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   1146篇
  免费   92篇
  国内免费   1篇
电工技术   6篇
综合类   3篇
化学工业   578篇
金属工艺   7篇
机械仪表   19篇
建筑科学   29篇
能源动力   27篇
轻工业   201篇
水利工程   4篇
石油天然气   1篇
无线电   79篇
一般工业技术   171篇
冶金工业   29篇
原子能技术   1篇
自动化技术   84篇
  2024年   2篇
  2023年   22篇
  2022年   173篇
  2021年   178篇
  2020年   40篇
  2019年   40篇
  2018年   50篇
  2017年   33篇
  2016年   54篇
  2015年   43篇
  2014年   53篇
  2013年   72篇
  2012年   59篇
  2011年   95篇
  2010年   50篇
  2009年   42篇
  2008年   42篇
  2007年   31篇
  2006年   28篇
  2005年   33篇
  2004年   20篇
  2003年   11篇
  2002年   12篇
  2001年   5篇
  2000年   3篇
  1999年   7篇
  1998年   4篇
  1997年   5篇
  1996年   8篇
  1995年   6篇
  1994年   3篇
  1993年   1篇
  1992年   1篇
  1991年   3篇
  1990年   2篇
  1989年   2篇
  1988年   1篇
  1986年   1篇
  1985年   1篇
  1983年   1篇
  1981年   1篇
  1976年   1篇
排序方式: 共有1239条查询结果,搜索用时 15 毫秒
991.
Several studies reported the association between total plasma phytosterol concentrations and the parenteral nutrition‐associated cholestasis (PNAC). To date, no data are available on phytosterol esterification in animals and in humans during parenteral nutrition (PN). We measured free and esterified sterols (cholesterol, campesterol, stigmasterol, and sitosterol) plasma concentrations during PN in 16 preterm infants (500–1249 g of birth weight; Preterm‐PN), in 11 term infants (Term‐PN) and in 12 adults (Adult‐PN). Gas chromatography–mass spectrometry was used for measurements. Plasma concentrations of free cholesterol (Free‐CHO), free phytosterols (Free‐PHY) and esterified phytosterols (Ester‐PHY) were not different among the three PN groups. Esterified cholesterol (Ester‐CHO) was statistically lower in Preterm‐PN than Adult‐PN. Preterm‐PN had significantly higher Free‐CHO/Ester‐CHO and Free‐PHY/Ester‐PHY ratios than Adult‐PN (Free‐CHO/Ester‐CHO: 1.1 ± 0.7 vs. 0.6 ± 0.2; Free‐PHY/Ester‐PHY: 4.1 ± 2.6 vs. 1.3 ± 0.8; *P < 0.05). Free‐CHO/Ester‐CHO and Free‐PHY/Ester‐PHY ratios of Term‐PN (Free‐CHO/Ester‐CHO: 1.1 ± 0.4; Free‐PHY/Ester‐PHY: 2.9 ± 1.7) were not different from either Preterm‐PN or from Adult‐PN. Plasma Free‐CHO/Ester‐CHO and Free‐PHY/Ester‐PHY were unchanged after 24 h on fat‐free PN both in Preterm‐PN and in Adult‐PN. Free‐PHY/Ester‐PHY did not correlate with phytosterol intake in Preterm‐PN. Free‐PHY/Ester‐PHY of Preterm‐PN was positively correlated with the Free‐CHO/Ester‐CHO and negatively correlated with gestational age and birth weight. In conclusion, PHY were esterified to a lesser extent than CHO in all study groups; the esterification was markedly decreased in Preterm‐PN compared to Adult‐PN. The clinical consequences of these findings warrant further investigations.  相似文献   
992.
993.
