粉末粒度对碳/碳基体上羟基磷灰石涂层的影响

采用等离子喷涂技术在碳纤维增强碳复合材料(简称碳／碳复合材料)上制备了羟基磷灰石涂层,研究了原始粉末粒度对涂层的表面形貌、剪切强度、相组成等方面的影响,并分析了涂层与基体的界面结合状况。结果表明：采用粗粉末喷涂后得到的涂层结合强度较高．结晶程度也较高,但涂层与基体的结合机制仍为机械嵌合。     

Effect of specimen thickness on the creep response of a Ni-based single-crystal superalloy

Creep tests on Ni-based single-crystal superalloy sheet specimens typically show greater creep strain rates and/or reduced strain or time to creep rupture for thinner specimens than predicted by current theories, which predict a size-independent creep strain rate and creep rupture strain. This size-dependent creep response is termed the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal, constant nominal stress creep tests were performed on uncoated PWA1484 Ni-based single-crystal superalloy sheet specimens of thicknesses 3.18 and 0.51 mm under two test conditions: 760 °C/758 MPa and 982 °C/248 MPa. The specimens contained initial microvoids formed during the solidification and homogenization processes. The dependence of the creep response on specimen thickness differed under the two test conditions: at 760 °C/758 MPa there was a reduction in the creep strain and the time to rupture with decreasing section thickness, whereas at 982 °C/248 MPa a decreased thickness resulted in an increased creep rate even at low strain levels and a decreased time to rupture but with no systematic dependence of the creep strain to rupture on specimen thickness. For the specimens tested at 760 °C/758 MPa microscopic analyses revealed that the thick specimens exhibited a mixed failure mode of void growth and cleavage-like fracture while the predominant failure mode for the thin specimens was cleavage-like fracture. The creep specimens tested at 982 °C/248 MPa in air showed the development of surface oxides and a near-surface precipitate-free zone. Finite-element analysis revealed that the presence of the alumina layer at the free surface imposes a constraint that locally increases the stress triaxiality and changes the value of the Lode parameter (a measure of the third stress invariant). The surface cracks formed in the oxide scale were arrested by further oxidation; for a thickness of 3.18 mm the failure mode was void nucleation, growth and coalescence, whereas for a thickness of 0.51 mm there was a mixed mode of ductile and cleavage-like fracture.     

Mechanism and dynamics of shrinking island grains in mazed bicrystal thin films of Au

This work investigates the mechanism and dynamics of grain boundary migration driven by capillary forces via in situ electron microscopy, complemented by molecular-dynamics simulations. Using thin films of Au with the mazed bicrystal geometry, the shrinkage of island grains with 90°〈1 1 0〉 tilt grain boundaries was observed by diffraction contrast and high-resolution imaging. The grains remained cylindrical throughout the shrinkage, and there was no measurable grain rotation even at very small sizes. The rate of shrinkage was found to be erratic and inconsistent with parabolic kinetics, accelerating before complete disappearance. Residual defects were found immediately after complete shrinkage, although the type and magnitude of the defects varied from grain to grain. Measurement of the grain boundary shape anisotropy showed a preference for facets on low-index planes of the crystals, including the mirror-symmetry planes of the bicrystal. These facets were also found directly on individual images extracted from high-resolution video recordings of shrinking grains at ～300 °C. The dynamics of boundary motion were found to be limited by nucleation and propagation of steps on these facets. The cylindrical geometry and size of the experimentally observed island grains allow direct comparison with molecular-dynamics simulations on the same length scale, which reproduced many of the experimentally observed features, including non-parabolic shrinkage, absence of systematic grain rotation, step-controlled migration and dislocation debris after complete grain shrinkage. Differences between model and experiment are discussed in terms of the possible role of impurities, surfaces and interfacial steps.     

Imaging of ferroelectric thin films by X-ray photoemission electron microscopy (XPEEM)

We performed X-ray photoemission electron microscopy (XPEEM) measurements at the Nanospectroscopy Beamline of the synchrotron light source ELETTRA, Trieste, Italy, to demonstrate the principal possibility of imaging ferroelectric thin films by low-energy photoelectrons. Due to the insulating properties of ferroelectric films, severe surface charging was the major experimental challenge to overcome. This was achieved by grounding an array of gold inter-digital electrodes (with 5 microm blank intervals between them) deposited on top of the films. The images taken with BaTiO(3) films revealed 50-100 nm-sized holes (material discontinuities) on the surface, an observation confirmed by high-resolution scanning electron microscopy (HRSEM). Finer details, e.g. a granular structure, which has been resolved with HRSEM, could not be observed in the XPEEM images. Our measurements indicate that despite some residual charging, a 50 nm lateral resolution can be achieved in XPEEM measurements with ferroelectric films.     

