全文获取类型
收费全文 | 192685篇 |
免费 | 27402篇 |
国内免费 | 19395篇 |
专业分类
化学 | 98946篇 |
晶体学 | 1712篇 |
力学 | 9534篇 |
综合类 | 836篇 |
数学 | 17386篇 |
物理学 | 56923篇 |
无线电 | 54145篇 |
出版年
2024年 | 740篇 |
2023年 | 4383篇 |
2022年 | 5743篇 |
2021年 | 7590篇 |
2020年 | 7207篇 |
2019年 | 6721篇 |
2018年 | 5785篇 |
2017年 | 5835篇 |
2016年 | 8226篇 |
2015年 | 8730篇 |
2014年 | 10623篇 |
2013年 | 13539篇 |
2012年 | 16107篇 |
2011年 | 16159篇 |
2010年 | 11665篇 |
2009年 | 11546篇 |
2008年 | 12200篇 |
2007年 | 10948篇 |
2006年 | 10246篇 |
2005年 | 8972篇 |
2004年 | 6392篇 |
2003年 | 5250篇 |
2002年 | 4773篇 |
2001年 | 3942篇 |
2000年 | 3677篇 |
1999年 | 4287篇 |
1998年 | 3692篇 |
1997年 | 3259篇 |
1996年 | 3452篇 |
1995年 | 2852篇 |
1994年 | 2623篇 |
1993年 | 2145篇 |
1992年 | 1948篇 |
1991年 | 1696篇 |
1990年 | 1316篇 |
1989年 | 939篇 |
1988年 | 758篇 |
1987年 | 594篇 |
1986年 | 586篇 |
1985年 | 508篇 |
1984年 | 367篇 |
1983年 | 281篇 |
1982年 | 222篇 |
1981年 | 157篇 |
1980年 | 116篇 |
1979年 | 69篇 |
1977年 | 58篇 |
1976年 | 62篇 |
1975年 | 60篇 |
1974年 | 61篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Chih-Wei Yang Yean-Kuan Fang Shih-Fang Chen Chun-Yu Lin Ming-Fang Wang Yeou-Ming Lin Tuo-Hung Hou Liang-Gi Yao Shih-Chang Chen Mong-Song Liang 《Electronics letters》2003,39(5):421-422
The electrical properties of polysilicon gate MOS capacitors with hafnium silicate (HfSiO) dielectric, with and without NH/sub 3/ nitridation, were investigated. The results show that with NH/sub 3/ nitridation prior to deposition of HfSiO can effectively tune the flatband voltage close to that of conventional oxide and significantly improve the leakage properties over SiO/sub 2/ (three orders reduction). Furthermore, the excellent interface quality has been evidenced by the result of immunity against soft breakdown with NH/sub 3/ nitridation. 相似文献
972.
973.
Wang Hongyu 《Microelectronics Reliability》1989,29(3)
The theory of reliability optimum link allocation (ROLA) is a theory through which the ROLA in a system can be achieved in accordance with the reliability size relations of the system's links. This paper deals with its theory and application in maintainable systems after the reliability index has been extended into the availability index. In conclution its feasibility has been proved with the illustration of a pratical power system. 相似文献
974.
对红外透射光谱法测定HgCdTe液相外延材料纵向组分分布技术进行了深入的研究.红外透射光谱的理论计算采用了王庆学提出的组分分布模型,并考虑了光穿越组分梯度区时产生的干涉效应.通过测量同一样品在不同外延层厚度下的一组红外透射光谱,该方法的有效性得到了实验验证.进一步对组分模型中参数(即外延总厚度、组分互扩散区厚度、材料表面组分和HgCdTe层组分梯度)的拟合方法进行了讨论,并确定了各拟合参数的拟合精度.结果显示,该方法可作为测定HgcdTe液相外延材料组分特性的一种有效的测试评价技术. 相似文献
975.
