首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   264篇
  免费   3篇
电工技术   1篇
化学工业   21篇
金属工艺   7篇
机械仪表   7篇
建筑科学   9篇
矿业工程   1篇
能源动力   9篇
轻工业   14篇
水利工程   1篇
石油天然气   40篇
无线电   22篇
一般工业技术   23篇
冶金工业   92篇
原子能技术   3篇
自动化技术   17篇
  2014年   2篇
  2013年   2篇
  2012年   4篇
  2011年   7篇
  2010年   5篇
  2009年   8篇
  2008年   7篇
  2007年   11篇
  2006年   4篇
  2005年   6篇
  2004年   2篇
  2002年   9篇
  1999年   8篇
  1998年   12篇
  1997年   14篇
  1996年   13篇
  1995年   4篇
  1994年   11篇
  1993年   9篇
  1992年   4篇
  1991年   6篇
  1990年   4篇
  1989年   5篇
  1987年   4篇
  1986年   6篇
  1985年   4篇
  1984年   6篇
  1983年   10篇
  1982年   4篇
  1981年   6篇
  1980年   3篇
  1979年   2篇
  1978年   4篇
  1977年   12篇
  1976年   11篇
  1975年   2篇
  1974年   6篇
  1973年   2篇
  1972年   3篇
  1971年   2篇
  1970年   2篇
  1969年   2篇
  1968年   1篇
  1967年   3篇
  1966年   3篇
  1959年   1篇
  1957年   1篇
  1956年   2篇
  1955年   1篇
  1954年   1篇
排序方式: 共有267条查询结果,搜索用时 15 毫秒
61.
The objective of this study was to explore the extent of microbial conversion of lesquerolic acid (14-hydroxy-cis-11-eicosenoic acid; LQA) by whole cell catalysis and to identify the newly converted products. Among compost isolates including NRRL strains B-23212 (Sphingobacterium multivorum), B-23213 (Acinetobacter sp.), B-23257 (Enterobacter cloacae B), B-23259 (Escherichia sp.) and B-23260 (Pseudomonas aeruginosa) the S. multivorum strain was the only microorganism that converted LQA to produce a new product identified as 14-oxo-cis-11-eicosenoic acid by GC-MS and NMR analyses. The conversion yield was 47.4% in 48 h at 200 rpm and 28°C in small shake flask experiments. In comparison, both Acinetobacter and Pseudomonas strains failed to convert LQA to major new products but used LQA apparently as an energy source during fermentation. For structural analysis, 6.88 g of 14-oxo-cis-11-eicosenoic acid was produced from converting 11 g LQA (a 62% yield) in 72 h at 200 rpm and 28 °C in Fernbach flasks using 18-h-old NRRL B-23212 cultures and an improved medium that also contained EDTA and glycerol in lieu of glucose as carbon source. NRRL B-23212 was further identified by 16S rRNA gene sequence analysis as a unique strain of S. multivorum. Therefore, S. multivorum NRRL B-23212 possesses an enzymatic activity presumably a secondary alcohol dehydrogenase for converting LQA to produce 14-oxo-cis-11-eicosenoic acid, a first report that demonstrates the functional modification of LQA by whole cell catalysis.  相似文献   
62.
63.
In this work, the effect of grain boundary engineering (GBE) on the structure and connectivity of networks of two types of boundaries was quantified. General high angle boundaries and “special” Σ = 3 and Σ = 9 coincident site lattice boundaries were considered. The effect of GBE processing was to increase the population and length of special boundaries and to disrupt the network of high-angle grain boundaries (HAGBs) in the microstructure. The GBE processing resulted in an increase in the population of special boundaries as determined by line length fraction from approximately 37% to approximately 57%. The connectivity of the special boundaries, as determined by topological analysis, increased by a factor of 4, while the connectivity of HAGBs decreased by an order of magnitude. Cluster sizes in the special boundary network increased across the range of sizes, and the maximum cluster size of HAGBs decreased significantly. The metrics reported here allow for a quantitative analysis of grain boundary connectivity in microstructures, as well as for a quantitative means of comparison of microstructures. These metrics will be used in simulations of diffusional creep, with the aim of quantifying structure–property relationships in grain boundary engineered systems.  相似文献   
64.
Shadows cast by trees and buildings can limit the solar access of rooftop solar-energy systems, including photovoltaic panels and thermal collectors. This study characterizes residential rooftop shading in Sacramento, San Jose, Los Angeles and San Diego, CA. Our analysis can be used to better estimate power production and/or thermal collection by rooftop solar-energy equipment. It can also be considered when designing programs to plant shade trees.High-resolution orthophotos and LiDAR (Light Detection And Ranging) measurements of surface height were used to create a digital elevation model of all trees and buildings in a well-treed 2.5–4 km2 residential neighborhood. On-hour shading of roofing planes (the flat elements of roofs) was computed geometrically from the digital elevation model. Values in future years were determined by repeating these calculations after simulating tree growth. Parcel boundaries were used to determine the extent to which roofing planes were shaded by trees and buildings in neighboring parcels.For the subset of S + SW + W-facing planes on which solar equipment is commonly installed for maximum solar access, absolute light loss in spring, summer and fall peaked about 2 to 4 h after sunrise and about 2 to 4 h before sunset. The fraction of annual insolation lost to shading increased from 0.07–0.08 in the year of surface-height measurement to 0.11–0.14 after 30 years of tree growth. Only about 10% of this loss resulted from shading by trees and buildings in neighboring parcels.  相似文献   
65.
