Bismuth layer–structured ferroelectric calcium bismuth niobate (CaBi2Nb2O9, CBN) is considered to be one of the most potential high‐temperature piezoelectric materials due to its high Curie ...temperature Tc of ∼940°C, but the drawbacks of low electrical resistivity at elevated temperature and low piezoelectric performance limit its applications as key electronic components at high temperature (HT). Herein, we report significantly enhanced dc electrical resistivity and piezoelectric properties of CBN ceramics through rare‐earth element Tb ions compositional adjustment. The nominal compositions of Ca1−xTbxBi2Nb2O9 (abbreviated as CBN‐100xTb) have been fabricated by conventional solid‐state reaction method. The composition of CBN‐3Tb exhibits a significantly enhanced dc electrical resistivity of 1.97 × 106 Ω cm at 600°C, which is larger by two orders of magnitude compared with unmodified CBN. The donor substitutions of Tb3+ ions for Ca2+ ions reduce the oxygen vacancy concentrations and increase the band‐gap energy, which is responsible for the enhancement of dc electric resistivity. The temperature‐dependent dc conduction properties reveal that the conduction is dominated by the thermally activated oxygen vacancies in the low‐temperature region (200–350°C) and by the intrinsic conduction in the HT region (350–650°C). The CBN‐3Tb also exhibits enhanced piezoelectric properties with a high piezoelectric coefficient d33 of ∼13.2 pC/N and a high Tc of ∼966°C. Moreover, the CBN‐3Tb exhibits good thermal stabilities of piezoelectric properties, remaining 97% of its room temperature value after annealing at 900°C. These properties demonstrate the great potentials of Tb‐modified CBN for high‐temperature piezoelectric applications.
The attitude control problem is addressed for a quadrotor system subject to the modeling uncertainties and unknown disturbances. A novel attitude control scheme is proposed based on nonsingular fast ...terminal sliding mode (NFTSM) technique. First, the tracking differentiator (TD) is designed to obtain the smooth tracking signal and its derivative. Then, the extended state observer (ESO) is constructed to provide the estimate of the modeling uncertainties and unknown disturbances. With the designed TD and ESO, a novel NFTSM controller is developed such that tracking error converges to zero in finite time. The transient-state and the steady-state performances are both achieved with the new controller. Finally, the simulation and real experiment results verify the effectiveness and superiority of the proposed control method.
Solar radiation is often shielded by terrain relief, especially in mountainous areas, before reaching the surface of the Earth. The objective of this paper is to study the spatial structures of the ...shielded astronomical solar radiation (SASR) and the possible sunshine duration (PSD) over the Loess Plateau. To this end, we chose six test areas representing different landforms over the Loess Plateau and the software package of Matlab was used as the main computing platform. In each test area, 5-m-resolution digital elevation model established from 1:10,000 scale topographic maps was used to compute the corresponding slope, SASR and PSD. Then, we defined the concepts of the slope-mean SASR spectrum and the slope-mean PSD spectrum, and proposed a method to extract them from the computed slope, SASR and PSD over rectangular analysis windows. Using this method, we found both spectrums in a year or in a season for each of the four seasons in the six test areas. Each spectrum was found only when the area of the corresponding rectangular analysis window was greater than the corresponding stable area of the spectrum. The values of the two spectrums decreased when the slope increased. Furthermore, the values of the stable areas of the spectrums in a year or in a season were positively correlated with the variable coefficients of the slope or the profile curvature. The values of the stable areas of the two spectrums in a year or in a season may represent the minimum value of test areas for corresponding future research on the spatial structures of the SASR or PSD. All the findings herein suggest that the spatial structures of the PSD and the SASR are caused by the interactions between solar radiation and terrain relief and that the method for extracting either spectrum is effective for detecting their spatial structures. This study may deepen our understanding of the spatial structure of solar radiation and help us further explore the distribution of solar energy in mountainous regions.
