Ni0.8Zn0.2Fe2O4和CO2Y添加Ni0.8Zn0.2Fe2O4磁性质对比研究

采用固相反应法制备了Ni0.8Zn0.2Fe2O4和2wt％ Co2Y添加Ni0.8Zn0.2Fe2O4铁氧体多晶样品,并对样品的磁性质进行了研究.实验发现,添加Co2Y后,铁氧体的磁损耗明显下降,相同条件下Q值增大到原来的3倍左右.对添加Co2Y引起铁氧体磁损耗下降的原因进行了讨论,认为主要是钴离子掺杂引起的铁氧体磁滞损耗明显降低所导致的.     

Cluster spin glass behaviour of mixed spinel ferrite Ga0.8Fe0.2NiCrO4

Mössbauer effect, d. c. magnetisation and a. c. susceptibility measurements in disordered spinel ferrite Ga_0.8Fe_0.2NiCrO₄ (0.3≦T≦300 K, ?12≦H (external field) ≦12 kOe) show it to be a cluster spin glass where Gabay-Toulouse phase diagram transitions occur not for individual spins but for groups of correlated spins (clusters) whose effective spin's transverse and longitudinal components successively freeze.     

(Ni0.8Zn0.2Fe2O4)epoxy-PZT双层膜中的磁电效应

讨论了Ni0.8Zn0.2Fe2O4(NZFO)与锆钛酸铅(PZT)的双层膜结构样品的磁电(ME)效应.NZFO粉料由溶胶-凝胶法制成,再经900℃热压,并高温烧结.在该双层膜中测量到了很强的磁电相互作用.发现横向的磁电效应比纵向效应大一个数量级,并且随NZFO烧结温度的提高而增加.当烧结温度从950℃上升到1380℃时,横向ME电压系数(αE)的最大值变化范围为25.6 mV Am-2≤αE≤199.6 mV Am-2.理论分析显示NZFO-PZT双层膜样品中ME效应源于NZFO与PZT之间相对良好的磁电耦合.     

On the origin of the ferromagnetism in Zn0.8Mn0.2O having a higher Curie temperature than Zn0.8Co0.2O

Zn_0.8Co_0.2O and Zn_0.8Mn_0.2O films were deposited on substrates by a sol–gel technique. X-ray diffraction, field-emission scanning electron microscopy, photoluminescence, and ferromagnetism measurements were used to characterize these dilute magnetic semiconductors. It is shown that the ferromagnetic properties might be related to the formation of acceptor-like defects in the Zn_0.8Co_0.2O and Zn_0.8Mn_0.2O films. It is found that ferromagnetic Zn_0.8Mn_0.2O has a higher Curie temperature than Zn_0.8Co_0.2O. In addition, the higher ratio of grain-boundary area to grain volume of Zn_0.8Mn_0.2O than Zn_0.8Co_0.2O indicates that grain boundaries and related acceptors are the intrinsic origin for ferromagnetism.     

Oxygen reduction at sol–gel derived La0.8Sr0.2Co0.8Fe0.2O3 cathodes

The perovskite material, La_0.8Sr_0.2Co_0.8Fe_0.2O₃ (LSCF), substituted by Sr and Fe at the A and B sites, was prepared using the sol–gel (SG) method, followed by heating at 900 °C for 4 h. The X-ray powder diffraction pattern for the SG derived LSCF material showed good agreement with the literature data. Scanning electron microscopy showed that the LSCF structure is highly porous, facilitating gas transfer and maximizing the number of active sites for the oxygen reduction reaction (ORR) at the cathode of a solid oxide fuel cell. Transmission electron microscopy (TEM) was employed to determine the SG–LSCF particle size and distribution. The kinetics of the ORR were investigated at SG–LSCF, deposited by screen-printing on a samarium-doped ceria (SDC) electrolyte, using electrochemical impedance spectroscopy and cyclic voltammetry at temperatures ranging from 400 to 700 °C. The results showed that the SG–LSCF cathode is stable and exhibits a high exchange current density (and low charge transfer resistance), yielding an apparent activation energy for the ORR of ca. 120 kJ/mol. It was also found that the SG–LSCF on SDC cathode was approximately one order of magnitude more active than standard manganite-based composite cathodes, deposited on yttria stabilized zirconia, studied under otherwise identical operating conditions.     

