全文获取类型
收费全文 | 105171篇 |
免费 | 1862篇 |
国内免费 | 486篇 |
专业分类
电工技术 | 1040篇 |
综合类 | 2353篇 |
化学工业 | 16344篇 |
金属工艺 | 5190篇 |
机械仪表 | 3454篇 |
建筑科学 | 3830篇 |
矿业工程 | 718篇 |
能源动力 | 1772篇 |
轻工业 | 5657篇 |
水利工程 | 1438篇 |
石油天然气 | 397篇 |
武器工业 | 1篇 |
无线电 | 10630篇 |
一般工业技术 | 20198篇 |
冶金工业 | 5323篇 |
原子能技术 | 390篇 |
自动化技术 | 28784篇 |
出版年
2023年 | 151篇 |
2022年 | 307篇 |
2021年 | 532篇 |
2020年 | 315篇 |
2019年 | 330篇 |
2018年 | 14806篇 |
2017年 | 13679篇 |
2016年 | 10378篇 |
2015年 | 1074篇 |
2014年 | 903篇 |
2013年 | 1685篇 |
2012年 | 4174篇 |
2011年 | 10767篇 |
2010年 | 9237篇 |
2009年 | 6491篇 |
2008年 | 7905篇 |
2007年 | 8753篇 |
2006年 | 1046篇 |
2005年 | 2045篇 |
2004年 | 1876篇 |
2003年 | 1835篇 |
2002年 | 1179篇 |
2001年 | 484篇 |
2000年 | 545篇 |
1999年 | 464篇 |
1998年 | 458篇 |
1997年 | 397篇 |
1996年 | 383篇 |
1995年 | 302篇 |
1994年 | 311篇 |
1993年 | 305篇 |
1992年 | 260篇 |
1991年 | 234篇 |
1990年 | 201篇 |
1989年 | 231篇 |
1988年 | 193篇 |
1987年 | 177篇 |
1986年 | 174篇 |
1985年 | 241篇 |
1984年 | 214篇 |
1983年 | 184篇 |
1982年 | 168篇 |
1981年 | 195篇 |
1980年 | 130篇 |
1979年 | 147篇 |
1978年 | 143篇 |
1977年 | 127篇 |
1976年 | 138篇 |
1975年 | 121篇 |
1974年 | 105篇 |
排序方式: 共有10000条查询结果,搜索用时 6 毫秒
51.
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
52.
液晶电视具有图像逼真、画质细腻、层次丰富、立体感强,以及高亮度、宽视角、高清晰等优点;而且轻薄、省电、无闪烁、无辐射,可有效减轻人们的视觉疲劳;液晶电视的接口也极为丰富,可接驳电脑、DVD等音视频设备,现在一些厂家还将读取Flash卡的功能整合进了液晶电视,这让液晶电视具备了更多的数码味道。在一些发达国家,液晶电视已向许多家庭的厨房、起居室和浴室渗透,这让液晶电视的超薄、壁挂、平面、节能环保的优点充分显示出来。而夏新LC-17HWT1液晶电视则是这一方面的佼佼者。 相似文献
53.
Dong-Soo Yoon Jae Sung Roh Sung-Man Lee Hong Koo Baik 《Journal of Electronic Materials》2003,32(8):890-898
The effect of a thin RuOx layer formed on the Ru/TiN/doped poly-Si/Si stack structure was compared with that on the RuOx/TiN/doped poly-Si/Si stack structure over the post-deposition annealing temperature ranges of 450–600°C. The Ru/TiN/poly-Si/Si
contact system exhibited linear behavior at forward bias with a small increase in the total resistance up to 600°C. The RuOx/TiN/poly-Si/Si contact system exhibited nonlinear characteristics under forward bias at 450°C, which is attributed to no
formation of a thin RuOx layer at the RuOx surface and porous-amorphous microstructure. In the former case, the addition of oxygen at the surface layer of the Ru film
by pre-annealing leads to the formation of a thin RuOx layer and chemically strong Ru-O bonds. This results from the retardation of oxygen diffusion caused by the discontinuity
of diffusion paths. In particular, the RuOx layer in a nonstoichiometric state is changed to the RuO2-crystalline phase in a stoichiometric state after post-deposition annealing; this phase can act as an oxygen-capture layer.
