Air Gap Formation Research Articles

Rhizosheath Formation and Its Role in Plant Adaptation to Abiotic Stress

The rhizosheath, the layer of soil tightly attached to the roots, protects plants against abiotic stress and other adverse conditions by providing a bridge from the plant root system to the soil. It reduces the formation of air gaps between the root and soil and facilitates the transportation of water at the root–soil interface. It also serves as a favourable niche for plant-growth-promoting rhizobacteria in the surrounding soil, which facilitate the absorption of soil water and nutrients. This review compares the difference between the rhizosheath and rhizosphere, and summarises the molecular and physiological mechanisms of rhizosheath formation, and identifying the causes of rhizosheath formation/non-formation in plants. We summarise the chemical and physical factors (root hair, soil-related factors, root exudates, and microorganisms) that determine rhizosheath formation, and focus on the important functions of the rhizosheath in plants under abiotic stress, especially in drought stress, phosphorus deficiency, aluminium stress, and salinity stress. Understanding the roles played by the rhizosheath and the mechanisms of its formation provides new perspectives for improving plant stress tolerance in the field, which will mitigate the increasing environmental stress conditions associated with on-going global climate change.

Mold material and dimension effects on the microstructural gradient in a glass-ceramic

Microstructural gradients can significantly affect the properties of glass-ceramics (GCs). Such spatial variations in glass-ceramic (GC) microstructure may result from a non-uniform cooling rate when the glass-forming liquid is cast into a mold. This study investigates the material and mold size influence on the cooling process, leading to a non-uniform distribution of crystalline nuclei in the parent glass. We used a high-nucleation rate, non-stoichiometric, lithium metasilicate GC as a model. Initially, the glass-forming liquid was cooled in cylindrical molds with diameters of 7 and 14 mm, made from copper and 304 stainless steel – materials with distinct thermal diffusivities. Simulated thermography, validated by thermocouple data, showed contrasting cooling profiles across the samples. Our key findings are: i) cooling rate: faster cooling in the smaller mold resulted in a lower crystal number density and a reduced crystallized volume fraction (αv) as expected; ii) mold material: surprisingly, copper molds led to quicker glass shrinkage, which facilitated the earlier formation of air gaps with reduced heat transfer and slower cooling than steel, favoring incipient crystallization during the cooling path; iii) microstructure: samples cooled in both molds exhibited higher αv in the central and upper (mold contact-free) regions due to slower cooling compared to the mold base and edges. Therefore, this study reveals how mold material and size affect microstructural gradients in GCs, paving the way for tailoring crystallization and microstructure through strategic mold design.

Influence of Interfering Parameters in Electrospark Testing of Paint Coatings

An analysis is made of interfering factors, such as base surface roughness and partial discharges, which can affect the process of continuity testing by the electrospark method, reducing the breakdown voltage of the interelectrode gap and leading to false alarms. To consider the influence of the coating base surface roughness, the distribution of the electric field in the system of two electrodes with a given roughness was constructed in the Ansys Electronics Desktop program. An experiment was carried out to determine the breakdown voltage of the air gap for different values of the surface roughness of one of the electrodes. A film 0.05 mm thick with a hole simulating a defect was placed on the roughness sample. An electrode was placed on the surface of the film in the region of the hole, to which a control voltage was applied. The voltage was increased to the value of the breakdown voltage and fixed. The experiment showed that the breakdown voltage of the interelectrode gap, simulating a through coating defect, practically does not change with a change in roughness. The process of electrospark testing can be affected by the waviness of the coating and the inhomogeneity of its thickness, which lead to the formation of air gaps between the electrode and the coating surface. It is shown that in this case the electric field intensity in the air gap will be higher than the field intensity in the dielectric coating, which, in turn, creates conditions for the formation of partial discharges in the interelectrode gap. Partial discharge creates a current pulse proportional to the capacitance of the air gap, which can be mistaken for the spark discharge current of the entire gap, and therefore as a defect, while the presence of a partial discharge does not mean the presence of a defect. Thus, it can be argued that partial discharges are an interfering parameter, the influence of which must be taken into account. Measures are proposed to reduce the influence of partial discharges on the continuity control process. To eliminate the possibility of false alarms from partial discharges, it is necessary to measure the parameters of the voltage pulse caused by the full discharge current and take them into account when making a decision about the presence of a defect.

