Infinite single-walled boron-nitride nanotubes studies by LGTO-PBC-DFT method

Localized Gaussian type orbital-periodic boundary condition-density functional theory (LGTO-PBC-DFT) method was used to determine the electronic and detailed geometrical structures of (n, 0) zigzag type for n = 6 – 33 and (n, n) armchair type for n = 3 – 15 single-walled boron-nitride (BN) nanotubes with infinite tubular lengths. The calculations reveal that the calculated E_g (band gap between HOCO and LUCO) increases with increasing tubular diameter and eventually converge to 5.03 eV for BN nanotubes of larger tubular diameter. According to the calculated E_gs, the BN nanotubes are semi-conductor and their conductivities are not sensitive to the tubular diameter. Theoretically, the calculated bond length decreases with increasing tubular diameter. Based on our calculations, the bond length and angle do converge to 1.45 Å and 120 degree, respectively. Thus, the structures of BN nanotubes with the infinite tubular length approach the perfect hexagonal network when the tubular diameter increases. The calculated results also indicate that zigzag BN nanotubes with the tubular diameter larger than 18 Å display 3n properties in the calculated E_g, which is also obtained for zigzag carbon nanotubes.     

Probabilistic simulation for developing likelihood distribution of engineering project cost

Inaccurate early project cost estimates can eliminate investment benefits. This study focuses on assisting estimators who are attempting to enhance the accuracy and reliability of engineering project cost in the pre-conceptual stage. This aim has recently garnered the attention of the transportation communities. Data from the Texas Department of Transportation (TxDOT) were utilized to develop an alternative approach that aids decision makers in terms of probability and confidence level. The proposed procedure comprises heuristic and practical simulation models that can be employed to calculate the probabilistic costs of highway bridge replacement projects. The simulation models utilize independent, correlated, and Latin Hypercube sampling approaches that incorporate major work items, roll-up work items, and project-level engineering contingencies. Cumulative distribution functions (CDFs) are then developed as a user-friendly chart for decision makers and these CDFs can be used to assess project risks during the pre-conceptual stage. Trial runs using these estimating procedures generate reliable pre-conceptual estimates. Additionally, these procedures can be extended to other project types along with programming techniques for developing an engineering project cost decision support system.     

Stability and bandwidth enhancement of difference frequencygeneration (DFG)-based wavelength conversion by pump detuning

The authors report on a method for enhancing the operating stability and signal bandwidth of difference-frequency-generation-based wavelength conversion by detuning the pump from the degenerate phase matching wavelength; both can be further enhanced by using nonuniform quasi-phase-matching structures     

Novel impacts of glycol-modified poly(ethylene terephthalate)(PETG) to crystallization behavior of polyethylene naphthalate (PEN) within stretched miscible blends

For studied blends of amorphous glycol-modified poly(ethylene terephthalate) (PETG) and semicrystalline polyethylene naphthalate (PEN), melt miscibility is understood from the linear variation of a single glass transition temperature (T_g) over the entire composition range. The diluent effect of PETG component severely retarded the crystallization of PEN component within blends. Nevertheless, after being through isothermal stretching at 120 °C, crystallization was able to progress efficiently during heating in a continuous manner. Instead of being thermally relaxed back to amorphous state, parallel sliding motions of stretched PEN segments toward crystallization appear rather dominant. Within stretched blends, the PETG content emerged as a critical factor to the crystallinity increase of PEN fraction and the absence of lattice defect, instead of behaving as a diluent component. Furthermore, as being indicated by in-situ small-angle X-ray experiments, regular lamellar stacking gradually developed within stretched blends through heating, which indicates the involvement of thermally activated self-association of randomly distributed crystalline lamellae. With including a higher fraction of PETG component, these secondary ordering processes including lamellar thickening can be activated at lower temperature. Hence, the accompanied thermal relaxation of flexible PETG segment is inferred able to lubricate the sliding of stretched PEN segments in amorphous regions via lowering encountered frictional hindrance, and thus enhance both primary and secondary ordering processes within stretched blends.     

