全文获取类型
收费全文 | 15418篇 |
免费 | 431篇 |
国内免费 | 113篇 |
专业分类
化学 | 9956篇 |
晶体学 | 237篇 |
力学 | 328篇 |
综合类 | 1篇 |
数学 | 2430篇 |
物理学 | 1793篇 |
无线电 | 1217篇 |
出版年
2022年 | 136篇 |
2021年 | 197篇 |
2020年 | 219篇 |
2019年 | 191篇 |
2018年 | 186篇 |
2017年 | 189篇 |
2016年 | 377篇 |
2015年 | 309篇 |
2014年 | 352篇 |
2013年 | 820篇 |
2012年 | 814篇 |
2011年 | 882篇 |
2010年 | 496篇 |
2009年 | 436篇 |
2008年 | 823篇 |
2007年 | 817篇 |
2006年 | 754篇 |
2005年 | 701篇 |
2004年 | 642篇 |
2003年 | 504篇 |
2002年 | 552篇 |
2001年 | 171篇 |
2000年 | 164篇 |
1999年 | 162篇 |
1998年 | 139篇 |
1997年 | 173篇 |
1996年 | 232篇 |
1995年 | 145篇 |
1994年 | 143篇 |
1993年 | 149篇 |
1992年 | 149篇 |
1991年 | 129篇 |
1990年 | 153篇 |
1989年 | 129篇 |
1988年 | 126篇 |
1987年 | 119篇 |
1986年 | 120篇 |
1985年 | 209篇 |
1984年 | 223篇 |
1983年 | 156篇 |
1982年 | 246篇 |
1981年 | 214篇 |
1980年 | 193篇 |
1979年 | 175篇 |
1978年 | 169篇 |
1977年 | 150篇 |
1976年 | 165篇 |
1975年 | 147篇 |
1974年 | 156篇 |
1973年 | 144篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Mladejovsky Michael G. Eddington Donald K. Evans Jerald R. Dobelle William H. 《IEEE transactions on bio-medical engineering》1976,(4):286-296
Electrical stimulation of the visual cortex of the brain results in punctate photic sensations called ``phosphenes.' This suggests the concept of producing artificial vision for the blind by implanting arrays of electrodes, producing scoreboard-like displays. Electrical stimulation of the cochlea, VIIIth nerve, and auditory cortex produces analogous auditory sensations called ``audenes,' which might be used to provide artificial hearing for the deaf. 相似文献
142.
对于空间环境中近乎无碰撞的等离子体,可采用Vlasov方程进行理论描述,基于Vlasov方程,讨论了等离子体湍流能量传输和耗散的过程:由亚格子应力引起的尺度间的能量传输,电场做功,压强张量做功(压强张量与速度梯度张量的相互作用).通过混合Vlasov-Maxwell(HVM)数值模拟,进一步研究了能量传输通道之间的联系... 相似文献
143.
High‐Performance Sodium‐Ion Hybrid Supercapacitor Based on Nb2O5@Carbon Core–Shell Nanoparticles and Reduced Graphene Oxide Nanocomposites 下载免费PDF全文
Eunho Lim Changshin Jo Min Su Kim Mok‐Hwa Kim Jinyoung Chun Haegyeom Kim Jongnam Park Kwang Chul Roh Kisuk Kang Songhun Yoon Jinwoo Lee 《Advanced functional materials》2016,26(21):3711-3719
Sodium‐ion hybrid supercapacitors (Na‐HSCs) have potential for mid‐ to large‐scale energy storage applications because of their high energy/power densities, long cycle life, and the low cost of sodium. However, one of the obstacles to developing Na‐HSCs is the imbalance of kinetics from different charge storage mechanisms between the sluggish faradaic anode and the rapid non‐faradaic capacitive cathode. Thus, to develop high‐power Na‐HSC anode materials, this paper presents the facile synthesis of nanocomposites comprising Nb2O5@Carbon core–shell nanoparticles (Nb2O5@C NPs) and reduced graphene oxide (rGO), and an analysis of their electrochemical performance with respect to various weight ratios of Nb2O5@C NPs to rGO (e.g., Nb2O5@C, Nb2O5@C/rGO‐70, ‐50, and ‐30). In a Na half‐cell configuration, the Nb2O5@C/rGO‐50 shows highly reversible capacity of ≈285 mA h g?1 at 0.025 A g?1 in the potential range of 0.01–3.0 V (vs Na/Na+). In addition, the Na‐HSC using the Nb2O5@C/rGO‐50 anode and activated carbon (MSP‐20) cathode delivers high energy/power densities (≈76 W h kg?1 and ≈20 800 W kg?1) with a stable cycle life in the potential range of 1.0–4.3 V. The energy and power densities of the Na‐HSC developed in this study are higher than those of similar Li‐ and Na‐HSCs previously reported. 相似文献
144.
