【作者】 马骥；

【导师】 李建功；

【作者基本信息】 兰州大学 ， 材料物理与化学， 2009， 博士

【摘要】 近年来,铁磁性金属-氧化物纳米复合材料由于其特殊的磁性和在高效微波吸收剂方面的潜在应用价值而受到广泛的关注和研究。在本工作中,我们以铁磁性金属-氧化物纳米复合材料中具有代表性的Fe／SiO2纳米复合材料为研究对象,开展了对其微结构、磁性及微波性能的研究。研究内容包括：(1)铁磁性金属颗粒间偶极相互作用对体系磁性的影响；(2)铁磁性金属-氧化物纳米复合材料在高频段的微波吸收机制。主要研究结果如下：(1)本工作通过Fe203与Si粉末的机械化学反应制备了不同Fe含量的Fe／SiO2纳米复合材料。对Fe／SiO2纳米复合材料微结构的分析表明,Fe颗粒为单晶并随机分散在非晶SiO2基体中。在非晶SiO2基体中,Fe颗粒的形貌为等轴或准球形；Fe颗粒尺寸分布较窄,平均颗粒尺寸在11nm左右,并且在Fe含量为22～51wt%之间,颗粒尺寸基体保持不变。(2)在Fe／SiO2纳米复合材料中,Fe颗粒之间的偶极相互作用不仅对Fe颗粒的磁性状态有影响,而且对颗粒磁矩的排布产生影响。这种影响主要表现在,随着Fe含量的增加,Fe颗粒之间偶极相互作用强度随之加强,从而使超顺磁Fe颗粒的磁化强度反转的能量壁垒增加。这使得超顺磁Fe颗粒的磁化强度有可能克服热扰动,从超顺磁态转变为铁磁态。与此同时,Fe颗粒的磁矩从随机取向逐渐形成局部的链状结构,然后又形成闭合磁路结构。这种磁矩排布的变化导致Fe／SiO2纳米复合材料的矫顽力与剩磁比随Fe含量增加表现出先增加后减小的变化规律。(3)不同Fe含量的Fe／SiO2纳米复合材料与石蜡混合物的磁导率虚部μ″在1～16 GHz的频率范围内表现出一个宽的共振峰。理论计算和分析表明这个宽的共振峰来源于自然共振和交换共振的叠加。影响自然共振频率和交换共振频率的主要因素是Fe颗粒的表面各向异性。对于自然共振,由于表面各向异性的存在,共振频率向高频移动。对于交换共振,表面各向异性改变了共振频率与颗粒尺寸之间的指数关系,使颗粒尺寸的指数增加。(4)对不同Fe含量的Fe／SiO2纳米复合材料与石蜡混合物的反射损耗(RL)计算表明,当混合物的厚度不变时,随着Fe含量的增加,混合物的RL值逐渐减小,说明混合物的微波吸收性能随Fe含量的增加逐渐提高。对于Fe含量为51wt%的样品,当混合物的厚度在1.4～2.0 mm之间时,RL小于-20 dB(99%的吸收率)的频宽为9.6～17.1 GHz。当该样品的厚度为1.5 mm时,在14.9 GHz,RL出现最小值-33 dB。更多还原

【Abstract】 Nanocomposites composed of ferromagnetic nanoparticles dispersed in oxide matrix have attracted much attentions because of their special magnetic properties and potential applications as microwave absorbers. In this work, the microstructure and magnetic properties of Fex(SiO2)1-x nanocomposties have been studied to understand the effect of interparticle dipolar interactions on the magnetic properties of system and the microwave absorption mechanism in ferromagnetic metal-oxide nanocomposites.The Fes(SiO2)1-x nanocomposites were synthesized by mechanochemical reduction reaction of Fe2O3 and Si powders. The Fex(SiO2)1-x nanocomposites of the single crystal Fe nanoparticles with equiaxed (or quasi-sphere) shape dispersed randomly in the amorphous SiO2 matrix were preprared. The Fe nanoparticles in the Fex(SiO2)1-x nanocomposites have a narrow particle size distribution; and the Fe average particle size of about 11 nm keeps almost unchanged for the Fe contents x between 22wt% and 51wt%.It has been shown that the interparticle dipolar interactions have an effect not only on the magnetic state of Fe particles but also on the moment arrangements of the Fe particles. With increasing x, the dipolar interaction strength among the Fe particles increases. The surperparamagnetic Fe particles in Fes(SiO2)1-x nanocomposites become blocked, which may be due to that the dipolar interaction increases the energy barrier for the magnetization reverse. The moment arrangements of the Fe particles change from the random orientation to the local "chain-like" arrangements and then to locally forming flux-closure arrangements, which leads to the first increase and then decrease of the coercivity and remanence ratio.The study of microwave absorption mechanism in the Fex(SiO2)1-x nanocomposites indicates that a broad resonance band in the 1-16 GHz range observed in these nanocompsites results from the overlap of the natural resonance and exchange resonance. The natural resonance and exchange resonance frequencies depend on the surface anisotropy of the Fe nanoparticles. The enhanced surface anisotropy resulting from the reduced Fe particle size shifts the natural resonance to high frequency. For exchange resonance, the surface anisotropy weakens the dependence of exchange resonance frequency on the square of the reciprocal particle size.The reflection loss (RL) values calculated for the Fex(SiO2)1-x nanocomposites-paraffin mixtures reveal that their microwave absorption performance is improved with increasing x when the thickness of the mixtures keeps unchanged. For the mixture with thickness of 1.4-2.0 mm and x=51wt%, the frequency band of the RL values smaller than-20 dB, corresponding to a microwave absorption of 99%, is from 9.6 to 17.1 GHz. A minimum RL of-33 dB is reached at 14.9 GHz for this mixture with the thickness of 1.5 mm.更多还原