聚对苯撑/LiNi0.5Fe2O4纳米复合热电材料的制备及其性能研究

本文以流变相反应法原位合成了聚对苯撑/LiNi_0.5Fe₂O₄纳米复合热电材料,并对其热电性能进行表征,研究了放电等离子烧结时保温时间对其热电性能的影响.结果发现,复合材料铁氧体颗粒粒径为100---300nm,其外部被一层聚对苯撑膜包覆.电子在Fe²⁺和Fe³⁺之间的跳跃机理在铁氧体电导中占主导作用,因此聚对苯撑/LiNi_0.5Fe₂O₄复合材料具有n型导电特性.随着保温时间增加,复合材料电导率基本不变,但热导率逐渐增大且Seebeck系数逐渐减小,导致热电优值系数降低.由于结合了有机物高电导率和低热导率以及无机材料高赛贝克系数的优点,所制备的复合材料热电性能较单一材料有较大提高.     

糠醇辅助的聚合凝胶法制备纳米Li4Ti5O12负极材料

以硝酸锂、钛酸正丁酯和糠醇为反应物,采用糠醇聚合凝胶法制备了纳米Li₄Ti₅O₁₂粉体．利用XRD、SEM和BET比表面测试对产物进行了表征,并研究了纳米Li₄Ti₅O₁₂粉体作为锂离子电池负极材料的电化学性能．在700℃或更高温度烧结时产物为纯相的尖晶石型．通过柠檬酸、聚乙烯吡咯烷酮、十六烷基三甲基溴化铵(CTAB)表面活性剂的加入能够减少产物颗粒的团聚程度,增大粉体的比表面积,提高其电化学性能．加入0.5 g CTAB、700℃烧结12 h的Li₄Ti₅O₁₂粉体展示出最高的比容量和最佳的循环性能,10 C下充电比容量高达156.7 mAh/g．     

超细Fe3O4的氧化过程

根据穆斯堡尔谱分析,Fe₃O₄转变为γ-Fe₂O₃是γ-Fe₂O₃成核生长过程。根据氧化过程磁矩的变化确定Fe₃O₄浓度随时间的变化,-(dc)/(dt)=mc³仅在氧化初始阶段适合。     

Al2O3薄膜/纳米Ag颗粒复合结构的光吸收谱及增强Raman散射光谱研究

Al₂O₃介质薄膜与纳米Ag颗粒构成的复合结构,被应用于表面增强Raman散射探测实验中,其中Al₂O₃介质薄膜对纳米Ag颗粒的吸收谱及增强Raman散射光谱的影响被特别关注.该复合结构的光学特性表征出纳米Ag颗粒的偶极振荡特性.从光吸收谱中可以看到,其共振吸收谱随Al₂O₃介质薄膜厚度增加而在整个谱域上发生红移,表明纳米Ag颗粒的周围介电常数随Al₂O₃介质薄膜厚度的增加而增大.采用罗丹明6G作为探针原子,6个Raman特征峰的平均增益值作为表征表面增强Raman散射衬底增益程度的量度.实验结果表明,Al₂O₃介质薄膜层的引入提高了纳米Ag颗粒的衬底介电常数,并引起了散射共振的增强,从而使表面增强Raman散射强度提高.     

压力对纳米CoFe2O4/SiO2复合材料结构的影响

 采用溶胶-凝胶工艺和高温高压实验技术,制备了纳米CoFe₂O₄/SiO₂复合材料。利用X射线衍射仪、扫描电子显微镜和振动样品磁强计,对样品的结构、微观形貌和磁性进行了研究,并对CoFe₂O₄中阳离子的占位情况进行了讨论。结果表明,随着处理压力的升高,样品的晶粒尺寸增大,晶格常数减小,比饱和磁化强度增大。通过计算结果可以推断,压力的升高导致CoFe₂O₄中的部分Fe³⁺从A位移向了B位,而部分Co²⁺则从B位移向了A位。     

利用氧化辅助气固法生长高磁性的α-Fe2O3和Fe3O4纳米线

探讨生长α-Fe₂O₃和Fe₃O₄纳米线的一个可控制的合成过程. 在研究中发现,高磁性的α-Fe₂O₃纳米线已经成功地利用氧化辅助气固法结晶生成于Fe_0.5Ni_0.5合金基板上;若基板事先浸泡于草酸溶液中,随草酸浓度的增加,所生长的纳米线晶相会逐转变为Fe₃O₄,当草酸浓度达到0.75 mol/L时,所生长的纳米线几乎全部转变成Fe₃O₄晶相. 此外,实验结果也显示所生长的纳米线长度及直径会随着气固过程中的温度上升而增加,生长密度则会随着气固过程中的流量加大而上升. 此过程所提出的合成程序可在2 h內完成.     

