A series of ferroelectric ceramic models with grain and grain‐boundary structures of different sizes are established via Voronoi tessellations. A phase‐field model is introduced to study the ...dielectric breakdown strength of these ferroelectric ceramics. Afterward, the relation between the electric displacement and electric field and the hysteresis loop are calculated using a finite element method based on a classical and modified hyperbolic tangent model. The results indicate that as the grain size decreases, the dielectric strength is enhanced, but the dielectric permittivity is reduced. The discharge energy density and energy storage efficiency of these ferroelectric ceramics extracted from the as‐calculated hysteresis both increase along with a decrease in their grain size at their breakdown points. However, under the same applied electric field, the ferroelectric ceramic with a smaller grain size possesses a lower discharge energy density but a higher energy storage efficiency. The results suggest that ferroelectric ceramics with smaller grain sizes possess advantages for applications in energy storage devices.
This study examines the influence of analytical particle size on the surface area and mesopore and micropore volume data obtained from low-pressure N2 and CO2 adsorption analyses in response to the ...crushing of coal and shale. Pennsylvanian high-volatile bituminous coal (Ro~0.57%) and Devonian to Lower Mississippian low-maturity (Ro~0.57%) and high-maturity (Ro~1.30%) shales from the Illinois Basin were progressively crushed from chunks (~7mm) to 4mesh (<4.78mm), 7mesh (<2.83mm), 18mesh (<1mm), 30mesh (0.595mm), 60mesh (<0.250mm), 200mesh (<0.074mm), and 230mesh (<0.063mm), and, subsequently, low-pressure N2 and CO2 adsorption analyses were performed on all the grain size fractions. Our results demonstrate that the values of both surface area and specific mesopore and micropore change with progressive crushing. For example, BET surface area of coal shows a steady increase from 2m2/g in the 4mesh fraction to 4.7m2/g in the 200mesh fraction. For comparable size ranges, BET surface area changes from 0.15 to 7.82m2/g in the low-maturity shale, and from 0.02 to 6.26m2/g in the high-maturity shale. Changes in mesoporosity and microporosity parameters indicate that the coarsest fractions (4mesh and larger) are not suitable for low-pressure adsorption analysis; the values are very low and not reproducible dominantly because of equilibration problems. Our results demonstrate that the 60mesh fraction for coal and the 200mesh fraction for shales seem to be optimal and the most practical sizes for performing low-pressure N2 and CO2 adsorption analysis; these analytical particle sizes yield results closest to the “real” values, unbiased by disequilibrium.
•Analytical particle size influences low pressure gas adsorption results.•Coarse fractions are unable to reach equilibrium.•60mesh is suggested as the optimal size for coal to use.•200mesh is the optimal size for shale.
0.70BiFeO3‐0.30BaTiO3 (0.70BF‐0.30BT) ceramics have been widely concerned because of their potential applications for high‐temperature piezoelectric devices. In this work, a series of dense ...0.70BF‐0.30BT ceramics with average grain size variation from 0.55 to 6.0 μm were prepared. XRD results indicate that 0.70BF‐0.30BT ceramics show the coexistence of rhombohedral and pseudo‐cubic phases and the volume fraction of the rhombohedral phase increase with the grain size. The dielectric, ferroelectric and piezoelectric properties increase with the grain size initially from 0.55 to 5.0 μm and then decrease slightly. Values of d33, Pr, and εr, of 0.70BF‐0.30BT ceramics with the grain size of 5.0 μm are 185 pC/N, 21.2 μC/cm2, and 638, respectively, about five times higher than those ceramics with fine‐grain of 0.55 μm. Of particular importance is that 0.70BF‐0.30BT ceramics with large grain sizes possess better piezoelectric thermal stability due to the much stabler poled domain state with the rising temperature. The detailed structural studies indicate that the enhanced electric properties are owing to the significantly improved domain motion and the increased lattice distortion. This clarifying the relationship between electrical properties and grain size offers a novel way of improving the performances of piezoceramics.
Abstract
Channel stability and sediment transport in gravel bed streams depend on temporally and spatially variable fluid forces, bed surface structures, armoring, and sediment supply/storage. Of ...particular interest here is the influence of sediment supply timing on bedload transport rate and grain size distribution, bed surface composition and channel morphology. We conducted flume experiments in a sediment feed flume with poorly sorted sediment. A symmetrical, identical stepped hydrograph was used with five different sediment feeding schemes: no feed, constant feed, rising‐limb only feed, falling‐limb only feed, and variable feed. The same sediment mass of 800 kg was fed during each experiment. Sediment transport rates ranged over five orders of magnitude regardless of feeding scheme. Clockwise hysteresis was observed for bedload transport rate and bedload grain size, that is, the transport rate was larger and coarser during the rising limb. Counterclockwise hysteresis was observed for the grain size distribution of the bed surface, that is, the bed surface was finest during the rising limb. In all experiments sediment yield during the rising was higher than during the falling limb, indicating that the rising limb is more capable to transport the supplied sediment. Our study provides insight on how timing of sediment supply influences sediment transport and bed surface during a single hydrograph, essential information for artificial sediment supply projects to restore and habilitate gravel bed streams.
