全文获取类型
收费全文 | 344847篇 |
免费 | 6305篇 |
国内免费 | 2767篇 |
专业分类
化学 | 175875篇 |
晶体学 | 5514篇 |
力学 | 16660篇 |
综合类 | 71篇 |
数学 | 40104篇 |
物理学 | 115695篇 |
出版年
2022年 | 2932篇 |
2021年 | 3702篇 |
2020年 | 3955篇 |
2019年 | 4351篇 |
2018年 | 5747篇 |
2017年 | 5932篇 |
2016年 | 8214篇 |
2015年 | 4886篇 |
2014年 | 7968篇 |
2013年 | 17041篇 |
2012年 | 13814篇 |
2011年 | 16215篇 |
2010年 | 11958篇 |
2009年 | 11808篇 |
2008年 | 14252篇 |
2007年 | 14091篇 |
2006年 | 12933篇 |
2005年 | 11242篇 |
2004年 | 10330篇 |
2003年 | 9102篇 |
2002年 | 9077篇 |
2001年 | 10705篇 |
2000年 | 7940篇 |
1999年 | 6278篇 |
1998年 | 5231篇 |
1997年 | 5046篇 |
1996年 | 4737篇 |
1995年 | 4125篇 |
1994年 | 4005篇 |
1993年 | 3847篇 |
1992年 | 4245篇 |
1991年 | 4449篇 |
1990年 | 4219篇 |
1989年 | 4107篇 |
1988年 | 3861篇 |
1987年 | 3983篇 |
1986年 | 3732篇 |
1985年 | 4670篇 |
1984年 | 4709篇 |
1983年 | 3910篇 |
1982年 | 3986篇 |
1981年 | 3713篇 |
1980年 | 3656篇 |
1979年 | 3913篇 |
1978年 | 3903篇 |
1977年 | 3906篇 |
1976年 | 3879篇 |
1975年 | 3675篇 |
1974年 | 3615篇 |
1973年 | 3650篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
172.
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
173.
Abstract Gastrodigenin, also known as 4-hydroxybenzyl alcohol (HBA), is one of the main components of Gastrodia elata, which is a perfect lead compound of natural products. In order to get new active compounds, we modified the structure of HBA through esterification with carboxylic acid, and got a series of derivatives in which 4-hydroxybenzyl alcohol 2-naphthoate (NHBA) showed stronger antidepressant activity than HBA. In this paper, we firstly evaluated the antidepressant activity of NHBA by tail suspension test (TST) and forced swimming test (FST). Then, we carried out the biochemical assay and western blot to determine its mechanism. The results displayed that NHBA could increase the content of serotonin, dopamine, norepinephrine, γ-aminobutyric acid, brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) in mice brain. It suggested that NHBA exhibited an antidepressant-like effect through monoaminergic system, GABAergic system and BDNF/TrkB signaling pathways. 相似文献
174.
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
175.
176.
Two novel 2′-hydroxychalcone derivatives (i.e., M1 and M2) are explored in this work. We mainly focus on investigating the effects of photoexcitation on hydrogen bonds and on the excited-state intramolecular proton transfer (ESIPT) process. On the basis of calculations of electrostatic potential surface and intramolecular interactions, we verify the formation of hydrogen bond O1 H2···O3 in both S0 and S1 states. Exploring the ultraviolet–visible spectra in the liquid phase, our simulated results reappear in the experimental phenomenon. Analyzing molecular geometry and infrared stretching vibrational spectra, we confirm O1 H2···O3 is strengthened for both M1 and M2 in the S1 state. We further confirm that charge redistribution facilitates ESIPT tendency. Constructing potential energy curves, we find the ultrafast ESIPT behavior for M1, which is because of the deficiency of side hydroxyl moiety comparing with M2. This work makes a reasonable affiliation of the ESIPT mechanism for M1 and M2. We wish this paper could facilitate understanding these two novel systems and promote their applications. 相似文献
177.
Paul S. Wesson 《General Relativity and Gravitation》2003,35(1):111-119
Results from 5D induced-matter and membrane theory with null paths are extended to show that a particle obeys the 4D Klein-Gordon equation but with a variable mass. The Dirac equation also follows, but raises concerns about 4D quantization in the two natural 5D gauges, and reopens the question of a Regge-like trajectory for the spin angular momenta and squared masses of gravitationally-dominated systems. 相似文献
178.
Ji-Dong Lou Chun-Ling Gao Li Li Zhi-Gang Fang 《Monatshefte für Chemie / Chemical Monthly》2006,41(10):1071-1074
A new procedure for the selective oxidation of alcohols to the corresponding aldehydes and ketones with potassium permanganate
supported on aluminum silicate at room temperature under solvent-free conditions and shaking is reported. 相似文献
179.
Using the simplest (sinusoidal) corrugation profile in the approximation of absence of losses in the material as an example, we study the regimes of complete transformation of an incident wave to the diffraction lobe by a corrugated metal surface or a corrugated interface of two dielectric media. A numerical-intuitive pattern of appearance and evolution of such regimes with increasing corrugation amplitude is revealed. It is shown that the regimes of complete transformation of an incident wave to the diffraction lobe are possible in both the case of autocollimation and the case of a considerable deviation from it. The examination is based on a numerical method of solving the integral equation by means of a specially created interactive processing system in Visual Fortran. 相似文献
180.
The European Physical Journal E - The isothermal gas adsorption of two hexane isomers (n-hexane and cyclohexane) in the mesopores of MCM-41 silica have been investigated by small angle neutron... 相似文献