The viscosity — Temperature relationship for silicate glasses

Glass and Ceramics -     

Studies of surface chemistry — physico-chemical aspects of porous silicate surfaces

A series of porous silicates are prepared by destabilization (precipitation) of an alkaline silicate with a polyvalent metal salt in aqueous medium. The experimental conditions are optimised in such a manner as to obtain almost all particles with precise control on their size. On an average 98 – 99% of the product is in the particle size range −300 +350 BSS mesh. For the source of silica and alumina, several starting materials of both chemical and natural sources have been used in the synthesis. The porous silicates thus developed are exhaustively characterised for bulk density, oil absorption, specific gravity, pore volume, surface area and particle size distribution. The results obtained show that the surface properties of porous silicates vary over a wide range, although the chemical composition of the products remains practically the same. It is discussed that mode of preparation, the reactants employed and the key process variables ultimately decide the surface properties and the end-use applications.     

Fluorescence of Cascade Blue™ inside nano-sized porous shells of silicate

Cascade Blue™ (CB) dye at a concentration as high as 0.227 M was encapsulated within nano-sized porous silicate shells, and its relative fluorescence yield determined over the pH range of 1.8–12.3, using 380 nm as the excitation wavelength. The results were compared with those obtained in aqueous solution using similar pH and total dye concentration. Near neutral pH, the relative fluorescence yields of CB inside the shells exhibited little fluorescence quenching, even though a high concentration of the dye was trapped inside the particles, while the peak wavelength of fluorescence was shifted from 420 nm in solution to 430 nm in shells. Both in shells and solution, the relative fluorescence intensity decreased as the solution pH was raised from 2 to 4, and in shells it nearly disappeared at about pH 3–4. As the pH was further increased, the red shift of fluorescence peak in the shell-trapped dyes was evident at pH 5 and its fluorescence intensity regained equal to that in acid. In the neutral pH range, the fluorescence intensity of CB in the shells was similar to that of the equivalent total concentration of the CB in solution. In solution, a similar red shift of the fluorescence maximum of CB to 430 nm was observed only above pH 9. These observations suggest that the fluorescence intensities of dyes trapped inside nano-sized porous silicate shells can be equal to or higher than that observed in solution under comparable conditions, leading to several hundreds times more fluorescent intensity when it is measured per single shell rather than per unit fluorophore.     

Preparation and phosphorus recovery performance of porous calcium–silicate–hydrate

Porous calcium–silicate–hydrate was synthesized and used to recover phosphorus from wastewater. The principal objective of this study was to explore the phosphorus recovery performance of porous calcium–silicate–hydrate prepared by different Ca/Si molar ratios. Phosphorus recovery mechanism was also investigated via Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectrum (EDS), Brunauer–Emmett–Teller (BET) and X-ray Diffraction (XRD). The law of Ca²⁺ release was the key of phosphorus recovery performance. Different Ca/Si molar ratios resulted in the changes of pore structures. The increase of specific surface area and the increase in concentration of Ca²⁺ release were well agreement together. The Ca/Si molar ratio of 1.6 for porous calcium–silicate–hydrate is more proper to recover phosphorus. The pore structure of porous calcium–silicate–hydrate provided a local condition to maintain a high concentration of Ca²⁺ release. Porous calcium–silicate–hydrate could release a proper concentration of Ca²⁺ and OH^? to maintain the pH values at 8.5–9.5. This condition was beneficial to the formation of hydroxyapatite. Phosphorus content of porous calcium–silicate–hydrate reached 18.64% after phosphorus recovery.     

Search of high-capacity cathode materials based on lithium–iron silicate compounds

The crystal structures of 197 lithium–silicate compounds have been analyzed using the method of crystal chemistry analysis (TOPOS software package). The compounds whose structures are characterized with a combination of high values of such parameters as the channel radius, stability, gravimetric capacity, and capacity per volume unit have been revealed: LiFeSiO₄ (R3?), Li₄Fe₂Si₃O₁₀ (C2/c), Li₂FeSiO₄ (Pc2₁ n), and Li₂FeSiO₄ (C222₁). It has been demonstrated that lithium–iron silicates of the monoclinic syngony have high values of the capacity per volume unit, as compared to those of the rhombic syngony. The structural stability of the Li₂FeSiO₄ (Pc2₁ n) framework has been corroborated using the method of computer simulation within the scopes of the electron density functional theory. The obtained information could facilitate creation of novel cathode materials of high capacity and specific accumulated energy.     

