Grain-Size Effects on Diffuse Phase Transitions of Batio 3 Ceramics Obtained by Alkoxide Precursors
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OBTAINED BY ALKOXIDE PRECURSORS R. P. S. M. LOBO, R. L. MOREIRA and N. D. S. MOHALLEM Departamentos de Fisica e de Quimica, ICEx, UFMG, C. P. 702, CEP 30161-970 Belo Horizonte, MG, Brazil
ABSTRACT Barium titanate ceramics have been obtained by sol-gel methods. The dielectric investigations of these materials revealed the existence of diffuse ferroelectric transitions. By using a phenomenological model, we could demonstrate the existence of a simple relationship between the diffuse character of the transition and the sample grain-size. This effect has been attributed to interactions between charged defects on the grain surfaces and the spontaneous polarization of the material.
INTRODUCTION Barium titanate (BT) ceramics are widely used in the production of ceramic capacitors, due to their extremely high dielectric susceptibility. In these materials, the dielectric constant shows an important dependence on the ceramic grain-sizes in the limit of small grains. This dependence appears to be more important in the ferroelectric phase than in the paraelectric one.[1] These effects can find their origin in internal stresses [2,3] together with domain wall contributions [4,5]. Among the various methods used to produce BT ceramics, sol-gel methods present some particular advanitages because they allow us to obtain materials with high purity and homogeneity, through a low temperature process avoiding contamination of the material.J[6,7] They also yield a better stoichiometric control and allow the preparation of thin films. Finally, another important feature of sol-gel processes is the possibility of grain-size and grain-shape control. The BT ceramics produced by these methods posses small grains, which give rise to a diffuse ferroelectric phase transition (DFPT).[8] This kind of transition occurs very often in solid solutions of tungsten-bronze and perovskite structures. Its origin is essentially associated with defects in the material, such as stoichiometric fluctuations, grain configuration, structural disorder and microscopic heterogeneities. [9-12] The main characteristic of such transitions is the absence of a specific critical temperature so that the phase transition (PT) occurs within a temperature interval -- yielding a smearing of the physical anomalies at the transition.[9-15] Moreover, concerning the dielectric properties, it is well known that the systems presenting DFPT show a deviation from the Curie-Weiss law for the reciprocal dielectric susceptibility. [9,13-14] The purpose of this work is to report on our investigations of the dielectric properties of small-grained BT ceramics obtained by alkoxide precursors. The results allow us to show the crossover between the Smolensky-Isupov and the Curie-Weiss behaviors and the dependence between the diffuse character of the transition and the sample grain-sizes. 309 Mat. Res. Soc. Symp. Proc. Vol. 346. ©1994 Materials Research Society
EXPERIMENTAL PROCEDURES The BT samples have been prepared by three different processes using alkoxide precursors (sol-gel methods)
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