Partner und Internationale Organisationen
(Englisch)
|
A, B, BG, CZ, DK, FIN, F, D, GR, H, IRL, I, LT, N, PL, P, RO, SK, SI, E, S, CH, TR, GB
|
Abstract
(Englisch)
|
Granular magnetic systems, consisting of nanometer-sized particles of a magnetic metal (eg cobalt) dispersed in a non-magnetic matrix (eg copper) is expected to exhibit giant magnetoresistance behaviour. These nanocomposite particles may be prepared by an oxalate co-precipitation followed by the appropriate thermal treatment. The copper oxalate particles are made up of self-assembled nanocrystallites and the cobalt oxalate is expected to exhibit a similar behaviour due to its crystallographic structure. The main objective of this work is to synthesize and characterize the self-assembly of size and shape-controlled nanocrystals. The understanding of the precipitation and growth processes of cobalt and copper oxalate nanoparticles to form metal nanocomposites with giant magnetoresistance is fundamental. One challenge will be the control of the thermal decomposition of the mixed copper - cobalt oxalate and the influence of the reduction process on the distribution, shape and size of the cobalt particles in the copper matrix. The electric, optical, transport and magnetic properties of the structures depend not only on the characteristics of the individual nanocrystallites, but also on the coupling and interaction among the nanocrystals arranged with long-range order with respect to translation and even orientation. It is also extremely important to be able to control the agglomeration step of the cobalt and copper oxalate nanocrystallites in order to influence the physical properties of the cobalt-copper metal composite. Finally it is also necessary to study and to understand the precipitation process of the single copper and cobalt oxalates as tools to know and control the mixed oxalate precipitation. The shape of the copper oxalate particles is influenced by the concentration of a polymer additive, hydroxypropylmethylcellulose (HPMC), when it is added to the precipitating agents. Without polymer the copper oxalate particles present a cushion-like morphology while with increasing the concentration of HPMC, the shape of the aggregates varies from cube to elongated rods. The presence of HPMC affects the copper oxalate nucleation step, crystal growth and aggregation. The first aim of our work is to understand the mechanism of copper oxalate precipitation by following the kinetics. During 2002 we studied the precipitation of copper oxalate in an instrumented mini-batch reactor to follow parameters such as pH. Using thermodynamic solution modelling and microscopic imaging we could identify the three steps of the precipitation: nucleation, crystalline growth and growth of particles by aggregation of nanocrystallites. By following the pH behaviour we were able to quantify the precipitation yield of copper oxalate as a function of time. Also the formation of a 0.2% wt of malachite (CuCO3·Cu(OH)2) otherwise undetectable was brought to light - which may have an important effect on the decomposition products and colloidal behaviour of the micron sized particles. The precipitated particles have been further characterized after stopping the reaction at specific times using various methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and atomic force microscopy (AFM) to help elucidate the growth mechanism.
|
