全文获取类型
收费全文 | 76363篇 |
免费 | 6190篇 |
国内免费 | 3279篇 |
专业分类
电工技术 | 4266篇 |
技术理论 | 7篇 |
综合类 | 4857篇 |
化学工业 | 13152篇 |
金属工艺 | 4206篇 |
机械仪表 | 4726篇 |
建筑科学 | 6261篇 |
矿业工程 | 2158篇 |
能源动力 | 2274篇 |
轻工业 | 4745篇 |
水利工程 | 1324篇 |
石油天然气 | 5243篇 |
武器工业 | 520篇 |
无线电 | 8773篇 |
一般工业技术 | 9548篇 |
冶金工业 | 3648篇 |
原子能技术 | 805篇 |
自动化技术 | 9319篇 |
出版年
2024年 | 222篇 |
2023年 | 1176篇 |
2022年 | 2021篇 |
2021年 | 2869篇 |
2020年 | 2151篇 |
2019年 | 1871篇 |
2018年 | 2136篇 |
2017年 | 2414篇 |
2016年 | 2186篇 |
2015年 | 2804篇 |
2014年 | 3830篇 |
2013年 | 4421篇 |
2012年 | 4849篇 |
2011年 | 5326篇 |
2010年 | 4643篇 |
2009年 | 4377篇 |
2008年 | 4354篇 |
2007年 | 4180篇 |
2006年 | 4291篇 |
2005年 | 3742篇 |
2004年 | 2500篇 |
2003年 | 2234篇 |
2002年 | 1932篇 |
2001年 | 1860篇 |
2000年 | 1919篇 |
1999年 | 2153篇 |
1998年 | 1725篇 |
1997年 | 1422篇 |
1996年 | 1298篇 |
1995年 | 1109篇 |
1994年 | 933篇 |
1993年 | 656篇 |
1992年 | 515篇 |
1991年 | 401篇 |
1990年 | 367篇 |
1989年 | 256篇 |
1988年 | 198篇 |
1987年 | 109篇 |
1986年 | 120篇 |
1985年 | 58篇 |
1984年 | 49篇 |
1983年 | 27篇 |
1982年 | 47篇 |
1981年 | 31篇 |
1980年 | 21篇 |
1979年 | 8篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1974年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
81.
82.
宽带DDS跳频源设计 总被引:1,自引:0,他引:1
直接数字合成(DDS)简单可靠、控制方便,具有很高的频率分辨率,高速转换,非常适合快速跳频的要求。在对DDS基本原理进行了简要介绍和分析后,提出宽带跳频源设计方案。 相似文献
83.
W. Wang J. Dai J. Tang D.-T. Jiang Y. Chen J. Fang J. He W. Zhou L. Spinu 《Journal of Superconductivity》2003,16(1):155-157
Ball-milling method was applied to dissolve Fe into titanium dioxide (TiO2). X-ray diffraction indicated the starting anatase changed to a rutile-type structure with oxygen deficiency after ball milling. Transmission electron microscopy and X-ray absorption experiments were conducted to examine the possible existence of magnetic impurities in the ball-milled powders after they were leached in HCl solutions. Temperature dependence of the resistivity shows semiconducting behavior and the magnetic hysteresis loops at 5 and 300 K exhibit ferromagnetic characteristics. Fe-doped TiO2 films were also prepared by pulsed laser deposition. The magnetic properties of the films are discussed. 相似文献
84.
Feng-Wen Sun Yimin Jiang Baras J.S. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(1):180-190
Many issues in signal processing involve the inverses of Toeplitz matrices. One widely used technique is to replace Toeplitz matrices with their associated circulant matrices, based on the well-known fact that Toeplitz matrices asymptotically converge to their associated circulant matrices in the weak sense. This often leads to considerable simplification. However, it is well known that such a weak convergence cannot be strengthened into strong convergence. It is this fact that severely limits the usefulness of the close relation between Toeplitz matrices and circulant matrices. Observing that communication receiver design often needs to seek optimality in regard to a data sequence transmitted within finite duration, we define the finite-term strong convergence regarding two families of matrices. We present a condition under which the inverses of a Toeplitz matrix converges in the strong sense to a circulant matrix for finite-term quadratic forms. This builds a critical link in the application of the convergence theorems for the inverses of Toeplitz matrices since the weak convergence generally finds its usefulness in issues associated with minimum mean squared error and the finite-term strong convergence is useful in issues associated with the maximum-likelihood or maximum a posteriori principles. 相似文献
85.
