Zhiyong Zhong

Sintering dense NiZn ferrite by two-step sintering process

A two-step sintering process has been adopted to produce NiZn ferrite with composition of (NiO)0.35(ZnO)0.57(CuO)0.08(Fe2O3). The densification, microstructure, and magnetic properties of the ferrite have been investigated and compared with those of ferrites produced by the traditional final-stage sintering process. It was found that the sample produced by a two-step sintering process with a high temperature of 1200 °C and a lower temperature of 1100 °C attained more than 96% of the theoretical density and had a uniform microstructure with a small average grain size. It also exhibited good performances in terms of permeability and Q-factor. Hence, this would seem to be an effective method for producing dense ferrites with a microstructure composed of small and even grains, which is a very effective strategy for improving the magnetic properties of ferrites.

Low-loss NiCuZn ferrite with matching permeability and permittivity by two-step sintering process

Magneto-dielectric materials with matched permeability and permittivity are promising candidates as loading materials to reduce the physical dimensions of low-frequency antennas. In this study, NiCuZn ferrites were prepared by both the traditional final-stage sintering process and the two-step sintering process to obtain the magneto-dielectric materials. It was found that the samples sintered by the two-step sintering process tended to obtain microstructure with dense, homogeneous, and small average grain size, which was favorable to obtain low magnetic and dielectric losses. The sample sintered by the two-step sintering process with high temperature 950 °C and holding temperature 900 °C could obtain almost equal permeability and permittivity of around 11.8. And the magnetic and dielectric loss tangents were lower than 0.015 in a frequency range from 10 to 100 MHz. These properties make the material useful to the design of miniaturized antennas.

Enhanced Visible-Photocatalytic Activity of Anodic TiO2 Nanotubes Film via Decoration with CuInSe2 Nanocrystals

The fabrication and photocatalytic properties of visible-light driven CuInSe2/TiO2 heterojunction films are reported. CuInSe2 nanoparticles (NCs) were synthesized using a solvothermal method and then decorated onto self-organized anodic TiO2 nanotube (NT) arrays through an electrophoretic deposition process, forming a CuInSe2 NC/TiO2 NT hetero-structure film. An increase in deposition time produced an increased amount of CuInSe2 NCs loaded onto the TiO2 NT arrays, expanding the light-absorption range of the CuInSe2 NCs/TiO2 NTs film from 400 nm to 700 nm. Photocatalytic degradation results show that activities of the CuInSe2 NCs/TiO2 NTs films were significantly enhanced compared to that of pure TiO2 NTs (degradation rate constant k increased from 3 × 10(-3) min(-1) to >1 × 10(-2) min(-1)). Particularly, the CuInSe2 NCs/TiO2 NTs with 50 min electrophoretic deposition show the highest degradation rate, k = 1.6 × 10(-2) min(-1) (more than 5 times greater than that of the pure TiO2 NTs film), due to optimization of CuInSe2 NCs loading and a well-maintained open TiO2 tube-mouth configuration.

Factors influencing epitaxial growth of three-dimensional Ge quantum dot crystals on pit-patterned Si substrate

We investigated the molecular beam epitaxy growth of three-dimensional (3D) Ge quantum dot crystals (QDCs) on periodically pit-patterned Si substrates. A series of factors influencing the growth of QDCs were investigated in detail and the optimized growth conditions were found. The growth of the Si buffer layer and the first quantum dot (QD) layer play a key role in the growth of QDCs. The pit facet inclination angle decreased with increasing buffer layer thickness, and its optimized value was found to be around 21°, ensuring that all the QDs in the first layer nucleate within the pits. A large Ge deposition amount in the first QD layer favors strain build-up by QDs, size uniformity of QDs and hence periodicity of the strain distribution; a thin Si spacer layer favors strain correlation along the growth direction; both effects contribute to the vertical ordering of the QDCs. Results obtained by atomic force microscopy and cross-sectional transmission electron microscopy showed that 3D ordering was achieved in the Ge QDCs with the highest ever areal dot density of 1.2 × 10(10) cm(-2), and that the lateral and the vertical interdot spacing were ~10 and ~2.5 nm, respectively.

Changing and reversing the exchange bias in a current-in-plane spin valve by means of an electric current

The authors show that a current flowing in a direction not perpendicular to the layer planes of an exchange-biased spin valve systematically changes the exchange bias. The direction of the exchange bias can be completely reversed when a large enough current is applied. This effect occurs only when the direction of the initial sweeping magnetic field is antiparallel to the exchange-bias field. The effect is attributed to the recently predicted current-induced torque in an antiferromagnet and provides evidence to support the prediction that the critical current is smaller in an antiferromagnet than the typical value for current switching in a ferromagnet.

