Amorphous SiO2 Research Articles

Giant Isotope Effect of Thermal Conductivity in Silicon Nanowires.

Isotopically purified semiconductors potentially dissipate heat better than their natural, isotopically mixed counterparts as they have higher thermal conductivity (κ). But the benefit is low for Si at room temperature, amounting to only ∼10% higher κ for bulk ^{28}Si than for bulk natural Si (^{nat}Si). We show that in stark contrast to this bulk behavior, ^{28}Si (99.92% enriched) nanowires have up to 150% higher κ than ^{nat}Si nanowires with similar diameters and surface morphology. Using a first-principles phonon dispersion model, this giant isotope effect is attributed to a mutual enhancement of isotope scattering and surface scattering of phonons in ^{nat}Si nanowires, correlated via transmission of phonons to the native amorphous SiO_{2} shell. The Letter discovers the strongest isotope effect of κ at room temperature among all materials reported to date and inspires potential applications of isotopically enriched semiconductors in microelectronics.

Effect of laser power on the microstructure and properties of additive manufactured 17-4 PH stainless steel in different fabrication atmosphere

The effect of laser power (400, 600, 800 W) on the microstructure and properties, such as porosity, strength and microhardness, of directed energy deposition 17-4 PH stainless steel in different fabrication atmosphere (atmosphere air and atmosphere Ar) was investigated via the view of thermal process. With the increase of laser power, the porosity of the samples decreased, and the Ar atmosphere further enhanced the effect through a power exponent law. The submicron (∼0.5 μm) amorphous SiO2 particles and the nanoscale (∼35 nm) Cu-rich precipitates dispersed in the matrix strengthened the samples, and the dimensions changed when different atmosphere and heat treatment were employed which in turn influenced the strength and microhardness. The mechanical properties were also affected by content of austenite and the release of oxygen, which were dominated by thermal process and heat treatment.

An investigation on the oxidation behavior of spatters generated during the laser powder bed fusion of 316L stainless steel

The characterization of the oxide islands on the surface of the spatter contributes to a fundamental understanding of the formation and oxidation of spatter in laser powder bed fusion (LPBF). In the present study, the surface and cross-section characteristics of spatters with oxide islands are investigated using high resolution scanning electron microscopy combined with energy dispersive spectroscopy, scanning transmission electron microscopy and selected area electron diffraction. The oxide islands with different configurations on the spatter surface are divided into three main categories: hemispherical, disc-like and film-like. Moreover, the primary component of oxide islands is an amorphous SiO2, which may also contain MnO, MnSiO3, and Mn2SiO4. Furthermore, a transition layer (enriched in Si, Mn, and O) is found at the oxide islands || spatter substrate interface, and the width of which is 4 μm and 20 nm on the spatter surface and within the spatter cross-section, respectively. Besides, the composition and sources of the circular oxide nano-inclusions (50-600 nm) internal to the spatters are also discussed in detail. Eventually, the driving forces affecting the formation of oxide islands on the surface of spatters are summarized, and a schematic diagram of the formation procedure is developed.

Amorphous silica glass nano-grooving behavior investigated using molecular dynamics method

This work uses molecular dynamics to simulate the grooving behavior of optical silica glass. The amorphous SiO2 (silica glass) was fabricated using a melting-quenching process, and the critical cut depth, and mechanism of the chip formation, were explored using a molecular dynamics simulation. The analytical results indicated that the tool edge radius affected the critical cut depth and roughness of the machined surface. A larger tool edge radius had a larger equivalent negative rake angle at the same depth of cut, causing a larger critical cut depth, while producing a smoother machined surface. In addition, the uncut chip thickness affected the tangential force and thrust force distribution weighting of the tool. The temperature field analysis revealed that a groove formed by the chip formation mechanism resulted in a higher workpiece temperature, causing the brittle material to exhibit ductile cutting behavior during the nanogrooving process. However, grooves formed by the scratching-indenting mechanism had a lower workpiece temperature.

