High-Entropy Strategy for Improved Mechanical and Energy Storage Properties in BaTiO3-BiFeO3-Based Ceramics.

Dielectric capacitors are employed extensively due to their exceptional performance, including a rapid charge-discharge speed and superior power density. However, their practical implementation is hindered by constraints in energy-storage density (ESD), efficiency (ESE), and thermal stability. To achieve domain engineering and improved relaxor behavior in 0.67BiFeO3-0.33BaTiO3-based Pb-free ceramics, the concerns have been addressed here by employing a synergistic high-entropy strategy involving the design of the composition of Sr(Mg1/6Zn1/6Ta1/3Nb1/3)O3 with B-site multielement coexistence and high configuration entropy. Remarkably, in (0.67-x)BiFeO3-0.33BaTiO3-xSr(Mg1/6Zn1/6Ta1/3Nb1/3)O3 ceramics with x = 0.08, a good ESE (η) of 75% and a recoverable ESD (Wrec) of 2.4 J/cm3 at 190 kV/cm were attained together with an ultrahigh hardness of ∼7.2 GPa. The high-entropy strategy, which is tailored by an increase in configuration entropy, can be attributed to the superior mechanical and ES properties. It also explains the enhanced random field and relaxation behavior, the structural coexistence of ferroelectric rhombohedral (R3c) and nonpolar pseudocubic (Pm-3m) symmetries, the decreased domain size, and evenly distributed polar nanoregions (PNRs). Moreover, improved thermal stability and outstanding frequency stability are also obtained. By boosting the configuration entropy, BiFeO3-BaTiO3 materials dramatically improved their complete energy storage performance. This suggests that designing high-performance dielectrics with high entropy can be a convenient yet effective technique, leading to the development of advanced capacitors.

Nonvolatile switchable half-metallicity and magnetism in the MXene Hf2MnC2O2/Sc2CO2 multiferroic heterostructure.

Nonvolatile electrical control of two-dimensional (2D) van der Waals (vdW) magnetism is important for spintronic devices. Here, using first-principles calculations, we systematically investigated the magnetic properties of the MXene Hf2MnC2O2 combined with the ferroelectric MXene Sc2CO2. When flipping the electric polarization of Sc2CO2, a transition between a semiconductor and a half-metal occurs in the Hf2MnC2O2 monolayer. Moreover, the ferromagnetic exchange parameter J1 can be enhanced to 9.67 meV under polarized P↑ of Sc2CO2, much larger than those of the pristine Hf2MnC2O2 monolayer and Hf2MnC2O2/Sc2CO2-P↓. In addition, the easy magnetization axis of the Hf2MnC2O2 monolayer is also dependent on the polarization orientation of Sc2CO2. Our results indicate a multiferroic heterostructure based on MXenes, offering an effective way for obtaining nonvolatile electrical control of electronic and magnetic properties.

Microstructure and Mechanical Properties of AlTiVCuN Coatings Prepared by Ion Source-Assisted Magnetron Sputtering.

The AlTiVCuN coatings were deposited by magnetron sputtering with anode layer ion source (ALIS) assistance, and the microstructure and mechanical properties were significantly affected by the ion source power. With increasing the ion source power from 0 to 1.0 kW, the deposition rate decreased from 2.6 to 2.1 nm/min, and then gradually increased to 4.0 nm/min at 3.0 kW, and the surface roughness gradually decreased from 28.7 nm at 0 kW to 9.0 nm at 3.0 kW. Due to the enhanced ion bombardment effect, the microstructure of the coatings changed from a coarse into a dense columnar structure at 1.0 kW, and the grain size increased at higher ion source powers. All the coatings exhibited c-TiAlVN phase, and the preferred orientation changed from the (220) to the (111) plane at 3.0 kW. Due to the low Cu contents (1.0~3.1 at.%), the Cu atoms existed as an amorphous phase in the coatings. Due to the microstructure densification and high residual stress, the highest hardness of 32.4 GPa was achieved for the coating deposited at 1.0 kW.

