全文获取类型
收费全文 | 1159篇 |
免费 | 33篇 |
国内免费 | 52篇 |
专业分类
化学 | 820篇 |
晶体学 | 4篇 |
力学 | 19篇 |
综合类 | 2篇 |
数学 | 190篇 |
物理学 | 209篇 |
出版年
2023年 | 27篇 |
2022年 | 6篇 |
2021年 | 11篇 |
2020年 | 6篇 |
2019年 | 27篇 |
2018年 | 23篇 |
2017年 | 34篇 |
2016年 | 19篇 |
2015年 | 15篇 |
2014年 | 63篇 |
2013年 | 120篇 |
2012年 | 35篇 |
2011年 | 80篇 |
2010年 | 62篇 |
2009年 | 77篇 |
2008年 | 76篇 |
2007年 | 88篇 |
2006年 | 88篇 |
2005年 | 68篇 |
2004年 | 89篇 |
2003年 | 17篇 |
2002年 | 24篇 |
2001年 | 18篇 |
2000年 | 13篇 |
1999年 | 15篇 |
1998年 | 23篇 |
1997年 | 27篇 |
1996年 | 17篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 2篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有1244条查询结果,搜索用时 31 毫秒
81.
Implementing of photonic sampling and quantizing analog-to-digital converters (ADCs) enable us to extract a single binary word from optical signals without need for extra electronic assisting parts. This would enormously increase the sampling and quantizing time as well as decreasing the consumed power. To this end, based on the concept of successive approximation method, a 4-bit full-optical ADC that operates using the intensity-dependent Kerr-like nonlinearity in a two dimensional photonic crystal (2DPhC) platform is proposed. The Silicon (Si) nanocrystal is chosen because of the suitable nonlinear material characteristic. An optical limiter is used for the clamping and quantization of each successive levels that represent the ADC bits. In the proposal, an energy efficient optical ADC circuit is implemented by controlling the system parameters such as ring-to-waveguide coupling coefficients, the ring’s nonlinear refractive index, and the ring’s length. The performance of the ADC structure is verified by the simulation using finite difference time domain (FDTD) method. 相似文献
82.
Stephen K. Taylor Dawn DeYoung Lloyd J. Simons James R. Vyvyan ‡ Mary A. Wemple Noelle K. Wood 《合成通讯》2013,43(9):1691-1701
The nucleophilic opening of epoxides by nitrile enolates using an efficient, convenient protocol is described. The diastereoselectivity of this reaction was explored and found to give syn:anti ratios ranging from 1.1:1.0 to 4.8:1.0. 相似文献
83.
A simple and convenient route is presented for the preparation of regio- and stereoisomers of novel azepane β-amino esters, starting from bicyclic β-lactam isomers. The synthetic procedure consists of dihydroxylation of the olefinic bond of the alicyclic amino esters, followed by NaIO4-mediated cleavage of the diol intermediate and reductive ring closure, which furnishes novel regioisomeric 5-aminoazepane-4-carboxylate and 3-aminoazepane-4-carboxylates. This method also allows the preparation of amino esters with an azepane skeleton in enantiomerically pure form. 相似文献
84.
This paper proposes the interesting nonlinear behavior of light known as bifurcation, where the use of such behavior in a micro ring resonator to form the secure digital codes for optical packet switching application is demonstrated. A new concept of the stop-start bits in an optical packet switching protocol is formed by using the bifurcation codes. Bifurcation is introduced when light is input into a nonlinear micro ring device, where the refractive index of an InGaAsP/InP is one of device parameters. The other parameters of the device are coupling coefficient (K) and the ring radius (R), where the ring radii used are ranged from 5 to 10 μm. Simulation results obtained have shown that the packet switching data can be secured by using the generated start-stop bits as the secured codes. 相似文献
85.
