Glassy SiO2 Research Articles

An inlaid interface of carbon/carbon composites to enhance the thermal shock resistance of SiC coating in combustion environment

Abstract To relieve the mismatch of thermal expansion coefficient and enhance the service reliability of SiC coating efficiently, blasting treatment of C/C composites conducted by oxyacetylene torch, as a surface modification method, was used. SiC coating was prepared on the treated C/C composites by pack cementation, and an inlaid interface with different structures between SiC coating and C/C substrate was achieved. High temperature test was conducted under oxyacetylene torch, and the oxyacetylene flame was parallel to the as-received SiC coated C/C composites. After 5 thermal cycles from 1750 °C to room temperature, a glassy SiO2 protective layer without macro-crack was formed on the surface of the coated C/C composites with pre-blasting treatment in the heat flux of 3.2 MW/m2. The good thermal cycle performance was attributed to the inlaid interface, which could relieve the thermal stress, improve the bonding strength and alleviate the mismatch of thermal expansion coefficient between C/C substrate and SiC coating efficiently.

Oxygen 2p Partial Density of States and Bond Angles around O Atoms in SiO2 Glass

The valence- and conduction-band O 2p partial densities of states (DOSs) in a typical oxide glass, SiO2, and the corresponding α-quartz crystal were investigated by soft X-ray emission spectroscopy and soft X-ray absorption spectroscopy, respectively. Both of the obtained spectra for glassy and crystal SiO2 surprisingly resemble each other, indicating closely similar local environments around the O atoms, in particular suggesting an average Si–O–Si bond angle of ∼150°. Ab initio molecular dynamics (MD) simulations were carried out, and the above experimental results were confirmed from the bond angle dependence of the O 2p partial DOS obtained from the ab initio MD simulation.

Ablation behavior of boron nitride based ceramic composites reinforced by continuous silicon oxynitride fiber

Continuous silicon oxynitride fiber reinforced boron nitride matrix (Si–N–Of/BN) composite has been recognized as a promising microwave transparent radome material. In the present study, the ablation behavior and properties of the Si–N–Of/BN composites under an oxyacetylene torch ablation environment were investigated. The average linear and mass ablation rates of the composites decreased with the increasing of the fabrication temperature and were 0.132mms−1 and 0.057gs−1 for the 1200°C fabricated composites. The ablation products mainly consisted of glassy SiO2 and B2O3, and highly crystallized BN. The ablation center involved three different actions: decomposition of the Si–N–O fibers and sublimation of the BN matrix, oxidations of the matrix and the reinforcing fibers, and thermal treatment of the BN matrix beneath the covering SiO2 film.

Crack-healing ability and strength recovery of different hot-pressed TZ3Y20A–SiC ceramics by heat treatment in air

Abstract Crack-healing ability and strength recovery of different ZrO2(Y2O3)–Al2O3–SiC (TZ3Y20A–SiC) ceramics have been investigated by tailoring heat treatment temperature, heat treatment time, SiC content and crack size. Heat treatment in air enhances distinctly the flexural strength of TZ3Y20A–SiC ceramics. A complete crack-healing of TZ3Y20A–10 vol%SiC ceramic can be realized by optional heat treatments at 1073 K for 30 h, 1273 K for 10 h or 1373 K for 5 h, respectively. With increasing the content of nanosize SiC particles from 10 to 20 vol%, the crack-healing process of TZ3Y20A–SiC ceramics speeds up at identical temperature levels. The pre-existing cracks with a width up to 0.45 μm are healed completely under the optimum heat treatment conditions, and the flexural strengths of crack-healed TZ3Y20A–SiC ceramics are completely recovered to the value of smooth specimens. Crack closure and rebonding of crack wall due to the formation of SiO2 glassy phase by oxidation reaction between SiC and the oxidizing atmosphere are dominant healing mechanisms in TZ3Y20A–SiC ceramics.

Effect of SiC-mullite coatings on oxidation resistance of graphite

Two layers oxidation protecting coatings of SiC and mullite were successfully created on the graphite substrate by pack cementation and plasma spray methods, respectively. Phase synthesis was studied by X-ray diffraction. Microstructure and morphology were investigated by optical and scanning electron microscopes (SEM). X-ray diffraction results of the first coating showed that the secondary SiC was completely synthesised by the heat treatment of Si, Al2O3, C and SiC at 1500°C. Alumina and kaolin reaction at 1400°C led to the formation of mullite as the second layer. The oxidation resistance of two layers coating was considerably improved in comparison with mono layer coating and raw graphite. Oxidation resistance was decreased at higher temperatures in all samples. Formation of SiO2 glassy phase improved the graphite oxidation resistance which was confirmed by SEM.

