Structure Of SiO2 Research Articles

Liquid–Crystal Structure Inheritance in Machine Learning Potentials for Network-Forming Systems

It has been studied whether machine learning interatomic potentials parameterized with only disordered configurations corresponding to liquid can describe the properties of crystalline phases and predict their structure. The study has been performed for a network-forming system SiO2, which has numerous polymorphic phases significantly different in structure and density. Using only high-temperature disordered configurations, a machine learning interatomic potential based on artificial neural networks (DeePMD model) has been parameterized. The potential reproduces well ab initio dependences of the energy on the volume and the vibrational density of states for all considered tetra- and octahedral crystalline phases of SiO2. Furthermore, the combination of the evolutionary algorithm and the developed DeePMD potential has made it possible to reproduce the really observed crystalline structures of SiO2. Such a good liquid–crystal portability of the machine learning interatomic potential opens prospects for the simulation of the structure and properties of new systems for which experimental information on crystalline phases is absent.

Effect of Pressure on the Structure of SiO2 at the Glass Transition Temperature

Effect of Pressure on the Structure of SiO2 at the Glass Transition Temperature

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.

Behavior of Hydrogarnet‐Type Defects in Hydrous Stishovite at Various Temperatures and Pressures

AbstractDense polymorphs of silica have been demonstrated experimentally to incorporate from 1.5 wt% to as much as 11.6 wt% H2O as OH groups, with implications for the hydrogen budgets of Earth and other planets. This OH is thought to enter the SiO2 structure via a charge‐balanced substitution in which silicon vacancies (VSi) are compensated by protonating four of the surrounding six oxygen atoms, often referred to as a hydrogarnet‐type defect. There are many possible configurations for this defect structure in dense silica, but the nature of these configurations and whether they can be distinguished experimentally is unknown. We present here density functional theory calculations that systematically assess the possible configurations of a hydrogarnet‐type defect in stishovite (rutile‐type SiO2), with direct comparisons to experimental vibrational spectroscopy data. We predict that stishovite synthesized at 450 K and 10 GPa quenched to room temperature is dominated by a single defect type with tetrahedral geometry. This leads to OH stretching modes (2,500–3,000 cm−1) and SiOH bending modes (∼1,400–1,450 cm−1) largely consistent with experimentally observed modes. One remaining issue is that our calculations produce results compatible with experimental data on H to D exchange, but do not explain why a considerable fraction of the 1,420 cm−1 mode shifts by only 40 cm−1 in deuterated samples. At elevated pressures and temperatures, we find that a second square planar defect configuration also becomes favorable, leading to modes that should allow differentiation from the tetrahedral configuration.

Optical switch of low-insertion-loss based on the Brewster angle effect

Optical switches are critical components for optical communication devices, and currently, they are now bottlenecking the development of optical telecommunications. In this study, we propose an innovative all-optical Kerr switch based on the Brewster angle effect, which exhibits a graphene/SiO2/graphene/SiO2 structure and an insertion loss (IL) that can be minimized by tuning the Fermi level of the graphene layers. A novel Brewster’s angle effect is achieved, resulting in a change in the incidence angle under nonlinear conditions as well as on–off switching of the phase change. The proposed switch has perfect absorption at an incident angle of 63 deg, where the phase of the reflected light is shifted from − π to 0. Furthermore, the Brewster angle of the switch was successfully shifted from 63 deg to 80 deg with an incident frequency of 0.8 THz. It was observed that an increase in Fermi level continuously shrank the nonlinear region of the switch. The minimum IL of the proposed optical switch was only 2 dB, which is indicative of excellent energy efficiency. The results of this study can aid in the development of key devices for broadband communications and electromagnetic sensing.

