全文获取类型
收费全文 | 2378篇 |
免费 | 45篇 |
国内免费 | 8篇 |
专业分类
化学 | 1380篇 |
晶体学 | 10篇 |
力学 | 57篇 |
数学 | 325篇 |
物理学 | 659篇 |
出版年
2022年 | 25篇 |
2021年 | 29篇 |
2020年 | 30篇 |
2019年 | 35篇 |
2017年 | 25篇 |
2016年 | 48篇 |
2015年 | 31篇 |
2014年 | 47篇 |
2013年 | 107篇 |
2012年 | 104篇 |
2011年 | 122篇 |
2010年 | 61篇 |
2009年 | 45篇 |
2008年 | 92篇 |
2007年 | 101篇 |
2006年 | 89篇 |
2005年 | 112篇 |
2004年 | 75篇 |
2003年 | 78篇 |
2002年 | 62篇 |
2001年 | 62篇 |
2000年 | 56篇 |
1999年 | 34篇 |
1998年 | 23篇 |
1997年 | 38篇 |
1996年 | 33篇 |
1995年 | 48篇 |
1994年 | 42篇 |
1993年 | 43篇 |
1992年 | 41篇 |
1991年 | 32篇 |
1990年 | 35篇 |
1989年 | 20篇 |
1988年 | 23篇 |
1987年 | 21篇 |
1986年 | 17篇 |
1985年 | 33篇 |
1984年 | 26篇 |
1983年 | 17篇 |
1982年 | 26篇 |
1981年 | 33篇 |
1980年 | 21篇 |
1979年 | 34篇 |
1978年 | 28篇 |
1977年 | 31篇 |
1976年 | 35篇 |
1975年 | 31篇 |
1974年 | 27篇 |
1973年 | 37篇 |
1972年 | 26篇 |
排序方式: 共有2431条查询结果,搜索用时 0 毫秒
871.
872.
873.
The transfer function of a multimode step-index fibre has been calculated by means of geometrical optics taking into account mode coupling and leaky rays. The light source may have axial symmetry (light emitting diode) or be line shaped (semiconductor laser). Radial displacement of the line source relative to the fibre axis is allowed. Calculations performed for a large numerical aperture fibre (N.A.=0.475) with a coupling length of 2 km show that leaky rays and radial displacement of the line source are significant for fibre lengths shorter than 200m. The influence of the width of the entrance beam is significant for lengths up to 5 km.Varying coupling length and steady state angle for a fibre length of 150 m calculations show that the bandwidth varies between 15 and 95 MHz for penalty losses between 8 and 18 dB.Calculations have been compared with direct baseband frequency measurements for a high-loss fibre with a large numerical aperture and excellent agreement has been found. 相似文献
874.
Abbott T Akiba Y Beavis D Bloomer MA Bond PD Chasman C Chen Z Chu YY Cole BA Costales JB Crawford HJ Cumming JB Debbe R Engelage J Fung SY Grodzins L Gushue S Hamagaki H Hansen O Hayano RS Hayashi S Homma S Huang HZ Ikeda Y Juricic I Kang J Katcoff S Kaufman S Kimura K Kitamura K Kurita K Ledoux RJ Levine MJ Miake Y Morse RJ Moskowitz B Nagamiya S Olness J Parsons CG Remsberg LP Sakurai H Sarabura M Shor A Stankus P Steadman SG Stephans GS Sugitate T Tanaka M Tannenbaum MJ Torikoshi M 《Physical review letters》1991,66(12):1567-1570
875.
The European Physical Journal H - The two main methodologies of computational Statistical Mechanics, namely the stochastic Monte Carlo and the deterministic Molecular Dynamic methods, were... 相似文献
876.
Mark J. Meijerink Cristiano Spiga Thomas W. Hansen Christian D. Damsgaard Krijn P. de Jong Jovana Ze
evi 《Particle & Particle Systems Characterization》2019,36(1)
Liquid phase transmission electron microscopy (LP‐TEM) is a novel and highly promising technique for the in situ study of important nanoscale processes, in particular the synthesis and modification of various nanostructures in a liquid. Destabilization of the samples, including reduction, oxidation, or dissolution by interactions between electron beam, liquid, and sample, is still one of the main challenges of this technique. This work focuses on amorphous silica nanospheres and the phenomena behind their reshaping and dissolution in LP‐TEM. It is proposed that silica degradation is primarily the result of reducing radical formation in the liquid phase and the subsequent accelerated hydroxylation of the silica, while alterations in silica solid structure, pH, and oxidizing species formation had limited influence. Furthermore, the presence of water vapor instead of liquid water also results in degradation of silica. Most importantly however, it is shown that the addition of scavengers for reducing radicals significantly improved amorphous silica stability during LP‐TEM imaging. Devising such methods to overcome adverse effects in LP‐TEM is of the utmost importance for further development and implementation of this technique in studies of nanoscale processes in liquid. 相似文献
877.
Gustav J. Wrmer Bente K. Hansen Johan Palmfeldt Thomas B. Poulsen 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(34):12044-12048
Cyclopropenes are an important new addition to the portfolio of functional groups that can be used for bioorthogonal couplings. The inert nature of these highly strained compounds in complex biological systems is almost counterintuitive given their established electrophilic properties in organic synthesis. Here we provide the first demonstration of a cyclopropene that is capable of direct conjugation to protein targets in cells and show that this compound preferentially alkylates the active site cysteine of glutathione S‐transferase omega‐1 (GSTO1). 相似文献
878.
Alex Hansen Santanu Sinha Dick Bedeaux Signe Kjelstrup Magnus Aa. Gjennestad Morten Vassvik 《Transport in Porous Media》2018,125(3):565-587
Based on thermodynamic considerations, we derive a set of equations relating the seepage velocities of the fluid components in immiscible and incompressible two-phase flow in porous media. They necessitate the introduction of a new velocity function, the co-moving velocity. This velocity function is a characteristic of the porous medium. Together with a constitutive relation between the velocities and the driving forces, such as the pressure gradient, these equations form a closed set. We solve four versions of the capillary tube model analytically using this theory. We test the theory numerically on a network model. 相似文献
879.
880.
Nguyen TT Le TN Duus F Hansen BK Hansen PE 《Magnetic resonance in chemistry : MRC》2007,45(3):245-252
A new class of compounds, the 2-hydroxythioacetophenones, and related compounds have recently been synthesized. The hydrogen-bond system has been characterized by NMR chemical shifts and deuterium isotope effects on these as well as by DFT calculations. Use of solid-state (13)C NMR has enabled measurements of the intrinsic deuterium isotope effects of the most abundant tautomer of beta-thioxoketones. The compounds show very interesting long-range deuterium isotope effects on the thiocarbonyl carbon. The intramolecular hydrogen bonds of o-hydroxythioacetophenones are found to be slightly stronger than those of the corresponding acetophenones. The reactivity and stability of the compounds can be related to hydrogen bonding and to the presence of electron donating substituents. 相似文献