The unique characteristics of opportunistic networks (ONs), such as intermittent connectivity and limited network resources, makes it difficult to support quality of service (QoS) provisioning, particularly to guarantee delivery ratio and delivery delay. In this paper, we propose a QoS-oriented packet scheduling scheme (QPSS) to make decisions for bundle transmissions to satisfy the needs for the delivery ratio and delivery delay constraints of bundles. Different from prior works, a novel bundle classification method based on the reliability and latency requirements is utilized to decide the traffic class of bundles. A scheduling algorithm of traffic class and bundle redundancy is used to maintain a forwarding and dropping priority queue and allocate network resources in QPSS. Simulation results indicate that our scheme not only achieves a higher overall delivery ratio but also obtains an approximate 14% increase in terms of the amount of eligible bundles. 相似文献
976.
977.
Most existing two dimensional(2-D)angles of arrival(AOAs)estimation methods are based on the assumption that the signal sources are point sources.However,in mobile communications,local scattering in the vicinity of the mobile results in angular spreading as seen from a base station antenna array.In this paper,we consider the problem of estimating the 2-D AOAs of spatially distributed sources.First we perform blind estimation of the steering vectors by exploiting joint diagonalization,then the 2-D AOAs are obtained through two fast Fourier transforming of the estimated steering vectors.Simulations are carried out to illustrate the performance of the method. 相似文献
978.
Spectrum sensing is one of the most challenging issues of Cognitive Radio communications. The possibility of extremely low signal-to-noise ratio (SNR) of the received signal poses a fundamental challenge to spectrum sensing. In this paper, pilot-based spectrum sensing for OFDM signals is investigated. It is shown that the existing pilot-based OFDM spectrum sensing algorithms suffer from the frequency offset between the transmitter and sensing devices, as well as the noise uncertainty in the sensing threshold design. We consequently propose a robust pilot-based spectrum sensing algorithm for low SNR OFDM signals using a sliding frequency correlator. The proposed algorithm processes additional bandwidth to eliminate the impact of frequency offset. In addition, considering the unknown noise statistics and its time-varying nature, a ratio threshold which is not sensitive to the noise power level is derived for spectrum sensing. Our theoretical analysis and simulation results show that this algorithm can achieve exceptionally good sensing performance at very low SNR, while being insensitive to time and frequency offsets and requiring no information of the noise statistics. 相似文献
979.
对射频放电产生单重态氧进行了实验研究,在不同掺杂物种、不同电极间距、不同稀释配比的情况下,研究了单重态氧相对产率的变化规律,分析了单位氧流量注入能量对单重态氧产率和电效率的影响。实验表明,NO和Hg蒸气的加入,使放电产生单重态氧的相对产率都有显著的提高。随着电极间距的缩小,可以实现高气压工作,放电总压可以达到22.6 kPa,氧气分压超过了4.0 kPa,产率方面也有大幅度提高。单位氧流量注入能量在150~400 J/mmol时,单重态氧产率比较高;电效率较高的区域对应的单位氧流量注入能量在150 J/mmol左右。 相似文献
980.
A rigorous modal analysis (MA) for the H-plane waveguide T-junction loaded with a partial-height conducting post is presented in this paper. The analysis is based on the classical resonator mode-matching technique for waveguide junction problems and a novel concept called extended eigenmode functions. The new concept can be used for constructing eigenmode functions of a complex resonator region as long as the modal solution for a subproblem is available. Particularly for the T-junction problem, the modal solution for the two-port in-line waveguide loaded with the post is used. The proposed MA has been extensively verified by a finite-element method software package. Excellent agreement can be observed. Numerical results obtained by the analysis reveal that by adjusting the dimension of the loading post, the usable band width of the T-junction for constructing a diplexer can be significantly expanded. Since the generalized scattering matrix is obtained, the proposed analysis can be integrated with other available waveguide key building-block models for system analysis 相似文献