全文获取类型
收费全文 | 445篇 |
免费 | 10篇 |
国内免费 | 2篇 |
专业分类
化学 | 210篇 |
晶体学 | 3篇 |
力学 | 34篇 |
数学 | 71篇 |
物理学 | 139篇 |
出版年
2022年 | 23篇 |
2021年 | 22篇 |
2020年 | 9篇 |
2019年 | 15篇 |
2018年 | 18篇 |
2017年 | 13篇 |
2016年 | 14篇 |
2015年 | 16篇 |
2014年 | 17篇 |
2013年 | 32篇 |
2012年 | 32篇 |
2011年 | 22篇 |
2010年 | 21篇 |
2009年 | 11篇 |
2008年 | 13篇 |
2007年 | 17篇 |
2006年 | 19篇 |
2005年 | 15篇 |
2004年 | 5篇 |
2003年 | 9篇 |
2002年 | 24篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1999年 | 1篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1994年 | 3篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 10篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1967年 | 1篇 |
1939年 | 2篇 |
排序方式: 共有457条查询结果,搜索用时 15 毫秒
371.
Nonlinear Dynamics - The extensive proliferation of recent coronavirus (COVID-19), all over the world, is the outcome of social interactions through massive transportation, gatherings and... 相似文献
372.
Saeed K. Amini Hoora Shaghaghi Alex D. Bain Ammar Chabok Mohsen Tafazzoli 《Solid state nuclear magnetic resonance》2010,37(1-2):13-20
The experimental 13C NMR chemical shift components of uracil in the solid state are reported for the first time (to our knowledge), as well as newer data for the 15N nuclei. These experimental values are supported by extensive calculated data of the 13C, 15N and 17O chemical shielding and 17O and 14N electric field gradient (EFG) tensors. In the crystal, uracil forms a number of strong and weak hydrogen bonds, and the effect of these on the 13C and 15N chemical shift tensors is studied. This powerful combination of the structural methods and theoretical calculations gives a very detailed view of the strong and weak hydrogen bond formation by this molecule. Good calculated results for the optimized cluster in most cases (except for the EFG values of the 14N3 and 17O4 nuclei) certify the accuracy of our optimized coordinates for the hydrogen nuclei. Our reported RMSD values for the calculated chemical shielding and EFG tensors are smaller than those reported previously. In the optimized cluster the 6-311+G** basis set is the optimal one in the chemical shielding and EFG calculations, except for the EFG calculations of the oxygen nuclei, in which the 6-31+G** basis set is the optimal one. The optimal method for the chemical shielding and EFG calculations of the oxygen and nitrogen nuclei is the PW91PW91 method, while for the chemical shielding calculations of the 13C nuclei the B3LYP method gives the best results. 相似文献
373.
R-function is a widely used tool when considering objects obtained through the Boolean operations start from simple base primitives. However, there is square root operation in the representation. Considering that the use of splines will facilitate the calculations within the CAD system, in this paper, we propose a system of R-functions represented in spline form called Spline R-function (SR). After transforming the function ranges of two base primitives to a new coordinate system, a series of sign constraints following a specific Boolean operation are derived and the spline R-function can be formulated as a piecewise function. Representation of SR in both Bézier form and B-spline form have been given. Among which the Bézier ordinates are determined with the help of the B-net method through setting up a series of relations according to the sign constraints and properties of R-functions. The construction processes for both Boolean intersection and union operations with different smoothness are discussed in detail. Numerical experiments are conducted to show the potential of the proposed spline R-function. 相似文献
374.
A consecutive(r, s)-out-of-(m, n):F lattice system which is defined as a two-dimensional version of a consecutive k-out-of-n:F system is used as a reliability evaluation model for a sensor system, an X-ray diagnostic system, a pattern search system, etc. This system consists of m × n components arranged like an (m, n) matrix and fails iff the system has an (r, s) submatrix that contains all failed components. In this paper we deal a combined model of a k-out-of-mn:F and a consecutive (r, s)-out-of-(m, n):F lattice system. Namely, the system has one more condition of system down, that is the total number of failed components, in addition to that of a consecutive (r, s)-out-of-(m, n):F lattice system. We present a method to obtain reliability of the system. The proposed method obtains the reliability by using a combinatorial equation that does not depend on the system size. Some numerical examples are presented to show the relationship between component reliability and system reliability. 相似文献
375.
This paper presents a co-evolutionary particle swarm optimization (PSO) algorithm, hybridized with noising metaheuristics,
for solving the delay constrained least cost (DCLC) path problem, i.e., shortest-path problem with a delay constraint on the
total “cost” of the optimal path. The proposed algorithm uses the principle of Lagrange relaxation based aggregated cost.
It essentially consists of two concurrent PSOs for solving the resulting minimization-maximization problem. The main PSO is
designed as a hybrid PSO-noising metaheuristics algorithm for efficient global search to solve the minimization part of the
DCLC-Lagrangian relaxation by finding multiple shortest paths between a source-destination pair. The auxiliary/second PSO
is a co-evolutionary PSO to obtain the optimal Lagrangian multiplier for solving the maximization part of the Lagrangian relaxation
problem. For the main PSO, a novel heuristics-based path encoding/decoding scheme has been devised for representation of network
paths as particles. The simulation results on several networks with random topologies illustrate the efficiency of the proposed
hybrid algorithm for the constrained shortest path computation problems. 相似文献
376.
We present a direct method for finding particular solutions of equations of sine-Gordon type, including single sinh-Gordon equation, single sine-Gordon equation, double sinh-Gordon and double sine-Gordon equations. 相似文献
377.
378.
379.
In this study, efficient preconcentration method has been developed for the determination of 2,4-dichlorophenoxyacetic acid (2,4-D) using biosorption based on microassemblies combined with core–shell magnetic molecularly imprinted polymers (mag-MIP) nanoparticles followed by high-performance liquid chromatography. The method involves three steps: (1) The bioaggregates is added to the sample. The analyte is extracted into the colloidal phase formed by aggregation of the rhamnolipid. (2) Mag-MIPs are added in order to selectively extract 2,4-D from the bioaggregate. (3) The mag-MIPs are then collected from the solution by a strong magnet placed underneath the test tube, and 2,4-D is desorbed from the surface of the mag-MIP with an acetonitrile: methanol mixture. This material has a high adsorption capacity and yields high upconcentrations, which is reusable and chemically stable. Under optimum conditions, the enrichment factor was 265. The linear dynamic range and limit of detection are 0.16–40 and 0.053 µg L?1, respectively. The relative standard deviations for six replicate measurements are 5.2 %. 相似文献
380.