全文获取类型
收费全文 | 201399篇 |
免费 | 2170篇 |
国内免费 | 635篇 |
专业分类
电工技术 | 4101篇 |
综合类 | 124篇 |
化学工业 | 27111篇 |
金属工艺 | 7733篇 |
机械仪表 | 5535篇 |
建筑科学 | 4160篇 |
矿业工程 | 835篇 |
能源动力 | 4765篇 |
轻工业 | 14668篇 |
水利工程 | 1723篇 |
石油天然气 | 2847篇 |
武器工业 | 7篇 |
无线电 | 27599篇 |
一般工业技术 | 37876篇 |
冶金工业 | 46407篇 |
原子能技术 | 3591篇 |
自动化技术 | 15122篇 |
出版年
2021年 | 1293篇 |
2019年 | 1323篇 |
2018年 | 2163篇 |
2017年 | 2143篇 |
2016年 | 2225篇 |
2015年 | 1528篇 |
2014年 | 2680篇 |
2013年 | 8142篇 |
2012年 | 4563篇 |
2011年 | 6238篇 |
2010年 | 5054篇 |
2009年 | 5908篇 |
2008年 | 6260篇 |
2007年 | 6328篇 |
2006年 | 5769篇 |
2005年 | 5396篇 |
2004年 | 5324篇 |
2003年 | 5170篇 |
2002年 | 4971篇 |
2001年 | 5361篇 |
2000年 | 4982篇 |
1999年 | 5496篇 |
1998年 | 15887篇 |
1997年 | 10486篇 |
1996年 | 8061篇 |
1995年 | 5866篇 |
1994年 | 5091篇 |
1993年 | 5119篇 |
1992年 | 3434篇 |
1991年 | 3306篇 |
1990年 | 3255篇 |
1989年 | 3080篇 |
1988年 | 2816篇 |
1987年 | 2272篇 |
1986年 | 2343篇 |
1985年 | 2644篇 |
1984年 | 2345篇 |
1983年 | 2099篇 |
1982年 | 1933篇 |
1981年 | 2079篇 |
1980年 | 1825篇 |
1979年 | 1684篇 |
1978年 | 1684篇 |
1977年 | 2077篇 |
1976年 | 2768篇 |
1975年 | 1447篇 |
1974年 | 1397篇 |
1973年 | 1346篇 |
1972年 | 1147篇 |
1971年 | 971篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Tantalum capacitor failure modes have been discussed both for the standard manganese dioxide cathode and the new conductive polymer (CP) type. For standard tantalum in the normal operation mode, an electrical breakdown can be stimulated by an increase of the electrical conductance in channel by an electrical pulse or voltage level. This leads to capacitor destruction followed by thermal breakdown. In the reverse mode, we have reported that thermal breakdown is initiated by an increase of the electrical conductance by Joule heating at a relatively low voltage level. Consequently, a feedback cycle consisting of temperature–conductivity–current–Joule heat–temperature, ending with electrical breakdown was created. Both of these breakdown modes possess a stochastic behavior and can be hardly localized in advance. CP capacitors have shown a slightly different current conductivity mechanism compared to standard tantalum capacitors. The breakdown of CP dielectrics is similar to avalanche and field emission breaks. It is an electromechanical collapse due to the attractive forces between electrodes, electrochemical deterioration, dendrite formation, and so on. However, some self-healing of the cathode film has been reported. This can be attributed to film evaporation, carbonizing or reoxidation. Not all of the breakdowns of CP capacitors can lead to self-healing or an open circuit state. Short circuits can also occur. 相似文献
992.
Marcus T. Schmitz Bashir M. Al-Hashimi Petru Eles 《Design Automation for Embedded Systems》2002,6(4):401-424
In this paper, we introduce the LOPOCOS (Low Power Co-synthesis) system, a prototype CAD tool for system level co-design. LOPOCOS targets the design of energy-efficient embedded systems implemented as heterogeneous distributed architectures. In particular, it is designed to solve the specific problems involved in architectures that include dynamic voltage scalable (DVS) processors. The aim of this paper is to demonstrate how LOPOCOS can support the system designer in identifying energy-efficient hardware/software implementations for the desired embedded systems. Hence, highlighting the necessary optimization steps during design space exploration for DVS enable architectures. The optimization steps carried out in LOPOCOS involve component allocation and task/communication mapping as well as scheduling and dynamic voltage scaling. LOPOCOS has the following key features, which contribute to this energy efficiency. During the voltage scaling valuable power profile information of task execution is taken into account, hence, the accuracy of the energy estimation is improved. A combined optimization for scheduling and communication mapping based on genetic algorithm, optimizes simultaneously execution order and communication mapping towards the utilization of the DVS processors and timing behaviour. Furthermore, a separation of task and communication mapping allows a more effective implementation of both task and communication mapping optimizationsteps. Extensive experiments are conducted to demonstrate the efficiency of LOPOCOS. We report up to 38% higher energy reductions compared to previous co-synthesis techniques for DVS systems. The investigations include a real-life example of an optical flow detection algorithm. 相似文献
993.
