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1.
Prediction of Rare-Earth A2 Hf2 O7 Pyrochlore Phases 总被引:1,自引:1,他引:0
Within the series of A2 O3 -HfO2 phase diagrams (A ranges in cationic radius from Lu3+ to La3+ ), only those from Gd2 Hf2 O7 to La2 Hf2 O7 are known to exhibit a pyrochlore phase field. For A cations smaller than Gd3+ , only a disordered fluorite phase field exists. Disorder energies for the entire series of pyrochlore oxides have been determined via atomistic simulation. These energies can be correlated to the order-disorder temperatures from the phase diagrams. Consequently, we predict the existence of hitherto unobserved pyrochlores: Dy2 Hf2 O7 , Ho2 Hf2 O7 , and Er2 Hf2 O7 . 相似文献
2.
Standard Enthalpy of Formation of Lanthanum Zirconate 总被引:1,自引:0,他引:1
Mark Bolech Erik H. E Cordfunke Frans J. J. G. Janssen Alexandra Navrotsky 《Journal of the American Ceramic Society》1995,78(8):2257-2258
The enthalpy of formation of lanthanum zirconate, La2 Zr2 O7 , which is shown to have a distorted pyrochlore structure, was determined. High-temperature solution calorimetry in molten lead borate was used to determine the heat of formation from the constituent oxides as −135.8 ± 6.4 kJ/mol at 974 K. Using additional data on the enthalpies of formation of the oxides and heat contents, this value was converted to a standard enthalpy of formation from the elements: Δf H°298.15 =−4130.4 ± 6.8 kJ/mol. 相似文献
3.
TERRENCE B. LINDEMER ALFRED L. SUTTON Jr. 《Journal of the American Ceramic Society》1988,71(7):553-561
The extensive nonstoichiometry in the 〈U1– z Gd z O2± x 〉 † phase was investigated experimentally and the data are represented by a chemical thermodynamic method. The experimental ranges of temperature, oxygen potential, and z were 1273 to 1773 K, 0 to −600 kJ/mol, and 0.1 to 0.8, respectively. For hypostoichiometry, ideal-solution thermodynamics for the equilibrium 3Gd4/3 O2 + 4UO2 + (O2 ) = 6U2/3 Gd2/3 O8/3 were used to represent the experimental data, while for hyperstoichiometry a nonideal solution was used for the equilibrium 4UO2 + (O2 ) = 2U2 O5 . The wide ranges in x and z led to an improvement of the previous analysis of literature data and led to partial molal Gibbs free energy values that are useful for any thermodynamic calculation involving the phase. 相似文献
4.
Anita Fossdal Mari-Ann Einarsrud Tor Grande 《Journal of the American Ceramic Society》2005,88(7):1988-1991
The phase relations in the pseudo-ternary system La2 O3 –SrO–Fe2 O3 have been investigated in air. Isothermal sections at 1100° and 1300°C are presented based on X-ray diffraction and thermal analysis of annealed samples. Extended solid solubility was observed for the compounds Sr n +1− v La v Fe n O3 n +1−δ ( n =1, 2, 3, and ∞) and Sr1− x La x Fe12 O19 , while only limited solubility of La in Sr4− z La z Fe6 O13±δ was observed. At high Fe2 O3 content, a liquid with low La2 O3 content was stable at 1300°C. 相似文献
5.
Hilde Lea Lein Kjell Wiik Mari-ann Einarsrud Tor Grande Edgar Lara-curzio 《Journal of the American Ceramic Society》2006,89(9):2895-2898
Steady-state compressive creep rate of La0.5 Sr0.5 Fe0.5 Co0.5 O3−δ (LSFC) and La0.5 Sr0.5 CoO3−δ (LSC) is reported in the temperature region 900°–1050°C and stress range 5–28 MPa. The stress exponents for the two materials were 1.71±0.18 and 1.24±0.15, respectively. The activation energy for creep was considerably higher for LSC (619±56 kJ/mol) than for LSFC (392±28 kJ/mol). The grain size exponent for LSC was 1.28±0.14. Considerably higher creep rates were observed for both materials in N2 compared with air. Relaxation by creep of chemical-induced stresses in oxygen-permeable membranes is addressed, especially at low partial pressure of oxygen. 相似文献
6.
The system ZrO2 -La2 O3 has been studied in the 0 to 15 mol% La2 O3 range using X-ray diffraction and thermal analysis. Two kinds of diagrams were elaborated: First is a phase equilibrium diagram where the eutectoid decomposition of ZrO2, ss (T) → ZrO2, ss (M) + Py ss occurs at 1100°C and 0.75 mol% La2 O3 (Py is the pyrochlore compound Zr2 La2 O7 ). The maximum solubility of La2 O3 in ZrO2, ss (M) is 0.5 mol% at room temperature. Second, a nonequilibrium diagram is determined showing the correlations between grain size, La2 O3 content, and the martensitic transformation temperature start ( Ms ). 相似文献
7.