Key Points
Sediment yield during a single hydrograph is dominated by sediment supply in the rising limb
Clockwise hysteresis characterizes sediment transport despite the sediment feed timing
Counterclockwise hysteresis was observed for bed surface grain size in most experiments
Ferroelectric Zr-doped HfO 2 (HZO) is a promising candidate component for the future transistors and memory devices applications. To address the deviceto-device variation in those highly scaled ...devices, the grain properties of 12-nm-thick HZO films are investigated and optimized by altering the atomic layer deposition (ALD) cycle ratio of HfO 2 and ZrO 2 at a constant Zr concentration. The largest remanent polarization 2Pr of 41μC/cm 2 , refined average grain radius from 16.6 nm to 13 nm, and improved grain size distribution with the standard deviation reduced from 5 to 3.3 are realized by adjusting the ALD cycle ratio to 5/5. Furthermore, the possible underlying mechanisms for the ferroelectric behaviors and the growth modes of the HZO films deposited with various cycle ratios are demonstrated.
NaNbO3 (NN) is considered to be one of the most prospective lead-free antiferroelectric energy storage materials due to the merits of low cost, nontoxicity, and low density. Nevertheless, the ...electric field-induced ferroelectric phase remains dominant after the removal of the electric field, resulting in large residual polarization, which prevents NN ceramics from obtaining superior energy storage performance. In this work, the relaxor ferroelectric Sr0·7Bi0·2TiO3 (SBT) was chosen to partially replace the NN ceramics, and the introduction of the nanodomain of the relaxor ferroelectric hinders the generation of field-induced ferroelectric phases, allowing the material to combine the large polarization strength of the relaxor ferroelectric with the near-zero residual polarization of the antiferroelectric. Large recoverable energy storage density (4.5 J cm−3) and ultra-high energy storage efficiency (90.3%) were gained in NN-20SBT under an electric field of 288 kV cm−1. Furthermore, superior temperature (25–120 °C) and frequency (1–500 Hz) stabilities were acquired. These performances demonstrate that NN-20SBT ceramics are potential candidates as dielectric materials for high energy storage density pulsed power capacitors.
In this study, we explored the effects of abrasive grain size, applied load and sliding speed on the particle distribution density of polytetrafluoroethylene/bronze composite at the worn surface. ...Wear tests were performed against different sandpapers under varied loads and speeds. Results show that the surface density can be divided into three types, namely, lesser than, close to and more than the original volume fraction value of particle (15%), as the grain size decreases. This finding can be explained by the change in wear mechanisms. Micro-ploughing is the main mechanism of all the abrasive wears. When the grain size is large, the peeling-off of bronze particles occurs. On the contrary, when the grain size is small, the ‘rolling-effect’ of debris happens.
•Abrasive grain size dominates the particle deposition behaviour at the worn surface.•Large-sized abrasive grains will peel the particles off and lead to a lack of surface particles.•Moderate-sized abrasive grains give a distribution density near the original volume fraction.•Small-sized abrasive grains cannot catch the debris which, then released, results in particles accumulation.•Applied load and sliding speed cannot affect the steady-state particle distribution density.
Grain refinement is perfectly efficient for improving strength, but accompanies by a dramatic loss of ductility. Here we report a bimodal grain size structure design by inducing ultra-high strength ...TiZrNbTa high entropy alloy (HEA) with nanoscale grain size into pure Ti with coarse grain size to achieve the strength-strain balance by establishing bimodal grain size distribution (BGSD). Through optimizing the TiZrNbTa HEA content, the 60 wt.% TiZrNbTa/Ti titanium matrix composite (TMC) exhibits a yield strength of 1426 ± 30 MPa, ultimate compressive strength of 2133 ± 88 MPa, and an acceptable compressive strain of 22.3% ± 1.8%, respectively. The compressive yield strength of 60 wt.% TiZrNbTa/Ti TMC is 3.6-fold for c.p. Ti (393.0 ± 7.1 MPa). The unusual high strength is attributed to the nanoscale grain size provided by TiZrNbTa HEA, and the acceptable compressive strain is obtained from the co-effect of coarse grain Ti and TiZrNbTa HEA. The excellent mechanical properties of 60 wt.% TiZrNbTa/Ti TMC undoubtedly greatly increase the possibility of its application as an orthopedic implant in the biomedical field.
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•A bimodal grain size structure design is beneficial to the strength-ductility balance.•The strength and plasticity of TiZrNbTa/Ti MMCs achieve the best combination by the composition optimization of TiZrNbTa HEA.•The yield strength (1426 ± 30 MPa) of 60 wt% TiZrNbTa/Ti MMC with plasticity of 22.3% ± 1.9% is 3.6-fold for c.p. Ti (393.0 ± 7.1 MPa).
