Nuclear Magnetic Resonance Studies of Silicon Carbide Polytypes

The ²⁹Si MAS NMR spectra of the 2H, 4H, 6H, and 3C polytypes of silicon carbide are presented. An attempt is made to correlate differences in the chemical shifts with local atomic environment. The results of the analysis of the spectra of pure polytypes are used as a basic for the interpretation of the spectra of mixed polytypes and a discussion of the crystallinity and impurity levels of different samples. Carbon-13 chemical shifts obtained from spectra of the same polytypes are also tabulated.     

Magnetic Resonance Visualisation of Flow and Pore Structure in Packed Beds with Low Aspect Ratio

Different NMR techniques were combined to obtain the structure and velocity information for a systematic investigation of fixed beds with low aspect ratio (tube diameter to particle diamter, d_t/d_p) in the range 1.4 to 32. The structure of the void space was determined for a variety of packed beds of glass beads or regular and irregular porous pellets by magnetic resonance imaging (MRI). Based on the images the radial distribution of the voids within the bed was obtained. Ordering effects were found even for non‐spherical and polydisperse particles, and a maximum of the fluid density near the tube wall was confirmed for all pellet geometries and sizes. By combining MRI with velocity encoding, velocity profiles and distributions of flow velocity components of a single fluid phase through packed beds have been acquired. The radial velocity distribution follows an oscillatory pattern which largely reflects the ordering of the particles, which can be accessed from the density distribution of the interparticle fluid. Maximum velocities of up to four times the average value were found to occur near the tube wall. This wall effect was observed for all but the smallest particles, where the aspect ratio was d_t/d_p = 32. Moreover, a visualisation of flow pattern in the presence of packed particles was achieved by using a tagging technique, and the stationary flow field could be identified for an experimental time of several hours.     

A Micellar Route to Ordered Arrays of Magnetic Nanoparticles: From Size‐Selected Pure Cobalt Dots to Cobalt–Cobalt Oxide Core–Shell Systems

Starting with Co‐salt‐loaded inverse micelles, which form if the diblock copolymer polystyrene‐block‐poly(2‐vinylpyridine) is dissolved in a selective solvent like toluene and CoCl₂ is added to the solution, monomicellar arrays of such micelles exhibiting a significant hexagonal order can be prepared on top of various substrates with tailored intermicellar distances and structure heights. In order to remove the polymer matrix and to finally obtain arrays of pure Co nanoparticles, the micelles are first exposed to an oxygen plasma, followed by a treatment in a hydrogen plasma. Applying in‐situ X‐ray photoelectron spectroscopy, it is demonstrated that: 1) The oxygen plasma completely removes the polymer, though conserving the original order of the micellar array. Furthermore, the resulting nanoparticles are entirely oxidized with a chemical shift of the Co 2p_3/2 line pointing to the formation of Co₃O₄. 2) By the subsequent hydrogen plasma treatment the nanoparticles are fully reduced to metallic Co. 3) By exposing the pure Co nanoparticles for 100 s to various oxygen partial pressures pequation/tex2gif-inf-5.gif, a stepwise oxidation is observed with a still metallic Co core surrounded by an oxide shell. The data allow the extraction of the thickness of the oxide shell as a function of the total exposure to oxygen (pequation/tex2gif-inf-7.gif × time), thus giving the opportunity to control the ferromagnetic–antiferromagnetic composition of an exchange‐biased magnetic system.     

