全文获取类型
收费全文 | 157801篇 |
免费 | 2386篇 |
国内免费 | 1137篇 |
专业分类
化学 | 86160篇 |
晶体学 | 2100篇 |
力学 | 6919篇 |
综合类 | 81篇 |
数学 | 15818篇 |
物理学 | 50246篇 |
出版年
2020年 | 1309篇 |
2019年 | 1384篇 |
2018年 | 1620篇 |
2017年 | 1597篇 |
2016年 | 2741篇 |
2015年 | 1830篇 |
2014年 | 2744篇 |
2013年 | 6770篇 |
2012年 | 5371篇 |
2011年 | 6660篇 |
2010年 | 4659篇 |
2009年 | 4601篇 |
2008年 | 6121篇 |
2007年 | 6126篇 |
2006年 | 5723篇 |
2005年 | 5301篇 |
2004年 | 4663篇 |
2003年 | 4061篇 |
2002年 | 4062篇 |
2001年 | 4502篇 |
2000年 | 3443篇 |
1999年 | 2665篇 |
1998年 | 2308篇 |
1997年 | 2239篇 |
1996年 | 2025篇 |
1995年 | 1935篇 |
1994年 | 1916篇 |
1993年 | 1768篇 |
1992年 | 2031篇 |
1991年 | 2153篇 |
1990年 | 1935篇 |
1989年 | 1930篇 |
1988年 | 1869篇 |
1987年 | 1768篇 |
1986年 | 1706篇 |
1985年 | 2205篇 |
1984年 | 2332篇 |
1983年 | 1901篇 |
1982年 | 2083篇 |
1981年 | 1966篇 |
1980年 | 1890篇 |
1979年 | 2033篇 |
1978年 | 2198篇 |
1977年 | 2069篇 |
1976年 | 2112篇 |
1975年 | 2021篇 |
1974年 | 2076篇 |
1973年 | 2061篇 |
1972年 | 1354篇 |
1971年 | 1211篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
61.
62.
Archiv der Mathematik - A generalized Baumslag–Solitar group is a finitely generated group that acts on a tree with infinite cyclic edge and vertex stabilizers. A group G is residually a... 相似文献
63.
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
64.
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
65.
Ortiz Adolfo Yang Jianhua Coccolo Mattia Seoane Jesús M. Sanjuán Miguel A. F. 《Nonlinear dynamics》2020,102(4):2323-2337
Nonlinear Dynamics - The main purpose of this paper is to study both the underdamped and the overdamped dynamics of the nonlinear Helmholtz oscillator with a fractional-order damping. For that... 相似文献
66.
Hejia Hu Linlin Wu Mei Li Cun Xue Guangcheng Wang Siying Chen Yong Huang Lin Zheng Aimin Wang Yueting Li Zipeng Gong 《Biomedical chromatography : BMC》2020,34(1):e4714
Eucommia ulmoides Oliv. (E. ulmoides) is a valuable and nourishing medicinal herb in China that has been used in the treatment of hypertension. Given the fact that most traditional Chinese medicine is mainly used to treat disease, investigating the pharmacokinetics of traditional Chinese medicines in the pathological state is more useful than that in the normal state. However, the differences in the absorption kinetics of active ingredients of E. ulmoides extract between pathological and physiological conditions have not been reported. Therefore, in this study, the rat intestinal in situ circulatory perfusion model was used to investigate the differences in absorption kinetics of seven active ingredients of E. ulmoides extract in normal and spontaneously hypertensive rats, namely, genipinic acid, protocatechuic acid, neochlorogenic acid, chlorogenic acid, cryptochlorogenic acid, (+)-pinoresinol di-O-β-D -glucopyranoside and (+)-pinoresinol 4′-O-β-D -glucopyranoside. Our results indicate that the pathological state of spontaneous hypertension may change the absorption of active components of E. ulmoides extracts, and these findings may provide a reference for improving the rational use of E. ulmoides in the clinic. 相似文献
67.
Umie F.M. Haziz Rosenani A. Haque Shun-Ze Zhan Hassan H. Abdallah Mohd R. Razali 《应用有机金属化学》2020,34(10):e5818
The synthesis and characterizations for a series of dinuclear gold (I)-di-NHC complexes, 1–8 through the trans-metalation method of their respective silver (I)-di-NHC complexes, i–viii are reported (where NHC = N-heterocyclic carbene). The successful complexation of a series of unusual non-symmetrical and symmetrical di-NHC ligands, 3,3'-(ethane-1,2-diyl)-1-alkylbenzimidazolium-1'-butylbenzimidazolium (with alkyl = methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, benzyl) with the gold (I) ions are suggested by elemental analysis, Fourier transform-infrared, 1H- and 13C-NMR data. The 13C-NMR spectra of 1–8 show a singlet sharp peak in the range of 190.00–192.00 ppm, indicating the presence of a carbene carbon that bonded to the gold (I) ion. From single crystal X-ray diffraction data, the structure of complex 6 with the formula of [di-NHC-Au (I)]2·2PF6 is obtained [where NHC = 3,3'-(ethane-1,2-diyl)-1-hexylbenzimidazolium-1'-butylbenzimidazolium]. The photophysical study in solid state of 6 displays an intense photoluminescence with a strong emission maxima, λem = 480 nm, upon excitation at 340 nm at room temperature. Interestingly, the emission maximum at 77 K shows a structural character with a strong peak at 410 nm, a medium at 433 nm and a weak at 387 nm, accompanied by a tail band to about 500 nm. 相似文献
68.
69.
In this work the results of the statistical topometric analysis of fracture surfaces of soda-lime-silica glass with and without ionic exchange treatment are reported. In this case, the mechanism of substitution is K+-Na+. atomic force microscopy (AFM) was employed to record the topometric data from the fracture surface. The roughness exponent (ζ) and the correlation length (ξ) were calculated by the variable bandwidth method. The analysis for both glasses (subjected and non-subjected to ionic exchange) for ζ shows a value ∼0.8, this value agrees well with that reported in the literature for rapid crack propagation in a variety of materials. The correlation length shows different values for each condition. These results, along with those of microhardness indentations suggest that the self-affine correlation length is influenced by the complex interactions of the stress field of microcracks with that resulting from the collective behavior of the point defects introduced by the strengthening mechanism of ionic exchange. 相似文献
70.
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