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1.
Hollow microspheres composed of phase-pure ZnFe 2O 4 nanoparticles (hierarchically structured) have been prepared by hydrothermal reaction. The unique hollow spherical structure significantly increases the specific capacity and improves capacity retention of this material. The product of each phase transition during initial discharge (ZnFe 2O 4 ? Li 0.5ZnFe 2O 4 ? Li 2ZnFe 2O 4 → Li 2O + Li–Zn + Fe) and their structural reversibility are recognized by X-ray diffraction and electrochemical characterization. The products of the deeply discharged (Li–Zn alloy and Fe) and recharged materials (Fe 2O 3) were clarified based on high resolution transmission electron microscopic technique and first-principle calculations. 相似文献
2.
High-temperature heat capacity measurements were obtained for Cr 2O 3, FeCr 2O 4, ZnCr 2O 4, and CoCr 2O 4 using a differential scanning calorimeter. These data were combined with previously available, overlapping heat capacity data at temperatures up to 400 K and fitted to 5-parameter Maier–Kelley Cp( T) equations. Expressions for molar entropy were then derived by suitable integration of the Maier–Kelley equations in combination with recent S∘(298) evaluations. Finally, a database of high-temperature equilibrium measurements on the formation of these oxides was constructed and critically evaluated. Gibbs free energies of Cr 2O 3, FeCr 2O 4, and CoCr 2O 4 were referenced by averaging the most reliable results at reference temperatures of (1100, 1400, and 1373) K, respectively, while Gibbs free energies for ZnCr 2O 4 were referenced to the results of Jacob [K.T. Jacob, Thermochim. Acta 15 (1976) 79–87] at T = 1100 K. Thermodynamic extrapolations from the high-temperature reference points to T = 298.15 K by application of the heat capacity correlations gave Δ fG∘(298) = (−1049.96, −1339.40, −1428.35, and −1326.75) kJ · mol −1 for Cr 2O 3, FeCr 2O 4, ZnCr 2O 4, and CoCr 2O 4, respectively. 相似文献
3.
Zinc ferrite (ZnFe 2O 4) y nanoparticles/Cu 0.5Tl 0.5Ba 2Ca 2Cu 3O 10−δ (CuTl-1223) superconductor composites with y = 0–2 wt.% were prepared by adding ZnFe 2O 4 nanoparticles into CuTl-1223 superconductor matrix and characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier transforms infrared (FTIR) spectroscopy and dc-resistivity ( ρ) measurements. The bulk CuTl-1223 superconductor matrix was synthesized by solid-state reaction and Zinc ferrite (ZnFe 2O 4) nanoparticles were separately prepared by sol–gel method. XRD analysis revealed the tetragonal and spinel structure of CuTl-1223 superconductor and ZnFe 2O 4 nanoparticles, respectively. The XRD analysis showed that increased concentration of ZnFe 2O 4 nanoparticles doesn't disturb the tetragonal structure of host CuTl-1223 superconductor matrix and has no appreciable effect on its lattice parameters. The SEM images confirm the granular structure of the host superconductor matrix. The presence of ZnFe 2O 4 nanoparticles in host superconductor matrix is confirmed by using FTIR study. Variation of zero resistivity critical temperature { Tc (0)} depends upon the concentration of the nanoparticles in the host superconductor matrix. The overall suppression of Tc (0) and diamagnetism with increasing nanoparticles concentration is most probably due to trapping of mobile free carriers and reflection of spin charge due to presence of paramagnetic ZnFe 2O 4 nanoparticles. There is possibility for the incorporation of Fe and Zn in the lattice sites during the synthesis process, which may also cause the reduction of Tc (0) of the final composites. 相似文献
4.
The low-temperature heat capacity of NiAl 2O 4 and CoAl 2O 4 was measured between T = (4 and 400) K and thermodynamic functions were derived from the results. The measured heat-capacity curves show sharp anomalies peaking at around T = 7.5 K for NiAl 2O 4 and at T = 9 K for CoAl 2O 4. The exact cause of these anomalies is unknown. From our results, we suggest a standard entropy for NiAl 2O 4 at T = 298.15 K of (97.1 ± 0.2) J · mol ?1 · K ?1 and for CoAl 2O 4 of (100.3 ± 0.2) J · mol ?1 · K ?1. 相似文献
5.
