ treatment, although the mechanisms responsible for the observed increase in the efficiency are different. This enables further improvements of the photovoltaic performance by combining the TiCl₄ treatment and porous layer deposition, so that the efficiency in the case of ∼5 μm long tubes increases on average from ∼1.6 to ∼2.2%.     

Optimization of field emission properties from multi-walled carbon nanotubes using ceramic fillers

The field emission properties of multi-walled carbon nanotubes were examined using a screen-printed thick film with a diode-type configuration in a vacuum. The effects of various concentrations of two different ceramic fillers, indium tin oxide (ITO) powder and a glass frit, on the emission current density and turn-on field were evaluated. The emission properties of both pastes were dependent on the amount of filler. Considerably enhanced emission properties were obtained with the paste containing 5–10 wt.% of either ITO or the glass frit compared with those without a filler. The paste containing the ceramic filler showed enhanced emission properties compared with that containing the 5 wt.% Ag conventionally used, which confirmed the importance of the filler. The paste containing 10 wt.% ITO represented an emission current density of 176.4 μA/cm² at 5 V/μA, a turn-on field of 1.87 V/μA for an emission current density of 1 μA/cm² and a field enhancement factor of 7580. The paste formulation was also found to be suitable for fine patterning using UV-lithography techniques. A long-term stability test for 110 h of a paste containing 10 wt.% ITO revealed a half-life of approximately 30000 h, which is appropriate for commercial applications.     

Shrinkage stress and mechanical properties of photoactivated composite resin using the argon ion laser

The objective of this study was to verify the influence of photoactivation with the argon ion laser on shrinkage stress (SS), followed by evaluation of Vickers microhardness (VM), percentage of maximum hardness (PMH), flexural strength (FS), and flexural modulus (FM) of a composite resin. The study groups were: L1-laser at 200 mW for 10 seconds; L2-laser at 200 mW for 20 seconds; L3-laser at 250 mW for 10 seconds; L4-laser at 250 mW for 20 seconds; H-halogen light at 275 mW for 20 seconds. Data were analyzed by ANOVA/Tukey’s test (α=5%). The values of SS (MPa) were statistically lower for the group L3 (1.3)c, followed by groups L1 (2.7)b, L4 (3.4)a, b, L2 (3.7)a, and H (4.5)a. There was no difference in the values of VM when the same time of photoactivation was used, with respective values being L1=70.1a, L2=78.1b, L3=69.9a, L4=78.1b and H=79.9b. All groups showed a PMH of at least 80%. Only the group L1 showed differences in FS (MPa) and FM (GPa), the respective values of 86.2 and 5.4 being lower. Therefore, the use of argon ion laser had influenced the composite resin polymerization. The L3 group presented adequate mechanical properties and minimum SS, reducing the clinical working time for photoactivation of restorations with the tested resin by 50%.     

Mechanical properties of nanotubes of polyelectrolyte multilayers

The elastic properties of nanotubes fabricated by layer-by-layer (LbL) assembly of polyelectrolytes in the nanopores of polycarbonate track-etched membranes have been investigated by resonant contact Atomic Force Microscopy (AFM), for nanotube diameters in the range of 100 to 200 nm. The elastic modulus of the nanotubes was computed from the resonance frequencies of a cantilever resting on freely suspended LbL nanotubes. An average value of 115 MPa was found in air for Young's modulus of these nanostructures, well below the values reported for dry, flat multilayers, but in the range of values reported for water-swollen flat multilayers. These low values are most probably due to the lower degree of ionic cross-linking of LbL nanotubes and their consequently higher water content in air, resulting from the peculiar mode of growth of nanoconfined polyelectrolyte multilayers.     

High repetition rate femtosecond laser nano-machining of thin films

Thin film laser micromachining has been utilized for repairing semiconductor masks, creating solar cells and fabricating MEMS devices. A unique high repetition rate femtosecond fiber laser system capable of variable repetition rates from 200 KHz to 25 MHz along with helium gas assist was used to study the effect of pulse repetition rate and pulse energy on femtosecond laser machining of gold-coated silicon wafer. It was seen that high repetition rates lead to smaller craters with uniform line width. Craters created at 13 MHz pulse repetition rate with 2.042 J/cm² beam energy fluence measured 110 nm in width and had a heat affected zone of 0.79 μm. It was found that pulse repetition rate only played a significant role in the size of the heat affected zone in the lower pulse energy ranges. In the future, a 1 W laser system will be acquired to find the optimal repetition rate that would create the minimal feature size with the least heat affected zone. Using this kind of setup along with techniques such as radial polarization and a different gas assist may enable us to create sub 100 nm feature size with good quality.     

Calculation by plane wave Born approximations of electron-impact ionization of silver and copper

The plane wave Born approximation is used to calculate total electron impact ionization cross section of silver and copper. Wavefunctions of the target and residual ions were modeled by non orthogonal Hartree-Fock and Dirac-Fock orbitals. The wave functions of the atom and residual ion are calculated with allowance for relaxation effects. The one-electron wavefunction of the continuous spectrum for the ejected electron is obtained using single-configuration Hartree-Fock and Dirac-Fock method. The orthogonalization of the ejected electron wave functions to all occupied orbitals of the target atom is performed. Results of calculations are compared to available experimental measurements and theoretical calculations performed by non relativistic one-electron PWBA, where the ejected electrons is modeled by the hydrogenic Coulomb wave function.     

Non-linear pressure dependence of A-state fluorescence lifetime of formaldehyde

Fluorescence lifetimes of formaldehyde excited at 352 nm ( A₂ – A₁ 4₀¹ band) were measured as a function of bath gas pressure. He, N₂, O₂, CO₂ and HCHO were investigated for the bath gas and the temperature dependence between 298 and 500 K for N₂ and O₂ bath gases was also examined. It was found that the non-linear pressure dependence of the lifetime is successfully reproduced by the model formula