Enhancement of microwave absorption of nanocomposite BaFe_(12)O_(19)/α-Fe microfibers

Nanocomposite BaFe12O19/α-Fe microfibers with diameters of about 1-5 μm are prepared by the organic gelthermal selective reduction process. The binary phase of BaFe12O19 and α-Fe is formed after reduction of the precursor BaFe12O19/α-Fe2O3 microfibers at 350℃ for 1 h. These nanocomposite microfibers are fabricated from α-Fe (16-22 nm in diameter) and BaFe12O19 particles (36-42 nm in diameter) and basically exhibit a single-phase-like magnetization behavior, with a high saturation magnetization and coercive force arising from the exchange-coupling interactions of soft α-Fe and hard BaFe12O19 . The microwave absorption characteristics in a 2-18 GHz frequency range of the nanocomposite BaFe12O19/α-Fe microfibers are mainly influenced by their mass ratio of α-Fe/BaFe12O19 and specimen thickness. It is found that the nanocomposite BaFe12O19/α-Fe microfibers with a mass ratio of 1:6 and specimen thickness of 2.5 mm show an optimal reflection loss (RL) of 29.7 dB at 13.5 GHz and the bandwidth with RL exceeding 10 dB covers the whole Ku-band (12.4-18.0 GHz). This enhancement of microwave absorption can be attributed to the heterostructure of soft, nano, conducting α-Fe particles embedded in hard, nano, semiconducting barium ferrite, which improves the dipolar polarization, interfacial polarization, exchange-coupling interaction, and anisotropic energy in the nanocomposite BaFe12O19/α-Fe microfibers.     

二元比例X射线荧光光谱法测定BaFe12O19中Fe和Ba的含量

采用二元比例X射线荧光光谱法测定铁钡氧体BaFe12O19中铁(Fe)和钡(Ba)的含量,为铁钡氧体BaFe12O19的定量分析建立了一种新的分析方法.选取纯Fe粉(分析纯)及纯BaCO3粉(分析纯),采用粉末压片法自制标准样品,建立了定标曲线.选用Fe含量为75％、Ba含量为17.4％的标准样品作为验证样品,对定标曲线的准确度进行验证.经分析得到验证样品中Fe和Ba含量分别为75.68％和16.92％,相对标准偏差分别为0.317％和1.11％,可知定标曲线具有良好的线性关系,且准确度高,精密度好.实验测得BaFe12O19样品中Fe和Ba的含量分别为57.28％和13.42％,相对标准偏差为1.05％和0.73％.     

Microwave absorption properties of a double-layer absorber based on nanocomposite BaFe12O19/α-Fe and nanocrystalline α-Fe microfibers

The nanocomposite BaFe12O19/α-Fe and nanocrystalline α-Fe microfibers with diameters of 1–5 μm, high aspect ratios and large specific areas are prepared by the citrate gel transformation and reduction process. The nanocomposite BaFe12O19/α-Fe microfibers show some exchange–coupling interactions largely arising from the magnetization hard(BaFe12O19) and soft(α-Fe) nanoparticles. For the microwave absorptions, the double-layer structures consisting of the nanocomposite BaFe12O19/α-Fe and α-Fe microfibers each exhibit a wide band and strong absorption behavior. When the nanocomposite BaFe12O19/α-Fe microfibers are used as a matching layer of 2.3 mm in thickness and α-Fe microfibers as an absorbing layer of 1.2 mm in thickness, the optimal reflection loss(RL) achieves-47 dB at 15.6 GHz, the absorption bandwidth is about 12.7 GHz ranging from 5.3 to 18 GHz, exceeding-20 dB, which covers 72.5% C-band(4.2–8.2 GHz)and whole X-band(8.2–12.4 GHz) and Ku-band(12.4–18 GHz). The enhanced absorption properties of these double-layer absorbers are mainly ascribed to the improvement in impedance matching ability and microwave multi-reflection largely resulting from the dipolar polarization, interfacial polarization, exchange–coupling interaction, and small size effect.     

