Magnetocaloric nanostructured La0.7Ca0.3-xBaxMnO3 (x < 0.3) ceramics produced by combining polyol process and Spark Plasma Sintering

Ferromagnetic La0.7Ca0.3-xBaxMnO3 (x < 0.3) ceramics were produced by an original pathway, combining soft chemistry and Spark Plasma Sintering (SPS), within a few minutes at a temperature of 800 degrees C (instead of several hours at 1300 degrees C with the conventional ceramic route). Their structural, chemical, magnetic and magnetocaloric properties were investigated by combined X-ray diffraction (XRD), Mn K-edge X-ray absorption near-edge structure (XANES) and routine magnetometry and compared to those of their conventionally solid-solid made counterparts. All the samples are single phase. A slight unit cell dilatation is observed in the produced ceramics when the Ba atomic content increases, due to the progressive replacement of Ca2+ cations by larger Ba2+ ones. In every case, the volume cell size is higher in the polyol-SPS samples than in their conventionally made counterparts. The qualitative and quantitative analysis of the XANES spectra allow to attribute this unit cell size change to a manganese electronic state modification in relation with the experimental material processing conditions: polyol-SPS samples exhibit higher Mn3+/Mn4+ atomic ratio. Moreover, the microstructure of these samples is highly dense and ultrafine grained with a compactness of about 94% and an average size grain of less than 150 nm. Interestingly, the para-to ferromagnetic transition in these ceramics is found to be significantly broadened at Curie temperatures, T-C, which are smaller than those measured on their bulk counterparts. As a consequence, the thermal variation of the magnetic entropy change is broader in these ceramics compared to in the conventionally made ones, making them particularly valuable for magnetocaloric applications. (C) 2016 Published by Elsevier B.V.