Deng Longjiang

Micromagnetic simulation on the dynamic susceptibility spectra of cobalt nanowires arrays: the effect of magnetostatic interaction

Micromagnetic simulations have been performed to obtain the dynamic susceptibility spectra of 4×4 cobalt nanowire arrays with different spatial configurations and geometries. The susceptibility spectra of isolated wires have also been simulated for comparison purposes. It is found that the susceptibility spectrum of nanowire array bears a lot of similarities to that of an isolated wire, such as the occurrences of the edge mode and the bulk resonance mode. The simulation results also reveal that the susceptibility spectrum of nanowire array behaves like that of single isolated wire as the interwire distance grows to an extent, which is believed due to the decrease of magnetostatic interaction among nanowires, and can be further confirmed by the static magnetic hysteresis simulations. In comparison with single nanowire, magnetostatic interaction may increase or decrease the resonance frequencies of nanowire arrays assuming a certain interwire distance when the length of array increases. Our simulation results are also analysed by employing the Kittel equation and recent theoretical studies.

Electromagnetic wave absorbing properties and hyperfine interactions of Fe—Cu—Nb—Si—B nanocomposites

The Fe—Cu—Nb—Si—B alloy nanocomposite containing two ferromagnetic phases (amorphous phase and nanophase phase) is obtained by properly annealing the as-prepared alloys. High resolution transmission electron microscopy (HR-TEM) images show the coexistence of these two phases. It is found that Fe—Si nanograins are surrounded by the retained amorphous ferromagnetic phase. Mossbauer spectroscopy measurements show that the nanophase is the D03-type Fe—Si phase, which is employed to find the atomic fractions of resonant 57Fe atoms in these two phases. The microwave permittivity and permeability spectra of Fe—Cu—Nb—Si—B nanocomposite are measured in the frequency range of 0.5 GHz–10 GHz. Large relative microwave permeability values are obtained. The results show that the absorber containing the nanocomposite flakes with a volume fraction of 28.59% exhibits good microwave absorption properties. The reflection loss of the absorber is less than −10 dB in a frequency band of 1.93 GHz–3.20 GHz.

Mössbauer studies on the shape effect of Fe84.94Si9.68Al5.38 particles on their microwave permeability

Ball milling for long time (such as 10, 20, and 30 h) can transform Fe84.94Si9.68Al5.38 alloy powders with irregular shapes into flakes. X-ray diffraction (XRD) and Mossbauer measurements have proven that the unmilled particles and the flakes obtained by milling for 10 h have the same D03-type superlattice structure. The flakes obtained by milling for 20 h and 30 h have the same disorder α-Fe(Si, Al) structure. There are more than 6 absorption peaks in the transmission Mossbauer spectra (TMSs) for the particles with D03-type superlattice structure, which can be fitted with 5 sextets representing 5 different Fe-site environments. However, only 6 TMS absorption peaks have been found for particles with a disorder α-Fe(Si, Al) structure, which can be fitted with the distributions of Mossbauer parameters (Bhf, isomer shift). The TMS results show that the flaky particles have a stronger tendency to possess the planar magnetic anisotropy. As the result, the flakes have larger microwave permeability values than particles with irregular shapes. The conversion electron Mossbauer spectra (CEMSs) also show the significantly different Fe-sites environments between the alloy surface and the inside.

Co-occurrence of Dominant Direct and Indirect Transitions in Low Temperature Sputtered Indium Tin Oxide Thin Films on Polymers

Indium tin oxide (ITO) layers are prepared on polymethylmethacrylate and polyethylene terephthalate by rf-magnetron sputtering at room temperature. Their 3D-AFM images and visible transmittance spectra are contrastively characterized and studied. An interesting morphological effect of the polymer substrates on their top ITO layers is observed. Dominant direct and indirect transition types deduced from optical spectra are surprisingly found in ITO films on different polymers. Furthermore, qualitative band structures are figured, and some theoretical discussions about the correlation of optical band structure, interface/surface morphology and carrier density are also presented.

Al/Cr 2 O 3 复合粉体的制备及红外反射特性研究

采用液相沉淀法, 制备了在片状铝粉表面包覆Cr 2 O 3 的复合粉体颜料, 利用XRD、SEM、UV/VIS/NIR分光光度计、FTIR红外光谱仪等方法对颜料相结构、形貌、光谱及红外反射特性等进行了测试表征。结果表明, Cr 2 O 3 包覆层对光谱反射特性的影响随波长变化而异, 波长越短, 包覆前后反射率的变化越大。当Cr 3+ :Al摩尔比为0.2:1.0时, Al/Cr 2 O 3 复合颜料在可见光波段的反射率下降接近50%, 颜色明度参数 L *降低15, 而8~14 μm波段红外反射率下降不到10%。分析表明, 包覆层对不同波长辐射的吸收系数以及表面粗糙度的差异导致了上述结果。该复合颜料对于低明度和光泽度的低红外发射率涂层设计有重要研究价值。采用液相沉淀法, 制备了在片状铝粉表面包覆Cr 2 O 3 的复合粉体颜料, 利用XRD、SEM、UV/VIS/NIR分光光度计、FTIR红外光谱仪等方法对颜料相结构、形貌、光谱及红外反射特性等进行了测试表征。结果表明, Cr 2 O 3 包覆层对光谱反射特性的影响随波长变化而异, 波长越短, 包覆前后反射率的变化越大。当Cr 3+ :Al摩尔比为0.2:1.0时, Al/Cr 2 O 3 复合颜料在可见光波段的反射率下降接近50%, 颜色明度参数 L *降低15, 而8~14 μm波段红外反射率下降不到10%。分析表明, 包覆层对不同波长辐射的吸收系数以及表面粗糙度的差异导致了上述结果。该复合颜料对于低明度和光泽度的低红外发射率涂层设计有重要研究价值。

Electromagnetic absorption properties of flowerlike cobalt composites at microwave frequencies

In this work, we report the electromagnetic absorption (EMA) properties of composites consisting of micrometer-sized cobalt with flowerlike architecture synthesized by a facile hydrothermal reduction method. Compared with the conventional spherical Co-paraffin composites, the flowerlike Co-paraffin composites are favorable with respect to EMA performance in the low frequency region, ascribing interfacial polarization loss and Ohmic loss to the improvement in the impedance match.

Electric tunability of microwave permeability dispersion behaviors of metamaterials

By embedding a capacity-variable diode into a split ring resonator (SRR), it is found that the effective capacitance of such an SRR element can be varied by the biased voltage of diode. Therefore the resonance frequency and effective permeability of an SRR element can be tuned. Based on these results, a smart microwave magnetic metamaterial is proposed. Employing the finite-difference time-domain and retrieval method, the variations of resonance frequency and permeability of the SRR element are simulated and calculated by changing the biased voltage. It is found that with the decrease of capacitance due to the increase of biased voltage, the resonance frequencies are shifted towards lower values, and so are the resonance frequencies of permeability dispersion. Finally, the difference in permeability dispersion behavior between SRR structure and natural magnetic material (for instance, ferrite) has been pointed out.