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

Abstract

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 °C (instead of several hours at 1300 °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, TC, 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.