Due to rapid solidification of melted powders in metal additive manufacturing processes and high thermal gradients, large residual stresses are created in the build. This can lead to undesired ...distortions as well as crack initiation. The main aim of this work is to optimize the Additive Manufacturing (AM) process parameters by finite element modelling of the entire process to minimize the resulting residual stresses and distortions. We focus on two most important metal AM processes: (a) Laser Direct Energy Deposition (LDED) and (b) Selective Laser Melting (SLM). The ABAQUS AM module is employed to simulate both processes as it provides an automated interface allowing the user to define event data, such as element activation and heat input, as a function of both position and time to achieve process simulation of complex 3D parts. For the LDED processes, thin wall components are simulated, and residual stresses predictions are compared with both FIB-DIC and XRD measurement results at different scales. For the SLM process, overhanging structures with different support thicknesses are simulated and compared with experimental part distortion after support removal. It is shown that the support thickness together with selected process and material properties play a key role in resulting distortions.
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•A computationally-efficient simulation framework is validated via residual stress and distortion measurements.•Separate time and spatial information for material deposition and heat input need to be provided via separate event series.•The main cause for cracking of thin-walled Inconel 718 structures is a high through-thickness residual stress at corners.•Simulation of surrounding powder bed is important to accurately predict temperature history for complex support geometries.•Predicted distortions of Inconel 718 cantilever structures were found to be highly sensitive to material anisotropy.
2D organic-inorganic lead iodide perovskites have recently received tremendous attention as promising light absorbers for solar cells, due to their excellent optoelectronic properties, structural ...tunability, and environmental stability. However, although great efforts have been made, no 2D lead iodide perovskites have been discovered as ferroelectrics, in which the ferroelectricity may improve the photovoltaic performance. Here, by incorporating homochiral cations, 2D lead iodide perovskite ferroelectrics R-1-(4-chlorophenyl)ethylammonium
PbI
and S-1-(4-chlorophenyl)ethylammonium
PbI
are successfully obtained. The vibrational circular dichroism spectra and crystal structural analysis reveal their homochirality. They both crystalize in a polar space group P1 at room temperature, and undergo a 422F1 type ferroelectric phase transition with transition temperature as high as 483 and 473.2 K, respectively, showing a multiaxial ferroelectric nature. They also possess semiconductor characteristics with a direct bandgap of 2.34 eV. Nevertheless, their racemic analogue adopts a centrosymmetric space group P2
/c at room temperature, exhibiting no high-temperature phase transition. The homochirality in 2D lead iodide perovskites facilitates crystallization in polar space groups. This finding indicates an effective way to design high-performance 2D lead iodide perovskite ferroelectrics with great application prospects.
Solar radiation is the primary energy source that drives many of Earth's physical and biological processes and determines the patterns of climate and productivity on the surface of the Earth. A ...fundamental proportion of solar radiation is composed of shielded extra-terrestrial solar radiation (SESR), which can be computed using the slope and aspect derived from a digital elevation model (DEM). The objective of this paper is to determine the influence of the grid spacing of the DEM (the influence of the scale of the DEM) on the errors of slope, aspect and SESR. This paper puts forward the concepts of slope representation error, aspect representation error, and SESR representation error and then studies the relations among these errors and the grid spacing of DEMs. We find that when the grid spacing of a DEM becomes coarser, the average SESR increases; the increase in SESR is dominated by the grid cells of the DEM with a negative slope representation error, whereas SESR generally decreases in the grid cells with a positive slope representation error. Although the grid spacing varies, the distribution of the percentages of positive SESR representation errors on the slope, which is classified into 11 slope intervals, is independent of the grid spacing; this distribution is concentrated across some slope intervals. Moreover, the average absolute value and mean square error of the SESR representation error are closely related to those of the slope representation error and the aspect representation error. The findings in this study may be useful for predicting and reducing the errors in SESR measurements and may help to avoid mistakes in future research and in practical applications in which SESR is the data of interest or plays a vital role in an analysis.
