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991.
Bubble-liquid turbulent flow has an excellent heat and mass transfer behaviors than single gas or liquid flow. In order to analyze the effects of normal and reduced gravity on cold bubble-liquid two-phase turbulent flow in bubble column a second-order moment cold bubble-liquid two-phase turbulent model was developed to disclose the bubble dispersion characteristics. Under the reduced gravity condition, volume fraction caused by the decrease of buoyance force is larger than normal gravity level due to bigger bubble solid volume. In addition, bubble frequency is also decreased by in decrease of buoyance force. Normal and shear stresses have strongly anisotropic characteristics at every directions and have larger values under normal gravity than reduced gravity. The liquid turbulent kinetic energy has the two-peak bimodal distribution and weaker than bubble turbulent kinetic energy with one peak unimodal, which is caused by vigorous wake fluctuations. The correlation of fluctuation velocities between bubble and liquid has clearly anisotropic behaviors Under reduced gravity, the bubble motion has a little impact on liquid turbulent flow caused by slight buoyancy force, however, it will greatly reduce the liquid turbulent intensity due to energy cascade transport, which was transformed into bubbles or dissipated by interface friction. Bubble formation and detachment mechanisms affected by gravity conditions lead to the different levels of bubble dispersion distributions. 相似文献
992.
轮胎空腔共振噪声对汽车NVH性能有决定性影响。为研究轮胎内部添加多孔材料的降噪机理,从轮胎力传递率出发,通过试验验证了轮胎力传递率与空腔噪声的一致性;建立了轮胎力传递率模型,并验证了模型的准确性;通过该模型研究了多孔材料对轮胎空腔共振噪声的降噪机理。结果表明:多孔材料对轮胎空腔共振降噪效果是耦合结构振动与声学降噪共同作用的结果,选择多孔材料时要综合考虑多孔材料的物理参数与声学参数,研究结果为改善汽车NVH性能和提高低噪声轮胎设计水平奠定理论基础和方法指导。 相似文献
993.
Isolated graphene nanoribbons(GNRs)usually have energy gaps,which scale with their widths,owing to the lateral quantum confinement effect of GNRs.The absence of metallic GNRs limits their applications in device interconnects or being one-dimensional physics platform to research amazing properties based on metallicity.A recent study published in Science provided a novel method to produce metallic GNRs by inserting a symmetric superlattice into other semiconductive GNRs.This finding will broader the applications of GNRs both in nanoelectronics and fundamental science. 相似文献
994.
Khaled?ElbassioniEmail author Amr?Elmasry Ibrahim?Kamel 《Distributed and Parallel Databases》2005,17(3):215-249
We consider the problem of indexing a set of objects moving in d-dimensional spaces along linear trajectories. A simple external-memory indexing scheme is proposed to efficiently answer
general range queries. The following are examples of the queries that can be answered by the proposed method: report all moving
objects that will (i) pass between two given points within a specified time interval; (ii) become within a given distance
from some or all of a given set of other moving objects. Our scheme is based on mapping the objects to a dual space, where
queries about moving objects are transformed into polyhedral queries concerning their speeds and initial locations. We then
present a simple method for answering such polyhedral queries, based on partitioning the space into disjoint regions and using
a B+-tree to index the points in each region. By appropriately selecting the boundaries of each region, we guarantee an average
search time that matches a known lower bound for the problem. Specifically, for a fixed d, if the coordinates of a given set of N points are statistically independent, the proposed technique answers polyhedral queries, on the average, in O((N/B)1−1/d⋅(log B N)1/d+K/B) I/O's using O(N/B) space, where B is the block size, and K is the number of reported points. Our approach is novel in that, while it provides a theoretical upper bound on the average
query time, it avoids the use of complicated data structures, making it an effective candidate for practical applications.
The proposed index is also dynamic in the sense that it allows object insertion and deletion in an amortized update cost of
log B(N) I/O's. Experimental results are presented to show the superiority of the proposed index over other methods based on R-trees.
recommend Ahmed Elmagarmid 相似文献
995.
996.
A bottom-up method for creating a hierarchy of entity-relationship diagrams (HERD) from a given, flat ER diagram (ERD) is proposed. The hierarchy consists of simple and interrelated diagrams—ER structures—with external relationships to other structures. The HERD-tree diagram, which provides the most general view of the conceptual schema, is located at the top of the hierarchy. The method is based on packaging operations, which group entities and relationships according to certain criteria. These operations are applied in several steps on a given (presumably large-scale) ERD. We describe the new constructs, which are added to the ER model to enable the creation of HERD, and a bottom-up method for creating HERD. We also evaluate HERD from the point of view of user comprehension and preference, based on an experimental comparison to flat ERDs.
