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1.
Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders   总被引:1,自引:0,他引:1  
Praseodymium-doped ceria (CeO2) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH4)0.16Ce1− x Pr x (CO3)1.58·H2O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO2 lattice, because of the oxidation of Pr3+ to smaller Pr4+, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.  相似文献   

2.
The effect of the addition of V2O5 on the structure, sintering and dielectric properties of M -phase (Li1+ x − y Nb1− x −3 y Ti x +4 y )O3 ceramics has been investigated. Homogeneous substitution of V5+ for Nb5+ was obtained in LiNb0.6(1− x )V0.6 x Ti0.5O3 for x ≤ 0.02. The addition of V2O5 led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]r= 66, Q × f = 3800 at 5.6 GHz, and τf= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V2O5-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.  相似文献   

3.
Reactive Ceria Nanopowders via Carbonate Precipitation   总被引:3,自引:0,他引:3  
Nanocrystalline CeO2 powders have been successfully synthesized via a carbonate precipitation method, using ammonium carbonate (AC) as the precipitant and cerium nitrate hexahydrate as the cerium source. The AC/Ce3+ molar ratio ( R ) affects significantly precursor properties, and spherical nanoparticles can be produced only in a narrow range of 2 < R ≤ 3. The precursor, having an approximate composition of Ce(OH)CO3·2.5H2O, decomposes to CeO2 at temperatures ≥300°C. The CeO2 powder calcined at 700°C exhibits high reactivity and can be densified to >99% of theoretical at 1000°C.  相似文献   

4.
(Yb x Gd1− x )2Zr2O7 (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 )2Zr2O7 ceramics exhibit a defective fluorite-type structure. The linear thermal expansion coefficients of (Yb x Gd1− x )2Zr2O7 ceramics increase with increasing temperature from room temperature to 1400°C. The measured thermal conductivity of (Yb x Gd1− x )2Zr2O7 ceramics first gradually decrease with increasing temperature and then slightly increase above 800°C because of the increased radiation contribution. YbGdZr2O7 ceramics have the lowest thermal conductivity among all the composition combinations studied.  相似文献   

5.
The synthesis, characterization, and bulk and lattice thermal expansions of a series of compounds with general composition Ce1– x Y x O2– x /2 (0.0 ≤ x ≤ 1.0) are reported. The XRD pattern of each product was refined to learn the solid solubility limit and the homogeneity range. The solid solubility limit of YO1.5 in CeO2 lattice, under the conditions of slow cooling from 1400°C, is represented as Ce0.55Y0.45O1.775 (i.e., 45 mol% of YO1.5). The subsequent compositions were biphase. There was no solubility of CeO2 into the lattice of YO1.5. The bulk thermal expansion measurements from ambient to 1123 K, as investigated using a dilatometer, revealed that the αl (293–1123 K) values, within the homogeneity range, decreased on increased Y3+ content. A similar trend was observed for average lattice thermal expansion coefficient, αa (293–1473 K), as investigated using high-temperature XRD. No ordered phases were obtained in this system under the used conditions. These studies on Ce1– x Y x O2– x /2 (0.0 ≤ x ≤ 1.0) system can be used to simulate the phase relation and thermal expansion behavior of Pu1– x Y x O2– x /2 (0.0 ≤ x ≤ 1.0), because CeO2 is widely used as a surrogate material for PuO2.  相似文献   

6.
Subsolidus phase relationships in the Ga2O3–Al2O3–TiO2 system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO2 (rutile), Ga2−2 x Al2 x O3 ( x ≤0.78 β-gallia structure), Al2−2 y Ga2 y O3 ( y ≤0.12 corundum structure), Ga2−2 x Al2 x TiO5 (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga4−4 x Al4 x Ti n −4O2 n −2 ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.  相似文献   

