The role of strontium and potassium on crystallization and bioactivity of Na2O-CaO-P2O5-SiO2 glasses

The effect of SrO/CaO and K₂O/Na₂O replacements on the crystallization process of glasses based on Na₂O-CaO-P₂O₅-SiO₂ system was investigated. The glasses were thermally treated through controlled heat treatment regimes to obtain glass ceramic materials. Combeite Na₂Ca₂Si₃O₉, sodium calcium silicate Na₂Ca₃Si₆O₁₆, wollastonite solid solution, and whitlockite Ca₃(PO₄)₂ were identified as major crystalline phases in the prepared thermally treated glasses. No potassium and strontium-containing phases could be detected in the glass-ceramics; potassium seems to be accommodated in the wollastonite structure, while strontium might be incorporated in the sodium calcium silicate structure.The surface reactivity of the prepared glass-ceramic specimens was also studied in vitro in Kokobo's simulated body fluid (SBF). EDAX, SEM, inductively coupled plasma ICP, and FTIR were used to examine the formation of apatite layer's surface and characterize the glass ceramic surface and SBF compositional changes. A decrease in the bioactivity of the glass ceramic was observed as Na₂O was replaced by K₂O. Strontium together with calcium ions in the apatite layer formed was detected with SrO/CaO replacement.The role played by the glass oxide constituents in determining the crystallization and bioactivity behaviour of the prepared thermally treated glasses was discussed.     

Impact of recently discovered sodium calcium silicate solutions on the phase diagrams of relevance for glass-ceramics in the Na2O-CaO-SiO2 system

Although phase relations in the Na₂O-CaO-SiO₂ system are vital to melting and thermal treatments in glass and glass-ceramics industries, the available data for thermodynamic modeling are mostly based on reports published in 1920s and 1950s. The present investigation verifies the formation of solid solutions of Na₂CaSiO₄ and Na₂Ca₂Si₂O₇ which have previously assumed to be stoichiometric compounds. The impact of these solid solutions on the features of the phase diagram were investigated using the equilibration-quenching-EDS/EPMA technique. The data were reported as liquidus projections and in isothermal sections within the temperature range of 1000 and 1400 °C. Ten primary phase fields were identified, namely SiO₂, Na₂Ca₃Si₆O₁₆, combeite, Na₄CaSi₃O₉ss, CaSiO₃, Na₂CaSiO₄ss, Na₂Ca₂Si₂O₇ss, Na₂Ca₆Si₄O₁₅, Ca₃Si₂O₇ and Ca₂SiO₄. In addition, some novel liquidus data and invariants points were examined in more detail. The fundamental data obtained can be employed for the thermodynamic reassessment of the Na₂O-CaO-SiO₂ system. The present study also discusses the findings and their impact on melting and annealing processes during the manufacture of glass and glass-ceramics.     

Magnetic and bioactive properties of MnO2/Fe2O3 modified Na2O-CaO-P2O5-SiO2 glasses and nanocrystalline glass-ceramics

Glass and in-situ nanocrystalline glass-ceramics of compositions 45SiO₂-25CaO-10Na₂O-5P₂O₅-xFe₂O₃-(15-x) MnO₂ are investigated for their magnetic and in-vitro bioactive properties. The ferrimagnetic character is observed in the high Fe₂O₃ containing in-situ nanocrystalline glass-ceramics. Saturation magnetization and coercivity increases with Fe₂O₃. After soaking in the simulated body fluid (SBF), the powdered as well as the bulk glasses and glass-ceramics are investigated using various characterization techniques. The presence of MnO₂ increases the leaching of Na⁺ ions from the glasses and also attracts the Ca²⁺ cations from the SBF as compared to Fe₂O₃ containing nano-crystalline glass-ceramics. It also increases the tendency to form hydroxyl apatite (HAp) layer. Microwave Plasma Atomic Emission Spectroscopy (MP-AES), Fourier Transform Infrared (FTIR) spectra, X-ray diffraction and Scanning electron micrographs (SEM) after soaking in the SBF confirm the HAp formation on the surface of all the glasses and glass-ceramics. Urbach energy also indicates the structural modifications on the surfaces of the glass and glass-ceramics after soaking in the SBF.     

