Preparation, Electrical Conductivity, and Thermal Expansion Behavior of Dense Nd1−xCaxCrO3 Solid Solutions

Nd_{1− x} Ca _x CrO₃ (0≤ x ≤0.25) solid solutions are synthesized using a two-step calcination solid-state reaction method and can potentially be used as interconnect applications in solid oxide fuel cells (SOFCs). The lattice parameters and unit cell volume decrease with an increase of the Ca²⁺ dopant concentration at the A-site. On the other hand, the relative density, electrical conductivity, and thermal expansion coefficient (TEC) increase with an increase of Ca²⁺ contents. Calcium doping significantly depresses the chromium vaporization of NdCrO₃, thus accelerating the sintering and densification of the oxides. Nd_{1− x} Ca _x CrO₃ with x =0.25 shows the best electrical conductivity of 28.8 and 1.1 S/cm at 850°C in air and hydrogen, respectively, with a high relative density of 98.0% and a good TEC of 9.19 × 10⁻⁶ K⁻¹ in the temperature range from 30° to 1000°C in air. This indicates that Nd_0.75Ca_0.25CrO₃ is a promising candidate as an interconnect material for application in SOFCs.     

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⁻     

Grain Growth in CeO2: Dopant Effects, Defect Mechanism, and Solute Drag

The effects of the dopants, Mg²⁺, Ca²⁺, Sr²⁺, Sc₃₊, Yb³⁺, Y³⁺, Gd³⁺, La³⁺, Ti⁴⁺, Zr⁴⁺, and Nb⁵⁺, on the grain boundary mobility of dense CeO₂ have been investigated from 1270° to 1420°C. Parabolic grain growth has been observed in all instances. Together with atmospheric effects, the results support the mechanism of cation interstitial transport being the rate-limiting step. A strong solute drag effect has been demonstrated for diffusion-enhancing dopants such as Mg2+ and Ca²⁺, which, at high concentrations, can nevertheless suppress grain boundary mobility. Severely undersized dopants (Mg, Sc, Ti, and Nb) have a tendency to markedly enhance grain boundary mobility, probably due to the large distortion of the surrounding lattice that apparently facilitates defect migration. Overall, the most effective grain growth inhibitor at 1.0 % doping is Y³⁺, while the most potent grain growth promoter is either Mg²⁺ (e.g., 0.1%) or Sc³⁺ at high concentration (greater than 1.0%).     

Solid Solution of Titanium Dioxide in Aluminum Oxide

The solid solution limit of titanium dioxide in Al₂O₃ was investigated using diffuse reflectance and cathodoluminescence measurements on polycrystalline Al₂O₃ doped with 1.0 mole % or less TiO₂. The samples had been fired in air at 1300°C. The diffuse reflectance spectra indicated that the solid solubility limit is between 0.25 and 0.30 mole %. Cathodoluminescence spectra indicated that both Ti³⁺ and Ti⁴⁺ are present in air-fired samples whereas only Ti³⁺ is present in hydrogen-fired samples.     

Incorporation of Er3+ into BaTiO3

The incorporation of Er³⁺ into BaTiO₃ ceramics was investigated on samples containing 0.25, 0.5, 1, 2, 8, and 10 at.% of dopant, after sintering at 1350–1550°C in air. For Er³⁺ concentrations ≤1 at.%, dense and large-grained ceramics with low room-temperature resistivity (10²–10³Ω·cm) were obtained. The observed properties are largely independent of stoichiometry. Simultaneous substitution of Er³⁺ at both cation sites, with higher preference for the Ba site, is proposed. The behavior of heavily doped ceramics depends on stoichiometry. When Ba/Ti < 1, the electrical properties change from slightly semiconducting to insulating as Er concentration increases from 2 to 8 at.%. The ceramics have tetragonal perovskite structure and contain a large amount of Er₂Ti₂O₇ pyrochlore phase. On the other hand, when Ba/Ti > 1, the ceramics are insulating, fine-grained, and single phase. In this case, incorporation of Er³⁺ predominantly occurs at the Ti site, with oxygen vacancy compensation. Incorporation is accompanied by a significant reduction of tetragonality and by expansion of the unit cell. The different results indicate that Er³⁺ solubility at the Ba site does not exceed 1 at.%, whereas solubility at the Ti site is at least 10 at.%. However, the incorporation of Er³⁺ and the resulting properties are also strongly affected by sintering conditions.     

