全文获取类型
收费全文 | 25214篇 |
免费 | 4516篇 |
国内免费 | 5202篇 |
专业分类
化学 | 18434篇 |
晶体学 | 559篇 |
力学 | 1738篇 |
综合类 | 588篇 |
数学 | 3391篇 |
物理学 | 10222篇 |
出版年
2024年 | 51篇 |
2023年 | 387篇 |
2022年 | 744篇 |
2021年 | 826篇 |
2020年 | 866篇 |
2019年 | 909篇 |
2018年 | 785篇 |
2017年 | 952篇 |
2016年 | 1007篇 |
2015年 | 1179篇 |
2014年 | 1420篇 |
2013年 | 1799篇 |
2012年 | 2152篇 |
2011年 | 2207篇 |
2010年 | 1796篇 |
2009年 | 1680篇 |
2008年 | 1969篇 |
2007年 | 1787篇 |
2006年 | 1771篇 |
2005年 | 1365篇 |
2004年 | 1097篇 |
2003年 | 945篇 |
2002年 | 941篇 |
2001年 | 891篇 |
2000年 | 819篇 |
1999年 | 622篇 |
1998年 | 421篇 |
1997年 | 378篇 |
1996年 | 399篇 |
1995年 | 301篇 |
1994年 | 296篇 |
1993年 | 270篇 |
1992年 | 256篇 |
1991年 | 233篇 |
1990年 | 193篇 |
1989年 | 180篇 |
1988年 | 125篇 |
1987年 | 128篇 |
1986年 | 118篇 |
1985年 | 91篇 |
1984年 | 77篇 |
1983年 | 68篇 |
1982年 | 51篇 |
1981年 | 46篇 |
1980年 | 44篇 |
1979年 | 37篇 |
1978年 | 30篇 |
1977年 | 32篇 |
1976年 | 27篇 |
1973年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
本文用AES研究了P-InP/TiPdAu热处理前后的界面特性,结果表明:TiPd层对Au的内扩散和In的扩散有阻挡作用。以TiPdAu作InGaAsP/InP双异质结发光管的p面电极、镀Au作热沉,采用In焊料,研究了器件的可靠性问题,在室温大气气氛中;70℃存储,70~80℃带电老化,三种条件下长时间考核结果表明:器件的I-V特性正常,末见正向压降明显变化。还比较了Au-Zn材料作p面电极用TiPdAu作肖脱基势垒限制层制成的器件和用TiPdAu作电极材料制成的深Zn扩散型器件在老化过程中的特性变化,后二种结构的器件,在长期老化过程中,有源区中有大面积DSD生长和增殖。 相似文献
92.
This work proposes and demonstrates the feasibility of gray image encryption using multiple polarization encoding, in which
the cascaded twisted nematic liquid crystal devices sandwiched between a crossed polarizer pair are used to implement exclusive
OR (XOR) logical encryption and decryption. The polarization encoding scheme provides a multichannel XOR logic operation to
secure bit-plane images derived from a gray image, according to the adiabatic following properties of twisted nematic liquid
crystal devices. Experimental data reveal that the multiple polarization encoding schemes for gray image encryption and decryption
are performed with acceptable image quality by selecting appropriate polarization orientation and modulation conditions. Both
analytic and experimental results are presented and discussed. 相似文献
93.
本文具体刻画有界Lipschitz域上的反射Brown运动跨过某给定时刻t的离开Martin边界的游程.在此基础上,给出Lipschitz域上的反射Brown运动的边界过程的Levy系统. 相似文献
94.
95.
In the present paper, a Lotka–Volterra type mutualism system with several delays is studied. Some new and interesting sufficient conditions are obtained for the global existence of positive periodic solutions of the mutualism system. Our method is based on Mawhin’s coincidence degree and novel estimation techniques for the a priori bounds of unknown solutions. Our results are different from the existing ones such as those in of Yang et al. [F. Yang, D. Jiang, A. Ying, Existence of positive solution of multidelays facultative mutualism system, J. Eng. Math. 3 (2002) 64–68] and Chen et al. [F. Chen, J. Shi, X. Chen, Periodicity in a Lotka–Volterra facultative mutualism system with several delays, J. Eng. Math. 21 (3) (2004) 403–409]. 相似文献
96.
97.
M F Anjos R C H Cheng C S M Currie 《The Journal of the Operational Research Society》2004,55(5):535-541
The paper describes a methodology that has been implemented in a major British airline to find the optimal price to charge for airline tickets under one-way pricing. An analytical model has been developed to describe the buying behaviour of customers for flights over the selling period. Using this model and a standard analytical method for constrained optimization, we can find an expression for the optimal price structure for a flight. The expected number of bookings made on each day of the selling period and in each fare class given these prices can then be easily calculated. A simulation model is used to find the confidence ranges on the numbers of bookings and these ranges can be used to regulate the sale of tickets. A procedure to update the price structure based on the remaining capacity has also been developed. 相似文献
98.
采用交替沉积磁控溅射工艺制备了超薄多层的FeCoB SiO2 磁性纳米颗粒膜 .利用x射线衍射仪、扫描探针显微镜、透射电子显微镜分析了薄膜的微结构和形貌特征 .采用振动样品磁强计、四探针法、微波矢量分析仪及谐振腔法测量薄膜试样的磁电性能和微波复磁导率 .重点对SiO2 介质相含量、薄膜微结构对电磁性能产生重要影响的机理做了分析和探讨 .结果表明 :这类FeCoB SiO2 磁性纳米颗粒膜具有良好的软磁性能和高频电磁性能 ,2GHz时的磁导率 μ′高于 70 ,可以应用于高频微磁器件或微波吸收材料的设计 相似文献
99.
本文详细讨论了李双代数胚中的Dirac结构、群胚上的Dirac结构。利用Dirac结构的特征对的概念,给出了作用不变Dirac结构,拉回Dirac结构等概念的新的刻画。最后利用Dirac结构的有关性质,讨论了泊松齐性空间和泊松群胚作用的约化。 相似文献
100.
With the advances in pulsed laser systems, microscopic imaging techniques such as multiphoton and pump-probe fluorescence microscopy have developed into effective tools for investigating intensity and time-resolved phenomena inside biological systems. However, pulsed lasers used in these techniques usually are commercial systems with repetition frequencies of around 80 MHz. While these systems have proven to be adequate for multiphoton and pump-probe microscopic imaging applications, the temporal separation of the laser pulse train (around 12.5 ns) is long compared to the fluorescence lifetimes of many common fluorescence species. In this work, we present the designs of repetition rate multipliers based on passive optical components that can be used to increase the efficiency in multiphoton and pump-probe fluorescence microscopy. Depending on the lifetime of fluorescence molecules under investigation, the passive repetition rate multiplier can increase the duty cycle of multiphoton or pump-probe microscopy up to fourfold. 相似文献