A new molybdenum complex (C 4H 12N 2) 2[(Mo V2O 4)(Mo VIO 4)(C 2O 4) 2]·2H 2O, was solvothermally synthesized and characterized by single-crystal X-ray diffraction. The structure of the compound consists of oxalate acid-coordinated mixed-valent [Mo V2O 4][Mo VIO 4] helical chains and protonated piperazine cations. The helical chains are built up from the [Mo V2O 4] units and [Mo VIO 4] tetrahedral. The central axis about helical chain is a 2-fold screw axis. The compound crystallizes in the space group P2 1/ n of monoclinic system with a = 11.396(2) Å, b = 14.107(3) Å, c = 15.805(3) Å, β = 102.09(3)°, V = 2484.6(9) Å 3, Z = 4. Other characterizations by elemental analysis, IR, and thermal analysis for this compound are also given. 相似文献
6.
Heterostructured ZnFe 2O 4–graphene nanocomposites are synthesized by a facile hydrothermal method. The as-prepared ZnFe 2O 4–graphene nanocomposites are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis and galvanostatic charge and discharge measurements. Compared with the pure ZnFe 2O 4 nanoparticles, the ZnFe 2O 4–graphene nanocomposites exhibit much larger reversible capacity up to 980 mAh g −1, greatly improved cycling stability, and excellent rate capability. The superior electrochemical performance of the ZnFe 2O 4–graphene nanocomposites could be attributed to the synergetic effect between the conducting graphene nanosheets and the ZnFe 2O 4 nanoparticles. 相似文献
7.
Ambient pressure CaV 2O 4 and high-pressure NaV 2O 4 crystallize in the CaFe 2O 4 structure type containing double chains of edge-sharing VO 6 octahedra. Recent measurements on NaV 2O 4 reveal low-dimensional metallicity and evidence of half-metallic ferromagnetism. In contrast, CaV 2O 4 is an antiferromagnetic insulator. To explore the evolution of these ground-state behaviors, we have prepared a series of Ca-doped NaV 2O 4 compounds with the formula Na 1?xCa xV 2O 4 ( x = 0–1) using high-pressure synthesis. Samples at the Na end ( x = 0–0.07) show a broad antiferromagnetic transition in the 120–160 K range in accordance with earlier reports. Transport measurements show an insulator–metal transition at x ~ 0.2. Samples with higher Ca concentrations ( x = 0.4–0.7) exhibit a metal–insulator transition around 150 K. The results for the Na 1?xCa xV 2O 4 solid solution is discussed in comparison to existing studies at the Ca- and Na-rich ends. 相似文献
8.
Conducting polymer composite films comprised of polypyrrole (PPy) and multiwalled carbon nanotubes (MWCNTs) [PPy–CNT] were synthesized by in situ polymerization of pyrrole on carbon nanotubes in 0.1 M HCl containing (NH 4)S 2O 8 as oxidizing agent over a temperature range of 0–5 °C. Pt nanoparticles are deposited on PPy–CNT composite films by chemical reduction of H 2PtCl 6 using HCHO as reducing agent at pH = 11 [Pt/PPy–CNT]. The presence of MWCNTs leads to higher activity, which might be due to the increase of electrochemically accessible surface areas, electronic conductivity and easier charge-transfer at polymer/electrolyte interfaces allowing higher dispersion and utilization of the deposited Pt nanoparticles. A comparative investigation was carried out using Pt–Ru nanoparticles decorated PPy–CNT composites. Cyclic voltammetry demonstrated that the synthesized Pt–Ru/PPy–CNT catalysts exhibited higher catalytic activity for methanol oxidation than Pt/PPy–CNT catalyst. Such kinds of Pt and Pt–Ru particles deposited on PPy–CNT composite polymer films exhibit excellent catalytic activity and stability towards methanol oxidation, which indicates that the composite films is more promising support material for fuel cell applications. 相似文献
9.
ZnO/ZnFe2O4@reduced graphene oxide (RGO) nanocomposites have been successfully synthesized through annealing treatment of Zn/Fe MOF-5@GO composites. The ZnO/ZnFe2O4 nanoparticles with a diameter of 12–15 nm are evenly distributed on the surface of RGO. The ZnO/ZnFe2O4@RGO nanocomposites show superior rate capacity and cyclic stability of 655 mAh/g after 200 cycles at 0.2 A/g for lithium ion battery (LIB) anode. The superior electrochemical property benefits from the unique structure of ZnO/ZnFe2O4@RGO nanocomposites, which can provide a buffer space for volume expansion, and enhance conductivity in the charge/discharge cycle. 相似文献
10.
