全文获取类型
收费全文 | 204525篇 |
免费 | 1970篇 |
国内免费 | 598篇 |
专业分类
化学 | 102709篇 |
晶体学 | 3348篇 |
力学 | 9318篇 |
综合类 | 5篇 |
数学 | 22540篇 |
物理学 | 69173篇 |
出版年
2021年 | 1667篇 |
2020年 | 1816篇 |
2019年 | 2007篇 |
2018年 | 2705篇 |
2017年 | 2607篇 |
2016年 | 3773篇 |
2015年 | 2307篇 |
2014年 | 3619篇 |
2013年 | 9122篇 |
2012年 | 7267篇 |
2011年 | 8763篇 |
2010年 | 6053篇 |
2009年 | 5928篇 |
2008年 | 7935篇 |
2007年 | 8104篇 |
2006年 | 7745篇 |
2005年 | 6912篇 |
2004年 | 6450篇 |
2003年 | 5678篇 |
2002年 | 5504篇 |
2001年 | 6436篇 |
2000年 | 4945篇 |
1999年 | 3799篇 |
1998年 | 3048篇 |
1997年 | 3059篇 |
1996年 | 2799篇 |
1995年 | 2622篇 |
1994年 | 2472篇 |
1993年 | 2347篇 |
1992年 | 2701篇 |
1991年 | 2676篇 |
1990年 | 2607篇 |
1989年 | 2401篇 |
1988年 | 2456篇 |
1987年 | 2454篇 |
1986年 | 2291篇 |
1985年 | 2979篇 |
1984年 | 3225篇 |
1983年 | 2639篇 |
1982年 | 2808篇 |
1981年 | 2779篇 |
1980年 | 2644篇 |
1979年 | 2737篇 |
1978年 | 2826篇 |
1977年 | 2831篇 |
1976年 | 2891篇 |
1975年 | 2683篇 |
1974年 | 2653篇 |
1973年 | 2783篇 |
1972年 | 1766篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
An M/G/1 retrial queueing system with additional phase of service and possible preemptive resume service discipline is considered. For an arbitrarily distributed retrial time distribution, the necessary and sufficient condition for the system stability is obtained, assuming that only the customer at the head of the orbit has priority access to the server. The steady-state distributions of the server state and the number of customers in the orbit are obtained along with other performance measures. The effects of various parameters on the system performance are analysed numerically. A general decomposition law for this retrial queueing system is established. 相似文献
82.
This paper deals mainly with generalizations of results in finitary combinatorics to infinite ordinals. It is well-known that for finite ordinals ∑bT<αβ is the number of 2-element subsets of an α-element set. It is shown here that for any well-ordered set of arbitrary infinite order type α, ∑bT<αβ is the ordinal of the set M of 2-element subsets, where M is ordered in some natural way. The result is then extended to evaluating the ordinal of the set of all n-element subsets for each natural number n ≥ 2. Moreover, series ∑β<αf(β) are investigated and evaluated, where α is a limit ordinal and the function f belongs to a certain class of functions containing polynomials with natural number coefficients. The tools developed for this result can be extended to cover all infinite α, but the case of finite α appears to be quite problematic. 相似文献
83.
84.
A. J. Parsons S. D. J. Inglethorpe D. J. Morgan A. C. Dunham 《Journal of Thermal Analysis and Calorimetry》1997,48(1):49-62
Using a system based on non-dispersive infrared (NDIR) detectors, evolved gas analysis (EGA) was able to identify and quantify the principal volatiles produced by heating powdered samples of UK brick clays. From these results, atmospheric emissions likely to result from brick production can be predicted. In addition, EGA results for extruded brick clay test pieces are significantly different from those of powdered samples. Within an extruded brick clay body, evolved gases are contained within a pore system and evolved gas-solid phase reactions also occur. This EGA study provides further evidence on the nature of firing reactions within brick clay bodies. The qualitative and quantitative influence of heating rate — a key process condition in brick manufacture — on gas release is also outlined.Dedicated to Dr. Robert Mackenzie on the occasion of his 75th birthday 相似文献
85.
86.
R.C. Viscovini E.M. Telles A. Scalabrin D. Pereira 《Applied physics. B, Lasers and optics》1997,65(1):33-35
2 laser of wide tunability to pump 13CD3OD we could observe 10 new far infrared laser lines ranging from 109 μm to 472 μm. The lines were generated by pumping infrared
absorbing transitions of large offset appertaining to the C-O stretching Q-branch. All lines were characterised in wavelength,
polarisation, optimum pressure of operation and precise offset measurements.
Received: 17 September 1996/Revised version: 18 November 1996 相似文献
87.
L. K. Aminov I. N. Kurkin D. A. Lukoyanov K. P. Chernov 《Physics of the Solid State》1997,39(8):1184-1186
Results are reported for measurements of the spin-lattice relaxation times of E′1 centers in quartz glass, produced by neutron irradiation, with the measurements made at two frequencies 9.25 and 24.0 GHz
over a wide temperature interval 1.5–300 K. The experimental data are interpreted on the basis of interaction mechanisms of
the spins with two-level systems with excitation energies ∼6, ∼26, and ∼420 cm−1. A small modification of the existing theory allows us to explain a number of features of the observed temperature and frequency
dependence of the relaxation rate. The results are compared with the data available in the literature on spin-lattice relaxation
of irradiation centers in crystalline quartz and quartz glass.
Fiz. Tverd. Tela (St. Petersburg) 39, 1335–1337 (August 1997) 相似文献
88.
89.
We provide a characterization of the limit periodic sets for analytic families of vector fields under the hypothesis that the first jet is non-vanishing at any singular point. Also, applying the family desingularization method, we reduce the complexity of some of these sets. 相似文献
90.
Whittum D.H. Ebihara K. Hiramatsu S. Kishiro J.-i. Monaka T. Ozaki T. Takayama K. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1993,21(1):136-141
Studies of microwave amplification with an in-focused electron beam drawn from an induction injector are reported. A free-electron laser (FEL) operating at 9.4 GHz and employing ion-focusing within the interaction region has achieved power in excess of 30 MW at 9.4 GHz, with a beam energy of 0.8 MeV and current of 0.7 kA. Peak gain is 20 dB/m, with no saturation after 15 wiggler periods. Also reported are the first evolution and detuning data for an ion-channel laser/maser (ICL). Two shortcomings of the prematurely halted ICL studies are poor frequency discrimination and a large axial plasma gradient. Prospects for operation with an upgraded 1.6 MeV accelerator are discussed 相似文献