The effect of Graphitization on the Mechanical Properties of twodimensional carbon – carbon composites

In this article the effect of graphitization in twodimensional c-c composite bodies prepared by impregnation of novalak resin is reported. Mechanical properties were determined by bending tests and microstructural features were studied by SEM and XRD. It was found that primary graphitization at 2300°C followed by graphitization at 2500°C improves the mechanical strength remarkably. Repeated graphitization and long – term treatment at 2500°C, however, found to have an adverse effect. Microstructural observations revealed that the level of strength is mainly controlled by a sheath developed around the fibre during graphitization. Applying an improper thermal regime affects the matrix – fibre adherence and may cause the destruction of the fibres. This enhances the crack propagation and causes early failure of composite body under loading.     

Macromodel of Spatial Smoothing in Wind Farms

Departing from aerodynamics (micromodel), the macromodel begins with the power output of a single wind turbine generator (WTG). The N units of WTGs in a wind farm are characterized by the time delays it takes wind, at average velocity, to traverse the distances separating them. Predictions from simulations are in agreement with recorded wind farm data. Smoothing of high-frequency power components is by a factor close to N^-1/2. Smoothing of low-frequency harmonic power components is small because the wind farm is limited in size. A theory, based on Fourier analysis, is presented to explain how the macromodel simultaneously copes with: 1) the high-frequency components of wind velocity (which have poor correlation) even for short distances and 2) the low-frequency components (which have some correlation)     

Chattering-free Fault-tolerant Attitude Control with Fast Fixed-time Convergence for Flexible Spacecraft

International Journal of Control, Automation and Systems - This paper is mainly dedicated to the challenging issue of fixed-time attitude control for a flexible spacecraft in the presence of...     

Prevention of deadlocks and livelocks in lossless backpressured packet networks

No packets will be dropped inside a packet network, even when congestion builds up, if congested nodes send backpressure feedback to neighboring nodes, informing them of unavailability of buffering capacity-stopping them from forwarding more packets until enough buffer becomes available. While there are potential advantages in backpressured networks that do not allow packet dropping, such networks are susceptible to a condition known as deadlock in which throughput of the network or part of the network goes to zero (i.e., no packets are transmitted). In this paper, we describe a simple, lossless method of preventing deadlocks and livelocks in backpressured packet networks. In contrast with prior approaches, our proposed technique does not introduce any packet losses, does not corrupt packet sequence, and does not require any changes to packet headers. It represents a new networking paradigm in which internal network losses are avoided (thereby simplifying the design of other network protocols) and internal network delays are bounded.     

An optimum selection strategy of reflective cracking mitigation methods for an asphalt concrete overlay over flexible pavements

Reflective Cracking (RC) has been a daunting challenge in pavement maintenance and rehabilitation (M&R), yet, still, after several decades of research, no exclusive solution prevails. Moreover, RC mitigation methods have shown significant variation in in situ performance. Therefore, a technique tailored to select an effective RC mitigation method is essential for the success of pavement M&R. In this study, a life cycle cost (LCC) and multi-criteria decision-making (MCDM) analyses were conducted to evaluate the effectiveness of currently available RC mitigation methods and to select the optimal method for an asphalt concrete overlay above flexible pavements. The MCDM includes three components: LCC, performance, and materials (recyclability). These criteria determine the selection ranking of each RC mitigation method. In addition, the effects of the priority level including cost, performance, and recyclability on the final decision were evaluated by conducting a series of sensitivity analysis under multiple scenarios; therefore, weight combination of the three criteria were recorded to define the measurements affecting the final decision.     

Characterization,microstructure and corrosion behavior of magnesia refractories produced from recycled refractory aggregates

This paper aims to report the results of some investigations carried out in Iranian steel industries to reuse the spent magnesia graphite refractory bricks in the forms of the new shaped and unshaped magnesia refractories. Economical aspects of recycling and minimizing the environmental effects of spent refractories landfills were the basic goals of this research. The spent MgO–C refractory bricks from electric arc (EAF) and ladle (LF) furnaces were analyzed in terms of microstructural and chemical properties. Different samples were prepared from natural sintered magnesia and 10–30 wt.% of recycled aggregates in the forms of magnesia refractory brick and ramming mix and their physical and mechanical properties were evaluated. Also the slag corrosion behavior and microstructural properties of corroded samples were investigated. The results showed that the addition of up to 30 wt.% of recycled aggregates had no negative effects on the properties of magnesia refractories.     

Phase-dependent intersubband optical properties of asymmetric AlGaAs/GaAs coupled quantum wells

In this article, we have investigated the optical properties of intersubband transitions in an asymmetric AlGaAs/GaAs coupled quantum wells system. We solved the Schrödinger equation numerically using homemade finite difference codes to determine the electronic structure. The susceptibilities were calculated in frame of density matrix equations. The effect of relative phase variation between applied fields, on the linear absorption coefficients, relative refractive index changes, and group velocity has been investigated. The results show that the linear optical properties are extremely phase-dependent. By adjusting the phase, it is possible to obtain absorption curves with dispersion-like profile and vice versa.     

The effect of processing parameters in the carbothermal synthesis of titanium diboride powder

The mechanism of the carbothermal method for synthesizing titanium diboride (TiB₂) powder has been studied. Mixtures of TiO₂, H₃BO₃ and carbon were heated in an argon atmosphere at 1000–1600 °C. The effect of the molar ratio and holding time on the phase evolution was studied by X-ray diffraction. The products were also characterized by scanning electron microscopy observations and particle size measurements.For a composition with a molar ratio of TiO₂:H₃BO₃:C = 1:2.4:5 heated for 1 h, the simultaneous presence of TiC and TiB₂ phases at 1100 °C and the transformation of TiO₂ to Ti₂O₃ at 1200 °C and higher confirms that TiB₂ synthesis is based on a TiC formation mechanism, in which TiC may be formed from a reaction between TiO₂ or Ti₂O₃ and carbon. Then TiC may react with liquid B₂O₃ and/or gaseous B₂O₂ to form the TiB₂ phase. The reaction is completed at 1500 °C. Also by increasing the molar ratio of boric acid to 3, the impurities decreased considerably and pressing of the material had an obvious effect on decreasing the impurities, due to an increase of the surface contact of particles, which causes an effective inhibition of boron escape from the reaction chamber. Under these experimental conditions, a relatively narrow size distribution of TiB₂ particles was produced. When the reaction time increased to 1.5–2 h, grain growth of particles occurred. Therefore, a wider distribution of particle size was obtained.