全文获取类型
收费全文 | 22811篇 |
免费 | 885篇 |
国内免费 | 83篇 |
专业分类
电工技术 | 293篇 |
综合类 | 44篇 |
化学工业 | 5151篇 |
金属工艺 | 478篇 |
机械仪表 | 460篇 |
建筑科学 | 1718篇 |
矿业工程 | 160篇 |
能源动力 | 690篇 |
轻工业 | 2141篇 |
水利工程 | 182篇 |
石油天然气 | 56篇 |
武器工业 | 1篇 |
无线电 | 1484篇 |
一般工业技术 | 4173篇 |
冶金工业 | 2773篇 |
原子能技术 | 137篇 |
自动化技术 | 3838篇 |
出版年
2023年 | 152篇 |
2022年 | 308篇 |
2021年 | 533篇 |
2020年 | 312篇 |
2019年 | 328篇 |
2018年 | 494篇 |
2017年 | 425篇 |
2016年 | 531篇 |
2015年 | 481篇 |
2014年 | 690篇 |
2013年 | 1506篇 |
2012年 | 1081篇 |
2011年 | 1518篇 |
2010年 | 1092篇 |
2009年 | 1043篇 |
2008年 | 1265篇 |
2007年 | 1099篇 |
2006年 | 939篇 |
2005年 | 866篇 |
2004年 | 775篇 |
2003年 | 690篇 |
2002年 | 660篇 |
2001年 | 392篇 |
2000年 | 376篇 |
1999年 | 412篇 |
1998年 | 411篇 |
1997年 | 372篇 |
1996年 | 342篇 |
1995年 | 294篇 |
1994年 | 302篇 |
1993年 | 297篇 |
1992年 | 250篇 |
1991年 | 214篇 |
1990年 | 201篇 |
1989年 | 230篇 |
1988年 | 186篇 |
1987年 | 178篇 |
1986年 | 174篇 |
1985年 | 242篇 |
1984年 | 214篇 |
1983年 | 185篇 |
1982年 | 168篇 |
1981年 | 195篇 |
1980年 | 130篇 |
1979年 | 148篇 |
1978年 | 144篇 |
1977年 | 128篇 |
1976年 | 138篇 |
1975年 | 120篇 |
1974年 | 106篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Barbara M. Johnston Peter R. Johnston 《International journal for numerical methods in engineering》2003,56(4):589-607
Accurate numerical evaluation of integrals arising in the boundary element method is fundamental to achieving useful results via this solution technique. In this paper, a number of techniques are considered to evaluate the weakly singular integrals which arise in the solution of Laplace's equation in three dimensions and Poisson's equation in two dimensions. Both are two‐dimensional weakly singular integrals and are evaluated using (in a product fashion) methods which have recently been used for evaluating one‐dimensional weakly singular integrals arising in the boundary element method. The methods used are based on various polynomial transformations of conventional Gaussian quadrature points where the transformation polynomial has zero Jacobian at the singular point. Methods which split the region of integration into sub‐regions are considered as well as non‐splitting methods. In particular, the newly introduced and highly accurate generalized composite subtraction of singularity and non‐linear transformation approach (GSSNT) is applied to various two‐dimensional weakly singular integrals. A study of the different methods reveals complex relationships between transformation orders, position of the singular point, integration kernel and basis function. It is concluded that the GSSNT method gives the best overall results for the two‐dimensional weakly singular integrals studied. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
22.
23.
Peter V. Sushko Alexander L. ShlugerKatsuro Hayashi Masahiro HiranoHideo Hosono 《Thin solid films》2003,445(2):161-167
Recently it has been discovered that a nano-porous main group oxide 12CaO·7Al2O3 (C12A7) can be converted from a wide-gap insulator to a good transparent conductor. Using ab initio modelling we explain good conductivity of this material by very small barriers for hopping of localised electrons between neighbouring positive cages. We show that optical absorption of C12A7 in infrared region and at energies higher than 2.7 eV is due to inter-cage and intra-cage electron transitions, respectively. The proposed mechanisms can be useful in further search for conducting transparent media. 相似文献
24.
