SiO2 Network Research Articles

3D Printed Optical Quality Silica and Silica–Titania Glasses from Sol–Gel Feedstocks

AbstractA method for fabricating optical quality silica and silica–titania glasses by three‐dimensional (3D) printing is reported. Key to this success is the combination of sol–gel derived silica and silica–titania colloidal feedstocks, direct ink writing (DIW) technology, and conventional glass thermal processing methods. Printable silica and silica–titania sol inks are prepared directly from molecular precursors by a simple one‐pot method, which is optimized to yield viscous, shear‐thinning colloidal suspensions with tuned rheology ideal for DIW. After printing, the parts are dried and sintered under optimized thermal conditions to ensure complete organic removal and uniform densification without crystallization. Characterizations of the 3D‐printed pure silica and silica–titania glasses show that they are equivalent to commercial optical fused silica and silica–titania glasses. More specifically, they exhibit comparable chemical composition, SiO2 network structure, refractive index, dispersion, optical transmission, and coefficient of thermal expansion. 3D‐printed silica and silica–titania glasses also exhibit comparable polished surface roughness and meet refractive index homogeneity standards within range of commercial optical grade glasses. This method establishes 3D printing as a viable tool to create optical glasses with compositional and geometric configurations that are inaccessible by conventional optical fabrication methods.

Interactions of atomic hydrogen with amorphous SiO2

Dozens of models are investigated by the first-principles calculations to simulate the interactions of an atomic hydrogen with a defect-free random network of amorphous SiO2 (a-SiO2) and oxygen vacancies. A wide variety of stable configurations are discovered due to the disorder of a-SiO2, and their structures, charges, magnetic moments, spin densities, and density of states are calculated. The atomic hydrogen interacts with the defect-free a-SiO2 in positively or negatively charged state, and produces the structures absent in crystalline SiO2. It passivates the neutral oxygen vacancies and generates two neutral hydrogenated E′ centers with different Si dangling bond projections. Electron spin resonance parameters, including Fermi contacts, and g-tensors, are calculated for these centers. The atomic hydrogen interacts with the positive oxygen vacancies in dimer configuration, and generate four different positive hydrogenated defects, two of which are puckered like the Eγ′ centers. This research helps to understand the interactions between an atomic hydrogen, and defect-free a-SiO2 and oxygen vacancies, which may generate the hydrogen-complexed defects that play a key role in the degeneration of silicon/silica-based microelectronic devices.

Porphyrin/SiO2/Cp*Rh(bpy)Cl Hybrid Nanoparticles Mimicking Chloroplast with Enhanced Electronic Energy Transfer for Biocatalyzed Artificial Photosynthesis

AbstractA biocatalyzed artificial photosynthesis system (APS) based on porphyrin/SiO2/Cp*Rh(bpy)Cl hybrid nanoparticles (TCPP/SiO2/Rh HNPs) to mimic chloroplasts in green plant is reported. The TCPP/SiO2/Rh HNPs are fabricated via sol–gel reaction of silica precursors functionalized with photosensitizer (porphyrin, TCPP) and electron mediator (Cp*Rh(bpy)Cl, M); while the integration of enzyme and coenzyme nicotinamide adenine dinucleotide (NAD)(H), on the outer surface of the HNPs is achieved through electrostatic‐interaction‐driven assembling under the entanglement of a negatively charged polyelectrolyte. The chloroplast‐mimicking, highly integrated APS exhibits remarkably superior performance over a free system such that the regeneration of NADH is improved from 11% to 75%, and the synthesis of formic acid from CO2 increased from 15 to 100 µmol. Based on the detailed investigations into the photochemical and electrochemical properties, it is speculated that the covalent linking of the photosensitizer and electron mediator via silicon hydride bonds, and the formed SiO2 network through sol–gel reaction, may form intramolecular and intermolecular electron transfer chains to direct more efficient electron transfer from TCPP to M. Such intramolecular and intermolecular electrons and energy transfer, cooperated with the integrated biocatalytic process, lead to the significantly enhanced overall reaction efficiency. Moreover, the integrated APS also allows facile recycling of expensive M, enzymes, and cofactors.

