Pulsed cathodoluminescence of two-alkali sodium potassium silicate glasses

Alkali silicate glasses of variable composition 22xNa₂O · 22(1?x)K₂O · 3CaO · 75SiO₂ with equimolecular replacement of sodium ions by potassium ions are investigated using pulsed cathodoluminescence. It is revealed that localized electronic states interact with vibrations of two types, namely, polarization vibrations of the silicon-oxygen network with the frequency v₀ = 820 cm^?1 and bending vibrations of the modifier sublattice. At low concentrations of one of the alkali components (x < 0.1), bending vibrations are observed at two frequencies. These frequencies coincide with those of the corresponding vibrations in one-alkali systems containing Na (530 cm^?1) and K (520 cm^?1). At higher concentrations (x in the range ～0.14–0.86), there occur bending vibrations of the cationic sublattice with a frequency of 420 cm^?1. This can be interpreted as a luminescence analog of two-alkali (mixed-alkali) effect.     

Titanium coordination in TiO2-Na2O-SiO2 glasses of xTiO2 · (100 ? x) [2Na2O · 3SiO2] (0 ≤ x ≤ 30) composition based on Raman spectroscopy

The structural position of Ti⁴⁺ ions in TiO₂-Na₂O-SiO₂ glasses of xTiO₂ · (100 ? x) [2Na₂O · 3SiO₂] (0 ?? x ?? 30) composition has been studied with Raman spectroscopy. The analysis of spectra recorded has demonstrated that Ti⁴⁺ ions in the studied glasses can be in two structural groups??TiO₅ and TiO₆. Titanium ions with a coordination number of 5 are present in the structure of all Ti-containing glasses, whereas the highly coordinated state exists at x > 10 mol % TiO₂.     

XRD Analysis of the Role of Cesium in Sodium‐Based Geopolymer

The purpose of this work was to study the role of cesium in sodium‐based geopolymer and its thermal stability for nuclear waste management. A series of mixed sodium and cesium geopolymer samples (Na_1?x Cs _x )₂O·Al₂O₃·SiO₂·12H₂O (referred to as (Na_{1? x} Cs _x )‐GP, where x = 0, 0.08, 0.15, 0.42, 1) have been prepared. All geopolymer samples were heated at 1100°C for 24 h. Pollucite (CsAlSi₂O₆) and feldspathoid (CsAlSiO₄) were crystallized from Cs‐GP. Nepheline (NaAlSiO₄) and a small amount of crystallized silica were obtained from Na‐GP. The other geopolymers (Na_{1? x} Cs _x )‐GP (x = 0.08, 0.15, 0.42) led to pollucite and nepheline main phases. Amorphous silica phase was observed in all the geopolymer samples with various amounts. Phase quantification and scanning electron microscope revealed that higher Cs concentrations in Na‐GP tend to decrease the amorphous phase while improving pollucite and nepheline phase quantification. The amorphous geopolymers have also been studied by pair distribution function analysis. Tetrahedral chains formed by T–O bonding (with T = Si, Al) were shown to be more tighten around Cs⁺ than around Na⁺. It led to shorter Cs–T bond than Na–T bond matching the higher solvation property of Na⁺. Furthermore, thermal study analysis pointed out the fact that geopolymer samples (Na_{1? x} Cs _x )‐GP, can be considered as solid solutions.     

Migration processes in the glass forming melts of the Na<Subscript>2</Subscript>O–BaO–Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system

The specific conductivity and tracer of diffusion of sodium and barium ions have been determined and the values of the transport numbers and correlation factor for diffusion have been calculated in two melts of the Na₂O–BaO–Ga₂O₃–SiO₂ system.     

Criterion for estimating the number of phases in phase-separated glass of the Na2O-K2O-B2O3-SiO2 system by dilatometric analysis

The data of dilatometry and electron microscopy of four series of xNa₂O-(8 ? x)K₂O-32B₂O₃-60SiO₂, xNa₂O-(8 ? x)K₂O-22B₂O₃-70SiO₂, xNa₂O-(6 ? x)K₂O-34B₂O₃-60SiO₂, and xNa₂O-xK₂O-(40 ? 2x)B₂O₃-60SiO₂ phase-separated glass heat-treated at 550°C for 144 h (for glass containing 70 mol % SiO₂) and 24 h (for glass containing 60 mol % SiO₂) for separation on phases are summarized. The comparison of dilatometric data and electron microscopy allow one to conclude that glass with a difference between the onset deformation temperature and a glass transition temperature of more than 100°C is phase-separated; and glass with a difference of less than 65°C is single-phase. Curves for the glass transition temperature as a function of the K₂O content reveal a mixed alkali effect, namely, minimums for glass containing 60% SiO₂, and maximums for glass containing 70% SiO₂.     

