Mohammad A. Rafiee

Enhanced Mechanical Properties of Nanocomposites at Low Graphene Content

In this study, the mechanical properties of epoxy nanocomposites with graphene platelets, single-walled carbon nanotubes, and multi-walled carbon nanotube additives were compared at a nanofiller weight fraction of 0.1 +/- 0.002%. The mechanical properties measured were the Youngs modulus, ultimate tensile strength, fracture toughness, fracture energy, and the materials resistance to fatigue crack propagation. The results indicate that graphene platelets significantly out-perform carbon nanotube additives. The Youngs modulus of the graphene nanocomposite was approximately 31% greater than the pristine epoxy as compared to approximately 3% increase for single-walled carbon nanotubes. The tensile strength of the baseline epoxy was enhanced by approximately 40% with graphene platelets compared to approximately 14% improvement for multi-walled carbon nanotubes. The mode I fracture toughness of the nanocomposite with graphene platelets showed approximately 53% increase over the epoxy compared to approximately 20% improvement for multi-walled carbon nanotubes. The fatigue resistance results also showed significantly different trends. While the fatigue suppression response of nanotube/epoxy composites degrades dramatically as the stress intensity factor amplitude is increased, the reverse effect is seen for graphene-based nanocomposites. The superiority of graphene platelets over carbon nanotubes in terms of mechanical properties enhancement may be related to their high specific surface area, enhanced nanofiller-matrix adhesion/interlocking arising from their wrinkled (rough) surface, as well as the two-dimensional (planar) geometry of graphene platelets.

Toughening in Graphene Ceramic Composites

The majority of work in graphene nanocomposites has focused on polymer matrices. Here we report for the first time the use of graphene to enhance the toughness of bulk silicon nitride ceramics. Ceramics are ideally suited for high-temperature applications but suffer from poor toughness. Our approach uses graphene platelets (GPL) that are homogeneously dispersed with silicon nitride particles and densified, at ∼1650 °C, using spark plasma sintering. The sintering parameters are selected to enable the GPL to survive the harsh processing environment, as confirmed by Raman spectroscopy. We find that the ceramics fracture toughness increases by up to ∼235% (from ∼2.8 to ∼6.6 MPa·m(1/2)) at ∼1.5% GPL volume fraction. Most interestingly, novel toughening mechanisms were observed that show GPL wrapping and anchoring themselves around individual ceramic grains to resist sheet pullout. The resulting cage-like graphene structures that encapsulate the individual grains were observed to deflect propagating cracks in not just two but three dimensions.

Superhydrophobic to superhydrophilic wetting control in graphene films.

Adv. Mater. 2010, 22, 2151–2154 2010 WILEY-VCH Verlag G T IO N Superhydrophobic materials with water contact angles above 1508 are the key enabler for antisticking, anticontamination, and self-cleaning technologies. Similarly, superhydrophilic materials with water contact angles below 108 have many important applications; for example, as a wicking material in heat pipes and for enhanced boiling heat transfer. In general, the wettability of a solid surface is strongly influenced both by its chemical composition and by its geometric structure (or surface roughness). Several experimental and modeling studies have focused on exploiting surface roughness to engineer superhydrophobicity or superhydrophilicity. Both microscale roughness features (e.g., micromachined silicon pillars) as well as nanoscale features (e.g., aligned arrays of carbon nanotubes) have been investigated. However, so far the wetting properties of graphene-based coatings have not been studied in detail. Graphene is a single-atom-thick sheet of sp hybridized carbon atoms. When deposited on a planer substrate, the individual graphene sheets form an interconnected film, which increases the surface roughness of the substrate by one to two orders of magnitude. We demonstrate here that this roughness effect in conjunction with the surface chemistry of the graphene sheets can be used to dramatically alter the wettability of the substrate. If hydrophilic graphene sheets are used (for example, by sonicating the as-produced graphene in water), the substrate acquires a superhydrophilic character. Conversely if hydrophobic graphene sheets are used (by sonicating the as-produced graphene in acetone) then the roughness effect imparts superhydrophobicity to the underlying substrate. By controlling the relative proportion of acetone and water in the solvent, the contact angle of the resulting graphene film can be tailored over a wide range (from superhydrophobic to superhydrophilic). Such graphene-based coatings with controllable wetting properties provide a facile and effective means to modify the wettability of a variety of surfaces. The graphene sheets used in this study were extracted from graphite using the method developed in Reference [19,20]. In this method, partially oxygenated graphene sheets are generated by the rapid thermal expansion (>2000 8C min ) of completely oxidized graphite oxide. The protocols used to oxidize graphite to graphite oxide and then generate graphene sheets (Fig. 1a) by the thermal exfoliation of graphite oxide are provided in the Experimental section. Figure 1b illustrates a transmission electron microscopy (TEM) image of a typical graphene flake synthesized by the above method and deposited on a standard TEM grid for imaging. The flake is several micrometers in dimension; note the wrinkled (rough) surface texture of the graphene flake. Figure 1c displays a high-resolution TEM (HRTEM) image of the edge of a typical graphene flake, indicating that each flake is comprised of 3 individual graphene sheets. The electron diffraction pattern (shown in inset) confirms the signature of few-layered graphene.

