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ON-state conductance properties of vertical GaN n-channel trench MISFETs manufactured on different GaN substrates and having different gate trench orientations are studied up to 200 °C ambient temperature. The best performing devices, with a maximum output current above 4 kA/cm2 and an area specific ON-state resistance of 1.1 mΩ·cm2, are manufactured on ammonothermal GaN substrate with the gate channel parallel to the a-plane of the GaN crystal. The scalability of the devices up to 40 mm gate periphery is investigated and demonstrated. It is found that, in addition to oxide interface traps, the semiconductor border traps in the p-GaN layer limit the available mobile channel electrons and that the channel surface roughness scattering limits the channel mobility. Both strongly depend on the gate trench orientation and on the GaN substrate defect density.

Laser powder bed fusion is a well-established additive manufacturing method that can be used for the production of net-shaped Nd Fe B magnets. However, low coercivity has been one of the drawbacks in the laser powder bed fusion processed Nd Fe B magnets. In this work, we have demonstrated that the grain boundary diffusion process using low-melting Nd Cu, Nd Al Ni Cu, and Nd Tb Cu alloys to the selective laser sintered NdFeB magnets can results in a substantial enhancement of coercivity from 0.65 T to 1.5 T. Detailed microstructure investigations clarified that the formation of Nd-rich grain boundary phase, the introduction of Tb-rich shell at the surface of Nd2Fe14B grains, and maintaining the grain size in nano-scale are responsible for the large coercivity enhancement.

Transport processes in porous media have been traditionally studied through the parameterization of macroscale properties, by means of volume-averaging or upscaling methods over a representative elementary volume. The possibility of upscaling results from pore-scale simulations, to obtain volume-averaging properties useful for practical purpose, can enhance the understanding of transport effects that manifest at larger scales. Several studies have been carried out to investigate the impact of the geometric properties of porous media on transport processes for solute species. However, the range of pore-scale geometric properties, which can be investigated, is usually limited to the number of samples acquired from microcomputed tomography images of real porous media. The present study takes advantage of synthetic porous medium generation to propose a systematic analysis of the relationships between geometric features of the porous media and transport processes through direct simulations of fluid flow and advection-diffusion of a non-reactive solute. Numerical simulations are performed with the lattice Boltzmann method on synthetic media generated with a geostatistically based approach. Our findings suggest that the advective transport is primarily affected by the specific surface area and the mean curvature of the porous medium, while the effective diffusion coefficient scales as the inverse of the tortuosity squared. Finally, the possibility of estimating the hydrodynamic dispersion coefficient knowing only the geometric properties of porous media and the applied pressure gradient has been tested, within the range of tested porous media, against advection-diffusion simulations at low Reynolds (, Geofluids, 11 (6), ISSN:1468-8115, ISSN:1468-8123

Dropwise condensation on rough structures with different wettability are numerically simulated using Lattice Boltzmann method. The latent heat during condensation process together with the energy equation is taken into consideration in this paper by solving the temperature distribution equation with a source term for the phase change. Typical droplet nucleation positions and wetting states are found out at different surface wettability through numerical results. It is concluded that with the increase of strength coefficient of the fluid-solid interaction or surface hydrophobicity, the nucleation position rises from the bottom to top of the pillar, and the wetting state of droplet changes from the wetting Wenzel state to the nonwetting Cassie one.

The development of high-performance, veneer-based wood composites is a topic of increasing importance due to the high design flexibility and the comparable mechanical performance to solid wood. Part of this improved mechanical performance can be contributed to the size effect present in wood. Based on previous findings in the literature, this size effect can be either strengthening or weakening. The presented study investigates the influence of thickness and load angle on the tensile strength and tensile stiffness of peeled veneers compared to thin sawn timber. Veneers with thicknesses of 0.5 &plusmn, 0.05 mm, 1.0 &plusmn, 0.05 mm and 1.5 &plusmn, 0.05 mm as well as sawn wood with thicknesses of 1.5 &plusmn, 0.1 mm, 3.0 &plusmn, 0.1 mm and 5.0 &plusmn, 0.1 mm were tested in tension under different load angles (0&deg, 45&deg, and 90&deg, ). The results only partly confirm a size effect for strength parallel to the grain. The strength perpendicular to the grain increased significantly between 0.5 mm and 1.5 mm, with a significant decrease between 1.5 mm and 5.0 mm. The presence of lathe checks diminished the strength perpendicular to the grain of the veneers by about 70% compared to solid wood, partly overshadowing a possible strengthening effect. It was concluded that a transition from a strengthening to a weakening behaviour lies in the range of multiple millimetres, but further investigations are needed to quantify this zone more precisely. The presented results provide a useful basis for the development of veneer-based wood composites with a performance driven layer-thickness.

