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   800篇
  免费   55篇
电工技术   7篇
化学工业   268篇
金属工艺   3篇
机械仪表   13篇
建筑科学   48篇
矿业工程   2篇
能源动力   12篇
轻工业   183篇
水利工程   61篇
石油天然气   4篇
无线电   31篇
一般工业技术   86篇
冶金工业   46篇
原子能技术   10篇
自动化技术   81篇
  2024年   3篇
  2023年   8篇
  2022年   42篇
  2021年   59篇
  2020年   29篇
  2019年   26篇
  2018年   33篇
  2017年   34篇
  2016年   34篇
  2015年   34篇
  2014年   34篇
  2013年   75篇
  2012年   52篇
  2011年   60篇
  2010年   35篇
  2009年   39篇
  2008年   32篇
  2007年   46篇
  2006年   34篇
  2005年   32篇
  2004年   22篇
  2003年   15篇
  2002年   17篇
  2001年   11篇
  2000年   5篇
  1999年   1篇
  1998年   3篇
  1997年   2篇
  1996年   5篇
  1995年   1篇
  1994年   3篇
  1992年   6篇
  1991年   1篇
  1990年   2篇
  1989年   1篇
  1987年   3篇
  1986年   5篇
  1985年   2篇
  1984年   3篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1977年   2篇
  1975年   1篇
排序方式: 共有855条查询结果,搜索用时 943 毫秒
81.
In Machiavelli's theory of power, the concept of ‘divide and rule’ forms the main theme: the ruler has absolute power and to maintain and increase such power all means are justified. When viewed against the background of this theory, the current debate in the Netherlands on the unbundling of energy (electricity, gas) companies can be observed as an example of ‘divide and rule’, in which the Dutch Minister of Economic Affairs plays a central role. Yet, contrary to Machiavelli in his time, the Dutch government does, in fact, aim principally at the greater welfare of the Dutch people. It is therefore noteworthy that, while important steps in the decision to unbundled have been taken, there is no evidence that the Dutch people will indeed benefit from the envisaged unbundling.  相似文献   
82.
Excessive nerve growth factor (NGF) production by the ovary, achieved via a transgenic approach, results in arrested antral follicle growth, reduced ovulatory capacity, and a predisposition to cyst formation in response to mildly elevated LH levels. Two salient features in these mutant mice (termed 17NF) are an elevated production of 17α-hydroxyprogesterone (17-OHP(4)), testosterone, and estradiol (E(2)) in response to gonadotropins, and an increased frequency of granulosa cell (GC) apoptosis. In this study, we show that the increase in steroidal response is associated with enhanced expression of Cyp17a1, Hsd17b, and Cyp19a1, which encode the enzymes catalyzing the synthesis of 17-OHP(4), testosterone, and E(2) respectively. Using a proteomic approach, we identified stathmin (STMN1), as a protein that is overproduced in 17NF ovaries. In its phosphorylated state, STMN1 mediates a cell death signal initiated by tumor necrosis factor α (TNF). STMN1 is expressed in GCs and excessive NGF increases its abundance as well as that of its forms phosphorylated at serine (Ser) 16, 25, and 38. TNF synthesis is also increased in 17NF ovaries, and this change is abolished by blocking neurotrophic tyrosine kinase receptors. Inhibiting TNF actions in vivo by administering a soluble TNF receptor prevented the increase in total and phosphorylated STMN1 production, as well as GC apoptosis in NGF-overproducing ovaries. These results indicate that an excess of NGF in the ovary promotes steroidogenesis by enhancing the expression of enzyme genes involved in 17-OHP(4), testosterone, and E(2) synthesis, and causes GC apoptosis by activating a TNF/ STMN1-mediated cell death pathway.  相似文献   
83.
Ostreopsis cf. ovata, a benthic dinoflagellate often blooming along the Mediterranean coasts, has been associated with toxic events ranging from dyspnea to mild dermatitis. In late September 2009, an Ostreopsis cf. ovata bloom occurred in the Gulf of Trieste (Northern Adriatic Sea; Italy), causing pruritus and mild dermatitis in beachgoers. An integrated study was initiated to characterize Ostreopsis cells by light and confocal microscopy, PCR techniques, immunocytochemistry, and high resolution liquid chromatography-mass spectrometry (HR LC-MS). The presence of Ostreopsis cf. ovata of the Atlantic/Mediterranean clade was unambiguously established by morphological and genetic analyses in field samples. Several palytoxin-like compounds (ovatoxin-a,-b,-c,-d,-e) were identified by HR LC-MS, ovatoxin-a being the most abundant (45-64 pg/cell). Surprisingly, no palytoxin was detected. For the first time, monoclonal and polyclonal antipalytoxin antibodies revealed the intracellular cytoplasmic localization of ovatoxins, suggesting their cross-reactivity with these antibodies. Since harmful dinoflagellates do not always produce toxins, the immunocytochemical localization of ovatoxins, although qualitative, can provide an early warning for toxic Ostreopsis cells before their massive diffusion and/or concentration in seafood.  相似文献   
84.
