Silicon oxycarbide glasses and glass‐ceramics: “All‐Rounder” materials for advanced structural and functional applications

Silicon oxycarbides can be considered as being carbon‐containing silicates consisting of glass networks in which oxygen and carbon share bonds with silicon. The carbon‐for‐oxygen substitution in silicate glass networks has been shown to induce significant changes in the network connectivity and consequently strong improvements in the properties of the silicate glass network. For instance, SiOC glasses exhibit Young's moduli, hardness values, glass transition, and crystallization temperatures which are superior to those of vitreous silica. Moreover, the silicon oxycarbide glass network exhibits unique structural features such as reduced mass fractal dimension and nano‐heterogeneity, which significantly affect and/or dictate its properties and behavior. In the present Review, a consideration of the current state of the art concerning the synthesis, processing, and various structural and functional properties of silicon‐oxycarbide‐based glasses and glass‐ceramics is done. Thus, the synthesis of silicon oxycarbides starting from macromolecular precursors such as polysiloxanes or alkoxysilanes‐based sol‐gel systems as well as current advances related to their processing will be critically reviewed. In addition, various structural and functional properties of silicon oxycarbides are presented. Specific emphasis will be put on the intimate correlation between the molecular architecture of the precursors and the structural features and properties of the resulting silicon oxycarbides.     

Structure,energetics and bioactivity of silicon oxycarbide-based amorphous ceramics with highly connected networks

Mg and Ca substituted silicon oxycarbides are investigated with respect to their network architecture, enthalpy of formation and bioactivity. The synthesized materials have highly connected structural network with an open architecture and a minor but critical fraction of depolymerized Q-species. This combination of structural features allows for providing bioactivity at the same time with enhanced thermo-mechanical robustness and crystallization resistance. It is argued that silicon oxycarbide amorphous ceramics may consequently serve as model compounds for developing rational design concepts for advanced bioactive glasses.     

High‐Temperature Creep Behavior of SiOC Glass‐Ceramics: Influence of Network Carbon Versus Segregated Carbon

Three silicon oxycarbide samples with different carbon contents are analyzed in the present study with respect to their high‐temperature creep behavior. The tests were performed in compression at 1100°C, 1200°C, and 1300°C; in this temperature range the mechanism of creep relies on viscoelastic flow within the samples and has been modeled with the Jeffreys viscoelastic model. After the release of the applied mechanical stress, a viscoelastic recovery behavior was observed in all samples. The creep behavior of the investigated samples indicates two rheological contributions in SiOC: (i) a high viscous answer, coming from the silica‐rich network, and (ii) an elastic response from the segregated carbon phase within the samples. Furthermore, two distinct effects of the carbon phase on the HT creep behavior of SiOC were identified and are discussed in the present paper: the effect of the carbon presence within the SiOC network (the “carbidic” carbon), which induces a significant increase in the viscosity and a strong decrease in the activation energy for creep, as compared to vitreous silica; and the influence of the segregated carbon phase (the “free” carbon), which has been shown to affect the viscosity and the activation energy of creep and dominates the creep behavior in phase‐separated silicon oxycarbides.     

Evolution of the sensitized Er3+ emission by silicon nanoclusters and luminescence centers in silicon-rich silica

The structural and optical properties of erbium-doped silicon-rich silica samples containing different Si concentrations are studied. Intense photoluminescence (PL) from luminescence centers (LCs) and silicon nanoclusters (Si NCs), which evolves with annealing temperatures, is obtained. By modulating the silicon concentrations in samples, the main sensitizers of Er³⁺ ions can be tuned from Si NCs to LCs. Optimum Er³⁺ PL, with an enhancement of more than two, is obtained in the samples with a medium Si concentration, where the sensitization from Si NCs and LCs coexists.     

