Vertical Sidewalls Research Articles

Enhancement of the part-load thermal charging performance of a latent heat thermal energy storage unit with variable length fins at effective locations

Rapid thermal charging of the latent heat thermal energy storage unit (LHTESU) based on phase change material (PCM) is restricted by the material's low thermal conductivity. The numerical investigation for the thermal charging process of LHTESU considering a rectangular enclosure under the high-temperature gradient is presented. Constant and variable length fins with different locations are considered inside the enclosure with one vertical sidewall at a constant temperature. For various part-load thermal charging operations, a two-dimensional transient model with enthalpy-porosity technique is employed with natural convection current to evaluate the thermal charging rate and melting characteristics. The simulation outcomes manifest that convective heat transfer significantly impacts PCM melting in the case of variable length fins. Uniform distribution of Variable length fins with the most extended fin near the base of the container has 20% superior thermal charging performance than of constant length in full load thermal charging operations. However, in half-load thermal charging operations, constant-length fins with uniform distribution in the enclosure reduce the maximum thermal charging time and the average temperature of the PCM domain. Further, the result indicates that Cases VF-2 (most extended fins placed significantly close to the bottom and remaining fins inserted with equal gap) and VF-3 (fins insertion only in the lower half of the container) minimized the sensible heating of liquid PCM by directing the heat towards solid PCM. Case VF-3 demonstrated the most robust thermal charging performance by taking the least melting time in all cases for a 90% thermal charging of the system.

Direct numerical simulation of “fountain filling box” flow with a confined weak laminar plane fountain

AbstractA “fountain filling box” flow produced by discharging a weak laminar plane fountain in a confined open channel is studied numerically. Two‐dimensional direct numerical simulations were performed for weak plane fountains. The development of the fountain flow experiences five stages; the initial upflow and the subsequent downflow after the fountain penetrates to the maximum height, followed by the outward movement of the intrusion of the fallen fountain fluid on the channel bottom, and then the wall fountain formed by the impingement of the intrusion on the vertical sidewall, which results in the reversed flow, and finally the gradual stratification of the fluid. The behavior of the intrusion can be approximately described with the plane gravity current theory. The period for the intrusion to reach the bounded side wall increases with increasing Re or decreasing Fr. Three regimes are found for the wall fountain behavior; “no‐falling,” “slumping down,” and “rolling down” behavior. Convection, mixing, conduction, and filling all contribute to the formation and development of stratification, but their effects vary at different stages. For the initial stages, convection and mixing play a key role, resulting in an increasing bulk entrainment rate, while conduction and filling are dominant after quasi‐steady stratification is created, presenting a decreasing bulk entrainment rate.

Controlling the sidewall verticality of a CVD diamond in Gaussian laser grooving

For the specific energy distribution of Gaussian laser, the obtained grooves always fall short in the sidewall verticality. To overcome this problem, the improvement of sidewall inclination in laser grooving of a CVD diamond is undertaken by the surface tilting motion control, where the insufficient material removal at the groove sidewall is relieved. Combined with finite element modelling, the influence of laser energy density, scanning speed, scanning times and scanning pitch on the inclination of sidewall are firstly analyzed, which shows that laser energy density is the main factor that dominates the sidewall inclination. The finite element simulation model (FEM) is established to achieve the contour evolution of the machining area under different tilted angles, where the generation mechanism of 'V' shape or 'W' shape at the bottom of the groove is analyzed. Finally, the inclination degree of the groove sidewall can be effectively controlled by optimizing the relative incident angle under the selected laser energy density.

High-Quality Dry Etching of LiNbO3 Assisted by Proton Substitution through H2-Plasma Surface Treatment.

The exceptional material properties of Lithium Niobate (LiNbO3) make it an excellent material platform for a wide range of RF, MEMS, phononic and photonic applications; however, nano-micro scale device concepts require high fidelity processing of LN films. Here, we reported a highly optimized processing methodology that achieves a deep etch with nearly vertical and smooth sidewalls. We demonstrated that Ti/Al/Cr stack works perfectly as a hard mask material during long plasma dry etching, where periodically pausing the etching and chemical cleaning between cycles were leveraged to avoid thermal effects and byproduct redeposition. To improve mask quality on X- and Y-cut substrates, a H2-plasma treatment was implemented to relieve surface tension by modifying the top surface atoms. Structures with etch depths as deep as 3.4 µm were obtained in our process across a range of crystallographic orientations with a smooth sidewall and perfect verticality on several crystallographic facets.