Effect of Na2MoO4 on bond strength of adhesive-bonded phosphate coated magnesium AZ31 sheets

A pretreat phosphate solution having the formulation of KMnO₄, K₂HPO₄, Na₂SiO₃, NaF and Na₂MoO₄ was developed to improve the strength of adhesive-bonded magnesium AZ31 joints. The phosphate coating which has the magnesium phosphate, MgO, Mg (OH)₂, MgF₂ and minor Al₂O_3,, Al(OH)₃ Al_0.35–0.55Si_0.10–0.48P_0.13–0.35O_2.1–2.2 and, Al_0.35Si_0.48P_0.18O_2.2, and Al_0.52P_0.48O_2.2 was formed on the surface of the magnesium AZ31 after treatment. The additives Na₂MoO₄ in a phosphate-permanganate solution mainly act as an accelerator to speed up the coating formation. The combination of Na₂MoO₄ and NaF improves significantly the corrosion resistance of the magnesium AZ31 and performance of adhesive bonded joints. The joint strength and durability of the bonded Mg AZ31 joints in hot-humid environmental condition are superior to that of the phosphate treated (RPT) and CrO₃ etching (CET) pretreated samples.     

Formation characteristics of sodium calcium silicate compounds based on the solid-state reaction

The formation thermodynamics, phase transition and stability of sodium calcium silicate compounds under different calcination parameters in the Na₂O–CaO–SiO₂ system were studied using XRD, FTIR and SEM-EDS methods. As the Na₂O/SiO₂ ratio increases from 0.3 to 0.7 when the CaO/SiO₂ ratio is 1.0, the formation sequence of sodium calcium silicate compounds is Na₂Ca₃Si₂O₈→Na₆Ca₃Si₆O₁₈→Na₂Ca₂Si₂O₇→Na₂CaSiO₄; as the CaO/SiO₂ ratio increases from 0.3 to 1.2 when the Na₂O/SiO₂ ratio is 0.5, the formation sequence is Na₆Ca₃Si₆O₁₈→Na₂Ca₂Si₂O₇→Na₂Ca₃Si₂O₈. As the most stable sodium calcium silicate compound, Na₆Ca₃Si₆O₁₈ forms by the solid-state reaction of preformed Na₂SiO₃ with CaO and SiO₂, while increasing the calcination temperature and holding time can promote its crystal stability. The decomposition of Na₆Ca₃Si₆O₁₈ in sodium aluminate solution follows the mixed control of the film diffusion and chemical reaction, and the corresponding activation energy is between 40 and 41 kJ/mol.     

Effect of metastable phase on the crystallization and mechanical properties of MgO-Al2O3-SiO2 glass-ceramics without nucleating agents

Glass-ceramics without nucleating agents usually undergo surface crystallization, which deteriorates the overall performance of the products. In this paper, we evaluated the effects of the metastable MgAl₂Si₃O₁₀ crystalline phase on the crystallization behavior of a MgO–Al₂O₃–SiO₂ (MAS) glass without nucleating agents and mechanical properties of the glass-ceramics obtained. The results demonstrated that the precipitation of metastable MgAl₂Si₃O₁₀ crystallites promotes the crystallization mechanism transformed from surface crystallization into volume crystallization with two-dimensional crystal growth. Furthermore, the grain size of MgAl₂Si₃O₁₀ near the surface of the prepared glass-ceramics was larger than that of MgAl₂Si₃O₁₀ inside, which helps to generate compressive stress and improves its mechanical properties. The glass-ceramics containing metastable MgAl₂Si₃O₁₀ phase exhibited an enhanced hardness in the range of 7.6 GPa–9.5 GPa for indentation loads ranging from 2.94 N to 98 N, and indentation size effect behavior was observed in Vickers hardness tests of both MAS glass and glass-ceramics. The load-independent hardness values for MAS glass and glass-ceramics were reliably evaluated by the modified proportional specimen resistance (MPSR) model of 7.1 GPa and 7.6 GPa, respectively, with a high correlation coefficient of more than 0.9999. This work reveals the unexploited potential of the metastable phase in improving the crystallization ability and mechanical properties of glass-ceramics.     

