全文获取类型
收费全文 | 2748篇 |
免费 | 228篇 |
国内免费 | 199篇 |
专业分类
化学 | 1706篇 |
晶体学 | 59篇 |
力学 | 85篇 |
综合类 | 26篇 |
数学 | 344篇 |
物理学 | 955篇 |
出版年
2023年 | 50篇 |
2022年 | 66篇 |
2021年 | 63篇 |
2020年 | 75篇 |
2019年 | 76篇 |
2018年 | 81篇 |
2017年 | 84篇 |
2016年 | 102篇 |
2015年 | 82篇 |
2014年 | 112篇 |
2013年 | 139篇 |
2012年 | 194篇 |
2011年 | 186篇 |
2010年 | 126篇 |
2009年 | 143篇 |
2008年 | 154篇 |
2007年 | 130篇 |
2006年 | 109篇 |
2005年 | 104篇 |
2004年 | 72篇 |
2003年 | 78篇 |
2002年 | 73篇 |
2001年 | 45篇 |
2000年 | 32篇 |
1999年 | 35篇 |
1998年 | 26篇 |
1997年 | 20篇 |
1996年 | 32篇 |
1995年 | 37篇 |
1994年 | 26篇 |
1993年 | 33篇 |
1992年 | 40篇 |
1991年 | 26篇 |
1990年 | 30篇 |
1989年 | 32篇 |
1988年 | 21篇 |
1987年 | 23篇 |
1986年 | 23篇 |
1985年 | 30篇 |
1984年 | 32篇 |
1983年 | 27篇 |
1982年 | 15篇 |
1980年 | 14篇 |
1979年 | 19篇 |
1977年 | 13篇 |
1976年 | 17篇 |
1975年 | 13篇 |
1974年 | 17篇 |
1973年 | 25篇 |
1972年 | 14篇 |
排序方式: 共有3175条查询结果,搜索用时 15 毫秒
991.
We made an attempt to study the propagation of multidimensional ion-acoustic waves in multicomponent plasmas that include negative ions by deriving a Kodomtsev-Petviashvili (K-P) equation. The negative ions, as expected, introduce the precursor on the propagation because of which the need arises to derive a modified K-P equation with higher order non-linearity. We have examined a number of solitons like waves and supported the results by numerical simulation. The results reveal the existence of compressive and rarefactive solitary waves which are comparable with those of one-dimensional solitary waves in plasmas with negative ions except that multidimensional propagation of ion-acoustic waves is more involved than unidirectional propagation. 相似文献
992.
U. C. Ray A. K. Gupta M. N. Sen 《International Journal of Infrared and Millimeter Waves》1989,10(9):1165-1179
Frequency response of W-band Publsed IMPATT diodes operating over varying ambient temperature has been discussed in details. A method has been suggested to compensate the chirp fluctuation with the ambient temperature. Also discussed quantiatively the control of chirp bandwidth by bias current ramping. 相似文献
993.
For a general class of unipolar, rotationally symmetric distributions on the multi-dimensional unit spherical surface, a characterization of locally best rotation-invariant test statistics is exploited in the construction of locally best rotation-invariant rank tests for modal location. Allied statistical distributional problems are appraised, and in the light of these assessments, asymptotic relative efficiency of a class of rotation-invariant rank tests (with respect to some of their parametric counterparts) is studied. Finite sample permutational distributional perspectives are also appraised. 相似文献
994.
Richard A. Register Ashish Sen Robert A. Weiss Chi Li Stuart Cooper 《Journal of Polymer Science.Polymer Physics》1989,27(9):1911-1925
Carboxy-terminated polybutadiene neutralized with Cu2+ (CuPBD) and its blend with poly(styrene-co-4-vinylpyridine) (SVP) were examined by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), small-angle x-ray scattering (SAXS), and extended x-ray absorption fine structure (EXAFS) spectroscopy. The DSC results indicate that in the blend substantial mixing occurs in the CuPBD-rich phase, although complete miscibility is not achieved, and the SVP-rich phase remains relatively pure. The TEM micrographs indicate that the morphology, while irregular, is reasonably described as bicontinuous, with a domain size of order 100 nm. The SAXS patterns show that the ionic aggregates present in CuPBD are destroyed upon blending, which is interpreted as being due to steric hindrances between ionic groups coordinated to vinylpyridine nitrogens. The EXAFS radial structure function of the blend exhibits a marked decrease in the Cu-Cu peak in comparison with CuPBD, indicating a change in local structure upon blending. The results indicate that some of the SVP is miscible with CuPBD owing to complexation between the pendant pyridine groups and the Cu2+ ions, which disrupts the ionic aggregates. However, the two materials are not fully miscible, leading to a rather coarse two-phase morphology. 相似文献
995.
