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   1494篇
  免费   88篇
  国内免费   5篇
电工技术   13篇
综合类   4篇
化学工业   319篇
金属工艺   15篇
机械仪表   33篇
建筑科学   73篇
矿业工程   1篇
能源动力   38篇
轻工业   317篇
水利工程   9篇
石油天然气   7篇
无线电   76篇
一般工业技术   223篇
冶金工业   268篇
原子能技术   11篇
自动化技术   180篇
  2024年   5篇
  2023年   20篇
  2022年   38篇
  2021年   59篇
  2020年   35篇
  2019年   37篇
  2018年   40篇
  2017年   55篇
  2016年   54篇
  2015年   42篇
  2014年   55篇
  2013年   93篇
  2012年   81篇
  2011年   126篇
  2010年   69篇
  2009年   69篇
  2008年   86篇
  2007年   73篇
  2006年   61篇
  2005年   56篇
  2004年   47篇
  2003年   30篇
  2002年   47篇
  2001年   32篇
  2000年   22篇
  1999年   20篇
  1998年   35篇
  1997年   43篇
  1996年   25篇
  1995年   14篇
  1994年   16篇
  1993年   9篇
  1992年   12篇
  1991年   8篇
  1990年   10篇
  1989年   9篇
  1988年   8篇
  1987年   4篇
  1985年   6篇
  1984年   3篇
  1983年   3篇
  1981年   4篇
  1980年   3篇
  1979年   2篇
  1978年   4篇
  1977年   3篇
  1976年   2篇
  1975年   2篇
  1974年   3篇
  1973年   2篇
排序方式: 共有1587条查询结果,搜索用时 31 毫秒
271.
The volatile composition of ‘Bravo de Esmolfe’ (BE) apple was achieved by headspace-solid phase microextraction (HS-SPME) followed by gas chromatography–mass spectrometry (GC–qMS) analysis using fruits from two different rootstocks (PAJAM2 and MM106), different trees of each rootstock, and different sections of each tree (north and south). Fruits were harvested in the maturity state defined for this variety and then stored for four months at 4 °C. During this period, analyses were made monthly. α-Farnesene was the compound found in the volatile profile of BE apple with the largest GC area. A large number of esters were also detected. Rootstocks, trees, and tree sections did not have statistical significance on the volatile composition of BE apples. However, the volatile composition changed throughout the storage period. The principal component analysis (PCA) allowed distinguishing between the apples obtained immediately after harvest and those stored for 4 months. In the first case, the volatile composition was related to the occurrence of aldehydes, aromatic hydrocarbons, and phenols and, in the second case, to esters and terpenoids. Ocymene, cymene, α-bergamotene, geranyl acetone, nerolidol, and ethyl 3-(methylthio)-propionate were identified for the first time in apples. This peculiar profile allows to propose them as varietal markers of BE apple variety. However, as this methodology has only been applied to BE apple, the occurrence of these compounds in other cultivars can not be excluded.  相似文献   
272.
Wireless Networked Control Systems (WNCS) are used to implement a control mechanism over a wireless network that is capable of carrying real-time traffic. This field has drawn enormous attention from current researchers because of its flexibility and robustness. However, designing efficient WNCS over Mobile Ad Hoc Networks (MANET) is still a challenging topic because of its less-predictable aspects, such as inconsistent delay, packet drop probability, and dynamic topology. This paper presents design guidelines for WNCS over MANET using the Network Simulator version 2, NS2 software. It investigates the impact of packet delay and packet drop under the AODV and DSR routing protocols. The simulation results have been compared to MATLAB results for validation.  相似文献   
273.
Smoking-related cancer and other disease account for more than 400,000 U.S. deaths annually. Smoking cessation reduces smoking-related disease rates, but relapse rates are high. Thus, interest in reducing the harm of continued smoking is growing. Potential reduced exposure products (PREPs) are marketed to reduce smokers' exposure to smoke toxicants such as carbon monoxide (CO) and carcinogens and may be harm reduction tools. New PREPs are proliferating, but past experience with "low-yield" cigarettes that failed to reduce smokers' toxicant exposure suggests that comprehensive evaluation is necessary to predict if these new products are likely to alter the harm caused by smoking. The purpose of the study was to develop clinical laboratory methods for PREP evaluation. Smokers (N = 35) completed four, 5-day conditions that differed by product used: Advance, Eclipse, own brand cigarettes, or no cigarettes. Carcinogen (as assessed by one nitrosamine and one polycyclic aromatic hydrocarbon biomarker) and nicotine exposure were assessed via thrice-weekly urine sampling. Withdrawal symptoms were measured daily, and smoking behavior was assessed on the first and last day of each condition. Relative to own brand, Advance reduced exposure to the nitrosamine NNK and CO, and Eclipse reduced exposure to nicotine and the nitrosamine NNK, increased exposure to CO, and resulted in larger, longer, and more frequent puffs. No smoking reduced exposure to the nitrosamine NNK, CO, and nicotine, whereas withdrawal was elevated (all p values <.05). Clinical laboratory evaluation of PREPs for smokers is valuable for measuring users' smoke toxicant exposure, withdrawal, and smoking behavior and should be incorporated into a comprehensive PREP evaluation strategy.  相似文献   
274.
