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   5257篇
  免费   222篇
  国内免费   35篇
电工技术   114篇
综合类   16篇
化学工业   819篇
金属工艺   104篇
机械仪表   160篇
建筑科学   245篇
矿业工程   15篇
能源动力   191篇
轻工业   256篇
水利工程   33篇
石油天然气   12篇
无线电   898篇
一般工业技术   881篇
冶金工业   878篇
原子能技术   42篇
自动化技术   850篇
  2023年   31篇
  2022年   60篇
  2021年   99篇
  2020年   66篇
  2019年   71篇
  2018年   132篇
  2017年   99篇
  2016年   97篇
  2015年   95篇
  2014年   138篇
  2013年   299篇
  2012年   204篇
  2011年   262篇
  2010年   218篇
  2009年   245篇
  2008年   271篇
  2007年   256篇
  2006年   219篇
  2005年   185篇
  2004年   178篇
  2003年   181篇
  2002年   139篇
  2001年   137篇
  2000年   121篇
  1999年   123篇
  1998年   308篇
  1997年   189篇
  1996年   140篇
  1995年   110篇
  1994年   96篇
  1993年   87篇
  1992年   54篇
  1991年   43篇
  1990年   64篇
  1989年   77篇
  1988年   53篇
  1987年   40篇
  1986年   47篇
  1985年   44篇
  1984年   29篇
  1983年   14篇
  1982年   20篇
  1981年   15篇
  1980年   17篇
  1979年   13篇
  1978年   17篇
  1977年   24篇
  1976年   26篇
  1973年   12篇
  1971年   7篇
排序方式: 共有5514条查询结果,搜索用时 15 毫秒
991.
In this article, the multi-objective flexible flow shop scheduling problem with limited intermediate buffers is addressed. The objectives considered in this problem consist of minimizing the completion time of jobs and minimizing the total tardiness time of jobs. A hybrid water flow algorithm for solving this problem is proposed. Landscape analysis is performed to determine the weights of objective functions, which guide the exploration of feasible regions and movement towards the optimal Pareto solution set. Local and global neighbourhood structures are integrated in the erosion process of the algorithm, while evaporation and precipitation processes are included to enhance the solution exploitation capability of the algorithm in unexplored neighbouring regions. An improvement process is used to reinforce the final Pareto solution set obtained. The performance of the proposed algorithm is tested with benchmark and randomly generated instances. The computational results and comparisons demonstrate the effectiveness and efficiency of the proposed algorithm.  相似文献   
992.
W. C. Ng  K. L. Mak  M. K. Li 《工程优选》2013,45(11):1039-1054
In a land-scarce container terminal, congestions in the terminal yard due to highly concentrated workload often lead to unsatisfactory terminal productivity. Currently, yard planners use their experience to design a yard template for determining the storage locations of export containers to be loaded onto vessels deployed to services with a cyclical calling pattern. This article studies the problem of designing a yard template that balances the workload in an export yard. The template design problem is formulated as an integer program. It is found that the computational time required to optimally solve realistic sizes of the template design problem is unacceptably long. This article proposes a simpler integer program as an approximate model. On the basis of the analysis on the approximate model, a heuristic is developed to solve the template design problem. Results of computational experiments show that the heuristic can find effective solutions for the template design problem.  相似文献   
993.
Lo PC  Fong WP  Ng DK 《ChemMedChem》2008,3(7):1110-1117
A series of silicon(IV) phthalocyanines with two axial isopropylidene-protected galactose moieties and one, two, or eight chloro group(s) on the periphery of the macrocycle have been synthesised and spectroscopically characterised. The photophysical properties and in vitro photodynamic activities of these compounds have been studied and compared with those of the nonchlorinated analogue. All the compounds, with the exception of the octachlorinated counterpart which has a limited solubility, are essentially nonaggregated in N,N-dimethylformamide. The fluorescence quantum yield decreases and the singlet oxygen quantum yield increases as the number of chloro substituent increases, which is in accord with the heavy-atom effect. The non-, mono-, and dichlorinated phthalocyanines formulated with Cremophor EL are all photodynamically active against HT29 human colon adenocarcinoma and HepG2 human hepatocarcinoma cells with IC(50) values ranging from 0.03 to 1.05 microM. The photocytotoxicity as well as the efficiency to generate intracellular reactive oxygen species decrease along this series because of the increase in aggregation tendency upon chloro substitution. The nonchlorinated analogue exhibits the highest potency and can target the lysosomes of HT29 cells, whereas the monochlorinated counterpart is not localised in the lysosomes.  相似文献   
994.
CH4/CO2 reforming over La2NiO4 and 10%NiO/CeO2–La2O3 catalysts under the condition of supersonic jet expansion was studied via direct monitoring of the reactants and products using the sensitive technique of cavity ring-down spectroscopy. Vibration–rotational absorption lines of CH4, H2O, CO2 and CO molecules were recorded in the near infrared spectral region. Our results indicated that La2NiO4 is superior to 10%NiO/CeO2–La2O3 in performance. In addition, we observed enhanced reverse-water-gas-shift reaction at augmented reaction temperature. The formation of reaction intermediates was also investigated by means of time-of-flight mass spectrometry and there was the detection of CHx+, OH+ and H+ species.  相似文献   
995.