Vibrational signatures of chemical- and density-induced structural changes in simulated amorphous silica

Molecular dynamics simulations show that vibrational modes in glassy SiO2 are affected differently by density-induced changes to the structural order compared with those induced by specific cationic substitutions (Al, P, Na). Using standard measures of local and midrange positional order, we find that P disrupts network order in terms of second nearest neighbours, but preserves local order. In contrast, Al decreases local order while maintaining network order. Increased density preserves structural correlations in the network although the length scale shrinks. The complex short-range and long-range structural trends in these modified glasses coincide with changes in distinct regions in the vibrational density of states (VDOS). This suggests that a greater role for VDOS as a tool to link simulation and experiment in amorphous materials.

Effect of iron on the dielectric properties of silicate glasses prepared by sol‐gel

AbstractThe sol‐gel method is a very flexible route for producing sieving matrix such as SiO2 with pore radii in nanoscale region to embed oxide particles like Fe2O3. In this work, iron oxide nanoparticles embedded in a SiO2 glassy matrix prepared by the sol‐gel method have been investigated as a function of Fe/Si ratio. Different composition of dried gels with 1, 2, 10 and 20 mol% of Fe2O3 in the SiO2 matrix were heat‐treated (HT) in air, at temperatures from 400 ºC up to 1400 °C. The structure and morphology of the obtained glasses and glass‐ceramics were studied by X‐ray powder diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). Impedance spectroscopy measurements and magnetic characterization were performed as function of temperature and the results are related with the microstructure. The X‐ray diffraction patterns showed that increasing the amount of Fe2O3 oxide promotes the formation of crystalline phases, in particular silicate phases. The increase of the heat‐treatment temperature has a similar behavior. The electrical and dielectric properties were related with these structural modifications and with the sample morphology. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improving high-rate performance of mesoporous Li2FeSiO4/Fe7SiO10/C nanocomposite cathode with a mixed valence Fe7SiO10 nanocrystal

Li2FeSiO4 is considered to be one of the most promising cathode materials for lithium-ion batteries due to its theoretical specific capacity as high as 330 mA h g−1, resource abundance, low cost, and environmental benignity. However, Li2FeSiO4 possesses a poor conductivity, and co-exists with a small amount of an insulating glassy SiO2 impurity, leading to severe capacity decay at high rates. Herein, we report a mesoporous Li2FeSiO4/Fe7SiO10 hetero-nanocrystal/C nanocomposite as a cathode material for high power lithium-ion batteries. By controlling the crystallization and composition of the materials, a smaller amount of mixed valence Fe7SiO10 nanocrystals intergrow with Li2FeSiO4 nanocrystals instead of glassy SiO2, thus minimizing the insulating SiO2 in the Li2FeSiO2 cathode. The conductive hybrid framework consisting of a three-dimensional carbon network and conductive Fe7SiO10 nanocrystals offers a continuous electron transport network in the material, and sponge-like mesoporosity allows efficient penetration, transport and storage of an electrolyte within a cathode. By intergrowing Li2FeSiO4/Fe7SiO10 hetero-nanocrystals, a remarkably improved high-rate performance is achieved with a reversible discharge capacity of 112 mA h g−1 over 80 cycles at a current rate of 10 C. Our finding represents a new approach towards designing conductive nanostructures of insulating oxides for high-power lithium-ion batteries on a mesoscopic scale.

Formation and oxidation resistance of germanium modified silicide coating on Nb based in situ composites

Ge-modified silicide coating was prepared on the Nb–Si based alloy by Si–Ge co-deposition at 1300°C for 10h. The pack powder for co-deposition consisted of 16Si–8Ge–5NaF–71Al2O3 (wt.%). It is observed that the coating is composed of an outer layer and a transitional layer. The mass gain of substrate and Ge-modified silicide coating are 190.08 and 5.42mgcm−2 after oxidation at 1250°C for 100h, respectively. The coating can prevent substrate from oxidizing efficiently. The good oxidation resistance is attributed to the formation of glassy GeO2 and SiO2.

Effects of ZrB2 and SiC dual addition on the oxidation resistance of graphite at 1600–2000°C

Abstract The effects of ZrB2 and ZrB2 + SiC additions on the oxidation kinetics of graphite at 1600–2000 °C in air were investigated. The ZrB2 + SiC dual addition improves the oxidation resistance of graphite more effectively than the ZrB2 single addition, because the oxide scale formed on C–ZrB2–SiC is denser and thinner due to the existence of glassy SiO2. As the oxidation temperature increases, the oxidation rate of C–ZrB2–SiC gradually increases and oxide scales with layered microstructures form on its surface due to the greatly enhanced active oxidation of SiC at higher temperatures.

