Si Angle Research Articles

Effects of La and Mg on Microstructure and Mechanical Property of Al‐15Si Alloy

The effects of La on the microstructures and mechanical properties of Al‐15Si alloy were investigated, and the results showed that 0.3% La could availably modify the Al‐15Si alloy. The morphology of the eutectic Si obviously changed into the short flake and the dot shape, and the angle of primary Si was refined to bluntness with the contents of La increasing from 0.1 % to 0.3%. In addition, the effects of Mg on the microstructures and mechanical properties Al‐15Si‐0.3La alloy were studied. The optical micrographs showed that the eutectic Si were further refined to the shorter bar the dot shape, and the amount of primary Si reduced with the increase of Mg addition. The formation of Mg2Si strengthening phase improved the mechanical properties of the modified Al‐15Si‐0.3La alloy. Compared with Al‐15Si alloy, the tensile strength and the elongation of Al‐15Si‐0.3La‐2.0Mg alloy increased 41.4% and 40%, respectively.

Ionic ASi2N3 (A=Li, Na, K and Rb) stabilized by the covalent Si–N bonding: First-principles calculations

The structural, elastic and electronic properties of LiSi2N3 and its substitutions by Na, K and Rb were investigated through first-principles computations. The expansion of lattice parameters of ASi2N3 from Li, Na, K to Rb is found to be determined by the bond angle of Si–N1–Si, which suggests a possible way to improve the lithium ionic conductivity by substitutions. ASi2N3 (A=Li, Na, K and Rb) shows the similar elastic behaviors, while the electronic band gap gradually decreases from 5.1 to 3.4eV from LiSi2N3 to RbSi2N3. Interestingly, the analysis of electronic structure, crystal orbital Hamiltonian populations and Bader charges shows that the covalence of Si–N bonding is critical for the stability of ASi2N3 phase. Among ASi2N3 phases, there is a relatively high ionicity in NaSi2N3; the Si–N bond strength in [Si2N3]− net for KSi2N3 and RbSi2N3 is comparable to LiSi2N3, but stronger than NaSi2N3.

Evolutions of molecular oxygen formation and sodium migration in Xe ion irradiated borosilicate glasses

The modifications of a commercial borosilicate glass induced by Xe ion irradiation have been studied by Raman spectroscopy and ToF-SIMS depth profiling. A decrease in the average Si–O–Si angle, an increase in the population of three-membered rings and an increase of the glass polymerization are evidenced. The molecular oxygen appears in the irradiated glasses after the irradiation fluence reaches approximately 1015ions/cm2. The O2 concentration decreases with the depth of irradiated glass at the ion fluence of 2×1016ions/cm2. A sodium depleted layer at the surface and a depleted zone at around the penetration depth of 5MeV Xe ions are observed. The thickness of the sodium depleted layer increases with the irradiation fluence. Moreover, comparing with previous results after electron and Ar ion irradiation, it can be concluded that the nuclear energy deposition can partially inhibit the formation of molecular oxygen and increase the threshold value of electron energy deposition for the molecular oxygen formation.

Quartz: structural and thermodynamic analyses across the α ↔ β transition with origin of negative thermal expansion (NTE) in β quartz and calcite.

The temperature variation, T, of the crystal structure of quartz, SiO2, from 298 to 1235 K was obtained with synchrotron powder X-ray diffraction data and Rietveld structure refinements. The polymorphic transformation from P3221 (low-T, α quartz) to P6222 (high-T, β quartz) occurs at a transition temperature, Ttr = 847 K. The T variations of spontaneous strains and several structural parameters are fitted to an order parameter, Q, using Landau theory. The change in Si atom coordinate, Six, gives Ttr - Tc = 0.49 K, which indicates an α ↔ β transition that is weakly first order and nearly tricritical in character (Q(4) ∝ T). Strains give higher Ttr - Tc values (≃ 7 K). Other fitted parameters are the oxygen Oz coordinate, Si-Si distance, Si-O-Si and ϕ angles, and intensity of the (111) reflection, I111. In α quartz, the Si-Si distance increases with T because of cation repulsion, so the Si-O-Si angle increases (and ϕ decreases) and causes the thermal expansion of the framework structure that consists of corner-sharing distorted rigid SiO4 tetrahedra. The Si-Si distances contract with T and cause negative thermal expansion (NTE) in β quartz because of increasing thermal librations of the O atom in the Si-O-Si linkage that occur nearly perpendicular to the Si-Si contraction. In calcite, CaCO3, the short Ca-Ca distance expands with T, but the next-nearest Ca-Ca distance, which is of equal length to the a axis, contracts with T and causes NTE along the a axis. The thermal librations of the atoms in the rigid CO3 group increase with T along the c axis.

