全文获取类型
收费全文 | 273篇 |
免费 | 37篇 |
国内免费 | 31篇 |
专业分类
化学 | 157篇 |
晶体学 | 6篇 |
综合类 | 2篇 |
数学 | 68篇 |
物理学 | 90篇 |
无线电 | 18篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 6篇 |
2021年 | 4篇 |
2020年 | 11篇 |
2019年 | 8篇 |
2018年 | 12篇 |
2017年 | 7篇 |
2016年 | 8篇 |
2015年 | 6篇 |
2014年 | 14篇 |
2013年 | 49篇 |
2012年 | 27篇 |
2011年 | 33篇 |
2010年 | 24篇 |
2009年 | 16篇 |
2008年 | 19篇 |
2007年 | 19篇 |
2006年 | 13篇 |
2005年 | 19篇 |
2004年 | 17篇 |
2003年 | 8篇 |
2002年 | 2篇 |
2001年 | 2篇 |
1999年 | 1篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1984年 | 1篇 |
排序方式: 共有341条查询结果,搜索用时 15 毫秒
41.
We theoretically investigate the electron spin transport properties through a δ-doped magnetic-barrier nanostructure, which can be realized experimentally by depositing two identical ferromagnetic stripes with the opposite in-plane magnetization on the top of a semiconductor heterostructure in parallel configuration and by using atomic layer doping technique. The δ-doping dependent transmission, conductance and spin polarization are calculated exactly by analytically solving Schrödinger equation of the spin electron. It is found that the electronic spin-polarized behavior in this device can be manipulated by changing the weight and/or the position of the δ-doping. Therefore, such a device can be used as a controllable spin filter, which may be helpful for spintronics applications. 相似文献
42.
Giangiacomo Beretta Fabrizio Gelmini Fabrizio Fontana Roberta Manuela Moretti Marina Montagnani Marelli Patrizia Limonta 《Natural product research》2018,32(10):1130-1135
In this work, we report a rapid and convenient HPLC-UV-DAD method for the isolation of δ-T3 on semi-preparative scale from two different vitamin E rich processed, commercially available products obtained from the fruits of Elaeis guineensis Jacq. (oil palm) and from the seeds of Bixa orellana L. (achiote tree). Chromatography was run using reverse phase (RP) C-18 columns and HPLC-grade acetonitrile as mobile phase. The purity of the isolated δ-T3, assessed by GC–MS and 1H NMR was above 98%. The δ-T3 cytotoxic activity found in vitro against the proliferation of human A375 melanoma cells compared to that of the other δ-T3 free tocols mixture suggest its primary role in the experimental anticancer activity observed for palm oil derived products. Taken altogether, the results of this study highlight the importance of the application of suitable purification systems for the preparations of tocotrienols prior to their experimental or clinical testing. 相似文献
43.
Enantiopure unnatural homologated amino acids, whereby there is?>?1 carbon atom between the C- and N- termini have found great utility in a number of applications. The enantiopure syntheses of β-amino acids are well documented, as increasingly are those of γ-amino acids. δ-Amino acids on the other hand are much less well-studied despite reports of their potential utility. This review attempts to summarise strategies that have been adopted towards the enantioselective synthesis of δ-amino acids and their precursors (e.g. nitrile/nitro/azido/ester/alcohol etc) and where appropriate demonstrate their utility. Only systems which are all carbon between the two termini are considered and only those where the shortest route between any given C-termini and any given N-termini is four carbons long (i.e. lysine derivatives are not considered). 相似文献
44.
Let X be a Banach space and Z a nonempty subset of X. Let J:Z→R be a lower semicontinuous function bounded from below and p?1. This paper is concerned with the perturbed optimization problem of finding z0∈Z such that ‖x−z0p‖+J(z0)=infz∈Z{‖x−zp‖+J(z)}, which is denoted by minJ(x,Z). The notions of the J-strictly convex with respect to Z and of the Kadec with respect to Z are introduced and used in the present paper. It is proved that if X is a Kadec Banach space with respect to Z and Z is a closed relatively boundedly weakly compact subset, then the set of all x∈X for which every minimizing sequence of the problem minJ(x,Z) has a converging subsequence is a dense Gδ-subset of X?Z0, where Z0 is the set of all points z∈Z such that z is a solution of the problem minJ(z,Z). If additionally p>1 and X is J-strictly convex with respect to Z, then the set of all x∈X for which the problem minJ(x,Z) is well-posed is a dense Gδ-subset of X?Z0. 相似文献
45.
Stereoselective isomerization of α,β-ynones was catalyzed by polymer-supported tertiaryphosphines under solvent-free conditions. (E,E)-α,β-γ,δ-Dienones were obtained with up to 93% isolated yields when JJ-TPP was employed. 相似文献
46.
从电化学阻抗谱和阴极极化等方面对Cr毒化La0.8Sr0.2MnO3-δ(LSM)阴极机理进行了研究. 相似文献
47.
李锦堂 《浙江大学学报(理学版)》1999,26(4):14-18
本文研究了出发流形为δ-Pinched流形的P调和映射的不稳定性,推广了文献[3]的相应结果。 相似文献
48.
依据近年来公开发表的有关单光子发射断层成像(SPECT)成像技术的文献,探索SPECT均匀衰减投影数据的反投影问题.主要采用指数型拉动变换的性质和构造特殊函数—–δ函数的方法,进而应用δ函数的特殊性得到了一个精确的反演公式.结果表明,由这种方法得到的反演公式能够更加有效和精确地实现图像的重建. 相似文献
49.
Thin Li1+xMn2O4−δ films were deposited on several substrate materials (stainless steel, p-doped silicon and glassy carbon) by pulsed laser deposition. To obtain the correct thin film stoichiometries, targets with a different amount of excess lithium were required (Li1.03Mn2O4 + xLi2O; x = 2.5 and 7.5 mol%). The resulting polycrystalline thin films were characterized with respect to their morphology and electrochemical activity. It was found that only thin Li1+xMn2O4−δ films deposited on stainless steel and glassy carbon showed the typical insertion and deinsertion peaks of Li+ during cycling. 相似文献
50.
δ-Pu为Pu的高温相,掺杂少量的Ga即可使其在室温下稳定存在.本文采用密度泛函理论方法,对不同掺杂量体系进行晶体结构和电子结构计算,主要包括体系的晶格常数、密度、形成能、态密度、电荷密度和Mulliken布居分析.结果表明:在研究范围内,Ga掺杂后,体系晶格常数降低,密度增大,6.25%(原子百分比,下同)掺杂量体系的稳定性高于3.125%和12.5%掺杂量的体系;Ga掺杂使得Pu周围体系电子的局域性增强,成键能力增强,揭示了Ga稳定δ-Pu的电子机制.Ga和Pu之间为金属键,发生的作用主要由Pu的7s、6p、6d和Ga的4s、4p轨道电子贡献,但这种成键作用相对较弱,使得掺杂体系可以保持原有的力学性能和机械加工性能.Ga对δ-Pu的稳定作用主要在于改善Pu原子的成键性能,而不是与Pu原子直接成键. 相似文献