超顺磁纳米γ-Fe_2O_3/SiO_2复合材料的制备和磁性能研究

本文以正硅酸乙酯(TEOS)、Fe(NO3)3.9H2O、无水乙醇(Eth)、盐酸(HCl)和去离子水作为原料,以十六烷基三甲基溴化铵(CTAB)为模板剂,采用溶胶-凝胶法制备了纳米γ-Fe2O3/SiO2复合材料.主要研究了CTAB、热处理温度以及Fe2O3的浓度对纳米γ-Fe2O3/SiO2复合材料的形成及磁性能的影响.用X射线衍射分析(XRD)对纳米粒子进行表征以及用Quantum Design Model物理性质测量系统(PPMS)测量纳米颗粒的零场冷却(ZFC)和加场冷却(FC)时的磁化强度随温度的变化关系.通过对XRD衍射图和ZFC/FC曲线分析,可知制备纳米γ-Fe2O3/SiO2复合材料最佳热处理温度为700℃左右,Fe2O3最佳浓度为30wt%左右.尤其加CTAB改性后,所得的纳米γ-Fe2O3/SiO2复合材料较纯正和表现出超顺磁性.     

基于磁性Fe_3O_4/Ag的表面增强拉曼光谱检测农药丙线磷

以共沉淀法合成Fe3O4纳米颗粒,再通过柠檬酸三钠还原AgNO3制备Fe3O4/Ag磁性复合材料。Fe3O4/Ag能够与溶液中的丙线磷形成吸附,通过磁性收集达到萃取富集的效果。测定吸附于Fe3O4/Ag表面的痕量丙线磷所产生的表面增强拉曼光谱(SERS),其检测过程的拉曼增强因子为1.48×105,极大地提高了检测灵敏度,建立了磁性Fe3O4/Ag萃取富集与SERS分析农药丙线磷的方法。采用紫外可见吸收光谱、能谱及透射电子显微镜对制备的材料进行了分析及形貌与结构的表征。并对丙线磷模型分子进行结构优化的密度泛函理论计算,得到了理论拉曼光谱和谱峰归属,以用于丙线磷的判断。结果表明,SERS峰强在富集15min后基本趋于稳定,其丙线磷浓度低至2×10-8 mol·L-1仍有明显响应,可以满足丙线磷农残检测的要求。其方法可推广至含硫有机磷农药残留的分析。     

Fe_3O_4纳米颗粒/聚二甲基硅氧烷复合材料磁电容效应的研究

提出了一种以Fe3O4纳米颗粒和聚二甲基硅氧烷(PDMS)组成的复合材料为介质的平行板磁电容结构,并对其产生的磁电容效应的特点以及影响磁电容效应的因素进行了研究.对不同粒径的Fe3O4纳米颗粒按不同比例与PDMS混合形成的复合材料进行了测试.研究表明,与无磁场情况相比,在外磁场作用下,Fe3O4纳米颗粒/PDMS复合材料的电容值和介电损耗均发生了改变,产生了磁电容效应.由该复合材料磁电容效应所产生的电容变化量随着纳米颗粒混合浓度的增大而增大,并且当纳米颗粒粒径尺寸大于常温超顺磁临界尺寸时,材料的电容变化量随着颗粒尺寸的减小而增大.     

铈基纳米材料水热合成及储放氧性能

" 采用水热方法合成汽车尾气三效催化剂助剂材料NiO-CeO2和Bi2O3-CeO2二元纳米化合物．首先,对NiO-CeO2二元材料进行X射线分析,结果显示颗粒平均尺寸在11~12 nm,这一结果与高分辨电镜结果观察的颗粒尺寸相吻合,同在12 nm左右．同时,对NiO-CeO2二元化合物进行BET比表面积分析,结果显示：水热合成材料的比表面积在54~75 m2/g．另外,对Bi2O3-CeO2二元化合物进行X射线分析,结果显示合成材料的晶粒尺寸在8~11 nm．最后,我们分别对两种材料进行了氧化还原条件下     

