全文获取类型
收费全文 | 38891篇 |
免费 | 1358篇 |
国内免费 | 66篇 |
专业分类
电工技术 | 397篇 |
综合类 | 29篇 |
化学工业 | 7644篇 |
金属工艺 | 770篇 |
机械仪表 | 773篇 |
建筑科学 | 2057篇 |
矿业工程 | 124篇 |
能源动力 | 1158篇 |
轻工业 | 3176篇 |
水利工程 | 448篇 |
石油天然气 | 125篇 |
武器工业 | 5篇 |
无线电 | 2718篇 |
一般工业技术 | 6451篇 |
冶金工业 | 7308篇 |
原子能技术 | 289篇 |
自动化技术 | 6843篇 |
出版年
2023年 | 220篇 |
2022年 | 454篇 |
2021年 | 717篇 |
2020年 | 477篇 |
2019年 | 636篇 |
2018年 | 817篇 |
2017年 | 722篇 |
2016年 | 871篇 |
2015年 | 798篇 |
2014年 | 1104篇 |
2013年 | 2473篇 |
2012年 | 1770篇 |
2011年 | 2193篇 |
2010年 | 1726篇 |
2009年 | 1631篇 |
2008年 | 1918篇 |
2007年 | 1878篇 |
2006年 | 1672篇 |
2005年 | 1518篇 |
2004年 | 1234篇 |
2003年 | 1188篇 |
2002年 | 1102篇 |
2001年 | 769篇 |
2000年 | 589篇 |
1999年 | 647篇 |
1998年 | 794篇 |
1997年 | 713篇 |
1996年 | 638篇 |
1995年 | 637篇 |
1994年 | 572篇 |
1993年 | 573篇 |
1992年 | 539篇 |
1991年 | 319篇 |
1990年 | 449篇 |
1989年 | 420篇 |
1988年 | 342篇 |
1987年 | 376篇 |
1986年 | 338篇 |
1985年 | 442篇 |
1984年 | 437篇 |
1983年 | 344篇 |
1982年 | 310篇 |
1981年 | 294篇 |
1980年 | 286篇 |
1979年 | 280篇 |
1978年 | 264篇 |
1977年 | 246篇 |
1976年 | 262篇 |
1975年 | 205篇 |
1973年 | 182篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Multipath routing protocols for Mobile Ad hoc NETwork (MANET) address the problem of scalability, security (confidentiality and integrity), lifetime of networks, instability of wireless transmissions, and their adaptation to applications.Our protocol, called MultiPath OLSR (MP-OLSR), is a multipath routing protocol based on OLSR [1]. The Multipath Dijkstra Algorithm is proposed to obtain multiple paths. The algorithm gains great flexibility and extensibility by employing different link metrics and cost functions. In addition, route recovery and loop detection are implemented in MP-OLSR in order to improve quality of service regarding OLSR. The backward compatibility with OLSR based on IP source routing is also studied. Simulation based on Qualnet simulator is performed in different scenarios. A testbed is also set up to validate the protocol in real world. The results reveal that MP-OLSR is suitable for mobile, large and dense networks with large traffic, and could satisfy critical multimedia applications with high on time constraints. 相似文献
992.
993.
Three-dimensional segmentation and growth-rate estimation of small pulmonary nodules in helical CT images 总被引:11,自引:0,他引:11
Kostis WJ Reeves AP Yankelevitz DF Henschke CI 《IEEE transactions on medical imaging》2003,22(10):1259-1274
Small pulmonary nodules are a common radiographic finding that presents an important diagnostic challenge in contemporary medicine. While pulmonary nodules are the major radiographic indicator of lung cancer, they may also be signs of a variety of benign conditions. Measurement of nodule growth rate over time has been shown to be the most promising tool in distinguishing malignant from nonmalignant pulmonary nodules. In this paper, we describe three-dimensional (3-D) methods for the segmentation, analysis, and characterization of small pulmonary nodules imaged using computed tomography (CT). Methods for the isotropic resampling of anisotropic CT data are discussed. 3-D intensity and morphology-based segmentation algorithms are discussed for several classes of nodules. New models and methods for volumetric growth characterization based on longitudinal CT studies are developed. The results of segmentation and growth characterization methods based on in vivo studies are described. The methods presented are promising in their ability to distinguish malignant from nonmalignant pulmonary nodules and represent the first such system in clinical use. 相似文献
994.
