Silica Thickness Research Articles

Temperature-Dependent Oxidation Behavior of Silicon Carbide Surface: Reactive Molecular Dynamics Simulations.

The high-temperature oxidation mechanism of silicon carbide (SiC) is crucial for designing thermal protection systems in aircraft. This study explored the oxidative chemical reaction processes and mechanism on SiC surface and interfaces within the temperature range of 300-2300 K by using reactive molecular dynamics simulation. The oxygen impact on the silica (SiOx) growth of the SiC surface was analyzed, which shows a progressive increase of silica thickness. The simulation results indicated that the oxidation process of SiC was a typical passive oxidation mechanism. With the environmental temperature rising and oxygen impact, the increase of oxidation thickness on the SiC surface undergoes three oxidizing reaction processes: little chemical adsorption of oxygen molecules on the initial surface, rapid oxidation of silicon and carbon, and dramatic oxidation of the interface between the oxidation layer and SiC. Additionally, this work studied the mechanism of oxidation thickness growth and chemical diffusion of oxygen. The oxidation rate is weakened according to the oxygen atom diffusion barrier effect of silica repulsion. Moreover, the kinetic parameters were statistically calculated by fitting the growth of Si-O bonds and their reaction rate constants. Subsequently, the activation energy and pre-exponential factors were derived by using the Arrhenius equation to model the chemical reaction kinetics of the thermal oxidation process. The chemical reaction behaviors of the two stages could be concluded as follows: (i) in stage I, the initial oxidation is reaction rate limiting; (ii) in stage II, SiC oxidation is limited by both the oxidation reaction rate and the oxygen diffusion coefficient of the oxidation layer. The activation energy of stage II increased compared with stage I due to the oxygen atoms diffusion barrier between the oxidation layers. This study on the oxidation and ablation mechanism of the SiC surface at the atomic scale would provide insight into understanding thermal oxidation behavior and the design of ceramic materials.

Impact of substrate thickness and surface roughness on Al/Ni multilayer reaction kinetics

Reactive multilayers comprising alternating nanoscale layers of Al and Ni, exhibit potential across various applications, including localized heating for welding and joining. Control over reaction properties is pivotal for emerging applications, such as chemical time delays or neutralization of biological or chemical weapons. This research offers insights into the intricate interplay between substrate thickness, surface roughness, and the behavior of Al/Ni reactive multilayers, opening avenues for tailored applications in various domains. To observe this interplay, silica with various thicknesses from 0.4 µm to 1.6 µm was deposited on polished single crystalline Si and rough poly‐Si base substrates. Additionally, to analyze the impact of varying silica thickness along the sample length on reaction behavior, silica in step‐like shape was fabricated. Subsequently, Al/Ni multilayers with 5 µm total thickness and 20 or 50 nm bilayer periodicities were deposited. Reaction velocity and temperature were monitored with a high‐speed camera and pyrometer. Results indicate that silica thickness significantly affects self‐propagation in multilayers. The reaction is not self‐sustained for silica layers ≤ 0.4 µm, depending on bilayer periodicity and substrate roughness. The velocity increases or decreases based on the direction of reaction propagation, whether it moves upwards or downwards, in relation to the thickness of silica.This article is protected by copyright. All rights reserved.

Interfacial nanolayer effect on thermophysical properties of silica-paraffin phase change material - A molecular dynamics simulation

