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1.
J. J. COMER N. C. TOMBS J. F. FITZGERALD 《Journal of the American Ceramic Society》1966,49(5):237-240
Single-crystal and polycrystalline films of Mg-Al2 O4 and MgFe2 O4 were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al2 O8 film. Above 540°C, the γ-Al2 O, and MgO reacted to form a single-crystal MgAl2 O4 film with {001} MgAl2 O4 ‖{001} MgO. Above 590°C, an additional layer of MgAl2 O4 , which is polycrystalline, formed between the γ-Al2 O3 and the single-crystal spinel. Polycrystalline Mg-Al2 O4 formed only when diffusion of Mg2+ ions proceeded into the polycrystalline γ-Al2 O3 region. Corresponding results were obtained for Mg-Fe2 O4 . MgAl2 O4 films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al2 O3 crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate. 相似文献
2.
K. Thomas Jacob Kampurath P. Jayadevan Yoshio Waseda 《Journal of the American Ceramic Society》1998,81(1):209-212
The standard Gibbs energy of formation of the spinel MgAl2 O4 from component oxides, MgO and α-Al2 O3 , has been determined in the temperature range 900 to 1250 K using a solid-state cell incorporating single-crystal CaF2 as the solid electrolyte. The cell can be represented as—Pt,O2 ,MgO+MgF2 |CaF2 |MgF2 +MgAl2 O4 +α-Al2 O3 ,O2 ,Pt—The standard Gibbs energy of formation from binary oxides, computed from the reversible emf, can be represented by the expression—capdelta G °f,ox =−23600 − 5.91 T (±150) J/mol—The 'second-law' enthalpy of formation of MgAl2 O4 obtained in this study is in good agreement with high-temperature solution calorimetric studies reported in the literature. 相似文献
3.
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
4.
Koji Morita Keijiro Hiraga Yoshio Sakka 《Journal of the American Ceramic Society》2002,85(7):1900-1902
High-strain-rate superplasticity is attained in a 3-mol%-Y2 O3 -stabilized tetragonal ZrO2 polycrystal (3Y-TZP) dispersed with 30 vol% MgAl2 O4 spinel: tensile elongation at 1823 K reached >300% at strain rates of 1.7 × 10−2 – 3.3 × 10−1 s−1 . The flow behavior and the microstructure of this material indicate that the MgAl2 O4 dispersion should enhance accommodation processes necessary for grain boundary sliding. Such an effect is assumed to arise from an enhancement of the cation diffusion by the dissolution of Al and Mg ions into the ZrO2 matrix and from stress relaxation due to the dispersed MgAl2 O4 grains. 相似文献
5.
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
6.
A MgAl2 O4 (MA) spinel layer was synthesized on Ti3 AlC2 substrate through the molten salt synthesis (MSS) method. The Ti3 AlC2 substrate was immersed in MgCl2 ·6H2 O powders and treated at 800°, 850°, and 900°C for 4 h in air. A continuous and 10-μm-thick MgAl2 O4 layer was obtained at 900°C, by which the surface hardness of Ti3 AlC2 can be effectively improved. The combined scanning electron microscopy observations and crystal morphology simulation further revealed that the as-formed MgAl2 O4 presents tetragonal bipyramids morphology with (400)-orientation. 相似文献
7.
Interdiffusion coefficients in single-crystal MgO were determined using an MgO-MgAl2 O4 diffusion couple. For a concentration of 1 mol% Al2 O3 in MgO, the interdiffusion coefficient can be expressed as D =2.0±0.2 exp (−76,000±3,000/ RT ) for the MgO-MgAl2 O4 couple. This relation compares well with previous measurements in the MgO-Al2 O3 system. The interdiffusion coefficients, which increased with the mol fraction of cation vacancies, were in the range of 10−8 to 10−10 cm2 s−1 for the concentrations and temperatures studied. Diffusion was enhanced below 1640°C if powdered MgAl2 O4 was used. Self-diffusion coefficients for Al3+ ions in MgO were calculated; Al3+ diffuses faster than Cr3+ in MgO. 相似文献
8.