Non-Boussinesq effects on natural convection airflows simulated in a tall rectangular cavity

Natural convection of air in a tall rectangular cavity is explored by solutions of the fully compressible transient Navier–Stokes equations in two-dimensional form, without the Boussinesq approximation. A temperature difference is imposed on the two vertical side walls, each isothermal, with the two horizontal boundaries adiabatic. Thermo-physical properties of air, including density, viscosity, thermal conductivity and specific heat, are all variable with temperature. Contrary to the conclusions in most previous numerical studies invoking the Boussinesq approximation, this study finds the instability of the conduction regime to be a traveling wave drifting downward in the cavity. The wave-drift speed is a strong function of the dimensionless wall temperature difference, ε, defined as the temperature difference between the vertical walls divided by twice the mean wall temperature. Wave-drift speeds and wave numbers are calculated over a range of ε less than 0.5 and Rayleigh numbers less than 9000. The dimensionless temperature difference ε is also found to have significant effects on local heat transfer and transient flow structure inside the cavity.

Benchmarked simulations of natural convection airflow in a square cavity

An open-source code, OpenFOAM, and two commercial finite volume packages are benchmarked for accuracy and convergence speed in solving steady natural convection airflows in a square cavity with adiabatic horizontal boundaries and with high temperature difference on the two vertical side walls. The compressible two-dimensional Navier–Stokes equations are solved at Rayleigh numbers 106 and 107 (steady flow for Rayleigh numbers less than 2 × 108) using seven different CFD algorithms, including both transient and steady flow solvers. Velocity fields, pressures, temperatures and Nusselt numbers are compared against well-established results for three benchmark test cases from the literature. Relative accuracy and CPU time are reported for each algorithm and for each of the three test cases, yielding benchmarked comparisons of OpenFOAM, STAR-CCM+, and ANSYS-Fluent solvers for low Mach number compressible airflows.

Three-Dimensional Kelvin Probe Force Microscopy.

Traditional Kelvin probe force microscopy (KPFM) is mainly limited to the characterization of two-dimensional (2D) surfaces, and in situ surface potential (SP) imaging along 3D device surfaces remains a challenge. This paper presents a multimode 3D-KPFM based on an orthogonal cantilever probe (OCP) that can achieve SP mapping of 3D micronano structures. It integrates three working modes: a bending mode for 2D horizontal surface imaging, a torsion mode for vertical sidewall imaging, and a vector tracking-based 3D scanning mode. The customized OCP has a nanoscale tip protruding from the side and underside of the cantilever, rather than the front, and the extended tip makes the proposed approach universally applicable for 3D detection from the nanometer to micrometer scale. The spatial resolution of the proposed method is analyzed by simulation, which shows it can reduce the cantilever homogenization effect. Linearity and energy resolution measurements show that the proposed method has comparable performance to traditional methods. A comparative experiment using a gold-silicon interface verifies the accuracy of the reported method in its bending and torsion modes. Further, the imaging ability of the 3D scanning mode is confirmed in the 3D characterization of a step grating. This technique is applied to the in situ characterization of a microforce sensor with microcomb structures. The experiment results show that this method can excellently achieve the 3D quantitative characterization of topography and SP, including critical dimensions and SP along a 3D device surface. This novel 3D-KPFM technique has many potential applications in the further exploration of 3D micronano devices.

Vertical and Lateral Etch Survey of Ferroelectric AlN/Al1-xScxN in Aqueous KOH Solutions.