Enhanced Piezoelectricity and Thermal Stability of Electrostrain Performance in BiFeO3-Based Lead-Free Ceramics.

BiFeO3-based ceramics possess an advantage over large spontaneous polarization and high Curie temperature, and are thus widely explored in the field of high-temperature lead-free piezoelectrics and actuators. However, poor piezoelectricity/resistivity and thermal stability of electrostrain make them less competitive. To address this problem, (1 - x) (0.65BiFeO3-0.35BaTiO3)-xLa0.5Na0.5TiO3 (BF-BT-xLNT) systems are designed in this work. It is found that piezoelectricity is significantly improved with LNT addition, which is contributed by the phase boundary effect of rhombohedral and pseudocubic phase coexistence. The small-signal and large-signal piezoelectric coefficient (d33 and d33*) peaks at x = 0.02 with 97 pC/N and 303 pm/V, respectively. The relaxor property and resistivity are enhanced as well. This is verified by Rietveld refinement, dielectric/impedance spectroscopy and piezoelectric force microscopy (PFM) technique. Interestingly, a good thermal stability of electrostrain is obtained at x = 0.04 composition with fluctuation η = 31% (Smax'-SRTSRT×100%), in a wide temperature range of 25-180 °C, which is considered as a compromise of negative temperature dependent electrostrain for relaxors and the positive one for ferroelectric matrix. This work provides an implication for designing high-temperature piezoelectrics and stable electrostrain materials.

Effect of V/Mo Atomic Ratio on the Microstructure and Mechanical Properties of MoVCuN Coatings.

To improve the gas ionization ratio, the Mo-V-Cu-N coatings were deposited by pulsed dc magnetron sputtering with assistance from an anode layer ion source, and the influence of the V/Mo atomic ratio was explored with regard to the microstructure and mechanical properties of the coatings. The findings of this study indicated that the MoVCuN coatings exhibited a solid solution phase of FCC B1-MoVN with a prominent (220) preferred orientation, and the deposition rate was found to decrease from 4.7 to 1.8 nm/min when the V/Mo atomic ratio increased. The average surface roughness of the MoVCuN coatings gradually decreased, and the lowest surface roughness of 6.9 nm was achieved at a V/Mo atomic ratio of 0.31. Due to the enhanced ion bombardment effect, the coatings changed from a coarse columnar to a dense columnar crystal structure, and promoted grain refinement at higher V/Mo atomic ratios, contributing to a gradual improvement in the compressive residual stress, hardness and adhesion strength of the coatings.

Cu2+-Ion-Substitution-Driven Microstructure and Microwave Dielectric Properties of Mg1-xCuxAl2O4 Ceramics.

In this work, Cu-substituted MgAl2O4 ceramics were prepared via solid-state reaction. The crystal structure, cation distribution, and microwave dielectric properties of Mg1-xCuxAl2O4 ceramics were investigated. Cu2+ entered the MgAl2O4 lattice and formed a spinel structure. The substitution of Cu2+ ions for Mg2+ ions contributed to Al3+ ions preferential occupation of the octahedron and changed the degree of inversion. The quality factor (Qf) value, which is correlated with the degree of inversion, increased to a maximum value at x = 0.04 and then decreased. Ionic polarizability and relative density affected the dielectric constant (εr) value. The temperature coefficient of the resonant frequency (τf) value, which was dominated by the total bond energy, generally shifted to the positive direction. Satisfactory microwave dielectric properties were achieved in x = 0.04 and sintered at 1550 °C: εr = 8.28, Qf = 72,800 GHz, and τf = -59 ppm/°C. The Mg1-xCuxAl2O4 solid solution, possessing good performance, has potential for application in the field of modern telecommunication technology.

Enhanced Energy Storage Performance in Na0.5Bi0.5TiO3-Based Relaxor Ferroelectric Ceramics via Compositional Tailoring.

