全文获取类型
收费全文 | 45929篇 |
免费 | 5070篇 |
国内免费 | 5027篇 |
专业分类
化学 | 20561篇 |
晶体学 | 425篇 |
力学 | 1289篇 |
综合类 | 305篇 |
数学 | 2971篇 |
物理学 | 9292篇 |
综合类 | 21183篇 |
出版年
2024年 | 137篇 |
2023年 | 640篇 |
2022年 | 1126篇 |
2021年 | 1284篇 |
2020年 | 1354篇 |
2019年 | 1314篇 |
2018年 | 1133篇 |
2017年 | 1254篇 |
2016年 | 1572篇 |
2015年 | 1880篇 |
2014年 | 2559篇 |
2013年 | 2943篇 |
2012年 | 3428篇 |
2011年 | 3626篇 |
2010年 | 3098篇 |
2009年 | 3075篇 |
2008年 | 3305篇 |
2007年 | 3190篇 |
2006年 | 2818篇 |
2005年 | 2352篇 |
2004年 | 1784篇 |
2003年 | 1529篇 |
2002年 | 1610篇 |
2001年 | 1400篇 |
2000年 | 1190篇 |
1999年 | 1099篇 |
1998年 | 765篇 |
1997年 | 729篇 |
1996年 | 626篇 |
1995年 | 586篇 |
1994年 | 505篇 |
1993年 | 377篇 |
1992年 | 318篇 |
1991年 | 322篇 |
1990年 | 275篇 |
1989年 | 223篇 |
1988年 | 190篇 |
1987年 | 130篇 |
1986年 | 106篇 |
1985年 | 60篇 |
1984年 | 22篇 |
1983年 | 20篇 |
1982年 | 22篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1971年 | 4篇 |
1957年 | 4篇 |
1955年 | 3篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
71.
Yan WANG Rong Quan FENG 《数学学报(英文版)》2005,21(4):773-778
A Cayley map is a Cayley graph embedded in an orientable surface such that. the local rotations at every vertex are identical. In this paper, balanced regular Cayley maps for cyclic groups, dihedral groups, and generalized quaternion groups are classified. 相似文献
72.
对用磷酸脲的结晶母液合成的氮-磷膨高效胀型阻燃剂性能进行了研究,测定了经该阻燃剂处理后的木材的耐火性能和使用性能。实验结果表明,该阻燃剂具有良好的木材阻燃性能,用该阻燃剂处理后的木材各项综合性能均较理想,是一种高效的木材阻燃剂,可以广泛用于木材阻燃。 相似文献
73.
以丙酮/水为溶剂,采用紫外光(UV)引发丙烯酰胺(AM)沉淀聚合,考察了聚合反应的特征以及溶剂、初始单体浓度、引发剂浓度、表面入射光强、液层厚度等参数对聚合物分子量的影响。结果表明,紫外光引发丙烯酰胺沉淀聚合过程中单体转化率和聚合物分子量都随反应时间的延长而增加,光照80min时,单体转化率可达90%以上。所得聚合物分子量为105~106,提高单体浓度可使聚合物分子量增加,增加引发剂浓度和提高光强导致聚合物分子量降低,液层厚度对聚合物的分子量影响不大。所得聚合物粒子粒径约为200~300nm左右,粒径分布较为均匀,溶剂组成对聚合物粒子相貌影响较大,增加水含量可使粒子粒经变大但分散性变差。 相似文献
74.
Wang Qing-Lun Zhao Bin Liao Dai-Zheng Yan Shi-Ping Cheng Peng Jiang Zong-Hui 《Transition Metal Chemistry》2003,28(3):326-330
The copper(II) complex Na2[Cu(pba)] · 6H2O has been synthesized and its structure determined. It consists of a novel two-dimensional network of [Cu(pba)]2– anions connected to each other through the sodium ion as a bridge. Spectroscopic and magnetic properties have been measured and a spin distribution calculation has been carried out with the GAUSSIAN-94. Theoretical calculations indicate the rather larger spin density on nitrogen atoms should be responsible for the satellite lines appearing beside the main hyperfine coupling signs of copper(II). The antiferromagnetic interaction may be due to the intermolecular interaction and/or different copper(II) ions through the Na+ bridge in the same layer. 相似文献
75.
76.
Ch Yan Poon 《Tetrahedron letters》2004,45(14):2985-2988
An efficient synthesis of the unique indolizino[7,6-c]quinoline carboskeleton of isaindigotidione has been achieved. This strategy employed l-proline and isatin as the main building blocks in the construction of the framework. Four transformations occurred in a one-pot operation to furnish the tetracyclic nucleus. 相似文献
77.
Hypersonic longitudinal sound velocities in five silicate and alumino-silicate liquids have been measured between 293 and 2550 K by Brillouin spectroscopy. Together with previous observations for four other glasses and liquids of the system SiO2-Al2O3-CaO-MgO, these results are used to discuss changes in hypersonic velocities in three adjacent temperature domains, i.e., below, in, and above the glass transformation range. The temperature dependence of Brillouin velocities is consistent with the observed variations with temperature of viscosity, density, and mean heat capacity for the same three temperature domains. These variations of physical properties of alumino-silicate liquids are qualitatively in agreement with the Inherent Structure Theory for liquids. 相似文献
78.
Density functional method (DFT) (B3p86) of Gaussian98 has been used to optimize the structure of the Tc_2 molecule. The result shows that the ground state for Tc_2 molecule is an 11-multiple state and its electronic configuration is {}^{11}Σ_g^-, which shows the spin polarization effect of Tc_2 molecule of a transition metal element for the first time. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state does not mingle with wavefunctions of higher energy states. So, that the ground state for Tc_2 molecule is an 11-multiple state is indicative of the spin polarization effect of Tc_2 molecule of a transition metal element: that is, there exist 10 parallel spin electrons. The non-conjugated electron is greatest in number. These electrons occupy different spacious tracks, so that the energy of Tc_2 molecule is minimized. It can be concluded that the effect of parallel spin of the Tc_2 molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell--Sorbie potential functions with the parameters for the ground state {}^{11}Σ_g^- and other states of Tc_2 molecule are derived. Dissociation energy D_e for the ground state of T_{c2} molecule is 2.266eV, equilibrium bond length R_e is 0.2841nm, vibration frequency ω_e is 178.52cm^{-1}. Its force constants f_2, f_3, and f_4 are 0.9200aJ·nm^{-2}, --3.5700aJ·nm^{-3}, 11.2748aJ·nm^{-4} respectively. The other spectroscopic data for the ground state of Tc_2 molecule ω_eχ_e, B_e, α_e are 0.5523cm^{-1}, 0.0426cm^{-1}, 1.6331×10^{-4}cm^{-1} respectively. 相似文献
79.
80.
不同维生素组合对南美白对虾生长性能的影响 总被引:1,自引:0,他引:1
选取我国海水养殖种类中的南美白对虾为研究对象,采用均匀设计法,在相同基础日粮中,分别添加不同配比的维生素组合,其中有VE(X1)、烟酸(X2)、肌醇(X3)、泛酸钙(X4)、VB6(X5),经过45 d的试验,以相对体重和相对体长为指标,建立了多元二次多项式回归模型,并利用SAS软件在计算机上拟合,用求极值的方法给出南美白对虾不同维生素组合的最佳配方. 相似文献