Oxidation-Reduction Equilibria in Molten Na2O.2SiO2 Glass

The oxidation-reduction equilibria in molten glasses having a soda/silica ratio of 1/2 and containing small amounts of variable-valence ions of the transition elements titanium, vanadium, iron, cobalt, and nickel, the post-transition elements tin and antimony, and the rare-earth element cerium were obtained by equilibrating the melts with various atmospheres. The simple mass expression
was always applicable. In the expression, n is the number of electrons involved in the valence change of the metal M. The value of n is 1 for all systems except for the ions of antimony and tin where n is 2. The ions found are those generally accepted as existing in glass melts which are Ti³⁺, Ti⁴⁺; Fez+, Fe³⁺; Ce³⁺, Ce⁴⁺; Mn²⁺, Mn³; Co²⁺, Co³⁺; Ni²⁺, Ni³; Sb³⁺, Sb⁵⁺; and Sn²⁺, Sn⁴⁺. Two mass action expressions were needed for vanadium-containing glasses to describe the equilibria between 5+, 4+, and 3 f species.     

Role of Cerium in Suppression of Gamma-Ray Induced Coloring of Borate Glasses

The role of cerium in the suppression of gamma-ray induced coloration in glass has been found to depend on the relative concentration of Ce³⁺ to Ce⁴⁺ ions as well as on the total cerium content. In a borate glass having high ultraviolet transmission, it has been found that both Ce³⁺ and Ce⁴⁺ ions are necessary to suppress the optical absorption bands induced in the visible region. The role of cerium can be explained on the basis of a change in its oxidation state as a result of gamma irradiation. It is postulated that the cerous ions, by the reaction Ce³⁺→ Ce⁴⁺+ e , suppress the induced visible band at 2.36 ev (525 mμ), which may result from positive hole centers. High cerous ion concentration results, however, in an induced center (Ce³⁺+ e ) which absorbs in the visible at about 1.9 ev (650 mμ). The presence of Ce⁴⁺ ions near Ce³⁺ prevents the formation of this center possibly by the reaction Ce⁴⁺+ e → Ce³⁺. These induced opposite changes in the oxidation state of cerium tend to maintain a balance in the ratio of Ce³⁺ to Ce⁴⁺ ions in the glass during irradiation, and the suppression of the visible bands depends on this ratio.     

Stannous-Stannic Equilibrium in Molten Binary Alkali Silicate and Ternary Silicate Glasses

The stannous-stannic equilibrium in binary alkali silicate and ternary silicate glasses was studied by equilibrating glassmelts with air at 1400°C. The Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the oxidized state with increasing ionic radii of the alkali ions or with increasing concentration of the alkali ions in the same series of glasses. The slope of the straight lines obtained on plotting log (Sn⁴⁺)/(Sn²⁺)( pO₂ )^n/2 vs mol% R₂O increased in the order Li→Na→K. In ternary silicate glasses having the base glass composition 20Na₂O·10RO·70SiO₂, the Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the reduced state, with increasing bond strength between the divalent cations and the nonbridging oxygens. With increasing temperature, the equilibrium shifts more toward the reduced state.     

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.     

Photochemical Reaction in a Reduced Soda-Lime-Silica Glass Containing Ce3+ and As3

The photochemical reaction between Ce³⁺ and As³⁺ in a reduced 16Na₂O. ¹1CaO.73SiO₂ glass (mol%) was examined by exposing it to an Xe or Hg arc. In addition to Ce⁴⁺, a paramagnetic As(II)OS center having C_2v symmetry and a trapped electron center were identified by ESR. The As(II)O defect is the origin of the unidentified ESR signal which correlates with the discoloration of a borosilicate glass developed for solar energy application .     

Phase Separation in 12 mol% Ceria-Doped Zirconia Induced by Heat Treatment in H2 and Ar

The phase separation in 12 mol% CeO₂─ZrO₂ ceramic heattreated in a mixture of H₂ and Ar was investigated by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and Raman scattering. After heat treatment at temperatures above 1200°C, the tetragonal solid-solution phase separated into Zr₂Ce₂O₇ and the monoclinic phase. Raman scattering measurements also provided supplementary evidence for the phase separation. XPS showed that the valence change from Ce⁴⁺ to Ce³⁺ predominantly occurred, whereas the reduction from Zr⁴⁺ to Zr³⁺ took place above 1200°C. It is concluded, that in the highly reduced sample, where the valence changes from Ce⁴⁺ (Zr⁴⁺) to Ce³⁺ (Zr³⁺), the phase separation is noticeably promoted. Below 1000°C the phase separation was suppressed because of no appreciable valence change to trigger the phase separation, and the single tetragonal phase was retained.     