To study the synthesis and characterization of photocatalyst TiO2/SiO2 nanocomposite prepared by Co-Precipitation method

We report here the synthesis of Nanocomposites of TiO2/SiO2 by the wet chemical method i.e. Coprecipitation method. The sample of TiO2 and SiO2 and composite of TiO2/SiO2 calcined at 600 °C and were structurally analyzed by X-Ray Diffraction. The values of cell parameter, Wyckoff position, sites etc. of prepared samples were also calculated. The size of the crystal of TiO2/SiO2 nanocomposites was obtained as 33.2 nm. The visualization of crystal structure of both TiO2 and SiO2 were also discussed in this paper.

Metamaterial ultra-wideband solar absorbers based on a multi-layer structure with cross etching.

In this paper, we design a solar absorber based on the Si3N4-W-Ti-SiO2 insulator-metal-insulator structure and demonstrate it using the finite difference time domain (FDTD) method. The absorption rate of the absorber consisting of a multi-layer structure with cross etching is over 90% in the bandwidth of 500 nm to 2995 nm with an average absorption rate of 98.3%. There are three peaks at 620 nm, 1002 nm and 1761 nm with peak heights of 99.8%, 99.8% and 99.0%, respectively. By analyzing the distribution of electric and magnetic fields in different sections of the absorber, it is found that the physical mechanism of the structure with high absorptivity is due to the interaction of propagating surface plasmon resonance and local surface plasma resonance. The effects of different structural parameters and the angle of incidence of a light source on the absorber absorption are discussed. The absorber can be used in solar thermal systems, thermal photovoltaics and thermoelectronic devices.

Methane direct conversion to olefins, aromatics, and hydrogen over silica entrapped bimetallic MeFe-SiO2 (Me = Co, Ni, Pd, Pt) catalysts

Here, MeFe-SiO2 (Me = Co, Ni, Pd, Pt) catalysts with bimetallic sites entrapped in a highly crystalline SiO2 structure were synthesized and used for the conversion of methane to olefins, aromatics, and hydrogen (MTOAH) at 1020°C. The MeFe-SiO2 catalysts showed polymorphic forms of cristobalite, quartz, and tridymite after reaction. Among the bimetallic catalysts, 0.5Pt1.0Fe-SiO2 exhibited the highest methane conversion (10.0%) with high hydrocarbon selectivity (79.9%) at 1020°C. In C2 (ethane, ethylene, acetylene) conversion with hydrogen co-feeding at 1020°C, acetylene was identified as a major coke precursor. MTOAH with different gas hourly space velocities (GHSV) showed that the 0.5Pt1.0Fe-SiO2 catalyst exhibited higher methane conversion and aromatics selectivity than the 1.0Fe-SiO2 catalyst. Density functional theory calculations showed that the Pt-Fe3C surface is energetically favorable for methane activation and inhibits graphitic coke deposition by C2 dehydrogenation. Consequently, a modification of the entrapped Fe sites by Pt addition improved the methane conversion and hydrocarbon selectivity of the catalyst.

Synthesis and characterization of nanostructured silica powders using rice hull ash-based sodium silicate solution by precipitation and calcination

Herein, characteristics of the nanostructured silica powders (NsSP) synthesized from Philippine rice hull ash-based sodium silicate solution by precipitation (pH 2, pH 6) and calcination (600 °C, 900 °C) were presented. Characterizations confirmed that the synthesized NsSP contains high amount of SiO2. NsSP obtained at pH 2 revealed higher mass losses by ∼ 12 % until 900 °C and higher intensities of –OH and Si–OH bonds than obtained at pH 6. Calcination of NsSP at 900 °C removes the Si–OH bond leading to a relative growth in the SiO2 primary particles but SiO2 structure remained amorphous. Morphology of NsSP obtained at pH 2 were cloud-like while NsSP obtained at pH 6 were bead-like and more distinct.