Magnetic isotropic NdFeB powder with a spherical morphology is used to 3D print magnets by stereolithography (SLA). Complex magnets with small feature sizes in a superior surface quality can be printed with SLA. The magnetic properties of the 3D printed bonded magnets are investigated and compared with magnets manufactured by fused filament fabrication (FFF), and selective laser sintering (SLS). All methods use the same hard magnetic isotropic NdFeB powder material. FFF and SLA use a polymer matrix material as binder, SLS sinters the powder directly. SLA can print magnets with a remanence of 388 mT and a coercivity of 0 . 923 T. A complex magnetic design for speed wheel sensing applications is presented and printed with all methods.

This paper presents surface micromachined MEMS Fabry–Perot interferometers (FPIs) with two released membrane mirrors. Fabrication of such FPIs with large-area membrane reflectors of up to 5 mm in diameter for first-order operation in the near infrared is shown. The mirrors are distributed Bragg reflectors based on silicon and silicon carbonitride. Single mirror membrane FPIs with the same mirror design are used as a benchmark in order to demonstrate the improvements achievable by the double membrane approach. To that end, the FPIs are characterized with respect to their top surface flatness by white light interferometry as well as to the variations of the optical gap width over the aperture area by spatially resolved transmittance measurements. It is shown that the upper mirror membranes are flat within the nanometer range for both approaches. Nevertheless, single membrane FPIs exhibit strongly increasing gap variations for increasing membrane diameters due to a bow in the substrate wafer, thus decreasing the filter resolution over the full aperture. On the other hand, for double-membrane FPIs, the mirrors are highly parallel with a standard deviation of the gap as low as 2 nm. [2018-0096]

A method to create a highly homogeneous magnetic field by applying topology optimized, additively manufactured passive shimming elements is investigated. The topology optimization algorithm can calculate a suitable permanent and nonlinear soft magnetic design that fulfills the desired field properties. The permanent magnetic particles are bonded in a polyamide matrix and they are manufactured with a low-cost, end-user 3D printer. Stray field measurements and an inverse stray field simulation framework can determine printing and magnetization errors. The customized shimming elements are manufactured by a selective melting process which produces completely dense soft magnetic metal parts. The methodology is demonstrated on a simple example of two axial symmetric cylindrical magnets, which generates a high inhomogeneous magnetic field. In this case, the maximum magnetic field density is 25 mT and the the homogeneity can be increased by a factor of 35 or down to 6‰. Simulation and measurement results point out a good conformity. Additional topology optimizations of more than one shimming element layer show the opportunity to make the manufactured magnetic system even more homogeneous.

We present a sol–gel processed hematite–titania-based photoanode, which exhibits a photocurrent of up to 2.5 mA/cm2 at 1.23 VRHE under simulated AM 1.5 G illumination (100 mW/cm2) thanks to the addition of an amorphous cocatalyst with the nominal composition Fe20Cr40Ni40Ox. To unveil the role of the cocatalyst interconnected to the photoanode, we performed impedance measurements. According to the one order of magnitude higher value for the capacitance associated with surface states (CSS) compared to the bare photoanode, the function of the catalyst−photoanode interface resembles that of a p−n-like junction. In addition, the charge transfer resistance associated with charge transfer processes from surface states (Rct,ss) was unchanged at potentials between 0.8 and 1.1 VRHE after adding the cocatalyst, indicating that the catalyst has a negligible effect on the hole transport to the electrolyte. The understanding of the role of oxygen evolution catalysts (OECs) in conjunction with the photoanodes is particularly important for water splitting because most OECs are studied separately at considerably higher potentials compared to the potentials at which photoanode materials are operated. ISSN:2470-1343

Amorphous silicon carbonitride deposited at temperatures between 200 °C and 395 °C by plasma-enhanced chemical vapor deposition has been characterized by Fourier transfrom infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, ultraviolet–visible–near-infrared reflectance-transmittance spectroscopy, residual stress measurements and etch tests in hydrofluoric acid (HF). The optical and mechanical properties have been measured both directly after deposition and after storage under ambient air. The bonding structure derived from FTIR spectra indicates that higher deposition temperatures lead to a denser material with a higher degree of crosslinking due to an increased dissociation of hydrogen molecules, thereby reducing the amount of hydrogen-terminated bonds. This results in an increased refractive index and a reduced compressive stress level as well as a better resistance against HF etching of the as-deposited layers. Diffusion of ambient moisture into the thin-films is identified as the main aging mechanism which affects layers deposited at lower temperatures more pronouncedly due to their higher degree of porosity. The resulting adsorbed water molecules cause an increase of layer thickness accompanied by a decrease of refractive index and an increase of compressive stress.

In this paper we use the multiphase multispecies Lattice Boltzmann method to investigate the influence of non-condensable gas on condensation. Condensation on a horizontal cold wall as well as that on a vertical wall with droplet movement is investigaged. The presence of non-condensable gas obviously reduces the condensation mass rate as well as the heat flux compared to condensation from pure vapor. The waiting time before nucleation is increased with non-condensable gas, and the wetting characteristics are also changed (the contact angles are increased), which further influences the heat transfer. Correlations of the relationship between droplet diameter and condensing time for different surface wettability, or contact angles, as well as different air mass fraction are obtained. As for condensation on a surface parallel to the gravitational force, it’s demonstrated that the presence of non-condensable gas reduces the droplet departure diameter and increases the period between subsequent droplet formation and departure.

