Microstructure and high-temperature corrosion behaviors of aluminide coatings by low-temperature pack aluminizing process

Aluminide coatings were produced on carbon steel and Fe-5Cr-Mo alloy at a relatively lower temperature below 600 °C in shorter treatment time by a combination of surface refinement process and pack aluminizing process. Repetitive ball impact, generated by mechanical vibration, caused the top-layer refinement of substrates in a conventional pack aluminizing process. The effects of temperature and treatment time on the formation of aluminide coatings were analyzed. The microstructure of the coatings was investigated by SEM, AFM and XRD. The aluminide coatings were one-layer, compacted structure with ultrafine grains and uniform elemental distribution. High-temperature oxidation and sulphidation tests were carried out at 600 °C in air for 200 h and 10% SO₂ + Ar gas mixture atmosphere for 50 h, respectively. The mass gains and spallation indicated that the aluminide coatings significantly improved the high-temperature oxidation and sulphidation resistance.     

快速辐射度算法的实现研究

当虚拟场景发生变化时,扩展的逐步求精算法作为一种快速的辐射度算法,大量减少了重新计算辐射度所花费的时间。对扩展的逐步求精算法进行研究,并利用OpenGL对算法进行了实现。通过在实验中对虚拟场景的表面反射率和几何属性进行改变,获得了一些证明算法有效性的数据,从而验证了该算法的有效性。     

Improvement of ITO-free inverted polymer-based solar cells by using colloidal zinc oxide nanocrystals as electron-selective buffer layer

In this study, we report on the improvement of ITO-free inverted polymer/fullerene solar cells by introducing a zinc oxide (ZnO) layer between the active layer and the cathode. The ZnO layers are deposited from solution, using colloidal ZnO nanocrystals with a rodlike shape, which are obtained using a wet-chemical synthesis route at low temperature. The nanocrystals are widely characterized with respect to their structural, optical, and electronic properties. In particular, simulations of powder X-ray diffraction data based on Rietveld refinement are shown to be a suitable method to characterize the average crystallite shape and particle size. Cyclic voltammetry reveals that nanocrystalline ZnO is an appropriate choice as electron-selective buffer layer in organic solar cells based on a bulk heterojunction of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). Using ITO-free inverted solar cells in substrate configuration with an opaque Cr/Al/Cr bottom electrode, we demonstrate that introducing a cathodic interlayer of ZnO nanocrystals leads to a notable enhancement in photovoltaic performance. The magnitude of the effect is found to depend on the solvents used to process the active layer. In case of absorber blends processed from o-dichlorobenzene, we show an almost threefold increase in efficiency from 0.8 to 2.2% at an active area of 1 cm².     

Microstructural evolution of AZ31 magnesium alloy subjected to sliding friction treatment

Microstructural evolution and grain refinement mechanism in AZ31 magnesium alloy subjected to sliding friction treatment were investigated by means of transmission electron microscopy. The process of grain refinement was found to involve the following stages: (I) coarse grains were divided into fine twin plates through mechanical twinning; then the twin plates were transformed to lamellae with the accumulation of residual dislocations at the twin boundaries; (II) the lamellae were separated into subgrains with increasing grain boundary misorientation and evolution of high angle boundaries into random boundaries by continuous dynamic recrystallisation (cDRX); (III) the formation of nanograins. The mechanisms for the final stage, the formation of nanograins, can be classified into three types: (i) cDRX; (ii) discontinuous dynamic recrystallisation (dDRX); (iii) a combined mechanism of prior shear-band and subsequent dDRX. Stored strain energy plays an important role in determining deformation mechanisms during plastic deformation.     

Adjoint-based h–p adaptive discontinuous Galerkin methods for the 2D compressible Euler equations

In this paper, we investigate and present an adaptive Discontinuous Galerkin algorithm driven by an adjoint-based error estimation technique for the inviscid compressible Euler equations. This approach requires the numerical approximations for the flow (i.e. primal) problem and the adjoint (i.e. dual) problem which corresponds to a particular simulation objective output of interest. The convergence of these two problems is accelerated by an hp-multigrid solver which makes use of an element Gauss–Seidel smoother on each level of the multigrid sequence. The error estimation of the output functional results in a spatial error distribution, which is used to drive an adaptive refinement strategy, which may include local mesh subdivision (h-refinement), local modification of discretization orders (p-enrichment) and the combination of both approaches known as hp-refinement. The selection between h- and p-refinement in the hp-adaptation approach is made based on a smoothness indicator applied to the most recently available flow solution values. Numerical results for the inviscid compressible flow over an idealized four-element airfoil geometry demonstrate that both pure h-refinement and pure p-enrichment algorithms achieve equivalent error reductions at each adaptation cycle compared to a uniform refinement approach, but requiring fewer degrees of freedom. The proposed hp-adaptive refinement strategy is capable of obtaining exponential error convergence in terms of degrees of freedom, and results in significant savings in computational cost. A high-speed flow test case is used to demonstrate the ability of the hp-refinement approach for capturing strong shocks or discontinuities while improving functional accuracy.     

