Bulk Material Parameters Research Articles

Towards efficiency enhancement of earth abundant Cu2BaSn(S,Se)4 chalcogenide solar cell using In2S3 as efficient electron transport layer

Cu2BaSn(S,Se)4 (CBTSSe) solar cells, are an intriguing kind of photovoltaic devices with theoretical efficiency close to 31 %. Their promise in the field of renewable energy is highlighted by their sustainable structure, as well as their low density of defects. Even with these benefits, the record efficiency for CBTSSe solar cells is now only 5.2 %. In order to identify limitations in performance and clear the path for obtaining improved practical efficiency, this emphasizes the vital requirement for deep analysis. In this work, we investigate a new structure based on Cd free In2S3/CBTSSe heterojunction in which an Indium (III) sulfide as ETL layer, and CBTSSe as absorber layer take the place of the traditional CdS/ CBTSSe structure. For the first time, an analytical model that incorporates a variety of recombination mechanisms occurred in CBTSSe solar cell, such as Auger, Shockley-Read-Hall (SRH), interface recombination, tunneling-enhanced recombination, and non-radiative recombinations is proposed. In our approach the reverse saturation mechanism is taken into account in the suggested model as a metric to identify the main recombination mechanism. Significantly, there is a good agreement between the outcomes of our model and the experiment. It is shown that CBTSSe bulk recombination, In2S3/CBTSSe interface recombination and resistances are dominating. Besides, the developed model serves as fitness function for MOGA approach to locate the optimal parameters design combination that led to optimal efficiency. We demonstrate the ability to obtain an efficiency of up to 10.12 % by carefully tuning both the CBTSSe bulk material parameters and the In2S3/CBTSSe interface features. Our findings demonstrate that the optimized design using In2S3/CBTSSe heterojunction outperforms the baseline, reaching a high JSC of 20.57 mA/cm2, VOC of 0.88 V, and FF of 55.54 %, with appropriate band alignment at the In2S3/CBTSSe interface and optimized physical and geometrical parameters. The suggested approach paves the way for additional design optimization while also making it possible to identify the degradation factors that are responsible.

Creep Properties of a Viscoelastic 3D Printed Sierpinski Carpet-Based Fractal

In this paper, the phenomenon of creep compliance and the creep Poisson’s ratio of a 3D-printed Sierpinski carpet-based fractal and its bulk material (flexible resin Resione F69) was experimentally investigated, as well as the quantification of the change in the viscoelastic parameters of the material due to the fractal structure. The samples were manufactured via a vat photopolymerization method. The fractal structure of the samples was based on the Sierpinski carpet at the fourth iteration. In order to evaluate the response of both the fractal and the bulk material under the creep phenomenon, 1 h-duration tensile creep tests at three constant temperatures (20, 30 and 40 °C) and three constant stresses (0.1, 0.2 and 0.3 MPa) were conducted. A digital image correlation (DIC) technique was implemented for strain measurement in axial and transverse directions. From the results obtained, the linear viscoelastic behavior regime of the fractal and the bulk material was identified. The linear viscoelastic parameters of both fractal and bulk materials were then estimated by fitting the creep Burgers model to the experimental data to determine the effect of the fractal geometry on the viscoelastic properties of the samples. Overall, it was found that the reduction in stiffness induced by the fractal porosity caused a more viscous behavior of the material and a reduction in its creep Poisson’s ratio, which means an increase in the compliance of the material.

Effect of Material-Dependent Boundaries on the Interference Induced Enhancement of the Surface Superconductivity Temperature.

Using the tight-binding Bogoliubov-de Gennes formalism, we describe the influence of the surface potential on the superconducting critical temperature at the surface. Surface details are taken into account within the framework of the self-consistent Lang-Kohn effective potential. The regimes of strong and weak coupling of superconducting correlations are considered. Our study reveals that, although the enhancement of the surface critical temperature, originating from the enhancement of the localized correlation due to the constructive interference between quasiparticle bulk orbits, can be sufficiently affected by the surface potential, this influence, nonetheless, strongly depends on the bulk material parameters, such as the effective electron density parameter and Fermi energy, and is likely to be negligible for some materials, in particular for narrow-band metals. Thus, superconducting properties of a surface can be controlled by the surface/interface potential properties, which offer an additional tuning knob for the superconducting state at the surface/interface.

