Phase Composition and Compatibilities in the Bi-Sr-Ca-Cu Quaternary Oxide System at 800°C in Air

Subsolidus phase relations in the system (Bi-Sr-Ca-Cu)-O at 800°C were investigated via powder X-ray diffraction and electron probe microanalysis of solid-state reaction products. Two recently reported (Bi-Sr-Ca)-O phases were detected in quaternary assemlages and had the approximate cation stoichiometries of 9 11 5 and 825. The only truly quaternary compound detected is the superconducting phase with the cation stoichiometry 2212. Tie lines, tie triangles, and compatibility tetrahedra involving this phase and the 2201 superconducting phase were determined. Both superconducting compounds exhibit small but detectable Sr-to-Ca solubility and are Sr-deficient with respect to the "ideal" formulas.     

Reaction between Sodium Carbonate and Silica Sand at 874°C < T < 1022°C

This paper is concerned with the mechanism of the reaction between sodium carbonate and silica sand at 874° to 1022deg;C. A reaction path which evolves in four stages, i.e., decarbonization, dissolution of orthosilicate, dissolution of metasilicate, and final dissolution of silica, is dealt with in detail, including mass balances. Formulas for dissolution rates in the first two stages are suggested, and the effect of wetting the grains by the melt, amount and distribution of the melt, and nonuniform dissolution (e.g., segregation) on the dissolution rate of silica grains are discussed. The need for more detailed experimental studies is stressed.     

Limits of the Thermodynamic Stability of Cobalt—Iron—Manganese Mixed Oxides at 1200°C

The thermodynamic stability of spinel and rock-salt structure phases has been investigated for the quasi-ternary mixed oxide system Co-Fe-Mn-O at 1200°C and at total pressures of the order of 1 atm (∼1.01 × 10⁵ Pa) using thermogravimetric and electrical conductivity measurements. The results reveal the stability limits of the spinel and rock-salt structure phases with varying cationic composition and oxygen partial pressure at 1200°C. The oxygen partial pressure was varied over 12 orders of magnitude, from log₁₀ a _o2= 0 to log₁₀ a _o2= -12, by using CO/CO₂ and N₂/O₂ gas mixtures and monitored by an electrochemical, stabilized zirconia-based EMF cell. The maximum cobalt mole fraction used in the investigation was about 0.33.     

Deposition mechanism of pyrolytic carbons at temperature between 800–1200 °C

The textures, growth features, microstructures and binding of carbon atoms of pyrolytic carbons prepared by chemical vapor deposition (CVD) at a temperature between 800–1200 °C on graphite substrate and carbon fibers were studied. The intermediate product phase of pyrolytic carbons was also investigated. Based on the present study a deposition model of viscous droplet was proposed in this paper. The viscous droplet here refers to all kinds of fine spheroids that are more or less viscous. The mechanism of the formation of three typical textures namely, smooth laminar, rough laminar and isotropic carbons can be satisfactorily explained by this model.     

Surface Energy Anisotropy of SrTiO3 at 1400°C in Air

Geometric and crystallographic measurements of grain-boundary thermal grooves and surface faceting behavior as a function of orientation have been used to determine the surface energy anisotropy of SrTiO₃ at 1400°C in air. Under these conditions, thermal grooves are formed by surface diffusion. The surface energy anisotropy was determined using the capillarity vector reconstruction method under the assumption that Herring's local equilibrium condition holds at the groove root. The results indicate that the (100) surface has the minimum energy. For surfaces inclined between 0° and 30° from (100), the energy increases with the inclination angle. Orientations inclined by more than 30° from (100) are all about 10% higher in energy and, within experimental uncertainty, energetically equivalent. A procedure for estimating the uncertainties in the reconstructed energies is also introduced. Taken together, the orientation dependence of the surface-facet formation and the measured energy anisotropy lead to the conclusion that the equilibrium crystal shape is dominated by {100}, but also includes {110} and {111} facets. Complex planes within about 15° of {100} and 5° of {110} are also part of the equilibrium shape.     