Synthesis and characterization of a 12. 6Å calcium silicate hydrate

A 12.6Å calcium silicate hydrate was synthesized by reacting amorphous silica and calcium oxide in the presence of a large excess of NaOH at 80–98°C. This new synthetic phase was characterized by microscopic and spectroscopic techniques. These techniques showed that the above phase belongs to the tobermorite family. The 12.6Å phase transformed to 10.6Å phase by dehydration at room temperature and further dehydration at 100°C resulted in a 9.3Å phase. The 12.6Å phase exhibited a cation exchange capacity of 130 meq/100g but showed little or no Cs selectivity.     

Catalytic monoepoxidation of butadiene over titanium silicate molecular sieves TS‐1

Titanium silicate molecular sieves TS‐1 have been prepared under hydrothermal conditions with tetrapropylammonium hydroxide as structure‐directing agent. The structures of TS‐1 were characterized by means of XRD, TEM, FT‐IR, DR UV‐Vis, ICP‐AES, and N₂ adsorption/desorption techniques. Its catalytic performances in the epoxidation of butadiene with hydrogen peroxide were investigated; 3,4‐epoxy‐1‐butene was obtained with high selectivity. The influences of various parameters such as temperature, solvent, and titanium content on the activities of the catalysts were studied systematically. This revised version was published online in July 2006 with corrections to the Cover Date.     

Additive manufacturing of Ca–Mg silicate scaffolds supported by flame-synthesized glass microspheres

Novel manufacturing techniques such as additive manufacturing also referred to as 3D printing hold a critical role in the preparation of novel bioactive three-dimensional glass-ceramic scaffolds. The present paper focuses on the use of Ca–Mg silicates microspheres (Ca₂MgSi₂O₇, i.e. 40 mol% CaO, 20% MgO and 40% SiO₂) for the fabrication of 3D structures by additive manufacturing. In the first step, the crystallization of the åkermanite system was avoided, by feeding nearly fully crystallized precursor powders prepared by conventional melt quenching into oxygen-methane (O₂/CH₄) torch, and solid glass microspheres (SGMs) with diameters bellow 63 μm were prepared. In the second step, the crystallization was utilized to control the viscous flow of SGMs during firing of reticulated scaffolds, obtained by digital light processing (DLP) of the SGMs suspended in a photocurable acrylate binder. The spheroidal shape facilitated a high solid content, up to 77 wt% of the SGMs in the suspension. After burn-out of the organic binder, a fast sintering treatment at 950 °C, for 30 min, led to scaffolds preserving the macro-porosity from 3D printing model (diamond cell lattice) but with well densified struts. The crystallization of 3D scaffolds during the sintering process led to 3D structures with adequate strength-to-density ratio.     

Preparation,structural and thermo-mechanical properties of lithium aluminum silicate glass–ceramics

Lithium aluminum silicate glasses of composition (wt%) 12.6Li₂O–71.7SiO₂–5.1Al₂O₃–4.9K₂O–3.2B₂O₃–2.5P₂O₅ were prepared by the melt quench technique. These glasses were converted to glass–ceramics based on DTA data. X-ray diffraction (XRD) and Fourier transform infra-red spectroscopy (FTIR) were used to discern the phases evolved in the glass–ceramics. Phase morphology was studied using scanning electron microscopy (SEM). Thermal expansion coefficient (TEC) and glass transition temperature (T_g) of all samples were measured using thermo-mechanical analyzer (TMA). It was found that 3 h dwell time at crystallization temperature yielded samples with good crystallinity with a TEC of 9.461 × 10⁻⁶ °C⁻¹. Glass–ceramic-to-metal compressive seal with SS-304 was fabricated using LAS glass–ceramic. The presence of metal housing and compressive stresses at the glass–ceramic-to-metal interface reduced average grain size and changed the overall microstructure.     

The Mechanism of corrosion of MgOCaZrO3–calcium silicate materials by cement clinker

The chemical reactions involved in the corrosion of MgOCaZrO₃–calcium silicate materials by cement clinker were studied using a hot-stage microscope up to 1600 °C. The phases formed at 1500 °C were characterized by RLOM and SEM–EDS of the crystalline phases conducted near the reaction front and on unreacted refractory area.The general corrosion mechanism of attack on MgOCaZrO₃–calcium silicate materials involves a mechanism of matter diffusion of the liquid clinker phase through the grain boundaries and pores into the refractory substrate. The liquid phases in the clinker mainly enriched in calcium, iron and aluminium are rapidly diffused and preferentially react with magnesium spinel, calcium zirconate and magnesia, which are the major constituents in the refractory substrates. The dissolution of the CaZrO₃ refractory phase produces the enrichment with zirconium of the liquid phase increasing its viscosity and hindering the liquid phase diffusion.     