Graphene Nanoribbon Composites

It is well established that pristine multiwalled carbon nanotubes offer poor structural reinforcement in epoxy-based composites. There are several reasons for this which include reduced interfacial contact area since the outermost nanotube shields the internal tubes from the matrix, poor wetting and interfacial adhesion with the heavily cross-linked epoxy chains, and intertube slip within the concentric nanotube cylinders leading to a sword-in-sheath type failure. Here we demonstrate that unzipping such multiwalled carbon nanotubes into graphene nanoribbons results in a significant improvement in load transfer effectiveness. For example, at ∼0.3% weight fraction of nanofillers, the Youngs modulus of the epoxy composite with graphene nanoribbons shows ∼30% increase compared to its multiwalled carbon nanotube counterpart. Similarly the ultimate tensile strength for graphene nanoribbons at ∼0.3% weight fraction showed ∼22% improvement compared to multiwalled carbon nanotubes at the same weight fraction of nanofillers in the composite. These results demonstrate that unzipping multiwalled carbon nanotubes into graphene nanoribbons can enable their utilization as high-performance additives for mechanical properties enhancement in composites that rival the properties of singlewalled carbon nanotube composites yet at an order of magnitude lower cost.

Buckling resistant graphene nanocomposites

An experimental study on buckling of graphene/epoxy nanocomposite beam structures is presented. Significant increase (up to 52%) in critical buckling load is observed with addition of only 0.1% weight fraction of graphene platelets into the epoxy matrix. Based on the classical Euler-buckling model, the buckling load is predicted to increase by ∼32%. The over 50% increase in buckling load observed in our testing suggests a significant enhancement in load transfer effectiveness between the matrix and the graphene platelets under compressive load. Such nanocomposites with high buckling stability show potential as lightweight and buckling-resistant structural elements in aeronautical and space applications.

Feature extraction of forearm EMG signals for prosthetics

This paper presents a new technique for feature extraction of forearm electromyographic (EMG) signals using a proposed mother wavelet matrix (MWM). A MWM including 45 potential mother wavelets is suggested to help the classification of surface and intramuscular EMG signals recorded from multiple locations on the upper forearm for ten hand motions. Also, a surface electrode matrix (SEM) and a needle electrode matrix (NEM) are suggested to select the proper sensors for each pair of motions. For this purpose, EMG signals were recorded from sixteen locations on the forearms of six subjects in ten hand motion classes. The main goal in classification is to define a proper feature vector able to generate acceptable differences among the classes. The MWM includes the mother wavelets which make the highest difference between two particular classes. Six statistical feature vectors were compared using the continuous form of wavelet packet transform. The mother wavelet functions are selected with the aim of optimum classification between two classes using one of the feature vectors. The locations where the satisfactory signals are captured are selected from several mounted electrodes. Finally, three ten-by-ten symmetric MWM, SEM, and NEM represent the proper mother wavelet function and the surface and intramuscular selection for recording the ten hand motions.

