全文获取类型
收费全文 | 1561篇 |
免费 | 98篇 |
国内免费 | 2篇 |
专业分类
化学 | 1236篇 |
晶体学 | 3篇 |
力学 | 32篇 |
综合类 | 1篇 |
数学 | 194篇 |
物理学 | 195篇 |
出版年
2024年 | 4篇 |
2023年 | 28篇 |
2022年 | 27篇 |
2021年 | 31篇 |
2020年 | 51篇 |
2019年 | 34篇 |
2018年 | 22篇 |
2017年 | 17篇 |
2016年 | 80篇 |
2015年 | 75篇 |
2014年 | 69篇 |
2013年 | 76篇 |
2012年 | 103篇 |
2011年 | 103篇 |
2010年 | 73篇 |
2009年 | 54篇 |
2008年 | 100篇 |
2007年 | 103篇 |
2006年 | 97篇 |
2005年 | 89篇 |
2004年 | 78篇 |
2003年 | 57篇 |
2002年 | 34篇 |
2001年 | 27篇 |
2000年 | 16篇 |
1999年 | 21篇 |
1998年 | 10篇 |
1997年 | 10篇 |
1996年 | 5篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1916年 | 3篇 |
排序方式: 共有1661条查询结果,搜索用时 15 毫秒
71.
Isothermal vapor–liquid equilibrium (VLE) and excess enthalpy (HE) data were measured for binary systems required for the design of reactive distillation processes for the methyl acetate production. The isothermal P–x data were measured with the help of a computer-operated static apparatus. A commercial isothermal flow calorimeter was used for the determination of the heats of mixing. Temperature-dependent interaction parameters for the UNIQUAC model were fitted simultaneously to the experimental data from this work and other authors. 相似文献
72.
The structures of the six isomers of 2,3,5,6-tetramethyl-morpholine have been determined by means of proton nuclear magnetic resonance studies at 100 MHz. The spectra have been analysed in terms of spin–spin coupling constants and population distributions of the possible conformers at low temperatures. Also included in this paper are results from n.m.r. studies on the six 4-benzyl-2,3,5,6-tetramethylmorpholine derivatives, which give information about the stereochemistry of the methyl groups α to the amine group. 相似文献
73.
It is shown that the recent observations of NASA's Explorer mission, "Wilkinson Microwave Anisotropy Probe," hint that our Universe may possess a nontrivial topology. As an example we discuss the Picard space which is stretched out into an infinitely long horn but with finite volume. 相似文献
74.
75.
Jonas
rtegren Philippe Busson Ulf W. Gedde Anders Hult Anders Eriksson Mikael Lindgren Per-Otto Arntzn David S. Hermann Per Rudquist Sven T. Lagerwall 《Macromolecular Symposia》1999,148(1):179-195
Highly oriented pyroelectric liquid-crystalline polymers were prepared by photopolymerization under the influence of a static electric field from binary mixtures of two acrylate monomers exhibiting chiral smectic C mesomorphism. Both monomers contained nitro groups to yield second order nonlinear optical properties (second harmonic generation) and one of the monomers had two functional groups to yield a crosslinked polymer. The room temperature second order nonlinear susceptibility of the polymers showed during the first two hours a 10 % decrease after which it remained constant during the next 48 days. At elevated temperatures there was a significant difference in the nonlinear optical properties over time between crosslinked and uncrosslinked polymers. The uncrosslinked polymer showed a pronounced loss of second order nonlinear optical activity with time at ≥38°C. The crosslinked polymer showed a much smaller and basically a temperature independent decrease rate in the second order nonlinear optical properties at all the ageing temperatures (23-130°C). Both the loss in mesogen order parameter, very evident for the uncrosslinked polymer, and conformational changes occurring within the mesogens (β mechanism), may account for the observations made. 相似文献
76.
Mang C Jakupovic S Schunk S Ambrosi HD Schwarz O Jakupovic J 《Journal of combinatorial chemistry》2006,8(2):268-274
The generation of a natural-product-based library starting from andrographolide is described. Utilizing andrographolide itself in parallel solution-phase synthesis leads to a 360-membered library. The initial transformation of the starting material via ozonolysis is followed by the conversion into a suitable template by introduction of a thiazole moiety. Subsequent decoration at two points of diversity yields the desired natural product derivatives. The selection of actually synthesized compounds is based on a virtually generated library and the assessment of its members with respect to physicochemical parameters, thus ensuring pharmacological relevance of the compounds. 相似文献
77.
A property of a system is called actual, if the observation of the outcome of the test that pertains to that property yields
an affirmation with certainty. We formalize the act of observation by assuming the outcome itself is an actual property of
the state of the observer after the act of observation and correlates with the state of the system. For an actual property
this correlation needs to be perfect. A property is called classical if either the property or its negation is actual. We
show by a diagonal argument that there exist classical properties of an observer that he cannot observe perfectly. Because
states are identified with the collection of properties that are actual for that state, it follows no observer can perfectly
observe his own state. Implications for the quantum measurement problem are briefly discussed.
PACS: 02.10-v, 03.65.Ta 相似文献
78.
The synthesis of imidazolinium salts from the reaction of formamidines and (2-bromoethyl)diphenylsulfonium triflate is described. A variety of symmetrical and unsymmetrical imidazolinium triflate salts were synthesized in high yield in short reaction times under mild conditions. Aromatic and aliphatic N-substituents work well. The reaction is proposed to proceed via generation of a vinyl sulfonium salt intermediate from the bromoethylsulfonium triflate. 相似文献
79.
Received December 31, 2001 / Published online May 21, 2002 相似文献
80.
McCarthy MC Gottlieb CA Thaddeus P Thorwirth S Gauss J 《The Journal of chemical physics》2011,134(3):034306
The rotational spectra of two small silicon sulfides, silanethione H(2)SiS and the disilicon sulfide ring Si(2)S, have been detected in the centimeter band by Fourier transform microwave spectroscopy of a molecular beam; lines of H(2)SiS were also observed in the millimeter band up to 377 GHz in a glow discharge. Precise rotational and centrifugal distortionconstants have been determined for the normal and a number of the more abundant rare isotopic species of both closed-shell molecules. Theoretical equilibrium (r(e)) structures of H(2)SiS and Si(2)S were derived from coupled-cluster calculations that included triple and quadruple excitations, core correlation, and extrapolation to the basis-set limit. The r(e) structures agree to within 5×10(-4) A? and 0.1(°) with empirical equilibrium (r(e)(emp)) structures derived from the experimental rotational constants, combined with theoretical vibrational and electronic corrections. Both H(2)SiS and Si(2)S are good candidates for radioastronomical detection in the circumstellar shells of evolved carbon-rich stars such as IRC+10216, because they are fairly polar and are similar in composition to the abundant astronomical molecule SiS. 相似文献