Fabrication and characterization of pyrolytic carbons from phenolic resin reinforced by SiC nanowires with chain-bead structures

SiC/SiO2 nanowires (NWs) were successfully synthesized via a simple chemical vapor deposition method at 1200 °C under an argon atmosphere by using phenolic resin (PF) and silicon (Si) and then catalyzed by copper citrate. The obtained SiC/SiO2 NWs as-calcined at 1200 °C had a chain-bead structure, and the SiC nanowire was wrapped by amorphous SiO2 nanospheres: The diameter of the SiO2 bead was in the range of 175–250 nm; the length of the SiC chain was 375–500 nm, and the diameter was 83 nm. The copper phase derived from copper citrate could dissolve with Si to form a Cu/Si alloy, and it could reduce the melting point of the Si particle and generate higher amount of SiO. The growth mechanism of the Cu-catalyzed SiC/SiO2 NWs was vapor-solid (V–S). The strength and oxidation resistance of the PF pyrolysis carbons were reinforced by SiC/SiO2 NWs. The cold crushing strength (CCS) of the PF pyrolysis carbons calcined at 1200 °C and 1300 °C increased from 0.08 MPa to 0.64 MPa and 0.36 MPa to 1.81 MPa, respectively. The starting and ending temperatures of weight loss temperatures of PF pyrolysis carbons were delayed from 513 °C and 678 °C to 568 °C and 723 °C, respectively. The enhancements in the mechanical properties and oxidation resistance properties of the PF pyrolysis carbons were because the C–SiC bonding phase of the PF resisted external stress and oxygen effectively.

Mechanochemical Strategies for the Preparation of SiO2-Supported AgAu Nanoalloy Catalysts.

Silver-gold nanoalloys were prepared from their metal salts precursors through bottom-up mechanochemical synthesis, using one-pot or galvanic replacement reaction strategies. The nanostructures were prepared over amorphous SiO2 as an inert supporting material, facilitating their stabilization without the use of any stabilizing agent. The nanomaterials were extensively characterized, confirming the formation of the bimetallic nanostructures. The nanoalloys were tested as catalysts in the hydrogenation of 2-nitroaniline and exhibited up to 4-fold the rate constant and up to 37% increased conversion compared to the respective single metal nanoparticles. Our approach is advantageous to produce nanoparticles with clean surfaces with available catalytic sites, directly in the solid-state and in an environmentally friendly manner.

Synthesis of Fe3O4-decorated SiO2 nanostructure using rice husk as a source by microwave combustion for the development of a magnetically recoverable adsorbent

In this study, we prepared Fe3O4-decorated SiO2 (Fe3O4@SiO2) nanostructure using rice husk as a silica source through a microwave-assisted combustion process. The results of the X-ray diffraction, Fourier transform infrared, energy-dispersive X-ray, scanning electron microscopy, and transmission electron microscope analyses corroborate that polycrystalline Fe3O4 nanoparticles with 5–10 nm size were decorated on the amorphous SiO2 nanosphere having a size of 50 nm in diameter. A vibrating sample magnetometer indicated that Fe3O4 decoration on rice husk–derived SiO2 (RH SiO2) was able to transform nonmagnetic into ferromagnetic material with saturation magnetization of 45 emu/g. The effects of pH, adsorbent dosage, and time on the removal of Hg2+ by RH SiO2 and Fe3O4@SiO2 samples were investigated and it was found that 100 mg adsorbent dosage at pH 6 for 20 min contact time is suitable for the sufficient removal (>90%) of Hg2+ ions with 100 mg/L concentration. The ferromagnetic characteristic, mesoporous structure, and high specific surface area (174.930 m2g−1) of Fe3O4@SiO2 nanostructure help develop a magnetically recoverable adsorbent for removal of Hg2+ ions from an aqueous solution.

Thermal conductivity of amorphous SiO2 by first-principles molecular dynamics

The approach-to-equilibrium molecular dynamics (AEMD) methodology implemented within a first-principles molecular dynamics (FPMD) scheme is applied to amorphous SiO2. In this disordered material, measurements of the thermal conductivity indicate no reduction down to 10 nm. In view of these premises, we calculate the thermal conductivity of amorphous SiO2 in the size range comprised between 2 and 8 nm via the AEMD/FPMD approach. The thermal conductivity agrees with experiments for the largest sizes we considered, while it is strongly reduced for values not accessible to experimental resolution (up to 50% for 2 nm). This behavior is close to that found in amorphous chalcogenides GeTe4 and Ge2Sb2Te5 within the same AEMD/FPMD approach. Taken together, these results show that the observed decrease of the thermal conductivity is a general feature of disordered networks and in any case cannot be taken as peculiar to a specific class of systems.