Air oxidation of Beypazari lignite at 50°C, 100°C and 150°C

Structural changes occurred in Beypazari lignite while oxidizing at 50°C, 100°C and 150°C in a dynamic air atmosphere was detected by infrared spectroscopy. Relative changes in the amount of various oxygenated functional groups with respect to time were determined by curve resolution procedures. Investigation of the spectra of the samples at different oxidation conditions reveal that as the duration and temperature of the oxidation reactions increased the absorption in the aliphatic C–H stretching region decreased, while the absorption in the C=O stretching region presented a significant increase.     

Phase Composition and Compatibilities at 930° to 950°C in the System Europium(III) Oxide-Barium Oxide-Copper(II) Oxide in Air

Phase composition and compatibilites at 930° to 950°C were determined for the system Eu₂O₃–BaO–CuO in air. The binary compound Eu₂CuO₄ dissolves Ba to the extent 0 x 0.02 in Eu_2-xBa_xCuO₄, whereas the other binary compounds, Eu₂BaO₄ and BaCuO₂, do not exhibit solid solubility. Three ternary compounds were obtained, Eu₂BaCuO₅ and two solid solution phases. The first contains the 90 K '123' superconductor and has solubility limits represented by Eu_1+xBa_2-xCu₃O_7±y, where 0 x 0.5. The second has a solubility limit represented as Eu_1+xBa_8-xCu₄O_y, where 0 x 0.44. The limited solid solution range of this phase provides insight concerning the probable solid solution range of the analogous phase in the Y₂O₃-BaO-CuO system.     

Delayed Failure of Hi-Nicalon and Hi-Nicalon S Multifilament Tows and Single Filaments at Intermediate Temperatures (500°–800°C)

Previous results have shown that tows of SiC Nicalon fibers are sensitive to the phenomenon of delayed failure, at temperatures below 700°C. The present paper examines the static fatigue of Hi-Nicalon and Hi-Nicalon S when subjected to constant load, at temperatures between 500° and 800°C in air. Multifilament tows and single filaments were tested. Experimental data show that the rupture times of tows depend on the applied stress according to the conventional power law t σ ⁿ = A . In contrast, the stress-rupture time data obtained on single filaments exhibit significant scatter. A model based on slow crack growth in single filaments shows that the stress-rupture of fiber tows follows the conventional time power law. The dependence on temperature was introduced. The model allowed sound calculations of tow lifetimes and characteristics of the slow crack growth phenomenon to be extracted from the tow stress-rupture time data.     

Mechanism of lithium transport through an MCMB heat-treated at 800-1200 °C

Mechanism of lithium transport through a mesocarbon-microbeads (MCMB) heat-treated at 800-1200 °C was elucidated in 1 M LiPF₆-ethylene carbonate-diethyl carbonate (50:50 vol.%) solution by the quantitative analysis of potentiostatic current transient considering the difference in the relative amount of lithium deintercalation sites having different activation energies for lithium deintercalation. From the coincidence between the current transients experimentally measured and theoretically calculated based upon the modified McNabb-Foster equation along with ‘cell-impedance-controlled’ constraint as the governing equation with the boundary condition, respectively, it is suggested that lithium transport through the MCMB electrode is limited by the ‘cell-impedance’, and at the same time the difference in the kinetics of lithium transport between through the four different lithium deintercalation sites is due to the difference in activation energy for lithium deintercalation between from the four different lithium deintercalation sites present within the MCMB. Moreover, it is realised that since the degree of microcrystallinity of the MCMB is increased with rising heat-treatment temperature, the relative charge amount of lithium deintercalated from the lattice-site is increased, but that amount from the extra-sites is decreased. Thus, the inflexion point, i.e. ‘quasi-current plateau’ in the current transient is less clearly observed with rising heat-treatment temperature.     