Management of multidimensional discrete data 总被引:1,自引:0,他引:1
Peter Baumann Ph.D. 《The VLDB Journal The International Journal on Very Large Data Bases》1994,3(4):401-444
Spatial database management involves two main categories of data: vector and raster data. The former has received a lot of in-depth investigation; the latter still lacks a sound framework. Current DBMSs either regard raster data as pure byte sequences where the DBMS has no knowledge about the underlying semantics, or they do not complement array structures with storage mechanisms suitable for huge arrays, or they are designed as specialized systems with sophisticated imaging functionality, but no general database capabilities (e.g., a query language). Many types of array data will require database support in the future, notably 2-D images, audio data and general signal-time series (1-D), animations (3-D), static or time-variant voxel fields (3-D and 4-D), and the ISO/IEC PIKS (Programmer's Imaging Kernel System) BasicImage type (5-D). In this article, we propose a comprehensive support ofmultidimensional discrete data (MDD) in databases, including operations on arrays of arbitrary size over arbitrary data types. A set of requirements is developed, a small set of language constructs is proposed (based on a formal algebraic semantics), and a novel MDD architecture is outlined to provide the basis for efficient MDD query evaluation. 相似文献
25.
26.
Concept for Scale-up on Suspension in a Stirred Tank. The relationships derived for the suspension of solids in a propeller loop can be applied to corresponding processes in a stirred tank by means of analogy considerations. Accordingly, the ratio of stirrer power to slip power of laminar flow state is inversely proportional to the vessel diameter. In contrast, the power ratio remains constant in turbulent flow. There is a pronounced transition range between these two limiting states. The author's own experimental studies performed over a wide range of Re numbers for the recycling flow confirm this relationship. The observations presented explain why numerous authors have determined scale-up rules lying between these two extremes. The experiments shows that the power input for suspension is always significantly greater than the slip power. Under otherwise identical conditions, the necessary stirrer power is smaller by a factor of three for a propeller stirrer than for a disk impeller. The suspension of small solids concentrations can also be approximated by the flow model presented. The somewhat different physical relations valid in this case are substantiated by the author's experiments. The model also provided information relevant to scale-up when the particle properties are unknown. This is demontrated for an example in a 21 m3 vessel. 相似文献
27.
We consider a language of operations which pass parameters by means of a stack. An algebra over the set of type signatures is introduced, which allows the type signature of a program to be obtained from the type signatures of its constituent operations.Although the theories apply in principle to any stack based language, they have been evolved with particular regard to the proposed ANSI Standard Forth language, which is currently implemented in a type free manner. We hope this work will stimulate an interest in Forth amongst those applying algebraic techniques in software engineering, and we hope to lay the theoretical foundations for implementing practical type checkers to support Forth. 相似文献
28.
Experiments showing the frequency and amplitude of the flow induced motion of the gate for a 2- and a 4-in. swing check valve have been performed. The gate motion is due to turbulence in approach flow. We have found the dominant turbulent frequency of the approach flow is about half the natural frequency of the valves. The valves appear to be almost critically damped. Because of this, the valves respond almost as they would to a static force of the magnitude characteristic of the turbulent fluctuation in the flow. Both the dimensionless exciting force and the damping ratio have been found to be independent of valve size so the above statements are true for larger valves also. The recommended valve oscillation amplitudes and frequencies are used to calculate the wear at the shaft and at the stop. For an unpegged check valve, such as one of the 10-in. valves which was used at the San Onofre Nuclear Generation Station, it was found that shaft bearing wear would amount to 0.27 in.3/year and stop wear to 0.03 in.3/year. 相似文献
29.
30.
We consider the problem of rescheduling trains in the case where one track of a railway section consisting of two tracks
in opposing directions is closed due to construction activities. After presenting an appropriate model for this situation
we derive a polynomial algorithm for the subproblem of finding an optimal schedule with minimal latenesss if the subsequences
of trains for both directions outside the construction site are fixed. Based on this algorithm we propose a local search procedure
for the general problem of finding good schedules and report test results for some real world instances.
Received: December 8, 1999 / Accepted: May 2, 2001 相似文献