全文获取类型
收费全文 | 3559篇 |
免费 | 247篇 |
国内免费 | 11篇 |
专业分类
化学 | 2120篇 |
晶体学 | 33篇 |
力学 | 75篇 |
数学 | 134篇 |
物理学 | 515篇 |
无线电 | 940篇 |
出版年
2024年 | 2篇 |
2023年 | 38篇 |
2022年 | 32篇 |
2021年 | 78篇 |
2020年 | 94篇 |
2019年 | 86篇 |
2018年 | 61篇 |
2017年 | 65篇 |
2016年 | 132篇 |
2015年 | 106篇 |
2014年 | 123篇 |
2013年 | 203篇 |
2012年 | 253篇 |
2011年 | 285篇 |
2010年 | 187篇 |
2009年 | 189篇 |
2008年 | 268篇 |
2007年 | 249篇 |
2006年 | 205篇 |
2005年 | 205篇 |
2004年 | 146篇 |
2003年 | 126篇 |
2002年 | 128篇 |
2001年 | 86篇 |
2000年 | 68篇 |
1999年 | 78篇 |
1998年 | 42篇 |
1997年 | 32篇 |
1996年 | 40篇 |
1995年 | 36篇 |
1994年 | 32篇 |
1993年 | 30篇 |
1992年 | 15篇 |
1991年 | 12篇 |
1990年 | 16篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1979年 | 2篇 |
1978年 | 5篇 |
1976年 | 4篇 |
1974年 | 5篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有3817条查询结果,搜索用时 15 毫秒
991.
An 8 bit current-mode folding and interpolation analog to digital converter (ADC) with three-level folding amplifiers is proposed in this paper. A current-mode three-level folding amplifier is employed not only to reduce the number of reference current sources, but also to decrease a power dissipation within the ADC. The designed ADC fabricated by a 0.8 m n-well CMOS double metal/single poly process occupies the chip area of 2.2 × 1.6. The experimental result shows the power dissipation of 33.6 mW with a power supply of 5 V. 相似文献
992.
[formula: see text] A mild and reliable one-pot protocol for the elaboration of sterically demanding carboxylic acids into alpha-diazoketones via acyl mesylates has been developed. Aside from delineating the reaction parameters which render this strategy quite general for hindered carboxylic acids, we have directly proven the existence of the fleeting acyl mesylate group as the reactive species in these reactions and shed light onto the differing mechanisms which are operative in the activation of hindered and simple carboxylic acids with methanesulfonyl chloride. 相似文献
993.
alpha,beta-Unsaturated nitroalkenes are readily reduced to the corresponding aldoximes and ketoximes in good yields, using a system of decaborane (B10H14) and DMSO in methanol in the presence of 10% Pd/C at room temperature under nitrogen. 相似文献
994.
Reported herein is a mild and efficient Pd(II) catalysis, leading to the formation of carbon-carbon bonds between a broad spectrum of organoboron compounds and alkenes. Molecular oxygen was employed to reoxidize the resultant Pd(0) species back to Pd(II) during catalytic cycles. This oxygen protocol promoted the desired Pd(II) catalysis, whereas it retarded competing Pd(0) catalytic pathways such as Heck or Suzuki couplings. [reaction: see text] 相似文献
995.
[reaction: see text] We have found a new synthetic method for the preparation of the alpha-bromoketones that are brominated in the less activated terminal position of unsymmetrical ketones. Brominations in short reaction times (kinetically controlled) provided internally brominated compounds as a major product. However, brominations in longer reaction times (thermodynamically controlled) gave more of the terminally brominated compound through the reversible reaction by Br(2) and produced hydrogen bromide. Several brominated compounds at the terminal position were successfully prepared through the new synthetic route. 相似文献
996.
We have synthesized low-molecular-weight, triazole ring-appended 2'-deoxyuridine hydrogelators that gelate pure water at a concentration as low as 0.2 wt% and these compounds display different aggregation and network structures as evidenced by their SEM images, FTIR spectra and minimum gel concentrations. 相似文献
997.
