全文获取类型
收费全文 | 35874篇 |
免费 | 6161篇 |
国内免费 | 4914篇 |
专业分类
化学 | 26681篇 |
晶体学 | 504篇 |
力学 | 1919篇 |
综合类 | 375篇 |
数学 | 4106篇 |
物理学 | 13364篇 |
出版年
2024年 | 84篇 |
2023年 | 680篇 |
2022年 | 993篇 |
2021年 | 1255篇 |
2020年 | 1428篇 |
2019年 | 1477篇 |
2018年 | 1244篇 |
2017年 | 1270篇 |
2016年 | 1733篇 |
2015年 | 1709篇 |
2014年 | 2123篇 |
2013年 | 2729篇 |
2012年 | 3280篇 |
2011年 | 3468篇 |
2010年 | 2541篇 |
2009年 | 2465篇 |
2008年 | 2578篇 |
2007年 | 2331篇 |
2006年 | 2179篇 |
2005年 | 1750篇 |
2004年 | 1308篇 |
2003年 | 1054篇 |
2002年 | 992篇 |
2001年 | 851篇 |
2000年 | 782篇 |
1999年 | 678篇 |
1998年 | 555篇 |
1997年 | 500篇 |
1996年 | 409篇 |
1995年 | 446篇 |
1994年 | 390篇 |
1993年 | 289篇 |
1992年 | 244篇 |
1991年 | 219篇 |
1990年 | 210篇 |
1989年 | 153篇 |
1988年 | 116篇 |
1987年 | 85篇 |
1986年 | 95篇 |
1985年 | 70篇 |
1984年 | 35篇 |
1983年 | 39篇 |
1982年 | 30篇 |
1981年 | 12篇 |
1980年 | 16篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1971年 | 5篇 |
1957年 | 7篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
71.
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
72.
Haibo Peng Boyu Wu Qiong Song Fei Lu Shuyun Luo 《International Journal of Infrared and Millimeter Waves》2003,24(9):1587-1596
Hybrid electrode combined coplanar waveguide (CPW) and microstrip lines in ultra-broadband electro-optic modulators. The characteristics of the modulators with hybrid electrode is like that of the modulator with the microstrip lines, in which the microstrip electrode is loaded above only one arm of Mach-Zehnder (M-Z) optical waveguide, besides the problem of microstrip line to coaxial transition and corona polarization in polymer modulators have been best solved at the same time. By using finite element method (FEM), the characteristic parameters such as half-wave voltage, modulation bandwidth characteristic impedance, effective microwave refractive index of polymer modulators with microstrip line and hybrid electrode are analysed and compared in this paper. The results present that velocity match, impendence match and lower microwave propagating loss is easily realized in modulators with hybrid electrode. 相似文献
73.
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
74.
Liuman Ou Guilie Luo Yiping Li 《Journal of Mathematical Analysis and Applications》2003,283(2):534-548
In this paper, we consider an autonomous predator-prey Lotka-Volterra system in which individuals in the population may belong to one of two classes: the immatures and the matures, the age to maturity is represented by a time delay. By using eigenvalue analysis, principal term analyze method, reduction to absurdity, and iterative method, we obtain some simple conditions for global asymptotic stability of the unique positive equilibrium point. Moreover, a condition that the prey population in the system get extinction and the predator population in the system get permanence will be obtained. Meanwhile the theorems extend the corresponding conclusions in which there have no two stage structures. 相似文献
75.
Pengbo Jiang Tiexin Cheng Yan Liu Xianghao Cui Yingli Bi Kaiji Zhen 《Reaction Kinetics and Catalysis Letters》2004,82(1):49-56
The novel phase transfer catalysts S-8 [4-(dimethyloctylammonium) propansultan] and DB-X [1,4-bis(triethylmethylammonium)benzene dibromide] were synthesized and employed for high conversion synthesis of dichlorocyclopropane from various olefins.This revised version was published online in December 2005 with corrections to the Cover Date. 相似文献
76.
77.
This Note addresses the derivation of an improved On-Surface Radiation Condition for the numerical solution of the exterior Helmholtz equation at high-frequencies. This condition is built as an approximation of the Neumann-to-Dirichlet map by using a local regularization of its principal classical symbol in the gliding zone for modelling the creeping waves. The numerical simulation of this pseudodifferential operator is efficiently realized with a linear cost according to the dimension of the boundary element approximation space using suitable complex Padé approximants. A numerical example is provided. To cite this article: X. Antoine et al., C. R. Acad. Sci. Paris, Ser. I 340 (2005). 相似文献
78.
从现有的PCDFs分子的正辛醇 /水分配系数 (logKow)实验数据出发 ,建立定量结构 性质关系方程(QSPR) .采用G98W程序包中的PM3方法对 13 5个多氯代二苯并呋喃 (PCDFs)分子和二苯并呋喃进行了优化计算 ,作业命令为 #pPM3optfreqscf(conver =9) ,以计算所得的分子轨道能量、碳原子电荷作为PCDFs分子结构描述符 ,运用多元线性回归技术建立了PCDFs的logKow与分子结构描述符的四元方程 ,最优相关系数为 0 .95 0 7,标准偏差为 0 .173 7,经检验该模型的稳健性好 ,并对未有实验数据的 85个PCDFs的logKow进行预测 相似文献
79.
L. C. Lew Yan Voon C. Galeriu M. Willatzen 《Physica E: Low-dimensional Systems and Nanostructures》2003,18(4):547-549
We argue that the two-dimensional elliptic quantum dot problem with finite barrier cannot be exactly solved, contrary to a recent assertion (van den Broek and Peeters, Physica E 11 (2001) 345. We also prove it explicitly by numerically calculating the correct energy spectrum. 相似文献
80.