The ‘miniature silo’ test: A simple experimental setup to estimate the effective friction coefficient between the granular solid and a horizontally-corrugated cylindrical metal silo wall

Corrugated walls are widely employed in the construction of metal silos. Despite the long history of testing to establish bulk material parameters for silo design, there does not appear to be an established test procedure to directly determine the effective friction coefficient for a corrugated wall other than an adaptation of a classical direct shear test. EN 1991-4: 2006 prescribes a simple weighted average formula for this coefficient based on the internal friction angle of the granular solid and a ‘flat’ wall friction coefficient. This paper describes a set of miniature-scale silo tests performed at the University of Bologna intended to establish a simple but useful procedure to directly estimate an average global friction coefficient between the granular solid and a corrugated silo wall. These ‘miniature silo’ tests are simple enough to be performed in any civil engineering laboratory with standard equipment. • Particle-wall friction coefficient is fundamental for structural design of silos. • No tests are available to measure the effective friction for a corrugated wall. • A novel straightforward experimental test is proposed to fill this gap. • The miniature-silo test can be built using basic tools easy to be found in any lab. • The obtained results are in accordance with EN-1991-4 average formula predictions.

Features for the design of a specialized information-measuring system for the study of thermoelectric properties of semiconductors

Methods for studying thermoelectric parameters of semiconductors that are optimal for the implementation of software and hardware have been analyzed and selected. It is based on the Harman method and its modifications, adapted for pulse measurements, which are convenient to implement on a modern element base. An important advantage of these methods is the absence of the need for accurate measurements of heat fluxes, which greatly simplifies and reduces the time for conducting experimental research. The required operating ranges for the voltage 10 µV–1 V, for the current 10 µA–300 mA and the element base performance at the processing level of 40–200 million samples per second have been determined. Structural and electrical circuits, as well as software for a specialized computer system for studying thermoelectric parameters of both bulk and thin-film thermoelectric materials, and express analysis of the operational characteristics of finished modules have been developed. It has been shown that the proposed scheme copes well with the task. And the use of FPGA and 32-bit microcontrollers provide sufficient processing speed up to 200 MSPS and the necessary synchronization modes for the implementation of the Harman pulse method even when studying films of nanometer thickness. Experimental studies of both bulk thermoelectric modules based on Bi2Te3 and thin-film thermoelectric material based on PbTe have been carried out. The effectiveness of the developed tools and techniques has been shown, which made it possible to more than halve the time for sample preparation and experiment. Based on the presented models, all the main thermoelectric and operational parameters have been determined, in particular, electrical conductivity, Seebeck coefficient, thermal conductivity, thermoelectric figure of merit. As a result of the development of specialized computer tools, it was possible to reduce the labor intensity of the process of measuring the main electrical and operational parameters of semiconductor thermoelectric materials and energy conversion modules based on them, as well as to automate the process of defects identification of thermoelectric modules. The labor intensity of the research process has decreased not only due to the automation of the measurement process, but also due to an optimized technique that allows research on a sample of one configuration, since the manufacture and preparation of samples are the most laborious

Modelling extracellular matrix and cellular contributions to whole muscle mechanics.

