全文获取类型
收费全文 | 1661篇 |
免费 | 62篇 |
国内免费 | 1篇 |
专业分类
化学 | 1119篇 |
晶体学 | 3篇 |
力学 | 35篇 |
数学 | 222篇 |
物理学 | 345篇 |
出版年
2023年 | 11篇 |
2022年 | 20篇 |
2021年 | 64篇 |
2020年 | 40篇 |
2019年 | 50篇 |
2018年 | 47篇 |
2017年 | 47篇 |
2016年 | 87篇 |
2015年 | 74篇 |
2014年 | 71篇 |
2013年 | 150篇 |
2012年 | 131篇 |
2011年 | 135篇 |
2010年 | 81篇 |
2009年 | 77篇 |
2008年 | 101篇 |
2007年 | 96篇 |
2006年 | 65篇 |
2005年 | 58篇 |
2004年 | 54篇 |
2003年 | 36篇 |
2002年 | 32篇 |
2001年 | 16篇 |
2000年 | 17篇 |
1999年 | 13篇 |
1998年 | 7篇 |
1997年 | 12篇 |
1996年 | 14篇 |
1995年 | 8篇 |
1994年 | 16篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1972年 | 5篇 |
1962年 | 2篇 |
排序方式: 共有1724条查询结果,搜索用时 312 毫秒
91.
Lana Mara Rodrigues dos Santos Philippe Michelon Marcos Nereu Arenales Ricardo Henrique Silva Santos 《Annals of Operations Research》2011,190(1):165-180
In this article we propose a 0-1 optimization model to determine a crop rotation schedule for each plot in a cropping area.
The rotations have the same duration in all the plots and the crops are selected to maximize plot occupation. The crops may
have different production times and planting dates. The problem includes planting constraints for adjacent plots and also
for sequences of crops in the rotations. Moreover, cultivating crops for green manuring and fallow periods are scheduled into
each plot. As the model has, in general, a great number of constraints and variables, we propose a heuristics based on column
generation. To evaluate the performance of the model and the method, computational experiments using real-world data were
performed. The solutions obtained indicate that the method generates good results. 相似文献
92.
M. Elena Mart��n M. Luz S��nchez Jos�� C. Corchado Aurora Mu?oz-Losa Ignacio Fdez. Galv��n Francisco J. Olivares del Valle Manuel A. Aguilar 《Theoretical chemistry accounts》2011,128(4-6):783-793
The possible influence of the solvent Stark effect (SSE) on the solvatochromic shift in electron transitions has been analyzed by using the ASEP/MD (averaged solvent electrostatic potential from molecular dynamics) method. With this purpose, four molecules, two polar (acrolein and formaldehyde) and two non-polar (p-difluorobenzene and trans-difluoroethene) have been studied in solvents of diverse polarity. Independently of the nature of the system we found that the contribution of SSE on the average value of the solvent shift or on the multipole moment values is negligible. In the case of centro-symmetric molecules, our results permit to discard the SSE as cause of the solvent shift found, which must be assigned to the electrostatic interaction of the solute quadrupole and higher multipoles with the solvent. As the SSE values provide also a measure of the errors introduced by the mean field approximation (MFA), these results indicate that MFA permits a very accurate determination of the solvent shift at the same time that it reduces drastically the computational cost. Finally, a new procedure suited to the ASEP/MD method has been presented that permits to estimate the inhomogeneous broadening of spectral bands, complementing the information provided by mean field theories. This procedure does not need additional quantum calculations and its computational cost is minimal. 相似文献
93.
B. R. L. Galvão J. A. Corzo‐Espinoza P. J. S. B. Caridade A. J. C. Varandas 《国际化学动力学杂志》2011,43(7):345-352
Quasiclassical trajectories have been run to study the fundamental one‐quantum vibrational transition formed from collisions of ground‐state nitric oxide with atomic oxygen at temperatures of 500, 750, and 1000 K. Two adiabatic potential energy surfaces of different symmetry (2A′ and 2A″ of NO2) have been utilized. The rate constant for the title process is given along with the rotational distributions, and the results shown to corroborate previous atmospheric models that describe the nascent state by a Maxwell–Boltzmann distribution at the local temperature. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 345–352, 2011 相似文献
94.
95.