Synthesis and characterization of ZnS@Fe3O4 fluorescent-magnetic bifunctional nanospheres

Herein, we synthesized and characterized fluorescent and super paramagnetic ZnS@Fe3O4 nanospheres. First, (3-mercaptopropyl) trimethoxysilane (MPS) capped ZnS quantum dots (QDs) and SiO2 coated Fe3O4 nanoparticles were synthesized separately by using solution growth and co-precipitation techniques. After synthesis and characterization of these two nanoparticles, they were conglutinated together in a nano sized sphere. The QDs were attached to the surface of the Fe3O4 nanoparticles by SiOSi bonds and so SiOSi bonds created a SiO2 network around the nanoparticles during the formation of the ZnS@Fe3O4 nanospheres. The synthesized MPS capped ZnS fluorescent QDs, SiO2 coated magnetite super paramagnetic nanoparticles and ZnS@Fe3O4 fluorescent-magnetic bifunctional nanospheres were characterized by using UV–Vis Absorption Spectroscopy, Fluorescence Spectroscopy, X-ray analysis, Vibrating Sample Magnetometer analysis, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy, Scanning Electron Microscope and Energy-dispersive X-ray spectroscopy. ZnS@Fe3O4 bifunctional nanospheres were shown to retain the magnetic properties of magnetite, while exhibiting the luminescent optical properties of ZnS nanoparticles. The combination of fluorescent and magnetic behaviors of nano composites make them useful for potential applications in the field of bio-medical and environmental.

Structural transformations in sodium silicate liquids under pressure: New static and dynamic structure analyses

The anomalous behavior displayed by silicate liquid/glass is a long-standing issue in physics, earth sciences, and glass technology. One such well-known unusual behavior is the contrary pressure dependence of silica-rich liquids on transport coefficients in comparison to other liquids. To investigate the origin of this anomalous behavior of silicate liquids, several analyses are newly developed and applied to molecular dynamics simulations of sodium silicate liquids. The cluster analyses performed on an SiO network and simplex sphere reveals obvious structural changes arising as a function of pressure and composition. At lower pressure and/or low silica content, all simplex oxygen spheres containing the modifier cations are connected together, and multiple SiO networks exist in the liquid structure. In contrast, at high pressure and/or high silica content, all SiO4 tetrahedra belong to a single cluster, and the cluster of simplex oxygen spheres containing the modifier cations is split into multiple clusters. The collective function, which is found to decrease with increasing pressure, reveals the pressure dependence of the dynamic structure. This change represents a change in the diffusive motion from large-group diffusion to small-group diffusion for the SiO network. The complexity factor enables us to analyze the changes in the network structure. The decrease in the complexity factor of sodium disilicate liquids with increasing pressure represents an increase in the complexity of the SiO network, mediated by the formation of additional connections with other domains via ring-opening reactions. These analyses reveal the pressure- and composition-dependent changes in the dominant network in silicate liquids. When the network of corner-sharing SiO4 tetrahedra is the dominant species in silicate liquids, the SiO bonds become weaker as a result of the SiOSi angle bending, and the shear viscosity decreases with increasing pressure. In contrast, the transport coefficients show normal pressure dependence when the cation domain is fully connected in silicate liquids.

Origins of a damage-induced green photoluminescence band in fused silica revealed by time-resolved photoluminescence spectroscopy

With combined lifetime and intensity spectra in time-resolved photoluminescence (PL) spectroscopy, the origins of a green PL band consisting of a 2.39eV-band with lifetime ~10μs and a 2.25eV-band with lifetime ~50μs, of high-purity fused silica under ArF excimer laser (6.4eV) excitation were revealed. The 2.39eV PL defects from the surface were annealed due to the deoxidation of the dioxasilyrane group ( = SiO2). The 2.25eV PL defects from bulk showed a UV induced growth. Mechanical and laser damage induced growth of the 2.25eV PL band confirmed that it was due to physical disorder from bending of Si-O-Si bonds. Theoretical calculations further assigned the 2.25eV PL band to silanone groups ( = Si = O), which can be created by relaxation process of strained Si-O bonds in SiO2 network.