The electrical conductivities of single crystal and polycrystalline MgAl2 O4 and Y3 Al5 O12 were measured to 1260 K using a three-contact, guard-ring technique. The electrical conduction mechanisms change with temperature, with anomalous oxygen pressure and time-dependent inflections in log σ versus T−1 curves between 900 to 1000 K. The conduction processes of Y3 Al5 O12 and MgAl2 O4 appear to be similar and possibly related to A13+ ion diffusion. 相似文献
9.
The effect of Al8 B4 C7 used as an antioxidant in MgO–C refractories and the behavior of Al8 B4 C7 in CO gas were investigated in the present study. Al8 B4 C7 was found to react with CO gas, to form Al2 O3 ( s ), B2 O3 ( l ), and C( s ), at temperatures >1100°C. The Al2 O3 reacts with MgO to form MgAl2 O4 near the surface of the material. At the same time, B2 O3 ( l ) evaporates and reacts with MgO, to form a liquid phase, at >1333°C, the eutectic point between 3MgO·B2 O3 and MgO. The coexistence of the liquid and MgAl2 O4 makes the protective layer more dense, thus inhibiting oxidation of the refractory. At >1333°C, the process apparently is controlled by oxygen diffusion, whereas it is controlled by chemical reaction when the temperature is <1333°C. 相似文献
10.
The elastic moduli and fracture toughnesses of a series of PbO-ZnO-B2 O3 glasses were measured for different PbO/ZnO ratios and for B2 O3 contents from 30 to 70 mol%. Substituting ZnO for PbO increased both the elastic modulus and fracture toughness at all B2 O3 levels with the fracture toughness being related to the elastic modulus. Structural effects on these properties are discussed. 相似文献
11.
Gualtiero Gusmano Giampiero Montesperelli Enrico Traversa Giulia Mattogno 《Journal of the American Ceramic Society》1993,76(3):743-750
Active elements for humidity sensors based upon MgAl2 O4 thin films or sintered pellets were investigated. Thin films were deposited on Si/SiO2 substrates by radiofrequency (rf) sputtering. Sintered MgAl2 O4 pellets were prepared by traditional ceramic processing. Scanning electron microscopy (SEM) analysis showed that the thin films were rather dense and homogeneous, made up of clustered particles of about 20–30 nm, while the pellets showed a wide pore-size distribution. X-ray photoelectron spectroscopy (XPS) demonstrated that the thin films have a stoichiometry close to that of MgAl2 O4 . Sintered MgAl2 O4 is crystalline, while it is disordered in thin-film form. The presence of two different components of the Al 2 p peaks was correlated with the structural difference between pellets and thin films. The relationship between good film–substrate adhesive properties and the chemical composition at the interface was studied. The electrical properties of the sensing elements were studied at 40°C in environments at different relative humidity (RH) values between 2% and 95%, using ac impedance spectroscopy. MgAl2 O4 thin films showed interesting characteristics in terms of their use in humidity-measurement devices. Resistance versus RH sensitivity values showed variations as high as 4 orders of magnitude in the RH range tested for thin films, and 5 orders of magnitude for pellets. The differences in the electrical behavior of MgAl2 O4 pellets and thin films were correlated with their different microstructures. 相似文献
12.