磁场下合成Fe3O4粉体的隧道磁阻

通过在半金属Fe₃O₄合成过程中外加磁场的方法，改变样品粒子的表面结晶状态和晶格缺陷，研究了由此引起的Fe₃O₄输运性质的变化.合成的Fe₃O₄粉体的主要导电机理均为自旋极化隧穿和高阶跃迁电导，电阻随温度升高成指数降低，电阻与电压显示了非线形相关性，磁阻与磁场的关系为蝴蝶形，是典型的隧道磁阻特征.与没有外加磁场时合成的样品比较，外加磁场合成的样品显示了更低的电阻和更高的磁阻.     

超声强化合成MgFe2O4纳米颗粒及其机理研究

用超声水解方法制备MgO纳米颗粒,用化学沉淀法制备α-Fe₂O₃纳米颗粒,将MgO/α-Fe₂O₃混合体常温下超声活化2h,400℃固相合成制备出MgFe₂O₄纳米颗粒.通过X射线衍射和透射电子显微镜测试产品的化学成分、晶体结构和形貌尺寸,分析声化学反应机理及其影响因素.研究结果表明:所制备的MgFe₂O₄为尖晶石铁氧体,颗粒尺寸分布在20-30nm之间,粒度分布均匀;超声空化效应提高了化学反应活性、增加反应物的比表面积和反应物之间的接触面积,促进固相合成反应速度,降低反应温度,实现了一般条件下难以完成的化学反应.     

Bi(Fe1-xMnx)O3的结构与磁性

使用快速烧结和溶胶-凝胶方法制备了Bi(Fe_1-xMnx)O₃的块材陶瓷,其x能够达到0.3,因而可以得到较细的粉末. 经X射线衍射谱和拉曼光谱研究发现其结构转变取决于锰的掺杂浓度. 当x=0.05和0.1时,Bi(Fe_1-xMnx)O₃的结构保持斜方六面体结构;当x=0.3时,Bi(Fe_1-xMnx)O₃的结构变化到正方六面体结构.在x=0.05和0.1时观测到弱铁磁性,但在x=0.3时观测到强顺磁性. 这显示了Bi(Fe_1-xMnx)O₃从反铁磁性到顺磁性的磁相变,同时也表明其结构从R3C变化到C222. 在30和140 K, x=0.05和0.1时,观察到Bi(Fe_1-xMnx)O₃的两个反常现象. 在30 K时,直流磁记忆和缓慢测量Bi(Fe_1-xMnx)O₃团族的反常现象与冷冻的旋转玻璃系统有关.     

锰位铁掺杂La0.67Sr0.33FexMn1-xO3薄膜的光诱导效应

分别采用溶胶-凝胶法和脉冲激光沉积的方法制备了La_067Sr_033Fe_xMn_1-xO₃(x=0, 005, 010, 015)系列块材和薄膜，研究了Fe部分替代对La_067Sr_033Fe_xMn_1-xO₃薄膜     

具有介孔结构的MnSiO3@Fe3O4@C纳米粒子的制备以及作为pH响应的T1-T2*双模MRI造影剂的研究应用

本文通过一个简单的、温和的方案制备了平均尺寸为120 nm,介孔结构的纳米粒子MnSiO₃@Fe₃O₄@C. 粒子的细胞毒性微小,可以用作T₁-T₂^*双模MRI造影剂. 酸性条件下MnSiO₃@Fe₃O₄@C释放出大量的Mn²⁺缩短T₁弛豫时间,提高成像分辨率. 超顺磁性的Fe₃O₄可以增强T₂对比成像,检测病变组织. 类似于肿瘤微环境/细胞器的酸性PBS(pH=5.0)中Mn²⁺的释放率达到31.66%,约为中性条件(pH=7.4)下的7倍. 释放的Mn²⁺通过内吞作用被细胞摄取,经肾脏排出,细胞毒性实验表明,MnSiO₃@Fe₃O₄@C具有低的细胞毒性,即使高浓度的200 ppm MnSiO₃@Fe₃O₄@C对HeLa细胞的毒性也相对较小. 对荷瘤小鼠静脉注射定量MnSiO₃@Fe₃O₄@C后,可以观察到一个快速增强的对比成像,给药24 h后,T₁MRI信号显著增强,达到132%,而T₂信号则明显降低至53.8%,活体MR成像证明了MnSiO₃@Fe₃O₄@C可以同时作为阳性和阴性造影剂. 此外,得益于介孔MnSiO₃优秀的酸敏感性,MnSiO₃@Fe₃O₄@C可以作为一种潜在的药物载体,实现肿瘤的诊疗一体化.     