Electronic structures and physical properties of Na2O doped silicate glass

Ab initio molecular dynamics has been applied to construct seven sodium silicate glass models with Na2O concentration ranging from 0 to 50 mol. %. The structures of the simulated (Na2O)x(SiO2)1-x glasses are critically analyzed and validated by comparing with available experimental data. Because the initial seed model is based on a near-perfect continuous random network model for amorphous SiO2 with periodic boundaries, the structures of these silicate glasses are highly reliable. The electronic structure, interatomic bonding, and the mechanical and optical properties of seven models are calculated using the first-principles density functional method. In particular, a single quantum mechanical metric, the total bond order density (TBOD), is used to characterize the internal cohesion of sodium silicate glass. This is a significant step beyond the traditional analysis of glasses based purely on the geometric parameters. The TBOD value is found to decrease with increasing Na content, indicating the destruction of silica network connectivity. The calculated mass density and refractive index increase with x are in good agreement with experiment. The elastic coefficients and bulk mechanical properties exhibit a nonlinear variation in the series and depend greatly on the internal bonding and cohesion of the glass. The calculated Poisson's ratio indicates that the glass becomes more ductile with the addition of Na2O. Our results indicate that sodium silicate glass tends to be unstable for x greater than 0.4 due to the total destruction of the SiO2 network.

Nanolayer-transfer method of TiO2 slot layers and its application for fabricating hybrid electro-optic polymer/TiO2 vertical slot waveguide modulators

We have presented a alternative soft process named nanolayer transfer (NLT), to fabricate hybrid electro-optic polymer/TiO2 vertical slot waveguide modulators. We have transfered successfully a 100nm thick TiO2 line pattern layer on sol-gel silica cladding layer, and at the same time we have created a line waveguide in sol-gel silica cladding layer with the widths up to 1μm. The sol-gel silica layer is hydrolyzed and cross-linked, creating a SiO2 network with a highly oriented polycrystalline film of TiO2 in which all crystals grow in the main preferential direction. NLT method also enables to control the high refractive index with better crystallization and lower optical propagation loss of transferred TiO2 slot layer, which allows us to have a higher optical mode confinement of EO polymer ensuring the feasibility of the EO device modulators.

Unexpected "Hammerlike Liquid" to Pulverize Silica Powders to Stable Sols and Its Application in the Preparation of Sub-10 nm SiO2 Hybrid Nanoparticles with Chirality.

Silane coupling agents are well-known as surface modifiers for various kinds of silica (SiO2). However, in the present research, it has been found that they can also work as “hammerlike liquid” to pulverize different kinds of bulk amorphous SiO2 in aqueous systems. This new function was typically clarified by using 3-aminopropyltrimethoxysilane (APS) and bundles of chiral SiO2 nanofibers (with average diameter of ∼10 nm) as raw materials. By a simple reflux of the mixture of SiO2 nanofibers and excessive APS in pure H2O, the solid-containing mixture turned into a completely clear solution that contained sub-10 nm, amine-modified, and water-soluble hybrid SiO2 sols (HS-sols). Moreover, this solution showed blue luminescence under ultraviolet irradiation. Furthermore, the circular dichroism and vibrational circular dichroism spectra revealed that the HS-sols are optically active even though the pristine chiral SiO2 nanofibers were completely destroyed. It was considered that the chirality of SiO2 nanofibers was due to the asymmetric arrangement of Si and O atoms in chiral domains (<10 nm) on the Si–O–Si network of SiO2, and these domains are still preserved in chiral HS-sols. This green method has high potential for the recycling of rich SiO2 sources to obtain functional SiO2 nanomaterials with applications such as optical display, imaging, and chiral recognition. Also, it offers a tool for the analysis of the structural properties of SiO2 on the molecular scale.