Therefore, it appears that the electrical properties of the Ru/TiN/poly-Si/Si contact system are better than those of the
RuOx/TiN/poly-Si/Si contact system. 相似文献
54.
55.
Ting Gang Zhu Uttiya Chowdhury Michael M. Wong Jonathan C. Denyszyn Russell D. Dupuis 《Journal of Electronic Materials》2002,31(5):406-410
In this paper, we report the study of the electrical characteristics of GaN and AlGaN vertical p-i-n junctions and Schottky
rectifiers grown on both sapphire and SiC substrates by metal-organic chemical-vapor deposition. For GaN p-i-n rectifiers
grown on SiC with a relatively thin “i” region of 2 μm, a breakdown voltage over 400 V, and forward voltage as low as 4.5
V at 100 A/cm2 are exhibited for a 60-μm-diameter device. A GaN Schottky diode with a 2-μm-thick undoped layer exhibits a blocking voltage
in excess of ∼230 V at a reverse-leakage current density below 1 mA/cm2, and a forward-voltage drop of 3.5 V at a current density of 100 A/cm2. It has been found that with the same device structure and process approach, the leakage current of a device grown on a SiC
substrate is much lower than a device grown on a sapphire substrate. The use of Mg ion implantation for p-guard rings as planar-edge
terminations in mesageometry GaN Schottky rectifiers has also been studied. 相似文献
56.
The purpose of this paper is to evaluate two methods of assessing the productivity and quality impact of Computer Aided Software Engineering (CASE) and Fourth Generation Language (4GL) technologies: (1) by the retrospective method; and (2) the cross-sectional method. Both methods involve the use of questionnaire surveys. Developers' perceptions depend on the context in which they are expressed and this includes expectations about the effectiveness of a given software product. Consequently, it is generally not reliable to base inferences about the relative merits of CASE and 4GLs on a cross-sectional comparison of two separate samples of users. The retrospective method that requires each respondent to directly compare different products is shown to be more reliable. However, there may be scope to employ cross-sectional comparisons of the findings from different samples where both sets of respondents use the same reference point for their judgements, and where numerical rather than verbal rating scales are used to measure perceptions. 相似文献
57.
Jan van Eijck 《Formal Aspects of Computing》1994,6(1):766-787
Presuppositions of utterances are the pieces of information you convey with an utterance no matter whether your utterance is true or not. We first study presupposition in a very simple framework of updating propositional information, with examples of how presuppositions of complex propositional updates can be calculated. Next we move on to presuppositions and quantification, in the context of a dynamic version of predicate logic, suitably modified to allow for presupposition failure. In both the propositional and the quantificational case, presupposition failure can be viewed as error abortion of procedures. Thus, a dynamic assertion logic which describes the preconditions for error abortion is the suitable tool for analysing presupposition. 相似文献
58.
59.
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
60.
The aditi deductive database system 总被引:2,自引:0,他引:2
Jayen Vaghanl BSc Kotagiri Ramamohanarao Ph.D. David B. Kemp Ph.D. Zoltan Somogyi Ph.D. Peter J. Stuckey Ph.D. Tim S. Leask BSc James Harland Ph.D. 《The VLDB Journal The International Journal on Very Large Data Bases》1994,3(2):245-288
Deductive databases generalize relational databases by providing support for recursive views and non-atomic data. Aditi is a deductive system based on the client-server model; it is inherently multi-user and capable of exploiting parallelism on shared-memory multiprocessors. The back-end uses relational technology for efficiency in the management of disk-based data and uses optimization algorithms especially developed for the bottom-up evaluation of logical queries involving recursion. The front-end interacts with the user in a logical language that has more expressive power than relational query languages. We present the structure of Aditi, discuss its components in some detail, and present performance figures. 相似文献