全文获取类型
收费全文 | 414453篇 |
免费 | 4505篇 |
国内免费 | 1218篇 |
专业分类
电工技术 | 6961篇 |
综合类 | 2481篇 |
化学工业 | 61117篇 |
金属工艺 | 21026篇 |
机械仪表 | 16032篇 |
建筑科学 | 9710篇 |
矿业工程 | 2972篇 |
能源动力 | 8274篇 |
轻工业 | 29509篇 |
水利工程 | 4964篇 |
石油天然气 | 8500篇 |
武器工业 | 16篇 |
无线电 | 44680篇 |
一般工业技术 | 85344篇 |
冶金工业 | 57804篇 |
原子能技术 | 8566篇 |
自动化技术 | 52220篇 |
出版年
2021年 | 2095篇 |
2018年 | 22576篇 |
2017年 | 22106篇 |
2016年 | 16009篇 |
2015年 | 3395篇 |
2014年 | 4504篇 |
2013年 | 12235篇 |
2012年 | 11391篇 |
2011年 | 23555篇 |
2010年 | 20607篇 |
2009年 | 18707篇 |
2008年 | 19625篇 |
2007年 | 21901篇 |
2006年 | 7978篇 |
2005年 | 11229篇 |
2004年 | 9316篇 |
2003年 | 8872篇 |
2002年 | 7605篇 |
2001年 | 6635篇 |
2000年 | 6432篇 |
1999年 | 6291篇 |
1998年 | 14796篇 |
1997年 | 11051篇 |
1996年 | 8611篇 |
1995年 | 6539篇 |
1994年 | 5948篇 |
1993年 | 5819篇 |
1992年 | 4504篇 |
1991年 | 4481篇 |
1990年 | 4284篇 |
1989年 | 4301篇 |
1988年 | 4264篇 |
1987年 | 3598篇 |
1986年 | 3595篇 |
1985年 | 4161篇 |
1984年 | 3965篇 |
1983年 | 3620篇 |
1982年 | 3422篇 |
1981年 | 3558篇 |
1980年 | 3422篇 |
1979年 | 3365篇 |
1978年 | 3445篇 |
1977年 | 3923篇 |
1976年 | 5037篇 |
1975年 | 3158篇 |
1974年 | 3007篇 |
1973年 | 3024篇 |
1972年 | 2651篇 |
1971年 | 2473篇 |
1970年 | 2104篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
151.
152.
The physical mechanism of highly efficient photoluminescence (PL) emission from p-type silicon is described by a comparative study of the effectiveness of the etching parameters in an electrochemical anodization technique. Two series of porous silicon samples were prepared in a combination of anodization current and time, to maintain the total amount of anodic charge transfer constant. Photoluminescence studies show that irrespective of the amount of charge transfer, the samples prepared with comparatively higher current density show an efficient PL as well as stronger blueshift in the emission energy vis-à-vis the samples prepared for longer durations. An overall decrease in crystallite size, as estimated by Raman spectral analysis, was observed for both series of samples with the progress of charge transfer. Comparative analysis shows a marginal difference in crystallite size for both series of samples in the initial state of charge transfer, whereas major differences arise at higher values. This is explained with the formation of silicon suboxide on the porous surface at higher current density, leading to initiation of side wall reaction, and higher reduction rate in crystallite size as well as strong luminescence due to the carrier quantum confinement effect. 相似文献
153.
154.
155.
V. M. Bozrov 《Journal of Machinery Manufacture and Reliability》2018,47(3):213-220
A methodology for estimation of the limit energy characteristics of volumetric pneumatic motors based on the Parameter Space Investigation method and a generalized mathematical model in dimensionless (normalized) form taking into account the dynamics of working processes is proposed. A calculated example is presented. The methodology is applicable for determination of the limit energy characteristics of newly created design solutions, of prospects to improve the known designs of pneumatic motors, of evaluation of energy capacity of several pneumatic motors supplied from a single source of compressed air, and of the initial data preparation for parametric synthesis of rotational pneumatic drives for two or more robots taking into account their interaction. 相似文献
156.
