首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6656篇
  免费   313篇
  国内免费   7篇
电工技术   64篇
综合类   10篇
化学工业   1824篇
金属工艺   112篇
机械仪表   116篇
建筑科学   238篇
矿业工程   16篇
能源动力   151篇
轻工业   1173篇
水利工程   40篇
石油天然气   38篇
无线电   259篇
一般工业技术   778篇
冶金工业   1299篇
原子能技术   36篇
自动化技术   822篇
  2024年   19篇
  2023年   54篇
  2022年   269篇
  2021年   333篇
  2020年   156篇
  2019年   192篇
  2018年   234篇
  2017年   197篇
  2016年   194篇
  2015年   156篇
  2014年   219篇
  2013年   438篇
  2012年   310篇
  2011年   359篇
  2010年   316篇
  2009年   277篇
  2008年   310篇
  2007年   263篇
  2006年   199篇
  2005年   187篇
  2004年   157篇
  2003年   127篇
  2002年   151篇
  2001年   93篇
  2000年   101篇
  1999年   120篇
  1998年   141篇
  1997年   103篇
  1996年   111篇
  1995年   81篇
  1994年   73篇
  1993年   84篇
  1992年   73篇
  1991年   31篇
  1990年   57篇
  1989年   78篇
  1988年   55篇
  1987年   59篇
  1986年   59篇
  1985年   59篇
  1984年   63篇
  1983年   46篇
  1982年   35篇
  1981年   48篇
  1980年   28篇
  1979年   25篇
  1978年   24篇
  1977年   24篇
  1976年   41篇
  1975年   31篇
排序方式: 共有6976条查询结果,搜索用时 15 毫秒
991.
China has set an ambitious target to increase its wind power capacity by 35 GW from 2007 to 2020. The country’s hunger for clean power provides great opportunities for wind energy investors. However, risks from China’s uncertain electricity market regulation and an uncertain energy policy framework, mainly due to uncertain Clean Development Mechanism (CDM) benefits, prevent foreign investors from investing in China’s wind energy. The objectives of this paper are to: (1) quantify wind energy investment risk premiums in an uncertain international energy policy context and (2) evaluate the impact of uncertain CDM benefits on the net present values of wind power projects. With four scenarios, this study simulates possible prices of certified emissions reductions (CERs) from wind power projects. Project net present values (NPVs) have been calculated. The project risk premiums are drawn from different and uncertain CER prices. Our key findings show that uncertain CDM benefits will significantly affect the project NPVs. This paper concludes that the Chinese government needs revising its tariff incentives, most likely by introducing fixed feed-in tariffs (FITs), and re-examining its CDM-granting policy and its wind project tax rates, to facilitate wind power development and enable China to achieve its wind energy target.  相似文献   
992.
The use of biomass for the production of electrical energy is a promising technological solution for those countries where there are problems with the disposal of agricultural waste and/or the production of low-cost energy. The gasification and/or pyrolysis of the biomass produces a gas rich in hydrogen that can be used in a fuel cell system to produce electrical energy with reduced environmental impact and significant energy recovery.In this work, a study of the pyrolysis of Brazilian sugarcane bagasse was carried out. The experimental process consisted of the pyrolysis of the biomass material in a batch pyrolysis reactor. In some runs the biomass was dry, while in others it was pre-treated by the addition of water. It was noted that the water added to the biomass before the pyrolysis process resulted in a decrease in the quantity of steam added to the fuel cell feeding gas, necessary to avoid carbon deposition, and in an increase in cell power, but, at the same time, caused a decrease in the quantity of syngas produced.Then, the composition of the gas obtained from the experimental pyrolysis of the sugarcane was inserted in a simulation tool of a molten carbonate fuel cell system in order to estimate the feasibility of the entire process in terms of operating conditions and electrical performance.The present study indicates that the syngas obtained from the sugarcane biomass (about 40%) can be converted into electricity using a fuel cell system with a high efficiency.  相似文献   
993.
Preliminary laboratory tests carried out on 20 mineral waters indicate that the infrared (IR) method for the determination of total dissolved inorganic carbon (TDIC) is more accurate than the pH-alkalinity method, especially for high-TDIC, naturally sparkling waters and low-TDIC waters. The results of the two analytical methods are similar for mineral waters whose TDIC ranges from 100 to 500 mg/L as HCO3, suggesting a relative accuracy of 0.3–2.9%. The limit of detection (LOD) of the IR method is close to 10 mg HCO3/L and independent of pH. This value is comparable with the LOD of the pH-alkalinity method in the pH range 6.5–9 whereas the LOD of the latter increases strongly with decreasing pH below pH 6.5. The IR method is quick, since a run takes a few minutes and, on average, 10 analyses are carried out in an hour, including the time spent for clean up and preparation between samples. It constitutes a sound alternative to the pH-alkalinity technique for the field measurement of TDIC in geothermal and other natural waters.  相似文献   
994.