A relationship between the collection efficiency and the normalized excess minority carrier distribution under forward bias in the dark, which was recently derived using the Green's function technique for a one-dimensional (1-D) quasi-neutral region of a solar cell, is generalized here to three dimensions. The link with other reciprocity theorems is also briefly discussed 相似文献
994.
995.
NITRIC oxide (NO) plays a role in the modulation of the predominant potassium current of type I vestibular hair cells, a low-voltage activated current called I(K1). Since many effects of NO are mediated via cGMP, patch-clamp recordings were made to evaluate the effects of cGMP on I(K1). In whole-cell recordings 1 mM cGMP shifted Vhalf of I(K1) by 15.0 +/- 2.4 mV (n = 6) to more positive. In cell-attached 'multichannel' recordings 1 mM 8-bromo-cGMP caused a reversible shift of Vhalf by 13.8 +/- 2.6 mV (n = 12) and in single channel recordings in the cell-attached configuration the open probability was reduced at -60 mV from 0.39 +/- 0.14 to 0.08 +/- 0.01. cGMP had no effect on excised inside-out patches, indicating that an intact cytosolic milieu with functioning phosphorylation cascades is necessary. cGMP seems to be an important second messenger which reduces the potassium conductance of vestibular hair cells. 相似文献
996.
A high-speed monolithic optical interface switch LSI is developed using a GaAs MSM photodetector and large-scale integrated electric circuits. This LSI operates universally as a 1.8 Gb/s optical-input/optical-output four-channel time-division switch, a 1.8 Gb/s optical-input/electrical-output 1:4 demultiplexer, a 2.0 Gb/s electrical-output 4:1 multiplexer, and a 2.8 Gb/s electrical-input/electrical-output 4×4 space-division switch. It uses a new multistage 2×2 switch block with small hardware and high-speed operation. It can be expanded to a 16×16 optical-input/optical-output time-division switch operating at up to 1.8 Gb/s for broadband-ISDN 相似文献
997.
Cardioprotective effects of a standardized extract from leaves with flowers of Crataegus (WS-1442; content of oligomeric procyandins [OPC]: 18.75%) have recently been demonstrated in an ischemia-reperfusion model in rats. Further studies were now conducted to clarify the mechanism of action and to identify active constituents involved in these effects of WS-1442. Exhausting partitioning between ethyl acetate/water and successive ultrafiltration of the aqueous layer led to the quantitative recovery of three fractions, which were tested for their in vitro radical scavenging (RS) and human neutrophil elastase (HNE) inhibitory activity. The lipophilic ethylacetate-soluble fraction A, enriched in flavone derivatives and constituting 14.9% of WS-1442, was as active as WS-1442 in inhibiting HNE. However, its RS activity was only about half that of the primary extract. Although 67.9% of WS-1442 was recovered in a water-soluble low molecular weight fraction B, this fraction displayed only weak RS and HNE inhibiting activity. In contrast, the RS and HNE inhibiting potencies of an essentially flavone-free and OPC-rich fraction C (21.3% of WS-1442) were significantly higher (inhibition of lipid peroxidation: IC50 0.3 microgram/ml; inhibition of HNE: IC50 0.84 microgram/ml) as those of WS-1442. The RS and HNE inhibitory activities of the extract and those of its fractions correlated well with their OPC-content but not with their concentration of flavonols. These results demonstrate that OPCs of Crataegus extracts possess stronger radical scavenging activities than flavone derivatives or other constituents. In addition, the oligomeric components are potent inhibitors of HNE. Oral administration of 20 mg/kg/d of the OPC-rich fraction C to rats afforded similar protection against ischemia-reperfusion induced pathologies as treatment with WS-1442 at a dose of 100 mg/kg/d. These observations indicate that radical scavenging and elastase inhibitory activities could indeed be involved in the observed cardioprotective effects of WS-1442, and demonstrate that OPCs are major orally active constituents of WS-1442. Thus, Crataegus extracts used therapeutically for cardiovascular diseases should be analyzed and standardized for their OPC-content. 相似文献
998.
Baker S. Sim T. Kanade T. 《IEEE transactions on pattern analysis and machine intelligence》2003,25(1):100-109
The complete set of measurements that could ever be used by a passive 3D vision algorithm is the plenoptic function or light-field. We give a concise characterization of when the light-field of a Lambertian scene uniquely determines its shape and, conversely, when the shape is inherently ambiguous. In particular, we show that stereo computed from the light-field is ambiguous if and only if the scene is radiating light of a constant intensity (and color, etc.) over an extended region. 相似文献
999.
Neogi A. Everitt H. Morkoc H. Kuroda T. Tackeuchi A. 《Nanotechnology, IEEE Transactions on》2003,2(1):10-14
Self-assembled GaN quantum dots (QDs), grown on AlN by molecular beam epitaxy, were investigated by time-resolved photoluminescence spectroscopy. We investigate the emission mechanism in GaN QDs by comparing the carrier recombination dynamics in single and multiple period QDs. At 100 K, the PL decay time in single period QD structures is considerably shorter than in stacked QDs. Compared to single period QDs, the room temperature PL efficiency is considerably enhanced in 20 period QDs due to the reduction in nonradiative recombination processes. 相似文献
1000.