摘要
采用溶胶-凝胶法及微波法,成功合成了双钙钛矿磁电阻材料Sr2FeMoO6以及A位取代系列化合物,并用X射线粉末衍射(XRD)、扫描电镜(SEM)、振动样品磁强计(VSM)等对合成产物进行了物相结构、粒度形貌、电磁性能进行了分析和表征。
首先,采用廉价的活性炭粉作为还原剂,利用溶胶-凝胶法成功合成了双钙钛矿Sr2FeMoO6单相,研究了合成样品的工艺条件对产物结构及磁性能的影响,确定了最佳的工艺条件。所得样品Sr2FeMoO6属四方晶系,I4/mmm空间群,晶胞参数为:a = 5.580?,c = 7.882?;晶粒尺寸在100nm以下;样品的居里温度高于室温,1T磁场下,样品的室温饱和磁化强度为13.59A·m2/kg,室温磁电阻变化率为-10.02%;样品的导电机制属于小极化子变程跃迁导电机制。
然后,分别以Li+、Na+、K+、Gd3+取代Sr2FeMoO6中的Sr2+,研究了A位取代对样品结构和磁性能的影响。结果表明:由于掺杂离子引起的空间效应和电子掺杂效应在不同体系中贡献有所差别,Li、Na、K取代样品的晶胞参数呈现减小的趋势,而Gd取代样品的晶胞参数较未掺杂样品的晶胞参数大;Li+、Na+、K+取代样品的饱和磁化强度均呈现随掺杂量的增加先增大后减小的趋势,而Gd3+取代的样品的饱和磁化强度则随着掺杂量的增加而减小。同时,以Sr1.9K0.1FeMoO6为例,提出了溶胶-凝胶法合成Sr1.9K0.1FeMoO6的反应机制。
最后,探索了一条合成双钙钛矿Sr2FeMoO6的新工艺—微波烧结法,与传统的高温固相烧结法以及溶胶凝胶法相比,具有合成时间短,节能高效等优点。该法所得样品仍属四方晶系,空间群为I4/mmm,晶胞参数为:a = 5.571?,c = 7.872?。电磁性能研究表明:微波烧结法有利于提高样品的饱和磁化强度及磁电阻变化率,1T磁场下,样品的室温饱和磁化强度为17.949A·m2/kg,室温磁电阻变化率为-15.63%,均高于溶胶凝胶法所得样品;其导电机制属于绝热小极化子导电机制。此外,结合XRD及红外分析结果,推测反应历程为:在微波场下,SrCO3、Fe2O3、MoO3首先反应生成前驱物Sr2Fe2O5和SrMoO4;此前驱物经活性炭还原,促成Fe-O-Mo键的形成,从而生成最终产物Sr2FeMoO6。
The magnetoresistance materials Sr2FeMoO6 with double perovskite structure and series of A site doped compounds were successfully synthesized by Sol-Gel method and microwave method. X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM) were used to analyze and characterize the phase structure, the size and morphology of particles, magnetic and electric properties of the samples.
First, with cheap activated carbon for reducing agent, the sol-gel method was used to prepare the giant magnetoresistance materials Sr2FeMoO6, which is the double perovskite oxide. The influence of process conditions on the structure and magnetic properties of the samples was investigated in detail and the appropriate conditions were determined. The as-synthesized sample is Sr2FeMoO6 with tetragonal crystal structure and I4/mmm space group, and unit cell parameter is a = 5.580?, c = 7.882?. The grain size is below 100nm. The Curie temperature is above room temperature and the saturation magnetization of the sample is 13.59A·m2/kg under 1T at room temperature. The room temperature magnetoresistance of the sample under 1T field is up to -10.02%. The sample exhibit typical semiconductor behavior and the conductive mechanism can be described by small polaron variable range hopping model.
Then, the series of Sr2-xAxFeMoO6 (A = Li+, Na+, K+ or Gd3+) were synthesized by sol-gel method, in order to investigate the influence of A site substitution on structure and magnetic properties of the samples. Because the contribution of the steric effect and electron-doping effect are different in different systems, the unit cell parameters of Li-, Na-, K-doped samples are less than those of undoped sample, while the unit cell parameters of Gd-doped samples are more than those of undoped sample. With the increase of doping amount, the saturation magnetization of Li-, Na- or K-doped samples increases firstly and then decrease, while that of Gd-doped samples decreases with the increase of doping amount. Meanwhile, the primary mechanism of the synthesis of Sr1.9K0.1FeMoO6 by sol-gel method is proposed according to the analysis of TG-DTA, XRD, infra-red spectrum and X-ray energy disperse spectrum.
Finally, a new process named microwave sintering method is explored to prepare double perovskite Sr2FeMoO6 with MnO2 for microwave absorbent and activated carbon for reducing agent. Compared with the traditional solid state reaction and sol-gel technique, this method has many advantages of short reaction time, saving energy and high efficiency. The as-synthesized sample has tetragonal crystal structure and I4/mmm space group, and unit cell parameter is a = 5.571?, c = 7.872?. The investigation of magnetic and electric properties shows that microwave sintering method is favor to improve the saturation magnetization and room temperature magnetoresistance of the sample. The Curie temperature is above room temperature and the saturation magnetization of the sample is17.949 A·m2/kg under 1T at room temperature. The room temperature magnetoresistance of the sample under 1T field is up to -15.63%, which is larger than that of the sample prepared by sol-gel technique. The sample exhibit typical semiconductor behavior and the conductive mechanism can be described by adiabatic small polaron model. According to XRD and infra-red spectra analysis, the reaction mechanism is proposed, as follows: firstly, under the radiation of microwave, the reaction of SrCO3、Fe2O3 and MoO3 produces the precursor Sr2Fe2O5 and SrMoO4; then, the precursor is reduced by activated carbon in order to facilitate the formation of Fe-O-Mo bond, i.e., the generation of final product Sr2FeMoO6.
引文
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