Fabrication and optical properties of the Si honeycomb structure via colloidal monolayer template

We use solution immersion method to prepare the SiO2 network structure. A single-layer PS sphere template was immersed in SiO2 precursor solution for a certain period of time. Filter paper was used to absorb the residual liquid, and then the PS nanospheres were removed by high temperature-annealing to form SiO2 network structure. The Si honeycomb structure was formed on the SiO2 network structure by chemical vapor deposition method. The longer growth time caused the smaller sizes of the Si apertures and produced various quantum size effects. When the growth time is 15 min, the thermal conductivity of the sample is the lowest. The heat conduction behavior of the 15 min sample is similar to that of a phonon crystal due to the aperture size of the sample is almost the same as the mean free path of phonons. Scattering at the interface of the isolated honeycomb microstructures also contributed to the low thermal conductivity. The position and number of the reflection peak of the sample can be controlled by the deposition time. The luminescence properties of the samples are greatly influenced by the surface topography.

Structural, thermal properties and chemical durability of aluminosilicate glasses prepared by Bayer red mud

Two series of glass which are based on the particularity of iron oxide in the Bayer red mud, dolomite, pyrophyllite, quartz powder, fluorite, and industrial Al2O3 were prepared. The structure, thermal properties, and chemical durability of the prepared glasses were investigated. X-ray diffraction patterns showed no peaks of crystallization. Result of Fourier transform infrared shows that the existence of ~992 cm−1 in SF series can be assigned to mixed anion structure of [TO4] (T = Si, Al) tetrahedra. For the AF series samples, the Si-O network gradually depolymerized as the decrease of Fe2O3 when the content of SiO2 remained unchanged. Differential scanning calorimetry analysis shows that, as the content of Si/Fe and Al/Fe increased, the glass-transition temperature of glass approximately showed an increase tendency. From the chemical durability experiment results, the presence of TiO2, Fe2O3, etc. in the Bayer red mud improved the resistance to chemical corrosion (especially in acid and alkali resistance) of the glasses, bringing about corrosion resistance close or be superior to that of E-glass and AR-glass, which can be applied in the preparation of aluminosilicate-based glass fiber.

Facile synthesis and characterization of novel rapid-setting spherical sub-micron bioactive glasses cements and their biocompatibility in vitro

Dental pulp vitality is extremely important for the tooth viability, since it provides nutrition and forms the dentin. Bioactive glasses (BGs) may be promising materials for pulp repair due to their excellent abilities of rapidly bonding to bone and stimulating new bone growth. However, the unsatisfied handling property, low plasticity, and poor rapid-setting property of traditional BGs limit its application in vital pulp therapy. Spherical bioactive glasses (SBGs) exhibited higher osteogenesis and odontogenic differentiation than irregular BGs. This study focuses on the application of SBGs with rapid setting property for dental pulp repair. Here, SBGs with various compositions were successfully synthesized by a sol-gel process using dodecylamine (DDA) served as both a catalyst and a template. The maximum content of CaO in SBGs was about 15%. The non-bridge oxygen amounts of the SiO network and the apatite-forming ability increased with the content proportion of CaO and P2O5. Bioactive glass pulp capping materials (BGPCMs) were prepared by mixing the SBGs powders and the phosphate buffer solution (PBS). The K3CaH(PO4)2 and hydroxyapatite (HA) formed between SBGs particles as soon as they were mixed with PBS solution. The compressive strengths of fully set BCPCM-2 molded were measured to be 31.76±1.9577MPa after setting for 24h. The K3CaH(PO4)2 and the low crystallinity HA phases at the initial stage of solidification transformed to crystalline HA for 3days, and the compressive strength was still higher than 10MPa. Additionally, SBG-2 with a designed molar composition of 35% SiO2, 55% CaO and 10% P2O5 more promoted dental pulp cell proliferation, and could be potential pulp capping applications.

