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   458363篇
  免费   8239篇
  国内免费   1483篇
电工技术   8350篇
综合类   418篇
化学工业   72191篇
金属工艺   16090篇
机械仪表   14928篇
建筑科学   12709篇
矿业工程   888篇
能源动力   13166篇
轻工业   47392篇
水利工程   3509篇
石油天然气   1920篇
武器工业   15篇
无线电   62511篇
一般工业技术   86213篇
冶金工业   78220篇
原子能技术   6142篇
自动化技术   43423篇
  2022年   2332篇
  2021年   4368篇
  2020年   3196篇
  2019年   3573篇
  2018年   5219篇
  2017年   5272篇
  2016年   5831篇
  2015年   4662篇
  2014年   7711篇
  2013年   22091篇
  2012年   13249篇
  2011年   18282篇
  2010年   13891篇
  2009年   15564篇
  2008年   16390篇
  2007年   16134篇
  2006年   14510篇
  2005年   13420篇
  2004年   12856篇
  2003年   12548篇
  2002年   12028篇
  2001年   12047篇
  2000年   11074篇
  1999年   11543篇
  1998年   25337篇
  1997年   18425篇
  1996年   14587篇
  1995年   11425篇
  1994年   10144篇
  1993年   9679篇
  1992年   7350篇
  1991年   6990篇
  1990年   6591篇
  1989年   6306篇
  1988年   6024篇
  1987年   5035篇
  1986年   4940篇
  1985年   6069篇
  1984年   5666篇
  1983年   4883篇
  1982年   4523篇
  1981年   4475篇
  1980年   4242篇
  1979年   4099篇
  1978年   3796篇
  1977年   4578篇
  1976年   6154篇
  1975年   3127篇
  1974年   3026篇
  1973年   2908篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
在韩国,网络购物(InternetShopping)的环境非常发达。因此,在这种竞争之下,传统的售货柜台不仅要担负展陈商品的基本功能,而且开始更加重视营造消费场所给消费者带来的空间体验,以此提升柜台销售的竞争力。正是在这样的背景之下,SSAMZIE公司策划了“我喜欢草莓(Ilikedalki)”的  相似文献   
53.
A steady-state model for substrate removal in a rotating biological disc reactor is presented. The model considers the consumption of substrate by micro-organisms in the biofilm attached to the rotating disc, and mass transfer from the attached liquid film to the biofilm. A mass balance on substrate over the liquid in the trough provides an expression for effluent substrate in terms of the microbial kinetic constants, the geometry of the system and the operating conditions. In order to simplify the solution of the equations in the model, first order kinetics are assumed for the rate of microbial growth and substrate utilization. This simplified model can be solved, for example, by using a programmable calculator. The model predicts that the fractional removal of substrate per stage is strongly dependent on the hydraulic loading rate per unit disc area but independent of feed substrate concentration. It predicts constant removal independent of disc size provided the hydraulic loading per unit area is kept constant. The rotational speed has only a slight predicted effect on the rate of substrate removal. The predictions of the model are compared with published data from the literature for both domestic wastewater and some industrial wastes. Trends observed in the field are predicted qualitatively by the model.  相似文献   
54.
Polycyclic aromatic hydrocarbons (PAHs) were determined in tissues of wild mussels (Mytilus galloprovincialis) from the Galicia coast (NW Spain) in order to assess the extent of the environmental impact caused by the Prestige oil spill (November 13, 2002). Three sampling campaigns were carried out in February, June and November 2003 at 24 stations along the Galicia coast, from La Guardia (Pontevedra) to Ribadeo (Lugo). The spatial distribution of PAHs found in the first sampling period, clearly revealed the central area (Costa da Morte) as the most affected by the oil spill. In these stations, concentrations up to 7780 microg/kg dw of the sum of 13 parent PAHs were found 2-3 months after the spill. Molecular parameters within the aliphatic and aromatic fractions confirmed the presence of the Prestige oil in these samples. The levels markedly decreased at most of the stations in the second sampling and recovered to levels found before the spill in November 2003, 1 year after the accident (29-279 microg/kg dw, av. 133+/-83 microg/kg dw). However, a certain increase was observed in some sites which could be related to the remobilization of oil residues from still unclean intertidal spots or sediments due to the winter marine weather conditions.  相似文献   
55.