Skeletal muscle tissue has a highly complex and heterogeneous structure comprising several physical length scales. In the simplest model of muscle tissue, it can be represented as a one dimensional nonlinear spring in the direction of muscle fibres. However, at the finest level, muscle tissue includes a complex network of collagen fibres, actin and myosin proteins, and other cellular materials. This study shall derive an intermediate physical model which encapsulates the major contributions of the muscle components to the elastic response apart from activation-related along-fibre responses. The micro-mechanical factors in skeletal muscle tissue (eg. connective tissue, fluid, and fibres) can be homogenized into one material aggregate that will capture the behaviour of the combination of material components. In order to do this, the corresponding volume fractions for each type of material need to be determined by comparing the stress-strain relationship for a volume containing each material. This results in a model that accounts for the micro-mechanical features found in muscle and can therefore be used to analyze effects of neuro-muscular diseases such as cerebral palsy or muscular dystrophies. The purpose of this study is to construct a model of muscle tissue that, through choosing the correct material parameters based on experimental data, will accurately capture the mechanical behaviour of whole muscle. This model is then used to look at the impacts of the bulk modulus and material parameters on muscle deformation and strain energy-density distributions.

Discrete Element Simulation of Powder Sintering for Spherical Particles

This paper presents a numerical simulation of powder sintering. The numerical model presented in this paper uses the discrete element method, which suggests that the material can be modeled by a large set of discrete elements (particles) of a spherical shape that interact with each other. A methodology has been developed to determine the DEM parameters of bulk materials based on machine vision and a neural network algorithm. The approach is suitable for obtaining the exact values of the DEM parameters of the investigated bulk material by comparing the visual images of the material’s behavior at the experimental stand in reality and in the model. Simulation of sintering requires an introduction of cohesive interaction between particles representing interparticle sintering forces. Numerical sintering studies were supplemented with experimental studies that provided data for calibration and model validation. The experimental results have shown a significant capability of the designed numerical model in modeling sintering processes. Evolution of microstructure and density during sintering have been studied under the laboratory conditions.

Surface Energy Effects on the Nonlinear Free Vibration of Functionally Graded Timoshenko Nanobeams Based on Modified Couple Stress Theory

An integrated nonlinear couple stress-surface energy continuum model is developed to study the nonlinear vibration characteristics of size-dependent functionally graded nanobeams for the first time. The nanobeam theory is formulated based on the Timoshenko kinematics, augmented by von Kármán’s geometric nonlinearity. The modified couple stress and Gurtin–Murdoch surface elasticity theories are incorporated to capture the long-range interaction and surface energy, respectively. Unlike existing Timoshenko nanobeam models, the effects of surface elasticity, residual surface stress, surface mass density and Poisson’s ratio, in addition to bending and axial deformations, are incorporated in the newly developed model. A power law function is used to model the material distribution through the thickness of the beam, considering the gradation of bulk and surface material parameters. A variational formulation of the nonlinear nonclassical governing equations and associated nonclassical boundary conditions is established by employing Hamilton’s principle. The generalized differential quadrature method is exploited in conjunction with either the Pseudo-arclength continuation or Runge–Kutta method to solve the problem with an exact implementation of the nonclassical boundary conditions. The formulation and solution procedure presented are verified by comparing the obtained results with available ones. Based on the parametric study, it is concluded that the nonclassical boundary conditions, material length scale parameter, residual surface stress, surface elasticity, bulk elasticity modulus, gradient index, nonlinear amplitude and thickness have important influences on the linear and nonlinear vibration responses of functionally graded Timoshenko nanobeams.

Above-light-line Nonlinear Surface Polaritons near a Conductive Interface: Threshold Case

We investigate the TM-polarized nonlinear surface polaritons (NLSP) propagating along aguided structure consisting of a magnetic optically linear medium and a non-magnetic opticallynonlinear medium with saturable permittivity separated by a flat conductive layer of zerothickness. We consider those values of hosting media bulk material parameters for which theNLSP existence (for zero sheet conductance) has threshold character with respect to the wavesintensity. Based on the exact solution of Maxwell's equations we show that the energy andpropagation properties of the NLSP near the above-light-line condition (1 > n > 0) dependconsiderably on the surface conductivity of the layer, even the threshold character of the NLSPcan be lost; for certain sheet conductance values these waves can exist in a linear limit. TheNLSP propagation constant is defined by both the surface conductivity and field intensity andcan be varied in a wide diapason, which gives an opportunity to obtain and control the importantfor quantum information processing 0 n  condition. For a chosen value of the NLSPpropagation constant the NLSP field intensity and energy flux decries when the surfaceconductivity grows; saturation of the nonlinear permittivity leads to an increase of the NLSPenergy flux compared with Kerr-like nonlinearity.