Tetragonal copper ferrite (CuFe 2O 4) nanofibers were fabricated by electrospinning method using a solution that contained poly(vinyl pyrrolidone) (PVP) and Cu and Fe nitrates as alternative metal sources. The as-spun and calcined CuFe 2O 4/PVP composite samples were characterized by TG-DTA, X-ray diffraction, FT-IR, and SEM, respectively. After calcination of the as-spun CuFe 2O 4/PVP composite nanofibers (fiber size of 89 ± 12 nm in diameter) at 500 °C in air for 2 h, CuFe 2O 4 nanofibers of 66 ± 13 nm in diameter having well-developed tetragonal structure were successfully obtained. The crystal structure and morphology of the nanofibers were influenced by the calcination temperature. After calcination at 600 and 700 °C, the nature of nanofibers changed which was possibly due to the reorganization of the CuFe 2O 4 structure at high temperature, and a fiber structure of packed particles or crystallites was prominent. Crystallite size of the nanoparticles contained in nanofibers increases from 7.9 to 23.98 nm with increasing calcination temperature between 500 and 700 °C. Room temperature magnetization results showed a ferromagnetic behavior of the calcined CuFe 2O 4 samples, having their specific saturation magnetization ( Ms) values of 17.73, 20.52, and 23.98 emu/g for the samples calcined at 500, 600, and 700 °C, respectively. 相似文献
11.
The molar heat capacities of GeCo 2O 4 and GeNi 2O 4, two geometrically frustrated spinels, have been measured in the temperature range from T=(0.5 to 400) K. Anomalies associated with magnetic ordering occur in the heat capacities of both compounds. The transition in GeCo 2O 4 occurs at T=20.6 K while two peaks are found in the heat capacity of GeNi 2O 4, both within the narrow temperature range between 11.4<( T/K)<12.2. Thermodynamic functions have been generated from smoothed fits of the experimental results. At T=298.15 K the standard molar heat capacities are (143.44 ± 0.14) J · K −1 · mol −1 for GeCo 2O 4 and (130.76 ± 0.13) J · K −1 · mol −1 for GeNi 2O 4. The standard molar entropies at T=298.15 K for GeCo 2O 4 and GeNi 2O 4 are (149.20 ± 0.60) J · K −1 · mol −1 and (131.80 ± 0.53) J · K −1 · mol −1 respectively. Above 100 K, the heat capacity of the cobalt compound is significantly higher than that of the nickel compound. The excess heat capacity can be reasonably modeled by the assumption of a Schottky contribution arising from the thermal excitation of electronic states associated with the CO 2+ ion in a cubic crystal field. The splittings obtained, 230 cm −1 for the four-fold-degenerate first excited state and 610 cm −1 for the six-fold degenerate second excited state, are significantly lower than those observed in pure CoO. 相似文献
12.
Zinc ferrite (ZnFe2O4) nanoparticles were successfully synthesized from Zn(NO3)2 · 6H2O and Fe(NO3)3 · 9H2O by microwave hydrothermal method at 150°C for 1 h. Cubic ZnFe2O4 with particle size below 7 nm was formed in the solution at pH ≥ 6. The crystallinity and particle size of ZnFe2O4 nanoparticles were increased after calcination. The effects of pH of the precursor solution and calcination on the particle size and crystallinity of the particles were studied. At room temperature the products show superparamagnetic and ferromagnetic properties, determined by their size. The formation mechanism of ZnFe2O4 was also discussed according to the experimental results. 相似文献
13.
Embedding structures of a metal nanoparticle in an oxide matrix were first achieved by electron beam irradiation. In the system of Al/ α-Al 2O 3. Al nanoparticles derived from θ-Al 2O 3 migrated and embedded in α-Al 2O 3 matrix having epitaxy relation, {1 1 2̄ 0} α-Al 2O 3//{2 0 0} Al. The driving force of the embedding is momentum transfer from electrons or ions to Al atoms of nanoparticles in the pole piece of transmission electron microscopy. 相似文献
14.
