共查询到20条相似文献,搜索用时 15 毫秒
1.
OSAMA H. EL-BAYOUMI CATHERINE J. SIMMONS EDWARD M. CLAUSEN JR. MICHAEL J. SUSCAVAGE JOSEPH H. SIMMONS 《Journal of the American Ceramic Society》1987,70(7):510-513
The techniques of scanning electron microscopy, energy dispersive X-ray microanalysis, and dark-field transmission electron microscopy were combined to study the phase stability of CdF2 -LiF-AlF3 -PbF2 glasses. The selected compositions, presumed to be exceptionally stable, exhibited a liquid-liquid immiscibility phase transition; this led to the formation of two amorphous phases consisting of a matrix near the original glass composition and an isolated spherical phase which was richer in CdF2 and poorer in PbF2 than the matrix. Subsequent secondary-phase separation and crystallization were observed following heat treatments near the glass transition temperature. 相似文献
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MICHAEL S. RANDALL JOSEPH H. SIMMONS OSAMA H. EL-BAYOUMI 《Journal of the American Ceramic Society》1988,71(12):1134-1141
Occurrence of primary and secondary phase separation in the CdF2 -LiF-AlF3 -PbF2 glass system is observed. A primary phase separation, which is characterized by the formation of a small volume fraction of Cd-rich, Pb-deficient spheres whose size is on the order of 5μ is found to form upon cooling. A secondary phase separation initiates with precipitation of Pb-rich microspheres (about 4nm (40 Å) in diameter) in a Cd-rich matrix. The Pb-rich spheres develop with heat treatment 35°C above the glass transition temperature. They increase in volume fraction while changing composition and then grow slightly. As the material is further heat-treated, the Pb-rich regions crystallize. A pseudobinary model explaining the occurrence of both primary and secondary decomposition phenomena is presented. 相似文献
4.
The solid phases formed at 1400°C. in air in the three-component systems BaO-SrO-TiO2 , BaO-CaO-TiO2 , and SrO-CaO-TiO2 are described. Besides solid solutions of components with known structures, some new ternary compounds have been studied. The dielectric constants and loss factors of a number of specimens are given. Crystallographic data of the compounds BaCaTiO4 , Ba3 Ca2 Ti2 O9 , and Ca3 Ti2 O7 and of the solid solution series (Ba, Sr), TiO4 are presented. The preparation of the new compounds is described in detail. 相似文献
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In earlier work, a prediction method of the immiscibility boundary of a ternary silicate glass system was developed involving two known binary immiscibility boundaries and a measured immiscibility temperature of one ternary glass composition. In the present work, the method is extended to the case where one of the two binary immiscibility boundaries is not known and is applied as an example to ternary silicate systems containing K2 O. First, the immiscibility boundary of the system K2 O-SiO2 is estimated by measuring the immiscibility temperatures of three glasses in the system K2 O-Li2 O (or Na2 O)-SiO2 . Using this result the immiscibility boundaries of the systems K2 O-Li2 O-SiO2 , K2 O-Na2 O-SiO2 , and K2 O-BaO-SiO2 are estimated. The results agree reasonably well with the experimentally determined immiscibility temperatures at selected compositions. 相似文献
6.