Transformation of porous structure under vacuum ultraviolet irradiation of the films based on silicon dioxide

Transformation of the films with the pores of different radii under the action of vacuum ultraviolet radiation was studied experimentally and theoretically. Simulation results showed that Si–O–Si angle depends on pore size. Fourier Transform infrared spectra provide indirect confirmation of this statement. The experimental data and calculation results suggest that methyl group decreases π bonding, which causes a decrease in Si–O–Si angle. The action of ultraviolet radiation is to be considered as a photochemical reaction; the fragments formed in this reaction (СН3⁎, CH2⁎) can participate in polymerization, which leads to pore sealing.

Trimethylsilylmethylcalcium Iodide, an Easily Accessible Grignard-Type Reagent of a Heavy Alkaline Earth Metal

The direct synthesis of iodomethyltrimethylsilane with calcium and magnesium in ether yields the corresponding ether adducts [(thf)4Ca(I)(CH2SiMe3)] (1a) and [(Et2O)2Mg(I)(CH2SiMe3)] (2a). The 1,4-dioxane method allows shifting the Schlenk equilibrium toward bis(trimethylsilylmethyl)magnesium 2b. After substitution of ligated thf ligands by tetrahydropyran (thp) crystalline [(thp)4Ca(I)(CH2SiMe3)] (1b) can be isolated. The Ca–C and Ca–I bond lengths of 252.7(3) and 319.11(3) pm represent characteristic values. Steric repulsion between ligated thp ligands and the rather bulky trimethylsilylmethyl group widens the Ca–C–Si angle to 131.19(14)°. NMR data and quantum chemical studies support that hyperconjugative effects might be operative, leading to a shortened Si–C bond of this Ca–C–Si fragment.

Effects of Pressure, Thermal Treatment, and O2 Loading in MCM41, MSU-H, and MSU-F Mesoporous Silica Systems Probed by Raman Spectroscopy

We present a Raman study of the effects induced by pressure, thermal treatments, and O2 loading in MCM41, MSU-H, and MSU-F representative mesoporous silica. We compared the starting powders with the mechanically pressed tablets produced applying pressures of ∼0.2 and ∼0.45 GPa. The spectra of the three untreated tablets evidence that the main value of the Si–O–Si angle decreases and that in the MCM41 and the MSU-H Si–O–Si hydrolysis occurs, whereas such a process is absent or much less efficient in the MSU-F. Despite their different networks, the three powders tend to crystallize in cristobalite when treatments are at 1000 °C. The MCM41 and MSU-H tablets exhibit behavior similar to their starting powders, whereas the MSU-F tablets tend to form the tridymite crystalline phase. Such a finding could be related by the differences in the Si–O–Si hydrolysis, occurring during the tablets production. Finally, we inserted O2 molecules within the interstices of the SiO2 wall only in MSU-F. By monitoring the O2 cont...

Atomic motion of resonantly vibrating quartz crystal visualized by time-resolved X-ray diffraction

Transient atomic displacements during a resonant thickness-shear vibration of AT-cut α-quartz are revealed by time-resolved X-ray diffraction under an alternating electric field. The lattice strain resonantly amplified by the alternating electric field is ∼104 times larger than that induced by a static electric field. The resonantly amplified lattice strain is achieved by fast displacements of oxygen anions and collateral resilient deformation of Si−O−Si angles bridging rigid SiO4 tetrahedra, which efficiently transduce electric energy into elastic energy.

First-principles simulation of Raman spectra and structural properties of quartz up to 5 GPa**Project supported by the Key Laboratory of Earthquake Prediction, Institute of Earthquake Science, China Earthquake Administration (CEA) (Grant No. 2012IES010201) and the National Natural Science Foundation of China (Grant Nos. 41174071 and 41373060).