Palladium-Silica Organic-Inorganic Membrane Materials via a Sol-Gel Process: Preparation and Properties Calcined under H&lt;sub&gt;2&lt;/sub&gt; Atmosphere

Pd-doped SiO2 organic-inorganic membrane materials were prepared via a sol-gel process. The sol stability is characterized by sol particle size and zeta potential analysis. And the properties of the Pd-doped SiO2 organic-inorganic membrane materials are investigated by X-ray diffraction (XRD), fourier transform infrared (FTIR), energy dispersive spectrum (EDS) analysis and N2 absorption-adsorption measurement. The results showed that the sol particle size distributions are relatively narrow. With the increase of Pd content, the sol particle sizes slightly increase and the zeta potentials decreases until the palladium/tetraethylorthosilicate molar ratio closes to the isoelectric point. After calcined at 350 °C under H2 atmosphere, the added PdCl2 is reduced to metallic Pd0 completely, and the hydrophobic Si−CH3 groups exist in the SiO2 network. The Pd atoms have a lower concentration on the surface and more Pd atoms embedded in the SiO2 matrix. The Pd-doped SiO2 membrane materials calcined at 350 °C with a palladium/tetraethylorthosilicate molar ratio of 0.15 exhibit large BET surface area (up to 683.36 m2/g) and total pore volume (up to 0.386 cm3/g), as well as a mean pore diameter of about 2.26 nm.

A DFT-Based Aspherical Ion Model for Sodium Aluminosilicate Glasses and Melts

Na + ions play important roles on the physical and chemical properties of aluminosilicate glasses. It is known that they strongly modify the network of tetahedral SiO 4 and AlO 4 units, but the microscopic details on how they alter the local structure remain to be fully established. Here we address this issue by performing classical molecular dynamics simulations over a wide range of glasses compositions. The simulations include atomic polarization and deformation effects through the use of a DFT-parametrized aspherical ion model (AIM), which is carefully validated against experimental data (bond lengths, neutron and X-ray diffraction, and NMR spectroscopy). We show that the structure of the glasses is a subtle interplay between the nature of bridging/nonbridging oxides and the arrangement of Na + ions around them. This reflects, in particular, on the oxide instantaneous dipole moments. ■ INTRODUCTION Durability and mechanical properties of oxide glasses at ambient pressure have attracted much attention from the perspective of material chemistry. 1−11 In particular, in the case of aluminosilicates, the mechanical strength can be further improved through the ion-exchanging procedure on the glass surface, such as in the well-known Corning Gorilla glass. 12,13 Since aluminosilicates in the form of polycrystalline ceramics also show excellent optical property thanks to structural disorder, they are practically used as transparent materials in electrical devices. 14 The dense packing structure of oxide glasses also enables to seal radioactive ions for a long period. 15−18 A key component of functional glasses are alkali and alkaline-earth ions. Because of the weaker character of the bonds they form with oxide ions, in a pure silicate network, they generally create nonbridging oxygens and modify the tetrahedral network topology; they are acting as network modifiers. However, when coexisting with Al 3+ or B 3+ , they can also play the role of charge compensators and assist the formation of four-coordinated cations (tetrahedral AlO 4 or BO 4 units) and bridging oxygens. 11 As a consequence, they increase the packing fraction of the network structure. Due to the difference of strengths between the Si−O/Al−O/B−O bonds on the one hand and the Na−O bonds on the other hand, these oxide materials are often called iono-covalent glasses, even if most of the interactions are very well captured by purely ionic models. 19 Recently, structural and mechanical properties of several aluminosilicate glasses have been extensively investigated using experimental techniques 4,5,8,20,21 and computer simulations. 7,13,22 One of the main remaining challenges is to fully understand the effect of network modifiers on the glass properties. Thereby, a precise atomistic understanding of the nature of ionic bonding in aluminosilicate glasses is important both for fundamental studies and because of their widespread usage in material design. The most intuitive approaches for the experimental determination of atomic structures are X-ray and neutron diffraction, together with EXAFS measurements. 23−37 They are indeed useful techniques for obtaining first coordination shell bond lengths and numbers, although it remains difficult to extract information on the second coordination shell. For such a task, NMR spectroscopy has proven to be a very powerful tool that gives access to the short-and intermediate-range

Influence of SiO2 reduction on the local structural order and fluidity of molten coke ash in the high temperature zone of a blast furnace. A molecular dynamics simulation investigation