Single crystals of a new phosphate AgCr 2(PO 4)(P 2O 7) have been prepared by the flux method and its structural and the infrared spectrum have been investigated. This compound crystallizes in the monoclinic system with the space group C2/c and the parameters are, a = 11.493 (3) Å, b = 8.486 (3) Å, c = 8.791 (2) Å, β = 114.56 (2)°, V = 779.8 (3) Å 3and Z = 4. Its structure consists of CrO 6 octahedra sharing corners with P 2O 7 units to form undulating chains extending infinitely along the [110] direction. These chains are connected by the phosphate tetrahedra giving rise to a 3D framework with six-sided tunnels parallel to the [101] direction, where the Ag + ions are located. The infrared spectrum of this compound was interpreted on the basis of P 2O 74? and PO 43? vibrations. The appearance of ν sP–O–P in the spectrum suggests a bent P–O–P bridge for the P 2O 74? ions in the compound, which is in agreement with the X-ray data. The electrical measurements allow us to obtain the activation energy of (1.36 eV) and the conductivity measurements suggest that the charge carriers through the structure are the silver captions. 相似文献
15.
A calorimetric and thermodynamic investigation of two alkali-metal uranyl molybdates with general composition A 2[(UO 2) 2(MoO 4)O 2], where A = K and Rb, was performed. Both phases were synthesized by solid-state sintering of a mixture of potassium or rubidium nitrate, molybdenum (VI) oxide and gamma-uranium (VI) oxide at high temperatures. The synthetic products were characterised by X-ray powder diffraction and X-ray fluorescence methods. The enthalpy of formation of K 2[(UO 2) 2(MoO 4)O 2] was determined using HF-solution calorimetry giving Δ fH° ( T = 298 K, K 2[(UO 2) 2(MoO 4)O 2], cr) = −(4018 ± 8) kJ · mol −1. The low-temperature heat capacity, Ср°, was measured using adiabatic calorimetry from T = (7 to 335) K for K 2[(UO 2) 2(MoO 4)O 2] and from T = (7 to 326) K for Rb 2[(UO 2) 2(MoO 4)O 2]. Using these Ср° values, the third law entropy at T = 298.15 K, S°, is calculated as (374 ± 1) J · K −1 · mol −1 for K 2[(UO 2) 2(MoO 4)O 2] and (390 ± 1) J · K −1 · mol −1 for Rb 2[(UO 2) 2(MoO 4)O 2]. These new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, Δ fG°, for both phases giving: Δ fG° ( T = 298 K, K 2[(UO 2) 2(MoO 4)O 2], cr) = (−3747 ± 8) kJ · mol −1 and Δ fG° ( T = 298 K, Rb 2[(UO 2) 2(MoO 4)], cr) = −3736 ± 5 kJ · mol −1. Smoothed Ср°( Т) values between 0 K and 320 K are presented, along with values for S° and the functions [ H°( T) − H°(0)] and [ G°( T) − H°(0)], for both phases. The stability behaviour of various solid phases and solution complexes in the (K 2MoO 4 + UO 3 + H 2O) system with and without CO 2 at T = 298 K was investigated by thermodynamic model calculations using the Gibbs energy minimisation approach. 相似文献
16.
An experimental study on metastable equilibria at T=288 K in the quinary system Li 2CO 3 + Na 2CO 3 + K 2CO 3 + Li 2B 4O 7 + Na 2B 4O 7 + K 2B 4O 7 + H 2O was done by isothermal evaporation method. Metastable equilibrium solubilities and densities of the solution were determined experimentally. According to the experimental data, the metastable equilibrium phase diagram under the condition saturated with Li 2CO 3 was plotted, in which there are four invariant points; nine univariant curves; six fields of crystallization: K 2CO 3 · 3/2H 2O, K 2B 4O 7 · 5H 2O, Li 2B 2O 4 · 16H 2O, Na 2B 2O 4 · 8H 2O, Na 2CO 3 · 10H 2O, NaKCO 3 · 6H 2O. Some differences were found between the stable phase diagram at T=298 K and the metastable one at T=288 K. 相似文献
17.
