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   869篇
  免费   35篇
  国内免费   5篇
电工技术   5篇
综合类   2篇
化学工业   133篇
金属工艺   33篇
机械仪表   34篇
建筑科学   33篇
矿业工程   3篇
能源动力   22篇
轻工业   73篇
水利工程   7篇
石油天然气   7篇
无线电   62篇
一般工业技术   135篇
冶金工业   267篇
原子能技术   26篇
自动化技术   67篇
  2022年   11篇
  2021年   7篇
  2020年   9篇
  2019年   10篇
  2018年   17篇
  2017年   19篇
  2016年   12篇
  2015年   15篇
  2014年   21篇
  2013年   59篇
  2012年   26篇
  2011年   31篇
  2010年   21篇
  2009年   27篇
  2008年   35篇
  2007年   27篇
  2006年   32篇
  2005年   21篇
  2004年   23篇
  2003年   20篇
  2002年   20篇
  2001年   16篇
  2000年   17篇
  1999年   16篇
  1998年   35篇
  1997年   21篇
  1996年   17篇
  1995年   15篇
  1992年   7篇
  1990年   7篇
  1989年   14篇
  1988年   10篇
  1987年   13篇
  1986年   11篇
  1985年   12篇
  1984年   15篇
  1982年   7篇
  1981年   7篇
  1980年   12篇
  1979年   12篇
  1977年   12篇
  1976年   11篇
  1975年   9篇
  1974年   14篇
  1973年   10篇
  1972年   10篇
  1971年   8篇
  1964年   8篇
  1956年   6篇
  1955年   7篇
排序方式: 共有909条查询结果,搜索用时 15 毫秒
901.
Winter navigation is a complex but common operation in north-European sea areas. In Finnish waters, the smooth flow of maritime traffic and safety of vessel navigation during the winter period are managed through the Finnish–Swedish winter navigation system (FSWNS). This article focuses on accident risks in winter navigation operations, beginning with a brief outline of the FSWNS. The study analyses a hazard identification model of winter navigation and reviews accident data extracted from four winter periods. These are adopted as a basis for visualizing the risks in winter navigation operations. The results reveal that experts consider ship independent navigation in ice conditions the most complex navigational operation, which is confirmed by accident data analysis showing that the operation constitutes the type of navigation with the highest number of accidents reported. The severity of the accidents during winter navigation is mainly categorized as less serious. Collision is the most typical accident in ice navigation and general cargo the type of vessel most frequently involved in these accidents. Consolidated ice, ice ridges and ice thickness between 15 and 40 cm represent the most common ice conditions in which accidents occur. Thus, the analysis presented in this article establishes the key elements for identifying the operation types which would benefit most from further safety engineering and safety or risk management development.  相似文献   
902.
The density of a symmetric statistic T = g(X 1, X 2, …, Xn ), for a random sample from a mixed population with density f(x) = pf 1(x) + pf 2), is a binomial mixture of the densities of the statist.ics Tk = g(Xk1 , Xk2 , Xkn ), k = 0, 1, … n. where Xki 's are independent with density f 1(x) if ik and density f 2(x) if i > k. It is shown how to find the distributions of some important symmetric statistics like sample mean, sample variance, and order statistics by using Tk 's. The results are applied to normal and exponential mixtures.  相似文献   
903.
Resistance of Helicobacter pylori to metronidazole often causes failure of commonly used combination drug treatment regimens. We determined the MICs of metronidazole and clarithromycin against 18 H. pylori strains from Peru using tetrazolium egg yolk (TEY) agar. The MIC results obtained by agar dilution with petri dishes were compared with the results found through a miniwell format. The results of the two protocols for measuring drug susceptibility differed by no more than 1 dilution in all cases. On TEY agar, bright-red H. pylori colonies were easy to identify against a yellow background. Sixty-one percent (11 of 18) of the strains were resistant to metronidazole (MIC, > or = 4 micrograms/ml) and 50% (9 of 18) were resistant to clarithromycin (MIC, > or = 0.125 micrograms/ml), whereas none (0 of 5) of the strains tested were resistant to tetracycline (MIC, > or = 1 micrograms/ml). Thus, the prevalence of metronidazole and clarithromycin resistance in Peru is higher than that in developed regions of the world. The miniwell plate with TEY agar allows easy H. pylori colony identification, requires about one-third less of the costly medium necessary for petri dish assaying, conserves space, and yields MICs equivalent to those with agar dilution in petri dishes.  相似文献   
904.
