Structural and pyroelectric properties of sol–gel derived multiferroic BFO thin films

Multiferroic BFO/PZT multilayer films were fabricated by spin-coating method on the (1 1 1)Pt/Ti/SiO₂/Si substrate alternately using PZT(30/70), PZT(70/30) and BFO alkoxide solutions. The structural and ferroelectric properties were investigated for uncooled infrared detector applications. The coating and heating procedure was repeated six times to form BFO/PZT multilayer films. All films showed the typical XRD patterns of the perovskite polycrystalline structure without presence of the second phase such as Bi₂Fe₄O₃. The thickness of BFO/PZT multilayer film was about 200–220 nm. The ferroelectric properties such as dielectric constant, remnant polarization and pyroelectric coefficient were superior to those of single composition BFO film, and those values for BFO/PZT(70/30) multilayer film were 288, 15.7 μC/cm² and 9.1 × 10^?9 C/cm² K at room temperature, respectively. Leakage current density of the BFO/PZT(30/70) multilayer film was 3.3 × 10^?9 A/cm² at 150 kV/cm. The figures of merit, F_V for the voltage responsivity and F_D for the specific detectivity, of the BFO/PZT(70/30) multilayer film were 6.17 × 10^?11 Ccm/J and 6.45 × 10^?9 Ccm/J, respectively.     

High-quality CH3NH3PbI3 thin film fabricated via intramolecular exchange for efficient planar heterojunction perovskite solar cells

The high-quality CH₃NH₃PbI₃ perovskite thin film with excellent coverage and uniformity was prepared using an intramolecular exchange technology via a low-temperature, two-step sequential deposition process. The PbI₂(DMSO) complex was synthesized at room temperature without any additives and was deposited, then the CH₃NH₃I solution was deposited subsequently. The further controllable thermal annealing process resulted in the complete formation of flat and uniform CH₃NH₃PbI₃ thin film with large-size grains and (110) preferred crystallographic orientation. The perovskite solar cells (PSCs) with a very simple inverted planar heterojunction structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Al and without other buffer layers, e.g., C₆₀, LiF, BCP, etc., were fabricated, resulting in a power conversion efficiency (PCE) as high as 14.26%. The results suggest that the low-temperature, two-step sequential deposition process with intramolecular exchange technology provides a good route to fabricate high-quality perovskite thin film and efficient PSCs, which would match with large-scale, high-output roll-to-roll (R2R) printing/coating techniques.     

热轧带钢卷取机夹送辊偏转角分析

通过数值解析法,分析热轧带钢在卷取机导板通道里面的运行轨迹。以上下导板为边界,以带钢头部不碰撞到导板为目标,推导出具体钢种和厚度的带钢卷取时的夹送辊最佳偏转角。该方法直观、准确,可为实际生产提供指导。     

Time-resolved Förster energy transfer in polymer blends

Sub-picosecond spectroscopy and pump–probe experiments show Förster energy transfer in blends from larger gap (blue or green-emitting) host polymers poly(2,3-diphenyl-5-hexyl-1,4-phenylenevinylene) (DP6-PPV) or poly[2-(meta-2′-ethylhexoxyphenyl)-1,4-phenylenevinylene) (m-EHOP-PPV) to the smaller gap, red-emitting guest polymer poly(2,5-bis(2′-ethylhexoxy)-1,4-phenylenevinylene) (BEH-PPV). The dynamics of the stimulated emission (SE) and photoinduced absorption (PA) of the blends indicate that 10–20 ps are required for complete energy transfer. Quantitative measurements of energy transfer rates give a Förster interaction range of 3–4 nm, 1.4 times longer than the theoretical values as calculated from the spectral overlap. We attribute this difference to delocalization of the excited state. Insufficient spectral overlap between the emission of the host and absorption of the guest is shown to be the cause for the absence of energy transfer in a blend with poly(2,5-bis(cholestanoxy)-1,4-phenylenevinylene) (BCHA-PPV) as the guest polymer.     

