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   1391篇
  免费   96篇
电工技术   1篇
化学工业   509篇
金属工艺   25篇
机械仪表   30篇
建筑科学   58篇
矿业工程   1篇
能源动力   34篇
轻工业   300篇
水利工程   8篇
石油天然气   6篇
无线电   58篇
一般工业技术   140篇
冶金工业   162篇
原子能技术   2篇
自动化技术   153篇
  2023年   10篇
  2022年   77篇
  2021年   92篇
  2020年   39篇
  2019年   46篇
  2018年   45篇
  2017年   40篇
  2016年   53篇
  2015年   47篇
  2014年   53篇
  2013年   97篇
  2012年   85篇
  2011年   83篇
  2010年   73篇
  2009年   76篇
  2008年   83篇
  2007年   60篇
  2006年   57篇
  2005年   38篇
  2004年   39篇
  2003年   21篇
  2002年   23篇
  2001年   27篇
  2000年   13篇
  1999年   16篇
  1998年   17篇
  1997年   19篇
  1996年   16篇
  1995年   12篇
  1994年   9篇
  1993年   9篇
  1991年   4篇
  1990年   7篇
  1989年   6篇
  1988年   4篇
  1987年   5篇
  1986年   3篇
  1985年   3篇
  1984年   5篇
  1983年   3篇
  1980年   3篇
  1979年   5篇
  1977年   3篇
  1974年   5篇
  1972年   5篇
  1931年   5篇
  1930年   5篇
  1929年   5篇
  1926年   5篇
  1925年   11篇
排序方式: 共有1487条查询结果,搜索用时 15 毫秒
81.
Chemical signals modulate aggressive behavior in mice. For example, male urinary cues enhance aggression against other adults: a resident mouse attacks a male but not a castrated intruder, unless it is anointed with male urine. Our purpose was to understand whether molecules excreted with urine also act as aggression triggers in a different context. Therefore, the effect of urine, or molecules purified from urine, voided by different animals (males or females), was tested on the aggression of male mice against pups. Latency to the first attack, percentage of pups receiving the first attack, and percentage of attacked pups after 5 and 15 min were recorded. At variance with intermale aggression, male urinary chemosignals sprayed on pups reduced infanticide, while female urine did not. Male urine also delayed infanticide when compared to female urine. Pups anointed with low molecular weight dialyzed urine and with the high molecular weight protein fraction were attacked later than controls. Pups anointed with Major Urinary Proteins (MUPs) also were attacked later. The volatiles retained by MUPs act in the same way as adult male urine. MUPs and their ligands did not modify biting of food items. The results show that mice do not perceive male chemosignals as compulsory aggression triggers but rather can consistently and differentially shape their behavior in response to the same molecules according to different contextual events.  相似文献   
82.
Identification of potent human anti-IL-1RI antagonist antibodies   总被引:4,自引:0,他引:4  
Interleukin-1 (IL-1) blockade by IL-1 receptor antagonist benefits some arthritis patients by reducing joint damage. This fact inspired us to develop antagonist human therapeutic antibodies against IL-1R(I) using phage libraries that display single-chain variable fragment (scFv) antibody fragments. Panning libraries against human IL-1R(I) generated 39 unique scFv-phage whose binding to IL-1R(I) was competed by IL-1 ligands. Fifteen of these scFv-phage, identified using IL-1R(I)-binding assays and dissociation rate ranking, were reformatted as scFv-Fc and IgG(4) molecules. The ease of producing antibodies in the scFv-Fc format permitted rapid identification of four lead clones (C10, C13, C14, C15) that inhibit NF-kappaB nuclear translocation induced by IL-1. Reformatting these clones as IgG(4) molecules increased their inhibition potency by 相似文献   
83.
We present C2 quasi-interpolating schemes with tension properties. The B-splines like functions used in the quasi-interpolanting schemes are parametric cubic curves and their shape can be easily controlled via tension parameters which have an immediate geometric interpretation. Applications to the problem of approximation of curves with shape-constraints are discussed.  相似文献   
84.
Poly(ethylene terephthalate) (PET) is one of the most commonly employed polymers in the textile industry. Its relevance as a source of evidence in the reconstruction of criminal cases is nevertheless very limited because the properties and morphologies of fibers from different producers tend to be very similar. By integrating bands, obtained on single fibers by infrared (IR) microscopy, associated with trans and gauche conformation and to the O-H end-groups of the molecules, a method is proposed that can discriminate otherwise similar PET fibers. The absorbancies at 1370 and at 846 cm(-1) relative, respectively, to the gauche and trans conformation, were measured and ratioed. The end-group content was evaluated by ratioing the absorbancies of the signals at 3440 and at 874 cm(-1). Relative standard deviation (R. S. D.) was 1% for repetitive analyses on the same location of the same single fiber. Precision was reduced if the ratios were measured along the length of a single fiber (R. S. D. = 3%) and even further when different fibers of the same sample were examined (R. S. D. varied from 2 to 10%). This simple method can greatly enhance the evidential value of PET fibers by subclassifying them, thus helping the Court to better assess their significance.  相似文献   
85.