Roberto D. Galvão 《European Journal of Operational Research》1981,6(2):162-165
The p-median problem is a minisium network location problem that seeks to find the optimal location of p centres in a network. In the present paper a graph-theoretical bound is developed for the problem. This bound is based on shortest spanning trees and arborescences and other graphical properties of the problem. It is shown that the graph-theoretical bound dominates a bound based on shortest distances. 相似文献
96.
Pessoa JC Correia I Galvão A Gameiro A Felix V Fiuza E 《Dalton transactions (Cambridge, England : 2003)》2005,(13):2312-2321
Several Ni(II) complexes derived from (S)-o-N-(N-benzylprolyl)aminobenzophenone ((S)-BBP) and amino acids of general formula [Ni((S)-BBP-L-(or D-)-aa)] were prepared. The crystal and molecular structures of [Ni((S)-BBP-Gly)], [Ni((S)-BBP-L-Ser)] and [Ni((S)-BBP-L-aaIm)](aaIm =L-2-amino-3-(imidazol-1-yl)propanoate were determined by X-ray diffraction analysis. In the three complexes the nickel atoms display a square-planar coordination and the overall structure around the metal indicates that the entire Schiff-base ligands form quite rigid frameworks. Molecular mechanics calculations were carried out for complexes [Ni((S)-BBP-Gly)], [Ni((S)-BBP-Ser)] and [Ni((S)-BBP-aaIm)] containing either the L- or D-amino acid forms, and the factors controlling the stereoselectivity are discussed. Several other [Ni((S)-BBP-L-aa)] complexes are also prepared and their circular dichroism spectra in solution and of the solids dispersed in KBr disks are measured and discussed. In agreement with other studies in solution with similar [Ni((S)-BBP-aa)] complexes, the Cotton effects for the bands with lambda(max) at 520--530 nm are positive when the amino acids have the L-configuration at the alpha-carbon. The same is observed in this work for the solid-state CD spectra of all compounds. 相似文献
97.
Oliveira JS Sousa EH Basso LA Palaci M Dietze R Santos DS Moreira IS 《Chemical communications (Cambridge, England)》2004,(3):312-313
The in vitro kinetics of inactivation of both wild-type and I21V InhA enzymes by [FeII(CN)5(INH)]3- indicate that this process requires no activation by KatG, and no need for the presence of NADH. This inorganic complex may represent a new class of lead compounds to the development of anti-tubercular agents aiming at inhibition of a validated target. 相似文献
98.
99.
Martinelli LA Gat JR de Camargo PB Lara LL Ometto JP 《Isotopes in environmental and health studies》2004,40(1):45-56
The stable isotope content of samples of precipitation and of the river water throughout the Piracicaba basin in Brazil was measured over a two-year period. The isotope values of precipitation follow a consistent pattern of relatively depleted values of both deuterium and oxygen 18 during the rainy summers and enriched ones during the dry winters, with all values aligned slightly above the Global Meteoric Water Line. The isotopic composition of the river water throughout the basin shows a remarkable spatial coherence and much smaller scatter of data than those of the precipitation. The isotope composition of river water is close to that of the precipitation in the rainy season, however, with a consistent lower d-excess value by 1/1000-2/1000. This is attributed to evaporative water loss in the basin, in part an expression of the recycling of water due to the anthropogenic activity in the region. The more divergent values are recorded during high-water stages in the rivers. In many cases, the floods during the beginning of the rainy season are characterized by an enrichment of the heavy isotopes and lower d-excess values when compared to the precipitation, with the opposite situation later in the rainy season. This is interpreted as resulting from the watershed/riverflow interaction pattern, and it thus suggests that the isotope composition can monitor the hydrologic situation in the basin and its changes. 相似文献
100.
We present an investigation of the excited-state absorption and laser emission of a 1.0-at. %-Nd3+-doped YVO4 single-crystal fiber grown by the low-cost and versatile laser-heated pedestal growth technique. Efficient laser emission at 1064 nm was achieved when the fiber was pumped, in an end-pump cavity, by a Ti:sapphire laser at 808 nm. A continuous-wave threshold of 10 mW was observed with an efficiency of 42% with respect to the absorbed pump power and the maximum output power of 200 mW. These results are excellent when compared with those of a commercial bulk crystal adapted to the same cavity (48% efficiency, 250-mW maximum output power). Thus the fibers are characterized as strong candidates for the construction of compact lasers that can also be pumped by low-cost diode lasers. 相似文献