OLAF MULLER RUSTUM ROY WILLIAM B. WHITE 《Journal of the American Ceramic Society》1968,51(12):693-699
Phase equilibria were determined for the systems NiO-Cr2 O3 −O2 , MgO-Cr2 O3 ,-O2 , and CdO-Cr2 O3 −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 NiCr2 O4 . 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 MgCr2 O4 and CdCr2 O4 . The cadmium system contains an additional phase, Cd2 CrO5 , which is primitive monoclinic. 相似文献
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Equilibrium phase diagrams for the systems MgCl2 -MgF2 , CaCl2 -MgF2 and NaCl-MgF2 were determined by differential thermal analysis, thermal analysis, and temperature-composition equilibrium techniques. Simple eutectics were observed at 78.0±0.5 mol% MgCl2 and 628°±2°C in the MgCl2 -MgF2 system, at 87.5±0.5 mol% CaCl2 and 694°±2°C in the CaCl2 -MgF2 system, and at 95.5±0.5 mol% NaCl and 786°±3°C in the NaCl-MgF2 system. The phase diagrams determined for these systems were compared with phase diagrams that were computed using Temkin's model. The phase diagrams of the CaCl2 -MgF2 and NaCl-MgF2 systems were also compared with diagrams that were computed using the expression suggested by Flood et al. for reciprocal systems. The experimentally determined and computed phase diagrams agreed for the MgCl2 -MgF2 system but not for the CaCl2 -MgF2 and NaCl-MgF2 systems. 相似文献
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Studies made on low-hafnium-content ZrO2 , show that the monoclinic-tetragonal inversion temperature is 1170°C., and it is raised to approximately 1190°C. in the "natural" ZrO2 , which contains approximately 2% HfO2 . No explanation could be found for the knownmarked hysteresis during cooling, when the reverse polymorphic transformation takes dace at 1040°C. In the system ZrO2 -ThO2 the monoclinic-tetragonal ZrO2 , inversion temperature is lowered to 1000°C., although the maximum solid solution extent of ZrO2 , in Thon and vice versa is approximately only 2% at this temperature. Below about 400°C. under hydrothermal conditions it was possible to prepare a continuous, although metastable series of solid solutions with the fluorite structurewith compositions varying from ThO2 , to nearly pure ZrO2 . Contrary to earlier work only 8 mole ZrO2 , dissolves in UO2 and less than 4 mole of UO, in ZrO2 at temperatures up to 13OO0C. A continuous series of solid solutions could be made between Th2 and UO2 at 13OO°C., and extensive defect fluorite solid solutions could be prepared between Tho2 and U3 O8 ; there is some evidence for exsolution into uranium-rich and thorium-rich members at low temperatures. 相似文献
9.
Equilibrium partial pressures of SiF4 were measured for the reactions 2SiO2 ( c )+2BeF2 ( d )⇋SiF4 ( g )+Be2 SiO4 ( c ) (log P siF4 (mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be2 SiO4 ( c ) +2BeF2 ( d )⇋SiF4 ( g ) +4BeO( c )(log P siF4 (mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF2 was present in solution with LiF as molten Li2 BeF4 . The solubility of SiF4 was low (∼0.04 mol kg-1 atm-1 ) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ Hf and Δ Gf values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO2 and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF4 generated showed an increasing positive deviation from the expression given for the equilibrium involving Be2 SiO4 and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO2 . 相似文献
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Suguru Suzuki Minoru Takahashi Yasio Hikich 《Journal of the American Ceramic Society》1987,70(9):213-C
The free-volume fraction (Vf ) defined by Simha and Boyer was measured for network-forming oxide glasses in the systems P2 O5 -(GeO2 , TeO2 ,Sb2 O3 .V2 O5 ). The Vf values varied from 0.06 to 0.25. The systems P2 O5 -TeO2 : and P2 O5 -Sb2 O3 have Vf ∼0.1, which is near the magnitude of the free-volume fraction for normal metaphosphate glasses and many organic high polymers. 相似文献
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TiO2 above 4 mol% is an effective nucleating agent for CaO–P2 O5 glass which also contains substantial SiO2 and Al2 O3 additions. Glass ceramics can be made from this glass using a single slow heating ramp with no need for a nucleating heat treatment step. Powder of this composition crystallizes rapidly to β-Ca2 P2 O7 , whereas bulk glass crystallizes from diphasic nuclei consisting of a central cubic Ca-P-Ti-Si-Al oxide phase surrounded by impure AlPO4 dendrites. Metastable calcium phosphate grows on the AlPO4 dendrites and later transforms to β-Ca2 P2 O7 . 相似文献
13.
The mechanism of parahydrogen conversion was studied on Gd2 O3 and Y2 O3 powders and on Gd and Y evaporated metal films at low and high temperatures (77° to 90°K and 298° to 418°K). Absolute rates of conversion are compared to theoretical values for 3 possible reaction mechanisms, and it is concluded that a paramagnetic vibrational mechanism is operative on Gd2 O3 , Gd, and Y. On Y2 O3 the reaction rate is enhanced by additional surface paramagnetic sites. The portion of the surface which is active is ∼1 for the metals and ∼0.01 for the oxides. 相似文献
14.
