Confecção de alvos de ferrita de cobalto

Abstract

This work aims the local development of techniques and/or methodologies for the preparation of targets, particularly of cobalt ferrite (CoFe2O4), whose domain should potentiate the local scienti c production, since there is a PED (Pulsed Electron Deposition) thin lm deposition system locally installed. Based on previous experiments of the group, the gel-combustion method was chosen to generate a homogeneous, single phase and nanostructured powder by chemical synthesis. The nanostructured powder was pressed using a conventional press, and pressure of 5 tons, to produce "green"(pressed but not sintered) pellets. The "green"/pellets were further sintered at temperatures of 1050, 1150, 1250 and 1350 oC, during 4 hours and in conventional atmosphere (ambient air), thereby obtaining the sintered pellets or prototypes of the targets. The densi cation of each pellet, as a function of the sintering temperature, was 25.9(7), 30.6(8), 40.2(7) and 44.7(6) vol.%, respectively. X-ray Di raction (XRD) was used to characterize the crystalline structure of nanostructured powders, green /pellets and sintered pellets. Only the cubic crystalline phase of cobalt ferrite was identi ed by XRD in the samples analyzed in this work. The nanostructured powder presented a lattice parameter lower than that expected for the bulk material, referring to the ICSD database, most probably due to the oxygen vacancies present in the sample structure, which are lled during conventional sintering performed up to 1150 oC and, consequently, increasing the lattice parameter. For higher sintering temperatures, there is a further reduction of the lattice parameter, probably due to the creation of new defects whose type could not be identi ed in this work but will be the subject of future research. The Williamson-Hall method was used in the microstructural analysis of the samples, allowing the determination of the mean crystallite size and strain of the di erent samples analyzed by XRD. The average crystallite size of the green (80 nm) and/or sintered (65 nm) pellets were determined to be equivalent, within the precision of the method for the analyzed samples (∼16 nm). On the other hand, the variation of the strain is evident, changing from expansive (0.32%), for green pellets and nanostructured powder, to compressive (-0.14%) after sintering, with imprecision estimated at around 0,05%. This information is consistent with the existence of defects di erent from those observed in green pellets and nanostructured powders. The pellet sintered at 1250 oC have shown densi cation very close to the highest densi - cation observed, but without deformation of the pellets after sintering (which happened for pellets sintered at 1350 oC). Also, among all the sintered pellets, the pellet sintered at 1250 oC presented the lattice parameter closer to that observed for the cobalt ferrite (ICSD) standard. This result does not exclude the presence of strain caused by defects in the lattice, but it certainly shows that the in uence of these defects on the lattice parameter of the material is almost non-existent for sinterization at 1250 oC, an interesting feature for a target material since its atomic structure characteristics would be equivalent to those of the bulk material. Thus, in the search for a pure material, which produces a dense, rigid, homogeneous and monophasic target, the best results obtained in this work point to a sintering temperature around 1250 oC. With the information obtained from this work, the manufacture of a tempered carbon steel template to produce 1-inch cobalt ferrite targets was requested.