全文获取类型
收费全文 | 147055篇 |
免费 | 6207篇 |
国内免费 | 3368篇 |
专业分类
电工技术 | 4700篇 |
技术理论 | 6篇 |
综合类 | 7385篇 |
化学工业 | 21509篇 |
金属工艺 | 8830篇 |
机械仪表 | 7356篇 |
建筑科学 | 7341篇 |
矿业工程 | 2336篇 |
能源动力 | 2780篇 |
轻工业 | 8497篇 |
水利工程 | 2576篇 |
石油天然气 | 3458篇 |
武器工业 | 634篇 |
无线电 | 15879篇 |
一般工业技术 | 23077篇 |
冶金工业 | 5198篇 |
原子能技术 | 947篇 |
自动化技术 | 34121篇 |
出版年
2024年 | 146篇 |
2023年 | 684篇 |
2022年 | 1518篇 |
2021年 | 2205篇 |
2020年 | 1723篇 |
2019年 | 1363篇 |
2018年 | 15822篇 |
2017年 | 14924篇 |
2016年 | 11475篇 |
2015年 | 3021篇 |
2014年 | 3307篇 |
2013年 | 4013篇 |
2012年 | 7524篇 |
2011年 | 14093篇 |
2010年 | 12740篇 |
2009年 | 9849篇 |
2008年 | 10932篇 |
2007年 | 11889篇 |
2006年 | 4166篇 |
2005年 | 4700篇 |
2004年 | 3626篇 |
2003年 | 3185篇 |
2002年 | 2575篇 |
2001年 | 1856篇 |
2000年 | 1671篇 |
1999年 | 1446篇 |
1998年 | 1074篇 |
1997年 | 878篇 |
1996年 | 845篇 |
1995年 | 616篇 |
1994年 | 525篇 |
1993年 | 402篇 |
1992年 | 323篇 |
1991年 | 259篇 |
1990年 | 146篇 |
1989年 | 151篇 |
1988年 | 123篇 |
1987年 | 61篇 |
1986年 | 51篇 |
1985年 | 32篇 |
1968年 | 43篇 |
1967年 | 33篇 |
1966年 | 42篇 |
1965年 | 44篇 |
1959年 | 36篇 |
1958年 | 39篇 |
1957年 | 36篇 |
1956年 | 34篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
51.
Video transmission over IEEE 802.11e wireless networks still shows poor performance for large bandwidth demand and frequently changed environments. Thus, several enhancements of IEEE 802.11e were proposed. On the other hand, big frames and simultaneous sending of adjacent frames always cause packet dropping for buffer overflow. In the past, we proposed an IEEE 802.11e enhancement named DFAA and a content aware mechanism to solve the above problems. The motivation of this paper is to find a proper way to integrate these two mechanisms. A DFAA enhancement (DFAA-E) is proposed to make up the insufficiency of content aware mechanism. Experiments results show that the combination of DFAA-E and content aware mechanism improves the video decoded quality greatly. And its performance can be further enhanced by selecting the suitable settings of certain parameters. 相似文献
52.
四川盆地作为国内页岩气开发的重点区域,通过近几年的不断攻关和实践,机械钻速得到了提高,但由于受地层复杂、可钻性差、非均质性强等地质因素的影响,导致井下钻柱系统不良振动剧烈,容易出现钻头损坏严重、钻速较低等问题,严重影响了钻井时效。为了解决上述难题,以该盆地涪陵工区上二叠统龙潭组—中二叠统茅口组为例,采用井下振动高频测量工具的实测手段,测量了钻头—钻柱系统的动态振动加速度参数,结合地层的岩性和矿物组分分析,研究钻头失效原因与对策,并开展了现场试验。研究结果表明:①在非均质地层中钻进的钻头—钻柱系统产生了大于40 m/s~2的高幅值瞬时冲击振动,高幅值的瞬时冲击是导致钻头先期失效的主要原因;②提出了抑制高幅值的瞬时冲击振动采用"减振+增压"工具组合和避免井下工具共振的钻井参数;③采用钻井新参数的试验井比邻井的高幅值瞬时振动降低了17%,单只钻头进尺增加24%,钻头工作环境得到了较大的改善,钻头使用数量减少。结论认为,该研究成果能够有效地改善钻头—钻柱系统的振动状态,有利于达成延长钻头使用寿命的目标。 相似文献
53.