Synthesis and pyrolysis of non-oxide precursors for ZrC/SiC and HfC/SiC composite ceramics

Multiphase ceramics like ZrC/SiC are promising candidates as ultra-high temperature ceramics for applications in extreme environments. In this work, non-oxide precursors for ZrC/SiC and HfC/SiC composite ceramics were synthesized by a one-pot reaction of three components – metal source, silicon source, and activating reagent. Molecular structures of the precursors were identified by ¹H NMR and FTIR. Transformation process of the precursors to the ZrC/SiC ceramics was investigated via XRD and SEM. After heat-treatment at 1600 °C under argon, the obtained ZrC/SiC and HfC/SiC ceramics features a particle size of 100–200 nm and high metal content without excess carbon. The elemental composition of pyrolyzed ceramics can be tuned by varying the ratio of the reagents in the synthesis of precursors. This strategy also inspires a facile fabrication of composite ceramics with other elemental compositions.     

Phase development in metal-dropped silicon oxycarbides under water vapor and argon hybrid atmosphere

In this study, metal-modified silicon oxycarbides ceramics (SiOC/M, M = Fe, Al, and Zr) were fabricated, and the thermal stability of the SiOC/M (M = Fe, Al, and Zr) ceramics was investigated under the water-vapor- argon hybrid atmosphere. The phase and microstructural analysis showed that the thermal stability was in the order of SiOC/Zr ＞ SiOC/Al > SiOC/Fe. The SiOC/Zr and SiOC/Al ceramics possessed better thermal stability than SiOC/Fe ceramics, because they formed metallic oxides (ZrO₂, mullite, or sillimanite). Additionally, the water-vapor-argon hybrid atmosphere increases the defects within the carbon clusters of SiOC/M ceramics and refines the lateral size of the nanocrystalline carbon. Understanding thermal stability and microstructural evolutions of metal-dropped polymer derived ceramics provided a new method for achieving high-temperature ceramics for application in an extreme environment.     

Carbon content and pyrolysis atmosphere effects on phase development in SiOC systems

In this study, bulk silicon oxycarbides (SiOCs) were fabricated from base polysiloxane (PSO) systems with different carbon content by using Ar or Ar + H₂O pyrolysis atmosphere. Compared to the Ar pyrolysis condition, the SiOC samples pyrolyzed with water vapor plus Ar generally show lower ceramic yield except for the Tospearl (polymethylsilsesquioxane) sample at 1400 °C. The SiOC ceramics contain significantly less SiC and carbon after pyrolysis under Ar + H₂O atmosphere compared to pure Ar atmosphere. The carbon-poor Tospearl sample shows a crystalline SiO₂ structure (cristobalite) after pyrolysis at 1400 °C in Ar + H₂O, which is also confirmed using TEM diffraction pattern analysis. TEM microstructures indicate little change in microstructures for the carbon-rich samples. The fundamentals, such as total Gibbs free energy, the driving force for crystallization, and phase contents at different pyrolysis temperatures can be calculated based on a Gibbs free energy minimization method. The phase content calculations predict considerable decrease in the amounts of SiC and C and significant increase in the percent of SiO₂ after pyrolysis in Ar + H₂O compared to Ar. The thermodynamic calculation results match with our experimental observations. This work provides a guided method to synthesize high temperature SiOCs with desired phases.     

High productive machining of C/SiC preceramics

High productive machining of C/SiC preceramics is investigated in relation to the fabrication of complex-shaped reaction-bonded silicon carbide ceramics. Machinability is analyzed at different manufacturing steps of ceramics preparation. Machining after the carbonization step is shown to be the most efficient. Great value of material removal rate of 360 mm³/s is achieved by high productive CNC-milling of carbonized preceramics at a feed rate of 50 mm/s without any defects upon the processed surfaces, edges, and corners. Diamond tool wears approximately .01% (weight loss per mass of material removed) in the process of green CNC-milling is two orders lower compared with the milling of sintered ceramics (2.8%). Specifics of surface processing are investigated depending on carbon content in preceramics. The increase of bonding carbon from 8 to 16 vol.% decelerates loose abrasive grinding three times, improves the accuracy of surface leveling, and leads to a change of fracture mechanism. The obtained results can be helpful for the advantageous manufacture of complex-shaped silicon carbide ceramics.     