Europium-doped ceria (EDC, Ce0.9Eu0.1O2−δ)/alkaline carbonate (LNC, (Li,Na)2CO3) composite ceramics prepared through a one-step citrate-based route were analyzed by powder X-ray diffraction, infrared and laser Raman spectroscopies as well as scanning and transmission electron microscopy. The electrochemical behavior of the electrolyte material was studied by impedance spectroscopy in air, CO2 and N2 + H2 (90/10 vol%, respectively) gas mixtures, in the temperature range 300–600 °C. The sub micrometric and even nanosized ceramic particles appeared as merged inside the mixed carbonates, with modest grain to grain necking. The EDC/LNC composite electrolytes showed a conductivity of 0.27 S cm−1 at 600 °C in air, amongst the best ever reported, exceeding the usual requirements for fuel cell applications.  相似文献   
995.
Silverpoint drawings from the Renaissance are among the most precious and rarest treasures of graphical art. Our research group is particularly interested in the analysis of silverpoint drawings by Albrecht Dürer (1471-1528). A very sensitive and non-destructive analytical method, either spatially resolved synchrotron-radiation induced X-ray fluorescence (SY-XRF) or proton-induced X-ray emission (PIXE), is needed to determine the chemical composition of the very faint silver marks on such drawings. Dürer drawings from the collection of the Albertina, Vienna, were analyzed to amend existing data on Dürer drawings. For this purpose an external-beam PIXE setup was installed at the Vienna Environmental Research Accelerator (VERA). It allows to analyze a spot of ∼0.15 mm on the object in air with 3 MeV protons, and to detect the emitted X-rays that are characteristic for the chemical composition with very good sensitivity and without harming the precious objects. After successful measurements on artificial test samples, four original silverpoint drawings were investigated: two portraits from Albrecht Dürer’s very early period (self-portrait and portrait of his father) and two drawings from Dürer’s sketch book of his travel to the Netherlands 1520/21.  相似文献   
996.
There are at least 607 thousand households in the Brazilian Amazon that need to be attended with some form of regular electricity service. These households are not attended by the electricity sector through its electricity distribution companies and most of them have some form of precarious decentralized electricity generation that is not registered or regulated in the institutional framework. Diverse initiatives were taken by Brazilian government to attend these household through alternatives that relied on locally available renewable energy. This paper accesses this initiatives of rural electrification in the Brazilian Amazon. First an overview of the problems of rural electricity are discussed and its specificities in the Brazilian Amazon. Then the Brazilian institutional framework that organizes the decentralized electricity generation is described with its various limitations. The diverse initiatives undertaken to attend the rural communities in the Amazon since the 1990s are described, as well as how these initiatives are linked to the policies for rural electrification. The results shows that it can be inferred that sole market mechanisms are not sufficient to guarantee economic sustainability of these projects. This can be one of the reasons why traditional electricity distribution companies showed the lack of interest in promoting rural electrification with other means than grid extension. The most successful projects had financed efforts to integrate the generation of electricity into local development initiatives in order to guarantee sustainability and used substantial part of funding for local mobilization and organization. It needs a paradigm chance by treating these initiatives as local development initiatives and promoting alternative ways for its implementation through partnership between local new actors in the electricity sector and government and implementing policy on a local municipal level.  相似文献   
997.
The investigation on incorporating nitrogen group into titanium dioxide in order to obtain powdered visible light-active photocatalysts is presented. The industrial hydrated amorphous titanium dioxide (TiO2·xH2O) obtained directly from sulphate technology installation was modified by heat treatment at temperatures of 100–800 °C for 4 h in an ammonia atmosphere. The photocatalysts were characterized by UV–VIS–DR and XRD techniques. The UV–VIS–DR spectra of the modified catalysts exhibited an additional maximum in the VIS region (, ) which may be due to the presence of nitrogen in TiO2 structure. On the basis of XRD analysis it can be supposed that the presence of nitrogen does not have any influence on the transformation temperature of anatase to rutile. The photocatalytic activity of the modified photocatalysts was determined on the basis of decomposition rate of phenol and azo-dye (Reactive Red 198) under visible light irradiation. The highest rate of phenol degradation was obtained for catalysts calcinated at 700 °C (6.55%), and the highest rate of dye decomposition was found for catalysts calcinated at 500 and 600 °C (ca. 40–45%). The nitrogen doping during calcination under ammonia atmosphere is a very promising way of preparation of photocatalysts which could have a practical application in water treatment system under broader solar light spectrum.  相似文献   
998.
Examined the long-term effects of participating in a field experiment on the effects of control and predictability-enhancing interventions. 40 retirement home residents who had initially benefited from being exposed to a specific positive predictable or controllable event (visits by college students) were assessed at 3 different intervals after the study was terminated. Health and psychological status data collected 24, 30, and 42 mo after the study indicated no positive long-term effects attributable to the interventions. In fact, groups that had initially benefited from the interventions exhibited precipitous declines once the study was terminated, whereas groups that had not benefited remained stable over time. Theoretical and ethical implications are discussed. (11 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
999.
Two studies, using a total of 109 undergraduates as Ss, tested the validity of the bogus pipeline procedure for eliciting truthful responses from Ss in social psychological experiments. Ss were illegitimately informed about how to perform well on an experimental test, were tested, and then were asked whether they had possessed this prior information. As compared to Ss responding to pencil-and-paper questions for face-to-face questioning by the experimenter, those in a bogus pipeline condition confessed more often. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
1000.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号