49 research outputs found
Nanosized Sodium-Doped Lanthanum Manganites: Role of the Synthetic Route on their Physical Properties
In this paper we present the results of the synthesis and characterisation of
nanocrystalline La1-xNaxMnO3+delta samples. Two synthetic routes were employed:
polyacrylamide-based sol-gel and propellant synthesis. Pure, single phase
materials were obtained with grain size around 35 nm for the sol-gel samples
and around 55 nm for the propellant ones, which moreover present a more broaden
grain size distribution. For both series a superparamagnetic behaviour was
evidenced by means of magnetisation and EPR measurements with peculiar features
ascribable to the different grain sizes and morphology. Preliminary
magnetoresistivity measurements show enhanced low-field (< 1 T)
magnetoresistance values which suggest an interesting applicative use of these
manganites.Comment: 31 Pages 10 Figures to appear in Chem. Mate
Correlations of structural, magnetic, and dielectric properties of undoped and doped CaCu3Ti4O12
The present work reports synthesis, as well as a detailed and careful
characterization of structural, magnetic, and dielectric properties of
differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this
purpose, neutron and x-ray powder diffraction, SQUID measurements, and
dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO
ceramics were investigated in great detail to document the influence of
low-level doping with 3d metals on the antiferromagnetic structure and
dielectric properties. In the light of possible magnetoelectric coupling in
these doped ceramics, the dielectric measurements were also carried out in
external magnetic fields up to 7 T, showing a minor but significant dependence
of the dielectric constant on the applied magnetic field. Undoped CCTO is
well-known for its colossal dielectric constant in a broad frequency and
temperature range. With the present extended characterization of doped as well
as undoped CCTO, we want to address the question why doping with only 1% Mn or
0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor
of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni
doping changes the dielectric properties only slightly. In addition,
diffraction experiments and magnetic investigations were undertaken to check
for possible correlations of the magnitude of the colossal dielectric constants
with structural details or with magnetic properties like the magnetic ordering,
the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that
while the magnetic ordering temperature and the effective moment of all
investigated CCTO ceramics are rather similar, there is a dramatic influence of
doping and tempering time on the Curie-Weiss constant.Comment: 10 pages, 11 figure
Apport de l’EXAFS à l’étude de l’effet alcalin mixte dans les verres silicates
Nous avons enregistré des spectres de photoabsorption sur des verres appartenant au système ternaire SIO2 - Cs2O - Rb2O au voisinage des seuils de la raie LIII du césium (5012 eV) et de la rale K du rubidium (15200 eV). La comparaison des spectres EXAFS décrit la variation de l’environnement de l’ion alcalin lorsqu’on remplace progressivement un Ion alcalin par un autre
Frittage de YBaCuO, implications du diagramme de phases
The motivations of this experimental work are to underline the implications between the phases diagram constitution and the sintering of YBaCuO superconductors. This preliminary work is focussed on the solid → liquid transformations of this system, in the vicinity of the (123) phase. Two transformations are observed at 915 and 935 °C depending of the composition of the compound. They both have an important role on the sintering process and the chemical homogeneity of the ceramic. No such transformations seems to occur in the domain (123)-(211)-BaCuO2, the sintered sample has therefore a better chemical homogeneity.Les motivations de ce travail sont d'examiner les implications du diagramme de phases du système pseudo-ternaire Y2O3-CuO-BaO sur le frittage de composés proches de la composition supraconductrice YBa 2Cu3O7-p. Dans ce travail préliminaire, on s'est intéressé à mettre en évidence les transformations solide → liquide de ce système, au voisinage de la composition (123). Deux transformations apparaissent à 915 et 935 °C, selon la composition du produit, elles jouent un rôle important sur le frittage et l'homogénéité chimique du produit fritté. Il ne semble pas y avoir de transformations liquides dans le domaine (123)-(211)-BaCuO2, il en résulte une homogénéité chimique plus grande des échantillons frittés