全文获取类型
收费全文 | 4550篇 |
免费 | 83篇 |
国内免费 | 18篇 |
专业分类
化学 | 2842篇 |
晶体学 | 33篇 |
力学 | 102篇 |
数学 | 667篇 |
物理学 | 1007篇 |
出版年
2022年 | 51篇 |
2021年 | 74篇 |
2020年 | 74篇 |
2019年 | 57篇 |
2018年 | 44篇 |
2017年 | 34篇 |
2016年 | 80篇 |
2015年 | 70篇 |
2014年 | 71篇 |
2013年 | 213篇 |
2012年 | 222篇 |
2011年 | 251篇 |
2010年 | 129篇 |
2009年 | 123篇 |
2008年 | 199篇 |
2007年 | 237篇 |
2006年 | 219篇 |
2005年 | 202篇 |
2004年 | 198篇 |
2003年 | 164篇 |
2002年 | 147篇 |
2001年 | 109篇 |
2000年 | 123篇 |
1999年 | 63篇 |
1998年 | 70篇 |
1997年 | 60篇 |
1996年 | 65篇 |
1995年 | 71篇 |
1994年 | 58篇 |
1993年 | 71篇 |
1992年 | 48篇 |
1991年 | 49篇 |
1990年 | 44篇 |
1989年 | 47篇 |
1988年 | 39篇 |
1987年 | 44篇 |
1986年 | 44篇 |
1985年 | 60篇 |
1984年 | 52篇 |
1983年 | 39篇 |
1982年 | 34篇 |
1981年 | 39篇 |
1980年 | 40篇 |
1979年 | 28篇 |
1978年 | 31篇 |
1977年 | 39篇 |
1976年 | 45篇 |
1975年 | 50篇 |
1974年 | 41篇 |
1973年 | 45篇 |
排序方式: 共有4651条查询结果,搜索用时 15 毫秒
1.
2.
João Marcos G. Barbosa Milenna K. Fernandes Rodrigues Lurian C. David Taynara C. e Silva Danielly A. Fortuna Lima Naiara Z. Pereira Emmanuel B. D'Alessandro Anselmo E. de Oliveira Paulo H. Jorge da Cunha Maria Clorinda S. Fioravanti Nelson R. Antoniosi Filho 《Biomedical chromatography : BMC》2020,34(11):e4935
An innovative volatolomic approach employs the detection of biomarkers present in cerumen (earwax) to identify cattle intoxication by Stryphnodendron rotundifolium Mart., Fabaceae (popularly known as barbatimão). S. rotundifolium is a poisonous plant with the toxic compound undefined and widely distributed throughout the Brazilian territory. Cerumen samples from cattle of two local Brazilian breeds (‘Curraleiro Pé-Duro’ and ‘Pantaneiro’) were collected during an experimental intoxication protocol and analyzed using headspace (HS)/GC–MS followed by multivariate analysis (genetic algorithm for a partial least squares, cluster analysis, and classification and regression trees). A total of 106 volatile organic metabolites were identified in the cerumen samples of bovines. The intoxication by S. rotundifolium influenced the cerumen volatolomic profile of the bovines throughout the intoxication protocol. In this way, it was possible to detect biomarkers for cattle intoxication. Among the biomarkers, 2-octyldecanol and 9-tetradecen-1-ol were able to discriminate all samples between intoxicated and nonintoxicated bovines. The cattle intoxication diagnosis by S. rotundifolium was accomplished by applying the cerumen analysis using HS/GC–MS, in an easy, accurate, and noninvasive way. Thus, the proposed bioanalytical chromatography protocol is a useful tool in veterinary applications to determine this kind of intoxication. 相似文献
3.
Bobby Wilson 《偏微分方程通讯》2015,40(8):1521-1542
This article explores the questions of long time orbital stability in high order Sobolev norms of plane wave solutions to the NLSE in the defocusing case. 相似文献
4.
The multilevel generalized assignment problem is a problem of assigning agents to tasks where the agents can perform tasks at more than one efficiency level. A profit is associated with each assignment and the objective of the problem is profit maximization. Two heuristic solution methods are presented for the problem. The heuristics are developed from solution methods for the generalized assignment problem. One method uses a regret minimization approach whilst the other method uses a repair approach on a relaxation of the problem. The heuristics are able to solve moderately large instances of the problem rapidly and effectively. Procedures for deriving an upper bound on the solution of the problem are also described. On larger and harder instances of the problem one heuristic is particularly effective. 相似文献
5.
6.
David B. Wilson 《Random Structures and Algorithms》2002,21(2):182-195
There has been much recent interest in the satisfiability of random Boolean formulas. A random k‐SAT formula is the conjunction of m random clauses, each of which is the disjunction of k literals (a variable or its negation). It is known that when the number of variables n is large, there is a sharp transition from satisfiability to unsatisfiability; in the case of 2‐SAT this happens when m/n → 1, for 3‐SAT the critical ratio is thought to be m/n ≈ 4.2. The sharpness of this transition is characterized by a critical exponent, sometimes called ν = νk (the smaller the value of ν the sharper the transition). Experiments have suggested that ν3 = 1.5 ± 0.1. ν4 = 1.25 ± 0.05, ν5 = 1.1 ± 0.05, ν6 = 1.05 ± 0.05, and heuristics have suggested that νk → 1 as k → ∞. We give here a simple proof that each of these exponents is at least 2 (provided the exponent is well defined). This result holds for each of the three standard ensembles of random k‐SAT formulas: m clauses selected uniformly at random without replacement, m clauses selected uniformly at random with replacement, and each clause selected with probability p independent of the other clauses. We also obtain similar results for q‐colorability and the appearance of a q‐core in a random graph. © 2002 Wiley Periodicals, Inc. Random Struct. Alg., 21: 182–195, 2002 相似文献
7.
8.
E. Rodriguez E. Jimenez G.J. Jacob A.A.R. Neves C.L. Cesar L.C. Barbosa 《Physica E: Low-dimensional Systems and Nanostructures》2005,26(1-4):361
Multilayer PbTe quantum dots (QDs) and SiO2 were grown by pulsed laser deposition (PLD) and Plasma enhanced chemical vapor deposition (PECVD) techniques. The crystalline structure, QD size and size dispersion were observed by high-resolution transmission electron microscopy (HRTEM) measurements. This technique allows one to grow PbTe QDs as small as 1.8 nm diameter and 0.6 nm size dispersion. The whole structure can be used in a Fabry–Perot cavity for an optical device operating at the mid-infrared region. 相似文献
9.
Quasi-wavelets (QWs) are a representation of turbulence consisting of self-similar, eddy-like structures with random orientations and positions in space. They are used in this paper to calculate the scattering, due to turbulent velocity fluctuations, of sound behind noise barriers as a function of the size and spatial location of the eddies. The sound scattering cross-section for QWs of an individual size class (eddy size) is derived and shown to reproduce results for the von Kármán spectrum when the scattered energies from a continuous distribution of QW sizes are combined. A Bragg resonance condition is derived for the eddy size that scatters most strongly for a given acoustic wavenumber and scattering angle. Results for scattering over barriers show that, for typical barrier conditions, most of the scattered energy originates from eddies in the size range of approximately one-half to twice the size of the eddies responsible for maximum scattering. The results also suggest that scattering over the barrier due to eddies with a line of sight to both the source and receiver is generally significant only for frequencies above several kilohertz, for sources and receivers no more than a few meters below the top of the barrier, and for very turbulent atmospheric conditions. 相似文献
10.
A computer construction of the Monster is used to prove thatPSL(2, 59) is a subgroup of the Monster. 相似文献