全文获取类型
收费全文 | 36353篇 |
免费 | 5045篇 |
国内免费 | 2378篇 |
专业分类
电工技术 | 2800篇 |
技术理论 | 1篇 |
综合类 | 4163篇 |
化学工业 | 1398篇 |
金属工艺 | 1129篇 |
机械仪表 | 5342篇 |
建筑科学 | 5526篇 |
矿业工程 | 1352篇 |
能源动力 | 1358篇 |
轻工业 | 1059篇 |
水利工程 | 1546篇 |
石油天然气 | 823篇 |
武器工业 | 506篇 |
无线电 | 2903篇 |
一般工业技术 | 7119篇 |
冶金工业 | 1960篇 |
原子能技术 | 384篇 |
自动化技术 | 4407篇 |
出版年
2024年 | 179篇 |
2023年 | 552篇 |
2022年 | 1288篇 |
2021年 | 1273篇 |
2020年 | 1470篇 |
2019年 | 1159篇 |
2018年 | 1151篇 |
2017年 | 1409篇 |
2016年 | 1542篇 |
2015年 | 1580篇 |
2014年 | 2319篇 |
2013年 | 2079篇 |
2012年 | 2836篇 |
2011年 | 2972篇 |
2010年 | 2223篇 |
2009年 | 2186篇 |
2008年 | 1969篇 |
2007年 | 2546篇 |
2006年 | 2226篇 |
2005年 | 1784篇 |
2004年 | 1481篇 |
2003年 | 1365篇 |
2002年 | 978篇 |
2001年 | 943篇 |
2000年 | 826篇 |
1999年 | 643篇 |
1998年 | 503篇 |
1997年 | 446篇 |
1996年 | 356篇 |
1995年 | 300篇 |
1994年 | 257篇 |
1993年 | 163篇 |
1992年 | 146篇 |
1991年 | 129篇 |
1990年 | 97篇 |
1989年 | 119篇 |
1988年 | 81篇 |
1987年 | 42篇 |
1986年 | 19篇 |
1985年 | 15篇 |
1984年 | 21篇 |
1983年 | 13篇 |
1982年 | 28篇 |
1981年 | 22篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1974年 | 3篇 |
1966年 | 3篇 |
1965年 | 4篇 |
1959年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
32.
Dynamics of liquid-filled spacecraft 总被引:1,自引:0,他引:1
A method is presented for simulating coupled liquid-solid dynamics. An important example of a coupled liquid-solid system is a satellite carrying fuel. The dynamics of the satellite and the onboard fuel influence each other, which may lead to satellite motion that is uncontrollable. For better understanding of the complex dynamics of coupled systems, a numerical model is developed. The model consists of two parts. The first part that solves the liquid motion is only briefly discussed here. The focus in this paper is on the way in which the dynamics of the liquid and the solid body are coupled. For this, the governing equations are presented in which terms appear that represent the force and torque on the solid body due to the sloshing liquid. The governing equations are rewritten such that the discrete approximation of these equations can be integrated in a stable manner for arbitrary liquid/solid mass ratios. Results are presented demonstrating the stability of the present model. A grid-refinement study and a time-step analysis are performed. Finally, the flat-spin motion of a satellite, partially filled with liquid, that flew in 1992 as part of the Wet Satellite Model experiment is studied. Results from the simulation are compared with the actual flight data. 相似文献
33.
Thirty patients who had undergone either a right or left unilateral temporal lobectomy (14 RTL; 16 LTL) and 16 control participants were tested on a computerized human analogue of the Morris Water Maze. The procedure was designed to compare allocentric and egocentric spatial memory. In the allocentric condition, participants searched for a target location on the screen, guided by object cues. Between trials, participants had to walk around the screen, which disrupted egocentric memory representation. In the egocentric condition, participants remained in the same position, but the object cues were shifted between searches to prevent them from using allocentric memory. Only the RTL group was impaired on the allocentric condition, and neither the LTL nor RTL group was impaired on additional tests of spatial working memory or spatial manipulation. The results support the notion that the right anterior temporal lobe stores long-term allocentric spatial memories. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
34.
通过对减振器结构设计的理论分析,建立了减振器的数学模型,同时通过设计的模拟装置对减振效果进行了实测,效果良好。 相似文献
35.
Dimitris Papadias M.Sc. Ph.D. candidate Timos Sellis Ph.D. 《The VLDB Journal The International Journal on Very Large Data Bases》1994,3(4):479-516
Various relation-based systems, concerned with the qualitative representation and processing of spatial knowledge, have been developed in numerous application domains. In this article, we identify the common concepts underlying qualitative spatial knowledge representation, we compare the representational properties of the different systems, and we outline the computational tasks involved in relation-based spatial information processing. We also describesymbolic spatial indexes, relation-based structures that combine several ideas in spatial knowledge representation. A symbolic spatial index is an array that preserves only a set of spatial relations among distinct objects in an image, called the modeling space; the index array discards information, such as shape and size of objects, and irrelevant spatial relations. The construction of a symbolic spatial index from an input image can be thought of as a transformation that keeps only a set of representative points needed to define the relations of the modeling space. By keeping the relative arrangements of the representative points in symbolic spatial indexes and discarding all other points, we maintain enough information to answer queries regarding the spatial relations of the modeling space without the need to access the initial image or an object database. Symbolic spatial indexes can be used to solve problems involving route planning, composition of spatial relations, and update operations. 相似文献
36.
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
37.
Wesley W. Chu Ph.D. Ion T. leong Ph.D. Ricky K. Taira Ph.D. 《The VLDB Journal The International Journal on Very Large Data Bases》1994,3(4):445-477
We introduce a semantic data model to capture the hierarchical, spatial, temporal, and evolutionary semantics of images in pictorial databases. This model mimics the user's conceptual view of the image content, providing the framework and guidelines for preprocessing to extract image features. Based on the model constructs, a spatial evolutionary query language (SEQL), which provides direct image object manipulation capabilities, is presented. With semantic information captured in the model, spatial evolutionary queries are answered efficiently. Using an object-oriented platform, a prototype medical-image management system was implemented at UCLA to demonstrate the feasibility of the proposed approach. 相似文献
38.
刀具工作前角时变系统切削稳定性的研究 总被引:4,自引:0,他引:4
理论分析和试验结果表明,刀具工作前角时变系统的振动响应幅值比刀具工作前角固定不变的系统的振动响应幅值明显减小.试验在普通车床上进行,刀具工作前角由微机通过步进电机按规定的时变参数控制。 相似文献
39.
40.
Free vibration of partially supported piles with the effects of bending moment, axial and shear force 总被引:1,自引:0,他引:1
The pile parts above the soil and embedded in the soil are called the first region and the second region, respectively. The fourth order partial differential equations of both regions for free vibration of partially supported pile subjected to bending moment, axial force and shear force are obtained using the small-displacement theory and the Winkler model. It is assumed that the behavior of the material is linear-elastic, and that axial force along the pile length and modulus of subgrade reaction for the second region to be constant. Shear effect is included in the partial differential equations by the second derivative of the elastic curve function with respect to shear deformation. Natural circular frequencies and relative stiffness of the pile are calculated for non-trivial solution of linear homogeneous system of equations obtained due to the values of axial forces acting on the pile, the shape factors, and the boundary conditions of the pile with both ends free and both ends simply supported; the results are presented in graphs. 相似文献