A reliability/availability approach to joint production and maintenance scheduling with multiple preventive maintenance services

In classical scheduling problems, it is often assumed that the machines are available during the whole planning horizon, while in realistic environments, machines need to be maintained and therefore may become unavailable within production periods. Hence, in this paper we suggest a joint production and maintenance scheduling (JPMS) with multiple preventive maintenance services, in which the reliability/availability approach is employed to model the maintenance aspects of a problem. To cope with the suggested JPMS, a mixed integer nonlinear programming model is developed and then a population-based variable neighbourhood search (PVNS) algorithm is devised for a solution method. In order to enhance the search diversification of basic variable neighbourhood search (VNS), our PVNS uses an epitome-based mechanism in each iteration to transform a group of initial individuals into a new solution, and then multiple trial solutions are generated in the shaking stage for a given solution. At the end of the local search stage, the best obtained solution by all of the trial solutions is recorded and the worst solution in population is replaced with this new solution. The evolution procedure is continued until a predefined number of iterations is violated. To validate the effectiveness and robustness of PVNS, an extensive computational study is implemented and the simulation results reveal that our PVNS performs better than traditional algorithms, especially in large size problems.     

Heat transfer and flow analysis of Al2O3-Water nanofluids in interrupted microchannel heat sink with ellipse and diamond ribs in the transverse microchambers

A three-dimensional numerical simulation of an interrupted microchannel heat sink is performed to investigate the pressure drop and heat transfer characteristics of a nanofluid flow. In the first part the effect of two different rib shapes on flow characteristics is studied to find the optimum design. In this part water is used as working fluid. For the second part of the present work, an Al₂O₃/water nanofluid with different volume fractions is modeled as the working fluid, and its performance is compared to water. Analysis shows that using ellipse ribs results in better performance of the microchannel rather than diamond ribs and no ribs. It was also shown that at least 2% of nanoparticles volume fraction is required to achieve an enhancement in heat transfer.     

Transport in nanoribbon interconnects obtained from graphene grown by chemical vapor deposition

We study graphene nanoribbon (GNR) interconnects obtained from graphene grown by chemical vapor deposition (CVD). We report low- and high-field electrical measurements over a wide temperature range, from 1.7 to 900 K. Room temperature mobilities range from 100 to 500 cm(2)·V(-1)·s(-1), comparable to GNRs from exfoliated graphene, suggesting that bulk defects or grain boundaries play little role in devices smaller than the CVD graphene crystallite size. At high-field, peak current densities are limited by Joule heating, but a small amount of thermal engineering allows us to reach ～2 × 10(9) A/cm(2), the highest reported for nanoscale CVD graphene interconnects. At temperatures below ～5 K, short GNRs act as quantum dots with dimensions comparable to their lengths, highlighting the role of metal contacts in limiting transport. Our study illustrates opportunities for CVD-grown GNRs, while revealing variability and contacts as remaining future challenges.     

Effects of hot pressing conditions on the performance of Nafion membranes coated by ink-jet printing of Pt/MWCNTs electrocatalyst for PEMFCs

In our previous work, a hydrothermal method was employed to prepare Pt/MWCNTs nanocomposites with 20 wt.% Pt, a low mean Pt nanoparticles size (2.8 nm) and a specific surface area of 99 m² g⁻¹. In this work, the membrane electrode assemblies (MEAs) with hydrothermally synthesized Pt/MWCNTs nanocatalysts were fabricated by catalyst-coated membrane (CCM) method. For this purpose, a commercial HP inkjet printer was used to deposit Pt/MWCNTs ink (as catalyst ink) directly on to the substrate (Nafion membrane or decal substrate) with a loading of 0.2 mg cm⁻² Pt for both the anode and cathode. The effects of hot-pressing conditions on the performance of MEAs were investigated through Taguchi design of experiments method using temperature (100 and 130 °C), pressure (800 and 1000 psi) and time (3 and 5 min) as effective experimental parameters. The compression ratios of MEAs were determined by testing the thicknesses before and after hot-pressing process. The performance of MEAs was characterized by the polarization curves and cyclic voltammetry (CV) and the surface morphologies of the electrodes were observed by scanning electron microscopy (SEM). The results showed that the most appropriate hot-pressing conditions were 800 psi, 100 °C, and 3 min. Electrochemical analysis and physical property examination revealed that the MEA fabricated by CCM method has a better performance compared to the one prepared by conventional decal transfer (DT) method.     

