全文获取类型
收费全文 | 19078篇 |
免费 | 3522篇 |
国内免费 | 3077篇 |
专业分类
化学 | 11984篇 |
晶体学 | 213篇 |
力学 | 1648篇 |
综合类 | 171篇 |
数学 | 1690篇 |
物理学 | 9971篇 |
出版年
2024年 | 36篇 |
2023年 | 243篇 |
2022年 | 393篇 |
2021年 | 593篇 |
2020年 | 812篇 |
2019年 | 639篇 |
2018年 | 620篇 |
2017年 | 682篇 |
2016年 | 814篇 |
2015年 | 758篇 |
2014年 | 1051篇 |
2013年 | 1704篇 |
2012年 | 1147篇 |
2011年 | 1234篇 |
2010年 | 1073篇 |
2009年 | 1349篇 |
2008年 | 1355篇 |
2007年 | 1397篇 |
2006年 | 1331篇 |
2005年 | 1073篇 |
2004年 | 970篇 |
2003年 | 875篇 |
2002年 | 711篇 |
2001年 | 626篇 |
2000年 | 590篇 |
1999年 | 495篇 |
1998年 | 438篇 |
1997年 | 358篇 |
1996年 | 314篇 |
1995年 | 301篇 |
1994年 | 253篇 |
1993年 | 208篇 |
1992年 | 172篇 |
1991年 | 171篇 |
1990年 | 111篇 |
1989年 | 110篇 |
1988年 | 95篇 |
1987年 | 76篇 |
1986年 | 82篇 |
1985年 | 78篇 |
1984年 | 54篇 |
1983年 | 28篇 |
1982年 | 43篇 |
1981年 | 43篇 |
1980年 | 34篇 |
1979年 | 37篇 |
1978年 | 14篇 |
1977年 | 24篇 |
1976年 | 14篇 |
1974年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
21.
周忠源 《原子与分子物理学报》2020,37(6):845-857
基于自旋相关局域Hartree-Fock (SLHF)势函数,本文提出了一种计算双原子分子激发态势能的密度泛函理论(DFT)方法,并将该方法应用于和的激发态势能曲线的计算。在只考虑交换能的情况下,本文的DFT计算结果与文献中精确方法和Hartree-Fock (HF)方法的结果符合的非常好,说明采用SLHF势函数作为交换势的DFT方法是一个很好的计算激发态势能的方法。本文还计算和探讨了电子的关联势函数和关联能,发现传统的近似方法在较大核间距的情况下大大低估了电子的关联能. 相似文献
22.
近来有些文章断言,在一个惯性参考系里能量守恒的物理系统,在别的参考系看来能量也一定守恒.实际上这些作者混淆了物理方程式的协变性和相对性原理.本文将澄清这一误解. 相似文献
23.
采用量子统计系综理论,研究了基态NO分子宏观气体摩尔熵、摩尔内能、摩尔热容等热力学性质.首先应用课题组前期建立的变分代数法(variational algebraic method, VAM)计算获得了基态NO分子的完全振动能级,得到的VAM振动能级作为振动部分,结合欧拉-麦克劳林渐进展开公式的转动贡献,应用于经典的热力学与统计物理公式中,从而计算得到了1000-5000 K温度范围内NO宏观气体的摩尔内能、摩尔熵和摩尔热容.将不同方法计算得到的摩尔热容结果分别与实验值进行比较,结果表明基于VAM完全振动能级获得的结果优于其他方法获得的理论结果.振动部分采用谐振子模型对无限能级求和计算热力学性质的方法有一定的局限性,应当使用有限的完全振动能级进行统计求和. 相似文献
24.
25.
