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1.
Titanium-doped α-Al2 O3 exhibits a high-temperature conductivity which is ionic at high oxygen pressures and electronic at low oxygen pressures. Both are isotropic. The temperature dependence of conductivity under conditions where equilibrium with the atmosphere is not maintained indicates both the position of the energy level of titanium (TiAl x ) in the forbidden gap and the temperature dependence of the mobility of the native ionic defects (Al vacancies, V Al m ). Optical absorption responsible for the pink color of the reduced crystals is measured as a function of p o2 and is used to determine concentrations of Ti3+ and Ti4+ . Parameters for the equilibrium constants of the reactions involving electrons by which the composition of Al2 O3 :Ti and undoped Al2 O3 is varied are determined. The chemical diffusion data by Jones et al. are described quantitatively. 相似文献
2.
Measurements of dc electrical conductivity and emf of single crystal and polycrystalline α-Al2 O3 doped, respectively, with 100 and 300 ppm yttrium show that yttrium, although isoelectronic with aluminum, is a donor. The electronic energy level of the donor is attributed to O2- next to Y3+ , its level being forced up as a result of the large size of the yttrium ion. The donor activity of Y supports the view that the favorable effect of Y on the oxidation of super alloys is due to reduced diffusion of Al in the bulk through a reduced concentration of Ali … without a marked increase in the concentration of V… Al . 相似文献
3.
Electrical conductivity and the ionic and electronic transference numbers were determined for two types of unintentionally doped single crystalline sapphire for current directions perpendicular to the r plane (1102). One was acceptor dominated and the other was initially donor (Hi x ) dominated, changing to acceptor domination after a prolonged anneal at 1600°C. The positions of the electronic energy levels of dominant impurities and the constants regulating the oxidation-reduction of these impurities and of pure Al2 O3 are determined. The latter shows a discrepancy with an expression reported previously. 相似文献
4.
Xue-Ning Huang Chu Kun Kuo Patrick S. Nicholson 《Journal of the American Ceramic Society》1995,78(4):892-896
An epitaxial β-alumina crystal growth method was used to modify α-AI2 O3 platelet surfaces before inclusion as a reinforcing phase in partially stabilized zirconia (3Y-TZP). The as-grown surface phase was Na-β"-AI2 O3 . This was converted to Ca-β"-AI2 O3 by ion exchange, as the latter is more temperature-stable at composite sintering temperatures. The conditions of formation, thermal stability, and chemical compatibility of these interfacial phases were examined. α-AI2 O3 platelets with Ca-β"-AI2 O3 film were incorporated into 3Y-TZP. The β"-AI2 O3 /ZrO2 interface was found to promote platelet debonding and pullout, thus enhancing the α-AI2 O3 platelet/crack interactions during the fracture process. 相似文献
5.
C. RICHARD A. CATLOW JOHN CORISH JOHN HENNESSY WILLIAM C. MACKRODT 《Journal of the American Ceramic Society》1988,71(1):42-49
Computer-modelling techniques are applied to the calculation of defect formation and migration energies in α-Fe2 O3 and α-Cr2 O3 : both electronic and lattice defects are considered. The results are used to predict Arrhenius energies for cation and anion migration in different composition and temperature regimes and show reasonable agreement with experimental data where these are available. 相似文献
6.
WILLIAM C. HAGEL P. J. JORGENSEN D. S. TOMALIN 《Journal of the American Ceramic Society》1966,49(1):23-26
Since the difference between oxygen-ion and cation diffusion coefficients is greater for α-Cr2 O3 than for α-Fe2 O3 or α-Al2 O3 , a study of initial-sintering kinetics was undertaken to show unequivocally which species is rate controlling. Fine powders of α-Cr2 O3 , obtained by thermal decomposition of reagent-grade (NH4 )2 Cr2 O7 , were lightly compacted and their isothermal rates of shrinkage were determined between 1050° and 1300°C. Resultant data follow volume-diffusion sintering models, and calculated diffusion coefficients agree with, those measured for oxygen ions in α-Cr2 O3 . There is little evidence that oxygen diffusion along grain boundaries becomes so enhanced that chromium ions are left in control of the process. 相似文献
7.
WILLIAM C. HAGEL 《Journal of the American Ceramic Society》1965,48(2):70-75
Oxygen-18 exchange between gaseous oxygen, held at a pressure of 125 mm Hg in a Pt10Rh chamber, and spheres of α-Cr2 O3 containing three or less grains was determined from 1100° to 1450°C. Isotope equilibrium on crystal surfaces appears to be quickly established, and the rate-determining factor is self-diffusion conforming to the relation D = 15.9 exp(-101,000/ RT) cm2 sec−1 . Changing sphere diameters caused no detectable variation in diffusion coefficients. Anions are the much slower diffusing species in this oxide. 相似文献
8.