Phase formation,microstructure and electrical properties of mica glass-ceramics containing Cr2O3 produced by heat treatment

The mica glass-ceramics containing Cr₂O₃ were produced via heat treatment process from the bulk parent glass specimens. The appropriate treatment temperatures were selected according to the information provided by the DTA measurement. XRD analysis demonstrated that co-existence of the mica and MgAl₂Si₃O₁₀ phases were found at the lower treatment temperature while pure mica phase could be formed at a higher temperature. The SEM investigations revealed the effect of Cr₂O₃ addition on mica crystals development. The electrical resistivity of the glass-ceramic samples showed some significant relation with the added ions. With small amount of addition, an increase of the electrical resistivity was observed. The present glass-ceramics could be a promising candidate as advanced insulating materials, as evidenced from the presence of crystalline phases with machinable properties and good mechanical strength coupled with its high resistivity.     

Fabrication and characterization of ferrimagnetic bioactive glass-ceramic containing BaFe12O19

Bioactivity of ferrimagnetic glass-ceramics is useful as thermo seeds for hyperthermia treatment of cancer. Ferrimagnetic glass-ceramics were prepared from the BaFe₁₂O₁₉(BF)–SiO₂–CaO–Na₂O–P₂O₅ system using the incorporation method. The mixture was then further sintered at 800 °C to form the glass-ceramic samples. The structure and microstructure of the samples were characterized by X-ray diffraction, energy dispersive X-ray analysis (EDXA) and scanning electron microscopy. Magnetic hysteresis loops of the glass-ceramic samples were obtained with maximum field of 10 kOe, in order to evaluate the potential of these samples for hyperthermia treatment of cancer. In vitro bioactivity was investigated in simulated body fluid (SBF) for 14 days. The results showed that Na₂Ca₂Si₃O₉ and BaFe₁₂O₁₉ were the main phases in the glass-ceramic samples. Apatite was formed on the surface layers of the glass-ceramics, confirming their biocompatibility. It was found that the bioactivity increased with an increase in BF contents.     

Enhanced Activity of Spinel-type Ga2O3–Al2O3 Mixed Oxide for the Dehydrogenation of Propane in the Presence of CO2

Catalytic performance of a series of Ga₂O₃–Al₂O₃ mixed oxides prepared by alcoholic-coprecipitation method for the dehydrogenation of propane in the presence of CO₂ was investigated. It is shown that the combination of Ga and Al oxides greatly improved the performance of the Ga₂O₃-based materials for catalytic dehydrogenation of propane, with the highest performance attainable at a Ga₂O₃–Al₂O₃ catalyst with a 20 mol% aluminum content. While the same tendency was observed for the specific activity normalized by BET surface area, significantly enhanced stability was achieved for Ga₂O₃–Al₂O₃ with higher aluminum content. X-ray diffraction (XRD) revealed that a homogeneous spinel-type Ga₂O₃–Al₂O₃ solid solution is uniformly formed by substitution of Ga³⁺ for Al³⁺ in the Al₂O₃ lattice. The enhanced activity of Ga₂O₃–Al₂O₃ mixed oxides was accounted for by the abundance of surface weak acid sites due to the synergetic interaction between Ga₂O₃ and Al₂O₃ in the solid solution systems.     

