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   441539篇
  免费   6584篇
  国内免费   1422篇
电工技术   8071篇
综合类   389篇
化学工业   67670篇
金属工艺   15023篇
机械仪表   12387篇
建筑科学   13342篇
矿业工程   1099篇
能源动力   12094篇
轻工业   46130篇
水利工程   3627篇
石油天然气   2032篇
武器工业   15篇
无线电   57372篇
一般工业技术   81164篇
冶金工业   80374篇
原子能技术   5852篇
自动化技术   42904篇
  2021年   3144篇
  2020年   2334篇
  2019年   2755篇
  2018年   4239篇
  2017年   4347篇
  2016年   4610篇
  2015年   3802篇
  2014年   6322篇
  2013年   21154篇
  2012年   11228篇
  2011年   15964篇
  2010年   12273篇
  2009年   13954篇
  2008年   15109篇
  2007年   15500篇
  2006年   13821篇
  2005年   12942篇
  2004年   12450篇
  2003年   12126篇
  2002年   11750篇
  2001年   11677篇
  2000年   10773篇
  1999年   11260篇
  1998年   24522篇
  1997年   18002篇
  1996年   14393篇
  1995年   11431篇
  1994年   10224篇
  1993年   9812篇
  1992年   7530篇
  1991年   7063篇
  1990年   6752篇
  1989年   6462篇
  1988年   6187篇
  1987年   5234篇
  1986年   5163篇
  1985年   6425篇
  1984年   6016篇
  1983年   5212篇
  1982年   4912篇
  1981年   4843篇
  1980年   4511篇
  1979年   4390篇
  1978年   4058篇
  1977年   4811篇
  1976年   6328篇
  1975年   3377篇
  1974年   3229篇
  1973年   3136篇
  1972年   2459篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
A short-term study of a commercially-available physical—chemical waste treatment plant is reported. Results indicate that an excellent quality of effluent can be consistently obtained despite wide fluctuations of influent wastewater characteristics. Physical, chemical, and biological parameters were monitored. The short-term data obtained offer some evidence of superior treatment efficiency in terms of COD, color, turbidity, and phosphorus removals when compared to more conventional treatment methods.  相似文献   
22.
本文用弦线模量计算了营业楼地基的压缩变形和湿陷变形,以及由它们引起的差异沉降值和局部倾斜。计算结果和楼房的裂缝情况吻合。计算湿陷变形时考虑地基浸水后的含水量为30.00%,这是由实测情况确定。根据浸水的实际状况计算建筑物的湿陷变形,分析工程事故在文献中似未见到。  相似文献   
23.
Toxicity bioassay of heavy metals in water using Tetrahymena pyriformis   总被引:1,自引:0,他引:1  
The toxicities of five heavy metal compounds on the survival of the ciliated protozoan Tetrahymena pyriformis were determined. Three-day old cultured cells were exposed to five concentrations of each test compound in one or more of three water series; distilled, soft, and hard water. A culture of each sample was placed in a plastic petri dish and covered with mineral oil to prevent evaporation. The cells were counted initially, then again at 0·5 h, 1 h, 2 h, 1 day, 2 days, 3 days, and on the fourth day. Lethal threshold concentrations and tolerance limit medians were determined by graphic methods. On a weight bases cadmium was most toxic, followed in decreasing toxicity by mercury, cobalt, zinc, and lead. Compared to similar fish data, all heavy metals were more toxic to T. pyriformis except lead. The toxicity of lead in soft versus hard water exemplified an antagonistic effect with greater than seven times the amount of lead necessary in hard water to produce comparable mortality as lead in soft water. On the other hand, the toxicity of mercury is about twice as great in hard water as in soft water (a synergistic effect). T. pyriformis appears to be a more sensitive indicator than fish of heavy metal contamination of water. T. pyriformis bioassay should prove a good means of determining the existence of many water pollutants.  相似文献   
24.
Brominated flame retardants (BFRs) were investigated in juvenile common sole from nursery zones situated along the French coast in 2007, 2008 and 2009. Extensive identification was performed with regard to PBDEs, novel BFRs 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) and decabromodiphenylethane (DBDPE), and other non-PBDE BFRs, namely, hexabromobenzene (HBB) and 2,2′,4,4′,5,5′-hexabromobiphenyl (BB-153). Polybrominated diphenyl ether (PBDE) concentrations (Σ 14 congeners) ranged from 0.01 ng/g to 0.16 ng/g wet weight (ww) in muscle, and 0.07 ng/g to 2.8 ng/g ww in liver. Concentrations were in the lower range of those reported in the literature in other European locations. Lower PBDE concentrations, condition indices and lipid contents were observed in the Seine estuary in 2009, possibly in relation to a lower water flow. The PBDE patterns and ratios we observed suggested that juvenile sole have a relative high metabolic degradation capacity. Non-PBDE BFRs were detected at lower levels than PBDEs, i.e., within the < method detection limit - 0.005 ng/g ww range in muscle, and < method detection limit - 0.2 ng/g ww range in liver. The data obtained is of particular interest for the future monitoring of these compounds in the environment.  相似文献   
25.
