全文获取类型
收费全文 | 14684篇 |
免费 | 556篇 |
国内免费 | 5篇 |
专业分类
化学 | 10814篇 |
晶体学 | 86篇 |
力学 | 270篇 |
数学 | 1993篇 |
物理学 | 2082篇 |
出版年
2023年 | 119篇 |
2022年 | 72篇 |
2021年 | 128篇 |
2020年 | 216篇 |
2019年 | 231篇 |
2018年 | 213篇 |
2017年 | 219篇 |
2016年 | 495篇 |
2015年 | 464篇 |
2014年 | 497篇 |
2013年 | 935篇 |
2012年 | 1132篇 |
2011年 | 1219篇 |
2010年 | 659篇 |
2009年 | 499篇 |
2008年 | 1068篇 |
2007年 | 1045篇 |
2006年 | 944篇 |
2005年 | 899篇 |
2004年 | 733篇 |
2003年 | 604篇 |
2002年 | 522篇 |
2001年 | 191篇 |
2000年 | 185篇 |
1999年 | 112篇 |
1998年 | 87篇 |
1997年 | 89篇 |
1996年 | 133篇 |
1995年 | 82篇 |
1994年 | 69篇 |
1993年 | 87篇 |
1992年 | 76篇 |
1991年 | 55篇 |
1990年 | 52篇 |
1989年 | 43篇 |
1988年 | 45篇 |
1987年 | 42篇 |
1986年 | 62篇 |
1985年 | 89篇 |
1984年 | 73篇 |
1983年 | 44篇 |
1982年 | 69篇 |
1981年 | 70篇 |
1980年 | 71篇 |
1979年 | 53篇 |
1978年 | 54篇 |
1977年 | 42篇 |
1976年 | 49篇 |
1975年 | 28篇 |
1974年 | 33篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
José A. Zapata 《General Relativity and Gravitation》1998,30(8):1229-1245
The canonical quantization of diffeomorphism invariant theories of connections in terms of loop variables is revisited. Such theories include general relativity described in terms of Ashtekar-Barbero variables and extension to Yang-Mills fields (with or without fermions) coupled to gravity. It is argued that the operators induced by classical diffeomorphism invariant or covariant functions are respectively invariant or covariant under a suitable completion of the diffeomorphism group. The canonical quantization in terms of loop variables described here, yields a representation of the algebra of observables in a separable Hilbert space. Furthermore, the resulting quantum theory is equivalent to a model for diffeomorphism invariant gauge theories which replaces space with a manifestly combinatorial object. 相似文献
122.
The reaction of 2,5-dimethylfuran (DMF) with H-atoms was studied using a potential energy surface calculated at the CBS-QB3 level of theory and master equation/RRKM modeling. Hydrogen abstraction by H-atom and hydrogen additions on DMF were considered. As the decomposition pathways of the initial adducts were unknown, a large number of decomposition routes was explored for these adducts. An important number of interconnected product channels were found and preliminary master equation calculations were performed to select the crucial wells and exit channels. The ipso substitution DMF + H → methylfuran (MF) + CH3 and the formation of 1,3-butadiene and acetyl radical (CH3CO) were found to be the major product channels in the addition process. The total calculated rate constant was found in good agreement with experimental data and is nearly pressure-independent. A small sensitivity to pressure was found for the computed branching ratios. At 1 bar, the yields of the two product channels of the addition process are maximal at 1100 K with computed branching ratios of 39% (MF + CH3) and 27% (1,3-C4H6 + CH3CO). Above 1300 K, hydrogen abstraction by H-atom becomes dominant and reaches a branching ratio of 56% at 2000 K. 相似文献
123.
Stanisław Lewanowicz Eduardo Godoy Iván Area André Ronveaux Alejandro Zarzo 《Numerical Algorithms》2000,23(1):31-50
We propose an algorithm to construct recurrence relations for the coefficients of the Fourier series expansions with respect
to the q-classical orthogonal polynomials pk(x;q). Examples dealing with inversion problems, connection between any two sequences of q-classical polynomials, linearization
of ϑm(x) pn(x;q), where ϑm(x) is xmor (x;q)m, and the expansion of the Hahn-Exton q-Bessel function in the little q-Jacobi polynomials are discussed in detail.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
124.
