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   3961篇
  免费   131篇
  国内免费   9篇
电工技术   79篇
化学工业   501篇
金属工艺   34篇
机械仪表   60篇
建筑科学   123篇
矿业工程   10篇
能源动力   98篇
轻工业   345篇
水利工程   26篇
石油天然气   16篇
无线电   472篇
一般工业技术   534篇
冶金工业   1275篇
原子能技术   62篇
自动化技术   466篇
  2022年   24篇
  2021年   54篇
  2020年   31篇
  2019年   57篇
  2018年   57篇
  2017年   57篇
  2016年   81篇
  2015年   44篇
  2014年   72篇
  2013年   189篇
  2012年   107篇
  2011年   134篇
  2010年   121篇
  2009年   110篇
  2008年   154篇
  2007年   132篇
  2006年   105篇
  2005年   92篇
  2004年   82篇
  2003年   81篇
  2002年   75篇
  2001年   93篇
  2000年   71篇
  1999年   93篇
  1998年   326篇
  1997年   203篇
  1996年   134篇
  1995年   84篇
  1994年   96篇
  1993年   108篇
  1992年   47篇
  1991年   36篇
  1990年   62篇
  1989年   57篇
  1988年   64篇
  1987年   56篇
  1986年   48篇
  1985年   63篇
  1984年   43篇
  1983年   37篇
  1982年   33篇
  1981年   42篇
  1980年   38篇
  1979年   27篇
  1978年   37篇
  1977年   50篇
  1976年   73篇
  1975年   40篇
  1974年   18篇
  1973年   19篇
排序方式: 共有4101条查询结果,搜索用时 703 毫秒
101.
Low‐income Hispanics are the most digitally underserved population in the U.S. This article examines the potential of community‐based participatory research approach to e‐health to decrease the disparities in access to technology and health information in low‐income Hispanic communities. To demonstrate this framework, we describe the process of designing a community‐based e‐health intervention to increase knowledge and parental self‐efficacy in coping with young children's mental health problems including mental health service utilization. Our model incorporates utilizing promotoras de salud (lay community health educators) and community media principles to create the content of e‐health interventions and train community members in using the technology. This case study illustrates the processes involved in using this approach, barriers for participatory e‐health interventions in bridging the Digital Divide, and lessons learned.  相似文献   
102.
We show for the first time that self-exciting and self-sensing piezoelectric cantilevers consisting only of lead zirconate titanate (PZT) measure resonance only if they are asymmetrically anchored. Symmetric-anchoring did not give rise to electrically measurable bending resonant modes in the 0-100 kHz range. Sensitivity of first and second bending mode resonances was characterized in a flow apparatus using small density changes in liquid (0.003-0.01 g/cm3) and by dodecanethiol chemisorption at 30 pM. Density change results were consistent with existing models of submerged cantilevers, and yielded mass-change sensitivity of ∼33 ng/Hz and 217 pg/Hz for the first two modes. In chemisorption experiments, where binding was localized to 1 mm2 distal tip of the PZT cantilever, sensitivity improved by an order of magnitude to 2 pg/Hz and 414 fg/Hz for the same two resonant modes.  相似文献   
103.
Epiphylls - lichens, fungi, liverworts, etc. infesting leaf surfaces - are found throughout humid forests of the world. It is well understood that epiphylls inhibit light interception by host plants, but their effect on remote sensing of colonized forests has not been examined. Incorporating leaf-level spectra from Terra Firme (primary forest) and Amazonian Caatinga (woodlands/forest growing on nutrient-deficient sandy soils), we used the GeoSAIL model to propagate leaf-level measurements to the canopy level and determine their effect on commonly used vegetation indices. In Caatinga, moderate infestations (50% leaf area epiphyll cover), lowered simulated Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) values by 6.1% and 20.4%, respectively, largely due to near infrared dampening. Heavy infestation (100% cover) simulations exhibited decreases 1.5-2 times greater than those of moderate infestations. For Terra Firme, which are generally less affected by epiphylls, moderate (20% leaf area) and heavy infestations (40%) lowered EVI by 4.4% (S.D. 0.8%) and 8.1% (S.D. 1.5%), respectively. Near infrared and green reflectance were most affected at the canopy level, showing mean decreases of 10.6% (S.D. 2.25%) and 9.5% (S.D. 3.49%), respectively, in heavy Terra Firme infestations. Time series of Moderate Resolution Imaging Spectrometer (MODIS) data corroborated the modeling results, suggesting a degree of coupling between epiphyll cover and the EVI and NDVI. These results suggest that, without explicit consideration of the presence of epiphylls, remote sensing-based methodologies may underestimate leaf area index, biomass and productivity in humid forests.  相似文献   
104.