THILO GIPSER HUBERT A. JAGER LUTZ RAPP Swiss Federal Institute of Technology Communication Technology Laboratory ETH Zentrum Zurich Switzerland 《Fiber and Integrated Optics》1998,17(1):21-40
All-optical networks are a sound answer to the huge data traffic demand expected for the forthcoming information society. Interesting networking aspects are the broadcasting, scalability, and reconfiguration in an all-optical environment. These issues are addressed in the present article for an all-optical network called MATRIX, which is time slotted and uses packet switching. The architecture enables the all-optical interconnection of a large number of nodes with both a small number of wavelengths and wavelength continuity based on a consequent exploitation of wavelength-division multiplexing and space-division multiplexing (i.e., multiple fibers per cable are used). For an efficient use of the resources, each node is equipped with multiple transmitters and receivers, respectively. Two different broadcasting schemes are investigated. An analytical model is developed, and the corresponding simulation results are in good agreement. It is further shown how different networks of arbitrary size can be interconnected to larger all-optical network clusters, thereby providing scalability not only with respect to the number of nodes but also to the geographical extension. Finally, wavelength reassignments allow change of optical paths through the network in case of changing traffic patterns or network failures in order to best exploit the available network resources. The total number of possible configurations is determined by the theory of Latin Squares. 相似文献
86.
Summary Synthesis of MBAMT (3-methyl-4-benzylideneamino-5-mercapto-1,2,4-triazole) and its IR and NMR spectral data are reported. The high stability of the characteristically coloured chelates with Cu(II), Co(II), Ni(II), Pd(II), Pt(IV) and Rh(III) has been made the basis for their efficient ascending TLC separations on silica gel G layers, when present together. Results of four different solvent systems are included to assess efficient resolution of the chelates along with their limits of identification and separation. TLC separations, followed by the ring colorimetric determination of the six metal ions (as ternary mixtures) are tabulated.MBAMT=3-methyl-4-benzylideneamino-5-mercapto-1,2,4-triazole. 相似文献
87.
88.
I. Redsven 《Applied Surface Science》2007,253(12):5536-5543
Detection of adenosine triphosphate (ATP) by bioluminescence is used, for instance, in the food industry and in hospitals to assess the hygiene status of surfaces. The aim of this laboratory study was to investigate the feasibility of the ATP method for estimating the cleanability of resilient floor coverings from biological soil. The surfaces were worn using a Soiling and Wearing Drum Tester, and soiled and cleaned with an Erichsen Washability and Scrubbing Resistance Tester. In the laboratory test carried out with the bioluminescence method, most of the new and worn floor coverings that were biologically soiled were cleaned efficiently. According to this study, the semiquantitative ATP screening method can be used for hygiene monitoring of flooring materials. No correlation was found between cleanability and contact angles or surface topography measured using a profilometer. However, by revealing local irregularities and damage on surfaces, scanning electron micrographs appeared useful in explaining differences in cleanability. 相似文献
89.
We present the second part of the investigation of the high sensitivity absorption spectrum of nitrous oxide by CW-Cavity Ring Down Spectroscopy near 1.5 μm. In a first paper [A.W. Liu, S. Kassi, P. Malara, D. Romanini, V.I. Perevalov, S.A. Tashkun, S.M. Hu, A. Campargue, J. Mol. Spectrosc. 244 (2007) 33-47] devoted to the 6000-6833 cm−1 region, more than 6000 line positions of five isotopologues (14N216O, 15N14N16O, 14N15N16O, 14N217O, and 14N218O), were rovibrationally assigned to a total of 68 bands. The achieved noise equivalent absorption (αmin ∼ 2 × 10−10 cm−1) allowed for the detection of lines with intensity weaker than 2 × 10−29 cm/molecule. In this contribution, the investigated region was extended down to 5905 cm−1 and additional recordings allowed accessing small spectral sections uncovered in our preceding recordings. A deeper analysis based on the predictions of the effective Hamiltonian model has allowed assigning a total of 3149 transitions and lowering the percentage of lines left unassigned from 51% to 28%. It led to the analysis of 35, 6, 7, and 6 bands for the 14N216O, 15N14N16O, 14N15N16O, and 14N218O isotopologues, respectively. Forty-two of these 54 bands are newly observed, while the rotational analysis of the twelve others is significantly extended and improved. Most of the bands were found unperturbed and their line positions could be reproduced within the experimental uncertainty (about 1 × 10−3 cm−1). The corresponding spectroscopic parameters are reported. Local rovibrational perturbations induced by either intrapolyad or interpolyad couplings were found to affect five hot bands of 14N216O. Their detailed analysis is presented. 相似文献
90.
《合成通讯》2013,43(21):3847-3864
Abstract Total synthesis of the nonaromatic cuparenoid sesquiterpenes (±)‐infuscol A and cuprenenol, and (±)‐infuscol B and neocuprenenol isolated from the Japanese liverwort Jungermannia infusca has been accomplished. Two ring‐closing metathesis reaction based strategies have been developed for the generation of the key intermediate of the sequence. 相似文献