Lane  Terran  Brodley  Carla E. 《Machine Learning》2003,51(1):73-107
This paper introduces the computer security domain of anomaly detection and formulates it as a machine learning task on temporal sequence data. In this domain, the goal is to develop a model or profile of the normal working state of a system user and to detect anomalous conditions as long-term deviations from the expected behavior patterns. We introduce two approaches to this problem: one employing instance-based learning (IBL) and the other using hidden Markov models (HMMs). Though not suitable for a comprehensive security solution, both approaches achieve anomaly identification performance sufficient for a low-level focus of attention detector in a multitier security system. Further, we evaluate model scaling techniques for the two approaches: two clustering techniques for the IBL approach and variation of the number of hidden states for the HMM approach. We find that over both model classes and a wide range of model scales, there is no significant difference in performance at recognizing the profiled user. We take this invariance as evidence that, in this security domain, limited memory models (e.g., fixed-length instances or low-order Markov models) can learn only part of the user identity information in which we're interested and that substantially different models will be necessary if dramatic improvements in user-based anomaly detection are to be achieved.  相似文献   
86.
We report the NMR solution structure of (+)-CPI-indole (CPI, 1,2,8,8a-tetrahydrocyclopropa[c]pyrrolo[3,2-e]indol-4(5H)-one), an agent belonging to the CC-1065/duocarmycin family of antitumor compounds. This (+)-CPI-indole structure is covalently bound to d(G(1)ACTAATTGTC(11))-d(G(12)TCAATTAGTC(22)), a synthetic DNA duplex containing a high-affinity binding site. The three-dimensional structure has been determined by several cycles of restrained molecular dynamics calculations with a total of 563 NMR-derived constraints, both in vacuo and by using the generalized Born solvent continuum model. In-depth analysis of the structure of this ligand-DNA complex led to a detailed knowledge of the bound state conformation of the CPI-indole, the most simplified agent related to CC-1065 and duocarmycins, the parent members of a family of extremely potent antitumor compounds. Comparison of the CPI-indole bound conformation with those previously found for (+)-duocarmycin SA (DSA), its unnatural enantiomer (-)-DSA, and the demethoxylated analogue (+)-DSI in their DNA complexes provided additional evidence of the tight correlation between the catalytic effect exerted by DNA on the alkylation reaction and the extent of angular twist between the two planar heteroaromatic subunits of these agents. Additionally, comparison of the structural features of the DNA-bound state of a "naked" ligand, such as CPI-indole, with those of various other duocarmycin agents provided useful information for the interpretation of the observed effects on chemical reactivity of the different substitution patterns at the hemispheres of these types of complex.  相似文献   
87.
Intuitively, there should be a relationship between the size of the design fee for a transportation project and the quality of the resulting design. This study sought that relationship by looking at the fee expressed as a percentage of the construction cost and the final construction cost growth from the engineer’s initial estimate of the construction cost at the time the design contract was awarded. The research team analyzed 31 projects from the Oklahoma Turnpike Authority with a total construction value of $90 million. The projects were divided into road and bridge projects. Based on the results of the analysis, it seems that as the design fee decreases, the absolute percentage of construction cost growth from the engineer’s early estimate increases. The relationship is strongest for bridge projects, which tend to be more technically complex to design than roadway projects. This confirms for U.S. projects the result of an earlier study in Saudi Arabia. This paper concludes that the design fee should be viewed as an investment at a point in time where the ability to impact the project is the highest and can accrue the benefit of reduced cost growth.  相似文献   
88.
89.
Portugal is faced with important challenges concerning the definition of policies to achieve energy and environmental targets, taking also into account the economic and social issues. A multi-sectoral economy-energy-environment model has been developed to perform a prospective analysis of the changes in the economic structure and the energy system, as well as to assess the corresponding environmental impacts, providing decision support in policy making. This model is a multi-objective linear programming model that allows for the explicit consideration of distinct axes of evaluation, generally conflicting and non-commensurate, of the merit of distinct policies. The policy recommendations obtained are subject to the inherent uncertainty associated with the model coefficients and, therefore, they may not be robust in face of changes of the input data. The specification of less energy or carbon-intensive technologies is done by considering pollutant/energy coefficients defined as intervals. This analysis is crucial for understanding the role of technology in carbon mitigation efforts and other energy system planning settings, allowing to explore the effects of distinct policies on the total system costs, the fuel and technology mix, and the levels of greenhouse gases and other emissions.  相似文献   
90.
Agroecus griseus is a serious corn pest in Brazil. Contents of the dorsal abdominal glands (DAGs) in nymphs, and the metathoracic gland (MTG) in adults of this species were characterized and quantified. Compounds found were similar to those of other Pentatomidae species and included aliphatic hydrocarbons, aldehydes, oxo-alkenals, and esters. However, two compounds were found in the MTG that have not been described previously for this family. Mass spectrometry, infrared spectroscopy, and gas chromatographic analysis using coinjection with authentic standards confirmed the identities of the compounds as enantiopure (S)-2-methylbutyl acetate and 3-methyl-2-butenyl acetate. The five nymphal instars showed significantly different ratios of components, mainly between those of the first and later instars. No significant differences were detected in MTG contents between sexes. Gas chromatography (GC) analysis of aeration extracts of males and females showed the presence of a compound released exclusively by males. Gas chromatography – electroantennographic detection (GC-EAD) assays indicated that the male-specific compound is bioactive in females, suggesting the presence of an attractant pheromone. The mass spectrum and infrared data for this compound matched with methyl 2,6,10-trimethyltridecanoate, a sex pheromone component previously detected in the stink bugs, Euschistus heros and E. obscurus. The synthetic standard coeluted with the natural pheromone on two different GC stationary phases, confirming the proposed structure. Y-tube olfactometer assays showed that the synthetic standard was strongly attractive to females, and GC-EAD tests produced responses with antennae from females similar to those of the natural pheromone.  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