Study of hydrated phases present in a MK-lime system cured at 60 °C and 60 months of reaction

This research presents the experimental results of a study carried out to determine the effect of curing temperature on the reaction kinetics in a metakaolin/lime mixture cured at 60 °C and after 60 months of hydration. The stabilities of hydrated phases formed during the pozzolanic reaction in these working conditions were evaluated. The results obtained in current paper showed that metastable hexagonal phases (C₂ASH₈ and probably C₄AH₁₃) coexist with stable cubic phase (hydrogarnet) in the absence of lime. Also, there is evidence of the possible presence of a calcium aluminum silicate hydroxide hydrate (vertumnite).     

Thermal Expansion Characteristics of Alumina–Magnesia and Alumina–Spinel Castables in the Temperature Range 800°–1650°C

The thermal expansion of Al₂O₃–MgO castables containing 5.5 wt% MgO and 1.36 wt% CaO and Al₂O₃–spinel castables containing 20 wt% spinel having 95 wt% Al₂O₃ and 1.7 wt% CaO was measured in the temperature range of 800–1650°C by dilatometry. A sharp increase in expansion from around 1425° to 1525°C, followed by a sharp decrease with further increasing temperature, is characteristic of Al₂O₃–MgO castables. The sharp increase in expansion is believed to be caused by the bond linkage between the CA₆ and spinel grains in the bonding matrix, while the sharp decrease is apparently related to liquid-phase sintering. The sharp increase and decrease in expansion were not observed in Al₂O₃–spinel castables because of the much lower MgO (around 1 wt% MgO) and impurity contents. The magnitude of thermal expansion of calcium aluminate bonded castables containing self-forming or preforming spinels or both is dictated by the MgO content of the castables.     

1000°C Phase Relations and Superconductivity in the (Nd-Ce-Cu)-O Ternary System

The phase relations involving the 24 K n -type Nd_{2- x} Ce _x CuO₄ superconductor were investigated at 1000°C in air. The terminal solid solubility was confirmed to be x = 0.2. This solid solution is the only ternary phase in the Nd₂O₃–CeO₂–CuO diagram. A binary (1 − y )CeO₂– y NdO_1.5 solid solution exists out to y = 0.4. Phase diagrams for NdO_1.5–CeO₂–CuO (1000°C) and NdO_1.5–CeO₂ (900° to 1500°C) are presented.     

The effect of high curing temperature on the reaction kinetics in MK/lime and MK-blended cement matrices at 60 °C

It is well known that the pozzolanic reaction between metakaolin (MK) and calcium hydroxide produces CSH, C₂ASH₈ (stratlingite), C₄AH₁₃ and C₃ASH₆ (hydrogarnet). However, the presence or absence of these hydrated phases depends on different parameters, such as curing temperature, matrix used, etc. This paper shows the results of a study in order to know the effect of high curing temperature (60 °C) on the kinetics of the pozzolanic reaction in different matrices. MK/lime (calcium hydroxide) and MK-blended cement matrices were studied in samples stored and cured at 60 °C and up to 123 days of hydration. The nature, sequence and crystallinity of the hydrated phases were analysed using differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques.Results showed that the sequence and formation of the hydrated phases was different in both matrices cured at 60 °C. In an MK/lime matrix, C₂ASH₈, C₄AH₁₃ and C₃ASH₆ were the main hydrated phases; while in an MK-blended cement, stratlingite was the sole hydrated phase issued from pozzolanic reaction. The DTA and XRD data also reveal an important fact: there is no evidence of the presence of hydrogarnet in blended cements.     

Influence of metastable hydrated phases on the pore size distribution and degree of hydration of MK-blended cements cured at 60 °C

When MK reacts with calcium hydroxide, cementitious products are formed. It has been reported that CSH, C₂ASH₈ and C₄AH₁₃ are the most important hydrated phases formed. These phases are stable at 20 °C. However, some of them (C₂ASH₈ and C₄AH₁₃) are metastable phases, converting to hydrogarnet (C₃ASH₆) for long curing times at elevated temperatures. The partial or total conversion reaction could produce a negative effect on the performance and durability of blended cements, due to a volume decrease associated with the process of transformation.Due to the influence that this conversion could have on the microstructure and durability of a cement paste containing MK, the current paper presents the results of a research programme carried out on blended cements containing 10%, 20% and 25% of MK, cured at 60 °C up to 124 days of hydration.The total, partial porosity and average pore diameter evolution vs. time is determined using mercury intrusion porosimetry (MIP). An estimated degree of hydration of MK-blended cements cured at 60 °C is proposed.The results show that there is no increase in porosity values and average pore diameters with time. Therefore, the hydrated phases produced in MK-blended cements under the test conditions used do not have a negative effect on the microporosity. A suitable correlation between porosity and degree of hydration has been found.     