Single source precursor-derived SiOC/TiOxCy as an anode component for Li-ion batteries

Amorphous silicon oxycarbides are known to be an effective anode material for lithium-ion batteries. Despite their exceptional properties and high charge capacities, however, their practical uses are limited by their significant first-cycle loss, considerable hysteresis, and low cyclic ability. Comparatively, SiOC/metal oxide materials have demonstrated increased rate capability and cyclic stability. This study utilized a liquid precursor-derived ceramic method to modify SiOC with titanium (IV) butoxide precursor to synthesize SiOC/TiO_xC_y. X-ray diffractograms confirmed the amorphous nature of SiOC/TiO_xC_y. The elemental composition and bonding properties were investigated using X-ray photoelectron spectroscopy, and electron microscopy was used to explore morphological features. In the first cycle, the reversible capacity of pyrolyzed SiOC/TiO_xC_y was 520 mAh g⁻¹, which then increased to 736 mAh g⁻¹ for the 1200°C annealed SiOC/TiO_xC_y due to the increased free carbon network and TiC conductive phases. The irreversible capacity of the first cycle was 568 mAh g⁻¹, which was lower than the annealed SiOC irreversible capacity of 695 mAh g⁻¹. Interestingly, the rate stability of the pyrolyzed SiOC/TiO_xC_y performed more stability than the annealed sample. Localized carbothermal reactions between amorphous SiOC/TiO_xC_y and free carbon at annealing temperatures resulted in loss of structure stability.     

Evaluation of citric acid added cleaning solution for removal of metallic contaminants on Si wafer surface

We have investigated cleaning solutions based on citric acid (CA) to remove metallic contaminants from the silicon wafer surface. Silicon wafers were intentionally contaminated with Fe, Ca, Zn, Na, Al and Cu standard solution by spin coating method and cleaned in various CA-added cleaning solutions. The concentration of metallic contaminants on the silicon wafer surface before and after cleaning was analyzed by vapor phase decomposition/inductively coupled plasma-mass spectrometry (VPD/ICP-MS). And the surface micro-roughness was also measured by atomic force microscopy (AFM) to evaluate the effect of cleaning solutions. It was found that acidic CA/H₂O solution has the ability to remove metallic contaminants from silicon surfaces. Fe, Ca, Zn and Na on silicon surface were decreased from the order of 10¹² atoms/cm² to the order of 10⁹ atoms/cm² even at low CA concentration, low temperature of CA solution and with short immersion time. CA was also effective in alkali cleaning solution. Fe, Ca, Zn, Na and Cu were reduced down to the order of 10⁹ atoms/cm² in CA added with NH₄OH/H₂O₂/H₂O solution without degradation of surface micro-roughness.     

X-ray photoelectron spectroscopy of aluminium-substituted tobermorite

We have synthesised 11-Å tobermorite hydrothermally, both pure and with increasing isomorphic substitution of aluminium for silicon. The samples were analysed by X-ray photoelectron spectroscopy (XPS). Aluminium was found, on the basis of its Al 2p binding energies, to be tetrahedrally coordinated. We observed no changes in Ca/(Si+Al) ratio upon aluminium substitution, implying that charge balancing does not occur via the incorporation of additional calcium into the tobermorite structure. Aluminium substitution into the silicate structure led to a decrease in Si 2p binding energies. This implies one of two alternatives. Firstly, that charge balancing occurs via substitution of OH⁻ for O²⁻ in the tobermorite structure. Secondly, the presence of aluminium in the tobermorite structure may negatively influence the degree of silicate polymerisation. Further work is required to determine which of these possibilities is the case.     

The influence of carbon atom distribution in precursors with 1:1 carbon to silicon atoms ratio on structure and thermal evolution of obtained silicon oxycarbide based materials

In this work, a number of precursors with 1:1 silicon to carbon atoms ratio and various carbon atom distributions were synthesized and pyrolyzed in order to obtain silicon oxycarbide based materials. The different carbon atom distributions were obtained using both simple monomers with only one silicon atom, as well as large monomers containing either four or sixteen silicon atoms with predefined carbon atom positions. The silicon oxycarbide based materials were investigated using IR, XRD, ²⁹Si MAS NMR and elemental analysis after annealing at various temperatures, as well as TG. The research shows that carbon atom distribution has great impact on the structure of final material and can be used to tailor the material for its projected uses.     