On effective behavior of microstructures embedding general interfaces with damage

The interface between constituents of a multiphase material exhibits properties different from those of the bulk and can lead to major alternation of the material response. Interface effects are particularly important for multiphase nano-materials where the area-to-volume ratio is significantly large. In this contribution, we study the influence of a degrading general interface. That is, we allow for the initiation and accumulation of damage on a generalized interface accounting for both jumps of the displacement and the traction across the interface. The applicability of the proposed framework is demonstrated through several numerical examples. We present a parametric study on the influence of a broad range of interface material parameters on the overall behavior of various microstructures subject to volumetric loading and unloading. The numerical results illustrate that the resistance along the interface plays a key role in the resulting damage mechanism and could potentially prevent the detachment of the inclusion from the matrix regardless of the resistance across the interface or bulk material parameters. This behavior is observed and shown for both two- and three-dimensional examples. Moreover, the size-effect due to the general interface model is examined and compared against other interface models. Finally, the influence of the boundary conditions on the effective response and damage initiation of several microstructures is studied.

DEM parameter calibration of cohesive bulk materials using a simple angle of repose test

The numerical effort of discrete element method (DEM) simulations results in a general idealisation of DEM models that makes the calibration crucial to obtaining realistic simulation results. The angle of repose test has become a standard test for the calibration of DEM parameters of cohesionless bulk materials and is extensively discussed in the literature. One of the most used test methods is the pull-up test of a hollow cylinder filled with bulk material. This paper presents how this basic pull-up test can also be used for the calibration of DEM parameters of cohesive materials by changing the analysis criteria. In contrast to analysing the static angle of repose, the focus lies on the macroscopic flow behaviour during the lifting of the cylinder. Reproducible phases of flow are identified in experiments: the build-up of a stable bulk material column, the convex bending of the column, and the beginning of collapse of the column. Furthermore, the phases are independent of the size and lifting velocity. The convex bending of the bulk-material column combined with the moment of first collapse are introduced as new measurable calibration criteria. These experimental results of wet sand are used for the calibration of related DEM parameters using a simplified JKR cohesion model. The DEM calibration including an analysis algorithm that fits the experimental flow behaviour and the final selection of the relevant parameters (i.e., sliding friction coefficient, rolling friction coefficient, and cohesion energy density) is presented in detail.

Characterization of Piezoelectric Energy Harvesters Using Frequency-Weighted Impedance Method

This paper presents a frequency-weighted impedance method for the characterization of piezoelectric bending beam energy harvesters. The method uses the frequency response of the beam to determine the beam model parameters. Existing methods often need to measure the beam tip deflection for the characterization of the parameters. This requires the accessibility of the tip and advanced measurement tools, such as a laser vibrometer. However, in the proposed method only the knowledge of the base acceleration is adequate to determine the electromechanical coupling coefficient of the beam. Furthermore, the rest of parameters including mechanical damping ratio can be determined based on a pure electrical test. The proposed method is useful for the characterization of all types of piezoelectric energy harvesters, particularly the vacuum-packed ones in which the tip of the beam is inaccessible. The validity of the method is experimentally verified using a test setup including a vacuum-packed piezoelectric energy harvester. The measured parameters of the beam are compared with the bulk material parameters showing that the proposed method can accurately estimate the model parameters without using the information of the tip deflection and with an error less than 4%.

