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   735篇
  免费   42篇
电工技术   13篇
综合类   6篇
化学工业   227篇
金属工艺   6篇
机械仪表   40篇
建筑科学   21篇
能源动力   23篇
轻工业   90篇
水利工程   5篇
石油天然气   1篇
无线电   33篇
一般工业技术   96篇
冶金工业   23篇
原子能技术   11篇
自动化技术   182篇
  2024年   1篇
  2023年   8篇
  2022年   35篇
  2021年   58篇
  2020年   25篇
  2019年   26篇
  2018年   21篇
  2017年   24篇
  2016年   37篇
  2015年   17篇
  2014年   31篇
  2013年   57篇
  2012年   59篇
  2011年   47篇
  2010年   40篇
  2009年   32篇
  2008年   30篇
  2007年   29篇
  2006年   23篇
  2005年   27篇
  2004年   14篇
  2003年   22篇
  2002年   21篇
  2001年   8篇
  2000年   9篇
  1999年   11篇
  1998年   4篇
  1997年   17篇
  1996年   8篇
  1995年   5篇
  1994年   5篇
  1993年   5篇
  1992年   2篇
  1991年   1篇
  1990年   1篇
  1987年   1篇
  1986年   2篇
  1984年   3篇
  1983年   3篇
  1982年   4篇
  1981年   1篇
  1980年   1篇
  1975年   1篇
  1971年   1篇
排序方式: 共有777条查询结果,搜索用时 156 毫秒
761.
BACKGROUND: Given the interest in the development of cultivation systems with low agronomic input and environmental impact, the aim of this study was to determine the influence of tillage system (conventional tillage (CT), two‐layer tillage (TT), surface tillage (ST) and minimum tillage (MT)) and nitrogen (N) fertilisation rate (0, 50 and 100 kg ha?1) on triticale grain and protein yields, chemical composition and nutritive value. RESULTS: There were no significant differences among tillage treatments in grain and protein yields. ST resulted in significantly higher crude protein (CP) and true soluble protein (TSP) contents as well as in vitro crude protein digestibility (CPD). Neutral detergent fibre concentration was significantly higher with ST than with MT, and this led to a small reduction (2 g kg?1 dry matter (DM)) in in vitro true DM digestibility (IVTDMD). N fertilisation significantly increased grain and protein yields as well as CP, non‐protein nitrogen and TSP contents and CPD. IVTDMD was significantly lower with 0 kg N ha?1 than with 50 and 100 kg N ha?1. CONCLUSION: Reducing tillage intensity improved the CP content and CPD of triticale grain. The application of 50 kg N ha?1 resulted in good grain quality parameters and grain and protein yields. Copyright © 2010 Society of Chemical Industry  相似文献   
762.
Hypogonadic subjects with insulin resistance (IR) showed different metabonomic profiles compared to normo-insulinemic subjects (IS). Testosterone replacement therapy (TRT) may have a different impact on the metabolisms of those with the presence or absence of insulin resistance. We evaluated the changes in the metabolism of IR hypogonadic patients before and after 60 days of TRT. The metabonomic plasma profiles from 20 IR hypogonadal patients were recorded using ultra-high-performance liquid chromatography (UHPLC) and high-resolution mass spectrometry (HRMS). Plasma metabolites, before and after 60 days of TRT, were compared. In hypogonadic patients, carnosine, which is important for improving performance during exercise, increased. Conversely, proline and lysine—amino acids involved in the synthesis of collagen—reduced. Triglycerides decreased and fatty acids (FFAs) increased in the blood as a consequence of reduced FFA β-oxidation. Glycolysis slightly improved, while the Krebs cycle was not activated. Gluconeogenesis (which is the main energy source for hypogonadal IR before TRT) stopped after treatment. As a consequence, lactate and acetyl CoA increased significantly. Both lactate and acetyl CoA were metabolized into ketone bodies which increased greatly, also due to leucine/isoleucine degradation. Ketone bodies were derived predominantly from acetyl CoA because the reaction of acetyl CoA into ketone bodies is catalyzed by mtHMGCoA synthase. This enzyme is inhibited by insulin, which is absent in IR patients but overexpressed following testosterone administration. Ketosis is an alternative route for energy supply and provides the same metabolic effects as insulin but at the metabolic or primitive control level, which bypasses the complex signaling pathway of insulin. After treatment, the hypogonadic patients showed clinical symptoms related to ketonuria. They presented similarly to those following a ketogenic diet, the so-called ‘keto flu’. This must be taken into account before the administration of TRT to hypogonadic patients.  相似文献   
763.
WiFi - short for "wireless fidelity" - is the commercial name for the 802.11 products that have flooded the corporate wireless local area network (WLAN) market and are becoming rapidly ingrained in our daily lives via public hotspots and digital home networks. Authentication and confidentiality are crucial issues for corporate WiFi use, but privacy and availability tend to dominate pervasive usage. However, because a technology's dependability requirements are proportional to its pervasiveness, newer applications mandate a deeper understanding of how much we can rely on WiFi and its security promises. In this article, we present an overview of WiFi vulnerabilities and investigate their proximate and ultimate origins. The intended goal is to provide a foundation to discuss WiFi dependability and its impact on current and future usage scenarios. Although a wireless network's overall security depends on the network stack to the application layer, this article focuses on specific vulnerabilities at the physical (PHY) and data (MAC) layers of 802.11 networks  相似文献   
764.