Electrodeposition of NiS/Ni2P nanoparticles embedded in amorphous Ni(OH)2 nanosheets as an efficient and durable dual-functional electrocatalyst for overall water splitting

To develop earth-abundant and cost-effective catalysts for overall water splitting is still a major challenge. Herein, a unique “raisins-on-bread” Ni–S–P electrocatalyst with NiS and Ni₂P nanoparticles embedded in amorphous Ni(OH)₂ nanosheets is fabricated on Ni foam by a facile and controllable electrodeposition approach. It only requires an overpotential of 120 mV for HER and 219 mV for OER to reach the current density of 10 mA cm⁻² in 1 M KOH solution. Employed as the anode and cathode, it demonstrates extraordinary electrocatalytic overall water splitting activity (cell voltage of only 1.58 V @ 10 mA cm⁻²) and ultra-stability (160 h @ 10 mA cm⁻² or 120 h @50 mA cm⁻²) in alkaline media. The synergetic electronic interactions, enhanced mass and charge transfers at the heterointerfaces facilitate HER and OER processes. Combined with a silicon PV cell, this Ni–S–P bifunctional catalyst also exhibits highly efficient solar-driven water splitting with a solar-to-hydrogen conversion efficiency of 12.5%.     

Metallography of a melt-quenched aluminum-cobalt alloy

Rapid quenching from the melt of an Al-5 wt.% Co alloy has resulted in the formation of a supersaturated solid solution at very high cooling rates as obtained in the gun technique. With decreasing cooling rate (and increasing thickness of the specimen), e.g., in melt-spun condition, a variety of microstructures has been produced consisting of fine dendritic patterns, cellular-dendritic structures, and precipitation of coarse intermetallic particles. Two metastable phases have also been detected—one with a bcc structure and a = 0.286 nm corresponding to β-AlCo, and the other having an fcc structure with a lattice parameter a = 0.52 nm. Electron microscopy, electron diffraction, and x-ray microanalysis have been extensively employed to fully characterize the phases present in the melt-quenched alloy.     

Fractography of silicon nitride based ceramics to guide process improvements

Fractography is an important tool to understand and identify the cause of the failure in materials. This understanding can be used to make changes in raw materials selection and processing to increase the strength of brittle materials. This study reports the fracture behavior of hot-pressed silicon nitride based ceramics, with focus on dominant flaw identification with respect to material and process parameters. Silicon nitride is an important material for structural applications which require high strength and wear resistance, such as bearings, nozzles, and cutting tools. Silicon nitride with a target base composition of Si_6-zAl_zO_zN_8-z (z = 0.5), along with varying boron dopant levels, was explored in this work. Detailed fractographic analyses revealed that the majority of fracture origins were internal flaws due to the foreign impurities introduced at various stages of processing. All materials were found to have reasonably high strength (800−1100 MPa). Strength was inversely proportional to the square root of the flaw size, however no correlation was found between measured flexural strength and fracture origin types. Mirror constants calculated from fracture mirror measurements ranged between 5.8 and 9.8 MPa.m^1/2.     

High-temperature strength of boron carbide with Pt grain-boundary framework in situ synthesized during spark plasma sintering

Grain boundaries, twins, and defects are considered to influence the thermomechanical behavior of any covalent ceramic, as a result, monolithic B₄C samples show different curve shapes of bending strength vs temperature and the present theoretical models fail to fit them over the entire temperature range. To overcome these issues, we fabricated a novel high-density boron carbide and evaluated its high-temperature bending strength. The as-obtained ceramic is composed of boron carbide grains and a fine grain-boundary metal Pt framework. The material shows a decreased strength, which is due to a non-linear increase in the volume expansion coefficient of the B₄C. Recovery in strength above 1000 °C is due to the presence of twins, their growth and rearrangements. We consider twins rearrangements are the pieces of evidence for a novel ‘micro’ mechanism of high-temperature stress accommodation for the boron carbide bulks.     

Intrinsic correlation between structure,vibration spectrum and microwave dielectric properties of ferroelectric-dielectric composite ceramics

A novel series of (1-x)Ba_0.45Sr_0.55TiO₃-xMgGa₂O₄ (x = 0, 10, 30, 50, 70 wt%) ceramics was prepared by a solid-state method to investigate the relationship between their dielectric properties and ion diffusion, composition effect, and lattice vibration. XRD refinement and DFT calculations of Ba_0.45Sr_0.55TiO₃ (BST45) revealed that the substitution of Ga³⁺ and Mg²⁺, both of which have small polarizability for Ti⁴⁺, reached the saturation state at x = 10 wt%, thus decreasing the quality factor (Q value). In contrast, the addition of MgGa₂O₄ (MG) with x > 10 wt% significantly reduced the relative permittivity and improved the Q value owing to the compositional effect. The vibration spectra (Raman and FT-IR) confirmed that the Q value initially decreased owing to ion diffusion at x < 10 wt% and then increased with increasing MG content according to the composition effect. Therefore, the Q value was remarkably improved in the Ba_0.45Sr_0.55TiO₃-MgGa₂O₄ composites, with good tunability and low relative permittivity.