Randall Claywell Laszlo Nadai Imre Felde Sina Ardabili Amirhosein Mosavi 《Entropy (Basel, Switzerland)》2020,22(11)
The accurate prediction of the solar diffuse fraction (DF), sometimes called the diffuse ratio, is an important topic for solar energy research. In the present study, the current state of Diffuse irradiance research is discussed and then three robust, machine learning (ML) models are examined using a large dataset (almost eight years) of hourly readings from Almeria, Spain. The ML models used herein, are a hybrid adaptive network-based fuzzy inference system (ANFIS), a single multi-layer perceptron (MLP) and a hybrid multi-layer perceptron grey wolf optimizer (MLP-GWO). These models were evaluated for their predictive precision, using various solar and DF irradiance data, from Spain. The results were then evaluated using frequently used evaluation criteria, the mean absolute error (MAE), mean error (ME) and the root mean square error (RMSE). The results showed that the MLP-GWO model, followed by the ANFIS model, provided a higher performance in both the training and the testing procedures. 相似文献
26.
27.
Structure of Amido Pyridinium Betaines: Persistent Intermolecular C−H⋅⋅⋅N Hydrogen Bonding in Solution 下载免费PDF全文
Dr. Robert J. Thatcher Dr. David G. Johnson Dr. John M. Slattery Dr. Richard E. Douthwaite 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(10):3414-3421
A hydrogen bond of the type C?H???X (X=O or N) is known to influence the structure and function of chemical and biological systems in solution. C?H???O hydrogen bonding in solution has been extensively studied, both experimentally and computationally, whereas the equivalent thermodynamic parameters have not been enumerated experimentally for C?H???N hydrogen bonds. This is, in part, due to the lack of systems that exhibit persistent C?H???N hydrogen bonds in solution. Herein, a class of molecule based on a biologically active norharman motif that exhibits unsupported intermolecular C?H???N hydrogen bonds in solution has been described. A pairwise interaction leads to dimerisation to give bond strengths of about 7 kJ mol?1 per hydrogen bond, which is similar to chemically and biologically relevant C?H???O hydrogen bonding. The experimental data is supported by computational work, which provides additional insight into the hydrogen bonding by consideration of electrostatic and orbital interactions and allowed a comparison between calculated and extrapolated NMR chemical shifts. 相似文献
28.
Alexandru Vlasceanu Dr. Søren L. Broman Anne S. Hansen Anders B. Skov Prof. Dr. Martina Cacciarini Prof. Dr. Anders Kadziola Prof. Dr. Henrik G. Kjaergaard Prof. Dr. Kurt V. Mikkelsen Prof. Dr. Mogens Brøndsted Nielsen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(31):10796-10800
The conversion and efficient storage of solar energy is recognized to hold significant potential with regard to future energy solutions. Molecular solar thermal batteries based on photochromic systems exemplify one possible technology able to harness and apply this potential. Herein is described the synthesis of a macrocycle based on a dimer of the dihydroazulene/vinylheptafulvene (DHA/VHF) photo/thermal couple. By taking advantage of conformational strain, this DHA–DHA macrocycle presents an improved ability to absorb and store incident light energy in chemical bonds (VHF–VHF). A stepwise energy release over two sequential ring‐closing reactions (VHF→DHA) combines the advantages of an initially fast discharge, hypothetically addressing immediate energy consumption needs, followed by a slow process for consistent, long‐term use. This exemplifies another step forward in the molecular engineering and design of functional organic materials towards solar thermal energy storage and release. 相似文献
29.
An efficient,mild and selective Ullmann‐type N‐arylation of indoles catalysed by Pd immobilized on amidoxime‐functionalized mesoporous SBA‐15 as heterogeneous and recyclable nanocatalyst 下载免费PDF全文
A wide range of N‐arylated indoles were selectively synthesized through intermolecular C(aryl)? N bond formation from the corresponding aryl iodides and indoles through Ullmann‐type coupling reactions in the presence of a catalytic amount of Pd immobilized on amidoxime‐functionalized mesoporous SBA‐15 (SBA‐15/AO/Pd(0)) under mild reaction conditions. These cross‐coupled products were obtained in excellent yields under mild conditions at extremely low palladium loading (ca 0.3 mol%), and the heterogeneous catalyst can be readily recovered by simple filtration and reused seven times with loss in its activity. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
30.