Magnetic Behaviour of Nano-Particles of Ni0.8Cu0.2Fe2O4

Nano-particles of Ni_0.8Cu_0.2Fe₂O₄ have been synthesized by the co-precipitation method. Langevin function fitting of the superparamagnetic M–N curve at 290 K provides a log-normal distribution with median diameter of 30 Å and standard deviation of 0.4. Outside a core of ordered spins, moments in the surface layer are disordered. Magnetization evolves over a long period of time t going linearly with log t. Magnetic anisotropy which was estimated by fitting the M–log t curve shows many fold increase over that of bulk particle sample. Major contribution to this enhancement comes form the disordered surface spins.     

Structure and oxygen stoichiometry of SrCo0.8Fe0.2O3−δ and Ba0.5Sr0.5Co0.8Fe0.2O3−δ

High temperature X-ray diffraction (HT-XRD), temperature programmed desorption (TPD), thermogravimetric analysis–differential thermal analysis (TGA/DTA) and neutron diffraction were combined to determine the structure and oxygen stoichiometry of SrCo_0.8Fe_0.2O_3−δ (SCF) and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) up to 1273 K in the pO₂ range of 1 to 10^− 5 atm. Formation of the vacancy-ordered brownmillerite phase, SrCo_0.8Fe_0.2O_2.5, was observed as a region of zero oxygen release in the TPD measurements and confirmed by HT-XRD and TGA/DTA. No ordering was observed in the BSCF system by any of the techniques utilized in this work. The oxygen vacancy concentration of BSCF was found to be considerably higher than that of SCF and always higher than that of the ordered brownmillerite phase of SCF, δ = 0.5. The combination of a high vacancy concentration and absence of ordering leads to higher oxygen permeation fluxes through BSCF membranes in comparison to SCF.     

Pinned and entropic disordering of mixed spinel ferrite Ga0.8Fe0.2NiCrO4

Mössbauer and magnetic hysteresis measurements (T=1.4 K and H=-12 to+12 kOe) confirm the existence of pinned and entropic spins in the frustrated disordered spinel ferrite (Ga³⁺_0.8Fe³⁺_0.2) [Ni²⁺Cr³⁺]O₄, as suggested by earlier studies.     

Synthesis,structural and complex impedance spectroscopy studies of Ni0.4Co0.4Mg0.2Fe2O4 spinel ferrite

Spinel ferrite having composition Ni_0.4Co_0.4Mg_0.2Fe₂O₄ was prepared by sol-gel method. X-ray diffraction result indicates that the ferrite sample has a cubic spinel type structure. FT-IR showed two absorption bands (ν₁ and ν₂) that are attributed to the stretching vibration of tetrahedral and octahedral sites. Complex impedance properties have been investigated in 200–420 K temperature range with varying frequency between 40 and 10⁷ Hz. Frequency and temperature dependency of imaginary part of permittivity (?″) and dielectric loss (tanδ) has been discussed in terms of hopping of charge carriers between Fe²⁺ and Fe³⁺ ions. Activation energy has been estimated from both temperature dependency of dc conductivity and relaxation time data, which indicates that the relaxation process and conductivity have the same origin. Nyquist plots of impedance show semicircle arcs for sample and an electrical equivalent circuit has been proposed to explain the impedance results.     

Spin glass behaviour of the spinel system GexCu1−xFe2O4

Ac-susceptibility measurements at various temperatures and hysteresis studies at various fields indicate that the system Ge_xCu_1?xFe₂O₄ (x ? 0.6) contains mixed multidomain, single domain or superparamagnetic states. Sharp susceptibility maxmima, resembling those of spin glasses, are observed for x = 0.1 and 0.2, which may be attributed to the single domain-superparamagnetic (SDSP) transition and the overall behaviour shows a cluster spin glass type of ordering.     

