全文获取类型
收费全文 | 51154篇 |
免费 | 5768篇 |
国内免费 | 3469篇 |
专业分类
电工技术 | 4538篇 |
技术理论 | 7篇 |
综合类 | 4546篇 |
化学工业 | 8754篇 |
金属工艺 | 2097篇 |
机械仪表 | 2004篇 |
建筑科学 | 4596篇 |
矿业工程 | 2116篇 |
能源动力 | 3208篇 |
轻工业 | 2761篇 |
水利工程 | 1638篇 |
石油天然气 | 3400篇 |
武器工业 | 358篇 |
无线电 | 3614篇 |
一般工业技术 | 5400篇 |
冶金工业 | 3992篇 |
原子能技术 | 837篇 |
自动化技术 | 6525篇 |
出版年
2024年 | 229篇 |
2023年 | 970篇 |
2022年 | 1773篇 |
2021年 | 2047篇 |
2020年 | 1976篇 |
2019年 | 1659篇 |
2018年 | 1548篇 |
2017年 | 1827篇 |
2016年 | 2027篇 |
2015年 | 2053篇 |
2014年 | 3248篇 |
2013年 | 3397篇 |
2012年 | 3794篇 |
2011年 | 4314篇 |
2010年 | 3213篇 |
2009年 | 3255篇 |
2008年 | 2979篇 |
2007年 | 3306篇 |
2006年 | 2818篇 |
2005年 | 2296篇 |
2004年 | 1834篇 |
2003年 | 1581篇 |
2002年 | 1392篇 |
2001年 | 1141篇 |
2000年 | 954篇 |
1999年 | 849篇 |
1998年 | 681篇 |
1997年 | 541篇 |
1996年 | 537篇 |
1995年 | 393篇 |
1994年 | 351篇 |
1993年 | 240篇 |
1992年 | 216篇 |
1991年 | 155篇 |
1990年 | 135篇 |
1989年 | 110篇 |
1988年 | 112篇 |
1987年 | 56篇 |
1986年 | 44篇 |
1985年 | 46篇 |
1984年 | 49篇 |
1983年 | 27篇 |
1982年 | 29篇 |
1981年 | 30篇 |
1980年 | 24篇 |
1979年 | 11篇 |
1964年 | 12篇 |
1961年 | 10篇 |
1959年 | 9篇 |
1955年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
Strength reduction factors (SRFs) continue to play a key role in obtaining design forces from elastic design spectra (via
response modification factors) in ductility-based earthquake-resistant design. Despite several years of sustained research
efforts, it has not been conclusively shown how SRF for a given singledegree-of-freedom structural system depends on various
source and site parameters. A parametric study is carried out here for the explicit dependence of SRF spectrum (describing
variation of SRF with system period for a given ductility demand) on strong motion duration, earthquake magnitude, geological
site conditions, and epicentral distance in case of (non-degrading) elasto-plastic oscillators. For this, scaled response
spectra are considered for different combinations of earthquake magnitude, site conditions and epicentral distance, and SRF
spectra are generated from 1274 accelerograms recorded in western USA after making those compatible with each of these spectra.
It is shown that there is no clear and significant dependence of SRF spectrum on strong motion duration. Further, the parametric
dependence on earthquake magnitude, site conditions, and epicentral distance broadly conforms to the trends reported by earlier
investigations. In particular, this study confirms that the dependence of SRF spectra on earthquake magnitude should not be
ignored.
This paper has been contributed in honour of Professor R N Iyengar, Indian Institute of Science, on the occasion of his formal
retirement. 相似文献
122.
Burns G. Leonard; Gomez Rapson; Walsh James A.; de Moura Marcela Alves 《Canadian Metallurgical Quarterly》2003,15(1):118
G. J. DuPaul (see record 2003-02033-012) offered two suggestions for additional research to understand the strong source effects reported by R. Gomez, G. L Burns, J. A. Walsh, and M. A. de Moura (see record 2003-02033-001) in attention-deficit/ hyperactivity disorder (ADHD) rating scales. The first suggestion was to determine whether the source effects represent mostly bias or accuracy. The second suggestion was to minimize source effects through the development of better ADHD rating scales. Because source effects can represent bias or accuracy, it is important to minimize the bias aspect through content validation procedures prior to attempts to determine whether source effects better reflect bias or accuracy. This comment offers various suggestions to reduce the bias in ADHD rating scales. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
123.
