Long-Lasting Phosphorescence of Transparent Surface-Crystallized Glass-Ceramics

Transparent surface-crystallized glass-ceramics with molar compositions of 43SiO₂–2B₂O₃–30SrO–25MgO–0.05Eu₂O₃–0.8Dy₂O₃ (sample GC-A) and 43SiO₂–2B₂O₃–24SrO–6CaO–25MgO–0.05Eu₂O₃–0.8Dy₂O₃ (sample GC-B) were prepared by heat-treating the mother glasses at 850°C for 10 h. The precipitated phases in both glass-ceramics were (Sr_{1− x} Ca _x )₂MgSi₂O₇, in which Eu²⁺ and Dy³⁺ ions were incorporated. Phosphorescence with emission peaks at 470 and 480 nm that is due to the 4 f ⁶5 d –4 f ⁷ transition of Eu²⁺ ions was observed from samples GC-A and GC-B, respectively. The phosphorescence lasted for >5 h at room temperature.     

Stability of CaNiSi2O6 ("Niopside") and Activity–Composition Relations of CaMgSi2O6– CaNiSi2O6 Solid Solutions at 1350°C

A complete solid-solution series exists between diopside (CaMgSi₂O₆) and its nickel analogue, "niopside"(CaNiSi₂O₆). Activity–composition relations within this solid solution, and the stability of the end member CaNiSi₂O₆, have been determined by equilibrating CaNiSi₂O₆ with SiO₂, CaSiO₃, and metallic Ni in atmospheres of known oxygen pressures. Within limits of accuracy of the experiments, the solution is ideal at 1350°C. From the experimental data obtained in the present investigation, the standard free energy (Δ G °) of formation of CaNiSi₂O₆ according to the equation CaO + NiO + 2SiO₂= CaNiSi₂O₆ is calculated to be Δ G °=−165862 + 42.40 T J. Experiments in the system CaO–NiO–SiO₂ have shown that the nickel analogue of the phase pseudo-enstatite (MgSiO₃) is unstable with respect to SiO₂ and nickel olivine (Ni₂SiO₄), and the nickel analogues of the phases akermanite (Ca₂MgSi₂O₇) and monticellite (CaMgSiO₄) are unstable relative to the phase assemblage pseudo-wollastonite (CaSiO₃) plus NiO. In the system CaO–MgO–NiO–SiO₂, however, substitution of Ni for Mg in these phases was observed. The percentage substitution of Ni for Mg in the phases is given in parentheses: diopside (100%), olivine (100%), enstatite (18%), akermanite (20%), and monticellite (57%).     

Synthesis of V2O3 Powder by Evaporative Decomposition of Solutions and H2 Reduction

The evaporative decomposition of solutions method was used to form V₂O₅. Spraying above the congruent melting temperature of V₂O₅ (690°C) resulted in dense spherical particles with a smooth surface. Spraying below the V₂O₅ melting temperature yielded porous V₂O₅ powder with a rough surface. Reduction of the V₂O₅ to V₂O₃ was done in a H₂ atmosphere. Spherical V₂O₃ powder was attained when the reduction temperature was low enough to reduce the V₂O₅ surface before partial sintering (necking) between V₂O₅ particles occurred. The resulting V₂O₃ particle size was smaller than the precursor V₂O₅ powder as expected by the differences in densities between V₂O₅ ( p = 3.36 g/cm³) and V₂O₃ ( p = 4.87 g/cm³).     

Pore Structure Evolution of Lanthana–Alumina Systems Prepared through Coprecipitation

Pure Al₂O₃ and different compositions of La₂O₃–Al₂O₃ samples have been prepared through coprecipitation. Even after heating at 1300°C, the compositions La₂O₃·11Al₂O₃ and La₂O₃·13Al₂O₃ had higher surface area compared to the pure Al₂O₃ and the La₂O₃·Al₂O₃ composition. Ethanol washing is an effective way for improving the textural stability of pure Al₂O₃ and La₂O₃–Al₂O₃ samples. The effect of steam on the thermal stability of La₂O₃·11Al₂O₃ has also been studied. La₂O₃·11Al₂O₃ sample is found to be stable in steam.     

