Bridging Oxygen Atoms Research Articles

Effect of varying phosphate content on the structure and properties of sol-gel derived SiO2-CaO-P2O5 bio-glass

In this work, biocompatible glass (bioglass) particles were prepared by low temperature, acid catalysed sol-gel method. The effect of varying phosphate (P2O5) content (10, 15 and 20 mol %) in the sol-gel derived glass composition were studied. The sol-gel derived bioglass particles were compacted into cylindrical pellets via hydraulic press machine and sintered at 600°C for 3 hours. The bioglass particulates were analysed by x-ray fluorescence (XRF), Fourier Transformed Infrared (FTIR), X-Ray Diffraction (XRD) and nitrogen gas adsorption. Meanwhile, the sintered bioglass pellets were analysed by FTIR, XRD and FESEM-EDX. Furthermore, in vitro bioactivity analysis was performed by immersion in simulated body fluid (SBF) for 14 days. Bioglass particulates with high glassy phase, high surface area and high porosities were obtained for all compositions. Increasing of phosphate content to 20 mol% particularly reduced the porous characteristics of the bioglass particulates. Furthermore, leads to higher bridging oxygen (BO) atoms, higher amorphous silicate networks, lower glass crystallinity and higher number of phosphate crystallites within the amorphous glassy matrix. Increased to 20 mol% of phosphate also reduced the ability of the bioglass surface to induce carbonated apatite formation when immersed in simulated body fluid (SBF) solution.

The influence of temperature on the Ti4+ coordination in sodium titanosilicate melts: In-situ high-temperature Raman spectroscopy study

The structure of xTiO2-(100-x)[40Na2O-60SiO2] (x = 10, 15, 25 and 30 mol%) glasses and melts was studied in situ in the temperature range of 20–1150 °C using high-temperature Raman spectroscopy. The fraction of five-fold coordinated Ti4+ ions gradually decreases as temperature increases above ∼ 600 °C in the samples with x ≤ 25 mol%. No similar changes were found for the sample with x = 30 mol%. The change in titanium coordination is related to the transformation of square-based TiO5 pyramids into TiO4 tetrahedra with the formation of additional non-bridging oxygen atoms (NBO) in the melt structure. This transformation can be described by [TiØ4O]2− + SiØ3O− ↔ TiØ3O− + SiØ2O22− and [TiØ4O]2− + SiØ4 ↔ TiØ3O− + SiØ3O− (Ø is bridging oxygen atom) disproportional reactions, which are shifted toward the right with increasing temperature. All studied melts exhibit a more uniform titanium distribution in comparison with glasses of corresponding composition.

Yttrium and rare-earth modified lithium orthoborates: Glass formation and vibrational activity

Glass formation and structure-property relations were explored in highly modified borate glasses containing high loads of rare-earth elements, whose crystalline analogues display a trigonal to tetrahedral borate phase transition (BO33− → BØ2O23−, where Ø and O− indicate bridging and non-bridging oxygen atoms). The resulting borate networks are completely depolymerized, where borate anions are crosslinked to rare-earth and modifier cations via ionic bonds. The borate structure was found to be based on a single structural unit, BO33− triangles, whose fundamental vibrations are all active in both the Raman and infrared. The local environment of the rare-earth ions in orthoborate glasses was studied with far infrared spectroscopy and, in some cases, by using terbium as a probe ion. A linear correlation was obtained between the effective force constant in the far infrared and the field strength of the rare-earth cation.