Monopyroxenic Basalt-Based Glass-Ceramics

Uniform, ultrafine, microcrystalline, hard, pyroxenic glass-ceramic materials have been obtained successfully from basalt rock; instead of adding nucleation catalysts, the FeO:Fe₂O₃, CaO:Na₂O, and CaO:MgO ratios have been rectified. This process has been accomplished by deliberately adding the smallest permissible amounts of oxidizers, limestone, dolomite, and soda ash (as additives) that are necessary to fulfill the monominerality requirements; these requirements affect the melting, workability, crystallization, and microstructure of the glass-ceramics. The melting temperature decreases as the ratios decrease (beyond certain limits); in addition, the workability, crystallization, and microstructure also improve as the ratios decrease. An almost-stable solid solution of augite or aegirine-augite composition is the only crystalline phase that is formed. The minimal FeO:Fe₂O₃ ratio and the likelihood of a greater affinity of the Na⁺ cation for the Fe³⁺ cation, rather than the Al³⁺ cation, may be responsible for increasing the stability and widening of the crystallization field of the complex aluminum-bearing pyroxene solid solution.     

Discoloration of Fired Kaolinitic Clays (Study of Fe3+ Coordination by Mössbauer and UV-ViS-NIR Spectroscopy)

The final color of a ceramic body is generally determined by the contents of Fe³⁺ ions. The Central European market accepts kaolinitic clays for the production of tableware, electro-insulators, and wall tiles only if the Fe₂O₃ in chemical analyses of the kaolin does not exceed 1.0 wt%. The chemical analyses calculate the content of all iron components as if they were in the form of Fe₂O₃ and then their quantity excludes even good-quality clay from the ceramic industry of white bodies. We found that oxidizing firing of the samples fired at over 1180°C changes the color in cases where the initial Fe³⁺ component is in the form of hydro ferric oxide [FeO(OH)] or if the ferric ions were added to the white kaolin samples in the form of ferric nitrate [Fe(NO₃)₃·9H₂O]. The Mössbauer spectroscopy confirms the presence of only diluted Fe³⁺ ions. The UV-ViS-NIR spectroscopy confirms that even if the concentration of Fe₂O₃ is above 3.0 wt%, these ions of Fe³⁺ in the case of their initial hydro ferric oxide formed in clay are incorporated into the mullitic structure in tetrahedral coordination. The iron-coloring effect depends on the coordination of Fe³⁺ ions with the studied discoloring effect of fired bodies—the very small and well-distributed particles enter into the formatted mullitic structure.     

The Iron-Oxygen Equilibrium in Glass: Effect of Platinum on the Fe2+/Fe3+ Equilibrium

A number of experiments were performed on iron-containing sodium disilicate melts in air. It was found that it was not possible to obtain an equilibrium between Fe²⁺ and Fe³⁺ in platinum crucibles owing to the reaction between platinum and iron, whereas in alumina crucibles the equilibrium was rapidly established. Thermodynamic calculations of the reaction 2FeO (in Na²O-2SiO²) +½O² (g) = Fe²O³ (in Na²O-2SiO²) showed that the equilibrium went more and more to the right with increasing temperature. The standard free energy, enthalpy, and entropy for the reaction were calculated.     

Barium Neodymium Titanate Electroceramics: Phase Equilibria Studies of Ba6–3xNd8+2xTi18O54 Solid Solution

A solid-solution phase with the general formula Ba_6-3x Nd_8+2x Ti₁₈O₅₄, where 0.25(5) ≤×≤ 0.75(5), has been characterized at 1250°C; this phase has been variously described as BaNd₂Ti₄O₁₂ and BaNd₂Ti₅O₁₄ in the literature. Variation in its stoichiometry is accommodated via the cation substitution mechanism, 3Ba²⇆2Nd³⁺. The location and extent of the solid solution were demonstrated by a combination of phase diagram studies and X-ray diffraction techniques, including lattice parameter measurements and electron microscopy. A combination of techniques was employed due to the insensitivity of secondary phase detection by X-ray diffraction in this system. Using this approach, a second possible solid-solution mechanism, Ba²⁺2Nd³⁺⇆2Ti⁴⁺, is discounted.     