An in-depth understanding about the transformation of molten coke ash is significant to uncover the mechanism of coke reaction at high temperature inside a blast furnace. The evolution of local structural order and fluidity of molten coke ash (SiO2-Al2O3-CaO) with the reduction of SiO2 at a fixed Al2O3/CaO ratio was investigated by means of molecular dynamics (MD) simulation. While [SiO4] tetrahedral was the main structural units forming Si-O network, due to the high levels of Al2O3 in coke minerals [AlO4] and a small amount of [AlO5] were the two key structural units forming the Al-O network. The effect of chemical composition on the bond length of Si-O, Al-O and Ca-O is very weak, while the coordination number of Al-O can be slightly influenced by the reduction of SiO2 due to the unstable structure of Al-centered polyhedron compared with [SiO4]. The concentration of bridging oxygen decreases, while that of tricluster oxygen increases with the reduction of SiO2 from the system. Al-centered polyhedrons prefer to be connected by O triclusters, while Si-centered polyhedron favor a link by bridging oxygen. With the decrease of SiO2, more [AlO4] will change from edge-sharing to corner-sharing with similar units. The angular distribution of Si-O-Si exhibits an asymmetric shape with the average value around 151°, while two peaks were observed in the Al-O-Al angular distribution because of the edge-sharing (∼90°) and corner-sharing (∼122°) features in Al-centered polyhedrons. With the reduction of SiO2 from the system, the diffusivity of all atoms increase considerably since the amount of strong Si-O bond decreases. Viscosity estimated from MD simulation decrease with the reduction of SiO2. The good agreement between MD simulated data and experimental data indicates that MD simulation can be adopted to estimate the property change of molten coke ash at high temperature.

Crystallization of ZrSiO4 from a SiO2–ZrO2 Binary System: The Concomitant Effects of Heat Treatment Temperature and TiO2 Additions

The concomitant effects of adding varied amounts of TiO2 and temperature of heat treatment on the crystallization of ZrSiO4 from SiO2–ZrO2 binary systems were investigated. The results showed that the t-ZrO2 phase is stabilized in the amorphous SiO2 network, and the simultaneous occurrence of SiO2 crystallization along with the tetragonal zirconia (t-ZrO2) → monoclinic zirconia (m-ZrO2) phase transition tends to activate the reaction between m-ZrO2 and SiO2 to yield ZrSiO4 at elevated temperatures. The formation of metastable ZrTiO4 is also witnessed in the intermediate temperatures that were dependent on TiO2 content, and its dissociation into their individual ZrO2 and TiO2 oxides resulted in ZrSiO4 formation. The Ti4+ occupancy at the Zr4+ lattice sites ensured enhanced crystallization of ZrSiO4, and the limit of Ti4+ occupancy is determined as 9%. Excess TiO2 discarded from the ZrSiO4 lattice gets crystallized into rutile TiO2 (r-TiO2).

Structural and viscosity properties of CaO-SiO2-Al2O3-FeO slags based on molecular dynamic simulation

Since CaO-SiO2-Al2O3-FeO is one of the most important slag systems in metallurgic processes, it is important to explore its microstructural characteristics and viscosity. Molecular dynamics (MD) simulation was carried out to investigate the effect of basicity (R=CaO/SiO2 mole ratio) on both the structural and viscosity properties of CaO-SiO2-Al2O3-FeO slags. The viscosities were estimated based on the atomic self-diffusion coefficients obtained from MD simulations, and were compared with mathematical models and experimental data. The results showed that both Si-O and Al-O network depolymerized into simple structure with increasing basicity, while atomic self-diffusion coefficients increased and viscosities decreased. The increase in basicity has a lower influence on the structure of Al2O3 rich samples. In addition to the basicity effect, the CaO-SiO2-Al2O3-FeO slag with high Al2O3 content trended to form more complex structures. Finally, a correlation between structural properties and viscosity of CaO-SiO2-Al2O3-FeO slags was established.

Molecular dynamics simulation of thermodynamic and structural properties of silicate glass: Effect of the alkali oxide modifiers

Molecular dynamics simulation was applied to elucidate the effect of adding alkali oxides (M2O)X(SiO2)(1−X)with M=(Na, Li or K) into silicate glass matrix. We are interested in the study of this effect particularly on structural and thermodynamic properties of the material. Some interesting results were obtained given a new insight on the bridging process and its reliability to the observed depolymerization phenomena affecting the existing SiO network and depending on both the kind of the alkali modifier and its molar fraction. We observed that the thermodynamic properties are influenced by these structural modifications. Indeed, the glass transition temperature Tg has been found to decrease as the molar fraction of modifier increases depending strongly on the alkali modifier kind. On the other hand, we extracted the fictive temperature from the calculated total energy of the system and determined the glass transition by studying the variation of the fictive temperature as a function of the conventional one using different cooling and heating rates.

