全文获取类型
收费全文 | 75480篇 |
免费 | 8242篇 |
国内免费 | 8754篇 |
专业分类
化学 | 37284篇 |
晶体学 | 2048篇 |
力学 | 10092篇 |
综合类 | 848篇 |
数学 | 24090篇 |
物理学 | 18114篇 |
出版年
2024年 | 77篇 |
2023年 | 876篇 |
2022年 | 1035篇 |
2021年 | 1887篇 |
2020年 | 2147篇 |
2019年 | 1968篇 |
2018年 | 1900篇 |
2017年 | 2297篇 |
2016年 | 2691篇 |
2015年 | 2393篇 |
2014年 | 3719篇 |
2013年 | 5694篇 |
2012年 | 4500篇 |
2011年 | 5173篇 |
2010年 | 4581篇 |
2009年 | 5088篇 |
2008年 | 5009篇 |
2007年 | 4976篇 |
2006年 | 4518篇 |
2005年 | 3994篇 |
2004年 | 3595篇 |
2003年 | 3212篇 |
2002年 | 2773篇 |
2001年 | 2252篇 |
2000年 | 2135篇 |
1999年 | 1883篇 |
1998年 | 1756篇 |
1997年 | 1444篇 |
1996年 | 1130篇 |
1995年 | 1149篇 |
1994年 | 1038篇 |
1993年 | 862篇 |
1992年 | 850篇 |
1991年 | 627篇 |
1990年 | 472篇 |
1989年 | 417篇 |
1988年 | 370篇 |
1987年 | 257篇 |
1986年 | 198篇 |
1985年 | 247篇 |
1984年 | 247篇 |
1983年 | 90篇 |
1982年 | 168篇 |
1981年 | 152篇 |
1980年 | 114篇 |
1979年 | 124篇 |
1978年 | 99篇 |
1977年 | 90篇 |
1976年 | 56篇 |
1975年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 463 毫秒
51.
52.
Rong Yin Jihui Zhang Xudong Shang 《Mathematical Methods in the Applied Sciences》2020,43(15):8736-8752
This paper is dedicated to studying the following Schrödinger–Poisson system Under some different assumptions on functions V(x), K(x), a(x) and f(u), by using the variational approach, we establish the existence of positive ground state solutions. 相似文献
53.
高能重带电粒子能直接穿透靶原子核外电子层,与原子核发生直接碰撞,发生散裂反应,产生一系列具有放射性的剩余产物核.重带电粒子诱发靶材放射性剩余核与辐射防护和人员安全有着密切联系,当前,大部分剩余核产额主要依靠蒙特卡罗粒子输运程序进行模拟计算,其准确程度亟需通过实验测量进行准确评估.本文利用能量为80.5 MeV/u的(12)^C6+粒子对薄铜靶开展了辐照实验与伽玛射线测量,结合伽玛谱学分析方法,得出了辐照产生的18种放射性剩余产物的初始活度和产生截面值,并与PHITS模拟结果进行对比.结果表明,PHITS模拟程序对放射性剩余核种类的估计具有较高可靠性,在其绝对产额方面,与实验测量仍具有较大偏差. 相似文献
54.
Flow physics of transvalvular flows in the aorta with bioprosthetic valves are investigated using computational modelling. For the efficient simulations of flow-structure-interaction in transvalvular flows, a simplified, reduced degree of freedom valve model is employed with a sharp interface immersed boundary based incompressible flow solver. Simulations are performed for normal as well as abnormal valves with reduced leaflet motion that models the effect of early leaflet thrombosis. The structure of the aortic jet and the hemodynamic stresses on the aortic wall are analysed to understand the hemodynamic impacts and possible long-term clinical implications of sub-clinical, reduced leaflet motion. The simulation results have shown that the reduced leaflet motion tilts the direction of aortic jet and generates stronger flow separation and re-attachment on the aortic wall downstream from the reduced motion leaflets. The modified flow pattern increases the wall pressure fluctuation and average wall shear stress on the downstream aortic wall, and results in the asymmetric oscillatory shear index distributions, which may have long-term clinical implications such as aortic wall damage and remodelling. 相似文献
55.
