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   1241篇
  免费   3篇
  国内免费   9篇
电工技术   8篇
综合类   4篇
化学工业   127篇
金属工艺   2篇
机械仪表   88篇
建筑科学   59篇
矿业工程   20篇
轻工业   304篇
水利工程   15篇
石油天然气   38篇
武器工业   2篇
无线电   77篇
一般工业技术   446篇
冶金工业   8篇
原子能技术   1篇
自动化技术   54篇
  2024年   1篇
  2023年   1篇
  2022年   2篇
  2021年   7篇
  2020年   1篇
  2018年   1篇
  2014年   1篇
  2013年   4篇
  2012年   101篇
  2011年   128篇
  2010年   35篇
  2009年   13篇
  2008年   88篇
  2007年   99篇
  2006年   83篇
  2005年   62篇
  2004年   72篇
  2003年   60篇
  2002年   73篇
  2001年   62篇
  2000年   56篇
  1999年   29篇
  1998年   30篇
  1997年   28篇
  1996年   26篇
  1995年   30篇
  1994年   27篇
  1993年   17篇
  1992年   16篇
  1991年   16篇
  1990年   15篇
  1989年   12篇
  1988年   3篇
  1987年   2篇
  1986年   6篇
  1985年   9篇
  1984年   8篇
  1983年   9篇
  1982年   4篇
  1981年   3篇
  1980年   3篇
  1979年   2篇
  1978年   3篇
  1972年   1篇
  1968年   1篇
  1966年   1篇
  1964年   1篇
  1947年   1篇
排序方式: 共有1253条查询结果,搜索用时 15 毫秒
1.
2.
3.
1 前言 西克利夫兰LTV钢公司的直接热装料成套设备于1993年9月7日投入使用。该设备年生产能力为2百万吨钢,由MANNESMAN DEMAG设计安装,安装期历时21个月。该连铸机为双流扁坯机,使用顶部插入引锭杆,快速变化传感器和电脑控制切割器和收料台。扁坯在现代化80英寸高温带材轧机上经一小时被  相似文献   
4.
李振民 《大氮肥》1997,20(6):406-408
针对转化系统的复杂反应机理,利用神经网络技术建立了操作优化模型。该模型是以BP网络为基础的多层网络,用实际操作数据对该模型进行了训练,获得了较好的过程模拟效果,该模型可用于大型氨厂转化系统操作分析。  相似文献   
5.
横截面为椭圆形的钢包一直是耐火材料过去三十余年中存在的一个问题。设计者之所以采用这种形状是因为耳轴的扁平部分可以提高钢包的容积,并不需改变昂贵的现有吊车悬梁。同时,椭圆形钢包又适合于熔炼车间和连铸机的建筑学结构(即.高架梁、塔等)。然而,这种钢包形状却可造成耐火材料工作衬结构减弱,同时又可能产生钢水渗入钢包里衬和烧穿包壳的问题。  相似文献   
6.
Purposes of these studies were first; to determine whether or not Calcium(II)(3) (3,5- diisopropylsalicylate)(6)(H(2)O)(6) [Ca(II)(3)(3,5-DIPS)(6)], a lipophilic calcium complex, could decrease activated-platelet aggregation, and second; to determine whether or not it is plausible that Ca(II)(3)(3,5-DIPS)(6) decreases activated-platelet aggregation by facilitating the synthesis of Nitric Oxide (NO) by Nitric Oxide Synthase (NOS). The influence of Ca(II)(3)(3,5-DIPS)(6) on the initial rate of activated-platelet aggregation was determined by measuring the decrease in rate of increase in transmission at 550 nm for a suspension of Thrombin-CaCl(2) activated platelets following the addition of 0, 50, 100, 250, or 500 muM Ca(II)(3)(3,5-DIPS)(6). To establish that the Ca(lI)(3)(3,5- DIPS)(6)-mediated decrease in aggregation was due to activation of NOS, the effect of L-NMMA, an inhibitor of NOS, on the inhibition of platelet aggregation by Ca(II)(3)(3,5-DIPS)(6) was determined using a suspension of activated platelets contaimng 0 or 250 muM Ca(II)(3)(3,5-DIPS)(6) without or with 1 mM L-NMMA. An in vitro Bovine Brain NOS reaction mixture, containing CaCl(2) for the activation of Phosphodiesterase-3' ,5'-Cyclic Nucleotide Activator required for the activation of NOS, was used to determine whether or not Ca(II)(3)(3,5-DIPS)(6) could be used as a substitute for the addition of Ca. The decrease in absorbance at 340 nm, lambda maximum for NADPH, was measured to determine NOS activity following the addition of NOS to the complete reaction mixture containing either CaCl(2), Ca(II)(3)(3,5-DIPS)(6), or neither Ca compound. Increasing the concentration of Ca(II)(3)(3,5-DIPS)(6) caused a concentration related decrease in activated platelet aggregation. The addition of L-NMMA to activated platelets, in the absence of Ca(II)(3)(3,5-DIPS)(6), caused a 129% increase in initial rate of platelet aggregation. The initial rate of platelet aggregation decreased 74% with the addition of 250 muM Ca(II)(3)(3,5-DIPS)(6) and the addition of L-NMMA plus 250 muM Ca(II)(3)(3,5-DIPS)(6) caused a 197% decrease in initial rate of aggregation compared to the initial rate observed width the presence of 1 mM L-NMMA alone. There was only a small, 27%, increase in initial rate of 0.4 mM NADPH oxidation when 0.9 mM CaCl(2) was added to the NOS reaction mixture in comparison to the initial rate of NADPH oxidation with no addition of CaCI(2). Addition of an equivalent amount of Ca