Effect of Yttria on Interfacial Reactions Between Titanium Melt and Hot-Pressed Yttria/Zirconia Composites at 1700°C

Various Y₂O₃/ZrO₂ samples were fabricated by hot pressing, whereby Y₂O₃ was mutually dissolved or reacted with ZrO₂ as a solid solution or Zr₃Y₄O₁₂. Hot-pressed samples were allowed to react with Ti melt at 1700°C for 10 min in argon. Microstructural characterization was conducted using X-ray diffraction and analytical electron microscopy. The Y₂O₃/ZrO₂ samples became more stable with increasing Y₂O₃ because Y₂O₃ was hardly reacted and dissolved with Ti melt. The incorporation of more than 30 vol% Y₂O₃ could effectively suppress the reactions in the Ti side, where only a very small amount of α-Ti and β'-Ti was found. When ZrO₂ was dissolved into Ti on the zirconia side near the original interfaces, Y₂O₃ reprecipitated in the samples containing 30%–70 vol% Y₂O₃, because the solubility of Y₂O₃ in Ti was very low. In the region far from the original interface, α-Zr, Y₂O₃, and/or residual Zr₃Y₄O₁₂ were found in the samples containing more than 50 vol% Y₂O₃ and the amount of α-Zr decreased with increasing Y₂O₃.     

Phase Equilibria in the Pseudoternary System ZrO2−Y2O3−Cr2O3 at 1600°C in Air

The pseudoternary system ZrO₂-Y₂O₃-Cr₂O₃ was studied at 1600°C in air by the quenching method. Only one intermediate compound, YCrO₃, was observed on the Y₂O₃−Cr₂O₃ join. ZrO₂ and Y₂O₃ formed solid solutions with solubility limits of 47 and 38 mol%, respectively. The apex of the compatibility triangle for the cubic ZrO₂, Cr₂O₃, and YCrO₃ three-phase region was located at =17 mol% Y₂O₃ (83 mol% ZrO₂). Below 17 mol% Y₂O₃, ZrO₂ solid solution coexisted with Cr₂O₃. Cr₂O₃ appears to be slightly soluble in ZrO₂(ss).     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Effect of Alumina Addition on the Tetragonal-to-Monoclinic Phase Transformation in Zirconia–3 mol% Yttria

The tetragonal-to-monoclinic martensitic phase transformation in ZrO–3 mol% Y₂O₃ (PSZ) containing 0 to 12 wt% Al₂O₃ was investigated by dilatometry, XRD, and SEM-EDS methods. The propagation of the transformation into the specimen interiors was suppressed by the addition of Al₂O₃. The grain size was independent of the addition of Al₂O₃. Both Y₂O₃ and Al₂O₃ segregated at grain boundaries. From this segregation behavior, it was suggested that a certain compound or phase of Y₂O₃–Al₂O₃ could be formed at grain boundaries, which would presumably prevent the propagation of the transformation into interiors of PSZ-containing Al₂O₃.     

Electrical Resistance of Some Refractory Oxides and Their Mixtures in the Temperature Range 600° to 1500°C.

Measurements of electrical resistance in the composition systems Al₂O₃–SiO₂, SiO₂–TiO₂, Al₂O₃–Cr₂O₃, and MgO–NiO were made using, in general, dry-pressed disks about 3 cm. in diameter and 0.4 cm. thick and fired to 1500°C. In the Al₂O₃–SiO₂ series minimum resistance was shown by the samples containing 50% SiO₂, 50% Al₂O₃. The resistance of Al₂O₃ was increased by the addition of small amounts of Cr₂O₃. The same effect was observed in the higher temperature range with small additions of NiO to MgO. In other instances the addition of the relatively inert SiO₂, Al₂O₃, and MgO to the semiconductors TiO₂, Cr₂O₃, and NiO resulted in a dilution effect. The resistance of Cr₂O₃ was decreased by the addition of a slight amount of MgO.     