Oxidation behavior of Nb–Si–B alloys with the NbSi2 coating layer formed by a pack cementation technique

The oxidation behavior of Nb–Si–B alloys with NbSi2 coating layers was investigated at 1100°C under ambient atmosphere. Continuous NbSi2 layers with a thickness of 40–60μm were coated by depositing Si on the alloy surfaces at 900°C using a pack cementation technique. A sharp interface was observed between the NbSi2 layer and the pure Nb, while a (Nb5Si3+T2) two-phase region formed between the NbSi2 layer and the alloy with the T2 phase composition. During oxidation, a glassy SiO2 layer formed significantly increasing the oxidation resistance of the coated alloys. However, evaporation of B occurred forming a porous SiO2 layer during the oxidation of the alloys with high B content. The oxidation behavior of the coated pure Nb and T2 phase alloys is discussed based on a microstructural analysis of the oxidized samples.

Microstructure Characteristics and Oxidation Behavior of Molybdenum Disilicide Coatings Prepared by Vacuum Plasma Spraying

In this study, molybdenum disilicide (MoSi2) coatings were fabricated by vacuum plasma spraying technology. Their morphology, composition, and microstructure characteristics were intensively investigated. The oxidation behavior of MoSi2 coatings was also explored. The results show that the MoSi2 coatings are compact with porosity less than 5%. Their microstructure exhibits typical lamellar character and is mainly composed of tetragonal and hexagonal MoSi2 phases. A small amount of tetragonal Mo5Si3 phase is randomly distributed in the MoSi2 matrix. A rapid weight gain is found between 300 and 800 °C. The MoSi2 coatings exhibit excellent oxidation-resistant properties at temperatures between 1300 and 1500 °C, which results from the continuous dense glassy SiO2 film formed on their surface. A thick layer composed of Mo5Si3 is found to be present under the SiO2 film for the MoSi2 coatings treated at 1700 °C, suggesting that the phenomenon of continuous oxidation took place.

Oxygen-excess-related point defects in glassy/amorphous SiO2 and related materials

An insight is given into recent experimental advances in the spectroscopic studies of oxygen-excess intrinsic defects, in glassy SiO2 and α-quartz. By controlling excess oxygen in a-SiO2, and the conditions of F2-laser irradiation, SiO2 glass samples can be obtained with optical absorption almost exclusively dominated by single defect, oxygen dangling bonds (“non-bridging oxygen hole centers” or NBOHCs), without the presence of complementary Si dangling bonds (generic “E′-centers”). This allows for a more accurate determination of the spectral shape of NBOHC optical absorption in UV and vacuum UV spectral regions. The temperature dependence of NBOHC electron paramagnetic resonance (EPR) signal intensity is stronger than predicted by Curie’s law (1/T) even at temperatures at and below 77K. Dangling bonds are characteristic of an amorphous state and do not exist in a regular crystal lattice. However, site-selective luminescence shows that highly ordered NBOHCs exist in particle-irradiated α-quartz, evidently either on the border between the damage tracks and the crystalline phase or as a part of Si vacancies. They are different from the common “glass-like” NBOHCs in a-SiO2 by giving distinct sharp zero-phonon lines with characteristic energies in luminescence spectra instead of a continuous distribution of lines. Two distinct types of luminescent NBOHCs, associated with the long and short Si–O bonds in α-quartz are suggested. EPR data corroborate the presence of oriented NBOHCs in neutron-irradiated α-quartz and confirm distinct NBOHCs at the sites of “long” and “short” Si–O bonds in α-quartz.

Sensitizing effects of ZnO quantum dots on red-emitting Pr3+-doped SiO2 phosphor

In this study, red cathodoluminescence (CL) (λemission=614nm) was observed from Pr3+ ions in a glassy (amorphous) SiO2 host. This emission was enhanced considerably when ZnO quantum dots (QDs) were incorporated in the SiO2:Pr3+ suggesting that the ZnO QDs transferred excitation energy to Pr3+ ions. That is, ZnO QDs acted to sensitize the Pr3+ emission. The sol–gel method was used to prepare ZnO–SiO2:Pr3+ phosphors with different molar ratios of Zn to Si. The effects of the ZnO QDs concentration and the possible mechanisms of energy transfer from ZnO to Pr3+ are discussed. In addition, the electronic states and the chemical composition of the ZnO–SiO2:Pr3+ phosphors were analyzed using X-ray photoelectron spectroscopy (XPS).

Oriented SrFe 12O 19 thin films prepared by chemical solution deposition

Thin films of SrFe 12O 19 (SrM) were prepared from a solution of iron and strontium alkoxides through the chemical solution deposition method on both amorphous (glassy SiO 2), and single crystal substrates (Si(100), Si(111), Ag(111), Al 2O 3(001), MgO(111), MgAl 2O 4(111), SrTiO 3(111)) substrates. The process of crystallization was investigated by means of powder diffraction, atomic force microscopy and scanning electron microscopy. Magnetization measurements, ferromagnetic and nuclear magnetic resonance were used for evaluation of anisotropy in the films. Whilst amorphous substrates enabled growth of randomly oriented SrM phase, use of single crystal substrates resulted in samples with different degree of oriented growth. The most pronounced oriented growth was observed on SrTiO 3(111). A detailed inspection revealed that growth of SrM phase starts through the breakup of initially continuous film into isolated grains with expressive shape anisotropy and hexagonal habit. A continuous film with epitaxial relations to the substrate was produced by repeating recoating and annealing.

