Sharif Digital Repository

Exceptional mechanical properties of carbon nanotubes (CNTs) such as high elastic modulus, stiffness and tensile strength have made them as promising reinforcement in polymer nanocomposite systems. The characteristics of CNTs/polymer interphase region directly affect the efficiency of nanotubes for improving the nanocomposite mechanical properties. In this work, the influence of chains alignment within the interphase region on elastic response of the nanocomposite is assessed using a novel ordered-inordered approach. The applicability of the presented approach is examined by implementing the approach on a series of reported data available in the literature. The effects of CNT content,... 

Microstructures and hardness of aluminum alloy ductile iron were investigated by SEM, XRD, EPMA and hardness measurement techniques. The results show, increasing the Al-alloying element leads to decrease of free ferrite and carbide, as well as increase in the pearlite volume fraction. It is also has been found that in the higher values of Al, about 85% of the matrix would be pearlite. It is indicated that an increase in the aluminum content also leads to a decrease in the spacing between pearlite layers. XRD results show, in the presence of Al, intermetallic compounds such as Al6Fe, AlFe3C0.5, Fe3Al and FeAl were produced. Furthermore the hardness measurements illustrate that by increasing... 

A temperature-induced degenerate p-type graphene nanopore/reduced graphene oxide (GNP/rGO) heterojunction-based nanodevice was prepared and studied for the first time, showing a robust negative differential resistance (NDR) feature. In this regard, cellulose-based perforated graphene foams (PGFs), containing numerous nanopores (with an average size of ∼2 nm surrounded by nearly six-layer rGO walls) were synthesized using bagasse as a green starting material. The PGFs with an essential p-type semiconducting property showed a band gap energy of ∼1.8 eV. The observed two-terminal NDR peak could present stable and reversible features at high temperatures of 586-592 K. It was demonstrated that... 

The effect of aluminium content on the formation mechanism, volume fraction, morphology, and particle size distribution of graphite has been investigated. Addition of aluminium to ductile iron causes some fundamental changes in iron-carbon phase diagrams and, as a result, improves graphite formation during eutectic transformation. Results reveal that aluminium compounds have been formed in the core of graphite nodules; thus aluminium plays an important role in the formation of graphite nodules. Furthermore, it is indicated that an increase in the aluminium content also leads to an increase in the number of graphite nodules and a decrease in the nodule size. By using electron probe... 

The exact symbol and bit error probabilities of linearly modulated signals in frequency non-selective Rician fading channels with Lth diversity branches are derived, where coherent detection with maximum ratio combining (MRC) is used at the receiver. For performance evaluation, the multiplicative distortion is combined with the additive Gaussian noise and form an additive noise is formed. This method allows computing the exact symbol and bit error probabilities of M-ary phase shift keying (M-PSK), M-ary quadrature amplitude modulation (M-QAM), M-ary amplitude modulation (M-AM) and M-ary amplitude modulation phase modulation (M-AMPM) for any arbitrary bit mappings. The results contain several... 

In this study, the effect of carboxyl-terminated butadiene-acrylonitrile liquid rubber on fracture toughness of glass-reinforced epoxy composites has been investigated. Fracture toughness tests were conducted in modes I, II and different mixed mode ratios. The data obtained from these tests have been analyzed by using different methods. Results indicated that interlaminar fracture toughness of composite specimens improved considerably through addition of carboxyl-terminated butadiene-acrylonitrile in modes I, II and mixed mode. Interlaminar fracture surfaces of the specimens were also characterized by field emission scanning electron microscope  

Carbon nanotubes (CNTs) are promising candidates for high-resolution mass nanosensors owing to their unique vibrational behavior. The structural characteristic (e.g. defect type and density) and working temperature have a significant effect on the natural frequency of CNT-based sensors. Herein, a stochastic approach based on novel finite element and molecular mechanics simulations is implemented to model the effect of temperature and structural characteristics of single-wall CNTs including defects (vacancy defect with different densities) and chirality (zigzag and armchair) on their vibrational behavior. The results show that the vacancy defects exert a significant deterioration of the... 

We consider a combined time-hopping (TH) and spread-time (ST) multiple-access technique that uses an internal code. In this method, the duration of each bit is divided into Ns frames. The outputs of the encoder and a pseudorandom (PN) sequence specify the number of the frame in which the data bit is transmitted in ST code-division multiple-access (ST-CDMA) form using the second PN sequence. We consider the correlator receiver, followed by the channel decoder. We obtain the performance of the combined method in additive white Gaussian noise (AWGN) and fading channels in the presence of multiple-access interference (MAI) and narrowband interference (NBI). We also consider the conventional... 

Celery residue modified with H2SO4 was utilized as a low-cost adsorbent for elimination of methylene blue cationic dye from aqueous solution in batch adsorption process. The adsorbent was characterized by Fourier transform infrared (FTIR) spectroscopy and Scanning Electron Microscopy (SEM). The efficacy of dye removal of the modified celery residue (MCR) was verifying by changing adsorbent dose, contact time, pH, initial dye concentration, and temperature. The isotherm models analysis shows that the experimental data can be better demonstrated by Freundlich isotherm model. In order to evaluate the best fit isotherm, three error analysis methods (χ2, ARE and MPSD) as well as correlation... 