Mitigation of explosions of hydrogen–air mixtures using bulk materials and aqueous foam

The objective of this work is to determine experimentally the effectiveness of protective barriers under conditions when blast waves are generated during premixed hydrogen– air combustion in various regimes. Experiments are conducted in a vertical tube having a diameter of 54 mm and a length of up to 2 m. Blast loads are produced by acceleration of premixed hydrogen–air flames in the tube with ring obstacles. Comparative tests are performed between protection barriers made of bulk materials with different densities and aqueous foams with different expansion ratios. It is demonstrated that the degree of blast load attenuation by an aqueous foam barrier increases with decreasing molecular weight of the filling gas and increasing density (decreasing expansion ratio) of the foam. An Aerosil barrier three times thicker than a titanium-dioxide one is found to have a similar attenuating effect on blast action. However, the mass per unit area of an Aerosil barrier is lower than titanium dioxide by a factor of 6 and is comparable to foam. The observed dependence of blast load attenuation on parameters of bulk materials and aqueous foams must be taken into account in systems designed to mitigate the consequences of accidental hydrogen release and combustion.

(Invited) Parameter Identification in Electrochemical Impedance Spectroscopy Applications: Analysis of Sensitivity

EIS is a powerful non-destructive technique that can access essential information about interfacial and bulk material parameters through the use of low energy time varying electrical excitations. When applied to an electrochemical system, IS can provide information on reaction parameters, corrosion rates [1], oxide characteristics, porosity, coating integrity [2-3], mass transport, and many other electrode/interface characteristics [4]. Software packages for the analysis of impedance data have also emerged [5–7]. The typical goal of an EIS experiment is to gather significant information regarding the physicochemical phenomena taking place in an electrochemical system, and the experimental data are then compared to a model that describes the system. The model can be purely empirical, being composed of resistors, capacitors, and generalized electrochemical elements, or otherwise it can be based on the physics of the system. Model parameters are evaluated through an inverse method. Minimization algorithms such as the Complex Non Linear Least Squares (CNLS) method, the Gauss-Newton method, the Levenberg-Marquardt method or the Downhill Simplex algorithm are used to determine the value of the parameters that minimize the sum of squared differences between the experimental and predicted data. Regardless of the method, the minimum value of the χ2 statistic is commonly used to assess the quality of the regression. χ2 is a non-weighted sum of squared differences: (1) In this equation, nf, Re, Im and |Z| represent the total number of frequencies constituting the impedance spectrum, the real and imaginary part and the modulus of the impedance, respectively. The subscripts e and c stand for the experimental and the calculated data, respectively. The standard deviation for the measurement is assumed to be Z= 0.01.|Ze| i.e.assuming a standard deviation for the impedance measurement of 1% of the modulus. For a complex regression, the fitting quality was determined by the standard deviation q. (2) Where n is the degree of freedom for the regression, i.e., the number of observations 2nf minus the number of adjustable parameters. The Smaller χ2 , the smaller the differences between the observed and modeled values and the better the fit. It is generally considered that a fit if satisfactory if the value of q is of the order of 1. A value smaller than 1 means that error was overestimated. χ2 provides a useful single numerical value for assessing the quality of the regression, but χ2does not provide any information on the accuracy of the different parameters evaluated. It is important to know if and how much the model, and subsequently the χ2or q are sensitive to the different parameters. Hence the sensitivity to the different parameters provides information on the accuracy of the identification technique. In this communication, we will show the results of the sensitivity analysis performed on three model cases: a blocking electrode and the ferri/ferrocyanide system, silicon nitride thin layers. Confidence intervals (sensitivity) for the fitted parameters were estimated empirically from the change (of +/- 5%) in a single parameter value yielding an increase of the standard deviation q characterizing the fit quality, the other parameters staying fixed to their optimal value or unfixed in order to estimate interactions between parameters. Basically, if a confidence interval is very wide, the data doesn’t define that parameter very well, or that the model proposed is not optimum. If a confidence interval is very narrow, this parameter becomes an important parameter in the overall model. This approach is helpful in order to evaluate the robustness of the model and to validate the physical interpretation of model Parameters. Keywords: electrochemical impedance spectroscopy, regression model, sensitivity, confidence interval References [1] Srisuwan, N., Ochoa, N., Pébère, N., Tribollet, B., Corrosion Science, Volume 50, Issue 5, May 2008, p 1245-1250 [2] NACE 2012 March 11 – Technology Exchange Group 097X, Electrochemical Measurements [3] Introduction to Corrosion Science, E. McCafferty [4] M.E. Orazem, B. Tribollet, Electrochemical Impedance Spectroscopy, The Electrochemical Society, Series, Ed. Wiley, 2008. [5] B.A. Boukamp, Solid State Ionics 18–19 (1986) 136. [6] J.R. Macdonald, L.D. Potter Jr., Solid State Ionics 24 (1987) 61. [7] J.R. Macdonald, CNLS (Complex Nonlinear Least Squares) lmmittance Fitting, Program LEVM Manual, Houston, TX, Version 7.11 Edition, 1999. Figure 1

