Piezoelectric Response and Cycling Fatigue Resistance of Low-Temperature Sintered PZT-Based Ceramics.

The preparation of low-cost multilayer piezoelectric devices requires using cheap internal electrodes between the dielectric layers. A general strategy is to reduce the sintering temperature Ts of the ceramic layer by sintering aids which can form a liquid phase. Here, 0.2 wt% Li2CO3 was added as a sintering aid to tailor the sinterability and piezoelectricity of the commercial PZT ceramics. As verified from experiments, the piezoelectric ceramics could be densified at a sintering temperature above 940 °C, suitable for co-firing with the cheap internal electrode. The optimized sintering temperature of 980 °C can be confirmed for the 0.2 wt% Li2CO3-modified PZT ceramics due to its high piezoelectric coefficient d33 ~ 701 pC/N, planar coupling factor kp ~ 66.7%, and a low mechanical quality factor Qm ~ 71 with a transition temperature of Tc ~ 226 °C, presenting the characteristics of typical soft piezoelectric ceramics. Moreover, both the potential piezoelectric strain ~0.13% under 20 kV/cm and the good cycling fatigue characteristic (>104 cycles) of the studied piezo compositions indicates strong competitiveness in the field of multilayer piezoelectric devices.

Synergistic Function via Amorphous and Nanoscale Polarization Heterogeneous Regions in (1-x)BaTiO3 -xBi(Ni0.5 Zr0.5 )O3 Thin Film with Ultrahigh Energy Storage Capability and Stability.

Dielectric film capacitors are considered as potential candidates for advanced power electronics technology due to their extremely high-power densities and outstanding mechanical and thermal stability, but the further improvement of energy storage density is still needed. Here, a strategy is proposed to enhance the energy storage properties by introducing nanoscale polarization regions into amorphous films, which can significantly improve the maximum polarization and maintain a high breakdown strength. The (1-x)BaTiO3 -xBi(Ni0.5 Zr0.5 )O3 ((1-x)BT-xBNZ) thin films are fabricated by the sol-gel method and the amorphous films with nanoscale polarization regions are obtained by adjusting the preparation process. Consistent with the conjecture, amorphous phase and nanoscale polarization regions in the (1-x)BT-xBNZ films are clearly observed by electron diffraction. Results show that giant recoverable energy density of 103.7 J cm-3 with high energy efficiency of 88.3% are simultaneously achieved at 8.3 MV cm-1 in 0.92BT-0.08BNZ thin films. Furthermore, the 0.92BT-0.08BNZ thin film exhibits excellent thermal stability in a wide temperature range of 20-200 °C, ∆Wrec /Wrec20 °C  < 2.2%. This work provides a novel method for dielectric thin film capacitors applied in high temperature and electric field.

Revealing the source of Jupiter's x-ray auroral flares.

Jupiter's rapidly rotating, strong magnetic field provides a natural laboratory that is key to understanding the dynamics of high-energy plasmas. Spectacular auroral x-ray flares are diagnostic of the most energetic processes governing magnetospheres but seemingly unique to Jupiter. Since their discovery 40 years ago, the processes that produce Jupiter's x-ray flares have remained unknown. Here, we report simultaneous in situ satellite and space-based telescope observations that reveal the processes that produce Jupiter's x-ray flares, showing surprising similarities to terrestrial ion aurora. Planetary-scale electromagnetic waves are observed to modulate electromagnetic ion cyclotron waves, periodically causing heavy ions to precipitate and produce Jupiter's x-ray pulses. Our findings show that ion aurorae share common mechanisms across planetary systems, despite temporal, spatial, and energetic scales varying by orders of magnitude.

How Jupiter's unusual magnetospheric topology structures its aurora.

Jupiter's bright persistent polar aurora and Earth's dark polar region indicate that the planets' magnetospheric topologies are very different. High-resolution global simulations show that the reconnection rate at the interface between the interplanetary and jovian magnetic fields is too slow to generate a magnetically open, Earth-like polar cap on the time scale of planetary rotation, resulting in only a small crescent-shaped region of magnetic flux interconnected with the interplanetary magnetic field. Most of the jovian polar cap is threaded by helical magnetic flux that closes within the planetary interior, extends into the outer magnetosphere, and piles up near its dawnside flank where fast differential plasma rotation pulls the field lines sunward. This unusual magnetic topology provides new insights into Jupiter's distinctive auroral morphology.

Plasmapause surface wave oscillates the magnetosphere and diffuse aurora.

Energy circulation in geospace lies at the heart of space weather research. In the inner magnetosphere, the steep plasmapause boundary separates the cold dense plasmasphere, which corotates with the planet, from the hot ring current/plasma sheet outside. Theoretical studies suggested that plasmapause surface waves related to the sharp inhomogeneity exist and act as a source of geomagnetic pulsations, but direct evidence of the waves and their role in magnetospheric dynamics have not yet been detected. Here, we show direct observations of a plasmapause surface wave and its impacts during a geomagnetic storm using multi-satellite and ground-based measurements. The wave oscillates the plasmapause in the afternoon-dusk sector, triggers sawtooth auroral displays, and drives outward-propagating ultra-low frequency waves. We also show that the surface-wave-driven sawtooth auroras occurred in more than 90% of geomagnetic storms during 2014-2018, indicating that they are a systematic and crucial process in driving space energy dissipation.

Homogeneous/Inhomogeneous-Structured Dielectrics and their Energy-Storage Performances.