Low-temperature SiO2 film coatings onto Cu particles using the polygonal barrel-plasma chemical vapor deposition method

Surface coating of Cu particles with SiO2 thin films was investigated at a low temperature of 100 °C using the polygonal barrel-plasma chemical vapor deposition (PB-PCVD) method. The Cu particles were treated at 250 W for 30 min while a hexagonal barrel containing the samples was oscillated at 5 rpm with an amplitude of ± 75°. The results show that the particle surfaces were uniformly coated with amorphous SiO2 films at the flow rates of vaporized hexamethyldisilazane (used as a precursor) and O2 gas of 6 and 120 ml/min, respectively. The thickness of the SiO2 films (77 nm) changed linearly with the treatment time and the RF power, whereas for longer treatment times (>40 min at 250 W) and higher RF powers (>300 W for 30 min), impurities were regenerated, which was prevented by inserting a barrel-cleaning process during the treatment. The deposition rate of the SiO2 films for PB-PCVD was 15 times higher than that of our original sputtering method, and the smoother films can be obtained by PB-PCVD, as compared to the sputtering method. Thus, this high-speed and controllable PB-PCVD method would be useful for ceramic coating researches such as electric device fields, although further investigations are required.

Interfacial Reactions between Si and SiO2 with Ceramic Additives

In this study, 10 wt.% ceramics—Al2O3, La2O3, Y2O3, MgO, and TiO2—were employed as additives for amorphous SiO2 after pressing and annealing at 1300 °C. The amorphous SiO2 changed to cristobalite SiO2. Through X-ray diffraction, scanning electron microscopy, and transmission electron microscopy with energy-dispersive spectrometry, the reaction phases of La2Si2O7, Y2Si2O7, and MgSiO3 (Mg2SiO4) were found in the SiO2 with 10 wt.% La2O3, Y2O3, and MgO additives. Cracks formed in the Si and SiO2–ceramic additive sites because of the difference in the coefficients of thermal expansion among the Si, SiO2, ceramic additives, and reaction phases. After Si came into contact with the SiO2–ceramics, two types of microstructures were found: those with and those without an amorphous SiO2 reaction layer at the interface. Amorphous SiO2 layer formation is due to the replacement of the Si position in SiO2 by Al3+ and Ti4+ impurities, which can break the bonds between Si atoms. The O content in the Si decreased from 6–9 × 1017 atoms/cm3 for SiO2 to less than ~1016 for SiO2–Al2O3 and SiO2–MgO. The average resistivity of the Si was 3 Ω·cm for SiO2 and decreased to 0.12–0.36 Ω·cm for the SiO2 with ceramic additives.

Evolution of Black Talc upon Thermal Treatment

Black talc is a natural silicate clay mineral with a typical 2:1 layered structure, low electrical conductivity, large specific surface area, and high thermal stability. The world’s largest black talc mine, with known reserves of one billion tons, is located in China’s Jiangxi province. Due to the restriction of its color, the application of black talc is only limited to ceramic raw materials, coating filler, waterproof materials, and other low-end application industries. Thermal treatment is a common method of clay mineral modification. It is vital to examine the structural and physical changes of black talc during calcination in order to prepare black-talc-based composites and to broaden their applications. This work discusses the evolution of black talc upon thermal treatment (30–1000 °C) and the corresponding structural changes. The thermal stability of minerals was analyzed via thermogravimetric (TG) analysis and thermogravimetry–mass spectrometry (TG-MS). The decomposition of minerals during calcination consists of four processes: dehydration, organic carbon decomposition, dihydroxylation, and phase transformation. In situ FTIR and in situ XRD were employed to track changes in black talc in real time during thermal treatment. At 800 °C, black talc was found to begin to go through dihydroxylation, and the crystallinity index decreased significantly. The XRD pattern of samples at 950 °C (T950) showed the reflection of the enstatite structure, and the relative crystallinity index was 27.3%, indicating that the mineral had undergone phase transformation. In addition, the Brunauer–Emmet–Teller (BET), laser particle size analyzer, Zeta potential, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques were used to systematically characterize the physicochemical properties of minerals at different temperatures. The results show that black talc’s particle size and specific surface area increase with the calcination temperature. The surface charge changes, and more amorphous SiO2 and MgO appear, indicating that thermal treatment could induce structural changes and activate the surface of black talc.