Unexpected and enhanced electrostatic adsorption capacity of oxygen vacancy-rich cobalt-doped In2O3 for high-sensitive MEMS toluene sensor

Benefiting from the enhanced electrostatic adsorption derived from surface oxygen vacancy, the reported MEMS based hollow-porous Co-In 2 O 3 gas sensor exhibits excellent selectivity and response, and toluene detection limits as low as 100 ppb at the lowest temperature so far. • The hollow-porous Co-doped In 2 O 3 microspheres with abundant electrostatic adsorption sites were synthesized by ultrasonic spray pyrolysis. • The MEMS toluene sensor reported in this work with the characteristic of low power consumption. • The Co-In 2 O 3 gas sensor exhibits high sensitivity and superior selectivity to ppb level of toluene at relatively lower temperature. • Demonstrating the impact of electrostatic adsorption on charge carrier transport and toluene sensing performance by DFT calculation. Semiconductor metal oxide (SMO) gas sensor has been widely researched in artificial olfactory for monitoring gases and odor. However, realizing the characteristics of low detection limit, immunity from interference capacity and low operating temperature still remains challenge. Herein, the synthesis of hollow-porous cobalt-doped In 2 O 3 sensing material with abundant surface oxygen vacancy (SOV) via one-pot and in-situ doping approach is reported. With their rich active sites, the Co-In 2 O 3 exhibits unexpected and high electrostatic adsorption of toluene at 175 °C, which obviously inducing the charge carrier transport. Accordingly, the final Microelectro Mechanical Systems (MEMS) based Co-In 2 O 3 gas sensor exhibits excellent selectivity and response, and toluene detection limits as low as 100 ppb at the lowest temperature so far. Density functional theory (DFT) calculations elucidated that the enhanced electrostatic adsorption is ascribed to the toluene preferentially bound to O 3c atom site (bridging oxygen atom near SOV) on Co-In_b-D 4 (110) surface, which will induce charge carrier accumulation layer and cause temperature-dependent abnormal pseudo p-type response. This work points out that the electrostatic adsorption derived from SOV could obviously modulate SMO sensing properties, and further supplements the defect engineering.

Synthesis and Oligonuclear Structures of Strontium and Barium Complexes with Protonated and Deprotonated N-Mesityl-P,P-diphenylphosphinic Amide Ligands.

Metalation of N-mesityl-P,P-diphenylphosphinic amide Ph2P(O)-NHMes (HL, I) with MgBu2 and Ae{N(SiMe3)2}2 (Ae = Ca, Sr, and Ba) yields alkaline-earth metal complexes with the compositions of [(thf)nAe(L·HL)2] [Ae/n = Mg/0 (II), Ca/2 (III)] as well as of [Sr2L3(L·HL)(HL)] (1), [Ba2L3(L·HL)(HL)] (2), [Ba3L6] (3), and [(thf)2Ba3L6] (4). In III, 1, 2, and 3, the alkaline-earth metal atoms are in severely distorted octahedral environments, and the structural distortions are partially caused by the small O-Ae-N bite angles of the chelating Ph2P(O)-NMes anions. The substructures (L·HL) contain N–H···N hydrogen bridges, stabilizing the arrangement of the ligands in complexes II, III, 1, and 2. In the trinuclear barium complex [Ba(μ-L)3Ba(μ-L)3Ba] (3), a rigid adjustment of the anionic L bases leads to a C3-symmetric molecule in the crystalline state with bridging oxygen atoms. Due to the small O–Ba–N bite angles of the chelating anions, vacant coordination sites are available at the outer barium centers. Coordination of thf bases in these gaps yields the complex [(thf)Ba(μ-L)3Ba(μ-L)3Ba(thf)] (4). However, THF is unable to deaggregate the trinuclear complexes into smaller barium-containing moieties. Increasing the radius of the alkaline-earth metals and increasing the nuclearity of these compounds lead to decreasing solubility in common organic solvents. NMR studies verify that the molecular structures of these alkaline-earth metal complexes are maintained in ethereal solvents and toluene.

