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1.
Srinivasan Raghavan Hsin Wang Ralph B. Dinwiddie Wallace D. Porter Robert Vaen Detlev Stöver Merrilea J. Mayo 《Journal of the American Ceramic Society》2004,87(3):431-437
Zirconia doped with 3.2–4.2 mol% (6–8 wt%) yttria (3–4YSZ) is currently the material of choice for thermal barrier coating topcoats. The present study examines the ZrO2 -Y2 O3 -Ta2 O5 /Nb2 O5 systems for potential alternative chemistries that would overcome the limitations of the 3–4YSZ. A rationale for choosing specific compositions based on the effect of defect chemistry on the thermal conductivity and phase stability in zirconia-based systems is presented. The results show that it is possible to produce stable (for up to 200 h at 1000°–1500°C), single (tetragonal) or dual (tetragonal + cubic) phase chemistries that have thermal conductivity that is as low (1.8–2.8W/m K) as the 3–4YSZ, a wide range of elastic moduli (150–232 GPa), and a similar mean coefficient of thermal expansion at 1000°C. The chemistries can be plasma sprayed without change in composition or deleterious effects to phase stability. Preliminary burner rig testing results on one of the compositions are also presented. 相似文献
2.
Jin-Ho Lee Masatomo Yashima Masato Kakihana Masahiro Yoshimura 《Journal of the American Ceramic Society》1998,81(4):894-900
The phase equilibria in the Y2 O3 -Nb2 O5 system have been studied at temperatures of 1500° and 1700°C in the compositional region of 0-50 mol% Nb2 O5 . The solubility limits of the C-type Y2 O3 cubic phase and the YNbO4 monoclinic phase are 2.5 (±1.0) mol% Nb2 O5 and 0.2 (±0.4) mol% Y2 O3 , respectively, at 1700°C. The fluorite (F) single phase exists in the region of 20.1-27.7 mol% Nb2 O5 at 1700°C, and in the region of 21.1-27.0 mol% Nb2 O5 at 1500°C, respectively. Conductivity of the Y2 O3 - x mol% Nb2 O5 system increases as the value of x increases, to a maximum at x = 20 in the compositional region of 0 ≤ x ≤ 20, as a result of the increase in the fraction of F phase. In the F single-phase region, the conductivity decreases in the region of 20-25 mol% Nb2 O5 , because of the decrease in the content of oxygen vacancies, whereas the conductivity at x = 27 is larger than that at x = 25. The conductivity decreases as the value of x increases in the region of 27.5 ≤ x ≤ 50, because of the decrease in the fraction of F. The 20 mol% Nb2 O5 sample exhibits the highest conductivity and a very wide range of ionic domain, at least up to log p O 2 =−20 (where p O 2 is given in units of atm), which indicates practical usefulness as an ionic conductor. 相似文献
3.
Phase equilibria have been determined in the system CaO-Al2 O3 -H2 O in the temperature range 100° to 1000°C. under water pressures of up to 3000 atmospheres. Only three hydrated phases are formed stably in the system: Ca(OH)2 , 3CaO·Al2 O3 ·6H2 O, and 4CaO·3Al2 O3 -3H2 O. Pressure-temperature curves delineating the equilibrium decomposition of each of these phases have been determined, and some ther-mochemical data have been deduced therefrom. It has been established that both the compounds CaO·Al2 O3 and 3CaO·Al2 O3 have a minimum temperature of stability which is above 1000°C. The relevance of the new data to some aspects of cement chemistry is discussed. 相似文献
4.
α-Al2 O3 -doped (8 mol % Sc2 O3 )ZrO2 composite solid electrolyte has been investigated in the fabrication of solid-state ceramic gas sensors. The microstructure and electrical conductivity of the composite solid electrolyte have been measured over a range of temperature from 240°C to 596°C. The composite solid electrolyte has been found to exhibit a higher conductivity compared with the commonly used (8 mol% Y2 O3 )ZrO2 at temperatures above ∼448°C. The sensing characteristics for NO2 detection have been studied in the temperature range of 500–650°C at the low concentration from 10 to 30 ppm and at high concentration from 100 to 500 ppm of NO2 . The NO2 sensor was found to respond reproducibly and rapidly to the variations of NO2 , concentration, indicating that the composite solid electrolyte has promising application as a solid electrolyte for on-board exhaust gas monitoring. 相似文献
5.