Owing to the high power density, excellent operational stability and fast charge/discharge rate, and environmental friendliness, the lead-free Na0.5Bi0.5TiO3 (NBT)-based relaxor ferroelectrics exhibit great potential in pulsed power capacitors. Herein, novel lead-free (1-x)(0.7Na0.5Bi0.5TiO3-0.3Sr0.7Bi0.2TiO3)-xBi(Mg0.5Zr0.5)O3 (NBT-SBT-xBMZ) relaxor ferroelectric ceramics were successfully fabricated using a solid-state reaction method and designed via compositional tailoring. The microstructure, dielectric properties, ferroelectric properties, and energy storage performance were investigated. The results indicate that appropriate Bi(Mg0.5Zr0.5)O3 content can effectively enhance the relaxor ferroelectric characteristics and improve the dielectric breakdown strength by forming fine grain sizes and diminishing oxygen vacancy concentrations. Therefore, the optimal Wrec of 6.75 J/cm3 and a η of 79.44% were simultaneously obtained in NBT-SBT-0.15BMZ at 20 °C and 385 kV/cm. Meanwhile, thermal stability (20-180 °C) and frequency stability (1-200 Hz) associated with the ultrafast discharge time of ~49.1 ns were also procured in the same composition, providing a promising material system for applications in power pulse devices.

Transient Heat Transfer Characteristics in a Flat Plate Heat Sink with Mini-Channels for Cooling High Heat Flux IGBT.

Effective cooling of a high heat flux IGBT electronic system is highly related to system efficiency and safety. A flat plate heat sink was designed to experimentally investigate the transient heat transfer characteristics of IGBT cooling. It is made of aluminum with 20 mini-channels of 249 mm × 3 mm × 4 mm dimensions, which were manufactured by milling machines and melt inert gas (MIG) welding technology to ensure no deformation. Experiments were conducted using deionized water at atmospheric pressure with flow rates of 3.2-9.5 L/min and heat fluxes of 104-347 W/cm2. It was found that instantaneous start-stop and transient heating power variation might cause IGBT failure, especially under low Reynolds and Nusselt number conditions. The temperature rise rate and cooling rate vary with different system parameters. Heating rate can be reduced by high flow rate due to local subcooled boiling. The concept of respond time (RT) based on the piecewise function is suggested to evaluate the influence of transient condition on heating rate. Analysis of flow fluctuation indicated that it would not be a threat to the system except for in extreme cases. These findings provide a reference for the considerations of the design and manufacture of IGBT cooling flat plate heat sinks with mini-channels.

Meta-Analysis of the Effectiveness and Safety of Intravenous Thrombolysis in Patients with Acute Cerebral Infarction.

In this paper, a meta-analysis of the effectiveness and safety of intravenous thrombolysis in patients with acute cerebral infarction was carried out, the original literature inclusion criteria and retrieval strategies were developed, and the collection deadline was about new oral anticoagulants and other methods for the antithrombotic intravenous thrombolytic treatment of patients with acute cerebral infarction for the relevant literature on the safety and effectiveness comparison. First, the quality of the literature is evaluated according to whether the included studies are randomized controlled trials, whether there is randomized concealment, whether blinding is used, and whether they are withdrawn or lost to follow-up, and the RevMan 5.2 software is used for meta-analysis. At the same time, grey literature databases such as dissertations were experimentally searched, and all randomized controlled studies (RCT), nonrandomized controlled studies, case-controlled studies, cohort studies, case series reports, etc. of Wingspan in the treatment of intracranial atherosclerotic stenosis were collected. In the prevention of myocardial infarction in patients with acute cerebral infarction, the difference between the two was not statistically significant (RR = 0.82, 95% CI (0.57, 1.17), P = 0.27). Compared with other methods, it can significantly reduce the all-cause mortality of patients with nonvalvular venous thrombolysis, and the difference is statistically significant (RR = 0.90, 95% CI (0.85, 0.96), P = 0.001). Experimental results show that in terms of safety, the new oral anticoagulant is better than other methods in reducing minor bleeding in patients with acute cerebral infarction, and the difference is statistically significant (RR = 0.87, 95% CI (0.76, 0 99), P = 0.03); the effect is better than other methods in reducing the incidence of serious bleeding events, and the difference is statistically significant (RR = 0.79, 95% CI (0.74, 0.85), P < 0.00001).

