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91.
The design of efficient and general methods for the selective oxyfunctionalization of unactivated carbon-hydrogen bonds continues to represent a major challenge for the community of chemists, despite the fact that the oxidation of alkanes is a major feature of the chemical economy. A low level of selectivity is characteristic of large-scale oxidation of hydrocarbons performed under customary industrial oxidizing conditions (e.g., the catalytic air oxidation of cycloalkanes); in these processes, selectivity is difficult to control, because they are often impacted by the usual problems associated with free-radical chain reactions. Thus, in the last decades much work has been devoted to the search for general methods of selective oxidation that could be applied to a variety of satured hydrocarbons. In this context, just a few leading methods appear encouraging at the present time. This Account addresses a new approach developed in our laboratory, consisting in the application of isolated dioxiranes, a class of powerful yet selective oxidants. We contend that the method shows promise to contribute resolution of a well-recognized general problem in the existing chemistry of alkanes, that is, to achieve efficient oxyfunctionalizations with high selectivity for simple as well as structurally complex targets. 相似文献
92.
João Pombo Jorge Ambrósio Manuel Pereira Roger Lewis Rob Dwyer-Joyce Caterina Ariaudo Naim Kuka 《Multibody System Dynamics》2010,24(3):347-366
The wear evolution of railway wheels is a very important issue in railway engineering. In the past, the reprofiling intervals
of railway vehicle steel wheels have been scheduled according to designers’ experience. Today, more reliable and accurate
tools in predicting wheel wear evolution and wheelset lifetime can be used in order to achieve economical and safety benefits.
In this work, a computational tool that is able to predict the evolution of the wheel profiles for a given railway system,
as a function of the distance run, is presented. The strategy adopted consists of using a commercial multibody software to
study the railway dynamic problem and a purpose-built code for managing its pre- and post-processing data in order to compute
the wear. The tool is applied here to realistic operation scenarios in order to assess the effect of some service conditions
on the wheel wear progression. 相似文献
93.
Two models concerning downward comparison are motivational; they predict that when people are unhappy, they make downward comparisons to self-enhance (e.g., Wills, 1981). In contrast, the affect-cognition priming model (Wheeler & Miyake, 1992) predicts that unhappy people make upward comparisons because negative affect makes mood-congruent comparisons more accessible. The authors propose that both motivational and accessibility factors influence social comparisons. A study of undergraduates' self-recorded everyday comparisons supported this view. In addition, results (a) pointed to motivational influences other than self-enhancement and an accessibility influence other than mood-congruent priming, (b) suggested that motivated and unintended comparisons may differ somewhat in their susceptibility to motivational and accessibility influences, and (c) identified challenges to both self-enhancement and priming models. (PsycINFO Database Record (c) 2011 APA, all rights reserved) 相似文献
94.
Simple and efficient methods for the synthesis of lipidic amino alcohols and diamines are described in this paper. Lipidic
2-amino alcohols and 1,3-diamines can be synthesized starting from synthetic lipidic α-amino acids. Alternatively, commercially
available lipidic 1,2-diols may be used as starting material for the synthesis of 2-amino alcohols. Initial experiments on
the in vivo antiinflammatory activity of the compounds synthesized gave promising results. 相似文献
95.
Forty lactating Comisana ewes were either exposed to or protected from solar radiation and fed either in the morning or afternoon during summer in a Mediterranean climate. Individual milk samples were taken on days 7, 21 and 42 of the study period to determine fatty acid composition by gas chromatography. Exposure to solar radiation resulted in higher proportions of short-chain and saturated fatty acids in milk, primarily because of increased contents of caproic, capric, lauric, myristic and stearic acids (by 3-18%), and decreased contents of oleic, linoleic and linolenic acids (by 2-9%). As a consequence, the long to short chain and the unsaturated to saturated fatty acid ratios were significantly higher by 4 and 13% respectively in the milk of the protected ewes compared with that of the exposed animals. Provision of shade also led to an increase in the 18:0+18:1 to 16:0 ratio, and to a decrease in the 12:0 + 14:0 + 16:0 fatty acid group, which are regarded as reliable indexes of the nutritional property of dietary fat in reducing cholesterol levels in human plasma. Feeding time had little impact on milk fat. Our findings suggest that high ambient temperature may markedly modify the lipid composition of ewe milk and that provision of shade, but not feeding management, can improve the milk fatty acid profile in dairy sheep raised in hot climates. 相似文献
96.