DABCOnium Ionic Liquid‐Immobilized Silica Gel for Solid Phase Extraction of Phenoxyacetic Acid Herbicides in Water Samples**

AbstractA novel sorbent (SiO2‐(CH2)3‐DABCO) for solid‐phase extraction (SPE) was successfully prepared by chemical‐bonding formation of diazabicyclo[2.2.2]octane ionic liquid onto silica gel. The structure of SiO2‐(CH2)3‐DABCO sorbent was characterized by FT‐IR, TGA, SEM, EDX, and BET methods. Cartridges packed with the synthetic SiO2‐(CH2)3‐DABCO were used for the preconcentration of trace‐level of phenoxyacetic acid herbicides in water samples for the first time. The factors influencing on the recovery were investigated, such as type of ion liquids, loading volume, eluents, elution volume, loading rate, and capacity. After the enrichment, the phenoxyacetic acid herbicides were analyzed by high‐performance liquid chromatography with an ultraviolet detector. Four model phenoxyacetic acid herbicides, including 2‐methyl‐4‐chlorophenoxyacetic acid (MCPA), 2,4,5‐trichlorophenoxyacetic acid (2,4,5‐T), fenoprop (2,4,5‐TP) and haloxyfop were well retained and separated by this material with a recovery efficiency from 77 to 103 %. The SiO2‐(CH2)3‐DABCO demonstrated a good capacity corresponding to 0.56 mg of the analyte adsorbed per 100 mg of the material. The mixture of phosphate buffer (pH 7) : MeOH (50 : 50, v/v) was used to elute the analytes and provided the highest recovery with the standard deviations from 2.4 to 6.5. The high sensitivity with detection limits of 1.070–9.587 μg/L can be achieved.

Yolk-shell SiO2 wrapped by reduced graphene oxide for high performance lithium-ion battery anode

In order to alleviate the rapid capacity attenuation caused by huge volume expansion and low electronic conductivity during SiO2 charge and discharge, the rGO@yolk-shell SiO2 structure is designed by using reduced graphene oxide (rGO) as the conductive network and loading the carbon-coated SiO2 as the yolk-shell structure. The well-designed yolk-shell structure can slow down the volume change without destroying the carbon layer. The conductive networks of rGO, in conjunction with this carbon layer, contribute to the efficient electron transport pathway. When measured as anode material for lithium-ion batteries, rGO@yolk-shell SiO2 exhibits a long cycle performance of 616 mAh g−1 after 358 cycles at 0.1 A g−1.

Preparation of Ni/SiO2 using silicon-rich-biomass as source for acetic acid steam reforming

In this paper, four kinds of silicon-rich biomass (rice husk, rice straw, peanut shell, corncob) were used as raw materials for SiO2 extraction by heat treatment method. And the extracted SiO2 was applied to prepare Ni/SiO2 catalyst for acetic acid (AcOH) steam reforming. Various characterization methods were used to analyze the purity, structure and specific surface area characteristics of SiO2 extracted from different sources. It was found that the silica prepared by acid washing and calcination (AWC) had larger specific surface area, abundant pore channels and larger pore volume. By water washing and calcination (WWC), K2O impurity would be remained in SiO2, which might dissolve SiO2 during the calcination process, generating an increase in the pore size and a decrease in the specific surface area. While the CaO impurity in SiO2 promoted the dispersion of the active components and enhanced the interaction between the active component and the carrier. A series of reforming catalysts were prepared by using the obtained SiO2 as carriers. It was found that the lifetime of catalyst from low to high was Ni/RS-SiO2 <Ni/PS-SiO2 <Ni/RH-SiO2, which was in the opposite order of K2O content. Acid washing was beneficial for the enhancement of Ni/RH-AC-SiO2 activity, and the increase of Ni/RS-AC-SiO2 stability. Among the four catalysts, the Ni/RH-AC-SiO2 catalyst exhibited the highest carbon conversion (100%) and hydrogen yield at 600 °C, which may be because that the acid washing removed the metal oxide impurity K2O. And its large specific surface area, the concentrated pore size distribution, and the suitable interactions between the support and carrier may also contribute to its high catalyst activity.

