全文获取类型
收费全文 | 34993篇 |
免费 | 5597篇 |
国内免费 | 5262篇 |
专业分类
化学 | 20672篇 |
晶体学 | 422篇 |
力学 | 1096篇 |
综合类 | 310篇 |
数学 | 2689篇 |
物理学 | 8606篇 |
无线电 | 12057篇 |
出版年
2024年 | 90篇 |
2023年 | 624篇 |
2022年 | 1048篇 |
2021年 | 1211篇 |
2020年 | 1173篇 |
2019年 | 1235篇 |
2018年 | 1092篇 |
2017年 | 1223篇 |
2016年 | 1507篇 |
2015年 | 1884篇 |
2014年 | 2218篇 |
2013年 | 2799篇 |
2012年 | 3313篇 |
2011年 | 3396篇 |
2010年 | 2671篇 |
2009年 | 2621篇 |
2008年 | 2713篇 |
2007年 | 2480篇 |
2006年 | 2277篇 |
2005年 | 1897篇 |
2004年 | 1452篇 |
2003年 | 1139篇 |
2002年 | 1194篇 |
2001年 | 1027篇 |
2000年 | 731篇 |
1999年 | 600篇 |
1998年 | 403篇 |
1997年 | 329篇 |
1996年 | 270篇 |
1995年 | 212篇 |
1994年 | 199篇 |
1993年 | 169篇 |
1992年 | 142篇 |
1991年 | 116篇 |
1990年 | 84篇 |
1989年 | 71篇 |
1988年 | 56篇 |
1987年 | 43篇 |
1986年 | 43篇 |
1985年 | 31篇 |
1984年 | 12篇 |
1983年 | 11篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1977年 | 3篇 |
1971年 | 2篇 |
1959年 | 3篇 |
1957年 | 4篇 |
1936年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
991.
992.
993.
Andreas Wieczorek Hua Yang Brendan Roycroft Frank H. Peters Brian Corbett 《Optical and Quantum Electronics》2013,45(4):365-371
Polarization insensitive optical coupling of a large spot size input waveguide to a small spot size photodiode is achieved by the introduction of a transition waveguide. The coupling efficiency to the photodetector and the related absorption efficiency of the photodiode depend mainly on the refractive indices of the different InGaAsP layers. Waveguide alignments, waveguide widths and the n-contact etch depth do not influence the absorption efficiency relative to a 1-dB tolerance. 相似文献
994.
Linghao He Lu Yao Debin Yang Qingmin Cheng Jing Sun Rui Song 《Journal of Macromolecular Science: Physics》2013,52(12):2454-2463
Composites comprised of chitosan (CS) and multiwalled carbon nanotubes (MWCNTs) were fabricated by milling and ultrasonication dispersion methods. Scanning electron microscopy images showed homogeneous dispersion of MWCNTs throughout the CS matrix for samples prepared by either ultrasonication or milling methods. Further, the crystallinity of the CS component was found to decrease with the addition of MWCNTs, although the decomposition temperature and the storage modulus (E′) of the samples were improved. The decomposition temperature for the composite prepared by milling was 7°C higher than that by the ultrasonication. Meanwhile, the E′ decreased relatively slowly with temperature in the dynamic mechanical analysis measurements. In addition, IR analysis implied an interaction between CS and MWCNTs, which likely originated from hydrogen bonds between the amino, hydroxyl, and carboxyl groups of the two components. Compared with the ultrasonication, milling was more effective to promote the formation of the hydrogen bonds between CS and the MWCNTs and thus enhance the thermal stability of CS. 相似文献
995.
In this paper, we propose a novel extending algorithm to design rate compatible (RC) QC-IRA-d codes through Tanner Graph extension. The structure of QC-IRA-d codes provides high efficiency and low complexity for graph extension. Thus, in the proposed method, there are three features: (1) close relation between the extended parts and original parity check matrix; (2) larger length of introduced cycles; (3) balanced check node degree. All of them guarantee good performance of the designed RC family codes. Simulation results show that the graph extension method outperforms existed algorithms from 0.05 to 0.47 dB for different rate codes.