In normal cell physiology, programmed death 1 (PD-1) and its ligand, PD-L1, play an immunoregulatory role in T-cell activation, tolerance, and immune-mediated tissue damage. The PD-1/PD-L1 pathway also plays a critical role in immune escape of tumor cells and has been demonstrated to correlate with a poor prognosis of patients with several types of cancer. However, recent reports have revealed that the immunohistochemical (IHC) expression of the PD-L1 in tumor cells is not uniform for the use of different antibodies clones, with variable specificity, often doubtful topographical localization, and with a score not uniquely defined. The purpose of this study was to analyze the IHC expression of PD-L1 on a large series of several human tumors to correctly define its staining in different tumor tissues.  相似文献   
994.
Green manure mixtures including legumes and forbs can help to increase N availability in organic arable systems. Anaerobic digestion of green manures may provide ammonium rich digestate, which can be redistributed as fertilizer. The aim of this study was to investigate the effect of plant species composition, cutting strategy and anaerobic digestion on the N fertilizer replacement value (NFRV) of different green manures. Digestates obtained from silages of pure stand lucerne (four cuts/year) and a mixture including lucerne, grass and forbs (two or four cuts/year) were used to fertilize winter wheat (surface banding) and spring barley (injection). In general, NFRV was 46–173% higher in spring barley than winter wheat, due to the different application method and timing, which reflect the common practices in Denmark. NFRV of digestates were 25–63% higher than the corresponding silages, with the largest increase with the most fibrous material (mixture at two cuts/year). Total N concentration (DM based) in the silages largely explained NFRV of the digestates. To obtain NFRV above 60%, total N concentration of silage should exceed 3.5 g 100 g?1 DM, achievable with silages from four-cut strategies. Silages of plant materials with different composition and N content may be similar in terms of biomethane production, but the fertilizer value of the digestates varies considerably depending on total N concentration.  相似文献   
995.
Formation of toxic alkylbenzenes, total polar compounds (%TPC) and degradation of tocochromanol are monitored. Analyses of the oil extracted from fried potatoes confirm the trend observed in the frying oil. The fresh oil has a TPC content of 3%, which increases with the frying time, exceeding the acceptable value (25%) after about 25 h for deep‐frying and 1.5 h for pan‐frying. During deep‐frying, total tocochromanol decreases to about half (25 mg per 100 g) of the initial value, pan‐frying shows faster, degradation (complete after 1.5 h). Toluene concentration increases with the frying time reaching a maximum, and afterwards gradually decreases. Except for butylbenzene during pan‐frying, pentylbenzene and butylbenzene concentration, increase with the frying time, but remain much lower than toluene. Practical Applications: This is the first systematic work comparing alkylbenzenes evolution under different frying conditions. Different from previous works, frying experiments are carried out following the indication of many European countries that recommend using temperature lower than 180 °C. The amount of alkylbenzenes assumed through a standard portion of fried potatoes (200 g) is assessed, which is relevant for evaluating dietary exposure to these contaminants.  相似文献   
996.
Inkjet printing technique is exploited for the synthesis of Ag nanoparticles (NPs) patterned on electrochemically etched silicon-based substrates. The nanostructure morphology, here analyzed by scanning electron microscopy, is dictated by the ink composition and the printing parameters. Under suitable excitation conditions, resonant surface-enhanced Raman scattering (SERS) performed on such metal-dielectric nanostructures can approach single-molecule detection as recently demonstrated on silvered porous silicon synthesized by immersion plating.

PACS

78.67.Bf; 78.30.-j  相似文献   
997.
Mitoxantrone (MTX) is an antineoplastic agent whose use is limited by serious side effects on non‐neoplastic cells. The aim of this study was the development of a new drug release system using an ionotropic gelation technique for microencapsulation of MTX in chitosan–carboxymethylinulin nanoparticles (CCInp), followed by evaluation of their cytotoxic effects on neoplastic MDA‐MB‐231 and non‐neoplastic NIH3T3 cell lines. The CCInp were prepared through a new reliable method for easy functionalization of both inulin and chitosan. Both unloaded and drug‐loaded nanoparticles were characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS) and showed a spherical morphology with an average hydrodynamic diameter between 40 and 80 nm. Both nanoparticles were stable and easily degraded by lysozyme. MTX‐loaded nanoparticles led to a greater mortality of MDA‐MB‐231 relative to free drug due to the ability of the nanoparticles to accumulate preferentially in neoplastic cells. The developed drug release system retains the ability to kill MDA‐MB‐231 cells in vitro, improving the survival of NIH3T3 cells.  相似文献   
998.
