Crystallization of Beta NaFeO2 from a Glass Along the Na2SiO3-Fe2O3 Join

Fine-particle beta sodium ferrite (β-NaFeO₂), rather than α-Fe₂O₃, may be responsible for superparamagnetic behavior in a glass of composition (in mole fractions) 0.37Na₂O-0.26Fe₂O₃-0.37SiO₂. The 700°C isothermal section of the phase diagram of the Na₂O-Fe₂O₃-SiO₂ system is given, showing a three-phase field bounded by Na₂SiO₃-NaFeO₂-Fe₂O₃; there is no evidence for the existence (at 700°C) of compounds of molar composition 6Na₂O-4Fe₂O₃-5SiO₂ or 2Na₂O-Fe₂O₃-SiO₂. The Moessbauer spectrum of β-NaFeO₂ has an internal magnetic field of 487 kOe at room temperature.     

Preparation and Properties of Nitrogen-Containing Na2O-B2O3 Glasses

Up to 3.3 wt% nitrogen can be incorporated into Na₂O-B₂O₃ glass melts. The melting procedure is described, and structure models are given. In contrast to N-containing silicate glasses, the borate glasses were transparent; however, micrographs of their fracture surfaces showed some crystallinity. Properties were determined as a function of the N and Na₂O contents of the glasses. Compared with N-containing silicate glasses, the properties of borate glasses are much less changed by the nitrogen introduced.     

Effect of Na2O Additions in the Crystallization of Barium Ferrite from a BaO-B2O3-Fe2O3 Glass

A glass crystallization method was utilized to synthesize nanosized BaO-6Fe₂O₃ platelets from a 0.412BaO-0.258B₂O₃-0.330Fe₂O₃ batch composition. Quenched ribbons were inhomogeneous, showing microclustering and ∼1 μm hematite crystals. Na₂O substitutions for BaO greatly enhanced the glass-forming tendency of quenched ribbons, though quenched-in ∼0.5 μm barium ferrite crystals were infrequently present. The improved homogeneity with Na₂O substitution was attributed to lower vapor pressure of BaO during batch melting, which increased its retention in the as-quenched ribbons. Quantities of BaO equal to or in excess of Fe₂O₃ allowed iron ions to adopt stable network positions in the glass melt. With Na₂O substitution, devitrification of dispersed ∼40 nm barium ferrite particles from phase-separated regions occurred after secondary heat treatment. 5 mol% Na₂O batch substitution showed the lowest crystallinity in the as-quenched ribbons, and the highest crystallinity after secondary heat treatment. After optimum devitrification, the maximum values of saturation magnetization and coercivity were 21.22 emu/g and 2.82 kOe, respectively.     

Phase Equilibria in the System Na2SiO3-Li2SiO3-SiO2

Ternary phase relations have been studied and several modifications made to Kracek's phase equilibrium diagram. These include location of the primary phase fields of Na₆Si₈O₁₉ and Li₂Si₂O₅. Metastable phases and polymorphs were often encountered, notably a primary phase, δ-Na₂Si₂O₅, and a new polymorph of Li₂Si₂O₅.     

Diffusion Studies of Na2O-SiO2-Fe2O3 Melts at 1200°C

The interdiffusion of ferric oxide and silica between Na₂O-SiO₂ and Na₂O-SiO₂-Fe₂O₃ melts containing the same mole % Na₂O was studied at 1200°C in air. Values in the range lO⁻⁵ to lO⁻⁸ cm² sec⁻¹ were obtained depending on the composition of the binary melt. The results were rationalized on the basis of the changing ionic nature of the iron with the soda-silica ratio of the ternary melt. It is concluded that significant concentrations of iron cations are present in the melt only if the mole fraction of soda is less than 0.3. A cursory examination of the relative electrical conductivity of a binary and a ternary melt tends to confirm this thesis.     

Stable and Metastable Equilibria in the Systems Y2O2-Al2O3, and Gd2O3-Fe2O3

The phase equilibrium relations in the systems Y₂O₃-Al₂O₃ and Gd₂O₃-Fe₂O₃ were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd₂O₃-Fe₂O₃ but only metastable in the system Y₂O₃-AI₂O₃. This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O₃. A new 2:1 molar ratio cubic phase has been found in the system Y₂O₃-A1₂O₃. Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.     

