全文获取类型
收费全文 | 16223篇 |
免费 | 2500篇 |
国内免费 | 1121篇 |
专业分类
化学 | 6120篇 |
晶体学 | 138篇 |
力学 | 1452篇 |
综合类 | 280篇 |
数学 | 3479篇 |
物理学 | 8375篇 |
出版年
2023年 | 151篇 |
2022年 | 406篇 |
2021年 | 366篇 |
2020年 | 383篇 |
2019年 | 393篇 |
2018年 | 356篇 |
2017年 | 519篇 |
2016年 | 582篇 |
2015年 | 496篇 |
2014年 | 800篇 |
2013年 | 1125篇 |
2012年 | 791篇 |
2011年 | 1097篇 |
2010年 | 897篇 |
2009年 | 1118篇 |
2008年 | 1006篇 |
2007年 | 900篇 |
2006年 | 878篇 |
2005年 | 689篇 |
2004年 | 572篇 |
2003年 | 608篇 |
2002年 | 496篇 |
2001年 | 412篇 |
2000年 | 408篇 |
1999年 | 364篇 |
1998年 | 275篇 |
1997年 | 246篇 |
1996年 | 210篇 |
1995年 | 147篇 |
1994年 | 142篇 |
1993年 | 121篇 |
1992年 | 122篇 |
1991年 | 92篇 |
1990年 | 78篇 |
1989年 | 90篇 |
1988年 | 74篇 |
1987年 | 65篇 |
1985年 | 443篇 |
1984年 | 312篇 |
1983年 | 250篇 |
1982年 | 228篇 |
1981年 | 162篇 |
1980年 | 129篇 |
1979年 | 146篇 |
1978年 | 83篇 |
1977年 | 91篇 |
1976年 | 93篇 |
1975年 | 90篇 |
1974年 | 117篇 |
1973年 | 136篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
During the design process of Blu-ray disc optical pick-up (BD OPU), the optical system and the collimator lens design is especially important. This paper designs an optical system and some lenses for the BD OPU, including collimator lens, beam shaping lens for laser beam shaping, and cylinder lens for signal detecting. In this OPU, we use a triplet lens to collimate the laser beam. At the same time, we build a series of assembly jigs to make sure that each lens can be put into the OPU basement properly. At last, we get the reading spot image and S-curve photo of OPU, which can be used to read Blu-ray disc (BD). 相似文献
912.
Road signs must provide a conspicuous signal to a wide variety of drivers over a broad range of environmental and geometric
conditions. Recently, there are an increasing number of applications in which light emitting diodes (LEDs) are used as the
light source, including critical transportation signaling. In the presence of fog, the resulting visual signal is disturbed
due to light scattering by airborne water droplets. By measuring LED brightness with human spectral sensitivity in various
densities and various droplet sizes (10, 30, 50, and 100 μm), it is understood that the particle size distribution (fog droplet
size) and density of fog does affect visibility in fog. The colored LEDs that contain a yellow component had high brightness
evaluation, blue component had low brightness evaluation in all densities and different droplet sizes. The result in this
paper can contribute to air and land traffic safety and the prevention of accidents. 相似文献
913.
We improve the isospin dependent quantum molecular dynamical model by including isospin effects in the Skyrme potential and the momentum dependent interaction to obtain an isospin dependent Skyrme potential and an isospin dependent momentum interaction. We investigate the isospin effects of Skyrme potential and momentum dependent interaction on the isospin fractionation ratio and the dynamical mechanism in intermediate energy heavy ion collisions. It is found that the isospin dependent Skyrme potential and the isospin dependent momentum interaction produce some important isospin effects in the isospin fractionation ratio. 相似文献
914.
WU Jin-Jie HENG Yue-Kun SUN Zhi-Jia WU Chong ZHAO Yu-Da YANG Gui-An JIANG Chun-Hua 《中国物理C(英文版)》2008,32(3)
A new charge measurement method, time over threshold (TOT), has been used in some gas detectors lately. Here TOT is studied for TOF system, made of plastic scintillator counter, which can simplify the electronics of the system. The signal characteristics are measured and analyzed with a high quality oscilloscope, including noise, pedestal, signal amplitude, total charge, rise time and the correlation between them. The TOT and charge are related and can be fitted by some empirical formula. The charge measurement resolution by TOT is given and this will help the design of TOF electronics. 相似文献
915.
降低轴流风机噪声的两种方法 总被引:2,自引:0,他引:2
本文对轴流风机的噪声源进行了理论和实验分析,对降低风机噪声的方法进行了总结。在此基础上,研究了降低风机噪声的两种新方法——叶片不等距分布方法和叶片穿孔方法。理论分析和实验结果表明,这两种方法为降低轴流风机噪声提供了新的途径。 相似文献
916.
917.
In this paper, the idea of random selection in the theorem on gambling system is extended to Markov chains, by using the notion likelihood ratio and an analytic technique. A strong limit theorem on the relative frequency of ordered couple under random selection is established.AMS Subject Classification (2001)
primary 60F15 Secondary 60J10 相似文献
918.
Takesi Hayakawa 《Annals of the Institute of Statistical Mathematics》2002,54(4):918-933
Let
i
be an i-tb population with a probability density function f(· |
i
) with one dimensional unknown parameter
i
= 1, 2, ... , k. Let n
i sample be drawn from each
i
. The likelihood ratio criteria
j|(j–1) for testing hypothesis that the first j parameters are equal against alternative hypothesis that the first (j – 1) parameters are equal and the j-th parameter is different with the previous ones are defined, j = 2, 3, ... , k. The paper shows the asymptotic independence of
j|(j–1)'s up to the order 1/n under a hypothesis of equality of k parameters, where n is a number of total samples. 相似文献
919.
Jnos Csirik Gerhard
J. Woeginger 《Journal of Algorithms in Cognition, Informatics and Logic》2002,44(2):1016
We study online bounded space bin packing in the resource augmentation model of competitive analysis. In this model, the online bounded space packing algorithm has to pack a list L of items in (0,1] into a small number of bins of size b1. Its performance is measured by comparing the produced packing against the optimal offline packing of the list L into bins of size 1.We present a complete solution to this problem: For every bin size b1, we design online bounded space bin packing algorithms whose worst case ratio in this model comes arbitrarily close to a certain bound ρ(b). Moreover, we prove that no online bounded space algorithm can perform better than ρ(b) in the worst case. 相似文献
920.
根据问题的特点 ,选择 Logistic判别模型 ,并借鉴普通线性回归的逐步向前法 ,用似然比统计量 ( Deviance)来筛选自变量 (因素 ) .结果显示 :在 1 1个因素中 ,有 3个是作用显著的 ,而其中舒张压值极为重要 .为降低再出血率 ,将其控制在 85 mm Hg以下是有效而可行的 相似文献