Influence of mould level instability on the unevenness of solidified shell deformations during continuous casting

ABSTRACT The influence of mould level instability during continuous casting on the unevenness of the initial solidified shell in hypo-peritectic steels has been investigated. Water model experiments and in-plant casting experiments revealed that mould level instability, which increases with increasing casting speed, is related to longitudinal cracking resulting from the unevenness of the solidification shell. A numerical model which considers the effect of mould level instability was developed and used to simulate the formation of the unevenness of the solidification shell and solidified shell deflections. The calculation results show that an increase in the non-uniform infiltration of mould flux increases the unevenness of the solidified shell. On the other hand, the unevenness of the solidified shell decreases due to mild cooling. Overall, the results of the plant tests and calculations show that non-uniform infiltration of mould flux dramatically affects air gap formation and the unevenness of the solidified shell.

Evaporative destabilization of a salt crust with branched pattern formation

The impact of salt crust formation over porous media on water evaporation is an important issue in relation with the water cycle, agriculture, building sciences and more. The salt crust is not a simple accumulation of salt crystals at the porous medium surface but undergoes complex dynamics with possible air gap formation between the crust and the porous medium surface. We report on experiments that allow to identify various crust evolution regimes depending on the competition between evaporation and vapor condensation. The various regimes are summarized in a diagram. We focus on the regime where dissolution–precipitation processes lead to the upward displacement of the salt crust and the generation of a branched pattern. It is shown that the branched pattern results from the crust upper surface destabilization whereas the crust lower surface remains essentially flat. We show that the resulting branched efflorescence salt crust is heterogeneous with a greater porosity in the salt fingers. This leads to the preferential drying of the salt fingers followed by a period in which the crust morphology change only occurs in the salt crust lower region. The salt crust eventually tends toward a frozen state where no visible change occurs in the salt crust morphology, but without blocking the evaporation. These findings provide in-depth insights into the salt crust dynamics and pave the way for the better understanding of the impact of efflorescence salt crusts on evaporation and the development of predictive models.

Airgap Insertion and Layer Reassignment Under Setup and Hold Timing Constraints

Airgap formed in intermetal dielectric (IMD) reduces coupling capacitance, and thus can be utilized for timing optimization. Metal layers with airgap are limited due to high cost of airgap formation. Layer reassignment is to relocate some timing critical wires in nonairgap layers to airgap layers while noncritical wires in airgap layers are reassigned to nonairgap layers. Airgap insertion is to determine the amount of airgaps that are inserted for each critical wires in airgap layers. The two problems are solved in unified fashion with a goal of maximizing setup total negative slack (TNS) while satisfying hold constraints and design rules. They can be formulated as mixed-integer quadratically constrained programming (MIQCP). So, for practical application, a heuristic algorithm is presented and is experimentally compared to MIQCP with small examples. The experiments demonstrate that setup TNS and setup worst negative slack (WNS) are improved by 37% and 8%, respectively; they are improved by 26% and 5% with a simple-minded approach. The algorithm is also parallelized for application to larger circuits; runtime is decreased by 69% with eight threads.

The Effect of Disinfectants Absorption and Medical Decontamination on the Mechanical Performance of 3D-Printed ABS Parts.

Producing parts by 3D printing based on the material extrusion process determines the formation of air gaps within layers even at full infill density, while external pores can appear between adjacent layers making prints permeable. For the 3D-printed medical devices, this open porosity leads to the infiltration of disinfectant solutions and body fluids, which might pose safety issues. In this context, this research purpose is threefold. It investigates which 3D printing parameter settings are able to block or reduce permeation, and it experimentally analyzes if the disinfectants and the medical decontamination procedure degrade the mechanical properties of 3D-printed parts. Then, it studies acetone surface treatment as a solution to avoid disinfectants infiltration. The absorption tests results indicate the necessity of applying post-processing operations for the reusable 3D-printed medical devices as no manufacturing settings can ensure enough protection against fluid intake. However, some parameter settings were proven to enhance the sealing, in this sense the layer thickness being the most important factor. The experimental outcomes also show a decrease in the mechanical performance of 3D-printed ABS (acrylonitrile butadiene styrene) instruments treated by acetone cold vapors and then medical decontaminated (disinfected, cleaned, and sterilized by hydrogen peroxide gas plasma sterilization) in comparison to the control prints. These results should be acknowledged when designing and 3D printing medical instruments.

A comprehensive review on effect of printing parameters on mechanical properties of FDM printed parts

Fused Deposition Modelling (FDM) is an additive manufacturing technique that is widely used for the fabrication and prototyping of parts and objects of different shapes and geometries. Several research and tests have been carried out over the past two decades concerning materials used and applications of the FDM process. Very few articles highlighted machine parameters associated with FDM. This article reviews the materials and machine parameters associated with FDM printing and highlights the effect of the mechanical and thermal properties of the material on the durability and strength of the printed objects. Machine parameters such as layer resolution, build orientation, raster angle, flow rate and speed, nozzle, platform temperature, and the air–gap formation in specimens of different materials have been discussed. This article provides an overview to researchers about the different process parameters involved, the applications of FDM 3D printed parts in various domains, and what the future holds for this technology.

