全文获取类型
收费全文 | 83589篇 |
免费 | 992篇 |
国内免费 | 408篇 |
专业分类
电工技术 | 773篇 |
综合类 | 2317篇 |
化学工业 | 11391篇 |
金属工艺 | 4782篇 |
机械仪表 | 3014篇 |
建筑科学 | 2153篇 |
矿业工程 | 563篇 |
能源动力 | 1099篇 |
轻工业 | 3574篇 |
水利工程 | 1266篇 |
石油天然气 | 341篇 |
无线电 | 9235篇 |
一般工业技术 | 16272篇 |
冶金工业 | 2625篇 |
原子能技术 | 254篇 |
自动化技术 | 25330篇 |
出版年
2022年 | 7篇 |
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 9篇 |
2018年 | 14454篇 |
2017年 | 13378篇 |
2016年 | 9962篇 |
2015年 | 608篇 |
2014年 | 225篇 |
2013年 | 193篇 |
2012年 | 3127篇 |
2011年 | 9397篇 |
2010年 | 8271篇 |
2009年 | 5542篇 |
2008年 | 6771篇 |
2007年 | 7771篇 |
2006年 | 116篇 |
2005年 | 1203篇 |
2004年 | 1120篇 |
2003年 | 1168篇 |
2002年 | 534篇 |
2001年 | 94篇 |
2000年 | 174篇 |
1999年 | 55篇 |
1998年 | 50篇 |
1997年 | 26篇 |
1996年 | 46篇 |
1995年 | 10篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 22篇 |
1988年 | 9篇 |
1969年 | 24篇 |
1968年 | 43篇 |
1967年 | 33篇 |
1966年 | 42篇 |
1965年 | 44篇 |
1964年 | 11篇 |
1963年 | 28篇 |
1962年 | 22篇 |
1961年 | 18篇 |
1960年 | 30篇 |
1959年 | 35篇 |
1958年 | 37篇 |
1957年 | 36篇 |
1956年 | 34篇 |
1955年 | 63篇 |
1954年 | 68篇 |
1949年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Nina Georgieva Kostova Elka Kraleva Alla A. Spojakina Erika Godocikova Peter Balaz 《Journal of Materials Science》2007,42(10):3321-3325
The Al-MCM-41 has been used as support to prepare Mo-containing catalysts. The 12- molybdophosphoric heteropoly acid (HPMo)
is used as initial compound. The catalysts are synthesized by two different methods: incipient impregnation with aqueous solution
of the acid and mechanochemical synthesis. The samples were tested in the reaction of the thiophene hydrodesulfurization after
activation with mixture H2 + H2S. The effect of the preparation method of the catalysts on their physicochemical and catalytic properties has been studied.
A partial destruction of the loaded compound is observed in mechanochemically treated sample whereas the aggregates are formed
from the particles of different size in the impregnated sample. The specific surface area of the sample prepared by mechanical–chemical
treatment decreases 2–3 times, while the total pore volume is about four times lower. The HDS activity is higher on the impregnated
sample than on the mechanochemically treated one. 相似文献
62.
Improved visible-light responsive photocatalytic activity of N and Si co-doped titanias 总被引:1,自引:0,他引:1
Thermal reaction of titanium tetraisopropoxide and tetraethyl orthosilicate in 1,4-butanediol afforded nanocrystalline silica-modified
titanias having large surface area and superior thermal stability. In this study, the thus-obtained silica-modified titanias
were treated in an NH3 flow at high temperatures, and their physical and photocatalytic properties were investigated. Compared with NH3-treated TiO2 without silica modification, the NH3-treated silica-modified titanias showed a stronger absorption in the visible region (400–500 nm) and had a larger peak at
396 eV in the N 1s XPS spectrum. These results indicate that a larger amount of nitrogen was stably doped in the silica-modified
titania. The obtained products exhibited a high photocatalytic activity for degradation of Rhodamine B and decomposition of
acetaldehyde under visible light irradiation.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
63.
Alexander Chroneos Nicholas J. Ashley Kaajal H. Desai John F. Maguire R. W. Grimes 《Journal of Materials Science》2007,42(6):2024-2029
The structures of a number of hydroxide and oxyhydroxide minerals have previously been reported without the hydrogen positions
explicitly defined. Here we use two atomic scale computer simulation techniques, one based on classical ionic potentials,
the other on density functional theory (DFT), to predict these positions. The aim is not only to provide data that can be
used as the basis for future experimental structure optimizations but also model parameters that can be used to predict complex
hydroxide structures. The efficacy of the approach is demonstrated through the comparison of predicted and experimental data
for minerals whose hydrogen positions are known. 相似文献
64.
Prabhakar R. Bandaru 《Journal of Materials Science》2007,42(5):1809-1818
A review on the syntheses and electrical characterization of Y-shaped multi-walled carbon nanotube morphologies is presented.
