Interfacial Reactions and Wetting Behavior of Glass-Iron Systems

Interfacial reactions and enhanced wetting or spreading of Na₂Fe_xSi₂O_5+x and Na_2–2xFe_xSi₂O₅ glasses on substrates of high-purity (Marz) and commercial (Armco) iron were observed at 1000°C at low partial pressures of O₂ and Na. Reactivity increased with increasing O/Si ratio in the glass, with the purity of the iron, and with decreasing gas pressure.     

Equilibrium Studies of Fe in Alkali Phosphate Glasses

The Fe²⁺-Fe³⁺ equilibrium in binary Na₂O-P₂O₅, glasses was studied by equilibrating glass melts at different temperatures in air. The enthalpy change (δH) of the reaction 1/2Fe₂O₃⇌FeO+1/4O₂ was calculated for 4 glasses. The results indicate that (1) the equilibrium shifts toward the oxidized state as the Na₂O content of the glass increases (plots of log ([Fe²⁺]/[Fe³⁺]) vs mol% alkali were linear) and (2) Δ H values for glasses of different composition are nearly equal but differ from the standard (calculated) value for the reaction. The experimental ΔH values were nearly equal to that for the reaction FePO₄→1/3 Fe₃(PO₄)²+ 1/6P₂O₅+1/4O₂, indicating that Fe forms phosphate or polyphosphate configurations in the Na₂O-P₂O₅ glasses. In certain of the glasses studied a faint-pink solid precipitated; its X-ray diffraction pattern indicated that its principal component is crystalline Na₂Fe¹¹¹P₃O₁₀.     

Redox Equilibria of Multivalent Ions in Silicate Glasses

Experimental studies were made on the compositional dependence of the redox equilibrium of Eu in synthetic silicate liquids, together with an empirical model describing the observed compositional dependence. Electron paramagnetic resonance (EPR) was used to measure the concentration ratio of Eu²⁺ to Eu³⁺ in various glasses formed by rapidly quenching silicate liquids. The compositional field studied comprised mixtures of SiO₂, TiO₂, Al₂O₃, CaO, MgO, and Na₂O. The proposed model describes the Eu²⁺/Eu³⁺ ratio over the entire compositional field in terms of parameters easily related to each glass composition. The general applicability and utility of the model is further demonstrated by its application to the Fe²⁺-Fe³⁺, Ce³⁺-Ce⁴⁺, and Cr³⁺-Cr⁶⁺ redox reactions in binary alkali oxide silicate glasses of Li, Na, and K.     

Systems of Na-Fe-SiO2 Glasses and Steel: I, Wetting and Adherence

Wetting behavior of molten Na₂O-Fe₂O₃-SiO₂ and Na₂O-FeO-SiO₂ glasses on very-low-carbon steel at 900°C in purified Ar was studied. The steel surface was dry-polished and cleaned by ion bombardment. Glass frit was then placed on the metal, and the system was introduced into the furnace without exposure to air. Dynamic advancing and receding contact angles were measured. Shear (push-off) tests were performed on the glass-steel composite at room temperature to determine adherence. The equilibrium contact angles were obtained from the arithmetic mean of the final advancing and receding angles. As the Fe oxide content of the glasses increased, the degree of wetting improved; Fe³⁺ was more effective than a combination of Fe³⁺ and Fe²⁺ in improving the wettability. Glasses very nearly saturated with Fe oxide exhibited high adherence values and chemically adhered to steel.     

Redox State of Iron and Its Related Effects in the CaO–P2O5–Fe2O3 Glasses

Iron exists in Fe²⁺ and Fe³⁺ states in CaO–P₂O₅–Fe₂O₃ glasses and they impart characteristic optical absorption bands that allow analysis of relative proportion of the two species. This communication reports the redox states of glasses melted under air, argon, and oxygen atmospheres and relates them to the dissolution rates. The dissolution rate was found to be related to the redox state and it is lowered if the glass is melted under oxidizing atmospheres.     