Trait and cultural psychology perspectives on cross-role consistency and its relation to adjustment were examined in 2 individualistic cultures, the United States (N = 231) and Australia (N = 195), and 4 collectivistic cultures, Mexico (N = 199), the Philippines (N = 195), Malaysia (N = 217), and Japan (N = 180). Cross-role consistency in trait ratings was evident in all cultures, supporting trait perspectives. Cultural comparisons of mean consistency provided support for cultural psychology perspectives as applied to East Asian cultures (i.e., Japan) but not collectivistic cultures more generally. Some but not all of the hypothesized predictors of consistency were supported across cultures. Cross-role consistency predicted aspects of adjustment in all cultures, but prediction was most reliable in the U.S. sample and weakest in the Japanese sample. Alternative constructs proposed by cultural psychologists-personality coherence, social appraisal, and relationship harmony-predicted adjustment in all cultures but were not, as hypothesized, better predictors of adjustment in collectivistic cultures than in individualistic cultures. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
905.
Surface reflectance obtained from remote-sensing data is the main input to almost all remote-sensing applications. The availability and special characteristics of Moderate Resolution Imaging Spectroradiometer (MODIS) products have led to their use worldwide. Validation of the MODIS reflectance product is then crucial to provid information on uncertainty in the reflectance data, and in other MODIS products and in the applied surface–atmosphere models. Compact Airborne Spectrographic Imager (CASI) and Système Pour l'Observation de la Terre (SPOT) data, collected during the Network for Calibration and Validation in Earth Observation (NCAVEO) 2006 Field Campaign, were applied to validate daily MODIS reflectance data over a site in the southern UK. The difference in the view geometry of at-nadir CASI and SPOT data and off-nadir MODIS data was dealt with using a semi-empirical bidirectional reflectance distribution function (BRDF) model. The validation results showed that for our particular study site, the absolute errors in the MODIS reflectance product were too large to allow the albedo data to be used directly in climate models. The errors were mainly related to the uncertainties in the MODIS atmospheric variables, the BRDF model, and undetected clouds and cloud shadows. More generally, the study highlights the extreme difficulty of achieving pixel-level validation of coarse spatial resolution satellite sensor data in an environment in which the atmosphere is constantly changing, and in which the landscape is characterized by high space–time heterogeneity.  相似文献   
906.
Both monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) play important roles in lipid metabolism, and diets enriched with either of these two fatty acids are associated with decreased cardiovascular risk. Conventional soybean oil (CSO), a common food ingredient, predominantly contains linoleic acid (LA; C18:2), a n-6 PUFA. Recently, a modified soybean oil (MSO) enriched in oleic acid (C18:1), a n-9 MUFA, has been developed, because of its improved chemical stability to oxidation. However, the effect of the different dietary soybean oils on cardiovascular disease remains unknown. To test whether diets rich in CSO versus MSO would attenuate atherosclerosis development, LDL receptor knock-out (LDLR-KO) mice were fed a Western diet enriched in saturated fatty acids (control), or a Western diet supplemented with 5% (w/w) LA-rich CSO or high-oleic MSO for 12 weeks. Both soybean oils contained a similar amount of linolenic acid (C18:3 n-3). The CSO diet decreased plasma lipid levels and the cholesterol content of VLDL and LDL by approximately 18% (p < 0.05), likely from increased hepatic levels of PUFA, which favorably regulated genes involved in cholesterol metabolism. The MSO diet, but not the CSO diet, suppressed atherosclerotic plaque size compared to the Western control diet (Control Western diet: 6.5 ± 0.9%; CSO diet: 6.4 ± 0.7%; MSO diet: 4.0 ± 0.5%) (p < 0.05), independent of plasma lipid level changes. The MSO diet also decreased the ratio of n-6/n-3 PUFA in the liver (Control Western diet: 4.5 ± 0.2; CSO diet: 6.1 ± 0.2; MSO diet: 2.9 ± 0.2) (p < 0.05), which correlated with favorable hepatic gene expression changes in lipid metabolism and markers of systemic inflammation. In conclusion, supplementation of the Western diet with MSO, but not CSO, reduced atherosclerosis development in LDLR-KO mice independent of changes in plasma lipids.  相似文献   
907.