Effect of Dopants on the Defect Structure of Single-Crystal Aluminum Oxide

Density and lattice parameter changes induced by dopants were studied in Czochralski rubies containing from 0.054 to 0.160 wt% Cr₂O₃, in Czochralski sapphires containing from 0.083 to 0.120 wt% TiO₂, and in verneuil crystals grown from powders containing 250 to 1000 ppm Ca, Mg, Si, and V. Densities were determined within at least ∼0.005% using a hydrostatic weighing technique; lattice parameter shifts were measured within a maximum of 0.25% using a step-scanning goniometer technique. Some crystals which appeared to be clear, transparent, and single-phase contained fine particles of a second phase. It is concluded that Si⁴⁺ and Ti⁴⁺ ions enter solution with the formation of cation vacancies to maintain charge neutrality, that verneuil crystals contain vanadium as V³⁺, that the solid solubility of Ca²⁺ is low (<340 ppm), and that MgO decomposes under verneuil growth conditions, resulting in formation of a second phase.     

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.     

Preparation and Faraday Rotation of Divalent Europium Glasses

A large glass-forming region was discovered in the system EUO–Al₂O₃–B₂O₃ and chemically stable glasses of good optical quality were produced. The concentration range extends from about 3 to 43 mole % EuO. Room-temperature Verdet constants as large as –2.55 min/oe-cm were measured on a glass containing 30.5 mole % Eu²⁺. This is somewhat higher than those reported for the trivalent rare-earth ions in similar glass matrices. The effective transition wavelength causing the rotation is 380 ± 20 mμ. Compared to Ce³⁺, Tb³⁺, and Pr³⁺ the large rotation of Eu²⁺ in the visible region is due primarily to a large J (total angular momentum) value as well as to a large effective transition wavelength.     

Optical Absorption and Oxidation States of Uranium in Alkali Aluminoborate Glasses

The optical absorption spectra of 22R₂O.13Al₂O₃.65B₂O₃ (mol%) glasses contain ing uranium indicate the presence of U⁶⁺, U⁵⁺, and U⁴⁺. The presence of U⁶⁺ in the uranyl group is favored as alkali ion (R) is replaced in the direction Li6+ coordinated diflerently from that in uranyl groups (possibly 4-coordinated). A suggestion for making use of such findings in estimating the uranium content of a sample is given.     

Compositional Dependence of Judd-Ofelt Parameters in Silicate, Borate, and Phosphate Glasses

Judd-Ofelt parameters Ω _t with t = 2,4, 6 for the rare-earth ions Pr³⁺, Nd³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, and Tm³⁺ in alkali and/or alkaline-earth silicate, borate, and phosphate glasses have been determined. The variations of Ω _t with the number of 4 f electrons of the rare-earth ions are demonstrated, and factors affecting the Judd-Ofelt parameter Ω₆are discussed. The intensity parameter Ω₆ depends on the ionic packing ratio of the glass host by changing modifier type in silicate and borate glasses, and it is independent of that in a series of borate glasses as a function of modifier content and phosphate glasses. The peak wavenumbers of the transitions whose intensities are determined mainly by the Ω₆<| U ⁽⁶⁾|>² term—where <| U ⁽⁶⁾|> is one of the reduced matrix elements—shift systematically with the values of Ω₆ for all the rare-earth ions.     

Electron Paramagnetic Resonance Spectroscopy Applied to Diffusion of Mn2+ in MgO

Electron paramagnetic resonance spectroscopy was used to measure the diffusion rates of Mn²⁺ in MgO single crystals at 1200 to 1520 K. It is shown that the spectral components of Mn²⁺ must be separated into one due to a high concentration and a set due to a low concentration of Mn²⁺ ions in the diffusion samples. It was found that the data fit a simple ion-diffusion model in which the activation energy E = 2.1±0.25 eV and D₀= (9.4±s2.8)× 10⁻⁶ cm² s ⁻¹.     