Heat Transfer Coefficient Determination in a Gravity Die Casting Process with Local Air Gap Formation and Contact Pressure Using Experimental Evaluation and Numerical Simulation

Anticipating the processes and parameters involved for accomplishing a sound metal casting requires an in-depth understanding of the underlying behaviors characterizing a liquid melt solidifying inside its mold. Heat balance represents a major factor in describing the thermal conditions in a casting process and one of its main influences is the heat transfer between the casting and its surroundings. Local heat transfer coefficients describe how well heat can be transferred from one body or material to another. This paper will discuss the estimation of these coefficients in a gravity die casting process with local air gap formation and heat shrinkage induced contact pressure. Both an experimental evaluation and a numerical modeling for a solidification simulation will be performed as two means of investigating the local heat transfer coefficients and their local differences for regions with air gap formation or contact pressure when casting A356 (AlSi7Mg0.3).

Underwater friction stir welding of PC: Experimental study and thermo-mechanical modelling

In underwater friction stir welding (UFSW), the thermal conditions between the tool and the plasticized polymeric materials and the relation between glass transition temperatures remain unclear. In this work, a developed coupled 3D thermo-chemical-mechanical numerical model was applied to simulate the UFSW process of polymeric materials. A computational fluid dynamic analysis was conducted for the evolution of the pressure, viscosity and temperatures properties on submerged situation. The numerical model is able to prediction of shrinkage percentage and formation of air gap on the joint stir zone (SZ) by evolution of the mixed slipping/sticking contact conditions at tool/workpiece interface. The simulation results were validated with experimental data on polycarbonate (PC) polymer. It was found that, higher cooling capability of water leads to lower heat concentration on the joint line and consequently, during traverse movement of UFSW tool, the materials viscosity in the leading edge was higher than FSW case. Higher viscosity materials caused the ∼6.8 % lower heat generation and smaller SZ at UFSWed sample. It is shown that the material stirring in FSW joint is higher than UFSW that leads the higher shrinkage percentage and air gap formation. On the other hand, the immersing situation caused the water droplet diffuse into SZ of the UFSW sample. The crystallinity and chemical structural changes of PC in submerged situations were lower than conventional FSW joint, and consequently, the tensile properties and hardness of the welded sample improves in UFSW joint.

A validated asymptotic thermomechanical model for air-gap formation in tapered moulds in the continuous casting of steel

Abstract A recent asymptotics-based thermomechanical model is adapted and applied to the mould region in the continuous casting of round steel billets, with a view to describing the complex interplay between air-gap formation, mould taper, cooling channel width and cooling water velocity. Although the situation is steady state, the analysis leads to what is mathematically a dual moving-boundary problem for the solid–melt and solid–air interfaces, where the distance from the top of the mould region is the time-like variable in the problem. Moreover, the two interfaces are initiated at different locations. In addition, the thermal and mechanical problems are found to decouple and it is possible to solve the first ahead of the second. The model equations are solved numerically using a finite-difference method, and the approach is subsequently successfully validated against a previous finite-element model and experimental data from temperature measurements taken within the mould.

Effect of Graphene Nanoplatelets on the Structure, the Morphology, and the Dielectric Behavior of Low-Density Polyethylene Nanocomposites.

The incorporation of graphene nanoplatelets (GnPs) within a polymer matrix can play an important role in the physical properties and the functionality of the composite material. Composites consisting of low-density polyethylene (LDPE) and GnPs of different concentrations were developed by mixing GnPs with a molten form of the polymeric matrix. The effect of the GnPs content on the morphological, structural, and electrical properties of the composites were investigated. As shown, graphene presence and its concentration significantly modified the polymer matrix properties, a behavior that can be employed for tailoring its applicability in electrical applications. It was found that the increase of the graphene platelets concentration seems to promote the formation of graphene agglomerates, air gaps, and inhomogeneities, while higher dielectric constant/lower dielectric losses can be achieved.

Dosimetric Comparison of Superflab and Specially Prepared Bolus Materials Used in Radiotherapy Practice.