M型钡铁氧体陶瓷微珠核壳腔结构吸波材料

利用自反应淬熄法制备了一种M型钡铁氧体空心陶瓷微珠材料,在此基础上,对其表面进行超声波化学镀Ni-Co复合层,从而形成了具有核/壳/腔结构的材料;通过扫描电子显微镜、能谱仪和X射线衍射仪分析表明,该材料具有中空结构,主要物相BaFe12O19为M型钡铁氧体,经过化学镀后,在其表面形成了一层Ni-Co复合层;经过吸波性能测试,化学镀Ni-Co复合层后,在2~18GHz范围内,当厚度为2.10mm时,最低反射率达到了-28.62dB,反射率小于-10dB的带宽为3.33GHz。     

BaAl12O19:Mn2+荧光粉的表面包覆与发光性能

采用传统的高温固相反应法合成出BaAl12O19：Mn^2 荧光粉,利用乳胶法对荧光粉颗粒表面包覆MgF2。对包覆过程中的反应物浓度、反应过程进行研究,得到最佳包覆条件,确定荧光粉表面包覆的制备工艺。通过X射线衍射、透射电镜图、红外光谱等对表面包覆的荧光粉颗粒的结构性能进行分析,并在彩色等离子体平板显示器(PDP)上进行涂屏老化实验。结果表明在BaAl12O19：Mn^2 荧光粉颗粒表面成功地包覆上MgF2层,包覆后的荧光粉的初始发光亮度下降1．6％,而老化8h后,包覆的荧光粉亮度衰减明显优于不包覆的材料。     

多酸-介孔分子筛复合材料的制备、表征及光催化活性

采用浸渍法将具有光催化活性的多金属氧酸盐(polyoxometalate,POM)磷钨酸和硅钨酸负载到介孔分子筛MCM-41,制备了H3PW12O40/MCM-41和H4SiW12O40/MCM-41两种复合材料,以红外光谱(FT-IR)、X射线衍射(XRD)、N2吸附、高分辨透射电镜(HRTEM)等分析手段对所合成的多酸-分子筛复合材料进行了表征,并以农药百草枯的光催化降解考察了复合材料的光催化活性。所合成的复合材料同时保留了母体多酸的Keggin特征结构和载体MCM-41分子筛均匀的六方孔道结构,比表面积超过200m2.g-1。用于农药百草枯的光催化降解实验表明,两种复合材料均具有较高的光催化活性。在365nm紫外光辐照下,以H3PW12O40/MCM-41和H4SiW12O40/MCM-41为催化剂催化反应14h后,百草枯(10mg.L-1)的降解转化率分别达到92.0%和87.6%,反应符合一级化学动力学模型,半衰期分别为3.7和4.6h。     

离子型钴铁氧体磁流体的稀土改性与机理研究

采用化学共沉淀法以柠檬酸三钠为表面改性剂制备了离子型稀土复合钴铁氧体磁流体.利用X衍射仪和透射电镜对磁粒子的组成、结构及粒径进行了分析.利用古埃磁天平和分光光度计研究了稀土离子改性对磁流体饱和磁化强度和磁感应的影响,实验结果表明:合成过程中添加稀土离子能明显降低磁性纳米粒子的粒径,制得的磁粒子均呈球形,钴铁氧体磁粒子的粒径为12～15nm,稀土钴铁氧体磁粒子的粒径为6～8nm.利用稀土改性的微观模型解释了粒径的降低.添加Dy3+能提高饱和磁化强度和磁感应,添加Y3+则能提高磁感应,却降低了饱和磁化强度.并从理论上对其改性机理进行了详细的分析.     

Magnetic nanocomposite thin films of BaFe12O19 and TiO2 prepared by sol-gel method

The composite films with different weight ratio of barium ferrite to titanium dioxide are successfully prepared using sol-gel method for the first time. The morphology, crystal structure and magnetic properties of composite films are investigated with atomic force microscopy, X-ray diffraction and vibrating sample magnetometry. The results show that the composite films are uniform with no microcracks. The grain diameters are less than 100 nm. With the increase of barium ferrite, the grain diameter decreases. The composite films are composed of M-type hexagonal barium ferrite and rutile titanium dioxide. The composite films possess the excellent magnetic properties. The specific saturation magnetization and coercivity reach 18.3 emu/g and 3350 Oe, respectively. The application of composite films in magnetic recording and electromagnetic absorption fields is promising.     

