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   2386篇
  免费   37篇
电工技术   39篇
综合类   2篇
化学工业   262篇
金属工艺   30篇
机械仪表   51篇
建筑科学   123篇
矿业工程   5篇
能源动力   46篇
轻工业   182篇
水利工程   37篇
石油天然气   9篇
无线电   160篇
一般工业技术   327篇
冶金工业   832篇
原子能技术   22篇
自动化技术   296篇
  2021年   24篇
  2020年   16篇
  2019年   23篇
  2018年   20篇
  2017年   22篇
  2016年   21篇
  2015年   26篇
  2014年   41篇
  2013年   138篇
  2012年   57篇
  2011年   74篇
  2010年   64篇
  2009年   54篇
  2008年   66篇
  2007年   91篇
  2006年   79篇
  2005年   72篇
  2004年   61篇
  2003年   44篇
  2002年   70篇
  2001年   37篇
  2000年   43篇
  1999年   57篇
  1998年   186篇
  1997年   139篇
  1996年   105篇
  1995年   75篇
  1994年   57篇
  1993年   51篇
  1992年   41篇
  1991年   32篇
  1990年   33篇
  1989年   45篇
  1988年   35篇
  1987年   24篇
  1986年   26篇
  1985年   38篇
  1984年   17篇
  1983年   22篇
  1982年   27篇
  1981年   32篇
  1980年   22篇
  1979年   15篇
  1978年   16篇
  1977年   41篇
  1976年   37篇
  1975年   11篇
  1973年   16篇
  1972年   14篇
  1971年   9篇
排序方式: 共有2423条查询结果,搜索用时 15 毫秒
31.
Several major physiological changes or disease states occur more frequently in alcohol abusers or prolonged high users. These include: cancer, immunosuppression, and nutritional deficiencies. They interact with potential enhancement of cancer growth or survival. Clearly alcohol abuse yielding cirrhosis is associated with immunosuppression of cellular immune functions. Severe, prolonged ethanol consumption frequently is associated with one or more nutrient deficiency. Such deficiencies are more common when there are other major problems such as liver or pancreas damage and in prolonged alcohol abusers should influence the host defenses against pathogens and tumors. The amount and frequency of nutritional and immunological deficiencies with "binge" or less severe alcohol abuse is less clear, but animal studies suggest they are frequently present. Clearly, nutrient deficiencies are common in alcohol abusers and could play significant roles in ethanol associated problems including immunosuppression. Data summarized suggest that immunosuppression caused by nutritional deficiencies and/or direct effects of ethanol and its' metabolites on lymphoid cells is a potential component of cancer in alcohol abusers. However the frequent occurrence of nutritional deficiencies, immunosuppression, and certain cancers in severe, prolonged ethanol abusers is an association, not proven cause and effect.  相似文献   
32.
TP53 gene mutations occur in 70% of oesophageal adenocarcinomas (OACs). Given the central role of p53 in controlling cellular response to therapy we investigated the role of mutant (mut-) p53 and SLC7A11 in a CRISPR-mediated JH-EsoAd1 TP53 knockout model. Response to 2 Gy irradiation, cisplatin, 5-FU, 4-hydroxytamoxifen, and endoxifen was assessed, followed by a TaqMan OpenArray qPCR screening for differences in miRNA expression. Knockout of mut-p53 resulted in increased chemo- and radioresistance (2 Gy survival fraction: 38% vs. 56%, p < 0.0001) and in altered miRNA expression levels. Target mRNA pathways analyses indicated several potential mechanisms of treatment resistance. SLC7A11 knockdown restored radiosensitivity (2 Gy SF: 46% vs. 73%; p = 0.0239), possibly via enhanced sensitivity to oxidative stress. Pathway analysis of the mRNA targets of differentially expressed miRNAs indicated potential involvement in several pathways associated with apoptosis, ribosomes, and p53 signaling pathways. The data suggest that mut-p53 in JH-EsoAd1, despite being classified as non-functional, has some function related to radio- and chemoresistance. The results also highlight the important role of SLC7A11 in cancer metabolism and redox balance and the influence of p53 on these processes. Inhibition of the SLC7A11-glutathione axis may represent a promising approach to overcome resistance associated with mut-p53.  相似文献   
33.
