Síntese e caracterização de ferrita de cobalto nanoestruturada pelo método hidrotermal assistido por micro-ondas

Abstract

Cobalt ferrite presents a positive saturation magnetic moment and considerable coercivity, characteristics that enable numerous applications, depending on the size of the formed particles. The methods that were adopted in the production of CoFe204, which may obtain control in the size of particles, has become more interesting for science and technology. In regards to this, this research has as its primary focus to produce CoFe204 ferrite through microwave-assisted hydrothermal synthesis with controlled wave size. Secondly, this work makes the reduction of the synthesized material in flow oven in hydrogen atmosphere (aiming to form the compound CoFe204-CoFe2). Lastly, it categorizes, structural and magnetically, nanoparticles of CoFe204 and core layer system of CoFe204-CoFe2. In the production of CoFe204, it has been produced 4 series. The structural categorization of the samples was done through X-ray diffraction supported by the Rietveld refinement and Transmission Electron Microscopy (TEM); however, only two of them present the pure cobalt ferrite. DRX’s have pointed out that the medium diameter of the crystallite that revolves the band goes from 5,2 to 21,3 nm. The series with potassium resulted in bigger mixed form crystallites (approximately, spherical or cubical) in comparison to ammonia hydroxide that resulted in smaller and spherical crystallites. For samples with sodium and ethylene glycol, the DRX’s have pointed out that besides the ferrite phase, the samples have presented a hematite phase. The samples were magnetically categorized through a vibrating sample magnetometer (VSM) in room temperature, by a Physical Property Measurement System (PPMS) in low temperatures and Mossbauer spectroscopy. These measures reveal a characteristic of magnetic blocked particles for samples with crystallites bigger than 10 nm and super pragmatic for smaller crystallites. The structural and magnetic measures relate. For example, the reason MR/MHmax grows with the increase in the average size of the crystallites. In the production of the CoFe204-CoFe2 system, a sample reduced in stabilized hydrogen atmosphere at 350 °C for 10 minutes was used and from this procedure it had become possible to obtain the CoFe2 alloy. The new reduced atmosphere had been analysed both, structurally and magnetically. The DRX’s and the MET images point out a formation of the core-layer system, indicating the medium size of crystallites of 33,7 nm (6,3 in CoFe204 and 27,4 nm in CoFe2). In conclusion, the magnetic properties, analyzed through a Hysteresis Curve and Mössbauer Spectrum were improved in comparison to the starting sample, for instance, the relation MR/MHmax increased to 0,356 from 0,299.