This study compares standard commercial bolus material (Superflab) to custom prepared silicone dental impression material (CDIM) and play dough material (PDM) with respect to dosimetric properties and applicability by using ion chamber measurement and calculated dose values. The CDIM bolus was prepared by mixing dental impression silicone material with enough water to maintain a density of about 1.0 g/cm3. The prepared bolus material is applied on an RW3 solid phantom by covering 10x10 cm2 area with 0.5-1 cm thickness. Ion chamber measurements were performed separately with and without bolus material application. The setup was scanned in CT and the same procedure was repeated in the TPS using the scan data, in which the Pencil Beam Convolution dose calculation algorithm was used. To compare the effect of bolus material on tissue, the Superflab bolus and CDIM bolus were applied with 1 cm of thickness on postmastectomy scar and dose calculations on TPS were performed. After comparison of the dosimetric values for Superflab, CDIM and PDM, we obtained statistically meaningful results between superflab and CDIM. For PDM, the results obtained with TPS and ion chamber measurements indicated that, it is not suitable to use in radiotherapy application due to its material properties. For the simulated skin dose values obtained at five random points on the scar tissue, the comparison of Superflab and CDIM TPS calculation results were not statistically significant. The CDIM is easy to prepare and apply on irregular mastectomy scar tissue and it prevents formation of air gaps in the application surface. Especially for curved anatomical regions such as scar tissue, inclusion of the bolus material in treatment planning protocol will reduce dose uncertainty in application. It is safe to use CDIM as an alternative to Superflab in radiotherapy application, whereas PDM is not useful in clinical practice due to its material properties.

A meshless thermomechanical travelling-slice model of continuous casting of steel

The travelling slice approximation of continuous casting process is widely used in industrial practice to model the heat transfer and solidification. Short computational times that are possible due to the reduction from three to two dimensions compensate for lower accuracy introduced by the approximation. In the same spirit, the travelling slice approximation is also used for the thermomechanical model in the present paper. The model includes contributions from metallostatic pressure, thermal contraction and viscoplastic deformation to describe the stress in the material slice. It incorporates simple models to predict the hot tearing and cracking. The model is solved with a novel strong-form meshless method. It allows flexible adaptive node distribution to accurately describe the behaviour in the solidifying region. In this paper we focus on the two-way coupling of the heat transfer and deformation of the strand to model the air-gap formation in the initial stage of the casting process and to study the emergent behaviour that is the result of this coupling. We find that the inclusion of the air-gap model significantly changes the stress distribution in the corners of the strand because of the reheating caused by the reduced heat-transfer.

Ultra-flexible nanofiber-based multifunctional motion sensor

Abstract Recently, human motion and respiration monitoring sensors have attracted significant research attention in the field of wearable electronics and healthcare applications. Fiber-based electronic sensors demonstrate significant potential for such applications due to their flexibility. However, the superior flexibility of fiber-based sensors is generally limited due to the partial use of rigid elements such as metal electrode layers. In this paper, a simple method is presented for the fabrication of all-nanofiber multifunctional sensors, which consist of triboelectric (TE), piezoresistive (PR), and thermoresistive (TR) sensing elements and demonstrate superior flexibility, excellent wearability, and multifunctionality. The TE sensing unit was fabricated by integrating two triboelectrically dissimilar nanofiber surfaces using a polymer mesh separator, for the formation of internal macro-scale air gaps; followed by the electrical polarization of nanofibers. The TR and PR sensing elements were realized using vapor-phase polymerized fiber electrode layers. Using the sensor developed in this study, different motions and breathing frequencies can be individually or simultaneously detected using the TE, PR, and TR sensing elements. This work therefore presents a novel scheme for human motion sensors and breathing sensors, which are widely applicable to wearable electronics, healthcare monitoring, and human-machine interfaces.

A Methodology for Dynamic Material Characterizations via Terahertz Time-Domain Spectroscopy

In this work, the challenges of terahertz (THz) time-domain spectroscopy on complex (multilayer) samples with time-varying (dynamic) characteristics are addressed. The challenges appear in characterizations of the refractive index and extinction coefficient, as etalon artifacts due to internal reflections, and are accentuated in multilayer structures having dynamic and low-loss materials, such as biomolecular materials. This is because nonidealities may form as airgaps at the interfaces and as inhomogeneity in the bulk. The proposed methodology addresses the challenges by introducing a generalized model that accommodates dynamic formation of airgaps and inhomogeneity. It is shown that the generality of the model allows it to mitigate etalon artifacts and yield a highly accurate representation of the material characteristics, with low systematic error, even for low-loss materials. The methodology is applied to characterizations of quartz and glucose in the THz spectrum to see fine detail in the characteristics of quartz and the crystallization of glucose.