At present, there is an unprecedented level of interest in the properties and structures of complexes consisting of DNA mixed with oppositely charged cationic liposomes (CLs). The interest arises because the complexes mimic natural viruses as chemical carriers of DNA into cells in worldwide human gene therapy clinical trials. However, since our understanding of the mechanisms of action of CL-DNA complexes interacting with cells remains poor, significant additional insights and discoveries will be required before the development of efficient chemical carriers suitable for long-term therapeutic applications. Recent studies describe synchrotron X-ray diffraction, which has revealed the liquid crystalline nature of CL-DNA complexes, and three-dimensional laser-scanning confocal microscopy, which reveals CL-DNA pathways and interactions with cells. The importance of the liquid crystalline structures in biological function is revealed in the application of these modern techniques in combination with functional transfection efficiency measurements, which shows that the mechanism of gene release from complexes in the cell cytoplasm is dependent on their precise liquid crystalline nature and the physical and chemical parameters (for example, the membrane charge density) of the complexes. In [section sign] 5, we describe some recent new results aimed at developing bionanotube vectors for gene delivery.  相似文献   
275.
Abstract: The near‐surface mounted (NSM) is one of the most recent techniques applied for the increase of the shear resistance of reinforced concrete (RC) beams. This technique involves the installation of carbon fibre reinforcement polymers (CFRP) laminates into thin slits open on the concrete cover of the elements to strengthen. The effectiveness of this technique for the shear strengthening of T cross‐section RC beams was assessed by experimental research. For this purpose, three inclinations of laminates were tested (45º, 60º and 90º) and, for each inclination, three percentages of CFRP were applied in RC beams with a percentage of steel stirrups of 0.10% (ρsw). The highest percentage of laminates was designed to provide a maximum load similar to the reference RC beam, which was reinforced with a reinforcement ratio of steel stirrups of 0.28% (ρsw = 0.28%). For each percentage of laminates, a homologous RC beam strengthened with unidirectional U‐shaped CFRP wet lay‐up sheets (discrete strips) applied according to the externally bonded reinforcement technique was also tested, with the purpose of comparing the effectiveness of these two CFRP‐strengthening techniques. To evaluate the influence of the percentage of steel stirrups in the effectiveness of the NSM technique, some of the abovementioned CFRP configurations were also applied in beams with ρsw = 0.17%.  相似文献   
276.
Electron field emission is a quantum tunneling phenomenon whereby electrons are emitted from a solid surface due to a strong electric field. Graphene and its derivatives are expected to be efficient field emitters due to their unique geometry and electrical properties. So far, electron field emission has only been achieved from the edges of graphene and graphene oxide sheets. We have supported graphene oxide sheets on nickel nanotip arrays to produce a high density of sharp protrusions within the sheets and then applied electric fields perpendicular to the sheets. Highly efficient and stable field emission with low turn-on fields was observed for these graphene oxide sheets, because the protrusions appear to locally enhance the electric field and dramatically increase field emission. Our simple and robust approach provides prospects for the development of practical electron sources and advanced devices based on graphene and graphene oxide field emitters.  相似文献   
277.
Objective: Describe the development and preliminary evaluation of a safety awareness program for women with disabilities. Design: Baseline and postintervention questionnaires administered to a sample of women who completed a safety awareness program. Participants: Seven women with diverse disabilities. Intervention: Eight interactive sessions designed to increase protective factors. Setting: Community site for persons with disabilities. Main Outcomes: Safety self-efficacy, safety skills, social support/isolation, and safety promoting behaviors. Results: Significant increases from baseline to postintervention were found on measures of self-efficacy and safety skills. Although not statistically significant, improvements were also found in safety promoting behavior. Conclusion: Despite the small sample, findings suggest that participation in a safety awareness program may improve protective factors. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
278.
Some countries now incorporate recommendations for increased consumption of whole grain (WG) into local dietary guidelines. Cereal and pseudo-cereal grains are good sources of complex carbohydrates, dietary fiber, proteins, phytochemicals, vitamins and minerals. However, research shows that the large majority of consumers are still falling short of WG consumption goals. To address this, we are actively involved in research to help increase the WG content of processed foods without compromising on taste and texture. In order to ensure consumer trust, the advancement of process technologies in incorporating WG to produce tasty food has to go hand in hand with well designed clinical trials that confirm the health benefits resulting from diets rich in WG.  相似文献   
279.
This study analyses and compares the cost efficiency of Japanese steam power generation companies using the fixed and random Bayesian frontier models. We show that it is essential to account for heterogeneity in modelling the performance of energy companies. Results from the model estimation also indicate that restricting CO2 emissions can lead to a decrease in total cost. The study finally discusses the efficiency variations between the energy companies under analysis, and elaborates on the managerial and policy implications of the results.  相似文献   
280.
Mauritia flexuosa L. (Arecaceae) is a popular Brazilian fruit known as “buriti” and belonging to the category of functional foods. This work reviewed the phytochemistry profile, nutritional and pharmacological activities of M. flexuosa. The main bioactive compounds reported to buriti were carotenoids, tocopherols, ascorbic acid, phenolic compounds, fiber, phytosterols, and mono‐ and poly‐unsaturated fatty acids. These compounds were mainly related to antioxidant, hypolipemiant, photoprotector, antiaggregant, antithrombotic, anti‐inflammatory, hypoglycemiant, antimicrobial, and antitumor activities. Furthermore, some compounds present in buriti fruit and its properties were tested in vitro and in vivo and showed biotechnology applications, especially for extraction of fiber, polysaccharides, pigments, antioxidants, and oil. Howerer, the buriti fruit shows great relevance to the development of new products in food, pharmaceutical and chemical industry, this fruit is still underexploited and it has need to expand its production chain and processing to encourage their consumption and utilization.  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