全文获取类型
收费全文 | 941篇 |
免费 | 19篇 |
国内免费 | 6篇 |
专业分类
化学 | 429篇 |
晶体学 | 3篇 |
力学 | 33篇 |
数学 | 249篇 |
物理学 | 252篇 |
出版年
2022年 | 16篇 |
2021年 | 22篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 8篇 |
2017年 | 10篇 |
2016年 | 13篇 |
2015年 | 14篇 |
2014年 | 15篇 |
2013年 | 41篇 |
2012年 | 33篇 |
2011年 | 39篇 |
2010年 | 27篇 |
2009年 | 28篇 |
2008年 | 43篇 |
2007年 | 39篇 |
2006年 | 40篇 |
2005年 | 35篇 |
2004年 | 20篇 |
2003年 | 22篇 |
2002年 | 23篇 |
2001年 | 11篇 |
2000年 | 19篇 |
1998年 | 6篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 15篇 |
1993年 | 12篇 |
1992年 | 14篇 |
1991年 | 8篇 |
1990年 | 19篇 |
1989年 | 10篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 16篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 18篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 10篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 19篇 |
1976年 | 12篇 |
1975年 | 12篇 |
1974年 | 11篇 |
1973年 | 10篇 |
1968年 | 7篇 |
排序方式: 共有966条查询结果,搜索用时 15 毫秒
61.
Xu J Mojet BL van Ommen JG Lefferts L 《The journal of physical chemistry. B》2005,109(39):18361-18368
The adsorption of oxygen and d2-propane (CH3CD2CH3) on a series of alkaline-earth-exchanged Y zeolite at room temperature was studied with in situ infrared spectroscopy. Surprisingly at room temperature, oxygen adsorption led to the formation of supercage M2+(O2) species. Further, at low propane coverage, propane was found to adsorb linearly on Mg2+ cations, but a ring-adsorption structure was observed for propane adsorbing on Ca2+, Sr2+, and Ba2+ cations. It is demonstrated that O2 and propane can simultaneously attach to one active center (M2+) to form a M2+(O2)(C3H8) species, which is proposed to be the precursor in thermal propane selective oxidation. Selectivity to acetone in the propane oxidation reaction decreases with increasing temperature and cation size due to the formation of 2-propanol and carboxylate ions. An extended reaction scheme for the selective oxidation of propane over alkaline earth exchanged Y zeolites is proposed. 相似文献
62.
A previous publication1 utilizing the INDO2 and CND0/23 semi-empirical SCF-MO techniques has shown that these methods predict anomalous minima in the potential surfaces of small molecules and molecular ions, particularly those of type 0-X-0. Two studies have bcmen published concerning the removal of such anomalous pre-dictiors. The first approach4 involved reparameterization of the INDO method – specifically involving the Slater exponents α, and the bonding parameters B° of carbon and oxygen – to remove the anomalous iinimum from the potential surface of C02. It was de-monstrated that reparameterization could be used to remove the Salse minimum in this case; but only at the expense of the other predictive qualities of INDO, or with the prediction of an unreal-istically broad energy minimum – neither of which could b e con-sidered an acceptable solution. 相似文献
63.
This paper describes seismic performance and evaluation for composite-moment frames (C-MF) with a new type of bolted connections. The study is purely analytical and explores the effort needed to establish new connection parameters without large-scale physical testing. The innovative aspects of this research are in the use of partial restraint (PR) connections between steel beams and concrete-filled tube (CFT) columns that utilize a combination of low-carbon steel and shape memory alloy (SMA) components as the main force transfer elements in the connections. The intent is to utilize the recentering provided by super-elastic shape memory tension bars to reduce building damage and residual drift after a major earthquake, and the energy dissipation of low-carbon steel components in parallel. Accurate modeling and computational efficiency were achieved through the use of a simplified joint element which includes all connection strength and deformation components. Four- and six-story C-MF models were designed for a high seismic zone in the western USA. Two connection types and three column systems installed at these prototype frame models were investigated through nonlinear pushover and dynamic analyses. The C-MF models with new bolted connections were compared to those with traditional welded connections. The results of numerical analysis demonstrate that C-MF with new PR connections show superior structural performance as indicated by small residual deformation and better distribution of the demand over the height of the structure. 相似文献
64.
Although much discussed, the original intent of Problem 10 of the Moscow Mathematical Papyrus continues to be an unresolved question. This essay proposes a possible solution to the issue by joining together aspects of two theories on the subject advanced in the 1930s by W. W. Struve and T. E. Peet. From this point of beginning it is shown that the problem’s “basket” may originally have been intended to be understood as being of a specific actual size, thus revealing a set of correlations and insights regarding early Middle Kingdom mathematical capabilities and grain measurement concerns. 相似文献
65.
66.
Arturo Leon Ligia Gargallo Arturo Horta Deodato Radi 《Journal of Polymer Science.Polymer Physics》1989,27(11):2337-2345
The monoester of itaconic acid with n-decyl alcohol was prepared and polymerized. The polymer was fractionated and characterized by viscometry, size-exclusion chromatography (SEC) and light scattering. Some of the common semiempirical relations for flexible and rigid polymers were used in order to obtain the unperturbed dimensions and the conformational parameters. Thermodynamic and dimensional parameters were determined and calculated. The results are compared with those reported previously for similar compounds. 相似文献
67.
68.
The Wigner distribution has proven to be a useful tool in many quantum problems requiring a joint distribution of position and momentum. In the present paper we develop a joint distribution for spin components within the spirit of the Wigner distribution. This distribution provides an insight into the quantum theory of measurement. We also discuss how one may write joint distributions for two arbitrary noncommuting operators.It is a pleasure to dedicate this paper to Professor John A. Wheeler—physicist, philosopher, poet—an inspiration to us all.Work supported by the Office of Naval Research. 相似文献
69.
Tim B. van Erp Chris T. Reynolds Ton Peijs Johannes A. W. van Dommelen Leon E. Govaert 《Journal of Polymer Science.Polymer Physics》2009,47(20):2026-2035
The time‐dependent yield and failure behavior of off‐axis loaded uniaxially oriented polypropylene tape is investigated. The yield and failure behavior is described with an anisotropic viscoplastic model. A viscoplastic flow rule is used with an equivalent stress, based on Hill's anisotropic yield criterion, and the Eyring flow theory combined with a critical equivalent strain definition. This model is based on factorization of the rate and draw ratio dependence and is capable of quantitatively predicting the rate, angle and draw ratio dependence of the yield stress as well as time‐to‐failure in various off‐axis tensile loading conditions characterized solely from the transverse direction. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2026–2035, 2009 相似文献
70.
A new wavelet multigrid method 总被引:1,自引:0,他引:1
The standard multigrid procedure performs poorly or may break down when used to solve certain problems, such as elliptic problems with discontinuous or highly oscillatory coefficients. The method discussed in this paper solves this problem by using a wavelet transform and Schur complements to obtain the necessary coarse grid, interpolation, and restriction operators. A factorized sparse approximate inverse is used to improve the efficiency of the resulting method. Numerical examples are presented to demonstrate the versatility of the method. 相似文献