全文获取类型
收费全文 | 4257篇 |
免费 | 297篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 80篇 |
综合类 | 21篇 |
化学工业 | 1050篇 |
金属工艺 | 86篇 |
机械仪表 | 70篇 |
建筑科学 | 337篇 |
矿业工程 | 18篇 |
能源动力 | 107篇 |
轻工业 | 287篇 |
水利工程 | 28篇 |
石油天然气 | 5篇 |
无线电 | 374篇 |
一般工业技术 | 860篇 |
冶金工业 | 406篇 |
原子能技术 | 17篇 |
自动化技术 | 812篇 |
出版年
2024年 | 6篇 |
2023年 | 59篇 |
2022年 | 139篇 |
2021年 | 152篇 |
2020年 | 122篇 |
2019年 | 102篇 |
2018年 | 131篇 |
2017年 | 116篇 |
2016年 | 165篇 |
2015年 | 170篇 |
2014年 | 199篇 |
2013年 | 280篇 |
2012年 | 266篇 |
2011年 | 335篇 |
2010年 | 268篇 |
2009年 | 220篇 |
2008年 | 226篇 |
2007年 | 217篇 |
2006年 | 172篇 |
2005年 | 168篇 |
2004年 | 112篇 |
2003年 | 92篇 |
2002年 | 88篇 |
2001年 | 65篇 |
2000年 | 57篇 |
1999年 | 68篇 |
1998年 | 102篇 |
1997年 | 71篇 |
1996年 | 77篇 |
1995年 | 40篇 |
1994年 | 24篇 |
1993年 | 32篇 |
1992年 | 15篇 |
1991年 | 13篇 |
1990年 | 14篇 |
1989年 | 20篇 |
1988年 | 12篇 |
1987年 | 7篇 |
1986年 | 19篇 |
1985年 | 15篇 |
1984年 | 7篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 13篇 |
1977年 | 8篇 |
1976年 | 12篇 |
1975年 | 6篇 |
1974年 | 6篇 |
1973年 | 6篇 |
排序方式: 共有4558条查询结果,搜索用时 15 毫秒
11.
Riikka Susitaival Ilmari Juva Markus Peuhkuri Samuli Aalto 《Telecommunication Systems》2006,33(1-3):67-88
In this paper we analyze measurements gathered from a 2.5~Gbps link in the Finnish university network (Funet) in 2004. The
traffic is broken down into origin-destination (OD) pair components based on source and destination IP address. We study the
traffic characteristics of these components, and identify four typical representative OD pairs. For these pairs we investigate
the validity of a moving IID Gaussian model. We find that the statistical properties of these OD pairs differ significantly
from each other, with only some of them close to Gaussian. The OD pairs are also found to have some cross-correlation between
each other, contradicting an often made assumption about OD pair independence. Furthermore, the existence of a mean-variance
relation between the OD pairs is studied. We find that there is a relation between mean and variance, but for some periods
of time it is rather weak. 相似文献
12.
We developed a stream classification system that is based on stream’s susceptibility to algal growth using a two-step approach. The model portrays algal biomass as a result of stream’s response to nutrient concentrations and the response is governed by various stream factors. In the first step, a nutrient-chlorophyll a relationship was developed to characterize nutrient’s effects on algal biomass. Residuals of the relationship were attributed to stream’s susceptibility to algal growth in response to nutrients and referred to as “observed” susceptibility. In the second step, conditions of other contributing factors were used to explain the variation in the residuals and the developed relationship was used to generate “predicted” susceptibility. Existing data compiled from various monitoring projects of Illinois streams and rivers were used to illustrate the approach. Streams were classified into three (high, medium, and low) categories based on their observed and predicted susceptibility values, respectively. With the available data, the model showed a 40-50% success rate for classifying the streams based on three observed and predicted susceptibility categories. Model entropy also was calculated for selecting the best model. The results show the important role of both nutrients and other contributing factors in explaining the variation of algal biomass. The study also suggests ways to fine tune the model and improve its accuracy, which would make the presented model a more viable tool for stream classification for establishing nutrient criteria to prevent surface streams from eutrophication. 相似文献
13.
Andrzej Banaszuk Andrzej Święch John Hauser 《Mathematics of Control, Signals, and Systems (MCSS)》1996,9(3):207-241
We study the problem of approximating one-dimensional nonintegrable codistributions by integrable ones and apply the resulting
approximations to approximate feedback linearization of single-input systems. The approach derived in this paper allows a
linearizable nonlinear system to be found that is close to the given system in a least-squares (L
2) sense. A linearly controllable single-input affine nonlinear system is feedback linearizable if and only if its characteristic
distribution is involutive (hence integrable) or, equivalently, any characteristic one-form (a one-form that annihilates the
characteristic distribution) is integrable. We study the problem of finding (least-squares approximate) integrating factors
that make a fixed characteristic one-form close to being exact in anL
2 sense. A given one-form can be decomposed into exact and inexact parts using the Hodge decomposition. We derive an upper
bound on the size of the inexact part of a scaled characteristic one-form and show that a least-squares integrating factor
provides the minimum value for this upper bound. We also consider higher-order approximate integrating factors that scale
a nonintegrable one-form in a way that the scaled form is closer to being integrable inL
2 together with some derivatives and derive similar bounds for the inexact part. This allows a linearizable nonlinear system
that is close to the given system in a least-squares (L
2) sense together with some derivatives to be found. The Sobolev embedding techniques allow us to obtain an upper bound on
the uniform (L
∞) distance between the nonlinear system and its linearizable approximation.
