全文获取类型
收费全文 | 350368篇 |
免费 | 6058篇 |
国内免费 | 2656篇 |
专业分类
化学 | 178021篇 |
晶体学 | 5711篇 |
力学 | 16890篇 |
综合类 | 80篇 |
数学 | 40835篇 |
物理学 | 117545篇 |
出版年
2021年 | 3712篇 |
2020年 | 4008篇 |
2019年 | 4459篇 |
2018年 | 5851篇 |
2017年 | 6000篇 |
2016年 | 8190篇 |
2015年 | 4970篇 |
2014年 | 8112篇 |
2013年 | 17336篇 |
2012年 | 14091篇 |
2011年 | 16704篇 |
2010年 | 12410篇 |
2009年 | 12161篇 |
2008年 | 14620篇 |
2007年 | 14427篇 |
2006年 | 13298篇 |
2005年 | 11451篇 |
2004年 | 10579篇 |
2003年 | 9313篇 |
2002年 | 9231篇 |
2001年 | 10968篇 |
2000年 | 8184篇 |
1999年 | 6357篇 |
1998年 | 5319篇 |
1997年 | 5115篇 |
1996年 | 4774篇 |
1995年 | 4097篇 |
1994年 | 4091篇 |
1993年 | 3890篇 |
1992年 | 4283篇 |
1991年 | 4464篇 |
1990年 | 4243篇 |
1989年 | 4139篇 |
1988年 | 3866篇 |
1987年 | 3974篇 |
1986年 | 3744篇 |
1985年 | 4694篇 |
1984年 | 4732篇 |
1983年 | 3913篇 |
1982年 | 4009篇 |
1981年 | 3729篇 |
1980年 | 3680篇 |
1979年 | 3936篇 |
1978年 | 3925篇 |
1977年 | 3923篇 |
1976年 | 3899篇 |
1975年 | 3696篇 |
1974年 | 3635篇 |
1973年 | 3665篇 |
1972年 | 2609篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
Dr. Felipe L. Coelho Eduarda S. Gil Dr. Paulo F. B. Gonçalves Dr. Leandra F. Campo Dr. Paulo H. Schneider 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(34):8157-8162
In this work, a series of 2-chalcogenylindoles was synthesized by an efficient methodology, starting from chalcogenoalkynes, including a previously unreported tellurium indole derivative. For the first time, these 2-substituted chalcogenylindoles were obtained in the absence of metal catalyst or base, under thermal conditions only. In addition, the results described herein represent a methodology with inverse regioselectivity for the chalcogen functionalization of indoles. 相似文献
202.
203.
We consider the problem of estimating a large rank-one tensor u ⊗k ∈ (ℝn)⊗k , k ≥ 3 , in Gaussian noise. Earlier work characterized a critical signal-to-noise ratio λ Bayes = O(1) above which an ideal estimator achieves strictly positive correlation with the unknown vector of interest. Remarkably, no polynomial-time algorithm is known that achieved this goal unless λ ≥ Cn(k − 2)/4 , and even powerful semidefinite programming relaxations appear to fail for 1 ≪ λ ≪ n(k − 2)/4 . In order to elucidate this behavior, we consider the maximum likelihood estimator, which requires maximizing a degree-k homogeneous polynomial over the unit sphere in n dimensions. We compute the expected number of critical points and local maxima of this objective function and show that it is exponential in the dimensions n , and give exact formulas for the exponential growth rate. We show that (for λ larger than a constant) critical points are either very close to the unknown vector u or are confined in a band of width Θ(λ−1/(k − 1)) around the maximum circle that is orthogonal to u . For local maxima, this band shrinks to be of size Θ(λ−1/(k − 2)) . These “uninformative” local maxima are likely to cause the failure of optimization algorithms. © 2019 Wiley Periodicals, Inc. 相似文献
204.
JPC – Journal of Planar Chromatography – Modern TLC - The genus Justicia (Acanthaceae) is a source of important pharmaceutical compounds. Justicia species are integral parts of several... 相似文献
205.
206.
207.
Tereshchenko E. D. Cherniakov S. M. Yurik R. Yu. Rietveld M. T. Häggström I. 《Radiophysics and Quantum Electronics》2020,62(10):667-676
Radiophysics and Quantum Electronics - We present the results of comparing the total electron content measurements based on GLONASS satellite signals and the EISCAT UHF incoherent scatter radar... 相似文献
208.
209.
Mathematical Notes - We prove the equivalence of a vector and a scalar equilibrium problem that naturally arise when studying the limit distribution of zeros of type I Hermite–Padé... 相似文献
210.
Shaobo Cheng Vidushi Sharma Altug S. Poyraz Lijun Wu Xing Li Amy C. Marschilok Esther S. Takeuchi Kenneth J. Takeuchi Marivi Fernndez-Serra Yimei Zhu 《Chemical science》2020,11(19):4991
Tunneled metal oxides such as α-Mn8O16 (hollandite) have proven to be compelling candidates for charge-storage materials in high-density batteries. In particular, the tunnels can support one-dimensional chains of K+ ions (which act as structure-stabilizing dopants) and H2O molecules, as these chains are favored by strong H-bonds and electrostatic interactions. In this work, we examine the role of water molecules in enhancing the stability of K+-doped α-Mn8O16 (cryptomelane). The combined experimental and theoretical analyses show that for high enough concentrations of water and tunnel-ions, H2O displaces K+ ions from their natural binding sites. This displacement becomes energetically favorable due to the formation of K2+ dimers, thereby modifying the stoichiometric charge of the system. These findings have potentially significant technological implications for the consideration of cryptomelane as a Li+/Na+ battery electrode. Our work establishes the functional role of water in altering the energetics and structural properties of cryptomelane, an observation that has frequently been overlooked in previous studies.Water displaces potassium ions and initiates the formation of a homonuclear dimer ion (K2+) in the tunnels of hollandite. 相似文献