全文获取类型
收费全文 | 18376篇 |
免费 | 1309篇 |
国内免费 | 23篇 |
专业分类
电工技术 | 361篇 |
综合类 | 69篇 |
化学工业 | 4122篇 |
金属工艺 | 578篇 |
机械仪表 | 477篇 |
建筑科学 | 871篇 |
矿业工程 | 133篇 |
能源动力 | 342篇 |
轻工业 | 2643篇 |
水利工程 | 150篇 |
石油天然气 | 26篇 |
无线电 | 1165篇 |
一般工业技术 | 5065篇 |
冶金工业 | 526篇 |
原子能技术 | 164篇 |
自动化技术 | 3016篇 |
出版年
2023年 | 249篇 |
2022年 | 160篇 |
2021年 | 407篇 |
2020年 | 355篇 |
2019年 | 338篇 |
2018年 | 670篇 |
2017年 | 676篇 |
2016年 | 748篇 |
2015年 | 622篇 |
2014年 | 766篇 |
2013年 | 1508篇 |
2012年 | 918篇 |
2011年 | 1093篇 |
2010年 | 943篇 |
2009年 | 861篇 |
2008年 | 959篇 |
2007年 | 961篇 |
2006年 | 670篇 |
2005年 | 522篇 |
2004年 | 439篇 |
2003年 | 462篇 |
2002年 | 500篇 |
2001年 | 407篇 |
2000年 | 326篇 |
1999年 | 241篇 |
1998年 | 206篇 |
1997年 | 213篇 |
1996年 | 185篇 |
1995年 | 179篇 |
1994年 | 150篇 |
1993年 | 123篇 |
1992年 | 116篇 |
1991年 | 99篇 |
1990年 | 85篇 |
1989年 | 73篇 |
1988年 | 80篇 |
1987年 | 75篇 |
1985年 | 149篇 |
1984年 | 165篇 |
1983年 | 103篇 |
1982年 | 109篇 |
1981年 | 129篇 |
1980年 | 102篇 |
1979年 | 84篇 |
1978年 | 90篇 |
1977年 | 74篇 |
1976年 | 77篇 |
1975年 | 83篇 |
1974年 | 63篇 |
1973年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Additional remarks are given for the data of auroras during the earlier centuries. 相似文献
62.
Review on testers for measuring flow properties of bulk solids 总被引:3,自引:2,他引:1
Jörg Schwedes 《Granular Matter》2003,5(1):1-43
The author was asked by the International Fine Particle Research Institue (IFPRI) to write a critical review on shear testers
for IFPRI-members. The review was delivered to IFPRI in summer 1999. Following the contract with IFPRI it was not allowed
to publish the review elsewhere for at least two years. Granular Matter invited the author to submit the review in the original
form; during the refereeing process (besides other changes) it turned out that some remarks should be added here for clarification:
- Since 1999, the author is not aware of a really new device for testing bulk solid properties, which could lead to a change
of the general comments and conclusions provided in the review.
It was argued, that the review is referring too much to the work of Jenike, while the works of Johanson and Peschl were not
adequately cited. Both are excellent engineers with a lot of experience, but their basic ideas are not available in published
form, and if, they are not set in relation to alternative approaches so that an objective comparison in detail would be a
future research issue rather than a topic in this report.
A discussion on the influence of electrostatic charges was missing. There hardly is an influence, since the particles are
in continuous contact. Only with non-conducting plastic particles electrostatic charges could cause problems. But no relevant
experiments and results are known. In closed systems, the effect of electric charges is thus mainly neglected, but it is clear
that electrostatic forces are eminent in flows with a free surface – an issue not addressed in this review.
Received: 1 November 2002 相似文献
63.
Discussed in this paper is the Cartesian stiffness matrix, which recently has received special attention within the robotics
research community. Stiffness is a fundamental concept in mechanics; its representation in mechanical systems whose potential
energy is describable by a finite set of generalized coordinates takes the form of a square matrix that is known to be, moreover,
symmetric and positive-definite or, at least, semi-definite. We attempt to elucidate in this paper the notion of “asymmetric
stiffness matrices”. In doing so, we show that to qualify for a stiffness matrix, the matrix should be symmetric and either
positive semi-definite or positive-definite. We derive the conditions under which a matrix mapping small-amplitude displacement
screws into elastic wrenches fails to be symmetric. From the discussion, it should be apparent that the asymmetric matrix
thus derived cannot be, properly speaking, a stiffness matrix. The concept is illustrated with an example. 相似文献
64.
65.