相似文献996.
Yang Guo Chen Corresponding author Zhang Zhi Guo Li Li Guan Rong Hua 《Liquid crystals》2013,40(12):1441-1447
The planar aligned nematic liquid crystal cell with different anchoring for the two substrates (i.e. a non-symmetric NLC cell) is investigated by an analytical method. We deduce the basic equations and the boundary conditions of the tilt angle θ of the LC director. Expressions for threshold and saturation magnetic field are obtained, and numerical results of these two quantities with variation in anchoring parameters of the two substrates are given. A symmetry breaking parameter Δ is introduced and the relations between Δ and applied field, as well as the two sets of anchoring parameters are discussed in detail. A feasible experimental plan for measurement of anchoring strengths of a series of different substrates is proposed. 相似文献
997.
A Pronounced Halogen Effect on the Organogelation Properties of Peripherally Halogen Functionalized Poly(benzyl ether) Dendrons 下载免费PDF全文
Prof. Dr. Yu Feng Hui Chen Zhi‐Xiong Liu Yan‐Mei He Prof. Dr. Qing‐Hua Fan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(14):4980-4990
An interesting halogen‐substituent effect on the organogelation properties of poly(benzyl ether) dendrons is reported. A new class of poly(benzyl ether) dendrons with halo substituents decorating their periphery was synthesized and fully characterized. A systematic study on the gelation abilities, thermotropic behaviors, aggregated microstructures, and mechanical properties of self‐assembled organogels was performed to elucidate the halogen‐substituent effects on their organogelation propensity. It was found that the exact halogen substitutions on the periphery of dendrons exert a profound effect on the organogelation propensity, and dendrons G n ‐Cl (n=2, 3) and G2‐I proved to be highly efficient organogelators. The cooperation of multiple π–π, dispersive halogen, CH–π, and weak C?H ??? X hydrogen‐bonding interactions were found to be the key contributor to forming the self‐assembled gels. Dendritic organogels formed from G n ‐Cl (n=2, 3) in 1,2‐dichloroethane exhibited thixotropic‐responsive properties, and such thixotropic organogels are promising materials for future research and applications. 相似文献
998.
With the rapid development of nanoscience and nanotechnology, various types of functional nanoreactors have been designed for diverse applications. Here, the recent evolution of the rational design of nanoreactors for chemical synthesis and biomedical applications are briefly summarized and discussed. The presence of nanoreactors provides constrained space isolated from the surrounding environment. Scientists are committed to studying changes in chemical reactions when the reaction system is confined to the nanosized space. Nanoreactors accelerate the reaction rate and even change mechanism of some chemical reactions. Cells and organelles as natural nanoreactors are also discussed. The development of intracellular synthesis makes it possible to realize various applications in biomedicine. The challenges on the rational design of nanoreactors and perspectives are also discussed. 相似文献
999.
Song Zhaorui Yu Lili Sun Yiyang He Hua 《Analytical and bioanalytical chemistry》2019,411(27):7293-7301
Analytical and Bioanalytical Chemistry - A simple, sensitive, and naked-eye assay of metformin (MET), based on the host–guest molecular recognition of cucurbit[6]uril (CB[6])-modified silver... 相似文献
1000.
This paper experimentally studies the natural convection heat transfer characteristics on horizontal and vertical plates at pressures lower than atmospheric value. Eight different pressures ranging from 10Pa to 101.325kPa, and three heating loads were used in the experiments. It is found that natural convection heat transfer characteristics at reduced pressures are independent of the plate arrangements and present different behaviors in two pressure ranges, 10Pa < p < 1000Pa and p > 1000Pa. The experimental heat transfer results are correlated into a non-dimensional equation to help the thermal design of stratosphere aircrafts and airborne equipment. 相似文献