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1.
MASAYUKI FUJIMOTO YET-MING CHIANG W. DAVID KINGERY 《Journal of the American Ceramic Society》1988,71(4):197-200
In a hot-pressed and deformed MgO single crystal, precipitates of Mg1-x Cax Al2 O4 spinel upon which MgAl2 O4 spinel subsequently precipitated were observed and analyzed using transmission electron microscopy and scanning electron microscopy. This behavior is related to the respective solubility limits of CaO and Al2 O3 in MgO at the hot-pressing temperature and may be aided by impurity segregation to the dislocations. The spinel selectively precipitated at the nodes of a dislocation network which was formed during [001] hot-pressing deformation, as a result of the reaction b3 = b1 + b2 = (1/2) [011] + (1/2) = [001]. The dislocation is sessile, and the precipitates have a <100>matrix ≨ <100>spinel coherent relationship. 相似文献
2.
Keith Rozenburg Ivar E. Reimanis Hans-Joachim Kleebe Ron L. Cook 《Journal of the American Ceramic Society》2008,91(2):444-450
The sintering kinetics of a pure magnesium aluminate spinel, MgAl2 O4 , and that doped with LiF were determined through the use of the master sintering curve technique developed by Su and Johnson. 20 Powders with 0%, 0.5%, and 1.0% by mass LiF were densified in a vacuum hot press under a range of unaxial pressures. After the sintering mechanisms in each temperature and pressure regime were determined, an optimized vacuum hot-pressing schedule was formulated for spinel powders doped with 1.0% by mass of LiF. In addition to forming a transient liquid phase, the presence of LiF leads to the formation of oxygen vacancies that promote late-stage sintering in MgAl2 O4 . 相似文献
3.
The effect of oxygen activity on the sintering of high-purity Cr2 O3 is shown. Theoretical density was approached at the equilibrium O2 partial pressure needed to maintain the Cr2 O3 phase ( P o2 =2×10−12 atm). The presence of N2 in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O2 activity, correlated with the changing predominant volatile species in the Cr-O system. 相似文献
4.
Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi2 O3 ceramic was investigated for A12 O3 additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI2 O3 reacted with the ZnO to form ZnAl2 O4 spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl2 O4 spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl2 O4 spinel particles, most probably controlled by the diffusion of O2- in the ZnAl2 O4 spinel structure. 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.
Some effects of CaO, Na2 O, Sb2 O3 , and ZrO2 additions on Li0.3 Zn0.4 Fe2.3 O4 ferrite were studied to improve its density and other material properties. The densification behavior of the ferrite depended on the amount and type of additive. A relative density of ∼98.5% was achieved with the addition of CaO. The grain size decreased with the addition of Na2 O, CaO, and Sb2 O3 . The permeability and electrical resistivity increased with additives. CaO remarkably increased resistvity, whereas, ZrO2 increased permeability. Na2 O and Sb2 O3 increased the Curie temperature, whereas CaO and ZrO2 decreased it. These effects were attributed to mainly additive segregation on the grain boundaries, which suppressed grain-size development during the sintering of lithium zinc ferrite. 相似文献
7.
Bertha Alicia Vázquez Ángel Caballero Pilar Pena 《Journal of the American Ceramic Society》2005,88(7):1949-1957
Solid-state compatibility and melting relations of MgAl2 O4 in the quaternary system Al2 O3 –CaO–MgO–SiO2 were studied by firing and quenching selected samples located in the 65 wt% MgAl2 O4 , plane followed by microstructural and energy dispersive X-ray analysis. A projection of the liquidus surface of the primary crystallization volume of MgAl2 O4 was constructed from CaO, SiO2 and exceeding Al2 O3 , not involved in stoichiometric MgAl2 O4 formation; those three amounts were recalculated to 100 wt%. The temperature and character of six invariant points, where four solids co-exist with a liquid phase, were defined. One maximum point was localized and the positions of the isotherms were tentatively established. The effect of CaO, SiO2 , and Al2 O3 impurities on the high temperature behavior of spinel materials was also discussed. 相似文献
8.
By a combination of solid-state sintering and quenching experiments the validity of calcium hexaluminate as a stable phase and the extent of its primary field in the system CaO–Al2 O3 –SiO2 have been established. The size of the primary field is considerably reduced from that suggested by earlier work. The anorthite-corundum-calcium hexaluminate invariant point has been relocated at 28.0% CaO, 39.7% Al2 O2 , and 32.3% SiO2 and at 1405°± 5°C. 相似文献
9.