Cation disorder and structural studies on Bi4−x Nd x Ti3O12 (0.0≤x≤2.0)

Here we report the results of combined powder X-ray and neutron diffraction studies of Bi_4?x Nd _x Ti₃O₁₂ (0.0 ≤ x ≤ 2.0) compositions. The parent Bi₄Ti₃O₁₂ has an orthorhombic lattice (space group: B2cb) with unit cell parameters a = 5.4432(5) Å, b = 5.4099(5) Å and c = 32.821(2) Å, and V = 966.5(1) ų. This orthorhombic lattice is retained in all the studied compositions. The unit cell parameters gradually decrease with Nd³⁺ ion concentration with a discontinuity at x = 0.75. Orthorhombicity of the lattice decreases with increase in Nd³⁺ content in the lattice. The orthorhombic unit cell parameters for a representative Bi₂Nd₂Ti₃O₁₂ composition are: a = 5.3834(9), b = 5.3846(9) and c = 32.784(1) Å. The observed orthorhombic distortion at x = 2.0 is very small and thus the crystal structure apparently has a pseudo-tetragonal lattice. In addition, Nd³⁺ preferentially substitutes in the perovskite slab of the Aurivillius structure. The fraction of Nd³⁺ in the fluorite slab increases with increase in Nd³⁺ contents.     

Temperature resolved X-ray diffraction as a tool of thermal analysis

Time and temperature resolved X-ray diffraction was used for thermal analysis. Series of diffraction patterns were measured, while the samples are heated/cooled stepwise or isothermally with freely selectable temperature programs.The method was applied for the investigation of the phase transitions of ammonium nitrate and HMX (1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane), when the identification of phases was required. Its capability in the field of kinetics is demonstrated with the isothermal investigation of the solid state reaction of ammonium nitrate with copper oxide and the non-isothermal investigation of the high temperature corrosion of nickel, which was performed by means of a difference procedure. For obtaining structural details peak fitting and Rietveld refinement were applied for the investigation of ammonium nitrate and HMX.We are indebted to Mr. K. O. Hartmann, Mr. H. Fietzek and Mr. H. G. Farr for their assistance during the measurements and for the maintenance of the measuring systems.     

Effect of strontium substitution on the composition and microstructure of sol–gel derived calcium phosphates

Sr-doped calcium phosphates have been prepared by sol–gel chemistry. All samples exhibit two phases: hydroxyapatite (HAp) and tricalcium phosphate (β-TCP). With respect to undoped sample, the Sr-doped samples exhibit higher proportion of β-TCP phase but the quantity appears to be quite independent of the doping level. To explain the mismatch with the nominal stoichiometry, the presence of amorphous CaO and SrO compounds have been postulated and their proportions evaluated. The insertion of Sr²⁺ ions in the two crystalline phases HAp and β-TCP is almost total for low doping levels but quite incomplete for the highest doping level. The majority of the inserted Sr²⁺ ions are in the β-TCP phase. Considering the acknowledged beneficial effect of Sr²⁺ on the bone regeneration process, the effective partial substitution of Sr in biphasic calcium phosphate makes these materials very interesting for clinical applications. The Sr-substituted HAp and β-TCP cell parameters agree fairly well with the Vegard’s law and Sr²⁺ ions substitute preferentially for Ca²⁺ in the Ca2 site for hydroxyapatite and in the Ca4 site for β-TCP. The microstructural parameters confirm the previous observation and give a new evidence of clear stabilizing effect of Sr²⁺ ions towards the β-TCP structure.     

Synthesis, structures and magnetic properties of n=3 Ruddlesden-Popper compounds Ca4Mn3−xTaxO10 (0≤x≤0.3)

The structural and magnetic properties of Ta-doped Ca₄Mn_3−xTa_xO₁₀ (0≤x≤0.3) compounds have been investigated. Structural refinement indicates that the Ta doping maintains the orthorhombic layered perovskite structure with space group Pbca as Ca₄Mn₃O₁₀ but induces an increase in both unit cell volume and octahedral distortion. The magnetization measurements reveal that the magnetization first increases and reaches to maximum for the x=0.1 sample and then gradually decreases with the increase of Ta content. There appear short-range ferromagnetic (FM) clusters in all the doped samples, which are caused by the double-exchange interaction between Mn⁴⁺ and Mn³⁺ that is induced by the charge compensation effect. As x is higher than 0.1, the overall results show evidence for the gradual appearance of a cluster glass behavior. When x increases to 0.3, the long-range antiferromagnetic (AFM) ground state is melted into the short-range magnetically ordered regions due to the increase of Ta⁵⁺ and Mn³⁺ at the expense of Mn⁴⁺. The competition between AFM regions and FM clusters makes the short-range magnetic components frustrate when the temperature falls to a frustrating point, and thus cluster glass transition occurs.