Frank Amoroso William W. Jones 《International Journal of Satellite Communications and Networking》1991,9(2):99-110
A geometric propagation model is developed for simulating the reception of direct sequence pseudonoise (DSPN) satellite signals by a directional or omni antenna in the dense scatterer mobile environment. The model is first validated for narrowband signals by a direct comparison of both simulated cumulative signal strength statistics and simulated diffuse Doppler spectra with classical theory. The model is then used to predict fading statistics for DSPN signaling, either with omni or directional antennas. As expected, the mitigation of fading is closely related to the ratio of DSPN chip duration to delay spread of the scatterer medium for both the omni and directional antennas. 相似文献
145.
Robert Dittmer Wook Jo Jürgen Rödel Sergei Kalinin Nina Balke 《Advanced functional materials》2012,22(20):4208-4215
Piezoresponse force microscopy (PFM) is used to afford insight into the nanoscale electromechanical behavior of lead‐free piezoceramics. Materials based on Bi1/2Na1/2TiO3 exhibit high strains mediated by a field‐induced phase transition. Using the band excitation technique the initial domain morphology, the poling behavior, the switching behavior, and the time‐dependent phase stability in the pseudo‐ternary system (1–x)(0.94Bi1/2Na1/2TiO3‐0.06BaTiO3)‐xK0.5Na0.5NbO3 (0 <= x <= 18 mol%) are revealed. In the base material (x = 0 mol%), macroscopic domains and ferroelectric switching can be induced from the initial relaxor state with sufficiently high electric field, yielding large macroscopic remanent strain and polarization. The addition of KNN increases the threshold field required to induce long range order and decreases the stability thereof. For x = 3 mol% the field‐induced domains relax completely, which is also reflected in zero macroscopic remanence. Eventually, no long range order can be induced for x >= 3 mol%. This PFM study provides a novel perspective on the interplay between macroscopic and nanoscopic material properties in bulk lead‐free piezoceramics. 相似文献
146.
William Fornaciari Donatella Sciuto Cristina Silvano Vittorio Zaccaria 《Design Automation for Embedded Systems》2002,7(1-2):7-33
In this paper, we propose a system-level design methodology for the efficient exploration of the architectural parameters of the memory sub-systems, from the energy-delay joint perspective. The aim is to find the best configuration of the memory hierarchy without performing the exhaustive analysis of the parameters space. The target system architecture includes the processor, separated instruction and data caches, the main memory, and the system buses. To achieve a fast convergence toward the near-optimal configuration, the proposed methodology adopts an iterative local-search algorithm based on the sensitivity analysis of the cost function with respect to the tuning parameters of the memory sub-system architecture. The exploration strategy is based on the Energy-Delay Product (EDP) metric taking into consideration both performance and energy constraints. The effectiveness of the proposed methodology has been demonstrated through the design space exploration of a real-world case study: the optimization of the memory hierarchy of a MicroSPARC2-based system executing the set of Mediabench benchmarks for multimedia applications. Experimental results have shown an optimization speedup of 2 orders of magnitude with respect to the full search approach, while the near-optimal system-level configuration is characterized by a distance from the optimal full search configuration in the range of 2%. 相似文献
147.