Size dependent and micromechanical modeling of strain gradient-based nanoparticle composite plates with surface elasticity

The bending and free vibration behaviors of strain gradient-based Kirchhoff plates made of nanoparticle composites are investigated with the incorporation of the plate's surface effects. The modified strain gradient theory is used to capture the first-order deformation of the material structure while the impacts of the surface energy and surface tension are represented according to Gurtin and Murdoch surface elasticity theory. The plate is made of a composite material with nanocrystalline matrix and nanoparticles reinforcement. A size-dependent micromechanical model for multi-phase materials is developed to estimate the elastic moduli of the composite plate. This model has the merit to account for the effects of the surface energy of the inhomogeneities inside the composite structure. In addition, the impacts of the interface, grain size of the nanocrystalline matrix, porosity, and size of the reinforcing particles are incorporated into the micromechanical model. A simply supported composite plate consisting of nanocrystalline-silicon as the matrix and Al2O3-nanoparticles as the reinforcement is considered to study the influences of the size of the Si-grains, Al2O3-particles, voids, strain gradient deformation, and plate surface energy and tension on the deflection and natural frequencies of the plate. The results show that a decrease in the plate's thickness is accompanied with a decrease in the plate's deflection and an increase in the plate's natural frequency. Furthermore, the plate's strain gradient and surface tension contribute to its stiffness through a hardening effect while the interface and porosities inside the material structure affect the plate's stiffness with a softening mechanism. The plate's surface energy could stiffen or soften the plate depending on the values of the surface Lame constants. The surface energy of the inhomogeneities inside the material structure may increase or decrease the plate's stiffness, deflection, and natural frequencies depending on the inhomogeneities' sizes, their bulk material parameters, their surface material parameters, and their volume fractions.

AlN–Ag based low-emission sputtered coatings for high visible transmittance window

Abstract Despite the availability of high performance glass technologies a preconceived idea that glass is the weak point in a building envelope still persists [1] . Thanks to the continuous improvement in thermal insulation performance combined with new methods of modulating solar heat and visible transmittance, glazing has strengthened its position as smart opportunity for energy savings in green buildings. In the field of smart glazing, low-emission (low-E) coatings have contributed to change glass from an energy liability to an energy asset, acting like filters to manage heat transfer, in a manner that some energy wavelengths pass through, while others are rejected. Low-E glass can retain furnace heat, when it is cold outside, or rejects excessive solar heat, when it is hot outside. In this work, by means of a semi-empirical method, low-E coatings based onto a multilayered structure have been designed, ideal for mild to hot climates. These coatings consist of thin silver layers sandwiched between two dielectric layers of aluminum nitride, obtained by sputtering deposition technique. Stacked structures with two or more dielectric/metal/dielectric sequences have been considered in order to maximize high visible transmittance and to minimize emissivity value at room temperature. Starting from optical parameters of bulk materials and by means of a simulation tool, thicknesses of the different layers and of the overall stacked structure have been determined and a highly effective low-E filter has been optimized having high visible transmittance (82%) and very low emissivity (8%).