A novel approach to fabrication of superparamagnetite hollow silica/magnetic composite spheres

We described a method for synthesizing hollow silica/magnetic composite spheres using sulfonic acid functionalized hollow silica spheres (SAFHSS) as templates. The Fe₃O₄ nanoparticles were deposited on or imbedded in the hollow silica shell by a precipitation reaction. The morphologies, composition and properties of the hollow composite spheres were characterized by transmission electron microscopy, Fourier transform infrared analysis, X-ray diffraction measurement and vibrating-sample magnetometry measurement. The results indicated crystal sizes and amount of the Fe₃O₄ nanoparticles on the SAFHSS. The magnetic properties of the hollow composite spheres were controlled by adjusting the proportion between Fe²⁺ and Fe³⁺ and iron ion total concentration. When appropriate loading species were added into the system, superparamagnetite hollow composite spheres were obtained. The method also could be applicable to prepare other superparamagnetite hollow silica/ferrite composite spheres.     

Phase formation and change of magnetic properties in mechanical alloyed Ni0.5Co0.5Fe2O4 by annealing

The effects of milling time and annealing temperature on phase formation, microstructure and magnetic properties of nickel-cobalt ferrite synthesized from oxide precursors by mechanical alloying were studied. The study of milling time effects on phase formation of milled materials showed that if milling continues up to 55 h, single phase nano-sized nickel-cobalt ferrite is obtained. Also, magnetic properties of powders versus milling time and annealing at different temperatures extensively changed, so that annealing at 1200 °C increased the magnetization saturation of the as-milled powder from 15.1 to 53.6 emu/g. X-ray powder diffraction technique (XRD) with Cu-Ka radiation was employed for phase identification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also used to determine the morphology and size of the particles. The magnetic properties were measured by a vibration sample magnetometer (VSM).     

Magnetic properties of substituted strontium ferrite nanoparticles and thin films

SrFe_12−x(Zr_0.5Mg_0.5)_xO₁₉ nanoparticles and thin films with x=0-2.5 were synthesized by a sol-gel method on thermally oxidized silicon wafer (Si/SiO₂). Structural and magnetic characteristics of synthesized samples were studied employing x-rays diffraction (XRD), transmission electron microscopy (TEM), magnetic susceptometer, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). TEM micrographs display that the narrow size distribution of ferrite nanoparticles with average particle size of 50 nm were fabricated. Fitting obtained data of effective magnetic susceptibility by Vogel-Fulcher law confirms the existence of strong magnetic interaction among fine particles. XRD patterns and FE-SEM micrographs demonstrated that single phase c-axis hexagonal ferrite films with rather narrow grain size distribution were obtained. AFM micrographs exhibited that the surface roughness increases with an increase in Zr-Mg content. It was found from the VSM graphs that with an increase in substitution contents the coercivity decreases, while the saturation of magnetization increases. The Henkle plots confirms the existence of exchange coupling among nano-grain in ferrite thin films.     

Synthesis and characterization of hexagonal ferrite Co2Sr2Fe12O22 with doped polypyrrole composites

Y-type hexagonal ferrite Co₂Sr₂Fe₁₂O₂₂ was prepared by microemulsion route. A conducting polymer, polypyrrole was chemically synthesized and doped with dodecylbenzenesulphonic acid (PPy·DBSA). FTIR and magnetic properties of Co₂Sr₂Fe₁₂O₂₂ were investigated and compared with composite made by mixing the Co₂Sr₂Fe₁₂O₂₂ with this conducting polymer PPy–DBSA by 1:1 molar ratio. Phase analysis was performed by using X-ray diffraction (XRD). The microstructure was examined by scanning electron microscopy (SEM) showing heterogonous distribution of grains. The field dependent magnetic properties of the prepared samples were investigated at room temperature by using vibrating sample magnetometer (VSM). It has been observed that mixing of polymer in ferrite plays a crucial role in enhancing the coercivity, while saturation magnetization (M_s) and remanance were observed to decrease. Due to fine grain size, the density of grain boundaries increases where the domain wall pinning can take place, hence the coercivity of the composite samples increases. Appreciable improved value of coercivity guaranteed the use of these synthesized materials in the recording media, which is =1896(Oe) in the present samples. The suitable signal-to-noise ratio in the high density recording media can be achieved due to small particles size of this synthesized composite, which is expected in the present ferrite–polymer composite sample.     