OBJECTIVE: Approximately 25% of patients with subarachnoid hemorrhage (SAH) have electrocardiographic (ECG) abnormalities consistent with myocardial ischemia or myocardial infarction (MI), and their cardiac prognosis remains unclear. The objective of this study was to determine the cardiac and all-cause mortality rate of a series of patients with SAH with ECG changes consistent with ischemia or MI. METHODS: Using an existing database of patients with SAH and predetermined ECG criteria for ischemia or MI, a study group of patients with abnormal ECG results within 3 days of presentation and before aneurysm surgery was identified. Database patients without abnormal ECG results served as a control group. Cardiac mortality, defined as death resulting from arrhythmia, congestive heart failure, or cardiogenic shock, was assessed by chart review. RESULTS: Of 439 patients with SAH in the database, 58 met the criteria for the study group. Forty-one of these patients were treated neurosurgically. No deaths resulting from cardiac causes occurred, and 20 patients died as a result of noncardiac causes. In a multivariable analysis, age older than 65 years and Hunt and Hess grade of at least 3 were predictive of all-cause mortality. ECG abnormalities, however, were not a statistically significant predictor. CONCLUSION: In patients with SAH and ECG readings consistent with ischemia or MI, the risk of death resulting from cardiac causes is low, with or without aneurysm surgery. The ECG abnormalities are associated with more severe neurological injury but are not independently predictive of all-cause mortality.  相似文献   
66.
Intelligent data analysis implies the reasoned application of autonomous or semi-autonomous tools to data sets drawn from problem domains. Automation of this process of reasoning about analysis (based on factors such as available computational resources, cost of analysis, risk of failure, lessons learned from past errors, and tentative structural models of problem domains) is highly non-trivial. By casting the problem of reasoning about analysis (MetaReasoning) as yet another data analysis problem domain, we have previously [R. Levinson and J. Wilkinson, in Advances in Intelligent Data Analysis, edited by X. Liu, P. Cohen, and M. Berthold, volume LNCS 1280, Springer-Verlag, Berlin, pp. 89–100, 1997] presented a design framework, MetaReasoning for Data Analysis Tool Allocation (MRDATA). Crucial to this framework is the ability of a Tool Allocator to track resource consumption (i.e. processor time and memory usage) by the Tools it employs, as well as the ability to allocate measured quantities of resources to these Tools. In order to test implementations of the MRDATA design, we now implement a Runtime Environment for Data Analysis Tool Allocation, RE:DATA. Tool Allocators run as processes under RE:DATA, are allotted system resources, and may use these resources to run their Tools as spawned sub-processes. We also present designs of native RE:DATA implementations of analysis tools used by MRDATA: K-Nearest Neighbor Tables, Regression Trees, Interruptible (Any-Time) Regression Trees, and Hierarchy Diffusion Temporal Difference Learners. Preliminary results are discussed and techniques for integration with non-native tools are explored.  相似文献   
67.
Modeling the Growth of Transportation Networks: A Comprehensive Review   总被引:1,自引:0,他引:1  
This paper reviews the progress that has been made over the last half-century in modeling and analyzing the growth of transportation networks. An overview of studies has been provided following five main streams: network growth in transport geography; traffic flow, transportation planning, and network growth; statistical analyses of network growth; economics of network growth; and network science. In recognition of the vast advances through decades in terms of exploring underlying growth mechanisms and developing effective network growth models, the authors also point out the challenges that are faced to model the complex process of transport development.  相似文献   
68.
Extensive narrowing of lumens of many cerebral arteries caused strokes with brain damage and chronic hemipareses in two children with Williams syndrome. Increased irritability, loss of consciousness, and seizures were initial signs. Arterial stenoses are not limited to the supravalvular aorta and pulmonary arteries in patients with Williams syndrome.  相似文献   
69.
目前在视频投影技术上,单片式DLP投影机已经出现了两种完全不同结构的机型,一种是采用白光光源加上色轮处理的传统机型,而另外一种就是采用三基色光源的新一代机型。早在一年多前,笔者已经从国外的相关资料中了解到单片式DLP投影机将出现全新的解决方案,也就是现在出现的无需色轮技术,只采用RGB三色LED光源背光,彻底解决了由于色轮而导致的彩虹现象。如今终于看到了采用这种技术的家庭影院投影机的出现,让不少影音爱好者相当期待。  相似文献   
70.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号