Nanoestructure cupric oxide properties

A. BIANCHI; T. PLIVELIC; S. STEWART; I. TORRIANI; L. CADIERNO; L. D. JUNCIEL ; G. PUNTE

Buenos Aires

Congreso; International Conference on Supramolecular Science & Technology; 2002

NANOSTRUCTURED CUPRIC OXIDE PROPERTIES A. Bianchi1-2, S. Stewart2, T. Plivelic3, I. Torriani3-4, L. Cadierno1-2,, L. Junciel 2,  G. Punte1-2 1 LANADI. 2 IFLP ( CONICET-UNLP) Dep. de Física, Facultad de Ciencias Exactas. Universidad Nacional de La Plata. CC 67.1900 La Plata. Argentina. 3. Laboratorio Nacional de Luz Síncrotron (LNLS). Campinas, Brazil. 4. Departamento de Estado Sólido e Ciencia dos Materiais. Universidad Estadual de Campinas. C.P. 6165, 13083-970, Campinas, SP, Brazil. punte@fisica.unlp.edu.ar. Many properties of nanostructured (NS) materials have been found to be essentially different from those of normal coarse-grained polycrystalline materials. This has been related to the interfacial structures and nanosized grains. Though the interfacial structures of the NS materials have been extensively investigated the results are still controversial. Two extreme pictures have been proposed1: the earliest, supported by some experiments and computer simulations suggests a nonequilibrium, highly disordered ??frozen-gas-like?? grain-boundary (GB) structure, substantially different from structures in coarse-grained (CG) polycrystalline materials. More recent experiments suggest that GB in nanocrystalline systems are not anomalous, but similar to those found in polycrystalline materials  Production and study of NS antiferromagnetic oxides have currently acquired great significance due to their potential technological application. These systems are also interesting from the basic physics point of view. NS cupric oxide (CuO) has shown a higher susceptibility than the CG polycristalls. This is more noticeable as the grain size becomes smaller2,3. In parallel, CuO single crystals have shown anomalies in their electric conductivity and magnetic susceptibility below  the Néel temperature4. To investigate if changes in the magnetic answer were correlated with changes in electrical conductivity when no long-range order is present, we have studied the electrical conductivity as a function of temperature of NS CuO. The nanocrystalline samples used in this work were produced by controlled ball milling (BM) and by solid state reaction (SSR). The results show different behaviour of BM and SSR samples. To tailor  materials with specific qualities the knowledge of the relationship between the nanostructure characteristics and their physico-chemical properties is essential. Therefore a microstructure characterisation of all the samples was performed. Strain and crystallite size were obtained by Rietveld analysis of the diffraction patterns of all samples. SAXS results were used to analyse the features of the interfaces and to find the volume fraction of the crystalline and the GB phases based on a two-phase model for the systems. In the case of the BM samples, the behaviour of the invariant indicates that the electron density contrast decreases as a function of milling time. Since an increase in the crystalline unit cell was verified from the XRD measurements, the results may indicate that ball-milling time increase causes a reduction of the crystalline phase density, with no change in the disordered phase. The SSR samples present different characteristics, with interfaces between the crystalline and GB regions becoming smoother. The results seem to indicate that conduction is help by the degree of the strain at the interface. 1.     Van Swygenhoven, H.; Farkas, D. Caro, A. Phys Rev B 62, 831 (2000) and references therein. 2.      R. A. Borzi, S. J. Stewart, R. C. Mercader, G. Punte y F. García. J. Mag. Mang. Mat. 226, 1513-1551, (2001). 3.     Punnoose, A.; Magnone, H.; Seerha, M. S.; Bonevich, J. Phys Rev B64, 174420 (2001). 4.    Zheng, X. G.; Tsutsumi, N.; Tanaka, S.; Suzuki, M.; Xu, C.N. Physica C321, 67 (1999).