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1.
The nonisothermal crystallization kinetic of Bi x Y3− x Fe5 O12 (0.25≤ x ≤1.00) powders prepared by coprecipitation process has been investigated. The activation energy of crystallization was calculated by differential scanning calorimetry at different heating rates. The activation energies of crystallization of Bi x Y3− x Fe5 O12 system are 492, 447, 377, and 353 kJ/mol and the Avrami constant n are 3.49, 2.25, 2.48, and 2.33 for x =0.25, 0.50, 0.75, and 1.00, respectively. The Avrami exponent values (1< n <3) were consistent with surface and internal crystallizations occurring simultaneously for 0.50≤ x ≤1.00, the value ( n >3) for the Avrami exponent was consistent with bulk crystallization domination in Bi x Y3− x Fe5 O12 system. The results reveal that increasing the substitution amount of bismuth for yttrium would significantly decrease activation energy in Bi x Y3− x Fe5 O12 system. 相似文献
2.
Low Thermal Conductivity in Garnets 总被引:3,自引:0,他引:3
The thermal conductivity of dense, polycrystalline garnet ceramics with compositions of Y3 Alx Fe(5−x) O12 (x = 0.0, 0.7, 1.4, and 5.0) was measured in the temperature range 23° to 1000°C. The high-temperature thermal conductivity of some of these garnets was found to be as low as 2.4 W·m-1 K-1 . The effects of temperature and composition on the observed thermal conductivity are discussed with reference to established theories of thermal conduction. The potential use of yttrium aluminum garnet or YAG (Y3 Al5 O12 ), in particular, for a specific application of advanced thermal barrier coatings (TBCs) with improved durability is considered. 相似文献
3.
Single crystals of yttrium iron garnet (Y3 Fe5 O12 ) have been grown using the flame fusion process, even though the compound is reported to melt incongruently. The growth of these single crystals involves a mechanism different from that which has been proposed for the growth of single crystals of incongruently melting mullite. Crystal boules were grown at varying linear growth rates and analyzed with chemical, X-ray, and metallographic techniques. With high linear growth rates, the samples are uniformly polycrystalline and three-phase, containing Fe2 O3 , YFeO3 , and Y3 Fe5 O12 . When slow linear growth rates are used, single-crystal Y3 Fe5 O12 can be grown. The mechanism is as follows: At the beginning of growth the first phase to precipitate is YFeO3 , and during this stage in growth the molten cap becomes enriched in Fe2 O3 , compared with the Y3 Fe5 O12 composition. The liquid cap composition thus changes to the limit of the peritectic on the Fe2 O3 -rich side, and Y3 Fe6 O12 then crystallizes from the bottom of the melt as Y3 Fe5 O12 powder is added to the top of the molten cap. The central sections of these boules are single-crystal yttrium iron garnet. 相似文献
4.
Kosuke Nagashio Okie Yamaguchi Taketoshi Hibiya Kazuhiko Kuribayashi 《Journal of the American Ceramic Society》2006,89(5):1504-1509
The droplet with Nd x Sm3− x Fe5 O12 composition was undercooled in an aerodynamic levitator and splat quenched by copper anvils in order to obtain a metastable garnet with a solubility limit larger than the phase equilibrium solubility limit, x =0.375, for the Nd x Sm3− x Fe5 O12 system. The peaks of the garnet were identified with the peaks of the perovskite by powder X-ray diffraction (XRD), although the peak intensity for the garnet decreased with increasing the Nd substitution from x =0.43 to 0.2, and finally disappeared at x =2.2. When the garnet was annealed at 1570 K for 24 h in air, it transformed into a mixture of perovskite and hematite, which indicates that the garnet obtained was the metastable phase. Moreover, the amorphous phase was found in the central part of all the samples even at x =2.2, which was confirmed by micro-focus XRD. The formation of the constituent phases in the as-quenched sample was discussed using a continuous cooling transformation diagram. 相似文献
5.
