REPOSITORY > RESULTS

Abstract

Presented work is a study of magnetic iron oxide (referred to as spinel) coating synthesis. The coatings were synthesized on the surfaces of selected core nanoparticles with heterogeneous nucleation during the co-precipitation of the Fe3+/Fe2+ ions. The work is divided into three parts. The first part comprise the synthesis of spinel nanoparticles with coprecipitation of Fe2+/Fe3+ ions under condition involving low supersaturation of nucleating species. The second part was aimed to coat spinel on the surfaces of different core nanoparticles involving heterogeneous nucleation, thus the spinel synthesis was done in the colloidal suspensions of the core nanoparticles. The third part of the thesis involves the study of the effect of the core nanoparticles size, size distribution, morphology, and crystal structure on the synthesis of spinel coatings. Here, the magnetic properties of synthesized nanocomposite particles were also evaluated.
First, the synthesis of the spinel nanoparticles under condition of low supersaturation was studied since such conditions favours heterogeneous nucleation when spinel is synthesized in the presence of core nanoparticles. The slow supersaturation was achieved with slow and homogeneous release of the reactants; the Fe3+ ions and hydroxyl ions. The highly reactive Fe3+ ions were immobilized into an urea-complex while hydroxyl ions needed for homogeneous precipitation of the Fe2+ ions were achieved with the addition of solid Mg(OH)2. The spinel synthesis proceeded at temperature of 60 °C where thermal decomposition of Fe3+- urea complex resulted in gradual release of Fe3+ ions that formed γ-FeOOH with thermal hydrolysis. The addition of Mg(OH)2 resulted in precipitation of the Fe2+ ions that after reacting with γ-FeOOH formed spinel. Here, the extend of thermal hydrolysis needed to be closely controlled since the γ-FeOOH phase required for final spinel formation formed only in a narrow time interval.
Second, the method described above was used to coat the spinel onto the surfaces of amorphous silica core nanoparticles. The analysis revealed that the spinel phase formed on the surfaces of the core nanoparticles with heterogeneous nucleation. As the result of the thermal hydrolysis of the Fe3+ ions, γ-FeOOH phase in the form of 3 nm nanoparticles nucleated as the initial phase. After the Mg(OH)2 additions the precipitated Fe2+ ions reacted with the γ-FeOOH phase on the silica surfaces resulting in the complete transformation into spinel. The formed spinel coating consisted of discrete, randomly-oriented spinel nanoparticles, approximately 7 nm in size.
To evaluate the effect of core nanoparticle properties on the spinel coating synthesis, the synthesis was done also in the suspension of crystalline platelet Ba-hexaferrite nanoparticles of different size and morphology. Here, the uniform and homogeneous spinel layer was obtained on the surface of hexaferrite basal planes. Due to the structural similarities between hexaferrite (HF) and spinel (S), the spinel layer grew epitaxially forming coherent interface with (0001)HF ║ (111)S. The spinel layer formed only on the basal planes, while the formation of spinel on the side faces of the hexaferrite nanoparticles was never observed due to lattice mismatch between the two phases along [0001]HF.
The magnetic measurement of nanocomposite particles where hard-magnetic core nanoparticles were coated with soft-magnetic spinel shell revealed the increase in remanence (28.2 Am2/kg) and in saturation magnetization (56 Am2/kg) due to the presence of the soft-magnetic spinel phase while coercivity only slightly decreed since the easy-axis magnetization of both phases is oriented in the same direction. The comparison of the hysteresis for magnetically oriented assemblies of the nanocomposite particles and the untreated Ba-hexaferrite core nanoparticles revealed the significant increase in the energy product for over 50 %.