Activity–Composition Relations in the Systems CaO–MnO and MgO–MnO at at 1500° and 1600°C

Activity–composition relations in CaO–MnO and MgO–MnO solid solutions were determined by equilibrating oxide solid solutions with platinum–manganese alloys under controlled oxygen partial pressures at 1500° and 1600°C. Both systems show considerable positive deviation from ideality.     

Experimental Determination and Thermodynamic Calculation of the Zirconia–Calcia–Magnesia System at 1600°, 1700°, and 1750°C

Phase relations in the ternary system ZrO₂–CaO–MgO were experimentally established at 1600°, 1700°, and 1750°C. The investigation was based on powder X-ray diffractometry, scanning electron microscopy–energy dispersive spectroscopy, and electron probe microanalysis, on 24 ternary compositions. The compositions were prepared using attrition milling of respective oxides and carbonates as raw materials. The results obtained allowed construction of the corresponding isothermal sections, which verified the existence of the cubic-ZrO₂–CaZrO₃ phase compatibility field at the three temperatures. Finally, experimental results also were compared with the thermodynamic assessment previously reported of the system ZrO₂–CaO–MgO.     

Properties of Modified Lead Zirconate Titanate Ceramics Prepared at Low Temperature (800°C) by Hot Isostatic Pressing

High-density lead zirconate titanate (PZT) ceramics were fabricated for the first time at a temperature as low as 800°C via the hot isostatic pressing (HIP) of a PZT powder with a modified composition of 0.92Pb(Zr_0.53Ti_0.47)O₃—0.05BiFeO₃—0.03Ba(Cu_0.5W_0.5)O₃ that contained 0.5 mass% MnO₂. The resultant PZT ceramics exhibited a microstructure that was denser and finer than that of PZT sintered at 935°C, which is the lowest temperature for the densification of the same composition via normal sintering. The relevant dielectric and piezoelectric properties of the HIPed PZT ceramics were as follows: coefficient of electromechanical coupling ( K ₃₁), 31.8%; mechanical quality factor ( Q _m), 1364; piezoelectric constant ( d ₃₁), −73.7 × 10⁻¹² C/N; relative dielectric constant (ɛ₃₃^T/ɛ₀), 633; dielectric loss factor (tan δ), 0.5%; Curie temperature ( T _c), 285°C; and density (ρ), 8.06 g/cm³. In addition to these reasonably good piezoelectric properties, the HIPed PZT exhibited better mechanical properties—particularly, higher fracture strength—than the normally sintered PZT.     

Interaction between BaCO3 and OPC/BFS composite cements at 20 °C and 60 °C

A BaCO₃ slurry, containing radioactive ¹⁴C, is produced during the reprocessing of spent nuclear fuel. This slurry is encapsulated in a Portland-blastfurnace slag composite cement. The effect of BaCO₃ on the hydration of OPC and Portland-blastfurnace slag cements has been studied in this work. Samples containing a simulant BaCO₃ slurry were cured for up to 720 days at 20 and 60 °C and analysed by XRD, SEM(EDX) and ICC. BaCO₃ reacted with OPC to precipitate BaSO₄ from a reaction between soluble sulfate and BaCO₃. Calcium monocarboaluminate subsequently formed from the carbonate released. The monocarboaluminate precipitated as crystals in voids formed during hydration. At 60 °C in OPC, it was not identified by XRD, suggesting the phase is unstable in this system around this temperature. In the Portland-blastfurnace slag cements containing BaCO₃, less monocarboaluminate and BaSO₄ were formed, but the hydration of BFS was promoted and monocarboaluminate was stable up to 60 °C.