全文获取类型
收费全文 | 3514篇 |
免费 | 97篇 |
国内免费 | 5篇 |
专业分类
电工技术 | 47篇 |
综合类 | 2篇 |
化学工业 | 600篇 |
金属工艺 | 108篇 |
机械仪表 | 55篇 |
建筑科学 | 123篇 |
矿业工程 | 27篇 |
能源动力 | 60篇 |
轻工业 | 317篇 |
水利工程 | 23篇 |
石油天然气 | 4篇 |
无线电 | 329篇 |
一般工业技术 | 618篇 |
冶金工业 | 904篇 |
原子能技术 | 39篇 |
自动化技术 | 360篇 |
出版年
2023年 | 19篇 |
2022年 | 18篇 |
2021年 | 58篇 |
2020年 | 34篇 |
2019年 | 52篇 |
2018年 | 66篇 |
2017年 | 58篇 |
2016年 | 54篇 |
2015年 | 42篇 |
2014年 | 71篇 |
2013年 | 164篇 |
2012年 | 112篇 |
2011年 | 154篇 |
2010年 | 120篇 |
2009年 | 109篇 |
2008年 | 117篇 |
2007年 | 101篇 |
2006年 | 111篇 |
2005年 | 83篇 |
2004年 | 75篇 |
2003年 | 84篇 |
2002年 | 80篇 |
2001年 | 75篇 |
2000年 | 67篇 |
1999年 | 81篇 |
1998年 | 268篇 |
1997年 | 179篇 |
1996年 | 143篇 |
1995年 | 82篇 |
1994年 | 93篇 |
1993年 | 80篇 |
1992年 | 44篇 |
1991年 | 37篇 |
1990年 | 36篇 |
1989年 | 52篇 |
1988年 | 39篇 |
1987年 | 29篇 |
1986年 | 31篇 |
1985年 | 27篇 |
1984年 | 26篇 |
1983年 | 22篇 |
1982年 | 17篇 |
1981年 | 23篇 |
1980年 | 17篇 |
1979年 | 21篇 |
1978年 | 29篇 |
1977年 | 31篇 |
1976年 | 66篇 |
1975年 | 28篇 |
1974年 | 19篇 |
排序方式: 共有3616条查询结果,搜索用时 15 毫秒
91.
Critical success factors revisited: success and failure cases of information systems for senior executives 总被引:1,自引:0,他引:1
The literature suggests the existence of critical success factors (CSFs) for the development of information systems that support senior executives. Our study of six organizations gives evidence for this notion of CSFs. The study further shows an interesting pattern, namely that companies either “get it right”, and essentially succeed on all CSFs, or “get it completely wrong”, that is, fall short on each of the CSFs. Among the six cases for which data were collected through in-depth interviews with company executives, three organizations seemed to manage all the CSFs properly, while two others managed all CSFs poorly. Only one organization showed a mixed scorecard, managing some factors well and some not so well. At the completion of the study, this organization could neither be judged as a success, nor as a failure. This dichotomy between success and failure cases suggests the existence of an even smaller set of “meta-success” factors. Based on our findings, we speculate that these “meta-success” factors are “championship”, “availability of resources”, and “link to organization objectives”. 相似文献
92.
The general label-placement problem consists in labeling a set of features (points, lines, regions) given a set of candidates
(rectangles, circles, ellipses, irregularly shaped labels) for each feature. The problem arises when annotating classical
cartographical maps, diagrams, or graph drawings. The size of a labeling is the number of features that receive pairwise nonintersecting
candidates. Finding an optimal solution, i.e., a labeling of maximum size, is NP-hard.
We present an approach to attack the problem in its full generality. The key idea is to separate the geometric part from
the combinatorial part of the problem. The latter is captured by the conflict graph of the candidates. We present a set of
rules that simplify the conflict graph without reducing the size of an optimal solution. Combining the application of these
rules with a simple heuristic yields near-optimal solutions.