Oxygen Evolution Reaction on the Fe3O4(001) Surface: Theoretical Insights into the Role of Terminal and Bridging Oxygen Atoms

Model catalysts, where the structure of single-crystal materials with well-defined surface terminations can be determined at the atomic level, are useful systems to perform fundamental studies on electrocatalysis. The magnetite Fe3O4(001) surface, in particular, has been the focus of several studies aimed at characterizing its structure in vacuum in the presence of a water layer and interfaced with liquid water-based electrolytes. Recently, this system has also been investigated as a catalyst for the oxygen evolution reaction (OER), with the goal of correlating structural properties of the interface with catalytic performance and mechanism. In this work, we use first-principles simulations based on density functional theory to establish the structural, thermodynamic, and electronic properties of the Fe3O4(001) surface in contact with water. We compute the phase diagram of the magnetite/water system and address some open issues on the structural transitions observed experimentally among different surface terminations. We then address the stability in electrochemical environments, and we investigate the OER mechanism, considering reaction paths involving both terminal and bridging oxygen atoms. We find that different surface reconstructions can promote OER via different reaction sites and potential-determining steps, albeit with a similar energy cost. In particular, the bulk-truncated termination promotes OER via terminal oxygen atoms, and the potential-determining step is the dehydrogenation of the *OH group. On the (2×2)R45° reconstruction, on the other hand, OER proceeds via the bridging oxygen atoms, and the potential-determining step is the formation of the hydroperoxo.

Structure of crystalline and amorphous materials in the NASICON system Na1+xAlxGe2-x(PO4)3.

The structure of crystalline and amorphous materials in the sodium (Na) super-ionic conductor system Na1+xAlxGe2-x(PO4)3 with x = 0, 0.4, and 0.8 was investigated by combining (i) neutron and x-ray powder diffraction and pair-distribution function analysis with (ii) 27Al and 31P magic angle spinning (MAS) and 31P/23Na double-resonance nuclear magnetic resonance (NMR) spectroscopy. A Rietveld analysis of the powder diffraction patterns shows that the x = 0 and x = 0.4 compositions crystallize into space group-type R3̄, whereas the x = 0.8 composition crystallizes into space group-type R3̄c. For the as-prepared glass, the pair-distribution functions and 27Al MAS NMR spectra show the formation of sub-octahedral Ge and Al centered units, which leads to the creation of non-bridging oxygen (NBO) atoms. The influence of these atoms on the ion mobility is discussed. When the as-prepared glass is relaxed by thermal annealing, there is an increase in the Ge and Al coordination numbers that leads to a decrease in the fraction of NBO atoms. A model is proposed for the x = 0 glass in which super-structural units containing octahedral Ge(6) and tetrahedral P(3) motifs are embedded in a matrix of tetrahedral Ge(4) units, where superscripts denote the number of bridging oxygen atoms. The super-structural units can grow in size by a reaction in which NBO atoms on the P(3) motifs are used to convert Ge(4) to Ge(6) units. The resultant P(4) motifs thereby provide the nucleation sites for crystal growth via a homogeneous nucleation mechanism.

Switching the O–O bond-formation mechanism by controlling water activity

Switching the O–O bond-formation mechanism by controlling water activity

Low-dimensional compounds containing bioactive ligands. Part XVI: Halogenated derivatives of 8-quinolinol N-oxides and their copper(II) complexes

Four N-oxides, 8-quinolinol N-oxide (8-HQNO), 5,7-dichloro-8-quinolinol N-oxide (HdClQNO), 5,7-dibromo-8-quinolinol N-oxide (HdBrQNO) and 7-iodo-8-quinolinol N-oxide (HIQNO) as well as their six copper complexes, CuCl2(8-HQNO)2(H2O) (1), CuCl2(HdClQNO)2(H2O)2 (2), Cu(dClQNO)2(CHCl3) (3), Cu(dClQNO)2(H2O) (4), {[Cu(dBrQNO)2]•2H2O}n (5) and CuCl2(HIQNO)2(H2O)4 (6) were synthesised as possible anticancer agents. Crystal structures of N-oxides contain planar molecules held together via hydrogen bonds involving oxygen atoms of N-oxide groups as acceptors. Crystal structure of 5 represents the first structure of a copper(II) complex with an N-oxide ligand derived from 8-HQNO and is formed by infinite chains. In the chain, the Cu(II) atom coordinates to six oxygen atoms from two bidentate chelating dBrQNO ligands occupying apexes of elongated tetragonal bipyramid with bridging oxygen atoms of N-oxide groups in axial positions. Antiproliferative activity of prepared N-oxides as well as their complexes was studied using in vitro MTT assay against the MDA-MB-231, HCT-116 and A549 cancer cell lines and their selectivity was verified on MSCs cells. Among the tested cancer cell lines, A549 and MDA-MB-231 cells were the most sensitive to the tested complexes. Complex 1 showed the highest cytotoxicity against both tumor cell lines. At concentration, which could be tested in animal models, 1 induced cell death in more than 50% of cancer cells and in 20% of MSCs indicating its selectivity.

