On free products in varieties of associative algebras

Varieties of associative algebras over a field of characteristic zero are considered. Belov recently proved that, in any variety of this kind, the Hilbert series of a relatively free algebra of finite rank is rational. At the same time, for three important varieties, namely, those of algebras with zero multiplication, of commutative algebras, and of all associative algebras, a stronger assertion holds: for these varieties, formulas that rationally express the Hilbert series of the free product algebra via the Hilbert series of the factors are well known. In the paper, a system of counterexamples is presented which shows that there is no formula of this kind in any other variety, even in the case of two factors one of which is a free algebra. However, if we restrict ourselves to the class of graded PI-algebras generated by their components of degree one, then there exist infinitely many varieties for each of which a similar formula is valid. Translated fromMatematicheskie Zametki, Vol. 65, No. 5, pp. 693–702, May, 1999.     

合成奈拉滨C-1位α-异构体的新方法

报道了一种合成奈拉滨C-1位α-异构体——2-氨基-6-甲氧基-9-α-D-阿拉伯糖呋喃糖基-9H-嘌呤(7α)的新方法。以D-阿拉伯糖为原料,经醚化,乙酰化和取代3步反应制得中间体1,2,3,5-O-四乙酰基-D-阿拉伯呋喃糖苷(4);4与甲氧基修饰的嘌呤经Vorbrüggen偶联反应后脱保护基合成了7α,其结构经1H NMR确证。     

Oxygen deficiency dependence of magnetic and magnetoresistive properties for La0.5Sr0.5CoO3-x thin films prepared by pulsed laser deposition

The complete (001)-oriented thin films of La_0.5Sr_0.5CoO_3-x (LSCO) are deposited on (001) SrTiO₃ substrates by pulsed laser deposition under reduced oxygen pressure. It is revealed that the c axis of the film stretches with depleting oxygen. The magnetic, electrical, and magnetoresistive properties of the films are characterized by means of various techniques. Significant dependence of these properties on oxygen deficiency in the films is demonstrated, with enhanced magnetoresistance recorded for the samples deposited over a wide range of reduced oxygen pressure. Received: 9 July 1998 / Accepted: 15 January 1999 / Published online: 31 March 1999     

Paternò-Büchi反应的区域选择性

Paternò-Büchi(P-B)反应,即羰基-烯的[2+2]光环化加成反应,利用其特殊的区域及立体选择性可以合成一些结构精巧的取代氧杂环丁烷。随着P-B反应在有机合成中越来越广泛的应用,人们对P-B反应的区域选择性的研究也越来越深入。在P-B反应发现之初,人们一直用“最稳定的双自由基规则”解释其区域选择性,这一规则对有些体系却不适用,近年来有人开始运用“自旋化学”理论来解释区域选择性。本论文结合作者的研究工作,评述了近年来P-B反应的区域选择性的研究进展,重点阐述了反应温度、反应物的取代基对P-B反应区域选择性的影响及其规律。     

Electron trapping center and SnO2-doping mechanism of indium tin oxide

Indium tin oxide (ITO) and Er³⁺-doped ITO powders were prepared by a conventional ceramic method. The density of ITO powders and optical absorption spectra of Er³⁺ ions in Er³⁺-doped ITO were measured as a function of the SnO₂ doping level. The results obtained were discussed in terms of the trapping center for immobile electrons in ITO. Observed densities of ITO powders were in good agreement with those calculated from their lattice parameters, assuming that the immobile electrons were trapped at the excess interstitial oxygen. The optical absorption spectra of Er³⁺-doped ITO indicated that some In³⁺ ions in ITO were surrounded by 7 and/or 8 oxygen ions; the increase in the coordination number of In³⁺ from 6 in In₂O₃ to 7 and/or 8 in ITO must be caused by the introduction of excess interstitial oxygen into the quasi-anion site in the C-typerare-earth lattice upon SnO₂ doping. It was concluded that the immobile electrons in ITO are trapped at the excess interstitial oxygen, and that the mechanism of conduction carrier generation and compensation upon SnO₂ doping into In₂O₃ can be expressed by the defect equation, 2SnO₂?2Sn_In·+2(1-z)e^′+zO_i ^′′+3O_O ^×+(1-z)/2O₂. Received: 26 November 1999 / Accepted: 20 April 2000 / Published online: 13 September 2000     

Gr"otzsch环与Ramanujan的模方程

本文建立了拟共形映照理论中平面Gr"otzsch环的模 (r)与由(8)式定义的出现于数论中Ramanujan广义模方程的函数a (r)之间的关系,并将(r)的若干已知结果推广到a(r),为研究模方程理论提供了一种方法.     

小尺寸低碳钢试件吕德斯效应的三维数字图像相关测量

吕德斯效应是多种金属和合金材料由于屈服阶段的不均匀变形而在材料表面产生条带状褶皱的现象,它会使冲压件表面质量降低.为了防止它的出现,对吕德斯效应进行研究变得非常重要.采用小视场(15 mm×15 mm)下三维数字图像相关方法对小尺寸低碳钢试件在单轴拉伸载荷作用下的变形场进行测量,实际观测了小尺寸试件的吕德斯效应,结合理论模型解释了其形成机理,并分析了吕德斯带传播过程中应变及应变率的变化规律.实验研究表明,运用三维数字图像相关方法测量试件表面变形场,实现了对小尺寸低碳钢试件的吕德斯带演化过程以及颈缩、断裂等细观力学行为的观测,该方法是研究材料变形细观机理的一种有效测量手段.     

Effective surface passivation of crystalline silicon using ultrathin Al2O3 films and Al2O3/SiNx stacks

We measure surface recombination velocities (SRVs) below 10 cm/s on p‐type crystalline silicon wafers passivated by atomic–layer–deposited (ALD) aluminium oxide (Al₂O₃) films of thickness ≥10 nm. For films thinner than 10 nm the SRV increases with decreasing Al₂O₃ thickness. For ultrathin Al₂O₃ layers of 3.6 nm we still attain a SRV < 22 cm/s on 1.5 Ω cm p‐Si and an exceptionally low SRV of 1.8 cm/s on high‐resistivity (200 Ω cm) p‐Si. Ultrathin Al₂O₃ films are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is extremely low compared to the frequently used plasma‐enhanced chemical vapour deposition of silicon nitride (SiN_x). Our experiments on silicon wafers passivated with stacks composed of ultrathin Al₂O₃ and SiN_x show that a substantially improved thermal stability during high‐temperature firing at 830 °C is obtained for the Al₂O₃/SiN_x stacks compared to the single‐layer Al₂O₃ passivation. Al₂O₃/SiN_x stacks are hence ideally suited for the implementation into industrial‐type silicon solar cells where the metal contacts are made by screen‐printing and high‐temperature firing of metal pastes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetocaloric effect: Overcoming the magnetic limit

We have studied anomalous peaks observed in magnetocaloric −ΔS(T) curves for systems that undergo first-order magnetostructural transitions. The origin of those peaks, which can exceed the conventional magnetic limit, R ln(2J+1), is discussed on thermodynamic bases by introducing an additional-exchange contribution (due to exchange constant variation arising from magnetostructural transition). We also applied a semiphenomenological model to include this additional-exchange contribution in Gd₅Si₂Ge₂- and MnAs-based systems, obtaining excellent results for the observed magnetocaloric effect.