Due to their favorable electromechanical properties, such as high sound velocity, low dielectric permittivity and high electromechanical coupling, Aluminum Nitride (AlN) and Aluminum Scandium Nitride (Al1−xScxN) thin films have achieved widespread application in radio frequency (RF) acoustic devices. The resistance to etching at high scandium alloying, however, has inhibited the realization of devices able to exploit the highest electromechanical coupling coefficients. In this work, we investigated the vertical and lateral etch rates of sputtered AlN and Al1−xScxN with Sc concentration x ranging from 0 to 0.42 in aqueous potassium hydroxide (KOH). Etch rates and the sidewall angles were reported at different temperatures and KOH concentrations. We found that the trends of the etch rate were unanimous: while the vertical etch rate decreases with increasing Sc alloying, the lateral etch rate exhibits a V-shaped transition with a minimum etch rate at x = 0.125. By performing an etch on an 800 nm thick Al0.875Sc0.125N film with 10 wt% KOH at 65 °C for 20 min, a vertical sidewall was formed by exploiting the ratio of the planes and planes etch rates. This method does not require preliminary processing and is potentially beneficial for the fabrication of lamb wave resonators (LWRs) or other microelectromechanical systems (MEMS) structures, laser mirrors and Ultraviolet Light-Emitting Diodes (UV-LEDs). It was demonstrated that the sidewall angle tracks the trajectory that follows the of the hexagonal crystal structure when different c/a ratios were considered for elevated Sc alloying levels, which may be used as a convenient tool for structure/composition analysis.

Interaction of Irregular Distribution of Submerged Rigid Vegetation and Flow within a Straight Pool

The interaction of bedform and vegetation cover significantly affects the turbulent flow parameters. To investigate this interaction, experiments were carried out in both a gravel-bed river and a laboratory flume. The purpose of field investigations was to find the slopes for both the entrance section and exit section of pools, the grain size of the bed material, and the flow condition. Based on field data, without considering any scaling analysis, a straight pool was constructed in a laboratory flume that was 0.9 m wide, 0.6 m deep, and 14 m long. The entry and exit slopes of the straight pool were 7.4° and 4°, respectively. The straight pool had vertical side walls and a gravel bed with a median grain size of d50 = 23.3 mm. Plastic cylinders planted in an irregular pattern in the channel beds were used to model rigid submerged vegetation. The velocity components were recorded by using an ADV at 200 Hz. In this study, the distributions of velocity, Reynolds stress, and TKE were investigated for flows in the presence of submerged rigid vegetation in channel beds with various area densities of vegetation. Results show that the shape of Reynolds stress distribution depends on the entrance and exit slopes of the pool, as well as the irregular distribution pattern of vegetated elements. Inside the pool with the presence of submerged vegetation in the channel bed, the maximum TKE appears above the bed surface with a larger distance depending on the area density of vegetation in the channel bed. However, the momentum exchange and turbulent energy are likely influenced by the secondary circulation of the flows associated with the irregular distribution of vegetated elements in the channel bed. Results of the quadrant analysis show that the momentum between the flow, bedform, and vegetated elements is mostly transferred by sweep and ejection events. The outward event tends to grow toward the water surface, reaching the highest amount near the water surface. At the pool entrance section where the flow is decelerating, the ejection event is dominant near the bed while the sweep event is strong near the water surface. With the decrease in the vegetation density in the pool bed, both the ejection and outward events become dominant.

MHD natural convective flow in a porous corrugated enclosure: Effects of different key parameters and discrete heat sources

Present work deals with an unsteady two-dimensional magneto-hydrodynamics natural convection flow problem in a porous-corrugated enclosure. The vertical side walls are insulated, and the top wall is non-uniformly heated; while square-shaped undulations at the bottom wall are discretely heated. Five different cases are considered depending on the discrete heat sources and various key parameters. A transformation-free higher-order compact (HOC) scheme is used to discretize the non-linear coupled transport equations, and an advanced iterative solver, like the hybrid-bi-conjugate-gradient stabilized technique, is used to solve the system of algebraic equations generated from the numerical discretization. Present higher-order compact scheme is fourth-order accurate in space coordinates and second-order accurate in the time variable. At first, the developed code is validated with existing experimental and numerical computed results for the case of a square enclosure associated with and without a heat source. Then, the computed results are analysed over a range of key parameters, like Rayleigh number (103≤Ra≤106), Darcy Number (10−5≤Da≤10−1), Hartmann number (25≤Ha≤150), with a fixed Prandtl number (Pr=6.1), to study the effects of these key parameters on the fluid flow behaviour and isotherm patterns in the porous-corrugated enclosure. It is found that at high Rayleigh–Darcy numbers with different Hartmann numbers, the effect of the permeability of the porous medium is significant; whereas the flow resistance from the boundary friction is gradually reduced, and the flow behaviour is similar to the pure buoyancy-induced flow. These simulated results are presented in the form of streamlines, isotherms, local and averaged Nusselt number plots, etc. The presented results show various flow features that have not been analysed before.