 In this paper we study the observability properties of nonlinear discrete-time systems. Two types of contributions are given. First we present observability criteria in terms of appropriate codistributions. For particular, but significant, classes of systems we provide criteria that require only a finite number of computations. Then we consider invertible systems (which includes discrete-time models obtained by sampling continuous-time systems) and prove that the weaker notion of forward–backward observability is equivalent to the stronger notion of (forward) observability. Date received: January 19, 2001. Date revised: May 14, 2002.  相似文献   
765.
A new criterion is established for global asymptotic stability of second-order systems modeled by equations of the type , where σ is the saturation function. The derivation is based on the Bendixon's theorem on limit cycles and a closer study of the trajectories of the systems. Applications to stabilization of more general cascade nonlinear systems are also discussed.  相似文献   
766.
Acute lymphoblastic leukemia type B (B-ALL) is the most common kind of pediatric leukemia, characterized by the clonal proliferation of type B lymphoid stem cells. Important progress in ALL treatments led to improvements in long-term survival; nevertheless, many adverse long-term consequences still concern the medical community. Molecular and cellular target therapies, together with immunotherapy, are promising strategies to overcome these concerns. Cannabinoids, enzymes involved in their metabolism, and cannabinoid receptors type 1 (CB1) and type 2 (CB2) constitute the endocannabinoid system, involved in inflammation, immune response, and cancer. CB2 receptor stimulation exerts anti-proliferative and anti-invasive effects in many tumors. In this study, we evaluated the effects of CB2 stimulation on B-ALL cell lines, SUP-B15, by RNA sequencing, Western blotting, and ELISA. We observe a lower expression of CB2 in SUP-B15 cells compared to lymphocytes from healthy subjects, hypothesizing its involvement in B-ALL pathogenesis. CB2 stimulation reduces the expression of CD9, SEC61G, TBX21, and TMSB4X genes involved in tumor growth and progression, and also negatively affects downstream intracellular pathways. Our findings suggest an antitumor role of CB2 stimulation in B-ALL, and highlight a functional correlation between CB2 receptors and specific anti-tumoral pathways, even though further investigations are needed.  相似文献   
767.
In the recent years a considerable effort has been devoted to foster the understanding of the basic mechanisms underlying the dynamical arrest that is involved in glass forming in supercooled liquids and in the sol-gel transition. The elucidation of the nature of such processes represents one of the most challenging unsolved problems in the field of material science. In this context, two important theories have contributed significantly to the interpretation of these phenomena: the Mode-Coupling theory (MCT) and the Percolation theory (PT). These theories are rooted on the two pillars of statistical physics, universality and scale laws, and their original formulations have been subsequently modified to account for the fundamental concepts of Energy Landscape (EL) and of the universality of the fragile to strong dynamical crossover (FSC). In this review, we discuss experimental and theoretical results, including Molecular Dynamics (MD) simulations, reported in the literature for colloidal and polymer systems displaying both glass and sol-gel transitions. Special focus is dedicated to the analysis of the interferences between these transitions and on the possible interplay between MCT and PT. By reviewing recent theoretical developments, we show that such interplay between sol-gel and glass transitions may be interpreted in terms of the extended F13 MCT model that describes these processes based on the presence of a glass-glass transition line terminating in an A3 cusp-like singularity (near which the logarithmic decay of the density correlator is observed). This transition line originates from the presence of two different amorphous structures, one generated by the inter-particle attraction and the other by the pure repulsion characteristic of hard spheres. We show here, combining literature results with some new results, that such a situation can be generated, and therefore experimentally studied, by considering colloidal-like particles interacting via a hard core plus an attractive square well potential. In the final part of this review, scaling laws associated both to MCT and PT are applied to describe, by means of these two theories, the specific viscoelastic properties of some systems.  相似文献   
768.
769.
In this paper we describe an efficient method for the synthesis of new heterocyclic systems: furo[2,3-c]-2,7-naphthyridines 6, as well as a new method for the preparation of 1,3-diamino-2,7-naphthyridines 11. For the first time, a Smiles rearrangement was carried out in the 2,7-naphthyridine series, thus gaining the opportunity to synthesize 1-amino-3-oxo-2,7-naphthyridines 4, which are the starting compounds for obtaining furo[2,3-c]-2,7-naphthyridines. The cyclization of alkoxyacetamides 9 proceeds via two different processes: the expected formation of furo[2,3-c]-2,7-naphthyridines 10 and the ‘unexpected’ formation of 1,3-diamino-2,7-naphthyridines 11 (via a Smiles type rearrangement).  相似文献   
770.
Thermal energy storage (TES) with phase change materials (PCMs) presents some advantages when shape-stabilization is performed with ceramic aerogels. These low-density and ultra-porous materials guarantee high energy density and can be easily regenerated through simple pyrolysis while accounting for moderate mechanical properties. However, the small pore size that typically characterizes these sorbents can hinder the crystallization of PCMs, slightly reducing the energy density of the stabilized compound. In this work, we present the use of polymer-derived mesoporous SiC and SiOC aerogels for the stabilization of polyethylene glycol and a fatty alcohol (PureTemp 23), having a melting temperature of 17 and 23°C, respectively. Their TES performances point out maximum thermal efficiency values of around 80%. These performances are discussed accounting for the results of thermogravimetric analysis, differential scanning calorimetry, and leaking tests.  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