Bulk Material Parameters Research Articles

Influence of granular coefficient of rolling friction on accumulation characteristics

The influence of friction coefficient, in terms of fixed static friction coefficient (sfc) and different rolling friction coefficients (rfc), on the accumulation characteristics of particle, including the angle of repose and accumulation body, is simulated using the discrete element method. The sfc of a particle is measured with a home-made cant instrument, and a linear equation is developed to describe the relationship between the rfc and the angle of repose. Results show that rfc has obviously an influence on the accumulation characteristics of particles: the angle of repose of particles increases as rfc increases, the gap between the boundary circle and the continuous circle decreases with increasing rfc. Based on the change of rotational kinetic energy in the process of accumulation of particles, the above results can be explained. And the linear equation can provide ideas for numerical measure of granular rfc. For some difficultly measured parameters of bulk materials, the simulation of accumulation process is a valid virtual calibration test.

Surface breaking cracks in resistance spot welds of dual phase steels with electrogalvanised and hot dip zinc coating

In normal production of resistance spot welded galvanised structures, it is difficult to completely avoid surface breaking cracks. Known key factors to cause cracking are zinc coating, electrode wear during subsequent welding and insufficient electrode cooling. In this report, an embrittlement mechanism was investigated that could be coupled to the galvanisation method for dual phase steels. With identical bulk material and weld parameters, the first 50 spot welds were crack free with electrogalvanised coating, while only 10 out of 50 were crack free with hot dip galvanised coating. Energy dispersive X-ray spectroscopy analysis of the worn electrode surfaces used for welding of the hot dip galvanised coating revealed areas of aluminium oxide. Since aluminium oxide is a very strong isolator, the electrical resistance will increase, which in turn is suggested to increase the surface temperature of the spot weld and thereby increase the probability for liquid metal embrittlement and surface cracks.

Simulation of fines migration using a non‐Newtonian lattice Boltzmann‐discrete element model

PurposeThe purpose of this paper is to present a novel computational framework based on the lattice Boltzmann method (LBM) and discrete element method (DEM) capable of simulating fines migration in three dimensions. Fines migration occurs in a block cave mine, and is characterised by the faster movement of fine and often low‐grade material towards the draw point in comparison to larger, blocky material.Design/methodology/approachThis study builds on the foundations and applications outlined in a companion paper, in which the non‐Newtonian LBM‐DEM framework is defined and applied in 2D simulations. Issues relevant to the extension to 3D, such as spatial discretisation, fluid boundary conditions and the definition of synthetic bulk material parameters using a power law model, are discussed.FindingsThe results of the 3D DEM percolation replication showed that migration is predominantly limited to within the draw zone, and that the use of a low‐cohesion material model resulted in a greater amount of fines migration. The draw sensitivity investigation undertaken with the two bell partial block cave analysis did not show a significant difference in the amount of migration, despite the two draw strategies being deliberately chosen to result in isolated and interactive draw of material.Originality/valueAlong with the companion paper, this paper presents a novel application of the developed non‐Newtonian LBM‐DEM framework in the investigation of fines migration, which until now has been limited to scale models, cellular automata or pure DEM simulations. The results highlight the potential for this approach to be applied in an industrial context, and indicate a number of potential avenues for further research.

Indirect absorption in germanium quantum wells

Germanium has become a promising material for creating CMOS-compatible optoelectronic devices, such as modulators and detectors employing the Franz-Keldysh effect (FKE) or the quantum-confined Stark effect (QCSE), which meet strict energy and density requirements for future interconnects. To improve Ge-based modulator design, it is important to understand the contributions to the insertion loss (IL). With indirect absorption being the primary component of IL, we have experimentally determined the strength of this loss and compared it with theoretical models. For the first time, we have used the more sensitive photocurrent measurements for determining the effective absorption coefficient in our Ge/SiGe quantum well material employing QCSE. This measurement technique enables measurement of the absorption coefficient over four orders of magnitude. We find good agreement between our thin Ge quantum wells and the bulk material parameters and theoretical models. Similar to bulk Ge, we find that the 27.7 meV LA phonon is dominant in these quantum confined structures and that the electroabsorption profile can be predicted using the model presented by Frova, Phys. Rev., 145 (1966).