Young׳s modulus,Vickers hardness and indentation fracture toughness of alumino silicate glasses

A wide range of alumino silicate glasses with different network modifier ions (Li, Mg, Na, Ca, Zn, La, Ba, Sr, and Pb) was prepared. The glasses were studied with respect to their mechanical properties: Poisson׳s ratio, Young׳s modulus, Vickers hardness and indentation fracture toughness. These properties were mostly affected by the field strength of network modifier ions. All determined properties increase with increasing field strength of the network modifier ions. The mixed modifier alumino silicate glasses with zinc and magnesium show a positive deviation from linearity with two maxima. Lanthanum containing glasses show larger values of mechanical properties for higher lanthanum concentrations. For magnesium alumino silicate glasses the mechanical properties get smaller with increasing SiO₂ concentration; an effect of the magnesium concentration is not observed. Furthermore, if up to 9 mol% MgO is replaced by MgF₂ the mechanical properties are not significantly affected. Compared to models predicting Young׳s moduli of all studied glass compositions, significant deviations are found.     

Influence of novel shrinkage-reducing polycarboxylate superplasticizer on the nature of calcium–silicate–hydrates

Currently, a novel shrinkage-reducing polycarboxylate superplasticizer (SR-PCA) is used to control cementitious shrinkage. To clarify its mechanism when applied in cementitious materials, the influence of SR-PCA on the composition, morphology, and structure of synthetic calcium–silicate–hydrate (C–S–H), together with the interaction between SR-PCA and C–S–H at the atomic level, is investigated. For comparison, a commercial polycarboxylate superplasticizer (PCA) is also employed. The results show PCA and SR-PCA can adsorb on the C–S–H surface rather than intercalate into the layers. Compared with PCA, SR-PCA has a milder impact on C–S–H crystallinity. SR-PCA refines the pore structure of C–S–H drastically, whereas PCA loosens the structure by increasing the mesopore volume. In addition, the adsorption effect of SR-PCA on the C–S–H surface is less significant than that of PCA. At the atomic level, this less adsorption of SR-PCA is attributed to the lower adhesion energy of the C–S–H/SR-PCA interface due to the weaker Ca–O bond strength.     

Vancomycin release kinetics from Mg–Ca silicate porous microspheres developed for controlled drug delivery

In this work, sol-gel derived bredigite (MgCa₇Si₄O₁₆), akermanite (MgCa₂Si₂O₇), and diopside (MgCaSi₂O₆) porous microspheres were loaded with vancomycin, and the release kinetics of this drug from them was evaluated. In this regard, mathematical models of Higuchi, Peppas, Baker-Lonsdale, Hixson-Crowell, first order, and zero order were considered in terms of linear and nonlinear regression analyses. Using the goodness of fitting correlation coefficients, the number of release stages was determined, and then diffusion- and degradation-controlled mechanisms were assigned to each stage. It was found that the diopside microspheres presented a two-step diffusional drug delivery. For akermanite and bredigite, an initial burst diffusion-controlled release followed by a sustained mixed-mode release mechanism of diffusion and degradation was identified, with the domination of the latter especially for bredigite.     

Role of carbon nanotubes on load dependent micro hardness of SWCNT–lead silicate glass composite

Single wall carbon nanotubes (SWCNTs) were successfully entrapped in lead silicate glass by the melt quench technique. Variation of micro hardness with load for lead silicate glass and the composites (having different wt% of SWCNTs) show Reverse Indentation Size Effect (RISE). Meyer's law, the proportional specimen resistance (PSR) model and the modified PSR model have been analyzed for the load dependent hardness of the glass and the composites. Incremented elastic recovery in the composites than that of the glass was explained by the appealing cushioning performance of entangled SWCNT bundles. In contrast, higher plastic deformation beneath the indenter in the composites was established by evaluating the recovery resistance of the materials.     

Enhanced photoluminescence quantum yield of Ce3+-doped aluminum–silicate glasses for scintillation application

Ce³⁺-doped 20Gd₂O₃–20Al₂O₃–60SiO₂ (GAS:xCe³⁺) glasses (x = 0.3, 0.7, 1.1, 1.5, 1.9 mol%) with Si₃N₄ as a reducing agent were prepared. The density of the glasses is around 4.2 g/cm³. With the increase in the Ce³⁺ concentration, both the photoluminescence (PL) and PL excitation peaks of GAS:xCe³⁺ glasses show a redshift because the 4f–5d energy levels of Ce³⁺ ions are narrowed. PL quantum yield and PL decay time of GAS:xCe³⁺ glasses are 28.32–50.59% and 43–64 ns, respectively. In addition, they both first increases and then decreases with the Ce³⁺ concentration increasing, reached the maximum when x = 1.1 mol%. The integrated X-ray excited luminescence (XEL) intensity of the GAS:1.1Ce³⁺ glass is 23.86% of that of Bi₄Ge₃O₁₂ (BGO) crystal, and the light yield reaches 1200 ph/MeV with an energy resolution of 22.98% at 662 keV when exposed to γ-rays. The PL and XEL thermal activation energies of GAS:xCe³⁺ glasses are independent of Ce³⁺ ions concentration. Scintillating decay time of the glasses exhibits two components consisting of nanosecond and microsecond levels, and the scintillating decay time gradually decreases with the Ce³⁺ concentration increasing. The difference between PL and scintillating decay time is discussed regarding the different luminescent mechanisms.     