Effect of Additives on the Electromechanical Properties of Pb(Zr,Ti)O3–Pb(Y2/3W1/3)O3 Ceramics

Dielectric and piezoelectric properties of 0.02Pb(Y_2/3W_1/3)O₃ 0.98Pb(Zr0.52Ti0.48)O3 ceramics doped with additives (Nb₂O₅, La₂O₃, MnO₂, and Fe₂O₃) were investigated. The grain sizes of these ceramics decreased with increasing amounts of additives. For additions of MnO₂ and Fe₂O₃, dielectric losses decreased, while for Nb₂O₅ and La₂O₃, these values increased. The maximum values of the mechanical quality factor Q_m were found to be 956 and 975 for additions of 0.9 wt% Fe₂O₃ and 0.7 wt% MnO₂, respectively, but donor dopants (Nb₂O₅ and La₂O₃) did not change the values of Q_m . On the other hand, the piezoelectric constant d₃₃ and the electromechanical coupling factor k_p decreased with additions of MnO₂ and Fe₂O₃, but improved with additions of Nb₂O₅ and La₂O₃.     

Reactive Hot-Pressed Alumina–Boron Nitride Composites with Y2O3 Sintering Additive

The effect of Y₂O₃ addition (0–5 wt%) on the densification and properties of reactive hot-pressed alumina (Al₂O₃)–boron nitride composites based on the reaction between aluminum borate (2Al₂O₃·B₂O₃) and aluminum nitride (AlN) was investigated. The densification process was very sensitive to the amount of Y₂O₃. Compared with a low relative density of 79.3 theoretical density (TD)% for material with no Y₂O₃ addition, the material density reached 98.6 TD% with 0.25% Y₂O₃ addition. High Y₂O₃ additions resulted in the formation of a new phase Al₅Y₃O₁₂. The grain growth of the Al₂O₃ matrix was promoted by the Y₂O₃ addition. Owing to the high density and the small Al₂O₃ particle size the sample with 0.25% Y₂O₃ addition demonstrated the highest bending strength of 540 MPa.     

Influence of the Y2O3 Content and Temperature on the Mechanical Properties of Melt-Grown Al2O3–ZrO2 Eutectics

The effect of Y₂O₃ content on the flexure strength of melt-grown Al₂O₃–ZrO₂ eutectics was studied in a temperature range of 25°–1427°C. The processing conditions were carefully controlled to obtain a constant microstructure independent of Y₂O₃ content. The rod microstructure was made up of alternating bands of fine and coarse dispersions of irregular ZrO₂ platelets oriented along the growth axis and embedded in the continuous Al₂O₃ matrix. The highest flexure strength at ambient temperature was found in the material with 3 mol% Y₂O₃ in relation to ZrO₂(Y₂O₃). Higher Y₂O₃ content did not substantially modify the mechanical response; however, materials with 0.5 mol% presented a significant degradation in the flexure strength because of the presence of large defects. They were nucleated at the Al₂O₃–ZrO₂ interface during the martensitic transformation of ZrO₂ on cooling and propagated into the Al₂O₃ matrix driven by the tensile residual stresses generated by the transformation. The material with 3 mol% Y₂O₃ retained 80% of the flexure strength at 1427°C, whereas the mechanical properties of the eutectic with 0.5 mol% Y₂O₃ dropped rapidly with temperature as a result of extensive microcracking.     