本文在分析张宇等及钱忠平等提出的高频噪声判别准则基础上,利用夏洪瑞等根据在窄档等频距小波分频之后识别、压制高频噪声的思路,提出在时频域将代表地震道主要能量的几个频带的平均振幅绝对值作为正常子波的平均振幅绝对值,以此来识别和压制高频噪声的异常振幅。实际处理结果表明,此法不仅可以压制高频谐振及高频突发噪声,而且可以保护弱地震反射信号不受损害。 相似文献
86.
姜建国 《信息安全与通信保密》2006,(2):20-22
研究意义和应用前景信息系统的安全威胁成为日益受到关注的严重问题。涉密内网虽然与因特网物理隔离,仍同样面临着漏洞、蠕虫、病毒、木马和后门等威胁。涉密信息可以通过文件共享、木马、邮件等方式被泄露,也可能被非授权用户登录访问,还可能通过外设(如:USB、光 相似文献
87.
Wavelet-based Rayleigh background removal in MRI 总被引:1,自引:0,他引:1
Rayleigh distribution governs noise in 'no signal' regions of magnetic resonance magnitude images. Large areas of background noise in MRI images will seriously affect their effective utilisation. A new wavelet-based algorithm is presented that can work efficiently either as a standalone procedure or couple with existing denoising algorithms to significantly improve their effectiveness. 相似文献
88.
Given Hong Kong's special circumstances of small physical size, advanced infrastructure, and low shopping cost, a survey is designed under which supply-side problems in Internet business-to-consumer (B2C) e-commerce are indirectly revealed by responses on the demand side. Difficulties arising from the reluctance to answer questionnaires on the part of e-firms wary about trade and innovation secrets and their small number at the outset are thereby overcome. Survey data on demand-side obstacle factors in the form of perceived low e-shopping comparability, e-shopping inconvenience, e-transaction insecurity, and poor Internet privacy, together with orientation toward social interaction and low awareness on the part of consumers, translate into information on notionally matching supply-side hurdles. Regression analysis and hypothesis testing indicate statistical significance for the above hurdle factors in terms of impact on individual unwillingness to shop online. These results add to the inductive basis for future research into a general demand-supply theory of Internet B2C e-commerce and offer an empirically-grounded position against which the effects of later supply-side changes can be evaluated. Useful information also follows for engineer-managers seeking to compare marginal improvements in supply-side problems, particularly in the form of estimated substitution ratios. 相似文献
89.
Jui-Ting Weng Jiunn Ru Lai Wanjiun Liao 《Wireless Communications, IEEE Transactions on》2006,5(7):1676-1684
In this paper, we analyze node mobility for reliable packet delivery in mobile IP networks. In mobile IP, packets destined to roaming nodes are intercepted by their home agents and delivered via tunneling to their care of addresses (CoA). A mobile node may roam across multiple subnets. At each boundary crossing, a handoff is initiated such that the CoA is updated and a new tunnel is established. We consider both basic mobile IP handoff and smooth handoff. We find that reliable packet delivery in mobile IP networks can be modeled as a renewal process, because the retransmission over a new tunnel after each boundary crossing is independent of the previous history. We then derive the probability distribution of boundary crossings for each successful packet, based on which the packet reliable delivery time can be obtained. Our analytical model is derived based on a general distribution of residence time in a subnet and a general distribution of successful retransmission attempts in each subnet. The results can be readily applied to any distributions for both items. We also provide numerical examples to calculate the probability distribution of boundary crossings, and conduct simulations to validate our analytical results 相似文献
90.