Yachao Dong Christos T. Maravelias Norman F. Jerome 《Optimization and Engineering》2018,19(4):937-976
We study a maritime inventory routing problem, in which shipments between production and consumption nodes are carried out by a fleet of vessels. The vessels have specific capacities and can be chartered under different agreements. The inventory levels of all consumption nodes and some production nodes should be maintained within specified bounds; for the remaining production nodes, orders should be picked up within pre-defined time windows. We propose a discrete-time mixed-integer programming model. In the face of new information and uncertainty, this optimization model has to be re-solved, as the horizon is rolled forward. We discuss how to account for different sources of uncertainty. We present a rolling-horizon reoptimization framework that allows us to study different policies that impact the quality of the implemented solution, so we can identify the optimal set of policies. 相似文献
54.
Natural Computing - The fiber stretching process plays the key role in the process of fiber production and its effects is measured by the stretching ratio. The stretching ratio is determined by the... 相似文献
55.
煤气化废水的特点是温度高、硬度高、NH3-N高、有机物含量相对不足。针对这些特点引起的运行不稳定,逐一提出解决方案,并形成一套完整的针对煤气化废水的处理工艺。 相似文献
56.
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
57.
最大塑性功原理在烧结体上限分析中的应用 总被引:1,自引:0,他引:1
最大塑性功原理又称第二塑性变分原理,在致密体塑性变形分析叶l是运用能量法进行变形力能计算的基础。本文从分析Drucker公设的亢要条件出发,证明了烧结体塑性变形最大塑性功原理的存在,片在该原理的荩础卜,推导出塑性变形的上限功率表达式,并以平面挤压条件下的上限法应用作为实例加以说明。 相似文献
58.
59.
Gert Schubring 《NTM》2003,11(2):90-99
Enno Heeren Dirksen (1788–1850) was one of several mathematicians originating from Eastern Frisia at the end of the eighteenth
century to acquire prominence in neohumanist reformed Prussia. While striving—like so many of his contemporaries—for more
rigour in the foundations of calculus, his work is significant due to his unusual emphasis on the semiotic aspects of the
basic concepts. As a clear-sighted propagator of Cauchy's conceptual innovations in Germany, his achievements in improving
sign expressions of such concepts enabled him to clarify hitherto not differentiated multiple limit processes—although less
successfully in constructive concept formation itself.
The contribution is based on the analysis of Dirksen's published texts and on the numerous texts which remained drafts for
his projected voluminousOrganon der gesamten Analysis.
Menso Folkerts zum 60. Geburtstag gewidmet 相似文献
Menso Folkerts zum 60. Geburtstag gewidmet 相似文献
60.
Yang Aiying Wang Ziyu Zhang Zhaoyi Chen Zhangyuan Wu Deming 《电子科学学刊(英文版)》2003,20(6):461-466
A 10Gbit/s recirculating system is configured with Chirped Fiber Bragg Grating (CFBG) for the dispersion compensation. For the first time, the transmission distance in the loop reaches 1000km with bit error rate of 10-9. The effect of the group delay ripple of the fiber grating is also investigated in the recirculating systems, and it is shown that the transmission distance is limited to 4 cycles (4×167.1km ) in the loop with the power penalty fluctuation below 1.0dB. Thus the group delay ripple should be reduced to allow for the wavelength drift of±5GHz. At the end of this letter, the principles are given for designing long haul recirculating systems with dispersion compensation CFBG. 相似文献