An isothermal section of the ternary system MgO–Al2 O3 -Cr2 O3 was determined at 1700°± 15°C to delineate the stability field for spinel crystalline solutions (cs). Crystalline solutions were found between the pseudobinary joins MgAl2 O4 –Cr2 O3 and MgCr2 O4 -Al2 O3 , and the binary join MgAl2 O4 -MgO. The first two crystalline solutions exhibit cation vacancy models while the latter can probably be designated as a cation interstitial model. Precipitation from spinel cs may proceed directly to an equilibrium phase, (Al1-x Crx )2 O3 , with the corundum structure or through a metastable phase of the probable composition Mg(Al1-x Cr )26 O40 . The composition and temperature limits were defined where the precipitation occurs via metastable monoclinic phases. The coherency of the metastable monoclinic phase with the spinel cs matrix can be understood by considering volume changes with equivalent numbers of oxygens and known crystallographic orientation relations. Electron probe and metallographic microscope investigations showed no preferential grain boundary precipitation. 相似文献
13.
ALEXANDRA NAVROTSKY BARRY A. WECHSLER KAREN GEISINGER FRIEDRICH SEIFERT 《Journal of the American Ceramic Society》1986,69(5):418-422
Solution calorimetry of MgAl2 O4 -Al8/3 O4 solid solutions was performed in a molten 2PbO · B2 O3 solvent at 975 K. The results indicate small negative heats of mixing, relative to spinel standard states for both end-members. These data were combined with information on the energetics of the α-γ transition in Al2 O3 and on the MgAl2 O4 -Al8/3 O4 (MgO-Al2 O3 ) subsolidus phase relations to estimate the partial molar entropy of mixing of γ-Al8/3 O4 in the solid solution. This entropy is much less positive than that calculated from several models for the configurational entropy of mixing of magnesium, aluminum, and vacancies on octahedral and/or tetrahedral sites. The data suggest a good deal of local order to be present in the solid solutions, consistent with negative enthalpies of mixing and entropies of mixing far less than ideal configurational values. 相似文献
14.
Chang Wei Zheng Shu Ya Wu Xiang Ming Chen Kai Xin Song 《Journal of the American Ceramic Society》2007,90(5):1483-1486
MgAl2 O4 microwave dielectric ceramics were modified by Zn substitution for Mg, and their dielectric characteristics were evaluated, along with their structures. Dense (Mg1− x Zn x )Al2 O4 ceramics were obtained by sintering at 1550°–1650°C in air for 3 h, and the (Mg1− x Zn x )Al2 O4 solid solution was determined in the entire composition range. With Zn substitution for Mg, the dielectric constant ɛ of MgAl2 O4 just varied from 7.90 to 8.56, while the Q × f value had significantly improved up to a maximal value of 106 000 GHz at x =1.0. Moreover, the τf of MgAl2 O4 ceramics had declined from −73 to −63 ppm/°C. 相似文献
15.
The subsolidus phase equilibrium diagram for the pseudobinary join MgAl2 O4 -Ga2 O3 was determined. The shape of the exsolution boundary was obtained by heat-treating samples pre- equilibrated at 1600°C. Crystalline solubility of Ga2 O3 in MgAl2 O4 decreased from 73 mole % at 1600°C to 55 mole % at 1200°C. The crystalline solution was formed by the replacement of Mg2+ ions by Ga3+ ions to produce a cation defect spinel. The phase precipitated was the mono-clinic δ-Ga2 O3 (=δ-Al2 O3 structure). Changes in the ratios of relative X-ray diffraction intensities indicated that the crystalline solutions also disorder with temperature. 相似文献
16.
K. Rozenburg I. E. Reimanis H.-J. Kleebe Ron L. Cook 《Journal of the American Ceramic Society》2007,90(7):2038-2042
Reactions between LiF and MgAl2 O4 at temperatures up to 1500°C are examined with a variety of tools, including differential scanning calorimetry, thermo-gravimetric analysis, X-ray diffraction, and scanning electron microscopy. LiAlO2 and MgF2 are found to be the active reaction products at these temperatures. A transient liquid phase comprising MgF2 and LiF forms at intermediate temperatures, but then is consumed at higher temperatures during the reformation of MgAl2 O4 . If processed as an uncompacted powder mixture, all of the initial LiF in the system eventually vaporizes at temperatures exceeding 1300°C. A new reaction sequence relevant to the densification of LiF-doped MgAl2 O4 spinel is proposed. 相似文献
17.
