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   834篇
  免费   44篇
  国内免费   1篇
电工技术   6篇
化学工业   225篇
金属工艺   21篇
机械仪表   31篇
建筑科学   32篇
矿业工程   5篇
能源动力   39篇
轻工业   144篇
水利工程   4篇
石油天然气   5篇
无线电   45篇
一般工业技术   119篇
冶金工业   45篇
原子能技术   3篇
自动化技术   155篇
  2024年   3篇
  2023年   9篇
  2022年   29篇
  2021年   30篇
  2020年   26篇
  2019年   22篇
  2018年   32篇
  2017年   34篇
  2016年   34篇
  2015年   21篇
  2014年   43篇
  2013年   67篇
  2012年   63篇
  2011年   67篇
  2010年   36篇
  2009年   37篇
  2008年   51篇
  2007年   39篇
  2006年   32篇
  2005年   25篇
  2004年   18篇
  2003年   20篇
  2002年   12篇
  2001年   5篇
  2000年   4篇
  1999年   3篇
  1998年   6篇
  1997年   8篇
  1996年   3篇
  1995年   11篇
  1994年   8篇
  1993年   4篇
  1992年   4篇
  1991年   5篇
  1990年   5篇
  1989年   3篇
  1988年   5篇
  1986年   4篇
  1985年   3篇
  1984年   4篇
  1983年   7篇
  1981年   7篇
  1980年   5篇
  1979年   4篇
  1978年   5篇
  1976年   5篇
  1974年   3篇
  1971年   2篇
  1958年   1篇
  1955年   2篇
排序方式: 共有879条查询结果,搜索用时 0 毫秒
1.
Polyamines are ubiquitous, low-molecular-weight aliphatic compounds, present in living organisms and essential for cell growth and differentiation. Copper amine oxidases (CuAOs) oxidize polyamines to aminoaldehydes releasing ammonium and hydrogen peroxide, which participates in the complex network of reactive oxygen species acting as signaling molecules involved in responses to biotic and abiotic stresses. CuAOs have been identified and characterized in different plant species, but the most extensive study on a CuAO gene family has been carried out in Arabidopsis thaliana. Growing attention has been devoted in the last years to the investigation of the CuAO expression pattern during development and in response to an array of stress and stress-related hormones, events in which recent studies have highlighted CuAOs to play a key role by modulation of a multilevel phenotypic plasticity expression. In this review, the attention will be focused on the involvement of different AtCuAOs in the IAA/JA/ABA signal transduction pathways which mediate stress-induced phenotypic plasticity events.  相似文献   
2.
3.
Oxygenated fuels increase fuel consumption due to their low enthalpy of combustion; however, their high antiknock index renders them suitable for use in engines with a high compression rate, increasing their thermal yield. This study evaluated the performance of biorenewable oxygenated fuels (ethanol and isoamyl alcohol) and partially renewable fuels (ETBE: ethyl tert-butyl ether, TAEE: tert-amyl ethyl ether and di-TAE: di-tert-amyl ether) with high degree of purity and in mixtures with automotive gasoline, based on tests with Otto cycle engines. Among the oxygenated fuels evaluated here, di-TAE was found to present the best characteristics of performance, both individually and in mixtures with gasoline.  相似文献   
4.
5.
Today middleware is much more powerful, more reliable and faster than it used to be. Nevertheless, for the application developer, the complexity of using middleware platforms has increased accordingly. The volume and variety of application contexts that current middleware technologies have to support require that developers be able to anticipate the widest possible range of execution environments, desired and undesired effects of different programming strategies, handling procedures for runtime errors, and so on. This paper shows how a generic framework designed to evaluate the usability of notations (the Cognitive Dimensions of Notations Framework, or CDN) has been instantiated and used to analyze the cognitive challenges involved in adapting middleware platforms. This human-centric perspective allowed us to achieve novel results compared to existing middleware evaluation research, typically centered around system performance metrics. The focus of our study is on the process of adapting middleware implementations, rather than in the end product of this activity. Our main contributions are twofold. First, we describe a qualitative CDN-based method to analyze the cognitive effort made by programmers while adapting middleware implementations. And second, we show how two platforms designed for flexibility have been compared, suggesting that certain programming language design features might be particularly helpful for developers.  相似文献   
6.
