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1.
Sintering of magnesium aluminate spinel of the MgO-excess, stoichiometric, and Al2 O3 -excess compositions has been investigated under vacuum and in air for the effect of low oxygen partial pressures. Densification enhancement of the surface layer is due to MgO evaporation which generates oxygen vacancies in the host crystal lattice. Regions of different grain sizes are observed from samples sintered under both conditions. Microstructural features of pairwise breakup of particle chains representing differential sintering are characteristic of the less-densified sample interior. The densification improved initially and yet was retarded in the intermediate sintering stage when the density exceeded 75% with vacuum-sintering owing to MgO evaporation. 相似文献
2.
S. J. Zinkle 《Journal of the American Ceramic Society》1989,72(8):1343-1351
Stoichiometric polycrystalline magnesium aluminate spinel has been irradiated at 25° and 650°C with 2.4-MeV Mg+ ions to a fluence of 1.4 × 1021 ions/m2 (∼35 dpa (displacement per atom) peak damage level). Microindentation hardness measurements and transmission electron microscopy combined with energy dispersive X-ray spectroscopy measurements were used to characterize the irradiation effects. The room-temperature hardness of spinel increased by about 5% after irradiation at both temperatures. There was no evidence for amorphization at either irradiation temperatures. Interstitial-type dislocation loops lying on {110} and {111} planes with Burgers vectors along 〈110〉 were observed at intermediate depths (∼1 μm) along the ion range. The 〈110〉{111} loops are presumably formed from 〈111〉{111} loops as a result of a shear on the anion sublattice. Only about 0.05% of the calculated displacements were visible in the form of loops, which indicates that spinel has a high resistance to aggregate damage accumulation. The peak damage region contained a high density of dislocation tangles. There was no evidence for the formation of voids or vacancy loops. The specimen irradiated at 650°C was denuded of dislocation loops within ∼1 μm of the surface. 相似文献
3.
Daniela Domanski Guillermina Urretavizcaya Facundo J. Castro Fabiana C. Gennari 《Journal of the American Ceramic Society》2004,87(11):2020-2024
MgAl2 O4 spinel was successfully synthesized using a mechanochemical route that avoided the formation and calcination of its precursors at high temperatures. The method involved a single step in which γ-Al2 O3 –MgO, AlO(OH)–MgO, and α-Al2 O3 –MgO mixtures were milled at room temperature under air atmosphere. The formation of MgAl2 O4 occurred faster with γ-Al2 O3 than with AlO(OH) or α-Al2 O3 . After 140 h, the mechanochemical treatment of the γ-Al2 O3 –MgO mixture yielded 99% of MgAl2 O4 . 相似文献
4.
Shinobu Hashimoto Shaowei Zhang William Edward Lee Akira Yamaguchi 《Journal of the American Ceramic Society》2003,86(11):1959-1961
Spinel platelets were formed from a powder mixture of 3–5 μm wide and 0.2–0.5 μm thick α-Al2 O3 and 1–8 μm (average 3 μm) MgSO4 heated 2 h at 1200°C. The hexagonal platelet shape of the original α-Al2 O3 platelet was maintained in the spinel, although their size was slightly increased and their surface roughened. When a mixture of α-Al2 O3 platelets and MgO powder was heated 3 h at 1400°C, the spinel formed lost the platelet morphology of the alumina. 相似文献
5.
Chuei-Tang Wang Lang-Sheng Lin Sheng-Jenn Yang 《Journal of the American Ceramic Society》1992,75(8):2240-2243
High-sinterability MgAl2 O4 powder has been produced from alkoxide precursors via a freeze-drying method. Clear alumina sol and magnesium methoxide were used as starting materials in the process. The spinel powders were characterized by various techniques, such as thermal analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The tap density and sinterability of the spinel power are affected by the ball-milling techniques. Highly dense, transparent, polycrystalline MgAl2 O4 has been obtained from these powders by sintering and hot isostatic pressing. Bimodal grain-size microstructure is observed in a HIPed sample. 相似文献
6.