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
6.
Danjela Kuer Darko Hanel Janez Holc Marko Hrovat Drago Kolar 《Journal of the American Ceramic Society》2001,84(5):1148-1154
La1− y Sr y Fe1− x Al x O3−δ perovskites were studied as potential materials for solid-oxide fuel cell (SOFC) cathodes. The phase relations in the LaFeO3 –SrFeO3−δ –LaAlO3 system were investigated by X-ray powder diffraction analysis. The defect structure of the La1− y Sr y Fe1− x Al x O3−δ perovskites was investigated by Mössbauer spectroscopy and weight-loss analysis. Relations between the nonstoichiometry and the conductivity of the La1− y Sr y Fe1− x Al x O3−δ perovskites were investigated. The incorporation of aluminum ( x ) into LaFe1− x Alx O3 was found to have no influence on the defect structure but to decrease the conductivity. The incorporation of strontium ( y ) into La1− y Sr y Fe1− x Al x O3−δ promotes the formation of anion vacancies and Fe4+ that lead to higher conductivity. 相似文献
7.
Novel Synthesis of Silica-Coated Ferrite Nanoparticles Prepared Using Water-in-Oil Microemulsion 总被引:2,自引:0,他引:2
Teruoki Tago Takatoshi Hatsuta Kanta Miyajima Masahiro Kishida Shizuka Tashiro Katsuhiko Wakabayashi 《Journal of the American Ceramic Society》2002,85(9):2188-2194
Ferrite nanoparticles (magnetite (Fe3 O4 ) and cobalt-ferrite (Co x Fe3− x O4 )) coated with silica (SiO2 ) were prepared using water-in-oil microemulsion of a polyoxyethylen(15)cetylether/cyclohexene system. Observation via transmission electron microscopy revealed that the ferrite nanoparticles were located nearly at the center of spherical SiO2 particles. The sizes of Fe3 O4 and Co x Fe3− x O4 nanoparticles were in the range of 8–12 nm and 10–14 nm, respectively, and the thickness of the SiO2 layer was ∼14.0 nm. The solid solution of cobalt ions into Fe3 O4 to form Co x Fe3− x O4 nanoparticles led to an increase in the coercivity of the SiO2 -coated ferrite nanoparticles. The coercivity increased with increasing amounts of added cobalt, obtaining a maximum value of 780 Oe around a Co/Fe ratio of 0.3–0.4. 相似文献
8.
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
9.
Yusuke Kaminaga Hisanori Yamane Takahiro Yamada 《Journal of the American Ceramic Society》2007,90(6):1917-1920
Compounds in a CaO–Y2 O3 –SnO2 system were prepared by a solid-state reaction at 1673 K. The phase relation in this system was investigated by powder X-ray diffraction. Besides the previously reported ternary compounds, CaSnO3 , Ca2 SnO4 , Y2 Sn2 O7 , and a quaternary compound Ca0.4 Y1.2 Sn0.4 O3 , solid-solution series of Ca2− x Y2 x Sn1− x O4 with 0≤ x ≤0.5, and Ca1− y Y2 y Sn1− y O3 with 0≤ y ≤0.2 and 0.95≤ y ≤1.0 were found. The cell parameters of these solid-solution series were refined. The changes of rhombohedral cell parameters in the samples prepared in the range 0.565< y <0.714 of Ca1− y Y2 y Sn1− y O3 suggested the existence of solid solutions of Ca0.4 Y1.2 Sn0.4 O3 , although their single phases could not be prepared, except at y =0.6. 相似文献
10.
