全文获取类型
收费全文 | 84648篇 |
免费 | 1499篇 |
国内免费 | 1105篇 |
专业分类
化学 | 29101篇 |
晶体学 | 849篇 |
力学 | 7076篇 |
综合类 | 29篇 |
数学 | 32614篇 |
物理学 | 17583篇 |
出版年
2023年 | 193篇 |
2022年 | 324篇 |
2021年 | 335篇 |
2020年 | 373篇 |
2019年 | 328篇 |
2018年 | 10633篇 |
2017年 | 10456篇 |
2016年 | 6418篇 |
2015年 | 1214篇 |
2014年 | 727篇 |
2013年 | 831篇 |
2012年 | 4411篇 |
2011年 | 11115篇 |
2010年 | 6039篇 |
2009年 | 6434篇 |
2008年 | 6975篇 |
2007年 | 9117篇 |
2006年 | 534篇 |
2005年 | 1590篇 |
2004年 | 1715篇 |
2003年 | 2144篇 |
2002年 | 1157篇 |
2001年 | 356篇 |
2000年 | 392篇 |
1999年 | 271篇 |
1998年 | 292篇 |
1997年 | 234篇 |
1996年 | 291篇 |
1995年 | 190篇 |
1994年 | 139篇 |
1993年 | 146篇 |
1992年 | 114篇 |
1991年 | 103篇 |
1990年 | 81篇 |
1989年 | 91篇 |
1988年 | 78篇 |
1987年 | 70篇 |
1986年 | 70篇 |
1985年 | 58篇 |
1984年 | 47篇 |
1983年 | 39篇 |
1982年 | 47篇 |
1981年 | 40篇 |
1980年 | 47篇 |
1979年 | 44篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
13.
Chunhong Wang Qi Cui Zhixin Zhang Prof. Zhu-Jun Yao Dr. Shaozhong Wang Prof. Zhi-Xiang Yu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(42):9821-9826
1,3-Azaprotio transfer of propargylic α-ketocarboxylate oximes, a new type of alkynyl oximes featuring an ester tether, has been explored by taking advantage of gold catalysis. The incorporation of an oxygen atom to the chain of alkynyl oximes led to the formation of two different oxa-cyclic nitrones. It was found that internal alkynyl oximes with an E-configuration deliver five-membered nitrones, whereas terminal alkynyl oximes with an E-configuration afford six-membered nitrones. DFT calculations on four possible pathways supported a stepwise formation of C−N and C−H bonds, in which a 1,3-acyloxy-migration competes with the 1,3-azaprotio-transfer, especially in the case of internal alkynyl oximes. The relative nucleophilic properties of oxygen in the carbonyl group and the nitrogen in the oxime, the electronic effects of alkynes, and the influence of the ring system have been investigated computationally. 相似文献
14.
15.
16.
17.
The structure, chemical composition, and magnetic properties of electrochemically deposited nanocrystalline Co-Ni-Fe films
were investigated using a number of techniques. A high saturation magnetic induction up to B
s
= 21 kG was attained. An enhancement of the saturation magnetization compared to the ideal anticipated one was revealed,
which correlated with the nonlinear behavior of the structural phase composition and lattice parameters with the change of
the composition.
The text was submitted by the authors in English. 相似文献
18.
A forecasting model is developed for the number of daily applications for loans at a financial services telephone call centre. The purpose of the forecasts and the associated prediction intervals is to provide effective staffing policies within the call centre. The model building process is constrained by the availability of only 2 years and 7 months of data. The distinctive feature of the data is that demand is driven in the main by advertising. The analysis given focuses on applications stimulated by press advertising. Unlike previous analyses of broadly similar data, where ARIMA models were used, a model with a dynamic level, multiplicative calendar effects and a multiplicative advertising response is developed and shown to be effective. 相似文献
19.
Data Envelopment Analysis (DEA) offers a piece-wise linear approximation of the production frontier. The approximation tends to be poor if the true frontier is not concave, eg in case of economies of scale or of specialisation. To improve the flexibility of the DEA frontier and to gain in empirical fit, we propose to extend DEA towards a more general piece-wise quadratic approximation, called Quadratic Data Envelopment Analysis (QDEA). We show that QDEA gives statistically consistent estimates for all production frontiers with bounded Hessian eigenvalues. Our Monte-Carlo simulations suggest that QDEA can substantially improve efficiency estimation in finite samples relative to standard DEA models. 相似文献
20.