Comparison of traditional and flash pyrolysis of different carbon content silicon oxycarbides

This study is to understand the effect of carbon content on the pyrolysis behaviors and phase contents of silicon oxycarbides (SiOCs). Flash pyrolysis conditions, evolution of different SiOC phases, and free carbon types/amounts are compared for C-rich and less C-rich precursors. The C-rich system experiences the flash event at a much lower pyrolysis temperature with a much higher current density even though the internal temperatures at flash are very similar. SiC formation is more obvious for the C-rich samples along with a much higher carbon content under both flash and traditional pyrolysis conditions. The phase contents of SiO₂, SiC, and other SiOC intermediates can be calculated using a Gibbs energy minimization method, showing that the C-rich sample has more C-rich SiOC intermediates while the less C-rich sample has more Si-rich intermediates. This research provides a general framework in assessing the pyrolysis behaviors of different SiOC materials.     

Construction of novel silicon‐phosphorus linear polymers with DDSQ and DOPO derivatives for effective flame retardancy of PC/ABS

Two kinds of novel silicon‐phosphorus linear polymers were synthesized by the hydrosilylation reaction of double‐decker‐shaped silsesquioxane (DDSQ) and 9,10‐dihydro‐9‐oxa‐10‐ phosphaphenanthrene‐10‐oxide (DOPO) derivatives and characterized using ¹H NMR, ³¹P NMR, and ²⁹Si NMR. Flame‐retardant polycarbonate/acrylonitric‐butadiene‐styrene (PC/ABS) blends were prepared with different contents of silicon‐phosphorus linear polymers. The flame‐retardant properties of silicon‐phosphorus linear polymers as well as the morphology were investigated in detail by using thermogravimetric analysis (TGA), limiting oxygen index (LOI), (Underwriters Laboratory) UL‐94, microscale combustion calorimetry (MCC), and scanning electron microscopy (SEM), respectively. The silicon‐phosphorus linear polymers containing DDSQ and DOPO units can synergistically improve the flame retardancy and thermal stability of PC/ABS blends. Flame‐retardant polymers with different linkers between DDSQ and phosphate units show comparable effect on the flame retardancy of PC/ABS.     

Color tuning of Lu3Al5O12:Dy3+ ceramic-based white light-emitting phosphors via Yb incorporation

Transparent Lu₃Al₅O₁₂:Dy³⁺ ceramics were fabricated for UV-pumped white light-emitting diodes (WLEDs) via solid-state sintering under vacuum. The color chromaticity of the ceramic-based phosphors were tuned by tailoring the Dy³⁺ concentration and incorporating Yb into the crystal lattice to form (Lu, Yb)₃Al₅O₁₂:Dy³⁺ solid solutions. Phase composition, microstructure, optical and photoluminescence properties of the ceramics were investigated in detail by X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV–vis-NIR spectrometer and fluorescence spectrophotometer, respectively. White light can be obtained by combining the UV-chip and the structure/property-optimized ceramic phosphors. The color hue was tuned from (0.4107, 0.4037) to (0.3647, 0.3299) with the increasing Yb content from 0 to 0.5 substituting Lu sites in the garnet structure. The (Lu_0.5Yb_0.5)₃Al₅O₁₂: 0.01Dy³⁺ ceramic-based phosphor showed a relative low correlated color temperature of 4137 K. The decrease in PL intensities with Yb incorporation was also discussed via microstructure and fluorescence lifetime characterizations.     

The peculiarities of transformation of organosilicon polymer into ceramic products under mechanochemical treatment

The effect of mechanical treatment on polygermasilethyne for different periods of time was studied. The structural and chemical transformations of polygermasilethyne into ceramic products induced during a high-energy ball milling were investigated by infrared and Raman spectroscopy, thermal analysis, X-ray analysis, as well as scanning electron microscopy analysis with using EDS spectrometer. It was shown that high-energy mechanical treatment leads to the cross-linking process including the formation of new covalent bonds between molecules and rigid three-dimensional network. Simultaneously an amorphous solid network is converted to nano- and microsized inorganic phases, namely, silicon carbide, carbon and germanium-containing phase.     