Mechanical properties of open-cell rhombic dodecahedron cellular structures

A series of analytical relationships is presented to predict the mechanical properties and response of open three-dimensional Voronoi tessellation of face-centered cubic structures called rhombic dodecahedrons. The cell edge material was assumed to be elastic–perfectly plastic, and the effective mechanical properties of the cellular structure were related to the cell edge material properties and the relative density of the cellular structure. Detailed finite element models were carried out to establish the validity of the analytical models. In the elastic regime, the monodisperse cellular structure is orthotropic and near-incompressible in all loading directions, and its response is governed by bending deformation of the cell edges. The yield strength of the cellular structure in all loading directions is equal. We also studied the role of irregularity in the organization of the cellular structure on its mechanical properties. The irregularity in the cellular structure organization was introduced by moving the vertices of a regular cellular structure in three orthogonal directions by a random value within a predefined range called the “irregularity index”. At a constant overall relative density, increasing the level of irregularity increases the effective elastic modulus and significantly decreases the effective yield strength of the cellular structure. We also studied the mechanical properties of the cellular structure tied to rigid plates, in view of the application of cellular structure as the core construction of sandwich panels. In this case, the cellular structure is significantly stiffer and its mechanical response is dominated by cell wall stretching.     

Comparison between the fatigue response of hot and warm mix asphalts based on the dissipated energy approach

This study focused on the fatigue behaviour of warm mix asphalt (WMA) based on the dissipated energy (DE) approach. Two conventional binders consisting of 60/70 and 85/100 penetration-grade bitumens were used to prepare the control mix. WMA was prepared by incorporating 2% Sasobit by weight of bitumen. First, the basic properties of WMA containing optimum bitumen content were compared with those of control hot mix asphalt (HMA). The main laboratory programme included four-point flexural fatigue test that was accomplished at different strain levels of 250, 500, 750 and 1000 microstrain. The studied WMA had comparable Marshall stability and indirect tensile strength to those of control HMA. Furthermore, based on the resilient modulus test results, the temperature susceptibility of WMA was slightly more than that of the control mix. Fatigue lives of studied mixes were evaluated using the conventional fatigue curves that were developed based on the initial strain level. Comparison between these curves revealed the predominant fatigue behaviour of WMAs at different examined strain levels. The initial DE and the cumulative DE of WMAs were lower than those of HMAs. The latter issue justifies the predominant fatigue response of WMA. By considering the DE curve, the ratio of dissipated energy change (RDEC) was calculated. Afterward, the plateau value (PV) was determined using the moving average of fatigue data in the plateau stage of the RDEC curve. Finally, PV-based fatigue models were developed which could precisely estimate the fatigue life regardless of the mix type and testing condition.     

Metallic Hydrogen

One of the great challenges in condensed matter physics has been to produce metallic hydrogen (MH) in the laboratory. There are two approaches: solid molecular hydrogen can be compressed to high density at extreme pressures of order 5–6 megabars. The transition to MH should take place at low temperatures and is expected to occur as a structural first-order phase transition with dissociation of molecules into atoms, rather than the closing of a gap. A second approach is to produce dense molecular hydrogen at pressures of order 1–2 megabars and heat the sample. With increasing temperature, it was predicted that molecular hydrogen first melts and then dissociates to atomic metallic liquid hydrogen as a first-order phase transition. We have observed this liquid–liquid phase transition to metallic hydrogen, also called the plasma phase transition. In low-temperature studies, we have pressurized HD to over 3 megabars and observed two new phases. Molecular hydrogen has been pressurized to 4.2 megabars. A new phase transition has been observed at 3.55 megabars, but it is not yet metallic.     

An agent-based system for bilateral contracts of energy

In this paper, an agent-based system for bilateral contracts of energy is proposed. The generating companies submit their offers to the demand companies. The demand companies also submit their bids to the generators. Each load or generator’s agent wants to match with an opponent, which offers the most valuable proposal. However, the problem of simultaneous decision-making causes decision conflicts among the agents. To overcome this conflict, we assume loads as the leaders and generators as the followers. We use Stackelberg game to match the seller and buyer agents. The negotiation process between a buyer and its potential seller will determine the power price between them. This process is carried out through a proposed combined time-behavioral protocol (TBP). With negligible changes in around the agreed price, this protocol can reduce the negotiation time considerably. After successful negotiation, the seller and buyer agents could sign a bilateral contract of energy if the market conditions allow it. The applicability of the proposed method is illustrated through a case study.