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   749372篇
  免费   8530篇
  国内免费   1687篇
电工技术   14089篇
综合类   639篇
化学工业   115832篇
金属工艺   29666篇
机械仪表   23497篇
建筑科学   17440篇
矿业工程   4822篇
能源动力   19846篇
轻工业   60707篇
水利工程   8477篇
石油天然气   16433篇
武器工业   49篇
无线电   85204篇
一般工业技术   150843篇
冶金工业   133162篇
原子能技术   17567篇
自动化技术   61316篇
  2021年   7277篇
  2020年   5507篇
  2019年   6579篇
  2018年   11239篇
  2017年   11472篇
  2016年   11965篇
  2015年   7569篇
  2014年   12889篇
  2013年   34350篇
  2012年   20003篇
  2011年   27119篇
  2010年   21816篇
  2009年   24346篇
  2008年   24922篇
  2007年   24629篇
  2006年   21471篇
  2005年   19287篇
  2004年   18451篇
  2003年   18330篇
  2002年   17536篇
  2001年   17461篇
  2000年   16517篇
  1999年   16711篇
  1998年   40278篇
  1997年   28685篇
  1996年   22327篇
  1995年   16972篇
  1994年   15313篇
  1993年   15056篇
  1992年   11298篇
  1991年   11017篇
  1990年   10786篇
  1989年   10371篇
  1988年   10031篇
  1987年   8930篇
  1986年   8702篇
  1985年   9760篇
  1984年   8816篇
  1983年   8513篇
  1982年   7716篇
  1981年   7787篇
  1980年   7504篇
  1979年   7378篇
  1978年   7145篇
  1977年   8162篇
  1976年   10504篇
  1975年   6350篇
  1974年   6146篇
  1973年   6128篇
  1972年   5308篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
43.
44.
An analysis is performed for the existing hypotheses and theories used for calculating the technological parameters of operations on methane extraction from coal seams. The priority of geodynamic processes in methane collector formation, as well as the methods applied for the morphometry and processing of the remote probing data are substantiated. The urgency is shown for the investigation into the autooxidation and other exothermic reactions proceeding in coal seams and leading to methane transition to the free state.  相似文献   
45.
The relevance of particle mass, surface area or number concentration as risk indicators for health effects in non-industrial buildings has been assessed by a European interdisciplinary group of researchers (called EUROPART) by reviewing papers identified in Medline, Toxline, and OSH. Studies dealing with dermal effects or cancer or specifically addressing environmental tobacco smoke, house dust-mite, cockroach or animal allergens, microorganisms and pesticides were excluded. A total of 70 papers were reviewed, and eight were identified for the final review: Five experimental studies involving mainly healthy subjects, two cross-sectional office studies and one longitudinal study among elderly on cardiovascular effects. From most studies, no definite conclusions could be drawn. Overall, the group concluded that there is inadequate scientific evidence that airborne, indoor particulate mass or number concentrations can be used as generally applicable risk indicators of health effects in non-industrial buildings and consequently that there is inadequate scientific evidence for establishing limit values or guidelines for particulate mass or number concentrations.  相似文献   
46.
Recently, the limit state design (LSD) or performance-based design have got popularity in the field of building design in Japan as well as in other countries. In the two design methods the structural reliability theory plays an essential role in setting design criteria as well as demonstrating the target reliability level to society. However, the conventional load and resistance factor design (LRFD) has been basically formulated supposing that safety checking is done on the basis of linear assumption of member forces and displacement. Therefore, when applying the LRFD for seismic design, for more accurate treatment of the non-linearity, a new procedure has to be explored especially for the ultimate limit state. Although several procedures for the structural reliability evaluation, treating non-linear displacement responses, have been proposed, they require complex procedures that may not be used in the practical design process. Accordingly, for applying it to a seismic LSD format based on the probabilistic concept, it is essential to manage two important requirements at the same time, accuracy and simplicity of procedure. In the present study, a new design format using the following two-step procedure is proposed to maintain both accuracy and simplicity; (1) a non-linear LRFD formulation, and (2) a formulation based on non-linear dynamic response analysis. Also, two design examples are presented.  相似文献   
47.
This paper characterizes the relationship between occupant activities and indoor air particulate levels in a non-smoking office building. Occupant activities were recorded on video. Particulate concentrations were monitored by three optical particle counters (OPCs) in five size ranges at three heights. Particulate mass concentrations were measured gravimetrically and bioaerosol concentrations were determined by impaction methods. Occupant activities and number concentrations were determined with 1-min resolution over a 1-week period. Occupant activities such as walking past or visiting the monitoring site explained 24-55% of the variation of 1- to 25-micron diameter particle number concentrations. Statistical models associating particulate concentrations with occupant activities depended on the size fraction and included an autocorrelative term. Occupant activities are estimated to contribute up to 10 micrograms m-3 in particulate concentrations per person. Number concentrations of particles smaller than 1 micron had little correlation with indoor activities other than cigarette smoking and were highly correlated with outdoor levels. The method can be used to characterize emissions from activities if rapid measurements can be made and if activities can be coded from the video record.  相似文献   
48.
49.
An extension of ASM2d including pH calculation   总被引:2,自引:0,他引:2  
Serralta J  Ferrer J  Borrás L  Seco A 《Water research》2004,38(19):4029-4038
This paper presents an extension of the Activated Sludge Model No. 2d (ASM2d) including a chemical model able to calculate the pH value in biological processes. The developed chemical model incorporates the complete set of chemical species affecting the pH value to ASM2d describing non-equilibrium biochemical processes. It considers the system formed by one aqueous phase, in which biochemical processes take place, and one gaseous phase, and is based on the assumptions of instantaneous chemical equilibrium under liquid phase and kinetically governed mass transport between the liquid and gas phase. The ASM2d enlargement comprises the addition of every component affecting the pH value and an ion-balance for the calculation of the pH value and the dissociation species. The significant pH variations observed in a sequencing batch reactor operated for enhanced biological phosphorus removal were used to verify the capability of the extended model for predicting the dynamics of pH jointly with concentrations of acetic acid and phosphate. A pH inhibition function for polyphosphate accumulating bacteria has also been included in the model to simulate the behaviour observed. Experimental data obtained in four different experiments (with different sludge retention time and influent phosphorus concentrations) were accurately reproduced.  相似文献   
50.
Contact flocculation-filtration of humic dispersions with alum in medium grain (0.62 mm) sand gave short runs to breakthrough, while in coarse (1.2 mm) media clarification was inefficient. Alum with cationic polymer gave in coarse media acceptable runs, but shorter then for mineral dispersions. Analysis of results by parameters of existing filtration models showed that using the Ives-Iwasaki model and plotting filtration coefficient λ vs specific deposit σ gave different λmax for different polymer dose, plot of λmax vs dose goes through maximum at optimum dose.Plotting the cube root of headloss vs square root of specific deposit gave a straight line corresponding to Shektman formula.Analysis by Adin-Rebhun model showed that for contact flocculation-filtration humic dispersions attachment coefficient k1 was similar to, detachment k2 was many fold higher then and theoretical maximum mass capacity much lower then in case of mineral dispersions.Calculations showed that the solid content and the density of alumo-organic deposits in bed are much lower than of alumo-mineral deposits. Polymers and minerals increase the solid content, the density of deposits and the mass capacity of filter beds.  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