Flame retardancy and thermal degradation mechanism of epoxy resin composites based on a DOPO substituted organophosphorus oligomer

A series of flame-retardant epoxy resins (EP) with different content of poly(DOPO substituted dihydroxyl phenyl pentaerythritol diphosphonate) (PFR) were prepared. The PFR was synthesized via the polycondensation between 10-(2,5-dihydroxyl phenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-BQ) and pentaerythritol diphosphonate dichloride (SPDPC). The structure of PFR was confirmed by Fourier transform infrared spectroscopy (FTIR) and ¹H nuclear magnetic resonance (¹H NMR). The flame retardancy and the thermal stability of the EP/PFR hybrids were investigated by limiting oxygen index (LOI) test and thermogravimetric analysis (TGA) in air. The results showed that the incorporation of PFR into EP can improve the thermal stability dramatically. The mechanical results demonstrated that PFR enhanced failure strain slightly accompanied by a decrease in tensile strength. The thermal oxidative degradation mechanisms of the EP/PFR hybrids were investigated by real time Fourier transform infrared spectra (RTFTIR) and direct pyrolysis/mass (DP-MS) analysis. X-ray photoelectron spectroscopy (XPS) was used to explore chemical components of the residual char of EP and EP/PFR hybrid. DP-MS analysis showed that the degradation process of EP/PFR hybrid was divided into two characteristic temperature regions, attributed to the decomposition of phosphate and aromatic structure.     

On the origin of emission and thermal quenching of SRSO:Er3+ films grown by ECR-PECVD

Silicon nanocrystals embedded in a silicon-rich silicon oxide matrix doped with Er³⁺ ions have been fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition. Indirect excitation of erbium photoluminescence via silicon nanocrystals has been investigated. Temperature quenching of the photoluminescence originating from the silicon nanocrystals and the erbium ions has been observed. Activation energies of the thermally activated quenching process were estimated for different excitation wavelengths. The temperature quenching mechanism of the emission is discussed. Also, the origin of visible emission and kinetic properties of Er-related emission have been discussed in details.     

Preparation and properties of borosiloxane gels

Hybrid gels were synthesized from modified silicon alkoxides (R‐Si(OEt)₃, R?H, Me, Vi) and triiso‐butylborate (B(O? R′)₃, R′ = isoBu). The obtained gels were characterized by Fourier‐transform infrared (FTIR), nuclear magnetic resonance (NMR), elemental analyses, X‐ray diffraction (XRD), and scanning electron microscopy (SEM). Boron atoms in the precursors were homogeneously dispersed in the siloxane network via Si? O? B bonds. The effect of the boron load on oxidation resistance of the gels, and the pyrolysis behavior of the borosiloxane gels with different substituents on silicon, were investigated using various techniques. The experimental results suggest that the addition of a proper fraction of boron alkoxide to the precursors can improve the oxidation stability of the gels, and decrease the weight loss of the samples to 6.9 wt % at 1000°C under air. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 719–724, 2006     

高炉镍铁渣粉在水泥基复合胶凝材料中的水化特性研究

通过对不同高炉镍铁渣掺量的水泥-高炉镍铁渣粉复合胶凝材料水化放热速率、高炉镍铁渣粉的反应程度、硬化浆体化学结合水含量以及水化产物中C-S-H凝胶Ca/Si的测定,分别研究了水泥-高炉镍铁渣粉复合胶凝材料的早期、中长期水化进程、浆体微观形貌以及水化产物特点等水化特性.研究结果表明:高炉镍铁渣的掺入会降低水化放热速率,并推迟水化加速期放热峰的出现时间;在复合胶凝体系中,随着高炉镍铁渣粉掺量的增大,其反应程度和硬化浆体中化学结合水含量将降低.复合胶凝材料水化生成的C-S-H凝胶的Ca/Si低于水泥,且随着水化的进行呈降低趋势;高炉镍铁渣粉中的Al,在水化过程中会取代部分Si进入C-S-H凝胶中,形成C-A-S-H凝胶.