全文获取类型
收费全文 | 123881篇 |
免费 | 7795篇 |
国内免费 | 5487篇 |
专业分类
化学 | 56654篇 |
晶体学 | 1089篇 |
力学 | 9436篇 |
综合类 | 342篇 |
数学 | 37341篇 |
物理学 | 32301篇 |
出版年
2023年 | 1006篇 |
2022年 | 990篇 |
2021年 | 1432篇 |
2020年 | 1990篇 |
2019年 | 1901篇 |
2018年 | 11600篇 |
2017年 | 11353篇 |
2016年 | 7897篇 |
2015年 | 2933篇 |
2014年 | 2954篇 |
2013年 | 3747篇 |
2012年 | 7997篇 |
2011年 | 14599篇 |
2010年 | 8496篇 |
2009年 | 8783篇 |
2008年 | 9527篇 |
2007年 | 11287篇 |
2006年 | 2758篇 |
2005年 | 3375篇 |
2004年 | 3286篇 |
2003年 | 3299篇 |
2002年 | 2258篇 |
2001年 | 1261篇 |
2000年 | 1208篇 |
1999年 | 1192篇 |
1998年 | 1043篇 |
1997年 | 945篇 |
1996年 | 1033篇 |
1995年 | 847篇 |
1994年 | 728篇 |
1993年 | 690篇 |
1992年 | 582篇 |
1991年 | 501篇 |
1990年 | 431篇 |
1989年 | 354篇 |
1988年 | 311篇 |
1987年 | 238篇 |
1986年 | 204篇 |
1985年 | 210篇 |
1984年 | 175篇 |
1983年 | 146篇 |
1982年 | 134篇 |
1981年 | 97篇 |
1980年 | 86篇 |
1979年 | 81篇 |
1978年 | 64篇 |
1976年 | 47篇 |
1975年 | 49篇 |
1973年 | 45篇 |
1914年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Nonlinear Dynamics - This paper concerns the problem of fixed-time synchronization of master–slave Lorenz systems. The adaptive control and fixed-time control strategies are successfully... 相似文献
2.
研究了具有死区输入的预设约束未知高阶严格反馈非线性系统的控制问题,提出了一种基于免疫函数的自抗扰预设漏斗约束自适应控制策略。首先,针对系统内部的未知问题,采用免疫函数与扩张状态观测器结合对系统内部未知项进行观测;其次,通过Lyapunov方法与漏斗控制相结合设计控制器,使得跟踪误差能够维持在预先设定的漏斗约束范围内;同时,利用双曲正切函数速率变化快这一特性设计自适应控制律,引入指令滤波器避免反步法中重复求导问题,分析证明了闭环系统所有信号的有界性。仿真实例表明了控制方法的有效性。 相似文献
3.
Dehydrogenation of an organic compound is the first and the most fundamental elementary reaction in many organic reactions. In ethanol oxidation reaction (EOR) to form CO2, there are a total of 46 pathways in C2HxO (x=1–6) species leading to the removal of all six hydrogen atoms in five C−H bonds and one O−H bond. To investigate the degree of dehydrogenation in EOR under operando conditions, we performed density function theory (DFT) calculations to study 28 dehydrogenation steps of C2HxO on Ir(100). An activation energy surface was then constructed and compared with that of the C−C bond cleavages to understand the importance of the degree of dehydrogenation in EOR. The results show that there are likely 28 dehydrogenations in EOR under fuel cell temperatures and the last two hydrogens in C2H2O are less likely cleaved. On the other hand, deep dehydrogenation including 45 dehydrogenations can occur under ethanol steam reforming conditions. 相似文献
4.