First-Principle Calculations on Structure, Thermodynamics and Electrochemistry of Si-O-C Electrodes

Siliconoxycarbides (SiOC) have emerged as promissing electrode material for Li-ion batteries, showing capacities of up to 800 mAh/g and reasonable cycling stability. SiOCs are amorphous networks that are typically obtained by pyrolysis of various precursors. Depending on the choice of the precursor and the pyrolysis temperature, SiOCs with different chemical compositions and thus different morphologies, including an additional free carbon phase, can be obtained. Moreover, Si or Sn nanoparticles can be embedded into SiOC networks, either by mixing or by modification of the precursor. In this way, a further increase of Li capacities has been demonstrated. Despite numerous experimental investigations our understanding of atomic structure and electrochemical behaviour of SiOCs is still limited. In this contribution, density functional theory (DFT) calculations are used to model lithiation of SiOC from a an atomistic and electronic perspective. We first address the challenge of designing realistic structure models. For pristine SiOC, the lack of the existence of a crystalline phase prevents straightforward structure-generation using a cook-and-quench approach. In recent literature, SiOC models that were obtained by replacing O by C in amorphous SiO2 networks have been presented. However, such models suffer from the fact that O is only 2-fold coordinated in SiO2, while C prefers a 4-fold coordination in the vicinity of Si. Therefore, the carbon in such SiOC models is expected to be more reactive than in reality. We present a strategy that results in model structures that consist of Si and C that are mainly 4-fold coordinated while O is 2-fold coordinated. In addition, a free carbon phase is included in our models. After structural optimization and thermodynamic characterization, we study Li insertion. In particular we present information about Li storage sites, energetic evolution (including voltage profiles), structural evolution (including specific volume changes) and elastic properties. We compare our results to experiment and previous modelling studies.

Comparative studies on the physical properties of TEOS, TMOS and Na2SiO3 based silica aerogels by ambient pressure drying method

In order to compare the various precursors of silica aerogels, three different precursors namely TMOS, TEOS and Na2SiO3 were studied in this paper. The property differences of the aerogels caused by the three precursors were discussed in terms of reaction process, gelation time, pore size distributions, thermal conductivity, SEM, hydrophobicity and thermal stability. It has been found that the gelation time of the silica gel is strongly dependent on the type of precursor used. During the surface modification process, organic groups were attached to the wet gel skeletons transforming the hydrophilic to the hydrophobic which were characterized by Fourier Transform Infrared spectroscopy (FTIR). It has been found that the contact angle of the Na2SiO3 and TMOS precursor based aerogels with water have the higher contact angle of 149° and whereas Na2SiO3 precursor based aerogel has the lower contact angle of 130°. The thermal conductivities of the Na2SiO3 and TMOS based aerogels have been found to be lower (0.025 and 0.030 W m−1 K−1, respectively) compared to the TEOS based (0.050 W m−1 K−1) aerogels. The pore sizes obtained from the N2 adsorption measurements varied from 40 to 180, 70 to 190, and 90 to 200 nm for the TEOS, TMOS and Na2SiO3 precursor based aerogels, respectively. The scanning electron microscopy studies of the aerogels indicated that the Na2SiO3 and TMOS based aerogels show narrow and uniform pores while the particles of SiO2 network are very small. On the other hand, TEOS aerogel show non-uniform pores such that the numbers of smaller size pores are less compared to the pores of larger size while the SiO2 particles of the network are larger as compared to both Na2SiO3 and TMOS aerogels. Hence, the surface are of the aerogels prepared using TEOS precursor has been found to be the lowest (~620 m2 g−1) compared to the Na2SiO3 (~868 m2 g−1) and TMOS (~764 m2 g−1) aerogels.