全文获取类型
收费全文 | 32123篇 |
免费 | 2231篇 |
国内免费 | 5039篇 |
专业分类
化学 | 26712篇 |
晶体学 | 548篇 |
力学 | 193篇 |
综合类 | 289篇 |
数学 | 6952篇 |
物理学 | 4699篇 |
出版年
2024年 | 47篇 |
2023年 | 323篇 |
2022年 | 520篇 |
2021年 | 649篇 |
2020年 | 800篇 |
2019年 | 1773篇 |
2018年 | 924篇 |
2017年 | 1718篇 |
2016年 | 1066篇 |
2015年 | 938篇 |
2014年 | 1255篇 |
2013年 | 2734篇 |
2012年 | 1840篇 |
2011年 | 2055篇 |
2010年 | 1557篇 |
2009年 | 1960篇 |
2008年 | 2145篇 |
2007年 | 2093篇 |
2006年 | 2031篇 |
2005年 | 1755篇 |
2004年 | 1710篇 |
2003年 | 1385篇 |
2002年 | 1203篇 |
2001年 | 1020篇 |
2000年 | 982篇 |
1999年 | 768篇 |
1998年 | 711篇 |
1997年 | 586篇 |
1996年 | 472篇 |
1995年 | 452篇 |
1994年 | 322篇 |
1993年 | 287篇 |
1992年 | 279篇 |
1991年 | 198篇 |
1990年 | 125篇 |
1989年 | 115篇 |
1988年 | 96篇 |
1987年 | 56篇 |
1986年 | 49篇 |
1985年 | 62篇 |
1984年 | 49篇 |
1983年 | 23篇 |
1982年 | 51篇 |
1981年 | 45篇 |
1980年 | 26篇 |
1979年 | 45篇 |
1978年 | 19篇 |
1977年 | 20篇 |
1976年 | 16篇 |
1975年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Zhijun Yi Tingyu LiuQiren Zhang Yuanyuan Sun 《Journal of Electron Spectroscopy and Related Phenomena》2006
The electronic structures of PbWO4 crystals containing F type color centers with the lattice structure optimized are studied within the framework of the fully relativistic self-consistent Direc–Slater theory, using a numerically discrete variational (DV-Xα) method. The calculated results show that F and F+ centers have donor energy level in forbidden band. Their optical transition energy are 1.84 eV, 2.21 eV, respectively, which corresponds to the 680 nm, 550 nm absorption bands. It predicts that the 680 nm, 550 nm absorption bands originate form the F and F+ centers in PbWO4 crystals. 相似文献
122.
Valentin Brimkov 《Discrete Applied Mathematics》2007,155(14):1812-1825
We consider the problem of computing the Lovász theta function for circulant graphs Cn,J of degree four with n vertices and chord length J, 2?J?n. We present an algorithm that takes O(J) operations if J is an odd number, and O(n/J) operations if J is even. On the considered class of graphs our algorithm strongly outperforms the known algorithms for theta function computation. We also provide explicit formulas for the important special cases J=2 and J=3. 相似文献
123.
Jin Hu Gang Liu Jixiu Zhang Lingchao Kong Weidong Fei 《Applied Surface Science》2006,253(5):2792-2795
The microstructure of a laser treated Al18B4O33w/2024Al composite has been investigated using transmission electron microscope (TEM), low-angle (glancing angle) X-ray diffraction (XRD) techniques. Various surface microstructures were observed in the laser treated composite. The Al18B4O33 whisker on the surface of the composite was decomposed during laser surface melting, various decomposition products were studied in the laser treated composite. Eutectic phases and the precipitation in the matrix of the composite with laser-treated were observed. The main phases detected in the molten zone were aluminum and decomposition products Al2O3. The effect of laser treatment on the hardness of the composite was also examined. A surface hardness of 400 Hv was noted. 相似文献
124.
125.
Hongrui Zhang Mingju Chao Mingyi Gao Liwen Zhang Jianquan Yao 《Optics & Laser Technology》2003,35(6):573-449
A fiber-coupled diode-single-end-pumped Nd:YVO4 laser with an Nd:YVO4 crystal of 0.3 at% doping concentration and 3×3×10 mm3 dimensions was reported. 14.850 W of continuous-wave output power in an M2 factor of 1.12 was obtained under pump power of 27.365 W, with an optical conversion efficiency of 60.49%, and a slope efficiency of 64.5%. 相似文献
126.
127.
Using Lagrange's multiplier rule, we find upper and lower bounds of the energy of a bipartite graph G, in terms of the number of vertices, edges and the spectral moment of fourth order. Moreover, the upper bound is attained in a graph G if and only if G is the graph of a symmetric balanced incomplete block design (BIBD). Also, we determine the graphs for which the lower bound is sharp. 相似文献
128.
To find an effective extraction and removal method for palladium(II), which is one of the main fission products from an acidic nuclear spent fuel solution, the extraction behavior of palladium(II) from a nitric acid medium by an acidic chelating extractant, 1-phenyl-3-methyl-4-trifluoroacetylpyrazolone-5-one (HPMTP) and a tertiary amine of high molecular weight, tri-n-octylamine (TOA), has been studied by spectrophotometry. A noticeable antagonistic extraction effect was observed in the extraction system under the given conditions. To understand this phenomenon, a preliminary investigation was performed to explain the mechanism of this reaction. According to the theory of corresponding solutions (TCS), the association reaction between HPMTP and TOA is proposed in the organic phase. An associated species, HPMTP·TOA, formed through hydrogen bonding in a chloroform medium might be the main reason why an antagonistic extraction effect occurred. The association constant between HPMTP and TOA was calculated to be 2.86±0.05. 相似文献
129.
An m‐covering of a graph G is a spanning subgraph of G with maximum degree at most m. In this paper, we shall show that every 3‐connected graph on a surface with Euler genus k ≥ 2 with sufficiently large representativity has a 2‐connected 7‐covering with at most 6k ? 12 vertices of degree 7. We also construct, for every surface F2 with Euler genus k ≥ 2, a 3‐connected graph G on F2 with arbitrarily large representativity each of whose 2‐connected 7‐coverings contains at least 6k ? 12 vertices of degree 7. © 2003 Wiley Periodicals, Inc. J Graph Theory 43: 26–36, 2003 相似文献
130.