We present a broadband tunable MEMS filter functional in a spectral range between 900 nm and 1650 nm. It is based on a double-membrane Fabry-Perot interferometer (FPI) operating in first interference order comprising two symmetric Si/Air Bragg mirrors and a long range electrostatic actuation. The double-membrane approach allows optical gap and actuation gap to be separated, enabling independent adjustment of initial transmission wavelength and maximum mirror travel range. Additionally, this approach improves mirror parallelism. To our best knowledge, it is the first time an FPI based on Si/Air mirrors is tuned over the first order free spectral range unlocking the full potential of these high quality broadband reflectors.

A viscosity model for suspensions of rigid particles with predictive capability over a wide range of particle volume fraction and shear conditions is of interest to quantify the transport of suspensions in fluid flow models. We study the shear viscosity of suspensions and focus on the effect of particle aspect ratio and shear conditions on the rheological behavior of suspensions of rigid bi-axially symmetric ellipsoids (spheroids). We propose a framework that forms the basis to microscopically parameterize the evolution of the suspension microstructures and its effect on the shear viscosity of suspensions. We find that two state variables, the intrinsic viscosity in concentrated limit and the self-crowding factor, control the state of dispersion of the suspension. A combination of these two variables is shown to be invariant with the imposed shear stress (or shear rate) and depends only on the particle aspect ratio. This self-similar behavior, tested against available experimental and numerical data, allows us to derive a predictive model for the relative viscosity of concentrated suspensions of spheroids subjected to low (near zero) strain rates. At higher imposed strain rates, one needs to constrain one of the state variables independently to constrain the state of dispersion of the suspension and its shear dynamic viscosity. Alternatively, the obtained self-similar behavior provides the means to estimate the state variables from the viscosity measurements made in the laboratory, and to relate them to microstructure rearrangements and evolution occurring during deformation.

The hydrometer test is one of the most reliable approaches to estimate the particle size distribution of fine-grained soil. The general approach neglects the particle hydrodynamic interactions which are known to hinder the settling rate of particles, and, as such, generally leads to an underestimation of the particle diameters. In this study, a theoretical hydrodynamic correction of the hydrometer test is proposed to account for the hindered velocity of particles settling through a suspension of monomodal spherical or non-spherical particles. The polydisperse nature of soil samples is then taken into consideration by introducing an effective volume fraction of particles for each partially sorted monomodal layer of particles. Using this approach, a model to correct for particle hydrodynamic interactions in conventional hydrometer tests is provided and tested successfully against results from laser diffraction measurements.

The possibility of producing polymer-bonded magnets with the aid of additive processes, such as 3D printing, opens up a multitude of new areas of application. Almost any structures and prototypes can be produced cost-effectively in small quantities. Extending the 3D printing process allows the manufacturing of anisotropic magnetic structures by aligning the magnetic easy axis of ferromagnetic particles inside a paste-like compound material along an external magnetic field. This is achieved by two different approaches: First, the magnetic field for aligning the particles is provided by a permanent magnet. Secondly, the 3D printing process itselfs generates an anisotropic behavior of the structures. An inexpensive and customizable end-user fused filament fabrication 3D printer is used to print the magnetic samples. The magnetical properties of different magnetic anisotropic Sr ferrite and SmFeN materials are investigated and discussed.

In this publication, we describe the extrusion process and the properties of polymer-bonded anisotropic SrFe 12 O 19 filaments for fused filament fabrication (FFF). Highly filled polyamide 12 filaments with a filling fraction from 40 vol. % to 55 vol. % are mixed and extruded into filaments with a diameter of 1.75 mm. Such filaments are processable with a conventional FFF 3D printer. No modifications of the 3D printer are necessary. Detailed mechanical and magnetic investigations of printed samples are performed and discussed. In the presence of an external alignment field, the Sr ferrite particles inside the PA12 matrix can be aligned along an external magnetic field. The remanence can be increased by 40% by printing anisotropic structures. For the 55 vol. % filled filament, a remanence of 212.8 mT and a coercivity of 307.4 mT are measured. The capabilities of printing magnetic anisotropic structures in a complex external field are presented with a Halbach-array arrangement. With the aim of an inverse field model, based on a finite element method, the orientation of the particles and the quality of the print can be estimated by a nondestructive method.

We present electrostatically actuated, surface-micromachined MEMS Fabry-Perot interferometers with two released membrane mirrors. Actuation of the lower mirror towards the substrate allows resonance tuning over the full free spectral range of the 1st interference order without the risk of reaching the pull-in point. We demonstrate a proof-of-concept device with a tuning range from 1130 nm to 1700 nm for actuation voltages below 10 V. Spatially resolved transmittance measurements show that the achieved spectral resolution is close to the expected theoretical value.