The grain-boundary mobility in magnesium aluminate spinel (MgO · nAl2 O3 ) of magnesia-rich ( n < 1) and alumina-rich ( n > 1) compositions has been measured from normal grain growth in dense, hot-pressed samples. Over the temperature range 1200° to 1800°C, the mobility in magnesia-rich compositions is found to be greater than that in alumina-rich compositions by a factor of 102 to 103 . Within the alumina-rich field, the mobility varies by less than a factor of 10 over the composition range 1 > n > 1.56. Nearly stoichiometric spinels (1 < n < 1.07) form a variety of sources and starting materials exhibit boundary mobilities within a factor of 5 at fixed temperature, showing an impurity tolerance which has not been found in other ionic solids. 相似文献
7.
The grain-boundary chemistry of magnesium aluminate spinel solid solutions MgO· n Al2 O3 in which grain growth measurements were reported in part I has been investigated in order to understand the mechanism of grain-boundary migration. It is found that although segregation of impurity Ca and Si is common, much larger deviations in grain-boundary stoichiometry are present. There is an excess of Al and O relative to Mg at grain boundaries in all compositions. Grain-boundary migration appears to be rate-limited by solute drag from intrinsic defects accommodating lattice nonstoichiometry, rather than by extrinsic solutes, consistent with the observed impurity tolerance of grain-boundary mobility. Different rate-limiting defects are proposed for magnesia-rich and alumina-rich spinels. 相似文献
8.
Hyoun-Ee Kim Steven J. Zinkle William R. Allen 《Journal of the American Ceramic Society》1991,74(6):1442-1444
Magnesium aluminate spinel (MgAl2 O4 ) with an 17 O enrichment (17 O/OTot ) of about 23 at.% was prepared by reacting fine mixtures of aluminum hydroxide (enriched with 17 O) and magnesium oxide of normal isotopic content. The material was prepared for experiments in which the radiation damage produced in a fusion reactor is simulated by fission reactor exposures. The powder mixtures were obtained by hydrolyzing, with water containing the 17 O iostope, a mixture of aluminum isopropoxide and magnesium oxide powder. The mixture was converted into pure spinel by a series of heat treatments and grindings. Essentially fully dense bodies, which contained about 45% of the 17 O isotope initially present in the water, were successfully fabricated provided that all thermal treatments were conducted in argon or vacuum atmospheres. 相似文献
9.
New structural and calorimetric data for samples of NiAl2 O4 quenched from 600° to 1560°C are presented The spinel remains stoichiometric for all heat treatments. Based on the refinement of X-ray powder patterns, it is shown that the degree of disorder, defined as the mole fraction of tetrahedral sites occupied by Al3+ , changes from x = 0.82 at 600°C to 0.78 at 1560°C. Simultaneously, the lattice parameter and enthalpy vary in a complex manner with quench temperature. The largest lattice parameter (0.80500 ± 0.00004 nm) and most exothermic enthalpy of annealing (heat released when sample is equilibrated at 780°C; -10.1 kJ/mol) occur for the sample quenched from 1100°C. A linear correlation exists between the heat of annealing and the lattice parameter. The results have been interpreted as a superposition of at least two effects: (1) the disordering of Ni2+ and Al3+ ions between octahedral (16d) and tetrahedral (8a) sites and (2) a second process, which may be a small amount of the disordering of ions into the usually empty (16c) sites. 相似文献
10.
The densification kinetics of magnesium aluminate spinel during vacuum hot-pressing were studied between 1175° and 1460° C and from 500 to 5100 psi. A phenomenologi-cal rate equation, which suggests a logarithmic relation between strain rate and porosity, excellently described the observed densification. Treating porosity as an independent variable was shown to be reasonable; it does not functionally restrict porosity as a modifier of the applied stress. The strain rate dependence on porosity decreased at a porosity of approximately 0.15. Below 1350° C the densification characteristics were similar to those reported for other oxide systems. At 1450°C an increase in the stress dependence of the densification rate and an interaction between stress and porosity suggested that plastic flow by dislocation motion was an operative mechanism during densification. 相似文献
11.