Methylene Blue Dye as Photosensitizer for Scavenger-Less Water Photo Splitting: New Insight in Green Hydrogen Technology.

In this study, hydrogen generation was performed by utilizing methylene blue dye as visible-light photosensitizer while the used catalyst is working as a transfer bridge for the electrons to H+/H2 reaction. Silica NPs-incorporated TiO2 nanofibers, which have a more significant band gap and longer electrons lifetime compared to pristine TiO2, were used as a catalyst. The nanofibers were prepared by electrospinning of amorphous SiO2 NPs/titanium isopropoxide/poly (vinyl acetate)/N, N-dimethylformamide colloid. Physicochemical characterizations confirmed the preparation of well morphology SiO2–TiO2 nanofibers with a bandgap energy of 3.265 eV. Under visible light radiation, hydrogen and oxygen were obtained in good stoichiometric rates (9.5 and 4.7 mL/min/gcat, respectively) without any considerable change in the dye concentration, which proves the successful exploitation of the dye as a photosensitizer. Under UV irradiation, SiO2 NPs incorporation distinctly enhanced the dye photodegradation, as around 91 and 94% removal efficiency were obtained from TiO2 nanofibers containing 4 and 6 wt% of the used dopant, respectively, within 60 min.

The Fast Track for Intestinal Tumor Cell Differentiation and In Vitro Intestinal Models by Inorganic Topographic Surfaces.

Three-dimensional (3D) complex in vitro cell systems are well suited to providing meaningful and translatable results in drug screening, toxicity measurements, and biological studies. Reliable complex gastrointestinal in vitro models as a testbed for oral drug administration and toxicity are very valuable in achieving predictive results for clinical trials and reducing animal testing. However, producing these models is time-consuming due to the lengthy differentiation of HT29 or other cells into mucus-producing goblet cells or other intestinal cell lineages. In the present work, HT29 cells were grown on an inorganic topographic surface decorated with a periodic pattern of micrometre-sized amorphous SiO2 structures for up to 35 days. HT29 cells on topographic surfaces were compared to undifferentiated HT29 in glucose-containing medium on glass or culture dish and with HT29 cells differentiated for 30 days in the presence of methotrexate (HT29-MTX). The cells were stained with Alcian blue for mucus, antibodies for mucus 2 (goblet cells), villin (enterocytes), lysozyme (Paneth cells), and FITC-labeled lectins to identify different cells, glycomic profiles, and cell features. We observed that HT29 cells on topographic surfaces showed more similarities with the differentiated HT29-MTX than with undifferentiated HT29. They formed islands of cell clusters, as observed for HT29-MTX. Already after 2 days, the first mucus secretion was shown by Alcian blue stain and FITC-wheat germ agglutinin. After 4–6 days, mucus was observed on the cell surface and in the intercellular space. The cell layer was undulated, and in 3D reconstruction, the cells showed a clear polarisation with a strong actin signal to one membrane. The lectins and the antibody-staining confirmed the heterogeneous composition of differentiated HT29 cells on topographic surfaces after 6–8 days, or after 6–8 days following MTX differentiation (30 days).