Effect of Molybdenum on the Structure and on the Crystallization of SiO2–Na2O–CaO–B2O3 Glasses

Crystallization of the poorly durable Na₂MoO₄ phase able to incorporate radioactive cesium must be avoided in SiO₂–Al₂O₃–B₂O₃–Na₂O–CaO glasses developed for the immobilization of Mo-rich nuclear wastes. Increasing amounts of B₂O₃ and MoO₃ were added to a SiO₂–Na₂O–CaO glass, and crystallization tendency was studied. Na₂MoO₄ crystallization tendency decreased with the increase of B₂O₃ concentration whereas the tendency of CaMoO₄ to crystallize increased due to preferential charge compensation of BO₄⁻ entities by Na⁺ ions. ²⁹Si MAS NMR showed that molybdenum acts as a reticulating agent in glass structure. Trivalent actinides surrogate (Nd³⁺) were shown to enter into CaMoO₄ crystals formed in glasses.     

Phase Equilibria and Crystallization of Melts in the System Na2O-BaO-SiO2

Phase-equilibrium studies of the liquidus surface of the Na₂O-BaO-SiO₂ system are presented. The system contains 5 ternary compounds, Na₂Ba₄Si₁₀O₂₅, Na₂Ba₄Si₂O₆, Na₂Ba₂Si₂O₇, X (probably Na₂Ba₁₈Si₂₈O₇₅), and Y (probably Na₂Ba₄₅Si₇₃O₁₉₂), all of which melt incongruently. Twenty-three liquidus invariant points, including 13 peritectics, 4 eutectics, and 6 thermal maxima, were located. Metastable crystallization reactions are commonly encountered, and some of the metastable equilibria are described.     

Electrical Resistivity Surface for FeO-Fe2O3-P2O5 Glasses

The dc electrical properties and microstructure of x (FeO-Fe₂O₃)-(100 – x )P₂O₅ glasses were investigated up to a maximum of x = 75 mol%. Results indicate that, in general, the minimum resistivity of the glass does not occur at equal Fe^2+] and Fe^3+] concentrations, although for the special case where x = 55 mol% the minimum does occur at Fe^2+]/Fe total = 0.5, as reported by other investigators. Evidence presented shows that the position of the minimum resistivity is a function of total iron content. The minimum shifts to glasses richer in Fe^2+] at higher total iron concentrations.     

Role of Iron in Calcium Phosphate Glasses

Some physical properties okf phosphate glasses con-taining up to about 26 mol% Fe₂O₂ were studied. Pronounced changes in properties were observed at compositions containingabout 6, 10, and 13 mol% Fe₂O₃. The X-ray diffraction spectra of devitrified (heat-treated) samples showed new compounds near these compositions. Electron spin resonance and optical studies confirmed the presence of Fe³⁺ and Fe²⁺ in both 4- and 6-coordination. An increase in total iron in these samples was associated with a decrease in the ratios Fe²⁺ 4-coordinated/Fe³⁺ 6-coordinated 6-coordinated and Fe³⁺ 4-coordinated/Fe³⁺ 6-coordinated up to about 2.0 mol% Fe₂O₃, as shown by the intensity of the optical absorption bands at about 2.0 and 1.0 μm and by the intensity of the ESR lines at g⋍4.2 and 2.0, respectively. Samples containing up to 4.3 mol% Fe₂O₃ showed an increase in Fe³⁺ concentration and a decrease in Fe²⁺ concentration after gamma irradiation. The electrical conductivity and activation energy decreased sharply with increasing Fe₂O₃ content.     