Lutz Kindermann Dasarathi Das Dhirendra Bahadur Reinhard Wei Hubertus Nickel Klaus Hilpert 《Journal of the American Ceramic Society》1997,80(4):909-914
Orthoferrite-based perovskites are of interest as materials for the cathode in solid oxide fuel cells (SOFCs). Therefore, the chemical compatibility between perovskites of the composition (La1−x Srx )z Fe1−y Mny O3−δ (0 # x # 0.3; 0.2 # y # 1; z = 0.90, 0.95, 1.00) and the solid electrolyte zirconia (ZrO2 ) doped with 8 mol% yttria (Y2 O3 ) (8YSZ) has been investigated. Powder mixtures of the two materials have been annealed at different temperatures. The formation of monoclinic ZrO2 at 1000°C, as well as of La2 Zr2 O7 and SrZrO3 at 1400°C, has been determined in some samples. The reactions that are observed are discussed, with respect to the thermodynamic activities, tolerance factor, and oxygen-ion migration energies. Some perovskite compositions seem to be compatible with Y2 O3 -stabilized ZrO2 (YSZ), thereby offering the possibility to use orthoferrite-based perovskites in SOFCs with a solid electrolyte made of YSZ. 相似文献
11.
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
12.
New titanium–niobium oxynitride (Ti1− z Nb z O x N y ) powders were synthesized by ammonolysis of nanosized TiO2 /Nb2 O5 composite powders at 700°–900°C for 5 h. The products were characterized by X-ray diffraction (XRD), chemical analysis, and transmission electron microscopy. The results indicated that the as-synthesized powders were pure cubic structures with sizes of 30–60 nm. With increasing value of z , XRD peaks of Ti1− z Nb z O x N y powders tended to shift toward low 2θ angle and the cell parameter showed a linear increase. 相似文献
13.
Microwave Synthesis of Yttrium Iron Garnet Powder 总被引:4,自引:0,他引:4
Teiichi Kimura Hirotsugu Takizawa Kyota Uheda Tadashi Endo Masahiko Shimada 《Journal of the American Ceramic Society》1998,81(11):2961-2964
A 28 GHz microwave heating method was used to react an Fe2 O3 + Y2 O3 powder mixture to form yttrium iron garnet (YIG, Y3 Fe5 O12 ) powder. The minimum temperature to form YIG was lower than the conventional (external) heating method. YIG began to form after only 70 s of irradiation, which means that the solid-state reaction proceeded very rapidly. The amounts of byproducts were controlled by the starting composition and by the Y2 O3 particle size. The resultant YIG particle size also was controlled by the Y2 O3 particle size. 相似文献
14.
Sridhar Komarneni Elizabeth Fregeau Else Breval Rustum Roy 《Journal of the American Ceramic Society》1988,71(1):26-C-
Ultrafine and nearly spherical ferrites such as NiFe2 O4 , ZnFe2 O4 , Nix Zn1−x Fe2 O4 , Mn0.5 Zn0.5 Fe2 O4 , and CoFe2 O4 were prepared under mild hydrothermal conditions by precipitating from metal nitrates with aqueous ammonia. These hydrothermal ferrite powders were shown to sinter to almost theoretical density at 1000°C without any sintering aids. 相似文献
15.
Yoshihito Mizoguchi Masahiro Kagawa Yasuhiko Syono Toshio Hirai 《Journal of the American Ceramic Society》2001,84(3):651-653
Thin films of yttrium aluminum garnet (YAG, Y3 Al5 O12 ) and yttrium iron garnet (YIG, Y3 Fe5 O12 ) were synthesized on single-crystal Al2 O3 substrates by a modification of spray pyrolysis using a high-temperature inductively coupled plasma at atmospheric pressure (spray–ICP technique). Using this technique, films could be grown at faster rates (0.12 μm/min for YAG and 0.10 μm/min for YIG) than using chemical vapor deposition (0.005–0.008 μm/min for YAG) or sputtering (0.003–0.005 μm/min for YIG). The films were dense and revealed a preferred orientation of (211). The growth of YIG was accompanied by coprecipitation of α-Fe2 O3 . The coprecipitation, however, could be largely suppressed by preliminary formation of a Y2 O3 layer on the substrate. 相似文献
16.
