Hydrogels with the ordered structures

A water-swollen hydrogel with a molecularly-ordered structure was prepared by copolymerizing acrylic acid and acrylic monomer containing hydrophobic long alkyl or mesogenic moiety. The gels with a long alkyl side chain formed the crystalline structure and undergo the reversible order–disorder transition with change in temperature or solvent composition which accompanied a dramatic change in Young’s modulus. These gels exhibited such chemomechanical behaviors as the shape memory and the rotational motility on the water surface. On the other hand, the gels with the mesogenic side chain formed the liquid crystalline structure with polymorphism in water, and mesophase diagram was established by changing copolymer composition and water content. Mechanism of these structural and chemomechanical behaviors has been explained in terms of order–disorder transition.     

Disorder-Induced Metal–Insulator Transition in Conducting Polymers

The metal–insulator (M-I) transition in conducting polymers is particularly interesting; critical behavior has been observed over a relatively wide temperature range in a number of systems, including polyacetylene, polypyrrole, poly (p-phenylene vinylene), and polyaniline. In each case, metallic, critical, and insulating regimes have been identified through Zabrodskii plots of the logarithmic derivative of the conductivity. The critical regime (in which the conductivity varies as T, where 1/3) is tunable by varying the extent of disorder and by applying external pressure and/or an external magnetic field. The transitions from metallic to critical behavior and from critical to insulating behavior have been induced with a magnetic field and from insulating to metallic behavior with applied pressure.     

Wittrick–Williams algorithm proof of bracketing and convergence theorems for eigenvalues of constrained structures with positive and negative penalty parameters

The well‐established Wittrick–Williams algorithm is used to derive novel and general proofs that show that the eigenvalues of systems with constraints can be bracketed by replacing the constraints by positive and negative pairs of either ordinary or inertial penalty parameters. It is also shown that convergence occurs from both above and below when the numerical values of these parameters are increased towards infinity. The proofs are applicable in many contexts but are derived in that of structural systems, for which the eigenvalues are either buckling load factors or the squares of natural frequencies of vibration; ordinary penalty parameters are stiffnesses of translational and rotational springs; and inertial penalty parameters are either masses or rotary inertias. The penalty parameters can be used to constrain a system or to impose constraints between systems. It is shown that the use of inertial penalty parameters has several advantages compared with using ordinary ones. Then the pth eigenvalue of a system with n constraints is bounded closely from above by the (p+n)th eigenvalue of the system with very large positive inertial penalty parameters and from below by the pth eigenvalue, when large negative values are used instead. This work is expected to enhance the versatility of numerical eigenproblem methods, e.g. the Rayleigh–Ritz method. Copyright © 2007 John Wiley & Sons, Ltd.     

Photoacoustic Spectroscopic Study of Optical Band Gap of Zn<Subscript>1-x</Subscript>Be<Subscript>x</Subscript>Se Semiconductors

Using the photoacoustic spectroscopic (PAS) technique for the first time, the composition-dependent optical absorption coefficient and band gap of Zn_1-xBe_xSe semiconductors (with x = 0.0–0.25) have been measured at room temperature. The band gap ₀estimated from the PAS spectra varies nonlinearly with Be concentration. The exchange interaction of electrons in conduction and valence bands, effects of polytypes, microstructures, and the mixed crystallization (zinc-blend and wurtzite structures) effect are considered for the analysis of the data. The observed exponential edge (Urbachs edge) can be considered as an internal Franz–Keldish effect arising from the charged impurity generated and frozen-in optical phonon-generated fields. The phonon-assisted indirect transition at the band tail regions for some samples is also observed in the present studies.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Turbulent transition simulation using the k–ω model

This paper describes a novel approach in simulating laminar to turbulent transition by using two-equation models. The Total Stresses Limitation (TSL) concept is used to make the two-equation model capable of predicting high-Reynolds-number transitional flow. In order to handle the transition triggered by laminar separation at a low Reynolds number location, which commonly occurs in high speed flow, a sensor is introduced to detect separation and trigger transition in the separated zone. Test cases include the classical flat-plate turbulent boundary flow, and low-pressure turbine cascade flows at design and off-design conditions. © 1998 John Wiley & Sons, Ltd.     

The effect of silicon content on long crack fatigue behaviour of aluminium–silicon piston alloys at elevated temperature

The microstructure of aluminium piston alloys comprises primary and eutectic silicon together with numerous intermetallics. Previous research has shown that primary silicon strongly influences both fatigue crack initiation and subsequent propagation behaviour, however, the detailed effects of varying silicon volume fraction and morphology have not been fully addressed. Therefore, the fatigue properties of a number of candidate piston alloys with varying volume fractions of silicon have been studied. Long crack fatigue tests have been performed at room and elevated temperature typical of the gudgeon pin boss (200 °C) using a test frequency of 15 Hz (a typical engine frequency at engine idle condition).Microstructural characterisation using image analysis approaches combined with optical profilometry has been used to assess the fracture surfaces of test samples. The role of primary Si in enhancing crack growth rates at high ΔK levels, whilst affording improvements in crack growth rates at lower ΔK levels due to local crack deflections and shielding, has been confirmed. In the absence of primary Si (lower Si content alloys) the low ΔK level crack growth behaviour is dominated by matrix properties (intra-dendritic crack growth pre-dominates) whilst the high ΔK level crack growth behaviour is inter-dendritic and occurs along the weak path of the eutectic Si and/or intermetallic network.     

