全文获取类型
收费全文 | 3891篇 |
免费 | 168篇 |
国内免费 | 47篇 |
专业分类
化学 | 2690篇 |
晶体学 | 31篇 |
力学 | 152篇 |
数学 | 354篇 |
物理学 | 879篇 |
出版年
2023年 | 14篇 |
2022年 | 19篇 |
2021年 | 63篇 |
2020年 | 72篇 |
2019年 | 58篇 |
2018年 | 46篇 |
2017年 | 43篇 |
2016年 | 100篇 |
2015年 | 84篇 |
2014年 | 140篇 |
2013年 | 232篇 |
2012年 | 245篇 |
2011年 | 287篇 |
2010年 | 180篇 |
2009年 | 202篇 |
2008年 | 240篇 |
2007年 | 199篇 |
2006年 | 191篇 |
2005年 | 177篇 |
2004年 | 171篇 |
2003年 | 156篇 |
2002年 | 164篇 |
2001年 | 105篇 |
2000年 | 112篇 |
1999年 | 60篇 |
1998年 | 55篇 |
1997年 | 48篇 |
1996年 | 62篇 |
1995年 | 50篇 |
1994年 | 45篇 |
1993年 | 44篇 |
1992年 | 29篇 |
1991年 | 27篇 |
1990年 | 24篇 |
1989年 | 32篇 |
1988年 | 16篇 |
1987年 | 25篇 |
1986年 | 26篇 |
1985年 | 33篇 |
1984年 | 28篇 |
1983年 | 29篇 |
1982年 | 19篇 |
1981年 | 21篇 |
1980年 | 12篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 16篇 |
1976年 | 20篇 |
1975年 | 17篇 |
1974年 | 10篇 |
排序方式: 共有4106条查询结果,搜索用时 15 毫秒
311.
We propose double pass fiber Raman amplifier schemes based on Raman fiber oscillator in order to amplify optical signal with wavelengths from 1610 to 1650 nm efficiently. We experimentally demonstrate that the proposed double pass amplifier scheme has enhanced gain characteristics compared to a conventional single pass scheme. We also demonstrate a scheme for the proposed double pass amplifier to have small gain variation over the wavelength range by using two fiber Bragg gratings with different center wavelengths. 相似文献
312.
A novel technique for collimation testing with a circular Dammann grating is proposed. When the beam under test is incident on a one-order circular Dammann grating with limited aperture, double-humped radial rings will be generated at the back focal plane of a focusing lens. If the beam is collimated, the separation between the two rings will reach its minimal, otherwise the two rings will be apart from each other. Therefore, the degree of collimation of the tested beam can be estimated from the separation. The principle and experimental results of the method are presented. Owing to the simplicity and low cost of the method, it is a promising method for quickly checking the collimation of a laser beam. 相似文献
313.
X.M. Liu Paul K. Chu C.Y. Chung K.W.K. Yeung K.M.C. Cheung 《Applied Surface Science》2007,253(6):3154-3159
Water plasma immersion ion implantation (PIII) was conducted on orthopedic NiTi shape memory alloy to enhance the surface electrochemical characteristics. The surface composition of the NiTi alloy before and after H2O-PIII was determined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) was utilized to determine the roughness and morphology of the NiTi samples. Potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) were carried out to investigate the surface electrochemical behavior of the control and H2O-PIII NiTi samples in simulated body fluids (SBF) at 37 °C as well as the mechanism. The H2O-PIII NiTi sample showed a higher breakdown potential (Eb) than the control sample. Based on the AFM results, two different physical models with related equivalent electrical circuits were obtained to fit the EIS data and explain the surface electrochemical behavior of NiTi in SBF. The simulation results demonstrate that the higher resistance of the oxide layer produced by H2O-PIII is primarily responsible for the improvement in the surface corrosion resistance. 相似文献
314.
Hyun Sik Yoon Chung Ho Jeon Jae Hwan Jung Bonguk Koo Changyoung Choi Sung Chul Shin 《国际流体数值方法杂志》2013,73(3):250-265
In the present paper, a direct forcing/fictitious domain (DF/FD)–level set method is proposed to simulate the twophase flow–body interaction. The DF/FD does not sacrifice accuracy and robustness by employing a discrete δ (Dirac delta) function to transfer quantities between the Eulerian nodes and Lagrangian points explicitly as the immersed boundary method. The advantages of this approach are the simple concept, the easy implementation and the utilization of original governing equation without modification. The main idea is to combine DF/FD method with the level set method in the Cartesian coordinates. We present the results of a number of test cases to illustrate the effectiveness of the proposed method for single‐phase flow–body interaction problem and the two‐phase flows with a stationary body. Eventually, the simulations of various water entry problems have been conducted to validate the capability and the accuracy of the present method on solving the twophase flow–body interaction. Consequently, the present results are found to be in good agreement with those of previous studies. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
315.
