Martensitic transformation, magnetic and magnetocaloric properties of Ni–Mn–Fe–Sn Heusler ribbons

Melt-spun ribbons of nominal composition Ni50Mn36-xFexSn14 (x = 0, 2, and 3) were prepared by melt-spinning. The alloys undergo a martensitic transformation from L21 austenite to an orthorhombic 4O martensite on cooling, as determined by X-ray powder diffraction analysis. Replacement of Mn by Fe linearly reduces the characteristic temperatures of the martensitic transformation (the equilibrium temperature decreases from 328 to 285 K) and reduces the Curie temperature of the austenite phase (from 336 to 300 K), whereas the effect of the applied magnetic field on the martensite transition temperatures is negligible. Magnetic measurements (zero-field cooled, ZFC, and field cooled, FC, curves, AC susceptibility measurements) hint the coexistence of two different ferromagnetic martensitic magnetic phases. Moreover, the AC susceptibility measurements and the irreversibility of the ZFC and FC curves point towards the presence of antiferromagnetic and ferromagnetic interactions in the martensitic phase. All samples exhibit spontaneous exchange bias at 2 K, with double-shifted loops, whereas the evolution of the conventional exchange bias with the temperature agrees quite well with the behavior of ferromagnetic regions surrounded by spin-glass regions or with the coexistence of ferromagnetic–antiferromagnetic interactions. Ni50Mn36-xFexSn14 ribbons present a moderate inverse magnetocaloric effect (with a maximum of the magnetic entropy change of 5.7 Jkg−1K−1 for μ0H = 3 T for x = 3). It is worth to note that these materials feature a significant reservoir (up to 44 Jkg−1K−1 for x = 2) of magnetic entropy change, linked to the proximity of the austenitic ferromagnetic transition to the martensitic transformation.Se prepararon cintas hiladas por fusión de composición nominal Ni 50 Mn 36-x Fe x Sn 14 (x = 0, 2 y 3) mediante hilatura por fusión. Las aleaciones experimentan una transformación martensítica de austenita L2 1 a una martensita ortorrómbica 4O al enfriarse, según lo determinado por análisis de difracción de rayos X en polvo. La sustitución de Mn por Fe reduce linealmente las temperaturas características de la transformación martensítica (la temperatura de equilibrio desciende de 328 a 285 K) y reduce la temperatura de Curie de la fase austenita (de 336 a 300 K), mientras que el efecto del campo magnético aplicado sobre las temperaturas de transición martensítica es despreciable. Las mediciones magnéticas (campo cero enfriado, ZFC y campo enfriado, FC, curvas, medidas de susceptibilidad de CA) sugieren la coexistencia de dos fases magnéticas martensíticas ferromagnéticas diferentes. Además, las medidas de susceptibilidad AC y la irreversibilidad de las curvas ZFC y FC apuntan hacia la presencia de interacciones antiferromagnéticas y ferromagnéticas en la fase martensítica. Todas las muestras exhiben un sesgo de intercambio espontáneo a 2 K, con bucles de doble desplazamiento, mientras que la evolución del sesgo de intercambio convencional con la temperatura concuerda bastante bien con el comportamiento de regiones ferromagnéticas rodeadas por regiones spin-glass o con la coexistencia de interacciones ferromagnéticas-antiferromagnéticas. Ni50 Mn 36-x Fe x Sn 14 Las cintas presentan un efecto magnetocalórico inverso moderado (con un cambio de entropía magnética máximo de 5,7 Jkg −1 K −1 para μ 0 H = 3 T para x = 3). Vale la pena señalar que estos materiales presentan un reservorio significativo (hasta 44 Jkg −1 K −1 para x = 2) de cambio de entropía magnética, vinculado a la proximidad de la transición ferromagnética austenítica a la transformación martensítica

Thermal Stability Investigation and the Kinetic Study of Folnak® Degradation Process Under Nonisothermal Conditions

The nonisothermal degradation process of Folnak® drug samples was investigated by simultaneous thermogravimetric and differential thermal analysis in the temperature range from an ambient one up to 810°C. It was established that the degradation proceeds through the five degradation stages (designated as I, II, III, IV, and V), which include: the dehydration (I), the melting process of excipients (II), as well as the decomposition of folic acid (III), corn starch (IV), and saccharose (V), respectively. It was established that the presented excipients show a different behavior from that of the pure materials. During degradation, all excipients increase their thermal stability, and some kind of solid–solid and/or solid–gas interaction occurs. The kinetic parameters and reaction mechanism for the folic acid decomposition were established using different calculation procedures. It was concluded that the folic acid decomposition mechanism cannot be explained by the simple reaction order (ROn) model (n = 1) but with the complex reaction mechanism which includes the higher reaction orders (RO, n > 1), with average value of <n > = 1.91. The isothermal predictions of the third (III) degradation stage of Folnak® sample, at four different temperatures (Tiso = 180°C, 200°C, 220°C, and 260°C), were established. It was concluded that the shapes of the isothermal conversion curves at lower temperatures (180–200°C) were similar, whereas became more complex with further temperature increase due to the pterin and p-amino benzoic acid decomposition behavior, which brings the additional complexity in the overall folic acid decomposition process