Doreen D. Edwards Pollyanna E. Folkins Thomas O. Mason 《Journal of the American Ceramic Society》1997,80(1):253-257
Subsolidus phase relationships in the Ga2 O3 –In2 O3 system were studied by X-ray diffraction and electron probe microanalysis (EPMA) for the temperature range of 800°–1400°C. The solubility limit of In2 O3 in the β-gallia structure decreases with increasing temperature from 44.1 ± 0.5 mol% at 1000°C to 41.4 ± 0.5 mol% at 1400°C. The solubility limit of Ga2 O3 in cubic In2 O3 increases with temperature from 4.X ± 0.5 mol% at 1000°C to 10.0 ± 0.5 mol% at 1400°C. The previously reported transparent conducting oxide phase in the Ga-In-O system cannot be GaInO3 , which is not stable, but is likely the In-doped β-Ga2 O3 solid solution. 相似文献
6.
Masanori Hirano Shinzi Watanabe Etsuro Kato Yasunobu Mizutani Masayuki Kawai Yasuhisa Nakamura 《Journal of the American Ceramic Society》1999,82(10):2861-2864
The microstructure, crystal phase, electrical conductivity, and mechanical strength of less than 7-mol%-Sc2 O3 -doped zirconia ceramics fabricated by comparatively low-temperature sintering at 1200–1300°C for 1 h were investigated. Zirconia ceramics having a uniform microstructure (grain size < 0.5 μm) stabilized with 6 mol% Sc2 O3 showed high electrical conductivity (0.15 S/cm at 1000°C) and high fracture strength (660 MPa). With the increase of Sc2 O3 content from 3.5 to 7 mol%, the grain size, fracture strength, and electrical conductivity at 1000°C changed from 0.2 to 0.5 μm, 970 to 440 MPa, and 0.07 to >0.2 S/cm, respectively. Sc2 O3 -doped zirconia polycrystals with high fracture strength and high electrical conductivity are promising candidates for the electrolyte material of solid oxide fuel cells. 相似文献
7.
The electrical conductivity of partially stabilized zirconia was investigated as a function of frequency (2 Hz to 100 kHz) and temperature (400° to 1000°C) by measuring ac admittance on a 2-probe cell using Lissajous figures. The dependence of the conductivity on annealing time was investigated with in situ conductivity measurements for prolonged annealing at 800°, 900°, and 1000°C. The aging behavior of two commercial zirconias was studied. At higher temperatures (7<750°C) the decrease in total conductivity arises mainly from changes within the grains, whereas at lower temperatures the decrease arises principally from the grain boundaries. Differences in initial conductivity and aging rates between samples from the two sources were related to grain size and impurity effects. 相似文献
8.
The electrical conductivity of polycrystalline TiTa2 O7 was measured at 1000° and 1050°C as a function of oxygen partial pressure from 10−1 to 10−21 MPa. In the apparent n-type region, the value of m in σn ∝PO2 −1/m was found to be ∼6 in the region >10−17 MPa and ∼4from 10−16 to 10−9 MPa. The conductivity appeared to be p-type for P02 <10−5 MPa. The measured data are explained on the basis of the presence of small amounts of acceptor impurities in the undoped sample. 相似文献
9.
Dan S. Perera Martin W. A. Stewart Huijun Li R. Arthur Day Eric R. Vance 《Journal of the American Ceramic Society》2002,85(12):2919-2924
The phase relationships in the CaHfTi2 O7 -Gd2 Ti2 O7 (zirconolite-pyrochlore) pseudobinary system were investigated, after heating at 1500°C, because of their importance in the design of pyrochlore-rich titanate ceramics for immobilization of impure surplus plutonium. Up to 15 mol% of MgTi2 O5 and Al2 TiO5 were added to CaHfTi2 O7 -Gd2 Ti2 O7 compositions to elucidate the effects of divalent and trivalent impurities on the phase stability within these systems. From X-ray diffractometry analysis, scanning electron microscopy, and energy dispersive X-ray spectrometry, phase formation and compositional stability limits were evaluated. The main phases observed in these systems were pyrochlore, perovskite, and polytypes of zirconolite. The formation of the 2 M -, 4 M -, and 3 O -zirconolite polytypes was dependent on the amount of aluminum or magnesium present. In the magnesium system, a large area of pyrochlore-only was observed, which indicated that divalent impurities of appropriate ionic size could be readily incorporated in the eightfold site of the pyrochlore. The locations of the tentative phase boundaries are discussed with respect to the chemical composition. 相似文献
10.