Fresnel reflection suppression from deterministic illumination diffusers using antireflection random nanostructures

Deterministic illumination diffractive-diffusers have nonperiodic short and medium-scale topography. Because of the deterministic locations of vertical sidewalls at the phase transition boundaries, over-coating diffractive diffusers with thin-film antireflection layers perturbs their function, resulting in performance deviations and nonuniformities. To mitigate these effects, we added antireflection random nanostructures on the surface of three different classes of fused-silica multiphase diffractive diffusers, using reactive-ion plasma etching. The diffusers were measured before and after the random nanostructures addition, using a polarized-laser scatterometer with a dynamic range of nine orders of magnitude. The bidirectional scatter distribution function was measured over the entire equatorial plane of incidence, to analyze the directionality of scattered light and the impact of the antireflective nanostructure presence on the optical performance of the diffusers. The overall reflectivity suppression was measured across the illumination patterns directions, as well as, across the entire 180-deg angle-sweep. The designed deterministic illumination patterns and their contrast were unaffected by the presence of the random antireflective structures, whereas Fresnel reflectivity was reduced by an order of magnitude on average.

Analysis of the local non-equilibria on the heat transfer and entropy generation during thermal natural convection in a non-Darcy porous medium

This paper addressed new and unique information on the influence of local thermal non-equilibrium (LTNE) effects on heat exchange and entropy generation during thermal-free convection in a non-Darcy porous domain. The simplest configuration was considered, which consisted of a square cavity in which the steady-thermal-free convection motion is generated by maintaining the vertical sidewalls at different and constant temperatures. The Darcy-Brinkman-Forchheimer design has been adopted. The governing equations, which include the two energy equations for the fluid and solid phases, along with momentum equations in their dimensionless form, are solved using the finite volume computational approach. Due to the application of two different heat transport equations under the LTNE modeling technique, sources of entropy generation owing to thermal diffusion in the fluid phase and in the solid phase in addition to the entropy generation owing to viscous dissipation were identified. Maps of the distribution pattern of LTNE sources were also visualized. Furthermore, to assess and compare the thermal diffusion irreversibility in the two components of the porous structure, for the first time, we introduced and examined a new parameter (Ty), which is the ratio of the thermal entropy generation rate in the fluid phase to that in the solid phase.

MHD natural convection in a corrugated enclosure with discrete isothermal heating

AbstractIn this study, we numerically examine the unsteady two‐dimensional natural convective heat transfer flow problem in a corrugated enclosure containing an electrically conducting fluid whose top wall is nonuniformly heated. The vertical sidewalls of the corrugated enclosure are kept isothermally cold, while square‐shaped undulations at the bottom wall are discretely heated. Five different cases are considered depending upon the location of the heat sources. A higher‐order compact scheme is used to discretize the governing equations, and an advanced iterative solver, like the hybrid biconjugate gradient stabilized method, is used to solve the system of algebraic equations generated by the numerical discretization. We first validate our results with existing experimental and numerically computed results for the case of a square cavity with and without a heat source. Then, the computed results are analyzed over a range of key parameters, like Rayleigh number (), Hartmann number (), and Prandtl number (), to study the effect of these parameters on the characteristics of heat transfer and the flow field in the corrugated enclosure. Computed results are presented in the form of streamlines, isotherms, local and averaged Nusselt number plots, and so forth. It is found that, with the increase in the heat sources, the convection rate becomes stronger in the core region of the corrugated enclosure at higher Rayleigh numbers. Variations in the Hartmann and Prandtl numbers also have significant effects on the temperature distribution and flow field. Our simulated results show various flow features that have not been studied before.

