全文获取类型
收费全文 | 50747篇 |
免费 | 8256篇 |
国内免费 | 6154篇 |
专业分类
化学 | 29129篇 |
晶体学 | 484篇 |
力学 | 2209篇 |
综合类 | 246篇 |
数学 | 4157篇 |
物理学 | 14749篇 |
无线电 | 14183篇 |
出版年
2024年 | 130篇 |
2023年 | 1135篇 |
2022年 | 1403篇 |
2021年 | 1941篇 |
2020年 | 1980篇 |
2019年 | 2006篇 |
2018年 | 1668篇 |
2017年 | 1589篇 |
2016年 | 2309篇 |
2015年 | 2474篇 |
2014年 | 3042篇 |
2013年 | 3723篇 |
2012年 | 4482篇 |
2011年 | 4550篇 |
2010年 | 3178篇 |
2009年 | 3228篇 |
2008年 | 3566篇 |
2007年 | 3035篇 |
2006年 | 2891篇 |
2005年 | 2484篇 |
2004年 | 1876篇 |
2003年 | 1514篇 |
2002年 | 1525篇 |
2001年 | 1271篇 |
2000年 | 977篇 |
1999年 | 1046篇 |
1998年 | 907篇 |
1997年 | 814篇 |
1996年 | 808篇 |
1995年 | 653篇 |
1994年 | 574篇 |
1993年 | 440篇 |
1992年 | 424篇 |
1991年 | 338篇 |
1990年 | 289篇 |
1989年 | 208篇 |
1988年 | 141篇 |
1987年 | 102篇 |
1986年 | 130篇 |
1985年 | 93篇 |
1984年 | 51篇 |
1983年 | 56篇 |
1982年 | 38篇 |
1981年 | 28篇 |
1980年 | 10篇 |
1979年 | 6篇 |
1977年 | 3篇 |
1975年 | 2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 687 毫秒
51.
Semiconductors - The performance of InGaN/GaN multiple quantum well (MQW) solar cells with five different Si-doping concentrations, namely 0, 4 × 1017 cm–3, 1 × 1018 cm–3, 3... 相似文献
52.
53.
Yu Wang Xin-jing Zhao Rong-Jing Wei Gui-jie Liang Kang Wang Yuan-zhi Tan Ye Yang 《化学物理学报(中文版)》2021,34(5):591-597
The intermolecular interaction determines the photophysical properties of the organic aggregates, which are critical to the performance of organic photovoltaics. Here, excitonic coupling, an important intermolecular interaction in organic aggregates, between the π-stacking graphene quantum dots is studied by using transient absorption spectroscopy. We find that the spectral evolution of the ground state bleach arises from the dynamic variation of the excitonic coupling in the excited π-stacks. According to the spectral simulations, we demonstrate that the kinetics of the vibronic peak can be exploited as a probe to measure the dynamics of excitonic coupling in the excited π-stacks. 相似文献
54.
55.
56.
Abstract Gastrodigenin, also known as 4-hydroxybenzyl alcohol (HBA), is one of the main components of Gastrodia elata, which is a perfect lead compound of natural products. In order to get new active compounds, we modified the structure of HBA through esterification with carboxylic acid, and got a series of derivatives in which 4-hydroxybenzyl alcohol 2-naphthoate (NHBA) showed stronger antidepressant activity than HBA. In this paper, we firstly evaluated the antidepressant activity of NHBA by tail suspension test (TST) and forced swimming test (FST). Then, we carried out the biochemical assay and western blot to determine its mechanism. The results displayed that NHBA could increase the content of serotonin, dopamine, norepinephrine, γ-aminobutyric acid, brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) in mice brain. It suggested that NHBA exhibited an antidepressant-like effect through monoaminergic system, GABAergic system and BDNF/TrkB signaling pathways. 相似文献
57.
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
58.
In this paper we investigated the BER performance of DS-CDMA using various chip-waveforms, which include three time-limited chip-waveforms and two band-limited chip-waveforms. Closed-form formulae were derived for evaluating the achievable bit-error rate performance with the aid of the standard Gaussian approximation, when communicating over a Nakagami-m channel. The time-limited waveforms impose a low implementational complexity, since they maybe over sampled and read from a look-up table. However, they are outperformed by the frequency-domain raised-cosine waveform as well as the optimum waveform specifically designed by Cho and Lehnert for achieving the lowest possible bit error rate 相似文献
59.
Zhao M. Shea J. D. Hagness S. C. van der Weide D. W. 《Microwave and Wireless Components Letters, IEEE》2006,16(12):675-677
We present a 3-12 GHz compact mixer-based reflectometer (CMR) and horn antenna system, and demonstrate its use in detecting backscatter signals with a free space calibration procedure. We evaluate the frequency-domain performance of the CMR-antenna system for measuring the complex reflection coefficient of a dielectric slab and compare it with that of a commercial vector network analyzer (VNA)-antenna setup. Time-domain responses are also investigated, and show the effectiveness of this calibration method. This low-cost, compact system eliminates the need for traditional mechanical standards and a VNA, is effective in reducing reflection artifacts, and allows for flexibility in the placement of reference planes; thus it is well suited for array-based imaging applications 相似文献
60.