全文获取类型
收费全文 | 4366篇 |
免费 | 152篇 |
国内免费 | 5篇 |
专业分类
电工技术 | 58篇 |
综合类 | 3篇 |
化学工业 | 606篇 |
金属工艺 | 87篇 |
机械仪表 | 104篇 |
建筑科学 | 186篇 |
矿业工程 | 7篇 |
能源动力 | 150篇 |
轻工业 | 440篇 |
水利工程 | 20篇 |
石油天然气 | 12篇 |
无线电 | 390篇 |
一般工业技术 | 748篇 |
冶金工业 | 1223篇 |
原子能技术 | 61篇 |
自动化技术 | 428篇 |
出版年
2023年 | 17篇 |
2022年 | 25篇 |
2021年 | 73篇 |
2020年 | 57篇 |
2019年 | 74篇 |
2018年 | 66篇 |
2017年 | 62篇 |
2016年 | 72篇 |
2015年 | 68篇 |
2014年 | 67篇 |
2013年 | 158篇 |
2012年 | 144篇 |
2011年 | 188篇 |
2010年 | 144篇 |
2009年 | 127篇 |
2008年 | 157篇 |
2007年 | 165篇 |
2006年 | 129篇 |
2005年 | 110篇 |
2004年 | 118篇 |
2003年 | 115篇 |
2002年 | 106篇 |
2001年 | 86篇 |
2000年 | 75篇 |
1999年 | 107篇 |
1998年 | 325篇 |
1997年 | 236篇 |
1996年 | 162篇 |
1995年 | 114篇 |
1994年 | 98篇 |
1993年 | 97篇 |
1992年 | 65篇 |
1991年 | 55篇 |
1990年 | 59篇 |
1989年 | 39篇 |
1988年 | 64篇 |
1987年 | 50篇 |
1986年 | 38篇 |
1985年 | 62篇 |
1984年 | 31篇 |
1983年 | 42篇 |
1982年 | 31篇 |
1981年 | 31篇 |
1980年 | 34篇 |
1979年 | 32篇 |
1978年 | 33篇 |
1977年 | 49篇 |
1976年 | 90篇 |
1974年 | 24篇 |
1973年 | 27篇 |
排序方式: 共有4523条查询结果,搜索用时 281 毫秒
61.
Although the banking industry plays a central role in the world economy and has traditionally been an innovative user of information technology, banks are struggling to develop mature Internet strategies. A model used to analyze the type of Internet activities among a sample of banks as well as their level of sophistication and customization is useful not only for revealing unexploited opportunities in the banking sector but also for guiding the strategy-building process of organizations in other industries. 相似文献
62.
OBJECTIVES: This study sought to explore the separate and combined effects of changes in preload, afterload and contractility on the dynamics of systolic bulging. BACKGROUND: The extent of ischemic systolic bulging has been shown to be mechanically disadvantageous to left ventricular pump performance. The factors that determine ischemic segmental wall motion have not been systematically studied. METHODS: Fourteen beagles were instrumented with sonomicrometers, micromanometer pressure gauges and a balloon in the inferior vena cava. Regional function was evaluated before and after 90 s of proximal left circumflex coronary artery occlusion. Occlusions were repeated after increasing systolic pressure by 5 to 10 (afterload I) and 15 to 20 mm Hg (afterload II) with graded aortic occlusion during inotropic stimulation with dobutamine (2.5 and 5 micrograms/kg body weight per min intravenously), with simultaneous 5 micrograms/kg per min dobutamine infusion and afterload II and during 2.5% halothane (negative inotrope) concentration. A 20-min recovery period was allowed between each stage of the experiment so that regional function returned to its preocclusion level. Ischemic wall motion was characterized by percent systolic bulging and its peak positive systolic lengthening rate (+dL/dt). RESULTS: Because bulging is markedly influenced by regional preload, systolic bulging was characterized over a wide range of end-diastolic lengths of the ischemic segment during caval balloon occlusion. During each intervention, a decrease in regional preload increased the extent of percent systolic bulging. This preload dependency was more pronounced with dobutamine infusions. An increase in afterload was not associated with increased percent systolic bulging at any given preload. At a predetermined preload, bulging was not appreciably altered when an increase in left ventricular systolic pressure was not associated with a change in peak positive first derivative of left ventricular pressure (+dP/dt) but was significantly worse when peak +dP/dt increased. Dobutamine caused a dose-dependent increase in percent systolic bulging and peak +dL/dt that was positively correlated with peak +dP/dt. CONCLUSIONS: By using different loading and inotropic interventions and analyzing the regional wall motion behavior over a range of regional preloads, we can conclude that preload and rate of pressure (tension) development are the principal determinants of systolic bulging. Increases in left ventricular pressure alone had a minimal effect on systolic bulging. 相似文献
63.
64.
Two macromodeling techniques using rational bases are investigated to accurately predict the natural frequencies of highly resonant microwave structures. Three methods are proposed and compared to calculate pole‐free solutions to the Thiele continued fraction and vector fitting pole‐residue models of the characteristic equation det[Z(s)]. © 2007 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2007. 相似文献
65.