Effect of Heat-Treatment on the Constitution and Mechanical Properties of Some Hydrated Aluminous Cements

The compound compositions of four aluminous cements were determined on anhydrous as well as hydrated specimens which had been heat-treated at temperatures between room temperature and 1400° C. Phases were identified by X-ray diffraction and differential thermal analysis. Specimens were also tested for transverse strength, dynamic modulus of elasticity, and thermal length change. A study of the dehydration characteristics of CaO - Al₂O₈ - 10H₂O₃ 3CaO.Al₂O₃. 6H₂O, and Al₂O₃. 3H₂O was included. The data indicated that CaO. Al₂O₃ 10H₂O was the primary crystalline hydrate formed in the cements at room temperature. At 50° C., 3 CaO Al₂O₃-6H₂O and Al₂O₃. 3H₂O were formed as by-products of the dehydration of CaO.Al₂O₃.10H₂O. When heated alone in an open system, CaO.Al₂O₃.10H₂O did not convert to 3CaO. Al₂O₃. 6H₂O and A1₂O₃. 3H₂O. A correlation between the mechanical properties and compound compositions was noted.     

The System CaO-Ga2O3

CaO and Ga₂O₃ form three compounds: 3CaO-Ga₂O₃, CaO-Ga₂O₃, and CaO-2Ga₂O₃. 3CaO-Ga₂O₃ melts incongruently to CaO plus liquid at 1263°C.; CaO Ga₂O₃ and CaO 2Ga₂O₃ melt congruently at 1369° and 1504°C. respectively. Eutectics are located at the following temperatures and compositions (in mole% Ga₂O₃): between 3CaO Ga₂O₃ and CaO Ga₂O₃, 1245°C. and 37.5%; between CaO Ga₂O₃ and CaO-2Ga₂O₃,1323^oC. and 57.0%; and between CaO -2Ga₂O₃ and β-Ga₂O₃,1457°C. and 68.0%. There is a peritectic at 1263°C. and 36.0%. Three polymorphs of CaO Ga₂O₃ are described. Compositions from approximately 35 to 70 mole% Ga₂O₃ can be quenched to yield homogeneous glasses.     

Synthesis of AlN Powder by Carbothermal Reduction-Nitridation of Various Al2O3 Powders with CaF2

We investigated the effect of characteristics of raw Al₂O₃ powder on the synthesis of AlN by the carbothermal reduction-nitridation method, in which CaF₂ was added as a catalytic material. Four types of Al₂O₃ powders were selected. An Al₂O₃/C molar ratio of 0.29 was fixed, and the amount of CaF₂ was varied from 3 to 30 wt%. The carbothermal reduction-nitridation was conducted from 1350° to 1450°C in N₂ flow. The nitridation rate tended to increase with decreasing particle size of raw Al₂O₃ and was found to depend on the Al₂O₃ synthesizing method. The particle sizes of the synthesized AlN increased somewhat with increasing reaction temperatures. However, even though different particle sizes of Al₂O₃ powders were used, AlN powders synthesized under the same conditions exhibited almost the same particle size, round shape, and narrow size distribution. From XRD analysis, CaO·6Al₂O₃ and CaO·2Al₂O₃ were identified as intermediate compounds during these reactions. The above phenomena suggest that the synthesis mechanism of AlN powder by carbothermal reduction-nitridation of Al₂O₃ with CaF₂ addition was the nitridation of the intermediate compounds through the liquid phase of the system CaF₂-CaO·6Al₂O₃-CaO·2Al₂O₃.     

Reactions and Microstructure Development in Mullite Fibers

Microstructural and compositional changes during heat treatment of sol–gel-derived mullite fibers with additions of 2 wt% B₂O₃, 2 wt% P₂O₅, 2 wt% Cr₂O₃, and (1 wt% P₂O₅+ 1 wt% Cr₂O₃) were compared with those of undoped mullite fibers. For all compositions the sequence of phase development was the crystallization of a spinel phase (†-Al₂O₃ or Al–Si spinel) from amorphous material, followed by the formation of mullite at higher temperatures. Differential thermal analysis showed that additions of B₂O₃ and P₂O₅ increased the temperature of spinel formation and that B₂O₃ significantly decreased the temperature of mullite formation. After 1 h at 1200°C, the size of mullite grains in fibers that contained B₂O₃ was less than 1000 Å the grains in fibers of other compositions were 6000 to 12000 Å. After 60 h at 1400°C, fibers modified with B₂O₃ had a grain size less than 2000 to 3000 Å the grains in fibers of other compositions were 6000 to 12000 Å. B₂O₃ was the most volatile additive.     