Enantioselective hydrogenation of ethyl pyruvate Long-term performance of chirally modified Pt/zeolite catalysts

The long-term performance of 5 wt.% Pt/zeolite catalysts (HNaY with different degrees of ion exchange) modified with (−)cinchonidine was studied. The enantioselective hydrogenation of ethyl pyruvate to R(+)ethyl lactate was used as the model reaction. The Pt/zeolites are effective catalysts for long-term use, if the chiral modifier is added before each cycle of hydrogenation. The optimal ratio between the amount of catalyst and chiral modifier essentially depends on both the specific Pt surface area of the catalyst and the solvent used. It is found that the use of acetic acid as solvent results in a higher long-term performance than cyclohexane. This is due to the competitive adsorption between the chiral auxiliary and acetic acid on the Pt surface. Furthermore, the acidity of the solvent results in a weakening of the (−)cinchonidine/Pt interaction, which prevents an overloading of the Pt surface area with (−)cinchonidine or its decay products. This effect was determined by means of differential thermal analysis and elemental analysis. When cyclohexane is used as the solvent, the long-term performance is significantly improved by raising the carrier acidity.     

An attempt to bridge the pressure and material gap with a disperse model catalyst for low-temperature alkane reforming

Pt black was exposed to n>-hexane/H₂ mixtures between 483 and 663 K followed by O₂ and H₂ treatments at 603 K. XP and UP spectra were measured without exposing the samples to air. 20–30% carbon accumulated after hydrocarbon exposures. O₂ removed most carbon. The surface C content increased after a subsequent contact with H₂, C 1s showing more atomic carbon as opposed to graphite after n-hexane exposure. Anisotropic recrystallization of Pt black favoring (220) and (311) lattice planes occurred under hydrogen-rich conditions. Both findings were attributed to a H₂-induced solid-state rearrangement; H atoms penetrating into the crystal lattice, force subsurface carbon and oxygen atoms to the surface and a concomitant restructuring would occur. Thus another hydrogen effect has been recognized, leading to structures favorable for skeletal reactions of alkanes.     

STUDIES ON FRYING KINETICS AND QUALITY OF FRENCH FRIES

Effects of temperature of oil (160, 170 and 180° C) and duration of pre-drying (0.00, 0.25, 0.50, 0.75 and 1.00 h) on the kinetics of moisture removal and oil uptake, and quality of French fries were studied. Frying times at each combination of temperature and pre-drying duration were standardized on the basis of sensory characteristics of the product in the preliminary trials. Results indicated that the rate of both moisture loss and oil uptake were higher in the beginning followed by a decrease in the later stages of frying. Mathematical models were developed to describe both the moisture removal and oil uptake by French fries. French fries prepared from potato fingers blanched in water for 4 min at 85° C and fried (without pre-drying) at 180° C were judged to be the most acceptable.     

Characterization and gamma radiation effects on poly(methyl methacrylate) doped with some benzylidene polymers

Anionic solution polmerization studies of benzylidene malononitrile, benzylidene ethyl cyanoacetate and their para chloro derivtives in dimethyl formamide (DMF) as a reaction medium using pipridene as initiator under nitrogen atmosphere at 100 °C were carried out. Infrared, ¹H NMR and ultraviolet-visible (UV-Vis) spectroscopic studies for the obtained polymers were investigated. The effect of γ-irradiation on PMMA films doped with 1 % of the benzylidene polymers was investigated by uv-vis spectroscopy. A change in the intensity of absorption bands with increasing irradiation dose was recorded. It is suggested that PMMA films doped with this type of benzylidene polymer can be used as dosimeter.     

On mechanical properties of sago starch/poly(ε‐caprolactone) composites

Four types of sago starch were incorporated into a poly(ε‐caprolactone) (PCL) matrix, native, predried, thermoplastic starch (TPS) granules and TPS. All systems were found to be phase‐separated. Tensile properties were obtained after formulation of various mixtures and processing of suitable test specimens. It was found that elongation at break of composites comprising native starch and thermoplastic starch decreases almost linearly with volume fraction of starch whereas tendencies to nonlinear dependencies were observed for predried and thermoplastic starch granules. Except for composites containing native starch, tensile strength was found to decrease linearly with volume fraction of starch. However, statistical analysis of the corresponding regression coefficients shows that the coefficients ruling the compostion dependence of tensile properties are not significantly different for the four starch types. One may conclude that in all cases, tensile properties decrease almost linearly with volume fraction and maximum volume fraction of starch loading is around 0.6. Scanning electron micrographs of fracture surfaces revealed weak interfacial adhesion between sago starch and the polymer matrix.