Ibram Ganesh Susana M. Olhero Paula M. C. Torres José M. F. Ferreira 《Journal of the American Ceramic Society》2009,92(2):350-357
A stoichiometric MgAl2 O4 spinel (MAS) powder was processed in aqueous media and consolidated by gelcasting from suspensions containing 41–45 vol% solids loading. The MAS powder was first obtained by heat treating a compacted mixture of α-Al2 O3 and calcined caustic MgO at 1400°C for 1 h, followed by crushing and milling. Then, its surface was passivated against hydrolysis using an ethanol solution of H3 PO4 and Al(H2 PO4 )3 . The as-treated surface MAS powder could then be dispersed in water using tetra methyl ammonium hydroxide and an ammonium salt of poly-acrylic acid (Duramax D-3005) as dispersing agents. The as-obtained stable suspensions were gelcast, dried, and sintered at 1650°C for 1–3 h. For comparison purposes, the treated powder was also compacted by die pressing of freeze-dried granules and sintered along with gelcast samples. Near-net-shape MAS components with 99.55% of the theoretical density could be fabricated by aqueous gelcasting upon sintering at 1650°C for 3 h. The MAS ceramics fabricated by gelcasting and die pressing exhibited comparable properties. 相似文献
12.
Yong Sheng Han Jian Bao Li Xiao Shan Ning Bo Chi 《Journal of the American Ceramic Society》2004,87(7):1347-1349
Nickel aluminate spinels were prepared by solid-state reaction. Their lattice parameters ( a 0 ) changed with preparation temperature, which was explained by cation distribution, vacancies formation, and cation entrance. When the preparation temperature increased from 1100° to 1200°C, a 0 decreased with temperature as the result of the uptake of Al2 O3 . After 1200°C, a 0 increased to temperature, which was attributed to some Ni2+ exchange with tetrahedral sites and some vacancies occupancy by Ni2+ and Al3+ . 相似文献
13.
Fabrication of Translucent Magnesium Aluminum Spinel Ceramics 总被引:5,自引:0,他引:5
Ji-Guang Li Takayasu Ikegami Jong-Heun Lee Toshiyuki Mori 《Journal of the American Ceramic Society》2000,83(11):2866-2868
A precursor for magnesium aluminum spinel powder, composed of crystalline ammonium dawsonite hydrate (NH4 Al(OH)2 CO3 ·H2 O) and hydrotalcite (Mg6 Al2 (CO3 )(OH)16 ·4H2 O) phases, was synthesized via precipitation, using ammonium bicarbonate as the precipitant. The precursor was characterized by differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. Reactive spinel powder, which could be densified to translucency under vacuum at 1750°C in 2 h without additives, was obtained by calcining the precursor at 1100°C for 2 h. 相似文献
14.
15.
Flavia N. Cunha-Duncan Richard C. Bradt 《Journal of the American Ceramic Society》2002,85(12):2995-3003
The synthesis of refractory Mg-Al spinel aggregates from bauxites and magnesias was addressed by applying the alumina–magnesia–silica ternary equilibrium phase diagram as the fundamental basis. Four different alumina sources (bauxites) and four different magnesia sources were investigated. These were fired in air through 1700°C and their reactions were monitored using X-ray diffraction. Evolution of their microstructure was observed by scanning electron microscopy. Initially, the periclase reacts with the free corundum of the bauxite to produce Mg-Al spinel. The periclase then reacts with the mullite in the bauxites to yield additional spinel and also some forsterite. Spinels were produced for all sixteen different raw-material combinations. 相似文献
16.