In this paper, we describe a lattice Boltzmann model to simulate chemical reactions taking place at the interface between two immiscible fluids. The phase-field approach is used to identify the interface and its orientation, the concentration of reactant at the interface is then calculated iteratively to impose the correct reactive flux condition. The main advantages of the model is that interfaces are considered part of the bulk dynamics with the corrective reactive flux introduced as a source/sink term in the collision step, and, as a consequence, the model’s implementation and performance is independent of the interface geometry and orientation. Results obtained with the proposed model are compared to analytical solution for three different benchmark tests (stationary flat boundary, moving flat boundary and dissolving droplet). We find an excellent agreement between analytical and numerical solutions in all cases. Finally, we present a simulation coupling the Shan Chen multiphase model and the interface reactive model to simulate the dissolution of a collection of immiscible droplets with different sizes rising by buoyancy in a stagnant fluid.

Although the relative velocity of a single crystal or bubble in a quiescent fluid (melt) is well characterized, the interplay of crystals/bubbles in multiparticle systems and its effect on their settling/rising velocity is poorly quantified. We propose a theoretical model for the hindered velocity of non-Brownian emulsions and suspensions of nondeformable fluid and solid particles in the creeping flow regime. The model is based on three sets of correction: two on the drag coefficient experienced by each particle to account for both return flow and Smoluchowski effects and a correction on the rheology to account for nonlocal interactions introduced as a mean-field effective viscosity. Our model is tested against new and published experimental data over a wide range of particle volume fraction and viscosity ratio between the fluids. We find an excellent agreement between our model and experiments. The model is then applied to show that hindered settling can increase mineral residence time by up to an order of magnitude in convecting magma chambers.

The macroscopic description of buoyancy-driven thermal convection in porous media is governed by advection-diffusion processes, which in the presence of thermophysical heterogeneities fail to predict the onset of thermal convection and the average rate of heat transfer. This work extends the classical model of heat transfer in porous media by including a fractional-order advective-dispersive term to account for the role of thermophysical heterogeneities in shifting the thermal instability point. The proposed fractional-order model overcomes limitations of the common closure approaches for the thermal dispersion term by replacing the diffusive assumption with a fractional-order model. Through a linear stability analysis and Galerkin procedure, we derive an analytical formula for the critical Rayleigh number as a function of the fractional model parameters. The resulting critical Rayleigh number reduces to the classical value in the absence of thermophysical heterogeneities when solid and fluid phases have similar thermal conductivities. Numerical simulations of the coupled flow equation with the fractional-order energy model near the primary bifurcation point confirm our analytical results. Moreover, data from pore-scale simulations are used to examine the potential of the proposed fractional-order model in predicting the amount of heat transfer across the porous enclosure. The linear stability and numerical results show that, unlike the classical thermal advection-dispersion models, the fractional-order model captures the advance and delay in the onset of convection in porous media and provides correct scalings for the average heat transfer in a thermophysically heterogeneous medium.

This contribution presents silicon carbonitride as a new low refractive index material for integration together with silicon in suspended distributed Bragg reflector membranes. We show how its benign mechanical stress properties can be used to fabricate inherently flat mirror membranes of up to 5 mm diameter. Due to the large refractive index contrast to silicon such mirror membranes exhibit high reflectance over a wide spectral range. Using these mirrors, we demonstrate a proof of concept for a large aperture Fabry-Perot filter with narrow bandwidth that could help miniaturize hyperspectral imaging sensors.

Within this work a passive and wireless magnetic sensor, to monitor linear displacements is proposed. We exploit recent advances in 3D printing and fabricate a polymer bonded magnet with a spatially linear magnetic field component corresponding to the length of the magnet. Regulating the magnetic compound fraction during printing allows specific shaping of the magnetic field distribution. A giant magnetoresistance magnetic field sensor is combined with a radio-frequency identification tag in order to passively monitor the exerted magnetic field of the printed magnet. Due to the tailored magnetic field, a displacement of the magnet with respect to the sensor can be detected within the sub-mm regime. The sensor design provides good flexibility by controlling the 3D printing process according to application needs. Absolute displacement detection using low cost components and providing passive operation, long term stability and longevity renders the proposed sensor system ideal for structural health monitoring applications., 4 pages, 5 figures, 1 table

