Key Engineering Materials Vols. 512-515

Abstract: In this paper, Ni_0.5Zn_0.5Fe₂O₄ferrite ceramics were produced with Ni_0.5Zn_0.5Fe₂O₄powders prepared by reaction in a molten salt using Fe₂O₃, NiO and ZnO powders as raw materials. Ni-Zn powders were characterized by X-ray diffraction (XRD). Density and shrinkage in diameter of Ni_0.5Zn_0.5Fe₂O₄ceramics were measured. MicroSubscript text structures were observed using field emission scanning electron microscopy (FESEM), and magnetic properties were examined by HP4291B impedance analyzer. XRD pattern indicated that the powders synthesized at 900°C for 1h were pure spinel Ni_0.5Zn_0.5Fe₂O₄phase. Via the measurement of density and shrink in diameter, optimal temperature was 1200°C and holding time was 2h, which was in accord with the results concluded from micrographs. FESEM images illustrated that the average grain size increased with increasing holding time, which followed the Ostwald liquid growth mechanism. At lower frequencies, the initial permeability (μ_i) increased from 86.65 to 183.48 with increasing holding time, while the threshold frequency decreased from 13.3MHz to 8.52MHz, which implied the initial permeability (μ_i) variation complied with the Snoek law to some extent.

Abstract: Ferrous micro-beads from fly-ash were used as starting material to prepare CoZn-Z type barium ferrite. As the main impurity in the beads, SiO₂ plays an important role on the electromagnetic properties of the ferrites. To investigate the effect of SiO₂ content, a series of CoZn-Z type ferrites with various contents of SiO₂ (1 wt%, 2 wt% and 5 wt%) were prepared with analytical pure Fe₂O₃ and SiO₂. The density, phase composition, microstructure and electromagnetic properties of the sintered samples were investigated. The results show that the permeability of samples alters little at a low content of SiO₂ about 1 wt%, but deteriorates dramatically when the addition of SiO₂ is above 2 wt%. The ferrous micro-beads were then treated by alkali cleaning to reduce the content of SiO₂ to about 1 wt%, and the samples prepared with these treated beads show a similar performance on the electromagnetic properties with pure CoZn-Z type ferrite.

Abstract: The carbodiimide-based non-oxide sol-gel process is a novel route to non-oxide nitride and carbide ceramics. This process has been applied to fabricate ternary or binary silicon based nitride and carbide ceramics. Based on this non-oxide sol-gel process, iron carbide and nitride have been fabricated by reaction of iron trichloride with bis(trimethylsilyl)carbodiimide to form FeCN gel followed by pyrolysis in argon flow at different temperatures. The iron carbide material obtained at 700 °C exhibits hard ferromagnetic properties whereas α-iron along with iron nitride formed at 1300 °C shows soft ferromagnetic properties. Therefore, iron carbide and nitride ceramics with controlled magnetic properties can be obtained along this novel non-oxygen sol-gel process by controlled pyrolysis. The pyrolysis behavior was investigated based on thermal gravimetric analysis coupled with differential scanning calorimetry. The phase structures of the iron carbide and nitride are identified by X-ray diffraction and the magnetic properties of the materials are measured by magnetometer.

Abstract: Bi0.95Eu0.05Fe0.95Co0.05O3 Nanoparticles sample was prepared by sol-gel process. The microstructure of samples was analysised by X-ray diffraction(XRD), the result indicated that it was the single phase rhombohedral perovskite structure. The morphology of samples was measured by scanning electron microsopy(SEM), the SEM photograph of samples indicated that the nanoparticles of Bi0.95Eu0.05Fe0.95Co0.05O3 sample were small than that of BiFeO3. The valence states of Fe ions in the samples was analysised by the X-ray absorption spectroscopy（XAS）. The XAS of Fe2p showed that it was the mixed valence states (Fe2+ and Fe3+) of Fe ions in samples, and the binding energy of Bi0.95Eu0.05Fe0.95Co0.05O3 was bigger than that of BiFeO3.The magnetic characteristics of the samples were measured by vibrating sample magnetometer (VSM)，the results showed that the weak metamagnetism were obtained from clear hysteresis loop and the magnetic saturation reached 0.408emu/g，compared with BiFeO3 sample, the magnetic properties were significantly enhanced.

Abstract: In this article, Co-doped SrTiO₃ nanofibres have been prepared by electrospinning from a sol-gel precursor and the following calcination at 923K. XRD results confirmed that no second phase was formed, and Co ions successfully occupied the Ti sites. By annealing in hydrogen, oxygen vacancies and (Co-H-Co) were formed, which both contributed to the magnetic ordering in SrTi_1-xCo_xO₃ nanofibres are at 300K.

Abstract: The physical properties of magnetic liquids are theoretically analyzed in this paper, such as the effect of the magnetic volume force on the free surface of magnetic liquids, the density spectrum of the magnetic liquids based on horizontal component of the magnetic volume force, the first order levitation principle and the second order levitation principle, the magneto-electric principle of magnetic liquids, the fluidity of magnetic liquids used as magnetic core and the fluxgate effect of magnetic liquids. Some methods to study the properties of magnetic liquids were provided by the authors, and the effects of the magnetic volume force on the free surface of magnetic liquids and the second order levitation effect were experimentally studied in this paper. The analysis methods that provided in this paper may be applied to the design of new sensors.

Abstract: Researchers have been studying the magnetic fluid seal of small gap in the past. In order to meet the static seal requirements of large gap in aviation, aerospace, metallurgy, and so on. We set up the experimental rig of magnetic fluid static seal and designed the static seal of magnetic fluid. In the experimental rig, we studied the static seal of magnetic fluid and obtained the relationships among the burst pressure and the amount of injection of magnetic fluid, seal gap, temperature and magnetization in the experimental rig. In theory, we calculated the distribution of magnetic field under the seal gap and reasoned sustained pressure and temperature and calculated the burst pressure in different gaps and different saturation magnetization. We analyzed the experimental and theoretical results in detail. The theoretical analysis and tests indicated that magnetic fluid seal of large gap and large diameter can sustain a definite pressure and meet the requirements of practical problems.

Abstract: In order to meet the requirements of the static seal at large diameter in chemical industry, aerospace industry and military industry, we designed a magnetic fluid static seal at large diameter flange and set up a test rig. We studied the effect of magnetic fluid static seal on the test rig. Finally, we obtained the relationship between the anti-pressure capacity and the number of the seal stages. We also calculated the distribution of the magnetic field in the seal gap and the anti-pressure capacity in theory. The theoretical analysis and the experiment result indicate that the design of the magnetic fluid static seal at large diameter flange is feasible and can be used to meet the practical problems.

Abstract: Two different structure of dampers based on the principle of second-order buoyancy were designed in this paper. In order to verify the reduction of the vibration, a test rig was set up. On the test rig, the reduction effect of two different dampers was studied by adjusting the amount of the magnetic fluid. The experiment results indicate the relationship between the damping effect of magnetic fluid damper and the amount of magnetic fluid and this analysis result is benefit to the design of the dampers.

Abstract: The buoyancy principle of magnetic fluid is one of the most attractive effects of magnetic fluid statics. But it is too difficult to calculate the buoyancy directly due to the complexity of magnetic field. Moreover, numerical calculation employs too many assumptions. So in this paper the experimental method is adopted to measure the second-order buoyancy received by permanent magnet immersed in magnetic fluid, which depends on the distance between the magnet and the wall of vessel. The influence of the diameter of cylindrical vessels, the size of magnets and the saturation magnetization of magnetic fluid on the buoyancy is also analyzed qualitatively.