全文获取类型
收费全文 | 22984篇 |
免费 | 3913篇 |
国内免费 | 2781篇 |
专业分类
化学 | 11304篇 |
晶体学 | 247篇 |
力学 | 928篇 |
综合类 | 153篇 |
数学 | 1663篇 |
物理学 | 6597篇 |
无线电 | 8786篇 |
出版年
2024年 | 109篇 |
2023年 | 557篇 |
2022年 | 764篇 |
2021年 | 1000篇 |
2020年 | 980篇 |
2019年 | 898篇 |
2018年 | 825篇 |
2017年 | 800篇 |
2016年 | 1073篇 |
2015年 | 1197篇 |
2014年 | 1404篇 |
2013年 | 1769篇 |
2012年 | 2183篇 |
2011年 | 2103篇 |
2010年 | 1599篇 |
2009年 | 1644篇 |
2008年 | 1646篇 |
2007年 | 1491篇 |
2006年 | 1309篇 |
2005年 | 1072篇 |
2004年 | 832篇 |
2003年 | 635篇 |
2002年 | 573篇 |
2001年 | 493篇 |
2000年 | 434篇 |
1999年 | 405篇 |
1998年 | 260篇 |
1997年 | 221篇 |
1996年 | 240篇 |
1995年 | 163篇 |
1994年 | 166篇 |
1993年 | 135篇 |
1992年 | 124篇 |
1991年 | 95篇 |
1990年 | 100篇 |
1989年 | 73篇 |
1988年 | 54篇 |
1987年 | 46篇 |
1986年 | 31篇 |
1985年 | 35篇 |
1984年 | 36篇 |
1983年 | 23篇 |
1982年 | 21篇 |
1981年 | 14篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1974年 | 6篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
1.
2.
Electromigration reliability of interconnect under bidirectional current stress has been studied in a wide frequency range (mHz to 200 MHz). Experimental results show that the AC lifetime rises with the stress current frequency. The current density exponent and the activation energy of AC lifetime are found to be twice that of DC lifetime. Pure AC current stress failure at extremely high current density is believed to result from thermal migration of metal at hot/cold transition points 相似文献
3.
In this paper, we investigate the problem of determining the optimal bandwidth allocation for a Dynamic Bandwidth Allocation Scheme (DBAS). The objective is to minimize the total amount of bandwidth required to satisfy the Quality of Service (QoS) requirements of all traffic streams. It is shown that when the performance functions satisfy a certain number of conditions, there exists a unique optimal bandwidth allocation such that, for each traffic stream, either its QoS is just satisfied or its QoS is over-satisfied and it is allocated zero bandwidth. Such an allocation is said to be efficient. It is also shown that there exists a unique efficient allocation in the entire feasible region. An iterative algorithm is developed to compute the efficient allocation employing its special properties. Numerical examples are presented to demonstrate how the algorithm works. Future extensions of this work are also discussed.Partially supported by NSERC of Canada through grants OGP14020 and STRIN-200. 相似文献
4.
5.
A Subcarriers Allocation Scheme for Cognitive Radio Systems Based on Multi-Carrier Modulation 总被引:1,自引:0,他引:1
Tao Luo Tao Jiang Weidong Xiang Hsiao-Hwa Chen 《Wireless Communications, IEEE Transactions on》2008,7(9):3335-3340
Cognitive radio (CR) is a dynamic spectrum access technology as a solution to spectrum under-utilization problem in some licensed bands. Operating over an exceedingly wide spectrum, CR systems usually adopt multi-carrier modulation (MCM) to implement flexible channelization. Consequently, efficient channel allocation scheme becomes extremely important to an MCM based CR (MCM-CR) system. In this paper, a maximum likelihood detection model is developed to detect the presence and locations of licensed users (LUs) signals in the frequency domain. Performance of the detection model, including the optimal detection region, detection probability and false alarm probability, is analyzed. A one-order two-state Markovian chain model is proposed to predict channel status information. In particular, a novel subcarrier allocation scheme for MCM-CR systems is proposed, taking into account the confidence of channel estimation, quality of services (QoS) of rental users (RUs) and throughput. To validate the analytical results, simulations have been conducted to show effectiveness of the proposed scheme. 相似文献
6.