    
Zusammenfassung Zur Bestimmung biogener Amine aus Schlachtgeflügel wird ein einfach durchzuführendes Analysenverfahren beschrieben. Die Amine wurden mit 0,6m-Perchlorsäure extrahiert, nach Dansylierung auf einer RP-18 Säule aufgetrennt und bei 254 nm detektiert. Innerhalb von 7 min konnte eine gute Auftrennung von 8 Aminen erreicht werden. Zur quantitativen Erfassung von Spermin und Spermidin mußte der Extrakt zusätzlich über Amberlite CG 50 gereinigt werden. Die Wiederfindungsraten betrugen 82% bis 96%, die Nachweisgrenzen 0,2 bis 0,5 g pro g Hühnerhaut. Bei Schlachtgeflügel eignen sich Putrescin und Cadaverin gut als Leitsubstanzen für den einsetzenden Verderb, da sie bereits ab Gesamtkoloniezahlen von etwa 105 pro cm2 nachweisbar sind, und ihre Konzentration mit zunehmendem Verderb rasch ansteigen.
Determination of biogenic amines by RP-HPLC for monitoring microbial spoilage of poultry
Summary A simple high-performance liquid chromatographic (HPLC) analysis is described for the determination of biogenic amines in broiler carcasses. The clean-up procedure consists of an extraction with 0.6M-perchloric acid, formation of dansyl derivatives, separation by a RP-18 column and UV detection at 254 nm. Within 7 min eight amines could be separated. The quantitative determination of spermidine and spermine requires an additional ion-exchange clean-up with Amberlite CG 50 after the extraction. This procedure gives recoveries of 82%–96% with detection limits of 0.2–0.5 g/g of broiler skin. Putrescine and cadaverine are good indicators for the onset of spoilage of poultry carcasses, since both amines could be detected at total colony counts of 105 cfu/cm2 and their concentration increases rapidly with advancing decomposition.
  相似文献   
7.
Considering the high levels of materials used in the fields of electronics and energy storage systems, it is increasingly necessary to take into consideration environmental impact. Thus, it is important to develop devices based on environmentally friendlier materials and/or processes, such as additive manufacturing techniques. In this work, poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are prepared by direct-ink-writing (DIW) by varying solvent evaporation temperature and fill density percentage. Different morphologies for both polymers are obtained, including dense films and porous membranes, as well as different electroactive β-phase content, thermal and mechanical properties. The dielectric constant and piezoelectric d33 coefficient for dense films reaches up to 16 at 1 kHz and 4 pC N−1, respectively for PVDF-HFP with a fill density of 80 and a solvent evaporation temperature of 50 °C. Porous structures are developed for battery separator membranes in lithium-ion batteries, with a highest ionic conductivity value of 3.8 mS cm−1 for the PVDF-HFP sample prepared with a fill density of 100 and a solvent evaporation temperature of 25 °C, the sample showing an excellent cycling performance. It is demonstrated that electroactive films and membranes can be prepared by direct-ink writing suitable for sensors/actuators and energy storage systems.  相似文献   
8.
It is known that nonadditive quantum codes can have higher code dimensions than stabilizer codes for the same length and minimum distance. The class of codeword stabilized codes (CWS) provides tools to obtain new nonadditive quantum codes by reducing the problem to finding nonlinear classical codes. In this work, we establish some results on the kind of non-Pauli operators that can be used as observables in the decoding scheme of CWS codes and propose a procedure to obtain those observables.  相似文献   
9.
Cu(In,Ga)Se2-based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se2 solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se2 does not depend on the Cu content in the explored composition range, while the absorption coefficient value is primarily determined by the Cu content. An expression for the absorption spectrum is proposed, with Ga and Cu compositions as parameters. This set of parameters allows accurate device simulations to understand remaining absorption and carrier collection losses and develop strategies to improve performances.  相似文献   
10.
In this article we discuss artificial neural networks‐based fault detection and isolation (FDI) applications for robotic manipulators. The artificial neural networks (ANNs) are used for both residual generation and residual analysis. A multilayer perceptron (MLP) is employed to reproduce the dynamics of the robotic manipulator. Its outputs are compared with actual position and velocity measurements, generating the so‐called residual vector. The residuals, when properly analyzed, provides an indication of the status of the robot (normal or faulty operation). Three ANNs architectures are employed in the residual analysis. The first is a radial basis function network (RBFN) which uses the residuals of position and velocity to perform fault identification. The second is again an RBFN, except that it uses only the velocity residuals. The third is an MLP which also performs fault identification utilizing only the velocity residuals. The MLP is trained with the classical back‐propagation algorithm and the RBFN is trained with a Kohonen self‐organizing map (KSOM). We validate the concepts discussed in a thorough simulation study of a Puma 560 and with experimental results with a 3‐joint planar manipulator. © 2001 John Wiley & Sons, Inc.  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