The modification of graphene with alcohols and its use in shape memory polyurethane composites

Graphene prepared by the thermal reduction of graphite oxide was modified by reactions with methanol or 1‐butanol using aqueous HBF₄ solution as a catalyst. Results showed that the reaction created hydroxyl groups on the graphene and at the same time reduced the number of defects. Gravimetry, thermogravimetry, X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy showed that the alcohols had reacted with epoxide groups on graphene. Raman spectroscopy showed that the defects in the graphene were repaired through other accompanying reactions. The reinforcing effect of graphene, observed in the tensile properties and the shape memory behavior of graphene/polyurethane composites, was increased when the graphene was modified with methanol. However, decreases in density and glass transition temperature were evident for the composites made with alcohol‐modified graphene. These results show that the newly created hydroxyl groups on graphene produce effective covalent bonds with the polyurethane chains of the matrix; however, the increased number of bonds restricts the rearrangement of the matrix molecules for dense packing. The covalent bonds between graphene and polyurethane chains enhanced shape recoverability and reduced the hysteresis brought about by repeated thermomechanical cycles. Copyright © 2012 Society of Chemical Industry     

New statistical method for detection and quantification ofrespiratory sinus arrhythmia

A statistical method with the advantages of 1) enabling graphical representation of within-respiratory cycle heart rate variations, 2) detecting the presence of respiratory sinus arrhythmia (RSA) in a moving window, and 3) providing breath-by-breath RSA amplitude and phase obtained from the fitting of a sinusoid to the instantaneous relative heart rate is presented.     

A systematic approach to the construction of bandwidth-efficient multidimensional trellis codes

We present a unified approach to the construction of multidimensional trellis codes that have small constellation expansion ratio (CER) and small peak-to-average power ratio (PAR) and hence are of high bandwidth efficiency. In the approach, we extend Wei's (1987) construction to dimensionalities other than a power of two and propose a low-complexity encoder. We also present a general partition technique that gives geometrically uniform partitions with a good distance profile. The proposed method can generate many good codes including previously reported Ungerboeck codes, Gallager-Calderbank-Sloane codes, and Wei codes with quadrature amplitude modulation signals. Some newly discovered six-dimensional trellis codes are compared with these known codes on their CER, PAR, effective coding gain, and encoder/decoder complexities.     

Calculation of thermodynamic quantities of metals and alloys by the statistical moment method

The thermodynamic properties of metals and alloys are studied using the moment method in the statistical dynamics, which allows us to take into account the anharmonicity of thermal lattice vibrations and size mismatch of constituent atoms, going beyond the quasi-harmonic approximation. Within the fourth-order moment approximation, the free energy and equilibrium lattice spacing of the binary alloys are given explicitly in terms of the effective pair potentials and the second- and fourth-order vibrational constants. The long-range order (LRO) parameter η and order-disorder transition temperatures (ODTs) T_c of binary alloys are obtained by solving the explicit transcendental equations. The numerical calculations of thermodynamic quantities for Cu₃Au and β-CuZn alloys show that the inclusion of the anharmonicity of lattice vibration plays an essentially important role in determining the phase stabilities of metals and alloys.     

Effect of fluid Prandtl number on heat transfer characteristics in internally heated liquid pools with Rayleigh numbers up to 10

This paper presents an analysis of effects of the fluid Prandtl number (Pr) on natural convection heat transfer in volumetrically heated liquid pools. Experimental and computational studies performed in the past are reviewed, with particular emphasis on the analysis of Pr number effects. As a practical exercise, numerical analysis is performed for two-dimensional square, semicircular and elliptical enclosures, and for three-dimensional semicircular and hemispherical cavities, to investigate the physics of the effect of the Pr number on heat transfer in internally heated liquid pools with Rayleigh numbers up to 10¹². It was found that the fluid Prandtl number has a small effect on heat transfer in the convection-dominated regions (near the top surface and side walls) of the enclosures. The decrease of the Pr number leads to the decrease of the top and side wall Nusselt (Nu) numbers. The effects of the Pr number on the Nu number at the bottom surface of the enclosures are found to be significant and they become larger with increasing Rayleigh numbers. Two physical mechanisms, i.e. thermal diffusivity and kinematic viscosity phenomena, have been proposed to explain the fluid Prandtl number effects. Calculational results have been used to quantify the significance and the area of influence for each mechanism. Also, strong dependence on the geometry (curvilinearity) of the downward cooled pool surface has been found.     