Thermophysical properties of composite phase change materials (PCMs) are influenced by the interfacial nanolayer. Molecular dynamics (MD) method is used to study the effect of interfacial nanolayer on thermophysical properties of silica-paraffin composite PCM. The simulation results show that the melting enthalpy is reduced from 197.9 J·g−1 to 29.1 J·g−1, and the thermal conductivity is enhanced from 0.142 W·m−1·K−1 to 0.268 W·m−1·K−1, when the silica thickness is increased from 0 to 15.0 Å. The interfacial nanolayer is observed in composite PCM, and its thickness increases with the increase of silica wall thickness. In the composite PCM with 7.0 Å silica wall, a loss of 62.9% in the melting enthalpy is observed, with 32.6% attributed to the silica mass and 30.3% attributed to the nanolayer. Based on that, a modified model is proposed to predict the melting enthalpy of composite PCMs by considering the effects of both silica mass fraction and nanolayer. To reveal the thermal conductivity enhancement mechanism, the total heat flux is decomposed into three terms (namely, kinetic energy term, potential energy term and interaction energy term). It is found that the interaction energy term has the largest contribution to the total heat flux. The presence of nanolayer strengths the interactions, leading to the thermal conductivity enhancement.

Adenosine-Monophosphate-Assisted Homogeneous Silica Coating of Silver Nanoparticles in High Yield.

In this study, we propose a novel approach for the silica coating of silver nanoparticles based on surface modification with adenosine monophosphate (AMP). Upon AMP stabilization, the nanoparticles can be transferred into 2-propanol, promoting the growth of silica on the particle surfaces through the standard Stöber process. The obtained silica shells are uniform and homogeneous, and the method allows a high degree of control over shell thickness while minimizing the presence of uncoated NPs or the negligible presence of core-free silica NPs. In addition, AMP-functionalized AgNPs could be also coated with a mesoporous silica shell using cetyltrimethylammonium chloride (CTAC) as a template. Interestingly, the thickness of the mesoporous silica coating could be tightly adjusted by either the silica precursor concentration or by varying the CTAC concentration while keeping the silica precursor concentration constant. Finally, the influence of the silica coating on the antimicrobial effect of AgNPs was studied on Gram-negative bacteria (R. gelatinosus and E. coli) and under different bacterial growth conditions, shedding light on their potential applications in different biological environments.

Capillary Rise: A Simple Tool for Simultaneous Determination of Porosity and Thickness of Thin Silica Coatings

Coating porosity is an important property that supports solid-gas and solid-liquid exchange that can either enhance various science and technological applications or promote damage if not properly controlled. However, non-destructive instrumental techniques for the measurement of porosity on coated walls or surfaces can be quite challenging. Here, a seamless capillary rise technique has been used to determine both the thickness and porosity of a thin silica coating. Uniform coatings were prepared from 5 wt% hydrophobic fumed silica in absolute ethanol and spin-coated at 500–8000 rpm on glass slides. Capillary imbibition of squalane was then controlled into known areas of the resulted hydrophobic nano-porous coatings. The mass of the solid (silica) and the infiltrated oil (squalane) were gravimetrically measured. The porosity of the material was calculated as the percentage fraction of the pore volume while the film thickness was determined as the ratio of the total volume to the area of coverage. Mean values of the porosity and coating thickness calculated from capillary impregnation technique were 86 ± 2% and 3.7 ± 0.2 μm, respectively. The coating thickness obtained was comparable with those revealed by SEM and Dektak profiler measurements. This study highlights the effectiveness of capillary rise as a simple and cost-effective non-destructive technique for assessment of coating thickness and porosity.

Determination of mechanical properties of nanocomposites reinforced with spherical silica nanoparticles using experiments, micromechanical model and finite elements method

In this research, experimental, numerical, and micromechanical methods are used to determine the effects of silica nanoparticle content on the elastic modulus of polymer matrix nanocomposites in tension and compression. The direct mixing method was used to prepare experimental samples containing 0.1, 0.25, and 0.5 volume fractions of silica nanoparticles. The elastic modulus of the nanocomposites is evaluated by the Einstein, Guth, Mori-Tanaka, Halpin-Tsai, Kerner, and Ji et al. micromechanical models. In addition, the elastic modulus of the nanocomposites is evaluated using the RVE method in ABAQUS software. The effects of volume fraction and diameter of silica nanoparticles, thickness, and adhesion exponent of the interphase on the elastic modulus of polymeric nanocomposite are extensively examined. Stiffer elastic modulus behavior is found in the presence of interphase region. The results indicate that the elastic modulus of nanocomposites reinforced with spherical silica nanoparticles is improved with increasing nanoparticle volume fraction, decreasing the nanoparticle diameter, increasing the interphase thickness, and decreasing the interphase adhesion exponent. The validity of the results of numerical and micromechanical models has been checked with experimental results. It can be seen that the results obtained from the experiments with 1 hour of ultrasonic waves in the tensile samples with volume fractions of 0.1, 0.25, and 0.5, compared to the pure sample, have increased by 5.4, 10.3, and 9.7 percent, respectively. Similarly, for compression samples with the same volume fractions, the results have increased by 7, 13, and 11 percent, respectively.

