全文获取类型
收费全文 | 145460篇 |
免费 | 29079篇 |
国内免费 | 13497篇 |
专业分类
化学 | 113385篇 |
晶体学 | 1502篇 |
力学 | 7814篇 |
综合类 | 737篇 |
数学 | 15663篇 |
物理学 | 48935篇 |
出版年
2024年 | 428篇 |
2023年 | 2486篇 |
2022年 | 4119篇 |
2021年 | 4361篇 |
2020年 | 5688篇 |
2019年 | 6490篇 |
2018年 | 4842篇 |
2017年 | 4307篇 |
2016年 | 8574篇 |
2015年 | 8567篇 |
2014年 | 9649篇 |
2013年 | 12154篇 |
2012年 | 12970篇 |
2011年 | 12592篇 |
2010年 | 9720篇 |
2009年 | 9429篇 |
2008年 | 9438篇 |
2007年 | 8104篇 |
2006年 | 7300篇 |
2005年 | 6357篇 |
2004年 | 5119篇 |
2003年 | 4231篇 |
2002年 | 4806篇 |
2001年 | 3745篇 |
2000年 | 3447篇 |
1999年 | 2728篇 |
1998年 | 2084篇 |
1997年 | 1992篇 |
1996年 | 2000篇 |
1995年 | 1703篇 |
1994年 | 1483篇 |
1993年 | 1242篇 |
1992年 | 1095篇 |
1991年 | 1005篇 |
1990年 | 819篇 |
1989年 | 654篇 |
1988年 | 450篇 |
1987年 | 390篇 |
1986年 | 410篇 |
1985年 | 337篇 |
1984年 | 209篇 |
1983年 | 154篇 |
1982年 | 119篇 |
1981年 | 74篇 |
1980年 | 52篇 |
1979年 | 27篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1975年 | 8篇 |
1957年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We demonstrate three-dimensional tomographic imaging using a Fresnel lens with broadband terahertzpulses.Objects at various locations along the beam propagation path are uniquely imaged on the sameimaging plane using a Fresnel lens with different frequencies of the imaging beam.This procedure allowsthe reconstruction of an object's tomographic contrast image by assembling the frequency-dependentimages. 相似文献
992.
993.
994.
Efficiencies of the maximum pseudolikelihood estimator and a number of related estimators for the case-cohort sampling design in the proportional hazards regression model are studied. The asymptotic information and lower bound for estimating the parametric regression parameter are calculated based on the effective score, which is obtained by determining the component of the parametric score orthogonal to the space generated by the infinite-dimensional nuisance parameter. The asymptotic distributions of the maximum pseudolikelihood and related estimators in an i.i.d. setting show that these estimators do not achieve the computed asymptotic lower bound. Simple guidelines are provided to determine in which instances such estimators are close enough to efficient for practical purposes. 相似文献
995.
Vitali Vogel Jean‐Franois Gohy Bas G. G. Lohmeijer Jacomina A. Van Den Broek Winfried Haase Ulrich S. Schubert Dieter Schubert 《Journal of polymer science. Part A, Polymer chemistry》2003,41(20):3159-3168
In aqueous solutions, amphiphilic block copolymers in which a polystyrene (PS) segment is connected to a poly(ethylene oxide) (PEO) block via a bis(2,2′:6′,2″‐ terpyridine ruthenium) complex can form micelles. Such micelles of the protomer type PS20‐[Ru]‐PEO70, according to the preparation procedure representing frozen micelles, were studied by sedimentation velocity and sedimentation equilibrium analysis in an analytical ultracentrifuge and by transmission electron microscopy, with different techniques applied for the sample preparation. The particles obtained were surprisingly multifarious in size. In ultracentrifugation experiments performed at relatively low salt concentrations, the distributions of the sedimentation coefficient s20,w showed a pronounced peak at 9.6 S and a broad, only partly separated second peak around 14 S. The molar mass of the particles at the peak was around 430,000 g/mol, corresponding to an aggregation number of approximately 85. The average hydrodynamic diameter of the particles in the peak fraction was approximately 13 nm. In electron micrographs of negatively stained samples, spheres of diameters between 10 and 25 nm were the most abundant particles, but larger ones with a wide size range were also visible. The latter particles apparently were composed of smaller ones. The data from both sedimentation analysis and electron microscopy showed that (1) the studied compound formed primary micelles of diameters around 20 nm and (2) the primary micelles had a tendency toward aggregation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3159–3168, 2003 相似文献
996.
This study investigates the robust output tracking problem for a class of uncertain linear systems. The uncertainties are assumed to be time invariant and to satisfy the matching conditions. According to the selected nominal parameters, an optimal solution with a prescribed degree of stability is determined. Then, an auxiliary input via the use of an adapting factor, connected to the nominal optimal control, is introduced to guarantee the robustness and prescribed degree of stability for the output tracking control of the uncertain linear systems. This method is very simple and effective and can reject bounded uncertainties imposed on the states. A maglev vehicle model example is given to show its effectiveness. 相似文献
997.
998.
999.
在虚拟仪器平台上演示负载与功率的关系 总被引:1,自引:0,他引:1
Lab VIEW又称实验室虚拟仪器工程平台,是美国NI公司推出的一种基于G语言的虚拟仪器软件开发工具,用Lab VIEW设计的虚拟仪器可脱离Lab VIEW开发环境,最终用户看见的是和实际的硬件仪器相似的操作面板. 相似文献
1000.
Ru Ju Wang Feng Ying LiTing Zhang Deng Ji Fang WangHong Chen Chang Qing Jin 《Solid State Communications》2003,127(1):47-50
Using pulse echo overlap measurement, the elastic behavior of amorphous carbon has been studied at ambient and low temperatures. The smaller ratio B/G of the bulk modulus to shear modulus and smaller Poisson's ratio σ at room temperature indicate that there is an intrinsic stiffening of transverse acoustic phonons in the amorphous carbon. The acoustic velocity and attenuation for longitudinal modes have been measured between 2.1 and 300 K at three frequencies of 7, 21 and 35 MHz, respectively. Their frequency and temperature dependence are observed. The elastic constant C11 increases with decreasing temperature and show enhanced stiffening at low temperatures. In the 130-220 K region, the abnormal change and effect of longitudinal velocity and attenuation with temperature and frequency, and a phase transition associated with structure relaxations are discussed. 相似文献