On the role of fibril mechanics in the work of separation of fibrillating interfaces

High values for the work of separation have been reported in peel tests on fibrillating interfacial systems. The exact origin of these high values is not properly understood, since it remains unclear which dissipative mechanisms related to fibrillation cause a significant increase in the work of separation. In this paper, the contribution of fibril mechanics to the work of separation is quantified. To this end, a micromechanical model of a single fibril is used, in which the growth of a nucleated fibril up to the moment of fracture is described. The initial geometry is varied to reflect the variability in the substrate profile. It is observed that the stresses and strains that occur in the model are well beyond typical bulk values. Given the large variation in measured stress–strain curves for rubber materials reported in literature, a small scale single fibril experiment is performed to assess the applicability of reported bulk material parameters to this problem. The obtained experimental response falls well within the model bandwidth for the range of material parameters from literature. Furthermore, the fibril fracture stress, which serves as the failure criterion in the model, is extracted from the experiment. From the micromechanical model results, it appears that, for the considered range of material properties, the work of separation is mainly influenced by the fracture stress. Furthermore the initial geometry has a profound influence on the obtained work of separation. It is concluded that the work of separation determined from the micro-model is significantly larger than the intrinsic adhesion energy, yet it remains still an order of magnitude smaller than the values reported in literature. For the considered system this indicates that, although it has a significant contribution, fibril deformation is not the only contribution to the high work of separation.

Heterointerface Potentials in the Effective-Mass Approximation for Wurtzite Semiconductor Structures

In the effective-mass approximation, the step-like crystal potential of a wurtzite semiconductor heterostucture should be supplemented by Dirac delta-function heterointerface terms. They stem from the difference in the Bloch functions of the semiconductors and remain finite even for structures with graded chemical composition, where the terms are presented by a smeared Dirac delta function. We find these heterointerface potentials by employing the k*p method, and evaluate their strength from band-structure parameters of bulk materials. These potentials are weak for semiconductors compliant with the cubic approximation, which forces the zinc-blende crystal symmetry upon the wurtzite lattice. Nevertheless, they can produce a noteworthy effect due to a strong built-in electric field usually present in wurtzite heterostructures. We estimate that for GaN/AlN [0001] heterojunctions their net contributions to the energy of conduction and valence band states are 3 meV and 10 meV, respectively. The presence of the interface potential can modify the shape of the valence-band spectrum calculated without the potential.

Mechanics of thick-shell microcapsules made by microfluidics

Double emulsion templates made by microfluidics allow for the production of tailored and monodisperse microcapsules. However, their mechanical properties cannot be predicted by traditional analytical models because of their relatively thick shells. In this work, we produce thick-shelled microcapsules with varying sizes and shell thicknesses and mechanically characterize them in single-capsule compression tests using Weibull statistics. We simulate their compression with finite element modeling and find a good agreement with the experimental results under linear elastic conditions, which enables the prediction of elastic properties based on bulk material parameters. Analysis of the simulated stresses show that capsules with a thickness-to-radius ratio above 20% consistently fail at a constant maximum principal stress in accordance with the brittle nature of their bulk material, enabling the extrapolation of their failure loads as well. Combining this structure-property correlation with processing-structure relationships found in previous studies provides a general predictive framework for the assembly of monodisperse microcapsules with tunable mechanics for protection and/or controlled release of encapsulants.

Propagation Length of Self-healing Slip Pulses at the Onset of Sliding: A Toy Model

Macroscopic sliding between two solids is triggered by the propagation of a micro-slip front along the frictional interface. In certain conditions, sliding is preceded by the propagation of aborted fronts, spanning only part of the contact interface. The selection of the characteristic size spanned by those so-called precursors remains poorly understood. Here, we introduce a 1D toy model of precursors between a slider and a track in which the fronts are quasi-static self-healing slip pulses. When the slider’s thickness is large compared to the elastic correlation length and when the interfacial stiffness is small compared with the bulk stiffness, we provide an analytical solution for the length of the first precursor, \(\varLambda\), and the shear stress field associated with it. These quantities are given as a function of the bulk material parameters, the frictional properties of the interface and the macroscopic loading conditions. Analytical results are in quantitative agreement with the numerical solution of the model. In contrast with previous models, our model predicts that \(\varLambda\) does not depend on the frictional breaking threshold of the interface. Our results should be relevant to the various systems in which self-healing slip pulses have been observed.