Improvement of structural and superconducting properties of Bi-2212 textured rods by substituting sodium

In this paper, the physical and superconducting properties of Bi₂Sr₂Ca_1−xNa_xCu₂O_8+δ with x=0.0, 0.05, 0.075, 0.1, and 0.20 textured superconducting fiber rods prepared by a laser floating zone (LFZ) technique were studied. The effects of Na⁺¹ substitution for Ca²⁺ have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transport measurements, dc-magnetization, magnetic hysteresis and magnetic critical current density. The powder XRD patterns of samples have indicated that Bi-2212 phase is the major one, independently of Na content. The best critical temperature, T_C, has been found as 93.3 K from M–T data for the sample with 0.075 Na substitution. The maximum magnetic J_C value has been calculated as 1.35×10⁵ A/cm² at 10 K for the 0.10 Na sample. The maximum transport critical current density has directly been measured as 1.3×10³ A/cm² at 77 K for the 0.05Na sample.     

Role of active slip systems induced with holmium impurity in Bi-2212 ceramics on mechanical design performance and morphological properties

Effect of Ho/Bi partial replacement in Bi_2.1-xHo_xSr_2.0Ca_1.1Cu_2.0O_y (Bi-2212) superconductors on the fundamental structural, morphological and mechanical performance properties are investigated by Scanning Electron Microscopy (SEM) and Vickers hardness (H_v) measurement techniques. Crystallinity quality and surface morphology including the microcrystal coalescence orientations, grain alignment distributions, microscopic structural problems, microvoids, internal defects, uniform surface view, porosity and particle growth distribution are visually examined with the aid of SEM. Basic mechanical performance and characteristic features of Bi/Ho substituted Bi-2212 superconducting ceramics (0.00 ≤ x ≤ 0.10) are also determined with Vickers tests conducted at various loads intervals 0.245–2.940 N. Experimental findings show that the characteristic features enhance seriously in case of x = 0.01 due to refinement of crystallinity quality and slip systems. Thus, the optimum Ho concentration presents the highest mechanical fracture strength to the load applied as a result of better uniform surface appearance and grain orientations, well-connected flaky layers, larger particle size distribution and denser structure, confirmed by the SEM investigations. Namely, much more load is required to accelerate the dislocation movement and crack propagation to the terminal velocity for critical size growth. The fracture predominantly takes place in the transcrystalline regions and hence the propagations are easily controlled with the optimum Ho dopant ions. On the other hand, the increase in the Ho ions in Bi-2212 structure induces the crack-initiating defects for new stress concentration sites. In conclusion, the permanent and non-recoverable deformations appear at even lower indentation test loads. All samples present indentation size effect feature depending on the dominant character of elastic recovery mechanism. Further, original hardness parameters are semi-empirically analyzed in the plateau limit regions using mechanical modelling approaches for the first time. Based on the analyses, Hays–Kendall model exhibits the closest results to the experimental findings.     

High-temperature properties and interface evolution of C/SiBCN composites prepared by precursor infiltration and pyrolysis

C/SiBCN composites with a density of 1.64 g/cm³ were prepared via precursor infiltration and pyrolysis and the bending strength and modulus at room temperature was 305 MPa and 53.5 GPa. The precursor derived SiBCN ceramics showed good thermal stability at 1600 °C and the SiC and Si₃N₄ crystals appeared above 1700 °C. The bending strength of the composites was 180 MPa after heat treatment at 1500 °C, and maintained at 40 MPa-50 MPa after heat treatment for 2 h at 1600 °C–1900 °C. In C/SiBCN composites, SiBCN matrix could retain amorphous up to 1500 °C and SiC grains appeared at 1600 °C but without Si₃N₄. The reason for no detection of Si₃N₄ was that the carbon fiber reacted with Si₃N₄ to form an interface layer (composed of SiC and unreacted C) and a polycrystalline transition layer (composed of B and C elements), leading to the degradation of the mechanical properties.     