Daijitsu Harada Yukio Hinatsu Nobuyuki Masaki Akio Nakamura 《Journal of the American Ceramic Society》2002,85(3):647-652
Powder X-ray diffractometry (XRD) and 151 Eu Mössbauer spectroscopy were performed for samples prepared in the temperature range 1500–1500°C for the hafnia–europia (HfO2 –Eu2 O3 ) system Eu x Hf1− x O2− x /2 (0 ≤ x ≤ 1.0). The XRD results showed that two types of solid solution phases formed in the composition range 0.25 ≤ x ≤ 0.725: an ordered pyrochlore-type phase in the middle-composition range (0.45 < x < 0.575) and a disordered fluorite-type phase, enveloping the pyrochlore-type phase on both sides, in the composition ranges 0.25 ≤ x ≤ 0.40 and 0.60 ≤ x ≤ 0.725. 151 Eu Mössbauer spectroscopy sensitively probes local environmental changes around trivalent europium (Eu3+ ) caused by the formation of these solid solution phases. In addition to the broad, single Mössbauer spectra observed in this study for the Eu3+ , the values of isomer shift (IS) exhibited a pronounced minimum in the neighborhood of the stoichiometric pyrochlore phase ( x ≈ 0.5). Such IS behavior of Eu3+ was interpreted based on the XRD crystallographic information that the ordered pyrochlore phase has a longer (in fact, the longest) average Eu–O bond length than those of the disordered fluorite phases on both sides or the monoclinic (and C-type) Eu2 O3 at x = 1.0. 相似文献
8.
Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating Applications 总被引:9,自引:0,他引:9
Jie Wu Xuezheng Wei Nitin P. Padture Paul G. Klemens Maurice Gell Eugenio García Pilar Miranzo Maria I. Osendi 《Journal of the American Ceramic Society》2002,85(12):3031-3035
Rare-earth zirconates have been identified as a class of low-thermal-conductivity ceramics for possible use in thermal barrier coatings (TBCs) for gas-turbine engine applications. To document and compare the thermal conductivities of important rare-earth zirconates, we have measured the thermal conductivities of the following hot-pressed ceramics: (i) Gd2 Zr2 O7 (pyrochlore phase), (ii) Gd2 Zr2 O7 (fluorite phase), (iii) Gd2.58 Zr1.57 O7 (fluorite phase), (iv) Nd2 Zr2 O7 (pyrochlore phase), and (v) Sm2 Zr2 O7 (pyrochlore phase). We have also measured the thermal conductivity of pressureless-sintered 7 wt% yttria-stabilized zirconia (7YSZ)—the commonly used composition in current TBCs. All rare-earth zirconates investigated here showed nearly identical thermal conductivities, all of which were ∼30% lower than the thermal conductivity of 7YSZ in the temperature range 25°–700°C. This finding is discussed qualitatively with reference to thermal-conductivity theory. 相似文献
9.
Alexandra Navrotsky Luc Benoist Herve Lefebvre 《Journal of the American Ceramic Society》2005,88(10):2942-2944
High-temperature differential thermal analysis provided data on phase transitions in zirconia and yttria. The tetragonal form of ZrO2 transforms to the cubic fluorite structure at 2311°±15°C with an enthalpy of 3.4±3.1 kJ/mol. Cubic C-type Y2 O3 transforms, probably to the fluorite structure, at 2308°±15°C with Δ H =47.7±3.0 kJ/mol. This high-temperature polymorph melts at 2382°±15°C with an enthalpy of fusion of 35.6±3.0 kJ/mol. 相似文献
10.
ERNEST M. LEVIN CARL R. ROBBINS JON L. WARING 《Journal of the American Ceramic Society》1961,44(2):87-91
The phase equilibrium diagram for the system La2 O3 -B2 O3 has been determined experimentally. The compounds La2 O3 -3B2 O3 and La2 O3 -B2 O3 melt congruently at 1141°± 5°C. and 1660°± 15°C, respectively. At 1488°± 5°C, La2 O3 -B2 O3 inverts from the aragonite-type structure to a high-temperature form. Trilanthanum borate, 3La2 O3 B2 O3 , melts incongruently at 1386°± 5°C. to give liquid and La2 O3 . No solid solutions exist in the system. A region of liquid immiscibility exists in the system and extends at 1136°± 5°C. from almost pure B2 O3 to 21.5 mole % La2 O3 . The experimental value for the extent of immiscibility agrees with that calculated from theoretical considerations. A second method for estimating immiscibility in the system is demonstrated, which requires experimentally only the determination of the index of refraction of the modifier-rich liquid. Principles governing immiscibility are discussed. 相似文献
11.
