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1.
Microwave dielectric ceramics in the Sr1−xCaxLa4Ti5O17 (0 ≤ x ≤ 1) composition series were prepared through a solid state mixed oxide route. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1450-1580 °C. Theoretical density and molar volume decreased due to the substitution of Ca2+ for Sr2+ which was associated with a decrease in the dielectric constant (?r) and temperature coefficient of resonant frequency (τf) but an increase in quality factor, Qfo. Optimum properties were achieved for Sr0.4Ca0.6La4Ti5O17 which exhibited, ?r ∼ 53.7, Qfo ∼ 11,532 GHz and τf ∼ −1.4 ppm/°C. 相似文献
2.
Nd1−xSmxNiO3−δ ceramics were studied by means of XRD, iodometric titration, DSC and resistivity measurements. The metal-insulator transition for all compositions was studied by two independent procedures: resistivity and DSC measurements. The reason for differences in transition temperatures, defined by these two techniques is discussed. It was shown that these materials are promising for practical applications in bolometric devices due to high values of temperature coefficient of resistivity at room temperature. 相似文献
3.
The present work reports the effects caused by barium on phase formation, morphology and sintering of lead magnesium niobate-lead titanate (PMN-50PT). Ab initio study of 0.5Pb(Mg1/3Nb2/3)O3-0.5(BaxPb(1−x)TiO3) ceramic powders, with x = 0, 0.20, and 0.40 was proposed, considering that the partial substitution of lead by barium can reestablish the equilibrium of monoclinic-tetragonal phases in the system. It was verified that even for 40 mol% of barium, it was possible to obtain pyrochlore-free PMN-PT powders. The increase of the lattice parameters of PMN-PT doped-powders confirmed dopant incorporation into the perovskite phase. The presence of barium improved the reactivity of the powders, with an average particle size of 120 nm for 40 mol% of barium against 167 nm for the pure sample. Although high barium content (40 mol%) was deleterious for a dense ceramic, contents up to 20 mol% allowed 95% density when sintered at 1100 °C for 4 h. 相似文献
4.
The electrical conductivity of SrSn1−xFexO3−δ increases with the Fe content and reaches a value of ∼10−1 S/cm at 25°C at x=1. Compounds with low Fe content exhibit both ionic and electronic conductivity, while the higher Fe content perovskites are mainly electronic conductors with a conductivity independent of the oxygen partial pressure over a wide range from 0.21 to 10−22 atm. 相似文献
5.
The rare-earth nickelates are among few perovskite oxides showing very sharp temperature-induced metal/insulator transition. The synthesis and study of oxygen-deficient NdNiO3−δ and SmNiO3−δ are described in this paper. The results obtained indicate that the oxygen nonstoichiometry greatly influences transport properties of these compounds. The sharpness of metal/insulator transition strongly rises with decrease of oxygen deficiency without any noticeable change of transition temperature. The boundaries of oxygen and cation homogeneity ranges of Nd1−xNiO3−δ were determined for the first time. The thermogravimetric study was used to explain certain difficulties in oxidation of as-synthesized oxygen-deficient NdNiO3−δ samples. 相似文献
6.
Lianqi Zhang 《Materials Research Bulletin》2009,44(3):707-713
LiMxMn2−xO4 (M = Ni2+, Co3+, and Ti4+; 0 ≤ x ≤ 0.2) spinels were prepared via a single-step ultrasonic spray pyrolysis method. Comparative studies on powder properties and high rate charge-discharge electrochemical performances (from 1 to 15 C) were performed. XRD identified that pure spinel phase was obtained and M was successfully substituted for Mn in spinel lattice. SEM and TEM studies confirmed that powders had a feature of ‘spherical nanostructural’, that is, powders consisted of spherical secondary particles with the size of about 1 μm, which were developed from close-packed primary particles with several tens of nanometers. Substitutions enhanced density of second particles to different extents, depending on M and its content. Charge-discharge tests showed that as-prepared LiMn2O4 could deliver excellent rate performance (around 100 mAh/g at 10 C). Ni substitution contributed to improving electrochemical performances. In the voltage range of 4.95-3.5 V, the materials showed much better electrochemical performances than LiMn2O4 in terms of capacity, cycleability and rate capability. 相似文献
7.