We report the synthesis of 16 new compounds obtained from kokusaginine and flindersiamine, the main alkaloids isolated from the bark of Balfourodendron riedelianum. The activity of the compounds against axenic cultures of Trypanosoma cruzi epimastigtotes and trypomastigotes, as well as intracellular amastigotes, is described, together with their cytotoxic activity against three different human cell lines. The synthetic strategy for the preparation of the new compounds was based on the reactivity at the C4 position of the furoquinoline core towards nucleophiles. The new derivatives were synthesized by a Buchwald-Hartwig reaction, in most cases under green, solvent-free conditions. Compounds 1 c and 1 e displayed better in-vitro activity against trypomastigotes than benznidazole and nifurtimox (positive controls) with IC50<4 μM. In addition, both compounds were not cytotoxic against the three human cell lines K562 (erytroleukimia), LM2 (breast cancer), and HaCat (keratinocyte). Interestingly, when evaluated against intracellular amastigotes, compound 1 c was able to significantly reduce the number of this parasite form, compared to the negative control.  相似文献   
908.
Phase formation and stability of five component compositionally complex rare earth zirconates (5RE2Zr2O7) were investigated by X-ray diffraction and electron microprobe analysis. Zirconates with different rare earth compositions (LaNdSmEuDy, LaNdSmEuYb, LaNdEuErYb, LaNdDyErYb, SmEuDyYHo, LaYHoErYb, and DyYHoErYb) were synthesized at 1700°C and 2000°C by the solid-state method to investigate the effect of A-site site disorder (δA) on phase stability. Increased site disorder results from mixed cation occupancy with localized crystallographic strain and bond disorder. Compositions LaNdSmEuDy (δA = 4.6) and LaNdSmEuYb (δA = 6.0) produced a single pyrochlore phase and compositions SmDyYHoErYb (δA = 2.8), LaYHoErYb (δA = 6.2), and DyYHoErYb (δA = 1.7) produced a single fluorite phase. High δA compositions LaNdEuErYb (δA = 6.9) and LaNdDyErYb (δA = 7.2) produced a pyrochlore and fluorite phase mixture at 1700°C. Single phase was obtained for the latter composition at 2000°C. Of the single phase compositions calcined at 1700°C, LaNdSmEuYb and LaYHoErYb (both with largest δA) showed decomposition to mixed fluorite and pyrochlore phases during lower temperature anneals, indicating entropic stabilization. Comparison with prior work shows a temperature dependence of the critical δA for phase stability, and compositions near it are expected to be entropy stabilized.  相似文献   
909.
The aim of this study is to examine the impact of motile gyrotactic microorganisms on three-dimensional (3D) cylindrical geometry attached to a Cross-fluid flow mathematical model. The motion of the microorganisms is assumed to be governed by gyrotaxis, which is the tendency of the organisms to orient and swim perpendicular to fluid flow gradients. The study will incorporate the effects of the Cross fluid flow model with infinite shear rate viscosity, 3D cylinder geometry, and microorganism behavior on the resulting distribution and concentration of the organisms. For the inspection of the velocity profile of the Cross nanofluid, the inclined magnetic field is scrutinized. The temperature of Cross nanofluid and its concentration is also studied with several facts. Mass flux and heat flux values for motile microorganisms and nanoparticles are calculated through statistical graphs. Brownian motion parameter gives a lower concentration of nanoparticles, about 81.19% and 77.53% reduction is found in the concentration of motile microorganisms. These results will provide insights into the behavior of these microorganisms in natural and engineered environments, as well as their potential applications in fields such as biotechnology, environmental science, and medicine.  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