This research was supported in part by NSF under Grant PYI ECS-9396296, by AFOSR under Grant AFOSR F49620-94-1-0183, and by
a grant from the Hughes Aircraft Company. 相似文献
14.
Phase Equilibria in the Quaternary System Ti-Al-C-N 总被引:2,自引:0,他引:2
The quaternary system Ti-Al-C-N and its binary and ternary boundary systems are investigated using powder methods and XRD analysis. Phase equilibria at 1375°C are presented in an isothermal network for alloys up to 50 at.% Ti. In the vertical section Ti2 AIC1-x -Ti2 AlN1-x a complete series of solid solutions exists at 1495°C, but a wide miscibility gap occurs at 1375°C. The vertical section Ti3 AlC1-x -Ti3 AlN1-x is more complex because of the occurrence of the quaternary, tetragonally distorted phase Ti3 Al(C,N)1-x ( a = 0.41135(4) nm, c = 0.41366(5) nm) and the transformation of perovskite-type Ti3 AlN1-x into filled Re3 B-type Ti3 AlN1-x below 1200°C. 相似文献
15.
16.
Verghese S. Hauser J.R. Wortman J.J. Kerns S.E. 《Electron Devices, IEEE Transactions on》1989,36(7):1311-1317
An analysis of forward I -V characteristics for Al Schottky contacts to polycrystalline thin films is presented. Experimental results for lateral structures with various n- doping levels show the expected twofold exponential characteristics representing a transition from electrode-limited to bulk-limited conduction. At higher doping levels the bulk-limited characteristic is not a perfect exponential. The thermionic emission theory has been modified to include crystallite resistivity between grain boundaries and successfully matches experimental data 相似文献
17.
In the following paper at first general concepts which have to be considered for the accomplishment of a 3D fatigue crack propagation simulation by the FE‐method in real structures are introduced. In doing so numerical aspects concerning the necessary mesh generation as well as fracture mechanical issues will be discussed. Furthermore the 3D crack simulation program ADAPCRACK3D, which has been developed at the Institute of Applied Mechanics at University of Paderborn, is presented in its general functionality. The simulation example of the crack propagation in a shutter ring of a hydraulic press shows the practical relevance of both the implemented algorithms and fracture mechanical concepts. 相似文献
18.
Modified (α‐diimine)nickel(II) catalysts are suitable for ethylene oligomerization reactions. Due to the mechanism (“chain running”), the formation of double‐bond isomers and branched olefins can be observed. The obtained oligomer mixtures show a Schulz–Flory distribution. Increasing ethylene pressure favors the selectivity of α‐olefin formation. The same effect can be observed when Lewis bases like thiophene or furane are added to the oligomerization reaction mixture. These results can be related to an interaction of the additives with the catalytic active species during the oligomerization process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1356–1361, 2003 相似文献
19.
Markus Stolze 《Knowledge》1994,7(4):271-273
System development is strongly influenced by the perspectives used by system developers. Current development methods for knowledge-based systems are based on an information processing perspectives of experts and users which has been criticized by a number of researchers. The paper argues that work-oriented development of knowledge-based systems requires the parallel use of multiple perspectives (including the information processing perspective), and that the use of any single perspective is potentially dangerous. As an example of the importance of using multiple perspectives, thepaper presents the situated action perspective, and shows how it complements the information processing perspective. 相似文献
20.
Dipl.-Phys. Andreas Neuber M. Sc. Guenther Carlos Krieger Dipl.-Ing. Markus Tacke Prof. Dr.-Ing Johannes Janicka 《Forschung im Ingenieurwesen》1996,62(7-8):188-194
Zusammenfassung Die laserspektroskopischen Me?methoden laserinduzierte Fluoreszenz und spontane Ramanspektroskopie werden eingesetzt zur Messung
der Konzentrationen von Stickoxid, Hydroxyl sowie der chemischen Hauptkomponenten und der Temperatur. Betrachtet wird die
thermische Umsetzung von Wasserstoff mit Luftsauerstoff. Es wird die experimentelle Vorgehensweise und Problematik dargestellt.
Ebenso wird der Weg der Modellierung und Berechnung turbulenter Verbrennungsvorg?nge am konkreten Beispiel skizziert. Die
Annahme chemischen Gleichgewichts stellt sich als ungenügend in der Berechnung der Stickoxid- und Hydroxylkonzentrationen
heraus. Eine deutlich bessere Vorhersage wird durch Einführen einer Reaktionsfortschrittsvariablen für das chemische Modell
erreicht. 相似文献