全文获取类型
收费全文 | 3239篇 |
免费 | 158篇 |
国内免费 | 19篇 |
专业分类
化学 | 2514篇 |
晶体学 | 28篇 |
力学 | 92篇 |
数学 | 383篇 |
物理学 | 399篇 |
出版年
2023年 | 26篇 |
2022年 | 61篇 |
2021年 | 80篇 |
2020年 | 60篇 |
2019年 | 78篇 |
2018年 | 66篇 |
2017年 | 36篇 |
2016年 | 126篇 |
2015年 | 91篇 |
2014年 | 106篇 |
2013年 | 220篇 |
2012年 | 255篇 |
2011年 | 251篇 |
2010年 | 119篇 |
2009年 | 138篇 |
2008年 | 206篇 |
2007年 | 213篇 |
2006年 | 148篇 |
2005年 | 174篇 |
2004年 | 148篇 |
2003年 | 123篇 |
2002年 | 161篇 |
2001年 | 45篇 |
2000年 | 42篇 |
1999年 | 32篇 |
1998年 | 29篇 |
1997年 | 45篇 |
1996年 | 39篇 |
1995年 | 23篇 |
1994年 | 23篇 |
1993年 | 24篇 |
1992年 | 11篇 |
1991年 | 16篇 |
1990年 | 14篇 |
1989年 | 15篇 |
1988年 | 6篇 |
1987年 | 10篇 |
1986年 | 16篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1974年 | 4篇 |
1973年 | 5篇 |
排序方式: 共有3416条查询结果,搜索用时 15 毫秒
991.
Few methods, such as coulometrics, have been developed to produce reagents in situ for analytical purposes. In this work the concept related to the generation of ultrasound-assisted reagents was exploited to yield oxidizing species in batch and flow systems of analysis. To evaluate the efficiency of ultrasound-assisted reagent generation, the conversion of Fe(2+) to Fe(3+) associated with the 1,10-o-phenantroline spectrophotometric method was tested to compare the oxidizing power of the produced species from pure water and aqueous solutions saturated with CCl(4) or CHCl(3) irradiated ultrasonic waves. Irradiation processes were conducted with an ultrasonic bath (40 kHz and 140 W). The borosilicate reactor was used in the batch studies, while the PTFE tube reactor was used for setting up the flow system, with the temperature during irradiation being controlled using a thermostatic bath. The sonochemical production of oxidizing agents was demonstrated to be efficient for chemical analysis in batch and flow systems. This technique was exemplified by oxidation of iodide and ferrous ions. It was observed that after 120 min of sonication approximately 40 microg of Fe(2+) was quantitatively oxidized to Fe(3+). Similar result was obtained by the irradiation of iodine in aqueous-organic medium. 相似文献
992.
Carbon dioxide-in-water microemulsions 总被引:2,自引:0,他引:2
Lee CT Ryoo W Smith PG Arellano J Mitchell DR Lagow RJ Webber SE Johnston KP 《Journal of the American Chemical Society》2003,125(10):3181-3189
Liquid and supercritical carbon dioxide swell potassium carboxylate perfluoropolyether (PFPE-K) cylindrical micelles in water to produce novel CO(2)-in-water (C/W) microemulsions. The swelling elongates the micelles significantly from 20 to 80 nm as the molar ratio of CO(2) in the micelles to surfactant (R(CO2)) reaches approximately 8. As the micelles swell to form microemulsions, the solubility of pyrene increases by a factor of ca. 10. Fluorescence spectra suggest that pyrene resides primarily in the low-polarity micelle core rather than in the palisade region. The results illustrate the ability of C/W microemulsions to solubilize both lipophilic and fluorophilic substances simultaneously. 相似文献
993.