The kinetics of the oxidation of styrene, cinnamic acid, and some of their substituted derivatives by hexachloroiridate(IV) in dimethyl formamide–water mixtures and in the presence of perchloric acid have been investigated. The reactions appear to proceed via the formation of an unstable intermediate 1:1 complex between iridium(IV) and the substrate, followed by the decomposition of the complex in the rate‐determining step. Correlation with σ yielded ρ values of ?4.0 and ?3.5 which suggests the formation of a cationic intermediate in the rate‐determining step of the reaction. Subsequent cleavage of the carbon–carbon bond yielded the product aldehydes. Thermodynamic and activation parameters associated with the equilibrium and the rate‐determining steps were also evaluated. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 411–417, 2002 相似文献
996.
M. De la Sen 《分析论及其应用》1997,13(3):29-46
The importance of the zeros of multivariable linear systems is well-known in terms of measure obstructions to the controllability
and the observability. In this paper, a recursive decomposition of interconnected systems is outlined by taking into account
the sequential structure of the connections. The paper extends the coordinate, module-theoretic studies from the elementary
algebraic systems theory to include the case of such linear interconnected systems which need not to be controllable or observable.
Also, the properties of controllability and observability, the decoupling zeros and the signal bloking issues are characterized. 相似文献
997.
采用量子化学的密度泛函理论(DFT),在B3LYP/6 31G*水平上研究了(4 溴甲基双环[4. 4. 1] 1, 3, 5,7, 9 十一碳五烯基3 )甲醇环氧化反应机理,在对反应物和产物几何结构优化的基础上,搜索优化了反应的过渡态结构,通过振动分析和内禀反应坐标(IRC)跟踪方法对过渡态及其所处的反应路径进行了确认,并用电荷密度拓扑分析方法考察了反应过程中旧键断裂和新键形成的细节.计算结果表明,该反应中的消除反应和成环反应是协同进行的,溴化氢消除反应中的氢来自与O( 22 )相连的羟基氢,反应的活化势垒为139. 2kJ/mol,反应放热25. 1kJ/mol. 相似文献
998.
Sen Yang Kang Sun William M. Risen 《Journal of Polymer Science.Polymer Physics》1990,28(10):1685-1697
The glass transition temperatures and heat capacity changes in the transition region are reported for six sulfonated linear polystyrenes in the hydrogen form, H-SPS, in the 3.4–20.1 mol % sulfonation range and 76 metal SPS ionomers in the 3.4–12.8 mol % range. The metals are those which interact predominantly ionically and include +1, +2, and +3 ions of the alkali metal, alkaline earth, and rare earth (lanthanide) series. The results show the effect of H2O or coordinating ligands on glass transition temperatures (Tg) and the importance of eliminating it to obtaining reproducible values for Tg and ΔCp. The Tg values of dry M-SPS ionomers depend only on the sulfonation level despite wide variation in metal ion charge and size. The variation of ΔCp with sulfonation level is interpreted as showing that at high levels a few unsulfonated styrene units adjacent to sulfonated ones are constrained, presumably by clustering, from participation in the polystyrene-like cooperative rearrangements in the transition region. 相似文献
999.
Jana Jurečková Pranab Kumar Sen 《Annals of the Institute of Statistical Mathematics》1990,42(2):345-357
For a general (real) parameter, let M
nbe the M-estimator and M
n
(1) be its one-step version (based on a suitable initial estimator M
n
(0)). It is known that, under certain regularity conditions, n(M
n
(1)-M
n)=O
p(1). The asymptotic distribution of n(M
n
(1)-M
n) is studied; it is typically non-normal and it reveals the role of the initial estimator M
n
(0).Work of this author was partially supported by the Office of Naval Research, Contract No. N00014-83-K-0387 相似文献
1000.