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.     

Glass Formation in the System SiO2–PbO Containing Transition-Metal Oxides

Glass-forming regions, valence states, and viscosities in SiO₂–PbO systems containing various transition-metal oxides as a third component were investigated. The glasses were prepared by melting in an open atmosphere. The glass-forming regions ranged as follows: MnO≡ZnO > FeO_1.5>NiO. The ratios Fe²⁺/(Fe²⁺+ Fe³⁺) and Mn³⁺/ (Mn³⁺+ Mn²⁺) in the glasses were determined by chemical analysis. The Fe²⁺/ (Fe²⁺+ Fe³⁺) ratio in SiO₂–PbO–FeO_1.5 glasses ranged from 0.016 to 0.050. The Mn³⁺/ (Mn³⁺+ Mn²⁺) ratio in SiO₂–PbO–MnO glasses ranged from 0.056 to 0.30. The fraction of manganese (III) ions in the glasses varies considerably with the glass composition. The effects of transitionmetal oxides on the viscosity are discussed.     

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.     

Influence of Fe3+/Fe2+ Ratio on the Crystallization of Iron-Rich Glasses Made with Industrial Wastes

The influence of the Fe³⁺/Fe²⁺ ratio on the crystallization of iron-rich glasses was investigated in this study. The glass batches were made from two hazardous industrial wastes: mud (goethite and jarosite) originating from the zinc hydrometallurgical process and electric arc furnace dust (EAFD). Glass compositions were prepared by adding different percentages of carbon powder. The crystallization process was investigated by a combined thermogravimetry/differential thermal analysis technique, in air or nitrogen atmospheres, using powder and bulk glass samples. The crystalline phases formed, i.e., pyroxene and spinels, and their relative ratio were determined by X-ray diffractometry. The experimental results indicated that melting temperature and crystallization behavior were influenced by the initial Fe³⁺/Fe²⁺ ratio and by the amount of carbon added to the glass batch. For goethite and jarosite glass compositions, decreasing the Fe³⁺/Fe²⁺ ratio increased the crystallization rate by favoring magnetite formation. For EAFD glass compositions, the addition of carbon to the batch inhibited chromite–magnetite spinel formation and favored the attainment of an amorphous glassy phase.     

Preparation of Cerium-Activated Silica Glasses: Phosphorus and Aluminum Codoping Effects on Absorption and Fluorescence Properties

Cerium-activated silica (SiO₂) glasses were prepared by plasma torch chemical vapor deposition (CVD). In Ce-doped SiO₂ glasses, most Ce exists as Ce⁴⁺ ions; the remaining small amount of Ce³⁺ ions exhibits a broad fluorescence spectrum with a large Stokes shift, ∼9600 cm^-1, from the excitation spectrum peak of 324 nm. Aluminum and phosphorus codoping considerably increases the Ce³⁺ ratio and shifts the peaks of both spectra to shorter wavelengths. P codoping is the more effective way to achieve this result and in some cases produces an absorption spectrum similar to that of a Ce-doped phosphate glass. These findings are consistent with the solvatiorn shell model for codoping, as previously proposed. To codope P, a soot remelting method was devised to deal with the highly volatile P₂O₅.     

Reaction Sequence and Microstructrual Development of CeO2-Doped Reaction-Bonded Mullite