Wavelet basis functions in biomedical signal processing

Research highlights? Daubechies 44 wavelet basis is the most similar function across various biosignals. ? High order Daubechies functions are useful to extract features from 1-D biosignals. ? For wavelet signal processing, selection of mother function similar to signal is not always a proper strategy. During the last two decades, wavelet transform has become a common signal processing technique in various areas. Selection of the most similar mother wavelet function has been a challenge for the application of wavelet transform in signal processing. This paper introduces Daubechies 44 (db44) as the most similar mother wavelet function across a variety of biological signals. Three-hundred and twenty four potential mother wavelet functions were selected and investigated in the search for the most similar function. The algorithms were validated by three categories of biological signals: forearm electromyographic (EMG), electroencephalographic (EEG), and vaginal pulse amplitude (VPA). Surface and intramuscular EMG signals were collected from multiple locations on the upper forearm of subjects during ten hand motions. EEG was recorded from three monopolar Ag-AgCl electrodes (Pz, POz, and Oz) during visual stimulus presentation. VPA, a useful source for female sexuality research, were recorded during a study of alcohol and stimuli on sexual behaviors. In this research, after extensive studies on mother wavelet functions, results show that db44 has the most similarity across these classes of biosignals.

An n-type, new emerging luminescent polybenzodioxane polymer for application in solution-processed green emitting OLEDs

Herein, we report polybenzodioxane polymer (PIM-1), a multifunctional n-type emitter with strong green luminescence, and its suitability as an electron transport layer for OLEDs devices. The Brunauer–Emmett–Teller (BET) test and photo-electrical properties of as-synthesized PIM-1 confirm the presence of large microporosity and excellent electron mobility. The photoluminescence (PL) spectroscopy shows the intense green emission at 515 nm upon 332 nm excitation wavelength. Moreover, the Hall effect study reveals the negative Hall resistivity, which indicates that PIM-1 possesses n-type semiconductor characteristics. It enables the highly-efficient polymer-based green LEDs with configuration; ITO (120 nm)/PEDOT:PSS (30 nm)/PIM-1 (100 nm)/LiF (1 nm)/Al (150 nm), which are fabricated by the sequential solution-processing method. The OLED incorporating PIM-1 thin layer achieves maximum current efficiency of 1.71 Cd A−1 and power efficiency of 0.49 lm W−1. Additionally, the electron mobility is found to be 4.4 × 10−6 cm2 V−1 s−1. Hence, these results demonstrate that PIM-1 could be an ultimate choice as an n-type emitter for the next generation of advanced electronic devices.

Female Sexual Responses Using Signal Processing Techniques

INTRODUCTION An automatic algorithm for processing vaginal photoplethysmograph signals could benefit researchers investigating sexual behaviors by standardizing interlaboratory methods. Female sexual response does not co-vary consistently in the self-report and physiological domains, making the advancement of measurements difficult. Automatic processing algorithms would increase analysis efficiency. Vaginal pulse amplitude (VPA) is a method used to measure female sexual responses. However, VPA are problematic because of the movement artifacts that impinge on the signal. This article suggests a real-time approach for automatic artifact detection of VPA signals. The stochastic changes (artifacts) of VPA are characterized mathematically in this research, and a method is presented to automatically extract the frequency of interest from VPA based on the autocorrelation function and wavelet analysis. Additionally, a calculation is presented for the vaginal blood flow change rate (VBFCR) during female sexual arousal using VPA signals. AIM The primary aim is to investigate the experimental VPA measures based on theoretical techniques. Particularly, the goal is to introduce an automatic monitoring system for female sexual behaviors, which may be helpful for experts of female sexuality. METHODS The methods in the research are divided into experimental and theoretical parts. The VPA in twenty women was measured by a common vaginal photoplethysmography system in two conditions. Each subject was tested watching a neutral video followed by an erotic video. For theoretical analysis, an approach was applied based on wavelet transform to process the VPA. MAIN OUTCOME MEASURES Introduction of an automatic and real-time monitoring system for female sexual behaviors, automatic movement artifact detection, VBFCR, first application of wavelet transform, and correlogram in VPA analysis. RESULTS The natural and significant frequency information of VPA signals was extracted to automatically detect movement artifacts and to investigate the effects of erotic videos on female sexual responses. CONCLUSIONS The computerized automatic systems based on advanced math and statistics have several advantages for human sexuality research such as: savings in time and budget; increase in the accuracy of results; and reduction in human errors for data analysis.