Crystallization behavior of iron- and boron-containing nepheline (Na2O·Al2O3·2SiO2) based model high-level nuclear waste glasses

This study focuses on understanding the relationship between iron redox, composition, and heat-treatment atmosphere in nepheline-based model high-level nuclear waste glasses. Glasses in the Na₂O–Al₂O₃–B₂O₃–Fe₂O₃–SiO₂ system with varying Al₂O₃/Fe₂O₃ and Na₂O/Fe₂O₃ ratios have been synthesized by melt-quench technique and studied for their crystallization behavior in different heating atmospheres—air, inert (N₂), and reducing (96%N₂–4%H₂). The compositional dependence of iron redox chemistry in glasses and the impact of heating environment and crystallization on iron coordination in glass-ceramics have been investigated by Mössbauer spectroscopy and vibrating sample magnetometry. While iron coordination in glasses and glass-ceramics changed as a function of glass chemistry, the heating atmosphere during crystallization exhibited minimal effect on iron redox. The change in heating atmosphere did not affect the phase assemblage but did affect the microstructural evolution. While glass-ceramics produced as a result of heat treatment in air and N₂ atmospheres developed a golden/brown colored iron-rich layer on their surface, those produced in a reducing atmosphere did not exhibit any such phenomenon. Furthermore, while this iron-rich layer was observed in glass-ceramics with varying Al₂O₃/Fe₂O₃ ratio, it was absent from glass-ceramics with varying Na₂O/Fe₂O₃ ratio. An explanation of these results has been provided on the basis of kinetics of diffusion of oxygen and network modifiers in the glasses under different thermodynamic conditions. The plausible implications of the formation of iron-rich layer on the surface of glass-ceramics on the chemical durability of high-level nuclear waste glasses have been discussed.     

Contribution of some transition metal oxides to crystallization and electro-thermal properties of glass-ceramics

Lithium di-silicate (LS₂) glass-ceramics modified with copper oxide using the formula: 34.83Li₂O–xCuO–(65.17-x)SiO₂ (where; x = 1, 2, 4 and 6 mol%) were prepared by melt-quenching followed by controlling heat-treatment. 6 mol% of MnO or Fe₂O₃ transition metal oxides was added instead of SiO₂ in the high CuO-content composition. The effect of the transition cations on phase formation, microstructure, density, thermal expansion, and electrical conductivity was investigated as a function of the controlled crystallization. Results show that up to 4 mol%, Cu⁺² was hosted in stable Li₂Si₂O₅ structure. This enhanced the crystal formation, including Li₂Si₂O₅ and its solid solution (ss), Li₂SiO₃, Li₂Cu₅(Si₂O₇)₂, CuMn₆SiO₁₂, LiFeSi₂O₆ (ss), and the orthosilicate Li₂FeSiO₄ (ss). The prepared materials had different density values ranged from 2.35 to 2.79 g/cm³ for glass and varied from 2.43 to 3.15 g/cm³ for glass–ceramics, whereas the α-values of glass-ceramics ranged in the 95–165 × 10⁻⁷/°C. The progress of electrical properties in glass-ceramics, as a function of composition, was studied. It was markedly improved by adding different transition cations especially, Fe⁺³. The study reveals that the incorporation of transition metal ions in LS₂ composition has a positive effect on the physical-chemical properties of the prepared glass-ceramics. Therefore, it constitutes to prepare future glass-ceramic applications as hermetic seals of metals as well solid electrolyte materials.     

Composition and growth of thin anodic oxides formed on InP (100)

Thin anodic oxides (<100 Å) were formed on p-InP (100) in phosphate solution (0.3 M NH₄H₂PO₄) and in sodium tungstate solution (0.1 M Na₂WO₄·2H₂O) at different temperatures (25 and 80 °C) and potentials (1-8 V). Thickness and composition were determined by different surface-analytical techniques including Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy and transmission electron microscopy. In general, it has been observed that double-layered films are obtained with an outer In-rich layer. The thickness of the outer layer, oxide morphology and roughness as well as the composition of the duplex structure are strongly dependent on the temperature and the composition of the electrolyte. It has been found that oxides formed in phosphate exhibit a higher stability against dissolution compared with oxides formed in tungstate. The latter contain a large amount of In₂O₃, which leads to poor electrical properties.     

