Characterization,prediction, and optimization of flexural properties of vapor‐grown carbon nanofiber/vinyl ester nanocomposites by response surface modeling

A design of experiments and response surface modeling were performed to investigate the effects of formulation and processing factors on the flexural moduli and strengths of vapor‐grown carbon nanofiber (VGCNF)/vinyl ester (VE) nanocomposites. VGCNF type (pristine, surface‐oxidized), use of a dispersing agent (no, yes), mixing method (ultrasonication, high‐shear mixing, and a combination of both), and VGCNF weight fraction (0.00, 0.25, 0.50, 0.75, and 1.00 parts per hundred parts resin (phr)) were selected as independent factors. Response surface models were developed to predict flexural moduli and strengths as a continuous function of VGCNF weight fraction. The use of surface‐oxidized nanofibers, a dispersing agent, and high‐shear mixing at 0.48 phr of VGCNF led to an average increase of 19% in the predicted flexural modulus over that of the neat VE. High‐shear mixing with 0.60 phr of VGCNF resulted in a remarkable 49% increase in nanocomposite flexural strength relative to that of the neat VE. This article underscores the advantages of statistical design of experiments and response surface modeling in characterizing and optimizing polymer nanocomposites for automotive structural applications. Moreover, response surface models may be used to tailor the mechanical properties of nanocomposites over a range of anticipated operating environments. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2087–2099, 2013     

Creep characterization of vapor‐grown carbon nanofiber/vinyl ester nanocomposites using a response surface methodology

The effects of selected factors such as vapor‐grown carbon nanofiber (VGCNF) weight fraction, applied stress, and temperature on the viscoelastic responses (creep strain and creep compliance) of VGCNF/vinyl ester (VE) nanocomposites were studied using a central composite design (CCD). Nanocomposite test articles were fabricated by high‐shear mixing, casting, curing, and post curing in an open‐face mold under a nitrogen environment. Short‐term creep/creep recovery experiments were conducted at prescribed combinations of temperature (23.8–69.2°C), applied stress (30.2–49.8 MPa), and VGCNF weight fraction (0.00–1.00 parts of VGCNF per hundred parts of resin) determined from the CCD. Response surface models (RSMs) for predicting these viscoelastic responses were developed using the least squares method and an analysis of variance procedure. The response surface estimates indicate that increasing the VGCNF weight fraction marginally increases the creep resistance of the VGCNF/VE nanocomposite at low temperatures (i.e., 23.8–46.5°C). However, increasing the VGCNF weight fraction decreased the creep resistance of these nanocomposites for temperatures greater than 50°C. The latter response may be due to a decrease in the nanofiber‐to‐matrix adhesion as the temperature is increased. The RSMs for creep strain and creep compliance revealed the interactions between the VGCNF weight fraction, stress, and temperature on the creep behavior of thermoset polymer nanocomposites. The design of experiments approach is useful in revealing interactions between selected factors, and thus can facilitate the development of more physics‐based models. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42162.     

CMOS second-harmonic quadrature voltage controlled oscillator using substrate for coupling

In this work, a new circuit configuration for second-harmonic quadrature voltage controlled oscillator (QVCO) with CMOS technology is proposed. The proposed QVCO is made by coupling two identical cross-connected VCOs. The coupling elements (two resistors and two capacitors) do not increase the power consumption of the core VCOs and do not disturb the resonant frequency of the tank circuit in the core VCOs and also, according to the simulations the coupling elements do not adversely affect the phase noise. The role of the substrate terminal of cross-connected MOSFETs of the core oscillators as common mode nodes is demonstrated. Using this node for coupling makes it possible to eliminate the tail transistors in the core oscillators and therefore the circuit can operate with supply voltages as low as 0.5 V. Using the same method to couple more than two core oscillators, results in a multiphase VCO.     

Graphite crystals grown within electromagnetically levitated metallic droplets

Various graphite morphologies were observed to grow within the electromagnetically levitated nickel–carbon melts, including primary flakes and spheres, curved surface graphite and eutectic flakes, as well as engulfed and entrapped particles. As the supersaturated metallic solutions were cooled within the electromagnetic (EM) levitation coil, the primary graphite flakes and spheres formed and accumulated near the periphery of the droplet due to EM circulation. The primary graphite islands, moreover, nucleated and grew on the droplet surface which eventually formed a macroscopic curved graphite crystal covering the entire liquid. Upon further cooling, the liquid surrounding the primary graphite went under a coupled eutectic reaction while the liquid in the center formed a divorced eutectic due to EM mixing. This brought about the formation of graphite fine flakes and agglomerated particles close to the surface and in the center of the droplet, respectively. The graphite morphologies, growth mechanisms, defects, irregularities and growth instabilities were interpreted with detailed optical and scanning electron microscopies.     

A remote interactive non-repudiation multimedia-based m-learning system

One of the current challenges regarding distance learning systems, from a performance point of view, is the efficient and timely delivery of multimedia-enriched learning materials. Providing guaranteed Class of Service (CoS) and Quality of Service (QoS) are also challenging especially for remote sites and rural areas where Internet coverage tends to be limited. On a different note, another challenge is to track the audience accessing the learning materials and more importantly to monitor the true identity of the examination attendees. This paper aims to investigate both of these issues simultaneously, with an introduction of a non-repudiation system that provides a security mechanism, as well as maintaining certain QoS measures. This system not only authenticates the intended party, but also integrates a digital signature scheme accompanied with the transmitted multimedia-based information. The included digital signature prevents a later dispute from the involved parties that the communication ever took place or they ever took part in the communication.     

