全文获取类型
收费全文 | 70689篇 |
免费 | 10757篇 |
国内免费 | 7440篇 |
专业分类
化学 | 37324篇 |
晶体学 | 641篇 |
力学 | 3075篇 |
综合类 | 328篇 |
数学 | 5955篇 |
物理学 | 20031篇 |
无线电 | 21532篇 |
出版年
2024年 | 306篇 |
2023年 | 1838篇 |
2022年 | 2453篇 |
2021年 | 3057篇 |
2020年 | 2933篇 |
2019年 | 2652篇 |
2018年 | 2267篇 |
2017年 | 2327篇 |
2016年 | 3122篇 |
2015年 | 3442篇 |
2014年 | 4056篇 |
2013年 | 5061篇 |
2012年 | 6053篇 |
2011年 | 5951篇 |
2010年 | 4408篇 |
2009年 | 4292篇 |
2008年 | 4548篇 |
2007年 | 3993篇 |
2006年 | 3691篇 |
2005年 | 3201篇 |
2004年 | 2372篇 |
2003年 | 1914篇 |
2002年 | 1766篇 |
2001年 | 1441篇 |
2000年 | 1289篇 |
1999年 | 1519篇 |
1998年 | 1221篇 |
1997年 | 1117篇 |
1996年 | 1083篇 |
1995年 | 947篇 |
1994年 | 814篇 |
1993年 | 696篇 |
1992年 | 578篇 |
1991年 | 500篇 |
1990年 | 397篇 |
1989年 | 294篇 |
1988年 | 216篇 |
1987年 | 181篇 |
1986年 | 181篇 |
1985年 | 156篇 |
1984年 | 103篇 |
1983年 | 98篇 |
1982年 | 62篇 |
1981年 | 45篇 |
1980年 | 30篇 |
1979年 | 19篇 |
1977年 | 21篇 |
1976年 | 21篇 |
1975年 | 23篇 |
1972年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
In this paper we investigated the BER performance of DS-CDMA using various chip-waveforms, which include three time-limited chip-waveforms and two band-limited chip-waveforms. Closed-form formulae were derived for evaluating the achievable bit-error rate performance with the aid of the standard Gaussian approximation, when communicating over a Nakagami-m channel. The time-limited waveforms impose a low implementational complexity, since they maybe over sampled and read from a look-up table. However, they are outperformed by the frequency-domain raised-cosine waveform as well as the optimum waveform specifically designed by Cho and Lehnert for achieving the lowest possible bit error rate 相似文献
32.
分析了基于家庭网络开展多网络业务融合的多种形式和优势,并对基于家庭网络的多网络业务融合的实现方式进行了探讨。 相似文献
33.
34.
The γcmc values of CTAB-SDS decrease from 63.67 mN/m at 10‡C to 36.38 mN/m at 90‡C, slightly lower than those of either CTAB or SDS.
Correspondingly, the CMC of CTAB-SDS decreases almost by half. The increase of surface activity of CTAB-SDS can be attributed
to the relatively weak electrostatic interaction at high temperature, which is supported by the increase of solubility of
CTAB-SDS with rise in temperature. Catalytic effect on oxidation of toluene derivatives with potassium permanganate follows
the order CTAB-SDS > SDS > CTAB. This is not caused by the dissociative effect of CTAB-SDS with low surface activity at low
temperature, as seen from the fact that almost all oxidative products can be retrieved for different toluene derivatives and
surfactants by mimicking the conditions of reaction. In the emulsifications of toluene derivatives at 90‡C, the time that
turbid water layers of surfactant solutions take to become clear is the same as that of the catalytic effect on oxidation
of toluene derivatives. Thus, it can be inferred that surfactants can improve the oxidation yields of toluene derivatives
by increasing the contact between two reacting phases. 相似文献
35.
Ultrawideband (UWB) transmissions induce pronounced frequency-selective fading effects in their multipath propagation. Multipath diversity gains can be collected to enhance performance, provided that the underlying channel can be estimated at the receiver. To this end, we develop a novel pilot waveform assisted modulation (PWAM) scheme that is tailored for UWB communications. We select our PWAM parameters by jointly optimizing channel estimation performance and information rate. The resulting transmitter design maximizes the average capacity, which is shown to be equivalent to minimizing the mean-square channel estimation error, and thereby achieves the Crame/spl acute/r-Rao lower bound. Application of PWAM to practical UWB systems is promising because it entails simple integrate-and-dump operations at the frame rate. Equally important, it offers a flexible UWB channel estimator, capable of striking desirable rate-performance tradeoffs depending on the channel coherence time. 相似文献
36.
37.
Yang C.W. Fang Y.K. Lin C.S. Tsair Y.S. Chen S.M. Wang W.D. Wang M.F. Cheng J.Y. Chen C.H. Yao L.G. Chen S.C. Liang M.S. 《Electronics letters》2003,39(21):1499-1501
A novel technique to form high-K dielectric of HfSiON by doping base oxide with Hf and nitridation with NH/sub 3/, sequentially, is proposed. The HfSiON gate dielectric demonstrates excellent device performances such as only 10% degradation of saturation drain current and almost 45 times of magnitude reduction in gate leakage compared with conventional SiO/sub 2/ gate at the approximately same equivalent oxide thickness. Additionally, negligible flatband voltage shift is achieved with this technique. Time-dependent dielectric breakdown tests indicate that the lifetime of HfSiON is longer than 10 years at V/sub dd/=2 V. 相似文献
38.
Ray-Guang Cheng Cun-Yi Wang Li-Hung Liao Jen-Shun Yang 《Communications Letters, IEEE》2006,10(2):123-125
This work presents a wireless token-passing protocol, named Ripple, for wireless mesh networks (WMNs). In contrast to existing random-access approaches, Ripple uses a decentralized controlled-access approach to protect nodes from unintentional packet collisions and maximize the spatial reuse. The performance of Ripple under an error-free wireless channel was investigated and the accuracy of the analysis was verified by simulation. Simulation results also indicated that Ripple achieved throughput, stability, and QoS enhancement than that of 802.11 DCF under a highly loaded situation. 相似文献
39.
有线网络行业垄断性的改变及竞争形势分析 总被引:2,自引:0,他引:2
有线网络行业既具有自然垄断性,又具有行政垄断性.但随技术进步和国家政策的调整,其自然垄断性被削弱,行政垄断也被打破.在位的有线网络企业面临着两个层面的竞争:一是基于不同视频传输方式的竞争,二是同为有线网络传输方式的竞争,而后者是直接的竞争.对有线网络行业的竞争进行了SWOT分析.最后讨论了竞争形势下在位的有线网络运营商的应对思路. 相似文献
40.
Shanchieh Jay Yang Gustavo de Veciana 《Networking, IEEE/ACM Transactions on》2004,12(2):349-360
With a view on improving user-perceived performance on networks supporting best effort flows, e.g., multimedia/data file transfers, we propose a family of bandwidth allocation criteria that depends on the residual work of on-going transfers. Analysis and simulations show that allocating bandwidth in this fashion can significantly improve the user-perceived delay, bit transmission delay, and throughput over traditional approaches, e.g., by 58% on an 80% loaded linear network. A simple implementation based on TCP Reno, exemplifies how one might approach practically realizing such gains. We discuss several other advantages of incorporating such differentiation at the transport level. In particular we make the case that favoring small transfers combined with user impatience or peak rate constraints, both of which are natural mechanisms for users to express the utility of completing transfers, offers a lightweight approach to achieving good overall network goodput and/or utility for best effort networks. 相似文献