7.
Phase stability studies were performed within the quasi-ternary system LaGaO3-SrGaO2.5-"LaMgO2.5". Emphasis was cast on the temperature dependence of the homogeneity region of La1− x Sr x Ga1− y Mg y O3−δ perovskite solid solutions. Isothermal sections were determined at 1100°, 1250°, 1400°, and 1500°C in a static air atmosphere. The single-phase homogeneity region was found to considerably diminish with decreasing temperature, indicating a reduction of the solid solubility of Sr and Mg, and below 1100°C the doped perovskite becomes unstable. Consequently, the cubic perovskite phase was found to exist only at elevated temperatures and for high Sr and Mg amounts. Sample preparation was performed by the mixed-oxide process as well as by a modified combustion synthesis.  相似文献   

8.
The effect of Mn doping on the cubic to hexagonal phase transition temperature in BaTiO3 has been determined by quenching samples with different Mn contents from a range of temperatures. Under conditions of equilibrating samples in air over the range 1000°–1400°C, cubic solid solutions BaTi1− x Mn x O3−δ form over the range 0≤ x ≤0.015(5), whereas hexagonal solid solutions form for x ≥0.02, depending on the temperature. The results are compared with those on doping BaTiO3 with Fe3+ and observations made concerning acceptor doping with Ti3+.  相似文献   

9.
The interfacial reaction layers in the Ti/ZrO2 diffusion couples, isothermally annealed in argon at temperatures ranging from 1100° to 1550°C for 6 h, were characterized using scanning electron microscopy and transmission electron microscopy, both attached with an energy-dispersive spectrometer. Very limited reaction occurred between Ti and ZrO2 at 1100°C. A β'-Ti(Zr, O) layer and a two-phase α-Ti(O)+β'-Ti(Zr, O) layer were found in the titanium side after annealing at T ≥1300°C and T ≥1400°C, respectively. A three-phase layer, consisting of Ti2ZrO+α-Ti(O, Zr)+β'-Ti (O, Zr), was formed after annealing at 1550°C. In the zirconia side near the original interface, β'-Ti coexisted with fine spherical c- ZrO2− x , which dissolved a significant amount of Y2O3 in solid solution at T ≥1300°C. Further into the ceramic side, the α-Zr was formed due to the exsolution of Zr out of the metastable ZrO2− x after annealing at T ≥1300°C: the α-Zr was very fine and dense at 1300°C, continuously distributed along grain boundaries at 1400°C, and became coarsened at 1550°C. Zirconia grains grew significantly at T ≥1400°C, with the lenticular t -ZrO2− x being precipitated in c -ZrO2− x . Finally, the microstructural development and diffusion paths in the Ti/ZrO2 diffusion couples annealed at various temperatures were also described with the aid of the Ti–Zr–O ternary phase diagram.  相似文献   

10.
Pb0.98(La1− x Li x )0.02(Zr0.55Ti0.45)O3(PLLZT with 0.1 ≤ x ≤ 0.7) thin films were sol-gel-grown on Pt(111)/Ti/SiO2/Si substrates, employing a thin lead zirconate titanate (PZT) template layer. Films annealed at >550°C showed a highly (111)-oriented preferential growth. Typical values of the switchable remanent polarization (2 P r) and the coercive field ( E c) of the PLLZT/PZT/Pt film capacitor for x = 0.3 were 50 μC/cm2 and 39 kV/cm, respectively, at 5 V. All the PLLZT/PZT/Pt capacitors (for 0.1 ≤ x ≤ 0.7) exhibited fatigue-free behavior up to 6.5 × 1010 switching cycles, a quite stable charge retention profile with time, and high 2 P rvalues, all which assure their suitability for nonvolatile ferroelectric memories.  相似文献   

