RESUMO
The immense potential of lead-free dielectric capacitors in advanced electronic components and cutting-edge pulsed power systems has driven enormous investigations and evolutions heretofore. One of the significant challenges in lead-free dielectric ceramics for energy-storage applications is to optimize their comprehensive characteristics synergistically. Herein, guided by phase-field simulations along with rational composition-structure design, we conceive and fabricate lead-free Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-Sr(Sc0.5Nb0.5)O3 ternary solid-solution ceramics to establish an equitable system considering energy-storage performance, working temperature performance, and structural evolution. A giant Wrec of 9.22 J cm-3 and an ultra-high Æ ~ 96.3% are realized in the BNKT-20SSN ceramic by the adopted repeated rolling processing method. The state-of-the-art temperature (Wrec ≈ 8.46 ± 0.35 J cm-3, Æ ≈ 96.4 ± 1.4%, 25-160 °C) and frequency stability performances at 500 kV cm-1 are simultaneously achieved. This work demonstrates remarkable advances in the overall energy storage performance of lead-free bulk ceramics and inspires further attempts to achieve high-temperature energy storage properties.
RESUMO
As an electromechanical coupling between strain gradients and polarization, flexoelectricity is largely enhanced at the nanoscale. However, directly observing the evolution of flexoelectric fields at the nanoscale usually suffers from the difficulty of producing strain gradients and probing electrical responses simultaneously. Here, we introduce nanocracks in SrTiO3, Ba0.67Sr0.33TiO3, and TiO2 samples and apply continuously varying mechanical loading to them, and as a result, huge strain gradients appear at the crack tip and result in a significant flexoelectric effect. Then, using atomic force microscopy, we successfully measure the evolution of flexoelectricity around the crack tips. For the case of SrTiO3, the maximum induced electric field reaches 11 kV/m due to the tensile load increasing. The proposed method provides a reliable way to identify the significance of the flexoelectric effect. It may also open a new avenue for the study of flexoelectricity involving multiple physics phenomena including flexoelectronics, the flexo-photovoltaic effect, and others.
RESUMO
The present study aimed to observe the effects of acute exposure to hypobaric hypoxia on learning and memory in adult Sprague-Dawley (SD) rats as well as the changes in N-methyl-D-aspartate receptor (NMDAR) subunits. Learning and memory abilities were evaluated with Morris water maze following simulated hypoxia at 5000 m or 7000 m for 2, 4 or 6 days. The number of red blood cells, the level of hemoglobin, oxidative stress markers, the extent of neuronal damage in the hippocampal CA1, and the expression of NMDAR subunits were all measured. In place navigation test, the escape latency significantly increased only in the 5000 M 6Day (P < 0.05) group than the latency in the plain group on the fourth day. In the spatial probe test, the times of platform passing showed no significant difference between the hypoxia and plain groups. Polycythemia, an increased ratio of superoxide dismutase/malondialdehyde and degeneration of neurons appeared in an elevation-dependent and duration-dependent manner in those hypoxia groups. Furthermore, the protein expression of NR1 increased and the protein expression levels of NR2A, NR2B, and NR3A were all decreased in the 5000M 6Day and 7000M 6Day groups. In summary, the feeble effect of acute exposure to simulated hypobaric hypoxia on learning and memory in adult SD rats may be attributed to increased antioxidative capacities and decreased expression of NR3A. And moreover, differences in the expressions of NMDAR subunits were closely related to the altitude and duration.