全文获取类型
收费全文 | 492598篇 |
免费 | 5613篇 |
国内免费 | 1843篇 |
专业分类
化学 | 266997篇 |
晶体学 | 7227篇 |
力学 | 20535篇 |
综合类 | 9篇 |
数学 | 59346篇 |
物理学 | 145940篇 |
出版年
2020年 | 3445篇 |
2019年 | 3665篇 |
2018年 | 4122篇 |
2017年 | 3965篇 |
2016年 | 7079篇 |
2015年 | 5308篇 |
2014年 | 7573篇 |
2013年 | 22656篇 |
2012年 | 16715篇 |
2011年 | 20856篇 |
2010年 | 13256篇 |
2009年 | 13189篇 |
2008年 | 18847篇 |
2007年 | 19152篇 |
2006年 | 18185篇 |
2005年 | 16714篇 |
2004年 | 14976篇 |
2003年 | 13289篇 |
2002年 | 13013篇 |
2001年 | 14628篇 |
2000年 | 11237篇 |
1999年 | 8984篇 |
1998年 | 7376篇 |
1997年 | 7217篇 |
1996年 | 7059篇 |
1995年 | 6511篇 |
1994年 | 6184篇 |
1993年 | 6059篇 |
1992年 | 6704篇 |
1991年 | 6645篇 |
1990年 | 6236篇 |
1989年 | 6071篇 |
1988年 | 6298篇 |
1987年 | 5970篇 |
1986年 | 5762篇 |
1985年 | 8171篇 |
1984年 | 8369篇 |
1983年 | 6848篇 |
1982年 | 7468篇 |
1981年 | 7364篇 |
1980年 | 7115篇 |
1979年 | 7198篇 |
1978年 | 7439篇 |
1977年 | 7285篇 |
1976年 | 7354篇 |
1975年 | 6944篇 |
1974年 | 6908篇 |
1973年 | 7253篇 |
1972年 | 4375篇 |
1971年 | 3296篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
In recent years there has been tremendous progress in lithium niobate-based photonic switches. The various switch configurations proposed and implemented, and their characteristics, are reviewed. A summary of the device technology is given. Performance analysis based on different architecture and the size of arrays that can be relalized are discussed.Indian Institute of Science, Bangalore, India 相似文献
162.
R A Altes 《The Journal of the Acoustical Society of America》1989,85(2):934-942
Target parameters such as reflectivity, range, velocity, and angular position are represented by ordered maps of tuned cortical neurons in insectivorous bats. It is suggested that the response of each neuron in such a map is determined by a hypothesis test conditioned on a particular value of the mapped parameter. The excitation of each neuron is then interpreted as a sample value of a conditional log-likelihood ratio or a log-likelihood function. Interpolation between the samples, which is needed to find the parameter that maximizes the mapped function (e.g., the maximum likelihood parameter estimate), can be accomplished with overlapped tuning curves. An attempt to portray a sharp peak by a weighted sum of relatively broad neuronal tuning curves or interpolation functions results in excitatory center/inhibitory surround behavior. Facilitation or antifacilitation of neurons that are likely to be excited by succeeding observations can be used for sequential detection and tracking. Interpolation and pulse-to-pulse data storage capability are required to explain range jitter sensitivity and to allow for moving target indication in bat sonar. If a cortical map represents an ordered array of hypothesis tests, then many such tests are implemented in parallel when target parameters are unknown. Detection performance is then degraded relative to the idealized situation in which all parameters are specified. Performance in noise may thus appear to be much worse than that of an ideal detector, even if each hypothesis test is optimally implemented. 相似文献
163.
Time structure of iodine laser pulse in the free running mode of operation was studied using the power amplifier of laser system PERUN as the laser oscillator. Two characteristic shapes of laser pulse correspond to different regimes of laser operation, the existence of which is ascribed to the critical concentration of I2 molecules. Simultaneously an additional chemical pumping was proved at least in the initial phase of photodissociation and in a late-time lasing after the end of flashlamps pumping light. 相似文献
164.
R. R. Zapatrin 《International Journal of Theoretical Physics》1989,28(11):1323-1332
A refined definition of basic concepts for logic describing physical systems is proposed. Within the suggested formalism of generating semigroups the active logic of questions and passive logic of answers are introduced. The objects for which both logics are isomorphic are called self-adequate. It is shown that the assumption of self-adequacy implies that the object is either quantum or classical. The possibility of application of the theory to non-self-adequate objects is discussed. 相似文献
165.
Z.J. Yang T.H. Johansen H. Bratsberg G. Helgesen A.T. Skjeltorp 《Physica C: Superconductivity and its Applications》1989,160(5-6):461-465
Based on a dipole-dipole interaction model, we discuss the levitation force and related vibration problems to understand the effects of flux pinning in type-II superconductors, and the applications of levitation in superconducting systems. 相似文献
166.
The influence of an electrical field on the charge transfer in crystals with orbital degeneration is investigated. An expression is obtained for the current density in crystals with narrow d-conduction bands in transition metal compounds. As the electrical field intensity increases, a current density maximum is observed with a subsequent section of negative differential conductivity. The results of the investigation agree qualitatively with experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 14–18, February, 1989. 相似文献
167.
The rotational spectra of 34SO2F2 and S18O16OF2 have been measured in their ground vibrational state between 9 and 110 GHz. Accurate rotational constants have been derived. Various experimental structures including the average structure have been determined. The ab initio structure has been calculated at the CCSD(T) level of theory. The different structures are compared and the best equilibrium structure is the ab initio structure: re(SO)=1.401 (3) Å, re(SF)=1.532 (3) Å, ∠e(OSO)=124.91(20)°, ∠e(FSF)=95.53 (20)°. 相似文献
168.
J.J. Pan Xiangdong Qiu Kejian Guan Liang Dong FengQing Zhou Wei Wang Mingjie Zhang James Guo Jun Yang Chunmeng Wu 《光学学报》2003,23(Z1)
A low cost, coolerless 980nm diode pumped, gain flattened L band EDFA with fast transient control, high pump efficiency and gain clamping effect was realized by using FBGs as C band seed generators. 相似文献
169.
R. K. Bairamov 《Russian Journal of Applied Chemistry》2003,76(10):1699-1701
Influence exerted by additives introduced into the working solution, by iron or copper formedin electric-spark dispersion of these metals and zinc, on the quality of the products obtained in electric erosionwas studied. 相似文献
170.
The fine-structure parameters of a series of configurations of the first ion of indium and the gyromagnetic ratios in intermediate coupling are calculated by a semiempirical method in the single-configuration approximation. 相似文献