Ahmad Nozad Golikand Elaheh Lohrasbi Mohammad Ghannadi Maragheh Mehdi Asgari 《Journal of Applied Electrochemistry》2008,38(6):869-874
The effect of carbon surface oxidation on platinum supported carbon particles (Pt/C) with nitric acid was investigated by
cyclic voltammetry, electrochemical impedance spectroscopy, polarization experiments and chronoamperometry. Cyclic voltammograms,
polarization curves and electrochemical impedance spectra showed that the treated catalyst had much larger active surface
area and higher ionic conductivity than the untreated catalyst, and provided enhanced performance for oxygen reduction. The
formation of acidic groups was examined by IR spectra. The Pt/C surface oxidation had a large effect on the performance of
a gas diffusion electrode for oxygen reduction reaction. 相似文献
124.
Namsin Park Takeyuki Shiraishi Kazuyoshi Kamisugi Yoshitaka Hara Keita Iizuka Takashi Kado Shuzi Hayase 《Journal of Applied Electrochemistry》2008,38(3):371-375
A direct ethanol fuel cell (DEFC), which is less prone to ethanol crossover, is reported. The cell consists of PtRu/C catalyst as the anode, Nafion® 117 membrane, and Ni–Co–Fe (NCF) composite catalyst as the cathode. The NCF catalyst was synthesized by mixing Ni, Co, and Fe complexes into a polymer matrix (melamine-formaldehyde resins), followed by heating the mixture at 800 °C under inert atmosphere. TEM and EDX experiments suggest that the NCF catalyst has alloy structures of Ni, Co and Fe. The catalytic activity of the NCF catalyst for the oxygen reduction reaction (ORR) was compared with that of commercially available Pt/C (CAP) catalyst at different ethanol concentrations. The decrease in open circuit voltage (Voc) of the DEFC equipped with the NCF catalysts was less than that of CAP catalyst at higher ethanol concentrations. The NCF catalyst was less prone to ethanol oxidation at cathode even when ethanol crossover occurred through the Nafion®117 film, which prevents voltage drop at the cathode. However, the CAP catalyst did oxidize ethanol at the cathode and caused a decrease in voltage at higher ethanol concentrations. 相似文献
125.
126.
详细介绍了ZPG系列盘式真空过滤机的主要特点、基本原理和主要技术参数。对盘式真空过滤机的过滤理论进行了系统的研究 ,以及为提高过滤系数和降低滤饼水分所采取的关键措施。与传统的真空过滤机进行了对比 ,表明ZPG系列盘式真空过滤机的滤饼水分可以降低 1 %~ 3 % ,过滤系数提高 3 0 %~5 0 % ,过滤每吨精矿的能耗可以降低 5 0 %左右 ,使得ZPG系列盘式真空过滤机在国内各大冶金矿山得到全面推广使用 ,创造了巨大的经济效益和社会效益。 相似文献
127.
The photovoltaic (PV) power systems in Japan made great strides in the past decade. The PV industry in the 1990s greatly depended on the research projects and dissemination programs carried out by the Ministry of International Trade and Industry. However, the industrial structure for full-scale deployment of the PV system is currently being established by the manufacturers’ continuous efforts to reduce the PV system cost and the government’s consecutive supports to create the initial market. It is expected that the synergetic effect of the cost reduction and incentives for introduction will activate the PV industry and its market more and more toward achieving 5000 MW of the capacity in FY 2010. 相似文献
128.
The reversible persistent changes of the fill factor (FF) induced by the illumination and voltage bias along with changes in the electronic properties of the ZnO/CdS/Cu(In,Ga)Se2 photovoltaic devices have been studied. Admittance spectroscopy and capacitance–voltage characterization reveal a correlation between the FF and the space charge distribution within the absorber. Our experiments provide evidence that a major source of FF loss in efficient devices is caused by excess negative charge close to the interface. We explain the persistent changes in the net acceptor concentration in the interface region by the relaxation effects due to compensating donors—the same mechanism, which leads to metastable changes of the doping level in the bulk of the absorber. 相似文献
129.
Flammability ranking of foliage species by factor analysis of physical and chemical pyric properties
In this paper, factor analysis is introduced to evaluate the flammability of 55 foliage species that may be used in China for construction of the fuel break network of forest strips with lower flammability. Six pyric parameters, i.e. air dry moisture content, absolute dry moisture content, ignition point, ash content, caloric value and extractive content, are measured and used as variables for factor analysis. The covariance analysis shows that four principal factors can be extracted to reflect the flammability in different physical and chemical senses. In terms of the contributions of the four factors to the variances and the physical significance of the relevant parameters, the four factors are, respectively, termed as ‘flaming factor,’ ‘air dry factor,’ ‘ash factor’ and ‘absolute dry factor.’ The stability of the factor analysis method is examined by a different number of samples considered, and the variation degrees of the orderings indicate that the method has high reliability to measure the total flammability of foliage species. The results of the flammability evaluation are verified by comparison with the recommended tree species in the Chinese technology standard. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
130.