全文获取类型
收费全文 | 339239篇 |
免费 | 4095篇 |
国内免费 | 1282篇 |
专业分类
化学 | 184487篇 |
晶体学 | 4759篇 |
力学 | 13986篇 |
综合类 | 7篇 |
数学 | 37892篇 |
物理学 | 103485篇 |
出版年
2019年 | 2558篇 |
2018年 | 2996篇 |
2017年 | 2869篇 |
2016年 | 4895篇 |
2015年 | 3440篇 |
2014年 | 5004篇 |
2013年 | 15061篇 |
2012年 | 10802篇 |
2011年 | 13519篇 |
2010年 | 8780篇 |
2009年 | 8530篇 |
2008年 | 12117篇 |
2007年 | 12227篇 |
2006年 | 11566篇 |
2005年 | 10946篇 |
2004年 | 9691篇 |
2003年 | 8723篇 |
2002年 | 8556篇 |
2001年 | 9823篇 |
2000年 | 7394篇 |
1999年 | 5906篇 |
1998年 | 4982篇 |
1997年 | 5022篇 |
1996年 | 4695篇 |
1995年 | 4530篇 |
1994年 | 4199篇 |
1993年 | 4210篇 |
1992年 | 4655篇 |
1991年 | 4671篇 |
1990年 | 4382篇 |
1989年 | 4360篇 |
1988年 | 4400篇 |
1987年 | 4340篇 |
1986年 | 4109篇 |
1985年 | 5644篇 |
1984年 | 5777篇 |
1983年 | 4771篇 |
1982年 | 5332篇 |
1981年 | 5098篇 |
1980年 | 5038篇 |
1979年 | 5060篇 |
1978年 | 5414篇 |
1977年 | 5164篇 |
1976年 | 5330篇 |
1975年 | 4923篇 |
1974年 | 4886篇 |
1973年 | 5370篇 |
1972年 | 3283篇 |
1971年 | 2525篇 |
1968年 | 2424篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
991.
992.
Digital image processing techniques have been applied to the analysis of cellular smoked foil patterns from gaseous detonations. In particular, the two-dimensional autocorrelation function is applied to digital cell pattern images and an orientational correlation parameter is calculated. Taking line profiles along the directions of highest correlation provides an unbiased method of determining the mean cell size in each of the two principal directions. By analyzing the width, amplitude and angular position of the orientational correlation plots, information can be extracted concerning the cellular pattern regularity, the relative angular correlation between two sets of transverse waves in two directions, and the mean shape or elongation of the cells within the pattern. The technique is applied to smoked foils from oxyacetylene mixtures with argon dilutions ranging from 0 to 75% to quantify the increase in regularity with argon dilution. This method provides a simple and useful way of analyzing cellular patterns and constitutes a promising technique for improving smoked foil diagnostics. 相似文献
993.
S K Samanta U C Gupta R K Sharma 《The Journal of the Operational Research Society》2007,58(3):368-377
This paper investigates a discrete-time single-server finite-buffer queueing system with multiple vacations in which arrivals occur according to a discrete-time renewal process. Service and vacation times are mutually independent and geometrically distributed. We obtain steady-state system length distributions at prearrival, arbitrary and outside observer's observation epochs under the late arrival system with delayed access and early arrival system. The analysis of actual waiting-time for both the systems has also been carried out. The model has potential application in high-speed computer network, digital communication systems and other related areas. 相似文献
994.
The selection of the branching variable can greatly affect the speed of the branch and bound solution of a mixed-integer or
integer linear program. Traditional approaches to branching variable selection rely on estimating the effect of the candidate
variables on the objective function. We present a new approach that relies on estimating the impact of the candidate variables
on the active constraints in the current LP relaxation. We apply this method to the problem of finding the first feasible
solution as quickly as possible. Empirical experiments demonstrate a significant improvement compared to a state-of-the art
commercial MIP solver. 相似文献
995.