Soil incubation studies were undertaken in controlled environment cabinets at 15°C to investigate the effect of increasing application rates of calcium ammonium nitrate (CAN) on net nitrification in two grassland soils. Granular CAN was applied to the surface of freshly collected, moist soil, at a rate equivalent to 0, 100, 200, 400, 800 and 1600µg NH 4 + -N and NO 3 - -N per gram of oven dry soil. In half the treatments finely ground CaCO3 was incorporated into the moist soil to raise the starting pH. Changes in soil mineral N and pH were measured at weekly intervals up to six-weeks. The most probable number (MPN) technique was used to enumerate the NH 4 + -N and NO 2 - -N oxidizers at the beginning and end of the incubation.At low rates of CAN application there was considerable NH 4 + -N oxidation to NO 3 - -N during the incubation of both soils. Lime stimulated this N transformation. At high application rates (i.e. 800 and 1600 ppm) there was little change in NH 4 + -N or NO 3 - -N on either soil during the 6 week incubation, in the presence or absence of lime. The rate of NO 3 - -N produced peaked at 5.6 and 3.8 mg NO 3 - -N kg–1 d–1 on soil 1 and 2 respectively, in the presence of lime. Above a level of 400 ppm CAN (equivalent to 38 kg N ha–1) the rate of NO 3 - -N produced decreased. The higher rate of net nitrification in soil 1 compared with soil 2 was probably due to a higher number of nitrifying bacteria. Although high rates of CAN decreased the nitrifying activity of both soils there was little difference between treatments in the actual numbers of NH 4 + -N and NO 2 - -N oxidizers determined by the MPN technique.The results showed that the rate of granular CAN applied to the soil surface can influence the local activity of nitrifying bacteria and subsequent N transformations. At application rates of CAN generally used agriculturally for grass production, it is likely that net nitrification of the NH 4 + -N in the fertilizer granule will be inhibited.  相似文献   
34.
The catalytic performance of cobalt catalysts supported on γ-Al2O3, TiO2, ZrO2 were studied for bio-ethanol steam reforming (BESR) reaction. The supported catalysts (10 wt%Co) were prepared by impregnation and characterized through Thermogravimetric analysis (TGA), H2 chemisorption, laser Raman Spectroscopy, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), and temperature-programmed reaction (TPRxn). The metallic cobalt sites were found to correlate with the BESR reaction activity. The reaction and H2 chemisorption showed that ZrO2 supported catalyst showed the best dispersion and best catalytic activity. Over the 10% Co/ZrO2 catalyst, using a H2O:EtOH:inert molar ratio of 10:1:75 and a GHSV = 5000 h−1, 100% ethanol conversion and a yield of 5.5 mol H2/mol EtOH were obtained at 550 °C and atmospheric pressure.  相似文献   
35.
Urea can be an inefficient N source due to rapid hydrolysis by soil urease leading to NH3 volatilization. The current study investigated the effect of the urease inhibitor phenylphosphorodiamidate (PPD) incorporated at two concentrations (0.5% and 1% w/w) within the fertilizer granule on NH3 volatilization from surface applied urea. The daily rates of NH3 loss from 20 soils of widely differing properties from Northern Ireland were measured over 14 days using ventilated enclosures under simulated spring conditions. Cumulative loss rates were calculated and fitted to a logistic model from which total NH3 loss (Amax) and the time to maximum rate of loss (Tmax) were determined. Stepwise multiple linear regression analysis related the effectiveness of PPD in reducing NH3 volatilization from urea to soil properties.The total cumulative loss of ammonia from unamended urea varied from 0.37 to 29.2% depending on soil type. Ammonia volatilization appeared to be greatest on a soil with a high pH (R2 = 0.65), a low titratable acidity (TA) (R2 = 0.63) and a soil that was drying out (R2 = 0.50). Soil pH was negatively correlated with TA (r = –0.826, P < 0.001) suggesting that soils with a low TA may have received recent lime. Including cation exchange capacity (CEC) and % N as well as pH-KCl in the multiple linear regression equation explained 86% of the variance.The effectiveness of PPD in reducing Amax varied between 0% to 91% depending on soil type, the average over all 20 soils being 30 and 36% for 0.5% and 1% PPD respectively. The most important soil properties influencing the effectiveness of the urease inhibitor were soil pH-H2O and TA accounting for 33% and 29% of the variance respectively. PPD was less effective on a soil with a high pH and low TA. These were the soil conditions that led to high NH3 volatilization from unamended urea and may explain why PPD had limited success in reducing ammonia loss on these soils. Multiple linear regression analysis indicated that 75% of the variation in the % inhibition of NH3 loss by PPD could be significantly accounted for by pH-H2O, initial soil NO 3 - -N concentration, % moisture content and % moisture loss.The delay in Tmax by PPD ranged from 0.19 to 7.93 days, the average over all 20 soils being 2.5 and 2.8 days for 0.5% and 1% PPD respectively. TA, % moisture content, urease activity and CEC were soil properties that significantly explained 83% of the variation in the % delay in Tmax by PPD in multiple linear regression analysis. However, none of these soil properties were significant on their own. As urea hydrolysis occurs rapidly in soil, delaying Tmax under field conditions would increase the chance of rain falling to move the urea below the soil surface and reduce NH3 volatilization. A urease inhibitor should be more effective than PPD on soils with a high pH and low TA to be successful in reducing high NH3 losses.  相似文献   
36.
