Expansion of the multifunctionality in off-stoichiometric manganites using post-annealing and high pressure: physical and electrochemical studies.

Prospects for the use of manganites in various areas of modern technologies require comprehensive studies of their physical and chemical properties. La0.9Mn1.1O3 (LMO) ceramics have been synthesized at an annealing temperature tann of 1150 °C with further post-annealing at 1250, 1350, and 1450 °C. As tann increases, the structure symmetry changes, and both the crystallite size and chemical defects increase. The post-annealing, on one hand, leads to a dramatic reduction of the magnetocaloric effect (MCE) |-ΔSmaxM| from 3.50 to 0.75 J (kg K)-1 at 2 T and a Curie temperature TC from 227 to 113 K with increasing tann. On the other hand, an external hydrostatic high-pressure P works oppositely enhancing ferromagnetic interactions. The saturation of -ΔSmaxM and TC is already achieved at a relatively low P of ≈ 0.4 GPa. LMO-1150 exhibits the best magnetocaloric characteristics compared with other studied samples. Moreover, the electrochemical characteristics of the LMO materials as electrocatalysts for overall water splitting (OER process) and features of their transformation in different 0.5 M K2SO4, 0.5 M K2HPO4, and 0.1 M K2B4O7 electrolytes have been studied thoroughly. After electrocatalysis of LMO, the magnetization M decreases and TC remains, which makes it possible to control the depletion of electrodes and predict their working time based on the magnetic measurements. All samples show the best OER activity in the 0.5 M K2HPO4 media. The obtained results demonstrate the ways for controlling the MCE of LMO under changing internal and external conditions, and an evaluation of the possibilities for their OER applications in electrocatalysts.

Multifunctionality of lanthanum-strontium manganite nanopowder.

Manganites are multifunctional materials which are widely used in both technology and devices. In this article, new prospects of their use as nanoparticles for various types of applications are demonstrated. For that, the ferromagnetic nanopowder of La0.6Sr0.4MnO3 has been synthesized by the sol-gel method with a subsequent annealing at 700-900 °C. The crystal structure, phase composition and morphology of nanoparticles as well as magnetic, magnetothermal and electrocatalytic properties have been studied comprehensively. The critical sizes of superparamagnetic, single-domain, and multi-domain states have been determined. It has been established that an anomalously wide temperature range of magnetocaloric properties is associated with an additional contribution to the magnetocaloric effect from superparamagnetic nanoparticles. The maximum values of the specific loss power are observed in the relaxation hysteresis region near the magnetic phase transition temperature. The electrochemical stability and features of the decomposition of nanoparticles in 1 M KOH and Na2SO4 electrolytes have been determined. A decrease in the particle size contributes to an increase in electrocatalytic activity for overall water splitting. Magnetocaloric and electrocatalytic results of the work indicate the prospects for obtaining the possibility of changing the temperature regime of electrocatalysis using contactless heating or cooling.

Smart magnetic nanopowder based on the manganite perovskite for local hyperthermia.

For many medical applications related to diagnosis and treatment of cancer disease, hyperthermia plays an increasingly important role as a local heating method, where precise control of temperature and parameters of the working material is strongly required. Obtaining a smart material with "self-controlled" heating in a desirable temperature range is a relevant task. For this purpose, the nanopowder of manganite perovskite with super-stoichiometric manganese has been synthesized, which consists of soft spherical-like ferromagnetic nanoparticles with an average size of 65 nm and with a narrow temperature range of the magnetic phase transition at 42 °C. Based on the analysis of experimental magnetic data, a specific loss power has been calculated for both quasi-stable and relaxation hysteresis regions. It has been shown that the local heating of the cell structures to 42 °C may occur for a short time (∼1.5 min.) Upon reaching 42 °C, the heating is stopped due to transition of the nanopowder to the paramagnetic state. The obtained results demonstrate the possibility of using synthesized nanopowder as a smart magnetic nanomaterial for local hyperthermia with automatic heating stabilization in the safe range of hyperthermia without the risk of mechanical damage to cell structures.