Quantification of interfacial structure at nanoscale and its relationship with viscoelastic glass transition of SiO2/elastomer nanocomposites

The interfacial structure and chain segment dynamics of nanofiller/polymer composites play an important role in their macroscopic properties. Nevertheless, the mechanisms on the effects of nanofiller on the chain segment dynamics and the glass transition temperature (Tg) of nanocomposites are still controversial. In this study, the interfacial structure (thickness and nanomechanical properties) of silica (SiO2)/solution polymerized butadiene styrene rubber (SSBR) composites were quantified by using quantitative nanomechanical mapping technique of atomic force microscopy. The quantitative relationship between the degree of graft of coupling agent (CA) on SiO2 and interfacial structure was built up. The formation mechanism of interfacial structure in SiO2/SSBR composites with different degree of graft on SiO2 was revealed. A new insight on the effect of interfacial interaction of polymer nanocomposites on their chain segment dynamics and Tg was proposed. With the increase in degree of graft, the interfacial thinkness largely increases, the dispersion of SiO2 is more homogeneous, and the total area of interfacial SSBR layer largely increases, resulting in the significant decrease in chain density of SSBR matrix, and thus a significant decrease in Tg. This study provides guidance for the interfacial design of high performance polymer nanocomposites for their wider application in industry.

Bio-Inspired Aramid Fibers@silica Binary Synergistic Aerogels with High Thermal Insulation and Fire-Retardant Performance

Flame-retardant, thermal insulation, mechanically robust, and comprehensive protection against extreme environmental threats aerogels are highly desirable for protective equipment. Herein, inspired by the core (organic)-shell (inorganic) structure of lobster antenna, fire-retardant and mechanically robust aramid fibers@silica nanocomposite aerogels with core-shell structures are fabricated via the sol-gel-film transformation and chemical vapor deposition process. The thickness of silica coating can be well-defined and controlled by the CVD time. Aramid fibers@silica nanocomposite aerogels show high heat resistance (530 °C), low thermal conductivity of 0.030 W·m-1·K-1, high tensile strength of 7.5 MPa and good flexibility. More importantly, aramid fibers@silica aerogels have high flame retardancy with limiting oxygen index 36.5. In addition, this material fabricated by the simple preparation process is believed to have potential application value in the field of aerospace or high-temperature thermal protection.

Experimental Study of the Effects of Silica-gel Bed Thickness on The Regeneration Process in a Desiccant Column

A desiccant column contains a silica gel as a solid moisture absorbent material has been used in this study, in which the regeneration or desorption process was discussed. The dimensions of the silica-gel column are 30 cm long*30 cm wide with three thickness of desiccant bed (5,10, &15) cm, respectively. The mass flow rate and the inlet temperature of the regeneration air used are 3.72 kg/min and 53 °C respectively. This air was obtained using solar air heater in addition to an electric heating unit for temperature control purpose. The results obtained showed that the regeneration period is affected by the bed thickness. Increasing the bed thickness of silica increase the regeneration period and vice versa For example, the complete drying process of silica gel from moisture took 21 min with thickness of bed (5 cm), while it took 42 min with thickness of bed (15 cm)

Revealing the mechanisms for inactive rolling and wear behaviour on chemical mechanical planarization