A promising technique for the fabrication of mullite ceramics and mullite-matrix composites with low dimensional changes ("near-net-shape processing") is reaction bonding using Si metal and α-Al₂O₃ as starting materials, because sintering-induced shrinkage is compensated by Si-oxidation-induced volume expansion. A mullite reaction bonding (RBM) route which proceeds at much lower temperatures (lessthan equal to1350°C) than in conventional RBM systems (greaterthan equal to1500°C) is based on Ce doping which provides accelerated Si oxidation and mullite formation due to the formation of transient, low-viscosity Ce-Al-Si-O liquids. The present study shows that the required Ce-Al-Si-O liquids form in a reducing environment with Ce occurring as Ce³⁺. In an oxidizing environment, Ce is present as Ce⁴⁺, giving rise to precipitation of crystalline CeO₂. Ce³⁺ left and right arrow Ce⁴⁺ redox reactions in the temperature range under consideration appear to be controlled by the presence of nonoxidized Si in the samples. According to the present investigation the amount of CeO₂ added to the starting powders must be tailored carefully: Exaggerated CeO₂ content produces large amounts of low-viscosity Ce-Al-Si-O liquids which may have the disadvantage of excessive sealing of the open porosity. This slows the oxygen diffusion velocity into the specimen considerably, with the consequence that nonoxidized Si and a residual Ce-Al-Si-O glass coexist in the ceramics after processing. A solution to this problem is to simultaneously enhance mullite crystal growth through seeding which works against excessive liquid-phase-induced shrinkage of the samples. This in turn enables complete oxidation and recrystallization of all liquid phases.     

High-Brightness LaPO4:Ce3+, Tb3+ Nanophosphors: Reductive Hydrothermal Synthesis and Photoluminescent Properties

This paper definitely reveals that LaPO₄:Ce³⁺, Tb³⁺ (LAP) nanophosphors prepared by normal hydrothermal method suffer significant loss of luminescence due to the oxidation of Ce³⁺–Ce⁴⁺ at hydrothermal stage. To effectively protect Ce³⁺ from oxidation, a reductive hydrothermal process using hydrazine hydrate as a protecting agent is proposed to synthesize LAP nanophosphors with different Ce³⁺ and Tb³⁺ concentrations, which exhibited much stronger green photoluminescence (PL) and longer lifetime than the products prepared by normal hydrothermal method. Furthermore, the high-brightness LAP nanophosphors exhibited high-quenching concentration of Tb³⁺; the La_0.4Ce_0.4Tb_0.2PO₄ nanophosphor showed almost the same PL intensity as that of the commercially used La_0.7Ce_0.2Tb_0.1PO₄ bulk powder.     

Behavior of Paramagnetic Iron during the Thermal Transformations of Kaolinite

Kaolinites with various degrees of structural order and iron content were heated and subsequently analyzed via electron paramagnetic resonance. Iron was present in two different states in the heated materials, either as dilute structural Fe³⁺ ions or in concentrated Fe³⁺ phases. During metakaolinization, the environment of dilute Fe³⁺ ions changed, following modifications of the Al³⁺ coordination, and the Fe³⁺ concentration increased. With the breakdown of metakaolinite, the diffusion of Fe³⁺ ions induced their exsolution in superparamagnetic iron-rich domains (Fe³⁺ clusters in γ-Al₂O₃ and/or Fe³⁺ oxide nanophases), which produced a decrease in the dilute Fe³⁺ concentration. The subsequent breakdown of γ-Al₂O₃ and the formation of mullite made the dilute Fe³⁺ concentration increase again, because of the incorporation of Fe³⁺ ions in the mullite structure.     

Oxidation-Reduction Equilibria in Iron-Containing Glass

The oxidation-reduction equilibrium between ferrous and ferric iron in Na₂O · 2Si0₂ glass melts was studied by equilibrating melts with various oxygen partial pressures. Exceedingly long equilibration times were required to obtain meaningful results. The reaction appears to be diffusion-controlled and may be expressed as 20^2-+ 4Fe³⁺$4 4Fe²⁺+ O₂. Data are presented in which the valence of iron varies from predominantly +3 to predominantly +2.     

A Study of Conductivity in the Sr(Fe1−xTix)O3−δ System

The conductivity and conductivity-temperature characteristics of the Sr(Fe_1−xTi_xO_3−δ system fired in air are studied by means of Mössbauer spectroscopy at room temperature and at 78 K. On the basis of quantitative analysis of the Fe⁴⁺ content in the solid solution, the concentrations of oxygen vacancies are calculated and the relationship between Fe⁴⁺ content and conductivity is found. It is also found that the turning points of conductivity and material constant B, as well as Fe⁴⁺ and the percentage of oxygen vacancies, occur at x=0.7 on the curve. When x=0.7, Fe³⁺ (II), which has never been reported before, disappears and the lattice parameters no longer vary with x. Besides Fe⁴⁺, Fe³⁺(II) is also a factor affecting the conductivity of this system.     