Riham Michelle Morcos Jean Tangeman Sergey Ushakov Alexandra Navrotsky 《Journal of the American Ceramic Society》2008,91(4):1088-1094
A series of La2 O3 –HfO2 –SiO2 glasses, approximately along the join 0.73SiO2 –0.27( x HfO2 –(1− x )La2 O3 ), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B2 O3 ) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO2 content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO2 content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO2 content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon. 相似文献
12.
Defeng Zhou Yanjie Xia Jianxin Zhu Wei Guo Jian Meng 《Journal of the American Ceramic Society》2009,92(5):1042-1046
A series of oxide ion conductors Ce6− x Gd x MoO15−δ (0.0≤ x ≤1.8) have been prepared by the sol–gel method. Their properties were characterized by differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), Raman, IR, X-ray photoelectron spectroscopy (XPS), and AC impedance spectroscopy. The XRD patterns showed that the materials were single phase with a cubic fluorite structure. The conductivity of Ce6− x Gd x MoO15−δ increases as x increases and reaches the maximum at x =0.15. The conductivity of Ce4.5 Gd1.5 MoO15−δ is σt =3.6 × 10−3 S/cm at 700°C, which is higher than that of Ce4.5/6 Gd1.5/6 O2−δ (σt =2.6 × 10−3 S/cm), and the corresponding activation energy of Ce4.5 Gd1.5 MoO15−δ (0.92 eV) is lower than that of Ce4.5/6 Gd1.5/6 O2−δ (1.18 eV). 相似文献
13.
P. DURAN 《Journal of the American Ceramic Society》1979,62(1-2):9-12
The phase relations in the system HfO2 -Sm2 O3 were studied by high-temperature and room-temperature X-ray diffraction. The areas of existence of the solid solutions based on HfO2 , Sm2 Hf2 O7 , and Sm2 O3 were determined using the lattice parameter method. Also, the influence of small additions of Sm2 O3 on the monoclinic-tetragonal transformation temperature of hafnia was studied. A tentative phase diagram is proposed. 相似文献
14.
Wioletta Kuncewicz-Kupczyk Dietmar Kobertz Miroslaw Miller Christian Chatillon Lorenz Singheiser Klaus Hilpert 《Journal of the American Ceramic Society》2002,85(9):2299-2305
The vaporization of the samples of the compositions Ga2 O3 + LaGaO3 , LaGaO3 + La4 Ga2 O9 , and La4 Ga2 O9 + La2 O3 was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O2 , Ga, GaO, Ga2 O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO3 ( s ) and La4 Ga2 O9 ( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol−1 , respectively. The results were compared with the literature data obtained using other methods. 相似文献
15.
Zhan-Guo Liu Jia-Hu Ouyang Yu Zhou Jing Li Xiao-Liang Xia 《International Journal of Applied Ceramic Technology》2009,6(4):485-491
(Yb x Gd1− x )2 Zr2 O7 (0≤ x ≤1.0) ceramic powders synthesized with the chemical-coprecipitation and calcination method were pressureless-sintered at 1550–1700°C to develop new thermal barrier oxides with a lower thermal conductivity than yttria-stabilized zirconia ceramics. (Yb x Gd1− x )2 Zr2 O7 ceramics exhibit a defective fluorite-type structure. The linear thermal expansion coefficients of (Yb x Gd1− x )2 Zr2 O7 ceramics increase with increasing temperature from room temperature to 1400°C. The measured thermal conductivity of (Yb x Gd1− x )2 Zr2 O7 ceramics first gradually decrease with increasing temperature and then slightly increase above 800°C because of the increased radiation contribution. YbGdZr2 O7 ceramics have the lowest thermal conductivity among all the composition combinations studied. 相似文献
16.
Ju Hong Miao Tsang-Tse Fang Han-Yang Chung Chao-Wei Yang 《Journal of the American Ceramic Society》2009,92(11):2762-2764
Undoped and La-doped Bi2 Fe4 O9 ceramics were synthesized using a soft chemical method. It is observed that in calcining La-doped Bi2 Fe4 O9 , Bi(La)FeO3 phase rather than Bi2− x La x Fe4 O9 gradually increases with increasing La doping content. The phase conversion from mullite-type structure of Bi2 Fe4 O9 to rhombohedrally distorted perovskite one of Bi(La)FeO3 with increasing La doping content indicates that La doping can stabilize the structure of BiFeO3 . This is further evidenced that Bi2 Fe4 O9 can be directly converted to Bi(La)FeO3 by heating the mixtures of nominal composition of Bi2 Fe4 O9 / x La2 O3 . Furthermore, the microstructure changes and the room temperature hysteresis loops and leakage current for Bi2− x La x Fe4 O9 with x =0 and 0.02 were characterized. 相似文献
17.
