全文获取类型
收费全文 | 308272篇 |
免费 | 3823篇 |
国内免费 | 1369篇 |
专业分类
电工技术 | 5141篇 |
综合类 | 553篇 |
化学工业 | 46366篇 |
金属工艺 | 11940篇 |
机械仪表 | 8986篇 |
建筑科学 | 8141篇 |
矿业工程 | 1820篇 |
能源动力 | 7892篇 |
轻工业 | 24915篇 |
水利工程 | 3324篇 |
石油天然气 | 5981篇 |
武器工业 | 45篇 |
无线电 | 35460篇 |
一般工业技术 | 61112篇 |
冶金工业 | 56848篇 |
原子能技术 | 7758篇 |
自动化技术 | 27182篇 |
出版年
2021年 | 2496篇 |
2020年 | 1921篇 |
2019年 | 2410篇 |
2018年 | 4023篇 |
2017年 | 4138篇 |
2016年 | 4390篇 |
2015年 | 2904篇 |
2014年 | 5019篇 |
2013年 | 13955篇 |
2012年 | 8187篇 |
2011年 | 11121篇 |
2010年 | 8599篇 |
2009年 | 9881篇 |
2008年 | 10391篇 |
2007年 | 10450篇 |
2006年 | 8942篇 |
2005年 | 8302篇 |
2004年 | 8143篇 |
2003年 | 7916篇 |
2002年 | 7640篇 |
2001年 | 7418篇 |
2000年 | 7040篇 |
1999年 | 7276篇 |
1998年 | 16843篇 |
1997年 | 12038篇 |
1996年 | 9474篇 |
1995年 | 7342篇 |
1994年 | 6505篇 |
1993年 | 6534篇 |
1992年 | 4847篇 |
1991年 | 4472篇 |
1990年 | 4472篇 |
1989年 | 4442篇 |
1988年 | 4104篇 |
1987年 | 3773篇 |
1986年 | 3735篇 |
1985年 | 4221篇 |
1984年 | 3908篇 |
1983年 | 3622篇 |
1982年 | 3448篇 |
1981年 | 3442篇 |
1980年 | 3358篇 |
1979年 | 3167篇 |
1978年 | 3169篇 |
1977年 | 3426篇 |
1976年 | 4342篇 |
1975年 | 2702篇 |
1974年 | 2596篇 |
1973年 | 2641篇 |
1972年 | 2163篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
851.
Chawki M.J. Auffret R. Le Coquil E. Pottier P. Berthou L. Paciullo H. Le Bihan J. 《Lightwave Technology, Journal of》1992,10(10):1388-1397
Characteristics of a two-electrode DFB laser filter are studied both theoretically and experimentally. Using a matrix analysis of spontaneous emission, a continuous tuning range of 6.7 Å is achieved by changing both net field gains of the two electrodes. A total discontinuous tuning range of over 10 nm comprising alternating mode jumps and continuous tuning range of 4 Å are measured experimentally. The laser filter presents a FWHM bandwidth of 5 GHz which depends on the optical input power. In addition, it is demonstrated that a DFB laser filter can act as a frequency discriminator/photodetector, i.e., a narrow-band FM receiver, with a uniform bandwidth of 1.5 GHz. Using the two-electrode DFB laser for both transmitter and receiver, a two-channel FSK-WDM transmission system utilizing the discontinuous tuning range is reported. The advantage of such a device is the simplicity as compared to the heterodyne technique 相似文献
852.
P Oroszlan S Wicar G Teshima S L Wu W S Hancock B L Karger 《Analytical chemistry》1992,64(14):1623-1631
This paper examines the retention behavior of recombinant DNA-derived human growth hormone (rhGH) in reversed-phase chromatography and its separation from the closely related N-methionyl variant (Met-hGH). It is first shown that retention for rhGH decreases with increasing column temperature when 1-propanol (1-PrOH) is used as organic modifier. On the other hand, retention increases with temperature when acetonitrile (CH3CN) is employed. The differences in behavior for the two organic modifiers could be related to conformational changes in the protein as determined by solution and adsorption intrinsic fluorescence spectroscopy. Specifically, desorption and elution of rhGH using 1-PrOH could be correlated with a solvent-induced conformational change, with retention decreasing with increasing temperature due to the increasing ease of structural alteration. On the other hand for CH3CN the increase in retention correlated with temperature rise was related to a partial structural change yielding a more hydrophobic species. In this case, a surface-driven process is suggested. The work then turned to the separation of rhGH and Met-hGH where it was found for both organic modifiers optimum separation occurred at 45 degrees C and pH 6.5. Separate studies revealed that during the conformational change Met-hGH appeared more hydrophobic than rhGH since protein-protein aggregation was observed at a lower 1-PrOH concentration. It is suggested that this hydrophobic difference, which was optimized under the conditions cited above, resulted in the separation. The study demonstrates the importance of conformational changes in retention behavior and separation of protein samples. 相似文献
853.
