Molecular orbital study for Na, Mg, and Al adsorption on the Si (111) surface

We investigated the interactions between the Si(111) surface and the Na, Mg, and Al atoms using cluster model calculations. Calculations were performed at levels of complete-active-space self-consistent-field (CASSCF) and multi-reference singly and doubly excited configuration interaction (MRSDCI) calculations using the model core potential method. Our calculations revealed that the most favorable sites of Na, Mg, and Al adsorption on Si(111) are on top (T₁), bridge (B₂), and 3-fold filled (T₄) sites, respectively. The nature of chemical bonds between these metal atoms and the dangling bonds of the surface Si atoms are found to be essentially covalent.     

Xyloccensin L, a novel limonoid from Xylocarpus granatum

The isolation and structure elucidation of xyloccensin L from the stem bark of Xylocarpus granatum is described. Xyloccensin L is a highly oxidized heptacyclic A, B, D-seco limonoid with an α-8, 30-epoxy ring and a rare 1, 29 oxygen bridge.     

On constrained multivariate splines and their approximations

Summary The variational formulation of multivariate spline functions is generalized to include cases where the function has to satisfy inequality constraints such as positivity and convexity. Condition for existence and uniqueness of a solution is given. Approximation to the solution can be obtained by solving the variational problem in a finite dimensional subspace. Conditions for convergence and error estimates of the approximations are presented, both for interpolation problems and smoothing problems. The general theory is illustrated by specific examples including the volume-matching problem and the one-sided thin plate spline.This research is partially supported by the U.S. Army Contract No. DAAG 29-77-G-0207, and by NSF Grant No. MCS-8101836Part of this paper is based on Chapters 2 and 3 of the author's Ph. D. thesis. The author would like to express his sincere thanks to Professor Grance Wahba and to two referees for many helpful comments     

Co-hydrolysis products of tetraethoxysilane and methyltriethoxysilane in the presence of tetramethylammonium ions: Part 2 [1]

²⁹Si NMR peaks due to species with the double four-membered ring siloxane backbone composed of both Si(O^–)_4/2 and CH₃Si(O^–)_3/2 units, (CH₃) _n Si₈O _{20 – n} ^{/(8 – n) –} (n=1–3), formed by co-hydrolysis of tetraethoxysilane and methyltriethoxysilane in the presence of tetramethylammonium ions in methanol have been assigned. It has been found that ²⁹Si NMR peaks due to Si(OSi)₃(O^–) units shift to lower frequencies by replacement of the adjacent Si(O^–)_4/2 units by CH₃Si(O^–)_3/2 units, in other words, with increasing m value in Si[OSi(O^–)₃]_{3 – m} [OSi(CH₃) (O^–)₂] _m (O^–) (m=0–2). Peaks from CH₃ Si(OSi)₃ units in the species have also appeared as separated due to the kind of neighbor structural units. On the basis of the assignments, positions of CH₃Si(O^–)_3/2 units in the cubic octameric siloxane framework of (CH₃) _n Si₈O _{20 – n} ^{/(8 – n) –} (n=2, 3), for both of which three isomers are present, have been estimated.     

锂离子电池用多孔硅/石墨/碳复合负极材料的研究

在两步高能球磨和酸蚀条件下制得了多孔硅/石墨复合材料，并对其进行碳包覆制成多孔硅/石墨/碳复合材料。通过TEM，SEM等测试手段研究了多孔硅材料的结构。作为锂离子电池负极材料，电化学测试结果表明多孔硅/石墨/碳复合材料相比纳米硅/石墨/碳复合材料有更好的循环稳定性。同时，改变复合体配比、热解碳前驱物、粘结剂种类和用量也会对材料的电化学性能产生较大的影响。其中使用质量分数为10％的LA132粘结剂的电极200次循环以后充电容量保持在649.9 mAh·g^-1，几乎没有衰减。良好的电化学性能主要归因于主活性体-多孔硅颗粒中的纳米孔隙很好地抑制了嵌锂过程中自身的体积膨胀，而且亚微米石墨颗粒和碳的复合也减轻了电极材料的体积效应并改善了其导电性。     