Alessandro Martucci Sabrina Sartori Alessandro Muffato Lidia Armelao Elisabetta Di Bartolomeo Enrico Traversa Massimo Guglielmi 《Journal of the American Ceramic Society》2003,86(11):1965-1968
Na2 O· x Al2 O3 ( x = 9, 11)films have been obtained by sol–gel method. Crystallization processes during heat treatments have been investigated by X–ray diffraction analysis. A metastable phase with the mullite structure, λ–Na2 O· x Al2 O3 , has been observed starting from 800°C. Films remained stable after a heat treatment at 1000°C for 250 h. Impedance spectroscopy measurements showed that the films of λ-Na2 O· x Al2 O3 possess a large three–dimensional ionic conductivity at 400°C. 相似文献
11.
The dependence of the degree of nonstoichiometry of YBa2 Cu3 07–x (123) on temperature and oxygen pressure has been determined by thermogravimetric analysis (TGA) in the temperature range 400° to 950°C and the oxygen pressure range 10–6 to 1 atm (1 atm = 105 Pa). The nature of the decomposition of 123 in the temperature range 750° to 950°C and the oxygen pressure range 10–6 to 10–2 atm has been determined by TGA and X-ray diffractometry (XRD). As the oxygen pressure decreases, the decomposition of 123 follows the sequence 123→ Y2 BaCuo5 (211) + BaCuO2 ° Cu2 O→ 211 ° BaCuO2 ° BaCu2 O2 → 211 ° YBa3 Cu2 Ox (132) ° BaCu2 O2 → 211 ° BaCu2 O2 °BaO. The incongruent melting temperatures have been determined in the oxygen pressure range 10–6 to 1 atm by differential thermal analysis, and the phases formed on solidification have been identified by XRD. The stability diagram for the composition 123 has been constructed. 相似文献
12.
The impedance of the cubic perovskite BaZr0.9 Y0.1 O3-δ has been systematically investigated in dry and wet atmospheres at high and low oxygen partial pressures. In the grain interior, conductivity contributions from oxygen ions, electron holes, and protons can be identified. Below 300°C, proton conduction dominates and increases linearly with the frozen-in proton concentration. The proton mobility, with an activation energy of 0.44 ± 0.01 eV is among the highest ever reported for a perovskite-type oxide proton conductor. For dry oxygen atmos-pheres, electron hole conduction dominates with an activation energy of ∼0.9 eV. At temperatures <500°C, the grain-boundary conductivity can be separated and increases upon incorporation of protons. The high electrical conductivity and chemical stability make acceptor-doped barium zirconate a good choice for application as a high-temperature proton conductor. 相似文献
13.
G. Bhattacharya S. Zhang D. D. Jayaseelan W. E. Lee 《Journal of the American Ceramic Society》2007,90(1):97-106
Lithium borate (Li2 B4 O7 ) and sodium borate (Na2 B4 O7 ) mineralize spinel formation from stoichiometric MgO and Al2 O3 between 1000° and 1100°C. Mineralization with both compounds is shown to be mediated by B-containing liquids which form glass on cooling. However, the liquid compositions depend on the type of mineralizer and temperature, suggesting that templated grain growth or dissolution–precipitation mechanisms are operating, one dominating over the other under certain conditions. Na2 B4 O7 -mineralized compositions show predominantly templated grain growth at 1000°C, which changes to dissolution–precipitation at 1100°C, whereas Li2 B4 O7 -mineralized compositions show dissolution–precipitation from 1000°C. Li2 B4 O7 is a stronger mineralizer as spinel formation is complete with 3 wt% Li2 B4 O7 at 1000°C and with ≥1.5 wt% addition at 1100°C, whereas Na2 B4 O7 -mineralized compositions are found to retain some unreacted corundum even at 1100°C. 相似文献
14.
Electrical conductivity, thermoelectric power, and weight change were measured for polycrystalline Ta2 O5 from 900° to 1400°C. The predominant ionic and electronic defects in this temperature range are oxygen vacancies and electrons. The oxygen-vacancy and electron mobilities are 8.1 × 103 exp (−1.8 eV/ k T) and ∼0.05 cm2 /V-s, respectively. At O2 partial pressures near 1 atm, the ionic-defect concentration is essentially fixed by the presence of lower-valence cation impurities, and the total electrical conductivity is predominantly ionic, whereas at low P o2 's the conductivity is electronic and proportional to P P o2 −1/6 . 相似文献
15.
