全文获取类型
收费全文 | 23911篇 |
免费 | 667篇 |
国内免费 | 41篇 |
专业分类
电工技术 | 264篇 |
综合类 | 31篇 |
化学工业 | 4924篇 |
金属工艺 | 549篇 |
机械仪表 | 452篇 |
建筑科学 | 1056篇 |
矿业工程 | 89篇 |
能源动力 | 630篇 |
轻工业 | 1576篇 |
水利工程 | 287篇 |
石油天然气 | 212篇 |
武器工业 | 1篇 |
无线电 | 1980篇 |
一般工业技术 | 3875篇 |
冶金工业 | 4989篇 |
原子能技术 | 180篇 |
自动化技术 | 3524篇 |
出版年
2022年 | 184篇 |
2021年 | 304篇 |
2020年 | 238篇 |
2019年 | 313篇 |
2018年 | 363篇 |
2017年 | 363篇 |
2016年 | 427篇 |
2015年 | 350篇 |
2014年 | 543篇 |
2013年 | 1422篇 |
2012年 | 871篇 |
2011年 | 1091篇 |
2010年 | 877篇 |
2009年 | 905篇 |
2008年 | 1041篇 |
2007年 | 1123篇 |
2006年 | 920篇 |
2005年 | 850篇 |
2004年 | 757篇 |
2003年 | 703篇 |
2002年 | 677篇 |
2001年 | 431篇 |
2000年 | 431篇 |
1999年 | 431篇 |
1998年 | 600篇 |
1997年 | 503篇 |
1996年 | 457篇 |
1995年 | 413篇 |
1994年 | 365篇 |
1993年 | 404篇 |
1992年 | 333篇 |
1991年 | 244篇 |
1990年 | 295篇 |
1989年 | 315篇 |
1988年 | 290篇 |
1987年 | 273篇 |
1986年 | 315篇 |
1985年 | 323篇 |
1984年 | 316篇 |
1983年 | 301篇 |
1982年 | 238篇 |
1981年 | 269篇 |
1980年 | 261篇 |
1979年 | 255篇 |
1978年 | 234篇 |
1977年 | 240篇 |
1976年 | 216篇 |
1975年 | 197篇 |
1974年 | 176篇 |
1973年 | 184篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
In this study, four similar bench-scale submerged Anoxic/Oxic Membrane Bioreactors (MBR) were used simultaneously to investigate the effects of solids retention time (SRT) on organic and nitrogen removal in MBR for treating domestic wastewater. COD removal efficiencies in all reactors were consistently above 94% under steady state conditions. Complete conversion of NH(4+)-N to NO(3-)-N was readily achieved over a feed NH(4+)-N concentration range of 30 to 50 mg/L. It was also observed that SRT did not significantly affect the nitrification in the MBR systems investigated. The average denitrification efficiencies for the 3, 5, 10 and 20 days SRT operations were 43.9, 32.6, 47.5 and 66.5%, respectively. In general, the average effluent nitrogen concentrations, which were mainly nitrate, were about 22.2, 27.6, 21.7 and 13.9 mg/L for the 3, 5, 10 and 20 days SRT systems, respectively. The rate of membrane fouling at 3 days SRT operation was more rapid than that observed at 5 days SRT. No fouling was noted in the 10 days and 20 days SRT systems during the entire period of study. 相似文献
42.
Methyl groups from chain scission and H-crosslinks have been identified by solid-state nuclear magnetic resonance in amorphous ethylene-propylene copolymers containing 23 and 36 mole % propylene after γ-irradiation to 10 MGy at 30°C. G (scission) and G (crosslink) values determined from the n.m.r. spectra and by extraction are in agreement, which suggests that the crosslinks are not clustered. This may differ from the situation in polyethylene where there is a substantial crystalline content. G(S). G(X) and the ratio G(S)/G(X) increase with increasing propylene content of the copolymers. 相似文献
43.
44.