Evaluation of the fraction of fourfold-coordinated boron in oxide glasses from their composition

A model for evaluating the fraction of fourfold-coordinated boronN ₄ in the oxide glasses of different systems has been proposed. The model involves simplified equations of hypothetical chemical equilibria between structural groups. The calculated N₄ values are compared with the NMR experimental data obtained by different authors for ∼350 glasses containing Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, Tl₂O, SrO, PbO, ZnO, A1₂O₃, Ga₂O₃, Fe₂O₃, In₂O₃, B₂O₃, and SiO₂ oxides. The mean deviation between the calculated and experimental values ofN ₄ is equal to about 0.05.     

Bulk nucleated fine grained mono-mineral glass-ceramics from low-silica fly ash

Cr₂O₃-nucleated fine grained mono-mineral glass-ceramics of augite were produced from low-silica fly ash and additives of SiO₂, Al₂O₃ and MgCO₃, via two steps of heat treatment for nucleation and crystal growth. The starting glass approached the composition of CaMg_0.75Al_0.4Fe_0.1Si_1.75O₆, derived from CaMg_0.75Al_0.5Si_1.75O₆, which belongs to diopside – Ca-Tschermak solid solutions. The influence of Cr₂O₃ (up to 0.75 wt.%) on the development of crystalline phases, the properties and the microstructure of the resultant glass-ceramics crystallized at different temperatures was investigated.     

Evaluation of the fraction of fourfold-coordinated boron in oxide glasses from their composition

A model for evaluating the fraction of fourfold-coordinated boronN ₄ in the oxide glasses of different systems has been proposed. The model involves simplified equations of hypothetical chemical equilibria between structural groups. The calculated N₄ values are compared with the NMR experimental data obtained by different authors for ∼350 glasses containing Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, Tl₂O, SrO, PbO, ZnO, A1₂O₃, Ga₂O₃, Fe₂O₃, In₂O₃, B₂O₃, and SiO₂ oxides. The mean deviation between the calculated and experimental values ofN ₄ is equal to about 0.05.     

Metal oxide catalysts for DME steam reforming: Ga2O3 and Ga2O3–Al2O3 catalysts

The steam reforming of dimethyl ether (DME) was performed on Ga₂O₃–Al₂O₃ mixed oxides prepared by sol–gel method. Ga₂O₃ significantly affects the catalytic performance with respect to the DME conversion and H₂ yield. The catalytic activity increases with the Ga concentration in Ga₂O₃–Al₂O₃ mixed oxides. It is very interesting that without the aid of an additional transition metal component, Ga₂O₃ and Ga₂O₃ containing Al₂O₃ mixed oxide system exhibit good activity in the reforming reaction. To the best of our knowledge, this is the first report that reveals the reforming ability of Ga₂O₃ for the production of H₂ from DME and/or methanol.     

Effects of sintering additives on hot corrosion behavior of γ-Y2Si2O7 ceramics in Na2SO4+V2O5 molten salt

Dense γ-Y₂Si₂O₇ ceramics were prepared by moulding and sintering of pure γ-Y₂Si₂O₇ powders synthesized by adding various additives. Effects of sintering additives on hot corrosion behavior of γ-Y₂Si₂O₇ ceramics in Na₂SO₄ + V₂O₅ molten salts were systematically investigated. Chemical kinetics of corrosion process was also calculated to illuminate the influence of different additives on chemical stability. Results showed that corrosion reaction started at grain boundary due to the loose microscopic network. When the intercrystalline glass phase was completely etched, the molten salts began to contact Y₂Si₂O₇, forming NaY₉Si₆O₂₆ and YVO₄. Compared with Li₂O and MgO, intercrystalline glass phase formed by Al₂O₃ additive had the most compact microscopic network structure, leading to the best chemical stability. Apparent activation energy for hot corrosion reaction of γ-Y₂Si₂O₇ ceramics in pure Na₂SO₄, Na₂SO₄ + 5 wt% V₂O₅, Na₂SO₄ + 10 wt% V₂O₅, Na₂SO₄ + 15 wt% V₂O₅ molten salts was calculated to be 408.16, 373.60, 310.62, and 249.63 kJ/mol, respectively.     

