全文获取类型
收费全文 | 534篇 |
免费 | 32篇 |
国内免费 | 1篇 |
专业分类
化学 | 261篇 |
晶体学 | 2篇 |
力学 | 26篇 |
数学 | 51篇 |
物理学 | 227篇 |
出版年
2023年 | 4篇 |
2022年 | 3篇 |
2021年 | 10篇 |
2020年 | 16篇 |
2019年 | 15篇 |
2018年 | 9篇 |
2017年 | 5篇 |
2016年 | 16篇 |
2015年 | 17篇 |
2014年 | 17篇 |
2013年 | 30篇 |
2012年 | 34篇 |
2011年 | 51篇 |
2010年 | 26篇 |
2009年 | 18篇 |
2008年 | 46篇 |
2007年 | 27篇 |
2006年 | 35篇 |
2005年 | 21篇 |
2004年 | 28篇 |
2003年 | 13篇 |
2002年 | 19篇 |
2001年 | 10篇 |
2000年 | 10篇 |
1999年 | 1篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 12篇 |
1995年 | 5篇 |
1994年 | 13篇 |
1993年 | 18篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1979年 | 1篇 |
1973年 | 1篇 |
1968年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有567条查询结果,搜索用时 15 毫秒
561.
First‐principles calculations were carried out on recently synthesized Re2 and Re3 as well as hypothetical Tc and Mn nitrides. It is found that structure and covalent bonds play an important role in determining mechanical properties. Under a large strain along (0001)〈101 0〉direction, Re2N undergoes a phase transformation with a slight increase in ideal shear strength. On the other hand, it is transformed into a phase with weaker mechanical properties, if the strain is along Re2〈1 21 0〉 direction. Mn2N can be synthesized under moderate conditions due to its more negative formation energy. Re2N, Re3N, and Mn2N show structure‐related mechanical property under larger strains to ReB2 but exhibit much lower ideal strengths, which is attributed to the larger ionicity of cation–anion bond. Three‐dimensional framework of strong covalent bonds is thus highly recommended to design superhard materials. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011 相似文献
562.
[reaction: see text] The stereoselectivity of the key epoxidation step in the synthesis of guanacastepene A is shown to be controlled by torsional steering. In this particular epoxidation reaction, the transition structure energetic difference is enhanced by the great asynchronicity of the forming C-O bonds that intensifies the torsional interactions. 相似文献
563.
Mitsumori S Zhang H Ha-Yeon Cheong P Houk KN Tanaka F Barbas CF 《Journal of the American Chemical Society》2006,128(4):1040-1041
The development of catalysts for Mannich-type reactions that afford anti-products with excellent diastereo- and enantioselectivities under mild conditions and low catalyst loadings (1-5 mol %) is reported. Based on principles gained from the study of (S)-proline-catalyzed Mannich-type reactions that afford enantiomerically enriched syn-products, (3R,5R)-5-methyl-3-pyrrolidinecarboxylic acid (RR35) has been designed to catalyze the direct enantioselective anti-selective Mannich-type reactions. Computational studies of the above reaction using HF/6-31G* level of theory suggested that this design would be highly effective. The catalyst was subsequently synthesized and studied in organocatalytic Mannich-type reactions between unmodified aldehydes and N-PMP-protected alpha-imino esters. In accord with the design principles and in quantitative agreement with the theoretical predictions, reactions catalyzed by this catalyst afforded anti-products in good yields with excellent diastereo- and enantioselectivities (anti:syn 94:6 to 98:2, >97 to >99% ee). 相似文献
564.
Hee-Kap Ahn Peter Brass Otfried Cheong Hyeon-Suk Na Chan-Su Shin Antoine Vigneron 《Computational Geometry》2006,33(3):152-164
Given a planar convex set C, we give sublinear approximation algorithms to determine approximations of the largest axially symmetric convex set S contained in C, and the smallest such set S′ that contains C. More precisely, for any >0, we find an axially symmetric convex polygon QC with area |Q|>(1−)|S| and we find an axially symmetric convex polygon Q′ containing C with area |Q′|<(1+)|S′|. We assume that C is given in a data structure that allows to answer the following two types of query in time TC: given a direction u, find an extreme point of C in direction u, and given a line ℓ, find C∩ℓ. For instance, if C is a convex n-gon and its vertices are given in a sorted array, then TC=O(logn). Then we can find Q and Q′ in time O(−1/2TC+−3/2). Using these techniques, we can also find approximations to the perimeter, area, diameter, width, smallest enclosing rectangle and smallest enclosing circle of C in time O(−1/2TC). 相似文献
565.
We describe a bottom–up framework, based on the identification of appropriate order parameters and determination of phase diagrams, for understanding progressively refined agent-based models and simulations of financial markets. We illustrate this framework by starting with a deterministic toy model, whereby N independent traders buy and sell M stocks through an order book that acts as a clearing house. The price of a stock increases whenever it is bought and decreases whenever it is sold. Price changes are updated by the order book before the next transaction takes place. In this deterministic model, all traders based their buy decisions on a call utility function, and all their sell decisions on a put utility function. We then make the agent-based model more realistic, by either having a fraction fb of traders buy a random stock on offer, or a fraction fs of traders sell a random stock in their portfolio. Based on our simulations, we find that it is possible to identify useful order parameters from the steady-state price distributions of all three models. Using these order parameters as a guide, we find three phases: (i) the dead market; (ii) the boom market; and (iii) the jammed market in the phase diagram of the deterministic model. Comparing the phase diagrams of the stochastic models against that of the deterministic model, we realize that the primary effect of stochasticity is to eliminate the dead market phase. 相似文献
566.
Jun-Yeong Yoon Yangjin Lee Dong-Gyu Kim Dong Gun Oh Jin Kyun Kim Linshuo Guo Jungcheol Kim Jeongheon Choe Kihyun Lee Hyeonsik Cheong Chae Un Kim Young Jai Choi Yanhang Ma Kwanpyo Kim 《Angewandte Chemie (International ed. in English)》2023,62(36):e202307102
Elemental phosphorus exhibits fascinating structural varieties and versatile properties. The unique nature of phosphorus bonds can lead to the formation of extremely complex structures, and detailed structural information on some phosphorus polymorphs is yet to be investigated. In this study, we investigated an unidentified crystalline phase of phosphorus, type-II red phosphorus (RP), by combining state-of-the-art structural characterization techniques. Electron diffraction tomography, atomic-resolution scanning transmission electron microscopy (STEM), powder X-ray diffraction, and Raman spectroscopy were concurrently used to elucidate the hidden structural motifs and their packing in type-II RP. Electron diffraction tomography, performed using individual crystalline nanowires, was used to identify a triclinic unit cell with volume of 5330 Å3, which is the largest unit cell for elemental phosphorus crystals up to now and contains approximately 250 phosphorus atoms. Atomic-resolution STEM imaging, which was performed along different crystal-zone axes, confirmed that the twisted wavy tubular motif is the basic building block of type-II RP. Our study discovered and presented a new variation of building blocks in phosphorus, and it provides insights to clarify the complexities observed in phosphorus as well as other relevant systems. 相似文献
567.