Segregation of Isovalent Impurity Cations at the Surfaces of MgO and CaO

Enthalpies of segregation for isovalent impurities in magnesium and calcium oxide as a function of surface concentration were calculated by using an atomistic computer simulation method. We have considered Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, and Ni²⁺, segregating to both (001) and (110) faces. The results obtained can be extrapolated to predict the behavior of other impurities including Mn²⁺, Fe²⁺, and Co²⁺, We find, for example, that Fe²⁺, Mn²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ will concentrate at the (001) surface of MgO, while Ni²⁺ will be depleted. The enthalpy of segregation is found to vary substantially with coverage particularly for the larger impurities. The enthalpy becomes less negative with increasing impurity concentration due to the increasing lattice strain until the surface is nearly saturated. Then additional stabilization is obtained by restructuring of the surface layer. We predict reconstructed surfaces for both the (001) and (110) faces, which contain a high concentration of a larger impurity ion. The enthalpy of segregation shows a maximum at around 50% surface coverage implying a bimodal surface distribution of segregant. The influence of segregation on surface energy suggests two unusual effects. The (001) surface energy of the impure crystal becomes negative for surface concentrations of impurity greater than 10% Ba²⁺ or 75% Sr²⁺ in MgO. This implies a thermodynamic barrier to sintering. At high coverages of Ba²⁺ in MgO the (110) surface becomes more stable than the (001) face suggesting that facetting may occur.     

Thermal Expansion of MgFe2O4, FeO, and MgO·2FeO

The thermal expansion of magnesium ferrite, wüstite, and magnesio-wüstite has been measured up to 1000°C. Good agreement has been found with published data obtained at lower temperatures, but the reported extrapolations have been shown to be low. The thermal expansion of MgFe₂O₄ has been interpreted to support Gilleo's hypothesis that the super-exchange energy for Fe³⁺-O^2--Fe³⁺ linkages is independent of the Fe³⁺-O^2- distance. This conclusion is supported by the fact that the energy per linkage is the same as that for Fe³⁺-O^2-Fe³⁺ linkages in a variety of crystal structures.     

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.     

Effects of M3+ Ions on the Conductivity of Glasses and Glass-Ceramics in the System Li2O—M2O3—GeO2—P2O5 (M = Al, Ga, Y, Dy, Gd, and La)

Glasses with compositions Li_1.2M_0.2Ge_1.8(PO₄)₃ (M = Al, Ga, Y, Gd, Dy, and La) were prepared and converted to glass-ceramics by heat treatment. The effects of the M³⁺ ions on the conductivity of the glasses and glass-ceramics were studied. The main phase present in the glass-ceramics was the conductive phase LiGe₂(PO₄)₃. Al³⁺ and Ga³⁺ ions entered the LiGe₂(PO₄)₃ structure by replacing Ge⁴⁺ ions, but lanthanide ions did not. The glass-ceramics exhibited much higher conductivity than the glasses. With increased ionic radius of the M³⁺ ions, the conductivity remained almost unchanged at ∼3 × 10⁻¹² S/cm for the glasses, but it decreased from 1.5 × 10⁻⁵ to 8 × 10⁻⁹ S/cm for the glass-ceramics at room temperature. The higher conductivity for Al³⁺- and Ga³⁺-containing glass-ceramics was suggested to result from the substitutions of Al³⁺ and Ga³⁺ ions for Ge⁴⁺ ions in the LiGe₂(PO₄)₃ structure.     

Reaction and Diffusion in Silver-Arsenic Chalcogenide Glass Systems

The diffusion of Ag from the metal or Ag₂Se in amorphous As₂S₃ and As₂Se₃ at 175°C is accompanied by the reduction of As from a valence of 3+ to 2+ or 2+ to 1 + to maintain charge neutrality in the glass. Only Ag⁺ diffuses at this temperature; all other ions are essentially immobile. An amorphous reaction-product phase is formed in the diffusion zone with a composition range of 28.6 to 44.4 at % Ag. The lower limit corresponds to all As cations of 2+ valence (equivalent to amorphous Ag₂As₂S₃); the upper limit, the maximum solubility of Ag in these glasses, corresponds to all As cations of 1 + valence (equivalent to amorphous Ag₁As₂S₃). The diffusivity of Ag in these glasses at 175°C for concentrations of 10 to 35 at.% Ag is
Sulfide 4× 10⁻¹⁴ exp[(+0.23±0.01)(at.% Ag)]cm²/s
Selenide 2' 10⁻¹¹ exp[(+0.14±0.01)(at.% Ag)]cm²/s