小角X射线散射应用研究若干进展

文章从小角X射线散射（small-ande X-ray scattering，SAXS）的理论分析出发，结合国内外SAXS理论和应用研究的最新动态，总结了文章作者所在的研究小组近年来在SAXS技术用于材料微结构表征方面的研究成果。主要包括以下三个方面的内容：（1）有机／无机杂化材料中电子密度波动的研究；（2）弦长度分布函数材料的周期结构的研究；（3）纳米粉末晶化过程的研究。     

Characteristics of cobalt-based alloy coating on tool steel prepared by powder feeding laser cladding

Co-based alloy coating was deposited on tool steel by powder feeding laser cladding. Sections of such coatings were examined to reveal their microstructures and phases using scanning electron microscope (SEM) and X-ray diffractometer (XRD). The results showed that the prime phase (γ-Co dendrite) and other phases, including Cr₂₃C₆, Co₇W₆, and CrNi existed in the coatings. Some different solidification morphologies, such as planar (at the interface), cellular and dendrite formed, varying from the interface to the surface. Fine microstructures of γ-Co dendrite and lamellar eutectic in dendritical regions strengthened the coatings. Besides, the effects of aged treatment on the microstructure and microhardness of the surface coating were studied. Aged treatment led to the precipitations of some carbide particles (Cr₇C₃ and Co₃C) and boride particles (Co₄B) from the cladded coating, causing an increase in microhardness in the laser-cladded coating.     

Controlled-synthesis,characterization, and magnetic properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanostructures

Fe₃O₄ nanostructures with different morphologies, including uniform nanoparticles, nanorods and nanowire bundles, have been successfully synthesized via a facile hydrothermal route. Based on the observation of TEM images, the growth mechanism of one-dimensional Fe₃O₄ nanostructures is in accordance with Ostwald ripening process. From the hysteresis loops of as-prepared Fe₃O₄ products, we found that the morphology has great influence on the magnetic properties. The uniform Fe₃O₄ nanoparticles have higher saturation magnetization and lower coercivity than that of Fe₃O₄ nanorods and nanowires bundles. These phenomena attribute to the high shape anisotropy of nanorods and nanowire bundles, which prevent them from magnetizing in directions other than along their easy magnetic axes. PACS 81.07.-b; 75.50.Bb; 75.30.Gw; 81.10.Dn; 81.16.Be     

Initiation of petroleum formation and antioxidant function – a DFT study of sulfur?sulfur bond dissociation enthalpies

Experimental studies showed that sulfur radicals play the vital role in petroleum formation. 1 Sulfur‐ centered radicals also exhibit activities in antioxidant functions. Here we conduct a theoretical investigation of their precursor‐disulfides. By investigation into substituent effect on sulfur? sulfur bond dissociation enthalpies (S? S BDEs), we would like to find the most effective provider for sulfur radicals. In the present work, 50 alpha‐substituted disulfides and 16 para‐substituted aryl disulfides are studied systematically, with the general formula XS‐SX or HS‐SX. The substituent effect on S? S BDEs is found to be very eminent, ranging from 33.2 to 75.0 kcal/mol for alpha‐substituted disulfide, and from 43.7 to 59.7 kcal/mol for para‐substituted phenyl disulfides. We also evaluate the performance of 44 density functional methods to get an accurate prediction. A further study indicates that substituents play a major role in radical energies, instead of molecule energies, which is substantiated by the good linearity between XS‐SX bond dissociation enthalpy (BDE) and HS‐SX BDE. Copyright © 2007 John Wiley & Sons, Ltd.     

Effects of duty ratio at low frequency on growth mechanism of micro-plasma oxidation ceramic coatings on Ti alloy

The aim of this work is to study the effects of duty ratio on the growth mechanism of the ceramic coatings on Ti-6Al-4V alloy prepared by pulsed single-polar MPO at 50 Hz in NaAlO₂ solution. The phase composition of the coatings was studied by X-ray diffraction, and the morphology and the element distribution in the coating were examined through scanning electron microscopy and energy dispersive spectroscopy. The thickness of the coatings was measured by eddy current coating thickness gauge. The corrosion resistance of the coated samples was examined by linear sweep voltammetry technique in 3.5% NaCl solution. The changes of the duty ratio (D) of the anode process led to the changes of the mode of the spark discharge during the pulsed single-polar MPO process, which further influenced the structure and the morphology of the ceramic coatings. The coatings prepared at D = 10% were composed of a large amount of Al₂TiO₅ and a little γ-Al₂O₃ while the coatings prepared at D = 45% were mainly composed of α-Al₂O₃ and γ-Al₂O₃. The coating thickness and the roughness were both increased with the increasing D due to the formation of Al₂O₃. The formation of Al₂TiO₅ resulted from the spark discharge due to the breakdown of the oxide film, while the formation of Al₂O₃ resulted from the spark discharge due to the breakdown of the vapor envelope. The ceramic coatings improved the corrosion resistance of Ti-6Al-4V alloy. And the surface morphology and the coating thickness determined the corrosion resistance of the coated samples prepared at D = 45% was better than that of the coated samples prepared at D = 10%.     