Mano N Fernandez JL Kim Y Shin W Bard AJ Heller A 《Journal of the American Chemical Society》2003,125(50):15290-15291
The first enzyme-based catalyst that is superior to platinum in the four-electron electroreduction of oxygen to water is reported. The smooth Pt cathode reached half and 90% of the mass transport-limited current density at respective overpotentials of -0.4 and -0.58 V in 0.5 M sulfuric acid, and only at even higher overpotentials in pH 7.2 phosphate buffer. In contrast, the smooth "wired" bilirubin oxidase cathode reached half and 90% of the mass transport-limited current density at respective overpotentials as low as -0.2 and -0.25 V. The mass transport-limited current density for the smooth "wired" enzyme cathode in PBS was twice that with smooth Pt in 0.5 M sulfuric acid. Under 1 atm O2 pressure, O2 was electroreduced to water on a polished carbon cathode, coated with the "wired" BOD film, in pH 7.2 saline buffer (PBS) at an overpotential of -0.31 V at a current density of 9.5 mA cm-2. At the same overpotential, the current density of the polished platinum cathode in 0.5 M H2SO4 was 16-fold lower, only 0.6 mA cm-2. 相似文献
994.
While N,N'-bis(4-pyridyl)urea affords a 5-fold interpenetrated diamondoid network, its positional isomer N,N'-bis(3-pyridyl)urea gave a net-to-net hydrogen-bonded (4,4) net (square grid network) displaying temperature-dependent anion-induced reversible single-crystal-to-single-crystal disorder-order phase transition, when the ligands are reacted with zinc perchlorate. 相似文献
995.
Jose?E.?CastilloEmail author Mark?Yasuda 《Journal of Mathematical Modelling and Algorithms》2005,4(1):67-82
Recent investigations by Castillo and Grone have led to a new method for constructing mimetic discretizations of divergence and gradient operators. Their technique, which employs a matrix formulation to incorporate mimetic constraints, is capable of producing approximations whose order at a grid boundary is equal to that in the grids interior. In this paper, we construct a second-order mimetic discretization using Castillo and Grones approach and compare it to other second-order discretizations by applying them to an elliptic boundary value problem in one dimension. A detailed perturbation analysis is provided to offer some insight into the two discretizations yielding the best numerical results in the study.Mathematics Subject Classifications (2000) 65D25, 65M06, 65G99.This paper is an expanded version of the article [2] presented at the International Conference on Parallel and Distributed Processing Techniques and Applications held in Las Vegas, Nevada in June of 2003. 相似文献
996.
997.
The asymmetric acetate-type aldol reaction using (S,S)-(+)-pseudoephedrine has been studied in detail. Experimental variables like the nature of the metal counterion of the enolate, the presence of additives and the structure of the aldehyde have been examined in order to reach to the highest possible yields and diastereoselectivities. 相似文献
998.
[graph: see text] In this paper, the use of aggregation processes in squaraine derivatives as signaling protocols in the development of colorimetric probes for certain neutral guest (aromatic hydrocarbons) has been studied. 相似文献
999.
Richard G. Belshoff Edgar E. Enochs Juan Ramon Garcí a Rozas 《Proceedings of the American Mathematical Society》2000,128(5):1307-1312
Let be a commutative noetherian ring and let be the minimal injective cogenerator of the category of -modules. A module is said to be reflexive with respect to if the natural evaluation map from to is an isomorphism. We give a classification of modules which are reflexive with respect to . A module is reflexive with respect to if and only if has a finitely generated submodule such that is artinian and is a complete semi-local ring.
1000.
Marie-Paule Simonnin Claude Charrier Ramon Burgada 《Magnetic resonance in chemistry : MRC》1972,4(1):113-120
The ΔG* values for the barrier to rotation around the P? N bond have been determined in some aminophosphines and are dependent on the substituents bound to the phosphorus atom. The comparison between 3J(P? N? C? H) and ΔG* values indicates that pπ—dπ overlapping between nitrogen and phosphorus is the main effect. The nucleophilic behaviour of the nitrogen atom has been measured by the rate of the chemical exchange between methyl-trifluoroacetate and various aminophosphines: the results of this kinetic study are in good agreement with the NMR conclusions. 相似文献