This paper examines the dynamic causal relationships between pollutant emissions, energy consumption, and output for France using cointegration and vector error-correction modelling techniques. We argue that these variables are strongly inter-related and therefore their relationship must be examined using an integrated framework. The results provide evidence for the existence of a fairly robust long-run relationship between these variables for the period 1960–2000. The causality results support the argument that economic growth exerts a causal influence on growth of energy use and growth of pollution in the long run. The results also point to a uni-directional causality running from growth of energy use to output growth in the short run. 相似文献
45.
Collection and evaluation of existing data: an ecological risk assessment perspective. 总被引:1,自引:0,他引:1
James T Markwiese 《Quality Assurance: Good Practice, Regulation, and Law》2003,10(3-4):107-116
All scientific disciplines rely to some degree upon existing data to design new studies, test hypotheses, and make decisions. Because existing data can take many forms, a framework for addressing the quality of these data must be general and comprehensive. By nature of this inclusiveness, quality categories for existing data are necessarily broad. Effective employment of existing data requires the development of specific acceptance criteria from broad data quality categories. A framework is presented for collecting and evaluating existing data with examples of Environmental Protection Agency projects employing a tiered data review. The systematic planning inherent in a tiered review is described and attendant data quality considerations are developed in the context of an ecological risk assessment; specifically, the process is illustrated by defining an ecologically protective concentration of a chemical in soil. 相似文献
46.
47.
Strategies to increase CdTe solar-cell voltage 总被引:1,自引:0,他引:1
There is a significant difference in performance between today's highest efficiency of CdTe solar cells and single-crystal cells of comparable band gap. The largest contribution to this difference is the voltage, where the values for the best CdTe cells are about 230 mV below the best GaAs cells when an appropriate adjustment is made for band gap. CdTe voltage and fill-factor are currently compromised by low recombination lifetime (near 1 ns), low hole density (near 1014 cm− 3), and in some cases an excessive back-contact barrier. Numerical simulation is used to evaluate how combinations of lifetime, carrier density, back electron reflection, and interfacial properties affect voltage and cell performance. Two different strategies for improving voltage and performance are explored. 相似文献
48.
ABSTRACT: Grain of the soft white wheat cultivar Harus was harvested weekly from anthesis to maturity and fructooligosaccharides (FOS) contents were determined by reversed-phase high-performance liquid chromatography. Tests were carried out to determine the effect of adding immature wheat meal to a base flour of cultivar Russ (hard red spring) on the quality characteristics of bread. FOS content was also analyzed in baked bread, and the effect of transglutaminase in improving bread quality was examined. Marked decreases in FOS contents, such as 1-kestose and nystose, were observed with grain maturation. The overall quality of bread appeared to be acceptable, and the added FOS were retained after baking. 相似文献
49.
This research produced evidence that an aspect of visual long-term memory—memory for lateral orientation of pictures—is constrained by a viewer-centered or egocentric reference frame. Subjects in Experiment 1 verbally encoded and then verbally recalled the locations of objects within scenic pictures. Recall of locations in terms of left-right directions (using a viewer-centered frame) exceeded recall of locations in terms of relative proximities to features of the room (using an environmental frame), even if the relative proximities had been verbalized at input. Subjects in Experiment 2 viewed half of a list of pictures directly and the remainder, reflected in a mirror. They then took a test in which they classified old pictures—all viewed directly— as "same-orientation" or "reversed." Performance was much better with a viewer-centered definition of same orientation (Does the picture appear the same way around?) than with an environmental definition (Is the picture the same way around on the screen?), even with forewarning of an environmental orientation test. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
50.
Methods for modeling and managing uncertainty in computer vision systems have received increased attention in recent years. Automatic target recognition is one application area of computer vision where the demands are particularly acute. In this article, fuzzy logic is proposed as a means of handling uncertainty in an expert system structure for automatic target recognition. A new technique for logical inference is described which is well-suited for this type of application. A prototype system has been developed and tested on multisensor and temporal images. the results are compared to a similar expert system which used a numeric uncertainty calculus. 相似文献