11.
Sintering behavior, phase evolution, and microwave dielectric properties of Bi(Sb1− x Ta x )O4 ceramics (0.05≤ x ≤0.60) were studied and their relationships were discussed in detail. Phase studies revealed that a pure monoclinic phase could be formed when x ≤0.20 and a pure orthorhombic phase could be obtained when x ≥0.50. As the x value increased from 0.05 to 0.60, the densified temperature of Bi(Sb1− x Ta x )O4 ceramics decreased from 1050° to about 960°C whereas the density increased from 8.07 to 8.41 g/cm3. The microwave dielectric constant increased from 20.5 to 34 whereas the Q × f value decreased from 60 000 to 29 000 GHz. In the monoclinic phase region, the temperature coefficients of resonant frequency shifted linearly from −58 to −45 ppm/°C as the x value increased from 0.05 to 0.2 and then remained constant at about −12 ppm/°C when x ≥0.40. The Bi(Sb1− x Ta x )O4 ceramics are promising for application of low-temperature cofired ceramics technology.  相似文献   

12.
Subsolidus phase relations have been determined in the CdO–InO1.5–SnO2 system at 1175°C. A cubic-bixbyite solution In2−2 x (Cd,Sn)2 x O3 (0 < x < 0.34), a cubic spinel solution (1− x )CdIn2O4– x Cd2SnO4 (0 < x < 0.75), and an orthorhombic-perovskite solution Cd1− x Sn1− x In2 x O3 (0 < x < 0.045) having the GdFeO3 structure have been discovered. The CdO phase field exists over a small range of InO1.5 (<3%) and SnO2 (<1%). Orthorhombic Cd2SnO4 (Sr2PbO4 structure) and rutile SnO2 appear to be point compounds with negligible solubility. The vertical section between spinel CdIn2O4 and orthorhombic Cd2SnO4 was determined between 900° and 1175°C. The spinel phase field (1− x )CdIn2O4– x Cd2SnO4 was found to extend between x = 0 and x = 0.75 at 1175°C or x = 0.78 at 900°C. All of the phases in this system appear to allow small excess quantities of the donors In and/or Sn (vs cation stoichiometry) which may be the source of the electrons that give these oxides their n-type character. The electrical and optical properties of bulk and thin-film specimens in this system are compared and contrasted with each other and the relative merits of each are assessed.  相似文献   

13.
The processing conditions, microstructure, and dielectric properties of strontium bismuth niobate vanadate ceramics, SrBi2(V x Nb1− x )2O9 (SBVN, with 0 ≤ x ≤ 0.3), were systematically studied. A relative density of >90% was obtained for all the samples using a two-step sintering process. XRD showed that a single phase with the layered perovskite structure of SrBi2Nb2O9 (SBN) was formed with a vanadium content of up to 30 at.%. SEM revealed that the average grain size decreased gradually with an increase in vanadium content. The Curie point was found to gradually increase from ∼418°C for SBN to ∼459°C for SBVN with 30 at.% vanadium. Dielectric constants at room temperature and their respective Curie points were found to peak at a composition with 10–15 at.%; vanadium. Moreover, a high concentration of vanadium (>5 at.%) resulted in a significant increase in tangent loss at low frequencies (<1000 Hz). The relationships between chemical composition, processing condition, microstructure, and dielectric properties of SBVN ferroelectric ceramics are discussed.  相似文献   

14.
Thermodynamics of the Na1−xLi x NbO3 system is investigated by high-temperature drop-solution calorimetry in molten 3Na2O–4MoO3 solvent at 973 K. Standard molar enthalpies of formation are derived. The estimated heats of transition between hypothetical and stable structures, lithium niobate and perovskite for NaNbO3 and vice versa for LiNbO3 are −6 kJ/mol and −10 kJ/mol, respectively. X-ray diffraction studies at room temperature showed for 0 ≤ x ≤ 0.14 there are three phases based on different ordering of the perovskite type lattice: orthorhombic with a quadrupled reduced perovskite cell at 0 ≤ x ≤ 0.02, orthorhombic with a doubled reduced perovskite cell at 0.015 ≤ x ≤ 0.14, and rhombohedral at 0.08 ≤ x ≤ 0.13. There are two two-phase (morphotropic) regions with coexistence of the two orthorhombic phases at 0.015 ≤ x ≤ 0.02 and with the second orthorhombic phase coexisting with the rhombohedral phase at 0.08 ≤ x ≤ 0.13. A reproducible anomaly in specific heat at ∼600 K, not reported previously, has been observed in pure NaNbO3. Heat-capacity measurements confirm a phase transition at 553 K for 0.07 ≤ x ≤ 0.09. With increasing lithium concentration, a gradual disappearance of high-temperature phase transitions associated with tilting of oxygen octahedra has been observed.  相似文献   