Meng Yao Zhang Hyunjun Yang Gloria Ortiz Michael J. Trnka Nektaria Petronikolou Alma L. Burlingame William F. DeGrado Danica Galoni Fujimori 《Chemical science》2022,13(22):6599
Chemical probes for chromatin reader proteins are valuable tools for investigating epigenetic regulatory mechanisms and evaluating whether the target of interest holds therapeutic potential. Developing potent inhibitors for the plant homeodomain (PHD) family of methylation readers remains a difficult task due to the charged, shallow and extended nature of the histone binding site that precludes effective engagement of conventional small molecules. Herein, we describe the development of novel proximity-reactive cyclopeptide inhibitors for PHD3—a trimethyllysine reader domain of histone demethylase KDM5A. Guided by the PHD3–histone co-crystal structure, we designed a sidechain-to-sidechain linking strategy to improve peptide proteolytic stability whilst maintaining binding affinity. We have developed an operationally simple solid-phase macrocyclization pathway, capitalizing on the inherent reactivity of the dimethyllysine ε-amino group to generate scaffolds bearing charged tetraalkylammonium functionalities that effectively engage the shallow aromatic ‘groove’ of PHD3. Leveraging a surface-exposed lysine residue on PHD3 adjacent to the ligand binding site, cyclic peptides were rendered covalent through installation of an arylsulfonyl fluoride warhead. The resulting lysine-reactive cyclic peptides demonstrated rapid and efficient labeling of the PHD3 domain in HEK293T lysates, showcasing the feasibility of employing proximity-induced reactivity for covalent labeling of this challenging family of reader domains.We describe the development of covalent cyclic peptide ligands which target a chromatin methylation reader domain using a proximity-reactive sulfonyl fluoride moiety. 相似文献
148.
Three-dimensional segmentation and growth-rate estimation of small pulmonary nodules in helical CT images 总被引:11,自引:0,他引:11
Kostis WJ Reeves AP Yankelevitz DF Henschke CI 《IEEE transactions on medical imaging》2003,22(10):1259-1274
Small pulmonary nodules are a common radiographic finding that presents an important diagnostic challenge in contemporary medicine. While pulmonary nodules are the major radiographic indicator of lung cancer, they may also be signs of a variety of benign conditions. Measurement of nodule growth rate over time has been shown to be the most promising tool in distinguishing malignant from nonmalignant pulmonary nodules. In this paper, we describe three-dimensional (3-D) methods for the segmentation, analysis, and characterization of small pulmonary nodules imaged using computed tomography (CT). Methods for the isotropic resampling of anisotropic CT data are discussed. 3-D intensity and morphology-based segmentation algorithms are discussed for several classes of nodules. New models and methods for volumetric growth characterization based on longitudinal CT studies are developed. The results of segmentation and growth characterization methods based on in vivo studies are described. The methods presented are promising in their ability to distinguish malignant from nonmalignant pulmonary nodules and represent the first such system in clinical use. 相似文献
149.
Nicholas Douglas Charles J. Neef Robert A. Rogers Jake A. Stanley Jacob Armitage Ben Martin Todd W. Hudnall William J. Brittain 《Journal of Physical Organic Chemistry》2013,26(8):688-695
The synthesis and reactivity of a tetrahydrochromeno[2,3‐b]indoles are reported. Evidence for reversible ring‐opening is based on H/D exchange and trapping experiments. These compounds readily undergo reaction with tetra‐n‐butylammonium cyanide. The cyanide reaction is 10–100× faster when the solution is irradiated with 350 nm light. Reaction with trimethylsilyl cyanide occurs only with UV irradiation demonstrating photoreactivity. The rate of tetrahydrochromeno[2,3‐b]indole ring‐opening is greater for (i) Me substitution at the hemiaminal carbon (compared to Ph), and (ii) substitution of fluorine at the 9‐position of the indole. Under acidic conditions, the ring‐opened indolium ion is observed. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
150.
Alfred Schmidpeter Siegfried Lochschmidt Günther Burget William S. Sheldrick 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-3):23-26
Abstract Reduction of PCl3 e.g. with trialkyl phosphines produces the well-known but ill-defined insoluble, amorphous, orange material approximating a phosphorus subchloride. In repeating this reduction with Ph3P and with AlCl3 as a third component we now obtained crystalline hexaphenyl triphosphenium tetrachloroaluminate. It may be thought to derive from the four electron cation P+ being complexed by two Ph3P. 相似文献