A novel airgap formation scheme by GO nanosheet gap sealing process with extreme low effective dielectric constant

In order to reduce the parasitic capacitance between the interconnect metal line, low-k material was developed and integrated into CMOS BEOL process, and airgap is the ultimate low-k material with the lowest dielectric constant close to 1. In this work, a novel airgap formation scheme was proposed to form the airgap by low-cost coating process, in which Graphene Oxide (GO) nanosheet was used as gap-filling suppressor to seal the gap between two adjacent metal line and form the airgap. Traditional non-conformal CVD gap sealing process was firstly developed and compared with the proposed scheme. It was found that the non-conformal CVD sealing process cannot achieve high performance airgap, and there will always be some material deposited into the gap between the metal interconnect line even for very narrow gap. For the proposed scheme, volume ratio of GO sealed airgap to gap region before sealed can reach to about 85% with much wider gap, and calculated effective relative dielectric constant of this region is about 1.57. When increasing the gap CD to value larger than GO nanosheet size, conformal film deposition can be got, which indicate that GO nanosheet with the size larger than gap CD can seal the gap in the very beginning and prevent further material depositing into the gap to make the large ratio airgap formation.

Analytical and numerical determination of the heat transfer coefficient between scrap and hot metal based on small-scale experiments

Dynamic modelling of the Linz-Donawitz oxygen steelmaking process (LD) is one of the most challenging tasks in the current economic situation for the optimization of integrated steel plants. One of the main influencing parameters is the melting and dissolution behaviour of scrap. Scrap is used as an iron source and coolant for the exothermic reactions inside the LD converter. Literature-based dissolution equations are commonly used in modelling. As a basis for developing a new numerical model for scrap melting with coupled heat and mass transfer, laboratory-scale experiments were conducted. The aim of the experiments was the determination of the heat transfer coefficient between scrap and liquid hot metal through a combination of thermocouple measurements with analytical and numerical solutions. The heat transfer coefficients achieved were in the range between 4.5 and 6.2 kW/m2 K. The heat transfer coefficients estimated in the present work are approximately 10 times smaller than those evaluated through existing Nusselt correlations. These discrepancies may be explainable through specific effects of scrap dissolution, e.g. shell freezing and successive melting or air-gap formation between solidified shell and mother scrap increasing the heat transfer resistance at the solid scrap to melt interface. The numerical solution to the heat transfer problem shows identical results to the analytical solution of the problem and provides a feasible basis for further research and development.

Flexible resistive pressure sensor with silver nanowire networks embedded in polymer using natural formation of air gap

A new type of flexible pressure sensor with air gap has been designed based on polydimethylsiloxane (PDMS) and silver nanowires (AgNWs). During fabrication, the interdigitated Ag electrode was fabricated on the bottom PDMS layer by doctor blading and AgNWs were embedded in the top PDMS layer, with a naturally formed air gap between the AgNW layer and the interdigitated electrodes. The thus-formed air gap was controlled by the number of AgNW layers and played an important role in enhancing sensitivity by decreasing the off current in the absence of pressure. The maximum sensitivity of 16.1 kPa−1 was obtained via optimization of the air gap thickness, the AgNW sensing area, and the channel length of the electrodes. The stability of the sensors was confirmed by durability, repeatability, bending, and hysteresis tests. The pressure sensor based on the highly sensitive AgNW network could provide a great opportunity for measuring various types of forces and displacements in wearable devices.

Pore-scale visualization and measurement of paraffin solidification in high porosity open-cell copper foam

In this work, a pore-scale experimental investigation on solidification of paraffin in high porosity open-cell copper foam is performed based on the visualization and measurement technique. With the use of high definition camera and infrared camera, the phase and temperature fields of the copper foam/paraffin sample are obtained to directly visualize the solidification process. Simultaneously, T-type thermocouples are employed to measure the local thermal response characteristic and thermal non-equilibrium effect of the copper foam/paraffin sample. For comparison, the corresponding solidification experiment with pure paraffin sample is also conducted. Local details of solidification phase change including the evolution of phase change interface within foam pore region, the liquid phase flow and air gap formation due to the volume shrinkage effect of paraffin are revealed. It is found that the highly conductive copper foam remarkably extends the phase change interface, avoids the formation of overlarge-size air gap that forms in the pure paraffin sample and enables a higher thermal response rate of the copper foam/paraffin sample. As a result, an attractive solidification enhancement (about 2.8 times) of paraffin in copper foam is achieved. Moreover, strong effect of local thermal non-equilibrium between copper foam and paraffin is detected during the solidification process. Finally, through generalizing the present experimental results, a correlation for dimensionless solidification time of the copper foam/paraffin sample is developed at pore scale.