José Luis Sánchez Bote Antonio Pedrero González Juan José Gómez Alfageme 《Applied Acoustics》2012,73(9):977-985
This paper presents a new verification procedure for sound source coverage according to ISO 140–5 requirements. The ISO 140–5 standard applies to the measurement of façade insulation and requires a sound source able to achieve a sufficiently uniform sound field in free field conditions on the façade under study. The proposed method involves the electroacoustic characterisation of the sound source in laboratory free field conditions (anechoic room) and the subsequent prediction by computer simulation of the sound free field radiated on a rectangular surface equal in size to the façade being measured. The loudspeaker is characterised in an anechoic room under laboratory controlled conditions, carefully measuring directivity, and then a computer model is designed to calculate the acoustic free field coverage for different loudspeaker positions and façade sizes. For each sound source position, the method provides the maximum direct acoustic level differences on a façade specimen and therefore determines whether the loudspeaker verifies the maximum allowed level difference of 5 dB (or 10 dB for façade dimensions greater than 5 m) required by the ISO standard. Additionally, the maximum horizontal dimension of the façade meeting the standard is calculated and provided for each sound source position, both with the 5 dB and 10 dB criteria. In the last section of the paper, the proposed procedure is compared with another method used by the authors in the past to achieve the same purpose: in situ outdoor measurements attempting to recreate free field conditions. From this comparison, it is concluded that the proposed method is able to reproduce the actual measurements with high accuracy, for example, the ground reflection effect, at least at low frequencies, which is difficult to avoid in the outdoor measurement method, and it is fully eliminated with the proposed method to achieve the free field requisite. 相似文献
125.
José Carlos Díaz-Ramos Miguel Domínguez-Vázquez Cristina Vidal-Castiñeira 《Annals of Global Analysis and Geometry》2018,53(2):205-216
We show that an isoparametric submanifold of a complex hyperbolic plane, according to the definition of Heintze, Liu and Olmos’, is an open part of a principal orbit of a polar action. We also show that there exists a non-isoparametric submanifold of the complex hyperbolic plane that is isoparametric according to the definition of Terng’s. Finally, we classify Terng-isoparametric submanifolds of two-dimensional complex space forms. 相似文献
126.
We give a -approximation algorithm for minimizing total weighted completion time on a single machine under release time and precedence constraints. This settles a recent conjecture on the approximability of this scheduling problem (Skutella, 2016). 相似文献
127.
128.
The interaction between electric and magnetic fields enables smart devices which can find applications in sensor technology and data storage. Materials showing magneto-electric (ME) coupling combine different ferroic characteristics. In the present contribution we focus on composites, which combine ferroelectric and ferromagnetic phases due to strain couplings, such that they generate a strain-induced ME coupling. We derive a two-scale homogenization approach for the determination of effective properties in consideration of microscopic morphologies. Furthermore, we demonstrate the strong influence of ferroelectric polarization states on the ME-coefficient by modeling the switching of remanent polarizations on the microscale. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
129.
José Luiz Boldrini Luís H. de Miranda Gabriela Planas 《Journal of Evolution Equations》2013,13(3):535-560
This article considers the issues of existence and regularity of solutions to the following doubly nonlinear differential inclusion $$\omega_t+\alpha (\omega_t)-\Delta \omega-\Delta_p{\omega} \ni f$$ where α is a maximal monotone operator in ${\mathbb{R}^2}$ and Δ p denotes the p-Laplacian with p > 2. The investigation on fractional regularity is based on the Galerkin method combined with a suitable basis for W 1,p , which we exhibit as a preliminary result. This approach also allows the obtaining of estimates in the so-called Nikolskii spaces, since it balances the interplay between the maximal monotone operator with the appearing higher order nonlinear terms. 相似文献
130.
Christopher M. Tollan Jon Echeberria Rebeca Marcilla José A. Pomposo David Mecerreyes 《Journal of nanoparticle research》2009,11(5):1241-1245
The synthesis and characterisation of gold nanorods have been carried out by reduction of the gold salt HAuCl4. This has been done using a single reducing agent, acetylacetone, rather than the two reducing agents, sodium borohydride
and ascorbic acid, normally required by standard wet chemistry methods of gold nanorod formation. Using this novel method,
the nanorods were synthesised at several different pH values which were found to greatly affect both the rate at which the
nanorods form and their physical dimensions. The concentrations of acetylacetone and silver nitrate used relative to the gold
salt were found to alter the aspect ratio of the nanorods formed. Rods with an average length of 42 nm and an aspect ratio
of 4.6 can be easily and reproducibly formed at pH 10 using this method. Nanorods formed under optimum conditions were investigated
using TEM.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献