Numerical studies were conducted to investigate the natural convection heat transfer around a uniformly heated thin plate
with arbitrary inclination in an infinite space. The numerical approach was based on the finite volume technique with a nonstaggered
grid arrangement. For handling the pressure–velocity coupling the SIMPLE-algorithm was used. QUICK scheme and first order
upwind scheme were employed for discretization of the momentum and energy convective terms respectively. Plate width and heating
rate were used to vary the modified Rayleigh number over the range of 4.8×106 to 1.87×108. Local and average heat transfer characteristics were compared with regarding to the inclination angle. The empirical expressions
for local and average Nusselt number were correlated. It has been found that for inclination angle less than 10∘, the flow and heat transfer characteristics are complicated and the average Nusselt number can not be correlated by one equation
while for inclination angle larger than 10∘, the average Nusselt number can be correlated into an elegant correlation.
Received on 18 April 2001 / Published online: 29 November 2001 相似文献
7.
提出一种极近距离雷达概念,并导出其作用距离公式,在此基础上研制出一台3mm波段极近距离多普勒雷达样机.试验结果表明该雷达结构简单,成本低廉,在作用距离为零点几米至几米范围内可获得较高的定距精度. 相似文献
8.
Y. M. Tao J. Nin G. Y. Delisle 《Journal of Infrared, Millimeter and Terahertz Waves》1995,16(10):1769-1772
GaAs Gunn diodes were fabricated for pulse source application at 8 mm wave band and operated with pulsewidths of 0.05 to 2.0 microseconds and duty cycles of 0.001 to 0.01. Peak pulse output power levels of 0.8–1.2W are achieved and the maximum available power is 1.6W with the highest efficiency of 6.5 percent. A simple and compact pulsed power combiner is also given in this paper. 相似文献
9.
An electromigration failure model which can be used to project the electromigration lifetime under pulsed DC and AC current stressing has been reported. The experimental results indicate that different metallization systems (Al-2%Si, Al4%Cu/TiW, and Cu) show similar failure behaviors, which can be explained and predicted by this model. The pulsed DC lifetime is found to be longer than DC lifetime, and the AC lifetime is found to be very much longer. This recognition can provide significant relief to circuit designs involving metals carrying pulsed DC and AC currents, and allow a more aggressive design to improve circuit density and speed 相似文献
10.
Xiao Huang Bishwa Ranjan Nayak Tao Lu Lowe 《Journal of polymer science. Part A, Polymer chemistry》2004,42(20):5054-5066
A series of novel multifunctional hydrogels that combined the merits of both thermoresponsive and biodegradable polymeric materials were designed, synthesized, and characterized. The hydrogels were copolymeric networks composed of N‐isopropylacrylamide (NIPAAM) as a thermoresponsive component, poly(L‐lactic acid) (PLLA) as a hydrolytically degradable and hydrophobic component, and dextran as an enzymatically degradable and hydrophilic component. The chemical structures of the hydrogels were characterized by an attenuated total reflection–Fourier transform infrared spectroscopy (ATR–FTIR) technique. The hydrogels were thermoresponsive, showing a lower critical solution temperature (LCST) at approximately 32 °C, and their swelling properties strongly depended on temperature changes, the balance of the hydrophilic/hydrophobic components, and the degradation of the PLLA component. The degradation of the hydrogels caused by hydrolytic cleavage of ester bonds in the PLLA component was faster at 25 °C below the LCST than at 37 °C above the LCST, determined by the ATR–FTIR technique. Due to their multifunctional properties, the designed hydrogels show great potential for biomedical applications, including drug delivery and tissue engineering. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5054–5066, 2004 相似文献