Effect of polymers on the nanostructure and on the carbonation of calcium silicate hydrates: a scanning transmission X-ray microscopy study

This study investigated the effects of organic polymers (polyethylene glycol and hexadecyltrimethylammonium) on structures of calcium silicate hydrates (C–S–H) which is the major product of Portland cement hydration. Increased surface areas and expansion of layers were observed for all organic polymer modified C–S–H. The results from attenuated total reflectance–Fourier transform infrared (ATR–FTIR) spectroscopic measurements also suggest lowered water contents in the layered structures for the C–S–H samples that are modified by organic polymers. Scanning transmission X-ray microscopy (STXM) results further supports this observation. We also observed difference in the extent of C–S–H carbonation due to the presence of organic polymers. No calcite formed in the presence of HDTMA whereas formation of calcite was observed with C–S–H sample modified with PEG. We suggest that the difference in the carbonation reaction is possibly due to the ease of penetration and diffusion of the CO₂. This observation suggests that CO₂ reaction strongly depends on the presence of organic polymers and the types of organic polymers incorporated within the C–S–H structure. This is the first comprehensive study using STXM to quantitatively characterize the level of heterogeneity in cementitious materials at high spatial and spectral resolutions. The results from BET, XRD, ATR–FTIR, and STXM measurements are consistent and suggest that C–S–H layer structures are significantly modified due to the presence of organic polymers, and that the chemical composition and structural differences among the organic polymers determine the extent of the changes in the C–S–H nanostructures as well as the extent of carbonation reaction.     

A new model to describe absorption kinetics of Mg-based hydrogen storage alloys

Based on Chou model and unreacted-core model, a new mixed rate controlling kinetic model has been derived in this paper to investigate the adsorption reaction time t for Mg-based hydrogen storage materials as a function of temperature T, particle radius R₀ and reaction fraction ζ. This new model could be predigested into individual single step and mixed two-connection step equations. The characters of this new model have also been discussed. Moreover, the new model is successfully applied for a real case and results indicate that this new model works very well and could reasonably deal with complex kinetics mechanism.     

Predicting high-tech equipment fabrication cost with a novel evolutionary SVM inference model

Accurately predicting fabricating cost in a timely manner can enhance corporate competitiveness. This study employs the Evolutionary Support Vector Machine Inference Model (ESIM) to predict the cost of manufacturing thin-film transistor liquid–crystal display (TFT-LCD) equipment. The ESIM is a hybrid model integrating a support vector machine (SVM) with a fast messy genetic algorithm (fmGA). The SVM concerns primarily with learning and curve fitting, while the fmGA is focuses on optimization of minimal errors. Recently completed equipment development projects are utilized to assess prediction performance. The ESIM is developed to achieve the fittest C and γ parameters with minimized prediction error when used for cost estimate during conceptual stages. This study describes an actionable knowledge-discovery process using real-world data for high-tech equipment manufacturing industries. Analytical results demonstrate that the ESIM can predict the costs of manufacturing TFT-LCD fabrication equipment with sufficient accuracy.     

Synthesis and characterization of the iron/copper composite as an electrode material for the rechargeable alkaline battery

Iron/copper composite particles were synthesized by a chemical reduction method and then used as the anode material for a rechargeable alkaline battery. The particle size and structure of the samples were characterized by SEM and XRD. Their electrochemical performance was also studied. The results showed that the iron/copper composite prepared by this method is nanosized. Copper improves the electron transfer between particles, and the nanosized iron/copper composite not only has a high electrochemical capacity of up to 800 mAh g⁻¹(Fe to Fe(III)), but also has an excellent rate-capacity performance at a current density of 3200 mA g⁻¹. Compared with the iron nanoparticle without copper, the iron/copper composite sample maintains a smaller particle size during electrochemical cycling, and therefore improves the cycling stability of the iron electrode.     

Local inhomogeneity and surface degradation of Fe1.15Te and Fe1.03Te0.62Se0.38 single crystals

We have investigated the structural and magnetic properties of Fe_1.15Te and Fe_1.03Te_0.62Se_0.38 single crystals grown by slow cooling method using a specially designed double-walled quartz ampoule. X-ray diffraction reveals significant local inhomogeneity as a result of fractional substitution of Te site by Se in single crystal Fe_1.03Te_0.62Se_0.38. The existence of superconductivity is confirmed for Se substitution to the Te site of Fe_1.03Te_0.62Se_0.38 but with persistent Fe₇Se₈ impurity phase. Raman spectroscopy confirmed the surface structural instability when the samples were exposed to moderate laser irradiation in the atmospheric environment. With high intensity laser irradiation, Fe(Se,Te) compounds can be decomposed into Fe₂O₃ hematite phase.