全文获取类型
收费全文 | 1487篇 |
免费 | 244篇 |
国内免费 | 120篇 |
专业分类
化学 | 414篇 |
晶体学 | 1篇 |
力学 | 76篇 |
综合类 | 42篇 |
数学 | 1108篇 |
物理学 | 210篇 |
出版年
2023年 | 39篇 |
2022年 | 94篇 |
2021年 | 80篇 |
2020年 | 92篇 |
2019年 | 77篇 |
2018年 | 70篇 |
2017年 | 68篇 |
2016年 | 88篇 |
2015年 | 59篇 |
2014年 | 106篇 |
2013年 | 121篇 |
2012年 | 79篇 |
2011年 | 93篇 |
2010年 | 68篇 |
2009年 | 91篇 |
2008年 | 66篇 |
2007年 | 77篇 |
2006年 | 55篇 |
2005年 | 62篇 |
2004年 | 47篇 |
2003年 | 53篇 |
2002年 | 39篇 |
2001年 | 26篇 |
2000年 | 28篇 |
1999年 | 18篇 |
1998年 | 30篇 |
1997年 | 26篇 |
1996年 | 18篇 |
1995年 | 3篇 |
1994年 | 10篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有1851条查询结果,搜索用时 15 毫秒
41.
Jun Araki Kousaku Ohkawa Yusuke Uchida Yoshihiko Murakami 《Journal of polymer science. Part A, Polymer chemistry》2012,50(3):488-494
A sliding graft copolymer (SGC) with poly(ethylene glycol) (PEG) side chains was prepared by ester formation between terminal carboxyl groups of oxidized PEG methyl ether with molecular weight of 2000 (mPEG2000‐COOH) and hydroxyl groups of a polyrotaxane consisting of PEG and cyclodextrins (CDs). Formation of the SGC structure was confirmed by 1H NMR, attenuated total reflectance Fourier‐transformed infrared, and gel permeation chromatography. The SGC was soluble in good solvents of PEG and insoluble in poor solvents of PEG. Estimation of the number of grafted mPEG chains suggested a “rope‐curtain” like structure, in which an mPEG chain is connected to each CD ring. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 相似文献
42.
In recent years,the transportation system has been faced by increasing challenge in congestion and inefciency,and research in trafc network has become a significant area of interest.In this paper,we introduce a dynamic-information-based(DIB)queueing strategy into network trafc model under the efcient routing strategy.DIB makes a packet with higher priority to be delivered if there are less packets travelling along its path from the current node to the destination.It is found that,compared with the traditional first-in-first-out(FIFO)queueing strategy,DIB can efectively balance the trafc load of the system via delaying packets to be delivered to congested nodes.Although the network capacity has no obvious changes,some other indexes which reflect transportation efciency are efciently improved in the congestion state.Besides,extensive simulation results and discussions are provided to explain the phenomena.The results may provide novel insights for research on trafc systems. 相似文献
43.
In this article, an ALE finite element method to simulate the partial melting of a workpiece of metal is presented. The model includes the heat transport in both the solid and liquid part, fluid flow in the liquid phase by the Navier–Stokes equations, tracking of the melt interface solid/liquid by the Stefan condition, treatment of the capillary boundary accounting for surface tension effects and a radiative boundary condition. We show that an accurate treatment of the moving boundaries is crucial to resolve their respective influences on the flow field and thus on the overall energy transport correctly. This is achieved by a mesh‐moving method, which explicitly tracks the phase boundary and makes it possible to use a sharp interface model without singularities in the boundary conditions at the triple junction. A numerical example describing the welding of a thin‐steel wire end by a laser, where all aforementioned effects have to be taken into account, proves the effectiveness of the approach.Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
44.
以学习自我效能感和学习策略量表为工具,通过方差分析、相关分析和路径分析等方法,重点考察了学习自我效能感和学习策略这两个因素对学业成就的影响路径以及相互间的关系.发现中学生的学习自我效能感与学习策略间存在着显著的相关关系;学习自我效能感和学习策略对初中生的学业成就有显著回归效应,而学习策略对高中生的学业成就有显著回归效应;学习自我效能感可通过影响学习策略来间接影响学业成就. 相似文献
45.
This paper studies the multi-agent tracking problem of a third-order maneuvering target under uncertain communication environments. Each tracking agent is assumed to be a third-order system and can only use its own and neighbors' position, velocity, and acceleration information to design its control input. In this work, the uncertain communication environments are modelled by a finite number of constant Laplacian matrices together with their corresponding scheduling functions. Sufficient conditions for the existence of a tracking strategy have been expressed in terms of the solvability of linear matrix inequalities. Finally, a numerical example is employed to demonstrate the effectiveness of the proposed tracking strategy. 相似文献
46.
In sensitivity experiments, the response is binary and each experimental unit has a critical stimulus level that cannot be observed directly. It is often of interest to estimate extreme quantiles of the distribution of these critical stimulus levels over the tested products. For this purpose a new sequential scheme is proposed with some commonly used models. By using the bootstrap repeated-sampling principle, reasonable prior distributions based on a historic data set are specified. Then, a Bayesian strategy for the sequential procedure is provided and the estimator is given. Further, a high order approximation for such an estimator is explored and its consistency is proven. A simulation study shows that the proposed method gives superior performances over the existing methods. 相似文献
47.
Zhen-Xing ZhangBaolin Wu Bin WangTie-Hai Li Peng-Fei ZhangLi-Na Guo Wen-jun WangWei Zhao Peng George Wang 《Tetrahedron letters》2011,52(29):3802-3804
A novel and facile synthesis of a series of the biologically significant iminosugar derivatives including 2-deoxynojirimycin, Miglustat and Miglitol is reported. The synthesis features a strategic double inversion mechanism for securing the desired stereochemistry at C5 position of such glucose-type carbohydrate mimetics, representing a practical and remarkable improvement on the previously reported method that suffers from the loss of the stereo-control during the reaction process. 相似文献
48.
49.
AbstractAn amide-assisted arene nitration is presented, and both mono- and dinitration of protected anilines could be effected by using NaNO2 and NaNO3 as the mono- and bisnitrating agents, respectively. This divergent synthesis is transition-metal- and acid-free, and features a broad substrate scope, low cost, and ortho–para selectivity. 相似文献
50.
Daoyuan Zheng Yurong Guo Mingxing Zhang Xia Feng Lina Zhu Lijuan Qiu Xiaoning Jin Guangjiu Zhao 《Journal of computational chemistry》2020,41(10):976-985
Organic semiconductors (OSCs) materials are currently under intense investigation because of their potential applications such as organic field-effect transistors, organic photovoltaic devices, and organic light-emitting diodes. Inspired by the selenization strategy can promote anisotropic charge carrier migration, and selenium-containing compounds have been proved to be promising materials as OSCs both for hole and electron transfer. Herein, we now explore the anisotropic transport properties of the series of selenium-containing compounds. For the compound containing Se Se bond, the Se Se bond will break when attaching an electron, thus those compounds cannot act as n-type OSCs. About the different isomer compounds with conjugated structure, the charge transfer will be affected by the stacking of the conjugated structures. The analysis of chemical structure and charge transfer property indicates that Se-containing materials are promising high-performance OSCs and might be used as p-type, n-type, or ambipolar OSCs. Furthermore, the symmetry of the selenium-containing OSCs will affect the type of OSCs. In addition, there is no direct relationship between the R groups with their performance, whether it or not as p-type OSCs or n-types. This work demonstrates the relationship between the optoelectronic function and structure of selenium-containing OSCs materials and hence paves the way to design and improve optoelectronic function of OSCs materials. 相似文献