CaO and Ga2 O3 form three compounds: 3CaO-Ga2 O3 , CaO-Ga2 O3 , and CaO-2Ga2 O3 . 3CaO-Ga2 O3 melts incongruently to CaO plus liquid at 1263°C.; CaO Ga2 O3 and CaO 2Ga2 O3 melt congruently at 1369° and 1504°C. respectively. Eutectics are located at the following temperatures and compositions (in mole% Ga2 O3 ): between 3CaO Ga2 O3 and CaO Ga2 O3 , 1245°C. and 37.5%; between CaO Ga2 O3 and CaO-2Ga2 O3 ,1323o C. and 57.0%; and between CaO -2Ga2 O3 and β-Ga2 O3 ,1457°C. and 68.0%. There is a peritectic at 1263°C. and 36.0%. Three polymorphs of CaO Ga2 O3 are described. Compositions from approximately 35 to 70 mole% Ga2 O3 can be quenched to yield homogeneous glasses. 相似文献
10.
The preparation of near stoichiometric spinel and alumina-rich spinel composites from Al2 O3 and MgO powders with the addition of Na3 AlF6 up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al2 O3 can be produced from the mixture of 72 wt% (50 at.%) Al2 O3 + 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na3 AlF6 fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al2 O3 can be produced from either the mixture of 90 wt% (78 at.%) Al2 O3 + 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al2 O3 + 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na3 AlF6 in the temperature range 1300°–1600°C by using a torch-flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al2 O3 . Microscopic studies indicate that the alumina-rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water. 相似文献
11.
Takeshi Shiono Koji Shiono Kazuyo Miyamoto Giuseppe Pezzotti 《Journal of the American Ceramic Society》2000,83(1):235-37
A precursor was synthesized from a heterogeneous alkoxide solution that contained fine MgO powder, which allowed the preparation of MgAl2 O4 spinel powder with high sinterability characteristics. The precursor consisted of a mixture of boehmite (AlO(OH)) and a mixed hydroxide (Mg4 Al2 (OH)14 · 3H2 O). The spinel phase formed through two steps: (i) decomposition of the mixed hydroxide at low temperature and (ii) solid-state reaction between MgO and γ-Al2 O3 at higher temperatures. Dense polycrystalline spinel could be obtained from the calcined powders at sintering temperatures as low as 1400°C. 相似文献
12.
Koji Watari Maria Cecilia Valecillos Manuel E. Brito Motohiro Toriyama Shuzo Kanzaki 《Journal of the American Ceramic Society》1996,79(12):3103-3108
Small amounts of Li2 O result in sintering in the AIN-Y2 O3 -CaO and AIN-CaO systems at firing temperatures <1600°C. The effect is ascribed to reduction of the liquidus temperature. Furthermore, Li2 O is removed by volatization at temperatures from 1300° to 1600°C, and its content decreases several ppm from the initial 0.3 wt%. Li2 O-doped AIN specimens containing Y2 O3 and CaO additives are well densified by firing at 1600°C for 6 h, and their thermal conductivity is 135 W. m−1. K−1 .The effect of Li2 O addition on sintering and thermal conductivity also is discussed through thermo-dynamic considerations. 相似文献
13.
The crystallization behavior of a glass with a composition of 40 wt% 3CaO · P2 O5 −60 wt% CaO · MgO · 2SiO2 was investigated. The primary crystalline phase was apatite with a dendritic form and ellipsoidal shape. β-(3CaO · P2 O5 ) and CaO · MgO · 2SiO2 were crystallized as samples heated to 990°C, and a three-layer structure was obtained. The development and morphology of this construction were explained by both the surface crystallization of the apatite and CaO · MgO · 2SiO2 and the bulk crystallization of apatite and the CaO · MgO · 2SiO2 -β-(3CaO · P2 O5 ) composite. 相似文献
14.
Solid-state reactions between Li2 O and Al2 O3 were studied in the region between Li2 O.Al2 O 3 and Al2 O 3 . The compound Li2 O Al2 O 3 melts at 1610°± 15°C. and undergoes a rapid reversible inversion between 1200° and 1300°C. Vaporization of Li2 O from compositions in the system proceeds at an appreciable rate at 1400°C, as shown by fluorescence. Lithium spinel, Li2 O -5Al2 O3 , was the only other compound observed. The effect of Li2 O on the sintering of alumina was investigated. 相似文献
15.