Effect of Tetravalent Dopants on Raman Spectra of Tetragonal Zirconia

Tetragonal zirconia solid solutions were prepared by the additions of ceria, titania, germania, and cassiterite into 2-mol%-Y₂O₃-stabilized t -ZrO₂ (2Y-TZP), and the effect of the dopant size on the changes in the Raman spectra of t -ZrO₂ was investigated. The frequencies of the Raman modes that were associated with Zr-O bond stretching decreased as doping with the oversized cation (Ce⁴⁺) increased and increased linearly as doping with the undersized dopants (Ti⁴⁺, Ge⁴⁺, Sn⁴⁺) increased within the solubility limits of the dopants in 2Y-TZP. The frequency of the 640 cm⁻¹ Raman band decreased as the square root of the unit-cell volume increased, suggesting that the ionic size of dopants governs the Raman mode that is related to the short ZrO bond between two ZrO bonds in t -ZrO₂ solid solutions. Although there was no consistent relationship between the change in the tetragonality of t -ZrO₂ solid solutions and the tetravalent cation size, the intensity ratio of the Raman modes at 609 and 640 cm⁻¹ ( I ₆₀₉/ I ₆₄₀) decreased as the ionic size decreased.     

Microwave Characteristics of (Pb,Ca)(Fe,Nb,Sn)O3 Dielectric Materials

The dielectric properties of (Pb_{1− x} Ca _x ){(Fe_1/2Nb_1/2)_{1– y} Sn _y }O₃ solid solutions, where 0.4 lessthan equal tox ≤ 0.6, y = 0.05, 0.1, have been investigated at microwave frequencies. The replacement of Fe³⁺/Nb⁵⁺ by Sn⁴⁺ at the B–site of the perov-skite structure considerably improves the loss quality factor Q and does not remarkably affect the dielectric constant epsilon_r and the temperature coefficient of resonant frequency tau_f. The tau_f value of nearly 0 ppm/°C can be realized for x= 0.55. New high-quality dielectric ceramics having epsilon_r of 85.3-89.9,Qf values of 7510-8600 GHz, and τ_f of 0-9 ppm/°C were obtained at 1150°C for 3 h sintering in air. The influence of the sintering atmosphere on dielectric properties was also investigated.     

Structural and Photoluminescence Properties of Ce3+- and Tb3+-Activated Lu-α-Sialon

The crystal structure and photoluminescence properties of undoped and Ce³⁺- or Tb³⁺-doped Lu-α-Sialon are reported. Lu-α-Sialon with the composition of Lu_0.367Si_9.9Al_2.1ON₁₅ crystallizes in a trigonal system with a =7.8013(1) Å, c =5.6827(1) Å, in the space group P 31 c . The Lu³⁺ ion is accommodated at the interstice site at 2 b and directly connected by seven (N, O) atoms with an average bond length of 2.624 Å. The incorporation of large Ce³⁺ and Tb³⁺ ions on small Lu³⁺ site results in the expansion of the unit cell as expected but keeping the average Lu_Ln-(N, O) (Ln=Ce, Tb) distances nearly constant with slight distortion in the lattices. The optical band gap of Lu-α-Sialon is about 5.25 eV determined by the diffused reflection spectrum. Lu-α-Sialon:Ce³⁺ (2 mol%) exhibits efficiently greenish-blue emission at about 486 nm under near-ultraviolet (UV) excitation originating from the 5 d →4 f ¹5 d ⁰ transition of Ce³⁺. Lu-α-Sialon:Ce³⁺ is a potential candidate phosphor for white UV-LED applications due to its high quantum efficiency and excellent thermal stability. Lu-α-Sialon:Tb³⁺ (2 mol%) emits strong mixed blue and green light in equivalent due to the transitions of ⁵D₃→⁷F_J and ⁵D₄→⁷F_J of Tb³⁺.     

Optical and Magnetic Properties of Some Transition Metal Ions in Barium Phosphate Glass

In barium phosphate glass containing first-row transition metal ions, the ions appear to possess normal oxidation states: Cr³⁺, Mn³⁺, Fe^2+,3+, Co²⁺, Ni²⁺, and Cu²⁺. Room temperature and 77°K spectra for Cr³⁺-, Mn³⁺-, Fe^2+,3+-, Ni²⁺-, and Cu²⁺-containing glasses can be interpreted, on the, basis of octahedrally coordinated metal ions, whereas the spectrum of the Co²⁺ glass correlates with a tetrahedrally coordinated metal ion. Magnetic moments for the glasses are similar to those for the spin-only values of the various metal ions.