全文获取类型
收费全文 | 18030篇 |
免费 | 2632篇 |
国内免费 | 3969篇 |
专业分类
化学 | 19549篇 |
晶体学 | 205篇 |
力学 | 605篇 |
综合类 | 141篇 |
数学 | 741篇 |
物理学 | 3390篇 |
出版年
2024年 | 46篇 |
2023年 | 269篇 |
2022年 | 488篇 |
2021年 | 707篇 |
2020年 | 946篇 |
2019年 | 767篇 |
2018年 | 644篇 |
2017年 | 767篇 |
2016年 | 960篇 |
2015年 | 874篇 |
2014年 | 1011篇 |
2013年 | 1622篇 |
2012年 | 1555篇 |
2011年 | 1249篇 |
2010年 | 1041篇 |
2009年 | 1151篇 |
2008年 | 1221篇 |
2007年 | 1222篇 |
2006年 | 1095篇 |
2005年 | 1022篇 |
2004年 | 1005篇 |
2003年 | 798篇 |
2002年 | 585篇 |
2001年 | 490篇 |
2000年 | 425篇 |
1999年 | 376篇 |
1998年 | 349篇 |
1997年 | 305篇 |
1996年 | 237篇 |
1995年 | 251篇 |
1994年 | 222篇 |
1993年 | 211篇 |
1992年 | 149篇 |
1991年 | 139篇 |
1990年 | 121篇 |
1989年 | 68篇 |
1988年 | 54篇 |
1987年 | 28篇 |
1986年 | 27篇 |
1985年 | 19篇 |
1984年 | 23篇 |
1983年 | 17篇 |
1982年 | 23篇 |
1981年 | 19篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1972年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Hiroshi Sugimoto Hiromitsu Ohshima Shohei Inoue 《Journal of polymer science. Part A, Polymer chemistry》2003,41(22):3549-3555
The first successful example of the formation of polycarbonate from 1-atm carbon dioxide and epoxide was demonstrated by the alternating copolymerization of carbon dioxide and epoxide with manganese porphyrin as a catalyst. The copolymerization of carbon dioxide and cyclohexene oxide with (porphinato)manganese acetate proceeded under the 1-atm pressure of carbon dioxide to give a copolymer with an alternating sequence. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3549–3555, 2003 相似文献
102.
103.
Chunlin Li Yilu Fu Guozhu Bian Tiandou Hu Yaning Xie Jing ZhanDepartment of Chemical Physics University of Science and Technology of China Hefei China Institute of High Energy Physics Chinese Academy of Sciences Beijing China 《天然气化学杂志》2003,12(3)
The Ni/CeO2-ZrO2-Al2O3 catalyst with different Al2O3 and NiO contents were prepared by hydrothermal synthesis method. The catalytic performance for CO2 reforming of CH4 reaction, the interaction among components and the relation between Ni content and catalyst surface basicity were investigated. Results show that the interaction between NiO and Al2O3 is stronger than that between NiO and CeO2-ZrO2.The addition of Al2O3 can prevent the formation of large metallic Ni ensembles, increase the dispersion of Ni, and improve catalytic activity, but excess Al2O3 causes the catalyst to deactivate easily. The interaction between NiO and CeO2 results in more facile reduction of surface CeO2. The existence of a small amount of metallic Ni can increase the number of basic sites. As metallic Ni may preferentially reside on the strong basic sites, increasing Ni content can weaken the catalyst basicity. 相似文献
104.
105.
Multiwalled carbon nanotubes were modified by carboxy groups. Four independent methods for the determination of the degree
of functionalization of the surface were proposed: 13C NMR spectroscopy, thermogravimetry, titrimetry, and fluorimetry. The first two methods show the total content of carboxy
groups in the sample, and the latter two methods give information about the content of the surface groups only.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 291–295, February, 2008. 相似文献
106.
U.Deva Priyakumar 《Tetrahedron letters》2004,45(12):2495-2498
A novel preference for planar tetracoordination was observed over the conventional tetrahedral arrangement in a new series of C5H2, C5H4, C5H41+/2+ and related compounds. The stability of these molecules is assessed with the ring-opening barriers, HOMO-LUMO gap, singlet-triplet energy differences and nucleus independent chemical shift values. 相似文献
107.
第二类变型Bessel函数Kn(z)在自变量趋于无穷时就是指数变小的,使用多项式逼近的方法求解往往误差很大.采用指数变换和J.P.Boyd的有理Chebyshev多项式计算第二类变型Bessel函数,得到了令人满意的在较大范围内有效的解. 相似文献
108.
In this paper, we consider hypothesis testing problems in which the involved samples are drawn from generalized multivariate modified Bessel populations. This is a much more general distribution that includes both the multivariate normal and multivariate-t distributions as special cases. We derive the distribution of the Hotelling's T2-statistic for both the one- and two-sample problems, as well as the distribution of the Scheffe's T2-statistic for the Behrens–Fisher problem. In all cases, the non-null distribution of the corresponding F-statistic follows a new distribution which we introduce as the non-central F-Bessel distribution. Some statistical properties of this distribution are studied. Furthermore, this distribution was utilized to perform some power calculations for tests of means for different models which are special cases of the generalized multivariate modified Bessel distribution, and the results compared with those obtained under the multivariate normal case. Under the null hypothesis, however, the non-central F-Bessel distribution reduces to the central F-distribution obtained under the classical normal model. 相似文献
109.
Yirang Yuan 《Numerical Methods for Partial Differential Equations》2003,19(5):665-681
For the coupled system of multilayer fluid dynamics in porous media, the modified characteristic finite difference fractional steps method applicable to parallel arithmetic is put forward and two‐dimensional and three‐dimensional schemes are used to form a complete set. Some techniques, such as calculus of variations, energy method, piecewise biquadratic interpolation, multiplicative commutation rule of difference operators, decomposition of high order difference operators and prior estimates are adopted. Optimal order estimates in L2 norm are derived to determine the error in the approximate solution. This method has already been applied to the numerical simulation of multilayer fluid dynamics in porous media. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 665–681, 2003. 相似文献
110.