Reactive processes associated with multiphase flows play a significant role in mass transport in unsaturated porous media. For example, the effect of reactions on the solid matrix can affect the formation and stability of fingering instabilities associated with the invasion of a buoyant non-wetting fluid. In this study, we focus on the formation and stability of capillary channels of a buoyant non-wetting fluid (developed because of capillary instabilities) and their impact on the transport and distribution of a reactant in the porous medium. We use a combination of pore-scale numerical calculations based on a multiphase reactive lattice Boltzmann model (LBM) and scaling laws to quantify (i) the effect of dissolution on the preservation of capillary instabilities, (ii) the penetration depth of reaction beyond the dissolution/melting front, and (iii) the temporal and spatial distribution of dissolution/melting under different conditions (concentration of reactant in the non-wetting fluid, injection rate). Our results show that, even for tortuous non-wetting fluid channels, simple scaling laws assuming an axisymmetrical annular flow can explain (i) the exponential decay of reactant along capillary channels, (ii) the dependence of the penetration depth of reactant on a local Péclet number (using the non-wetting fluid velocity in the channel) and more qualitatively (iii) the importance of the melting/reaction efficiency on the stability of non-wetting fluid channels. Our numerical method allows us to study the feedbacks between the immiscible multiphase fluid flow and a dynamically evolving porous matrix (dissolution or melting) which is an essential component of reactive transport in porous media.

Microelectromechanical systems (MEMS) currently see a trend towards inclusion of optical functionalities, i.e., towards microoptoelectromechanical systems (MOEMS). However, typical MEMS materials such as silicon, silicon oxide and silicon nitride limit the choice of available refractive indices and spectral operating regions. Particularly, a low refractive index material is highly desirable which is usable in conjunction with high refractive index silicon, e.g., in distributed Bragg reflectors (DBRs) and which is not etched in an HF release step for releasing movable structures. Furthermore, MEMS applications typically need a careful control of the residual mechanical stress in free-standing structures. For this purpose, we present hydrogenated amorphous silicon carbonitride (a-SiCN:H) thin-films deposited by plasma-enhanced chemical vapour deposition (PECVD) from SiH 4 , CH 4 and NH 3 and show its beneficial properties for optical layers in mOeMS.

Changes in band gap energy of the buffer layer of kesterite Cu 2 ZnSn(S,Se) 4 solar cells were analyzed in dependence of different post-annealing temperatures and times using electroreflectance (ER). In order to measure the band gap of the buffer, the ER experiments were performed in “Brewster geometry” (angle of incidence and angle of detection equal Brewster's angle) allowing to reduce line shape distortions induced by interference effects and hence allowing reliable analysis of the ER spectra. Due to post-annealing, an irreversible reduction of the buffer layer's band gap is detected that increases for longer annealing times and higher temperatures. The decrease of band gap energy is attributed to an interdiffusion of atoms between the buffer and absorber layers.

The present study focuses on the transition between steady convective patterns in fluid-saturated porous media. We conduct experiments to identify the transition point from the single- to double-cell pattern in a two-dimensional porous medium. We then perform a basin stability analysis to assess the relative stability of different convective modes. The resulting basin stability diagram not only provides the domains of coexistence of different modes, but it also shows that the likelihood of finding convective patterns depends strongly on the Rayleigh number. The experimentally observed transition point from single- to double-cell mode agrees well with the stochastically preferred mode inferred from the basin stability diagram.

The present study investigates the role of thermal nonequilibrium on natural convection in a fluid-saturated porous medium heated from below. We conduct high-resolution direct numerical simulation at the pore scale in a two-dimensional regular porous structure by means of the thermal lattice-Boltzmann method (LBM). We perform a combination of linear stability analysis of continuum-scale heat transfer models, and pore-scale and continuum-scale simulations to study the role of thermal conductivity contrasts among phases on natural convection. The comparison of pore-scale lattice-Boltzmann simulations with linear stability analysis reveals that traditional continuum-scale models fail to capture the correct onset of convection, convection mode, and heat transfer when the thermal conductivity of the solid obstacles does not match that of the fluid.

Additive manufacturing of polymer-bonded magnets is a recently developed technique, for single-unit production, and for structures that have been impossible to manufacture previously. Also, new possibilities to create a specific stray field around the magnet are triggered. The current work presents a method to 3D print polymer-bonded magnets with a variable magnetic compound fraction distribution. This means the saturation magnetization can be adjusted during the printing process to obtain a required external field of the manufactured magnets. A low-cost, end-user 3D printer with a mixing extruder is used to mix permanent magnetic filaments with pure polyamide (PA12) filaments. The magnetic filaments are compounded, extruded, and characterized for the printing process. To deduce the quality of the manufactured magnets with a variable magnetic compound fraction, an inverse stray field framework is developed. The effectiveness of the printing process and the simulation method is shown. It can also be used to manufacture magnets that produce a predefined stray field in a given region. This opens new possibilities for magnetic sensor applications. This setup and simulation framework allows the design and manufacturing of polymer-bonded permanent magnets, which are impossible to create with conventional methods.

