首页 | 官方网站   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   559篇
  免费   46篇
  国内免费   6篇
工业技术   611篇
  2024年   4篇
  2023年   11篇
  2022年   61篇
  2021年   54篇
  2020年   25篇
  2019年   27篇
  2018年   12篇
  2017年   19篇
  2016年   33篇
  2015年   26篇
  2014年   35篇
  2013年   35篇
  2012年   35篇
  2011年   42篇
  2010年   23篇
  2009年   27篇
  2008年   20篇
  2007年   19篇
  2006年   14篇
  2005年   21篇
  2004年   11篇
  2003年   11篇
  2002年   10篇
  2001年   5篇
  2000年   3篇
  1999年   1篇
  1998年   4篇
  1997年   3篇
  1996年   2篇
  1995年   1篇
  1994年   2篇
  1993年   2篇
  1992年   2篇
  1990年   3篇
  1985年   2篇
  1984年   1篇
  1980年   1篇
  1975年   1篇
  1973年   1篇
  1957年   1篇
  1956年   1篇
排序方式: 共有611条查询结果,搜索用时 15 毫秒
1.
Biochemical conversion of crude oils is a multi-step process proceeding through a series of biochemical reactions. These reactions can be characterized by a set of chemical markers which are associated with the chemical composition of crude oils. Reactions with heavy crude oils indicate that there is an overall decrease in the concentration and chemical speciation of organic sulfur compounds, and a redistribution of hydrocarbons and organometallic species. The contents of trace metals in the crude oils, such as nickel and vanadium, also decrease. Further, heavy ends of crudes, containing the asphaltenes and the polar nitrogen, sulfur, and oxygen containing fractions, as well as the organometallic compounds and complexes, are biochemically converted to lower molecular weight chemical species. In the studies reported in this paper, microorganisms used to mediate such reactions were thermophilic ( > 60°C) and pressure tolerant (up to 2500 psi). These organisms are also capable of biochemical conversion of bituminous and lignite coals in an analogous manner to their action on crude oils and follow similar trends characterized by chemical markers. For example, X-ray absorption near-edge structural (XANES) analyses of biotreated crude oils and low grade coals show that biochemical reactions lead to decreases in organic sulfides and thiophenes with a concurrent increase in sulfoxide contents. Chemically related constituents present in heavy crude oil fractions and low grade coals are the asphaltenes. Asphaltenes are complex structures containing heteroatoms and metals involved in inter- and intra-molecular bridges and stereochemical configurations. The chemical markers associated with the biochemical conversion of oils and coals indicate multiple biochemical processes involving chemical reactions at sites containing heteroatoms and metals leading to a breakdown of the structure(s) to smaller molecular weight units. Thus, using chemical markers as diagnostic tools, the extent and the efficiency of fossil fuel bioconversion may be predicted and monitored, allowing for better cost-efficient field trials. Recent results in this area will be presented and discussed in this paper.  相似文献   
2.
Supplementation with anthocyanins, which are a type of flavonoids mainly found in various berries, is hypothesized to be a promising approach to lower the risk of developing cognitive decline. The aim of this systematic review was to provide a comprehensive overview of dietary intervention trials describing effects of berry anthocyanins on cognitive performance in humans, while also addressing potential underlying mechanisms. A total of 1197 articles were identified through a systematic search, and 49 studies reporting effects on cognitive performance (n = 18), vascular function (n = 22), or cardiometabolic risk markers (n = 32) were included. Significant improvements were observed on memory, while some of the studies also reported effects on attention and psychomotor speed or executive function. Vascular function markers such as brachial artery flow-mediated vasodilation were also affected and consistent evidence was provided for the beneficial effects of berry anthocyanins on endothelial function. Finally, studies reported improvements in blood pressure, but effects on metabolic risk markers (e.g. carbohydrate and lipid metabolism) were less consistent. In conclusion, this review provides evidence for the beneficial effects of berry anthocyanins on cognitive performance as memory improved. Whether observed anthocyanin-induced improvements in vascular function and blood pressure underlie beneficial effects on cognitive performance warrants further study.  相似文献   
3.
Auger electron spectroscopy in combination with depth profiling by Ar ion sputtering was employed as a surface analytical technique to determine the positions of thin (3–5 nm) inert Pd markers during growth of oxide films on Al, Ni, and -NiAl. Alumina films 35 and 120 nm thick formed on Al and -NiAl, respectively, grew by inward diffusion of oxygen. Metal migration accounted predominantly for the growth of a NiO film 150 nm thick on the (111) crystal face of Ni. However, an assessment could not be made with respect to oxygen diffusion, if any, in this film due to the limitations imposed on precise depth profiling of the marker position caused by uneven sputtering of the oxide and metal phases.  相似文献   
4.
加强汉语复句信息处理研究对中文信息处理具有重要意义。该文介绍了汉语复句信息工程的概况,将复句知识建模、关系标记识别、非分句识别、复句句法语义关系判定、复句知识库建设等成果条理化,结合已有成果的研究视角、方法及关注焦点来分析复句信息处理的研究现状,并对其发展趋势进行了展望。  相似文献   
5.
Objective: Objective and subjective methods have been used in the past to assess workplace fatigue, but little is known about correlations between them. We examine correlations between subjective and objective measures, including measures collected in a workplace scenario. Methods: 15 young and 17 older participants were assessed before and after work with four types of fatigue measure: objective physical (posturography), objective mental (psychomotor vigilance task), subjective physical and mental (self-assessment), objective and subjective realistic (oculomotor behaviour, observer-rated facial expression, typing performance). Results: Pre- and post-test scores were analysed with an ANOVA, significant differences were submitted to a factor analysis. It yielded three factors: one representing posturography, the second self-rated mental and physical fatigue and the third observer-rated facial expression. Conclusions: Results advocate the existence of three independent fatigue components: Objective physical fatigue, introspective and extrospective fatigue.