The main aim of the paper to the synthesis of Mn (x)-doped NiCr 2O 4 nanoparticles by varying Mn content ( x = 0.00%, 0.01%, 0.02%, and 0.03%) by microwave method for correlating the effect of NiCr 2O 4 on structural, optical, and magnetic properties of the materials. Understanding the optical, magnetic, and structural properties of huge reservoir factors has essential applications in various aspects of materials science. Our study is to relate the reduction of grain size of Mn content in NiCr 2O 4 host material. The XRD results revealed that there was an apparent decrease in the characteristic peaks of Mn in the MnNiCr 2O 4 nanostructure. Particularly, the peak position of (2 2 0) and (3 1 1) planes was decreased. This decrease in peak position is attributed to the creation of defects or disorders due to the Mn ions in the chromite lattice structure. This inter-site Mn cation migration is responsible for the breaking of long-range cation order and the introduction of defects at both the T-site and O-sublattices site simultaneously. 相似文献
18.
The chemical potentials of CaO in two-phase fields (TiO 2 + CaTiO 3), (CaTiO 3 + Ca 4Ti 3O 10), and (Ca 4Ti 3O 10 + Ca 3Ti 2O 7) of the pseudo-binary system (CaO + TiO 2) have been measured in the temperature range (900 to 1250) K, relative to pure CaO as the reference state, using solid-state galvanic cells incorporating single crystal CaF 2 as the solid electrolyte. The cells were operated under pure oxygen at ambient pressure. The standard Gibbs free energies of formation of calcium titanates, CaTiO 3, Ca 4Ti 3O 10, and Ca 3Ti 2O 7, from their component binary oxides were derived from the reversible e.m.f.s. The results can be summarised by the following equations: CaO(solid) + TiO 2(solid) → CaTiO 3(solid), Δ G° ± 85/(J · mol ?1) = ?80,140 ? 6.302( T/K); 4CaO(solid) + 3TiO 2(solid) → Ca 4Ti 3O 10(solid), Δ G° ± 275/(J · mol ?1) = ?243,473 ? 25.758( T/K); 3CaO(solid) + 2TiO 2(solid) → Ca 3Ti 2O 7(solid), Δ G° ± 185/(J · mol ?1) = ?164,217 ? 16.838( T/K).The reference state for solid TiO 2 is the rutile form. The results of this study are in good agreement with thermodynamic data for CaTiO 3 reported in the literature. For Ca 4Ti 3O 10 Gibbs free energy of formation obtained in this study differs significantly from that reported by Taylor and Schmalzried at T = 873 K. For Ca 3Ti 2O 7 experimental measurements are not available in the literature for direct comparison with the results obtained in this study. Nevertheless, the standard entropy for Ca 3Ti 2O 7 at T = 298.15 K estimated from the results of this study using the Neumann–Koop rule is in fair agreement with the value obtained from low-temperature heat capacity measurements. 相似文献
19.
LiMn 2O 4 spinel nanorods prepared from nanowire MnO 2 templates were capped with polyvinyl pyrrolidone (PVP) and coated with ZrC 2O 4 precursors in aqueous solution. Upon annealing at 600 °C in air, an amorphous ZrO 2 nanoscale coating layer was obtained on the spinel nanoparticles with a particle size of <100 nm that formed from the splitting of the original spinel nanorods. The electrochemical cycling results clearly showed that nanoscale ZrO 2 coating significantly improved the rate capability and cycle life at 65 °C in spite of very high surface area of the spinel nanoparticles. 相似文献
20.
Using solid complex molecular precursor [bis(salicylaldehyde)ethylenediiminecobalt(II)], [Co(salen)], a simple and surfactant-free method to synthesize Co 3O 4 nanoparticles was proposed. Cubic-phase Co 3O 4 nanoparticles of size 30–50-nm could be produced by thermal treatment of the Co(salen) in the air at 500 °C for 5 h. The as-prepared samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The optical absorption spectrum indicates that the direct band gaps of Co 3O 4 nanoparticles are 1.53 and 2.02 eV. The optical property test indicates that the absorption peak of the nanoparticles shifts towards short wavelengths, and the blue shift phenomenon might be ascribed to the quantum effect. The hysteresis loops of the obtained samples reveal their ferromagnetic behavior, an enhanced coercivity ( Hc) and a decreased saturation magnetization ( Ms) as compared to their respective bulk materials. 相似文献
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