Ana Rita Gomes Nasim Bahram Sangani Tiago G. Fernandes M. Margarida Diogo Leopold M. G. Curfs Chris P. Reutelingsperger 《International journal of molecular sciences》2020,21(24)
The central nervous system (CNS) is the most complex structure in the body, consisting of multiple cell types with distinct morphology and function. Development of the neuronal circuit and its function rely on a continuous crosstalk between neurons and non-neural cells. It has been widely accepted that extracellular vesicles (EVs), mainly exosomes, are effective entities responsible for intercellular CNS communication. They contain membrane and cytoplasmic proteins, lipids, non-coding RNAs, microRNAs and mRNAs. Their cargo modulates gene and protein expression in recipient cells. Several lines of evidence indicate that EVs play a role in modifying signal transduction with subsequent physiological changes in neurogenesis, gliogenesis, synaptogenesis and network circuit formation and activity, as well as synaptic pruning and myelination. Several studies demonstrate that neural and non-neural EVs play an important role in physiological and pathological neurodevelopment. The present review discusses the role of EVs in various neurodevelopmental disorders and the prospects of using EVs as disease biomarkers and therapeutics. 相似文献
157.
Pavel Strunz Debashis Mukherji Přemysl Beran Ralph Gilles Lukas Karge Michael Hofmann Markus Hoelzel Joachim Rösler Gergely Farkas 《Metals and Materials International》2018,24(5):934-944
An addition of boron largely increases the ductility in polycrystalline high-temperature Co–Re alloys. Therefore, the effect of boron on the alloy structural characteristics is of high importance for the stability of the matrix at operational temperatures. Volume fractions of ε (hexagonal close-packed—hcp), γ (face-centered cubic—fcc) and σ (Cr2Re3 type) phases were measured at ambient and high temperatures (up to 1500 °C) for a boron-containing Co–17Re–23Cr alloy using neutron diffraction. The matrix phase undergoes an allotropic transformation from ε to γ structure at high temperatures, similar to pure cobalt and to the previously investigated, more complex Co–17Re–23Cr–1.2Ta–2.6C alloy. It was determined in this study that the transformation temperature depends on the boron content (0–1000 wt. ppm). Nevertheless, the transformation temperature did not change monotonically with the increase in the boron content but reached a minimum at approximately 200 ppm of boron. A probable reason is the interplay between the amount of boron in the matrix and the amount of σ phase, which binds hcp-stabilizing elements (Cr and Re). Moreover, borides were identified in alloys with high boron content. 相似文献
158.
In order to improve the process effectiveness and joint quality, ultrasonic vibrations were integrated with friction stir lap welding. Effect of ultrasonic exertion on the process and joint quality of AA 6061-T6 were investigated. Upon ultrasonic exertion, joints owned larger effective lap width, shorter hooks and improved strength. Weld fracture mode changed from a ductile–brittle mixed mode to a more ductile mode while the fracture path shifted from lap interface to beyond the stir zone. Material flow and interface defects were characterised using lap welded dissimilar aluminium alloy joints. Ultrasonic vibration improved the material flow and reduced the interfacial defects. Variations in failure load of joints were found in accordance with the variations in material flow and interfacial defects. 相似文献
159.
Nano crystalline pure and Mg doped ceriaparticles were synthesized by simple chemical co-precipitation method using cerium nitrate hexahydrate as a source material and magnesium nitrate as doping precursor at room temperature. The effect of doping were investigated by X-ray diffraction pattern(XRD), FT-Raman,fourier transform infrared spectroscopy(FTIR), Ultraviolet spectroscopy(UV), photoluminescence spectroscopy(PL), field emission scanning electron microscope(FESEM) and high resolution transmission electron microscopy with energy dispersive spectroscopy (HRTEM &EDS). The X-ray diffraction pattern and FT-Raman studies showed that the prepared samples were nano particulates with cubic fluorite structure. The XRD pattern analysis showed that the size of the particles ranged from 13 to 20?nm, however 4?wt% Mg doping results in reduction of particle size compared with other doping concentrations. The effects of Mg concentration on various structural parameters of the prepared samples were also determined. The slight blue shift observed upon doping in UV–Vis absorption region around 330–360nmrecorded for reduction in particle size. The FTIR unveils the presence of Metal oxygen bonds below 700?cm?1in the prepared samples. All samples showed a broad emission band at 430?nm with linearly increasing intensity with respect to dopant concentrations. The Spherical morphology with weak agglomeration was identified through FESEM and HRTEM analysis. The elemental analysis of Ce, O and Mg were confirmed through EDS analysis. 相似文献
160.