全文获取类型
收费全文 | 49289篇 |
免费 | 5248篇 |
国内免费 | 8105篇 |
专业分类
化学 | 38443篇 |
晶体学 | 1980篇 |
力学 | 2914篇 |
综合类 | 428篇 |
数学 | 4135篇 |
物理学 | 14742篇 |
出版年
2024年 | 84篇 |
2023年 | 418篇 |
2022年 | 783篇 |
2021年 | 990篇 |
2020年 | 1341篇 |
2019年 | 1285篇 |
2018年 | 1096篇 |
2017年 | 1344篇 |
2016年 | 1855篇 |
2015年 | 1748篇 |
2014年 | 2223篇 |
2013年 | 3508篇 |
2012年 | 3801篇 |
2011年 | 2832篇 |
2010年 | 2417篇 |
2009年 | 3065篇 |
2008年 | 3261篇 |
2007年 | 3364篇 |
2006年 | 3184篇 |
2005年 | 2805篇 |
2004年 | 2738篇 |
2003年 | 2364篇 |
2002年 | 2491篇 |
2001年 | 1628篇 |
2000年 | 1704篇 |
1999年 | 1361篇 |
1998年 | 1227篇 |
1997年 | 1007篇 |
1996年 | 957篇 |
1995年 | 837篇 |
1994年 | 792篇 |
1993年 | 613篇 |
1992年 | 652篇 |
1991年 | 394篇 |
1990年 | 318篇 |
1989年 | 256篇 |
1988年 | 255篇 |
1987年 | 178篇 |
1986年 | 151篇 |
1985年 | 231篇 |
1984年 | 215篇 |
1983年 | 134篇 |
1982年 | 168篇 |
1981年 | 138篇 |
1980年 | 85篇 |
1979年 | 80篇 |
1978年 | 63篇 |
1977年 | 48篇 |
1976年 | 32篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 765 毫秒
111.
区域经济的发展受许多不同因素的影响,本结合西藏社会经济发展的特点,选取了总产值指标、第一产业指标、第二产业指标和第三产业指标等四大类指标体系,采用灰色关联分析法对西藏的区域经济系统进行了简要的分析评价,以研究西藏区域经济的发展状况,从社会经济的角度探讨西藏经济发展落后的原因,以期能为西藏经济发展政策提供参考依据。 相似文献
112.
M/G/1非空竭服务休假排队系统随机分解的简化算法 总被引:2,自引:0,他引:2
本文根据M/G/1非空竭服务休假排队系统稳态队长随机分解的结构特征提出一种统一算法,该方法简洁高效,避免了再生循环方法繁杂的运算。运用该方法得出的结果与已知的用再生循环方法得出的结论一致。并且修正了Levy(1989)关于Bernoulli闸门服务休假排队系统随机分解的一个错误。 相似文献
113.
114.
Early stages of rare-earth metal (Yb and Eu) growth on a vicinal, single-domain Si(1 0 0)2 × 1 surface have been studied in the coverage range of 0.1-0.3 monolayer (ML) by low energy electron diffraction, scanning tunneling microscopy, and synchrotron radiation photoemission spectroscopy. We show that Yb induces the 2 × 3 periodicity in the whole range of coverage studied. The 2 × 3 reconstruction coexists with the local 3 × 2/4 × 2 structure at about 0.2 ML of Yb. In contrast, Eu forms the 3 × 2 periodicity at 0.1-0.2 ML, whereas this structure is converted into the 2 × 3 phase at about 0.3 ML. The atomic arrangement and electronic properties of these reconstructions and the adsorbate-mediated modification of surface morphology are investigated. 相似文献
115.
Using the DFT/B3LYP method with a 6-31G(d) basis set, the structure, normal vibration frequencies, and the absolute band intensities in the IR spectra of the chlorin molecule and its four symmetric isotopomers have been calculated. Scaling of the force field by the Pulay method in independent and natural coordinates has been carried out. A method for obtaining effective force fields without using experimental data on the fundamental vibration frequencies is proposed. By comparing the vibration modes and constructing special matrices, complete assignment of the fundamental frequencies of porphin and chlorin has been carried out. It has been shown that the majority of porphin macroring vibrations upon pyrrolenine ring hydrogenation are frequency-characteristic and only 12 vibrations change considerably. A frequency correlation with regard for the mode transition between chlorin and all its isotopomers under consideration has been established. Comparative analysis of the force fields of porphin and chlorin in dependent natural coordinates has revealed the unique nonlocal character of the change in force constants of the macroring upon hydrogenation of one pyrrolenine ring. Modeling of the IR spectra of chlorin and its isotopomers has been performed. Assignment and interpretation of the normal vibrations of the molecules under consideration have been carried out. 相似文献
116.
Based on the Fourier transform method, a simple closed-form expression for the on-axis power spectrum of ultrashort Gaussian pulsed beams in diffraction at a circular aperture is derived, which permits us to study spectral changes both analytically and numerically. It is shown that for diffracted pulsed beams there exist spectral red and blue shifts, spectral narrowing, and spectral switches in the near field. The aperture diffraction plays an important role in spectral switching, but both the truncation parameter and bandwidth (or equally, Fourier transform limited pulse duration) affect the behavior of spectral switches. 相似文献
117.
Kazuaki Okamoto Suprakas Sinha Ray Masami Okamoto 《Journal of polymer science. Part A, Polymer chemistry》2003,41(24):3160-3172
A series of new poly(butylene succinate) (PBS)/layered silicate nanocomposites were prepared successfully by simple melt extrusion of PBS and organically modified layered silicates (OMLS). Three different types of OMLS were used for the preparation of nanocomposites: two functionalized ammonium salts modified montmorillonite and a phosphonium salt modified saponite. The structure of the nanocomposites in the nanometer scale was characterized with wide-angle X-ray diffraction and transmission electron microscopic observations. With three different types of layered silicates modified with three different types of surfactants, the effect of OMLS in nanocomposites was investigated by focusing on four major aspects: structural analysis, materials properties, melt rheological behavior, and biodegradability. Interestingly, all these nanocomposites exhibited concurrent improvements of material properties when compared with pure PBS. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3160–3172, 2003 相似文献
118.
119.
通过用毛细管测量纯水表面张力系数实验,得到管内径d与接触角θ的相对误差之间的关系曲线,由此说明实验测得的表面张力系数α和理论公式相比呈现一定的偏差,并简单地讨论了实验的理想状态与实际情况的差别. 相似文献
120.
The title compound, [Cd(SeCN)2(pyCN)2]∞, adopts an extended one‐dimensional chain structure in which the neighboring cadmium atoms are bridged by two selenocyanate ions. The central cadmium atom has a distorted octahedral geometry defined by two isonicotinonitrile and four selenocyanate ions in a 4N2Se fashion. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献