M ThirumalI.N Jawahar K.P SurendiranP Mohanan A.K Ganguli 《Materials Research Bulletin》2002,37(14):2321-2334
Oxides belonging to the families Ba3ZnTa2−xNbxO9 and Ba3MgTa2−xNbxO9 were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba3MgTa2−xNbxO9 over the entire range (0≤x≤1) of composition, while for Ba3ZnTa2−xNbxO9 the ordered structure exists in a limited range (0≤x≤0.5). The dielectric constant is close to 30 for the Ba3ZnTa2−xNbxO9 family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5-7 GHz) dielectric constant increases with increase in niobium concentration (22-26) for Ba3ZnTa2−xNbxO9; for Ba3MgTa2−xNbxO9 it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues. 相似文献
8.
Xiaoliang Zhou 《Materials Research Bulletin》2007,42(8):1582-1588
In this paper, an interconnecting ceramic for solid oxide fuel cells was developed, based on the modification from La0.7Ca0.3CrO3−δ by addition of Ce0.8Sm0.2O1.9. It is found that addition of small amount Ce0.8Sm0.2O1.9 into La0.7Ca0.3CrO3−δ dramatically increased the electrical conductivity. For the best system, La0.7Ca0.3CrO3−δ + 5 wt.% Ce0.8Sm0.2O1.9, the electrical conductivity reached 687.8 S cm−1 at 800 °C in air. In H2 at 800 °C, the specimen with 3 wt.% Ce0.8Sm0.2O1.9 had the maximal electrical conductivity of 7.1 S cm−1. With the increase of Ce0.8Sm0.2O1.9 content the relative density increased, reaching 98.7% when the Ce0.8Sm0.2O1.9 content was 10 wt.%. The average coefficient of thermal expansion at 30-1000 °C in air increased with Ce0.8Sm0.2O1.9 content, ranging from 11.12 × 10−6 to 12.46 × 10−6 K−1. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating it is still an electronically conducting ceramic. Therefore, this material system will be a very promising interconnect for solid oxide fuel cells. 相似文献
9.
A.V. Shlyakhtina A.V. Levchenko V.Yu. Bychkov V.A. Rassulov O.K. Karyagina 《Materials Research Bulletin》2007,42(4):742-752
The Er2+xTi2−xO7−δ (x = 0.096; 35.5 mol% Er2O3) solid solution and the stoichiometric pyrochlore-structured compound Er2Ti2O7 (x = 0; 33.3 mol% Er2O3) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10−3 S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10−10 Pa, the Er2+xTi2−xO7−δ (x = 0, 0.096) samples are purely ionic conductors. 相似文献
10.
Bi2O4−x, a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K2S2O8/Na2S2O8. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O2 losses prior to finally transforming into γ-Bi2O3. The as-prepared powders are 50-60 nm in size with a narrow size distribution. Optical spectra of Bi2O4−x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi2O3. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi2O3.65 ± 0.10. 相似文献
11.
Stoichiometric lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN), perovskite ceramics produced by reaction-sintering process were investigated. Without calcination, a mixture of PbO, Nb2O5, and Mg(NO3)2 was pressed and sintered directly. Stoichiometric PMN ceramics of 100% perovskite phase were obtained for 1, 2, and 4 h sintering at 1250 and 1270 °C. PMN ceramics with density 8.09 g/cm3 (99.5% of theoretical density 8.13 g/cm3) and Kmax 19,900 under 1 kHz were obtained. 相似文献
12.
R.V.K. Mangalam 《Materials Research Bulletin》2009,44(3):576-36
The ferromagnetic metallic oxide, SrRuO3 (TC ∼ 165 K) undergoes structural, magnetic and metal-insulator transitions upon substitution of Cu at the Ru-site. For x = 0.2 in SrRu1−xCuxO3, the structure becomes a tetragonal with the space group I4/mcm and there is a signature of both ferromagnetic (TC = 65 K) and antiferromagnetic (TN = 32 K) ordering due to possible magnetic phase separation. The antiferromagnetism arises due to short range ordering of Cu- and Ru-moments. Jahn-Teller distortion of (Ru,Cu)-O6 octahedra indicates that the copper ions are in 2+ oxidation state with 6t2g3eg electronic configuration. For x ≥ 0.1, narrowing of Ru-4d bandwidth by the substitution of Cu ions results in semiconducting behavior. For x = 0.3, the ac and dc susceptibility measurements indicate a spin glass behavior. The origin of spin glass behavior has been attributed to competing ferromagnetic and antiferromagnetic interactions. 相似文献
13.
