共查询到20条相似文献,搜索用时 31 毫秒
1.
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
2.
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
3.
Using a tracer sectioning technique, the self-diffusion of Er in pure and HfO2 -doped polycrystalline Er2 O3 was measured at 1614° to 1900°C. Up to ≊ 10 mol% HfO2 dopant level, the Er self-diffusion coefficients followed a relation based on cation vacancies as the principal mobile defects present and available for cation diffusion. Above 10 mol% HfO2 , deviation from this relation occurred, apparently due to clustering of cation vacancies and oxygen interstitials around the dopant hafnium ions. The activation energy for the self-diffusion of Er in pure Er2 O3 was 82.2 kcal/mol and increased with the HfO2 dopant level present. 相似文献
4.
OLAF MULLER RUSTUM ROY WILLIAM B. WHITE 《Journal of the American Ceramic Society》1968,51(12):693-699
Phase equilibria were determined for the systems NiO-Cr2 O3 −O2 , MgO-Cr2 O3 ,-O2 , and CdO-Cr2 O3 −O2 from 450° to above 850° C and at oxygen pressures of from 2 to 3500 atm. Only two intermediate phases were found in the nickel system: NiCrO., (CrVO4 structure) and the spinel NiCr2 O4 . The magnesium and cadmium systems are similar in that they have three analogous phases: the low-temperature α-MgCrO4 and α-CdCrO4 (both with the CrVO4 structure), the high-temperature β-MgCrO4 and β-CdCrO4 (both with the α-MnMoO4 structure), and the spinels MgCr2 O4 and CdCr2 O4 . The cadmium system contains an additional phase, Cd2 CrO5 , which is primitive monoclinic. 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.
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
7.
P. C. HAYES 《Journal of the American Ceramic Society》1980,63(7-8):387-391
The sintering kinetics of submicrometer Fe3 O4 and Fe2 O3 powders were investigated at 300° to 500°C. Using measurements of the rate of reduction of surface area, the coefficients of surface diffusion on the oxides are estimated for a range of oxygen partial pressures. The surface-diffusion coefficients appear to be independent of P O2 for magnetite and only slightly dependent on P O2 for hematite. 相似文献
8.
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
9.
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
10.
Thommy Ekström Harald Herbertsson Matthew James Ian Fleck 《Journal of the American Ceramic Society》1994,77(12):3087-3092
β-sialon and Nd2 O3 -doped α-sialon materials of varying composition were prepared by sintering at 1775° and 1825°C and by glass-encapsulated hot isostatic pressing at 1700°C. Composites were also prepared by adding 2–20 wt% ZrO2 (3 mol% Nd2 O3 ) or 2–20 wt% ZrN to the β-sialon and α-sialon matrix, respectively. Neodymium was found to be a fairly poor α-sialon stabilizer even within the α-phase solid solution area, and addition of ZrN further inhibited the formation of the α-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO2 (Nd2 O3 ) or ZrN were seen in both the HIPed β-sialon/ZrO2 (Nd2 O3 ) composites and the HIPed Nd2 O3 -stabiIized α-sialons with ZrN additions. 相似文献
11.
Thermal reactions in 93% Al2 O3 -7% MgO and 95.8% Al2 O3 -4.2% MgO gels seeded with α-Al2 O3 , MgAl2 O4 , α-Fe2 O3 , and SiO2 , sols were investigated by differential thermal analysis to determine the extent of nucleation catalysis of solid-state reactions. Seeding with α-Al2 O3 lowered the α-Al2 O3 crystallization temperature in these xerogels by 100° to 150°C. Spinel seeds have much less effect on the γ-α transition, and α-Fe2 O3 and SiO2 seeds do not affect it significantly. Isostructural seeding of gels may therefore permit lower ceramic processing temperatures. 相似文献
12.
HIROSHI HIRASHIMA DAIKICHI ARAI TETSURO YOSHIDA 《Journal of the American Ceramic Society》1985,68(9):486-489
The dc conductivities (α) of PbO-P2 O5 -V2 O5 glasses containing up to 80 mol% V2 O5 were measured at T = 100°C to T = 10°C below the glass transition temperature. Dielectric constants at 1 MHz, densities, and the fraction of reduced V ion were measured at room temperature. The conduction mechanism of glasses containing >10 mol% V2 O5 was considered to be small-polaron hopping, as previously reported for other vanadate glasses. The temperature dependence of α was exponential, with α= (αo/ T ) exp(− W/kT ). When the V2 O5 content was ≥50 mol%, W decreased and α increased with increasing V2 O5 content, and the adiabatic approximation could be applied. In the composition range between 10 and 50 mol% V2 O5 , α increased with increasing V2 O5 content, but W varied little. In this region, the hopping conduction was characterized as nonadiabatic. The effect of dielectric constants and V ion spacing on W is discussed. 相似文献
13.
Jana Bezjak Aleksander Renik Botjan Janar Philippe Boullay Ivana Radosavljevi Evans Danilo Suvorov 《Journal of the American Ceramic Society》2009,92(8):1806-1812
Polymorphic phase transitions in Ba4 Nb2 O9 were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba4 Nb2 O9 results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba4 Nb2 O9 becomes moderately sensitive to a loss of BaO. In air the surface of Ba4 Nb2 O9 grains decomposes to nanocrystalline Ba5 Nb4 O15 and BaO, which instantly reacts with atmospheric CO2 to form BaCO3 . Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba3 Nb2 O8 phase is observed above 1150°C. 相似文献
14.