Classification of boreal forest cover types using SAR images

Mapping forest cover types in the boreal ecosystem is important for understanding the processes governing the interaction of the surface with the atmosphere. In this paper, we report the results of the land-cover classification of the SAR (synthetic aperture radar) data acquired during the Boreal Ecosystem Atmospheric Study's intensive field campaigns over the southern study area near Prince Albert, Canada. A Bayesian maximum a posteriori classifier was applied on the national Aeronautics and Space Administration/Jet Propulsion Laboratory airborne SAR images covering the region during the peak of the growing season in July 1994. The approach is supervised in the sense that a combination of field data and existing land-cover maps are used to develop training areas for the desired classes. The images acquired were first radiometrically and absolutely calibrated, the incidence angle effect in airborne images was corrected to an acceptable accuracy, and the images were used in a mosaic form and geocoded and georeferenced with an existing land-cover map for validation purposes. The results show that SAR images can be classified into dominant forest types such as jack pine, black spruce, trembling aspen, clearing, open water, and three categories of mixed strands with better than 90% accuracy. The unispecies stands such as jack pine and black spruce are separated with 98% accuracy, but the accuracy of mixed coniferous and deciduous stands suffers from confusing factors such as varying species composition, surface moisture, and understory effects. To satisfy the requirements of process models, the number of cover types was reduced from eight to five general classes of conifer wet, conifer dry, mixed deciduous, disturbed, and open water. Reduction of classes improved the overall accuracy of the classification over the entire region from 77% to 92%.     

Linear Neural Circuitry Model for Visual Receptive Fields

The current state of art in the literature indicates that linear visual receptive fields are Gaussian or formed based on Gaussian kernels in biological visual systems. In this paper, by employing hypotheses based on the anatomy and physiology of vertebrate biological vision, we propose a neural circuitry possessing Gaussian-related visual receptive fields. Here, we present a plausible circuitry system matching the characteristic properties of an ideal visual front end of biological visual systems and then present a condition under which this circuit demonstrates a linear behaviour to model the linear receptive fields observed in the biological experimental data. The objective of this study is to understand the hardware circuitry from which various visual receptive fields in biological visual system can be deduced. In our model, a nonlinear neural network communicating with spikes is considered. The condition under which this neural network behaves linearly is discussed. The equivalent linear circuit proposed here employs some anatomical and physiological properties of the early biological visual pathway to derive the visual receptive field profiles for linear cells such as neurons with isotropic separable, non-isotropic separable and non-separable (velocity-adapted) Gaussian receptive fields in the LGN and striate cortex. In the model presented here, the theory of transmission lines for linear distributed electrical circuits is employed for two-dimensional transmission grids to model cell connectivities in a neural layer. The model presented here leads to a formulation similar to the Gaussian scale-space theory for the transmission of visual signals through various layers of neurons. Our model therefore presents a new insight on how the convolution process with Gaussian kernels can be implemented in vertebrate visual systems. The comparison of the numerical simulations of our model presented in this paper with the data analysis of receptive field profiles recorded in the biological literature demonstrates a complete agreement between our theoretical model and experimental data. Our model is also in good agreement with the numerical results of the Gaussian scale-space theory for the visual receptive fields.     

Hot-carrier reliability of submicron NMOSFETs and integrated NMOS low noise amplifiers

The effects of hot-carrier stress (HCS) on the performance of NMOSFETs and a fully integrated low noise amplifier (LNA) made of NMOSFETs in a 0.18 μm CMOS technology are studied. The main effects of HCS on single NMOSFETs are an increase in threshold voltage and a decrease in channel carrier mobility, which lead to a drop in the biasing current of the transistors. In the small-signal model of the transistor, hot-carrier effects appear as a decrease in the transconductance and an increase of the output conductance. No clear change was observed in the parasitic gate–source and gate–drain capacitances in the devices under test due to hot carriers. The main effects of hot carriers in the LNA were a drop of the power gain and an increase of its noise figure. The input and output matching, S₁₁ and S₂₂, slightly increased after hot-carrier stress. The third- order input-referred intercept point (IIP3) of the LNA improved after stress. This is believed to be due to the improvement of the linearity of the current–voltage (I–V) characteristics of the transistors in the LNA at the particular operating point where they were biased.     

Remote sensing of heart rate and patterns of respiration on a stationary subject using 94-GHz millimeter-wave interferometry

Using continuous wave, 94-GHz millimeter-wave interferometry, a signal representing chest wall motion can be obtained that contains both the heart rate and respiration patterns of a human subject. These components have to be separated from each other in the received signal. Our method was to use the quadrature and in-phase components of the signal, after removing the mean of each, to find the phase, unwrap it, and convert it to a displacement measurement. Using this, the power spectrum was examined for peaks, which corresponded to the heart rate and respiration rate. The displacement waveform of the chest was also analyzed for discrete heartbeats using a novel wavelet decomposition technique.