Evaluation of Lattice Site and Valence of Mn and Fe in Polycrystalline PbTiO3 by Electron Spin Resonance and Thermogravimetry

The oxides of Mn and Fe or combinations of La₂O₃ with these oxides were incorporated into PbTiO₃ under defined thermodynamic conditions. The substitution reaction of these additives was examined gravimetrically. At low partial pressures of O₂ and PbO vapor, Mn²⁺ and Fe²⁺ are formed at the A sites, and Mn³⁺ and Fe³⁺ enter both A and B sites. At higher partial pressures of PbO, Mn⁴⁺ is most probably formed at the B sites.
The Mn²⁺, Mn⁴⁺, and Fe³⁺ were determined by ESR.     

Effect of Oxidation State of Iron on Phase Separation in Sodium Silicate Glasses

The effect of the oxidation state of iron on the phase separation of x Na₂O·(100 – x )SiO₂ glasses, x = 18.56 and 13, containing 0.5 mol% Fe₂O₃ was studied. The oxidation state of iron in the glasses was varied by changing the melting conditions, such as melting temperature and melting atmosphere. The oxidation states of the iron ion were determined using colorimetric and UV–VIS–NIR spectrophotometric methods, and a comparison was made between the results obtained using these two methods. Immiscibility temperatures of the glasses were determined using opalescence and clearing methods. The immiscibility temperature of the sodium silicate binary glasses decreased ∼25°C with the addition of 0.5 mol% Fe₂O₃. The immiscibility temperature of the doped glasses increased slightly with increased concentration of Fe²⁺ ion in the glass. The prediction of immiscibility tendency on the addition of a minor amount of third component was made using models proposed by Tomozawa and Obara and Nakagawa and Izumitani. The Tomozawa and Obara model showed good agreement with measured immiscibility values.     

Kinetics and Mechanism of Hot Corrosion of γ-Y2Si2O7 in Thin-Film Na2SO4 Molten Salt

γ-Y₂Si₂O₇ is a promising candidate material both for high-temperature structural applications and as an environmental/thermal barrier coating material due to its unique properties such as high melting point, machinability, thermal stability, low linear thermal expansion coefficient (3.9 × 10⁻⁶/K, 200°–1300°C), and low thermal conductivity (<3.0 W/m·K above 300°C). The hot corrosion behavior of γ-Y₂Si₂O₇ in thin-film molten Na₂SO₄ at 850°–1000°C for 20 h in flowing air was investigated using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS). γ-Y₂Si₂O₇ exhibited good resistance against Na₂SO₄ molten salt. The kinetic curves were well fitted by a paralinear equation: the linear part was caused by the evaporation of Na₂SO₄ and the parabolic part came from gas products evolved from the hotcorrosion reaction. A thin silica film formed under the corrosion scale was the key factor for retarding the hot corrosion. The apparent activation energy for the corrosion of γ-Y₂Si₂O₇ in Na₂SO₄ molten salt with flowing air was evaluated to be 255 kJ/mol.     

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.     

Oxidation Equilibrium of Iron in Borosilicate Glass

Ferrous/ferric equilibria were determined in alkali-alkaline-earth borosilicate glass as a function of temperature, oxygen partial pressure, and glass composition. Expected linear relations are found between log(Fe²⁺/Fe³⁺) and log( p O₂) or 1/ T . The slopes of the correlation with 10g( p O₂) are near the expected value of -0.25, but are found to decrease with increasing temperature. Reaction enthalpies determined from the correlation with 1/ T of –100 to –116 kJ/mol are similar to those reported for other silicate glasses. The ferroudferric equilibrium is not dependent on total iron content in the range 0.5 to 0.09% Fe₂O₃. More reducing conditions are required at lower temperatures to stabilize the amber chromophore. The ferroudferric equilibria are correlated to the number of bridging and nonbridging oxygen ions in the glass. The results suggest that the oxidation-reduction reaction can be written as: Fe²⁺+ (¹/₄)O₂+ (³/₂)O^2-= FeO₂⁻     

Phase Relations in the System Na2O-TiO2-SiO2

Liǵuid us and subsolidus phase relations were studied using the quenching technique. The system contains four ternary compounds stable to liquidus temperatures: Na₂TiSi₄O₁₁, Na₂TiSi₂O₇, Na₂TiSiO₅, and Na₂Ti₂Si₂O₉. Six eutectics, eight peritecties, and eight thermal maxima were located and a region of liquid immiscibility was delineated. X-ray powder data are given for the stable and metastable crystalline phases. Glass-transition temperatures were determined by thermal analysis. The relation between physical properties of melts and glasses and the configuration of the liquidus is discussed.     