Effect of acid treated HY zeolites in adsorption of mesosulfuron-methyl.

Pollution of surface water and groundwater by bulky molecules such as pesticides has been recognized as a major problem in many countries due to their persistence in aquatic environment and potential adverse health effects. The main purpose of this study is the development of a capable adsorbent to remove these bulky molecules from wastewater such as the pesticide Mesosulfuron-Methyl (MM) by reducing the diffusion path, to overcome the problems of diffusional limitations on microporous adsorbents. The adsorption of mesosulfuron-methyl (MM) from aqueous solution is curried out using treated acid HY zeolite. Batch sorption equilibrium and kinetic experiments are conducted to evaluate the efficiency of these materials. Parent zeolites and their derivatives have been characterized by nitrogen adsorption-desorption, pyridine chemisorption followed by infrared spectroscopy and X-ray fluorescence. The acid treatment leads to an increase in the specific surface from 691 to 853 m2 g- 1 for HY(30) and from 631 to 806 m2 g- 1 for the HY(16.6) zeolites. It also leads to a reduction in Lewis acidity from 74 to 25 µmol g- 1 and from 135 to 31 µmol g- 1 for HY(30) and HY(16.6) zeolites respectively, and increases the adsorbent-adsorbate interaction. The adsorption capacity increased from 83 to 99 % after acid treatment. The equilibrium adsorption time is decreased from 15h to 10min for the HY(30)_A and from 20h to 20min for the HY(16.6)_A for an initial concentration of 20mg L- 1. The adsorption capacity depends on the pH solution, and the neutral form of the MM is more easily adsorbed into zeolite than the dissociated form via the framework bridged oxygen atoms. For all the samples, the pseudo-second-order kinetic model fits very well with the experimental data. In the case of the modified zeolites, the approaching equilibrium factor R w decreases from 0.08183 to 0.00008 when the Lewis acid sites decrease; indicating that the equilibrium is reached more quickly. S-shape adsorption isotherms indicates that cooperative adsorption phenomena. Nevertheless, the shape of acid treated zeolites evolves to an L type indicating a significant enhancement of the adsorbent - adsorbate interactions inducing better adsorption efficiency. Mesosulfuron-methyl adsorption has been successfully enhanced after acid treatments of zeolites HY.

Congener Substitution Reinforced Li7P2.9Sb0.1S10.75O0.25 Glass-Ceramic Electrolytes for All-Solid-State Lithium-Sulfur Batteries.