Hongkun He Mingjiang Zhong Dominik Konkolewicz Karin Yacatto Timothy Rappold Glenn Sugar Nathaniel E. David Jeff Gelb Naomi Kotwal Arno Merkle Krzysztof Matyjaszewski 《Advanced functional materials》2013,23(37):4720-4728
The design and preparation of porous materials with controlled structures and functionalities is crucial to a variety of absorption‐ or separation‐relevant applications, including CO2 capture. Here, novel functional polymeric materials with three‐dimensionally ordered macroporous (3DOM) structures are prepared by using colloidal crystals as templates using relatively simple, rapid, and inexpensive approaches. These ordered structures are used for the reversible CO2 capture from ambient air by humidity swing. Typically, the colloidal crystal template is synthesized from polymer latex particles of poly(methyl methacrylate) (PMMA) or polystyrene (PS). To maintain the functionality of the material, it is important to prevent the porous structure collapsing, which can occur by the hydrolysis of the ester bonds in conventional crosslinkers under basic conditions. This hydrolysis can be prevented by using a water‐soluble crosslinker containing two quaternary ammonium moieties, which can be used to prepare stable porous crosslinked polymers with the monomer (vinylbenzyl)trimethylammonium chloride (VBTMACl) and using a PMMA‐based colloidal crystal template. The hydroxide‐containing monomer and dicationic crosslinker are synthesized from their chloride precursors, avoiding the ion‐exchange step which causes shrinkage of the pores. An analysis of different methods for infiltrating the monomer solution into the colloidal crystal template shows that infiltration using capillary forces leads to fewer defects than infiltration under a partial vacuum. In addition, functional macroporous films with micrometer thickness are prepared from a template of PS‐based colloidal crystals in a thin film. In general, the colloidal crystal templated materials showed improved CO2 absorption/desorption rates and swing sizes compared to a commercially available material with similar functional groups. This work could easily be extended to create a new generation of ordered macroporous polymeric materials with tunable functionalities for other applications. 相似文献
995.
J.S. Ng H.Y. Liu M.J. Steer M. Hopkinson J.P.R. David 《Microelectronics Journal》2006,37(12):1468-1470
Photoluminescence measurements were carried out to investigate the origin of long wavelength emissions (1.6 μm at room temperature) observed from wafers with InAs quantum dots capped with GaAsSb layers. For wafers with high Sb content (22% and 26%) photoluminescence peak energies were found to be linearly proportional to third root of optical excitation power, a characteristic of emission due to a type-II band alignment. This work therefore presents unambiguous evidence that the long wavelength emission of the wafers comes from type-II band alignment between the InAs quantum dots and the GaAsSb capping layers. 相似文献
996.
EEG complexity as a measure of depth of anesthesia for patients 总被引:22,自引:0,他引:22
Zhang X.-S. Roy R.J. Jensen E.W. 《IEEE transactions on bio-medical engineering》2001,48(12):1424-1433
A new approach for quantifying the relationship between brain activity patterns and depth of anesthesia (DOA) is presented by analyzing the spatio-temporal patterns in the electroencephalogram (EEG) using Lempel-Ziv complexity analysis. Twenty-seven patients undergoing vascular surgery were studied under general anesthesia with sevoflurane, isoflurane, propofol, or desflurane. The EEG was recorded continuously during the procedure and patients' anesthesia states were assessed according to the responsiveness component of the observer's assessment of alertness/sedation (OAA/S) score. An OAA/S score of zero or one was considered asleep and two or greater was considered awake. Complexity of the EEG was quantitatively estimated by the measure C(n), whose performance in discriminating awake and asleep states was analyzed by statistics for different anesthetic techniques and different patient populations. Compared with other measures, such as approximate entropy, spectral entropy, and median frequency, C(n) not only demonstrates better performance (93% accuracy) across all of the patients, but also is an easier algorithm to implement for real-time use. The study shows that C(n) is a very useful and promising EEG-derived parameter for characterizing the (DOA) under clinical situations. 相似文献
997.
