影响磁性纳米颗粒膜吸波性能的主要因素

基于Landau-Lifshitz-Gilbert方程、Bruggeman有效媒质理论及纳米颗粒膜的电输运和介电理论,通过计算磁性纳米颗粒膜的有效磁导率、有效介电常数、计算分析饱和磁化强度、各向异性场、电导率和阻尼系数对纳米颗粒膜吸波性能的影响.结果表明,饱和磁化强度、各向异性场、电导率和阻尼系数均对纳米颗粒膜的微波吸收特性产生显著影响,通过调控纳米颗粒膜的电磁特性可以有效提高其吸波性能,可以应用于薄层吸波材料的设计中.     

前向大角度法测量亚微米级颗粒

采用Mie散射理论,分析了亚微米级颗粒散射光角度分布特征,研究了粒径、折射率等参数对散射光分布特点的影响.研究发现,将散射光强度的角度分布进行归一化后,前向30°部分范围内的分布特性近似为线性,且呈现斜率与颗粒粒径相关,而与折射率无关的特点.在此基础上,本文中提出一种新的测量方法--前向大角度法,即通过测量散射光在前向30°范围内的光强线性变化的斜率来直接得到亚微米级颗粒的平均粒径.文中讨论了该方法的适用范围、近似表达式、误差分析等.     

磁性纳米膜微波高磁导率理论与实验研究

磁性纳米膜的应用涉及磁记录头、微电感、微变压器、电磁噪声消除器、高频磁传感器等，其核心材料是软磁薄膜，要求其具有高的磁导率并能保持到几百MHz甚至GHz微波频段．为了获得满足要求的高磁导率和频率响应特性磁性纳米膜，本文分别从机理和实验研究角度，系统研究了磁性纳米膜的高磁导率理论。首先从理论研究角度，系统研究了单层纳米膜和纳米颗粒膜的微波高磁导率机理．单层纳米膜高磁导率主要取决于高饱和磁化强度和面内可调的各向异性．从纳米层次对纳米颗粒膜的交换耦合开展了深入的研究，应用微磁学分析了交换耦合作用的物理图像，交换耦合对克服退磁效应、提高微波磁导率和磁损耗、提高电阻率等方面都起到关键作用．其次从计算模拟角度，深入分析了纳米膜的交换耦合、各向异性、逾渗阈值、电阻率等主要因素对纳米膜微波磁导率作用。为获得具有微波磁导率和兼顾高电阻率的磁性纳米膜，在逾渗阈值附近，进行了纳米膜的电阻率与磁导率相关性研究。最后，从典型磁性纳米膜制备工艺角度，研究了溅射沉积的Fe40Co40B20合金薄膜单层纳米膜和采用顺序沉积Co40Fe40820/SiO2不连续多层纳米颗粒膜，对高磁导率机理进行了实验验证。     

GHz高磁导率铁磁薄膜研究进展

主要介绍了微波(GHz)频率下,高磁导率铁磁性薄膜近年来的最新研究进展。对于制备在柔性基片上的铁磁薄膜,重点介绍了Fe基、Co基合金薄膜,并分析了各成分对薄膜的影响;纳米晶薄膜的重点集中在Fe—N及FeXN系薄膜上,介绍了各类FeN相在薄膜中所起的作用;对于T—M—O系纳米磁性颗粒膜,分析了磁性颗粒膜的微波高磁导率机理。同时提出了目前探索GHz频率下高磁导率磁性薄膜主要面临的问题。     

超细颗粒物近场散射光场分布特性研究

为研究超细颗粒物近场散射光强分布的特性,应用了时域有限差分方法系统对不同尺寸微小颗粒的近场散射光强进行分析。对于单颗粒的散射光场,颗粒越小,散射光主要表现为各向同性,随着颗粒增大,前向散射光将占主要部分,这与传统的Mie散射理论相一致。对于紧靠的两个颗粒,由于多重散射的作用,散射光场表现出更多的复杂性,两个颗粒的排列方式不同也会对结果有较大影响。这些结果为进一步分析复杂颗粒系统的光散射特性提供了基础。     

颗粒层状结构对散射特性及激光粒度测量的影响

根据Aden-Kerker理论,对双层颗粒前向散射特性进行了研究。在此基础上,分别用衍射理论、Mie理论和Aden-Kerker理论计算了散射系数矩阵并采用非独立模式算法进行反演。数值计算结果表明,双层颗粒的散射光强由颗粒折射率和内外径决定,在实际的激光前向散射粒度测量中,如果沿用Fraunhofer衍射理论或Mie理论计算系数矩阵,在测量双层颗粒时,反演结果会有一定的误差。相比于Mie理论,衍射理论的反演误差更大。     