Glass-ceramic sulfide solid electrolytes like Li7P3S11 are practicable propellants for safe and high-performance all-solid-state lithium-sulfur batteries (ASSLSBs); however, the stability and conductivity issues remain unsatisfactory. Herein, we propose a congener substitution strategy to optimize Li7P3S11 as Li7P2.9Sb0.1S10.75O0.25 via chemical bond and structure regulation. Specifically, Li7P2.9Sb0.1S10.75O0.25 is obtained by a Sb2O5 dopant to achieve partial Sb/P and O/S substitution. Benefiting from the strengthened oxysulfide structural unit of POS33- and P2OS64- with bridging oxygen atoms and a distorted lattice configuration of the Sb-S tetrahedron, the Li7P2.9Sb0.1S10.75O0.25 electrolyte exhibits prominent chemical stability and high ionic conductivity. Besides the improved air stability, the ionic conductivity of Li7P2.9Sb0.1S10.75O0.25 could reach 1.61 × 10-3 S cm-1 at room temperature with a wide electrochemical window of up to 5 V (vs Li/Li+), as well as good stability against Li and Li-In alloy anodes. Consequently, the ASSLSB with the Li7P2.9Sb0.1S10.75O0.25 electrolyte shows high discharge capacities of 1374.4 mAh g-1 (0.05C, 50th cycle) at room temperature and 1365.4 mAh g-1 (0.1C, 100th cycle) at 60 °C. The battery also presents remarkable rate performance (1158.3 mAh g-1 at 1C) and high Coulombic efficiency (>99.8%). This work provides a feasible technical route for fabricating ASSLSBs.

Electric-field-promoted photo-electrochemical production of hydrogen from water splitting

Given that conversion efficiencies of incident solar radiation to liquid fuels, e.g., H2, are of the order of a few percent or less, as quantified by ‘solar to hydrogen’ (STH), economically inexpensive and operationally straightforward ways to boost photo-electrochemcial (PEC) H2 production from solar-driven water splitting are important. In this work, externally-applied static electric fields have led to enhanced H2 production in an energy-efficient manner, with up to ~30–40% increase in H2 (bearing in mind field-input energy) in a prototype, open-type solar cell featuring rutile/titania and hematite/iron-oxide (Fe2O3), respectively, in contact with an alkaline aqueous medium (corresponding to respective relative increases of STH by ~12 and 16%). We have also performed non-equilibrium ab-initio molecular dynamics in both static electric and electromagnetic (e/m) fields, for water in contact with a hematite/iron-oxide (001) surface, observing enhanced break-up of water molecules, by up to ~70% in the linear-response régime. We discuss the microscopic origin of such enhanced water-splitting, based on experimental and simulation-based insights. In particular, we external-field direction at the hematite surfaces, and scrutinise properties of the adsorbed water molecules and OH– and H3O+ species, e.g., hydrogen bonds between water-protons and the hematite surfaces’ bridging oxygen atoms, as well as interactions between oxygen atoms in adsorbed water molecules and underlying iron atoms.

Water Breakup at Fe2O3-Hematite/Water Interfaces: Influence of External Electric Fields from Nonequilibrium Ab Initio Molecular Dynamics.

The dynamical properties of physically and chemically adsorbed water molecules at pristine hematite-(001) surfaces have been studied by means of nonequilibrium ab initio molecular dynamics (NE-AIMD) in the NVT ensemble at room temperature, in the presence of externally applied, uniform static electric fields of increasing intensity. The dissociation of water molecules to form chemically adsorbed species was scrutinized, in addition to charge redistribution and Grotthus proton hopping between water molecules. Dynamical properties of the adsorbed water molecules and OH– and H3O+ ions were gauged, such as the hydrogen bonds between protons in water molecules and the bridging oxygen atoms at the hematite surface, as well as the interactions between oxygen atoms in adsorbed water molecules and iron atoms at the hematite surface. The development of Helmholtz charge layers via water breakup at Fe2O3–hematite/water interfaces is also an interesting feature, with the development of protonic conduction on the surface and more bulk-like water.

Synthesis, crystal structure, thermal analysis and molecular interactions of bis(3,5-dimethyl-3H-1,2,4-triazole)octamolybdate

