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1.
H. OKADA H. KAWAKAMI M. HASHIBA E. MIURA Y. NURISHI T. HIBINO 《Journal of the American Ceramic Society》1985,68(2):58-63
The rate of ZnA12 O4 formation for binary powder mixtures of ZnO and α-Al2 O3 (dense coarse particles and weak agglomerates of fine powder) fired in air or O2 atmospheres was measured and the microstructures of those systems were observed by scanning electron microscopy. With dispersed dense particles of α-Al2 O3 , the Al2 O3 surfaces were covered with ZnO and the spinel grew into the particles maintaining essentially a constant reaction interface area. Calculations based on geometric measurements and use of Jander's equation gave a similar high activation energy, 354 kJ/mol, which corresponds to the activation energy of volume diffusion of Zn2+ in ZnAl2 O4 . An oxygen atmosphere had no effect. With a matrix of fine α-Al2 O3 powder and dispersed granules of ZnO, a higher reaction rate occurred because of an increase in reaction interface area due to penetration of the powder compact matrix by ZnO vapor, which was enhanced by an O2 atmosphere. The reaction layer grew into the alumina matrix adjoining the ZnO granules with a parabolic rate law. Apparent activation energies below ∼200 kJ/mol were calculated. 相似文献
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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 Na2 O-2SiO2 ) +½O2 (g) = Fe2 O3 (in Na2 O-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. 相似文献
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L. C. F. BLACKMAN 《Journal of the American Ceramic Society》1959,42(3):143-145
The formation of ferrous iron in the system MgO-Fe2 O3 MgFe2 O4 is of interest in connection with its deleterious effect on the microwave performance of magnesium ferrite, MgFe2 O4 . The partial reduction of ferric iron in the system at relatively low temperatures is discussed in terms of the preferential diffusion of iron, and its partial stabilization as ferrous iron from lattice energy considerations. 相似文献
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R. Todd Hunter Mark Edge Aristotle Kalivretenos Kathy M. Brewer Nancy A. Brock Alice E. Hawkes James C. Fanning 《Journal of the American Ceramic Society》1989,72(6):943-947
The Fe2+ /Fe3+ ratios of 47 simulated nuclear waste glass samples with ratios varying from 0.01 (oxidized) to 1.6 (reduced) were determined by wet-chemical and Mössbauer spectral analyses. The wet-chemical method involved the spectrophotometric determination of Fe2+ and total iron using remote spectroscopy with fiber optic chemical sensing. Interferences from other species present in these glasses were examined and alternative analytical techniques were investigated. Results of wet-chemical and Mössbauer spectral analysis were comparable; however, the wet-chemical method is probably preferable for the analysis of highly radioactive glasses until such glasses have been shown to have satisfactory Mössbauer spectra. 相似文献
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Perovskites of the system SrFeO3 -SrTiO3 were prepared, and measurements were made of their magnetic and electrical behavior. Chemical analysis showed that the percentage of Fe4+ varied from 72.5% for SrFeO2.86 to about zero for Sr(Fe0.1 Ti0.9 )O2.95 ; the remainder of the iron was in the Fe3+ state and electrical balance was achieved by oxygen loss. Sr(Fe1- x Ti x )O3 was antiferromagnetic between x = 0 and x = 0.9, with a Néel temperature below 60°K. A parasitic ferromagnetic component developed when these compounds were cooled in a magnetic field, the magnitude of this component being dependent on the cooling field. The conductivity of these perovskites ranged from 10−8 ohm−1 cm−1 for x = 1.0 to 10−2 for x = 0.0 and showed a marked change at x = 0.8. The break corresponded to a change in slope of the lattice parameter and the disappearance of Fe4+ . The Fe4+ content depended on the heat treatment and atmosphere during formation. 相似文献
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Admittance spectroscopy was used to characterize the bulk electron traps in polycrystalline ZnO-Bi2 O3 and ZnO-BaO systems. Temperatures from 30 to 350 K and a frequency range from 1.0 to 33.3 kHz were used. Admittance spectra for the ZnO-Bi2 O3 system showed large variations with sintering atmosphere and heat treatment. A trap is observed at 0.33 eV below the conduction band edge in both systems. This trap is likely to be associated with an ionized oxygen vacancy. A possible explanation of primitive varistor characteristics in terms of the roles of the electronic defects is presented. 相似文献
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Electron paramagnetic studies showed that Ti3+ and Fe2+ occur in mullites taken from a refractory material which was fused-cast under a reducing atmosphere. Exposure of the mullite samples to temperatures >1600°C caused oxidation of Ti3+ and Fe2+ to Ti4+ and Fe3+ , respectively. 相似文献
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Influence of Fe3+ /Fe2+ Ratio on the Crystallization of Iron-Rich Glasses Made with Industrial Wastes
Alexander Karamanov Paola Pisciella Carlo Cantalini Mario Pelino 《Journal of the American Ceramic Society》2000,83(12):3153-3157
The influence of the Fe3+ /Fe2+ ratio on the crystallization of iron-rich glasses was investigated in this study. The glass batches were made from two hazardous industrial wastes: mud (goethite and jarosite) originating from the zinc hydrometallurgical process and electric arc furnace dust (EAFD). Glass compositions were prepared by adding different percentages of carbon powder. The crystallization process was investigated by a combined thermogravimetry/differential thermal analysis technique, in air or nitrogen atmospheres, using powder and bulk glass samples. The crystalline phases formed, i.e., pyroxene and spinels, and their relative ratio were determined by X-ray diffractometry. The experimental results indicated that melting temperature and crystallization behavior were influenced by the initial Fe3+ /Fe2+ ratio and by the amount of carbon added to the glass batch. For goethite and jarosite glass compositions, decreasing the Fe3+ /Fe2+ ratio increased the crystallization rate by favoring magnetite formation. For EAFD glass compositions, the addition of carbon to the batch inhibited chromite–magnetite spinel formation and favored the attainment of an amorphous glassy phase. 相似文献
