The super-efficiency procedure for outlier identification,not for ranking efficient units

In this paper, we conduct simulation experiments to evaluate the performance of two alternative uses of the super-efficiency procedure in Data Envelopment Analysis (DEA). The first is for outlier identification and the second is for ranking efficient units. We find that the ranking procedure does not perform satisfactorily. In fact, the correlations between the true efficiency and the estimated super-efficiency are negative for the subset of efficient observations, and the conventional DEA model performs as well as the super-efficiency DEA model when all observations are considered. However, when data are contaminated with outliers, the use of the super-efficiency model to identify and remove outliers results in more accurate efficiency estimates than those obtained from the conventional DEA estimation model.     

Symmetric Weighing Matrices Constructed using Group Matrices

A weighing matrix of order n and weight m² is a square matrix M of order n with entries from {-1,0,+1} such that MM^T=m²I where I is the identity matrix of order n. If M is a group matrix constructed using a group of order n, M is called a group weighing matrix. Recently, group weighing matrices were studied intensively, especially when the groups are cyclic and abelian. In this paper, we study the abelian group weighing matrices that are symmetric, i.e.M^T=M. Some new examples are found. Also we obtain a few exponent bounds on abelian groups that admit symmetric group weighing matrices. In particular, we prove that there is no symmetric abelian group weighing matrices of order 2p^r and weight p² where p is a prime and p≥ 5.Communicated by: K.T. Arasu     

Density,viscosity, conductance,and transference number of concentrated aqueous magnesium chloride at 25°C

The density, viscosity, conductance, and transference number of aqueous magnesium chloride at 298.15°K are reported for the concentration range 0 to 5.7m. The transference numbers were determined using the emf method. It was found that the viscosity, conductance, and transference number of aqueous magnesium chloride are similar to those for aqueous nickel chloride. It was therefore tentatively concluded that incomplete dissociation and complex formation was unlikely to be of importance in nickel chloride solutions.     

Influence of Sm<Superscript>3+</Superscript> ion in structural,morphological, and electrochemical properties of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> synthesized by microwave calcination

LiSm_xMn_2–xO₄ samples were synthesized via co-precipitation technique. The structural properties of the synthesized materials were studied using X-ray diffraction analysis and it confirmed the cubic spinel structure for all the compounds. The lattice parameter of LiMn₂O₄ was observed to be 8.2347 Ǻ and it decreased with Sm³⁺ concentration, due to the shrinkage in cell volume aided by higher binding energy between Sm-O bond. The SEM micrographs were analyzed using Image processing software (Image-J) to ascertain the pore and grain properties. The microwave synthesis had been observed to control the bulk grain formation and had yielded lesser porous and nanoparticles. The particle size distributions obtained through photocross correlation laser diffraction analysis had shown that LiMn₂O₄ with 60 nm and Sm-doped compounds with ∼30 nm, respectively. The cyclic voltammetry studies had revealed the decrease in electrocatalytic behavior in the initial cycle for compounds doped with Sm³⁺ ion. The initial capacities of LiMn₂O₄, LiSm_0.05Mn_1.95O₄ and LiSm_0.10Mn_1.90O₄ substituted compounds were observed to be 134.87 mAhg⁻¹, 132.22 mAhg⁻¹ and 126.41 mAhg⁻¹, respectively. The cells were simulated using 1D model namely Dualfoil5.1 program. The simulated results coincide well with the measured results. The cycle life studies reveal 93% capacity retention of samarium-0.05-doped samples when compared with 78.4% of the LiMn₂O₄.     

Effect of nanoporous GaN templates with different pore diameters on the subsequent thick GaN layers by HVPE

A set of GaN films were overgrown by hydride vapor phase epitaxy (HVPE) on nanoporous GaN templates with different pore diameters. These samples have various properties as seen from the measurements of X-ray diffraction (XRD) and photoluminescence (PL). Cross-sectional observations under a scanning electron microscopy (SEM) reveal that the overgrowth mechanism and process are strongly related to the dimension of nanopores, indicating that an optimum diameter exists for the properties of subsequent HVPE–GaN layers. When the diameters of nanopores are less than the optimum value, the pores on top of GaN templates can be left, and the properties of HVPE–GaN films show significant improvement. In contrast, the pores are almost stuffed with HVPE–GaN films, which obviously limit the improvement degree of HVPE–GaN films.     