Si(CH3)2Cl2分子的电离电势和化学键能的测定

有机硅化合物是半导体工业中产生硅元件的基本原料和有机合成中的重要试剂，是多年来大家研究较多的分子体系之一．本文报导了用同步辐射加速器产生的真空紫外光，电离St（CH3hCI。分子．在50－120n－m波长范围内，测量了各种离子产物与真空紫外光波长的关系，推算得它的绝热电离电势和离子中几个化学键的键能．1实验装置和方法本工作在国家同步辐射实验室光化学实验站进行．进行分子真空紫外光电离研究的实验系统已在文献山中详细描述．同步辐射加速器产生的真空紫外光波长用Ne气的电离势标定，其误差＜士0－Inln．单色仪的分辨率为河凸…     

On the Existence and Hydrolytic Stability of Titanosiloxane Bonds in the System: Glycidoxypropyltrimethoxysilane-Water-Titaniumtetraethoxide

Heterometal materials based on glycidoxypropyltrialkoxysilane and titaniumalkoxide are used for optical applications and require a high homogeneity on the molecular level. The presence of heterometal titanosiloxanes, their distribution and hydrolytic stability should influence the homogeneity of these materials. 29Si and 17O NMR spectroscopy has been used to investigate sols with molar ratios Si : Ti = 1 and H2O : OR (H) = 0.5 – 2.0 and their gels after heat treatment at 130°C. The presence of Si—O—Ti bonds in sols with a low water content (H < 0.2) and in the corresponding gels was identified by the high-field shift of the 29Si NMR signals of T1 and T2 units of up to 2–3 ppm compared to corresponding signals of homo-condensed Si—O—Si bonds. The existence of Si—O—Ti bonds in the sols is supported by 17O NMR spectra which show a characteristic signal around 340 ppm. A cleavage of the Si—O—Ti bonds occurs with increasing water/OR ratio in the sols. The cleavage of the heterometal bonds and the building up of homo-condensed species leads to a separation into areas with predominantly Ti—O—Ti and Si—O—Si bonds resulting in a decreased molecular homogeneity of the materials.     

R?ntgenspektrometrische Bestimmung von Eisen-55 in Luftstaub- und Niederschlagsproben

Zusammenfassung Eisen-55 wurde in Neuherberg bei München seit März 1970 in monatlich gesammelten Luftstaubproben und seit Januar 1971 in Monatsniederschlagsproben bestimmt. Nach dem Veraschen der Proben wurde Eisen durch Extraktion' mit Di-iso-propyläther abgetrennt. Zur weiteren Entfernung störender Radionuklide des Fallouts wurde eine Ionenaustauschertrennung angeschlossen. Nach der elektrolytischen Abscheidung des Eisens auf Kupferplättchen wurden die Röntgenspektren mit einem Proportionalzähler sowie einem Si(Li)-Detektor aufgenommen.Die mittlere spezifische Luftaktivität betrug in Neuherberg 1,0 fCi/m³ im Jahr 1970 (ab März), 1,2 fCi/m³ im Jahr 1971 und 0,36 fCi/m³ im Jahr 1972. Die dem Boden durch den Niederschlag zugeführte Aktivität wurde zu 697 pCi/m² · Jahr für 1971 und 178 pCi/m² · Jahr für 1972 bestimmt.Vortrag anläßlich der Tagung Spurenanalyse, 2. bis 5. April 1973 in Erlangen.     

XRD and Si MAS-NMR spectroscopy across the β-Lu2Si2O7-β-Y2Si2O7 solid solution

Samples in the system Lu_2−xY_xSi₂O₇ (0?x?2) have been synthesized following the sol-gel method and calcined to 1300 °C, a temperature at which the β-polymorph is known to be the stable phase for the end-members Lu₂Si₂O₇ and Y₂Si₂O₇. The XRD patterns of all the compositions studied are compatible with the structure of the β-polymorph. Unit cell parameters are calculated as a function of composition from XRD patterns. They show a linear change with increasing Y content, which indicates a solid solubility of β-Y₂Si₂O₇ in β-Lu₂Si₂O₇ at 1300 °C. ²⁹Si MAS NMR spectra of the different members of the system agree with the XRD results, showing a linear decrease of the ²⁹Si chemical shift with increasing Y content. Finally, a correlation reported in the literature to predict ²⁹Si chemical shifts in silicates is applied here to obtain the theoretical variation in ²⁹Si chemical shift values in the system Lu₂Si₂O₇-Y₂Si₂O₇ and the results compare favorably with the values obtained experimentally.