全文获取类型
收费全文 | 12476篇 |
免费 | 2131篇 |
国内免费 | 1313篇 |
专业分类
化学 | 6866篇 |
晶体学 | 85篇 |
力学 | 577篇 |
综合类 | 52篇 |
数学 | 1091篇 |
物理学 | 4021篇 |
无线电 | 3228篇 |
出版年
2024年 | 63篇 |
2023年 | 358篇 |
2022年 | 409篇 |
2021年 | 533篇 |
2020年 | 503篇 |
2019年 | 462篇 |
2018年 | 451篇 |
2017年 | 377篇 |
2016年 | 616篇 |
2015年 | 565篇 |
2014年 | 714篇 |
2013年 | 962篇 |
2012年 | 1112篇 |
2011年 | 1040篇 |
2010年 | 769篇 |
2009年 | 798篇 |
2008年 | 792篇 |
2007年 | 685篇 |
2006年 | 682篇 |
2005年 | 515篇 |
2004年 | 374篇 |
2003年 | 320篇 |
2002年 | 279篇 |
2001年 | 235篇 |
2000年 | 257篇 |
1999年 | 303篇 |
1998年 | 237篇 |
1997年 | 223篇 |
1996年 | 245篇 |
1995年 | 195篇 |
1994年 | 165篇 |
1993年 | 129篇 |
1992年 | 124篇 |
1991年 | 76篇 |
1990年 | 88篇 |
1989年 | 59篇 |
1988年 | 37篇 |
1987年 | 30篇 |
1986年 | 41篇 |
1985年 | 39篇 |
1984年 | 13篇 |
1983年 | 19篇 |
1982年 | 9篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
11.
社会经济的快速发展促进了科学技术水平的显著提升,社会生产生活各个领域当中对计算机网络的应用程度不断提升,当前,网络已经成为了人们日常生活与工作的重要组成部分。为此,加强对网络安全等因素影响研究就显得格外重要。网络环境关系着用户的使用质量,更加影响人们的工作效率,加强对网络安全技术的研究时解决相关问题的核心内容。本文针对网络安全威胁因素以及常见的网络安全技术进行了简要分析。 相似文献
12.
Xiao Sun Li Zhang Yu Cao Jinhua Li Atanas G. Atanasov Linfang Huang 《Biomedical chromatography : BMC》2020,34(12):e4949
Balanophora involucrata J. D. Hooker has been known to possess potential anti-inflammatory and antibacterial activities; however, its antiviral activity has not been evaluated so far. In order to find new neuraminidase inhibitors (NAIs), the neuraminidase (NA) inhibition activity of different B. involucrata extracts was evaluated. In this study, an in vitro NA inhibition assay was performed to identify which extract of B. involucrata exhibits (maximal) inhibitory activity against NA. Ultra high performance liquid chromatography/quadrupole time-of-flight–tandem mass spectroscopy (MS/MS) and molecular docking techniques were used to identify the specific compounds responsible for the anti-influenza activity of the extract, and to explore the potential natural NAIs. The ethyl acetate extract of B. involucrata exhibited significant inhibitory activity against NA with 50% inhibitory concentration (IC50) value of 159.5 μg/mL. Twenty compounds were identified according to the MS/MS spectra; among them two compounds (quercitrin and phloridzin) showed obvious inhibitory activity against NA, with IC50 of 311.76 and 347.32 μmol/L, respectively. This study suggested that B. involucrata can be a potential natural source of NAIs and may be useful in the fight against ferocious influenza viruses. 相似文献
13.
Studies on metabolism of total glucosides of paeony from Paeoniae Radix Alba in rats by UPLC‐Q‐TOF‐MS/MS 下载免费PDF全文
Wenli Cao Xinguo Wang Haojie Li Xuliang Shi Wencheng Fan Shaohua Zhao Minyan Liu Liying Niu 《Biomedical chromatography : BMC》2015,29(11):1769-1779
Total glucosides of paeony are the active constituents of Paeoniae Radix Alba. In this study, a novel strategy was proposed to find more metabolites and the differences between paeoniflorin, albiflorin and total glucosides of paeony (TGP). This strategy was characterized as follows: firstly, the animals were divided into three groups (paeoniflorin, albiflorin and TGP) to identify the source of TGP metabolites from paeoniflorin or albiflorin; secondly, a generic information‐dependent acquisition scan for the low‐level metabolites was triggered by the multiple mass defect filter and dynamic background subtraction; thirdly, the metabolites were identified with a combination of data‐processing methods including mass defect filtering, neutral loss filtering and product ion filtering; finally, a comparative study was used in the metabolism of paeoniflorin, albiflorin and TGP. Based on the strategy, 18 metabolites of TGP, 10 metabolites of paeoniflorin and 13 metabolites of albiflorin were identified respectively. The results indicated that the hydrolysis, conjugation reaction and oxidization were the major metabolic pathways, and the metabolic sites were the glycosidic linkage, the ester bond and the benzene ring. This study is first to explore the metabolism of TGP, and these findings enhance our understanding of the metabolism and the interactions of paeoniflrin and albiflorin in TGP. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
14.
15.
Xiang Cao Sheng Zhong 《Communications Letters, IEEE》2006,10(8):580-581
Lin et al., (2003) proposed a remote user authentication scheme for multi-server architecture. In this paper, we breaks this scheme by giving an attack. Our attack allows an adversary to impersonate any user in the system, as long as a single authentication message of that user is observed. 相似文献
16.
Single-crystalline gallium nitride nanobelts have been synthesized through the reaction of gallium vapor with flowing ammonia using nickel as a catalyst. The as-synthesized products were characterized using X-ray powder diffraction (XRD), scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected-area electron diffraction (SAED). XRD and SAED results revealed that the products are pure, single-crystalline GaN with hexagonal structure. The widths and thickness of the nanobelts ranged from 80 to 200 nm, and 10 to 30 nm, respectively. The lengths were up to several tens of micrometers. The nanobelts had smooth surface with no amorphous sheath, and a sharp-tip end. The growth mechanism of nanobelts was discussed. 相似文献
17.
Qunsheng Cao Yinchao Chen 《Antennas and Propagation, IEEE Transactions on》2003,51(2):350-357
We apply an anisotropic perfectly matched layer (APML) absorber for open boundary truncation in implementation of the multiresolution time domain (MRTD) scheme. We develop an APML update algorithm to handle a general APML region in the MRTD formulations with the content of the leapfrog algorithm applied in the conventional finite-difference time-domain method. We also discuss the boundary truncations for both perfectly electric conductor (PEC) and perfectly magnetic conductor (PMC) walls using the image techniques. We validate the algorithm by analyzing various guided wave and antenna structures. It is found that the APML performs well for absorbing electromagnetic waves in the MRTD algorithm. 相似文献
18.
19.
论文将Fermat素性检验的思想运用于不可约多项式的判断,给出了一个对于不可约判断问题的Monte Carlo 算法,分析了该算法的计算复杂度问题,并且给出了次数在200以内的检验结果。 相似文献
20.
传统的数据采集器多采用Inte151系列或Inte196系列的CPU,运算能力差,速度慢,精度低,尤其是浮点运算能力限制了技术指标的进一步提高。有鉴于此,各科研单位及数据采集器产家正积极开发新一代的数据采集器。如采用高性能的嵌入式芯片,或带有数字信号处理功能的芯片等。TI公司的TMS320VC33数字信号处理芯片已能支持高达150MFLOPS的运行速度,是需浮点运算的电子产品应用场合中一种较理想的微处理器件。 相似文献