全文获取类型
收费全文 | 2132篇 |
免费 | 56篇 |
国内免费 | 6篇 |
专业分类
化学 | 1301篇 |
晶体学 | 20篇 |
力学 | 80篇 |
数学 | 230篇 |
物理学 | 563篇 |
出版年
2023年 | 18篇 |
2022年 | 33篇 |
2021年 | 31篇 |
2020年 | 54篇 |
2019年 | 55篇 |
2018年 | 53篇 |
2017年 | 46篇 |
2016年 | 74篇 |
2015年 | 45篇 |
2014年 | 73篇 |
2013年 | 163篇 |
2012年 | 165篇 |
2011年 | 198篇 |
2010年 | 111篇 |
2009年 | 99篇 |
2008年 | 127篇 |
2007年 | 113篇 |
2006年 | 106篇 |
2005年 | 91篇 |
2004年 | 51篇 |
2003年 | 36篇 |
2002年 | 44篇 |
2001年 | 25篇 |
2000年 | 16篇 |
1999年 | 16篇 |
1998年 | 10篇 |
1997年 | 13篇 |
1996年 | 9篇 |
1995年 | 17篇 |
1994年 | 15篇 |
1993年 | 11篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 14篇 |
1983年 | 6篇 |
1982年 | 17篇 |
1981年 | 18篇 |
1980年 | 9篇 |
1979年 | 16篇 |
1978年 | 9篇 |
1977年 | 19篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1973年 | 13篇 |
排序方式: 共有2194条查询结果,搜索用时 62 毫秒
1.
Mixed integer programming (MIP) models are extensively usedto aid strategic and tactical decision making in many businesssectors. Solving MIP models is a computationally intensive processand there is a need to develop solution approaches that enablelarger models to be solved within acceptable timeframes. Inthis paper, we describe the implementation of a two-stage parallelbranch and bound (PB & B) algorithm for MIP. In stage 1of the algorithm, a multiple heuristic search is implementedin which a number of alternative search trees are investigatedusing a forest search in the hope of finding a good solutionquickly. In stage 2, the search is reorganized so that the branchesof a chosen tree are investigated in parallel. A new heuristicis introduced, based on a best projection criterion, which evaluatesalternative B & B trees in order to choose one for investigationin stage 2 of the algorithm. The heuristic also serves as away of implementing a quality load balancing scheme for stage2 of the algorithm. The results of experimental investigationsare reported for a range of models taken from the MIPLIB libraryof benchmark problems. 相似文献
2.
3.
The problem of estimating change points in various non-monotonic aging models is considered. A general methodology for consistent estimation of the change point is developed and applied to non-monotonic aging models based on the hazard rate function as well as on the mean residual life function. 相似文献
4.
The frequency and the amplitude of the driving AC current dependence on the harmonic behavior of the giant magneto-impedance (GMI) voltage of Co-rich amorphous wire have been studied. Single-peak, two-peak and three-peak behavior in the GMI characteristics was studied with the change of amplitude and frequency of the AC current. GMI ratio was calculated from the first harmonic signals which were initially increased and then decreased monotonically with the increase of frequency (f) and the amplitude (IAC) of the driving current. The response of the GMI voltage was found non-linear with the generation of higher harmonics at higher amplitude of the driving current. The second and third harmonic signals were also evaluated and their dependence on the driving current amplitude and frequency were presented in the paper. The experimental results were analyzed using single domain rotational model. The voltage harmonics were calculated through Fourier transform of the time derivative of the estimated circular magnetization of the sample. 相似文献
5.
Sonya V. Roberson Albert J. Fahey Amit Sehgal Alamgir Karim 《Applied Surface Science》2002,200(1-4):150-164
We present a simple method for chemical modification of chlorosilane self-assembled monolayers (SAMs) on Si surfaces by exposure to a gradient of UV-ozone radiation to create stable substrates with a range of contact angles (θH2O≈5–95°) and surface energies on a single substrate. These gradient energy substrates are developed to potentially generate libraries for combinatorial studies of thin film phenomenology, where a systematic variation of interfacial surface energy represents one of the significant parameters along one axis. The graded oxidation process presents a systematic variation of surface chemical composition. We have utilized contact angle measurements and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to investigate this variation for a series of ions, among which are SiCH3+, SiOH+ and COOH−. We show that the macroscopic measurements of surface free energy/contact angle correlate with the detailed analysis of surface chemistry (as assessed by ToF-SIMS) on these test substrates. 相似文献
6.
The use of multigrid methods in complex fluid flow problems is still under development. In this paper a full multigrid procedure has been incorporated in a finite volume solution for predicting fully developed fluid flow in a streamwise periodic geometry. Steady computations in two-dimensional body fitted co-ordinates have shown considerable savings in computation time by this multigrid method. 相似文献
7.
G. Jyothi Chanchal K. Mitra G. Krishnamoorthy 《Journal of Electroanalytical Chemistry》1991,321(3):395-402
The exchange of gramicidin between liposomes made of two different kinds of phospholipids has been studied using a fluorescent probe (pyranine). The experimentally observed rate of exchange is compared with that of nigericin, which is a simple carrier. Possible reasons for the difference in the rate of exchange are discussed and probable mechanisms suggested. It is proposed that gramicidin exchanges between vesicles by a contact mechanism. 相似文献
8.
Proton and alpha particle spectra have been measured in the 12C+93Nb and 12C+58Ni reactions at E(12C)=40 and 50 MeV and in the 16O + 93Nb reaction at E(16O) =75 MeV. The spectra are compared with the statistical model calculations. The shapes of the calculated spectra are in
agreement with experimental data except for the alpha spectrum in the 12C + 93Nb reaction at 40 MeV. The observed evaporation bump is at ∼2 MeV lower energy compared to the calculated one. This discrepancy
could imply alpha particle emission from a deformed configuration before compound nucleus formation at this near Coulomb barrier
beam energy. 相似文献
9.
The Schiff base [1,2-bis(salicylidene amino)phenylene]cobalt(II) complex, chemically bonded to a carbamate-modified silica gel catalyst, has been prepared by a four step procedure. The oxidation of cyclohexane was studied in the presence of this catalyst under relatively mild conditions (150–200 °C, 15–20 atm) using molecular oxygen. The catalyst was found to be very selective for the production of cyclohexanol, with cyclohexanone formed in only a small amount (45:1). This is in contrast to the commercially available processes in which cyclohexanol and cyclohexanone are both formed in appreciable amounts. The t.g.a. analysis shows the catalyst to be stable up to 211 °C and atomic absorption spectroscopy indicated negligible metal loss during 50 h use of the catalyst up to 180 °C. 相似文献
10.
Paital AR Mitra T Ray D Wong WT Ribas-Ariño J Novoa JJ Ribas J Aromí G 《Chemical communications (Cambridge, England)》2005,(41):5172-5174
Based on a combined theoretical-experimental study, we propose that substituted m-phenylene ligands (m-N-Phi-N) can act as tuneable strong ferromagnetic couplers connecting Cu(II) ions; a new complex presenting that bridge with J close to +15 cm(-1) has been suggested and synthesized. 相似文献