We study competing algorithms and do a thorough empirical comparison on point-labeling data. The new algorithm we suggest
is fast, simple, and effective.
Received December 21, 1998; revised October 13, 1999. 相似文献
93.
Application of autoregressive spectral analysis to cepstral estimation of mean scatterer spacing 总被引:4,自引:0,他引:4
Wear KA Wagner RF Insana MF Hall TJ 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1993,40(1):50-58
The problem of estimation of mean scatterer spacing in an object containing regularly spaced structures is addressed. An autoregressive (AR) spectral estimation method is compared with a conventional fast Fourier transform (FFT)-based approach for this task. Regularly spaced structures produce a periodicity in the power spectrum of ultrasonic backscatter. This periodicity is manifested as a peak in the cepstrum. A phantom was constructed for comparison of the two methods. It contained regularly spaced nylon filaments. It also contained randomly positioned glass spheres that produced incoherent backscatter. In an experiment in which this target was interrogated using broadband ultrasound, the AR spectral estimate offered considerable improvement over the FFT when the analysis gate length was on the order of the structural dimension. Advantages included improved resolution, reduction in bias and variance of scatterer spacing estimates, and greater resistance to ringing artifacts. Data were also acquired from human liver in vivo. AR spectral estimates on human data exhibited a decreased dependence on gate length. These results offer promise for enhanced spatial resolution and accuracy in ultrasonic tissue characterization and nondestructive evaluation of materials. 相似文献
94.
The use of information technology to enhance teaching and learning processes has been practiced for a number of years now.
However, the rapid growth in the use of the Internet has led to a new dimension in interactive and collaborative learning
anytime and anyplace dynamically. With the explosion of “virtual education initiatives”, the question of the feasibility and
success criteria for such projects quickly arises. To address the question in an organized way, we propose a project assessment
based on critical success factors. Hence, in this article, we draw on a widely recognized critical success factor framework.
We (slightly) adjust the framework to fit the special characteristics of virtual education initiatives, and apply to one case
study, namely the virtual education initiative at the Faculty of Business at the City University of Hong Kong. The results
suggest that the past success of the case is due to the adherence to the large majority of critical success factors. However,
it also outlines some areas of concern. The paper concludes with a discussion on the strength and limitations of virtual learning
environment as well as future directions.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
95.
96.
S. Heitsch G. Wagner A. Rahm G. Benndorf M. Schubert M. Grundmann 《Thin solid films》2006,496(2):234-239
C-axis oriented ZnO thin films were grown on silicon (100) and (111) substrates by pulsed laser deposition. Low temperature photoluminescence spectra show besides the peaks of free excitons, of defect bound excitons, and of a donor-acceptor pair transition a new doublet at 3.328/3.332 eV. The doublet seems to originate from the columnar textured ZnO film structure. A corresponding structural dependence of the broadening parameter of the infrared dielectric functions was derived from spectroscopic ellipsometry in the spectral range from 380 to 1200 cm− 1. The wave numbers of the E1 transverse optical and A1 longitudinal optical phonon modes of the ZnO films on silicon are determined to be 406 and 573 cm− 1, respectively. These values are slightly smaller than those of single-crystalline ZnO thin films on sapphire. 相似文献
97.
