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31.
Thomas Neumann Matthias Bender Sebastian Michel Ralf Schenkel Peter Triantafillou Gerhard Weikum 《Distributed and Parallel Databases》2009,26(1):3-27
Top-k query processing is a fundamental building block for efficient ranking in a large number of applications. Efficiency is a
central issue, especially for distributed settings, when the data is spread across different nodes in a network. This paper
introduces novel optimization methods for top-k aggregation queries in such distributed environments. The optimizations can be applied to all algorithms that fall into the
frameworks of the prior TPUT and KLEE methods. The optimizations address three degrees of freedom: 1) hierarchically grouping
input lists into top-k operator trees and optimizing the tree structure, 2) computing data-adaptive scan depths for different input sources, and
3) data-adaptive sampling of a small subset of input sources in scenarios with hundreds or thousands of query-relevant network
nodes. All optimizations are based on a statistical cost model that utilizes local synopses, e.g., in the form of histograms,
efficiently computed convolutions, and estimators based on order statistics. The paper presents comprehensive experiments,
with three different real-life datasets and using the ns-2 network simulator for a packet-level simulation of a large Internet-style
network. 相似文献
32.
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34.
Universal Access in the Information Society - Helping blind people to build cognitive maps of an environment is one of the aims of several assistive systems. In order to evaluate such assistive... 相似文献
35.
German Shegalov Michael Gillmann Gerhard Weikum 《The VLDB Journal The International Journal on Very Large Data Bases》2001,10(1):91-103
Advanced e-services require efficient, flexible, and easy-to-use workflow technology that integrates well with mainstream
Internet technologies such as XML and Web servers. This paper discusses an XML-enabled architecture for distributed workflow
management that is implemented in the latest version of our Mentor-lite prototype system. The key asset of this architecture
is an XML mediator that handles the exchange of business and flow control data between workflow and business-object servers
on the one hand and client activities on the other via XML messages over http. Our implementation of the mediator has made
use of Oracle's XSQL servlet. The major benefit of the advocated architecture is that it provides seamless integration of
client applications into e-service workflows with scalable efficiency and very little explicit coding, in contrast to an earlier,
Java-based, version of our Mentor-lite prototype that required much more code and exhibited potential performance problems.
Received: 30 October 2000 / Accepted: 19 December 2000 Published online: 27 April 2001 相似文献
36.
City councils hold meetings several times a week. There is a need for computer support at certain meetings. This paper examines the potential for group support systems for use in city council meetings and shows in what ways they can be helpful in pre-meeting and post-meeting activities. The study is based on 17 computer-supported city council meetings, carried out in Stuttgart, Kornwestheim and other cities as part of the Cuparla Project between 1996 and 1998. Three of these meetings are described in the paper as brief case studies. Following this, all 17 meeting sequences are evaluated and analysed. Guidelines have been developed from the results of the study for the introduction of group support systems in city council meetings. 相似文献
37.
38.
In June 2003, a large scale injection experiment started at the Continental Deep Drilling site (KTB) in Germany. A tiltmeter array was installed which consisted of five high resolution borehole tiltmeters of the ASKANIA type, also equipped with three dimensional seismometers. For the next 11 months, 86 000 m(3) were injected into the KTB pilot borehole 4000 m deep. The average injection rate was approximately 200 l/min. The research objective was to observe and to analyze deformation caused by the injection into the upper crust at the kilometer range. A new data acquisition system was developed by Geo-Research Center Potsdam (GFZ) to master the expected huge amount of seismic and tilt data. Furthermore, it was necessary to develop a new preprocessing software called PREANALYSE for long-period time series. This software includes different useful functions, such as step and spike correction, interpolation, filtering, and spectral analysis. This worldwide unique installation offers the excellent opportunity of the separation of signals due to injection and due to environment by correlation of the data of the five stations with the ground water table and meteorological data. 相似文献
39.
Physical workload of flight attendants when pushing and pulling trolleys aboard aircraft 总被引:2,自引:2,他引:0
Ulrich Glitsch Hans Jürgen Ottersbach Rolf Ellegast Karlheinz Schaub Gerhard Franz Matthias Jger 《International Journal of Industrial Ergonomics》2007,37(11-12):845
The musculoskeletal loads from moving trolleys aboard aircraft were assessed by observation of trolley handling on planes and by physical workload analyses of pushing and pulling of trolleys in a laboratory set up. Trolley handling by a total of 15 female flight attendants was observed on 10 short- and medium-distance flights in different types of aircraft. About 25 selected flight attendants (22 females; 3 males) of five German airlines took part in the laboratory study, which comprised three-dimensional (3D) measurements of posture and hand forces during pushing and pulling of trolleys in a variety of configurations. From the on-flight observations performed, between 150 and 250 trolley movements can be projected for a work shift. The greatest physical workload is to be expected at the beginning of service: The trolleys are fully laden then, and the cabin floor can still be inclined up to 8°, as the aircraft is still climbing, particularly on short-distance flights. The laboratory investigation revealed that the musculoskeletal workload from pushing and pulling depends essentially on the trolley load and on the gradient of the cabin floor. In addition, the degree of stressing depends significantly on the trolley type, mode of handling and personal dexterity. The up/down force component perpendicular to the direction of motion often achieved considerable amplitudes-in some cases equal to or exceeding the force in the direction of movement. The posture analysis demonstrated that pulling forced the flight attendants to adopt ergonomically unfavourable postures such as pronounced flexion of the back, particularly among female subjects. The highest values for flexion of the back occurred while pulling the half-size trolley. The results demonstrate that female flight attendants are likely to overload themselves if they frequently have to move heavily laden trolleys unaided on an inclined cabin floor.
Relevance to industry
On short distance flights, flight attendants have been complaining increasingly of high physical workload from manoeuvring trolleys. On the basis of the presented data airline companies may improve the trolley handling skills of their flight attendances by practical trainings and may ergonomically optimize the general service procedures aboard aircrafts. 相似文献40.
Data partitioning and load balancing in parallel disk systems 总被引:13,自引:0,他引:13
Peter Scheuermann Gerhard Weikum Peter Zabback 《The VLDB Journal The International Journal on Very Large Data Bases》1998,7(1):48-66
Parallel disk systems provide opportunities for exploiting I/O parallelism in two possible ways, namely via inter-request
and intra-request parallelism. In this paper, we discuss the main issues in performance tuning of such systems, namely striping
and load balancing, and show their relationship to response time and throughput. We outline the main components of an intelligent,
self-reliant file system that aims to optimize striping by taking into account the requirements of the applications, and performs
load balancing by judicious file allocation and dynamic redistributions of the data when access patterns change. Our system
uses simple but effective heuristics that incur only little overhead. We present performance experiments based on synthetic
workloads and real-life traces.
Received May 17, 1994 / Accepted June 9, 1997 相似文献