This paper proposes a method based on near-infrared hyperspectral imaging for discriminating between terrestrial and fish species in animal protein by-products used in livestock feed. Four algorithms (Mahalanobis distance, Kennard-Stone, spatial interpolation, and binning) were compared in order to select an appropriate subset of pixels for further partial least squares discriminant analysis (PLS-DA). The method was applied to a set of 50 terrestrial and 40 fish meals analyzed in the 1000-1700 nm range. Models were then tested using an external validation set comprising 45 samples (25 fish and 20 terrestrial). The PLS-DA models obtained using the four subset-selection algorithms yielded a classification accuracy of 99.80%, 99.79%, 99.85%, and 99.61%, respectively. The results represent a first step for the analysis of mixtures of species and suggest that NIR-CI, providing valuable information on the origin of animal components in processed animal proteins, is a promising method that could be used as part of the EU feed control program aimed at eradicating and preventing bovine spongiform encephalopathy (BSE) and related diseases.  相似文献   
85.
Field investigations on the treatment of MTBE and benzene from contaminated groundwater in pilot or full-scale constructed wetlands are lacking hugely. The aim of this study was to develop a biological treatment technology that can be operated in an economic, reliable and robust mode over a long period of time. Two pilot-scale vertical-flow soil filter eco-technologies, a roughing filter (RF) and a polishing filter (PF) with plants (willows), were operated independently in a single-stage configuration and coupled together in a multi-stage (RF + PF) configuration to investigate the MTBE and benzene removal performances. Both filters were loaded with groundwater from a refinery site contaminated with MTBE and benzene as the main contaminants, with a mean concentration of 2970 ± 816 and 13,966 ± 1998 μg L−1, respectively. Four different hydraulic loading rates (HLRs) with a stepwise increment of 60, 120, 240 and 480 L m−2 d−1 were applied over a period of 388 days in the single-stage operation. At the highest HLR of 480 L m−2 d−1, the mean concentrations of MTBE and benzene were found to be 550 ± 133 and 65 ± 123 μg L−1 in the effluent of the RF. In the effluent of the PF system, respective mean MTBE and benzene concentrations of 49 ± 77 and 0.5 ± 0.2 μg L−1 were obtained, which were well below the relevant MTBE and benzene limit values of 200 and 1 μg L−1 for drinking water quality. But a dynamic fluctuation in the effluent MTBE concentration showed a lack of stability in regards to the increase in the measured values by nearly 10%, which were higher than the limit value. Therefore, both (RF + PF) filters were combined in a multi-stage configuration and the combined system proved to be more stable and effective with a highly efficient reduction of the MTBE and benzene concentrations in the effluent. Nearly 70% of MTBE and 98% of benzene were eliminated from the influent groundwater by the first vertical filter (RF) and the remaining amount was almost completely diminished (∼100% reduction) after passing through the second filter (PF), with a mean MTBE and benzene concentration of 5 ± 10 and 0.6 ± 0.2 μg L−1 in the final effluent. The emission rate of volatile organic compounds mass into the air from the systems was less than 1% of the inflow mass loading rate. The results obtained in this study not only demonstrate the feasibility of vertical-flow soil filter systems for treating groundwater contaminated with MTBE and benzene, but can also be considered a major step forward towards their application under full-scale conditions for commercial purposes in the oil and gas industries.  相似文献   
86.
Native cassava starch was modified by dry heating treatment, and sodic montmorillonite was modified using an alkylpolyglucoside biosurfactant to produce a novel montmorillonite-filled starch bionanocomposites with improved performance. The bionanocomposites were produced in two extrusion steps using a co-rotating twin-screw extruder. The structural properties of the bionanocomposites were studied by X-ray diffraction. The reinforcement effect and material performance were evaluated by hydrophobicity, solubility, opacity, barrier, thermal and mechanical properties. In general, the dispersion of the nanofillers resulted in bionanocomposites with intercalated structures and improved properties compared with unfilled bioplastics. The main results exhibited that modified montmorillonite showed better compatibility with starch, improving nanofiller dispersion and interaction than the native one. As a result, it increased the hydrophobicity and reduced solubility, water, and oxygen permeabilities in comparison with bionanocomposites based on native starch by 36%, 48%, and 68%, respectively. Also, the tensile strength and Young modulus increased from 0.60 to 2.56 MPa and from 2.99 to 15.68 MPa, respectively. In conclusion, modification of MMT by the biosurfactant is a good approach to enhance dispersion/interaction with the starch matrix.  相似文献   
87.
An investigation was made on whether the optimal number of tubes in a dry-expansion evaporator, for a given area or a given cost, will change when R22 is replaced with the non-azeotropic mixture R407C. A comprehensive computer program was used to simulate the tube-and-shell evaporator, under several sets of conditions. The simulations show that there are only small differences between R22 and R407C in the optimal number of tubes. However, the results indicate that one can allow a somewhat higher pressure drop for R407C, and that the number of tubes for a given area therefore can be smaller when R407C is used. For both fluids, the evaporator performance is more sensitive to the optimization for a given cost than for a given area.  相似文献   
88.
89.
90.
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