Electromechanical properties of Co0.6 Zn0.4 Fe2O4 ferrite transducer

Co_0.6Zn_0.4Fe₂O₄ ferrite has been prepared by the general ceramic method. The resonance frequency of the disk for the radial and thickness mode was investigated as a function of mechanical stress. The increase of the resonance frequency of the transducer is attributed to domain wall motion inhibition. The increase of ultrasonic velocity with increasing mechanical stress can be explained as due to retarding the oscillation of the ferrimagnetic domains under applied high-frequency electric field. The induced strain increases the resonance frequency. The high values of the electromechanical coupling factor for the radial mode K_p, and thickness mode, K_t, indicate that this composition is useful for producing high-frequency ultrasonic waves that can be used in modern technology.     

Study of Co0.5Zn0.5Fe2O4 nanoparticles for magnetic hyperthermia

Physics of the Solid State - The structural characteristics, magnetic properties, and processes of magnetic heating in an alternating magnetic field of magnetic nanoparticles (MNPs) Co0.5Zn0.5Fe2O4...     

Structural, thermal and electrochemical properties of chromium-substituted LiNi0.8Co0.2O2

LiCr_yNi_0.8−yCo_0.2O₂ compositions, where y=0.000, 0.010, 0.025, 0.040, 0.050, 0.075 and 0.100, were synthesized via a conventional ceramic route. X-ray diffraction studies indicated cation mixing for the compositions with y ≥ 0.05. Cyclic voltammetric studies revealed that the systems were reversible only when y was lower than 0.05. High levels of substitutions with Cr resulted in highly irreversible systems, either due to cation mixing or the displacement of the substituent ions to the lithium inter-slab regions, or both. The charge-discharge characteristics of LiCr_yNi_0.8−yCo_0.2O₂ were similar to those of the unsubstituted material over ten cycles. All the other substituted compositions showed much lower capacities and reduced cyclability. LiCr_0.025Ni_0.775Co_0.2O₂ gave a first-cycle capacity of 169 mAh/g in the 3.0 to 4.4 V window at a 0.1 C rate, fading to 156 mAh/g in the tenth cycle. Differential scanning calorimetric studies revealed that substituting with chromium produced no benefit to thermal stability. The structural, thermal and electrochemical properties of the pristine and Cr-substituted LiNi_0.8Co_0.2O₂ compositions are discussed.     

Oxygen permeability of CaTi0.8Fe0.2O3-δ

The oxygen permeation through a dense ceramic membrane with the composition CaTi_0.8Fe_0.2O_3-δ has been measured as a function of the oxygen partial pressure gradient across the membrane at high temperatures (1000, 1050 °C). The permeation current dependence on the oxygen pressure gradient indicates that bulk diffusion controls the oxygen transport rate in the membrane under moderate oxygen pressure gradients, while the surface exchange limits the oxygen permeation at high oxygen pressure gradients. An oxygen concentration drop appears at the membrane-gas boundary.     

Synthesis and electric properties of perovskite Pr0.6Ca0.4Fe0.8Co0.2O3 for SOFC applications

In this study, polycrystalline powder Pr_0.6Ca_0.4Fe_0.8Co_0.2O₃ (PCFC) was synthesized by a sol–gel process. This oxide was analyzed by X-ray powder diffraction. Synthesized Pr_0.6Ca_0.4Fe_0.8Co_0.2O₃ showed up to be single phase and belongs to the orthorhombic crystalline system with a Pbnm space group. The microstructural features of the synthesized products display particles having an irregular morphology and a size in the range of 50–100 nm. X-ray diffraction (XRD) analysis shows the chemical compatibility between the PCFC cathode and the electrolyte Sm-doped ceria since no reaction products were honored when the material was mixed and co-fired at 1,000 °C for 168 h. The thermal expansion coefficient of PCFC 16.9?×?10^?6 °C^?1 is slightly higher than that of Ce_0.8Sm_0.2O_1.9 (SDC) over the studied temperature range. The greater contribution to the total resistance of the electrode is the electrochemical resistance associated with oxygen exchange in the cathode surface (0.96 Ωcm²). The dc four-probe measurement indicated that PCFC exhibits fairly high electrical conductivity, over 100 S cm^?1 at T?≥?500 °C, making this material promising as a cathode material for intermediate temperature solid oxide fuel cells.     