纳米磁性多层膜的微波电磁特性研究

采用射频磁控溅射法制备了FeCoB／SiO2纳米磁性多层膜,并对其微波特性进行研究。用微波谐振腔法测量了该薄膜的复磁导率．2GHz频率处复磁导率实部及虚部分别可达251和22．7。重点研究了多层膜结构中介质层厚度及薄膜总厚度对磁导率的影响,同时结合直流电阻率ρ、静态磁参数Bs、Hc的测量结果分析．探讨了纳米磁性多层膜的微波高磁导率机理．并提出了微波频率下获得高磁导率及低磁损耗的途径。实验结果也表明这种纳米磁性多层膜材料有望在高频平面变压器、平面电感器、薄膜电磁干扰抑制器等领域获得应用。     

微米/纳米TiO2对高频MnZn功率铁氧体的影响

研究了普通微米/纳米TiO2对MnZn功率铁氧体性能的影响,结果表明:普通微米TiO2可提高铁氧体起始磁导率和晶界电阻率,降低材料的损耗;而纳米TiO2则会促进晶粒异常长大,增大材料的气孔率,降低材料的磁导率,增大材料的损耗.     

经典Mie散射的数值计算方法改进

在光散射颗粒测量技术中,Mie散射理论的计算非常重要。本文介绍一种改进的Mie散射数值计算方法,通过对Mie散射系数进行重新构造,找到参量来控制Mie计算的收敛和计算精度。对各有关参量选用合适、稳定的递推关系进行计算。数值计算结果表明该方法具有快速、稳定的优点,可以在极大的颗粒粒径和折射率范围内得到合理结果。     

磁性纳米颗粒膜微波物性研究

采用磁控溅射工艺和复合靶技术制备FeCoB-SiO2磁性纳米颗粒膜;利用X射线衍射仪、扫描探针显微镜分析这类薄膜的微结构和形貌特征;采用振动样品磁强计、四探针法、微波矢量分析仪及谐振腔法测量薄膜试样的磁电性能和微波复磁导率;重点对SiO2介质相含量、薄膜微结构对电磁性能产生重要影响的机理做了分析和探讨.结果表明,这类FeCoB-SiO2磁性纳米颗粒膜具有良好的软磁性能和高频电磁性能,2GHz时,磁导率μ＞50,可以应用于高频微磁器件中.     

基于Mie散射理论的微球体颗粒数值模拟计算和实验研究

应用Mie散射理论,在散射角0～π范围内,模拟微球体颗粒的散射光强度与粒径大小的变化关系,分析了相对折射率的大小对散射光强度的影响;在实验装置条件下,分别模拟了散射光强度与粒径大小的变化关系及2um、5um、10um三种粒子的散射光强度与波长的变化关系。实验验证了数值模拟与实验结果基本一致,也证明实验中粒子散射遵从Mie散射理论模型。     

亚微米级扁椭球颗粒群的散射特性

采用T矩阵方法计算亚微米级扁椭球随机取向分布颗粒群的散射特性,研究消光截面、散射截面、吸光截面、单散射反照率、非对称因子以及散射矩阵元素与颗粒的大小、折射率、长短轴比之间的关系。结果表明,随颗粒粒径增大,消光截面、散射截面、吸光截面、非对称因子都单调增加,散射相函数F11的角分布曲线特征可以区分颗粒的大小;颗粒越偏离球形,颗粒对入射光的衰减效率越低,后向散射光强越强,在轴比不大时,前向50°内的F22/F11值可以区分颗粒的形状;折射率变化主要是对后向散射光的分布产生影响,实部、虚部的变化可分别通过F34/F11的角分布曲线、F12/F11的第一个峰值来体现。     

Light scattering by a coated infinite cylinder in an absorbing medium

The scattering formulation for a coated infinite cylinder in an absorbing medium is presented in this paper. The cylinder is subjected to an arbitrarily polarized plane wave propagating in a general direction at the cylinder. The refractive index and magnetic permeability of the host medium, as well as those for the core and coating of the cylinder, can be real or complex. The scattering and extinction efficiencies and the scattering amplitudes are derived for both the near field and the far field. As the medium is absorbing, the "true" extinction and scattering efficiencies are derived based on the radiative energy outflow at the surface of the cylinder. The radiative efficiencies in the far field are denoted as "apparent" properties because they include absorption by the intervening medium. The influence of the refractive index and permeability of the host medium on the scattering properties of a coated cylinder is illustrated by numerical examples.     

