全文获取类型
收费全文 | 40748篇 |
免费 | 2524篇 |
国内免费 | 1926篇 |
专业分类
化学 | 22201篇 |
晶体学 | 236篇 |
力学 | 3935篇 |
综合类 | 547篇 |
数学 | 8469篇 |
物理学 | 9810篇 |
出版年
2024年 | 48篇 |
2023年 | 354篇 |
2022年 | 653篇 |
2021年 | 1093篇 |
2020年 | 1024篇 |
2019年 | 959篇 |
2018年 | 874篇 |
2017年 | 1115篇 |
2016年 | 1506篇 |
2015年 | 1116篇 |
2014年 | 1603篇 |
2013年 | 2869篇 |
2012年 | 2318篇 |
2011年 | 2073篇 |
2010年 | 1794篇 |
2009年 | 2346篇 |
2008年 | 2313篇 |
2007年 | 2636篇 |
2006年 | 2186篇 |
2005年 | 1957篇 |
2004年 | 1887篇 |
2003年 | 1585篇 |
2002年 | 1262篇 |
2001年 | 1040篇 |
2000年 | 1031篇 |
1999年 | 901篇 |
1998年 | 842篇 |
1997年 | 748篇 |
1996年 | 675篇 |
1995年 | 658篇 |
1994年 | 569篇 |
1993年 | 473篇 |
1992年 | 434篇 |
1991年 | 381篇 |
1990年 | 335篇 |
1989年 | 231篇 |
1988年 | 233篇 |
1987年 | 149篇 |
1986年 | 128篇 |
1985年 | 157篇 |
1984年 | 114篇 |
1983年 | 61篇 |
1982年 | 99篇 |
1981年 | 65篇 |
1980年 | 47篇 |
1979年 | 74篇 |
1978年 | 40篇 |
1977年 | 19篇 |
1976年 | 34篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 357 毫秒
131.
Butyltins were analyzed in waters from California river and lake marinas and were detected at the part per trillion (ppt) level in most locations. The tributyltin:dibutyltin (TBT/DBT) ratios in the fresh waters were similar to those in their saline counterparts indicating like rates of degradation in the two salinity regimes. 相似文献
132.
Joseph Y.-T. Leung 《Discrete Applied Mathematics》2007,155(8):945-970
We consider the problem of scheduling orders for multiple different product types in an environment with m dedicated machines in parallel. The objective is to minimize the total weighted completion time. Each product type is produced by one and only one of the m dedicated machines; that is, each machine is dedicated to a specific product type. Each order has a weight and may also have a release date. Each order asks for certain amounts of various different product types. The different products for an order can be produced concurrently. Preemptions are not allowed. Even when all orders are available at time 0, the problem has been shown to be strongly NP-hard for any fixed number (?2) of machines. This paper focuses on the design and analysis of efficient heuristics for the case without release dates. Occasionally, however, we extend our results to the case with release dates. The heuristics considered include some that have already been proposed in the literature as well as several new ones. They include various static and dynamic priority rules as well as two more sophisticated LP-based algorithms. We analyze the performance bounds of the priority rules and of the algorithms and present also an in-depth comparative analysis of the various rules and algorithms. The conclusions from this empirical analysis provide insights into the trade-offs with regard to solution quality, speed, and memory space. 相似文献
133.
The decay process J/ψ→p X X→p P,where p,p and P are the proton,antiproton and pseudoscalar states,respectively,has been studied in terms of the angular distribution and the generalized moment analysis methods.The result shows that we can identify the spin,but cannot determine the parity of the baryon resonance state X produced in the process J/ψ→p X X→p P. 相似文献
134.
Frank H.G.M. Wijnands Charles G. Crookes Paul M. Charles Richard M. Ash Ian F. Lealman Michael J. Robertson Anthony E. Kelly Kevin A. Williams Aeneas B. Massara Richard V. Penty Ian H. White 《Optical and Quantum Electronics》2002,34(10):959-973
An anomalous modulation in the wavelength spectrum has been observed in lasers with spot-size converters. This intensity modulation is shown to be caused by beating between the fundamental lasing mode and radiation modes in the taper. This results in a periodic modulation in the net gain spectrum, which causes wavelength jumps between adjacent net gain maxima, and a drive current dependent spectral width that is expected to affect system performance. The amplitude of this spectral modulation is reduced significantly by either using an angled rear-facet which reflects the beating radiation modes away from the laser axis, or by using a nonlinear, adiabatic taper. 相似文献
135.
136.
137.
138.
基础房价的相关指标及其走势一直是大众关心的热门话题.本文通过对上海基础房价相关指标的分析,建立了市场房价走势的两个数学模型.模型一:在相关性分析的基础上利用主成分分析消除指标间的共线性,再用回归拟合房价模型并进行预测;模型二:在相关性分析的基础上利用核估计方法预测出房价.继呵对2005年下半年的房价走势进行了预测,得出的结果与实际情况相吻合. 相似文献
139.
L. Torrisi A.M. Mezzasalma J. Badziak J. Wolowski G. Franco 《Applied Surface Science》2006,252(24):8533-8538
High energy laser plasma-produced Cu ions have been implanted in silicon substrates placed at different distances and angles with respect to the normal to the surface of the ablated target. The implanted samples have been produced using the iodine high power Prague Asterix Laser System (PALS) using 438 nm wavelength irradiating in vacuum a Cu target. The high laser pulse energy (up to 230 J) and the short pulse duration (400 ps) produced a non-equilibrium plasma expanding mainly along the normal to the Cu target surface. Time-of-flight (TOF) technique was employed, through an electrostatic ion energy analyzer (IEA) placed along the target normal, in order to measure the ion energy, the ion charge state, the energy distribution and the charge state distribution. Ions had a Boltzmann energy distributions with an energy increasing with the charge state. At a laser fluence of the order of 6 × 106 J/cm2, the maximum ion energy was about 600 keV and the maximum charge state was about 27+.In order to investigate the implantation processes, Cu depth profiles have been performed with Rutherford backscattering spectrometry (RBS) of 1.5 MeV helium ions, Auger electron spectroscopy (AES) with 3 keV electron beam and 1 keV Ar sputtering ions in combination with scanning electron microscopy (SEM). Surface analysis results indicate that Cu ions are implanted within the first surface layers and that the ion penetration ranges are in agreement with the ion energy measured with IEA analysis. 相似文献
140.
Goran Pavi? 《Applied Acoustics》2006,67(9):864-881
A method is formulated for the identification of an unknown physical parameter of a fluid-filled pipe using the measurement of sound speed in the pipe. The method uses a simple formula which provides the relationship between the sound speed and a few physical parameters of the pipe: thickness, diameter, wall material constants and fluid constants. Once the sound speed in the pipe is measured, the simple formula can be used to extract one of the unknown parameters providing the remaining ones are known.The sound speed in the pipe is measured using a 3-transducer array. In order to demonstrate the potential of the technique the results of several measurements obtained in a water-filled steel pipe are presented.The required accuracy of the measurement of sound speed and of the specification of known parameters is analysed. The accuracy depends on the unknown parameter which is to be identified. For example, if the pipe thickness is the unknown parameter, the other parameters have to be known within a very narrow margin of error. On the contrary, if the fluid properties have to be identified the needed accuracy of known parameters gets much lower. 相似文献