Efficient,robust and cost-effective electrocatalysts that catalyze hydrogen evolution/oxidation reaction(HER/HOR)in alkaline media are highly demanded.Recently,single-atom catalysts(SACs)have emerged as new promising candidates;however,the rational design of supports and the optimization of coordination environment between supports and metal atoms are challenging.In this work,we successfully fabricate atomically dispersed ruthenium(Ru)species,which are strongly coordinated by N and S dual heteroatoms on holey graphene(RuSA/NSG),as an excellent bifunctional catalyst for HER/HOR.In alkaline media,the developed catalyst exhibits high catalytic performance with a low overpotential of 57.3 mV to drive a current density of 10 mA cm-1 for HER,and its mass activity is about 5.8 times higher than that of commercial Pt/C and Ru/C catalysts at an overpotential of 100 mV.Similarly,considerable HOR performance of Ru SA/NSG is verified to be superior to Pt/C and Ru/C.Furthermore,X-ray-based spectroscopy measurements and density-functional theory calculations have confirmed that,compared with Ru–N4,the tailored Ru–N4–S2 with nearby S dopants can act as more active centers to greatly accelerate the sluggish HER/HOR kinetics in alkaline media.The present work provides a new atomic-level engineering strategy to modulate catalytic activities of SACs via the coordination design using dual heteroatoms on the carbon support. 相似文献
5.
6.
The rotator cuff repaired construct must establish a contiguous and functioning tendon-bone junction to provide adequate stability. However, fibrocartilage deficiency and bone loss were hardly reversed after physical suture, especially in chronic rotator cuff tears. In this study, we synthesized an injectable methylcellulose/polyvinyl alcohol/polyvinylpyrrolidone-based thermo-sensitive hydrogel, which delivered kartogenin-loaded mesoporous bioactive glass nanoparticles. Physicochemical studies the revealed phase transition temperatures of 35 °C and its ability to induce chondrogenesis and osteogenesis differentiation of tendon-derived stem cells. Furthermore, experiments in rabbit chronic rotator cuff tears model confirmed the fibrocartilage and bone layer regenerative capability of the injected bioactive hydrogel, which could, in turn, support the ultimate tensile stress of the repaired rotator cuff. The bioactive agents-loaded hydrogel reported in this study is a valuable addition to the arsenal of biomaterials in applications to chronic tendon-bone junction injuries. 相似文献
7.
8.
通过改装多旋翼无人机(UAV)和搭载各类载荷以及联合地基观测设备对大柴旦地区大气、环境以及气溶胶参数进行测量。利用获得的数据资料,对该地区近地层气溶胶粒子数浓度(即单位体积空气中气溶胶粒子的数目)、消光系数以及气象要素等特征进行了分析。结果表明,在大柴旦地区,近地层气溶胶粒子数浓度日变化显著,呈现双峰形态,气溶胶粒子数浓度的变化范围为75~220 cm-3,消光系数的变化范围为0.004~0.038 km-1;当风速小于6 m/s时,气溶胶粒子数浓度与风速呈负相关关系;当风速大于6 m/s时,二者呈正相关关系;相对湿度对气溶胶粒子的影响较小,这可能是由于该地区以沙尘型气溶胶为主,吸湿性较弱。本研究基于多旋翼无人机探测平台,可以有效地获得近地层精细化大气、环境结构,有助于研究人员了解该地区气溶胶的结构、变化特征以及建立气溶胶模式,同时也为气溶胶及大气环境参数探测方法提供了技术支撑及思路拓展。 相似文献
9.
Hu Duo-Duo Gao Qian Dai Jing-Cheng Cui Ru Li Yuan-Bo Li Yuan-Ming Zhou Xiao-Guo Bian Kang-Jie Wu Bing-Bing Zhang Kai-Fan Wang Xi-Sheng Li Yan 《中国科学:化学(英文版)》2022,65(4):753-761
Science China Chemistry - A light-induced, nickel-catalyzed three-component arylsulfonation of 1,3-enynes in the absence of photocatalyst is reported. This methodology exhibited mild conditions,... 相似文献
10.
Science China Chemistry - Human health is always under global spotlight, but now it suffers from severe environmental issue and various diseases. Developing highly selective and effective... 相似文献