Microscopic simulation of superconductor/topological insulator proximity structures

We present microscopic, self-consistent calculations of the superconducting order parameter and pairing correlations near the interface of an $s$-wave superconductor and a three-dimensional topological insulator with spin-orbit coupling. We discuss the suppression of the order parameter by the topological insulator and show that the equal-time pair correlation functions in the triplet channel, induced by spin-flip scattering at the interface, are of ${p}_{x}\ifmmode\pm\else\textpm\fi{}i{p}_{y}$ symmetry. We verify that the spectrum at subgap energies is well described by the Fu-Kane model. The subgap modes are viewed as interface states with spectral weight penetrating well into the superconductor. We extract the phenomenological parameters of the Fu-Kane model from microscopic calculations, and find they are strongly renormalized from the bulk material parameters. This is consistent with previous results of Stanescu et al. for a lattice model using perturbation theory in the tunneling limit.

Coverage induced regulation of Au nanoparticles during pulsed laser deposition

The effects induced during the covering/embedding of metal nanoparticles (NPs) produced by pulsed laser deposition (PLD) and their impact on the structural and optical properties have been studied by producing pairs of samples containing Au NPs that are either uncovered (i.e., at the surface) or covered (i.e., embedded in an amorphous a-Al2O3 host). The main result is that covering species can sputter up to 100% of the Au atoms, the smaller the NPs the higher the sputtered fraction. This fraction has been simulated using standard models for ion bombardment and taking into account the kinetic energy distribution of arriving species and the cohesive energy dependence on NPs dimensions. Although all models well predict the order of magnitude of the sputtering yield, the calculated values are generally smaller than the experimental ones and do not account for the experimental dependence on NPs dimensions. This disagreement is discussed in terms of the limitations of standard models that do not take into account the lower adhesion of small NPs to the substrate, the high flux of species involved in PLD and, possibly to lesser extent, the use of some bulk material parameters. The metal sputtering during the coverage regulates the NPs morphology, through a reduction of dimensions and dimension dispersion. Most changes of structural features and optical spectra when covering the NPs are directly related to the variation in the amount of metal with the exception of a strong blueshift of the surface plasmon resonance when NPs are covered. This shift could be consistent with mixing of covering layer species and metal at the surface of the NPs.

Acoustic and optical phonon scattering in a single In(Ga)As quantum dot

Coupling of acoustic and optical phonons to excitons in single InGaAs/GaAs quantum dots is investigated in detail experimentally and theoretically as a function of temperature. For the theoretical description of the luminescence spectrum, including acoustic and optical phonon scattering, we used the exactly solvable independent boson model. Surprisingly, only GaAs bulk-type longitudinal-optical (LO) phonons are detected in experiment. A quantitatively correct theoretical description of the optical-phonon replica is obtained by including a limited lifetime of the phonons and the dispersion of the LO phonon energy. Similarly, a numerically correct description of the acoustic phonon wings is again based on GaAs bulk material parameters for the phonon dispersion and deformation coupling. In addition, the line shape of the calculated spectra agrees with experiment only when realistic wave functions (e.g., based on eight-band $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ theory) are used for the electron-phonon coupling matrix elements. Gaussian wave functions describing the ground state of a harmonic oscillator fail to describe high-energy tails. Thus, fundamental insights of importance for the correct prediction of properties of nonclassical light sources, based on semiconductor nanostructures, are obtained.