J-substitution algorithm in magnetic resonance electrical impedance tomography (MREIT): phantom experiments for static resistivity images 总被引:4,自引:0,他引:4
Khang HS Lee BI Oh SH Woo EJ Lee SY Cho MH Kwon O Yoon JR Seo JK 《IEEE transactions on medical imaging》2002,21(6):695-702
Recently, a new static resistivity image reconstruction algorithm is proposed utilizing internal current density data obtained by magnetic resonance current density imaging technique. This new imaging method is called magnetic resonance electrical impedance tomography (MREIT). The derivation and performance of J-substitution algorithm in MREIT have been reported as a new accurate and high-resolution static impedance imaging technique via computer simulation methods. In this paper, we present experimental procedures, denoising techniques, and image reconstructions using a 0.3-tesla (T) experimental MREIT system and saline phantoms. MREIT using J-substitution algorithm effectively utilizes the internal current density information resolving the problem inherent in a conventional EIT, that is, the low sensitivity of boundary measurements to any changes of internal tissue resistivity values. Resistivity images of saline phantoms show an accuracy of 6.8%-47.2% and spatial resolution of 64 x 64. Both of them can be significantly improved by using an MRI system with a better signal-to-noise ratio. 相似文献
998.
999.
Jong-Woon Ha Hyeong Ju Eun Byoungwook Park Hyungju Ahn Dong Ryeol Hwang Yeong Seok Shim Junseok Heo Changjin Lee Sung Cheol Yoon Jong H. Kim Seo-Jin Ko 《Advanced functional materials》2023,33(8):2211486
Near-infrared organic photodetectors (NIR OPDs) comprising ultra-narrow bandgap non-fullerene acceptors (NFA, over 1000 nm) typically exhibit high dark current density under applied reverse bias. Therefore, suppression of dark current density is crucial to achieve high-performance of such NIR OPDs. Herein, cyano (CN) with a strong electron-withdrawing property is introduced into alkoxy thiophene as a π-bridge to adjust its optoelectronic characteristics, and the correlation between dark current density and charge injection barrier is investigated. Compared with their motivated NFA (COTH), the novel CN-substituted NFAs, COTCN and COTCN2, exhibited deeper-lying highest occupied molecular orbital energy levels and narrower optical bandgap (<1.10 eV), owing to the strong inductive and resonance effect of CN. The dark current and total noise currents are minimized as the number of substituted CN increases because of the larger hole injection barrier. Consequently, PTB7-Th:COTCN2 exhibited the best shot-noise limited detectivity (D*sh, 1.18 × 1012 Jones) and total noise detectivity (D*n, 1.33 × 1011 Jones) compared with those of PTB7-Th:COTH (D*sh, 2.47 × 1011 Jones and D*n, 1.96 × 1010 Jones). 相似文献
1000.
Iqra Moeez Dieky Susanto Jae-Ho Park Ji-Young Kim Hee-Dae Lim Kyung Yoon Chung 《Advanced functional materials》2023,33(6):2210370
Sodium-rich metallic Nax+z has received significant attention as a low-cost alternative to the conventional electrode materials used in Li-ion batteries. However, the poor cyclability of NaxCl remains a major challenge to its practical application. Here, a simple method is developed for improving the electrochemical performance of NaxCl by controlling the upper limit of cut-off voltage. It is demonstrated that additional Na-vacancy defects can be introduced in the NaCl structure during the high-voltage activation process at 4.5 V. The structure then accommodates more sodium ions during the next discharge, resulting in increased capacity. At the same time, Cl-ions released by NaCl decomposition are oxidized to form Cl-based organic species at the active material interfaces. This plays a crucial role in protecting the NaCl electrode from undesired side reactions at high voltage. In short, this control of the charging protocol helps to induce more vacancies in the NaCl structure, as well as form stable interphases on the electrode surface, contributing to the increased capacity and enhanced cycle stability. This study will help in exploring a new approach for developing low-cost and high-capacity electrode material, which can potentially be applied in future energy-storage systems. 相似文献