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931.
Y0.2CoFe1.8O4 nanopowders were prepared using a sol–gel combustion method. Metal nitrates, such as yttrium nitrate, cobalt nitrate and ferric nitrate, were used as the source materials. Citric acid and polyvinyl alcohol were used as the burning agent and agglomeration reducing agent, respectively. The pH of the precursor was maintained at 7. The mean crystallite size of the prepared ferrite was in the range of ∼20–70 nm. The inverse spinel structure, cubic morphology, and the identification of functional groups of the yttrium-doped cobalt ferrite were analyzed systematically using several analytical tools. 相似文献
932.
Changzhen Liu Dawei Meng Haixia Pang Xiuling Wu Jing Xie Xiaohong Yu Long Chen Xiaoyang Liu 《Journal of magnetism and magnetic materials》2012
Zn1–xFexO (x=0–0.05) nanoparticles were synthesized without a catalyst by a two-step method. Fe was doped into ZnO by a source of metallic Fe sheets in a solid–liquid system at 80 °C, and the Zn1−xFexO nanoparticles were obtained by annealing at 300 °C. X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy were used to characterize the structural properties of the as-grown Zn1−xFexO. The optical properties were determined by Infrared and Ultraviolet–visible spectroscopy. The results confirm that the crystallinity of the ZnO is deteriorated due to Fe-doping. XPS results show that there is a mixture of Fe0+ and the Fe3+ in the representative Zn0.95Fe0.05O sample. The optical band gap of Zn1−xFexO is enhanced with increasing of Fe-doping. Room temperature ferromagnetism was observed in all the Fe-doped ZnO samples. 相似文献
933.
A kinetic study of the reaction of hydroxide ion with (CO)5MoC(XCH2CH2OH)(C6H5) (X = O for Mo-OR, and X = S for Mo-SR), and (CO)5WC(OCH2CH2OH)(C6H4-Z) (W-OR(Z)) is reported. The results are consistent with a pathway in basic solution that involves rapid deprotonation of the OH group followed by rate-limiting cyclization. The parameter k1KOH for the reaction of W-OR(Z) was determined as a function of the phenyl substituents. They were found to correlate well with the Hammett equation. The dependence of the reactivity on the metal atoms in the complexes M-OR (M = Cr, Mo and W) shows that the reactivity decreases slightly down the group of the Periodic Table, while for M-SR the reactivity increases slightly down the group. A plausible explanation of these results is offered based on electronegativity values of the metal atoms. The much higher ρ(k1KOH) value for W-OR(Z) over W-SR(Z) arises mainly due to the stabilization of the reactant carbene complex by the stronger π-donor effect of oxygen over sulfur. 相似文献
934.
Evgeny V. Alekseev Sergey V. Krivovichev W. Depmeier H. Katzke Evgeny V. Chuprunov 《Journal of solid state chemistry》2006,179(10):2977-2987
Three new uranyl tungstates, A8[(UO2)4(WO4)4(WO5)2] (A=Rb (1), Cs (2)), and Rb6[(UO2)2O(WO4)4] (3), were prepared by high-temperature solid-state reactions and their structures were solved by direct methods on twinned crystals, refined to R1=0.050, 0.042, and 0.052 for 1, 2, and 3, respectively. Compounds 1 and 2 are isostructural, monoclinic P21/n, (1): a=11.100(7), b=13.161(9), , β=90.033(13)°, , Z=8 and (2): , , , β=89.988(2)°, , Z=8. There are four symmetrically independent U6+ sites that form linear uranyl [O=U=O]2+ cations with rather distorted coordination in their equatorial planes. There are six W positions: W(1) and W(2) have square-pyramidal coordination (WO5), whereas W(3), W(4), W(5), and W(6) are tetrahedrally coordinated. The structures are based upon a novel type of one-dimensional (1D) [(UO2)4(WO4)4(WO5)2]4− chains, consisting of WU4O25 pentamers linked by WO4 tetrahedra and WO5 square pyramids. The chains run parallel to the a-axis and are arranged in modulated pseudo-2D-layers parallel to (0 1 0). The A+ cations are in the interlayer space between adjacent pseudo-layers and provide a 3D integrity of the structures. Compounds 1 and 2 are the first uranyl tungstates with 2/3 of W atoms in tetrahedral coordination. Such a high concentration of low-coordinated W6+ cations is probably responsible for the 1D character of the uranyl tungstate units. The compound 3 is triclinic, P1¯ a=10.188(2), b=13.110(2), , α=97.853(3), β=96.573(3), γ=103.894(3)°, , Z=4. There are four U positions in the structure with a typical coordination of a pentagonal bipyramid that contain uranyl ions, UO22+, as apical axes. Among eight W sites, the W(1), W(2), W(3), W(4), W(5), and W(6) atoms are tetrahedrally coordinated, whereas the W(7) and W(8) cations have distorted fivefold coordination. The structure contains chains of composition [(UO2)2O(WO4)4]6− composed of UO7 pentagonal bipyramids and W polyhedra. The chains involve dimers of UO7 pentagonal bipyramids that share common O atoms. The dimers are linked into chains by sharing corners with WO4 tetrahedra. The chains are parallel to [−101] and are arranged in layers that are parallel to (1 1 1). The Rb+ cations provide linkage of the chains into a 3D structure. The compound 1 has many structural and chemical similarities to its molybdate analog, Rb6[(UO2)2O(MoO4)4]. However, the compounds are not isostructural. Due to the tendency of the W6+ cations to have higher-than-fourfold coordination, part of the W sites adopt distorted fivefold coordination, whereas all Mo atoms in the Mo compound are tetrahedrally coordinated. Distribution of the WO5 configurations along the chain extension does not conform to its ‘typical’ periodicity. As a result, both the chain identity period and the unit-cell volume are doubled in comparison to the Mo analog, which leads to a new structure type. 相似文献
935.