Large-scale mapping of boreal forest in SIBERIA using ERS tandem coherence and JERS backscatter data 总被引:3,自引:0,他引:3
Wolfgang Wagner Adrian LuckmanJan Vietmeier Kevin Tansey Heiko BalzterChristiane Schmullius Malcolm Davidson David Gaveau Michael Gluck Thuy Le ToanShaun Quegan Anatoly ShvidenkoAndreas Wiesmann Jiong Jiong Yu 《Remote sensing of environment》2003,85(2):125-144
Siberia's boreal forests represent an economically and ecologically precious resource, a significant part of which is not monitored on a regular basis. Synthetic aperture radars (SARs), with their sensitivity to forest biomass, offer mapping capabilities that could provide valuable up-to-date information, for example about fire damage or logging activity. The European Commission SIBERIA project had the aim of mapping an area of approximately 1 million km2 in Siberia using SAR data from two satellite sources: the tandem mission of the European Remote Sensing Satellites ERS-1/2 and the Japanese Earth Resource Satellite JERS-1. Mosaics of ERS tandem interferometric coherence and JERS backscattering coefficient show the wealth of information contained in these data but they also show large differences in radar response between neighbouring images. To create one homogeneous forest map, adaptive methods which are able to account for brightness changes due to environmental effects were required. In this paper an adaptive empirical model to determine growing stock volume classes using the ERS tandem coherence and the JERS backscatter data is described. For growing stock volume classes up to 80 m3/ha, accuracies of over 80% are achieved for over a hundred ERS frames at a spatial resolution of 50 m. 相似文献
98.
We consider the problem of predictive control of uncertain stochastic discrete I/O systems. Given a model identification procedure able to give accurate output system estimates, e.g. a neural network approximation, we use another feedforward neural network to generate at each time step a constrained optimal control. Dynamic backpropagation is used to improve when necessary the controller network parameters. Both system and controller neural structures are first selected off-line by a statistical Bayesian procedure in order to make the predictive control minimizing process more efficient. The issue of stochastic stability of the closed-loop is considered. We developed this approach for the tracking control of such uncertain systems as biotechnological processes. Actual and simulated predictive neuro-control case studies in this field of application are proposed as illustrations. A comparison with a more classic quasi-Newton-based approach is also proposed, showing the interest of this neuro-control approach. 相似文献
99.
An Autonomous Spacecraft Agent Prototype 总被引:12,自引:0,他引:12
Barney Pell Douglas E. Bernard Steve A. Chien Erann Gat Nicola Muscettola P. Pandurang Nayak Michael D. Wagner Brian C. Williams 《Autonomous Robots》1998,5(1):29-52
This paper describes the New Millennium Remote Agent (NMRA) architecture for autonomous spacecraft control systems. The architecture supports challenging requirements of the autonomous spacecraft domain not usually addressed in mobile robot architectures, including highly reliable autonomous operations over extended time periods in the presence of tight resource constraints, hard deadlines, limited observability, and concurrent activity. A hybrid architecture, NMRA integrates traditional real-time monitoring and control with heterogeneous components for constraint-based planning and scheduling, robust multi-threaded execution, and model-based diagnosis and reconfiguration. Novel features of this integrated architecture include support for robust closed-loop generation and execution of concurrent temporal plans and a hybrid procedural/deductive executive.We implemented a prototype autonomous spacecraft agent within the architecture and successfully demonstrated the prototype in the context of a challenging autonomous mission scenario on a simulated spacecraft. As a result of this success, the integrated architecture has been selected to fly as an autonomy experiment on Deep Space One (DS-1), the first flight of NASA';s New Millennium Program (NMP), which will launch in 1998. It will be the first AI system to autonomously control an actual spacecraft. 相似文献
100.
We define and examine several probabilistic operators ranging over sets (i.e., operators of type 2), among them the formerly
studied ALMOST-operator. We compare their power and prove that they all coincide for a wide variety of classes. As a consequence,
we characterize the ALMOST-operator which ranges over infinite objects (sets) by a bounded-error probabilistic operator which
ranges over strings, i.e., finite objects. This leads to a number of consequences about complexity classes of current interest.
As applications, we obtain (a) a criterion for measure 1 inclusions of complexity classes, (b) a criterion for inclusions
of complexity classes relative to a random oracle, (c) a new upper time bound for ALMOST-PSPACE, and (d) a characterization
of ALMOST-PSPACE in terms of checking stack automata. Finally, a connection between the power of ALMOST-PSPACE and that of
probabilistic circuits is given.
Received: 16 April 1996 相似文献