The oxygen deficient Ruddlesden–Popper La3Ni2O7−δ (δ = 0.65) phase: Structure and properties

La₃Ni₂O_7−δ (δ = 0.65) was synthesized by hydrogen reduction of the parent La₃Ni₂O₇ Ruddlesden–Popper nickelate. The crystal structure of La₃Ni₂O_6.35 (space group: I4/mmm, a = 3.8742 (1) Å and c = 20.055 (1) Å) has been determined from powder neutron diffraction data by the Rietveld method for the first time. The oxygen vacancies are located in the LaO_x planes between two of the NiO₂ layers. Removal of these oxygen atoms from the parent phase results in a significant (∼0.4 Å) shrinkage of the perovskite block along c-direction and splitting of the Ni position. The major part of Ni cations is surrounded by five oxygen atoms forming square pyramids, while the rest are coordinated to six octahedrally arranged oxygen atoms. Over the 170–400 K temperature range, the conductivity of La₃Ni₂O_6.35 follows Mott's variable range hopping model modified for a 2D case.     

Room temperature multiferroicity in Aurivillius compounds Bi6Fe2−xNixTi3O18 (0≤x≤1)

We investigate the structural, magnetic, ferroelectric, and dielectric properties of Bi₆Fe_2−xNi_xTi₃O₁₈ (0≤x≤1). The coexistence of ferroelectricity and ferromagnetism were observed at room temperature for the Ni-doped samples. The ferromagnetism in Bi₆Fe_2−xNi_xTi₃O₁₈ can be understood in terms of spin canting of the antiferromagnetic coupling of the Fe-based and Ni-based sublattices via Dzyaloshinsky-Moriya interaction. Moreover, the substitution of Ni for Fe was effective for the enhancement of ferroelectric properties. The x=0.6 sample exhibits a maximum remnant polarization P_r of 37.8 μC/cm² because of a lower leakage current. The rather large activation energy in the x=0 and 0.2 samples implies that the relaxation process may be not associated with the thermal motion of oxygen vacancies inside the bulk.     

聚苯乙烯薄膜表面粘弹性行为的纳米压痕研究

目的通过研究聚苯乙烯薄膜不同深度下的加载力曲线,分析得到其随着深度的不同而导致测量硬度和模量的差异,研究聚苯乙烯薄膜表面粘弹性行为,从而为高分子薄膜受限效应的理论和应用探索提供参考和借鉴。方法采用纳米压痕方法对聚苯乙烯薄膜样品(厚度约500 nm)进行了系列纳米压痕测量与分析。结果加载速率较低时,其加载曲线与P-h~2关系会发生不同程度的偏离。加载速率为0.01、0.02、0.03、0.06、0.1 mN/s时,P-h~2关系中的指数从1.609逐渐增大到1.628。通过在最大载荷时保载60 s,得到了压入深度随时间变化的关系,进而计算得到蠕变柔量,并根据Zener模型拟合得到压入深度为300~400 nm时,薄膜样品存在具有不同粘弹性的两种结构。而根据不同压入深度的卸载曲线,得到样品的硬度和折合弹性模量都在压入深度约为200~400 nm的区域存在粘弹性的非均匀现象。结论在不同压入深度处存在两种不同的粘弹性结构,根据当前人们普遍接受的高分子薄膜层模型,这两种结构分别对应聚苯乙烯薄膜样品的自由表面层和本体层。     

Fabrication of Ta3N5ZnO direct Z-scheme photocatalyst for hydrogen generation

Based on the Z-scheme mechanism, the combination of two semiconductors with suitable bandgaps can reduce the recombination rate of electrons and holes in a single material to enhance photocatalytic hydrogen evolution. Ta₃N₅ with suitable band gap positions is a potentially promising material for photocatalysis. In order to raise the hydrogen production rate, ZnO nanocrystals were deposited by atomic layer deposition (ALD) on Ta₃N₅ to form a direct Z-scheme structure, ZnO@Ta₃N₅. The ALD cycle number varied from 200 to 500. All of the direct Z-scheme samples exhibited much higher hydrogen evolution efficiencies than Ta₃N₅, ZnO, and the indirect Z-scheme, with the order of ZnO300@Ta₃N₅>ZnO200@Ta₃N₅>ZnO400@Ta₃N₅>ZnO500@Ta₃N₅. Because of the uniform distribution, discrete particles, and proper size of ZnO, ZnO300@Ta₃N₅ showed the highest hydrogen evolution rate, being about 500 μmol/g-h. With 400 or 500 ALD cycles, the larger particles of ZnO would overlap with each other to form a continuous layer on Ta₃N₅, thus reducing the exposure of Ta₃N₅ to the light and water for producing hydrogen.     

Confined crystallization of polymeric materials

The interest in confined crystallization has greatly increased with the development and progress of nanotechnology applications. Polymeric confined crystallization has been studied in droplets, ultrathin films, nanolayers, nanostructures from solutions, blends, copolymers, polymers infiltrated within AAO templates and nanocomposites. As confinement increases, the crystallization temperature first decreases, then splits into several fractions (i.e., fractionated crystallization) and finally occurs in one step at the maximum possible supercooling, near the glass transition temperature. Two factors are responsible for these effects: (a) a change in nucleation mechanism, from heterogeneous nucleation to surface nucleation (or in extreme cases, homogeneous nucleation), (b) the dependence of the crystallization temperature on the volume or the surface (or interphase) of the crystallizable micro or nanodomains. The melting point also decreases with confinement but to a lesser degree. A preferential orientation of polymeric crystals is generally induced by one or two dimensional confinement. Avrami indexes decrease with confinement until values of 1 (or even lower) are achieved in the limit of isolated domains, as the material approaches a first order crystallization kinetics. This type of kinetics reflects that nucleation is the rate determining step in the overall crystallization of ideally confined polymers.     

Infiltration processing of fibre reinforced composites: governing phenomena

All three classes of fibre reinforced composite materials (polymer, metal and ceramic matrix) may be produced by flow of liquid matrix into the open spaces left within pores of a fibre preform. Even though several specific issues arise from the nature of each composite matrix class, governing phenomena apply to all infiltration processes, and include in particular: (i) capillary phenomena, (ii) transport phenomena, and (iii) the mechanics of potential fibre preform deformation. These phenomena and their governing laws are reviewed for the case of isothermal infiltration with no phase transformations. Four basic functional quantities, which need to be known to model the processes, are identified, and addressed in turn. The paper concludes with some examples of modelling methodologies and comparison with experimental data.