Heat Capacity and Structural Relaxation of Enthalpy in As2Se3 Glass

The evolution of enthalpy of As₂Se₃ glass during structural relaxation in the glass transition region was measured via differential scanning calorimetry for two types of time-temperature programs: rate-heating at 10 K/min following a cool at a constant rate (-20 to -0.31 K/min) and isothermal annealing following a temperature step from an equilibrium state. The rate-heating data yield kinetic parameters for the structural relaxation which predict accurately the evolution of the enthalpy during isothermal annealing. The glass heat capacities were independent of cooling rate within experimental precision (≤0.2%). In this respect, As₂Se₃ is unlike previously studied glasses whose heat capacities are more dependent on thermal history.     

X-Ray Study of the Solid Solution of TiO, Fe2O3, and G, O3 in Mullite (3A12O3a2Si02)

A study of the solid solution of TiO₂, Fe₂O₃, and Cr₂0₃ in mullite was made by measuring the changes in lattice parameters and unit-cell volume. Synthetic mullite (3O₃-2SiO₂) was reacted with up to 12 weight % of the oxides at temperatures ranging from 1000° to 17000C. The approximate minimum temperature required for the formation of solid solution was 12000C. for Fe₂0₃ and 1400°C. for Cr₂O₃ and TiO₃. The maximum amount of solid solution found was 2 to 4% TiO₂ at 1600°C., 10 to 12% Fe₂O_s at 1300°C., and 8 to 10% Cr_ZO₃ at 1600OC. Lattice parameters and unit-cell volumes for each solid solution series increased with increasing amounts of foreign oxide. There was good agreement between the calculated and observed increase in cell dimensions for the iron oxide series. Except in the case of titania, there was good agreement between X-ray data and petrographic observations.     

Tensile Strengths of Se, As2S3, As2Se3, and Ge30As15Se55 Glass Fibers

The strengths of as-drawn, abraded, and pristine Se, As₂Se₃, As₂S₃, and Ge₃₀As₁₅Se₅₅ fibers are reported. Pristine strength values range from an average of 80 MPa for Se and Ge₃₀As₁₅Se₅₅ to 100 and 180 MPa for As₂Se₃ and As₂S₃, respectively. Although these values are considerably higher than those reported previously for bulk samples, in no case does the maximum observed strength reach more than about 6% of that expected from simple theoretial calculations. Several factors that can control the strength are discussed. The overall behavior is interpreted in terms of the glass structures which permit localized stress relaxation effects in regions of weak, nondirectional bonds. These effects occur at temperatures well below the glass-transition region.     

CrO2-Cr2O3 Phase Boundary Under High O2 Pressures

The phase boundary between CrO₂ and Cr₂O₃ was reinvestigated under high O₂ pressures by using a new type of gas compressor. The boundary curve can be represented as log Po₂= 7.16-(3579/ T ). Using the observed data, Δ G °, Δ H °, and Δ S ° for the reaction 2CrO₂⇋Cr₂O₃+½O₂ were calculated to be: Δ G °= -(1.55/100) T +7.60 kcal/mol, Δ H °= -8.19 kcal/mol, and Δ S °= (-15.8/ T )+0.0155 kcal/mol.     

Effect of Nd2O3 Concentration on the Defect Structure of CeO2–Nd2O3 Solid Solution

An extensive X-ray study of CeO₂–Nd₂O₃ solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO_1.5 were measured using several techniques. Solid solutions containing 0–80 mol% NdO_1.5 were synthesized by coprecipitation from Ce(NO₃)₃ and Nd(NO₃)₃ aqueous solutions, and the coprecipitated samples were sintered at 1400°C. A fluorite structure was observed for CeO₂–NdO_1.5 solid solutions with 0–40 mol% NdO_1.5, which changed to a rare earth C-type structure at 45–75 mol% NdO_1.5. The change in the lattice parameters of CeO₂–NdO_1.5 solid solutions, when plotted with respect to the NdO_1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C-type regions. The maximum solubility limit for NdO_1.5 in CeO₂ solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0–70 mol% NdO_1.5) of solid solution.     

Phase Equilibria in the Systems NiO-Cr2,O3,-O2, MgO-Cr2O3−O2, and CdO-Cr2,O3,-O2, at High Oxygen Pressures

Phase equilibria were determined for the systems NiO-Cr₂O₃−O₂, MgO-Cr₂O₃,-O₂, and CdO-Cr₂O₃−O₂ from 450° to above 850° C and at oxygen pressures of from 2 to 3500 atm. Only two intermediate phases were found in the nickel system: NiCrO., (CrVO₄ structure) and the spinel NiCr₂O₄. The magnesium and cadmium systems are similar in that they have three analogous phases: the low-temperature α-MgCrO₄ and α-CdCrO₄ (both with the CrVO₄ structure), the high-temperature β-MgCrO₄ and β-CdCrO₄ (both with the α-MnMoO₄ structure), and the spinels MgCr₂O₄ and CdCr₂O₄. The cadmium system contains an additional phase, Cd₂CrO₅, which is primitive monoclinic.     

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.     

Effects of Sm2O3 and Gd2O3+ Nd2O3 on Electromechanical Properties of PbTiO3 Ceramics

The electromechanical properties of PbTiO₃ ceramics, modified by substitution of Sm or Gd + Nd (same average atomic radius as Sm) for Pb, were studied in the range of 6% to 14% substitution. The modified PbTiO₃ ceramics were stable, and the Curie temperature decreased linearly over this composition range. The 10% Sm composition had a large anisotropy in the coupling factor ratio, k _t / k _p , and a similar, but weaker, effect developed for 12% (1/2 Gd + 1/2 Nd). This indicates that other than average ion size may influence the electromechanical coupling factor ratio.     