Probabilistic bounded non-linear control algorithm for linear structures

Non-linear control algorithms with limited control force has been widely explored and has shown promising results, but the probabilistic analysis of such control algorithms has been made restrictively due to their non-linear nature and corresponding difficulty in obtaining analytical solution of the joint probability density function (PDF). In this paper, the method for the probabilistic analysis on the bounded non-linear control algorithm is proposed based on the equivalent non-linear system method and additional regression analysis. Numerical examples show that the proposed approximate joint PDF of the closed-loop system subjected to a Gaussian white noise and a Kanai–Tajimi filtered Gaussian white noise matches closely with the joint PDF obtained statistically. The effectiveness of the bounded non-linear control is also investigated utilizing the calculated approximate joint PDF. Time history analysis results indicate that the same control performance level as the linear controller is achieved when 50% of the maximum control force of the linear controller is used for the non-linear controller.     

Superconductivity of the Induced Pairing Model in the Presence of Diagonal Disorder

We analyze the influence of randomness of local pair (LP) site energies on the superconducting properties of a mixture of coexisting local pairs and itinerant electrons coupled via charge exchange mechanism. It has been found that the diagonal disorder effects depend in a crucial way on the total particle concentration n and the LP level position ₀. Depending on the parameters, the system can exhibit various types of superconducting behaviors, including the LP-like, intermediate (MIXED), and the BCS-like.     

Nucleation and Boiling Near the Gas–Liquid Critical Point

Nucleation near the gas–liquid critical point depends sensitively on whether the pressure or the volume is fixed. We consider near-critical fluids close to the coexistence curve. (i) Upon decompression to a constant pressure with a fixed boundary temperature, bulk nucleation can well be induced from a gas state, whereas from a liquid state boiling is easily triggered in the thermal diffusion layer near the boundary. In this case, bulk nucleation in a metastable gas is described by the classical Lifshitz–Slyozov theory. (ii) Upon cooling of the boundary temperature under the fixed-volume condition, bulk nucleation can be realized in a liquid and a modified Lifshitz–Slyozov theory follows. However, if a gas is cooled from the boundary at a fixed volume, liquid droplets readily appear in the thermal diffusion layer, apparently suggesting no metastability in a gas in agreement with previous experiments. (iii) On the other hand, if a liquid is heated at the boundary wall, boiling readily occurs both at a fixed volume and at a fixed pressure.     

Giant Oxygen Isotope Effect on the Metal-Insulator Transition of RNiO3 Perovskites

The metal to insulator transition displayed by all the members of the perovskite family RNiO₃ (R=4f rare earth different from La) has attracted a lot of interest since it constitutes one of the few examples of this phenomenon in perfectly stoichiometric compounds. In spite of the great deal of work performed during the last 6 years, the mechanism responsible for the electronic localization is still a matter of controversy. The observation of unusually large O isotope shifts on the metal-insulator temperature T_MI reported in this study represents an important advance since it clearly proves the dominant role of the electron lattice interaction as driving force for the transition. Moreover, the good agreement between this observation and a simple model based on the existence of Jahn–Teller polarons in the metallic state gives further qualitative and quantitative support to the polaronic picture recently suggested to account for O isotope effects in other 3-D transition metal oxides containing Jahn–Teller ions.     

A coupled two‐scale shell model with applications to layered structures

In this paper a coupled two‐scale shell model is presented. A variational formulation and associated linearization for the coupled global–local boundary value problem is derived. For small strain problems, various numerical solutions are computed within the so‐called FE ² method. The discretization of the shell is performed with quadrilaterals, whereas the local boundary value problems at the integration points of the shell are discretized using 8‐noded or 27‐noded brick elements or so‐called solid shell elements. At the bottom and top surface of the representative volume element stress boundary conditions are applied, whereas at the lateral surfaces the in‐plane displacements are prescribed. For the out‐of‐plane displacements link conditions are applied. The coupled nonlinear boundary value problems are simultaneously solved within a Newton iteration scheme. With an important test, the correct material matrix for the stress resultants assuming linear elasticity and a homogeneous continuum is verified.Copyright © 2013 John Wiley & Sons, Ltd.     

Thermo-optical effect of Nd0.3Sm0.7NiO3 ceramic in the infrared range

Nd_0.3Sm_0.7NiO₃ exhibits a reversible metal semiconductor phase transition which is, in the infrared range, responsible of a thermo-optical contrast. The variation of the thermal emittance for Nd_0.3Sm_0.7NiO₃ ceramic was measured, in the mid-infrared range, by using reflectance measurements and on a benchtest. The thermo-optical effect of the sample was measured in situ by thermal imaging camera. The emissivity contrast obtained is 18% between 293 and 373 K which induced a difference of 27 K between the real and the visible temperature at 373 K.     

An analytical model to study the effective stiffness of the composites with periodically distributed sphere particles

In order to analytically study the overall elastic stiffness of the composite containing periodically dispersed sphere particles, a new micro-mechanics model is developed in this paper. Three kinds of typical particle packing arrangements in the form of simple cubic lattice, body-centered cubic lattice and face-centered cubic lattice are considered and compared. The special characteristics of regular distribution are fully considered by incorporating the necessary geometrical symmetry conditions into strain Green’s function. It is found that particle arrangement obviously affects the macroscopic elastic response of such the kind of composite. Moreover, most of the predictions by the present model are in good agreement with the FEM computations. The effective Young’s modulus of BCC composite the effective shear modulus of SC composite are not in the range of the Hashin–Shtrikman bounds. The present model is also useful to verify some other numerical results mainly obtained by the unit-cell model, for instance, damage variables, matrix plasticity, etc.