ИНТЕГРАЛЬНЫЕ ОПТИЧЕСКИЕ ВОЛНОВОДЫ НА ОСНОВЕ ПЛЕНОК Si3N4

The paper presents studies at developing a technology for the formation of a rib integrated optical waveguide based on a Si3N4 film. The effect of ICP deposition modes on the structure of SiO2 and Si3N4 films and the effects of RIE etching of the waveguide on the rib profile and surface roughness are studied.

Effects of process parameters on preparation of Ti@SiO2 particles during fluidized bed chemical vapor deposition via design of experiments

Fluidized bed chemical vapor deposition (FBCVD) is a promising technology for preparing photocatalysts. However, the photocatalyst performance depends on numerous parameters, and the effects of process parameters on photocatalyst performance are not well understood. In this work, the effects of deposition time, fluidization number, reactant molar ratio and deposition temperature on photocatalysts prepared by FBCVD are investigated via a full factor factorial design approach, which is a kind of Design of Experiments. The results show that the deposition time, reactant molar ratio and deposition temperature are the main factors affecting the photocatalytic performance, while the fluidization number and the interactions between different parameters are insignificant factors. The characterization of the elemental content, elemental distribution and crystalline structure of Ti-deposited SiO2, Ti@SiO2, particles reveals that the formation of Ti-O-Si bonds and Ti-based nanoparticles on the Ti@SiO2 surface enhances the photocatalytic performance of the particles. Finally, the formation mechanisms of Ti@SiO2 particles are summarized which can be distinguished into two stages. At around 120 °C, intermediates such as Ti(OH)4 are generated and reactions consist of homogeneous and heterogeneous reactions. At around 400–600 ℃, the Ti@SiO2 particles are formed, which is dominated by heterogeneous reactions. The results of this study can improve understanding and control of photocatalyst preparation.

A robust superhydrophobic and oleophobic coating with short chain perfluoroalkyl group and flower-shaped SiO2 nanoparticles

Low-surface-energy fluorocarbon-based materials are widely used in superhydrophobic and oleophobic coatings. The fabrication of oil-repellant coatings, however, is particularly challenging due to the low surface tension of oils and the nondegradable of long-fluorocarbon compounds (CnF2n+1, n ≥ 8). In this work, superhydrophobic and oleophobic functional nanoparticles were prepared based on flower-shaped silica and environmentally friendly short chain perfluoroalkyl group. The experimental results show that a low-surface-energy coating surface (water contact angle of 152.5 ± 0.4°, oil contact angle of 142.2 ± 0.8°) can be obtained by using triethoxy(1H,1H,2H,2H-nonafluorohexyl)silane (CH3CH2O)3SiCH2CH2(CF2)3CF3 (C4FTES) and methyltriethoxysilane (MTES) to co-modified flower-shaped SiO2 nanoparticles (flower-shaped MF-SiO2). The analysis of morphology and chemical components of flower-shaped MF-SiO2 displayed a synergistic effect by providing a multi-scale rough structure of flower-shaped SiO2 which minimize contact at the liquid/solid interface, and promotes the distribution and accumulation of the outermost -CF3 groups of nanoparticles as evidenced by the angle-resolved X-ray photoelectron spectrum (AR-XPS) tests. Furthermore, flower-shaped MF-SiO2 nanoparticles have good mechanical stability under the action of adhesive, which means the flower-shaped MF-SiO2 nanoparticles as a promising candidate for superhydrophobic and oleophobic coating materials.

