全文获取类型
收费全文 | 4669篇 |
免费 | 300篇 |
国内免费 | 8篇 |
专业分类
电工技术 | 58篇 |
综合类 | 8篇 |
化学工业 | 924篇 |
金属工艺 | 55篇 |
机械仪表 | 74篇 |
建筑科学 | 176篇 |
矿业工程 | 5篇 |
能源动力 | 159篇 |
轻工业 | 326篇 |
水利工程 | 54篇 |
石油天然气 | 35篇 |
无线电 | 391篇 |
一般工业技术 | 1188篇 |
冶金工业 | 586篇 |
原子能技术 | 13篇 |
自动化技术 | 925篇 |
出版年
2024年 | 13篇 |
2023年 | 59篇 |
2022年 | 97篇 |
2021年 | 140篇 |
2020年 | 113篇 |
2019年 | 123篇 |
2018年 | 126篇 |
2017年 | 136篇 |
2016年 | 162篇 |
2015年 | 134篇 |
2014年 | 188篇 |
2013年 | 331篇 |
2012年 | 309篇 |
2011年 | 384篇 |
2010年 | 295篇 |
2009年 | 281篇 |
2008年 | 306篇 |
2007年 | 243篇 |
2006年 | 214篇 |
2005年 | 185篇 |
2004年 | 152篇 |
2003年 | 133篇 |
2002年 | 128篇 |
2001年 | 65篇 |
2000年 | 60篇 |
1999年 | 59篇 |
1998年 | 46篇 |
1997年 | 43篇 |
1996年 | 47篇 |
1995年 | 40篇 |
1994年 | 38篇 |
1993年 | 35篇 |
1992年 | 37篇 |
1991年 | 20篇 |
1990年 | 26篇 |
1989年 | 13篇 |
1988年 | 13篇 |
1987年 | 27篇 |
1986年 | 16篇 |
1985年 | 28篇 |
1984年 | 14篇 |
1983年 | 6篇 |
1982年 | 12篇 |
1981年 | 20篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1974年 | 6篇 |
1973年 | 6篇 |
1964年 | 4篇 |
排序方式: 共有4977条查询结果,搜索用时 31 毫秒
131.
Practical and financial constraints associated with traditional field-based lithological mapping are often responsible for the generation of maps with insufficient detail and inaccurately located contacts. In arid areas with well exposed rocks and soils, high-resolution multi- and hyperspectral imagery is a valuable mapping aid as lithological units can be readily discriminated and mapped by automatically matching image pixel spectra to a set of reference spectra. However, the use of spectral imagery in all but the most barren terrain is problematic because just small amounts of vegetation cover can obscure or mask the spectra of underlying geological substrates. The use of ancillary information may help to improve lithological discrimination, especially where geobotanical relationships are absent or where distinct lithologies exhibit inherent spectral similarity. This study assesses the efficacy of airborne multispectral imagery for detailed lithological mapping in a vegetated section of the Troodos ophiolite (Cyprus), and investigates whether the mapping performance can be enhanced through the integration of LiDAR-derived topographic data. In each case, a number of algorithms involving different combinations of input variables and classification routine were employed to maximise the mapping performance. Despite the potential problems posed by vegetation cover, geobotanical associations aided the generation of a lithological map - with a satisfactory overall accuracy of 65.5% and Kappa of 0.54 - using only spectral information. Moreover, owing to the correlation between topography and lithology in the study area, the integration of LiDAR-derived topographic variables led to significant improvements of up to 22.5% in the overall mapping accuracy compared to spectral-only approaches. The improvements were found to be considerably greater for algorithms involving classification with an artificial neural network (the Kohonen Self-Organizing Map) than the parametric Maximum Likelihood Classifier. The results of this study demonstrate the enhanced capability of data integration for detailed lithological mapping in areas where spectral discrimination is complicated by the presence of vegetation or inherent spectral similarities. 相似文献
132.
Katherine T. Moorhead Jonathan V. HillJ. Geoffrey Chase Christopher E. HannJennifer M. Scotter Malina K. StorerZoltan H. Endre 《Computer methods and programs in biomedicine》2011,101(2):173-182
This paper compares three methods for estimating renal function, as tested in rats. Acute renal failure (ARF) was induced via a 60-min bilateral renal artery clamp in 8 Sprague-Dawley rats and renal function was monitored for 1 week post-surgery. A two-compartment model was developed for estimating glomerular filtration via a bolus injection of a radio-labelled inulin tracer, and was compared with an estimated creatinine clearance method, modified using the Cockcroft-Gault equation for rats. These two methods were compared with selected ion flow tube-mass spectrometry (SIFT-MS) monitoring of breath analytes. Determination of renal function via SIFT-MS is desirable since results are available non-invasively and in real time. Relative decreases in renal function show very good correlation between all 3 methods (R2 = 0.84, 0.91 and 0.72 for breath-inulin, inulin-creatinine, and breath-creatinine correlations, respectively), and indicate good promise for fast, non-invasive determination of renal function via breath testing. 相似文献
133.
