全文获取类型
收费全文 | 4956篇 |
免费 | 129篇 |
国内免费 | 6篇 |
专业分类
化学 | 3938篇 |
晶体学 | 7篇 |
力学 | 42篇 |
数学 | 638篇 |
物理学 | 466篇 |
出版年
2019年 | 40篇 |
2018年 | 37篇 |
2017年 | 27篇 |
2016年 | 101篇 |
2015年 | 67篇 |
2014年 | 100篇 |
2013年 | 142篇 |
2012年 | 150篇 |
2011年 | 158篇 |
2010年 | 134篇 |
2009年 | 119篇 |
2008年 | 162篇 |
2007年 | 159篇 |
2006年 | 179篇 |
2005年 | 198篇 |
2004年 | 167篇 |
2003年 | 139篇 |
2002年 | 181篇 |
2001年 | 115篇 |
2000年 | 107篇 |
1999年 | 105篇 |
1998年 | 85篇 |
1997年 | 95篇 |
1996年 | 96篇 |
1995年 | 76篇 |
1994年 | 88篇 |
1993年 | 90篇 |
1992年 | 92篇 |
1991年 | 86篇 |
1990年 | 67篇 |
1989年 | 89篇 |
1988年 | 87篇 |
1987年 | 84篇 |
1986年 | 71篇 |
1985年 | 96篇 |
1984年 | 111篇 |
1983年 | 62篇 |
1982年 | 86篇 |
1981年 | 112篇 |
1980年 | 83篇 |
1979年 | 101篇 |
1978年 | 68篇 |
1977年 | 68篇 |
1976年 | 60篇 |
1975年 | 78篇 |
1974年 | 50篇 |
1973年 | 71篇 |
1972年 | 51篇 |
1971年 | 43篇 |
1970年 | 52篇 |
排序方式: 共有5091条查询结果,搜索用时 16 毫秒
971.
Florian Bruckner Christoph Vogler Michael Feischl Dirk Praetorius Bernhard Bergmair Thomas Huber Markus Fuger Dieter Suess 《Journal of magnetism and magnetic materials》2012
3D magnetostatic Maxwell equations are solved using the direct Johnson–Nédélec FEM–BEM coupling method and a reduced scalar potential approach. The occurring BEM matrices are calculated analytically and approximated by H-matrices using the ACA+ algorithm. In addition a proper preconditioning method is suggested that allows to solve large-scale problems using iterative solvers. 相似文献
972.
J Demaison NC Craig EJ Cocinero JU Grabow A Lesarri HD Rudolph 《The journal of physical chemistry. A》2012,116(34):8684-8692
N-Methylpiperidone (MPIP) and tropinone, which contain a structural motif found in numerous alkaloids, are too large to determine an accurate equilibrium structure either by ab initio methods or by experiment. However, the ground state rotational constants of the parent species and of all isotopologues with a substituted heavy atom ((13)C, (15)N, (18)O) are known from microwave spectroscopy. These constants have been corrected for the rovibrational contribution calculated from an ab initio cubic force field. These semiexperimental equilibrium rotational constants have been supplemented by carefully chosen structural parameters from medium level ab initio calculations. The two sets of data have been used in a weighted least-squares fit to determine reliable equilibrium structures for both molecules. This work shows that it is possible to determine reliable equilibrium structures for large molecules (34 degrees of freedom in the case of tropinone) at a detailed level of accuracy, and the method could be applied without too much difficulty to still larger molecules. 相似文献
973.
Pasinszki T Vass G Klapstein D Westwood NP 《The journal of physical chemistry. A》2012,116(13):3396-3403
Cyanoformyl chloride and cyanoformyl bromide, XC(O)CN (X = Cl and Br), have been investigated in the gas phase by UV photoelectron and mid-infrared spectroscopies. The ground-state geometries of the neutral molecules have been obtained from quantum-chemical calculations at the B3LYP and CCSD(T) levels using the aug-cc-pVTZ basis set. The individual spectroscopies provide a detailed investigation into the vibrational and electronic character of the molecules and are supported by quantum-chemical calculations. The results are compared to data for structurally and chemically related molecules. 相似文献
974.
Wim Fremout Maarten Dhaenens Steven Saverwyns Jana Sanyova Peter Vandenabeele Dieter Deforce Luc Moens 《Analytica chimica acta》2012
In recent years, the use of liquid chromatography tandem mass spectrometry (LC–MS/MS) on tryptic digests of cultural heritage objects has attracted much attention. It allows for unambiguous identification of peptides and proteins, and even in complex mixtures species-specific identification becomes feasible with minimal sample consumption. Determination of the peptides is commonly based on theoretical cleavage of known protein sequences and on comparison of the expected peptide fragments with those found in the MS/MS spectra. In this approach, complex computer programs, such as Mascot, perform well identifying known proteins, but fail when protein sequences are unknown or incomplete. Often, when trying to distinguish evolutionarily well preserved collagens of different species, Mascot lacks the required specificity. Complementary and often more accurate information on the proteins can be obtained using a reference library of MS/MS spectra of species-specific peptides. Therefore, a library dedicated to various sources of proteins in works of art was set up, with an initial focus on collagen rich materials. This paper discusses the construction and the advantages of this spectral library for conservation science, and its application on a number of samples from historical works of art. 相似文献
975.