Optimization designs and performance comparison of two CUSUM schemes for monitoring process shifts in mean and variance

In statistical process control (SPC), when dealing with a quality characteristic x that is a variable, it is usually necessary to monitor both the mean value and variability. This article proposes an optimization algorithm (called the holistic algorithm) to design the CUSUM charts for this purpose. It facilitates the determination of the charting parameters of the CUSUM charts and considerably or significantly increases their overall detection effectiveness. A single CUSUM chart (called the ABS CUSUM chart) has been developed by the holistic algorithm and fully investigated. This chart is able to detect two-sided mean shifts and increasing variance shifts by inspecting the absolute value of sample mean shift. The results of performance studies show that the overall performance of the ABS CUSUM chart is nearly as good as an optimal 3-CUSUM scheme (a scheme incorporating three individual CUSUM charts). However, since the ABS CUSUM chart is easier for implementation and design, it may be more suitable for many SPC applications in which both mean and variance of a variable have to be monitored.     

Using one EWMA chart to jointly monitor the process mean and variance

Two control charts are usually used to monitor the process mean and variance separately. The mean is monitored using the [`(X)]{\bar{{X}}} chart while the variance using either the standard deviation, S chart, or the range, R chart. Recently, numerous single variable charts are proposed to jointly monitor the mean and variance. Most approaches transform the sample mean and sample variance into two statistics, each having a standard scale, and either plotting them on the same chart or combining them into a single statistic to be plotted on a chart. The R chart is more widely used than the S chart but no attempt is made to combine the [`(X)]{\bar{{X}}} and R charts in the construction of a single variable chart and to study its properties and performance. In this paper, we transform the [`(X)]{\bar{{X}}} and R statistics into two standard normal random variables, used in the computation of two corresponding exponentially weighted moving average (EWMA) statistics, which are then merged into a single plotting statistic for the proposed chart, called the EWMA [`(X)]-R{\bar{{X}}-R} chart.     

Simulated fractal permeability for porous membranes

Fractal permeability model for bi-dispersed porous media is developed based on the fractal characteristics of pores in the membrane. The fractal permeability model is found to be a function of the tortuosity fractal dimension, pore area fractal dimension, sizes of particles and clusters, micro-porosity inside clusters, and the effective porosity of a medium. The pore area fractal dimension and the tortuosity fractal dimension of the porous membranes are determined by the box counting method. To verify the validity of the model, the predicted permeability were compared with the experimental data utilizing H₂ gas permeating through porous Pd-alumina, silicalite-1 and B-ZSM-5, and O₂ across perovskite-alumina membranes form the past effort.     

Effective synthesis of carbon nanotubes via catalytic decomposition of methane: Influence of calcination temperature on metal–support interaction of Co–Mo/MgO catalyst

The present work investigated the influence of calcination temperature for bimetallic Co–Mo/MgO catalyst on the synthesis of carbon nanotubes (CNTs) via catalytic chemical vapor deposition (CCVD) of methane. The experimental results showed that variation in the catalyst calcination temperature affected carbon yield, diameter distribution and quality of the CNTs. Increasing the catalyst calcination temperature enabled Co–Mo/MgO catalysts in growing CNTs at higher yield, narrower diameter distribution and better degree of graphitization, credited to the strong metal–support interaction (MSI) formed between CoO species and MgO support. We also discovered that the catalysts of weak MSI were beneficial to the nucleation and growth of CNTs, meanwhile the catalysts with strong MSI provoked the growth of CNTs with narrow diameter distribution. The catalyst calcined at 700 °C, possessing moderate MSI, was found to be the most suitable catalyst for the growth of high quality CNTs with the diameter of 7.70±0.77 nm and the carbon yield of as high as 647.4%.     

Propionic Anhydride Modification of Cellulosic Kenaf Fibre Enhancement with Bionanocarbon in Nanobiocomposites

The use of chemical modification of cellulosic fibre is applied in order to increase the hydrophobicity, hence improving the compatibility between the fibre and matrix bonding. In this study, the effect of propionic anhydride modification of kenaf fibre was investigated to determine the role of bionanocarbon from oil palm shell agricultural wastes in the improvement of the functional properties of bionanocomposites. The vinyl esters reinforced with unmodified and propionic anhydride modified kenaf fibres bio nanocomposites were prepared using 0, 1, 3, 5 wt% of bio-nanocarbon. Characterisation of the fabricated bionanocomposite was carried out using FESEM, TEM, FT-IR and TGA to investigate the morphological analysis, surface properties, functional and thermal analyses, respectively. Mechanical performance of bionanocomposites was evaluated according to standard methods. The chemical modification of cellulosic fibre with the incorporation of bionanocarbon in the matrix exhibited high enhancement of the tensile, flexural, and impact strengths, for approximately 63.91%, 49.61% and 54.82%, respectively. The morphological, structural and functional analyses revealed that better compatibility of the modified fibre–matrix interaction was achieved at 3% bionanocarbon loading, which indicated improved properties of the bionanocomposite. The nanocomposites exhibited high degradation temperature which signified good thermal stability properties. The improved properties of the bionanocomposite were attributed to the effect of the surface modification and bionanocarbon enhancement of the fibre–matrix networks.