This paper describes the synthesis of {bis(3,5-dimethyl-3H-1,2,4-triazole)octamolybdate}(2:1) molecule by mixing bis(acetylacetonato)dioxo molybdenum(VI) and 3-(E)-{1-(furan-2-yl)ethylidene)amino]-1-phenylurea. The structure of the molecule was determined by using single crystal X-ray diffraction technique. Structure determination reveals that the title complex Mo 8 O 26 .2(C 4 H 9 N 3 ) contains two moieties namely an octamolybdate [Mo 8 O 26 ] complex, and 3,5-dimethyl-1,2,4-triazole [2(C 4 H 9 N 3 )] in the unit cell. The [Mo 8 O 26 ] unit consists of two {Mo 4 O 13 } subunits linked together by bridging oxygen atoms with an approximate C 2h symmetry under Triclinic system, P space group with lattice parameters a = 10.1007(2), b = 10.3887(2), c = 13.0380(3) Å, α = 66.921(3) β = 69.466(3 ) γ = 62.892(2)°. The octamolybdate units are linked by methylmethanamine as chains via N-H…O interactions. The 1 H- 1 H COSY NMR recorded in DMSO-d 6 spectral data indicates the absence of the coupling between the protons. The thermal analysis indicates that the complex is thermally stable. The Hirshfeld surface of the compound mapped with d norm (-1.0 a.u. to 1.0 a.u.) and the deep red spot on the Hirshfeld surface depicts the formation of strong N-H···O hydrogen bond.

CaMn3IVO4 Cubane Models of the Oxygen‐Evolving Complex: Spin Ground States S<9/2 and the Effect of Oxo Protonation

AbstractWe report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn3IVO4 and YMn3IVO4 complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen‐evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin‐state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn3IVO4 complexes and variants with protonated oxo moieties need not be S=9/2. Desymmetrization of the pseudo‐C3‐symmetric Ca(Y)Mn3IVO4 core leads to a lower S=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S=3/2 spin ground state is observed in CaMn3IVO3(OH). Our studies complement the observation that the interconversion between the low‐spin and high‐spin forms of the S2 state is pH‐dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin‐state changes.

CaMn3IV O4 Cubane Models of the Oxygen-Evolving Complex: Spin Ground States S

We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn3IV O4 and YMn3IV O4 complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen-evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin-state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn3IV O4 complexes and variants with protonated oxo moieties need not be S=9/2. Desymmetrization of the pseudo-C3 -symmetric Ca(Y)Mn3IV O4 core leads to a lower S=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S=3/2 spin ground state is observed in CaMn3IV O3 (OH). Our studies complement the observation that the interconversion between the low-spin and high-spin forms of the S2 state is pH-dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin-state changes.

A 2D Cd-containing Keggin-type heteropolyoxometalate with electro-catalytic behaviors

A layer compound, [Cd2(bibp)2.5(DMA)3(DMF) 2 (SiW12O40)]n (1), which was synthesized via a self-assembly procedure by the reaction of silicotungstic acid, cadmium ion and bichelate-bridging ligand (4,4′-bis(imidazolyl)biphenyl, bibp) in the solution of DMF and DMA, has been characterized by IR, elemental analysis, TGA, X-ray single crystal diffractometer and electrochemistry. Single crystal structure analysis shows that the structure for 1 represents hoop-club chains by the cadmium ions and the bibp ligands, extends to layers by the [SiW12O40]4− polyanions and further spreads to 3D by hydrogen bond via the bridge oxygen atoms in [SiW12O40]4− units. The electrochemical investigation shows that compound 1 exhibits POM-based electro-catalytic behavior for the reduction of nitrite.

Isothermal Reduction of IrO2(110) Films by Methane Investigated Using In Situ X-ray Photoelectron Spectroscopy