Interconnected power systems with superconducting magnetic energy storage

The objective of this work is to discuss the concept of back‐to‐back interconnection systems with energy storage, especially with a Superconducting Magnetic Energy Storage (SMES) incorporated into a back‐to‐back DC link. In this case, each converter of the back‐to‐back system is used as a power conditioning system for the SMES coils. Since the AC–DC converter can be designed independently of the frequency of the power system, a two‐way switch is connected to the AC side of each converter. This two‐way switch can select the interconnected power systems. By using the two‐way switches, this system can provide the stored energy in the SMES system to each interconnected power system through two AC–DC converters. For instance, lower‐cost power of each power network can be stored through two converters during the off‐peak hours and made available for dispatch to each power network during periods of demand peak. Then this system increases the reliability of electric power networks and enables the economical operations depending on the power demand. This paper describes the unique operations of the back‐to‐back interconnection with SMES and discuses the optimal SMES configuration for a 300‐MW‐class back‐to‐back interconnection. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(2): 37–43, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20482     

锰锌铁氧体纳米粉体的液相制备及磁性能

以δ-FeOOH为前驱体,用氨水调节溶液的pH值,分别采用(1)90℃水浴加热动态转化、(2)沸腾回流动态转化,(3)90℃静态转化及(4)200℃水热法四种方法合成了锰锌铁氧体纳米粉体.采用X射线衍射仪(XRD),透射电子显微镜(TEM)、振动样品磁强计(VSM)和X射线荧光光谱仪(XRF)等方法对粉体进行了表征,对四种液相法制备的锰锌铁氧体纳米粉体的结构和性能等进行了对比和分析.结果表明,四种方法中沸腾回流相转化法得到的产物具有磁性能较好、形状较规则、粒径可控等优点.     

Composition and morphology of char particles of fly ashes from industrial burning of high-ash coals with different reactivity

A comparative investigation of the composition and the morphology of char particles was conducted: char particles were recovered from fly ashes of two power stations in Russia from burning of high- and low-reactivity high-ash coals; the known results of studies of char particles generated in laboratory conditions from coals characterized according to the ASTM D388-98a standard were also used. The composition of organic and mineral components of different fraction char particles was studied. An inverse correlation between the content of the organic substance and the iron content in char particles was identified. The morphology of the char particles for the three main types (Cenospheric, Network and Solid) and the influence of coal reactivity and temperature on char morphology were investigated. The morphology of the mineral component of char particles of the two varieties of coals was also studied. It was shown that the high-temperature of industrial burning of Ekibastuz coal results in melting of the mineral substance and forming of micron-scale microspheres located in the lamellar porous structure of the carbon matrix.     

Synthesis and magnetic properties of novel fully conjugated polymeric complexes containing 1,10‐phenanthroline

A novel fully conjugated polymer containing 1,10‐phenanthroline (DAPcDOD) was first synthesized by the polycondensation of 2,7‐dimethyl‐2,4,6‐octatriene‐1,8‐dial with 5,6‐diamine‐1,10‐phenanthroline. Three polymeric complexes were first prepared by the reaction of DAPcDOD with NiSO₄, CoCl₂, and FeSO₄, respectively. The structures of the polymer and the complexes were characterized by IR, ¹H‐NMR, and elemental analysis. The magnetic behaviors of these complexes were measured as a function of magnetic field strength (0–50 kOe) at 5 K and as a function of temperature (5–300 K) at a magnetic field strength of 30 kOe. The results show that DAPcDOD–Ni²⁺ and DAPcDOD–Co²⁺ were soft ferromagnets, whereas DAPcDOD–Fe²⁺ exhibited the features of an antiferromagnet. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

机械合金化的机理及其在稀土永磁材料中的应用

简述了机械合金化的机理及其在稀土永磁材料中的应用。指出了今后用机械合金化技术制备稀土永磁合金需要解决的主要问题。     

Techniques and Apparatus for Measuring Rotational Core Losses of Soft Magnetic Materials

In many situations such as the cores of a rotating electrical machine and the T joints of a multiphase transformer, the local flux density varies with time in terms of both magnitude and direction, i.e. the flux density vector is rotating. Therefore, the magnetic properties of the core materials under the rotating flux density vector excitation should be properly measured, modeled and applied in the design and analysis of these electromagnetic devices. This paper presents an extensive review on the development of techniques and apparatus for measuring the rotational core losses of soft magnetic materials based on the experiences of various researchers in the last hundred years.