in the form of Ca(II)(3)(3,5-DIPS)(6), 333 muM, caused a 37% increase in initial rate of NADPH oxidation compared to the addition of 0.9 mM CaCl(2). Addition of increasing concentrations of L-NMMA plus 0.9 mM CaCl(2) or 333 muM Ca(II)(3)(3,5-DIPS)(6) to the NOS reaction mixture caused a concentration related increase in initial rate of NADPH oxidation. Addition of L-NMMA while expected to decrease NADPH oxidation actually increased the rate of NADPH oxidation. Additions of 133 muM or 267 muM Ca(II)(3)(3,5- DIPS)(6) also caused concentration related increases in initial rate of NADPH oxidation in the presence of 113 muM L-NMMA. However, the addition of 533 muM Ca(II)(3)(3,5-DIPS)(6) caused a dramatic decrease in initial rate of NADPH oxidation by NOS. It is concluded that: 1) Ca(II)(3)(3,5- DIPS)(6) activates platelet NOS in preventing platelet aggregation, 2) in vitro NOS activity can be observed spectrophotometrically by following the consumption of NADPH as a decrease in absorbance at 340 nm, 3) Ca(II)(3)(3,5-DIPS)(6) plays a role in enhancing Bovine Brain NOS activity resulting in an increased rate of NADPH oxidation by NOS, 4) Ca(II)(3)(3,5-DIPS)(6) is a useful form of Ca in activating NOS and superior to CaCl(2) with regard to the facilitation of a NADPH oxidation, and 5) L-NMMA stimulates Bovine Brain NOS activity rather than causing an inhibition of this enzyme and must serve as a reducible substrate for Bovine Brain NOS.  相似文献   
7.
Why Phenolic Acids Are Unlikely Primary Allelochemicals in Rice   总被引:21,自引:0,他引:21  
Allelopathy in rice (Oryza sativa, L.) effective against weeds has been found in about 3.5% of tested rice germplasm in both laboratory and field experimentation. However, the allelochemicals responsible for growth inhibition of rice-associated weeds have not yet been identified. In the literature, phenolic acids are often mentioned as putative allelochemicals. If phenolic acids commonly reach growth inhibitory concentrations in rice ecosystems, it must be expected that the degree of tolerance to phenolic acids will vary among traditional rice cultivars or plant species adapted to rice environments having inherently different phenolic acid concentrations. Phenolic acids concentrations are normally greater in submerged than in aerobic soils. A dose–response study, however, showed that seedlings of rice cultivars adapted to submerged anaerobic soils did not have higher level of tolerance against p-hydroxybenzoic acid than did seedlings of varieties adapted to aerobic upland soils. Moreover, traditional rice cultivars had no greater tolerance than did improved cultivars that were recently bred for traits other than tolerance of phenolic acids. Similarly, there were no differences in tolerance of p-hydroxybenzoic acid between two Echinochloa weed species adapted to either anaerobic or aerobic growth conditions. Thus, neither the rice cultivars nor weed species had evolved different tolerance levels against the phenolic acid. However, all rice cultivars had significantly greater tolerance of p-hydroxybenzoic acid than did either weed species. In a second experiment, the rates at which rice plants released phenolic acids into solution cultures were measured for at least one month, the time period of greatest allelopathic activity following planting under field conditions. The maximum release rate of phenolic acids during the first month of growth was approximately 10 g/plant/day. At a conventional plant density, the release rate of phenolic acids would be approximately 1 mg/m2day. This order of release rate cannot provide concentrations remotely close to phytotoxic levels determined for these rice cultivars and weed species. The results presented in this paper do not preclude the possibility that phenolic acids might be one component in a mixture of chemicals that, when present simultaneously, are allelopathic.  相似文献   
8.