This paper presents a blind watermarking approach to protecting vector geo-spatial data from illegal use. By taking into account usability, invisibility, robustness, and blindness, the approach firstly determines three feature layers of the geo-spatial data and selects the key points from each layer as watermark embedding positions. Then it shuffles the watermark and embeds it in the least significant bits (LSBs) of the coordinates of the key points. A similar process for selecting the feature layers and the key points in the watermark embedding process is carried out to detect the watermark followed by obtaining the embedded watermark from the LSBs of the coordinates of the key points. Finally, the similarity degrees of three versions of the watermark from three feature layers are calculated to check if the data contains the watermark. Our experiments show that the method is rarely affected by data format change, random noise, similarity transformation of the data, and data editing. 相似文献
134.
Martin Dietzfelbinger Jonathan E. Rowe Ingo Wegener Philipp Woelfel 《Algorithmica》2011,59(3):301-322
We investigate the effects of precision on the efficiency of various local search algorithms on 1-D unimodal functions. We
present a (1+1)-EA with adaptive step size which finds the optimum in O(log n) steps, where n is the number of points used. We then consider binary (base-2) and reflected Gray code representations with single bit mutations.
The standard binary method does not guarantee locating the optimum, whereas using the reflected Gray code does so in Θ((log n)2) steps. A(1+1)-EA with a fixed mutation probability distribution is then presented which also runs in O((log n)2). Moreover, a recent result shows that this is optimal (up to some constant scaling factor), in that there exist unimodal
functions for which a lower bound of Ω((log n)2) holds regardless of the choice of mutation distribution. For continuous multimodal functions, the algorithm also locates
the global optimum in O((log n)2). Finally, we show that it is not possible for a black box algorithm to efficiently optimise unimodal functions for two or
more dimensions (in terms of the precision used). 相似文献
135.
Extending IP to low-power, wireless personal area networks (LoWPANs) was once considered impractical because these networks are highly constrained and must operate unattended for multiyear lifetimes on modest batteries. Many vendors embraced proprietary protocols, assuming that IP was too resource-intensive to be scaled down to operate on the microcontrollers and low-power wireless links used in LoWPAN settings. However, 6LoWPAN radically alters the calculation by introducing an adaptation layer that enables efficient IPv6 communication over IEEE 802.15.4 LoWPAN links. 相似文献
136.
The accurate measurement of the execution time of Java bytecode is one factor that is important in order to estimate the total execution time of a Java application running on a Java Virtual Machine. In this paper we document the difficulties and solutions for the accurate timing of Java bytecode. We also identify trends across the execution times recorded for all imperative Java bytecodes. These trends would suggest that knowing the execution times of a small subset of the Java bytecode instructions would be sufficient to model the execution times of the remainder. We first review a statistical approach for achieving high precision timing results for Java bytecode using low precision timers and then present a more suitable technique using homogeneous bytecode sequences for recording such information. We finally compare instruction execution times acquired using this platform independent technique against execution times recorded using the read time stamp counter assembly instruction. In particular our results show the existence of a strong linear correlation between both techniques. 相似文献
137.
Jonathan Alexander Author Vitae 《Computers and Composition》2008,25(1):1-8
The development, dissemination, and proliferation of multimedia and media convergent texts raise a number of pressing questions for literacy scholars in general and compositionists in particular. What kinds of literacy practices are students developing through their use and composition of multimodal and new media texts? What genres are used in the creation of such texts, and why? Are there particular genres that are favored? How are older genres remediated or recast through media convergence? What research methodology challenges are posed when attempting to study multimodal and new media texts? How might compositionists use media convergence to teach students about academic literacies, about research, about the changing nature of “writing?” What might media convergence look like in the future? Perhaps most immediately, the phrase itself—“media convergence”—begs a question: what, exactly, is converging? This special issue of Computers and Composition on “Media Convergence” poses answers—sometimes tentative, sometimes provocative—to these questions. 相似文献
138.
Mashburn DN Hinkson SJ Woods MC Gilligan JM Holcomb MR Wikswo JP 《The Review of scientific instruments》2007,78(10):104302
Defibrillators are a critical tool for treating heart disease; however, the mechanisms by which they halt fibrillation are still not fully understood and are the subject of ongoing research. Clinical defibrillators do not provide the precise control of shock timing, duration, and voltage or other features needed for detailed scientific inquiry, and there are few, if any, commercially available units designed for research applications. For this reason, we have developed a high-voltage, programmable, capacitive-discharge stimulator optimized to deliver defibrillation shocks with precise timing and voltage control to an isolated animal heart, either in air or in a bath. This stimulator is capable of delivering voltages of up to 500 V and energies of nearly 100 J with timing accuracy of a few microseconds and with rise and fall times of 5 micros or less and is controlled only by two external timing pulses and a control computer that sets the stimulation parameters via a LABVIEW interface. Most importantly, the stimulator has circuits to protect the high-voltage circuitry and the operator from programming and input-output errors. This device has been tested and used successfully in field shock experiments on rabbit hearts as well as other protocols requiring high voltage. 相似文献
139.
Re-entrant flow manufacturing lines, such as occur in semiconductor wafer fabrication, are characterized by a product routing that consists of multiple visits to a workstation or group of workstations during the manufacturing process. In this paper, a modeling approach is based on the use of generalized Petri nets for a re-entrant flow manufacturing line is presented. Specifically, three Petri net models representing a re-entrant flow line with three work centers and six machines are modeled. How these models may be used to represent a variety of queuing disciplines and work release policies is discussed. 相似文献
140.
Assessing the quality of use case descriptions 总被引:1,自引:0,他引:1