Shuo Jin 《Materials Research Bulletin》2011,46(3):432-437
Sr2−xCaxBi4Ti5O18(x = 0, 0.05) powders synthesized by solid state route were uniaxially pressed and sintered at 1225 °C for 2 h. The obtained dense ceramics exhibited crystallographic anisotropy with a dominant c axis parallel to the uniaxial pressing direction which was quantified in terms of the Lotgering factor. Microstructure anisotropy of both compositions (x = 0, 0.05) consisted of plate like grains exhibiting their larger surfaces mostly perpendicular to the uniaxial pressing direction. Dielectric and ferroelectric properties of Sr2−xCaxBi4Ti5O18 ceramics were measured under an electric field (E) applied parallel and perpendicularly to uniaxial pressing direction. The obtained dielectric ?R(T) and polarization (P-E) curves depended strongly on E direction thus denoting a significant effect from microstructure and crystallographic texture. Sr2−xCaxBi4Ti5O18 properties were also significantly affected by Ca content (x): Curie temperature increased from 280 °C (x = 0) to 310 °C (x = 0.05) while ?R and remnant polarization decreased for x = 0.05. The present results are discussed within the framework of the processing and crystal structure-properties relationships of Aurivillius oxides ceramics. 相似文献
14.
We have carried out photoacoustic experiments to study the layered electroceramics Bi4−xRxTi3O12 (Rx=Pr, Nd, Gd, Dy), with x=0.8. Results are shown in terms of successive analyses performed on functions, PA(t, Ti), which result from the interaction of the laser beam with the crystalline lattice. Previous permittivity experiments performed in the materials suggested anomalous ferroelectric behavior. Using a pulsed Nd:YAG laser (10 Hz, 5 ns pulse width), photoacoustic experiments were run from room temperature up to 800 °C. Perturbations in the resultant correlation curves can be interpreted as the existence of a wide set of different transition temperatures in the material, such as are believed to occur in relaxors. From these experiments, we conclude that the temperature dependence of non-classical ferroelectrics can be more closely monitored. 相似文献
15.
Ping-Hua Xiang Xian-Lin DongChu-De Feng Heng ChenYong-Ling Wang 《Materials Research Bulletin》2003,38(7):1147-1154
Piezoelectric Pb(ZrxTi1−x)O3 (PZT) ceramics with small amount (0.5-2.0 wt.%) of In2O3 are prepared by conventional sintering method. Based on X-ray diffraction analysis, the tetragonality of PZT matrix decreases with In2O3 content, indicating that In2O3 diffuses into PZT matrix. The microstructure of PZT matrix is significantly refined by doping small amounts of In2O3. The grain size reduction and the matrix grain boundary reinforcement are the probable mechanism responsible for the high strength and hardness in the PZT/In2O3 materials. The enhancement in Young’s modulus is attributed to In3+ substitution. The decreased tetragonality with In2O3 addition results in less crack energy absorption by domain switching and, hence, causes the small reduction in fracture toughness. 相似文献
16.
CdS nanoparticles coating SrZrxTi1−xO3 (x=0.27) microparticles were obtained by sonochemical reaction, and the as-prepared composites products were characterized by TEM, XRD, FT-IR and other techniques and the fabrication mechanism was also discussed. Moreover, the photocatalystic properties of the final products were investigated by degradation of methylene blue. By coating CdS nanoparticles, the absorption of the UV-Vis spectrum of SrZrxTi1−xO3 red-shifted, but the emission fluorescence spectrum blue-shifted. Adjusting the reactant concentration can control the thickness of the CdS layer. 相似文献
17.
Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb3+ ions is conspicuous in the 5D3 → 7F4 transition due to the cross-relaxation in BaLa4(WO4)7. A partial substitution of Ba2+ by Sr2+ can not only enhance the emission intensity but also increase the solid solubility of Tb3+ in Ba1−ySryLa4−xTbx(WO4)7. The emission intensity of the 5D4 → 7FJ (J = 4, 5, 6) transitions can be enhanced by increasing Sr2+ and Tb3+ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba0.6Sr0.4La2.8Tb1.2(WO4)7). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba1−ySryLa4−xTbx(WO4)7 vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4. 相似文献
18.
Thermal and reduction-oxidation stability of substituted LaMn1−yCoyO3 perovskite-type oxides (0.0 ≤ yCo ≤ 1.0) prepared by the citrate route have been studied by means of surface area, X-ray diffraction, FTIR spectroscopy and magnetic properties. The perovskite orthorhombic structure is found for yCo ≤ 0.5, with the exception of yCo = 0.1, which corresponds better to rhombohedral LaMnO3.15. For yCo > 0.5 the diffraction profiles are quite similar to the cobaltite’s rhombohedral structure. Magnetic iso-field studies (ZFC-FC) reveal that, for yCo ≤ 0.50, the system presents an antiferromagnetic canted-like ordering of the Mn/Co sublattice, in which the presence of divalent Co ion creates Mn3+-Mn4+ pairs that interact ferromagnetically through the oxygen orbital. This interpretation is confirmed by the magnetization loops, in which the magnetic moment increases when substituting Mn for Co. Therefore, the general trend is: for yCo ≤ 0.5, the Co ions are inserted in the manganite structure and for yCo > 0.5, the Mn ions are inserted in cobaltite structure. The enhancement of the ferromagnetic properties and the thermal stability against reduction for yCo = 0.5 is attributed to optimized Co2+-Mn4+ interactions. 相似文献
19.
Srinivas Janaswamy G. Sreenivasa Murthy V.R.K. Murthy 《Materials Research Bulletin》2008,43(3):655-664
The Ba3ZnTa2O9 (BZT) and Ba3MgTa2O9 (BMT) ceramics, a family of A3B2+B5+2O9 complex perovskites, are extensively utilized in mobile based technologies due to their intrinsic high unloaded quality factor, high dielectric constant and a low (near-zero) resonant frequency temperature coefficient at microwave frequencies. The preparation conditions as well as size and nature of B cations have a profound effect on the final dielectric properties. In this article, we report the effect of Nb5+ at the Ta5+ site on the BMT structure prepared at four synthesis temperatures (1300, 1400, 1500 and 1600 °C). The analysis has been carried out using the Rietveld technique on the X-ray powder diffraction data. Results suggest that both the preparation temperatures and Nb5+ content have significant effect on the ordering of B cations in the Ba3Mg(Ta1−xNbx)2O9 solid solution. A disordered (cubic) structure is preferred by the 1300 °C compounds. The weight percentage of the ordered (trigonal) phase escalates, for a given composition, with increasing calcination temperature. A fully ordered trigonal arrangement exists only for x = 0.0 and 0.2 compounds calcined at 1600 °C, and the rest are biphasic (cubic and trigonal). The increase in the cubic fraction upon Nb5+ augmentation suggests that the solid solution leans more toward the disordered structural arrangement of B2+ and B5+ cations. 相似文献
20.
We prepared Ba(Ti1−xSnx)O3 powders and ceramics by means of the sol-gel process, with dibutyltin dilaurate as the Sn precursor. The samples were characterized by means of Fourier-transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, and scanning electron microscopy, and also determined the dielectric properties of the ceramics. The powders synthesized by means of the sol-gel process had a grain size on the nanometer scale, with the grains mainly composed of a cubic BaTiO3 phase. Sn can disperse into BaTiO3 more uniformly in the sol-gel technique using dibutyltin dilaurate as the Sn precursor. With increasing Sn concentration, the grain size of the Ba(Ti1-xSnx)O3 ceramics increased and the maximum dielectric constant (?max) first increased and then decreased. At a Sn concentration of 5 mol%, ?max reached its maximum value (19,235). 相似文献