Preparation of dense La0.7Ca0.3MnO3 ceramics from freeze-dried precursors

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Abstract

Sinterability of La0.7Ca0.3MnO3 precursors, obtained by the freeze-drying method, is studied in order to develop a technique for preparation of dense (>95%) ceramics for CMR measurements and sputtering applications. Single phase powders, obtained by thermal decomposition at 650°C, were subjected to deagglomeration by ultrasonic or mechanical treatment. Sintering of deagglomerated powders for several hours at T=1200–1300°C allowed to achieve densities up to 97–98%. The best sinterability is demonstrated by mechanically processed powder, but further sintering of ceramics, obtained from this precursor, results in significant dedensification (up to 85% at 1300°C). Analysis of precursors and dedensified samples shows at high temperature decomposition of carbonates in closed pores to be the most probable reason for the observed process.

Introduction

The recent discovery of colossal magnetoresistance in perovskite complex oxides resulted in a number of new applications of these materials.1 The most significant of them until now are thin-film devices, while a large number of these films are prepared by the laser ablation method. It makes feasible the development of techniques for preparation ceramic sputtering targets in order to ensure planar and spatial homogeneity of ablated films. Features of the laser ablation method set some specific demands for ceramic samples being used as sputtering targets, first of all, the maximum chemical homogeneity and minimum porosity of used material.

The preparation of manganite ceramics is mentioned in several papers devoted to the study of magnetoresistive properties of perovskite CMR materials.2, 3, 4 The details of dense material preparation from manganite powders were not usually considered; in most cases the density or any other parameters of prepared materials are not mentioned at all. At the same time, more detailed study of densification processes may be important not only for optimization of target preparation processes. While the CMR effect in high (several Tesla) magnetic fields is controlled mostly by crystallochemical features of a particular phase,5 the effects in smaller fields are strongly related to the microstructure of the powder, ceramic or thin film samples.6 Some papers refer to the significant enhancement of CMR ratio in polycrystalline samples compared to single crystals or epitaxial films,7, 8 while properties of high density CMR ceramics remain underestimated. These considerations give additional reasons to study the sintering processes of fine powders of La–Ca manganite. The freeze-drying method demonstrated its efficiency in obtaining many other similar dense ceramic materials,9 and has been selected for precursor powder preparation.

Section snippets

Experimental

A stock solution for freeze-drying (FD) synthesis was prepared by mixing of appropriate amounts of preanalyzed 0.3–0.5 M solutions of La and Mn acetates and Ca nitrate. The solution was sprayed by pneumatic nozzle (mean diameter of droplet=100–200 μm) into liquid nitrogen under intensive stirring, then tray-dried in a semi-industrial freeze-drier SMH-15 (Usifroid) at P=4×10−2 mBar for 48 h; the temperature of the heating plates was changed from −50 to +40°C. Thermal decomposition of salt

Conclusions

It should be noted that submicron manganite powders, prepared by the freeze drying method, demonstrated excellent activity in the process of free sintering, large enough to obtain ceramics with density over 97% at reasonable temperatures (1300°C). The character of sintering curves and their variation for powders with various prehistory and microstructure show the possibility of further enhancement of sinterability by means of optimization of processing conditions. At the same time, the present

Acknowledgements

The work is supported by Korea Institute of Science and Technology Evaluation and Planning (KISTEP) grant. O.A. Shlyakhtin is grateful to V.V. Ischenko (MPI Stuttgart) for his kind help and fruitful discussion.

References (18)

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