Microwave synthesis of magnetic Fe3O4 nanoparticles used as a precursor of nanocomposites and ferrofluids

Methods to synthesize magnetic Fe₃O₄ nanoparticles and to modify the surface of particles are presented in the present investigation. Fe₃O₄ magnetic nanoparticles were prepared by the co-precipitation of Fe³⁺ and Fe²⁺, NH₃·H₂O was used as the precipitating agent to adjust the pH value, and the aging of Fe₃O₄ magnetic nanoparticles was accelerated by microwave (MW) irradiation. The obtained Fe₃O₄ magnetic nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The average size of Fe₃O₄ crystallites was found to be around 8–9 nm. Thereafter, the surface of Fe₃O₄ magnetic nanoparticles was modified by stearic acid. The resultant sample was characterized by FT-IR, scanning electron microscopy (SEM), XRD, lipophilic degree (LD) and sedimentation test. The FT-IR results indicated that a covalent bond was formed by chemical reaction between the hydroxyl groups on the surface of Fe₃O₄ nanoparticles and carboxyl groups of stearic acid, which changed the polarity of Fe₃O₄ nanoparticles. The dispersion of Fe₃O₄ in organic solvent was greatly improved. Effects of reaction time, reaction temperature and concentration of stearic acid on particle surface modification were investigated. In addition, Fe₃O₄/polystyrene (PS) nanocomposite was synthesized by adding surface modified Fe₃O₄ magnetic nanoparticles into styrene monomer, followed by the radical polymerization. The obtained nanocomposite was tested by thermogravimetry (TG), differential scanning calorimetry (DSC) and XRD. Results revealed that the thermal stability of PS was not significantly changed after adding Fe₃O₄ nanoparticles. The Fe₃O₄ magnetic fluid was characterized using UV–vis spectrophotometer, Gouy magnetic balance and laser particle-size analyzer. The testing results showed that the magnetic fluid had excellent stability, and had susceptibility of 4.46×10⁻⁸ and saturated magnetization of 6.56 emu/g. In addition, the mean size d (0.99) of magnetic Fe₃O₄ nanoparticles in the fluid was 36.19 nm.     

Template-free synthesis of ZnV2O4 hollow spheres and their application for organic dye removal

Hollow ZnV₂O₄ spheres with the shell aggregated by small nanoparticles were successfully synthesized through a facile one-pot template-free solvothermal method. The as-prepared product was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Brunauer-Emmett-Teller N₂ adsorption-desorption analyses. The formation of ZnV₂O₄ hollow spheres was based on flowerlike intermediate products supported reduction-dissolution-aggregation process at the expense of consumption of all the flowerlike products. The obtained ZnV₂O₄ hollow spheres showed a good adsorption capacity of methylene blue (MB) organic dye, which might be attributed to their special structural feature with large surface area. The adsorption kinetics and isotherm of MB on ZnV₂O₄ hollow spheres were also studied.     

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.     

Silane treatment of Fe3O4 and its effect on the magnetic and wear properties of Fe3O4/epoxy nanocomposites

In this study, the effect of silane treatment of Fe₃O₄ on the magnetic and wear properties of Fe₃O₄/epoxy nanocomposites was investigated. Fe₃O₄ nanopowders were prepared by coprecipitation of iron(II) chloride tetrahydrate with iron(III) chloride hexahydrate, and the surfaces of Fe₃O₄ were modified with 3-aminopropyltriethoxysilane. The magnetic properties of the powders were measured on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites using SQUID magnetometer. Wear tests were performed on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites under the same conditions (sliding speed: 0.18 m/s, load: 20 N).The results showed that the saturation magnetization (M_s) of surface-modified Fe₃O₄/epoxy nanocomposites was approximately 110% greater than that of unmodified Fe₃O₄/epoxy nanocomposites. This showed that the specific wear rate of surface-modified Fe₃O₄/epoxy nanocomposites was lower than that of unmodified Fe₃O₄/epoxy nanocomposites. The decrease in wear rate and the increase in magnetic properties of surface-modified Fe₃O₄/epoxy nanocomposites occurred due to the improved dispersion of Fe₃O₄ into the epoxy matrix.     

Fabrication, characterization and measurement of thermal conductivity of Fe3O4 nanofluids

Magnetite Fe₃O₄ nanoparticles were synthesized by a co-precipitation method at different pH values. The products were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electronic microscopy. Their magnetic properties were evaluated on a vibrating sample magnetometer. The results show that the shape of the particles is cubic and they are superparamagnetic at room temperature. Magnetic nanofluids were prepared by dispersing the Fe₃O₄ nanoparticles in water as a base fluid in the presence of tetramethyl ammonium hydroxide as a dispersant. The thermal conductivity of the nanofluids was measured as a function of volume fraction and temperature. The results show that the thermal conductivity ratio of the nanofluids increases with increase in temperature and volume fraction. The highest enhancement of thermal conductivity was 11.5% in the nanofluid of 3 vol% of nanoparticles at 40 °C. The experimental results were also compared with the theoretical models.     