The crystal structure and Raman spectra of quartz are calculated by using first-principles method in a pressure range from 0 to 5 GPa. The results show that the lattice constants (a, c, and V) decrease with increasing pressure and the a-axis is more compressible than the c axis. The Si–O bond distance decreases with increasing pressure, which is in contrast to experimental results reported by Hazen et al. [Hazen R M, Finger L W, Hemley R J and Mao H K 1989 Solid State Communications 725 507–511], and Glinnemann et al. [Glinnemann J, King H E Jr, Schulz H, Hahn T, La Placa S J and Dacol F 1992 Z. Kristallogr. 198 177–212]. The most striking changes are of inter-tetrahedral O–O distances and Si–O–Si angles. The volume of the tetrahedron decreased by 0.9% (from 0 to 5 GPa), which suggests that it is relatively rigid. Vibrational models of the quartz modes are identified by visualizing the associated atomic motions. Raman vibrations are mainly controlled by the deformation of the tetrahedron and the changes in the Si–O–Si bonds. Vibrational directions and intensities of atoms in all Raman modes just show little deviations when pressure increases from 0 to 5 GPa. The pressure derivatives (dνi/dP) of the 12 Raman frequencies are obtained at 0 GPa–5 GPa. The calculated results show that first-principles methods can well describe the high-pressure structural properties and Raman spectra of quartz. The combination of first-principles simulations of the Raman frequencies of minerals and Raman spectroscopy experiments is a useful tool for exploring the stress conditions within the Earth.

Impact of Aqueous Medium on Zeolite Framework Integrity

In this work, Al K-edge extended X-ray absorption fine structure and 27Al MAS NMR spectroscopies in combination with DFT calculations are used to determine both qualitative and quantitative structural changes of two well-characterized samples with the BEA structure. The effects of various properties, including Al concentration, Al distribution, particle size, and structural defects, on zeolite stability are explored. As the samples are degraded by treatment in hot liquid water, the local structure about the Al T-site remains mostly intact, including the Al–O–Si angles and bond distances, while the crystalline structure as measured by XRD and STEM is disrupted. The combined data suggests the crystallinity decreases via selective hydrolysis of the T1- and T2-sites that form the 4-member rings of the zeolite framework. The hydrolysis eventually leads to the dissolution of the T-sites followed by reprecipitation on the particle surface resulting in amorphization of the sample.

First-Principles Calculations and Analysis of29Si Nuclear Magnetic Resonance Chemical Shifts in Silicon Oxycarbide Ceramics

We calculate 29Si NMR chemical shifts for silicon atoms in different chemical environments observed in amorphous silicon oxycarbide ceramics using a library of optimized structural models and the gauge-including projector augmented wave (GIPAW) method. For each type of mixed tetrahedral environment, {Si}O4, {Si}O3C, and {Si}O2C2, we develop linear angular correlation functions that relate the 29Si NMR shift of the central silicon atom to the Si–O–Si angles surrounding it. The bonding nature of carbon atoms, whether 4-fold (sp3) or 3-fold (sp2) connected, impacts the chemical shift of {Si}O3C units and can be used to distinguish between Si–C bonding in the “glassy” SiCO host structure and at the interface to so-called “free” carbon. With the derived correlation functions, we analyze two representative experimental 29Si NMR spectra and extract their angle distributions. Our results are in agreement with X-ray and neutron diffraction data. Probing for an interface between “free” carbon and “glassy” SiCO host in a C-rich SiCO material, we find no evidence of significant bonding between Si and three-connected (sp2) C. Rather, the material exhibits a large fraction of wide Si–O–Si angles, which are typical of cage-like and zeolitic structures.

Distinctive Spectral and Microscopic Features for Characterizing the Three-Dimensional Local Aluminosilicate Structure of Perlites

Expanded perlite and raw perlites of various origins were examined using solid-state NMR and Fourier transform infrared (FTIR) spectroscopies, as well as scanning electron microscopy, in an attempt to correlate the characteristics of aluminosilicate framework with the material’s expansion properties. The 27Al magic angle spinning (MAS) NMR spectra indicated that aluminum atoms in perlite are mainly tetrahedrally coordinated in Q4 structures not participating in the expansion process; two different octahedral Al species were also identified in perlite. Owing to the tetrahedra−octahedra transformations, perlite may be classified among the metastable materials. The 29Si MAS NMR and 1H–29Si CPMAS NMR spectra showed that Q3 and Q4 units are mostly abundant, a fact that explains the low cation exchange capacity of perlites. Deconvoluted FTIR and 29Si MAS NMR spectra are consistent regarding the relative Qn intensities which, together with the network geometry, determine the expansibility of raw perlites. The calculated variations of the Si–O–Si angle are more significant than those of the Si–O bond length, remarkably affecting the structure of the aluminosilicate network. Thus, perlite samples of increasingly fragmentary framework exhibit the highest expansion ratios. SEM micrographs monitored the expansion process differentiating between the low- and high-expansibility perlites.

Constructing covalent interface in rubber/clay nanocomposite by combining structural modification and interlamellar silylation of montmorillonite.