Concentration dependence of electric conductivity for fluorine-containing sodium borate glasses

The influence of sodium fluoride additives on the physicochemical properties of glasses in the Na₂O-B₂O₃ systems is investigated. The introduction of sodium fluoride into the Na₂O · 2B₂O₃ and Na₂O · 3B₂O₃ glasses leads to an increase in the electric conductivity. The temperature-concentration dependence of the electric conductivity has been investigated. It is shown that, in glasses of the NaF-Na₂O · 2B₂O₃ system, an increase in the volume concentration of sodium ions from 2.4 × 10⁻² to ∼3 × 10⁻² mol/cm³ is accompanied by an insignificant decrease in the activation energy E_σ from 1.44 to 1.38 eV and a sharp (by a factor of ∼30) increase in the electric conductivity. In glasses of the NaF-Na₂O · 3B₂O₃ system, an increase in the concentration of sodium ions from 1.8 × 10⁻² to ∼2.3 × 10⁻² mol/cm³ brings about an increase in the electric conductivity by a factor of approximately 100 and an increase in E_σ from 1.6 to 1.83 eV. A further increase in the concentration of sodium ions (up to 2.5 × 10⁻² mol/cm³) virtually does not affect the electric conductivity and E_σ. At the same concentration of sodium ions (∼2.46 × 10⁻² mol/cm³) in the 9.8NaF · 90.2[Na₂O · 2B₂O₃] and 57.1NaF · 42.9[Na₂O · 3B₂O₃] glasses, the electric conductivity and the activation energy are considerably higher in the glass with a larger fluorine content. The regularities revealed are interpreted in the framework of the microinhomogeneous glass structure.     

Concentration dependence of electric conductivity for fluorine-containing sodium borate glasses

The influence of sodium fluoride additives on the physicochemical properties of glasses in the Na₂O-B₂O₃ systems is investigated. The introduction of sodium fluoride into the Na₂O · 2B₂O₃ and Na₂O · 3B₂O₃ glasses leads to an increase in the electric conductivity. The temperature-concentration dependence of the electric conductivity has been investigated. It is shown that, in glasses of the NaF-Na₂O · 2B₂O₃ system, an increase in the volume concentration of sodium ions from 2.4 × 10⁻² to ∼3 × 10⁻² mol/cm³ is accompanied by an insignificant decrease in the activation energy E_σ from 1.44 to 1.38 eV and a sharp (by a factor of ∼30) increase in the electric conductivity. In glasses of the NaF-Na₂O · 3B₂O₃ system, an increase in the concentration of sodium ions from 1.8 × 10⁻² to ∼2.3 × 10⁻² mol/cm³ brings about an increase in the electric conductivity by a factor of approximately 100 and an increase in E_σ from 1.6 to 1.83 eV. A further increase in the concentration of sodium ions (up to 2.5 × 10⁻² mol/cm³) virtually does not affect the electric conductivity and E_σ. At the same concentration of sodium ions (∼2.46 × 10⁻² mol/cm³) in the 9.8NaF · 90.2[Na₂O · 2B₂O₃] and 57.1NaF · 42.9[Na₂O · 3B₂O₃] glasses, the electric conductivity and the activation energy are considerably higher in the glass with a larger fluorine content. The regularities revealed are interpreted in the framework of the microinhomogeneous glass structure.     

