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   2836篇
  免费   94篇
  国内免费   1篇
电工技术   58篇
综合类   4篇
化学工业   989篇
金属工艺   50篇
机械仪表   29篇
建筑科学   189篇
矿业工程   30篇
能源动力   32篇
轻工业   355篇
水利工程   10篇
石油天然气   1篇
无线电   147篇
一般工业技术   393篇
冶金工业   221篇
原子能技术   27篇
自动化技术   396篇
  2022年   26篇
  2021年   36篇
  2020年   30篇
  2019年   29篇
  2018年   41篇
  2017年   40篇
  2016年   65篇
  2015年   84篇
  2014年   72篇
  2013年   115篇
  2012年   113篇
  2011年   152篇
  2010年   105篇
  2009年   106篇
  2008年   109篇
  2007年   133篇
  2006年   112篇
  2005年   105篇
  2004年   99篇
  2003年   70篇
  2002年   66篇
  2001年   50篇
  2000年   50篇
  1999年   50篇
  1998年   68篇
  1997年   54篇
  1996年   54篇
  1995年   52篇
  1994年   64篇
  1993年   37篇
  1992年   42篇
  1991年   40篇
  1990年   26篇
  1989年   36篇
  1988年   21篇
  1987年   28篇
  1986年   30篇
  1985年   36篇
  1984年   36篇
  1983年   31篇
  1982年   31篇
  1981年   26篇
  1980年   27篇
  1978年   24篇
  1977年   27篇
  1976年   25篇
  1975年   37篇
  1974年   20篇
  1973年   23篇
  1970年   30篇
排序方式: 共有2931条查询结果,搜索用时 15 毫秒
991.
Scope: Acrylamide (AA), classified as a genotoxic carcinogen, is generated by heating foods. We studied whether the food matrix modulates bioavailability and/or biotransformation and investigated kinetics and biological effectiveness of AA in rats. Methods and results: AA was given to the animals at a daily intake level of AA containing foods for up to 9 days, resulting in an exposure of 50 or 100 μg AA/kg body weight (b.w.)/day. Positive controls received the same dosages of AA in water, negative controls just water. As biomarkers urinary mercapturic acids, hemoglobin adducts, plasma levels of AA and glycidamide (GA) and DNA integrity in white blood cells and hepatocytes were measured. Altogether, no significant differences in bioavailability of AA from water and the different food matrices were observed. Only with bread crust, biomarkers indicated a slightly reduced bioavailability. Monitoring glycidamide valine adduct adducts did not provide evidence for treatment‐related significantly enhanced GA‐haemoglobin adduct formation in blood although glycidamide mercapturic acid excretion in urine indicated significant GA formation. Conclusions: The results suggest AA at dietary intake levels, exceeding estimated human mean intake by a factor of at least 100 to become detoxified in Sprague–Dawley rats to a major extent through glutathione coupling.  相似文献   
992.
The existence of localized vibrational modes both at the positive and at the negative LiNbO3 (0001) surface is demonstrated by means of first-principles calculations and Raman spectroscopy measurements. First, the phonon modes of the crystal bulk and of the (0001) surface are calculated within the density functional theory. In a second step, the Raman spectra of LiNbO(3) bulk and of the two surfaces are measured. The phonon modes localized at the two surfaces are found to be substantially different, and are also found to differ from the bulk modes. The calculated and measured frequencies are in agreement within the error of the method. Raman spectroscopy is shown to be sensitive to differences between bulk and surface and between positive and negative surface. It represents therefore an alternative method to determine the surface polarity, which does not exploit the pyroelectric or piezoelectric properties of the material.  相似文献   
993.
Photocatalytic TiO2 layers are deposited with different methods available within the Fraunhofer Photocatalysis Alliance. These are: Magnetron sputtering, evaporation, sol‐gel, laquer coating, and CVD. The photocatalytic activity under UV illumination is determined by decomposition of stearic acid. The optical properties of the films are compared and discussed.  相似文献   
994.
We report on the growth of AlGaInP quantum dots (QDs) with Al contents between 0% and 10% on GaP substrate by gas-source molecular beam epitaxy and the investigation of their morphological and low temperature photoluminescence properties. These high areal density QDs show short wavelength emission between 575 and 612 nm depending on their composition. The authors interpret the QD emission as originating from indirect type-II transitions. This interpretation is supported by a single-band effective-mass model, which allows us to describe the role of differing barrier composition in the QD emission. Time-resolved photoluminescence measurements are performed and discussed with respect to the calculations.  相似文献   
995.