Asish Ghosh Kenneth W. White Michael G. Jenkins Albert S. Kobayashi Richard C. Brad 《Journal of the American Ceramic Society》1991,74(7):1624-1630
The fracture resistance of a fully dense, transparent, polycrystalline magnesium aluminate spinel was measured from room temperature to 1400°C using the chevron-notched beam and the straight-notched beam macroflaw techniques, as well as the indentation-induced, controlled-microflaw test method, all in three-point bending. Flexural strengths were also measured for the same range of temperatures to compare with the fracture toughness measurements. From the load vs load-line displacement ( P-u ) curves of the chevronnotched test specimens, the crack growth resistance curves ( R -curves) and the total work-of-fracture were determined. It was observed that polycrystalline MgAl2 O4 exhibits rising R -curve behavior which increases with increasing test temperature. The R -curve increases are attributed to the geometric constraints due to grain bridging and grain wedging phenomena as well as secondary grain boundary microcracking processes, all of which occurred in the wake region behind the advancing crack. The work-of-fracture and the R -curves increased rapidly above 800°C coincident with the onset of increased secondary grain boundary microcracking. 相似文献
17.
Nicolas Nuns Franck Béclin Jacques Crampon 《Journal of the American Ceramic Society》2009,92(4):870-875
The grain-boundary chemistry of fine-grained spinel MgO· n Al2 O3 (mean grain size below micron) has been investigated by STEM microanalysis. We have quantified the concentration of each element across the grain boundaries. Stoichiometry variations are observed from the grain-boundary region to the bulk. The Al/Mg ratio increases from 2.1 in the bulk to 2.35 at the grain-boundary regions. X-ray quantification allows us to reveal and to characterize the space-charge layer in the subgrain boundary. The grain-boundary cores are negatively charged due to vacancies in excess, and in the subgrain-boundary region, an opposite, positive space-charge layer is obtained. The point defect composition and the characteristic (sign, space-charge potential Φ∞ ) of the space-charge layer are discussed. 相似文献
18.
Pressureless sintering studies have been conducted for excess Al2 O3 , stoichiometric, and excess MgO compositions of MgAl2 O4 at 1500-1625°C. Initial powders of various compositions are prepared by solid-state reaction of MgO and Al2 O3 . A Brouwer defect equilibrium diagram is constructed that assumes intrinsic defects of the Schottky type. The densification rate derived from sintering kinetics is compared with the compositions investigated when the concentration is converted to the activity of the two oxide components in MgAl2 O4 . The grain-size exponent of p similar/congruent 3 suggests that densification takes place by a lattice-diffusion mechanism in the solid state. Determined activation enthalpies of 489-505 kJmol-1 are close to those obtained from oxygen self-diffusion derived in previous sintering studies. It is, therefore, proposed that oxygen lattice diffusion through vacancies is the rate-controlling mechanism for the sintering of nonstoichiometric MgAl2 O4 compositions. The discrepancy between densification-rate ratios in experimental results and oxygen vacancy concentration in the Brouwer diagram is accounted for by the defect associates formed in the nonstoichiometric compositions. 相似文献
19.
Siddhartha Bandyopadhyay Georg Rixecker Fritz Aldinger Himadri Sekhar Maiti 《Journal of the American Ceramic Society》2004,87(3):480-482
The reaction sequences of the formation of nitrogen-containing magnesium aluminate spinel from MgO, Al2 O3 , and AlN were investigated as a function of temperature through dilatometric study and as a function of time through isothermal heat treatments. The natures of reactions are described through the appearance of phases in conjunction with densification behavior and the change in lattice parameter of the spinel phase. Although the dilatometric study provides the detail insights of the formation sequence, the isothermal runs reveal new information about the differential rate of reactivity of the reacting species that suggested a tentative controlling mechanism. Through the initial formation of magnesium aluminate, oxygen-rich solid solution (MgAlON) forms, which ultimately reacts with the rest of AlN to reach its nominal composition. Nitrogen diffusion through MgAlON lattice seems to be rate controlling. 相似文献