全文获取类型
收费全文 | 388439篇 |
免费 | 5345篇 |
国内免费 | 3131篇 |
专业分类
化学 | 218572篇 |
晶体学 | 5160篇 |
力学 | 15513篇 |
综合类 | 76篇 |
数学 | 45647篇 |
物理学 | 111947篇 |
出版年
2021年 | 3020篇 |
2020年 | 3363篇 |
2019年 | 3599篇 |
2018年 | 4568篇 |
2017年 | 4553篇 |
2016年 | 7294篇 |
2015年 | 4942篇 |
2014年 | 6888篇 |
2013年 | 17131篇 |
2012年 | 14658篇 |
2011年 | 17471篇 |
2010年 | 11854篇 |
2009年 | 11817篇 |
2008年 | 15783篇 |
2007年 | 15829篇 |
2006年 | 14934篇 |
2005年 | 13717篇 |
2004年 | 12495篇 |
2003年 | 10732篇 |
2002年 | 10467篇 |
2001年 | 10849篇 |
2000年 | 8424篇 |
1999年 | 6347篇 |
1998年 | 5385篇 |
1997年 | 5249篇 |
1996年 | 5139篇 |
1995年 | 4576篇 |
1994年 | 4592篇 |
1993年 | 4421篇 |
1992年 | 4825篇 |
1991年 | 4882篇 |
1990年 | 4555篇 |
1989年 | 4396篇 |
1988年 | 4288篇 |
1987年 | 4169篇 |
1986年 | 4102篇 |
1985年 | 5420篇 |
1984年 | 5573篇 |
1983年 | 4609篇 |
1982年 | 4950篇 |
1981年 | 4666篇 |
1980年 | 4365篇 |
1979年 | 4666篇 |
1978年 | 4967篇 |
1977年 | 4898篇 |
1976年 | 4983篇 |
1975年 | 4627篇 |
1974年 | 4743篇 |
1973年 | 4800篇 |
1972年 | 3544篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
The aim of this paper is to investigate laminar-turbulent transition in a mixed convection phenomenon occurring in a horizontal rectangular duct. Indeed, laminar-turbulent transition is well known in the case of forced convection but the presence of secondary flow induced by natural convection on this transition is not well highlighted. In this study, we will not be concerned by determining a critical threshold value of a Reynolds number of transition but only to estimate the degree of turbulence in the transition regime, i.e. weak turbulence in the case of a mixed convection phenomenon. This is possible thanks to the application of the wavelet transform. The calculation of the Hölder exponent, associated with the maximum value of the singularity spectrum for various experimental conditions allows the degree of turbulence to be measured. The variation of the Hölder exponent versus heat flux and Reynolds number enables us to show that there are two ways to go towards turbulence: thermal by increasing heat flux and hydrodynamic by increasing fluid velocity. 相似文献
62.
A. V. Ivanov M. Yu. Tsentalovich E. G. Kogan L. G. Tomilova N. S. Zefirov 《Russian Chemical Bulletin》2008,57(8):1676-1679
The chlorination of benzene, toluene, and o-xylene with molecular chlorine in the presence of the phthalocyanine complexes of different structures was studied. The transformations
of the catalysts during the reaction were investigated.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1644–1647, August, 2008. 相似文献
63.
A. Ya. Samuilov L. A. Zenitova I. N. Bakirova Ya. D. Samuilov 《Russian Journal of Applied Chemistry》2008,81(8):1419-1422
Thermodynamic parameters of reactions of methyl and phenyl isocyanates with a series of compounds were determined by quantum-chemical calculations. The products of these reactions model for various functional groups present in commercial polyurethanes. The thermodynamic stability series for compounds formed from aliphatic and aromatic isocyanates were constructed. 相似文献
64.
65.
In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore
glasses, with mean diameters ranging from 7.9 to 23.9 nm, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR),
and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radiusR follows the simplified Gibbs-Thompson equation ΔT=k
p/(R−s) with ak
p value of 74 K · nm and ans value of 1 nm. To our knowledge, this is the first time thek
p value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T
1), spin-spin relaxation times (T
2), and diffusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular
dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component
at the pore walls and a crystalline solid at the center of the pores. However, theT
2 measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores.
The diffusivity of the major narrow-line component is approximately three orders of magnitude larger than that in the plastic
bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region,T
1 of the narrow line is significantly shorter thanT
1 of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of
the pore. 相似文献
66.
W M Niessen M A de Kraa E R Verheij P J Bergers G F La Vos U R Tjaden J van der Greef 《Rapid communications in mass spectrometry : RCM》1989,3(1):1-4
On-line sample pretreatment by means of the phase-system switching approach is an interesting technique for the analysis of aqueous samples, e.g., plasma, by means of supercritical-fluid chromatography. In order to analyse plasma samples the following analytical procedure is used. The plasma sample is injected on to a short precolumn, which is washed with water and subsequently dried with nitrogen. Next, the solutes are desorbed with the supercritical mobile phase, analysed with packed-column supercritical-fluid chromatography and detected with either a UV detector or a mass spectrometer, equipped with a moving-belt interface. The herbicide diuron is selected as a test compound to study the feasibility of this approach. Using a selective detector the procedure is sufficiently sensitive to detect diuron in plasma, but not appropriate to detect the diuron metabolites in a post-mortem plasma sample. These have been identified with liquid chromatography/mass spectrometry. The detection limit of diuron in plasma using the procedure described is about 30 ng/mL. 相似文献
67.
68.
Operators on manifolds with corners that have base configurations with geometric singularities can be analysed in the frame of a conormal symbolic structure which is in spirit similar to the one for conical singularities of Kondrat'ev's work. Solvability of elliptic equations and asymptotics of solutions are determined by meromorphic conormal symbols. We study the case when the base has edge singularities which is a natural assumption in a number of applications. There are new phenomena, caused by a specific kind of higher degeneracy of the underlying symbols. We introduce an algebra of meromorphic edge operators that depend on complex parameters and investigate meromorphic inverses in the parameter-dependent elliptic case. Among the examples are resolvents of elliptic differential operators on manifolds with edges. 相似文献
69.
V. Ballarini G. Barucca E. Bennici C. F. Pirri C. Ricciardi E. Tresso F. Giorgis 《Physica E: Low-dimensional Systems and Nanostructures》2003,16(3-4):591
All amorphous silicon-nitride planar optical microcavities operating in the visible range have been grown by plasma enhanced chemical vapor deposition. The luminescence intensity of the N-rich silicon-nitride layer from a microcavity with 6 period distributed Bragg reflectors (DBRs) is two order of magnitude higher than that of the luminescent layer without the cavity. Moreover, a strong directionality of the microcavities emission can be observed. Such results can be ascribed to the anisotropic optical density of states induced in the Fabry–Perot structure. The quality factors of the resonators are strictly correlated to the number of periods of the DBRs. 相似文献
70.
F. Grunewald B. Kunyavskii D. Nikolova E. Plotkin 《Journal of Mathematical Sciences》2003,116(1):2972-2981
We study two-variable Engel-like relations and identities characterizing finite-dimensional solvable Lie algebras and, conjecturally, finite solvable groups and introduce some invariants of finite groups associated with such relations. Bibliography: 29 titles. 相似文献