Polymer Mixtures in Na2O.Sb2O3.GeO2 Glasses

Ternary Na₂O.Sb₂O₃.GeO₂ glasses (with various [Na]/[Na + Sb] ratios) that contained ≥65 mol% GeO₂ were prepared. Their densities (volumes), refractive indices, and infrared spectra were determined and their colors noted. The ternary glasses with ≥88 mol% GeO₂ exhibit nearly additive volumes, refractivities, and frequencies for the main Ge-O vibration. Ternary glasses with lesser amounts of GeO₂ exhibit a variety of behaviors, depending on the [Na]/[Na + Sb] ratio. Small amounts of Sb₂O₃ cause significant volume and refraction deviations, as well as changes in ν_Ge-O, that can be associated with gradual elimination of GeO₆ octahedra. All the information supports a model for the glasses with 65 to 88 mol% GeO₂ that involves a degree of depolymerization that is greater when Na₂O and Sb₂O₃ are present together than when either is present alone.     

Effect of Small Additions of Al2O3 and Ga2O3 on the Immiscibility Temperature of Na2O-SiO2 Glasses

Alumina and gallia were substituted separately for Na₂O in amounts of 0.2, 0.5, 1.0, 1.5, 2.0, and 3.0 wt% in three Na₂O-SiO₂ glass compositions (82, 84, and 86 wt% SiO₂) within the immiscibility region. The immiscibility regions for each system extend to ∼1.5 mol% of the added oxide. In general, the addition reduced the immiscibility temperature ( T _m), but at the edge of the immiscibility region (82% SiO₂) the Na₂O loss effect initially increased T _m. A structural model of the miscibility of Al₂O₃ added to silicate glasses is presented.     

Formation of Sm2+ lons in Sol-Gel-Derived Glasses of the System Na2 O-Al2O3-SiO2

Samarium ions (Sm²⁺) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na₂O-Al₂O₃SiO₂ synthesized by the hydrolysis of Si(OC₂H₅)₄, Al(OC₄H₉)₃, CH₃ COONa, and SmCl₃·6H₂O were heated in air at 500°C, then reacted with H₂ gas to form Sm²⁺ ions. Whereas Al³⁺ ions effectively dispersed the Sm³⁺ ions in the glass structure, Na⁺ ions were not effective. The Al₂O₃-SiO₂ glasses proved appropriate for reacting the Sm³⁺ ions with H₂ gas and exhibited the intense photoluminescence of Sm²⁺ ions. The reaction of Sm³⁺ ions with H₂ in the Al₂O₂-SiO₂ glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm²⁺ ions was achieved within 20 min.     

Activities and Structure of Some Melts in the System Na2SiO3-Na2Si2O5

Different structural models for melts between Na₂SiO₃ and Na₂Si₂O₅ were tested by comparison of activities computed from the models with activities determined from the calorimetric heat of fusion. Data were used from the liquidus curve of the phase diagram for the system Na₂SiO₃-Na₂Si₂O₅ (Kracek and Morey and Bowen). A group model involving (SiO₃)_x^2x- rings or chains, with a random distribution of (SiO_2.5)₂ pairs which bridge between rings or chains, gives activities in good agreement with those determined from the calorimetric heat of fusion.     

Nucleation and Crystallization of NaNbO3 from Glasses in the Na2O-Nb2O5-SiO2 System

Transparent glass-ceramics, in which the major phase was NaNbO₃, were obtained by heat treatment of glasses in the Na₂O-Nb₂O₅-SiO₂ system. The structure of the glass and the changes occurring during crystallization as a function of temperature and heating rate were examined by X-ray diffraction, transmission and replication electron microscopy, density, and other measurements. On heating, a rather abrupt formation of uniformly dispersed particles was observed. In the early stages of crystallization, these particles contained NaNbO₃ as loose, radially grown dendrites of identical crystal orientation which became dense during later stages of crystallization. The particle sizes ranged from 200 to 10,000 A, depending on the SiO₂ content of the glass. Transparency of the crystallized material was dependent on the particle size rather than on the amount of NaNbO₃ formed. The temperature at which crystallization occurred increased with the heating rate whereas the viscosity at crystallization decreased. For a given value of the rate of crystal formation per °C of temperature increase, the product (viscosity)ⁿ× (heating rate) was constant. The nucleation and growth phenomena which occurred in these glasses was attributed to microheterogeneities of higher Nb₂O₅ content which formed part of the glass structure.     