Lars-Olof Kihlstrm Nils Viklund David Bridgewater 《International Journal of Satellite Communications and Networking》1990,8(6):461-490
This paper discusses the systematization effort undertaken to generate an equipment specification of an extremely detailed and new nature for DCMEs in accordance with the INTELSAT specification IESS-501. The main areas of importance addressed are additional to what has already been defined within the IESS-501. A discussion of important aspects when introducing DCME into actual network conditions is also contained and results based on simulations of performance under simulated network environments are included. 相似文献
998.
Wu D Tan DM Baird M DeCampo J White C Wu HR 《IEEE transactions on medical imaging》2006,25(3):335-344
A novel perceptually lossless coder is presented for the compression of medical images. Built on the JPEG 2000 coding framework, the heart of the proposed coder is a visual pruning function, embedded with an advanced human vision model to identify and to remove visually insignificant/irrelevant information. The proposed coder offers the advantages of simplicity and modularity with bit-stream compliance. Current results have shown superior compression ratio gains over that of its information lossless counterparts without any visible distortion. In addition, a case study consisting of 31 medical experts has shown that no perceivable difference of statistical significance exists between the original images and the images compressed by the proposed coder. 相似文献
999.
Ishay JS Plotkin M Ermakov NY Volynchik S Barkay Z Bergman DJ 《Journal of electron microscopy》2006,55(1):41-49
In the social wasps Vespa orientalis and Paravespula germanica (Hymenoptera, Vespinae), a thermogenic center has been found in the dorsal part of the first thoracic segment. The temperature in this region of the prothorax is higher by 6-9 degrees C than that at the tip of the abdomen, and this in actively flying hornets outside the nest (workers, males or queens) as well as in hornets inside the nest that attend to the brood in the combs. On viewing the region from the outside, one discerns a canal or rather a fissure in the cuticle, which commences at the center of the dorsal surface of the prothorax and extends till the mesothorax. Thus the length of this canal or fissure is approximately 5-7 mm and it is seen to contain numerous thin hairs whose shape varies from that of the hairs alongside the structure. Beneath the cuticle in this region there are dorsoventral as well as longitudinal muscles in abundance, much the same as the musculature in the remaining thoracic segments (i.e. the meso- and metathorax), which activate the two pairs of wings. The canal-bearing segment is of course devoid of wings, and its dorsoventral muscles are attached to the cuticle, which in this region resembles a bowl harboring several layers of epithelium that boasts numerous butterfly-shaped tracheal branches. Additionally there are layers that display lymph-filled spaces and also perforated layers and depressions, and beneath all these is a lace-like layer that also coats the cuticle's hollows. Underneath the cuticle proper, there are numerous large mitochondria and tracheae, which occupy a considerable part of the cuticular epithelium surface. These abundant mitochondria are, most probably, the main element of heat production in the thermogenic center. 相似文献
1000.
Although the behavioral component was an integral part of the original formulation of the third-person effect hypothesis, little research has examined the impact of biased media effect perceptions on behaviors that assume others have already been affected. The present study examines how third-person perceptions (the belief that other people are particularly vulnerable to media messages) contributed to intentions to prepare for problems stemming from Y2K, also known as the "millennium bug." Analyses using data from 2 regional probability samples taken in late 1999 show that perceptions about the potential effects of news messages influenced public anxiety about the Y2K situation and beliefs that other people would overprepare for the new year. These 2 variables, in turn, predicted intentions to stockpile supplies of food, water, gasoline, and cash. The article discusses the implications of these findings for understanding the relationship between perceptions of media effects and an extended range of behaviors. 相似文献