HYDROTHERMAL SYNTHESIS AND ION EXCHANGE PROPERTIES OF THE NOVEL FRAMEWORK SODIUM AND POTASSIUM NIOBIUM SILICATES

ABSTRACT

Two novel metastable sodium niobium silicates of the empirical formula: Na_l+x?yH_y(Nb_1?xSi_x)O₃ nH₂O, where x=0.33?0.38, y<l+x, n=0,7-l.l (NbSi-Na, 6.0 Å phase), and Na_3-x H_xNb₃Si₂O₁₃ nH₂O, where x<1.5, n=2.5?3.5 (NbSi-Na, 12.6 Å phase), and two novel potassium niobium silicates: K_4?xH_xNb₄SijO₂₂nH₂O, where x<l, n=3.5-4.0 (NbSi-K., 10.0 Å phase), and K_1?xH_xNbSi₄O₁₁nH₂O, where x<0.2, n=0.4-0.5 (NbSi-K, 6.05 Å phase), were synthesized in the homogeneous alkaline reaction system NbCl₅ - SiO₂ - NaOH (KOH) -H₂O₂ - H₂O under mild hydrothermal conditions. The compounds were characterized by elemental analysis, FTIR, TGA, MAS ²⁹Si NMR and X-ray diffraction. It was found that alkali metal niobium silicates have open framework structures. Their ion exchange affinity towards alkali, alkaline earth and some transition metal ions was studied. All alkali metal niobium silicates are moderately acidic ion exchangers. Both sodium niobium silicates show a distinct affinity for Cs⁺ ion among alkali metal ions, whereas potassium niobium silicate, the NbSi-K, 10.0 Å phase, exhibits affinity for Rb⁺ ion. The affinity of the sodium niobium silicate, NbSi-Na, 6.0 Å, toward strontium ion in neutral solutions is equal or superior to the best Sr-selective inorganic ion exchangers. The sodium niobium silicate (NbSi-Na, 12.6 Å phase) exhibits extremely high affinity for Pb²⁺ ion in acidic and neutral media, and both sodium niobium silicates also show a moderate affinity for Hg²⁺ ion in neutral and highly alkaline media. These exchangers could be promising for the treatment of some specific nuclear waste and contaminated environmental and biological liquors containing lead, mercury and radioactive strontium.     

Synthesis and evaluation of a ruthenium–copper oxide/silica catalyst derived from microemulsion processing

Bimetallic Ru–Cu catalysts supported on SiO₂ have been synthesized in microemulsions using sodium metasilicate (Na₂SiO₃), copper nitrate (Cu(NO₃)_2·3H₂O) and ruthenium chloride (RuCl₃) at 28 °C. The microemulsion system consists of sodium 1,4‐bis(2‐ethylhexyl) sulfosuccinate (AOT) and sodium dodecyl sulphate (SDS), cyclohexane, and an aqueous solution of sodium metasilicate or metal salts. The catalysts have been characterized by XPS, EDX/SEM with line scanning and they possess a very narrow pore size distribution (around 38 Å) and relatively high specific surface areas (around 400 m²/g). The catalytic results of the N₂O decomposition reveal that higher conversions of N₂O can be achieved by the catalysts synthesized from the microemulsion process at lower temperatures (around 400 °C).     

Simple and novel synthesis of zirconia whiskers from a phosphate flux

High quality zirconia whiskers have been successfully prepared by molten salt method, using zirconium oxychloride (ZrOCl₂·8H₂O) and sodium phosphate tribasic dodecahydrate (Na₃PO₄·12H₂O) as precursor and molten salt, respectively. The effects of types of molten salt and heat treatment temperature on the formation of zirconia whiskers were characterized by XRD, Raman, DTA-TG, FE-SEM, TEM, SAED and HR-TEM. When Na₃PO₄·12H₂O is utilized as molten salt and the heat treatment temperature is 900?°C, the as-prepared zirconia whiskers with length ranging from 4?µm to 8?µm show an average aspect ratio of 25. The obtained ZrO₂ whiskers with monoclinic structure are elongated along [010] direction and exhibit a smooth surface with no distinct defects. The XRD and Raman results reveal that the phase transformation from tetragonal zirconia to monoclinic zirconia occurs with the increased crystal size and the water quenching treatment can significantly reduce the content of sodium zirconium phosphate [Na_9–4×Zr_x(PO₄)₃] in the final product. The growth mechanism of zirconia whiskers is supposed to be a dissolution-precipitation process. Since the sodium zirconium phosphate [Na_9–4×Zr_x(PO₄)₃] effectively promotes the dissolution of zirconia in liquid molten salt, zirconia can grow into zirconia whiskers according to its anisotropy.     

The Effects of Na2O and SiO2 on Liquid Phase Sintering of Bayer Al2O3

To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na₂O and 603 ppm SiO₂ and heated at 1525°C up to 8 h. Na₂O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al₂O₃, Na₂O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO₂ concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20?400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na₂O/SiO₂ ratios, densification of Bayer Al₂O₃ at 1525°C is controlled by diffusion of Al³⁺ through the grain‐boundary liquid, whereas for high Na₂O/SiO₂ ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al₂O₃ or diffusion of Al³⁺. Increasing Na₂O in SiO₂‐doped samples increases diffusion of Al³⁺ and Al₂O₃ solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al₂O₃ densification can be manipulated by adjusting the Na₂O to SiO₂ ratio.     