Details of the vorticity field structure associated with the 3D Tollmien-Schlichting waves have been examined based upon the
recent numerical studies of the subject. First, a single obliquet-s wave has been found to have the velocity component parallel to the wave front playing an overall dominant role, in particular,
to create the longitudinal vorticity. The so-called Benney-Lin longitudinal vortices are then demonstrated to be, in fact,
a minor consequence compared with the localized longitudinal vorticity field and its periodic pumping. Finally, the formation
of the longitudinal vorticity field in the fundamental- and subharmonic-mode interactions is explained.
The research reported in this paper has been supported in part by Bundesministerium für Forschung und Technologie and by Deutsche
Forschungsgemeinschaft. The major part of the paper has been presented at the Third Asian Congress of Fluid Mechanics, 1–5
September 1986, in Tokyo, as a General Lecture by the senior author, FRH. 相似文献
66.
Male and female mice of the BALB/c strain were placed on the floor of a hut built at a site of the Permian Basin of Lodève (Southwest France) where the dose rate of γ radiation amounts to about 10 mrad/h. In previous experiments such high natural radioactivity was shown to induce point mutations in plant material as well as chromosome aberrations in somatic cells of experimental mammals. Controls were kept near the radioactive site under comparable conditions. After the exposure period, the animals were mated in our laboratory, with control mice of the same strain.Due to the high radiosensitivity of mouse oocytes, the fertility of the exposed females was drastically reduced in spite of the fact that the dose received did not exceed 13.8 rad. The fertility of the males exposed to 13 rad, 15 rad or to 45 rad was increased above the control values but was reduced for the animals receiving 63 rad. Analysis of the results show that the differences in fertility are mainly due to a decrease in the number of sterile pairs when males have been exposed to doses upto 45 rad and to an inverse effect when animals received 63 rad. Since histology and weight of the testes suggest that the germ cell population was normal, one has to conclude that the variations in fertility result from physiological effects. 相似文献
67.
Possibilities and problems with the use of models as a communication tool in water resource management 总被引:2,自引:1,他引:1
Politicians and policy-makers, as well as modellers, often nurses an expectation that model derived results is an objective
source of information that can be used to support decisions. However, several prerequisites have to be dealt with in order
to ensure that models can be used as legitimate and efficient tools in water resource management. Based on empirical material
from recent studies on the use of models in stakeholder dialogues, mainly focusing on catchment nutrient transport, two central
problems are identified: (a) Models are laden with choices and thus depend on assumptions and priorities of modellers. (b)
There are several factors that influence ability and willingness of stakeholders (as information recovers) to criticize or
accept results of the modelling exercise. Recognized factors likely to influence stakeholders' acceptance of model derived
results include issues at stake, stakeholders' ability to criticize model derived information, and their trust in the institutions
that have developed or applied the used models. Identified prerequisites for successful use of models in integrated water
resource management include: consideration of user relevance, awareness of and preparedness to handle constraints linked to
communication of model-based results, transparency of used models and data and of involved uncertainties, mutual respect between
experts and stakeholders and between involved stakeholder groups, a robust institutional network, and sufficient time for
dialogues. Development and use of strategies for participatory modelling, based on a continuous dialogue between experts and
stakeholders is recommended as a way to facilitate that the prerequisites for a successful use of models in water resource
management are fulfilled. 相似文献
68.
Prolyl-dipeptidyl-peptidase activity was detected in cell extracts of 21 lactic acid bacteria tested. Using disc electrophoresis and various substrates it was possible to distinguish it from proline iminopeptidase and proline endopeptidase. Generally the activity was high and was greater than that of proline iminopeptidase of proline endopeptidase at neutral pH and at 25 degrees C. 相似文献
69.
Johanna Bakker Colin F. Timberlake 《Journal of the science of food and agriculture》1985,36(12):1315-1324
The distribution of anthocyanins in methanolic skin extracts of 16 grape cultivars used for port wine production and grown at five sites in the Douro Valley in Northern Portugal has been assessed by high performance liquid chromatography. Results are confined to the seven most readily separated and identified anthocyanins and are expressed as percentages of their total (88–99% of the total integrated area). Anthocyanins based on malvidin (Mv) predominated. Of these, Mv 3-glucoside was the major pigment (33–60%), being exceeded by Mv 3-p-coumarylglucoside (2–51%) in only two cultivars; Mv 3-acetylglucoside (1–15%) was consistently the lowest. Peonidin 3-glucoside (1–27%) was prominent in four cultivars, but delphinidin 3-glucoside (1–11%), petunidin 3-glucoside (2–11%) and cyanidin 3-glucoside (trace-6%) were of low proportions throughout. The ratio Mv 3-acetylglucoside/total Mv glucosides appeared characteristic of cultivar, independent of site, and a useful aid to identification of grape cultivars. The coloured pulp of some cultivars contained peonidin 3-glucoside as a major component, present in greater proportions than in the skin; the percentage of Mv 3-p-coumarylglucoside was lower in pulp than in skin. The identities of six of the seven anthocyanins were confirmed by fast atom bombardment mass spectrometry and the structures of the Mv derivatives were determined by nuclear magnetic resonance. The formation of formyl and acetyl anthocyanin artefacts is described. 相似文献
70.
Low-power monolithic RF peak detector analysis 总被引:2,自引:0,他引:2
Closed-form expressions are derived for the transfer characteristic of a low-power monolithic RF peak detector. These are compared with computer simulation and experimental measurements 相似文献