Influence of Surface Chemistry on the Surfactant Organization and Interfacial Structure of Mesoporous Silica and Organosilica

Mesoporous silica (SiO2) and periodic mesoporous organosilica (PMO) with divinylbenzene (DVB)- and divinylaniline (DVA)-bridge groups are studied by solid-state NMR and molecular dynamics simulations to examine the influence of the surface chemistry on the interfacial interactions and the assembly structures of the surfactants. The NMR signals of the surfactants inside the mesopores appear at different chemical shift values depending on the organic bridging groups. Two-dimensional NMR spectroscopy shows that the surfactants assemble as micellar structures, where the polar head groups directly interact with the pore walls, and the interaction strength of the surfactants with pore wall is in the order of SiO2 >DVA-PMO > DVB-PMO. To elucidate the functional dependence of the interfacial structure of SiO2 and PMOs, the radial distribution function is calculated from molecular dynamics simulations, which provides individual atomic correlations quantitatively. For SiO2, the surfactant head is preferentially adsorbed to the inorganic layer with extended aliphatic tail group packing, whereas the adsorption of the surfactant head group to the pore wall is weaker for PMOs. We find that the hydrophobic interaction between DVB and the surfactant tail, which is stronger than that in the case of polar DVA, weakens the interaction between the surfactant head and organic layer for DVB-PMO more than for DVA-PMO. From the conformational analysis, we directly observe a more abundant gauche defect of the surfactant in DVA-PMO than in DVB-PMO. The upfield NMR shift in DVB-PMO is revealed by more planar aromatic ring orientation, which is caused by the steric effect between the organic groups. Our study suggests that the hydrophobicity and bulkiness of the organic bridge can be exploited in the design of PMOs for molecular machines, absorbents, and enzyme immobilization materials.

Structure, Morphology and Bioactivity of Bioactive Glasses SiO2–CaO–P2O5 Doped with ZnO Synthesized by Green Synthesis

Structure, Morphology and Bioactivity of Bioactive Glasses SiO2–CaO–P2O5 Doped with ZnO Synthesized by Green Synthesis

Molecular dynamics study of crystal lattice effect on mechanical properties of SiO2: A density functional tight binding investigation

In this study, molecular dynamics simulations (MD) based on quantum mechanical method in which the interactions were expressed by self-consistent charge density functional tight binding (SCC-DFTB) to investigate the mechanical properties of four different SiO2 structures. The Young's, shear and bulk modulus of systems are calculated and the results compared to experimental and other computational findings. We have observed that the hexagonal SiO2 exhibits more different mechanical behavior than other SiO2 structures because of its negative bulk modulus value. While the length of a-lattice parameters for four SiO2 crystal structures increase with the change of MD time, the length of the b and c-lattice vectors shortens. Moreover, it is analyzed the stress-strain behavior and yield points are determined from this curve of SiO2 structures. It is revealed that the plastic deformation occurs at higher stress for hexagonal SiO2 than other SiO2 structures because of its rigid solid property. These results can provide a reference for understanding the mechanical properties of various SiO2 crystal structures.

Effect of B2O3 content on viscosity and structure of SiO2−MgO−FeO-based slag

The effect of B2O3 content on the viscosity of SiO2−MgO−FeO-based molten slag system was investigated using the rotating cylinder method. The evolution process of the melt structure under different contents of B2O3 was comprehensively studied via FTIR spectroscopy and a model for calculating the degree of polymerization was developed. The results showed that the viscosity of the molten slag decreased with the addition of B2O3, which had a slight effect when its content exceeded 3 wt.%. As the addition of B2O3 increased from 0 to 4 wt.%, the break temperature of the slags decreased from 1152 to 1050 °C and the apparent activation energy decreased from 157.90 to 141.84 kJ/mol. The addition of B2O3 to the molten slag destroyed the chain silicate structure to form a more cyclic borosilicate structure. The Urbain model was improved to calculate the viscosity of the SiO2−MgO−FeO-based slags, and the values were in good agreement with the experimentally measured values.

Theoretical Investigation of Coupling of Surface Plasmons in One-dimensional Periodic Structure of SiO2 and TiO2 with Defect of Nanocomposite of Liquid Crystals and Core–Shell Nanoparticles

Theoretical Investigation of Coupling of Surface Plasmons in One-dimensional Periodic Structure of SiO2 and TiO2 with Defect of Nanocomposite of Liquid Crystals and Core–Shell Nanoparticles