Generalized Universal Calculation of SiO2 Diffusion Coefficients in Melts of the Na2O – SiO2 System

Experimental data are summarized, and a generalized formula for the calculation of SiO₂ diffusion coefficients in melts of the Na₂O – SiO₂ system is obtained. The formula is recommended for the practical calculations of the process of melting impure sodium disilicate.     

Preparation of Mg–Zn bimetallic doped Na-containing bioceramic from sodium metasilicate

Glass–ceramic materials in the composition (mol%) 23.5 Na₂O–23.5CaO–xMgO–xZnO–47SiO₂–(6-2x) P₂O₅ (x = 0 and 1.5) were prepared by the sol–gel technique using sodium metasilicate (Na₂SiO₃) as source of Na₂O and SiO₂. Bioactivity was investigated in vitro by examining apatite formation on the surface of the samples after treatment in acellular simulated body fluid (SBF). Formation of bioactive apatite layer on the samples was confirmed by using scanning electron microscope, X-ray diffraction and Fourier transform infrared spectroscopy. Reactivity of the materials in SBF was based on changes in pH and ionic concentration of the solution as a function of immersion duration. Results obtained indicated that the sample doped with Mg and Zn exhibited greater crystallinity containing phases such as CaMgSi₂O₆, Ca₂MgSi₂O₇, CaSiO₃, Na₂CaSi₃O₉, Na₂CaSi₃O₈, CaZnSi₂O₇ and a new secondary phase, clinohedrite (CaZnSiO₄·H₂O), while the undoped sample contained a single crystalline phase composed of Na₂Ca₂Si₃O₉. After 7 days in SBF, phase transformation occurred at different rates on the materials leading to formation of apatite layer. The result shows that controlled amounts of MgO and ZnO may be useful in tuning up the mechanical properties of bioactive glasses while maintaining bioactivity.     

Conductance and spectroscopy of protonic beta and beta″-aluminas

Five protonic beta and beta″-aluminas; viz hydrated sodium beta-alumina (1,24Na₂O·11Al₂O₃), hydronium beta-alumina (1.24H₂O·11Al₂O₃·2.6H₂O), partially dehydrated hydronium beta-alumina (1.24H₂O·11Al₂O₃·1.3H₂O), hydrogen beta-alumina (1.24H₂O·11Al₂O₃) and hydronium beta″-alumina (0.84H₂O·0.8MgO·5Al₂O₃·2.8H₂O) were examined by broad band nuclear magnetic resonance from ?196°C to 200°C. The spectra of hydronium beta-alumina and hydronium beta″-alumina are consistent with a mixed composition of H₂O, H₃O⁺ and H⁺ species in the conducting plane. Hydrogen beta-alumina and partially dehydrated hydronium beta-alumina appear to contain only relatively isolated (2.6–2.7Å) protons; no evidence of molecular water or hydronium ions is found. Water molecules intercalated into the conduction plane of sodium beta-alumina do not appear to be in rapid motion, even at 167°C, but are relatively stationary. The onset of motional narrowing in hydronium beta″-alumina occurs at ?40°C but not until +30°C in hydronium beta-alumina. This is consistent with the higher conductivity reported for hydronium beta″-alumina, 10^?3–10^?5 (ohm-cm)^?1 at 25°C, in comparison to 10^?10–10^?11 (ohm-cm)^?1 for hydronium beta-alumina at 25°C.     

Synthesis of fluorozirconate ZBNL glass with partial or complete substitution of europium difluoride for barium (lanthanum) fluorides

The vitrification in two sets of fluorozirconate glass 53.5ZrF₄ · 20NaF · 20BaF₂ · (6.5 ? x)LaF₃ · xEuF₂ (0 ≤ x ≤ 6.5 mol %) (I) and 53.5ZrF₄ · 20NaF · (20 ? x)BaF₂ · 6.5LaF₃ · xEuF₂ (0 ≤ x ≤ 20 mol %) (II) has been studied. It has been determined that, in set I, at full or partial replacement of lanthanum fluoride by europium difluoride (1–6.5 mol %), colorless transparent glass is formed. In the samples of set II, at the substitution of BaF₂ in order to obtain stable glass under analogous conditions, the initial batch should not contain more than 7.0 mol % EuF₂. The methods of differential-thermal and X-ray phase analyses and IR and electron spectroscopy were used to study the properties and phase composition of the obtained glass and vitrocrystalline materials. Using the Grubi criterion, the optimal, with respect to crystallization, EuF₂ content has been deter-mined in the glass of both systems. In the devitrified glass of series II, the low-temperature modification of compound EuZrF₆ is formed, in addition to sodium, barium, and lanthanum fluorozirconates. For the vitro-crystalline samples, the absorption bands of Eu-F bonds are observed in infrared spectra, in addition to the broad band (～500 cm^?1), which is intrinsic for all fluorozirconate glass. The electron diffuse reflection spectra confirm the presence of both double and triple-charged europium ions in glass.     