The demand for dielectric capacitors with higher energy-storage capability is increasing for power electronic devices due to the rapid development of electronic industry. Existing dielectrics for high-energy-storage capacitors and potential new capacitor technologies are reviewed toward realizing these goals. Various dielectric materials with desirable permittivity and dielectric breakdown strength potentially meeting the device requirements are discussed. However, some significant limitations for current dielectrics can be ascribed to their low permittivity, low breakdown strength, and high hysteresis loss, which will decrease their energy density and efficiency. Thus, the implementation of dielectric materials for high-energy-density applications requires the comprehensive understanding of both the materials design and processing. The optimization of high-energy-storage dielectrics will have far-reaching impacts on the sustainable energy and will be an important research topic in the near future.

Auroral streamer and its role in driving wave-like pre-onset aurora.

The time scales of reconnection outflow, substorm expansion, and development of instabilities in the terrestrial magnetosphere are comparable, i.e., from several to tens of minutes, and their existence is related. In this paper, we investigate the physical relations among those phenomena with measurements during a substorm event on January 29, 2008. We present conjugate measurements from ground-based high-temporal resolution all-sky imagers and in situ THEMIS measurements. An auroral streamer (north-south aligned thin auroral layer) was formed and propagated equatorward, which usually implies an earthward propagating plasma flow in the magnetotail. At the most equatorward part of the auroral streamer, a wave-like auroral band was formed aligning in the east-west direction. The wave-like auroral structure is usually explained as a consequence of instability development. Using AM03 model, we trace the auroral structure to magnetotail and estimate a wavelength of ~0.5 R E. The scale is comparable to the drift mode wavelength determined by the in situ measurements from THEMIS-A, whose footpoint is on the wave-like auroral arc. We also present similar wave-like aurora observations from Cassini ultraviolet imaging spectrograph at Saturn and from Hubble space telescope at Jupiter, suggesting that the wave-like aurora structure is likely a result of fundamental plasma dynamics in the solar system planetary magnetospheres.

Using ultra-low frequency waves and their characteristics to diagnose key physics of substorm onset.

Substorm onset is marked in the ionosphere by the sudden brightening of an existing auroral arc or the creation of a new auroral arc. Also present is the formation of auroral beads, proposed to play a key role in the detonation of the substorm, as well as the development of the large-scale substorm current wedge (SCW), invoked to carry the current diversion. Both these phenomena, auroral beads and the SCW, have been intimately related to ultra-low frequency (ULF) waves of specific frequencies as observed by ground-based magnetometers. We present a case study of the absolute and relative timing of Pi1 and Pi2 ULF wave bands with regard to a small substorm expansion phase onset. We find that there is both a location and frequency dependence for the onset of ULF waves. A clear epicentre is observed in specific wave frequencies concurrent with the brightening of the substorm onset arc and the presence of "auroral beads". At higher and lower wave frequencies, different epicentre patterns are revealed, which we conclude demonstrate different characteristics of the onset process; at higher frequencies, this epicentre may demonstrate phase mixing, and at intermediate and lower frequencies these epicentres are characteristic of auroral beads and cold plasma approximation of the "Tamao travel time" from near-earth neutral line reconnection and formation of the SCW.

High-Temperature Dielectrics in BNT-BT-Based Solid Solution.

Bi0.5Na0.5TiO3-BaTiO3 (BNT-BT)-based ternary solid solutions were investigated for high-temperature capacitor applications. Through a comprehensive investigation of the (1 - x )(0.92Bi0.5Na0.5TiO3-0.08BaTiO3)- x NaNbO3 [(1 - x )(BNT-BT)- x NN, x = 0 -0.45] system, 0.85(BNT-BT)-0.15NN was selected as the parent matrix due to its relatively high permittivity (>1800) and favorable energy-storage density (0.56 J/cm3 at 7 kV/mm). The effect of bismuth substitution on the dielectric properties of the matrix was further characterized. The introduction of bismuth greatly broadened the operational temperature range of 0.85(BNT-BT)-0.15Na1-3yBiyNbO3 ceramics to over 327 °C for a ±15% tolerance. The dc resistivities were of the order of 108 - [Formula: see text] magnitude from room temperature to 300 °C. An activation energy of 1.1-1.2 eV in 200-350 °C was obtained from dc resistivity data, suggesting that the conduction process in this temperature range may be associated with oxygen vacancy migration. Furthermore, the energy-storage properties were largely improved by the addition of bismuth. When the substitution of Bi over Na achieved was up to 7%, the energy-storage density and efficiency reached 0.62 J/cm3 and 88% at 7 kV/mm, respectively. These results confirm that a BNT-BT-based solid solution is a promising candidate for lead-free high-temperature capacitor applications.

The effect of grain boundary on the energy storage properties of (Ba0.4Sr0.6M)TiO3 paraelectric ceramics by varying grain sizes.

(Ba0.4Sr0.6)TiO3 (BST) ceramics with various grain sizes (0.3-3.4 µm) were synthesized by the oxalate coprecipitation method and prepared by plasma activated sintering and conventional solid-state sintering process. The effect of grain boundary on the energy storage properties and the dielectric relaxation characteristics of BST paraelectric ceramics (Curie point ≈ -67°C) with various grain sizes were investigated. The dielectric breakdown strength (simplified as BDS) is obviously improved and then deteriorated with decreasing grain size, accounting for the energy density variation. The enhancement of interfacial polarization at grain boundary layers has a negative effect on the BDS, leading to the decreased values for samples with grain size smaller than 0.7 µm. In addition, the insulation effect of grain boundary barriers was discussed based on the complex impedance spectroscopy analysis, which was found to play a dominant role in controlling the BDS with coarser grain size. Among them, the sharply decreased BDS for BST with grain size of 1.8 µm was believed to be attributed to the combination of lower grain boundary density and higher interfacial polarization, due to the significant increase of oxygen vacancies at higher sintering temperature.