Effect of sodium substitution on the physical properties of sol-gel made La0.65Ca0.35MnO3 ceramics

Samples of the La0.65Ca0.35-xNaxMnO3 (0 <= x <= 0.20) solid solution were prepared by the Pechini method and studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and magnetization measurements versus temperature and magnetic field. All the samples are ferromagnetic single phases. The sodium poorest ones exhibit an orthorhombic structure (Pbnm) while the richest a rhombohedral (R $(3) over bar $c) one. Their Curie temperature increases from 275 to 316 K when x increases from 0 to 0.20. Compared to their bulk counterparts, produced by the solid-solid method, the sol gel made samples exhibit a finer microstructure. Their average grain size ranges in the submicrometer scale, while it ranges in the micrometer one for the former. The magnetocaloric effect (MCE) of the sol-gel solid solution appear to be quite comparable to that of the solid-solid made counterpart, with relative cooling power values ranging between 230 and 260 J kg(-1) at 5 T. This result is very important because it establishes, for this system at last, that the grain size reduction from the micrometer to the submicrometer range does not affect drastically the MCE, underlining the efficiency of the Pechini method to process at relatively low synthesizing temperature for short reacting time valuable devices for domestic magnetic refrigeration. (C) 2014 Elsevier B.V. All rights reserved.