全文获取类型
收费全文 | 5443篇 |
免费 | 808篇 |
国内免费 | 327篇 |
专业分类
化学 | 2057篇 |
晶体学 | 66篇 |
力学 | 597篇 |
综合类 | 68篇 |
数学 | 800篇 |
物理学 | 2990篇 |
出版年
2024年 | 11篇 |
2023年 | 36篇 |
2022年 | 123篇 |
2021年 | 151篇 |
2020年 | 160篇 |
2019年 | 136篇 |
2018年 | 134篇 |
2017年 | 191篇 |
2016年 | 223篇 |
2015年 | 168篇 |
2014年 | 285篇 |
2013年 | 456篇 |
2012年 | 285篇 |
2011年 | 323篇 |
2010年 | 279篇 |
2009年 | 339篇 |
2008年 | 382篇 |
2007年 | 368篇 |
2006年 | 339篇 |
2005年 | 270篇 |
2004年 | 228篇 |
2003年 | 228篇 |
2002年 | 166篇 |
2001年 | 163篇 |
2000年 | 131篇 |
1999年 | 161篇 |
1998年 | 120篇 |
1997年 | 100篇 |
1996年 | 79篇 |
1995年 | 95篇 |
1994年 | 55篇 |
1993年 | 53篇 |
1992年 | 52篇 |
1991年 | 40篇 |
1990年 | 30篇 |
1989年 | 21篇 |
1988年 | 33篇 |
1987年 | 16篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 15篇 |
1983年 | 9篇 |
1982年 | 17篇 |
1981年 | 18篇 |
1980年 | 15篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 13篇 |
1974年 | 4篇 |
1973年 | 4篇 |
排序方式: 共有6578条查询结果,搜索用时 484 毫秒
991.
Thomas Schücker 《General Relativity and Gravitation》2009,41(1):67-75
The effect of the cosmological constant on the curvature of light due to an isolated spherical mass is recalculated without
using the lens equation and compared to a lensing cluster.
Unité Mixte de Recherche (UMR 6207) du CNRS et des Universités Aix–Marseille 1 et 2 et Sud Toulon–Var, Laboratoire affilié
à la FRUMAM (FR 2291). 相似文献
992.
Shaukat Mahmood Nek M. Shaikh M.A. Kalyar M. Rafiq N.K. Piracha M.A. Baig 《Journal of Quantitative Spectroscopy & Radiative Transfer》2009,110(17):1840-1850
In the present work emission and absorption spectroscopy have been used to determine the plasma parameters of neon in a hollow cathode discharge lamp. The excitation temperature is determined using the intensity ratio method and Boltzmann's plot method whereas the electron density is determined from the Stark broadening of the spectral lines. The behavior of the optogalvanic signal as a function of laser energy has been studied for three transitions from the 2p53s [1/2]2 metastable state following ΔJ=ΔK=0, ±1 dipole selection rules. The saturation technique has been used to measure the photoionization cross section from three intermediate states 2p53p′ [1/2]1, 2p53p′ [3/2]2 and 2p53p [5/2]3 up to the 2p5 2P1/2 ionization threshold. 相似文献
993.
In three-dimensional (3D) freehand ultrasound (US), reconstructing a set of B-scans into a regular voxel array is the key procedure for consequent visualization and analysis. This paper presents a new adaptive interpolation algorithm for computing the voxel array to suppress speckle noises and enhance contrast. The local statistics of homogeneous regions including mean and variance were measured and the ratio of variance to mean was used as homogeneity criteria. For the computation of each voxel, the interpolation method was adaptively determined with respect to its local statistics. If the neighbouring pixels of a voxel satisfied the homogeneity criterion, its value was computed with an arithmetic mean filter. Otherwise, the voxel was probably locating in an inhomogeneous region and an adaptive distance-weighted (ADW) interpolation method was employed to compute its value. A resolution phantom and a subject’s forearm were reconstructed using the proposed algorithm and two other well-known methods - conventional distance-weighted (DW) and voxel nearest neighbourhood (VNN) interpolations. The comparison results demonstrated that the adaptive interpolation algorithm was able to suppress speckles, preserve edges and enhance contrast effectively for the volume reconstruction. 相似文献
994.