L. B. Kong Z. W. Li G. Q. Lin Y. B. Gan 《Journal of the American Ceramic Society》2007,90(10):3106-3112
The densification, grain growth, and microstructure development of Mg–Cu–Co ferrite ceramics (MgFe1.98 O4 , Mg1− x Cu x Fe1.98 O4 , with x =0.10–0.30 and Mg0.90− x Co x Cu0.10 Fe1.98 O4 , with x =0.05–0.20) were studied. The primary objective was to develop magneto-dielectric materials for miniaturization of high frequency and very-high frequency antennas. It was found that magnesium ferrite (MgFe1.98 O4 ) is a promising magneto-dielectric material. However, due to its poor densification, it could not be fully sintered at a temperature below 1200°C. High-temperature sintering resulted in undesirable electrical and dielectric properties, due to the formation of Fe2+ ions. The poor densification and slow grain growth rate of MgFe1.98 O4 can be considerably improved by incorporating Cu, due to the occurrence of liquid-phase sintering at a high temperature. A critical concentration of Cu was observed for Mg1− x Cu x Fe1.98 O4 , above which both densification and grain growth were maximized or saturated. The presence of Co did not have a significant influence on the densification and grain growth of the Mg-based ferrite ceramics. 相似文献
17.
Single-phase garnet solid solutions can be synthesized between the composition limits of x =4.18 and x =4.22 in Y2.66 Gd0.34 Fe x Al0.677 Mn0.09 O12 at temperatures between 1340° and 1500°C in O2 . Solid solutions occur only on the Y2 O3 -excess side of the stoichiometric garnet composition. Electromagnetic properties and microstructural features of sintered garnets depend critically on small changes in Fe content in the vicinity of the garnet solid-solution region. An intergranular spinel-type second phase exists for compositions when x >4.22 and has a deleterious effect on remanent induction and magnetic loss at 3 GHz. The relative density of powder compacts sintered for 16 h at 1500°C in O2 increases with increasing Fe content (i.e. as x increases) in the garnet solid solution. 相似文献
18.
19.
A. E. PALADINO E. A. MAGUIRE JR. L. G. RUBIN 《Journal of the American Ceramic Society》1964,47(6):280-282
The oxygen ion self-diffusion coefficient was determined for single-crystal and polycrystalline yttrium iron garnet (Y3 Fe5 O12 ). The rate of exchange between oxygen gas enriched with the stable isotope 18 O and solid yttrium iron garnet was measured. Oxygen ion diffusion rates were found to be the same in single-crystal and 8μ polycrystalline Y3 Fe5 O12 between 1100° and 1400°C. This is in contrast to previous measurements of anion diffusion in several alkali halides and in AI2 O3 where a strong dependence of diffusion rates on the presence of grain boundaries was found. Enhancing oxidation rates in dense, reduced yttrium iron garnet at low temperature by minimizing the final fired grain size in the sintering process does not appear to be possible on the basis of the results obtained in this investigation. The temperature dependence of the diffusion coefficient of oxygen measured at 100 torr can be represented by D = 0.40 exp (-65.4/RT) cm2 per second. 相似文献
20.
Jianghong Gong Ying Li Zhongtai Zhang Zilong Tang 《Journal of the American Ceramic Society》2000,83(3):648-650
The ionic conductivity in the system (1 − 0.08 x − 0.12 y )ZrO2 –0.08 x Y2 O3 –0.12 y CaO (where x + y = 1 and y = 0–1) has been investigated using the complex impedance technique at 523–973 K. Doping CaO in the ZrO2 –Y2 O3 system may result in an increase in the activation energy for lattice conduction and depress the grain-boundary effect on conduction. Analysis of the temperature dependence of the lattice conductivity, according to the familiar Arrhenius equation, predicts that the examined ternary system may exhibit greater lattice conductivity than the ZrO2 –Y2 O3 binary system at higher temperatures. Preliminary explanations for these experimental phenomena and predictions also have been presented. 相似文献