Sub-Solidus Synthesis and Microwave Dielectric Characterization of Plagioclase Feldspars

Na _x Ca_{1− x} Al_{2− x} Si_{2+ x} O₈ plagioclase solid solutions (0≤ x ≤1) were synthesized under sub-solidus conditions using a solid-state reaction technique. The plagioclase formation and the sintering temperature decreased with an increase in x from the anorthite (CaAl₂Si₂O₈; x =0) to the albite (NaAlSi₃O₈; x =1).
Microwave (MW) dielectric measurements revealed that slow-cooled ( P 1 ) anorthite exhibited higher Q × f values than fast-cooled ( I 1 ) anorthite. Slow cooling also considerably improved the Q × f values of the sodium-rich Na _x Ca_{1− x} Al_{2− x} Si_{2+ x} O₈ solid solutions (0.8≤ x ≤1), where the highest Q × f value of 17 600 GHz was obtained for slow-cooled Na_0.8Ca_0.2Al_1.2Si_2.8O₈. The temperature coefficient of resonant frequency (τ_f) approached zero for 0.8≤ x ≤1.     

Properties and Structure of Glasses in the Binary Systems Alkali—TiO2

Glasses corresponding to mole formulas R₂TiO₃ and R₂Ti₂O₅ were prepared in 1- to 5-g quantities by quenching in a platinum crucible. K₂O, Rb₂O, and Cs₂O formed fairly stable glasses with TiO₂. On heat treatment, these glasses nucleated readily and formed opal-like glasses. Li₂O and Na₂O, however, did not form glasses with TiO₂ in 1-g quantities. Hygroscopicity increased with the alkali content and decreased with the increase in TiO₂ concentration. The refractive indices of the glasses ranged from 1.66 to 1.90. These facts indicate that TiO₂ is a glass former in its own right and that Ti⁴⁺ exists in sixfold coordination in these glasses.     

Further Studies on Thermal Evolution of Glass-Forming Gels

Glasses of composition Na₂O·2SiO₂ containing iron oxide are obtained by the gel technique. The results show that the iron atoms accelerate the formation of the glass, whereas when they are present in low percentage (∼2%), the temperature necessary to obtain the glass phase is higher. During heat treatment at 250°C, two silicates are formed: Na₂SiO₃ and Na₄SiO₄. When the iron content is low, they change at 650°C into Na₂Si₂O₅, but when the iron content is higher (∼6%), this silicate does not form and the system becomes vitreous.     

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.     

Spectroscopic Properties and Thermal Stability of Er3+-Doped Germanotellurite Glasses for Broadband Fiber Amplifiers

The thermal stability and spectroscopic properties of Er₂O₃-doped TeO₂–GeO₂–ZnO–Na₂O–Y₂O₃ glasses for 1.5 μm fiber amplifiers were investigated. The thermal stability of the 75TeO₂·20ZnO· 5Na₂O glass was improved by introducing GeO₂ and Y₂O₃. The radiative transition and the nonradiative transition have a dominant influence on the ⁴I_13/2 level lifetime of Er³⁺ in high- and low-GeO₂ regions, respectively. Adding Y₂O₃ increases the ⁴I_13/2 level lifetime of Er³⁺ significantly. The Judd–Ofelt (J-O) parameter Ω₆ shows a strong correlation with the 1.5 μm emission bandwidth; and the larger the Ω₆, the wider the bandwidth.