Si-O Bond Length Research Articles

Corrosion resistance of multicomponent disilicate (Ho0.2Er0.2Tm0.2Yb0.2Lu0.2)2Si2O7 against CMAS and volcanic ash

With the increasing operating temperature of gas turbine engines, calcium-magnesium-aluminosilicate (CMAS) poses a serious threat on environmental barrier coatings (EBCs) applied on hot-sections of aero-engines. Here, we have synthesized a novel multicomponent disilicate—(Ho0.2Er0.2Tm0.2Yb0.2Lu0.2)2Si2O7 (brief to (5RE0.2)2Si2O7), and comparatively studied its performance in the presence of synthesized CMAS and natural volcanic ash at 1400ºC. In comparison with Yb2Si2O7, (5RE0.2)2Si2O7 has a shorter Si-O bond length and a larger RE-O bond length because of the larger average RE3+ radius. After CMAS corrosion, some apatite grains precipitate at the CMAS/(5RE0.2)2Si2O7 interface to develop a loose reaction layer, exhibiting a higher corrosion resistance than Yb2Si2O7. Meanwhile, the consumption of CaO and release of SiO2 during the chemical reaction process increase the viscosity of CMAS to some extent and thus weaken its infiltration propensity. For the volcanic ash case, it directly infiltrates into the interior of (5RE0.2)2Si2O7 along grain boundaries without any reaction due to the relatively low CaO content, exhibiting a more serious attacking behavior. In addition, (5RE0.2)2Si2O7 effectively increases the contact angle of molten volcanic ash due to its lower surface energy. These finds here provide a better understanding for the design and application of next-generation EBC material.

Evaluation of the reactivity of dense lanthanum‑gadolinium zirconate ceramics with Colima volcanic ashes

The effects of ingestion of airborne particles from pyroclastic events of active volcanoes by aircraft turbines and their subsequent reaction with thermal barrier coatings have attracted the attention of the scientific community in recent years. The reaction products of infiltration experiments of lanthanum‑gadolinium zirconate (LGZO) ceramics with molten ashes from the active Colima volcano at 1250 °C for 10 h are presented and discussed as a function of the Gd3+ content. Five ceramic compositions, varying the Gd3+ content in solid solution were synthesized by the chemical coprecipitation and calcination method of pressed powders. These compositions include pure lanthanum and gadolinium zirconates, LZO, and GZO, respectively. Penetration depth and identification, and in some cases quantification of the reaction products between the molten ash and LGZO ceramics were performed by scanning electron microscopy, chemical composition with energy dispersive X-ray spectroscopy, grazing incident X-ray diffraction as well as micro-Raman spectroscopy. The LZO ceramic exhibited the greatest infiltration resistance with an infiltration depth of approximately 23 μm from the surface. The phase characteristics of the reaction layers were dependent on the gadolinium content. LZO, LGZO25, and LGZO50 (x = 0, 0.25, and 0.5) showed the presence of apatite as well as monoclinic and tetragonal zirconia, while LGZO75 and GZO (x = 0.75 and 1), additionally showed the presence of cubic zirconia and anorthite. As the Gd3+ content increases in the LGZO solid solutions, the wavenumber value corresponding to the stretching vibrational mode of the silicon tetrahedra in apatite shifts from 862 to 877 cm−1, which is associated with a decrease in SiO bond lengths. These findings indicate that the amount and kind of rare earth cations dissolved in the melt plays an important role in the precipitation of the reaction products.

The effect of intra-molecular bonds on the liquid-liquid critical point in modified-WAC models.

To obtain a better understanding of liquid-liquid critical points (LLCPs) in one-component liquids, we extend the modified-WAC model by E. Lascaris, Phys. Rev. Lett. 116, 125701 (2016) which is known to have a LLCP. The original WAC model is a model for silica (SiO2) and consists of a mixture of non-bonded Si and O ions. By adding explicit intra-molecular Si-O bonds to the model, we are able to study how several parameters (Si-O bond length, O-Si-O angle, and bond stiffness) affect the existence and location of the LLCP. We find that for this model, only the Si-O bond length has a strong effect on the LLCP, while the bond angle and bond stiffness have no significant effect on the LLCP. An analysis of the relevant coordination numbers indicates that increasing the bond length decreases the ratio RSi/O of additional Si ions per additional O ion in the first coordination shell of the Si, which causes the LLCP to move to higher, more accessible temperatures. The behavior of the RSi/O parameter shows a strong correlation with the behavior of the LLCP and might be a useful tool to determine if a LLCP exists at low, hard-to-reach temperatures in other models.