High strength concretes, in particular at early ages (more than 40 MPa at 3 days and up to 80 MPa at 7 days), are obtained by replacement of certain granular size ranges of sand or gravel by cement-active artificial aggregates in a given concrete composition elaborated with a local supply of aggregates and cement. The strengthening of the paste-aggregate bond by chemical reactions between the artificial aggregates and the cement also involves an improvement of other concrete performances and not only of mechanical strengths. The cracking ability is decreased, the wear strength is higher, and water permeability is low or even nil for certain compositions. The laboratory results are confirmed by tests on the site and show the possibility of achieving high strength concretes without any particular selection of cement and natural aggregates.  相似文献   
56.
Mixed microbial films were grown on the inner walls of a tubular reactor with recirculation of the reactor contents and continuous flow-through of nutrient solution. The loss of total oxidized nitrogen was correlated to the film population, the nitrite concentration and the dissolved oxygen concentration in the reactor. When film population was greater than 0.5 × 109 cells cm−2, reactor dissolved oxygen concentration greater than 1 mg l−1 had little effect on the nitrate loss. Nitrate loss declined for film populations greater than 2 × 109 cells cm−2. Models based on Monod and zero-order microbial kinetics were calibrated using these data and a nonlinear least squares method. There was little difference in the residual errors with these methods.  相似文献   
57.
Until some thirty years ago tunnelling in southern Africa for civil engineering purposes had been on a relatively small scale and of a sporadic nature. The first major tunnel to be built in the region was the 82 km long Orange-Fish Tunnel. Since 1970 more than 175 tunnels with an aggregate length of just over 400 km have been built. Much of the first phase of the Lesotho Highlands Water Project, which will have more than 100 km of tunnels, is about to be completed. The last thirty years represent the busiest period of tunnelling by civil engineers that southern Africa has ever known. The paper starts by summarising the early history of tunnelling in the region, and goes onto describe recent and current tunnelling projects. It also gives an indication of the possible demand for tunnels in the future.  相似文献   
58.
The decomposition of ClO2 and ClO2 by u.v. radiation leads to the production of chlorate, chloride and oxygen as end-products via complex reactions which are initiated by the products generated by the primary reactions of photolysis (Buxton and Subhani, 1972a; Mialocq et al., 1973; Karpel Vel Leitner et al., 1992). As far as the rate of decomposition is concerned, Bowen and Cheung (1932) and Zika et al. (1985) have shown that the quantum yield of photodecomposition of chlorine dioxide (overall reaction) increases when the wavelength decreases [Zika et al. (1985): φ = 0.46 at 366 nm and 1.4 at 296.7 nm]. However, the values of the quantum yield of photodecomposition of ClO2 and ClO2 at 253.7 nm as well as the quantum yields for the primary reactions of photolysis of ClO2 and ClO2 at different wavelengths are not given in the literature.The aim of this work was to study the kinetics of photodecomposition of chlorine dioxide and of chlorite by u.v. irradiation.  相似文献   
59.
High radon concentration (average 410 kBq m-3) has been measured in a tunnel of a uranium mine, located 15-55 m below the village of Kovágószolos, Hungary. The mine was closed in 1997; the artificial ventilation of the tunnel was then terminated and recultivation works begun. In this paper, a study has been made as to whether the tunnel has an influence on the radon concentration of surface dwellings over the mining tunnel. At different distances from the surface projection of the mining tunnel, radon concentration, the gamma dose, radon exhalation and radon concentration of soil gas were measured. The average radon concentration in the dwellings was 483 Bq m-3. Significantly higher radon concentrations (average 667 Bq m-3) were measured in houses within +/-150 m from the surface projection of the mining tunnel +50 m, compared with the houses further than the 300-m belt (average 291 Bq m-3). The average radon concentration of the soil gas was 88.8 kBq m-3, the average radon exhalation was 71.4 Bq m-2 s-1 and higher values were measured over the passage as well. Frequent fissures crossing the passage and running up to the surface and the high radon concentration generated in the passage (average 410 kBq m-3) may influence the radon concentration of the houses over the mining tunnel.  相似文献   
60.
When quantifying inequality in health, the excess fraction (i.e. the proportional reduction in the outcome in question that would occur if all groups had the same, lowest, risk rate) is an appealing measure. If the population contains a natural, healthiest reference group, the excess fraction is straightforward to estimate. In the absence of an a priori reference group, calculating the excess fraction requires reflection, since using the group with the lowest observed risk rate as reference can lead to erroneous conclusions. This paper proposes a simple simulation procedure for calculating the excess fraction, without fixing a reference group.  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