We present analyses to provide a generalized rheological equation for suspensions and emulsions of non-Brownian particles. These multiparticle systems are subjected to a steady straining flow at low Reynolds number. We first consider the effect of a single deformable fluid particle on the ambient velocity and stress fields to constrain the rheological behavior of dilute mixtures. In the homogenization process, we introduce a first volume correction by considering a finite domain for the incompressible matrix. We then extend the solution for the rheology of concentrated system using an incremental differential method operating in a fixed and finite volume, where we account for the effective volume of particles through a crowding factor. This approach provides a self-consistent method to approximate hydrodynamic interactions between bubbles, droplets, or solid particles in concentrated systems. The resultant non-linear model predicts the relative viscosity over particle volume fractions ranging from dilute to the the random close packing in the limit of small deformation (capillary or Weissenberg numbers) for any viscosity ratio between the dispersed and continuous phases. The predictions from our model are tested against published datasets and other constitutive equations over different ranges of viscosity ratio, volume fraction, and shear rate. These comparisons show that our model, is in excellent agreement with published datasets. Moreover, comparisons with experimental data show that the model performs very well when extrapolated to high capillary numbers (C a≫1). We also predict the existence of two dimensionless numbers; a critical viscosity ratio and critical capillary numbers that characterize transitions in the macroscopic rheological behavior of emulsions. Finally, we present a regime diagram in terms of the viscosity ratio and capillary number that constrains conditions where emulsions behave like Newtonian or Non-Newtonian fluids.

A simple model to simulate the measurement of layered structures with confocal micro X-ray fluorescence (micro-XRF) was developed and implemented as a computer program. The model assumes monochromatic excitation, considers at the moment only K lines, and simplifies the volume defined by excitation and detection foci as a circle area. First simulation results and comparison with data acquired using the Atominstitut confocal micro-XRF spectrometer are very promising. The simulation software enables us to perform parameter studies to have a better understanding of the analysis of layered structures with confocal micro-XRF. Copyright © 2014 John Wiley & Sons, Ltd.

[1] We study the development of capillary instabilities during the invasion of a buoyant nonwetting phase in a saturated porous media. Capillary instabilities are generally attributed to heterogeneities in the porous medium resulting in the existence of fluid pathways opposing different resistance to the flow (“passive control”). We use a simple macroscale theoretical model based on the postulate that the nonwetting fluid will be distributed in the porous medium to minimize the resistance to transport. This theoretical argument is used to show that after their formation, some capillary instabilities can grow at the expense of others. The competitive growth between capillary channels arises because of pore-scale fluid interactions that occur even in a porous medium offering identical pathways at the pore scale. The evolution of the pore volume fraction of nonwetting fluid in capillary fingers is therefore dynamically controlled by fluctuations in the nonwetting phase saturation and its effect on the relative permeability (“active control”). The theoretical model predicts (1) the growth of heterogeneities in nonwetting fluid saturation among competing capillary channels if the second derivative of the invading phase relative permeability with respect to its saturation is positive, and (2) that the amplitude of the perturbation in nonwetting fluid content between competing fingers increases with the interfacial tension. We use a pore-scale multiphase flow numerical model to test the validity of the postulate for optimal transport of nonwetting fluids and the two ensuing predictions. We observe that the numerical calculations are in excellent agreement with the theoretical predictions.

3D print is a recently developed technique, for single-unit production, and for structures that have been impossible to build previously. The current work presents a method to 3D print polymer bonded isotropic hard magnets with a low-cost, end-user 3D printer. Commercially available isotropic NdFeB powder inside a PA11 matrix is characterized, and prepared for the printing process. An example of a printed magnet with a complex shape that was designed to generate a specific stray field is presented, and compared with finite element simulation solving the macroscopic Maxwell equations. For magnetic characterization, and comparing 3D printed structures with injection molded parts, hysteresis measurements are performed. To measure the stray field outside the magnet, the printer is upgraded to a 3D magnetic flux density measurement system. To skip an elaborate adjusting of the sensor, a simulation is used to calibrate the angles, sensitivity, and the offset of the sensor. With this setup a measurement resolution of 0.05\,mm along the z-axes is achievable. The effectiveness of our novel calibration method is shown. With our setup we are able to print polymer bonded magnetic systems with the freedom of having a specific complex shape with locally tailored magnetic properties. The 3D scanning setup is easy to mount, and with our calibration method we are able to get accurate measuring results of the stray field.

The advancement of 3D-printing opens up a new way of constructing affordable custom terahertz (THz) components due to suitable printing resolution and THz transparency of polymer materials. We present a way of calculating, designing and fabricating a THz waveplate that phase-modulates an incident THz beam ({\lambda}=2.14 mm) in order to create a predefined intensity profile of the optical wavefront on a distant image plane. Our calculations were performed for two distinct target intensities with the use of a modified Gerchberg-Saxton algorithm. The resulting phase-modulating profiles were used to model the polyactide elements, which were printed out with a commercially available 3D-printer. The results were tested in an THz experimental setup equipped with a scanning option and they showed good agreement with theoretical predictions., Comment: 4 pages, 3 figures

