全文获取类型
收费全文 | 288919篇 |
免费 | 4456篇 |
国内免费 | 878篇 |
专业分类
电工技术 | 5450篇 |
综合类 | 361篇 |
化学工业 | 43051篇 |
金属工艺 | 11012篇 |
机械仪表 | 9049篇 |
建筑科学 | 7773篇 |
矿业工程 | 1316篇 |
能源动力 | 7326篇 |
轻工业 | 25284篇 |
水利工程 | 2846篇 |
石油天然气 | 4092篇 |
武器工业 | 70篇 |
无线电 | 36637篇 |
一般工业技术 | 54565篇 |
冶金工业 | 52403篇 |
原子能技术 | 5715篇 |
自动化技术 | 27303篇 |
出版年
2021年 | 2359篇 |
2019年 | 2383篇 |
2018年 | 3912篇 |
2017年 | 3996篇 |
2016年 | 4190篇 |
2015年 | 2733篇 |
2014年 | 4693篇 |
2013年 | 13251篇 |
2012年 | 7649篇 |
2011年 | 10269篇 |
2010年 | 8037篇 |
2009年 | 8995篇 |
2008年 | 9960篇 |
2007年 | 9903篇 |
2006年 | 8914篇 |
2005年 | 8163篇 |
2004年 | 7800篇 |
2003年 | 7645篇 |
2002年 | 7284篇 |
2001年 | 7047篇 |
2000年 | 6716篇 |
1999年 | 6777篇 |
1998年 | 14777篇 |
1997年 | 10714篇 |
1996年 | 8646篇 |
1995年 | 6893篇 |
1994年 | 6180篇 |
1993年 | 6048篇 |
1992年 | 4983篇 |
1991年 | 4489篇 |
1990年 | 4428篇 |
1989年 | 4162篇 |
1988年 | 3963篇 |
1987年 | 3559篇 |
1986年 | 3414篇 |
1985年 | 4077篇 |
1984年 | 3834篇 |
1983年 | 3438篇 |
1982年 | 3267篇 |
1981年 | 3355篇 |
1980年 | 3078篇 |
1979年 | 2976篇 |
1978年 | 2796篇 |
1977年 | 3216篇 |
1976年 | 3801篇 |
1975年 | 2608篇 |
1974年 | 2530篇 |
1973年 | 2561篇 |
1972年 | 1993篇 |
1971年 | 1844篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
WKB近似下的Fourier衍射成象方法 总被引:1,自引:0,他引:1
对于介质目标微波衍射成象,本文引入了WKB近似来模拟目标内部总场。基于这种近似,我们导出了Fourier衍射公式,并采用了广义滤波逆传播方法由目标空间谱实现目标特性的重建。计算机模拟结果表明采用WKB近似重建目标特性较Born近似有明显改善。 相似文献
992.
993.
This paper provides a preliminary examination of present and projected land use in Africa to estimate the potential availability of land in 2025 for use in producing biomass energy. Fifty countries are included in the analysis. Future cropland requirements are projected on the basis of average African cereal crop yield improvements since 1972, and minimum nutritional requirements are assumed to be met in 2025 without increasing imports above present absolute levels. Cropland, natural forests and other wilderness areas are excluded from consideration for biomass energy use. Woody biomass energy yields are estimated on the basis of nationally averaged precipitation, using a yield-precipitation correlation for commercial eucalyptus plantations in Brazil. The total African bioenergy production potential in 2025 is estimated to be about 18 EJ per year for a set of baseline assumptions that includes planting only 10% of the available non-crop, non-forest, non-wilderness area with biomass energy crops. A preliminary cost assessment suggests that much of this biomass could be produced for $1–2 GJ−1. A number of uncertainties in the modelling assumptions are examined through a sensitivity analysis. Despite limitations in the model used here, one robust conclusion is that Africa as a whole has a significant biophysical potential for producing biomass energy. This result suggests that more detailed country and sub-country level assessments would be worthwhile to understand better the practical prospects for future biomass energy production in Africa. 相似文献
994.
995.
996.
997.
998.
The effect of na and Sr modification on surface tension and volumetric shrinkage of A356 alloy and their influence on porosity formation 总被引:2,自引:0,他引:2
D. Emadi J. E. Gruzleski J. M. Toguri 《Metallurgical and Materials Transactions B》1993,24(6):1055-1063
Modification of the eutectic Si in Al-Si foundry alloys by adding strontium or sodium is, unfortunately, accompanied by an
increase of porosity in the casting. In an attempt to understand the nature of this problem, this study used a sessile-drop
method to investigate the effect of Sr and Na on surface tension and volumetric shrinkage, two probable causes of porosity
occur-rence. The addition of 0.01 wt pct Sr and 0.005 wt pct Na to A356 alloy decreases the surface tension of the liquid
by about 19 and 10 pct, respectively, and may increase the volume shrink-age by about 12 pct. These changes to surface tension
and volumetric shrinkage promote the early formation of the pores during solidification and give the availability of a longer
period of growth prior to complete solidification, resulting in a larger pore size. The effect of surface tension on the pores
is more significant than volumetric shrinkage. Although the predicted pore diameter increases with lower surface tension or
higher volumetric shrinkage, these two effects alone do not seem able to completely account for the observed increase in porosity
that is associated with modification. 相似文献
999.
Recent work concerning the interstitial modification of permanent magnet alloys based on rare-earth iron intermetallic compounds is reviewed. The crystal structure of the new materials is discussed, then the thermodynamic and statistical aspects of the low-temperature interstitial modification process are considered. Finally, reaction kinetics and thermal stability of the new compounds are discussed and future prospects assessed. 相似文献
1000.
Numerical methods for solving integral equations of an axisymmetric problem of torsion of an elastic space with cracks on the surface of revolution are suggested for the cases of cracks crossing the axis of symmetry and cracks that have no common points with this axis. We also present relations for calculating the stress intensity factors at crack tips. Numerical results are obtained for a conic or paraboloidal simply connected crack and for a doubly connected crack lying on a surface formed by the revolution of an arbitrarily oriented straight segment or a parabolic arc. The crack faces are either subjected to a constant load or free of any forces; the body is subjected to torsion at infinity.Karpenko Physico-Mechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 6, pp. 87–93, November–December, 1993. 相似文献