Structure-dependent re-dispersibility of graphene oxide powders prepared by fast spray drying

The graphene oxide powder (GOP) obtained from the spray drying process often exhibits poor re-dispersibility which is considered due to the partial reduction of GO sheets. The reduction of drying tem-perature can effectively increase the re-dispersibility of GOP, but result in a decreased drying efficiency. Herein, we found that the re-dispersibility of GOP is strongly affected by its microstructure, which is determined by the feed concentration. With the increase of feed concentration, the GO nanosheet assem-bly varies from the disordered stacking to relatively oriented assembly, making the morphology of the GOP transform from ball-like (the most crumpled one) to flake-like (the least crumpled one), and the 0.8 mg·ml-1 is the threshold concentration for the morphology, structure, and re-dispersibility change. Once the feed concentration reaches 0.8 mg·ml-1, the appearance of the nematic phase in droplet ensures the relatively oriented assembly of GO sheets to form the layered structure with a low crumpling degree, which greatly improves the polar parts surface tension of the solid GOP, making the GOP easier to form hydrogen bonding with water during the redispersion process, thus stabilizing dispersion. This work pro-vides useful information for understanding the relationships between the morphology, microstructure, and final re-dispersibility of GOPs.     

Phase evolution and properties of (VNbTaMoW)C high entropy carbide prepared by reaction synthesis

(VNbTaMoW)C high entropy carbide was fabricated by reaction synthesis using the constituent metal and graphite powders as raw materials. Phase formation and composition uniformity are analyzed using XRD, EDS and SEM. The phase composition for samples sintered at 1600℃ includes TaC, NbC, WC, MoC and VC according to XRD peak fitting and deconvolution. Samples sintered at 1850℃ form a single phase high entropy carbides, which exhibit high microhardness and relatively lower thermal conductivity of 9.2 W/m.K^-1 at room temperature compared with binary carbides. Severe lattice distortion and high concentration of point defects are responsible for the low thermal conductivity of the high entropy (VNbTaMoW)C.     

Effect of precursor, ambient pressure, and temperature on the morphology, crystallinity, and decomposition of powders prepared by spray pyrolysis and drying

The effect of reactor pressure and temperature on the morphology, crystallinity, and decomposition behavior of various powders produced by spray pyrolysis or drying is investigated. Zirconia, magnesium sulphate, and sodium chloride powders are produced at the reactor pressures of 760, 400, 250, and 120 Torr, and at the reactor temperatures of 100 °C, 200 °C, and 400 °C. Zirconia and magnesium sulphate powders are spherical, whereas the sodium chloride powders are cubic. Regardless of the pressure, the powders produced at 100 °C and 200 °C appear solid, and powders produced at 400 °C are hollow and disrupted. The experimental data and the calculations indicate that the evaporation rate, which is a function of pressure and temperature, controls the solute distribution within the droplet and as such determines the morphology of the powders. In addition, the shape and morphology of the powders are strong functions of the precursor type. The decomposition and crystallinity of the powders are determined using XEDS and XRD analyses, respectively. The crystallinity and decomposition of the powders are weak functions of pressure and strong functions of temperature.     

钢渣基高反应性焦炭气孔结构的溶损演化行为

在工业配合煤中添加1%钢渣制备高反应性焦炭,采用低温氮气吸附法分析高反应性焦炭（HRC）和普通焦炭（BC）在1100℃下溶损不同比率碳素（5%~50%）后焦炭的气孔结构,并结合分形理论研究焦炭溶损反应过程中孔结构的演化特性。结果表明,随着碳素的溶损,HRC的吸脱附曲线的变化幅度比BC的大,吸附等温线由Ⅰ型向Ⅱ型的转变较晚;焦炭的比表面积和微孔孔容随碳素的溶损先增大后减小、总孔容逐渐增大,而HRC的比表面积增率（ΔS _BET /Δx）比BC的大,孔径分布也相对较宽;HRC的分形维数D ₁和D ₂随碳素溶损而变化趋势与BC的有较大差异。这说明高反应性焦炭中的钢渣通过增加了焦炭表面上活性点影响焦炭溶损过程中气孔结构的演化行为。