Influence behavior of phosphorus slag and fly ash on the interface transition zone in concrete prepared by cement-red mud

Cement production requires a large amount of raw materials, such as clay, limestone, gypsum, and other materials, and its production process causes a certain degree of pollution and destruction of the ecological environment. Using industrial solid waste as a mineral admixture to partially replace cement in the preparation of concrete is helpful in saving cement consumption, absorbing industrial solid waste, and optimizing the structure of the interfacial transition zone (ITZ) in the concrete. Compressive and flexural strength, mercury intrusion porosimetry (MIP), scanning electron microscope (SEM), X-ray diffraction (XRD) microzone testing, and microcalorimeter were performed to investigate the influence behaviors of phosphorus slag (PS) and fly ash (FA) on the ITZ in concrete prepared with cement partially replaced by red mud (RM), respectively. PS and FA are industrial wastes produced at high temperatures, and PS and FA contain a certain amount of gelling active substances (such as amorphous SiO2). The inactive substances in PS and FA mainly play filling roles and have a microaggregate effect in the matrix and in the ITZ, while the amorphous SiO2 reacts with calcium hydroxide (CH) produced from cement hydration to generate calcium silicate hydrate (C–S–H) gel to fill the porous area near the ITZ, resulting in a reduction in pore size near the ITZ. Moreover, the C–S–H gel squeezes the growing space for CH to weaken the CH growth and its directional distribution, which is beneficial to the compactness of the ITZ in RM–cement concrete.

TEM-based crystal structure analysis of body-centered tetragonal structure in non-epitaxial FeCo film with added V and N

FeCo with a tetragonally distorted, body-centered tetragonal (bct) structure has been grown epitaxially for investigating the basic magnetic properties; however, it is necessary to form the bct-FeCo without epitaxial growth for allowing their use in permanent magnet-based applications. In this study, we investigated the effect of adding V and N to non-epitaxial FeCo films by directly depositing FeCoVN films on an amorphous SiO2 substrate. Crystal structure analysis performed through transmission electron microscopy revealed a bct lattice with an axial ratio of 1.07 at 10 at.% V and 1.5 at.% N. The tetragonal distortion stability was evaluated using first-principles calculations. The results indicated that bct-FeCo was formed via non-epitaxial growth, and the addition of V and N stabilized the bct structure.

Inclusions of Amorphous and Crystalline SiO2 in Minerals from Itrongay (Madagascar) and Other Evidence for the Natural Occurrence of Hydrosilicate Fluids

Experimental studies increasingly often report low-temperature (200–800 °C) and low-pressure (0.05–3 kbar) hydrosilicate fluids with >40 wt.% of SiO2 and >10 wt.% of H2O. Compositionally similar fluids were long suggested to potentially exist in natural systems such as pegmatites and hydrothermal veins. However, they are rarely invoked in recent petrogenetic models, perhaps because of the scarcity of direct evidence for their natural occurrence. Here we review such evidence from previous works and add to this by documenting inclusions of hydrosilicate fluids in quartz and feldspar from Itrongay. The latter comprise opal-A, opal-CT, moganite and quartz inclusions that frequently contain H2O and have negative crystal shapes. They coexist with inclusions of CO2- and H2O-rich fluids and complex polycrystalline inclusions containing chlorides, sulphates, carbonates, arsenates, oxides, hydroxides and silicates, which we interpret as remnants of saline liquids. Collectively, previous studies and our new results indicate that hydrosilicate fluids may be common in the Earth’s crust, although their tendency to transform into quartz upon cooling and exhumation renders them difficult to recognise. These data warrant more comprehensive research into the nature of such hydrosilicate fluids and their distribution across a wide range of pressure and temperature conditions and geological systems.

Interfacial thermal conductance between gold and SiO2: A molecular dynamics study

ABSTRACT Silica coating on a gold nanoparticle can improve its thermal application in cancer thermotherapy. In this paper, the interfacial thermal conductance between gold and silica is calculated utilizing classical non-equilibrium molecular dynamics. It is revealed that the results of molecular dynamics are different from what has been predicted by the conventional diffuse mismatch model. Furthermore, the interfacial thermal conductance between amorphous SiO2 and gold is approximately twice that of crystalline silica, which is explained by calculating the vibrational density of state mismatches. The interfacial thermal conductance variations in terms of van der Waals interaction strength between gold and silica are also investigated. It is revealed that the conductance increases by about 30% by increasing the simulation temperature from 300 to 700 K. The results of this paper can be useful in nanofluid systems, in addition to the application of silica-coated gold nanoparticles in cancer thermal therapy.