Electromodulated reflectance (ER) is a standard characterization method to determine critical points such as the band gap in the band structure of semiconductors. These critical points show up as spectrally narrow features in ER and are typically evaluated using Aspnes's third-derivative functional form. ER spectra of stratified semiconductor systems such as thin-film solar cells, however, are significantly distorted by optical interference due to their layered structure. Furthermore, strong built-in electric fields result in a deviation from the typically assumed low-field conditions. We present here simulations of ER spectra from stratified systems based on transfer matrices using the Franz-Keldysh theory in its general form. For realistic thin-film solar cell conditions, distortions of ER line shapes due to the above-mentioned interferences and strong electric fields appear in the simulations. Furthermore, the results show good agreement with measured ER spectra of a structurally well-characterized $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ (CIGS) solar cell. Our analysis points out the restrictions on the determination of energetic position and number of critical points from ER spectra of stratified systems.

We investigate the impact of order-disorder related band gap changes on finished Cu2ZnSn(S,Se)4 solar cells. Solution-processed solar cells are post-annealed in a tube furnace in order to modify the Cu-Zn disorder within the kesterite unit cell. A decrease in this disorder leads to an increased band gap of the absorber. The latter is detected using electroreflectance spectroscopy. In our experiments the change in the open-circuit voltage follows the change in the absorber band gap. However, the open-circuit voltage deficit of the solar cell remains unaltered. Still, power conversion efficiencies could be increased by 1%, mainly due to an increased open-circuit voltage.

Active fiber composites (AFC) made from piezoelectric fibers show promising potential, not only as actuators integrated into smart composites but also as sensors. Selected sensor properties of AFC elements are investigated with methods from acoustic emission sensor characterization and verification and from acousto-ultrasonic testing. Model experiments with AFC elements integrated in simple composite structures show that the various approaches for analysis of acousto-ultrasonic data yield indications of damage accumulation.

Active fiber composite (AFC) composed of lead zirconate titanate (PZT) fibers with interdigitated electrodes (IDEs) has been integrated into orthotropic glass fiber reinforced plastic (GFRP) laminates to characterize the performance of AFC as a smart material component in laminated materials. Monotonic cyclic tensile loading was performed on integrated specimens at different strain levels. The AFC output was monitored to determine the effect of applied strain level on the AFC performance. It was found that the AFC sensitivity degraded beyond strains of 0.20% and approached a minimum at 0.50% strain. The degradation in the AFC performance appears to be attributed to the dominating effect of PZT fiber fragmentation during testing, as opposed to depolarization. Acoustic emission (AE) monitoring was used to detect damage in laminates during testing and was correlated with crack evidence from microscopy observations during testing to characterize damage evolution in response to strain levels.

Active fibre composite elements (AFCs) based on piezoelectric fibres are used as acoustic emission (AE) sensors on composites (unidirectional glass-fibre and carbon-fibre reinforced epoxy) and polymer (polymethyl-methacrylate, PMMA) plates. AFCs are mounted reversibly or permanently bounded to the plates. Their performances are compared with that of commercial AE sensors. The sensitivity of both sensors is characterised using signal attenuation curves and polar diagrams. The data indicate a large anisotropy behaviour of the AFCs in comparison with conventional AE sensors, leading to higher sensitivity of AFCs along the direction of the piezoelectric fibres. The emission of simulated AE by AFCs and AE sensors is also studied. Transient electric impulses applied to the AFC yield short vibrations transmitted to the testing object, that can be sensed by other AFCs or AE sensors. Permanently bonded AFCs show promising sensing and emitting properties for possible acousto-ultrasonic applications in composite laminates.

The present paper reports the synthesis of π-conjugated organometallic polymer networks based on poly[2,5-diotyloxy-1,4-diethynyl-phenylene-alt-2,5,-bis-(2'-ethylhexyloxy)-1,4-phenylene] (EHO-OPPE), a soluble poly(p-phenylene ethynylene) (PPE) derivative. The ethynylene moities of the polymer coordinate to Pt 0 centers, which act as conjugated cross-links. These materials are readily accessible through ligand-exchange reactions between the linear PPE and Pt(styrene) 3 . The in-situ NMR investigation of model reactions of Pt(styrene) 3 with diphenylacetylene (DPA) has shown that the ethynylene moieties comprised in the PPE readily coordinate to Pt 0 centers under release of the relatively weakly-bound styrene ligands. Spin-coating has resulted in cross-linked films of good optical quality. We also have been able to produce PPE-Pt-gels with high solvent content (>95 wt.-%). As expected, the coordination of Pt markedly influences the photophysical characteristics of the pPE. The photoluminescence is efficiently quenched, and the absorption maximum in the visible regime experiences a hypsochromic shift

We present a crowding-based model to predict the shear viscosity of suspensions of rigid bimodal-sized particles. In this model, the mutual crowding factor is defined to explicitly account for the change in the amount of fluid trapped in the interstices formed by particles upon mixing particles with two different sizes. Through this factor, we cancel the effect of size interference by mapping the bimodal suspensions to a suspension of noninterfering size ratio. This approach provides a set of decorrelated particle fractions that depend on crowding and size distribution. The shear viscosity of the resultant suspension is then directly estimated based on the viscosity of corrected components using a stiffening function that accounts for the self-crowding in each size class individually. We tested the proposed model against published experiments over a wide range of particle volume fractions, and we observe an excellent agreement.