Rabindra N. Das Amit Bandyopadhyay Susmita Bose 《Journal of the American Ceramic Society》2001,84(10):2421-2423
Nanocrystalline α-Al2 O3 ceramic powders have been prepared from an aqueous solution of aluminum nitrate and sucrose. Soluble Al ion-sucrose solution forms the precursor material once it is completely dehydrated. Heat treatment of the dehydrated precursors at low temperature (600°C) results in the formation of porous single-phase α-Al2 O3 . The precursor and heat-treated powders have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and BET surface area analysis. The phase-pure nanocrystalline α-Al2 O3 particles had an average specific surface area of >190 m2 /g, with an average pore size between 18 and 25 nm. 相似文献
9.
Refractory Y-α-SiAlON with elongated grain morphology was obtained by utilizing La2 O3 as a densification aid, which resulted in excellent room-temperature and high-temperature strength. Room-temperature strength of 1000 MPa was achieved when La2 O3 was augmented by adding Y2 O3 or removing AlN. With only La2 O3 , a temperature-independent strength of 800–950 MPa was maintained up to 1100°C, then gradually decreasing by 25% when reaching 1300°C. The R-curve measurements of fracture toughness showed relatively little dependence on microstructure, consistent with a strong interface that suppresses grain boundary decohesion. Compared with other densification aids such as SiO2 , Al2 O3 , Sc2 O3 , Y2 O3 , and Lu2 O3 , a finer microstructure was obtained by using La2 O3 . High nitrogen content in the residual La–Si–Al–O–N glass in equilibrium with the nitrogen-rich α-SiAlON is suggested to be the cause of these findings. 相似文献
10.
Polycrystalline AI2 O3 containing a glassy aluminosilicate second phase displays a low-activation-energy conductivity when Na is present. The conductivity is ionic in nature at high oxygen pressure and electronic at low oxygen pressure and is attributed to the migration of Na+ ions and electrons through the second phase present at triple junctions. A smaller low-activation-energy conductivity is found in the absence of sodium, but in that case the low-activation-energy branch has different properties. AI2 O3 :Na without silicon bas no low-temperature low-activation-energy branch. 相似文献
11.
Martine Gautier Gilles Fenaud Laurent Pham Van Bruno Villette Maud Pollak Nathalie Thromat François Jollet Jean-Paul Duraud 《Journal of the American Ceramic Society》1994,77(2):323-334
New results on the R ± 9° reconstructed α-Al2 O3 (0001) surface, which can be obtained after heating at high temperature (1400°C) under vacuum, are presented. The atomic structure has been studied by combining low-energy electron diffractometry and grazing incidence X-ray scattering. The surface structure is found to be perfectly commensurable with the underlying bulk lattice. The surface consists of hexagonal zones of two, nearly perfect, close-packed Al (111) planes separated by a defect of hexagonal periodicity with a parameter of 26.44 Å. This model is consistent with previous surface studies of this reconstruction. The electronic structure has been investigated using valence band photoemission spectroscopy, X-ray absorption spectroscopy at the O K edge, electron energy loss spectroscopy, and X-ray-induced Auger electron spectroscopy. Interpretation of these experimental data in the frame of a self-consistent, tight-binding calculation leads to the conclusion that the R ± 9° reconstructed surface is more covalent than the (1 × 1) surface. Significant changes in the; Al-O hybridizations are observed; these are likely due to a difference in the interatomic distances along the [0001] axis (relaxations). The increase of covalent character is mainly due to a strong decrease of the Madelung field on the reconstructed surface. 相似文献
12.
The effect of Cr and Fe in solid solution in γ-Al2 O3 on its rate of conversion to α-Al2 O3 at 1100°C was studied by X-ray diffraction. The δ form of Al2 O3 was the principal intermediate phase produced from both pure γ-Al2 O3 and that containing Fe3+ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr6+ in γ-Al2 O3 and as Cr3+ in α-Al2 O3 , with θ-Al2 O3 as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al2 O3 were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α-Al2 O3 formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ- or θ-Al2 O3 , within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O2- ions by synchro-shear. 相似文献
13.
MASARU MIYAYAMA SHINICHI KATSUTA YOSHIHIRO SUENAGA HIROAKI YANAGIDA 《Journal of the American Ceramic Society》1983,66(8):585-588
Electrical conduction in tetragonal β-Bi2 O3 doped with Sb2 O3 was investigated by measuring electrical conductivity, ionic transference number, and Seebeck coefficient. The β-Bi2 O3 doped with 1 to 10 mol% Sb2 O3 was stable up to 600°C and showed an oxygen ionic and electronic mixed conduction, where the electron conduction was predominant at low oxygen pressures. The oxygen-ion conductivity showed a maximum at 4 mol% Sb2 O3 , whereas the activation energy for the ionic conduction remained unchanged for 4 to 10 mol% Sb2 O3 -doped specimens. These results were interpreted in terms of the oxygen vacancy concentration and the distortion of the tetragonal structure. The electron conductivity and its oxygen pressure dependence decreased with increasing Sb2 O3 content. The fact that Sb5+ is partially reduced by excess electrons in heavily doped β specimens at low oxygen pressures is explained. 相似文献
14.