Modified thermal CVD processes, using Ti precursors, are used to grow Y-junctions of different geometries and distribution
of catalyst particles. It has been established that novel electrical switching behavior is feasible, where any one of the
three branches of the Y-junction can be used for modulating the electrical current flow through the other two branches. Current
blocking behavior, leading to perfect rectification, is seen which could be related to the interplay of the carrier lifetime
and the transit time. The overall goal is to investigate the possibility of obtaining novel functionality at the nanoscale,
which can lead to new device paradigms. 相似文献
65.
The average positron lifetime has been determined non-destructively and in-situ during fully symmetric push-pull fatigue experiments
in the bulk material of the aluminium alloys 2024 T3 and 7075 T6 using a servo-hydraulic fatigue testing system equipped with
a mobile positron beam produced by a 72Se/72As generator (initial activity of ≈ 0.9 MBq; average positron penetration depth ≥1 mm). Contrary to earlier investigations
on stainless steel using the same experimental approach, no variation of the average positron lifetime could be observed during
fatigue and neither early nor late stages of fatigue damage could be revealed. It is concluded that fatigue induced changes
of the defect spectra in technologically relevant aluminium alloys are masked for the present method by saturation trapping
in precipitates. A sufficiently high increase of the dislocation density and the creation of vacancy clusters must be confined
to the vicinity of propagating fatigue cracks or the fatal fatigue crack. Therefore the zone with sufficient detectable fatigue
damage has not enough statistical weight to modify the average positron lifetime of the aluminium alloy bulk material. 相似文献
66.
Dielectric relaxation behaviour of (1 − x)PMN − xPZ, for x = 0.10, 0.30 and 0.40 have been studied. The nature of relaxational behaviour was found to change with PZ concentration.
A crossover from a static freezing to critical slowing down like behaviour is observed with increase in Zr4+ concentration. We have used Glazounov and Tangastev criterion to distinguish freezing and critical slowing down like behaviour. 相似文献
67.
Daniel X. Gouveia Odair P. Ferreira Antonio G. Souza Filho M. G. da Silva J. A. C. de Paiva Oswaldo L. Alves Josué Mendes Filho 《Journal of Materials Science》2007,42(2):534-538
The thermal decomposition properties of Mg–Fe hydrotalcites were studied through in situ 57Fe Mössbauer spectroscopy and in situ X-ray diffraction. Abrupt changes in the quadrupolar splitting measured in the Mössbauer spectra revealed a phase transition from the starting lamellar structure to a new crystalline arrangement. By analyzing the Mössbauer parameters we show that the material is highly disordered in the 300–400 °C temperature range. This hypothesis is confirmed by the X-ray results whose diffractograms indicated the collapse of the lamellar structure and the formation of a solid solution. 相似文献
68.
Modelling of effects of ultrastructural morphology on the hygroelastic properties of wood fibres 总被引:1,自引:0,他引:1
Wood fibres constitute the structural framework of e.g. wood, paper, board and composites, where stiffness and dimensional
stability are of importance. An analytical modelling approach has been used for prediction of hygroelastic response, and assessment
of the stresses in thick-walled cylinder models of wood fibres. A wood fibre was idealised as a multilayered hollow cylinder
made of orthotropic material with helical orientation. The hygroelastic response of the layered assembly due to axisymmetric
loading and moisture content changes was obtained by solving the corresponding boundary value problem of elasticity. A simple
solution scheme based on the state space approach and the transfer matrix method was employed. This was combined with an analytical
ultrastructural homogenisation method, used to link hygroelastic properties of constituent wood polymers to properties of
each layer. Predicted hygroelastic response captured experimentally measured behaviour. Fibres that were constrained not to
twist showed a stiffer response than fibres allowed twisting under uniaxial loading. It was also shown that the ultrastructure,
i.e. the microfibril angle, will control the hygroexpansion in the same way as it controls the compliance of the cell wall.
Qualitative failure trends comparable with experimental observations could be established with stress analysis and a simple
plane-stress failure criterion. 相似文献
69.
Tanu Sharma Sanjeev Aggarwal Shyam Kumar V. K. Mittal P. C. Kalsi V. K. Manchanda 《Journal of Materials Science》2007,42(4):1127-1130
The UV-Visible absorption spectra of virgin and gamma irradiated (20–800 kGy) CR-39 polymer have been deduced by using Shimadzu
Double beam Double Monochromator UV-Visible Spectrophotometer (UV-2550). The existence of the peaks, their shifting and broadening
as a result of gamma irradiation has been discussed. Finally the indirect and direct band gap in virgin and gamma irradiated
CR-39 has been determined. The values of indirect band gap have been found to be lower than the corresponding values of direct
band gap. 相似文献
70.
Igor Studart Medeiros Erika R. M. Andreeta Antonio C. Hernandes 《Journal of Materials Science》2007,42(11):3874-3877