Thiourea-treated graphene aerogel as a highly selective gas sensor for sensing of trace level of ammonia

As a result of this study, a new and simple method was proposed for the fabrication of an ultra sensitive, robust and reversible ammonia gas sensor. The sensing mechanism was based upon the change in electrical resistance of a graphene aerogel as a result of sensor exposing to ammonia. Three-dimensional graphene hydrogel was first synthesized via hydrothermal method in the absence or presence of various amounts of thiourea. The obtained material was heated to obtain aerogel and then it was used as ammonia gas sensor. The materials obtained were characterized using different techniques such as Fourier transform infra red spectroscopy (FT-IR), thermal gravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The thiourea-treated graphene aerogel was more porous (389 m² g⁻¹) and thermally unstable and exhibited higher sensitivity, shorter response time and better selectivity toward ammonia gas, compared to the aerogel produced in the absence of thiourea. Thiourea amount, involved in the hydrogel synthesis step, was found to be highly effective factor in the sensing properties of finally obtained aerogel. The sensor response time to ammonia was short (100 s) and completely reversible (recovery time of about 500 s) in ambient temperature. The sensor response to ammonia was linear between 0.02 and 85 ppm and its detection limit was found to be 10 ppb (3S/N).     

Modeling of a MED-TVC desalination system by considering the effects of nanoparticles: energetic and exergetic analysis

In this study, the energetic and exergetic analysis of a multi-effect desalination system with a thermal vapor compression desalination system has been numerically evaluated. For this purpose, the mass, energy, and exergy balance equations for the thermo-compressor, first effect as well as middle effects, and condenser have been developed. The effects of motive steam pressure and number of effects on yield, gained output ratio (GOR), performance ratio (PR) and irreversibility have been examined. Nanoparticles were used to improve the heat transfer properties at different stages. The highest rate of exergy destruction with 61.67% is concerned with thermo-compressor, owing to the large difference between the motive steam pressure and the entrained steam. The lowest exergy losses rate among the various components was 4.89% for the condenser, due to the fact that much of the final distillate steam entrained the thermo-compressor. As the number of effects increased from 1 to 7, the yield, GOR as well as PR, improved by approximately 590% and the irreversibility reduced by 1.88%. As the motive steam pressure increased from 400 to 1290 kPa, the yield decreased by 25.45% while the GOR and PR improved by 12.62 and 14.8%, respectively. From the second law viewpoint, irreversibility intensified by 16.11% which in turn diminished the second efficiency by 3.17%.

     

Effect of geometric nonlinearity on dynamic pull-in behavior of coupled-domain microstructures based on classical and shear deformation plate theories

This paper investigates the dynamic pull-in behavior of microplates actuated by a suddenly applied electrostatic force. Electrostatic, elastic and fluid domains are involved in modeling. First-order shear deformation plate theory and classical plate theory are used to model the geometrically nonlinear microplates. The equations of motion are descritized by the finite element method. The effects of nonlinearity, fluid pressure, initial stress and different geometric parameters on dynamic behavior are examined. In addition, the influences of initial stress and actuation voltage on oscillatory behavior of microplates are evaluated.     

Application of homotopy analysis method in studying dynamic pull-in instability of microsystems

In this study, homotopy analysis method is used to derive analytic solutions to predict dynamic pull-in instability of electrostatically-actuated microsystems. The model considers midplane stretching, initial stress, distributed electrostatic force and fringing fields effect. Influences of different parameters on dynamic pull-in instability are investigated. Results are in good agreement with numerical and experimental findings.     

Hopf Bifurcation and Hunting Behavior in a Rail Wheelset with Flange Contact

An analytical investigation of Hopf bifurcation and hunting behavior of a rail wheelset with nonlinear primary yaw dampers and wheel-rail contact forces is presented. This study is intended to complement earlier studies by True et al., where they investigated the nonlinearities stemming from creep-creep force saturation and nonlinear contacts between a realistic wheel and rail profile. The results indicate that the nonlinearities in the primary suspension and flange contact contribute significantly to the hunting behavior. Both the critical speed and the nature of bifurcation are affected by the nonlinear elements. Further, the results show that in some cases, the critical hunting speed from the nonlinear analysis is less than the critical speed from a linear analysis. This indicates that a linear analysis could predict operational speeds that in actuality include hunting.     

Nonlinear modeling and experimental characterization of hydraulic dampers: effects of shim stack and orifice parameters on damper performance

A nonlinear model of monotube hydraulic dampers is presented with an emphasis on the shim stack properties and their effects on the overall damper performance. There has been no published detailed analysis of the effects of shim stack design in a hydraulic damper to date. Other damper models have used simplifying assumptions for the shim stack deflection and effects of the shim stack have not been completely studied. Various parameters affecting the nonlinear characteristics of monotube dampers such as the hysteresis region are studied. The model presented in this paper can be used for design purposes and helps in developing controllable valvings based on shim stacks. It can also be used to design controllable bypasses in hydraulic dampers. The mathematical model is validated by comparison against experimental test results carried out on an OHLINS CCJ 23/8 monotube damper, in CVeSS test facilities.     

BIANCA: a genetic algorithm to solve hard combinatorial optimisation problems in engineering

The genetic algorithm BIANCA, developed for design and optimisation of composite laminates, is a multi-population genetic algorithm, capable to deal with unconstrained and constrained hard combinatorial optimisation problems in engineering. The effectiveness and robustness of BIANCA rely on the great generality and richness in the representation of the information, i.e. the structure of populations and individuals in BIANCA, and on the way the information is extensively exploited during genetic operations. Moreover, we developed proper and original strategies to treat constrained optimisation problems through the generalisation of penalisation methods. BIANCA can also treat constrained multi-objective problems based on the construction of the Pareto frontier. Therefore, BIANCA allows us to approach very general design problems for composite laminates, but also to make a step forward to the treatment of more general problems of optimisation of materials and structures. In this paper, we describe specifically the case of optimal design of composite laminates, concerning both the theoretical formulation and the numeric resolution.     

Half-metallic ferromagnetism in the zincblende TiTe

The first-principle calculations based on spin-polarized density functional theory were performed to investigate the structural, electronic and magnetic properties of TiTe compound. The results showed that the ground state phase of TiTe is a non-magnetic NiAs structure and the zincblende (ZB) TiTe structure becomes stable at −5.2 GPa. It was predicted that the ZB structure is a half-metal ferromagnet with a magnetic moment of per formula unit for the equilibrium lattice parameter. The minority- spin and spin-flip gaps were calculated equal to 2.84 eV and 0.2 eV, respectively. In addition, the reasons for appearance of half-metallicity and magnetism in the ZB TiTe were discussed. It was noted that the half-metallicity characteristic exists within a wide range of lattice constant which makes the ZB TiTe an interesting material in the field of spintronics.