Jafar F. Al-Sharab Frederic Cosandey Amit Singhal Ganesh Skandan James Bentley 《Journal of the American Ceramic Society》2006,89(7):2279-2285
Nanostructured MgAl2 O4 spinel was synthesized by a direct conversion process from cubic γ-Al2 O3 . The effect of post-annealing temperature (300°, 500°, and 800°C) on MgAl2 O4 phase formation was investigated using transmission electron microscopy, selected area electron diffraction (SAED), electron energy loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). Relative diffraction intensities as well as lattice parameter measurements from SAED revealed that MgAl2 O4 spinel structure starts forming at temperatures as low as 300°C. EELS and EDS spectrum images also revealed an increase in elemental homogeneity with increasing annealing temperature. The degree of ordering of Mg and Al between octahedral and tetrahedral sites has been determined from relative diffraction intensities. Results show that annealing to 800°C leads to a spinel phase with an order parameter of 0.78. 相似文献
16.
MgO addition to 3 mol% Y2 O3 –ZrO2 resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO2 in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO2 grains with tetragonal ZrO2 precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule. 相似文献
17.
THOMAS L. BARRY V. S. STUBICAN RUSTUM ROY † 《Journal of the American Ceramic Society》1966,49(12):667-670
Phase equilibrium relations in the system CaO-Yb2 O3 were studied. Results of this work demonstrated the existence of four crystalline phases: Yb2 O3 .3CaO, Yb2 O3 .2CaO, Yb2 O3 °CaO, and 2Yb2 O3 °CaO. The 2Yb2 O3 °CaO phase is metastable at all temperatures and was obtained only by rapid quenching from the melt. The crystalline solubility limit of Yba O3 in CaO at 1850°C is slightly greater than 8 mole %, whereas no solubility of CaO in Yb2 O3 was detected. All four compounds have subsolidus minimums of stability and dissociate into the component oxides below 1800°C. Data are also presented for the systems CaO-Gd2 O3 and CaO-La2 O3 . 相似文献
18.
Jow-Lay Huang Si-Yau Sun Yung-Chao Ko 《Journal of the American Ceramic Society》1997,80(12):3237-3241
This work examines the effects of LiF or LiF/CaCO3 additives on the phases, microstructure, sintering temperature, and activation energy of MgO-Al2 O3 spinel formation. According to these results, the spinel formation temperature decreases and the spinel phase content increases by incorporating sintering additives. Such an effect is due to the formation of liquid phases at sintering temperatures. The Al composition in spinel is substantially higher in samples containing LiF or LiF/CaCO3 than in samples without sintering additives. Results obtained from the Arrhenius equation and Jander's model indicate that the activation energy for forming spinel decreases by incorporating LiF or LiF/CaCO3 . 相似文献
19.
Jean-Celestin Yamegni-Noubeyo Genevieve Pourroy Jacques Werckmann Antoni Malatsi Riera Gabrielle Ehret Paul Poix 《Journal of the American Ceramic Society》1996,79(8):2027-2032
The metal–ferrite composite (Fe0.2 Co0.8 )0.8 (Fe2.38 Co0.62 O4 ) has been studied by X-ray diffractometry measurements and high-resolution transmission electron microscopy. Spinel ferrite occurs in highly crystalline domains 100–150 nm in size, and the iron–cobalt alloy occurs in smaller and less-crystalline domains (10–20 nm). Both phases are heterogeneous in composition. The metal is embedded in the spinel phase, located near the edges, and overlaid by a poorly crystallized layer or misshapen regions containing small spinel crystals and amorphous phases. By annealing under vacuum up to 800°C, the misshapen regions disappear and the size of the metallic regions increases. The concentration of iron in the metallic regions decreases and their structure changes to face-centered cubic, while the spinel becomes enriched in iron. 相似文献
20.
Tania Bhatia Kamanio Chattopadhyay Vikram Jayaram 《Journal of the American Ceramic Society》2001,84(8):1873-1880
The response of the MgO–20 and 30 mol% Al2 O3 compositions to rapid solidification has been studied. The oxides were twin-roller quenched and the resulting flakes were characterized by X-ray diffraction and transmission electron microscopy. The results indicate that metastable extensions of spinel and periclase occurred and the microstructural pathway was determined from the final microstructure. The flakes having MgO–20 mol% Al2 O3 show a dendritic structure consisting of periclase and spinel. In the MgO–30 mol% Al2 O3 composition, the liquid transforms to spinel partitionlessly. The spinel is believed to undergo decomposition by a modulation in composition, and the resulting microstructure consists of spinel and periclase. Kinetic and thermodynamic aspects of phase selection have been rationalized based on the metastable extensions of the different phase fields in the phase diagram. It has been proposed that composition fluctuations in spinel are stabilized because of the formation of disordered phases with a continuous range of order parameter on the tetrahedral sublattice. 相似文献