Hulled, or ancient, wheats were the earliest domesticated wheats by mankind and the ancestors of current wheats. Their cultivation drastically decreased during the 1960s; however, the increasing demand for a healthy and equilibrated diet led to rediscovering these grains. Our aim was to use a non-targeted metabolomic approach to discriminate and characterize similarities and differences between ancient Triticum varieties. For this purpose, 77 hulled wheat samples from three different varieties were collected: Garfagnana T. turgidum var. dicoccum L. (emmer), ID331 T. monococcum L. (einkorn) and Rouquin T. spelta L. (spelt). The ultra high performance liquid chromatography coupled to high resolution tandem mass spectrometry (UHPLC-QTOF) metabolomics approach highlighted a pronounced sample clustering according to the wheat variety, with an excellent predictability (Q2), for all the models built. Fifteen metabolites were tentatively identified based on accurate masses, isotopic pattern, and product ion spectra. Among these, alkylresorcinols (ARs) were found to be significantly higher in spelt and emmer, showing different homologue composition. Furthermore, phosphatidylcholines (PC) and lysophosphatidylcholines (lysoPC) levels were higher in einkorn variety. The results obtained in this study confirmed the importance of ARs as markers to distinguish between Triticum species and revealed their values as cultivar markers, being not affected by the environmental influences.  相似文献   
999.
The aim of this work is to evaluate the cleanability performances of lapped porcelain stoneware surfaces with and without protective treatments. The knowledge of the material cleanability is an important selection parameter in buying tiles. However, an objective basis for testing and assessing this property still is lacking. In this study, standardized and laboratory tests to evaluate the cleanability performance of lapped porcelain stoneware products have been carried out and compared. Cleanability tests were performed by applying two different soiling suspensions, optimized for tile surfaces and reproducing chemical and mechanical actions simulating the working conditions and traditional cleaning operations. Surface characteristics (microstructure, topography, roughness) were analyzed before and after cleanability tests. The results showed a good correlation between different behavior in terms of cleanability and the microscopic and topographic characteristics of lapped tile. Respect to international standardized tests specific for ceramic tiles, such as stains resistance test, the laboratory procedures to evaluate the surface cleanability are able to better show cleanability improvements of lapped tiles with protective treatments.  相似文献   
1000.
The design, synthesis, spectroscopic and photochemical properties, and biological evaluation of a novel molecular hybrid that is able to deliver nitric oxide (NO) into mitochondria are reported. This molecular conjugate unites a tailored o‐CF3p‐nitroaniline chromophore, for photo‐regulated NO release, and a rhodamine moiety, for mitochondria targeting, in the same molecular skeleton via an alkyl spacer. A combination of steady‐state and time‐resolved spectroscopic and photochemical experiments demonstrate that the two chromogenic units preserve their individual photophysical and photochemical properties in the conjugate quite well. Irradiation with violet light triggers NO release from the nitroaniline moiety and photoionization in the rhodamine center, which also retains considerable fluorescence efficiency. The molecular hybrid preferentially accumulates in the mitochondria of A549 lung adenocarcinoma cells where it induces toxicity at a concentration of 1 μm , exclusively upon irradiation. Comparative experiments, carried out with ad‐hoc‐synthesized model compounds, suggest that the phototoxicity observed at such a low concentration is probably not due to NO itself, but rather to the formation of the highly reactive peroxynitrite that is generated from the reaction of NO with the superoxide anion.  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