Practitioner Summary: This study analyses correlations between different subjective and objective fatigue markers to better understand the complex nature of workplace fatigue. Measurements were conducted directly at the workplace. Results reveal that fatigue comprises three independent fatigue components: Objective physical fatigue, introspective and extrospective fatigue.  相似文献   

6.
在交际过程中,为了让听话人更好地理解话语,说话人可以向听话人提供明示导向的话语标记,从而对一系列的语境假设加以制约以减少听话人处理话语所需付出的努力。以关联理论为框架,通过对一份近五万字的法庭庭审语料进行分析,阐述了话语标记语对语境的制约作用主要表现在三个方面,即加强现时语境、消除现时语境以及产生新的语境暗含。  相似文献   
7.
Evolutionary behavioral biology suggests that certain characteristics of the human face and body are important for mate preferences and are therefore subject to sexual selection. J. Weeden and J. Sabini (see record 2005-11504-001) identify a number of weaknesses in the association between traits' attractiveness and health. In contrast, the authors argue that (a) studies on preferences for physical characteristics that rely on 1 trait permit only limited interpretation, (b) limitations placed on J. Weeden and J. Sabini's review exclude important associations, (c) there are misconceptions in their treatment of some traits, and (d) their selected literature provides an inaccurate picture regarding effect size. The authors suggest that future research in this field should seek conceptual and methodological constancy in trait selection and in the evaluation of attractiveness- and health-related traits. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
8.
9.
Therapy with mesenchymal stem cells (MSCs) is promising in many diseases. Evaluation of their efficacy depends on adequate follow-up of MSCs after transplantation. Several studies have shown that MSCs can be labeled and subsequently visualized with magnetic nanoparticles (NPs). We investigated the homing of MSCs labeled with magnetic cobalt ferrite NPs in experimentally induced acute kidney injury in mice. To explore the homing of MSCs after systemic infusion into mice, we developed a pre-infusion strategy for optimal tracing and identification of MSCs with polyacrylic acid-coated cobalt ferrite (CoFe2O4) NPs by light and transmission electron microscopy (TEM) in various organs of mice with cisplatin-induced acute kidney injury and control mice. By correlative microscopy, we detected MSCs labeled with NPs in the lungs, spleen, kidney, and intestine of cisplatin-treated mice and in the lungs and spleen of control mice. Our results confirm that labeling MSCs with metal NPs did not affect the ultrastructure of MSCs and their ability to settle in various organs. This study demonstrates the usefulness of cobalt ferrite NPs in ex vivo visualization of MSCs and offers correlative microscopy as a useful method in routine histopathology laboratories for tracing MSCs in paraffin-embedded tissue.  相似文献   
10.
The calcium-binding proteins S100A4, S100A8, and S100A9 are upregulated in chronic lymphocytic leukemia (CLL), while the S100A9 promotes NF-κB activity during disease progression. The S100-protein family has been involved in several malignancies as mediators of inflammation and proliferation. The hypothesis of our study is that S100A proteins are mediators in signaling pathways associated with inflammation-induced proliferation, such as NF-κB, PI3K/AKT, and JAK/STAT. The mononuclear cells (MNCs) of CLL were treated with proinflammatory IL-6, anti-inflammatory IL-10 cytokines, inhibitors of JAK1/2, NF-κB, and PI3K signaling pathways, to evaluate S100A4, S100A8, S100A9, and S100A12 expression as well as NF-κB activation by qRT-PCR, immunocytochemistry, and immunoblotting. The quantity of S100A4, S100A8, and S100A9 positive cells (p < 0.05) and their protein expression (p < 0.01) were significantly decreased in MNCs of CLL patients compared to healthy controls. The S100A levels were generally increased in CD19+ cells compared to MNCs of CLL. The S100A4 gene expression was significantly stimulated (p < 0.05) by the inhibition of the PI3K/AKT signaling pathway in MNCs. IL-6 stimulated S100A4 and S100A8 protein expression, prevented by the NF-κB and JAK1/2 inhibitors. In contrast, IL-10 reduced S100A8, S100A9, and S100A12 protein expressions in MNCs of CLL. Moreover, IL-10 inhibited activation of NF-κB signaling (4-fold, p < 0.05). In conclusion, inflammation stimulated the S100A protein expression mediated via the proliferation-related signaling and balanced by the cytokines in CLL.  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-23

京公网安备 11010802026262号