SUGURU SUZUKI MINORU TAKAHASHI YASUO HIKICHI HIROYASU SAKAMURA 《Journal of the American Ceramic Society》1987,70(4):254-256
The internal friction of only network-forming oxide glasses containing a P2 O5 component, i.e., in the systems P2 O5 -GeO2 , P2 O5 -TeO2 , P2 O5 -Sb2 O3 , and P2 O5 -V2 O5 , was measured as a function of temperature by a free torsional vibration method. P2 O5 -TeO2 and P2 O5 -Sb2 O3 glasses exhibited clearly high-temperature peaks in a plot of internal friction vs temperature in spite of the absence of nonbridging oxygens and network modifiers. Therefore, we conclude that the high-temperature peaks appeared when strong and weak parts coexisted in the network structure. 相似文献
15.
Stoichiometric NiAl2 O4 , CuAl2 O4 , and ZnAl2 O4 spinels were prepared and equilibrated at temperatures from 600° to 1400°C. The parameters u and x , denoting the oxygen position and fraction of divalent cations on tetrahedral sites, respectively, were determined from a detailed X-ray diffraction analysis. In NiAl2 O4 , x increased from 0.07 at 595° to 0.26 at 1391°C; in CuAl2 O4 , x decreased from 0.68 at 613° to 0.64 at 1195°C; and in ZnAl2 O4 , x decreased from 0.96 at 905° to 0.94 at 1197°C. The form of the temperature dependence of x could not be described using theoretically based equations advanced in the literature. A more general equation which allows for a non-distributional contribution to the configurational entropy was derived and observed to properly describe the temperature dependence; the results indicate that short-range order is of definite significance in these intermediate aluminate spinels. 相似文献
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Single crystals of phenacite (Be2 SiO4 ), bromellite (BeO), and tridymite (SiO2 ) were grown from an Li2 MoO4 -MoO3 flux. Phenacite, with rhombohedral symmetry, grew in three distinct shapes with aspect ratios (length/width) as follows: needles (>3), rods (>1.1 to 1.5), and rhombohedral-faced crystals (=1). The latter grew as single crystals; the others were twinned on the . For most experiments the temperature was held constant at 1165°C and the Li2 MoO4 /MoO3 ratio at 1/16. The growth mechanism for crystallization was the evaporation of MoO3 . The system produced one to three phases, depending on the BeO/SiO2 ratio. Bromellite grew until a BeO/SiO2 ratio of 0.8 was attained. It grew as a hemipyramidal crystal having a short prism with a curved top or as a hexagonal plate. The pyramid- and prism-shaped crystals were twinned, although a few hexagonal plates were single. Tridymite grew in small hexagonal plates when the BeO/SiO2 ratio was less than 1.5. The effect of temperature, nucleation, and flux composition on crystal shape, twinning, and occurrence is discussed. 相似文献
17.
Effect of Ta2 O5 , Nb2 O5 , and HfO2 Alloying on the Transformability of Y2 O3 -Stabilized Tetragonal ZrO2
Dae-Joon Kim 《Journal of the American Ceramic Society》1990,73(1):115-120
The addition of Ta2 O5 , Nb2 O5 , and HfO2 enhanced the transformability of Y2 O3 -stabilized tetragonal ZrO2 polycrystal (Y-TZP), which was indicated by an increase in phase transformation temperatures and fracture toughness of Y-TZP. Comparison of the alloying effects of these oxides on the transformability and crystal structure of Y-TZP suggested that an alloying oxide which increases the c/a axial ratio (tetragonality) of TZP also increases the transformability. Empirical equations to predict the tetragonality are proposed. Calculated tetragonalities showed good agreement with measured values in the systems ZrO2 -Y2 O3 -Ta2 O5 , -Nb2 O5 , and -HfO2 . 相似文献
18.
W. ZDANIEWSKI 《Journal of the American Ceramic Society》1975,58(5-6):163-169
Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al2 O3 -SiO2 system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO2 or TiO2 . Up to 10 wt% CeO2 was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce2 Ti2 O4 (Si2 O7 ). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO2 , but no crystalline TiO2 was detected. In the presence of CeO2 , TiO2 was a more effective catalyst than ZrO2 . Although CeO2 lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization. 相似文献
19.
Shyan-Lung Chung Yu-Cheng Sheu Ming-Shyong Tsai 《Journal of the American Ceramic Society》1992,75(1):117-123
SiO2 , Al2 O3 , and 3Al2 O3 .2SiO2 powders were synthesized by combustion of SiCl4 or/and AlCl3 using a counterflow diffusion flame. The SiO2 and Al2 O3 powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al2 O3 .2SiO2 powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame. 相似文献