Estimating Siberian timber volume using MODIS and ICESat/GLAS   总被引:4,自引:0,他引:4  
Geosciences Laser Altimeter System (GLAS) space LiDAR data are used to attribute a MODerate resolution Imaging Spectrometer (MODIS) 500 m land cover classification of a 10° latitude by 12° longitude study area in south-central Siberia. Timber volume estimates are generated for 16 forest classes, i.e., four forest cover types × four canopy density classes, across this 811,414 km2 area and compared with a ground-based regional volume estimate. Two regional GLAS/MODIS timber volume products, one considering only those pulses falling on slopes ≤ 10° and one utilizing all GLAS pulses regardless of slope, are generated. Using a two-phase(GLAS-ground plot) sampling design, GLAS/MODIS volumes average 163.4 ± 11.8 m3/ha across all 16 forest classes based on GLAS pulses on slopes ≤ 10° and 171.9 ± 12.4 m3/ha considering GLAS shots on all slopes. The increase in regional GLAS volume per-hectare estimates as a function of increasing slope most likely illustrate the effects of vertical waveform expansion due to the convolution of topography with the forest canopy response. A comparable, independent, ground-based estimate is 146 m3/ha [Shepashenko, D., Shvidenko, A., and Nilsson, S. (1998). Phytomass (live biomass) and carbon of Siberian forests. Biomass and Bioenergy, 14, 21-31], a difference of 11.9% and 17.7% for GLAS shots on slopes ≤ 10° and all GLAS shots regardless of slope, respectively. A ground-based estimate of total volume for the entire study area, 7.46 × 109 m3, is derived using Shepashenko et al.'s per-hectare volume estimate in conjunction with forest area derived from a 1990 forest map [Grasia, M.G. (ed.). (1990). Forest Map of USSR. Soyuzgiproleskhoz, Moscow, RU. Scale: 1:2,500,000]. The comparable GLAS/MODIS estimate is 7.38 × 109 m3, a difference of less than 1.1%. Results indicate that GLAS data can be used to attribute digital land cover maps to estimate forest resources over subcontinental areas encompassing hundreds of thousands of square kilometers.  相似文献   
105.
This paper surveys fitness functions used in the field of evolutionary robotics (ER). Evolutionary robotics is a field of research that applies artificial evolution to generate control systems for autonomous robots. During evolution, robots attempt to perform a given task in a given environment. The controllers in the better performing robots are selected, altered and propagated to perform the task again in an iterative process that mimics some aspects of natural evolution. A key component of this process–one might argue, the key component–is the measurement of fitness in the evolving controllers. ER is one of a host of machine learning methods that rely on interaction with, and feedback from, a complex dynamic environment to drive synthesis of controllers for autonomous agents. These methods have the potential to lead to the development of robots that can adapt to uncharacterized environments and which may be able to perform tasks that human designers do not completely understand. In order to achieve this, issues regarding fitness evaluation must be addressed. In this paper we survey current ER research and focus on work that involved real robots. The surveyed research is organized according to the degree of a priori knowledge used to formulate the various fitness functions employed during evolution. The underlying motivation for this is to identify methods that allow the development of the greatest degree of novel control, while requiring the minimum amount of a priori task knowledge from the designer.  相似文献   
106.
107.
108.
The purpose of this study was to determine if a change in forward head posture and occipital extension occurred in participants who wore multifocal lenses vs. those persons with non-multifocal lenses while performing an 8-min visual reading task on a visual display unit (VDU). Forty-two healthy human participants were recruited for this study. Thirty-three participants completed the study. Fourteen participants wore multifocal lenses and 19 wore frames with non-multifocal lenses. To evaluate the degree of change of forward head posture and occipital extension digital photographs of cervical posture were taken at four different time intervals: prior to performing the reading task and at 3, 5 and 8 min during the reading task. The digital photographs were analysed utilizing a computer program. Two one-way ANOVA were utilized to determine the degree of change of forward head posture and occipital extension between groups. A significant difference was identified between groups for changes in degrees of forward head posture while performing a visual reading task on a VDU. However, no significant difference between groups was found for occipital extension while performing the same task. Multifocal wearers exhibit greater degrees of change in forward head posture and occipital extension than non-multifocal wearers. These postural changes may place them at a greater risk for musculoskeletal disorders and headaches.  相似文献   
109.
A method of computing a basis for the second Yang–Baxter cohomology of a finite biquandle with coefficients in QQ and ZpZp from a matrix presentation of the finite biquandle is described. We also describe a method for computing the Yang–Baxter cocycle invariants of an oriented knot or link represented as a signed Gauss code. We provide a URL for our Maple implementations of these algorithms.  相似文献   
110.
For service-oriented architectures that span multiple businesses, organizations must transfer information back-and-forth about their available services. Because of the potential large volume, it is unreasonable and impractical to expect human practitioners to handle the scale of interactions desired and/or required on a continual basis. Intelligent agents offer the adaptability and flexibility to handle the knowledge transfer that must occur in order to share service offerings. Effectively transferring service-oriented information in this domain requires autonomous systems that adapt to heterogeneous environments. This work introduces an architecture and specialized communication procedures designed for this sort of knowledge sharing environment. We show that these procedures perform reasonably when evaluated with current agent communication technologies.
M. Brian Blake (Corresponding author)Email:
  相似文献   
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