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   24711篇
  免费   958篇
  国内免费   33篇
电工技术   329篇
综合类   38篇
化学工业   5656篇
金属工艺   380篇
机械仪表   476篇
建筑科学   1307篇
矿业工程   100篇
能源动力   622篇
轻工业   1898篇
水利工程   233篇
石油天然气   71篇
无线电   1818篇
一般工业技术   4503篇
冶金工业   4534篇
原子能技术   196篇
自动化技术   3541篇
  2023年   214篇
  2022年   359篇
  2021年   567篇
  2020年   338篇
  2019年   431篇
  2018年   535篇
  2017年   463篇
  2016年   633篇
  2015年   558篇
  2014年   749篇
  2013年   1419篇
  2012年   1161篇
  2011年   1510篇
  2010年   1075篇
  2009年   1015篇
  2008年   1148篇
  2007年   1086篇
  2006年   947篇
  2005年   764篇
  2004年   746篇
  2003年   659篇
  2002年   564篇
  2001年   428篇
  2000年   363篇
  1999年   402篇
  1998年   752篇
  1997年   554篇
  1996年   443篇
  1995年   412篇
  1994年   377篇
  1993年   362篇
  1992年   269篇
  1991年   195篇
  1990年   268篇
  1989年   260篇
  1988年   215篇
  1987年   224篇
  1986年   226篇
  1985年   265篇
  1984年   224篇
  1983年   223篇
  1982年   207篇
  1981年   194篇
  1980年   168篇
  1979年   193篇
  1978年   164篇
  1977年   185篇
  1976年   232篇
  1975年   160篇
  1974年   123篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
The primary objective of this project is to identify gaps, whether real or perceived, that hinder effective groundwater management in New Zealand. These gaps show as gaps in information, gaps in implementation, gaps in technological and management tools, and gaps in understanding of fundamental processes. The secondary objective is to propose a management strategy to close the identified gaps. Several methods are used to meet these objectives: surveys distributed to selected staff in each regional council; the review of various written reports; the analysis of land-use databases; and private consultation within each regional council. Results show that groundwater management in New Zealand is generally reactionary with the main gaps being in strategic planning and national guidelines. Most gaps appear to be predominantly information and implementation issues. In some cases there are gaps in the understanding of fundamental processes within an aquifer system, including the long-term effects of land-use on groundwater quality. An adaptive management approach is suggested as a means of closing these gaps.  相似文献   
32.
A simple, yet realistic physics-based model is introduced to describe the subthreshold drain current of a MOSFET taking into account the body- and drain-voltage dependencies, including the short channel effects. This model, verified by SPICE simulations, describes adequately the pseudotriode and pseudosaturation regions of MOS transistors operated below V/sub T/. It can be applied for predicting bulk- or partially depleted (PD) SOI CMOS circuit operation. Analytical expressions derived for the logic switching threshold and delay are applied to predict the performance of CMOS-SOI inverters.  相似文献   
33.
In four-color fluorescence-based automated DNA sequencing, a 4×4 filter matrix parameterizes the relationship between the dye-intensity signals of interest and the data collected by an optical imaging system. The filter matrix is important because the estimated DNA sequence is based on the dye intensities that can only be recovered via inversion of the matrix. Here, the authors present a calibration method for the estimation of the columns of this matrix, using data generated through a special experiment in which DNA samples are labeled with only one fluorescent dye at a time. Simulations and applications of the method to real data are provided, with promising results  相似文献   
34.
College readers read and answered questions on 12 short essays. Essays formatted so that points between phrases had fractional extra space added to them were comprehended better than normally formatted text. These improvements were specific to average readers. Practically, the results justify classroom research on the benefits of phrase-sensitive formatting; theoretically, the results add to existing evidence that poor to average readers specifically lack perceptual strategies for grouping word sequences into phrases. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
35.
The pipelined architecture and parallel organization of the AT&T Pixel Machine image computer are described and demonstrated with applications for the visualization of multidimensional fractals, particularly linear fractals and quaternion/ stacked Julia sets. Techniques for pushing the Pixel Machine to its peak abilities are described and apply to more recent parallel image computers as well.  相似文献   
36.
The purpose for this study was to evaluate the results of silicone oil use in patients who have undergone vitrectomy for proliferative vitreoretinopathy (PVR). The authors reviewed the 5-year results of 50 consecutive patients (52 eyes) with grade C1/D3 PVR operated on in 1987. Silicone oil has been removed in 42 eyes. The retina remained attached in 38 eyes. Visual acuities were finger counting in 12 eyes and 0.1 or greater in 19 eyes. Glaucoma was thought to be the major cause of loss of visual acuity after an initial improvement (9 eyes), as well as the most frequent complication of surgery (13 eyes). Silicone oil was deemed nonremovable in 10 operated eyes. A questionnaire was developed to determine the patients' perceptions of the use of silicone oil, and 46 patients responded to the questionnaire. Four patients indicated that they would not have elected to have surgery again. Six patients thought that surgery was not worthwhile, but would permit treatment again. Most patients thought that a modest amount of improvement had taken place and that vision was stable. Subjectively, most patients believed that surgery was worthwhile.  相似文献   
37.
38.
39.
40.
The expression patterns of laminin A, B1, B2, and nidogen genes were identified by in situ hybridization in postimplantation mouse extraembryonic tissues and maternal decidua during the period when the chorioallantoic placenta is established. Laminin and nidogen genes were not coordinately expressed either in the decidua or in trophoblast cells, indicating that these genes are regulated independently in these cell types during the establishment of the placenta. Laminin A mRNA was absent from the decidua except in the outer layer of cells adjacent to the myometrium and in the central decidual zone adjacent to the remnant of the uterine epithelium on Day 9. At this stage laminin B1, B2, and nidogen genes were strongly expressed in these cells and also in other regions of the decidua. Laminin B1 mRNA was present at higher levels in the decidua capsularis than in the decidua basalis, while nidogen mRNA showed highest expression in the decidua basalis. Laminin B2 mRNA was produced uniformly throughout the decidua at very high levels, suggesting that laminin B2 chains may be an important component of the decidual matrix. By Day 11, the nidogen gene was expressed only in endothelial cells lining the maternal blood spaces within the decidua. Laminin B1 and nidogen mRNAs were found at high levels within trophoblast giant cells at all stages, while laminin A mRNA was detected in trophoblast giant cells at later stages and laminin B2 mRNA was not produced in high levels by these cells. The patterns of gene expression show a very high degree of regional specialization, suggesting that the extracellular matrices in different regions of the decidua and extraembryonic membranes are likely to be composed of quite different ratios of laminin and nidogen polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS)  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