The pure rotational spectrum of ZnS (X1Σ+) has been measured using direct-absorption millimeter/sub-millimeter techniques in the frequency range 372–471 GHz. This study is the first spectroscopic investigation of this molecule. Spectra originating in four zinc isotopologues (64ZnS, 66ZnS, 68ZnS, and 67ZnS) were recorded in natural abundance in the ground vibrational state, and data from the v = 1 state were also measured for the two most abundant zinc species. Spectroscopic constants have been subsequently determined, and equilibrium parameters have been estimated. The equilibrium bond length was calculated to be re 2.0464 Å, which agrees well with theoretical predictions. In contrast, the dissociation energy of DE 3.12 eV calculated for ZnS, assuming a Morse potential, was significantly higher than past experimental and theoretical estimates, suggesting diabatic interaction with other potentials that lower the effective dissociation energy. Although ZnS is isovalent with ZnO, there appear to be subtle differences in bonding between the two species, as suggested by their respective force constants and bond length trends in the 3d series. 相似文献
995.
Hexagonal-pillar shaped pure Fe single crystal whiskers with six {1 1 0} side planes were obtained by means of chemical vapor deposition. Atomically resolved scanning tunneling microscopy images obtained on the {1 1 0} surface showed a quasi-hexagonal atomic array with mesoscopic-range periodic height modulation of about 1/3 of an atomic step. This height modulation was found to be a result of an interference between the quasi-hexagonal top-layer and the sub-surface bcc-Fe{1 1 0} layer. Unit vectors of the mesoscopic-range modulation turned out to be expressed as , where and are the primitive vectors of the two-dimensional atomic array in the top-layer and those in the sub-surface layer, respectively. The two-dimensional density of atoms in the top-layer is slightly higher by 0.46% than that in the sub-surface layer. 相似文献
996.
This study investigates ultra-thin potassium chloride (KCl) films on the Si(1 0 0)-2 × 1 surfaces at near room temperature. The atomic structure and growth mode of this ionic solid film on the covalent bonded semiconductor surface is examined by synchrotron radiation core level photoemission, scanning tunneling microscopy and ab initio calculations. The Si 2p, K 3p and Cl 2p core level spectra together indicate that adsorbed KCl molecules at submonolayer coverage partially dissociate and that KCl overlayers above one monolayer (ML) have similar features in the valance band density of states as those of the bulk KCl crystal. STM results reveal a novel c(4 × 4) structure at 1 ML coverage. Ab initio calculations show that a model that comprises a periodic pyramidal geometry is consistent with experimental results. 相似文献
997.
Madhumita Mukherjee Tanya Das Saswata Bose Chapal Kumar Das Alexander P. Kharitonov Kaushik Banik Tham Nguyen Chung Gunter Mennig 《Macromolecular Symposia》2009,277(1):24-35
Kevlar fiber was fluorinated and oxy-fluorinated directly in presence of undiluted fluorine and fluorine gas mixture and processed with Polycarbonate and LCP at 320 °C under 20 rpm in a twin-screw extruder. The composites were then injection molded into dumbbell shaped specimens under different conditions like various mold temperatures, injection temperatures, injection speeds and mold filling rates. Various physico-chemical characterizations have been performed under definite processing parameter. Orientation of fibers under different injection parameters was evaluated using mold flow simulation technique. Most injection molded or extruded structures however, exhibit non-uniform fiber orientation across the final parts, with a diverging variety of different local fiber orientation states. Distinct skin and core regions were observed in the injection molded parts and it has also been found out that fiber orientation is different in skin and core region for both unmodified and modified derivative, which affects the flow behavior. Processing parameters significantly affect the fiber orientation pattern in the skin and core region for all blended materials. It is worth mentioning that the maximum fiber orientation occurred during the extrusion process at the wall but different extent of fiber orientation is observed during the injection molding depending on the shape of the dumbbell specimen. This fibrillation has been corroborated by the SEM study in both the skin and core region. 相似文献
998.
随机参数结构的统计特征对 总被引:3,自引:1,他引:2
结合非正交多项式展式和传统的摄动技巧研究了随机参数结构的统计特征对问题,建立了和摄动法类似的一系列确定的递推方程,并用有限元方法进行了求解,得到了包括特征值和特征向量的特征对的统计值.最后,用算例验证了此方法的正确性. 相似文献
999.
1000.
一种正交二视角光学层析重建算法 总被引:1,自引:0,他引:1
提出了一种基于最大熵原理的正交二视角重建算法,该算法能够较好地重建多峰非对称待测场,且只需两个正交方向的投影数据。算法中融合了含有轴对称因子的先验知识,该先验知识可由两个正交方向的投影数据迭代算出,并分析了正交投影方向这个因素对重建结果的影响。通过计算机数值模拟,结果表明,融合先验知识的正交二视角重建算法与没有融合先验知识的正交二视角重建算法相比重建精度全面超出。其中,在两峰随机余弦高斯模拟待测场的情况下均方根误差减少了73%。在三峰随机高斯模拟待测场的情况下均方根误差减少了47%。该算法充分显示了在重建多峰非对称待测场时的优越性。同时,由于只需要两个正交方向的投影数据,可使实验系统得到简化。 相似文献