Investigation of Role of Beta Alumina in the System Na2O-Al2O3-SiO2

Beta alumina was once considered to be merely a metastable polymorph of corundum. Its status as a distinct and stable compound (Na₂O.-11Al₂O₅ and analogous formulas) is now quite firmly established. Yet the published phase diagram for the system soda-alumina-silica takes no cognizance of its existence. A study of the alumina-rich portion of the system was conducted to determine whether or not beta alumina should be incorporated in the diagram. Conventional methods of investigation were used, with certain innovations designed to overcome difficulties inherent in the system. Solid-state sintering and modified quenching techniques were utilized to minimize soda loss. Sintering was at 1400°C. or thereabout, in platinum envelopes which were immersed in buffering mixtures so as to reduce volatilization. To the same end, extra-large quenching charges were employed, and these were held at 1600°C., far below the liquidus temperatures of most of the mixtures studied. Phase identification was primarily by the X-ray powder method. The results of these studies confirm the status of beta alumina as a discrete compound, and are believed to justify the insertion of an additional tie line, linking beta alumina to carnegieite, in the soda-alumina-silica diagram. A large primary field of beta alumina, occupying a portion of the present corundum field, is also strongly indicated. The phase diagram accordingly has been modified so as to incorporate these latest findings. The revised diagram, although in some respects tentative, is believed to be in fuller accord with technological experience. It is believed also to be in complete agreement with all recent laboratory studies in the system lime-soda-alumina-silica.     

Effects of MoO3, on Phase Separation of Na2O-B2O3-SiO2 Glasses

Immiscibility temperatures of Na₂O-B₂O₃-SiO glasses, with andwithout 1 mol% MoO₃, additions, were determined and the effect of MoO₃ additions on the 65O°C immiscibility isotherms was established. In addition, immiscibility temperature and phase-separation morphology of an Na₂O-B₂O₃-SiO₂ glass with progressive additions of MoO₃, were investigated. It was found that the addition of small amounts of MoO₃ extends the immiscibility boundary of the system and raises the immiscibility temperature by ∼l8°C for each mol % MoO₃, addition. Analysis of phase-separation morphology suggests that the MoO₃, additions do not significantly alter the tie lines of phase separation in the system, although such additions cause a lowering of the viscosities and the glass-transition temperatures of these glasses.     

Anodic Spark Deposition from Aqueous Solutions of NaAlO2 and Na2SiO3

Coatings were deposited on Cu⁰, Ni⁰, and Al⁰ anodes from aqueous solutions of NaAlO₂ and Na₂SiO₃ by anodic spark deposition. Some coatings formed at constant current density (1.0 A/cm²) were characterized by scanning electron microscopy, electron beam microprobe analysis, X-ray diffraction, and emission spectroscopy. A relation for the current decay during a multiple-spark, constant-voltage process was determined. This relation, which describes the behavior of most of the systems studied, is

where φ_t, φ₀, and φ_∞ are the current densities at time t , at the start of the decay ( t =0), and at the steady state, respectively. K is a constant that is voltage and system dependent.     

Bioactivity of Na2O-CaO-SiO2 Glasses

Bioactivities of Na₂O-CaO-SiO₂ glasses were evaluated by examining the formation of bonelike apatite, which is responsible for their bonding to living bone, on their surfaces in a simulated body fluid, using thin-film X-ray diffraction and Fourier-transform infrared reflection spectroscopy. It was found that glasses in a wide compositional region in the P₂O₅-free Na₂O-CaO-SiO₂ system can show bioactivity, as those in the P₂O₅-containing system. The rate of apatite formation on the surfaces of glasses varied largely with the composition of the glasses. Under a constant SiO₂ content of 50 mol%, a glass containing equimole of Na₂O and CaO showed the highest rate of the apatite formation. Variation in the rate of apatite formation with the glass composition corresponded well with the rate of increase in the degree of the supersaturation of the simulated body fluid with respect to the apatite due to dissolution of sodium and calcium ions from the glasses. Little difference was observed in the rates of ion dissolution and of apatite formation between P₂O₅-containing Bioglass 45S5-type and a corresponding P₂O₅-free Na₂O-CaO-SiO₂ glass. It is believed that P₂O_s-free Na₂O-CaO-SiO₂ glasses also show bioactivity as high as that of Bioglass.     

Glass Formation and Properties of Glasses in the System Na2O-B2O3-SiO2-TiO2

An investigation was made of the effect of TiO₂ on the glassforming region and on the physical properties of glasses in the system Na₂O-B₂O₃-SiO₂TiO₂. Glasses containing up to 45 mole % TiO₂ may be formed with an alkali content of 30 mole %. At lower alkali contents (10 mole % Na₂O) glasses may be formed containing up to 22 mole % TiO₂. The way in which the coefficient of linear thermal expansion and the transformation and softening temperatures are affected by TiO₂ additions has been determined.