Drug Delivery: Thrombin‐Responsive Transcutaneous Patch for Auto‐Anticoagulant Regulation (Adv. Mater. 4/2017) 下载免费PDF全文
97.
Predicting categorical forest variables using an improved k-Nearest Neighbour estimator and Landsat imagery 总被引:2,自引:0,他引:2
Erkki O. Tomppo Caterina Gagliano Flora De Natale Matti Katila Ronald E. McRoberts 《Remote sensing of environment》2009,113(3):500-3174
The k-Nearest Neighbour (k-NN) estimation and prediction technique is widely used to produce pixel-level predictions and areal estimates of continuous forest variables such as area and volume, often by sub-categories such as species. An advantage of k-NN is that the same parameters (e.g., k-value, distance metric, weight vector for the feature space variables) can be used for all variables, whether continuous or categorical. An obvious question is the degree to which accuracy can be improved if the k-NN estimation parameters are tailored for specific variable groups such as volumes by tree species or categorical variables. We investigated prediction of categorical forest attribute variables from satellite image spectral data using k-NN with optimisation of the weight vector for the ancillary variables obtained using a genetic algorithm. We tested several genetic algorithm fitness functions, all derived from well-known accuracy measures. For a Finnish test site, the categorical forest attribute variables were site fertility and tree species dominance, and for an Italian test site, the variables were forest type and conifer/broad-leaved dominance. The results for both test sites were validated using independent data sets. Our results indicate that use of the genetic algorithm to optimize the weight vector for prediction of a single forest attribute variable had a slight positive effect on the prediction accuracies for other variables. Errors can be further decreased if the optimisation is done by variable groups. 相似文献
98.
In this paper, we address the challenge of overlay topology design by considering which overlay topology best minimizes cost function, taking into account overlay link creation cost and routing cost. First, we formulate the problem as Integer Linear Programming (ILP) given a traffic matrix and assuming cooperative behavior of nodes. Then, we propose some heuristics to find near-optimal overlay topologies with a reduced complexity. The solutions to the ILP problem on real network topologies have been analyzed, showing that the traffic demands between the nodes affect the decision to create new overlay links. Next, the obtained optimal and near-optimal overlay topologies are thoroughly analyzed and the heuristics are compared through extensive numerical evaluations. Finally guidelines for the selection of the best heuristic as a function of the cost parameters are also provided. 相似文献
99.
2002年夏天,卡泰丽娜·费克(Caterina Fake)、斯图尔特·巴特菲尔德(StewartButterfield)和杰森·克拉森(Jason Classon)创办了网络软件公司Ludicorp。Ludicorp公司的第一款产品是永不结束的游戏(Game Neverending),这是一种大型的多人在线角色扮演游戏。在2004年,他们为原来的游戏增加了一个新功能,就是即时交换照片功能的多人聊天室,正是这项新的功能,使得这款游戏迅速火爆起来,新加功能的受欢迎程度甚至超过了游戏本身。此时,整个开发团队意识到了其间的价值,于是他们决定抓住永不结束的游戏这个机会,在其基础上开发一款新的照片交换社区网站,网站的名字就叫做Flickr。果然,Flickr迅速崛起,成为最著名的Web2.0公司之一。尽管它在2005年3月被雅虎公司所收购,但卡泰丽娜·费克的创业激情永远不会停止。 相似文献
100.
How to deal with strangers efficiently is a big challenge for designing P2P systems, as ineffective stranger policies will cause the degradation of system performance and fairness. In this work, we focus on an open P2P file-sharing environment with reciprocity incentive mechanisms, and we evaluate the impact of different stranger policies to system performance and fairness using both numerical analyses and agent-based simulations. The results reveal that there exist tradeoffs between system performance and fairness; the optimal performance and fairness of the system cannot be reached simultaneously; free-riders will survive when system performance is optimal. The accuracy of this evaluation is verified through the analysis of stranger policies in BitTorrent and eMule/eDonkey. Therefore, this evaluation can effectively help P2P designers select appropriate stranger policies according to their individual design goals. 相似文献