Hyperfine interactions and site population in the spinel compounds Ca x Co1−x Fe2O4

Mössbauer spectra of the spinel series Ca _x Co_1–x Fe₂O₄, withx=0.0, 0.05 and 0.10, have been collected between 80 and 300 K and in an applied field of 6 T at 4.2 K. From the shape-independent hyperfine field distribution fits of the external field spectra, and from the binomial distribution fits of the temperature-dependent spectra, the hyperfine field and the center shift could be unambiguously assigned to octahedrally and tetrahedrally coordinated ferric ions. The importance of the application of an external field in conjunction with a reasonable physical model to obtain a correct assignment of hyperfine parameters to a specific site is demonstrated.     

Ba0.5Sr0.5Co0.8Fe0.2O3−δ thin film microelectrodes investigated by impedance spectroscopy

The electrochemical properties of geometrically well-defined Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) microelectrodes have been investigated by impedance spectroscopy. The microelectrodes of 20–100 μm diameter and 100 nm thickness were prepared by pulsed laser deposition (PLD), photolithography and argon ion beam etching. The oxygen reduction reaction at these model electrodes is limited by interfacial processes, i.e. by the oxygen surface exchange and/or by the transfer of oxide ions across the electrode/electrolyte boundary, whereas the resistance associated with the transport of oxide ions through the bulk of the thin film electrode is negligible. The experiments revealed an extremely low absolute value of the electrochemical surface exchange resistance of only 0.09 (± 0.03) Ω cm² at 750 °C in air, which is more than a factor of 50 lower than the corresponding value measured for La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) microelectrodes of the same geometry. The dependence of this and other electrochemical quantities such as the chemical bulk capacitance or the BSCF/YSZ interfacial resistance on temperature has been studied between 500 and 750 °C.     

Malonic acid-assisted synthesis of LiNi0.8Co0.2O2 cathode active material for lithium-ion batteries

Polycrystalline LiNi_0.8Co_0.2O₂ was synthesized by a solution route with malonic acid as the complexing agent. The effects of temperature, duration of heat treatment, pH of the precursor solution, and the nature of the solvent employed on the performance characteristics of the product were studied. It was observed that a 12-hour 800 °C heat treatment protocol was necessary to obtain products with optimal electrochemical characteristics. Furthermore, an excess lithium stoichiometry of 1.05 was found to be detrimental to the performance of the cathode material. The beneficial effect of ethanol as a solvent over water on the product characteristics is explained by the presence of solvent molecules in the coordination sphere of the cations. A pH of 7, at which malonic acid is complexed completely with the cations without interference from other nucleophiles, was found to be ideal for the synthesis of the cathode active material from aqueous solutions. With ethanol as the medium, the product formed by a 12-h calcination at 800 °C yielded a first-cycle capacity of 173 mAh/g and a tenth-cycle capacity of 169 mAh/g.     

The magnetic cluster blocking in Fe0.35V2O5 spin glass

The Wohlfarth method applied to the semiconducting spin glass Fe_0.35V₂O₅ indicates that the most intensive blocking of magnetic clusters occurs in low magnetic field at 7.5 K just below the temperature of the susceptibility maximum. However, the blocking process is diffused and several classes of clusters are blocked at higher temperatures.     

Effects of excessive Zn2+ ions on intrinsic magnetic and structural properties of Ni0.2Zn0.6Cu0.2Fe2O4 powder prepared by chemical coprecipitation method

Zn(OH)₂ is a kind of amphoteric compound. Therefore, for chemical coprecipitation method, the precipitation of Zn²⁺ ions may be incomplete if using NaOH as precipitator. In this study, single-phase powder specimens with a nominal composition Ni_0.2Zn_0.6Cu_0.2Fe₂O₄ were prepared with chemical coprecipitation method, and the effects of excessive Zn²⁺ content (x, x = 3%, 5%, 7%, 9%) in working solution on intrinsic magnetic and structural properties were studied by vibrating sample magnetometer and X-ray diffractometer, respectively. It was found that the magnetization when H_m = 398 kA/m (5000 Oe) reached a maximum when x = 5%, and then decreased with the increase of x, which was attributed to the effect of different amount of Zn²⁺ in A sites on the A–B and B–B exchange interaction. Moreover, it was found that the lattice parameter was affected by the Zn²⁺ and Fe³⁺ ions due to their different ion radius to a certain extent.