Green synthesis and characterization of superparamagnetic Fe3O4 nanoparticles

In this paper, we have first demonstrated a facile and green synthetic approach for preparing superparamagnetic Fe₃O₄ nanoparticles using α-d-glucose as the reducing agent and gluconic acid (the oxidative product of glucose) as stabilizer and dispersant. The X-ray powder diffraction (XRD), X-ray photoelectron spectrometry (XPS), and selected area electron diffraction (SAED) results showed that the inverse spinel structure pure phase polycrystalline Fe₃O₄ was obtained. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results exhibited that Fe₃O₄ nanoparticles were roughly spherical shape and its average size was about 12.5 nm. The high-resolution TEM (HRTEM) result proved that the nanoparticles were structurally uniform with a lattice fringe spacing about 0.25 nm, which corresponded well with the values of 0.253 nm of the (3 1 1) lattice plane of the inverse spinel Fe₃O₄ obtained from the JCPDS database. The superconducting quantum interference device (SQUID) results revealed that the blocking temperature (T_b) was 190 K, and that the magnetic hysteresis loop at 300 K showed a saturation magnetization of 60.5 emu/g, and the absence of coercivity and remanence indicated that the as-synthesized Fe₃O₄ nanoparticles had superparamagnetic properties. Fourier transform infrared spectroscopy (FT-IR) spectrum displayed that the characteristic band of Fe-O at 569 cm⁻¹ was indicative of Fe₃O₄. This method might provide a new, mild, green, and economical concept for the synthesis of other nanomaterials.     

Ag-deposited silica-coated Fe3O4 magnetic nanoparticles catalyzed reduction of p-nitrophenol

In this paper, a novel approach was successfully developed for advanced catalyst Ag-deposited silica-coated Fe₃O₄ magnetic nanoparticles, which possess a silica coated magnetic core and growth active silver nanoparticles on the outer shell using n-butylamine as the reductant of AgNO₃ in ethanol. The as-synthesized nanoparticles have been characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), Fourier transform infrared spectra (FT-IR), vibration sample magnetometer (VSM), and have been exploited as a solid phase catalyst for the reduction of p-nitrophenol in the presence of NaBH₄ by UV-vis spectrophotometry. The obtained products exhibited monodisperse and bifunctional with high magnetization and excellent catalytic activity towards p-nitrophenol reduction. As a result, the as-obtained nanoparticles showed high performance in catalytic reduction of p-nitrophenol to p-aminophenol with conversion of 95% within 14 min in the presence of an excess amount of NaBH₄, convenient magnetic separability, as well as remained activity after recycled more than 6 times. The Fe₃O₄@SiO₂-Ag functional nanostructure could hold great promise for various catalytic reactions.     

Strongly interacting superspins in Fe3O4 nanoparticles

In this paper we report structural and magnetic properties of Fe₃O₄ nanoparticles synthesized by thermal decomposition of ball milled iron nitrate and citric acid in N₂ and air ambient. The XRD pattern of samples which are prepared in air shows some impurity phases, while the samples synthesized in the N₂ atmosphere are almost pure Fe₃O₄ phase. The result shows that by increasing the particle size, the magnetization of the samples increases. The increase of magnetization by increasing the particle size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. The results of ac magnetic susceptibility measurements show that the susceptibility data are not in accordance with the Néel -Brown model for superparamagnetic relaxation, but fit well with conventional critical slowing down model which indicates that the dipole-dipole interactions are strong enough to cause superspin-glass like phase in these samples.     

Biosynthesis and magnetic properties of mesoporous Fe3O4 composites

Magnetic Fe₃O₄ materials with mesoporous structure are synthesized by co-precipitation method using yeast cells as a template. The X-ray diffraction (XRD) pattern indicates that the as-synthesized mesoporous hybrid Fe₃O₄ is well crystallized. The Barrett-Joyner-Halenda (BJH) models reveal the existence of mesostructure in the dried sample which has a specific surface area of 96.31 m²/g and a pore size distribution of 8-14 nm. Transmission electron microscopy (TEM) measurements confirm the wormhole-like structure of the resulting samples. The composition and chemical bonds of the Fe₃O₄/cells composites are studied by Fourier transform infrared (FT-IR) spectroscopy. Preliminary magnetic properties of the mesoporous hybrid Fe₃O₄ are characterized by a vibrating sample magnetometer (VSM). The magnetic Fe₃O₄/cells composites with mesoporous structure have potential applications in biomedical areas, such as drug delivery.