Nondestructive surface crack detection is an important issue in many industrial, manufacturing and structural environments. Although there exist several standard nondestructive techniques, each possesses their respective advantages and disadvantages. Recent research activities in using open-ended microwave sensors has proved to be a viable practical approach. Microwave techniques offer many advantages including the fact that they may be used in a noncontact fashion while detecting exposed, filled and covered surface cracks. This paper explores the noncontact characteristics of employing an open-ended rectangular waveguide sensor at K-band. The results show the possibility of detecting thin and long machined slots/ cracks on aluminum plates at standoff distances of several millimeters. In addition, the results show that these cracks/ slots are readily detected when exposed, filled with rust and covered with several layers of common paint. A discussion of the findings is also presented.

Electromagnetic properties of a system formed by an open-ended rectangular waveguide and a surface crack/slot in a metallic specimen are described in this paper. Scanning a crack on a metal surface changes the reflection coefficient of the incident dominant mode. A model as a function of relative crack location within the waveguide aperture (i.e., crack moving with respect to the waveguide aperture) is desired to describe and optimize practical crack detection applications. Hence, the change in the reflection coefficient for a generalized system encompassing empty, filled, and finite cracks located at an arbitrary position inside the waveguide aperture, is evaluated. A moment solution approach is employed, and a magnetic current density M is introduced over the common aperture formed by the waveguide and the crack. Subsequently, the junction formed by the waveguide and the cracked metallic surface is separated into two systems. A numerical solution employing the method of moments is obtained, and the reflection coefficient at the waveguide aperture is expressed in terms of the generalized scattering matrix. The convergence behavior is studied to determine an optimized set of basis functions and the optimal number of higher order modes for a fast and accurate solution. Numerical results presented in this paper include the evaluation of the field distribution over the waveguide aperture. Finally, the theoretical and measured crack characteristic signals are compared.

A passive wireless radio frequency-identification (RFID) stress/strain sensor is presented. Stress is transformed into a change of magnetic field by utilizing an amorphous metal ribbon. This magnetic field change is measured by a giant magnetoresistance magnetic field sensor and converted into a digital value with a RFID chip for wireless access. Standard metal foil strain gauges have a gauge factor GF from around 2 to 5 and suffer from the disadvantage of a physically connected power supply and measurement equipment. For the presented sensor, a strain range of −10 μmm to 190 μmm results in a linear sensor response, a gauge factor of GF ≈ 245, and a detectivity of 4.10 nmm1Hz. The detectivity of the presented sensor is similar to the detectivity of a reference metal foil strain gauge. Due to low power consumption and easy signal analysis, this sensor is well suited for long term strain measurement inside closed spaces. RFID adds features like multiple tag detection, wireless passive operation and a user d...

The rise behavior of single air bubbles of different size in viscous liquids (water/glycerol mixtures) is investigated using a 3D Direct Numerical Simulation method. The simulations where performed by the ITLR inhouse code Free Surface 3D (FS3D) which solves the incompressible Navier-Stokes equations using a Volume-of-Fluid (VOF) technique. The computational effort is reduced by using a moving frame of reference. This moving frame allows the simulation of a large scale rising trajectory. The numerical results, which comprise the terminal rise velocity, the aspect ratio of the deformed bubbles and the resulting bubble shape are compared to experimental data from literature. The simulations were performed on the NEC SX-8 and NEC SX-9 platforms of the HLRS.

In this response, we clarify some of the misconceptions that were brought up by Pal's Comments about our manuscript [S. A. Faroughi and C. Huber, J. Appl. Phys. 117, 055104 (2015)]. The main issue that was raised is that we account for the correction on the excluded volume (the net volume of the continuous phase in a finite composite decreases with increasing filler content) twice. We show here that this statement is incorrect and that our model is consistent with other models derived from a different standpoint. We also address the other comments raised by Pal.

In this study, we propose a theoretical model to compute the effective thermal conductivity of metal and dielectric spherical particle reinforced composites with interfacial thermal resistance. We consider a wide range of filler volume fraction with sizes ranging from nano- to macro-scale. The model, based on the differential effective medium theory, accounts for particle interactions through two sets of volume fraction corrections. The first correction accounts for a finite volume of composite and the second correction introduces a self-crowding factor that allows us to develop an accurate model for particle interaction even for high volume fraction of fillers. The model is examined to other published models, experiments, and numerical simulations for different types of composites. We observe an excellent agreement between the model and published datasets over a wide range of particle volume fractions and material properties of the composite constituents.