V. M. Sreekumar R. M. Pillai B. C. Pai M. Chakraborty 《Journal of the American Ceramic Society》2007,90(9):2905-2911
An aluminum/MgAl2 O4 in situ metal matrix composite has been synthesized using silica gel containing ∼98% SiO2 in an Al–5Mg alloy. The thermodynamics and kinetics of MgAl2 O4 formation have been discussed in detail. A transition phase of composition between MgO and MgAl2 O4 has been detected in the SEM-EDS analysis of the particles extracted from the composite by a 25% NaOH solution. This confirms the gradual transformation of MgO to MgAl2 O4 by the reaction 3SiO2 ( s )+2MgO( s )+4Al( l )→2MgAl2 O4 ( s )+3Si( l ). The stoichiometry, n , of MgAl2 O4 has been found to sustain close to 1 and the crystallite growth of MgAl2 O4 has been stopped at D ∼30 nm in the composites held at 750°C up to 10 h. 相似文献
18.
Antonio H. de Aza Pilar Pena Salvador de Aza 《Journal of the American Ceramic Society》1999,82(8):2193-2203
Using a novel experimental procedure in the field of ceramics/materials science that is based on the precise microanalysis (scanning electron microscopy, coupled with wavelength-dispersive spectroscopy) of the phases that are present in equilibrated specimens, the solid-state compatibility relations in the subsystem MgAl2 O4 -CaAl4 O7 -CaO-MgO and the melting relationships in the subsystems MgAl2 O4 -CaAl2 O4 -MgO and MgAl2 O4 -CaAl2 O4 -CaAl4 O7 were established. The primary phase field of crystallization of spinel in the above-mentioned subsystem, MgAl2 O4 -CaAl4 O7 -CaO-MgO, was determined subsequently. The temperature, composition, and character of the ternary invariant points of the subsystem were established, and the ranges of the solid solutions in periclase, spinel, monocalcium aluminate, and dicalcium aluminate also were studied and determined, up to 1725°C. 相似文献
19.
Antonio H. de Aza Juan E. Iglesias Pilar Pena Salvador De Aza 《Journal of the American Ceramic Society》2000,83(4):919-927
Solid-state compatibility and melting relationships in the subsystem Al2 O3 —MgAl2 O4 —CaAl4 O7 were studied by firing and quenching selected samples located in the isopletal section (CaO·MgO)—Al2 O3 . The samples then were examined using X-ray diffractomtery, optical microscopy, and scanning and transmission electron microscopies with wavelength- and energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant points of the subsystem were established. The existence of two new ternary phases (Ca2 Mg2 Al28 O46 and CaMg2 Al16 O27 ) was confirmed, and the composition, temperature, and peritectic character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem were plotted, and the solid-solution ranges of CaAl4 O7 , CaAl12 O19 , MgAl2 O4 , Ca2 Mg2 Al28 O46 , and CaMg2 Al16 O27 were determined at various temperatures. The experimental data obtained in this investigation, those reported in Part I of this work, and those found in the literature were used to establish the projection of the liquidus surface of the ternary system Al2 O3 —MgO—CaO. 相似文献
20.
Zhen-Kun Huang Steve D. Nunn Irene Peterson Tseng-Ying Tien 《Journal of the American Ceramic Society》1994,77(12):3251-3254
Formation of N-phase in the system Mg,Si,Al/N,O was studied. Its composition was confirmed to be MgAl2 Si4 O6 N4 (2Si2 N2 OMgAl2 O4 ). Subsolidus phase relationships in the MgO–Si2 N2 O-Al2 O3 system were determined. The results are discussed by comparing with two similar systems, CaO-and Y2 O3 –Si2 N2 O–Al2 O3 . 相似文献