Synthesis and properties of hybrid hydroxyapatite–ferrite (Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>) particles for hyperthermia applications

Hybrid ceramics consisting of hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ and ferrite Fe₃O₄ were synthesized using a two-stage procedure. The first stage included the synthesis of Fe₃O₄ ferrite particles by co-precipitation and the synthesis of hydroxyapatite. In the second stage, the magnetic hybrid hydroxyapatite–ferrite bioceramics were synthesized by a thorough mixing of the obtained powders of carbonated hydroxyapatite and Fe₃O₄ ferrite taken in a certain proportion, pressing into tablets, and annealing in a carbon dioxide atmosphere for 30 min at a temperature of 1200°C. The properties of the components and hybrid particles were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Mössbauer spectroscopy. The saturation magnetization of the hybrid ceramic composite containing 20 wt % Fe₃O₄ was found to be 12 emu/g. The hybrid hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂)–ferrite Fe₃O₄ ceramics, which are promising for the use in magnetotransport and hyperthermia treatment, were synthesized and investigated for the first time.     

Surface modification of M-Ba-ferrite powders by polyaniline: Towards improving microwave electromagnetic response

A composite of polyaniline (PANI)-coated M-type hexagonal barium ferrite (M-Ba-ferrite) powder was prepared by an in situ polymerization of an aniline monomer in the presence of M-Ba-ferrite particles. The obtained composite was characterized by Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The structure and microwave response properties were investigated. The continuous coverage of polyaniline has been produced on the platelet M-Ba-ferrite particle surface, and a core–shell structure has been formed. The results show that the coverage of polyaniline has a great influence on microwave response of M-Ba-ferrite particles. A polyaniline thin layer formed on the surface of a barium ferrite particle changes the character of frequency dispersion of microwave absorption. The results indicate the existence of an interaction at the interface of polyaniline macromolecule and barium ferrite particle, which influences the physical and chemical properties of the composite. The interaction and interfacial polarization are seen as important factors contributing to the influence on microwave response of the PANI-coated ferrite composite powders.     

Structural,magnetic, and dielectric properties of Ni–Zn ferrite and Bi_2O_3 nanocomposites prepared by the sol-gel method

Ni–Zn ferrite and Bi_2O_3 composites were developed by the sol-gel method. The structural, magnetic, and dielectric properties were studied for all the prepared samples. X-ray diffraction(XRD) was performed to study the crystal structure.The results of field emission scanning electron microscopy(FE-SEM) showed that the addition of Bi_2O_3 can increase the grain size of the Ni–Zn ferrite. Magnetic properties were analyzed by a hysteresis loop test and it was found that the saturation magnetization and coercivity decreased with the increase of Bi_2O_3 ratio. In addition, the dielectric properties of the Ni–Zn ferrite were also improved with the addition of Bi_2O_3.     

Magnetic permeability and microstructure of the Bi,Si oxides-doped NiZnCu ferrite composite material

Bi,Si oxides-doped NiZnCu ferrite composite material has been fabricated with different sintering times. The microstructure of the samples was investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The complex permeability has been measured up to 1 GHz by the impedance analyzers. The complex permeability has been simulated based on the magnetic circuit model, and the result was compared with the experimental results. In the low-frequency region, the real part of the permeability of the composite material is lower than the one of non-doped NiZnCu ferrite, on the other hand it is higher than non-doped sample in the high-field region. The peak position of the imaginary part is shifted to higher frequency in the composite material.