Supersaturated polycrystals of solid-solution spinel were heat-treated below the solvus temperature. Precipitation of α-alumina was examined by X-ray diffraction, microprobe, and scanning transmission electron microscopy. Precipitation kinetics followed classical time-temperature-transformation behavior: the precipitation rate was growth controlled at large undercoolings and nucleation controlled at low undercoolings. Scanning transmission electron microscopy and microprobe analysis revealed that the growth of precipitates was limited by interface reaction. Nucleation occurred throughout the polycrystal and was probably promoted by the grain boundaries. Precipitation was accompanied by the growth of pores at interfaces, presumably to accommodate the change in volume required by the phase transformation. Our observations in polycrystals contrast sharply with single-crystal work, where it has been reported that nucleation occurs at the free surface and that the formation of α-alumina is preceded by metastable phases. 相似文献
15.
Direct current conductivity was measured for polycrystalline Alz03 doped with silicon, which is found to act as a single donor, the donor level lying ∼165 kJ/mol (∼1.7 eV) below the conduction band. Silicon in excess of the solubility limit (∼220 ppm at 1500°C, 300 ppm at 1600°C) is present as a glassy aluminosilicate second phase. Silicon dissolved in Al2 O3 tends to segregate at grain boundaries. 相似文献
16.
Oxygen diffusion coefficients have been determined for polycrystalline samples of NiCr2 O4 and α-Fe2 O3 by exchange measurements with oxygen gas containing the stable isotope18 O, using mass spectrometer analysis. Oxygen diffusion in NiCr2 O4 can be represented by the equation D = 0.017 exp (-65,400/RT); oxygen diffusion in α-Fe2 O3 can be represented by the equation D = 1 × 1011 exp (-146,000/RT). The large difference between D0 and activation energy for these materials suggests that different diffusion mechanisms are involved. 相似文献
17.
Koji Tsukuma 《Journal of the American Ceramic Society》2000,83(12):3219-3221
Composites of β-Ce2 O3 ·11Al2 O3 and tetragonal ZrO2 were fabricated by a reductive atmosphere sintering of mixed powders of CeO2 , ZrO2 (2 mol% Y2 O3 ), and Al2 O3 . The composites had microstructures composed of elongated grains of β-Ce2 O3 ·11Al2 O3 in a Y-TZP matrix. The β-Ce2 O3 ·11Al2 O3 decomposed to α-Al2 O3 and CeO2 by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce2 O3 ·11Al2 O3 divided into several grains of α-Al2 O3 and ZrO2 doped with Y2 O3 and CeO2 . High-temperature bending strength of the oxygen-annealed α-Al2 O3 composite was comparable to the β-Ce2 O3 ·11Al2 O3 composite before annealing. 相似文献
18.
Pei-Ching Yu Rung-Je Yang Ya-Ting Chang Fu-Su Yen 《Journal of the American Ceramic Society》2007,90(8):2340-2346
The possibility of eliminating finger or vermicular growth of α-Al2 O3 particles obtained by calcination of boehmite was examined. Heterogeneous precipitation of boehmite in a well-dispersed θ-Al2 O3 suspension was first prepared, in which the mass ratio of boehmite to θ-crystallite was evaluated to form agglomerates of similar sizes that will form α-Al2 O3 crystallites of <100 nm in diameter. θ- to α-phase transformation of alumina experiences a nucleation and growth mechanism, with the critical size of nucleation being ∼25 nm for θ-Al2 O3 and the size for accomplishment of transformation followed by finger growth being ∼100 nm. Hence, fabricating agglomerates that would form α-Al2 O3 crystallites with sizes <100 nm accompanied with appropriate thermal treatments can be a method for obtaining α-Al2 O3 crystallites free of finger growth. It is found that proper preparation of the agglomerate with appropriate size may initiate a simultaneous and lower temperature θ- to α-Al2 O3 phase transformation for such powder systems, substantially limiting the mass transfer among the newly formed α-Al2 O3 particles. Moreover, α-Al2 O3 crystallites free of finger growth can be obtained. 相似文献
19.
Oxygen-diffusion coefficients were determined in single crystals of MgO and α-Fe2 O3 by exchanging the samples with 18 O enriched gas at 1 atm and measuring 18 O profiles using a proton activation technique. For MgO, in the temperature range 1580 to 1820 K, the diffusion coefficient is represented by: 相似文献
20.
Synthesis of monodispersed nanophase α-Fe2 O3 (hematite) powder to be used as a red pigment in porcelains was investigated using microwave-hydrothermal and conventional-hydrothermal reactions using 0.018 M FeCl3 ·6H2 O and 0.01 M HCl solutions at 100°–160°C. Acicular and yellow β-FeOOH (akaganite) particles 300 nm in length and 40 nm in thickness were dominantly formed at 100°C after 2–3 h, while spherical α-Fe2 O3 particles 100–180 nm in diameter were preferentially formed after 13 h using a conventional-hydrothermal reaction. However, a microwave-hydrothermal reaction at 100°C led to monodispersed and red α-Fe2 O3 particles 30–66 nm in diameter after 2 h without the formation of β-FeOOH particles. In this paper, the effect of microwave radiation during hydrothermal treatment at 100°–160°C on the formation yield, kinetics, morphology, phase type, and color of α-Fe2 O3 was investigated. 相似文献