M. Chiba  N. Yamaki  J. Tani 《Thin》1984,2(3):265-284
Theoretical analyses are presented for the linear free vibration of a clamped-free cylindrical shell partially filled with an incompressible, inviscid liquid. For the vibration of the shell itself, the dynamic version of the Donnell equations was used and the problem was solved with the modified Galerkin procedure, taking the effect of the axisymmetric deformation due to the static liquid pressure into consideration. Concerning the vibration relevant to the liquid motion, the solution for the velocity potential was assumed as a sum of two sets of linear combinations of the suitable harmonic function, the unknown parameters of which were imposed to satisfy both boundary conditions along the wetted shell wall and the free liquid surface in a sense of appropriate series expansions. The procedure stated in the foregoing leads to a determinantal equation for the determination of the natural frequencies of the present shell-liquid system. To compare with the experimental results which will be stated in a companion paper,14 detailed numerical results will be presented in another companion paper13 on the free vibration characteristics of the two test cylinders partially filled with water.  相似文献   
26.
Abstract: A generally applicable mathematical model, tailored for use on a Personal Computer, as the design tool to simulate unsteady flood flow in a network of interconnected rivers has been developed and applied to the Sham Chun River along the border between Hong Kong and the People's Republic of China. The effect on the improved Sham Chun River due to the proposed river training in the River Indus, one of its tributaries, is evaluated. The one-dimensional unsteady flow model itself is broadly applicable to a wide range of hydrologic conditions and varying field situations. It incorporates real hydraulic features including branched channels and 'tidal flats' flooding and drying. The time history of water stages and discharges are computed at any desired location throughout the connected essentially one-dimensional channel network subject to initial and boundary conditions. Details are given of techniques adapted in using the computational scheme to quantify the unsteady flow condition and the means to assure an efficient numerical solution as well as to verify the accuracy of the computed outputs by subjection to carefully selected schematic simplified test cases and real prototype cases with numerical solution method is a useful predictive engineering tool in river design.  相似文献   
27.
What are the effects of flame behavior of a number of fires burning in close proximity to one another? The results of measurements of burning rates, heat feedback, flame height, and flame trailing are reported for fires involving liquid pools.  相似文献   
28.
The Institute of Fire Safety Engineering Research and Technology Centre (FireSERT) at the University of Ulster in Northern Ireland has been a major provider of higher technical fire safety education, fire related research and services to industry for over 20 years. Its internationally recognised research spans the fields of fire dynamics, structural fire engineering, human behaviour in fire and more recently fire (CFD) modelling. Since 1991, it has offered the only PG Dip/MSc Fire Safety Engineering programme in the United Kingdom. In September 2000 FireSERT was the recipient of a £5.7 m Joint Infrastructure Fund (JIF) Award to build new fire safety engineering laboratories on the University's Jordanstown campus. These new facilities unparallel in the University sector in the UK offer greater scope and depth to the FireSERT's research portfolios. The new facilities will be extensively used in support of FireSERT's suite of MSc Fire Safety Engineering and DPhil programmes. This paper presents a brief history of FireSERT and its achievements, together with an update on the new facility, current research themes, taught programmes, research opportunities and visions for the future.  相似文献   
29.
An improved phenol-hypochlorite method for determining ammonia in water using nitroprusside as catalyst has been investigated. Unlike other recently published methods, the method is simple (and sufficiently sensitive for low-level analyses) and does not require accurate time intervals between reagents, not expensive equipment. The method is well suited for routine use.

Beer's Law was obeyed over the whole ammonia range investigated. Reproducibility was 1·3 per cent with 10 μg N, and 5·7 per cent with 1 μg N.  相似文献   

30.
Full- and quarter-scale experimental fires in corridor-room assemblies, using various combinations of floor, wall, and ceiling lining materials, yielded some interesting results. Between the small-and large-scale tests, there were marked differences in the relative effects of floor coverings and ceiling linings. Note: This paper is a contribution from the Division of Building Research, National Research Council of Canada and is published with the approval of the Director of the Division.  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