A method to determine effective metamaterial properties based on stratified metafilms

Metamaterials’ properties may be quantified through the assignment of bulk material parameters such as magnetic permeability and electric permittivity. These assignments rely on the assumption that metamaterials are effective media, in this context meaning that their electromagnetic properties are independent of the sample size. This assumption is not always valid, particularly for metafilms, the surface equivalents of metamaterials. However, metamaterials comprised of numerous metafilm and dielectric layers can be uniquely characterized in the effective medium approximation when all aspects of the metamaterial are properly integrated. We present a simple model based on stratified systems to illustrate this fact. The model further assists in the interpretation of novel metamaterial behaviors and offers a rapid and accurate method to determine the functionality of bulk metamaterial-based devices.

Asperity creep under constant force boundary conditions

This work presents an analysis of the transient creep deformation of a hemisphere in contact with a rigid flat, loaded by a constant force. The analysis is based on extensive finite element simulations, using a Garofalo creep law. Motivated by the simulations, an analytical framework is derived. Starting from the trivial case of a cylinder, the analytical framework can be generalized by exchanging a few functionals; this will describe the spherical geometry under analysis. The necessary functionals are derived by using a combination of analytical and empirical models. The resulting model accurately predicts the creep evolution of arbitrary asperities for a wide parameter range, requiring only the bulk material parameters. The results are interpreted in view of transient friction effects with creep as their possible cause.

Material parameters of metamaterials (a Review)

A theory of the homogenization of a certain class of metamaterials is stated. These metamaterials are volume lattices of electric and magnetic dipoles that are resonant with frequencies that are considerably lower than that of the first Bragg resonance of the lattice. It was shown that, for plates of a metamaterial, which are described by bulk material parameters, transition layers play an important role, and the known Drude notion of transition layers is significantly revised. The paper also discusses a more widespread method of determining the material parameters of metamaterials based on the extraction of the refractive index and the characteristic impedance from the scattering matrix of the plate of the metamaterial. The physical meaning of the material parameters obtained in this way is clarified, and the concept of Bloch lattices related to it is discussed. It is shown that the bulk material parameters and the parameters of transition layers can also be extracted from components of the scattering matrix of plates.

Indentation and imprint mapping for the identification of interface properties in film-substrate systems

Indentation tests are frequently employed at present for the identification of material parameters at different scales. An innovative inverse analysis technique, recently proposed by the Authors, combines the traditional indentation test with the mapping of the residual deformations (imprint), thus providing experimental data apt to be used to identify material parameters in film-substrate systems. In this paper, such methodology is enhanced to permit the identification of the fracture properties of the interface between a coating and its substrate once the bulk material parameters are known. In order to make the inverse problem well posed, a further set of experimental data, namely the horizontal displacement field measured on the film external surface, is considered as available experimental information. The sought material parameters are recovered through recursive calculations of the mechanical response of the film-substrate system, performed by a finite strain numerical simulation. The coating and a significant portion of the underlying bulk material are incorporated in the finite element models built up to this purpose, while delamination is accounted for through cohesive elements. The inverse analysis procedure rests on a batch, deterministic approach and conventional optimization algorithms are employed for the minimization of a suitably defined discrepancy norm. Extensive numerical computations have been performed in order to test the performance of the proposed methodology in terms of result accuracy and computational effort.

Effective Piezoelectric Coefficients of Ferroelectric Thin Films on Elastic Substrates