S. Wik  S. Ng  Z. Li  L.S. Kotlyar 《Fuel》2008,87(7):1413-1421
Variability in ore composition and process parameters is known to affect bitumen recovery from natural oilsands. In this work, we extend our earlier studies with model oilsands (MOS) systems to investigate the effects caused by the presence of asphaltene and divalent cations on the interaction between bitumen and silica particles. The absence of both asphaltene and fines was found to have significant effects on bitumen recovery. With a typical oilsands, addition of small amounts of caustic improves recovery but overdosing causes a drastic loss of yield through bitumen emulsification. Without asphaltene and in the presence of fines, the typical drop in bitumen recovery above pH 10 did not occur, indicating that the source of emulsifying agents had been eliminated. With deasphalted bitumen, recovery in the absence of silica fines was very poor at all NaOH dosages. The strong, negative effect of divalent cations on “normal” bitumen recovery was almost completely eliminated when asphaltene was absent.  相似文献   
996.
OBJECTIVES: An experiment was conducted to assess the effects of retrospective and combined retrospective and concurrent feedback on driver performance and engagement in distracting activities. BACKGROUND: A previous study conducted by the authors showed that concurrent (or real time) feedback can help drivers better modulate their distracting activities. However, research also shows that concurrent feedback can pose additional distractions due to the limited time and resources available during driving. Retrospective feedback, which is presented at the end of a trip (i.e., post-drive), can include additional information on safety critical situations during a trip and help the driver learn safe driving habits. METHOD: A driving simulator study was conducted with 48 participants and 3 conditions: retrospective feedback, combined feedback (both retrospective and concurrent), and no feedback (baseline case). RESULTS: The feedback conditions (retrospective and combined) resulted in faster response to lead vehicle braking events as depicted by shorter accelerator release times. Moreover, combined feedback also resulted in longer glances to the road. CONCLUSIONS: The results suggest that both feedback types have potential to improve immediate driving performance and driver engagement in distractions. APPLICATION: Combined feedback holds the most promise for mitigating the effects of distraction from in-vehicle information systems.  相似文献   
997.
Studies show that teenage drivers are at a higher risk for crashes. Opportunities to engage in technology and non-technology based distractions appear to be a particular concern among this age group. An ordered logit model was developed to predict the likelihood of a severe injury for these drivers and their passenger using a national crash database (the 2003, U.S. DOT-General Estimate System [GES]). As one would expect, speeding substantially increases the likelihood of severe injuries for teenage drivers and their passengers. The results of the analysis also reveal that teenage drivers have an increased likelihood of more severe injuries if distracted by a cell phone or by passengers than if the source of distraction was related to in-vehicle devices or if the driver was inattentive. Additionally, passengers of teenage drivers are more likely to sustain severe injuries when their driver is distracted by devices or passengers than with a non-distracted or inattentive driver. This supports the previous literature on teenage drivers and extends our understanding of injuries for this age group related to distraction-related crashes.  相似文献   
998.
This study addresses the simulation of ion chromatographic (IC) separations performed under conditions where the elution profile consists of a sequence of isocratic and gradient elution steps (referred to as "complex elution profiles"). First, models for prediction of retention under gradient elution conditions in IC were evaluated using an extensive database of gradient elution retention data. It is shown that one such model is preferred on the basis that it can be used to predict gradient retention times on the basis of isocratic input data. A method is then proposed for using this model for complex elution profiles whereby each step of the elution profile is treated separately and analyte movement through the column is mapped. An empirically based algorithm for predicting peak width under complex elution conditions is also proposed. Evaluation of the suggested approaches was undertaken on a set of 24 analyte anions and 13 analyte cations on 5 different Dionex columns using a range of 5-step complex elution profiles that gave R2 values for correlations between predicted and observed retention times of 0.987 for anions and 0.997 for cations. The simulation of separations of anions and cations using a 3-step complex elution profile is demonstrated, with good correlation between observed and predicted chromatograms. The proposed approach is useful for the rapid development of separations when complex elution profiles are used in IC.  相似文献   
999.
For the first time femtosecond-laser writing has inscribed low-loss optical waveguides in Schott BK7 glass, a commercially important type of borosilicate widely used in optical applications. The use of a variable repetition rate laser enabled the identification of a narrow processing window at 1 MHz repetition rate with optimal waveguides exhibiting propagation losses of 0.3 dB/cm and efficient mode matching to standard optical fibers at a 1550 nm wavelength. The waveguides were characterized by complementary phase contrast and optical transmission microscopy, identifying a micrometer-sized guiding region within a larger complex structure of both positive and negative refractive index variations.  相似文献   
1000.
In this study,waste tire was used as raw material for the production of activated carbons through pyrolysis.Tire char was first produced by carbonization at 550℃ under nitrogen.A two factorial design was used to optimize the production of activated carbon from tire char.The effects of several factors controlling the activation process,such as temperature(850-950 ℃),time(2-6 h) and percentage of carbon dioxide(70%-100%) were investigated.The production was described mathematically as a function of these three factors.First order modeling equations were developed for surface area,yield and mesopore volume.It was concluded that the yield,BET surface area and mesopore volume of activated carbon were most sensitive to activation temperature and time while percentage of carbon dioxide in the activation gas was a less significant factor.  相似文献   
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