Adnan Köksal 《Journal of Infrared, Millimeter and Terahertz Waves》1997,18(8):1525-1537
A parametric study of linearly tapered slot antenna (LTSA) in air and a method for optimization of its pattern are presented. A method of moments code is utilized to investigate the behavior of LTSA’s as the length, height and the taper angle varies. It is shown that the antenna pattern can be improved using a top layer of dielectric material with varying permittivity. 相似文献
66.
The self-organizing map (SOM) [5] provides a general data approximation method which is suitable for several application domains. The topology preservation is an important feature in data-analysis and may also be advantageous for the evaluation of the data in a function approximation or regression task. For this reason the interpolated self-organizing map (I-SOM) adds an output layer to the SOM architecture which computes a real valued output vector. This paper presents an extension of I-SOM towards a continuous interpolation. It is compared to RBF and to the parametrized self-organizing map. 相似文献
67.
68.
Bernd Stöckel 《Journal of Infrared, Millimeter and Terahertz Waves》1993,14(10):2131-2148
A two beam interferometer in the Martin-Puplett configuration is used to determine the complex dielectric constant at 300 GHz of teflon, TPX-plastics, SPECTRALON and paraffin waxes with melting temperatures of 48° C and 72° C, respectively. The design of the quasi-optical system leads to a constant beam diameter at the power detector independent of path delay and frequency. The power detector signal is recorded not only along one period but over about 50 periods. A spectrum estimation routine allows to determine more exactly amplitude and phase angle of the signal. A basic problem is noticed: imperfect detector and source match cause harmonic distortion of the power detector signal. The effects on processing the loss tangent and the invalidation are shown. Finally loss tangent and dielectric constant are determined indirectly by optimizing an equivalent microwave circuit using a commercial available microwave design system to take multiple reflections and losses in consideration. 相似文献
69.
Ö. Jonsson T. Larsson M. Rönnqvist 《Structural and Multidisciplinary Optimization》1994,7(1-2):103-111
An efficient and commonly used approach to structural optimization is to solve a sequence of approximate design problems that are constructed iteratively. As is well-known, a major part of the computational burden of this scheme lies in the sensitivity analysis needed to state the approximate problems. We propose a possibility for reducing this burden by streamlining the calculations in a combined approximation and duality scheme for structural optimization. The difference between this scheme and the traditional one is that, instead of calculating all the constraint gradients to state an approximate design problem explicitly, linear combinations of these gradients are generated as they are needed during the solution of the approximate problem by the dual method. We show, by analysing some typical scenarios of problem characteristics, that this rearrangement of the calculations may be a computationally viable alternative to the traditional scheme. An advantage of streamlining the calculations is that there is no need to incorporate an active set strategy in the scheme, as is usually done, since all the design constraints may be taken into consideration without any loss of computational efficiency. This may, clearly, enhance the practical rate of convergence of the overall approximation scheme. Moreover, the proposed rearrangement of the calculations may make it computationally viable to apply iterative equation solvers to the structural analysis problem. Numerical results with direct as well as iterative equation solvers show that the streamlined scheme is a feasible and promising approach to structural optimization. 相似文献
70.
Dr.-Ing. Ch. Dörnemann Prof. Dr.-Ing. E. Handschin 《Electrical Engineering (Archiv fur Elektrotechnik)》1992,76(1):33-40
Übersicht Der Betriebszustand eines Verteilungsnetzes wird durch die Strombelastung aller Betriebsmittel beschrieben. Mit Hilfe einer statischen Estimationsmethode werden aus einzelnen Messungen und der Kenntnis der Verbrauchsdaten typische Verbraucherkurven (TVK) berechnet, die das elektrische Verhalten verschiedener Verbrauchergruppen widerspiegeln. Auf dieser Basis ist eine betriebsmittelbezogene Lastmodellierung an beliebigen Betriebsmitteln in Abhängigkeit der Verbraucherzusammensetzung unter Berücksichtigung externer Einflüsse wie der Temperatur möglich. Die so gewonnenen Belastungsverläufe erlauben eine Beschreibung des Netzzustands in den Verteilungsnetzen.
Bus load modelling in distribution systems
Contents The operating state of a distribution system is determined by the bus load current. Based on individual substation measurements and the load composition typical load curves are calculated using statistical estimation theory. These typical load curves describe the electrical behaviour of the different load types (e.g. household, storage heating, commerce etc.). As soon as the typical load curves are determined it is possible to calculate the bus load current in any substation taking external influences such as temperature into consideration. The bus load current serve as load models which completely describe the operating state of the distribution system.相似文献