The rising level of atmospheric CO2 has stimulated several recent studies attempting to predict the effects of increased CO2 on ecological communities. However, most of these studies have been conducted in the benign conditions of the laboratory and in the absence of herbivores. In the current study, we utilized large octagonal chambers, which enclosed portions of an intact scrub-oak community to investigate the interactive effects of CO2 and insect herbivory on myrtle oak, Quercus myrtifolia. Specifically, we assessed the effects of ambient and elevated CO2 (2x current concentrations) on percent foliar nitrogen, C:N ratio, total relative foliar tannin content, and the presence of leaf damage caused by leaf mining and leaf chewing insects that feed on myrtle oak. Total foliar N declined and C:N ratios increased significantly in oaks in elevated CO2 chambers. The percentages of leaves damaged by either leafminers or leaf chewers tended to be lower in elevated compared to ambient chambers, but they co-occurred on leaves less than expected, regardless of CO2 treatment. Leaves that had been either mined or chewed exhibited a similar wounding or defensive response; they had an average of 25 and 21% higher protein binding ability, which is correlated with tannin concentration, compared to nondamaged control leaves, respectively. While the protein-binding ability (expressed as total percent tannin) of leaves from elevated CO2 was slightly higher than from leaves grown in ambient chambers, this difference was not significant.  相似文献   
9.
Image cytometry is an important technique in affordable healthcare and cellular research. Some efforts toward establishing a personal, low-cost cytometer have been described in the literature. However, a self-assembled fluorescence microscope requires software for cytometric analysis. There are some open-source image-based software analysis applications available. However, for a quantitative analysis of images, software that can generate data comparable to those of previously evaluated cytometric analyses programs is required. Hence, the aim of this study is to compare results of a commercially available image cytometry program to data obtained using the open-source software CellProfiler (CP). Leukocytes and fluorescent bead images obtained using a Laser Scanning Cytometer were analyzed by CP and the results compared with those of conventional cytometric analyses' programs. Algorithms were developed enabling the analysis of leukocytes and beads by CP. CP provided similar results to those obtained by the cytometer software. Hallmark parameters, including cell count and fluorescence intensity, revealed a high correlation in the analysis of both programs. Therefore, CP is appropriate for cellular analysis on a self-assembled microscope, thereby enabling affordable cytometry.  相似文献   
10.
Knowledge of three-dimensional skeletal kinematics during functional activities such as walking, is required for accurate modelling of joint motion and loading, and is important in identifying the effects of injury and disease. For example, accurate measurement of joint kinematics is essential in understanding the pathogenesis of osteoarthritis and its symptoms and for developing strategies to alleviate joint pain. Bi-plane X-ray fluoroscopy has the capacity to accurately and non-invasively measure human joint motion in vivo. Joint kinematics obtained using bi-plane X-ray fluoroscopy will aid in the development of more complex musculoskeletal models, which may be used to assess joint function and disease and plan surgical interventions and post-operative rehabilitation strategies. At present, however, commercial C-arm systems constrain the motion of the subject within the imaging field of view, thus precluding recording of motions such as overground gait. These fluoroscopy systems also operate at low frame rates and therefore cannot accurately capture high-speed joint motion during tasks such as running and throwing. In the future, bi-plane fluoroscopy systems may include computer-controlled tracking for the measurement of joint kinematics over entire cycles of overground gait without constraining motion of the subject. High-speed cameras will facilitate measurement of high-impulse joint motions, and computationally efficient pose-estimation software may provide a fast and fully automated process for quantification of natural joint motion.  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