John R. Clem Zhidong Hao Ljiljana Dobrosavljević-Grujić Zoran Radović 《Journal of Low Temperature Physics》1992,88(3-4):213-219
We present a generalized variational model of an isolated vortex, valid in the anisotropic case when the external magnetic field is along one of the symmetry directions. We discuss the effects of the field anisotropy on the core and magnetic pinning of individual vortices. 相似文献
854.
855.
856.
We report on first computations considering effects of a rough wall on the counterflow state in superfluid 3He-B for high flow velocities. Using the quasiclassical Green's-function formalism supplemented by the boundary conditions for a diffusive wall, we calculate the order-parameter field and the supercurrent near a container wall for various pressures and temperatures. One of our results is that the current density at the wall as a function of the flow has a maximum at the velocity which is about half of the pair breaking velocity. 相似文献
857.
P. S. Lawson 《Biomass & bioenergy》1992,2(1-6):319-330
In recent years it has been recognised by an increasing number of nations that there is considerable energy potential within MSW. As a result many countries have established R,D& D programmes to examine methods of exploiting this potential. The IEA's MSW Conversion Activity was set up in 1986 to provide an infrastructure for sharing information and co-ordinating work in this area internationally. This Activity was extended in 1989 and currently a total of 9 nations participate in it.
To cope with the wide scope of the area (encompassing both biological and thermal processing of MSW) the Activity was divided into three subgroups or Expert Working Groups (EWGs). Each of these dealt with a distinct area of expertise:
- 1. •Downstream effects of source separation and screening of MSW
- 2. •Sampling and analytical protocols
- 3. •Landfill gas
In addition to these groups a central secretariat based at Harwell (UK) has provided guidance, established and administered databases of contacts and produced a series of national reports.
This paper describes the achievements of the Activity and discusses work proposed for the future. 相似文献
858.
The effect of heat treatments on the tensile properties and microstructures of wrought Mg–Zn–Zr-Rare earth alloys MB25 and MB26 have been studied in this paper. It was shown that the homogenization of the cast ingots decreased the strength of the extruded bars because some grain boundary phases were dissolved during the process of this treatment, while the ageing treatment of the extruded bars increased the strength due to the dispersive precipitation of MgZn2 phase. The quenching + ageing treatments of the extruded bars decreases the strength and plasticity because of the growing up of the grains.Abbreviations ED
electron diffraction
- GB
grain boundary
- RE
rare earth
- TEM
transmission electron microscope 相似文献
859.
860.
Deterioration of concrete or reinforcing steel through excessive contaminant concentration is often the result of repeated
wetting and drying cycles. At each cycle, the absorption of water carries new contaminants into the unsaturated concrete.
Nuclear Magnetic Resonance (NMR) is used with large concrete samples to observe the shape of the wetting profile during a
simple one-dimensional wetting process. The absorption of water by dry concrete is modelled by a nonlinear diffusion equation
with the unsaturated hydraulic diffusivity being a strongly nonlinear function of the moisture content. Exponential and power
functions are used for the hydraulic diffusivity and corresponding solutions of the diffusion equation adequately predict
the shape of the experimental wetting profile.
The shape parameters, describing the wetting profile, vary little between different blends and are relatively insensitive
to subsequent re-wetting experiments allowing universal parameters to be suggested for these concretes.
Résumé La détérioration du béton ou du béton armé à travers la concentration excessive de contaminant est souvent le résultat de cycles humides et secs répétés. A chaque cycle, l'absorption de l'eau amène de nouveaux contaminants à l'intérieur du béton non saturé en eau. Pour observer le profil du processus d'humidification, on peut utiliser la Résonance Magnétique Nucléaire (RMN) sur de grands échantillons de béton dans le cadre d'une simple étude unidimensionnelle de ce profil. Il est démontré que l'absorption d'eau par du béton sec peut être précisément modélisée par une équation de diffusion non linéaire où la diffusivité hydraulique non saturée est fonction hautement non linéaire de l'humidité du contenant. Des fonctions exponentielles et puissances peuvent définir la diffusivité hydraulique. Les solutions résultant de l'équation modélisant cette diffusion permettent de prédire avec précision la forme du profil de l'humidification expérimentale. Les paramètres de forme qui décrivent le profil de l'humidification varient peu entre les différents mélanges de béton et sont relativement insensibles à des expériences de réhumidification ultérieures. Cela permet de suggérer que ces bétons suivent des paramètres d'humidification universels.相似文献