This study introduces a new and bio-friendly adsorbent based on natural and cetyltrimethylammonium bromide (CTAB) modified adsorbent, celery (Apium graveolens) residue for removal of organic pollutants. The adsorbent was characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and pHpzc techniques. Removal applicability and efficiency of this biosorbent were studied using Congo red (CR) as a sample organic pollutant. To reach the best results, main parameters such as pH, adsorbent mass, contact time, initial dye concentration and temperature were optimized through the adsorption experiments in a batch system. The kinetic and isotherm of CR removal... 

We consider an internally coded multiple access technique for Ultra Wide-Band systems which combines time-hopping (TH) and spread-time (ST) techniques using a super-orthogonal encoder. In this method, the duration of each bit is divided into Ns frames. The outputs of a superorthogonal encoder and a PN sequence specify the frame number, in which the data is transmitted in spread-time CDMA form using the second PN sequence. We consider soft decision before Viterbi decoding at the receiver. We obtain the performance of the proposed method. We also consider the uncoded ST-CDMA system and compare its performance with our method. The results indicate that our method has much more better... 

A new internally coded scheme that combines frequency hopping (FH) and multicarrier (MC) code division multiple access (CDMA) techniques using a super-orthogonal encoder is proposed. In this method, the total bandwidth is partitioned into Ns1 disjoint bands and each band is also segmented into Ns2 subbands. On the basis of a super-orthogonal encoder output and a pseudorandom sequence, one of the Ns1 bands is selected. The data bit is then transmitted in the band in the form of the multicarrier FH (MC-FH) CDMA scheme, that is, Ns2 carriers are transmitted in the Ns2 subbands of the selected band. At the receiver, both hard and soft detectors are used. The performance of the proposed method in... 

We consider an internally coded scheme for multicarrier frequency-hopping CDMA (MC-FH-CDMA) communication systems. In this method, the total bandwidth is partitioned into Ns1 disjoint bands and each band is also segmented into Ns2 subbands. Based on the super-orthogonal encoder output and a pseudorandom (PN) sequence, one of the Ns1 bands is selected. Then, the data is transmitted in that band in the form of multicarrier frequency-hopping technique using BPSK modulation. At the receiver front end, we use soft detector, then we apply Viterbi decoding algorithm. We evaluate the performance of the proposed method. The results indicate that our method outperforms the previously presented uncoded... 

In this paper, we consider the internally coded time-hopping spread-time multiple access scheme introduced in [1]. We evaluate the performance of the correlator receiver followed by the Viterbi algorithm in fading channel. We analyze the effect of multiple access interference (MAI) and narrowband interference (NBI). We also provide simulation results and further evaluate the performance of the system for the ultra wide band (UWB) application with the UWB channel model specified by the IEEE 802.15.4a group. For comparison, we consider the conventional ST-CDMA system. The results indicate that our method has much better performance while it has the same bandwidth and spectral efficiency as the... 

We propose a new multiple access technique for Ultra Wide-Band (UWB) systems based on time-hopping (TH) and direct sequence (DS) spread spectrum techniques. In this method, the duration of each bit is divided into N s frames each one consisting of Nh chips. The outputs of a super-orthogonal encoder and a PN sequence specify the frame number, in which the data is transmitted in DS spread-spectrum form using the second PN sequence. We consider hard decision with threshold and soft decision before Viterbi decoding at the receiver. We evaluate the performance of the proposed method and compare it with the conventional DS-CDMA system. The results show that the proposed method has the best... 

We consider multirate time-hopping ultra-wideband (TH-UWB) communication systems where each user can have several multirate services. We investigate both constant spreading factor scheme, which uses multicode, and variable spreading factor scheme, which uses constant chip time and variable frame number. For multicode structure, we consider two methods. In the first, one kind of signature (PN) code is used to distinguish users and their diverse services alike. Whereas in the second one different users and different services of the same user are distinguished through two kinds of signature codes, namely PN code and Walsh code, respectively. For variable spreading factor scheme, one signature... 

In this study, an efficient handover algorithm based on the received signal strength (RSS) prediction is presented for two-tier macro-femtocell networks in which, because of the fading effects of channel and short coverage range of femtocells, ping-pong handovers may take place. In the proposed approach, first each mobile station (MS) uses the recursive least square algorithm for predicting the RSS from the candidate base stations (BSs) including both femtocell and macrocell BSs. Then, according to the predicted RSS values, several future values of signal-to-interference plus noise ratio (SINR) are calculated. Afterwards, the candidate list of BSs is pruned according to the estimated future... 

This study presents an efficient fractional frequency reuse scheme for macrocell network which uses six directional antennas to cover the outer region and analyses the performance. Cross-tier interference between macrocell and femtocell networks leading to throughput and outage probability degradation is a major issue in two-tier networks when femtocells are deployed in co-channel with macrocells. In this study, the received interference from the macrocell network to femtocell network is analysed. Then, based on the degree of received interference, an algorithm for orthogonal resource allocation to femtocells with low cross-tier interference is presented. According to the introduced scheme,... 

Identification of modulation and space-time block code (STBC) is an important task of receivers in applications such as military, civilian, and commercial communications. Here, we consider multiple-input multiple-output (MIMO) systems. We propose two methods for STBC identification when the modulation is known. We also introduce a method for joint identification of code and modulation. Additionally, we present an enhanced zero-forcing (ZF) equaliser to improve the separation between the features of different classes. Higher order cumulants are used as the statistical features. In the first method of STBC identification, after the proposed equalisation, received data samples are segmented,...