Structural Behavior and In Vitro Bioactivity Evaluation of Sol-Gel Derived Glass-Ceramics Based on SiO<Subscript>2</Subscript>- CaO- P<Subscript>2</Subscript>O<Subscript>5</Subscript>- ZnO System

Glass and glass-ceramics, which are important bioceramics, have established an essential group of silicon-based materials having wide applications in medicine. Bioactive glass-ceramics have proved to be able to chemically bond to living bones due to forming an apatite-like layer on their surfaces. In this study, bioactive glass-ceramics of (64-X) SiO₂- 31 CaO- 5 P₂O₅- X ZnO based on mol% were synthesized by the sol–gel process. The bioactive glass-ceramics were analyzed using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) before and after being exposed to simulated body fluid (SBF) solution. The results show that an increase in ZnO will cause the ability of calcium phosphate apatite formation on the glass-ceramic surfaces to first rise by 8 % but subsequently to decline by 12 %. The bioactivity of synthesized glass-ceramics can be confirmed by SEM and XRD due to the presence of a rich bone-like apatite layer.     

The nature of the active phase of the Fe/K-catalyst for dehydrogenation of ethylbenzene

The industrial catalyst for high temperature dehydrogenation of ethylbenzene based on iron and potassium oxides undergoes, under reaction conditions, essentially a transformation into magnetite, Fe₃O₄, and a mixture of ternary oxides containing trivalent iron, viz. K₂Fe₂₂O₃₄ and KFeO₂. The latter compound constitutes the outside of the catalyst particles and is indeed the catalytically active phase.     

Synthesis and Crystal Structure of Na2Ba9Si20O50—An Intermediate Phase Along the Join Na2Si2O5‐BaSi2O5

Single crystals of Na₂Ba₉Si₂₀O₅₀ were obtained from solid state reactions performed along the join Na₂Si₂O₅‐BaSi₂O₅. The crystal structure has been determined from a data set collected at ambient temperatures and subsequently refined to a residual of R(|F|) = 0.0328 for 2211 independent reflections. The compound belongs to the group of phyllosilicates and adopts the monoclinic space group C2/m with the following lattice parameters: a = 39.111(3) Å, b = 7.6566(6) Å, c = 8.2055(6) Å, β = 97.319(6)°, V = 2437.2(3) Å³, Z = 2. Furthermore, weak one‐dimensional diffuse streaks running parallel to a* as well as a very small number of low intensity reflections at b*/3, indicating the presence of a superstructure, were observed. Basic buiding units are silicate layers parallel to (40‐1) which can be obtained from the condensation of single chains with a periodicity of four running along [010]. The sheets can be partitioned into two kinds of consecutive strips containing (i) a sequence of four‐ and eight‐membered rings and (ii) a four‐ring wide “zig‐zag shaped” unit consisting of exclusively six‐membered rings. The sodium and barium cations—distributed among six crystallographically independent positions—are sandwiched between subsequent layers and are linked to seven to nine nearest oxygen neighbors. The structure of Na₂Ba₉Si₂₀O₅₀ is closely related to that of K₂Ba₅Si₁₂O₃₀ and K₂Ba₇Si₁₆O₄₀, respectively. There are strong arguments that the previously claimed phase Na₄Ba₈Si₂₀O₅₀ is actually misinterpreted Na₂Ba₉Si₂₀O₅₀ and that the composition of the intermediate phase along the join Na₂Si₂O₅–BaSi₂O₅ is slightly different from that described in the literature.