Thermal Expansion of Substituted Copper Aluminosilicate Glasses

The effects of various components on the thermal expansion coefficient of low-expansion Cu₂O-Al₂O₃-SiO₂ glasses were examined. When a component of glass was substituted by another oxide, the expansion coefficient always increased, except for substitution of Al₂O₃ by B₂O₃. This result indicates that the essential components to maintain the low expansivity are Cu₂O, such trivalent oxides as Al₂O₃ and B₂O₃, and tetravalent oxide SiO₂. Glasses of the systems Cu₂O-Al₂O₃-B₂O₃, Cu₂O-Al₂O₃-GeO₂, and Cu₂O-Al₂O₃-P₂O₅, which were derived by replacing SiO₂ by other network-forming oxides, showed fairly low expansion coefficients compared with other conventional borate, germanate, and phosphate glasses. It was also found that the valence state of copper ions is important for the thermal expansion characteristics of these glasses; Cu⁺ ions contribute to the low expansion coefficient.     

Raman Spectroscopy of Tetragonal Zirconia Solid Solutions

Raman spectra of tetragonal zirconia were studied for the systems ZrO₂-Y₂O₃, ZrO₂-CeO₂, ZrO₂-Y₂O₃-Nb₂O₅, and ZrO₂-Y₂O₃-Ta₂O₅ There was no change in the Raman bands for tetragonal ziraconia in the ZrO₂-Y₂O₃ system as a function of Y₂O₃ content . On the other hand, in the ternary systems, the Raman lines corresponding to the stretching modes of two sets of cation-oxygen bonds shifted to greater wave numbers as either Nb₂O₅ Contents increased in Y₂O₃-stabilized tetragonal zirconia. This difference was interpreted by local bonding environments of the pentavalent cations, which were supposed to occupy tetrahedral sites in teteragonal zirconia solid solutions.     

Energetics of Defect Fluorite and Pyrochlore Phases in Lanthanum and Gadolinium Hafnates

Aerodynamic levitation combined with laser heating was used to prepare melts in the HfO₂–La₂O₃ (–Gd₂O₃) systems. All melts crystallized upon quenching in oxygen. Hf₂La₂O₇ pyrochlore and Gd_0.5Hf_0.5O_1.75 fluorite phases were identified. Gd_0.5Hf_0.5O_1.75 fluorite was transformed into the pyrochlore structure by annealing at 1450°C. Pyrochlore that crystallized from HfO₂- La₂O₃ melts contained 31.6–34.2 mol% La₂O₃. The unit cell parameter increased linearly with La content from 10.736 to 10.789 Å. Drop solution calorimetric experiments were performed in 3Na₂O·4MoO₃ melt at 702°C. The enthalpies of formation from the oxides for pyrochlore phases are −107.0±5.0 kJ/mol for Hf₂La₂O₇ and −48.8±4.7 kJ/mol for Hf₂Gd₂O₇. The enthalpy of the pyrochlore–fluorite phase transition in Hf₂Gd₂O₇ is 23.6±3.1 kJ/mol.     

Investigation of Grain-Boundary Segregation in Ceramic Oxides by Analytical Scanning Transmission Electron Microscopy

Analytical scanning transmission electron microscopy (AEM) was used to study grain-boundary solute segregation in the systems MgO-NiO, A1₂O₃-Y, O₃, NiO-Cr₂O₃, and NiO-Al₂O₃. Electron beam spreading within the specimen was incorporated into a model to quantitatively measure solute segregation. Grain-boundary segregation occurred in A1₂O₃-Y₂O₃ and NiO-Cr₂O₃ but was not detected in MgO-NiO and NiO-Al₂O₃ specimens. These results and the quantitative measurements agree with equilibrium solute segregation theories. Microhard-ness measurements indicate no difference in hardness between the grain boundary and the matrix for Cr-doped NiO, a system in which segregation was detected.