Interface-layer-assisted reliable ferroelectricity in BiFeO3 thin films by chemical solution deposition

BiFeO₃ thin films, specifically those fabricated by chemical solution deposition, suffer from severe leakage that hinder the acquirements of their intrinsic high polarizations and are thus normally not considered for use in practical electronics. The controlled fabrication of thin films with reduced leakage is of vital importance. In the present work, BiFeO₃ films (with thicknesses below ~300 nm), assisted by an interfacial amorphous layer, were fabricated by chemical solution deposition on Pt/Ti/SiO₂/Si substrates. This facile method facilitates the growth of the mentioned amorphous layer, and the ferroelectric properties of the obtained films were greatly enhanced. The conducting mechanisms of both types of thin films were systematically investigated to understand the impact of the designed interface. The results not only advance the potential use of BiFeO₃ thin films in electromechanical devices but also promote chemical solution deposition as a promising methodology for the fabrication of high-quality ferroelectric films with compressed leakage.     

Achieving high energy storage performance of Pb(Lu1/2Nb1/2)O3 antiferroelectric ceramics via equivalent A-site engineering

Manipulating the critical switching field between antiferroelectric (AFE) state and ferroelectric (FE) is an important concept for tuning the energy storage performance of AFEs. As one of the lead-based AFE systems, Pb(Lu_1/2Nb_1/2)O₃ promises high potential in the miniaturization of pulsed power capacitors, but the extremely high critical switching field and low induced saturated polarization demonstrate severe drawbacks with respect to temperature stability and flexibility. Here, A-site Ba²⁺ doping engineering is used to effectively reduce the critical switching field and improve the saturated polarization in Ba_xPb_1-x(Lu_1/2Nb_1/2)O₃ (0.01 ≤ x ≤ 0.08, abbreviated as xBa-PLN) ceramics. We found the AFE-FE phase transition can be occurred at 80ºC with a high energy storage density of 4.03 J/cm³ for Ba_0.06Pb_0.94(Lu_1/2Nb_1/2)O₃ ceramic. Our results show that Ba²⁺ additions destroy the antiparallel structure of AFE phase, and finally reduce the critical switching field, demonstrating a potential alternative to modulate the energy storage performance of AFEs.     

High-strength Ti2AlN ceramics prepared by pulse electric current sintering based on powders synthesized by molten salt method

Ti₂AlN powders were synthesized through molten salt method and re-calcination process using TiH₂, Al and TiN powders as raw materials at 1100 ℃. The composition of final composite was directly influenced by the initial Al and TiH₂ content in the starting mixture. The purity of the synthesized Ti₂AlN powder could reach 97.1 wt% when the Al molar ratio was 1.05. Then high strength Ti₂AlN ceramics were successfully prepared in different modes, including two forms of pulse electric current sintering (PECS/SPS) and hot-pressing sintering (HP). A record-high flexural strength of 719 MPa was obtained for the PECS/SPS with an electrical insulating die (PECS/SPS II) sintered sample, based on the synthesized powder in which the initial molar ratio of Al was 1.1. The sintering behaviors in various modes were analyzed, confirming the shrinkage of particles starting at lower temperature in PECS/SPS II. The density, microstructure, Vickers hardness and elastic modulus of sintered ceramics were also investigated. Therefore, the present work provided the new methods about powder preparation and ceramic sintering of Ti₂AlN, making it possible to be used as high strength structural ceramics.     

Relaxor antiferroelectric-like characteristic boosting enhanced energy storage performance in eco-friendly (Bi0.5Na0.5)TiO3-based ceramics

Ideal relaxor antiferroelectrics (RAFEs) have high field-induced polarization, low remnant polarization and very slim hysteresis, which can generate high recoverable energy storage W_rec and high energy storage efficiency η, thus attracting much attention for energy storage applications. True RAFEs, on the other hand, are extremely rare, and the majority of them contain environmentally hazardous lead. In this work, we use a viscous polymer rolling process to synthesize a novel and eco-friendly 0.65Bi_0.5Na_0.4K_0.1TiO₃-0.35[2/3SrTiO₃-1/3Bi(Mg_2/3Nb_1/3)O₃] (BNKT-ST-BMN) dielectric material, which possesses a very typical RAFE-like characteristic. As a result, this material has a high W_rec of 4.43 J/cm³ and a η of 86% at an electric felid of 290 kV/cm, as well as a high thermal stability of W_rec (>3 J/cm³) over a wide range of 30–140 °C at 250 kV/cm. Our findings suggest that the BNKT-ST-BMN material could be a potential candidate for use in energy storage pulse capacitors.     

空间超大幅宽低畸变红外成像扫描控制

本文以空间超大幅宽低畸变红外变焦扫描成像系统为研究对象,分析给出地面畸变与成像系统瞬时视场角的关系,提出变速扫描成像并推导了扫描角速度公式。为解决匀速360°旋转扫描效率低和双向摆动扫描成像需安装扫描线矫正器所导致的系统复杂性高、可靠性低的缺点,设计了一种正弦加速度快速回扫的方法。对变速扫描以及正弦加速度快速回扫方法进行了仿真及实验,结果表明扫描控制系统慢速扫描与快速回扫之间状态切换稳定,扫描起止角度误差仅为1.44角秒,扫描速度稳定度为±0.5%,扫描成像过程时间误差为83μs,回扫时间误差为250μs,整个扫描周期时间偏差小于1倍像元积分时间(355μs),扫描效率达86%,在提高了扫描效率的同时减小对扫描机构的冲击与振动,满足成像要求。正弦加速度快速回扫方法对机载红外扫描成像系统快速回扫运动设计也具有一定指导意义。     