Sodium silicate activated slag-fly ash binders: Part III—Composition of soft gel and calorimetry

Sodium silicate activated, slag-fly ash binders are potential alternative binders to Portland cement. In this study, the early age properties of slag-fly ash binders namely, set time, and heats of reaction were investigated. Set time was investigated using a combination of two methods namely, the ASTM C403 penetration testing, and s-wave ultrasonic wave reflectometry (SUWR). The discrepancy in set time identified by these two methods suggested the presence of a soft gel which eventually hardened with time. The composition of this soft gel was analyzed by suspending the chemical reaction of the binder after the soft gel formed, but before it hardened. In order to analyze the composition of the soft gel, selective chemical extractions were performed on the binder. ²⁹Si Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR), and FTIR spectroscopy were performed on binders and extraction residues. The soft gel contained a modified calcium silicate hydrate gel (C–N–S–H where N=Na), with a short mean chain length and no observable Al incorporation. Orthosilicate units were also found to be present in relatively high proportions when compared to hardened binders at later ages.     

Influence of additives on nano-SiC whisker formation in alumina silicate–SiC–C monolithic refractories

This study describes the effect of additives on increasing cold crushing strength (CCS) and bulk density (BD) of alumina silicate–SiC–C monolithic refractories. Two series of carbon-containing monolithics were prepared from Iranian chamotte (sample A) and Chinese bauxite (sample B), as 65 wt.% in each case together with, 15 wt.% SiC-containing material regenerates (crushed sagger) and 10 wt.% fine coke (a total of 90% aggregate) and 10 wt.% resole (phenol formaldehyde resin) as binder. Different values of additives (such as silicon and ferrosilicon metal) are added to the mixture and BD, apparent porosity and CCS are measured after tempering at 200 °C for 2 h and firing at 1100 °C and 1400 °C for 2 h. At low temperature of 200 °C, Si and ferrosilicon contribute to the formation of stronger cross-linking in the resit structure and provide CCS values of as high as about 65 MPa. At 1400 °C, SiC whiskers of nano-sized diameter are formed due to the presence of Si and FeSi₂ and improve CCS values of as high as about 3–4 times in sample containing 6 wt.% ferrosilicon metal as additive.     

Porous scaffold prepared from α′L-Dicalcium silicate doped with phosphorus for bone grafts

α′_L-Dicalcium silicate ceramic doped with phosphorus in subsystem Ca₂SiO₄?7CaO·P₂O₅·2SiO₂ was used as a template to prepare a porous scaffold by the burnout technique using cellulose as porogen. A phase composition study of the ceramics prepared after porogen removal was performed by XRD and SEM-WDS in contrast to the base material. An analysis of the inter- and intraparticle pores network was run by mercury porosimetry. Density was measured by helium pycnometry. The scaffold's porosity was measured using Archimedes’ method by immersion in mercury. The in vitro behavior in SBF of the monophasic scaffold was analyzed by SEM-WDS, FTIR-ATR and TEM for several time periods. The processing technique allowed scaffolds to be easily and cheaply obtained with porosities of up to 81% by combining macro- and microporosity. The specific area effectively increased which, together with the good bioactivity rate obtained, make this material a promising candidate for bone grafts in tissue engineering.     

Studies on the liquid-phase ammoximation of cyclohexanone over a titanium silicate sieve using on-line ATR–FTIR spectroscopy

The liquid-phase ammoximation of cyclohexanone over a titanium silicate sieve is followed in real time using on-line ATR–FTIR spectroscopy. We have discovered the formation and the vanishment of an intermediate during the reaction with the aid of on-line ATR–FTIR spectroscopy. A new possible reaction mechanism is proposed according to the experimental results. Hydrogen peroxide is firstly adsorbed on titanium silicate sieves, forming a complex peroxidic species of titanium, which can react with cyclohexanone to form an intermediate. Then the intermediate reacts with ammonia to give cyclohexanone oxime. The infrared spectra indicate that the intermediate has strong absorption of C–O bond.