PEDRO DURAN CARMEN PASCUAL JEAN-PIERRE COUTURES SAMUEL R. SKAGGS 《Journal of the American Ceramic Society》1983,66(2):101-106
The system erbia-hafnia was investigated using thermal expansion measurements and room-temperature X-ray diffraction of quenched and annealed samples. The range of existence of the solid solutions based on HfO2 and Er2 O3 was determined by the precision lattice parameter method. Three compounds with hexagonal structure are reported: the first is an ordered fluorite-related phase with the ideal formula Er4 Hf3 O12 and decomposes with increasing temperature at ∼1500°C into a solid solution of the fluorite type; the second is formed at ∼55 mol% Er2 O3 , corresponding closely to the formula Er5 Hf2 O11.5 , and undergoes a transformation at ∼1650°C into cubic solid solutions to fluorite and C type. The third compound is formed within the concentration range between 60 and 90 mol% Er2 O3 . This compound, Er6 HfO11 , decomposes at ∼1700°C into a cubic solid solution of the C type. A proposed phase diagram for the system erbia-hafnia is presented. 相似文献
18.
Dong-Huyn Peck Miroslaw Miller Dietmar Kobertz Hubertus Nickel Klaus Hilpert 《Journal of the American Ceramic Society》1996,79(12):3266-3272
The vaporization of LaCrO3 (s) and samples of the composition LaCrO3 + La2 O3 was investigated in the temperature range of 1887-2333 K by Knudsen effusion mass spectrometry using Knudsen cells made of tungsten lined completely with iridium. The species Cr(g), CrO(g ), CrO2 (g), and LaO(g) were identified in the vapor. Their partial pressures were determined by calibration with pure platinum solid. The thermodynamic activity of Cr2 O3 , a cr2 o3 in LaCrO3 for the Cr2 03 -poor phase boundary of this phase was In aCr2 o3 = -(17953/T) - 0.485 (temperature T given in K) for the temperature range of the measurements with a probable overall error of ± 13%. The following values and temperature dependence of ΔG°f,T resulted for the formation of LaCrO3 (s) according to the reaction 0.5Cr2 O3 (s) + 0.5La2 O3 (s) → LaCrO3 (s): ΔG°f,2100 = -78.9 ± 1.1 kj/mol, Δ H°f,298 = -76.8 ± 5.2 kj/mol, and ΔG°r (kJ/mol) = -74.7 - 0.00202 T . Computations for the vaporization of LaCrO3 were conducted to show the volatility of this material in different atmospheres at high temperatures. 相似文献
19.
The phase diagram for the CuO-rich part of the La2 O3 ─CuO join was redetermined. La2 Cu2 O5 was found to have a lower limit of stability at 1002°± 5°C and an incongruent melting temperature of ∼1035°C. LagCu7 O19 had both a lower (1012°± 5°C) and an upper (1027°± 5°C) limit of stability. Subsolidus phase relations were studied in the La2 O3 ─CuO─CaO system at 1000°, 1020°, and 1050°C in air. Two ternary phases, La1.9 Ca1.1 Cu2 O5.9 and LaCa2 Cu3 O8.6 , were stable at these temperatures, with three binary phases, Ca2 CuO3 , CaCu2 O3 , and La2 CuO4 . La2 Cu2 O5 and La8 Cu7 O19 were stable only at 1020°C, and did not support solid-solution formation. 相似文献
20.
Jooho Moon Masanobu Awano Kunihiro Maeda 《Journal of the American Ceramic Society》2001,84(11):2531-2536
Well-defined La2 Sn2 O7 with a phase-pure pyrochlore structure was produced by hydrothermal synthesis at temperatures as low as 200°C. Production of phase-pure La2 Sn2 O7 requires a pH above 10, and higher pH accelerates the crystallization process. The synthesis produced spherical particles of average particle size ∼0.59 μm (±0.12) and surface area ∼14.1 m2 /g. SEM and TEM observation for morphologic evolution and kinetic analysis during crystallization indicated that La2 Sn2 O7 formation probably proceeds via a two-step reaction. First a transient dissolution–precipitation occurs. Then the primary crystallites aggregate because of their colloidal instability, and heterocoagulation with the lanthanum hydrous oxide precursor particles also occurs. The sluggish reaction rate at the later stage of reaction is characterized by an in situ transformation, where the soluble tin species is diffused through the porous La2 Sn2 O7 aggregates to react with entrapped lanthanum precursors. 相似文献