Fabrication of SiGe/Ge microbridges based on Ge-on-Si(1 1 0) and observation of resonant light emission

Strained SiGe/Ge microbridges are fabricated based on Ge-on-Si(110). The Si(110) substrate is employed to fabricate the microbridge along [111] direction as uniaxial strain in the [111] direction is expected to be the most effective to shift the direct valley of the conduction band. As a result, very strong room temperature photoluminescence is obtained from the fabricated Ge microbridge. It is remarkable that Fabry-Perot resonant peaks are seen in this emission, which is attributed to the vertical side wall of the bridge realized by the anisotropic dry etching process. Moreover, for the purpose of controlling of the emission wavelength, a SiGe layer is regrown on the Ge microbridge. We observe strong emissions from the SiGe at shorter wavelength, demonstrating that the emission wavelength can be controlled using SiGe/Ge hetero structures on the microbridge and this structure is very promising for realization of efficient light sources for Si photonics.

Designer Metasurfaces for Antireflective Applications Enabled by Advanced Nanoparticle Technology

AbstractOptic processing advances have shifted system limitations, with current constraints consisting of antireflective (AR) coatings applied to optics and the inability to reduce size, weight, and price. Metasurfaces, and more broadly metaoptics, provide an avenue to overcome these limitations. The ability to generate substrate‐engraved metasurfaces that can be tailored is demonstrated within a broad design space to satisfy large‐area AR applications for bandpass or broadband needs, demonstrating an optically equivalent surface roughness standard deviation of only 5 Å. Using an advanced nanoparticle masking approach, two metasurfaces are fabricated: 1) vertical sidewall nanofeature metasurface (MS) for AR applications at 351 nm, and 2) sloped sidewall MS designed for broadband AR applications. For the 351 nm MS, the reflectance is measured to be 0.127%. In the case of the sloped sidewall MS, the reflectance is measured to be <0.25% over 400–700 nm and <1% over 400–1100 nm. To the best of the authors’ knowledge, this is the first demonstration of a broadband AR capable of this performance. The tunability of these structures, with a parameter space spanning even more remarkable designs such as broader bands and acceptance angles, coupled with high laser‐induced damage durability and mechanical robustness, displays the strength of metasurfaces for AR applications.

Controlled SOI nanopatterning for GaN pendeo-epitaxy

Nanopatterning of GaN/AlN layers on Silicon-On-Insulator (SOI) substrates is discussed with the aim of fabricating nanopillar arrays that can be used for subsequent GaN pendeo-epitaxy. The principle of the developed epitaxy process is that GaN crystallites are grown on deformable nano-pedestals able to rotate at GaN growth temperature. The objective is to control the complete pattern profile of GaN/AlN/Si/SiO 2 pillars, both in lateral and vertical directions. Since a smaller pillar diameter should enhance the pillar rotation at growth temperature, special emphasis will be given to nanometer-scale pillars. In this context, the control of GaN/AlN vertical sidewalls is still an issue for high-resolution nanopillars. In this paper, we report on the optimization of the plasma etching processes of GaN/AlN/Si/SiO 2 stacks to reduce the slope that widens the pillar base. Overall, we achieve near perfect arrays of 100 nm diameter nanopillars obtained by combining NanoImprint Lithography (NIL) and plasma etching, with bases of 150 nm in diameter. • Nanopatterning of GaN/AlN layers on SOI substrates • Controlled GaN/AlN etched sidewalls for high-resolution nanopillars • NIL Combined with ICP to achieve near perfect arrays of 100 nm diameter nanopillars

Fabrication of ultra-thin laser induced graphene electrodes over negative photoresist on glass for various electronic applications