We present a method to fabricate multiple metal nanogap electrodes of tailored width and distance in parallel, on the cleaved plane of a GaAs/AlGaAs heterostructure. The three-dimensional patterned structures are obtained by a combination of molecular-beam-epitaxial regrowth on a crystal facet, using the cleaved-edge-overgrowth (CEO) method, and subsequent wet selective etching and metallization steps. SEM and AFM studies reveal smooth and co-planar electrodes of width and distance of the order of 10 nm. Preliminary electrical characterization indicates electrical gap insulation in the 100 MΩ range with kΩ lead resistance. We propose our methodology to realize multiple electrode geometries that would allow investigation of the electrical conductivity of complex nanoscale objects such as branched organic molecules.  相似文献   
996.
The graphite calorimeter of the Federal Office of Metrology and Surveying (BEV-Bundesamt für Eich- und Vermessungswesen) was established in the 1980s as the primary standard for the absorbed dose to water for (60)Co gamma ray beams. To maintain the primary standard at an international level the graphite calorimeter and its corresponding components had to undergo a refurbishment and modernisation process. The correction factors of the graphite calorimeter were re-evaluated with Monte Carlo and experimental methods to obtain improved values. These are the correction for the effect of the gaps (1.0061), the scaling correction (0.9998), the correction for the difference in air attenuation (0.9971) and the corrections for the effective measurement depths in the graphite phantom for the graphite calorimeter (0.9886) and the CC01-105 ionisation chamber (0.9913). Consequently, it was necessary to change the reference value for the absorbed dose rate to water of the (60)Co teletherapy unit used for the calibration of secondary standard dosemeters.  相似文献   
997.
Sensory-guided fractionation of a roasted coffee brew by means of sequential solvent extraction, ultrafiltration, and RP-HPLC demonstrated a group of ethyl acetate soluble compounds formed from O-hydroxycinnamoyl quinic acid derivatives upon coffee roasting as the key compounds contributing to the bitter taste of roasted coffee beverages. LC-MS/MS studies, 1D- and 2D-NMR spectroscopy, syntheses, and model roast experiments with 5-O-caffeoyl- and 5-O-feruloylquinic acid led to the unequivocal identification of 3-O-caffeoyl-γ-quinide (2a), 4-O-caffeoyl-γ-quinide (3a), 5-O-caffeoyl-epi-δ-quinide (4a), 4-O-caffeoyl-muco-γ-quinide (5a), 5-O-caffeoyl-muco-γ-quinide (6a), 3-O-feruloyl-γ-quinide (2b), and 4-O-feruloyl-γ-quinide (3b) as intense coffee bitter tastants. Besides these individual bitter compounds, a highly complex and intensely bitter HPLC fraction was isolated from the ethyl acetate extractables of coffee brew. Application of COSY spectroscopy and alkaline hydrolytic degradation gave strong evidence that the bitter taste of that fraction is due to a multiplicity of rather complex quinic acid lactone isomers multiply esterified with p-coumaric acid, caffeic acid, ferulic acid, 3,4-dimethoxycinnamic acid, and quinic acid, respectively. As representatives of this fraction, 3,4-O-dicaffeoyl-γ-quinide (10), 3,5-O-dicaffeoyl-epi-δ-quinide (11), and 4,5-O-dicaffeoyl-muco-γ-quinide (12) have been isolated, purified, and identified as strongly bitter-tasting compounds in roasted coffee. For the first time, bitter taste recognition thresholds were determined for the individual compounds showing that, strongly depending on their chemical structure, the bitter threshold levels ranged between 9.8 and 180 μmol/l (water).  相似文献   
998.
999.
Panels of pine sapwood coated with 30 different coating systems were exposed to natural weathering in Vienna as well as artificial weathering using fluorescent UV-lamps and water. The aim was to compare coating durability in natural and artificial weathering in terms of the exposure time until the panels reached a defined limit state where the coatings required maintenance. For both weathering methods the durability of the coating systems was influenced by film thickness and for the semi-transparent systems also by pigmentation. Three opaque coating systems lasted over 10,000 h of artificial weathering. Comparison of the natural and artificial weathering regarding durability of the coating systems (time to limit state) with the present results (after 30 months natural weathering) revealed a non-linear correlation. The collected data can provide a basis for rough service life estimation of exterior wood coatings based on standardised weathering methods.  相似文献   
1000.
The aim of the study was to investigate if differences in acidification, biogenic amines (BA) and volatile compounds (VC) occurred when Lactobacillus sakei KTU05-6, Pediococcus acidilactici KTU05-07 and Pediococcus pentosaceus KTU05-8 are used as starters in solid state fermentation of flaxseed and the impact of flaxseed sourdough on the wheat bread making process assessed.  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