The effect of trap design, trap color, and a mixture of isothiocyanates on the capture of Ceutorhynchus assimilis (Paykull) was studied in a series of field experiments. Unbaited yellow water traps, a yellow sticky box trap, and a yellow sticky card trap, mounted vertically, or at 45° to the vertical with the yellow card facing upwards were effective for capturing seed weevils, but a horizontal yellow sticky card trap caught few. White, green, and black traps were unattractive. During migratory periods, trap catch could be enhanced two to four times with a lure consisting of a mixture of allyl, 3-butenyl, 4-pentenyl, and 2-phenylethyl isothiocyanates, but this bait did not attract in a trap with ineffective design. The sticky card trap, mounted at 45° to the vertical and baited with the isothiocyanate mixture, may be useful for monitoring movement of C. assimilis during migratory periods. However, during the colonization phase, lures of either a mixture of isothiocyanates or of allyl isothiocyanate alone were not attractive.  相似文献   
37.
The15N isotope was used to study the mode of action of individual nitrogen sources in a 30% urea:30% ammonium nitrate: 10% ammonium sulphate:30% filler (w/w) granular fertilizer for perennial ryegrass in a greenhouse pot experiment. The fertilizer consisted of two types of granules, one containing 80% urea and 20% filler and the second containing 48% ammonium nitrate (AN), 16% ammonium sulphate (AS) and 36% filler. In addition the effect of dolomite compared with silica as the filler was investigated on nitrogen recovery from the 30:30:10:30 formulation.Dolomite adversely affected the recovery of nitrate N from the system and evidence suggested that MgCO3 was the active component. Granules containing dolomite resulted in a lower dry-matter yield than those containing silica, however the difference was not significant as nitrate contributed only 20% of the N in the formulation. AN gave the greatest DM yield and urea the lowest with AS being intermediate. The15N budget in shoots, roots and soil indicated that only 65% of the N from urea was recovered at the end of the experiment compared with 86% for AN and 91% for AS. The dry-matter yield of the 30:30:10:30 formulation using silica as the filler was intermediate between urea and AN; however, the apparent N recovery was significantly higher than expected from the sum of the individual components. The use of15N labelling indicated that using separate granules for ammonium N and urea the recovery of urea was improved by 11% in the triple N mixture when both AN and AS were present in the second granule compared to the recovery on its own. The enhanced recovery of urea appeared to be a function of AN and AS acting together as neither source in double combination with urea had any effect on urea N recovery.Urea enhanced the recovery of nitrate N by 10% but decreased the recovery of AS by 6% (in the 30:30:10:30 formulation) in comparison with the single sources on their own. The results indicate that interactions can occur between N sources even when they are physically separated by being in different granules.  相似文献   
38.
Liu  Xinke  Kao  Evan  Haraldsson  Henrik  Ballweber  Megan  Martin  Alastair  Li  Youxiang  Wang  Yuting  Saloner  David 《Magma (New York, N.Y.)》2021,34(5):659-666
Magnetic Resonance Materials in Physics, Biology and Medicine - To determine the intra-individual flow variation in serially acquired studies, and the influence of this variation on subsequent...  相似文献   
39.
40.
Yellowbank Creek is a small stream in coastal central California being assessed for salmonid habitat limiting factors and restoration potential. Yellowbank flows through low‐density marine mudstone bedrock, which is the gravel source for the stream. To assess the potential effects of the low‐density substrate on spawning gravels, a tracer stone study comparing the incipient motion of low‐density mudstone particles and typical density granitic particles was used to populate a logistic regression particle entrainment model. A model comparison approach was used to test the strength of the model. Results demonstrate partial mobility of both mudstone and granitic particles under boundary shear conditions ranging from 6.9 to 42.2 N m‐2. The modelling results quantify the strong negative correlation between particle entrainment and particle density. Mudstone gravel was three times more likely to be entrained than granitic gravel, within the context of the experimental conditions. The effect of density difference on partial mobility was greater in smaller grain size fractions. This work has implications for salmonid spawning success in atypical geologic settings and may assist in prioritization of restoration efforts. Copyright © 2013 John Wiley & Sons, Ltd.  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