全文获取类型
收费全文 | 2311篇 |
免费 | 21篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 8篇 |
综合类 | 1篇 |
化学工业 | 149篇 |
金属工艺 | 30篇 |
机械仪表 | 19篇 |
建筑科学 | 9篇 |
能源动力 | 16篇 |
轻工业 | 33篇 |
石油天然气 | 1篇 |
无线电 | 66篇 |
一般工业技术 | 171篇 |
冶金工业 | 1757篇 |
原子能技术 | 3篇 |
自动化技术 | 70篇 |
出版年
2022年 | 4篇 |
2021年 | 17篇 |
2020年 | 7篇 |
2019年 | 5篇 |
2018年 | 10篇 |
2017年 | 8篇 |
2016年 | 18篇 |
2015年 | 11篇 |
2014年 | 14篇 |
2013年 | 35篇 |
2012年 | 22篇 |
2011年 | 24篇 |
2010年 | 20篇 |
2009年 | 22篇 |
2008年 | 16篇 |
2007年 | 24篇 |
2006年 | 21篇 |
2005年 | 21篇 |
2004年 | 17篇 |
2003年 | 22篇 |
2002年 | 18篇 |
2001年 | 11篇 |
2000年 | 12篇 |
1999年 | 50篇 |
1998年 | 487篇 |
1997年 | 279篇 |
1996年 | 205篇 |
1995年 | 128篇 |
1994年 | 111篇 |
1993年 | 109篇 |
1992年 | 24篇 |
1991年 | 23篇 |
1990年 | 26篇 |
1989年 | 36篇 |
1988年 | 38篇 |
1987年 | 46篇 |
1986年 | 24篇 |
1985年 | 30篇 |
1984年 | 13篇 |
1983年 | 17篇 |
1982年 | 15篇 |
1981年 | 13篇 |
1980年 | 24篇 |
1978年 | 11篇 |
1977年 | 66篇 |
1976年 | 136篇 |
1975年 | 8篇 |
1973年 | 6篇 |
1972年 | 5篇 |
1970年 | 5篇 |
排序方式: 共有2333条查询结果,搜索用时 15 毫秒
1.
2.
3.
DH Kang CL Coe DO McCarthy NN Jarjour EA Kelly RR Rodriguez WW Busse 《Canadian Metallurgical Quarterly》1997,17(8):481-487
T cell cytokines play an important role in mediating airway inflammation in asthma. The predominance of a Th2 cytokine profile, particularly interleukin (IL)-4 and IL-5, is associated with the pathogenesis and course of asthma. The aim of this study was to test the hypothesis that a stressful life event alters the pattern of cytokine release in asthmatic individuals. Thirteen healthy controls and 21 asthmatic adolescents gave blood samples three times over a semester: midsemester, during the week of final examinations, and 2-3 weeks after examinations. Interferon-gamma (IFN-gamma), IL-2, IL-4, and IL-5 were measured from supernatants of cells stimulated with PHA/PMA for 24 h. Cells from asthmatic subjects released significantly more IL-5 during the examination and postexamination periods, whereas cells from healthy controls released significantly more IL-2 during the midsemester and examination periods, thereby indicating a bias for a Th2-like pattern in asthmatics and a Th1-like pattern in healthy controls. IL-4 and IL-5 production showed a marked decrease during and after examinations in healthy controls, whereas this decline was absent in asthmatics. The ratios of IFN-gamma:IL-4 and IFN-gamma:IL-5 also revealed significant changes in the profile of cytokine release across the semester. These results indicate differential cytokine responses in asthmatics that may become pronounced during periods of cellular activation. 相似文献
4.
5.
6.
Conversion of acrylonitrile-based precursors to carbon fibres 总被引:1,自引:0,他引:1
Mukesh K. Jain M. Balasubramanian P. Desai A. S. Abhiraman 《Journal of Materials Science》1987,22(1):301-312
The progress of stabilization of two compositions of acrylic fibres with various orientations has been followed by a variety of techniques. The thermooxidative treatments for stabilization have been carried out in a continuous process and also in a batch process under free shrinkage, constant length and constant tension conditions. The morphological model of acrylic fibres consists of an alternating sequence of laterally ordered and laterally disordered regions along the fibre direction. This structure is consistent with the observations based on small-angle X-ray scattering of copper- impregnated precursor fibres and thermomechanical response, thermal stress development, calorimetry, wide- and small-angle X-ray scattering and sonic modu-lus measured at different extents of stabilization. Lateral as well as orientational order in these fibres can be increased markedly through a high-temperature deformation process prior to stabilization. An increase in perfection and extent of order is observed in the early stages of stabilization. There is also a simultaneous decrease in the orientation of the disordered phase at this stage and the extent of this decrease depends on the axial constraints imposed on the fibre. Little difference in the rate of stabilization is observed as measured by density or oxygen uptake for fibres with different extents of orientation, lateral order or restraint. Fibres containing itaconic add, a stabilization catalyst did show an increased rate of stabilization. Inferences have been drawn regarding additional research pertaining to achieving high order in precursor fibres, minimizing orientational relaxation during oxidative stabilization, and the techniques for monitoring the extents of the stabilization treatment and the changes in relevant morphological parameters. 相似文献
7.
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
8.
This paper examines the feasibility of extracting three-dimensional (3-D) or topographic information in spotlight mode stereo synthetic aperture radar (SAR). A display of a SAR (intensity) image has two axes: range and cross-range. Elevated scatterers appear closer in range; this phenomenon is called radar image layover. How the height of each scatterer can be computed from the difference in its layover between two images is investigated. This is analogous to computing height from disparity distance (triangulation) in optical stereo. The same procedure can be applied on pixel by pixel basis for terrain elevation mapping. A general expression is derived for the accuracy of the height estimate as a function of the range resolution and the angular difference between the image planes. Accuracy increases as the angle between the image planes increases, but the bright scatterers in one image tend to fade in the other image. This limited angular persistence of radar scatterers is also discussed. Trajectories for data collection are examined that provide near-optimal height estimates while eliminating the scatterer persistency problem. 相似文献
9.
An amplified primer extension assay has been developed for quantitatively mapping the sites of psoralen photoaddition to DNA. This assay was applied to a torsionally tuned Z-DNA-probe that was specifically designed for the primer extension assay. The torsionally tuned Z-DNA forming sequence, (CG)6TA(CG)2(TG)8, forms Z-DNA in vitro at negative superhelical density: sigma = -0.05. The internal 5'-TA dinucleotide was reactive to psoralen when it existed as B-DNA. Upon the formation of Z-DNA, the internal 5'-TA no longer photobound psoralen. The torsionally tuned sequence was synthesized as an EcoRI fragment such that, when Z-DNA formed, the central 5'-AATT of the EcoRI sites was part of the B-Z junctions. The 5'-AATT sequence was not reactive with psoralen when it existed as B-DNA. When the 5'-AATT sequence existed as a B-Z junction, one strand of each junction became hyperreactive to psoralen. The TT directly 5' to the B-DNA-Z-DNA junction photobound psoralen in a strand-specific fashion. Quantitation of the relative rate of psoralen photobinding to the internal 5'-TA and the 5'-AATT at the B-Z junctions provides relationships that are characteristic of the level of supercoiling in DNA. 相似文献
10.