Linear mixed 0–1 integer programming problems may be reformulated as equivalent continuous bilevel linear programming (BLP)
problems. We exploit these equivalences to transpose the concept of mixed 0–1 Gomory cuts to BLP. The first phase of our new
algorithm generates Gomory-like cuts. The second phase consists of a branch-and-bound procedure to ensure finite termination
with a global optimal solution. Different features of the algorithm, in particular, the cut selection and branching criteria
are studied in details. We propose also a set of algorithmic tests and procedures to improve the method. Finally, we illustrate
the performance through numerical experiments. Our algorithm outperforms pure branch-and-bound when tested on a series of
randomly generated problems.
Work of the authors was partially supported by FCAR, MITACS and NSERC grants. 相似文献
996.
A. Puiu G. Giubileo G. Addolorato L. Revelli G. Gasbarrini R. Bellantone A. D’Amore C. P. Lombardi C. Carrozza 《Laser Physics》2007,17(4):448-452
Nowadays, there is high demand for sensitive gas sensors both for human and environmental monitoring. This paper deals with a high-resolution (0.2 ppb) laser-based photoacoustic spectroscopic system realized at ENEA Frascati, Italy, applied for monitoring stress in scuba divers during sustained immersion by analyzing breath samples. Blood tests and psychometric tests for scuba divers were performed at Catholic University in Rome. Results will be reported and discussed. 相似文献
997.
998.
E. Kapon M. Walther J. Christen M. Grundmann C. Caneau D.M. Hwang E. Colas R. Bhat G.H. Song D. Bimberg 《Superlattices and Microstructures》1992,12(4)
Quantum wire (QWR) heterostructures suitable for optoelectronic applications should meet a number of requirements, including defect free interfaces, large subband separation, long carrier lifetime, efficient carrier capture. The structural and opticl properties of GaAs/AlGaAs and InGaAs/GaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on nonplanr substrates, which satisfy many of these criteria, are described. These crescent-shaped QWRs are formed in situ during epitaxial growth resulting in virtually defect free interfaces. Effective wire widths as small as 10nm have been achieved, corresponding to electron subband separations greater than KBT at room temperature. The enhanced density of states at the QWR subbands manifests itself in higher optical absorption and emission as visualized in photoluminescence (PL), PL excitation, amplified spontaneous emission and lasing spectra of these structures. Effective carrier capture into the wires via connected quantum well regions, which is important for enhancing the otherwise extremely small capture cross section of these wires, has also been observed. Room temperature operation of GaAs/AlGaAs and strained InGaAs/GaAs QWR lasers with threshold currents as low as 0.6mA has been demonstrated. 相似文献
999.
Zboril R. Mashlan M. Machala L. Walla J. Barcova K. Martinec P. 《Hyperfine Interactions》2004,156(1-4):403-410
Hyperfine Interactions - The natural garnets from almandine (Fe3Al2Si3O12)–pyrope (Mg3Al2Si3O12) series with the iron to magnesium atomic ratio ranging from 0.2 to 1 were characterised and... 相似文献
1000.
R. Kudrawiec M. Syperek J. Misiewicz R. Paszkiewicz B. Paszkiewicz M. Tlaczala 《Superlattices and Microstructures》2004,36(4-6):633
Undoped AlGaN/GaN heterostructures with different content and thickness of AlGaN layer are investigated by photoreflectance (PR) spectroscopy. We have observed PR resonances related to an absorption in both GaN and AlGaN layers. The character of these resonances has been analyzed, and PR lines associated with excitonic and band-to-band absorption in the GaN layer and band-to-band absorption in the AlGaN layer have been identified. The transition related to band-to-band absorption possesses characteristic Franz–Keldysh oscillations (FKOs) associated with a built-in electric field. The electric field in the AlGaN layer obtained on the basis of the analysis of FKOs has been found to be in the range of 244–341 kV/cm. The value of the field has been found to decrease with the increase in AlGaN thickness and to increase with the increase in Al concentration. The surface potential for AlGaN layers has been found to increase with the increase in Al mole fraction and has been estimated to be in the range of 1.0–1.7 eV. 相似文献