The paper examines the influences of abrasive conditions, silica thickness, and motion parameters on the polishing process of a silicon substrate covered by a thin silica film. The silica film minimizes the direct collision of the abrasive on the silicon substrate. During the polishing process, the adhesion phenomenon plays the main role in atomic removal. Notably, the abrasive sliding motion can generate an inactive rolling motion. The inactive rolling motion can be used to explain the stick-slip motion by showing the system of periodic high-strain marks. In rolling, the deformation behaviors of the workpiece rely on the correspondence between the rolling and the moving speeds. In the fixed diamond abrasive (FDA) cases, the atomic sticking rate is lower but the workpiece deformation is higher than the loose diamond abrasive (LDA) cases. Besides, the FDA cannot create the inactive rolling motion and the stick-slip motion as the LDA. In general, the paper points out the existence of the inactive rolling motion and its role in the atomic removal process through the stick-slip motion phenomenon at the nanoscale, the advantage of this type of motion in a conventional chemical mechanical polishing (CMP) process, and the padding role of the silica layer.

Comparison of interfaces, band alignments, and tunneling currents between crystalline and amorphous silica in Si/SiO2/Si structures

Recently, to improve the performance of an integrated metal-oxide-semiconductor (MOS) device, an attempt has been made in the industry to replace the amorphous oxide with a crystalline oxide. However, various characteristics caused by the difference between amorphous and crystalline oxide in the MOS structure have not been systematically investigated. Therefore, we demonstrate the difference in atomic interface structures, electronic structures, and tunneling properties concerning varied oxide phases in a representative system, Si/SiO2/Si structures, with sub-3 nm-thick silica from first-principles. We investigate two oxide phases of amorphous (a-) and crystalline (c-) SiO2 with and without H passivation at the interface. Si/a-SiO2 exhibits a smooth interface layer, whereas Si/c-SiO2 exhibits an abrupt interface layer, resulting in the thicker interface layer of Si/a-SiO2 than Si/c-SiO2. Thus for a given total silica thickness, the adequate tunneling-blocking thickness, where all the Si atoms form four Si–O bonds, is thinner in a-SiO2 than c-SiO2, originating more tunneling current through a-SiO2 than c-SiO2. However, the effects of dangling bonds at Si/c-SiO2 rather than Si/a-SiO2 on tunneling currents are crucial, particularly in valence bands. Furthermore, when the dangling bonds are excluded by H atoms at Si/c-SiO2, the tunneling current dramatically reduces, whereas the H-passivation effect on the tunneling blocking at Si/a-SiO2 is insignificant. Our study contributes systematic knowledge regarding oxide phases and interfaces to promote for high performance of MOS devices.

Thickness control of the silica shell: a way to tune the plasmonic properties of isolated and assembled gold nanorods

By combining photophysical measurements with transmission electron microscopy, we proved that the thickness of the silica shell around gold nanorods determines the position of the longitudinal plasmonic band when they are isolated in solution or assembled in solid. The silica thickness has been tuned by modulating the reaction time and the ratio between CTAB-coated gold nanorods and TEOS concentration, obtaining gold nanorods covered by a silica shell with a thickness varying from 3.5 to 24 nm. Considering this shell as a spacer between the gold cores, it is possible to modulate the coupling of the localized surface plasmon resonance (LSPR) of neighboring nanorods. Moreover, the comparison between the extinction spectra in solution and in solid, recorded from nanorods covered by silica shell with different thickness, can be used to estimate the inter-nanoparticles distance required for plasmon interaction. We found that LSPR coupling is effective when the distance between the gold cores is no more than 10 nm. When the distance is greater, the nanorods do not interact with each other.

Electrochemical grafting of organic films on silica.

Traditionally, self-assembled monolayers formed on silicon require the removal of the insulating and chemically inert silica layer that naturally forms on the surface of crystalline silicon. The removal of silica is thought to be necessary in order to expose the conducting Si-H surface, which is reactive towards molecules. Here we report the unexpected result of electrochemical formation of thin organic films on silica-terminated silicon with silica thickness up to 20 nm. The process is facilitated by the electrochemical generation of aryl radicals that react with silanol groups at the distal end of silica.