Strong interfacial interaction and nanodispersion are necessary for polymer nanocomposites with expectations on mechanical performance. In this work, montmorillonite (MMT) was first structurally modified by acid treatment to produce more silanol groups on the layer surface. This was followed by chemical modification of γ-methacryloxy propyl trimethoxysilane molecule (KH570) through covalent grafting with the silanol groups. (29)Si and (27)Al magic angle spinning (MAS) NMR results revealed the microstructural changes of MMT after acid treatment and confirmed the increase of silanol groups on acid-treated MMT surfaces. Thermogravimetric analysis indicated an increase in the grafted amount of organosilane on the MMT surface. X-ray diffraction (XRD) showed that the functionalization process changed the highly ordered stacking structure of the MMT mineral into a highly disordered structure, indicating successful grafting of organosilane to the interlayer surface of the crystalline sheets. The styrene-butadiene rubber (SBR)/MMT nanocomposites were further prepared by co-coagulating with SBR latex and grafted-MMT aqueous suspension. During vulcanization, a covalent interface between modified MMT and rubber was established through peroxide-radical-initiated reactions, and layer aggregation was effectively prevented. The SBR/MMT nanocomposites had highly and uniformly dispersed MMT layers, and the covalent interfacial interaction was finally achieved and exhibited high performance.

Molecular simulation of “hydrolytic weakening”: A case study on silica

Water molecules have a great influence on the mechanical performance of silicate materials. In this study, the hydrolytic weakening mechanisms of crystal α-quartz and amorphous silica glass were investigated in light of molecular simulation. A reactive force field was used to simulate the structure, dynamics and mechanical properties of the silica with water molecules during a uniaxial tensile process with loading rates from 0.02 to 0.16ps−1. Dry silica samples, obtained by the temperature quenching method, and wet silica samples, formed by Grand Canonical Monte Carlo water adsorption, were prepared for mechanical tests. Structurally, water penetrating the silica cavity can form H-bonds with the neighboring bridging oxygen in the silicate network, leading to the elongation of the Si–O bonds and the enlargement of the Si–O–Si angles. Dynamically, the confined water molecules, with a high diffusion rate, collide with the silicate network, reducing the stability of the siloxane bonds. The reactive force field, coupled with the chemical and mechanical responses, provides new insights into the molecular structural evolution. During the tensile process, the siloxane bonds and Si–O–Si angles are initially stretched in the elastic region. Subsequently, the silica network depolymerizes into branch structures and, finally, the structure undergoes a tensioned fracture in a catastrophic manner. When the stress exceeds 30% of the tensile strength, water molecules in the silica system begin to dissociate into two reaction pathways, i.e. one governed by the chemical adsorption of water at a low stress level and the other dominated by the breakage of siloxane bonds at a high stress level. The stress-dependent water dissociation accelerates the depolymerization of the silica network, and brings about further irreversible deformation, which weakens the cohesive force of the silica.

Alpha and deuteron irradiation effects on silica nanoparticles

We present an experimental investigation focused on the effects of alpha and deuteron irradiation on different silica nanoparticles. The study has been devoted also to characterize the induced point defects and the eventual structural modifications to evaluate the effects of the different irradiation source in comparison with the bulk materials. After irradiation up to about 1016 ions cm−2, we performed electron paramagnetic resonance (EPR), photoluminescence (PL), infrared (IR) absorption, Raman, and atomic force microscopy (AFM) measurements. We found that the two types of irradiation qualitatively induce comparable effects. Furthermore, irradiation generates the so-called twofold coordinated Si and the H(I) point defects, originating from the reaction of the former with hydrogen atoms. The occurrence of these defects is a not trivial and interesting finding due to its connection to irradiation-induced oxygen deficiency, not yet evidenced by other irradiation of silica nanoparticles. We also detected the E′Si paramagnetic centers and found that their lineshape at the highest fluence, independently from the nanoparticles size and irradiation source, is different from the one observed in the bulk. Furthermore, the integral of the E′Si signal does not depend significantly on the nanoparticles size differing from previous irradiation with β-ray of the same nanoparticles. AFM images indicate the absence of significant radiation induced sintering between the nanoparticles in the plan orthogonal to the irradiation direction suggesting the absence of morphological changes, whereas IR measurements suggest the occurrence of some structural modifications in all the nanoparticles, which consist in the decrease of the peak value of the Si–O–Si angle distribution. Finally, irradiation effects are induced not uniformly along the irradiation direction, as supported also by micro-Raman investigation of an irradiated bulk silica material and simulations of ions penetration profiles. These results suggest the occurrence of some irradiation effects due to light ion bombardment specific of nanoparticles.

