全文获取类型
收费全文 | 6321篇 |
免费 | 1334篇 |
国内免费 | 481篇 |
专业分类
化学 | 3174篇 |
晶体学 | 78篇 |
力学 | 194篇 |
综合类 | 69篇 |
数学 | 611篇 |
物理学 | 4010篇 |
出版年
2024年 | 21篇 |
2023年 | 92篇 |
2022年 | 209篇 |
2021年 | 253篇 |
2020年 | 350篇 |
2019年 | 233篇 |
2018年 | 188篇 |
2017年 | 253篇 |
2016年 | 279篇 |
2015年 | 230篇 |
2014年 | 335篇 |
2013年 | 533篇 |
2012年 | 384篇 |
2011年 | 475篇 |
2010年 | 337篇 |
2009年 | 401篇 |
2008年 | 373篇 |
2007年 | 405篇 |
2006年 | 395篇 |
2005年 | 300篇 |
2004年 | 263篇 |
2003年 | 246篇 |
2002年 | 229篇 |
2001年 | 183篇 |
2000年 | 186篇 |
1999年 | 108篇 |
1998年 | 138篇 |
1997年 | 123篇 |
1996年 | 94篇 |
1995年 | 75篇 |
1994年 | 68篇 |
1993年 | 73篇 |
1992年 | 66篇 |
1991年 | 44篇 |
1990年 | 44篇 |
1989年 | 39篇 |
1988年 | 25篇 |
1987年 | 16篇 |
1986年 | 11篇 |
1985年 | 13篇 |
1984年 | 13篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 3篇 |
排序方式: 共有8136条查询结果,搜索用时 31 毫秒
991.
基于V形三能级模型运用密度矩阵方程研究了非对称耦合量子阱退局域化现象,并具体分析了导带中电子作阱间振荡所产生的亚毫米辐射和激子复合所产生的近红外发射物理过程.理论结果表明:阱间电子波包振荡等效为一经典阻尼振子,其寿命由载流子与纵向光学声子散射时间和电子穿过势垒的渡越时间决定,所产生的亚毫米波辐射强度衰减仅与阱间阻尼系数有关,其线性服从洛仑兹分布.由带间激子复合所产生的近红外辐射频率随电子阱间振荡频率增加而减小,表现为量子限域斯塔克红移,即非对称量子阱在近红外发光区特性随反向偏压而变.这一结果预示着耦合量子
关键词:
非对称耦合量子阱
电子波包振荡
亚毫米波辐射
近红外发射 相似文献
992.
《Photonics and Nanostructures》2005,3(2-3):63-74
We discuss recent progress and the exciting potential of scanning probe microscopy methods for the characterization and control of photonic crystals. We demonstrate that scanning near-field optical microscopy can be used to characterize the performance of photonic crystal device components on the sub-wavelength scale. In addition, we propose scanning probe techniques for realizing local, low-loss tuning of photonic crystal resonances, based on the frequency shifts that high-index nanoscopic probes can induce. Finally, we discuss prospects for on-demand spontaneous emission control. We demonstrate theoretically that photonic crystal membranes induce large variations in spontaneous emission rate over length scales of 50 nm that can be probed by single light sources, or nanoscopic ensembles of light sources attached to the end of a scanning probe. 相似文献
993.
介质势垒放电(DBD)等离子体中NO荧光发射谱研究 总被引:2,自引:1,他引:1
利用介质阻挡放电 (DBD)等离子体技术对大气污染物NO分子进行了光谱研究 ,得到了低气压条件下放电等离子体在 2 1 0~ 2 80nm光谱范围内的荧光发射谱。该谱明显的表现为双峰结构 ,谱线均成对出现 ,强度分布符合Frank Condon原理 ,且最大峰值位置出现在 2 36nm处 ,将该组谱线归属为NO分子的A2 ΣA →X2 Π1 / 2 ,2 / 3 跃迁。荧光产生过程为 :基态NO分子与高能电子发生非弹性碰撞被激发至激发态A2 Σ 后自发跃迁回基态同时辐射出荧光。通过测量等离子体中NO分子和N2 分子 337nm谱线强度随时间的变化关系 ,初步证实了放电等离子体中存在的NO分子的分解机制为 :e NO→N O e,N NO→N2 O ,O NO→NO2 hν。 相似文献
994.
D. K. Chakraborty J. Kurian J. P. Kennedy W. L. Mattice 《Colloid and polymer science》1991,269(8):807-811
The steady-state fluorescence emission spectrum of poly(pentafluorostyrene) in dilute fluid solution shows no excimer emission. An atomic level modeling study explains why this polymer cannot form an excimer. Repulsive Coulombic interactions prohibit the attainment of the extensive overlap of the two rings in the classic face-to-face sandwich conformation of a singlet excimer. 相似文献
995.
996.
997.
998.
We study the message queueing delays in a node of a communication system, where a message consists of a block of consecutive packets. The message delay is defined as the time elapsing between the arrival epoch of the first packet of the message to the system until after the transmission of the last packet of that message is completed. We distinguish between two types of message generation processes. The message can be generated as abatch or it can bedispersed over time. In this paper we focus on the dispersed generation model. The main difficulty in the analysis is due to the correlation between the system states observed by different packets of the same message. This paper introduces a new technique to analyze the message delay in such systems for different arrival models and different number of sessions. For anM/M/1 system with variable size messages and for the bursty traffic model, we obtain an explicit expression for the Laplace-Stieltjes transform (LST) of the message delay. Derivations are also provided for anM/G/1 system, for multiple session systems and for fixed message sizes. We show that the correlation has a strong effect on the performance of the system, and that the commonly usedindependence assumption, i.e., the assumption that the delays of packets are independent from packet to packet, can lead to wrong conclusions. 相似文献
999.
Giovanna Idone 《Journal of Global Optimization》2004,28(1):45-53
A continuum model of transportation network is considered in presence of capacity constraints on the flow. The equilibrium conditions are expressed in terms of a Variational Inequality for which an existence theorem is provided. 相似文献
1000.
Sabry Mohamed Khalil 《Mikrochimica acta》2007,158(3-4):233-238
A simple, accurate, precise and sensitive method has been developed for the determination of Vardenafil (Vd) using direct
coupled plasma atomic emission spectrometry. The ion associates formed as a result of the reaction of vardenafil with ammonium
reineckate, Mn(II) and Co(II) thiocyanates and sodium cobaltinitrite are precipitated. The solubility of the solid complexes
under the optimum conditions of pH and ionic strength was studied. Saturated solutions of each ion associate at different
temperatures under the optimum precipitation conditions were prepared, and the metal ion content in the supernatant was determined.
The solubility products were thus calculated at different temperatures, and the thermodynamic parameters ΔH, ΔG and ΔS were
calculated. The method has been used for the determination of vardenafil (1.36–68.32 μg·mL−1) in pure solutions and in Levitra tablets. 相似文献