全文获取类型
收费全文 | 104841篇 |
免费 | 1861篇 |
国内免费 | 485篇 |
专业分类
电工技术 | 1032篇 |
综合类 | 2352篇 |
化学工业 | 16097篇 |
金属工艺 | 5166篇 |
机械仪表 | 3453篇 |
建筑科学 | 3822篇 |
矿业工程 | 718篇 |
能源动力 | 1771篇 |
轻工业 | 5647篇 |
水利工程 | 1437篇 |
石油天然气 | 396篇 |
武器工业 | 1篇 |
无线电 | 10625篇 |
一般工业技术 | 20186篇 |
冶金工业 | 5318篇 |
原子能技术 | 390篇 |
自动化技术 | 28776篇 |
出版年
2023年 | 141篇 |
2022年 | 109篇 |
2021年 | 520篇 |
2020年 | 314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 | 1687篇 |
2012年 | 4172篇 |
2011年 | 10766篇 |
2010年 | 9237篇 |
2009年 | 6491篇 |
2008年 | 7905篇 |
2007年 | 8752篇 |
2006年 | 1043篇 |
2005年 | 2044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 | 304篇 |
1992年 | 246篇 |
1991年 | 228篇 |
1990年 | 199篇 |
1989年 | 225篇 |
1988年 | 188篇 |
1987年 | 177篇 |
1986年 | 171篇 |
1985年 | 235篇 |
1984年 | 212篇 |
1983年 | 182篇 |
1982年 | 166篇 |
1981年 | 195篇 |
1980年 | 127篇 |
1979年 | 144篇 |
1978年 | 141篇 |
1977年 | 123篇 |
1976年 | 137篇 |
1975年 | 116篇 |
1974年 | 103篇 |
排序方式: 共有10000条查询结果,搜索用时 730 毫秒
51.
As an aid towards improving the treatment of exchange and correlation effects in electronic structure calculations, it is
desirable to have a clear picture of the errors introduced by currently popular approximate exchange-correlation functionals.
We have performed ab initio density functional theory and density functional perturbation theory calculations to investigate
the thermal properties of bulk Cu, using both the local density approximation (LDA) and the generalized gradient approximation
(GGA). Thermal effects are treated within the quasiharmonic approximation. We find that the LDA and GGA errors for anharmonic
quantities are an order of magnitude smaller than for harmonic quantities; we argue that this might be a general feature.
We also obtain much closer agreement with experiment than earlier, more approximate calculations. 相似文献
52.
Effect of doping of carbon nanotubes by magnetic transition metal atoms has been considered in this paper. In the case of
semiconducting tubes, it was found that the system has zero magnetization, whereas in metallic tubes the valence electrons
of the tube screen the magnetization of the dopants: the coupling to the tube is usually antiferromagnetic (except for Cr). 相似文献
53.
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
54.
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
55.
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
56.
57.
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
58.
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
59.
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
60.