Enzyme-Inspired Chiral Secondary-Phosphine-Oxide Ligand with Dual Noncovalent Interactions for Asymmetric Hydrogenation
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Caiyou Chen Zhefan Zhang Shicheng Jin Dr. Xiangru Fan Mingyu Geng Yan Zhou Songwei Wen Xinrui Wang Dr. Lung Wa Chung Dr. Xiu-Qin Dong Prof. Xumu Zhang 《Angewandte Chemie (International ed. in English)》2017,56(24):6808-6812
Inspired by the unique character of enzymes, we developed novel chiral SPO (secondary-phosphine-oxide) ligand (SPO-Wudaphos) which can enter into both ion pair and H-bond noncovalent interactions. The novel chiral SPO-Wudaphos exhibited excellent results in the asymmetric hydrogenation of α-methylene-γ-keto carboxylic acids, affording the chiral γ-keto acids with up to over 99 % ee. A series of control experiments and DFT calculations were conducted to illustrate the critical roles of both the ion pair and H-bond noncovalent interactions. 相似文献
316.
317.
318.
Kyung Hwan Kim Dae Sung Chung Chan Eon Park Dong Hoon Choi 《Journal of polymer science. Part A, Polymer chemistry》2011,49(1):55-64
New dithienothiophene‐containing conjugated polymers, such as poly(2,6‐bis(2‐thiophenyl‐3‐dodecylthiophene‐2‐yl)dithieno[3,2‐b;2′,3′‐d]thiophene, 4 and poly(2,6‐bis (2‐thiophenyl‐4‐dodecylthiophene‐2‐yl)dithieno[3,2‐b;2′,3′‐d]thiophene, 8 have been successfully synthesized via Stille coupling reactions using dodecyl‐substituted thiophene‐based monomers, bistributyltin dithienothiophene, and bistributyltin bithiophene; these polymers have been fully characterized. The main difference between the two polymers is the substitution position of the dodecyl side chains in the repeating group. Grazing‐incidence X‐ray diffraction (GI‐XRD) gave clear evidence of edge‐on orientation of polycrystallites to the substrate. The semiconducting properties of the two polymers have been evaluated in organic thin film transistors (OTFTs). The two conjugated polymers 4 and 8 exhibit fairly high hole carrier mobilities as high as μave = 0.05 cm2/Vs (ION/OFF = 3.42 × 104) and μave = 0.01 cm2/Vs, (ION/OFF = 1.3 × 105), respectively, after thermal annealing process. The solvent annealed films underwent reorganization of the molecules to induce higher crystallinity. Well‐defined atomic force microscopy (AFM) topography supported a significant improvement in TFT device performance. The hole carrier mobilities of the solvent annealed films are comparable to those obtained for a thermally annealed sample, and were one‐order higher than those obtained with a pristine sample. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010 相似文献
319.
Jung-Kyen Moon Jung-Won Park Woo Song Lee Young-Jin Kang Hyun-A Chung Mi-Seon Shin Yong-Jin Yoon Ki Hun Park 《Journal of heterocyclic chemistry》1999,36(3):793-798
This paper presents the synthesis of 2-amino-, 2-acetamido- and 2-benzamidothioxanthones and their 10,10-dioxides. 相似文献
320.
Saram Lee Segyeong Joo Sejin Park Soyoun Kim Hee Chan Kim Taek Dong Chung 《Electrophoresis》2010,31(10):1623-1629
In this study, in situ surface‐enhanced Raman scattering (SERS) decoding was demonstrated in microfluidic chips using novel thin micro gold shells modified with Raman tags. The micro gold shells were fabricated using electroless gold plating on PMMA beads with diameter of 15 μm. These shells were sophisticatedly optimized to produce the maximum SERS intensity, which minimized the exposure time for quick and safe decoding. The shell surfaces produced well‐defined SERS spectra even at an extremely short exposure time, 1 ms, for a single micro gold shell combined with Raman tags such as 2‐naphthalenethiol and benzenethiol. The consecutive SERS spectra from a variety of combinations of Raman tags were successfully acquired from the micro gold shells moving in 25 μm deep and 75 μm wide channels on a glass microfluidic chip. The proposed functionalized micro gold shells exhibited the potential of an on‐chip microfluidic SERS decoding strategy for micro suspension array. 相似文献