Atomistic insight into the ferroelastic post-stishovite transition by high-pressure single-crystal X-ray diffraction

Abstract The post-stishovite transition is a classic pseudo-proper typed ferroelastic transition with a symmetry-breaking spontaneous strain. This transition has been studied using high-pressure spontaneous strains, optic modes, and elastic moduli (Cij) based on the Landau modeling, but its atomistic information and structural distortion remain poorly understood. Here we have conducted synchrotron single-crystal X-ray diffraction measurements on stishovite crystals up to 75.3 GPa in a diamond-anvil cell. Analysis of the data reveals atomic positions, bond lengths, bond angles, and variations of SiO6 octahedra across the transition at high pressure. Our results show that the O coordinates split at ~51.4 GPa, where the apical and equatorial Si-O bond lengths cross over, the SiO6 octahedral distortion vanishes, and the SiO6 octahedra start to rotate about the c axis. Moreover, distortion mode analysis shows that an in-plane stretching distortion (GM1+ mode) occurs in the stishovite structure at high pressure while a rotational distortion (GM2+ mode) becomes dominant in the post-stishovite structure. These results are used to correlate with elastic moduli and Landau parameters (symmetry-breaking strain e1–e2 and order parameter Q) to provide atomistic insight into the ferroelastic transition. When the bond lengths of two Si-O bonds are equal due to the contribution from the GM1+ stretching mode, C11 converges with C12, and the shear wave VS1[110] polarizing along [110] and propagating along [110] vanishes. Values of e1–e2 and Q are proportional to the SiO6 rotation angle from the occurrence of the GM1+ rotational mode in the post-stishovite structure. Our results on the pseudo-proper type transition are also compared with that for the proper type in albite and improper type in CaSiO3 perovskite. The symmetry-breaking strain, in all these types of transitions, arises as the primary effect from the structural angle (such as SiO6 rotation or lattice constant angle) and its relevant distortion mode in the low-symmetry ferroelastic phase.

Precisely control the ultraviolet to blue light conversion for plant growth: Rigid crystal structure, lattice substitution and flux effect in the Ca1.1Sr0.9SiO4:Ce3+, Li+ phosphor

A series of Ce3+-doped Ca1.1Sr0.9SiO4-based phosphors has led to the development of a fingerprint spectrum to precisely tune the UV to blue light conversion for application in the photosynthesis of different plants in agriculture. The addition of MX2 flux (M = Ca, Ba; X = Cl, Br) not only remarkably enhances the luminescence efficiency, but also fine tunes the photoluminescence (PL) wavelength and photoluminescent excitation (PLE) peaks. With increasing Al content doping in the Si site, the excitation and emission peaks of the Ca1.08Sr0.88Si1-zAlzO4:0.02Ce3+,0.02Li+,0.04CaCl2 (CSSA:CeLiCaCl) phosphor shift to longer wavelengths, i.e. from 367 (z = 0) to 373 (z = 0.04) nm and from 429 (z = 0) to 448 (z = 0.04) nm, respectively, along with an increase of the luminescence intensity. Rietveld refinement of the XRD data for Ca2-ySrySiO4 (y = 0.8, 0.9, 1.0) revealed that Ca1.1Sr0.9SiO4 has the highest rigidity, which can be correlated with the shortest Si-O bond length and strongest bond energy along the series. The crystal structure and unit cell parameters of CSSA:CeLiCaCl (z = 0, 0.03 and 0.05) were also obtained, revealing that the efficient and tunable blue emission of the CSSA:CeLiCaCl phosphors can be attributed to the crystal structure distortions induced by the [(Si/ Al)O4] tetrahedron, while high rigidity is maintained. The combined effects of the Si4+ substitution by Al3+ and flux addition on the crystal structure and luminescence properties of the Ce3+-doped Ca1.1Sr0.9SiO4 phosphors are discussed in detail.