Effect of Al2O3 and Bi2O3 on the Formation Mechanism of Sn-Doped Ba2Ti9O20

High-performance Ba₂Ti₉O₂₀ ceramics are attracting great attention, but their formation mechanism still is somewhat unclear. The present investigation shows that the formation of Ba₂Ti₉O₂₀ can be promoted strikingly by the participation of Bi₂O₃ and Al₂O₃. The effect of Bi₂O₃ on the formation of Ba₂Ti₉O₂₀ is attributed to the fact that migration of the involved reactants is accelerated by liquid which forms from the melting of Bi₂O₃ above 830°C. This migration, however, is not the only rate-limiting factor. A high potential-energy barrier, resulting from stress that arises along the crystal-structured layers, also heavily restricts the formation of Ba₂Ti₉O₂₀. The participation of Al₂O₃, on the other hand, can reduce the height of this potential-energy barrier and effectively improve the kinetics of the formation of Ba₂Ti₉O₂₀ by causing the formation of BaAI₂Ti₆O₁₆ crystals; these crystals intergrow with Ba₂Ti₉O₂₀ crystals and result in decreased stress.     

Glass Formation Range, Acid Resistivity, and Surface Charge Density of ZnO-B2O3-SiO2 Passivation Glass Containing Al2O3

The glass formation range in the system ZnO-B₂O₃-SiO₂ increases when 5% Al₂O₃ is added and then decreases with further Al₂O₃ additions. The acid resistivity of the glass also increases when Al₂O₃ is added. An observed increase in negative charge with Al content until the system contains equal amounts of Al and Si (in forms of mole %) is explained by the formation of AlO₄⁻ tetrahedra which substitute in the SiO₄ network. Alkaline-earth oxides cause a positive charge which compensates for the negative charge formed by Al₂O₃. Antimony oxide and lanthanum oxide result in a negative charge in the glass. The formation of a negative or positive charge in the glass is thought to reflect the acidity or basicity of the glass, respectively.     

Sub-Sub-Tg Enthalpy Relaxation in a B2O3 Glass

Enthalpy relaxation processes in a B,O₃ glass were investigated calorimetrically in the temperature region from well below to near the glass-transition temperature T_g . The low-temperature ( T_aT_g- 100 K) or sub-sub- T_g anneals stabilize the glass structure. On heating, the annealed sample shows an excess endothermic peak above the annealing temperature T_a , and gradually recovers the original enthalpy without heating through T_g . The enthalpy relaxation evolves in a continuous manner with annealing time as seen in the recovery process. As T_a approaches T_g , a gradual transformation of the sub-sub- T_g behavior to that characteristic of the sub- T_g anneal occurs. The relaxation spectrum exhibits a broad distribution of relaxation times with activation energies of ∼ 10⁵ J/mol. The so-called β distribution commonly used in the analysis of the sub- T_g relaxation processes is seen to be too narrow to describe the sub-sub- T_g behavior. Possible mechanisms for the sub-sub- T_g relaxation phenomena are proposed.     

Influence of As2O3 on the Optical Properties of Ultrapure Multicomponent Silicate Glasses and Optical Fibers

To obtain low transmission loss optical fibers from ultrapure multicomponent silicate glasses, it is necessary to add small quantities of As₂O₃ (or Sb₂O₃). In optical fibers prepared from glasses without these agents, a significant increase in loss is observed. To investigate this effect, the influence of As₂O₃ (added to the batch and present in the glass as As₂O₅) on the optical properties of ultrapure silicate glasses was studied. These properties are the Rayleigh scattering loss coefficient, transition metal absorption, and position of the uv absorption edge. This study showed that the increase in loss of As₂O₃-free glass cannot be assigned to any of these contributions and was attributed to absorption by electrons, trapped in relatively shallow traps in the glass network. The As⁵⁺ ions serve as deep traps and therefore remove the additional absorption. The same phenomenon, although much more pronounced, was observed in optical fibers prepared from alkali borosilicate glasses.     

Characterization of Freeze-Dried Al2O3 and Fe2O3

Oxide crystallite formation and growth from freeze-dried sulfates were studied for the representative materials Al₂O₃ and Fe₂O₃. Transmission and scanning electron micrographs showed the formation and growth of chainlike aggregates of crystallites. Aggregation occurred as part of the nucleation and growth of the oxide, and discrete oxide particles were never present. Orientation of the chain aggregates was related to the ice structure formed during freezing. X-ray line broadening data showed that crystallite size is a function of the 1/5 to 1/7 power of time for isothermal treatments. A qualitative analysis of material transport favored the surface diffusion mechanism.     

Comment on "Effect of Al2O3 and Bi2O3 on the Formation Mechanism of Sn-Doped Ba2Ti9O20"

in a recent article of the Journal , Yu et al .¹ reported their experimental results on the effect of Al₂O₃ and Bi₂O₃ on the formation mechanism of Sn-doped Ba₂Ti₉O₂₀. They claimed that both Al₂O₃ and Bi₂O₃ can dramatically assist the formation of Sn-doped Ba₂Ti₉O₂₀ but are based on different mechanisms. They concluded that first, Bi₂O₃ melts above 830°C and accelerates the migration of the involved reactants to form Ba₂Ti₉O₂₀; second, Al₂O₃ can reduce the height of the potential energy barrier of the formation of Ba₂Ti₉O₂₀ due to the intergrowth of BaAl₂Ti₆O₁₆ phase. They explained their results from a point of view that the formation of Ba₂Ti₉O₂₀ is controlled by (1) the migration of reactants to the interfaces and (2) the height of the potential-energy barrier of the reaction at the interfaces. However, based on their results, we feel their conclusions are incautious and may be misleading, as will be discussed later.