K. Peter D. Lagerlof Terence E. Mitchell Arthur H. Heuer 《Journal of the American Ceramic Society》1989,72(11):2159-2171
The shrinkage rates of small prismatic dislocation loops in thin foils of sapphire (α-Al2 O3 ) have been determined using transmission electron microscopy; between observations the thin foils were annealed ex situ in the temperature range 1200° < T < 1500°C. The shrinkage rates of individual loops were used to determine the diffusivity of the rate-controlling species, assumed to be oxygen. The loop annealing results agree well with an extrapolation of oxygen self-diffusion data for undoped crystals obtained by conventional tracer techniques in the temperature range 1400° < T < 1900°C. The oxygen diffusion rate was slower in 600-ppm-Ti-doped and faster in 250-ppm-Mg-doped crystals compared to undoped sapphire, consistent at first sight with a "classical" picture of diffusion via oxygen vacancies in α-Al2 O3 . However, consideration of the experimental activation energies in terms of the concentration of free point defects suggests that substantial modification of the classical picture may be necessary. 相似文献
15.
Subsolidus equilibrium relations in a portion of the system Li2O-Fe2 O3 -Al2 O3 in the temperature range 500° to 1400°C. have been determined near po2 = 0.21. Of particular interest in this system is the LiFe5 O8 -LiAl5 O8 join, which shows complete solid solution above 1180°C. Below this temperature the solid solution exsolves into two spinel phases. At 600°C. approximately 15 mole % of each compound is soluble in the other. The high-temperature solid solution and the low-temperature exsolution dome extend into the ternary system from the 1:5 join. There is no appreciable crystalline solubility of LiFeO2 or of α-Fe2 O3 in LiFe5 O8 . An attempt to confirm HFe5 O8 as the correct formulation of the magnetic ferric oxide "γ-Fe2 O3 " was inconclusive, but in the absence of positive evidence, the retention of γ-Fe2 O3 is recommended. All the metallic oxides of the Group IV elements increase the temperature of the monotropic conversion of -γ-Fe2 O3 to α-Fe2 O3 . Silica and thoria have a greater effect on this conversion than does titania or zirconia. 相似文献
16.
Mark I. Pownceby Keri K. Constanti-Carey Michael J. Fisher-White 《Journal of the American Ceramic Society》2003,86(6):975-980
Subsolidus phase equilibria in the system Fe2 O3 –Al2 O3 –TiO2 were investigated between 1000° and 1300°C. Quenched samples were examined using powder X-ray diffraction and electron probe microanalytical methods. The main features of the phase relations were: (a) the presence of an M3 O5 solid solution series between end members Fe2 TiO5 and Al2 TiO5 , (b) a miscibility gap along the Fe2 O3 –Al2 O3 binary, (c) an α-M2 O3 ( ss ) ternary solid-solution region based on mutual solubility between Fe2 O3 , Al2 O3 , and TiO2 , and (d) an extensive three-phase region characterized by the assemblage M3 O5 +α-M2 O3 ( ss ) + Cor( ss ). A comparison of results with previously established phase relations for the Fe2 O3 –Al2 O3 –TiO2 system shows considerable discrepancy. 相似文献
17.
A tracer sectioning technique was used to measure cation self-diffusion coefficients in fully dense polycrystalline Ya O3 and Er2 Os under oxidizing conditions. The results are described by the relations for Y2 O3 (1400° to 1670°C), and for Er2 O3 (1400° to 1700°C). The greater activation energy for erbium diffusion in erbia may be partly attributable to a mass effect. 相似文献
18.
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
19.
Priyadarshi G. Desai Zhengkui Xu Jennifer A. Lewis 《Journal of the American Ceramic Society》1995,78(11):2881-2888
Novel microcomposite powders, consisting of inert cores (αAL-Al2 O3 ) surrounded by reactive cement-based coatings (CaAl2 O4 ), were synthesized by a modified Pechini process. The evolution of the crystalline CaAl2 O4 phase during calcination was studied using multiple analytical techniques, including DRIFTS,13 C and 27 AlMAS FT-NMR, and XRD, for both pure CaAl2 O4 and CaAl2 O4 -coated Al2 O3 precursor powders. In both powders, decomposition proceeded via hydrocarbon chain scission and removal of ester groups at low temperatures ( T < 450°C), followed by the formation of inorganic carbonates at higher temperatures ( T > 450°C). These decomposition processes were accelerated by the underlying Al2 O3 cores. Transmission electron microscopy (TEM) of the fully calcined powders showed that the inert αAL-Al2 O3 particles were surrounded by relatively uniform CaAl2 O4 coatings ranging in thickness from approximately 10 to 100 nm. 相似文献
20.
Studies of the oxidation of Gd and Dy at P O2 's from 10−0.3 to 10−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P 02 semiconducting domain yielded oxygen diffusion coefficients in Dy2 O3 in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd2 O3 which are considerably below literature values for monoclinic single-crystal Gd2 O3 . At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P O2 , the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10−6 to 10−7 Ω−1 cm−1 . 相似文献