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   16058篇
  免费   573篇
  国内免费   8篇
电工技术   111篇
综合类   10篇
化学工业   5805篇
金属工艺   184篇
机械仪表   369篇
建筑科学   533篇
矿业工程   24篇
能源动力   447篇
轻工业   3862篇
水利工程   137篇
石油天然气   84篇
无线电   643篇
一般工业技术   2094篇
冶金工业   829篇
原子能技术   56篇
自动化技术   1451篇
  2024年   173篇
  2023年   231篇
  2022年   1023篇
  2021年   1165篇
  2020年   538篇
  2019年   556篇
  2018年   551篇
  2017年   606篇
  2016年   654篇
  2015年   505篇
  2014年   662篇
  2013年   1063篇
  2012年   1001篇
  2011年   1106篇
  2010年   843篇
  2009年   784篇
  2008年   727篇
  2007年   684篇
  2006年   525篇
  2005年   394篇
  2004年   337篇
  2003年   308篇
  2002年   277篇
  2001年   180篇
  2000年   121篇
  1999年   150篇
  1998年   214篇
  1997年   170篇
  1996年   147篇
  1995年   107篇
  1994年   80篇
  1993年   87篇
  1992年   77篇
  1991年   61篇
  1990年   42篇
  1989年   41篇
  1988年   49篇
  1987年   33篇
  1986年   48篇
  1985年   44篇
  1984年   31篇
  1983年   28篇
  1982年   21篇
  1981年   20篇
  1980年   28篇
  1979年   25篇
  1978年   22篇
  1977年   19篇
  1976年   19篇
  1975年   14篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
    
Conditions for maximal batch culture production of extracellular-L-rhamnosidase byAspergillus terreus have been investigated. Production of the enzyme appeared to be inducible by rhamnose and rutin, reaching a maximal level after an incubation period of 162 h when the fungus was grown at 37°C on either of these compounds as the carbon source and on ammonium phosphate as the nitrogen source. Nonionic surfactants did not enhance-L-rhamnosidase secretion. Under optimal conditions,A. terreus produced only one-L-rhamnosidase of approximate molecular weight 90 kDa (SDS-PAGE) and isoelectric point of 4.6. Onp-nitrophenyl--L-rhamnopyranoside as substrate, the enzyme showed pH and temperature optima of 6–8 and 45–50°C, respectively. Neither divalent cations nor ethylenediaminetetraacetate (EDTA) inhibited or stimulated enzyme activity. The enzyme was active at the concentrations of glucose found in must or of ethanol in wine.  相似文献   
212.
The effects of natural fermentation upon phytic acid and less phosphorylated inositol phosphates ofLens culinaris var vulgaris cultivar Magda-20 were investigated. Seven fermentation runs were made following a 22 complete factorial design with three replicated centre points to study the effect of different conditions of temperature (28, 35 and 42°C) and broth concentration (79, 150 and 221 g/l). Samples were taken for each of them at daily intervals (0, 24, 48, 72 and 96 h). The pH value declined sharply in the first 24 h of fermentation, becoming stabilized from this time. The relation between lactic acid and titratable acidity presented important differences between the different fermentations, ranging from 30–80%. Phytic acid (IP 6), inositol pentakis (IP 5), tetrakis (IP 4) and tris-(IP 3) phosphates were quantitatively determined. The content of total inositol phosphates showed a maximum reduction of 63% at 72 h under the fermentation conditions of 42°C and 79 g/l.  相似文献   
213.
