Syntheses And Magnetic Properties Of ε-Fe₂O₃/SiO₂ Nanocomposites

This dissertation was focused on magnetic properties,syntheses and potential applications ofε-Fe₂O₃/SiO₂ composites.In the aspect of magnetic research,the effect of phase compositions and content on the magnetic properties were studied via alkali etching process.As a result,controllable magnetic properties ofε-Fe₂O₃/SiO₂composite could be achieved by adjusting etching conditions.On the preparation side,paramagnetic-classε-Fe₂O₃/SiO₂compositewassuccessfullypreparedby milling-etching route and explored its new application fields.Besides,the phase stability of etchedε-Fe₂O₃ was studied through post-treatment method.At the same time,theε-Fe₂O₃-based bi-magnetic composites were prepared and studied their exchange bias fields.Mainly research contents are shown as follows:1 Alkaline-etching procedure was utilized to tailor magnetic properties ofε-Fe₂O₃/SiO₂ composite.It was found that the saturation magnetization,coercivity and exchange bias field could be readily changed and tailored by altering the etching time and frequency in a set of sodium hydroxide solutions.The relative quantity ofε-Fe₂O₃phase,the proximity or pinning effect derived from SiO₂ phase as well as the phase transformation fromε-Fe₂O₃ toα-Fe₂O₃ during etching treatment were three main factors to its controllable magnetic properties.2 The milling-etching route was designed to synthesize self-assemblingε-Fe₂O₃/SiO₂ nanocomposite,which is environmental friendly and more desirable for energy-efficient mass production ofε-Fe₂O₃ phase than conventional thermal treatment methods.More strikingly,this product conformed to the characteristics of paramagnetic-class properties,and our study shown that the unique property derived from large lattice strain and a few of small grains ofε-Fe₂O₃.Its relaxivities study confirmed that the paramagnetic-class product has a potential to become a candidate as contrast agent in magnetic resonance imaging application filed.3 The phase stability of etchedε-Fe₂O₃ was researched via tempering treatment and hydrogenous reduction.It was found thatε-Fe₂O₃ transformed toα-Fe₂O₃ phase partly and formedε-Fe₂O₃/α-Fe₂O₃ bi-magnetic composite during tempering treatment at 900 ^oC.Meanwhile,the composite ofε-Fe₂O₃/α-Fe₂O₃ yielded an exchange bias field（ca.-465 Oe）at 5 K after zero-field cooling process.On the other hand,ε-Fe₂O₃gradually transformed to Fe_21.34O₃₂2 phase in the hydrogenous reduction process at 350^oC.Similarly,the formed bi-magnetic composite ofε-Fe₂O₃/Fe_21.34O₃₂2 exhibited an exchange bias field（ca.-290 Oe）at 5 K after field-cooling process with 1 T.