全文获取类型
收费全文 | 21438篇 |
免费 | 2785篇 |
国内免费 | 1139篇 |
专业分类
化学 | 2854篇 |
晶体学 | 70篇 |
力学 | 812篇 |
综合类 | 239篇 |
数学 | 6252篇 |
物理学 | 8194篇 |
无线电 | 6941篇 |
出版年
2024年 | 38篇 |
2023年 | 163篇 |
2022年 | 242篇 |
2021年 | 405篇 |
2020年 | 471篇 |
2019年 | 403篇 |
2018年 | 411篇 |
2017年 | 667篇 |
2016年 | 809篇 |
2015年 | 660篇 |
2014年 | 1307篇 |
2013年 | 1434篇 |
2012年 | 1490篇 |
2011年 | 1527篇 |
2010年 | 1235篇 |
2009年 | 1359篇 |
2008年 | 1667篇 |
2007年 | 1648篇 |
2006年 | 1548篇 |
2005年 | 1270篇 |
2004年 | 1056篇 |
2003年 | 933篇 |
2002年 | 669篇 |
2001年 | 583篇 |
2000年 | 503篇 |
1999年 | 432篇 |
1998年 | 385篇 |
1997年 | 313篇 |
1996年 | 289篇 |
1995年 | 234篇 |
1994年 | 197篇 |
1993年 | 173篇 |
1992年 | 152篇 |
1991年 | 127篇 |
1990年 | 97篇 |
1989年 | 83篇 |
1988年 | 55篇 |
1987年 | 36篇 |
1986年 | 44篇 |
1985年 | 43篇 |
1984年 | 38篇 |
1983年 | 24篇 |
1982年 | 25篇 |
1981年 | 19篇 |
1980年 | 16篇 |
1979年 | 18篇 |
1978年 | 12篇 |
1977年 | 15篇 |
1976年 | 8篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 335 毫秒
51.
52.
53.
Based on the beam coherence-polarization (BCP) matrix, the polarization property of coherent and incoherent Gaussian beam combinations is studied in detail. The general expressions for the degree of polarization P of the resulting beam in case of incoherent and coherent combinations are derived. It is shown that P is dependent on the incoherent or coherent combination, propagation distance, separation, azimuth of the polarization plane and numbers of beamlets in general. The irradiance distribution of the resulting beam for the coherent cases depends on the azimuth of the polarization plane of beamlets. However, for the incoherent case it does not. 相似文献
54.
Philip A. Wheeler Michael Brungs Graham R. Atkins 《Journal of Sol-Gel Science and Technology》2003,27(3):321-326
Organically modified silicates (ormosils) have been viewed as possible materials for optical signal processing devices for communications networks. One significant problem obstructing their use in this application is the absorption of infrared light by hydroxy species (present for example in silanol groups (SiOH) and water) around the communications wavelengths (1310 and 1550 nm). We have investigated several ormosil compositions, synthesised via an aqueous sol-gel method, to determine if residual hydroxy groups can be removed more effectively via reducing the connectivity of the material by increasing the number of organic groups (such as Si—CH3). Furthermore, we have investigated the effect of different inert atmospheres on the drying process. It was found that reducing the connectivity does promote the removal of hydroxy absorption, mainly via the condensation of residual silanol groups, but also by removal of residual water, depending on the other precursors in the system. It was also found that the drying atmosphere used (nitrogen or helium) had no effect on the drying process. 相似文献
55.
A solution to the phase problem in optics is considered within the context of the analysis of signals changing in time and of the effect of an object under investigation on the probing radiation. The solution is based on the use of a twin-wave interferometer with phase modulation in one of the channels and a spectrometer. The analysis is concerned, in particular, with signals and processes of ultrashort duration. To shift the frequency in the interferometer channel, an electrooptical crystal of ferroelectric perovskite is used as the modulator. Errors of amplitude and phase structures determined are analyzed. 相似文献
56.
The crystallization of poly(ethylene terephthalate) under uniaxial tensile strain at different extension rates was investigated with optical polarimetry in a temperature range between the glass-transition temperature and the quiescent crystallization temperature. The evolution of the optical properties of the polymer, including the turbidity, birefringence, and dichroism, were monitored simultaneously with the mechanical parameters. To complete the semicrystalline microstructure characterization of the polymer under strain, an online wide-angle X-ray diffraction (WAXD) technique was used in separate experiments, which were performed under the same thermomechanical conditions. For real-time measurements, a high-energy synchrotron radiation source was used. The optical properties provided information about both the crystalline and amorphous phases, whereas the WAXD patterns essentially gave information about the crystalline phase. The two experimental techniques were then used in a complementary way to characterize the semicrystalline microstructure. Significant deviations from the stress-optical rule were found. This was attributed to both transient effects and the appearance of crystallites, which consisted of highly oriented molecular segments that could contribute to the optical anisotropy but not necessarily to the stress. The behavior of the optical dichroism was found to be qualitatively different from that of the birefringence. The latter monotonically increased with the strain, whereas the former first increased with the strain, passed through a maximum, and then decreased to a steady-state value. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1915–1927, 2004 相似文献
57.
58.
59.
In this paper, we study the stability conditions of the MMAP[K]/G[K]/1/LCFS preemptive repeat queue. We introduce an embedded Markov chain of matrix M/G/1 type with a tree structure and identify conditions for the Markov chain to be ergodic. First, we present three conventional methods for the stability problem of the queueing system of interest. These methods are either computationally demanding or do not provide accurate information for system stability. Then we introduce a novel approach that develops two linear programs whose solutions provide sufficient conditions for stability or instability of the queueing system. The new approach is numerically efficient. The advantages and disadvantages of the methods introduced in this paper are analyzed both theoretically and numerically. 相似文献
60.
本文给出了 n阶 r-不可分矩阵的本原指数的上界 ,即任 n阶 r—不可分矩阵 A的本原指数 (A)≤n+(r- ) 2r (1≤ r相似文献