Growth of cubic MgxZn1−xO alloy films by electron beam evaporation

We report the growth of cubic Mg_xZn_1−xO alloy thin films on quartz by electron beam evaporation. It can be found that all the samples have sharp absorption edges by the absorption measurements. X-ray diffraction measurements indicate the Mg_xZn_1−xO films are cubic phase with preferred orientation along the (1 1 1) direction. Energy dispersive spectrometry (EDS) demonstrates that the Mg concentration in Mg_xZn_1−xO films is much higher than the ceramic target used, and the composition can be tuned in a small scope by varying the substrate temperature and the beam electric current. The reasons of this phenomenon are also discussed.     

Surface oxidation behavior of MgNd alloys

The oxidation kinetics of MgNd alloys oxidized in pure O₂ at high temperatures has been investigated. The results revealed two stages of the reaction: A fast initial oxidation was followed by a slow oxide growth with a parabolic kinetics. For MgNd alloys (Nd = 25 wt.%), the oxidation rate increased with the enhancement of the oxidation temperature. A sudden ignition was found for this alloys oxidized at 873 K up to about 80 min. Moreover, the increase of the Nd content would harm the oxidation resistance of the MgNd alloys. By Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis, it was found that a triplex structure of oxide film formed. The outer layer was composed of MgO, Nd₂O₃ and Nd(OH)₃, the middle layer mainly consisted of MgO and Nd₂O₃, and the inner layer was the transitional layer made of MgO, Nd₂O₃ and the content of the substrate. The protective oxidation was associated with the formation of the dense Nd₂O₃/MgO layer during isothermal oxidation process. The oxidation mechanisms for the formation of oxide film are discussed.     

R值测量中强子事例选择的改进

强子事例的选择和强子探测效率是在BEPC/BES上进行R值测量的两项主要误差来源. 过去实验只选取等于或者大于2叉的强子事例, 因而0叉和1叉事例的丢失将导致强子事例数和强子探测效率的较大误差. 试图提出在R值测量中选取包含1叉强子事例在内的样本, 这将有助于更合理地调节强子事例产生器LUARLW的参数, 减小强子探测效率和R值测量的系统误差.     

The electrical properties and phase transformation of PLZST 2/85/13/2 antiferroelectric thin films on different bottom electrode

Pb_0.97La_0.02(Zr_0.85Sn_0.13Ti_0.02)O₃ (PLZST 2/85/13/2) antiferroelectric thin films were deposited on Pt(111)/Ti/SiO₂/Si and LaNiO₃(LNO)/SiO₂/Si substrates through a modified sol-gel process. The phase structure and microstructure of PLZST 2/85/13/2 antiferroelectric thin films were analysed by x-ray diffraction (XRD), scanning electron microcopy (SEM) and field-emission SEM (FE-SEM). The antiferroelectric nature of the PLZST 2/85/13/2 thin films on two electrodes was demonstrated by the C-V (capacitance-voltage) and P-E (polarization-electric field) measurement. The maximum polarizations for PLZST 2/85/13/2 films on Pt and LNO electrodes were 42 and 18 μC/cm², respectively. The temperature dependence of the dielectric property of the PLZST 2/85/13/2 films was measured under different dc electric fields. Also, the phase transformation of the PLZST 2/85/13/2 films was studied in detail as a function of temperature and dc electric field.     

Widely tunable, high-repetition-rate, dual signal-wave optical parametric oscillator by using two periodically poled crystals

This paper reports on a high-repetition-rate dual signal-wave (DSW) optical parametric oscillator (OPO) operating at the 1.5 μm band with tunable wavelength intervals from 2.5 nm to 69.1 nm. Two periodically poled crystals, a periodically poled lithium niobate (PPLN) with multiple gratings and a single grating MgO-doped PPLN (PPMgOLN), are cascaded in the same OPO cavity to generate dual signal-waves by using quasi-phase-matched (QPM) technique. The pump source was a Q-switched diode-pumped Nd:YVO₄ laser operating at 50 kHz. At an incident pump power of 3 W, an average output power of 169.6 mW at 1489.2 nm and 1558.3 nm has been achieved.