In micro-electromechanical systems consisting of a piezoelectric thin film on a substrate, due to the clamping by the substrate, the effective piezoelectric film properties are different from the bulk material behavior. However, it is of particular difficulty to determine the transverse piezoelectric parameters for such a system. A simple theoretical model by Muralt et al. (1996) allows calculation of the transverse piezoelectric coefficients in terms of the bulk parameters of the piezoelectric material. Relying on the assumption of a rigid wafer this model is a reasonable first approximation, but on the other hand, for the high accuracy needed in technical applications, it may not always be sufficient. Therefore, a more complex theoretical model for a piezoelectric thin film on an elastic substrate was derived, which delivers more realistic results for the transverse piezoelectric coefficients. This model takes the elasticity of the substrate into account, while the PZT layer is fully covered by an electrode and the vertical displacements are suppressed at the bottom of the substrate. In this way, the model represents a system of infinite lateral extent with no overall bending. As the next step, finite element simulations were carried out to verify the simple theoretical model and the new developed model, and there was a good agreement between the theoretical models and the numerical results. Furthermore, a parametric study was performed considering the influence of various bulk material parameters. Finally, the newly proposed theoretical model was compared to more realistic models where the PZT layer possesses an isolated electrode spot instead of being covered fully by an electrode. Different options were used for the boundary conditions at the bottom of the substrate. First, the same boundary conditions as for the new theoretical model were chosen (suppression of the vertical displacements at the bottom of the substrate). Second, the bottom of the substrate was free to move such that overall bending was no longer prevented. As the main result, the comparison to the new theoretical model taking into account the elastic substrate showed only negligible differences, and, thus, it is suggested for the determination of the effective piezoelectric parameters of piezoelectric layers on elastic substrates.

Generation of TeraHertz radiation assisted by optical phonons in nitride‐based quantum wells and heterolayers

AbstractTo analyze the main features of THz radiation generation caused by optical‐phonon transit‐time resonance in the case of bidimensional transport, a simplified analytical model is developed in terms of generalized parameters of bulk materials and 2D structures. In the framework of such a model and direct Monte Carlo simulations, it is shown that for a given material: (i) the generation conditions are the same in quantum wells and heterolayers, and they can be characterized by the same parameter related to the effective width of the 2D‐channel and with the optical phonon momentum, (ii) a considerable increase up to 5 times of the high‐frequency cut‐off for THz radiation generation is predicted in going from 3D to 2D transport. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Frequency limits of terahertz radiation generated by optical-phonon transit-time resonance in quantum wells and heterolayers

We develop an analytical model based on a few generalized parameters of bulk materials and two-dimensional (2D) structures which enables us to analyze the main features of terahertz (THz) radiation generation caused by optical-phonon transit-time resonance. In the framework of such a model and direct Monte Carlo simulations it is shown that for the same material (i) generation conditions are the same in quantum wells and heterolayers characterized by the same parameter of electron localization in the 2D channel and (ii) a considerable increase up to 5 times of the high-frequency cutoff for THz radiation generation is predicted in going from 3D to 2D transport.

Temperature coefficient of elastic constants of SiO2 over-layer on LiNbO3 for a temperature stable SAW device

The influence of sputtered SiO2 over-layer on the SAW propagation characteristics of a 128° rotated Y-cut X-propagating lithium niobate SAW filter has been studied. Experimentally measured SAW phase velocity and temperature coefficient of delay (TCD), with varying SiO2 over-layer thickness, show a significant deviation from the theoretically calculated values using the bulk material parameters of SiO2. The observed deviation is attributed to the differences in the material parameters (density, elastic and dielectric constants and their temperature coefficient) of the deposited SiO2 over-layer. The density and the dielectric constant of the deposited SiO2 layer were determined separately, and the elastic constants and their temperature coefficients were estimated by fitting the experimental velocity and TCD data, respectively. The deviation in the dielectric constant and the density in comparison to the bulk was insignificant, and the estimated values of the elastic constants (C11 = 0.75×1011 N m−2 and C44 = 0.225×1011 N m−2) were found to be lower, and the respective temperature coefficients (5.0×10−4 °C−1 and 2.0×10−4 °C−1) were high in comparison to the bulk material parameters.