Aqueous batteries are promising devices for electrochemical energy storage because of their high ionic conductivity, safety, low cost, and environmental friendliness. However, their voltage output ...and energy density are limited by the failure to form a solid‐electrolyte interphase (SEI) that can expand the inherently narrow electrochemical window of water (1.23 V) imposed by hydrogen and oxygen evolution. Here, a novel (Li4(TEGDME)(H2O)7) is proposed as a solvation electrolyte with stable interfacial chemistry. By introducing tetraethylene glycol dimethyl ether (TEGDME) into a concentrated aqueous electrolyte, a new carbonaceous component for both cathode−electrolyte interface and SEI formation is generated. In situ characterizations and ab initio molecular dynamics (AIMD) calculations reveal a bilayer hybrid interface composed of inorganic LiF and organic carbonaceous species reduced from Li+2(TFSI−) and Li+4(TEGDME). Consequently, the interfacial films kinetically broaden the electrochemical stability window to 4.2 V, thus realizing a 2.5 V LiMn2O4−Li4Ti5O12 full battery with an excellent energy density of 120 W h kg−1 for 500 cycles. The results provide an in‐depth, mechanistic understanding of a potential design of more effective interphases for next‐generation aqueous lithium‐ion batteries.
A novel “ether‐in‐water” electrolyte is demonstrated by introducing the non‐aqueous co‐solvent TEGDME into an aqueous electrolyte. The designed Li4(TEGDME)(H2O)7 solvation sheath structure with stable interfacial chemistry dynamically expands the electrochemical stability window to 4.2 V. Meanwhile, the high‐quality solid electrolyte interphase (SEI) and cathode–electrolyte interface (CEI) derived from the reduction of Li+2(TFSI−) and Li+4(TEGDME) effectively suppress hydrogen/oxygen evolution and electrode dissolution.
The laser powder bed fusion (L-PBF) process involves a large number of processing parameters. Extending the intricate relationship between processing and structure to mechanical performance is ...essential for structural L-PBF materials. The high cycle fatigue properties of L-PBF parts are very sensitive to process-induced porosities which promote premature failure through the crack initiation mechanisms. Results from this work show that for stainless steel 316L, porosity does not impinge on the high cycle fatigue properties when processing is kept within a ±30% tolerance band. In this ‘optimum’ processing region, crack initiation takes place due to defects at the solidification microstructure level. Beyond the ‘optimum’ processing region, over-melting and under-melting can lead to porosity-driven cracking and inferior fatigue resistance. In addition, regardless of the processing condition, fatigue resistance was found to follow a direct linear relationship with ductility and tensile strength in the low and high stress fatigue regimes respectively.
This paper presents a comprehensive study conducted to optimize the selective laser melting (SLM) parameters and subsequent heat-treatment temperatures for near-α high-temperature titanium alloy ...Ti-6Al-2Zr-1Mo-1 V (TA15), which is widely used in the aerospace industry. Based on the surface morphology and relative density analysis, the optimized process parameters were: laser power from 230 W to 380 W, scan speed from 675 mm/s to 800 mm/s, scan spacing of 0.12 mm, and layer thickness of 0.03 mm. The effects of the laser power and the layer thickness on the phase constitutions, microstructure features, as well as room-temperature and high-temperature (500 °C) tensile properties, were then studied to obtain an in-depth understanding of SLM-built TA15. Six typical temperatures (650, 750, 850, 950, 1000 and 1100 °C) covering three representative temperature ranges, i.e., martensite partial decomposition temperature range, martensite complete decomposition temperature range and above β transus temperature, were subsequently selected as heat-treatment temperatures. The heat treatment-microstructure-mechanical property relationships of SLM-built TA15 were elucidated in detail. These results provide valuable information on the development of SLM-built TA15 alloy for industrial applications, and these findings are also beneficial to additive manufacturing of other near-α Ti alloys with desirable high-temperature properties.
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