Large-scale ab initio simulation of light–matter interaction at the atomic scale in Fugaku

In the field of optical science, it is becoming increasingly important to observe and manipulate matter at the atomic scale using ultrashort pulsed light. For the first time, we have performed the ab initio simulation solving the Maxwell equation for light electromagnetic fields, the time-dependent Kohn-Sham equation for electrons, and the Newton equation for ions in extended systems. In the simulation, the most time-consuming parts were stencil and nonlocal pseudopotential operations on the electron orbitals as well as fast Fourier transforms for the electron density. Code optimization was thoroughly performed on the Fujitsu A64FX processor to achieve the highest performance. A simulation of amorphous SiO2 thin film composed of more than 10,000 atoms was performed using 27,648 nodes of the Fugaku supercomputer. The simulation achieved excellent time-to-solution with the performance close to the maximum possible value in view of the memory bandwidth bound, as well as excellent weak scalability.

Reactivity of bi- and monometallic trifluoroacetates towards amorphous SiO2.

The reactivity of alkali-manganese(II) and alkali trifluoroacetates towards amorphous SiO2 (a-SiO2) was studied in the solid-state. K4Mn2(tfa)8, Cs3Mn2(tfa)7(tfaH), KH(tfa)2, and CsH(tfa)2 (tfa = CF3COO-) were thermally decomposed under vacuum in fused quartz tubes. Three new bimetallic fluorotrifluoroacetates of formulas K4Mn3(tfa)9F, Cs4Mn3(tfa)9F, and K2Mn(tfa)3F were discovered upon thermolysis at 175 °C. K4Mn3(tfa)9F and Cs4Mn3(tfa)9F feature a triangular-bridged metal cluster of formula [Mn3(μ3-F)(μ2-tfa)6(tfa)3]4-. In the case of K2Mn(tfa)3F, fluoride serves as an inverse coordination center for the tetrahedral metal cluster K2Mn2(μ4-F). Fluorotrifluoroacetates may be regarded as intermediates in the transformation of bimetallic trifluoroacetates to fluoroperovskites KMnF3, CsMnF3, and Cs2MnF4, which crystallized between 250 and 600 °C. Decomposition of these trifluoroacetates also yielded alkali hexafluorosilicates K2SiF6 and Cs2SiF6 as a result of the fluorination of fused quartz. The ability to fluorinate fused quartz was observed for monometallic alkali trifluoroacetates as well. Hexafluorosilicates and heptafluorosilicates K3SiF7 and Cs3SiF7 were obtained upon thermolysis of KH(tfa)2 and CsH(tfa)2 between 200 and 400 °C. This ability was exploited to synthesize fluorosilicates under air by simply reacting alkali trifluoroacetates with a-SiO2 powder.

Novel cordierite-acicular mullite composite for diesel particulate filters

Cordierite-acicular mullite composites containing 0, 25, 50, 75 and 100 wt% of mullite were fabricated from waste MoSi 2 and commercial powders of Al 2 O 3 and spinel (MgAl 2 O 4 ). Careful oxidation of pulverised waste MoSi 2 rendered a precursor mixture of MoO 3 and amorphous SiO 2 , which served as pore forming agent and SiO 2 source, respectively. Evaporation of MoO 3 at ∼750 °C allowed production of highly porous cordierite-mullite ceramic composite after sintering in air at 1350 °C for 4 h. The combination of equiaxed cordierite grains and elongated (prism-like) mullite grains, resulted in unique microstructure with open porosity between 53.3 and 55.6 vol% which makes the obtained composite convenient for application as diesel particulate filter material. The presence of mullite affected four key thermo-mechanical properties which determine the thermal shock resistance of cordierite-mullite composite. The best thermal shock resistance was measured in composite containing 75 wt% of mullite. It was a result of improved thermal conductivity (1.081 W/mK) and bending strength (3.62 MPa) and relatively low values of coefficient of thermal expansion (3.8 × 10 −6 K −1 ) and elastic modulus (2.27 GPa).