The compositions of the essential oils of Rosmarinus officinalis L, Salvia lavandulifolia V and Lavandula latifolia M, obtained by steam distillation from plants cultivated in the northeast of Spain, have been studied by gas chromatography and gas chromatography/mass spectrometry. The essential oil of rosemary appears to be more complex and richer in flavour notes than other previously studied Spanish rosemary oils, and has an intermediate ratio of α-pinene and of 1,8-cineole in relation to essential oils of rosemary of different geographical origins, but higher proportions of camphor, verbenone and linalool. The essential oil of sage has proportions of 1,8-cineole, camphor, endo-borneol, trans-caryophyllene and α-humulene closer to commercial Salvia officinalis L than to other Spanish sages of different geographical locations, and the difference from commercial Salvia officinalis L is due principally to the higher ratio of terpene hydrocarbons and the lower proportion of thujone in the Spanish sage studied here. The essential oil of the spike lavender has the same main components and similar proportions as other Spanish wild and commercial spike lavender essen-tial oils, but shows differences in the nature of the minor components (rhodinol fraction).  相似文献   
214.
Summary In the present work, the composition of commercial samples of Spanish red vermouths was analysed. Physicochemical parameters such as alcoholic concentration, pH, total sugars, dry extract and ashes were determined following Spanish Official Methods. Most of the samples comply with Spanish legislation for fortified wines. The major volatile components were evaluated by direct gas chromatography using 4-methyl-2-pentanol (Merck) as internal standard. It has been estimated that the concentration of higher alcohols in red vermouth is about twice the concentration in white wines.
Charakterisierung der spanischen roten Wermut-Weine
Zusammenfassung In dieser Arbeit wurde die Zusammensetzung von Handelsproben spanischer, roter Wermut-Weine analysiert. Die chemisch-physikalischen Parameter wie Alkoholkonzentration, pH, wie Zucker-, Extrakt- und Asche-Gehalt wurden durch amtliche, spanische Methoden bestimmt. Die meisten Wermutproben kommen den spanischen Gesetzen für aromatische Weine nach. Die mengenmäßig-dominierenden flüchtigen Komponenten wurden durch Direkt-Gaschromatographie mit 4-methyl-2-penthanol (interner Standard) geschätzt. Danach enthalten die roten Wermutweine ungefähr die doppelte Konzentration an höheren Alkoholen wie die weißen Weine.
  相似文献   
215.
The relationship between cobalt, copper and zinc content of soils and the vegetables grown in them is discussed. Samples of soil and edible vegetables were taken from 15 different sites, corresponding to four agricultural areas exposed to different degrees of environmental pollution: high industrial and traffic, high industrial and urban, and low industrial and urban pollution. Elements were determined by atomic absorption spectroscopy. Variance analysis (ANOVA) was applied to cobalt, copper and zinc content of vegetables and soils in order to detect significant differences among soils of different areas and the vegetables grown there. The possible relationship between the heavy metal content of soils (total and extractable) and of the vegetables grown in them was estimated by the correlation coefficient value. Statistically significant correlations were observed between the total and extractable cobalt content of soils and of roots and bulbs (p < 0.1) and between the total copper content of soils and of inflorescences (p < 0.05). In the case of zinc no correlation was found.  相似文献   
216.
The study compared the growth capability of probiotic (Lactobacillus acidophilus La05, Lactobacillus casei Lc01 and Bifidobacterium animalis Bb12) and non-probiotic (Lactobacillus delbrueckii subsp bulgaricus and Streptococcus thermophilus) cultures on twenty-one culture media grouped according to selectivity: non-selective agars, selective agars without antibiotics and MRS agars containing different combinations of lithium chloride, cystein, bile salts and antibiotics. Four of these media were selected for quantitative enumeration of L. acidophilus La05, L. casei Lc01, and B. animalis Bb12. The best culture media and incubation conditions for enumeration of the probiotic cultures were: B. animalis: MRS agar with dicloxacillin, 37 °C or 42 °C, anaerobiosis; L. acidophilus: MRS agar with bile salts, 37 °C or 42 °C, aerobiosis; L. casei: MRS agar with lithium chloride and sodium propionate, 37 °C or 42 °C, aerobiosis or anaerobiosis. Plating on MRS with glucose replaced by maltose, 37 °C or 42 °C, anaerobiosis, will distinguish probiotic from non-probiotic cultures. For enumeration of each probiotic in a mixed culture, the following media and incubation conditions were recommended: B. animalis: 4ABC-MRS, 42 °C, anaerobiosis, L. acidophilus: LC medium, 42 °C, aerobiosis or anaerobiosis and L. casei: LP-MRS, 42 °C, aerobiosis or anaerobiosis. In all experiments, differences in counts using pour plating or surface plating were not significant (P ≤ 0.05).  相似文献   
217.