Herwig Bruneel Willem Mélange Bart Steyaert Dieter Claeys Joris Walraevens 《European Journal of Operational Research》2012
This paper considers a simple discrete-time queueing model with two types (classes) of customers (types 1 and 2) each having their own dedicated server (servers A and B resp.). New customers enter the system according to a general independent arrival process, i.e., the total numbers of arrivals during consecutive time slots are i.i.d. random variables with arbitrary distribution. Service times are deterministically equal to 1 slot each. The system uses a “global FCFS” service discipline, i.e., all arriving customers are accommodated in one single FCFS queue, regardless of their types. As a consequence of the “global FCFS” rule, customers of one type may be blocked by customers of the other type, in that they may be unable to reach their dedicated server even at times when this server is idle, i.e., the system is basically non-workconserving. One major aim of the paper is to estimate the negative impact of this phenomenon on the queueing performance of the system, in terms of the achievable throughput, the system occupancy, the idle probability of each server and the delay. As it is clear that customers of different types hinder each other more as they tend to arrive in the system more clustered according to class, the degree of “class clustering” in the arrival process is explicitly modeled in the paper and its very direct impact on the performance measures is revealed. The motivation of our work are systems where this kind of blocking is encountered, such as input-queueing network switches or road splits. 相似文献
976.
Dirk Mortag Thomas Theeg Katharina Hausmann Lars Grüner-Nielsen Kim Giessmann Jespersen Uwe Morgner Dieter Wandt Dietmar Kracht Jörg Neumann 《Optics Communications》2012,285(5):706-709
An all-fiber-integrated linear chirped-pulse amplifier system generating microjoule pulse energies is reported. It is seeded by an all-fiber dissipative-soliton laser and incorporates a newly developed fiber stretcher, whose dispersive properties match that of the grating compressor. Pulse durations of 189 fs with energies of 2.2 μJ were achieved after compression. The average power was 9.8 W at the repetition rate of 4.5 MHz. 相似文献
977.
TADDOL (=α,α,α′,α′‐Tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol) and the corresponding dichloride are converted to TADDAMINs (=(4S,5S)‐2,2,N,N′‐tetramethyl‐α,α,α′,α′‐tetraphenyl‐1,3‐dioxolan‐4,5‐dimethanamines) (Scheme 2) and ureas, 12 – 15 , and to TADDOP derivatives with seven‐membered O? P? O ester rings (Schemes 3 and 4). Cl/P‐Replacement via the Michaelis? Arbuzov reaction (Scheme 7) on mono‐ and dichlorides, derived from TADDOL, are described. It was not possible to obtain phosphines with the P‐atom attached to the benzhydrylic C‐atom of the TADDOL skeleton (Schemes 6 and 7). The X‐ray crystal structures (Figs. 1 and 2) of ten of the more than 30 new TADDOL derivatives are discussed. Full experimental details are presented. 相似文献
978.
Two neutral group 15-pentaazides dmap-As(N(3))(5) (1) and dmap-Sb(N(3))(5) (2) were synthesized and structurally characterized for the first time (dmap = 4-dimethylaminopyridine). Base-stabilization was confirmed to be very suitable for the kinetic stabilization of highly explosive covalent main group polyazides. 相似文献
979.
Information on the electronic structure of a molecule and its chemical bonds is encoded in the molecular normal vibrational modes. However, normal vibrational modes result from a coupling of local vibrational modes, which means that only the latter can provide detailed insight into bonding and other structural features. In this work, it is proven that the adiabatic internal coordinate vibrational modes of Konkoli and Cremer [Int. J. Quantum Chem. 67, 29 (1998)] represent a unique set of local modes that is directly related to the normal vibrational modes. The missing link between these two sets of modes are the compliance constants of Decius, which turn out to be the reciprocals of the local mode force constants of Konkoli and Cremer. Using the compliance constants matrix, the local mode frequencies of any molecule can be converted into its normal mode frequencies with the help of an adiabatic connection scheme that defines the coupling of the local modes in terms of coupling frequencies and reveals how avoided crossings between the local modes lead to changes in the character of the normal modes. 相似文献
980.
Herbert Meier Michael Schmidt Axel Mayer Dieter Schollmeyer Bernhard Beile 《Journal of heterocyclic chemistry》2012,49(3):516-520
The reaction of benzothiete ( 1 ) and the bicyclic alkenes 5 , 7 , 9 , or 12 shows a very high π side selectivity (de > 95%) in the formation of the polycyclic thiopyrans 6 , 8 , 10 , 11 , 13 , and 14 . 相似文献