Structure and electrical properties evolution of B-site complex ions (Li1/4Nb3/4) modification BNT–BT ceramics

Abstract

B-site complex ions (Li_1/4Nb_3/4)⁴⁺ modification (Bi_1/2Na_1/2)_0·94Ba_0·06TiO₃ ceramics with compositions of (Bi_1/2Na_1/2)_0·94Ba_0·06Ti_1?x(Li_1/4Nb_3/4)_xO₃ (x?=?0, 0·01, 0·03 and 0·06) have been synthesised via the conventional solid state reaction. The effect of (Li_1/4Nb_3/4)⁴⁺ content and sintering temperature on structures and electrical properties were investigated. It was found that both compositions and sintering temperatures have no significant effect on the crystal structure, and trace (Li_1/4Nb_3/4)⁴⁺ addition and sintering temperatures have a great influence on the microstructure. Two obvious dielectric anomaly peaks (T_d and T_m) were observed and dielectric constant for all poled specimens displayed significant frequency dispersion at T_d and diffusion phase transition at T_m. The piezoelectric properties of the ceramics are insensitive to the sintering temperatures, and the composition with x?=?0·03 sintering at 1150°C exhibits favourable piezoelectric properties of d₃₃?=?155 pC N^?1 and k_p?=?0·312.     

Hydrogen evolution reaction on sodium-tungsten bronzes

The hydrogen evolution reaction has been investigated on sodium-tungsten bronzes (Na_xWO₃) of various stoichiometries (0·34 < × < 0·89) in sulfuric acid solutions. Steady-state current-potential relationships and capacity-potential profiles were examined. The first cathodic steady-state current-potential curve in the ascending direction indicates the presence of two distinct Tafel lines with slopes of about ?2RT/F and ?RT/2F. This behavior is interpreted in terms of alternative reactions. At low overvoltages the mechanism is fast discharge-slow electrochemical desorption, while at higher overvoltages a fast discharge-slow recombination mechanism is indicated. The change of the rate-determining step from one potential range to the other is attributed to a change in the crystallographic structure of the electrode surface, eg, a lattice dimensional change of the hydrogen tungsten bronze. The Tafel slope for an electrode pre-polarized cathodically is close to ?RT/F. In this case the rate-determining step is proposed to be the surface diffusion of adsorbed hydrogen atoms to recombination sites. The x-value in Na_xWO₃ and the platinum content of the electrode were found to have no significant effect on the rate of the hydrogen evolution reaction.     

Elucidating the Effect of Iron Speciation (Fe2+/Fe3+) on Crystallization Kinetics of Sodium Aluminosilicate Glasses

We report on the influence of Fe₂O₃ on the crystallization kinetics of nepheline (Na₂O·Al₂O₃·2SiO₂)‐based sodium aluminosilicate glasses. A series of glasses with varying Al₂O₃/Fe₂O₃ content were synthesized in the system 25Na₂O–(25–x) Al₂O₃–xFe₂O₃–50SiO₂ (x varies between 0 and 5 mol%) through melt‐quench technique. A systematic set of experiments were performed to elucidate the influence of iron speciation (Fe²⁺/Fe³⁺) on the crystallization kinetics of these glasses including: (1) obtaining the details of nonisothermal crystallization kinetics by differential scanning calorimetry, (2) determining the influence of heat treatment on the structure and iron coordination in glasses by X‐ray photoelectron spectroscopy and wet chemistry, and (3) following the crystalline phase evolution in glasses in air and inert environments by X‐ray diffraction and scanning electron microscopy. The crystallization of two polymorphs of NaAlSiO₄—carnegieite (orthorhombic) and nepheline (hexagonal)—was observed in all the glasses, wherein the incorporation of iron promotes the formation of nepheline over carnegieite while shifting the crystallization mechanism from surface to volume. The influence of environment (air versus inert) and iron content on the crystallization kinetics of these glasses is contextualized from the perspective of the devitrification problem usually observed in sodium‐ and alumina‐rich high level nuclear waste glasses.