Strong exchange bias with the (110)-oriented BiFeO3 films

Epitaxial BiFeO3 films were grown on the (001)-, (110)-, and (111)-oriented SrTiO3 substrates. Using CoFe as ferromagnetic layer, we have shown that both large exchange bias and coercive field enhancement can be observed in the (001)- and (110)-oriented BiFeO3. But no exchange bias was found in the (111)-oriented BiFeO3 films, which can be understood by the lack of either 109° or 71° ferroelectric domain walls and the preservation of the spatial modulated cycloid spin structure. The observation of large exchange bias field with the (110)-oriented BiFeO3 film may lead to an alternative choice towards reversible control exchange bias by electrical field.

Effects of

The influences of Nb2O5 addition on the microstructure and magnetic properties, especially DC-bias-superposition characteristic of the low-temperature-fired NiCuZn ferrites, were investigated. It was found that Nb ions entered into the lattice when Nb2O5 content was less than 0.3 wt%, after that, a second phase, bismuth iron (zinc or nickel) niobium oxide appeared. The second phase consumed some sintering aid Bi2O3 and had obvious influences on sintered density, grain size and magnetic properties, such as permeability of the ferrites. All of the sintered density, saturation magnetic flux density and average grain size gradually decreased with Nb2O5 content. Initial permeability first increased, and then continuously decreased with Nb2O5 content. The sample with 0.1 wt% Nb2O5 displayed the highest initial permeability, which was mainly due to the decrease of magnetocrystalline anisotropy constant. Under relatively low superposition magnetic field H ≤ 375 A/m, a higher initial permeability favored to obtain a higher incremental permeability. However, when the superposition magnetic field continued to increase, some samples with a lower initial permeability displayed a higher incremental permeability. The normalized incremental permeability was mainly determined by the coercivity of the samples. A higher coercivity favored the attainment of a better DC-bias-superposition characteristic on normalized permeability. So the sample with 0.5 wt% Nb2O5, which displayed the largest coercivity, had the best performance. Possible mechanisms contributing to the above results were discussed in detail.

{\hbox {Nb}}_{2}{\hbox {O}}_{5}

The influence of microstructure on the magnetic properties, especially the dc-bias-superposition characteristics, of NiZn ferrites has been investigated. The value of saturation magnetic flux density (<i>B</i>_s) first increased and then changed little with variation of the sintered density, and showed little relation to the microstructure. Coercivity (<i>H</i>_c) decreased continuously with increasing average grain size. Δ<i>B</i> (<i>B</i>_s-remanence) first increased with increasing average grain size, but then gradually decreased when abnormal grain growth prevailed. The incremental permeability (permeability with magnetic field superposition) was influenced by both Δ<i>B</i> and <i>H</i>_c. A sample with higher Δ<i>B</i> and <i>H</i>_c favored the attainment of a higher incremental permeability. The <i>Q</i>-factor was mainly determined by the sintered density and microstructure. To sum up, a uniform and dense microstructure with relatively small average grain size is favorable for obtaining better dc-bias-superposition characteristics, including permeability and <i>Q</i>-factor.

on DC-Bias-Superposition Characteristic of the Low-Temperature-Fired NiCuZn Ferrites

Fe/NixZn1-xFe2O4 ( x=0.1, 0.3, 0.5, 0.7, 0.9) soft magnetic composites were prepared by the chemical co-precipitation method. The effect of Ni content on the magnetic properties was discussed. The results of SEM, EDS and X-ray diffraction analysis indicate that an uniform and dense Ni-Zn ferrite was coated on the surface of bared Fe. The maximum of saturation magnetization of soft magnetic composites was obtained at x=0.7, however its magnetic loss is largest at 10-100 kHz. Analyzes of permeability spectra of the soft magnetic composites show that the contribution of spin rotation to the spectra becomes comparable with that of domain wall movement due to the magnetic insulating ferrite coating, which helps improve the working frequency of the soft magnetic composites drastically.

Influence of Microstructure on the DC-Bias-Superposition Characteristics of NiZn Ferrites

This study reports the room-temperature ferromagnetism in Cr-doped TiO2 nanotubes (NTs) synthesized via the electrochemical method followed by a novel Cr-doping process. Scanning electron microscopy and transmission electron microscopy showed that the TiO2 NTs were highly ordered with length up to 26 μm, outer diameter about 110 nm, and inner diameter about 100 nm. X-ray diffraction results indicated there were no magnetic contaminations of metallic Cr clusters or any other phases except anatase TiO2. The Cr-doped TiO2 NTs were further annealed in oxygen, air and argon, and room-temperature ferromagnetism was observed in all Cr-doped samples. Moreover, saturation magnetizations and coercivities of the Cr-doped under various annealing atmosphere were further analyzed, and results indicate that oxygen content played a critical role in the room-temperature ferromagnetism.