Seasonal variations in total fatty acid compositions of tucunaré, Cichla sp. in the Janauacá Lake of the Amazon basin were determined by GC. Saturated fatty acids (SFAs) were found to be at higher concentrations than monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) in two seasonal periods. Major fatty acids were palmitic (16:0, 48.5–51.6% of total SFA), oleic (18:1ω9, 43.9–50.2% of total MUFA), docosahexaenoic (DHA, 22:6ω3, 13.5–27.9% of total PUFA) and arachidonic acid (AA, 20:4ω6, 16.0–19.6% of total PUFA). Polyunsaturated fatty acids were the most abundant in muscle during the flood period, and more especially fatty acids 22:6ω3 (10%) and 20:4ω6 (7%). There were no significant variations amongst total lipid SFA, MUFA, and PUFA from eye sockets. The ω6 fatty acid percentages were always higher than the ω3 fatty acid percentages in tucunaré, with ω6/ω3 ratios varying between 1.02 and 1.6 in the flood and drought periods.  相似文献   
218.
The antioxidant capability and total phenolic contents of methanolic extracts of Agaricusbrasiliensis in two stages of maturity, young (YB) and mature (MB), were evaluated in this work. Four complementary assays, reducing power, radical scavenging capacity, inhibition of lipid peroxidation and chelating ability for ferrous ions were used to screen the antioxidant properties of extracts. Minor differences in the composition of phenolic compounds were detected, but the extracts showed similar antioxidant activities, except for the chelating ability for ferrous ions, higher in MB than in YB. Our results support the use of both young and mature fruiting bodies of Agaricus blazei as sources of antioxidant compounds.  相似文献   
219.
This study investigated lipid oxidation in spaghetti enriched in long chain n−3 polyunsaturated fatty acids (LC n−3 PUFA) by the addition to semolina of an integrator containing eicosapentaenoic acid and docosahexaenoic acid. Two oxidative parameters were evaluated: peroxide value (PV), to assess primary oxidation and oxidised fatty acids to quantify secondary oxidation products. Functional spaghetti had a shelf life comparable to control pasta. LC n−3 PUFA were not significantly implicated in the onset of oxidation in spaghetti stored under daylight and accelerated oxidation in a laboratory heater. Storage decidedly affected shelf life: PV in functional spaghetti increased from 7.1 to 43.4 meq O2/kg of fat under light exposure over 12 months, and from 7.1 to 16.2 meq O2/kg under accelerated ageing at 55 °C for 27 days, reproducing about 18 months at room temperature. Oxidised fatty acids increased in fortified spaghetti from 4.8 to 13.8 g/100 g of fat under light exposure over 12 months and from 4.8 to 7.8 g/100 g of fat at 55 °C in 27 days. The high sensitivity of spectrophotometric and chromatographic analytical methods permitted the evaluation of primary and secondary oxidative derivatives in small amounts of fat.  相似文献   
220.
The selenium content in fruit and vegetable samples from two regions in Portugal were analysed using hydride generation atomic fluorescence spectrometry (HG-AFS) and radiochemical nuclear activation analysis (RNAA) – two analytical methods with very low limits of detection. The lower detection limits of HG-AFS, 3 μg kg−1 and 8 μg kg−1 (according to conditions used for digestion), and for RNAA, 10 μg kg−1, meant that it was possible to determine selenium in samples previously analysed using the replicate sample instrumental nuclear activation analysis (RSINAA) with a higher detection limit associated.  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