全文获取类型
收费全文 | 174310篇 |
免费 | 1572篇 |
国内免费 | 451篇 |
专业分类
化学 | 83173篇 |
晶体学 | 2033篇 |
力学 | 6718篇 |
综合类 | 7篇 |
数学 | 15492篇 |
物理学 | 48955篇 |
无线电 | 19955篇 |
出版年
2020年 | 1240篇 |
2019年 | 1295篇 |
2018年 | 1607篇 |
2017年 | 1612篇 |
2016年 | 2721篇 |
2015年 | 1683篇 |
2014年 | 2703篇 |
2013年 | 6787篇 |
2012年 | 5293篇 |
2011年 | 6683篇 |
2010年 | 4688篇 |
2009年 | 4966篇 |
2008年 | 6587篇 |
2007年 | 6785篇 |
2006年 | 6369篇 |
2005年 | 5971篇 |
2004年 | 5395篇 |
2003年 | 4723篇 |
2002年 | 4725篇 |
2001年 | 5187篇 |
2000年 | 4130篇 |
1999年 | 3321篇 |
1998年 | 2964篇 |
1997年 | 2914篇 |
1996年 | 2627篇 |
1995年 | 2543篇 |
1994年 | 2502篇 |
1993年 | 2288篇 |
1992年 | 2538篇 |
1991年 | 2620篇 |
1990年 | 2392篇 |
1989年 | 2365篇 |
1988年 | 2271篇 |
1987年 | 1956篇 |
1986年 | 1914篇 |
1985年 | 2404篇 |
1984年 | 2511篇 |
1983年 | 2092篇 |
1982年 | 2247篇 |
1981年 | 2129篇 |
1980年 | 2052篇 |
1979年 | 2158篇 |
1978年 | 2311篇 |
1977年 | 2187篇 |
1976年 | 2227篇 |
1975年 | 2122篇 |
1974年 | 2161篇 |
1973年 | 2160篇 |
1972年 | 1434篇 |
1971年 | 1288篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Providing application-level QoS in 3G/4G wireless systems: a comprehensive framework based on multirate CDMA 总被引:3,自引:0,他引:3
Fitzek F. Kopsel A. Wolisz A. Krishnam M. Reisslein M. 《Wireless Communications, IEEE》2002,9(2):42-47
The emerging applications for 3G and 4G wireless systems typically require highly heterogeneous and time-varying quality of service from the underlying protocol layers. The wireless links, however, provide only an unreliable communication channel that suffers from temporal outages. As a consequence, protocol mechanisms are needed that, based on the unreliable wireless links, provide the different service qualities required by the emerging applications. We identify the emerging IP-based applications for 3G and 4G wireless systems and categorize their QoS requirements. We discuss the wireless access mechanisms that show promise as the basis for supporting these applications. We then propose a set of protocol mechanisms that, based on the discussed wireless access mechanisms, provide the required QoS for the different application categories. 相似文献
62.
A new bipolar differential input/output current-controlled current source (CCCS) is described. The basic cell consists of a translinear array of six transistors with two bipolar inputs, and is suited for the input stage of a differential current-mode operational amplifier. 相似文献
63.
An analytical current-voltage (I -V ) model for planar-doped HEMTs is developed. This compact model covers the complete range of I -V characteristics, including the current saturation region and parasitic conduction in the electron-supplying layer. Analytical expressions for the small-signal parameters and current-gain cutoff frequency are derived from the I -V model. Modeling results for a 0.1-μm-gate planar-doped AlInAs-GaInAs HEMT show excellent agreement with measured characteristics. Threshold voltages and parasitic conduction in planar-doped and uniformly doped HEMTs are also compared and discussed 相似文献
64.
65.
66.
For any finite dimensional control system with arbitrary cost, Pontryagin's Maximum Principle (PMP) [N. Bensalem, Localisation des courbes anormales et problème d'accessibilité sur un groupe de Lie hilbertien nilpotent de degré 2, Thèse de doctorat, Université de Savoie, 1998. [6]] gives necessary conditions for optimality of trajectories. In the infinite dimensional case, it is well known that these conditions are no more true in general. The purpose of this paper is to establish an “approached” version of PMP for infinite dimensional bilinear systems, with fixed final time and without constraints on the final state. Moreover, if the set of control is contained in a closed bounded convex subset with operators defining its dynamics are compact, or if it is contained in a finite dimensional space, we get an “exact” version of PMP. We also give two applications of these results. The first one deals with sub-Riemannian geometry on nilpotent Hilbertian Lie groups for which we can define a sub-Riemannian distance. The second one deals with heat equation for which we analyse the necessary conditions to give the optimal controls. 相似文献
67.
68.
Summary High-purity indium was analysed by spark source mass spectrometry, using electrical and photoplate detection. For the calibration of the differences in elemental sensitivity, a standard sample was prepared in which 10 impurities were determined by graphite furnace atomic absorption spectrometry. In this way accuracies of less than 40% were obtained for relatively homogeneous elements at ppm and sub-ppm level. About 40 elements could be determined with detection limits of 10 to 30 ppb. Two pattern recognition methods, principal component analysis and clustering analysis, were applied to obtain information on trace element distribution, which indicated that a number of elements were strongly spatially correlated in the analysed sample.
On leave from: Department of Chemistry, Nanjing Normal University, Nanjing, People's Republic of China 相似文献
Chemische Analyse und Verteilungsbestimmung von Spurenelementen in Indium-Matrix durch Funkenquellen-Massenspektrometrie
On leave from: Department of Chemistry, Nanjing Normal University, Nanjing, People's Republic of China 相似文献
69.
T. Yu. Gumerov A. F. Dobrynina R. A. Yusupov V. P. Barabanov 《Russian Journal of Applied Chemistry》2006,79(12):1935-1939
The behavior of real disperse systems with organic impurities was examined under conditions of destabilization by inorganic coagulants. A number of physicochemical methods of analysis and a simulation method were used to identify the character and type of the complexes formed and to calculate their stability constants. 相似文献
70.
The electro-optic response of ferroelectric smectic C* liquid crystals has been studied. Anomalous switching behaviour of such materials which possess a negative dielectric anisotropy has been reported. These materials show a minimum in response time at a sufficiently high field. We present results showing the dependency of this minimum upon spontaneous polarisation and the effect of AC bias. Calculations based upon the equation of motion of the director around the cone are presented which describe this effect and its dependence on the relative magnitudes of the spontaneous polarization and dielectric anisotropy of the material. Good agreement with the experimental results is found. 相似文献