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The densification behavior of Al2 O3 -MgO (0.1 wt%) has been studied in O2 and N2 atmospheres. Powder compacts have been sintered at 1600°C for 0.5 to 8 h. For some specimens the sintering atmosphere has been changed after 30 min of sintering. Irrespective of sintering atmosphere, sintered densities are approximately the same up to 99% relative density, implying that the capillary pressure effect dominates the atmosphere effect for most of the densification stage. During extended sintering treatment the density of specimens sintered in O2 becomes higher than that in N2 . When the sintering atmosphere is changed from O2 to N2 , enhanced densification results, and vice versa. Such an effect of sintering atmosphere is explained by the diffusiveness of gases entrapped in pores, as well as by oxygen potential differences inside and outside of the specimen. Differences in grain growth rate in various atmospheres are discussed on the basis of different densification rates. 相似文献
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A possibility to produce microwave (MW) dielectric materials by liquid-phase sintering of fine particles was investigated. Zn3 Nb2 O8 powders with a grain size 50–300 nm were obtained by the thermal decomposition of freeze-dried Zn–Nb hydroxides or frozen oxalate solutions. The crystallization of Zn3 Nb2 O8 from amorphous decomposition products was often accompanied by the simultaneous formation of ZnNb2 O6 . Maximum sintering activity was observed for single-phase crystalline Zn3 Nb2 O8 powders obtained at the lowest temperature. The sintering of as-obtained powders with CuO–V2 O5 sintering aids results in producing MW dielectric ceramics with a density 93%–97% of the theoretical, and a Q × f product up to 36 000 GHz at sintering temperature ( T s )≥680°C. The high level of MW dielectric properties of ceramics was ensured by intensive grain growth during the densification and the thermal processing of ceramics. 相似文献
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Equilibrium relations in isothermal sections through the system MgO-FeO-Fe2 O3 have been determined in the temperature range 1400° to 1800°C. Oxide mixtures prepared from reagent-grade chemicals were equilibrated and subsequently quenched to room temperature while sealed in 80Pt20Rh tubes. The data obtained for the isothermal sections are used to infer equilibrium relations existing along the liquidus surface of the system. 相似文献
16.
Marta Prades Nahum Masó Héctor Beltrán Eloisa Cordoncillo Anthony R. West 《Journal of the American Ceramic Society》2008,91(7):2364-2366
The effect of Mn doping on the cubic to hexagonal phase transition temperature in BaTiO3 has been determined by quenching samples with different Mn contents from a range of temperatures. Under conditions of equilibrating samples in air over the range 1000°–1400°C, cubic solid solutions BaTi1− x Mn x O3−δ form over the range 0≤ x ≤0.015(5), whereas hexagonal solid solutions form for x ≥0.02, depending on the temperature. The results are compared with those on doping BaTiO3 with Fe3+ and observations made concerning acceptor doping with Ti3+ . 相似文献
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JEONG-JOO KIM BYUNG-KI KIM BYUNG-MOO SONG DOH-YEON KIM DUK N. YOON 《Journal of the American Ceramic Society》1987,70(10):734-737
The shrinkage behavior of isolated pores during the liquidphase sintering of MgO-CaMgSiO4 at 1650°C in O2 and N2 atmospheres has been studied. When 90MgO- 10Ca MgSiO4 specimens containing artificially produced large spherical pores are sintered in O2 , the liquid and grains flow into the pores as oxygen diffuses out. When sintered in N2 the pores remain intact even after a long time, because the N2 gas entrapped in them does not diffuse out. The effect of the sintering atmosphere has also been studied in a fine powder mixture of 80MgO 20CaMgSiO4 composition. Changing the atmosphere from O2 to N2 during the sintering treatment reduces the porosity, probably because of the enhanced oxygen diffusion from the pores. The pores grow when the sintering atmosphere is changed from N2 to O2 , probably because of oxygen diffusion into the pores from the specimen surface. The practical implication of these results is that changing the atmosphere from O2 to air during the liquid-phase sintering of oxide ceramics can greatly reduce the porosity. 相似文献
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M. L. KEITH 《Journal of the American Ceramic Society》1954,37(10):490-496
Studies of phase equilibria by the method of quenching show that there are no ternary compounds at liquidus temperatures in the system MgO-Cr2 O3 -SiO2 . There is an extensive two-liquid region, and the stability fields of periclase, picrochromite, and chromic oxide extend across most of the diagram. Temperatures of liquid formation are sufficiently high that a number of different compositions can be suggested for refractory purposes. Fundamental explanations are brought out for the improvement of chromite and chromite-forsterite refractories by the addition of MgO and for the improvement of chromite-silica refractories by the addition of chromic oxide or some other high-chromium compound. 相似文献
20.
Liqiong An Jian Zhang Min Liu Shiwei Wang 《Journal of the American Ceramic Society》2005,88(4):1010-1012
Yb3+ , Ho3+ : Lu2 O3 nanocrystalline powders were synthesized by a reverse-strike co-precipitation method using NH4 HCO3 and NH3 ·H2 O as precipitators. X-ray diffraction analysis and field emission scanning electron microscopy observation showed that the phase composition of the powders was cubic and the particle size was 30–50 nm. Under the excitation of a 980 nm continuous wave diode laser, green and red emissions centered around 548 and 667 nm, respectively, were observed and the green emission dominated the upconversion spectrum. Power studies revealed that a two-photon process was involved in the upconversion emissions and the possible upconversion mechanisms were discussed. 相似文献