NH3 Sensor Based on 3D Hierarchical Flower-Shaped n-ZnO/p-NiO Heterostructures Yields Outstanding Sensing Capabilities at ppb Level.

Hierarchical three-dimensional (3D) flower-like n-ZnO/p-NiO heterostructures with various ZnxNiy molar ratios (Zn5Ni1, Zn2Ni1, Zn1Ni1, Zn1Ni2 and Zn1Ni5) were synthesized by a facile hydrothermal method. Their crystal phase, surface morphology, elemental composition and chemical state were comprehensively investigated by XRD, SEM, EDS, TEM and XPS techniques. Gas sensing measurements were conducted on all the as-developed ZnxNiy-based sensors toward ammonia (NH3) detection under various working temperatures from 160 to 340 °C. In particular, the as-prepared Zn1Ni2 sensor exhibited superior NH3 sensing performance under optimum working temperature (280 °C) including high response (25 toward 100 ppm), fast response/recovery time (16 s/7 s), low detection limit (50 ppb), good selectivity and long-term stability. The enhanced NH3 sensing capabilities of Zn1Ni2 sensor could be attributed to both the specific hierarchical structure which facilitates the adsorption of NH3 molecules and produces much more contact sites, and the improved gas response characteristics of p-n heterojunctions. The obtained results clear demonstrated that the optimum n-ZnO/p-NiO heterostructure is indeed very promising sensing material toward NH3 detection for different applications.

Thermal Analysis of High-Entropy Rare Earth Oxides.

Phase transformations in multicomponent rare earth sesquioxides were studied by splat quenching from the melt, high-temperature differential thermal analysis and synchrotron X-ray diffraction on laser-heated samples. Three compositions were prepared by the solution combustion method: (La,Sm,Dy,Er,RE)2O3, where all oxides are in equimolar ratios and RE is Nd or Gd or Y. After annealing at 800 °C, all powders contained mainly a phase of C-type bixbyite structure. After laser melting, all samples were quenched in a single-phase monoclinic B-type structure. Thermal analysis indicated three reversible phase transitions in the range 1900-2400 °C, assigned as transformations into A, H, and X rare earth sesquioxides structure types. Unit cell volumes and volume changes on C-B, B-A, and H-X transformations were measured by X-ray diffraction and consistent with the trend in pure rare earth sesquioxides. The formation of single-phase solid solutions was predicted by Calphad calculations. The melting point was determined for the (La,Sm,Dy,Er,Nd)2O3 sample as 2456 ± 12 °C, which is higher than for any of constituent oxides. An increase in melting temperature is probably related to nonideal mixing in the solid and/or the melt and prompts future investigation of the liquidus surface in Sm2O3-Dy2O3, Sm2O3-Er2O3, and Dy2O3-Er2O3 systems.

Site-related broadband sensitization in La3Ga5.5Nb0.5O14: Cr3+, Ln3+(Ln = Yb, Er) phosphors.

Yb3+ 1000 nm and Er3+ 1536 nm emission can be efficiently sensitized by broadband absorption of Cr3+ in almost the whole visible region in La3Ga5.5Nb0.5O14(LGNO): Cr3+, Ln3+ (Ln = Yb, Er) phosphor. Between the two kinds of Cr3+ sites, tetrahedral Cr(II) mainly behaves as the broadband sensitizer for Er3+ or Yb3+. Meanwhile octahedral Cr(I) may energy transfer (ET) to Cr(II), thereby influences the luminescence decay of Cr(II) as increasing Er3+ or Yb3+ content. This kind of site-related broadband sensitization may propose a strategy of designing tunable ET process between transition metal ions and rare earth ions.