Investigating metal‐enhanced fluorescence effect on fluorescein by gold nanotriangles and nanocubes using time‐resolved fluorescence spectroscopy

AbstractGold nanotriangles (AuNTs) and nanocubes (NCs) coated with silica shell and fluorescein were synthesized to study the effects of metal‐enhanced fluorescence. The interaction between gold nanoparticles (GNPs) and fluorescein with silica shells of different thicknesses as spacers was investigated by using time‐resolved fluorescence spectroscopy. From the biexponential decay of fluorescence of fluorophore and the kinetic mechanism for the interaction, we obtained the rate constants of energy transfer processes. Both energy transfer rates from fluorophore to the bright and dark modes of gold nanoparticles decreased with distance d (= silica thickness) with a dependence ∝ d−n and n ≈ 2. The rate constant of nanosurface energy transfer kNEST, considering dipolar interaction between fluorophore and metal surface, has an estimated value 1.7 times slower at d = 9 nm and about four times slower at 25 nm than the obtained rate constant for energy transfer from fluorophore to GNP dipolar mode in the AuNT system. For the AuNC system, the rate constants are greater than the AuNT system because of better spectral overlap between emission of fluorescein and surface plasmon resonance.

Influence of the thickness of silica layer on the radiative relaxation of AuNR@SiO2 core–shell nanostructures upon photoexcitation

AbstractSilica‐coated gold nanorods with different silica thicknesses (AuNR@X‐SiO2, X = 20, 35, 50 and 65, denoting the SiO2 thickness in nm on the longitudinal side) were excited with a 7‐ns pulsed 1064‐nm laser. The infrared emissions of AuNR@X‐SiO2, probed with a step‐scan Fourier‐transform interferometer, enveloped the optical phonon modes of the Si‐O‐Si bridge (1250–1000 cm−1) and adsorbed water (1700–1550 cm−1) within silica pores and minute blackbody radiation, indicating the capability of populating the optical phonon energy of capping layers in core‐shelled nanostructures upon photoexcitation. The decay of the emission at 1250–1000 cm−1 was decelerated as the thickness of silica increased. The kinetic analysis on the emission evolutions provided the thermalization properties of SiO2 on a microsecond timescale, including the intrinsic spontaneous radiation and the non‐radiative thermal conduction. The vibrational energy stored in the solid SiO2 and the embedded water might be capable of serving as an energy donor, that is, an optically‐energized catalyst, for vibrationally‐assisted chemical reactions.

Design, Simulation, and Analysis of Optical Microring Resonators in Lithium Tantalate on Insulator

In this paper we design, simulate, and analyze single-mode microring resonators in thin films of z-cut lithium tantalate. They operate at wavelengths that are approximately equal to 1.55 μm. The single-mode conditions and transmission losses of lithium tantalate waveguides are simulated for different geometric parameters and silica thicknesses. An analysis is presented on the quality factor and free spectral range of the microring resonators in lithium tantalate at contrasting radii and gap sizes. The electro-optical modulation performance is analyzed for microring resonators with a radius of 20 μm. Since they have important practical applications, the filtering characteristics of the microring resonators that contain two straight waveguides are analyzed. This work enhances the knowledge of lithium tantalate microring structures and offers guidance on the salient parameters for the fabrication of highly efficient multifunctional photonic integrated devices, such as tunable filters and modulators.