Calculation of Mie derivatives

Analytical expressions are found for the derivatives of commonly used Mie scattering parameters, in particular the absorption and the scattering efficiencies, and for the angular intensity functions. These expressions are based on the analytical derivatives of the Mie scattering amplitudes a(n) and b(n) with respect to the particle size parameter and complex refractive index. In addition, analytical derivatives are found for the volume absorption and scattering coefficients, as well as for the intensity functions of a population of particles with log normal size distribution. These derivatives are given with respect to the total number density, to the median radius and spread of the distribution, and to the refractive index. Comparison between analytically and numerically computed derivatives showed the analytical version to be 2.5 to 6.5 times as fast for the single-particle and particle-distribution cases, respectively.     

两种方法测量大气气溶胶折射率的对比分析

针对气溶胶折射率在分析大气气溶胶光学特性中的重要性,介绍两种综合利用黑碳仪、浊度计、光学粒子计数器和微脉冲激光雷达测量大气气溶胶折射率的新方法。两种方法都是根据球形粒子的Mie散射理论计算大气气溶胶的折射率,使用以上两种方法对厦门地区气溶胶折射率进行了计算和对比分析,证明了它们的合理性,分析了它们的测量精度和误差来源。     

核壳型铁钴复合材料的制备及其微波吸收性能的研究

采用多元醇还原工艺和自组装技术,通过在微米级Fe粉表面包覆纳米Co粒子,制备了一种具有核壳结构的复合磁性微球,表征了它们的相组成结构,测试了它们的微波电磁参数,研究了它们的微波吸收性能。结果表明：用该法制备的核壳型Fe／Co复合材料能够实现表面包覆致密,将其作为微波吸收剂,可以改善传统羰基铁粉吸收剂的频散特性,可使吸收量大于8dB的频带宽度达到7GHz。     

Scattering and Internal Fields of a Microsphere that Contains a Saturable Absorber: Finite-Difference Time-Domain Simulations

Illumination intensities that are used to induce scattering and fluorescence in aerosols can be large enough to cause variations in the refractive index. Methods used to calculate the scattering from homogeneous particles may not be valid for these systems. We use the finite-difference time-domain method and an iterative technique to model scattering by microspheres that contain a saturable absorber. We illustrate this technique by calculating the scattering from spheres that contain tryptophan. We show the Mueller scattering matrices along with the internal intensity distributions for different incident intensities. The backscattering increases as the illumination intensity becomes large enough to saturate the absorption in regions of the sphere.     

Magnetic alignment and patterning of cellulose fibers

The alignment and patterning of cellulose fibers under magnetic fields are reported. Static and rotating magnetic fields were used to align cellulose fibers with sizes ranging from millimeter to nanometer sizes. Cellulose fibers of the millimeter order, which were prepared for papermaking, and much smaller fibers with micrometer to nanometer sizes prepared by the acid hydrolysis of larger ones underwent magnetic alignment. Under a rotating field, a uniaxial alignment of fibers was achieved. The alignment was successfully fixed by the photopolymerization of a UV-curable resin precursor used as matrix. A monodomain chiral nematic film was prepared from an aqueous suspension of nanofibers. Using a field modulator inserted in a homogeneous magnetic field, simultaneous alignment and patterning were achieved.     

Magnetic alignment and patterning of cellulose fibers

Abstract

The alignment and patterning of cellulose fibers under magnetic fields are reported. Static and rotating magnetic fields were used to align cellulose fibers with sizes ranging from millimeter to nanometer sizes. Cellulose fibers of the millimeter order, which were prepared for papermaking, and much smaller fibers with micrometer to nanometer sizes prepared by the acid hydrolysis of larger ones underwent magnetic alignment. Under a rotating field, a uniaxial alignment of fibers was achieved. The alignment was successfully fixed by the photopolymerization of a UV-curable resin precursor used as matrix. A monodomain chiral nematic film was prepared from an aqueous suspension of nanofibers. Using a field modulator inserted in a homogeneous magnetic field, simultaneous alignment and patterning were achieved.     

球形粒子对平面偏振光散射的数值计算与分析

论文在Mie理论基础上,给出了球形粒子对平面偏振光的散射强度和散射系数公式,利用连分式递推算法进行了编程计算,重点对1.06μm激光的模拟结果进行了分析.从得到的散射图像可以看出,散射强度角分布与散射粒子尺度有密切关系,随着粒子尺度的逐渐增大,散射光强主要集中到前、后向散射方向,集中的角度越来越窄,模拟结果明显出现了散射强度最弱的极值角,且该极值角随粒径的增大而增大,最后逼近90°方向.散射强度角分布与波长有关,当它们在同一数量级时达到最大值,与散射粒子折射率无关.该递推算法因为每一步计算都是独立的,与前后项的准确性没有关系,不存在不稳定、发散等情况,能够计算粒径参数范围从10-4开始,对上限不受任何限制.