全文获取类型
收费全文 | 3626篇 |
免费 | 56篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 30篇 |
化学工业 | 438篇 |
金属工艺 | 35篇 |
机械仪表 | 77篇 |
建筑科学 | 119篇 |
矿业工程 | 5篇 |
能源动力 | 66篇 |
轻工业 | 184篇 |
水利工程 | 19篇 |
石油天然气 | 11篇 |
无线电 | 236篇 |
一般工业技术 | 409篇 |
冶金工业 | 1726篇 |
原子能技术 | 46篇 |
自动化技术 | 285篇 |
出版年
2021年 | 23篇 |
2020年 | 19篇 |
2019年 | 22篇 |
2018年 | 31篇 |
2017年 | 25篇 |
2016年 | 28篇 |
2015年 | 37篇 |
2014年 | 33篇 |
2013年 | 106篇 |
2012年 | 64篇 |
2011年 | 83篇 |
2010年 | 73篇 |
2009年 | 68篇 |
2008年 | 95篇 |
2007年 | 97篇 |
2006年 | 70篇 |
2005年 | 70篇 |
2004年 | 67篇 |
2003年 | 61篇 |
2002年 | 60篇 |
2001年 | 58篇 |
2000年 | 62篇 |
1999年 | 92篇 |
1998年 | 459篇 |
1997年 | 302篇 |
1996年 | 208篇 |
1995年 | 114篇 |
1994年 | 95篇 |
1993年 | 106篇 |
1992年 | 47篇 |
1991年 | 49篇 |
1990年 | 35篇 |
1989年 | 50篇 |
1988年 | 51篇 |
1987年 | 33篇 |
1986年 | 40篇 |
1985年 | 55篇 |
1984年 | 42篇 |
1983年 | 35篇 |
1982年 | 41篇 |
1981年 | 57篇 |
1980年 | 31篇 |
1979年 | 30篇 |
1978年 | 31篇 |
1977年 | 78篇 |
1976年 | 152篇 |
1975年 | 30篇 |
1974年 | 18篇 |
1973年 | 31篇 |
1972年 | 20篇 |
排序方式: 共有3686条查询结果,搜索用时 15 毫秒
11.
An incremental iterative process based on direct energy minimization is presented for a limit analysis of nonlinear elastic lateral displacements and twists of reinforced and prestressed beams. Problems encountered with the constitutive relations are discussed and two possible material models are presented. 相似文献
12.
We propose a new, less costly, design to test the equivalence of digital versus analogue mammography in terms of sensitivity and specificity. Because breast cancer is a rare event among asymptomatic women, the sample size for testing equivalence of sensitivity is larger than that for testing equivalence of specificity. Hence calculations of sample size are based on sensitivity. With the proposed design it is possible to achieve the same power as a completely paired design by increasing the number of less costly analogue mammograms and not giving the more expensive digital mammograms to some randomly selected subjects who are negative on the analogue mammogram. The key idea is that subjects who are negative on the analogue mammogram are unlikely to have cancer and hence contribute less information for estimating sensitivity than subjects who are positive on the analogue mammogram. To ascertain disease state among subjects not biopsied, we propose another analogue mammogram at a later time determined by a natural history model. The design differs from a double sampling design because it compares two imperfect tests instead of combining information from a perfect and imperfect test. 相似文献
13.
Human 11 beta-hydroxysteroid dehydrogenase-types 1 and 2 and human 15-hydroxyprostaglandin dehydrogenase belong to a large family of oxidoreductases that includes human dihydropteridine reductase and Streptomyces hydrogenans 20 beta-hydroxysteroid dehydrogenase, for which 3D structures are available. Almost all of these enzymes are either dimers or tetramers. The dimer interface of rat dihydropteridine reductase consists of alpha-helices E and F from each monomer arranged in a four alpha-helix bundle [Varughese et al. (1992) Proc. Natl. Acad. Sci. USA 89, 6080-6084]. Alpha-helix F contains tyrosine-146 and lysine-150, residues that are highly conserved in this protein superfamily and have been proposed to be at the catalytic site. We have examined the dimer interface between alpha-helix F in human and rat dihydropteridine reductase and Streptomyces hydrogenans 20 beta-hydroxysteroid dehydrogenase as well as modeled 3D structures of steroid and prostaglandin dehydrogenases and homologs for stabilizing interactions. We find a site in the middle of alpha-helix F that stabilizes the dimer. This anchor is adjacent to conserved lysine on alpha-helix F. Our analysis suggests that sequence variation in the anchor may be important in substrate specificity. 相似文献
14.