Impact of the dielectric duty factor on magnetic resonance in Ag-SiO2-Ag magnetic absorber

Magnetic absorber in optical frequency can be fulfilled through metamaterials designing. Therein, magnetic resonance in metal-dielectric-metal metasurfaces can be manipulated conveniently, and studying the parameters impacts is the primary for applications. In this work, through changing the grating width and the thickness of silica, the magnetic resonance modes have been studied, the conditions of the phase change zone from magnetic resonance (MR) to Fabry-Perot (FP) are given out in Ag−SiO2−Ag grating magnetic metasurfaces. The results indicate that the MR mode in metal-dielectric-metal configuration is mainly decided on the dielectric duty factor other than the sole behaviors of the thickness of dielectric and size of nanostructures. The physical mechanism is elucidated through simulated electromagnetic field distributions using finite difference time domain (FDTD) solution, and numerical analysis of effective refraction index of Ag−SiO2−Ag magnetic metasurfaces. This study may prompt development of metamaterials in basic research in condensed physics and in optical devices applications.

Modifying\xa0the thickness, pore size, and composition of diatom frustule in Craspedostauros sp. with Al3+ ions

Diatoms are unicellular photosynthetic algae that produce a silica exoskeleton (frustule) which exposes a highly ordered nano to micro scale morphology. In recent years there has been a growing interest in modifying diatom frustules for technological applications. This is achieved by adding non-essential metals to the growth medium of diatoms which in turn modifies morphology, composition, and resulting properties of the frustule. Here, we investigate the frustule formation in diatom Craspedostauros sp., including changes to overall morphology, silica thickness, and composition, in the presence of Al3+ ions at different concentrations. Our results show that in the presence of Al3+ the total silica uptake from the growth medium increases, although a decrease in the growth rate is observed. This leads to a higher inorganic content per diatom resulting in a decreased pore diameter and a thicker frustule as evidenced by electron microscopy. Furthermore, 27Al solid-state NMR, FIB-SEM, and EDS results confirm that Al3+ becomes incorporated into the frustule during the silicification process, thus, improving hydrolysis resistance. This approach may be extended to a broad range of elements and diatom species towards the scalable production of silica materials with tunable hierarchical morphology and chemical composition.

In-situ synthesis of silica coated titania nanoparticles in an aerosol reactor

A simple less temperature method for in situ combination of silica coated titania nano particles in an aerosol reactor is proposed in this work. The coating of silica on titania nano particles has been carried out using a low temperature vapor phase hydrolysis process in an aerosol reactor using Ticl4, and SiCl4, vapors as precursors. This novel process thus reduces the cost and complexity of the individual manufacturing units. Particle size and coating thickness of silica coated titania nano particles are characterized using HR TEM, XRD, and FT-IR. Various reaction parameters likeH2O Ticl4, hydrolysis reaction temperature, molar ratio. Ticl4, concentration, Sicl4/Ticl4, molar ratio and mixing positions in reactor for obtaining uniform coating of Sicl4, over Ticl4, are also investigated. The FT-IR spectrum and EDS clearly showed the formation of an interfacial chemical bond between TiO2, and SiO2.

The effects of graphene oxide-silica nanohybrids on the workability, hydration, and mechanical properties of Portland cement paste

Although graphene-based materials have attracted much attention as nanoreinforcements for cementitious materials, their effectiveness is significantly dependent on their dispersibility in cement matrix. In particular, graphene hybridization (e.g. with silica) has shown considerable potential to meet this challenge within the broader family of graphene-based materials. In this study, we investigated the effects of silica coated graphene oxide (GOS) nanohybrids on the workability and the mechanical and microstructure properties of Portland cement paste composites. GOS with three different thicknesses of silica coating (5, 10 and 30 nm) were designed via a sol-gel method and characterized using microscopic techniques. We found that the increase in thickness of silica coating resulted in reduced workability of cement paste and a corresponding increase in cement mechanical properties with nanohybrid addition. With the incorporation of GOS, the compressive strength of cement paste was increased by 3.2–34.6% at 7 days and by 7–12.4% at 28 days, proportional to the increasing coating thicknesses. X-ray diffraction and mercury intrusion porosimetry indicated that the reaction of portlandite with the silica coating of the GOS nanohybrids led to pore refinement of the cement matrix, with more pronounced effects in samples with thicker coatings. These results demonstrate that rational design of graphene oxide nanohybrids presents an effective method to obtain stronger and more durable Portland cement composites.