“共核”法制备高Ms锌铁氧体纳米颗粒的形成机理

Soft ferrites are widely used in electronic devices and microwave absorbing materials. In such applications, the saturation magnetization Ms is a fundamental parameter for performance criteria of ferrites. Ms is closely related to the crystal structure and the distribution of metal cations in the structure. Spinel ferrites possess face-centered cubic structure, in which the metal cations tend to distribute in either tetrahedral interstices (A site) or octahedral interstices (B site). However, the cations distribution in nanoparticles is complicated, which is difficult to control by existing preparation techniques, resulting in low Ms. Thus, the applicant designed a new method, named as “coherent nucleation” method because of the simultaneous dissolution and the coherent nucleation process. This method is able to prepare ferrimagnetic Zn ferrite, of which the Ms value is 30% higher than that of traditional ferrites. The causes of high magnetization and the formation mechanism are still unclear. Further studies in this project will be done to get the details of “coherent nucleation” method. The rules of how the technical parameters affect the formed nanoparticles, including the morphology, the particle size, the crystallinity and the composition and so on, will be investigated. The valence state of metal cations and the distribution will also be explored to reveal the formation mechanism of nanoparticle and its Ms. The ultimate aim of this project is to create a microstructure- and magnetization-controlled synthesis route for Zn ferrite nanoparticles. The studies in this project will promote the development and application of soft ferrites theoretically and technically.

软磁铁氧体在电子器件和微波吸收材料领域有着广泛的应用,而磁饱和强度Ms是衡量铁氧体材料的重要性能指标。Ms的大小与材料的晶体结构和金属离子分布密切相关。尖晶石型铁氧体具面心立方结构，研究显示不同金属阳离子在结构中的四面体间隙（A位）和八面体间隙（B位）存在占位倾向，但对于纳米颗粒，金属离子的分布复杂并难以通过现有合成工艺来控制，导致Ms值较低。为此，申请者设计了一种新的制备方法——“共核”法，应用多种前驱物同溶解、共形核的原理，制备出亚铁磁性的锌铁氧体纳米颗粒，其超高的Ms值超传统材料达30%，但形成机制尚不明确。本项目拟对“共核”法进一步研究，考察颗粒的形貌、粒度、结晶度、成分等随工艺参数的变化规律，结合对金属离子价态及分布规律的研究，揭示纳米颗粒及其高Ms的形成机制，最终实现锌铁氧体纳米颗粒在微观结构及磁性上的可控制备。本研究能为高性能软磁铁氧体的开发及应用提供一定的理论和技术支撑。

本项目通过不同的系列实验完成了“共核”法制备高磁性锌铁氧体纳米颗粒的技术探索，在实验中发现，当表面活性剂选用油酸或硬脂酸时，均可实现高磁性锌铁氧体纳米颗粒的制备，不同的是，硬脂酸对制备过程中需要有充足的低温排气过程才可以使反应顺利进行，而油酸的制备过程中，更容易用来实现大尺寸和结晶度高的纳米颗粒的制备。在使用油酸作为表面活性剂的情况下，很容易通过控制锌和铁前驱物的用量来实现纳米颗粒中锌的含量，因为锌原子优先占有A位，并因此可以调控锌铁氧体中Fe3+在A位上的数量，进而调控锌铁氧体纳米颗粒的磁性。与其他制备方法不同的是，“共核”法中制备的锌铁氧体并没有出现金属离子混合分布的现象，这对控制锌铁氧体纳米颗粒的磁性非常关键。“共核”法可以用来制备具有高磁饱和强度的锌铁氧体纳米颗粒，其磁饱和强度可以达到110 emu/g，在本项目拓展实验中发现，“共核”法还可以用来制备三元铁氧体材料，如锰锌铁氧体，所制备的锰锌铁氧体同样可以展示很高的磁饱和强度，这也证明了“共核”法是一种优良的铁氧体材料合成技术，这对铁氧体材料发展具有非常重要的意义。