15.
A multi-component substitution of Co and Ni was incorporated into ZnTiO3 to form pure hexagonal Zn1− x (Co1/2Ni1/2) x TiO3 ( x =0,0.8,0.9,1.0) dielectric ceramic powders by a modified sol–gel route, following heat treatments at 600°C for 3 h and at 800°C for 6 h. Differential scanning calorimetry measurements revealed that the order of increasing thermal stability of solid solution compound Zn1− x (Co1/2Ni1/2) x TiO3 was ZnTiO3 (945°C), Zn0.1Ni0.9TiO3 (1346°C), Zn0.1(Co1/2Ni1/2)0.9TiO3 (1390°C), and Zn0.1Co0.9TiO3 (>1400°C). Both the dielectric constant and loss tangent reached a maximum at x =0.8 and then decreased with solubility, x , and measurement frequency.  相似文献   

16.
The microwave dielectric properties of CaTi1− x (Al1/2Nb1/2) x O3 solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO3. The substitution of Ti4+ by Al3+/Nb5+ improved the quality factor Q of the sintered specimens. A small addition of Li3NbO4 (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛr) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τf) of −2 ppm/K. Li3NbO4 as a sintering additive had no harmful influence on τf of ceramics.  相似文献   

17.
Compositionally homogeneous indium tin oxide (ITO) ceramics with low porosity were obtained successfully by sintering hydrothermally prepared powders. The fabrication technique began with the preparation of microcrystalline, homogeneously tin-doped (5 wt%) indium oxyhydroxide powder, under hydrothermal conditions. Low-temperature (∼500°C) calcination of the hydrothermally derived powder led to the formation of a substitutional-vacancy-type solid solution of In2Sn1− x O5− y , and further heating of this phase at temperatures of >1000°C resulted in the formation of the tin-doped indium oxide phase, which had the C -type rare-earth-oxide structure. The sintering of uniformly packed, calcined powder compacts at 1450°C for 3 h in air resulted in low-porosity (∼0.7%) ITO ceramics.  相似文献   

18.
Ceramics of the melilite-type compound La1+ x Sr1− x Ga3O7−δ were prepared by conventional ceramic processing. Samples prepared represented the entire homogeneity region of the phase (i.e., x =−0.15 to 0.60). Electrochemical characterization under variable temperature and atmospheric conditions in the vicinity of air entailed four-point direct-current conductivity measurements and electromotive force measurements. La1+ x Sr1− x Ga3O7−δ samples exhibited a p -type behavior with generally increased conductivity with increased substitution of lanthanum for strontium, which reached a saturation value of ∼10−1 S·cm−1 at 950°C.  相似文献   

19.
Pr3+-doped YF3 (orthorhombic), YO0.80F1.40 (orthorhombic), YOF (rhombohedral), and Y2O3 (cubic) films were synthesized on quartz-glass substrates through pyrolysis of a single-source trifluoroacetate precursor at temperatures between 400° and 900°C in air. Phase-selective deposition was achieved by controlling heating temperature and time. YF3, which formed first from the precursor, was transformed to YO0.80F1.40, YOF, and Y2O3. Photoluminescent properties of Pr3+-doped films were examined using ultraviolet excitation. An intense green photoluminescence was observed in the YOF:Pr3+ film, which was deposited at 700°C, through an efficient charge transfer (O2−–Pr3+) excitation.  相似文献   

20.
Powder X-ray diffractometry (XRD) and 151Eu Mössbauer spectroscopy were performed for samples prepared in the temperature range 1500–1500°C for the hafnia–europia (HfO2–Eu2O3) 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. 151Eu 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) Eu2O3at x = 1.0.  相似文献   

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