Compression tuned crystalline and amorphous phases of Gd2Si2O7: Raman spectroscopic and first-principles studies

Compounds that have polymorphism at ambient conditions exhibit interesting structural transitions under different thermodynamic conditions. The compound Gd2Si2O7 is an interesting silicate that exhibits polymorphism at ambient conditions. We have investigated high pressure behavior of orthorhombic Gd2Si2O7 using Raman spectroscopy and identified structural changes at pressures 2 and 8 GPa followed by transformation to a disordered/glassy phase above 13 GPa. While the transitions are reversible from 12 GPa, releasing from higher pressures leads to recovery of a glassy phase. We have carried out density functional theory (DFT) simulations for the high-pressure structural optimization of the different phases of Gd2Si2O7. The enthalpy cross over as obtained from DFT simulations predicts the high-pressure phase above 2 GPa to be triclinic and the phase above 8 GPa to be monoclinic Gd2Si2O7 which support the experimental findings. The random change of Si-O bond length above 20 GPa points towards possible amorphization for the monoclinic phase.

Probing 29Si-17O connectivities and proximities by solid-state NMR

The measurement of dipolar and J- couplings between 29Si and 17O isotopes is challenging owing to (i) the low abundance of both isotopes and (ii) their close Larmor frequencies, which only differ by 19%. These issues are circumvented here by the use of isotopic enrichment and dedicated triple-resonance magic-angle spinning NMR probe. The surface of 29Si-enriched silica was labelled with 17O isotope and heated at 80 and 200 °C. 29Si-17O connectivities and proximities were probed using two-dimensional (2D) through-bond and through-space heteronuclear multiple-quantum coherences (J- and D-HMQC) experiments between 17O and 29Si nuclei. The simulation of the build-up of the J- and D-HMQC signals allowed the first experimental measurement of J- and dipolar coupling constants between 17O and 29Si nuclei. These HMQC experiments allow distinguishing two distinct siloxane (SiOSi) oxygen sites: (i) those covalently bonded to Q3 and Q4 groups, having a hydroxyl group as a second neighbour and (ii) those covalently bonded to two Q4 groups. The measured J- and dipolar coupling constants of siloxane 17O nucleus with Q4 29Si nuclei differ from those with Q3 29Si nuclei. These results indicate that the 29Si-17O one-bond J-coupling and Si-O bond length depend on the second neighbours of the Si atoms.

First-principles calculation of iron and silicon isotope fractionation between Fe-bearing minerals at magmatic temperatures: The importance of second atomic neighbors

In order to elucidate the processes involved in iron and silicon isotopes partitioning during magmatic differentiation, it is essential to know the precise value of equilibrium fractionation factors between the main minerals present in the evolving silicic melts. In this study, we performed first-principles calculations based on the density functional theory to determine the equilibrium iron and silicon isotopes fractionation factors between eleven relevant silicate or oxide minerals in the context of magmatic differentiation, namely: aegirine, hedenbergite, augite, diopside, enstatite, fayalite, hortonolite, Fe-rich and Fe-free forsterites, magnetite and ulvospinel. Results show that Fe2+-bearing silicate minerals display significant differences in iron isotope fractionation factors that cannot be neglected, even at high temperature (1000 °C). Various physical and chemical parameters control the iron isotopic fractionation of silicate minerals. However, the main parameter, after temperature and the iron oxidation state, is the nature and number of iron second neighbors (i.e. the local chemical composition around Fe atoms). This conclusion is also valid for silicon isotopes. In the investigated nesosilicates and inosilicates, silicon isotope reduced partition function ratios (also called β-factors) show no correlation with the average Si-O bond length, which remains almost constant, but Si β-factors are correlated with the local chemical composition of the minerals. Fractional crystallization is one of the mechanisms, which could explain the evolution of iron isotopic compositions during magmatic differentiation. Using the present theoretical set of equilibrium fractionation factors allows us to assess the impact of inter-mineral isotopic fractionations, and shows that pyroxene appears to be the main mineral phase driving the isotopic evolution to a heavier signature in the most evolved lavas.