Two step sintering of a novel calcium magnesium silicate bioceramic: Sintering parameters and mechanical characterization

Two-step sintering (TSS) was applied to control the grain growth during sintering of a novel calcium magnesium silicate (Ca₃MgSi₂O₈ – Merwinite) bioceramic. Sol–gel derived nanopowders with the mean particle size of about 90 nm were sintered under different TSS regimes to investigate the effect of sintering parameters on densification behavior and grain growth suppression. Results showed that sintering of merwinite nanopowder under optimum TSS condition (T₁ = 1300 °C, T₂ = 1250 °C) yielded fully dense bodies with finest microstructure. Merwinite compacts held at T₂ = 1250 °C for 20 h had the average grain size of 633 nm while the relative density of about 98% was achieved. Mechanical testing was performed to investigate the effect of grain growth suppression on the hardness and fracture toughness. Comparison of mechanical data for samples sintered under two sintering regimes, including TSS and normal sintering (NS), showed TSS process resulted in significant enhancement of fracture toughness from 1.77 to 2.68 MPa m^1/2.     

An active noise cancellation technique for the CUORE Pulse Tube cryocoolers

The Cryogenic Underground Observatory for Rare Events (CUORE) experiment at Gran Sasso National Laboratory of INFN searches for neutrinoless double beta decay using TeO₂ crystals as cryogenic bolometers. The sensitivity of the measurement heavily depends on the energy resolution of the detector, therefore the success of the experiment stands on the capability to provide an extremely low noise environment. One of the most relevant sources of noise are the mechanical vibrations induced by the five Pulse Tube cryocoolers used on the cryogenic system which houses the detectors. To address this problem, we developed a system to control the relative phases of the pulse tube pressure oscillations, in order to achieve coherent superposition of the mechanical vibrations transmitted to the detectors. In the following, we describe this method and report on the results in applying it to the CUORE system.     

Study on a high frequency pulse tube cryocooler capable of achieving temperatures below 4 K by helium-4

High-frequency pulse tube cryocooler (HPTC) has advantages of compact structure, low vibration, high reliability and long operation time. In this study, Theoretical analysis and experimental tests have been conducted in four aspects based on a developed 4 K HPTC. Firstly, a compressor with larger power output capability was employed and the impedance match between the cold head and the compressor was discussed. Secondly, simply using inertance tube configuration to replace the traditional inertance tube-gas reservoir structure. Then, the type and the size of the regenerator materials working at 4–20 K have been experimentally optimized. Finally, the performance of double-inlet working at as low as 20 K has also been tested for the first time for the HPTC. The present prototype achieved a no-load temperature of 3.6 K, which is the lowest temperature record ever reported for HPTC using helium-4 as working gas. A cooling power of 6 mW/4.2 K was also obtained with 250 W input power and a precooling power of 12.1 W/77 K.     

2 μm emission properties and hydroxy groups quenching of Tm3+ in germanate-tellurite glass

Tm³⁺ activated germanate-tellurite glasses with good thermal stability and anti-crystallization ability were prepared. Efficient 2 μm fluorescence was observed in the optimal concentration Tm³⁺ doped glass and the corresponding radiative properties were investigated. For Tm³⁺: ³F₄ → ³H₆ transition, high spontaneous radiative transition probability (260.75 s⁻¹) and large emission cross section (7.66 × 10⁻²¹ cm²) were obtained from the prepared glass. According to Dexter's and Forster's theory, energy transfer microscopic parameters were computed to elucidate the observed 2 μm emissions in detail. Besides, the effect of hydroxy groups quenching was also quantificationally investigated based on simplified rate equations. Results demonstrate that the optimal concentration Tm³⁺ doped germanate-tellurite glass possessing excellent spectroscopic properties might be an attractive candidate for 2 μm laser or amplifier.     

Cobalt-doped cerium oxide nanoparticles: Enhanced photocatalytic activity under UV and visible light irradiation

CeO₂ nanoparticles (NPs) were synthesized by coprecipitation using cerium(III) nitrate hexahydrate as the precursor and ethanol as the solvent. Different concentration of cobalt-doped cerium oxide NPs (3mol % and 6 mol %) were prepared by adding various concentrations of cobalt chloride to cerium nitrate. The as-synthesized NPs were characterized through X-ray diffraction (XRD) measurements, ultraviolet (UV)–visible spectroscopy, Photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM). XRD results reveal that the as-prepared CeO₂ NPs had a face-centered cubic structure with crystallite size in the range of 5–8 nm. TEM analyses showed that the CeO₂ NPs and Co-doped CeO₂ NPs had a homogenous size distribution (sizes were within 5–12 nm). Band-edge absorption of CeO₂ NPs redshifted upon increasing the Co concentration as compared to undoped CeO₂ NPs. PL spectra reveal a peak shift of CeO₂ emission upon cobalt doping, which were due to an increase in oxygen defects localized between the Ce4f and O2p energy levels (i.e., via formation of Ce³⁺ states). Photocatalytic degradation of methylene blue in aqueous solution under UV and visible (sunlight) irradiation in the presence of pure CeO₂ NPs and of Co-doped CeO₂ NPs was investigated. The efficiency of photocatalytic degradation of CeO₂ NPs increased with the Co concentration both under UV irradiation and under visible light. Co-doped CeO₂ NPs (6 mol%) showed degradation efficiencies of 98% and 89% at 420 min of exposure to UV irradiation and to visible light, respectively.