Rigorous Coupled Wave Analysis of Radially and Azimuthally-Inhomogeneous, Elliptical, Cylindrical Systems - Abstract

Rigorous Coupled Wave Analysis (RCWA) (used for electromagnetic (EM) analysis of planar diffraction gratings) has been applied to solve EM scattering and diffraction problems for spatially inhomogeneous, cylindrical, elliptical systems. The RCWA algorithm and an appropriate method for matching EM boundary conditions in the elliptical system are described herein. Comparisons of the eigenfunctions determined by RCWA (found in spatially homogeneous elliptical regions) and Mathieu functions are presented and shown to agree closely with one another. Numerical results of scattering from a uniform elliptical shell system (excited by an electrical surface current) obtained by using both a Mathieu function expansion method and by using the RCWA algorithm are presented and also shown to agree closely with one another. The RCWA algorithm was used to study EM scattering and diffraction from an elliptical, azimuthally inhomogeneous dielectric permittivity, step profile system. EM field matching and power conservation were shown to hold for this step profile example. A comparison of the EM fields of the step profile elliptical shell example and that of a uniform profile elliptical shell having the same excitation and bulk material parameters (permittivity and permeability) was made and significant differences of the EM fields of the two systems were observed.

Rigorous Coupled Wave Analysis of Radially and Azimuthally-inhomogeneous, Elliptical, Cylindrical Systems

Rigorous Coupled Wave Analysis (RCWA) (used for electromagnetic (EM) analysis of planar diffraction gratings) has been applied to solve EM scattering and diffraction problems for spatially inhomogeneous, cylindrical, elliptical systems. The RCWA algorithm and an appropriate method for matching EM boundary conditions in the elliptical system are described herein. Comparisons of the eigenfunctions determined by RCWA (found in spatially homogeneous elliptical regions) and Mathieu functions are presented and shown to agree closely with one another. Numerical results of scattering from a uniform elliptical shell system (excited by an electrical surface current) obtained by using both a Mathieu function expansion method and by using the RCWA algorithm are presented and also shown to agree closely with one another. The RCWA algorithm was used to study EM scattering and diffraction from an elliptical, azimuthally inhomogeneous dielectric permittivity, step profile system. EM field matching and power conservation were shown to hold for this step profile example. A comparison of the EM fields of the step profile elliptical shell example and that of a uniform profile elliptical shell having the same excitation and bulk material parameters (permittivity and permeability) was made and significant differences of the EM fields of the two systems were observed.

The field-induced cholesteric-nematic phase transition and its dependence on layer thickness, boundary conditions, and temperature

Chiral-nematic liquid-crystal layers which have a short pitch, homeotropic boundary conditions, and bistable properties are investigated with respect to the temperature dependence of their field-induced cholesteric-nematic phase transition. Hereto the threshold field needed for the transition is studied as a function of the layer thickness. This is done for various temperatures. The threshold field hardly depends on the layer thickness. Moreover, it sharply rises below −5 °C. Both the value of the threshold field and its temperature dependence can be fully described in terms of liquid-crystal bulk material parameters, the conditions at the homeotropic boundary layer used do not play a role. From the threshold field data the twist elastic constant is determined as a function of temperature. This temperature dependence is compared to the temperature dependence as determined from the square of the long range order parameter calculated from temperature dependent refractive index data. A good correlation is found between both data sets of the twist elastic constant.

Thickness dependence of polarization and response characteristics in thin FLC films

The effects of cell thickness on the pretransitional behaviour of ferroelectric liquid crystals are studied experimentally and theoretically for thin cells. The temperature and field dependences of the tilt angle, polarization, polarization to tilt angle ratio, dielectric permeability and relaxation frequency are considered in detail in the framework of a simple model for the surface tension. Apparent values of the phenomenological coefficients in the free energy expansion and some bulk and surface material parameters are estimated.

InSb: A Key Material for IR Detector Applications

ABSTRACTInSb has served as an important mid-wave IR (λ=3−5μm) detector material for several decades. In this presentation, we will briefly review General Electric's InSb Charge Injection Device technology. Emphasis will be placed on device performance as a function of material parameters. A new InSb materials technology utilizing liquid phase epitaxy will be described. This epitaxial growth technology improves InSb material parameters and increases minority carrier lifetimes by more than two orders of magnitude to near the Auger limit. Comparisons will be made between available bulk material parameters and that of the epitaxial material.