Subsurface structural change of silica upon nanoscale physical contact: Chemical plasticity beyond topographic elasticity

Surface defects or flaws on materials made by physical contacts with foreign objects can deteriorate their mechanical properties and limit technical applications. Thus, understanding the contact-induced subsurface damage is of great importance. Using nanoscale infrared spectroscopy and reactive molecular dynamics simulations, the subsurface structural changes of silica upon nanoindentation and nanoscratch are investigated. The results reveal an elongation of the SiO bond length distribution even after the topographically-elastic contact, indicating a “chemical plasticity” at the sub-Angstrom level. In the plastic region with subsurface densification, the SiO bond is found to be slightly longer than the pristine region, indicating the decrease in molar volume is accompanied with the elongation, not shortening, of the SiO bond. These results elucidate the structural damage of a material upon physical contact cannot be delineated based on the topographic deformation of the surface.

Continuous random network avoidance and microscopic relaxation time in As2Se3 glass

Recent nuclear magnetic resonance (NMR) studies on SiO2 structural glass has shown a positive correlation exists between Si-O bond lengths and Si-O-Si bond angles. This indicates that when the glass solidified it avoided forming as a continuous random network (CRN). A microscopic tight binding (TB) model was used some years ago to predict the nuclear quadrupole resonance (NQR) distribution of powdered As2Se3 glass. Similar bond length- bond angle correlations were found. Here we extend this model and show that these correlations are directly attributable to a competition between valence electronic energy and emergent frustration forces as the glass solidifies into a locally preferred structure (LPS). Recent studies have shown that fast atomic motion occurs in metallic glasses at temperatures far below the glass temperature. Using this model we calculate a microscopic relaxation time and show it is a product of CRN avoidance.

Coordination environment of Si in calcium silicate hydrates, silicate minerals, and blast furnace slags: A XANES database

Understanding the silicate polymerization of calcium silicate hydrate (C-S-H) gel and its crystalline polymorphs is important in cement science. NMR can determine Si environments, but the measurement can be time-consuming and provides no spatial information. X-ray absorption near-edge structure (XANES) spectroscopy is a fast tool for probing Si coordination, possibly with spatial information. However, there lacks an understanding of Si K-edge XANES spectra of cement-related silicate phases. Here, a Si K-edge XANES spectral database of nanocrystalline C-S-H, C-S-H minerals, blast-furnace slags, and metakaolin is provided. Si K-edge of C-S-H minerals shifts to higher energies with higher polymerized Si and lower CaSi connectivity in the Si second nearest neighbor shell. Si K-edge energy shows weak correlations with Ca/Si ratio, average SiO bond length, and SiO4 distortion due to the structural complexity of silicates. The substitution of Al for Si shifts the Si K-edge of tobermorite and slags to higher energies.

Identifying and explaining vibrational modes of sanbornite (low-BaSi2O5) and Ba5Si8O21: A joint experimental and theoretical study

We report here the analysis of vibrational properties of the sanbornite (low-BaSi2O5) and Ba5Si8O21 using theoretical and experimental approaches, as well as results of high temperature experiments up to 1100–1150 °C. The crystal parameters derived from Rietveld refinement and calculations show excellent agreement, within 4%, while the absolute mean difference between the theoretical and experimental results for the IR and Raman vibrational frequencies was <6 cm−1. The temperature-dependent Raman study renders that both sanbornite and Ba5Si8O21 display specific Ba and Si sites and their BaO and SiO bonds. In the case of the stretching modes assigned to specific Si sites, the frequency dependence on the SiO bond length exhibited very strong correlations. Both phases showed that for a change of 0.01 Å, the vibrational mode shifted 10 ± 2 cm−1. These results are promising for using Raman spectroscopy to track in situ reactions under a wide variety of conditions, especially during crystallization.