Four-wave mixing between pump and signal in a distributed Raman amplifier

We observed experimentally four-wave mixing (FWM) between a 14xx-nm pump and a 15xx-nm signal in a forward-pumped distributed Raman amplifier (DRA) over 50 km of nonzero dispersion-shifted fiber with a zero dispersion wavelength of 1497 nm. The 100-mW pump Fabry-Perot (FP) spectra centered at 1440, 1450, and 1460 nm are reproduced via FWM around the single-wavelength probe signal around 1558, 1548, and 1538 nm, respectively. The suppression of DRA gain by about 2-3 dB was experimentally observed with peak FWM at minimum phase mismatching between two 14xx-nm FP pump wavelengths and two 15xx-nm signal wavelengths. This DRA gain suppression, together with the reproduced pump FP spectrum at 15xx-nm signal band, may limit the usefulness of the forward-pumped DRA, generating spectrally nonuniform FWM-induced noise floors and crosstalk in wavelength-division-multiplexed fiber-optic transmission systems.     

Plasticity theory of ductile fracture by void growth and coalescence

The axisymmetric cold forging is used in the automobile industry to manufacture the formed parts en masse. But, this process is often limited by the onset of surface cracking which reduces the quality of the final product. In this paper, the theoretical investigation of the initiation of the ductile fracture in axisymmetric upsetting processes is carried out using a simple method of analysis. The developed method is based on the theory of plasticity and a fracture criterion, in which the influence of the growth and coalescence of the cylindrical and spherical voids is considered. The cracking at the free surface of the cylindrical billet is modelled. Dedicated to Professor Theordor Lehmann, Bochum     

Optimization of the dye-sensitized solar cell performance by mechanical compression

In this study, the P25 titanium dioxide (TiO₂) nanoparticle (NP) thin film was coated on the fluorine-doped tin oxide (FTO) glass substrate by a doctor blade method. The film then compressed mechanically to be the photoanode of dye-sensitized solar cells (DSSCs). Various compression pressures on TiO₂ NP film were tested to optimize the performance of DSSCs. The mechanical compression reduces TiO₂ inter-particle distance improving the electron transport efficiency. The UV–vis spectrophotometer and electrochemical impedance spectroscopy (EIS) were employed to quantify the light-harvesting efficiency and the charge transport impedance at various interfaces in DSSC, respectively. The incident photon-to-current conversion efficiency was also monitored. The results show that when the DSSC fabricated by the TiO₂ NP thin film compressed at pressure of 279 kg/cm², the minimum resistance of 9.38 Ω at dye/TiO₂ NP/electrolyte interfaces, the maximum short-circuit photocurrent density of 15.11 mA/cm², and the photoelectric conversion efficiency of 5.94% were observed. Compared to the DSSC fabricated by the non-compression of TiO₂ NP thin film, the overall conversion efficiency is improved over 19.5%. The study proves that under suitable compression pressure the performance of DSSC can be optimized.     

Using Genetic Algorithms to Optimize Stopping Patterns for Passenger Rail Transportation

In a passenger railroad system, the stopping pattern optimization problem determines the train stopping strategy, taking into consideration multiple train classes, station types, and customer origin‐destination (OD) demand, to maximize the profit made by a rail company. The stopping pattern is traditionally decided by rule of thumb, an approach that leaves much room for improvement. In this article, we propose an integer program for this problem and provide a systematic approach to determining the optimal train stopping pattern for a rail company. Commonly used commercial optimization packages cannot solve this complex problem efficiently, especially when problems of realistic size need to be solved. Therefore, we develop two genetic algorithms, namely binary‐coded genetic algorithm (BGA) and integer‐coded genetic algorithm (IGA). In many of the past evolutionary programming studies, the chromosome was coded using the binary alphabet as BGA. The encoding and genetic operators of BGA are straightforward and relatively easy to implement. However, we show that it is difficult for the BGA to converge to feasible solutions for the stopping pattern optimization problem due to the complex solution space. Therefore, we propose an IGA with new encoding mechanism and genetic operators. Numerical results show that the proposed IGA can solve real‐world problems that are beyond the reach of commonly used optimization packages.