相似文献
Peretz ShovalEmail: |
997.
This essay examines a politically engaged research genre, which follows the biography of the author who founded two journals: one on mathematical models published in English (Quality and Quantity) and one on politically committed social and economic research published in Italian (Inchiesta). The research considered focuses on Italy in the 1950s, the research by Lazarsfeld in Vienna in the 1920s and in the United States in the 1950s and 1960s, and post-1968 politically committed research in Italy. The analysis of such politically committed types of research (all very different from one another in terms of the interpretative model of reality, the methods used and the relations with the tendering party and the people observed) allows one to understand how this kind of research can be performed with very different methods—all of which do, however, seek to modify reality on the grounds of the research results. The metaphor of the crystal and the flame presented by Italo Calvino in order to classify facts and ideas and styles and feelings can also be used to visualise the polarity (and the difficulty/impossibility of contact) that has always been present in sociology between two different tendencies: towards a discipline that offers the most abstract and objective interpretative model of reality possible with the use of mathematical models (the crystal), and, instead, towards a discipline in which an interpretative model of reality is central, one that produces a social change and in which the qualitative and quantitative methods utilised are at the service of this change (the flame). This polarity thus allows us to separate first and foremost all the politically committed research performed by others for different purposes (market research, research to verify some theoretical hypotheses, etc.) but the same polarity also lies within the politically committed research. We can, indeed, identify a type of politically committed research in an area that has, at one extreme, research tending towards the utmost objectivity and scientific quality of the results and where the co-ordinators try to have all the information on the subjects maintaining the greatest possible distance (the subjects must not know they are being observed); at the other extreme, there is a type of research in which the coordinator completely involves the subjects in the gathering of information that will serve to realise a piece of research whose political aims are explicitly shared. For the identification of a sufficiently articulated typology it has been important to consider five dimensions of politically committed research: (a) the characteristics of the research coordinator (his/her belonging to the type and the academic/non-academic professional status) and his/her interpretative model of reality (b) some general characteristics of the research (the tender and funding, the topic choice, the aims, the contributions to the sociological theory, the duration); (c) the research coordinators relations with the political actors (the intensity of the relations, the political use of the research); (d) the relations of the research co-ordinator with the research subjects; (e) the methodological choices (the basic strategies, the methods used, the use of mathematical procedures). The research that exemplifies this typology is all politically committed to the left and was realised in different times and contexts. I start by considering a kind of politically engaged research (co-research) that appeared in Italy in the 1950s; I then analyse two types of politically engaged research realised in Austria at the end of the 1920s and one in the United States in the 1950s; there then follow two kinds of research realised in the 1960s and 1970s in the United States and a further two kinds realised in Italy in the 1970s and early 1980s. In order to have other kinds of research please refer to the two Journals of which I am the editor: one is an English language journal of mathematical models (Quality and Quantity, International Journal of Methodology, founded in 1966 and published today by Kluwer of Amsterdam), and a politically engaged review (Inchiesta, founded in 1971 and still published today by Dedalo of Bari). 相似文献
998.
999.
WANG Xiangyun LI Yongchao & CAI Kaiyuan Department of Automatic Control Beijing University of Aeronautics Astronautics Beijing China Correspondence should be addressed to Wang Xiangyun 《中国科学F辑(英文版)》2004,47(4):437-457
1IntroductionReactivesystemsareregardedessentialintheareaofcomputerscience[1].Quiteafewapproacheshavebeenproposedtodevelopreactivesystems,includingthestatemateapproach[2],thetemporal-logicapproach[3],thesynchronousapproach[4],amongoth-ers[5,6].Usually,thepropertiesofthereactivesoftwareunderdevelopmentarecheckedaposteriori(aftersoftwaredesignorevensoftwareimplementation)usingpropertyveri-ficationand/orsimulationtechniquessuchasthetemporal-logicapproach[7]andtheSmoochesapproach[5].However,thisi… 相似文献
1000.
Product development involves many experts collaborating to the same design goal. Every expert has his own formalisms and tools leading to a high heterogeneity of information systems supporting design activities. Interoperability became a major challenge to avoid information incompatibility along the product life cycle. To synchronise heterogeneous representations of product will be a major step to integrate expert activities. In this paper, the authors propose a meta-model framework to connect together heterogeneous design models. This meta-model framework is used to formalise possible interactions between heterogeneous representations. Interaction formalisation is considered as a key point to synchronise heterogeneous models and to provide more interoperability between various computer assisted systems. The synchronisation loop is also presented as a major sequence of activities to manage collaborative design. Tools to support synchronisation are proposed. However, through a basic case study, authors highlight what can be automated and where human intervention is still expected. 相似文献