Gas-phase 21Ne nuclear magnetic resonance spectra were measured at the natural abundance of 21Ne isotope for samples consisting of pressurized neon up to 60 bar at room temperature and applying the magnetic field of the strength B0 = 11.7574 T. It showed that the nuclear magnetic resonance frequency is linearly dependent on the density of gaseous neon. The resonance frequency was extrapolated to the zero-density point, and it permitted the determination of the 21Ne nuclear magnetic moment, μ(21Ne) = 0.6617774(10) μN. The present value of μ(21Ne) is not influenced by the bulk magnetic susceptibility of neon and interactions between neon atoms; therefore, it is more precise and reliable than the previous result obtained for μ(21Ne). 相似文献
56.
Michiel Janssen Prof. Dirk E. De Vos 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(45):10724-10734
The practical application of Shilov-type Pt catalysis to the selective hydroxylation of terminal aliphatic C−H bonds remains a formidable challenge, due to difficulties in replacing PtIV with a more economically viable oxidant, particularly O2. We report the potential of employing FeCl2 as a suitable redox mediator to overcome the kinetic hurdles related to the direct use of O2 in the Pt reoxidation. For the selective conversion of butyric acid to γ-hydroxybutyric acid (GHB), a significantly enhanced catalyst activity and stability (turnover numbers (TON)>30) were achieved under 20 bar O2 in comparison to current state-of-the-art systems (TON<10). In this regard, essential reaction parameters affecting the overall activity were identified, along with specific additives to attain catalyst stability at longer reaction times. Notably, deactivation by reduction to Pt0 was prevented by the addition of monodentate pyridine derivatives, such as 2-fluoropyridine, but also by introducing varying partial pressures of N2 in the gaseous atmosphere. Finally, stability tests revealed the involvement of PtII and FeCl2 in catalyzing the non-selective overoxidation of GHB. Accordingly, in situ esterification with boric acid proved to be a suitable strategy to maintain enhanced selectivities at much higher conversions (TON>60). Altogether, a useful catalytic system for the selective hydroxylation of primary aliphatic C−H bonds with O2 is presented. 相似文献
57.
裂纹前端的断裂过程区是引起岩石非线性断裂及尺寸效应的主要原因。利用数字图像相关技术对砂岩开展了三点弯曲梁实验,获得观测区域高精度的全场位移和应变数据,根据断裂韧带区域水平位移和水平应变的分布特征,结合裂尖岩石颗粒变化的微观分析,提出采用裂纹尖端水平位移波动性和水平应变突变性所得到的波动系数和水平应变突变值,确定断裂过程区形状和临界尺寸的方法。结果表明:砂岩断裂过程区的形状为不规则的狭长带状区域,断裂过程区的临界长度为11~13mm,临界宽度为1.58~2.36mm。断裂过程区区域内形变在趋向裂尖时呈指数增加,但其单位区域内的形变增量呈波动状态。该方法能够更加准确判断岩石断裂过程区的范围,有助于分析岩石的非线性断裂特性。 相似文献
58.
对石英音叉增强型光声光谱(QEPAS)系统中常用的石英音叉进行了有限元模态计算,获得石英音叉前6阶振型与模态频率,认知了第4阶对称摆动振型为有效振动,利用单因素法分析了石英音叉的音臂长度l1、音臂宽度w1、音臂厚度t、音臂切角θ、音臂圆孔直径d及音臂圆孔高度h对低阶有效共振频率(Fre)的影响,敏感度依次为: l1> w1>d>θ>t>h,考虑实际设计情形,筛选出了l1,w1,d与h四个石英音叉设计变量,采用Box-Behnken实验设计方案与RSM(response surface methodology)方法,以Fre为函数目标,建立l1,w1,d与h的二次回归响应面模型,得到了参数之间的交互作用,利用Design-Expert软件对响应面模型进行设计参数反求,结果表明,在15 000 Hz≤Fre≤25 000 Hz计算区域内误差较小,基本满足QEPAS系统的计算需求,所提出的研究与设计方法具有一定通用性,可为QEPAS系统中石英音叉结构参数设计提供参考。 相似文献
59.
60.
Ping Chen Yanhong Hao Xinxin Wang Prof. Dr. Dan Yuan Prof. Dr. Yingming Yao Prof. Dr. Lutz Ackermann 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(30):7292-7297
Highly position selective alkylations of N-alkylindoles at C7-positions have been enabled by cationic zirconium complexes. The strategy provides a straightforward access to install alkyl groups at C7-positions of indoles without a complex directing group. Mechanistic studies provided support for the importance of Brønsted acids in the catalytic manifold. 相似文献