全文获取类型
收费全文 | 30815篇 |
免费 | 194篇 |
国内免费 | 279篇 |
专业分类
化学 | 10514篇 |
晶体学 | 262篇 |
力学 | 1407篇 |
综合类 | 12篇 |
数学 | 9021篇 |
物理学 | 8129篇 |
无线电 | 1943篇 |
出版年
2021年 | 62篇 |
2020年 | 36篇 |
2018年 | 1226篇 |
2017年 | 1505篇 |
2016年 | 790篇 |
2015年 | 657篇 |
2014年 | 528篇 |
2013年 | 752篇 |
2012年 | 3305篇 |
2011年 | 2465篇 |
2010年 | 1843篇 |
2009年 | 1590篇 |
2008年 | 636篇 |
2007年 | 658篇 |
2006年 | 653篇 |
2005年 | 4806篇 |
2004年 | 3967篇 |
2003年 | 2342篇 |
2002年 | 514篇 |
2001年 | 323篇 |
2000年 | 104篇 |
1999年 | 180篇 |
1998年 | 126篇 |
1997年 | 74篇 |
1996年 | 52篇 |
1995年 | 43篇 |
1994年 | 44篇 |
1992年 | 173篇 |
1991年 | 157篇 |
1990年 | 160篇 |
1989年 | 125篇 |
1988年 | 123篇 |
1987年 | 89篇 |
1986年 | 74篇 |
1985年 | 66篇 |
1984年 | 52篇 |
1983年 | 48篇 |
1982年 | 39篇 |
1979年 | 44篇 |
1976年 | 88篇 |
1975年 | 46篇 |
1974年 | 49篇 |
1973年 | 62篇 |
1972年 | 47篇 |
1971年 | 34篇 |
1970年 | 48篇 |
1969年 | 38篇 |
1968年 | 44篇 |
1967年 | 33篇 |
1966年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Torbj?rn Eltoft 《IEEE transactions on image processing》2005,14(11):1722-1735
In this paper, we introduce a new statistical distribution for modeling non-Rayleigh amplitude statistics, which we have called the Rician inverse Gaussian (RiIG) distribution. It is a mixture of the Rice distribution and the inverse Gaussian distribution. The probability density function (pdf) is given in closed form as a function of three parameters. This makes the pdf very flexible in the sense that it may be fitted to a variety of shapes, ranging from the Rayleigh-shaped pdf to a noncentral chi2-shaped pdf. The theoretical basis of the new model is quite thoroughly discussed, and we also give two iterative algorithms for estimating its parameters from data. Finally, we include some modeling examples, where we have tested the ability of the distribution to represent locale amplitude histograms of linear medical ultrasound data and single-look synthetic aperture radar data. We compare the goodness of fit of the RiIG model with that of the K model, and, in most cases, the new model turns out as a better statistical model for the data. We also include a series of log-likelihood tests to evaluate the predictive performance of the proposed model. 相似文献
42.
Fabiano Severo Rodembusch Ndya Pesce da Silveira Dimitrios Samios Leandra Franciscato Campo Valter Stefani 《Journal of Polymer Science.Polymer Physics》2003,41(4):341-350
The solution behavior of new copolymers of methyl methacrylate and benzazole dyes emitting fluorescence because of an intramolecular proton‐transfer mechanism in the electronically excited state has been investigated by static light scattering, fluorescence spectroscopy, ultraviolet–visible, and gel permeation chromatography. In the dilute regime, with tetrahydrofuran (THF) and chloroform as solvents, the copolymers behave as typical polydisperse linear chains in good solvents. The analysis of the osmotic modulus for concentrated solutions in THF (c ≥ 60 g L?1) indicates the existence of an interchain association mechanism. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 341–350, 2003 相似文献
43.
The reaction of a new fluorinated unsymmetrical allenylazine with dimethyl acetylenedicarboxylate and phenylacetylene affords the combined intra-intermolecular criss-cross cycloaddition products, 2,3-disubstituted-1,10-diazatricyclo[5.2.1.04,10]deca-2,6-diene derivatives. The products contain three fused five-membered rings with two nitrogen atoms within an unsaturated heterocyclic system. The structures were assigned using 2D NMR correlations and in the case of the phenylacetylene adduct by X-ray structure analysis. 相似文献
44.
Ewa Stró?yna 《Journal of Differential Equations》2003,193(1):239-259
We solve completely the problem of classification of germs of complex planar vector fields with nilpotent singularity with respect to formal orbital equivalence. 相似文献
45.
A monolithic microwave frequency divider IC with an operating range of 1.4?5.3 GHz was developed and fabricated in a standard bipolar technology. The circuit operates on the principle of `regenerative frequency division?. Compared to the most popular divider concepts based on a master-slave D-flip-flop, an almost twice as high input frequency can be divided, provided that the same technology is used. A further advantage is the low power consumption. 相似文献
46.
Parameters of Gaussian multivariate models are often estimated using the maximum likelihood approach. In spite of its merits, this methodology is not practical when the sample size is very large, as, for example, in the case of massive georeferenced data sets. In this paper, we study the asymptotic properties of the estimators that minimize three alternatives to the likelihood function, designed to increase the computational efficiency. This is achieved by applying the information sandwich technique to expansions of the pseudo-likelihood functions as quadratic forms of independent normal random variables. Theoretical calculations are given for a first-order autoregressive time series and then extended to a two-dimensional autoregressive process on a lattice. We compare the efficiency of the three estimators to that of the maximum likelihood estimator as well as among themselves, using numerical calculations of the theoretical results and simulations. 相似文献
47.
In this paper, by using elementary analysis, we establish some new Lyapunov-type inequalities for nonlinear systems of differential equations, special cases of which contain the well-known equations such as Emden-Fowler-type and half-linear equations. The inequalities obtained here can be used as handy tools in the study of qualitative behaviour of solutions of the associated equations. 相似文献
48.
49.
E. Beinrohr M. Németh P. Tsch?pel G. T?lg 《Fresenius' Journal of Analytical Chemistry》1992,344(3):93-99
Summary Flow-through electrochemical cells with porous working electrodes made of crushed reticulated vitreous carbon and plated with mercury were used for absolute analysis of trace amounts of lead by anodic stripping coulometry with collection (ASCWC) in a flow system. The role of mercury coating, flow rate and pH were investigated. The coulombic content of the collection peak corresponded to the theoretical values calculated by Faraday's law in a concentration range from about 10–9 to 10–6 mol/l. The relative error and the relative standard deviation was +0.15% and 0.8%, respectively for 2×10–6 mol/l analyte concentration. The absolute detection limit (3 s) was 0.1 ng of Pb, the linear response range 7×104.
One leave from: Department of Analytical Chemistry, Slovak Technical University, CS-812 37 Bratislava, Czechoslovakia 相似文献
50.
Celik Tarimci Anda? Karamustafa Kutalmi? Güven Mehmet Kabak Akgül Ye?ilada 《Analytical sciences》2003,19(9):1347-1348
The crystal structure of 1-[3-(3,4-dimethoxyphenyl)-2-propenoyl]pyrrolidine (C15H19NO3) (I) has been determined by X-ray analysis. It crystallizes orthorhombic space group Pbca with a = 24.295(3), b = 15.086(3), c = 7.552(3)A, V = 2768(1)A3, Z = 8, Dcalc = 1.254 g/cm3, mu = (Mo K(alpha)) = 0.87 cm(-1). The title compound has analgesic activity of cycloaliphatic amine part. The molecule is deviated from planar configuration. 相似文献