全文获取类型
收费全文 | 109343篇 |
免费 | 17828篇 |
国内免费 | 14253篇 |
专业分类
化学 | 76812篇 |
晶体学 | 1327篇 |
力学 | 6281篇 |
综合类 | 867篇 |
数学 | 13516篇 |
物理学 | 42621篇 |
出版年
2024年 | 359篇 |
2023年 | 2130篇 |
2022年 | 3871篇 |
2021年 | 4071篇 |
2020年 | 4354篇 |
2019年 | 4141篇 |
2018年 | 3891篇 |
2017年 | 3602篇 |
2016年 | 5206篇 |
2015年 | 5183篇 |
2014年 | 6394篇 |
2013年 | 8218篇 |
2012年 | 9710篇 |
2011年 | 9899篇 |
2010年 | 7035篇 |
2009年 | 6882篇 |
2008年 | 7198篇 |
2007年 | 6448篇 |
2006年 | 6084篇 |
2005年 | 5090篇 |
2004年 | 3991篇 |
2003年 | 3146篇 |
2002年 | 2920篇 |
2001年 | 2492篇 |
2000年 | 2270篇 |
1999年 | 2294篇 |
1998年 | 1873篇 |
1997年 | 1675篇 |
1996年 | 1654篇 |
1995年 | 1446篇 |
1994年 | 1312篇 |
1993年 | 1070篇 |
1992年 | 972篇 |
1991年 | 826篇 |
1990年 | 710篇 |
1989年 | 563篇 |
1988年 | 424篇 |
1987年 | 384篇 |
1986年 | 378篇 |
1985年 | 298篇 |
1984年 | 223篇 |
1983年 | 183篇 |
1982年 | 142篇 |
1981年 | 94篇 |
1980年 | 70篇 |
1979年 | 36篇 |
1978年 | 27篇 |
1976年 | 23篇 |
1975年 | 23篇 |
1957年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Baker‘s yeast mediated reduction of optically active diketone is described. The two keto groups are efficiently differentiated and the ee value of the recovered material is considerably raised. It affords highly optically active key intermediates efficiently for the synthesis of natural polyhydroxylated agarofuran products. 相似文献
62.
Ming Zhao WANG Guan Liang CAI Ling XIA Jun Jian YAO Hong Yan CHEN Zhao Xing MENG Bo Li LIUDepartment of Chemistry Beijing Normal University Beijing Chemical Defense Institute PLA Beijing 《中国化学快报》2004,15(4):495-497
The crystal structures of two potential tumor imaging agents and therapeutic agents -copper(Ⅱ) complexes with salicylidene-tyrosinato Schiff base and nitrogen-donor chelating Lewis base, [Cu(sal-tyr)(bipy)] 1 and [Cu(sal-tyr)(phen)]2CH3OH 2 are presented. Our work is helpful to get deep understanding of novel 64Cu tumor imaging agents and therapeutic agents. 相似文献
63.
In this paper, we study the stability conditions of the MMAP[K]/G[K]/1/LCFS preemptive repeat queue. We introduce an embedded Markov chain of matrix M/G/1 type with a tree structure and identify conditions for the Markov chain to be ergodic. First, we present three conventional methods for the stability problem of the queueing system of interest. These methods are either computationally demanding or do not provide accurate information for system stability. Then we introduce a novel approach that develops two linear programs whose solutions provide sufficient conditions for stability or instability of the queueing system. The new approach is numerically efficient. The advantages and disadvantages of the methods introduced in this paper are analyzed both theoretically and numerically. 相似文献
64.
The paper discusses the relationship between weights and control vertices of two rational NURBS curves of degree two or three with all weights larger than zero when they represent the same curve parametrically and geometrically, and gives sufficient and necessary conditions for coincidence of two rational NURBS curves in non-degeneracy case. 相似文献
65.
In-time motion adjustment in laser cladding manufacturing process for improving dimensional accuracy and surface finish of the formed part 总被引:2,自引:0,他引:2
This paper presents in-time motion adjustment in laser cladding manufacturing process as a means to improve dimensional accuracy and surface finish of the built part. Defects occurring during laser cladding degrade the part quality such as dimensional accuracy and surface finish. In this paper, in-time motion adjustment strategy was presented to remedy and eliminate defects occurring during laser cladding to improve the dimensional accuracy and surface finish. Based on the relationship between the motion of laser head relative to the growing part and other parameters in effects on clad profile, the laser traverse speed, stand-off distance and laser approach orientation to the existing clad layer were adjusted by instructions from a close-loop control system in real time to remedy and eliminate defects. The results of the experiments verified the effects of in-time motion adjustment on dimensional accuracy and surface finish. 相似文献
66.
LiAlO2 single crvstals doped with Ti at concentration 0.2at.% are grown by the Czochralskl technique with dimensions φ42×55mm. Ti ions in the crystal are quadrivalence proven by comparing the absorption and fluorescence spectra of pure LiAlO2 and Ti: LiAlO2. After air and Li-rich atmosphere annealing, the absorption peaks in the range of 600-800nm disappear. We conclude that 682 and 756nm absorption peaks are attributed to the VLi and Vo absorptions, respectively: The peaks at 716nm and 798nm may stem from the VLi^+ and absorptions. The colour-centre model can be applied to explain the experimental phenomena. Ti^4+-doping produces more lithium vacancies in the LiAlO2 crystal. The intensities of [LiO4] and the associated bonds remain unchanged, which improves the anti-hydrolyzation and thermal stability of LiAlO2 crystals. 相似文献
67.
Jensen and Toft 8 conjectured that every 2‐edge‐connected graph without a K5‐minor has a nowhere zero 4‐flow. Walton and Welsh 19 proved that if a coloopless regular matroid M does not have a minor in {M(K3,3), M*(K5)}, then M admits a nowhere zero 4‐flow. In this note, we prove that if a coloopless regular matroid M does not have a minor in {M(K5), M*(K5)}, then M admits a nowhere zero 4‐flow. Our result implies the Jensen and Toft conjecture. © 2005 Wiley Periodicals, Inc. J Graph Theory 相似文献
68.
Science China Mathematics - We construct an unconditional basis in the Banach space L p(Ω) for p 1 by using the refinement equation and the basic operation of translation and scale, where... 相似文献
69.
基于改进遗传算法的布局优化子问题 总被引:2,自引:0,他引:2
本针对子问题,构造了布局子问题(关于同构布局等价类)的改进遗传算法。将该算法应用于二维布局优化子问题,数值实验表明该算法能够在很好地保持图元的邻接关系的前提下找到子问题的最优解。由于布局优化问题可分解为有限个子问题,所以利用该算法可以找到整个布局优化问题的全局最优解。 相似文献
70.
An all-optical fiber nonlinear Fabry-Perot resonator is proposed. The steady-state model for describing this resonator is developed. The transmission and refection bistabilities are shown numerically. Our results show that the bistability in this system is similar to that in a normal nonlinear Fabry-Perot etalon, except that the characteristics of the bistability can be changed not only by the cavity length but also by the phase shifts of the fiber loop mirrors. 相似文献