Photoresists play a pivotal role in pattern transfer applications to achieve microstructures with high aspect ratios and vertical sidewalls for MEMS and semiconductor industries. Negative photoresists (NPR) have been leading the photoresist industry ever since their inception with excellent chemical resistance and high thermal and mechanical stability. However, owing to its low conductivity, NPR is restricted only to structural and sacrificial applications. For this first time, this work showcases the utilization of NPR to obtain ultra-thin and highly conductive laser Induced graphene (LIG) over glass substrates through laser ablation. Different combinations of NPR samples were considered at different spin speeds (2000 rpm to 4000 rpm) to obtain various thickness values to comprehend the effect of parameters, such as UV exposure, baking conditions, and photoresist thickness over obtained conductivity of the LIG. It was observed that samples which were coated with NPR at 4000 rpm followed by hard baking without UV exposure exhibited a conductivity of 529.7 S/m upon repeated ablation of laser. The functionality of the fabricated thin film electrodes was tested for biofuel cell and electrochemical applications. The novel method to create conductive patterns over glass substrates not only pave a possible way for realizing glass-based electronic devices but also open up new avenues for microelectronic industry.

Vertical oxide thin-film transistor with interfacial oxidation

A vertical oxide thin-film transistor was developed with interfacial oxidation for low voltage operation. The gate metal was used as a spacer for the definition of the transistor’s channel as well as the gate electrode. After definition of the vertical side wall, an IGZO (In-Ga-Zn Oxide) layer was deposited, followed by the interfacial oxidation to form a thin gate insulator. Ta was used for the gate material due to the low Gibbs free energy and high dielectric constant of tantalum oxide. A 15 nm tantalum oxide layer was obtained by the interfacial oxidation of Ta at 400 °C under oxygen atmosphere. The thin gate oxide made it possible to operate the transistor under 1 V. The low operation voltage enables low power consumption, which is essential for mobile application.

Numerical study on combined convection heat transfer flow problem in a porous corrugated enclosure

AbstractIn this study, we numerically analyzed the unsteady two‐dimensional combined convection flow problem in a porous‐corrugated enclosure whose upper wall is moving with uniform velocity associated with sinusoidal temperature distribution. The vertical sidewalls of the porous‐corrugated enclosure are kept at constant cold temperature while square‐shaped undulations at the bottom wall are discretely heated. Five different cases are considered depending on the discrete isothermal heating. In this paper, we have used the vorticity‐stream‐function formulation of the Brinkmann‐extended Darcy model to numerically simulate the momentum transfer in a porous‐corrugated enclosure. A transformation‐free higher‐order compact (HOC) scheme is used to discretize the nonlinear coupled transport equations, and an advanced iterative solver, like the hybrid‐bi‐conjugate‐gradient stabilized technique, is used to solve the system of algebraic equations generated from the numerical discretization. The present higher‐order compact scheme is fourth‐order accurate in space coordinates and second‐order accurate in the time variable. The numerically simulated results are analyzed over a range of key parameters, like Darcy number , Reynolds Number , Prandtl number , and with fixed high Grashof number , to study the effects of these leading parameters on the characteristics of heat transfer, fluid flow and isothermal distributions in the porous‐corrugated enclosure. These numerically computed results are presented in the form of streamlines, isotherms, Nusselt number plots, and so forth. Our computed results show numerous flow features which have not been studied previously.

A microfluidic biosensor for rapid simultaneous detection of waterborne pathogens

A microfluidic based biosensor was investigated for rapid and simultaneous detection of Salmonella, Legionella, and Escherichia coli O157:H7 in tap water and wastewater. The biosensor consisted of two sets of focusing electrodes connected in parallel and three sets of interdigitated electrodes (IDE) arrays. The electrodes enabled the biosensor to concentrate and detect bacteria at both low and high concentrations. The focusing region was designed with vertical metal sidewall pairs and multiple tilted thin-film finger pairs to generate positive dielectrophoresis (p-DEP) to force the bacteria moving toward the microchannel centerline. As a result, the bacterial pathogens were highly concentrated when they reached the detection electrode arrays. The detection IDE arrays were coated with three different antibodies against the target bacterial pathogens and a cross-linker to enhance the binding of antibodies to the detection electrode. As the binding of bacterial pathogen to its specific antibodies took place, the impedance value changed. The results demonstrated that the biosensors were capable of detecting Salmonella, Legionella, and E. coli 0157:H7 simultaneously with a detection limit of 3 bacterial cells/ml in 30 − 40 min.