BaTi2 O5 (BT2 ) is thermodynamically stable over a very narrow temperature range between 1220° and 1230°C: a modification to the BaO–TiO2 phase diagram is proposed. This thermodynamic stability was shown by constructing a time–temperature transformation diagram for the decomposition of BT2 . Once formed, BT2 appears to be stable indefinitely at 1220°–1230°C; at higher temperatures, the decomposition rate increases with temperature; at lower temperatures, the decomposition rate increases with decreasing temperature and passes through a maximum at ∼1200°C; below ∼1150°C, BT2 has long-lived kinetic stability. Kinetic considerations show a nucleation and growth mechanism for decomposition, with a nucleation induction period that is very temperature dependent. BT2 can be prepared by various routes, including solid-state reaction of oxides below ∼1100°C; because it is metastable at all temperatures other than 1220°–1230°C, its formation is an example of Ostwald's rule of successive reactions. Discrepancies in the literature concerning the reported stability range of BT2 can be explained by the complex dependence on temperature and time of both its formation and decomposition, for both of which, the nucleation stage is rate limiting. 相似文献
16.
The electrical conductivity and ion/electron transference numbers in Al3 O3 were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O2 partial pressure over single-crystal Al2 O3 at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O2 (1 atm) the conductivity above 1400°C was σ≃3×103 exp (–80 kcal/ RT ) Ω−1 cm−1 , which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al2 O3 and MgO-doped Al2 O3 showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O2 partial pressure (below 10−10 atm). 相似文献
17.
C. C. Khaw C. K. Lee Z. Zainal G. C. Miles A. R. West 《Journal of the American Ceramic Society》2007,90(9):2900-2904
The subsolidus phase diagram of the system Bi2 O3 –ZnO–Ta2 O5 in the region of the cubic pyrochlore phase has been determined at 1050°C. This phase forms a solid solution area that includes the ideal composition P, Bi3 Zn2 Ta3 O14 ; possible solid solution mechanisms are proposed, supported by density measurements of Zn-deficient solid solutions. The general formula of the solid solutions is Bi3+ y Zn2− x Ta3− y O14− x − y , based on the creation of Zn2+ , O2− vacancies in Zn-deficient compositions and a variable Bi/Ta ratio. 相似文献
18.
La2 Ti2 O7 powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements. 相似文献
19.
Toshiyuki Mori Hiroshi Yamamura Syoshichi Saito 《Journal of the American Ceramic Society》1996,79(12):3309-3312
The ionic conductivity of the ceria-samaria (CeO2 -Sm2 O3 ) system is higher than that of yttria-stabilized zirconia and other CeO2 -based oxides. In this study, a small amount of alkali-element-doped CeO2 -Sm2 O3 solid solution was prepared. This solid solution was characterized by measuring the powder density and the chemical composition. Moreover, its electrochemical properties were investigated in the temperature range from 700° to 1000°C. It was found that a small amount of alkali-element-doped CeO2 solid solution enhanced the ionic conductivity. The power density of an oxygen-hydrogen fuel cell for alkali-element-doped CeO2 -Sm2 O3 ceramics exhibited high values at low temperatures such as 700° to 800°C. It is concluded that the improved fuel cell performance can be attributed to the high stability of this composition in the fuel atmosphere. 相似文献
20.
The electrical conductivity of polycrystalline Y2 O3 has been studied as a function of the partial pressure of oxygen (10–14 to 105 Pa) at 900° to 1500°C in atmospheres saturated with water vapor at 12°C or dried with P2 O5 . Yttria is a p -conductor at high oxygen activities. The p -conductivity increases with increasing P O2 and decreases with increasing PH2O . At low oxygen activities the oxide is a mixed ionic/electronic conductor. The ionic conductivity is approximately independent of P O2 and increases with increasing P H2O . In the Y2 O3 samples, excesses of lower-valent cation impurities (in the 10 to 100 mol-ppm range) are the dominating negatively charged defects, and in the presence of water vapor they are compensated by interstitial protons. At high P H2O levels additional protons are probably compensated by interstitial oxygen ions. At high temperatures (±1100°C) and for high P O2 and low P H2O , the protons are no longer dominant, and the lower-valent cations are mainly compensated by electron holes. The electrical conductivity exhibits hysteresis-like effects which are interpreted in terms of segregation/desegregation of impurities at grain boundaries. The mobility of electron holes in yttria at 1500°C is estimated to be of the order of magnitude of 0.05 cm2 . s–1 . V–1 相似文献