Continuous exposure to methane causes IrO2(110) films on Ir(100) to undergo extensive reduction at temperatures from 500 to 650 K. Measurements using in situ X-ray photoelectron spectroscopy (XPS) confirm that CH4 oxidation on IrO2(110) converts so-called bridging oxygen atoms (Obr) at the surface to HObr groups while concurrently removing oxygen from the oxide film. Reduction of the IrO2(110) film by methane is mildly activated as evidenced by an increase in the initial reduction rate as the temperature is increased from 500 to 650 K. The XPS results show that subsurface oxygen efficiently replaces Obr atoms at the IrO2(110) surface during CH4 oxidation, even after the reduction of multiple layers of the oxide film, and that metallic Ir gradually forms at the surface as well. The isothermal rate of IrO2(110) reduction by methane decreases continuously as metallic Ir replaces surface IrO2(110) domains, demonstrating that IrO2(110) is the active phase for CH4 oxidation under the conditions studied. A key finding is that the replacement of Obr atoms with oxygen from the subsurface is efficient enough to preserve IrO2(110) domains at the surface and enable CH4 to reduce the ∼10-layer IrO2(110) films nearly to completion. In agreement with these observations, density functional theory calculations predict that oxygen atoms in the subsurface layer can replace Obr atoms at rates that are comparable to or higher than the rates at which Obr atoms are abstracted during CH4 oxidation. The efficacy with which oxygen in the bulk reservoir replenishes surface oxygen atoms has implications for understanding and modeling catalytic oxidation processes promoted by IrO2(110).

Insight Into the Strengthening Mechanism of the Al-Induced Cross-Linked Calcium Aluminosilicate Hydrate Gel: A Molecular Dynamics Study

Understanding and controlling the mechanical properties of calcium aluminosilicate hydrate (C-A-S-H) gel is essential to the performance improvement of cementing materials. This study characterizes the mechanical properties and failure mechanism of cross-linked C-A-S-H that have Al/Si ratios ranging from 0 to 0.20 by employing the reactive molecular dynamics simulation. In these constructed C-A-S-H models, the Al-induced cross-linking effect on the aluminosilicate chains is well reproduced. With the incorporation of aluminate species, layered C-S-H structure gradually transforms into three-dimensional C-A-S-H. The uniaxial tensile tests show that Al-induced cross-links significantly increase the cohesive force and stiffness of C-A-S-H along both y- and z-directions. In the C-A-S-H model with the Al/Si ratio equal to 0.2, in which all the bridging sites are cross-linked, the toughness along y-direction significantly improves the interlayer mechanical properties compared to those within the layers. The deformation mechanism of the C-A-S-H structure is also studied. Results show that the depolymerization of the calcium aluminosilicate skeleton is the main route to uptake the loading energy. Both the increase of y- and z-directional strength of the structure can be related to the increasing polymerization of aluminosilicate chains along that direction. This demonstrates the important role of aluminosilicate chains in resisting the external tensile loading. Besides, during the failure process in C-A-S-H elongation, the hydrolysis reactions of calcium silicate skeleton are caused by the coupling effect of loading and interlayer water “attack.” While the Al-O-Si bond breakage results from the protonation of bridging oxygen atom, the hydrolytic reaction of Si-O-Si is initiated by five-coordinate silicon formation. Both pathways weaken the bridging bond and thus result in the breakage of T-O-Si, where T is Al or Si.

Study of the Crystallization and structural behavior of Bismuth Barium Titanate Glass-Ceramics

Bismuth Barium titanate glass-ceramics with composition (100-x)Bi2O3-xBaTiO3 synthesized via controlled crystallization have been explored for their structural and optical properties. X-Ray Diffraction data clearly confirmed the presence of crystalline phases in the prepared glass-ceramics with orthorhombic Ba2Ti13O22 phase being the leading and stable one. Additionally, the commencement of hexagonal BaTiO3 phase has been found in higher compositions above x ≥ 20 with increase in BaTiO3 content. FTIR and Raman spectroscopy revealed the conversion of Non-bridging oxygen atoms (NBO's) to Bridging oxygen atoms (BO's) takes place when BaTiO3 content increases in the sample and thus resulting in compactness of the network. UV-Visible spectroscopy has been performed to calculate their optical band gap energies (Eg). An increase in optical band gap energy has been found and same has also been confirmed using Urbach energy approach. The lower values of Urbach energy propose the creation of (BO's), decrease in disorder and hence support structural stability of the system. The electronegativity and electronic polarizability values reveal the covalent character of glass-ceramic samples.