RESUMEN
The structure of a material is an important factor in determining its physical properties. Here, we adjust the structure of the Ni50Mn37Ga13 spun ribbons by changing the wheel speed to regulate the exchange bias effect of the material. The characterization results of micromorphology and structure show that as the wheel speed increases, the martensite lath decreases from 200 nm to 50 nm, the structure changed from the NM to a NM and 10M mixed martensitic structure containing mainly NM, then changed to NM and 10M where 10M and NM are approaching. Meanwhile, HE first increased and then decreased as the wheel speed increased. The optimum exchange bias effect (HE = 7.2 kOe) occurs when the wheel speed is 25 mâs-1, mainly attributed to the enhanced ferromagnetism caused by part of 10M in NM martensite, which enhanced the exchange coupling of ferromagnetism and antiferromagnetism. This work reveals the structural dependence of exchange bias and provides a way to tune the magnitude of the exchange bias of Heusler alloys.
RESUMEN
Ceramics, often exhibiting important functional properties like piezoelectricity, superconductivity, and magnetism, are usually mechanically brittle at room temperature and even more brittle at low temperature due to their ionic or covalent bonding nature. The brittleness in their working temperature range (mostly from room down to cryogenic temperatures) has been a limiting factor for the usefulness of these ceramics. In this Letter, we report a surprising "low-temperature toughening" phenomenon in a La-doped CaTiO_{3} perovskite ceramic, where a 2.5× increase of fracture toughness K_{IC} from 1.9 to 4.8 MPa m^{1/2} occurs when cooling from above room temperature (323 K) down to a cryogenic temperature of 123 K, the lowest temperature our experiment can reach. In situ microscopic observations in combination with macroscopic characterizations show that this desired but counterintuitive phenomenon stems from a reentrant strain-glass transition, during which nanosized orthorhombic ferroelastic domains gradually emerge from the existing tetragonal ferroelastic matrix. The temperature stability of this unique microstructure and its stress-induced transition into the macroscopic orthorhombic phase provide a low-temperature toughening mechanism over a wide temperature range and explain the observed phenomenon. Our finding may open a way to design tough ceramics with a wide temperature range and shed light on the nature of reentrant transitions in other ferroic systems.
RESUMEN
In ferroelectric and relaxor-ferroelectric materials, piezoelectric and dielectric properties are significantly enhanced at the morphotropic phase boundary (MPB), a boundary between different ferroelectric phases with different macroscopic symmetries. By contrast, in relaxor systems, such an MPB does not exist because relaxors of different compositions possess the same macroscopic symmetry. Here, we report the existence of a morphotropic relaxor boundary (MRB) in the single phase relaxor region of a K_{0.5}Na_{0.5}NbO_{3}-xBaTiO_{3} system, which is a composition-induced boundary between two relaxors with different local polar symmetries (tetragonal versus rhombohedral) but with the same macroscopic cubic symmetry. At the MRB the electrostrain increases by â¼3 times and the permittivity increases by â¼1.5 times over a wide temperature range of more than 100 K, as compared with off-MRB compositions. Our Letter demonstrates that the MRB may become an effective mechanism to enhance the dielectric and electrostrictive properties of relaxors.
RESUMEN
With the development of magnetic refrigerant technology, magnetic substances with a large magnetocaloric effect (MCE) and nearly zero thermal hysteresis are desired. Although Ni-Mn based Heusler alloys have been found to produce large MCEs and have attracted increasing attention recently, the occurrence of thermal hysteresis accompanying MCE due to the nature of first-order phase transition limits its applications with magnetic refrigeration. Up to now, an effective theory or method to eliminate this thermal hysteresis is still lacking. Here, we propose to utilize the feature of magnetic transition at the morphotropic phase boundary (MPB) to eliminate thermal hysteresis and thus design a MPB-involved phase diagram in Heusler alloys of Ni50Mn36Sb14-xInx (x = 0-14). As theoretically expected, the magnetic transition at MPB really yields a MCE with a negligible thermal hysteresis (â¼0 K) and the refrigerant capacity arrives at a maximum value of 108.2 J kg-1 at the composition of x = 9. Our findings provide an effective way to design large MCE materials with zero thermal hysteresis.