First-principles insights into atomic oxygen diffusion inside polyhedral oligomeric silsesquioxane cages

Atomic oxygen erosion is a significant threat to exterior spacecraft polymers. Polyhedral oligomeric silsesquioxane (POSS) is a potential kind of material in resisting AO diffusion erosion, but lack of mechanism studies for application supports. In this research, we used the first-principles calculations to investigate the mechanism based on two types of nominally hydrated POSS named T8 (Si8O12H8) and T10 (Si10O15H10). According to the results, we found that the minimum energy path (MEP) of AO diffusing inside POSS is the adjacent-edge-path rather than the center-path. We also found the energy barriers that hinder the diffusion motion of AO inside POSS are originated from the structural distortion of POSS, which are negatively correlated to Si-O bond length and positively correlated to structural compactness. Thus, compacter POSS structures with shorter Si-O bond length are expected to give better AO diffusion resistance properties.

Study on the Structure and Properties of High-Calcium Coal Ash in the High-Temperature Zone of a Blast Furnace: A Molecular Dynamics Simulation Investigation

Molecular dynamics simulations were carried out to study the behavior of high-calcium coal ash in the high-temperature zone of a blast furnace. The radius distribution function, coordination number, structural unit, transform performance, and fluidity were analyzed. The results showed that CaO did not cause significant variation of the Si-O bond length in the aluminosilicate, except when the CaO content reaches 40%. The simulation results indicated that, as the CaO content increases, the transport properties of the coal ash will become better and the viscosity will accordingly be lower. The reason for this phenomenon is that CaO depolymerized the microscopic network structure of the aluminosilicate, which caused more bridge oxygen to convert to non-bridge oxygen, thereby reducing the degree of polymerization of the system. Meanwhile, through the analysis of potential energy, it was also proved that CaO has the function of improving the system energy and reducing stability. Our studies have fully related the viscosity change and microstructure of high-calcium coal ash in the high-temperature zone of a blast furnace.

Viscosity of molten CaO[sbnd]K2O[sbnd]SiO2 woody biomass ash slags in relation to structural characteristics from molecular dynamics simulation

Molten compositions in the CaOK2OSiO2 system relevant to woody biomass ash slags were simulated with molecular dynamics to extract structural characteristics. Multivariate analysis elucidated correlations of these structural characteristics with viscosity measurements. The simulations show SiO4/silicate tetrahedral units (STUs) diffusing slowly and forming flexible networks via oxygen bridges. The degree of STU polymerization varies linearly with the (K2O + CaO)/SiO2 ratio. Ca depolymerises stronger than K, but K diffuses quicker. Depolymerization and diffusion cause network disruptions and agitations that promote collective atomic mobility of the system. This imposes structural characteristics in the slag that correlate with viscosity. The inter-STU SiOSi angle narrows with decreasing viscosity, while the SiO bond length of these bridges increases. Attributes related to atomic mobility, such as the variations in the SiOSi angle and the distance of nearest SiSi pairs, also correlate with viscosity. The discussion provides insight into the connection between structural characteristics and viscosity.

Structure, hydration, and chloride ingress in C-S-H: Insight from DFT calculations

The structure of Calcium-Silicate-Hydrate (C-S-H) and the effect of variations in its water content have been investigated using density functional theory (DFT) calculations. Trends for calculated densities as a function of hydration are in good agreement with experimental values, and in line with what is found using molecular mechanics in the literature. While we observe very little variation in SiO and CaO bond lengths between different structures, structural diversity is otherwise great, in accordance with experimental observations, as we see no obvious correlation between structural features and material system stability. A mapping of energetics of hydroxyl substitution with chloride reveals, unsurprisingly, that chloride preferentially coordinates to calcium. More specifically, it was found that the most stable sites for chlorine substitution involves at least two adjacent calcium atoms. Computed chloride substitution energies indicate that the C-S-H phase may bind chloride from aqueous solution, potentially influencing chloride diffusion in concrete.