We compared the effects of an allergen challenge on airway responsiveness to methacholine, the slope of the dose-response curve (DRC) and post-methacholine fall in forced vital capacity (FVC) or forced expiratory volume in 1 s (FEV1)/FVC, and determined whether any changes in these parameters were related to the presence and magnitude of the late asthmatic response (LAR) in mild stable asthma. Twenty-three allergic asthmatic subjects had an allergen challenge, preceded and followed 24 (n = 12) and/or 48 (n = 22) h later by a methacholine challenge. Sixteen subjects had a dual asthmatic response to the allergen. On the post-allergen methacholine challenge, as compared with the pre-allergen test, differences in mean fall in FVC or FEV1/FVC at 20% fall in FEV1 and the slope of the DRC did not achieve statistical significance, even in the group with LAR, which showed a significant increase in airway responsiveness at 24 h. There was, however, a correlation between allergen-induced changes in PC20 and (1) the change in post-methacholine FVC fall in the LAR group at 48 h, and (2) the change in the slope of the DRC in the early-asthmatic-response group at 24 h. In conclusion, allergen-challenge-induced changes in airway response to methacholine are heterogeneous among asthmatic subjects and although it may increase airway responsiveness (PC20), particularly in late responders, it minimally affects the other aspects of airway response to methacholine, suggesting that a more powerful or sustained allergic stimulus is required to modify the latter. 相似文献
15.
16.
17.
Baker S. Sim T. Kanade T. 《IEEE transactions on pattern analysis and machine intelligence》2003,25(1):100-109
The complete set of measurements that could ever be used by a passive 3D vision algorithm is the plenoptic function or light-field. We give a concise characterization of when the light-field of a Lambertian scene uniquely determines its shape and, conversely, when the shape is inherently ambiguous. In particular, we show that stereo computed from the light-field is ambiguous if and only if the scene is radiating light of a constant intensity (and color, etc.) over an extended region. 相似文献
18.
19.
The extraction of nitrogenous material from Brewers' spent grain (BSG) by solvents (water, sodium chloride solution, aqueous alcohol, acidic and alkaline solutions) commonly used to extract cereal proteins, resulted in the relatively low (0·2 to 8·5%) recovery of the total nitrogen. The recovery of nitrogenous material by the use of aqueous alkaline solutions was increased from 3–4% to 41% when the BSG was subjected to cellulase incubation prior to extraction. Approximately 75% of the nitrogen was solubilized with trifluoracetic acid. Detergent solutions (acid, neutral) solubilized more than 80% of the total nitrogen of the BSG. The low solubility of the nitrogenous constituents by conventional protein solvents could be the result of association between cellulosic material and protein. 相似文献
20.
Two experiments addressed the issue of whether endorsement of a position by a numerical majority or a minority leads to greater scrutiny of the information presented in a persuasive message. In Exp 1, a counterattitudinal position was endorsed by a majority or a minority and was supported by strong or weak arguments. Argument quality had a larger impact on attitudes with majority than with minority endorsement. In Exp 2, a proattitudinal or a counterattitudinal message was endorsed by a majority or a minority and was supported by strong or weak arguments. When the source and message position were unexpected (i.e., majority-counter and minority-pro messages), argument quality had a larger impact on attitudes than when the source and message position were expected (i.e., majority-pro and minority-counter messages). Thus, either majority or minority endorsement can enhance message scrutiny if the source-position pairing is surprising. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献