Electronic mechanism of thermal destruction of radiation-induced E'-centers in crystalline and glassy SiO 2

The thermal decay regularities for radiation-induced E'-centers in crystalline and glassy SiO 2 were investigated. The results obtained point out that the destruction of E'-centers can be described as ionization process of deep centers in electric field. In terms of used model, the electric field and electron–vibration coupling parameters are sensitive to structural disorder. The most weak electron–phonon coupling in E'-centers is observed for amorphous systems.

Some comments on fluctuating-elasticity and local oscillator models for anomalous vibrational excitations in glasses

An overview is given on the present status of the theoretical description of vibrational spectra of glasses, as seen by inelastic neutron, X-ray and light (Raman) scattering. Using the language of Green's/response functions the merits and shortcomings of a local oscillator and a generalized elasticity-theory point of view are discussed. It is pointed out that in both cases the interaction of phonons with disorder-induced irregularities leads to Rayleigh scattering (mean free path ℓ ∝ ω − 4 ) at low enough frequencies and temperatures. In disordered solids at ambient temperature the Rayleigh scattering is usually masqued by Akhiezer-like anharmonic scattering ℓ ∝ ω − 2 , but it can be made visible by lowering the temperature. Using a combination of fluctuating-elasticity theory with an incoherent spectrum of local oscillators a fair description of the vibrational spectrum of glassy SiO 2 can be achieved.

Cu x Ni1−x alloy nanoparticles embedded SiO2 films: synthesis and structure

Cu–Ni fcc alloy nanoparticles (NPs) of tunable atomic ratios were generated in SiO2 films. The films were prepared using the Cu(NO3)2 and Ni(NO3)2 co-doped inorganic–organic hybrid silica sols by single dipping. Transparent, crack-free, glassy SiO2 films of 310 ± 10 nm in thickness embedded with high mol percent of Cu–Ni alloy NPs were yielded after annealing at 750 °C in 10% H2-90% Ar atmosphere. Nominal compositions of the films were 20 mol% (Cu–Ni)-80 mol% SiO2. Optical spectral study of the heat-treated films showed disappearance of Cu plasmon bands due to Cu–Ni alloy formation. Grazing incidence X-ray diffraction (GIXRD) studies revealed the formation of Cu–Ni alloy (2:1, 1:1 and 1:2) NPs inside the SiO2 film. GIXRD showed a systematic shifting of the diffraction peaks with respect to the fcc Cu–Ni alloy composition, maintaining the nominal ratios. Transmission electron microscopy (TEM) studies of the representative Cu0.5Ni0.5-doped film showed existence of homogeneously dispersed Cu–Ni alloy NPs of average size 6.35 nm inside the SiO2 matrix. The energy dispersive X-ray scattering (EDX) analysis of the individual NPs using the nano-probe (scanning TEM mode) confirmed the presence of both the Cu and Ni with the desired atomic ratio.

MoSi 2-based oxidation protective coatings for SiC-coated carbon/carbon composites prepared by supersonic plasma spraying

To protect carbon/carbon (C/C) composites against oxidation, MoSi 2-based oxidation protective coatings for SiC-coated carbon/carbon composites were prepared on them by supersonic plasma spraying. The MoSi 2-based coatings primarily consist of MoSi 2, Mo 5Si 3 and glassy SiO 2. Only a few pinholes and some microcracks are observed on the surface and no through-thickness cracks penetrate the cross-section. Weight loss of the MoSi 2-based coated specimens is only 1.14% after 400 h oxidation in air at 1773 K and the coated C/C composites remain intact after 11 thermal cycles between 1773 K and room temperature. The outstanding anti-oxidation ability is mainly attributable to the formation of SiO 2-based layer on the surface of MoSi 2-based coatings.

Statics and dynamics of excited states of oxygen-deficient centers in SiO2

The parameters of excited states of oxygen-deficient centers (ODCs) in high-energy-electron irradiated crystalline and glassy SiO2 have been studied using optical absorption, luminescence, and photoelectron emission spectroscopy. Additional evidence has been gained in support of the model of a neutral oxygen vacancy in ODCs, the diagram of electronic transitions has been refined, and their characteristics have been quantified. The possibility of ionization of the singlet and triplet defect states at a transition to the anomalously relaxed configuration has been demonstrated using the particular example of α-ODCs. Nonradiative excitation transfer from nonbridging oxygen centers to the triplet ODC state has been observed.