Evaluation of DFT methods to calculate structure and partial atomic charges for zeolite N

Zeolite N is a synthetic zeolite of the EDI framework type with chemical formula K12Al10Si10O40Cl2·8H2O Experimental and computational investigations verify the valuable ion-exchange capability of zeolite N. In this study, we assess the effects of Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) DFT models on zeolite structural parameters and on partial atomic charges of framework atoms. We applied these functionals with different quality of convergence and SCF tolerances, numerical basis sets and dispersion correction schemes. Optimized zeolite N structures are evaluated by comparing the atom positions and framework TO bond lengths with experimental data. The obtained SiO and AlO bond lengths of optimized structures in this study are in agreement with previous experimental and computational studies on zeolite N and other zeolites. The values of Mulliken partial atomic charges are sensitive to the choice of numerical basis sets. Results show that the GGA-PBE functional with DNP-4.4 basis set and TS dispersion correction scheme is a reliable DFT model in order to optimize and establish the structural parameters of zeolite N for further MD simulations.

Effects of water on the mechanical properties of silica glass using molecular dynamics

Understanding the effects of water on the mechanical properties of silica glass is important for many applications of silicate glasses. In this study, the effects of water on both elastic and plastic properties of pure silica glass are investigated. The introduction of molecular water leads to an increase of Young's modulus of silica glass at low water content, while hydroxyl exhibits an opposite effect. While hydroxyl groups decrease both strength and fracture toughness of the glass via destructing the silica network connectivity, molecular water undermines these properties by effectively driving the silica network to a “strained” configuration in absence of external stress. The water effect can be characterized in terms of the change of Si-O bond length and Si-O-Si bond angle. The plateau in the stress-strain curves of silica with the existence of molecular water in compact tension is associated with the Si-O bond breaking followed by formation of silanol groups. Moreover, the introduction of molecular water lowers the critical tensile stress where the plateau occurs.

A novel method for surface wettability modification of talc through thermal treatment

Silica tetrahedron (also called siloxane) is an important structure of clay minerals, the percentage of which influences the hydrophobicity, polarity and other surface properties on clay mineral surfaces. In this study, a novel method for surface wettability modification of talc through thermal treatment has been put forward. In this method, the symmetrical siloxane structures on talc basal surfaces were broken by transforming the nonpolar siloxane structure into a polar structure during the thermal treatment process, thus to decrease the hydrophobic components on talc surfaces, and therefore achieve the goal of surface wettability modification. This study was performed through the measurements of thermosgravimetric analysis and differential scanning calorimetry (TG-DSC), contact angle and wetting heat measurements, fourier transform infrared spectroscopy (FTIR), and 29Si-nuclear magnetic resonance spectroscopy (29Si NMR) as well as molecular dynamic simulations (MDs). The results indicate that the thermal treatment at 960 °C for only a very short time could greatly decrease the contact angle of water on talc surfaces. The mechanism could be attributed to that the thermal treatment leads to the disorder of some of the SiO bond length and O-Si-O bond angle in talc silica tetrahedron structure, which finally results in the disorder of the symmetrical siloxane structure and the formation of the amorphous silica phase. And the decrease of hydrophobic siloxane structures on talc surface gives rise to the relatively strong hydrophilicity on talc surfaces. This current method improves the wettability of talc without any addition of modifiers, which would be helpful of many applications of clay minerals, such as clay-polymer composites, filler materials, clay mineral dispersion in aqueous suspensions or mineral processing, etc.

Evidence for non-thermal microwave effect in processing of tailing-based glass-ceramics

Tailing-based glass-ceramics were crystallized via conventional and microwave heating at 720 °C for 30 min and 820 °C without holding and compared in order to obtain evidence for a non-thermal microwave effect. The comparative analytical results showed that the microstructural uniformity was greatly enhanced and the crystallization activation energy was significantly reduced by microwave processing compared to conventional heating, suggesting accelerated grain growth during crystallization. Microwave radiation affected the crystal orientation and induced corresponding changes in the SiO bond lengths and SiOSi angles, thus enhancing the formation of the diopside crystal structure. In addition, the samples under microwave processing exhibited superior physicochemical performance, including greater relative density, bending strength, microhardness, and resistance to acids and alkali, compared to conventionally processed samples. All these results provided positive evidence supporting the existence of a genuine microwave non-thermal effect, allowing deepened understanding for fine control over microwave-assisted metallurgy.