Ab Initio Calculations of the Energy Dependence of Si–O–Si Angles in Silica and Ge–O–Ge Angles in Germania Crystalline Systems

Cristobalite is the only corner-sharing tetrahedral framework that can maintain perfectly regular SiO4 tetrahedra throughout the entire 180° range of Si–O–Si angles. It is, thus, the ideal system for a study of the energy dependence of the Si–O–Si angle in a crystalline framework. Using the VASP first principles density functional code, we have derived energy versus Si–O–Si angle curves for pure silica and versus Ge–O–Ge angles for pure germania models of cristobalite. In addition, the frameworks of quartz, tridymite, and the zeolites sodalite (SOD) and metavariscite (BCT) were studied. The range of angles with low energies is larger for silica, though the lowest-energy Ge–O–Ge angle is always lower than the corresponding lowest-energy Si–O–Si angle in the same framework type. We discuss which framework types are possible for pure-silica and pure-germania based on a rigid tetrahedron model.

Effect of 10B(n, α)7Li irradiation on the structure of a sodium borosilicate glass

The effects of the nuclear reaction 10B(n, α)7Li on the properties and structure of a sodium borosilicate glass were analysed by density, hardness and fracture toughness measurements, Raman and Nuclear Magnetic Resonance spectroscopy and Transmission Electronic Microscopy (TEM) characterization. The TEM observations showed a homogeneous irradiated glass structure up to the nanometer scale. Modifications of the local order around the main cations were noticed, mainly a slight decrease of the mean boron coordination number and an increase of non-bridging oxygen concentrations. At the glass medium range order, the appearance of the D2 Raman band and a modification of the Si–O–Si angle distribution were also observed after irradiation.A comparison with other irradiation conditions with Swift Heavy Ions (Kr with 74MeV) and Gold irradiation (with energies ranging from 1 to 7MeV) is presented. Raman spectroscopy showed a similar final structure for irradiation conditions under which the glass evolutions are controlled by electronic energy loss in the ion tracks formation regime or nuclear energy loss. Despite important differences in energy deposition regimes, the similarities observed between the final glass structures suggest that structural evolutions are controlled by the glass relaxation mechanisms during the high quenching rate step that follows the energy deposition step.

BaYb6(Si2O7)2(Si3O10) ? the first silicate containing both Si2O7 and Si3O10 groups: synthesis, crystal chemistry and topology

Colourless crystals of the novel compound BaYb6(Si2O7)2(Si3O10) were prepared using a high-temperature flux-growth technique in air. The crystal structure was solved and refined to R ( F ) = 2.50 % using single-crystal X-ray diffraction data collected at room temperature. The silicate is monoclinic, space group P 21/ m ( Z = 2), with a = 5.5173(11), b = 27.260(6), c = 6.8150(14) A, β = 106.73(3)°, V = 981.6(3) A3. BaYb6(Si2O7)2(Si3O10) represents the first silicate containing both (Si2O7) and (Si3O10) groups in the ratio 2:1 and is isotypic with (NH4)Cd6(P2O7)2(P3O10). Its framework topology is characterised by one horseshoe-shaped trisilicate (Si3O10) group, two symmetrically equivalent (Si2O7) groups with the Si–O–Si angle of 165.3° and staggered conformation, and zigzag chains of edge-sharing distorted YbO6 octahedra ( ). The Ba atoms occupy narrow channels extending parallel to [100]. The topological features of BaYb6(Si2O7)2(Si3O10) are compared to those of other compounds containing mixed groups ( M 2O7) and ( M 3O10). A similarity to BaY4(Si2O7)(Si3O10) is pointed out.

Influence of miscut angle of Si(111) substrates on the performance of InGaN LEDs

As the angle of the miscut of a Si(111) substrate increases, the luminous efficiency of InGaN LEDs decreases dramatically, along with a considerable blue shift of emission wavelength. The blue shift of the wavelength and the decrease in the efficiency are found to originate from the reduced indium incorporation and the poor uniformity of the quantum wells. The origin of the nonuniform indium incorporation of the samples is attributed to the surface morphologies formed by step bunching, as confirmed by atomic force microscopy measurements.

Local Ordering Around Tetrahedral Co2+ in Silicate Glasses

Alkali lime silicate glasses containing 5 wt% of CoO were investigated by Co K‐edge XANES and EXAFS and optical absorption spectroscopy. Our results reveal the presence of tetrahedral Co2+ connected with the glass network, with a IVCo–O–Si angle of 134°. Changing the alkali from K+ to Na+ induces an increase of the local disorder around Co2+, as shown by a decrease of the contribution from the second neighbors in the EXAFS signal. We propose two models for interpreting the structure of the second shell of neighbors. Our results provide a structural basis for rationalizing the optical properties of Co2+ species in glasses.