全文获取类型
收费全文 | 40450篇 |
免费 | 5313篇 |
国内免费 | 4438篇 |
专业分类
化学 | 18148篇 |
晶体学 | 363篇 |
力学 | 1589篇 |
综合类 | 325篇 |
数学 | 3142篇 |
物理学 | 10141篇 |
综合类 | 16493篇 |
出版年
2024年 | 160篇 |
2023年 | 632篇 |
2022年 | 1111篇 |
2021年 | 1198篇 |
2020年 | 1274篇 |
2019年 | 1146篇 |
2018年 | 1077篇 |
2017年 | 1089篇 |
2016年 | 1407篇 |
2015年 | 1673篇 |
2014年 | 2163篇 |
2013年 | 2580篇 |
2012年 | 3027篇 |
2011年 | 3226篇 |
2010年 | 2487篇 |
2009年 | 2660篇 |
2008年 | 2836篇 |
2007年 | 2632篇 |
2006年 | 2391篇 |
2005年 | 2045篇 |
2004年 | 1676篇 |
2003年 | 1316篇 |
2002年 | 1416篇 |
2001年 | 1246篇 |
2000年 | 1006篇 |
1999年 | 1041篇 |
1998年 | 766篇 |
1997年 | 730篇 |
1996年 | 714篇 |
1995年 | 595篇 |
1994年 | 554篇 |
1993年 | 414篇 |
1992年 | 339篇 |
1991年 | 347篇 |
1990年 | 293篇 |
1989年 | 231篇 |
1988年 | 193篇 |
1987年 | 149篇 |
1986年 | 107篇 |
1985年 | 49篇 |
1984年 | 33篇 |
1983年 | 35篇 |
1982年 | 31篇 |
1981年 | 23篇 |
1980年 | 20篇 |
1979年 | 6篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1959年 | 5篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
The classical (first integral) methods of studying restrictions on the possible motion of gravitationally interacting bodies are reviewed. The simple and flexible inequality method is extended to a similar approach to the relationship between possible forbidden motion and (asymptotic) symmetries in the relativistic few-body problem. This extended method is used to reproduce the standard results of bounded motion for a test particle in a Schwarzschild geometry. The existence of bounded motion is shown for the general relativistic few-body problem and the difficulties in determining such bounded motion is analysed. The use of this approach to obtaining a relativistic alternative to the classical Roche lobe analysis in contact binaries is discussed. 相似文献
43.
Dirichlet boundary value problems for perturbed second-order differential equations on a half line are investigated in this paper. The methods mainly depend on the calculus of variations to the classical functionals. Sufficient conditions are obtained for the existence of the solutions. 相似文献
44.
45.
This paper investigates mutual influence of duct and room acoustics in the whole fan-duct-plenum-room integrations. Applying the parametric design language of finite element software ANSYS (APDL), dimensional and positional influence on system acoustics has been studied. Models with different room dimensions, duct lengths, duct cross-sections, duct locations, duct discharges and duct elbow were constructed, and their characteristics were compared qualitatively. Results show that small rooms, short ducts, large duct cross-sections and bell mouth duct discharges help to increase room sound pressure levels (SPLs); SPLs in ducts and plenums are sensitive to duct dimensions and duct discharge types but insensitive to duct locations and room dimensions; duct elbows have relatively indistinct acoustic influence in each component. Based on the calculation results, a semi-experimental method was proposed for simply and approximately evaluating indoor acoustic spectra of fan-duct-plenum-room integrations, then an example was used to demonstrate the prediction process. Finally, by adopting several ideal models, sound field constitutions, duct and room wall admittances and duct end reflection were explored quantitatively. This study may give a detailed understanding of fan-duct-plenum-room acoustics for researchers, also it might provide a new, simple and approximate prediction method for professionals to evaluate and improve fan-ducted acoustics. 相似文献
46.
The phase boundary theory and the contact rule of phase regions are compared, and some weaknesses of the latter are manifested.
The comparison between the Gupta’s method and the boundary theory method for constructing multicomponent isobaric sections
is also presented. 相似文献
47.
Recently, molecular imaging has been rapidly developed to studyphysiological and pathological processes in vivo at the cellularand molecular levels. Among molecular imaging modalities, opticalimaging has attracted a major attention for its unique advantages.In this paper, we establish a mathematical framework for multispectralbioluminescence tomography (BLT) that allows simultaneous studiesof multiple optical reporters. We show solution existence, uniquenessand continuous dependence on data as well as the limiting behaviourswhen the regularization parameter approaches zero or when thepenalty parameter approaches infinity. Then, we propose twonumerical schemes for multispectral BLT and derive error estimatesfor the corresponding solutions. 相似文献
48.
Z.G. Xiao R.J. HuH.Y. Wu G.M. JinZ.Y. Li L.M. DuanH.W. Wang B.G. ZhangS.F. Wang Z.Y. WeiH.S. Xu Y.T. ZhuS.L. Li F. FuX.H. Yuan Z.Q. Feng 《Physics letters. [Part B]》2006
The inclusive reduced velocity correlation functions of the intermediate mass fragments were measured in the reactions of 36Ar + 112,124Sn at 35 MeV/u. The anti-correlation is observed to be stronger in 36Ar + 124Sn system than that in 36Ar + 112Sn. The difference of the correlation functions between the two reactions is mainly contributed by the particle pairs with high momenta. A three body Coulomb repulsive trajectory model is employed to calculate the emission time scale of the IMFs for the two systems. The time scale is 150 fm/c in 36Ar + 112Sn and 120 fm/c in the 36Ar + 124Sn, respectively. 相似文献
49.
Characterization of iron surface modified by 2-mercaptobenzothiazole self-assembled monolayers 总被引:1,自引:0,他引:1
A self-assembled monolayer of 2-mercaptobenzothiazole (MBT) adsorbed on the iron surface was prepared. The films were characterized by electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared reflection spectroscopy (FT-IR) and scanning electron microscopy (SEM). Besides, the microcalorimetry method was utilized to study the self-assembled process on iron surface and the adsorption mechanism was discussed from the power-time curve. The results indicated that MBT was able to form a film spontaneously on iron surface and the presence of it could protect iron from corrosion effectively. However, the assembling time and the concentration influence the protection efficiency. Quantum chemical calculations, according to which adsorption mechanism was discussed, could explain the experimental results to some extent. 相似文献
50.
In this paper, multi-objective models for designing 3D trajectory of horizontal wells are developed in a fuzzy environment. Here, the objectives of minimizing the length of the trajectory and the error of entry target point are fuzzy in nature. Some parameters, such as initial value, end value, lower bound and upper bound of the curvature radius, tool-face angle and the are length of each curve section, are also assumed to be vague and imprecise. The impreciseness in the above objectives have been expressed by fuzzy linear membership functions and that in the above parameters by triangular fuzzy numbers. Models have been solved by the fuzzy non-linear programming method based on Zimmermann [1] and Lee and Li [2]. Models are applied to practical design of the horizontal wells. Numerical results illustrate the accuracy and efficiency of the fuzzy models. 相似文献