Oriental lacquer is the natural resin obtained by tapping lac trees. It hardens into a tough and insoluble film. The extreme hardness and insolubility are some of the most important functions, which are required for industrial coating materials. In this study, two kinds of oriental lacquer films, traditionally named Kiurushi (raw urushi) and Kuromeurushi produced by two different pretreatments, were analyzed during hardening with Fourier transform infrared spectroscopy (FT-IR), thermogravimetry/differential thermal analysis-mass spectrometry (TG/DTA-MS) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) to investigate their functional expression process. Typical functional groups of the lacquer films were detected by FT-IR. The TG/DTA-MS curves clarified that the thermal degradation of the lacquer films gradually began at around 200 °C, and reached the fastest rate at 400-500 °C. Apparently, FT-IR and TG/DTA-MS could not reveal any difference between the films. On the other hand, Py-GC/MS revealed differences between the films in the peak area ratios of 3-pentadecenylcatechol to 3-pentadecylcatechol and 3-pentadecadienylcatechol to 3-pentadecylcatechol. The ratios of Kiurushi lacquer film were higher than those of Kuromeurushi lacquer film. Both ratios, furthermore, decreased during hardening due to polymerization of the alkenylcatechols into an urushiol polymer skeleton comprising nucleus-side chain and side chain-side chain cross-linkages with 3-pentadecylcatechol at the terminal. The present results suggest that the reaction rate of these cross-linkages in Kuromeurushi lacquer film is faster than that in Kiurushi lacquer film. A good correlation was found between the peak area ratios obtained by Py-GC/MS and hardness obtained by pencil hardening testing. Oriental lacquer expresses the functions - an extreme hardness and insolubility - accelerating the nucleus-side chain and side chain-side chain cross-linkages. Furthermore, it has become clear that the traditional treatments called Nayashi and Kurome effectively accelerate the hardening rate by activating the cross-linkages. 相似文献
936.
Sheng-Ping Yang 《Tetrahedron letters》2006,47(7):1129-1132
A highly complex sesquiterpenoid dimmer, chloramultilide A (1), was isolated from Chloranthus multistachys. Its structure and relative stereochemistry were mainly established by using 1D NMR and 2D NMR spectra. 相似文献
937.
The crystal structure of the low-temperature forms of Rb2KCrF6 and Rb2KGaF6 has been solved on single crystal. The symmetry is tetragonal with F4/m space group; the unit cell parameters are: , for Rb2KCrF6 at and , for Rb2KGaF6 at . The relationships between the parameters of the prototype cubic elpasolite, which is stable at high temperature, and the tetragonal superlattice of the low temperature form have been established. Considering the general formulation A2BB′F6, the cationic positions in the A and (B,B′) sublattices remain identical in the two allotropic varieties. The main originality of the structure concerns the environment of 4/5 of the potassium atoms (B sublattice) which is transformed from octahedra into pentagonal bipyramids sharing edges with adjacent B′F6 octahedra containing Cr or Ga. The displacive phase transition is simply explained by the rotation of 45° in the (a,b) plane of 1/5 of the B′F6 (B′=Cr, Ga) octahedra. The similarity of this phase transition and the transformation of perovskite into tetragonal tungsten bronze (TTB) will be discussed. 相似文献
938.
V.B. Naik 《Journal of magnetism and magnetic materials》2010,322(18):2754-2757
We report radio-frequency (rf) electrodynamics in polycrystalline La0.67Ba0.23Ca0.1MnO3 as a function of temperature and magnetic field using a home-built LC resonant circuit powered by an integrated chip oscillator. The resonance frequency (fr) of the oscillator and the power (P) absorbed by the sample are measured simultaneously. The paramagnetic to ferromagnetic phase transition in the absence of an external magnetic field is accompanied by a rapid decrease in both P and fr around the Curie temperature TC=300 K. However, much below TC, the fr shows a step-like anomaly around 165 K (195 K) while cooling (warming), which we attribute to a structural phase transition from high temperature rhombohedral () to low temperature orthorhombic (Imma) phase. The step-like anomaly in fr versus T disappears in a field of 300 G. Fractional changes as large as 19% in Δfr/fr and 10% in ΔP/P are observed under H=1 kG around TC. Our study suggests that the rf resonance technique is a versatile tool to study the magnetization dynamics as well as to investigate the structural phase transition in manganites. 相似文献
939.
940.
Thin films of zinc (Zn) were deposited onto glass substrates (maintained at room temperature) by thermal evaporation under vacuum. The metallic zinc films were submitted to thermal oxidation in air at 670 K and 770 K, respectively, for 5–90 min, in order to obtain zinc oxide (ZnO) thin films. X-ray diffraction patterns revealed that the ZnO thin films were polycrystalline and had a wurtzite (hexagonal) structure. The morphology of the prepared ZnO thin films was investigated using atomic force microscopy and scanning electron microscopy techniques. Transmission spectra were recorded in the spectral domain from 300 nm to 1400 nm. The optical energy bandgap calculated from the absorption spectra (supposing allowed direct transitions) was in the range 3.05–3.30 eV. 相似文献