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   59387篇
  免费   2687篇
  国内免费   201篇
电工技术   884篇
综合类   94篇
化学工业   11587篇
金属工艺   2145篇
机械仪表   3279篇
建筑科学   1275篇
矿业工程   54篇
能源动力   2289篇
轻工业   4275篇
水利工程   260篇
石油天然气   210篇
武器工业   1篇
无线电   10428篇
一般工业技术   11799篇
冶金工业   5887篇
原子能技术   667篇
自动化技术   7141篇
  2023年   555篇
  2022年   868篇
  2021年   1472篇
  2020年   1063篇
  2019年   1111篇
  2018年   1412篇
  2017年   1417篇
  2016年   1743篇
  2015年   1398篇
  2014年   2169篇
  2013年   3676篇
  2012年   3398篇
  2011年   4156篇
  2010年   3138篇
  2009年   3301篇
  2008年   3078篇
  2007年   2561篇
  2006年   2351篇
  2005年   2036篇
  2004年   1936篇
  2003年   1787篇
  2002年   1720篇
  2001年   1345篇
  2000年   1245篇
  1999年   1236篇
  1998年   2231篇
  1997年   1470篇
  1996年   1224篇
  1995年   976篇
  1994年   739篇
  1993年   691篇
  1992年   505篇
  1991年   502篇
  1990年   430篇
  1989年   417篇
  1988年   330篇
  1987年   288篇
  1986年   261篇
  1985年   240篇
  1984年   204篇
  1983年   154篇
  1982年   152篇
  1981年   130篇
  1980年   129篇
  1979年   102篇
  1978年   94篇
  1977年   123篇
  1976年   159篇
  1975年   80篇
  1974年   74篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Objective: The ability to select what is important to remember, to attend to this information, and to recall high-value items leads to the efficient use of memory. The present study examined how children with and without attention-deficit/hyperactivity disorder (ADHD) performed on an incentive-based selectivity task in which to-be-remembered items were worth different point values. Method: Participants were 6–9 year old children with ADHD (n = 57) and without ADHD (n = 59). Using a selectivity task, participants studied words paired with point values and were asked to maximize their score, which was the overall value of the items they recalled. This task allows for measures of memory capacity and the ability to selectively remember high-value items. Results: Although there were no significant between-groups differences in the number of words recalled (memory capacity), children with ADHD were less selective than children in the control group in terms of the value of the items they recalled (control of memory). All children recalled more high-value items than low-value items and showed some learning with task experience, but children with ADHD Combined type did not efficiently maximize memory performance (as measured by a selectivity index) relative to children with ADHD Inattentive type and healthy controls, who did not differ significantly from one another. Conclusions: Children with ADHD Combined type exhibit impairments in the strategic and efficient encoding and recall of high-value items. The findings have implications for theories of memory dysfunction in childhood ADHD and the key role of metacognition, cognitive control, and value-directed remembering when considering the strategic use of memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
992.
A modular upgrade design for packet transport switch nodes is presented where packet loss is dramatically reduced by intra‐module and inter‐module buffer sharing. This modular design offers significant cost and power reduction in a high‐data‐rate system where buffers are highly costly and power‐greedy.  相似文献   
993.
Manganese oxide nanocrystals are combined with aluminum oxide nanocrystals to improve their crystallinity via calcination without a significant increase of crystal size. A nanocomposite, consisting of two metal oxides, can be synthesized by the reaction between permanganate anions and aluminum oxyhydroxide keggin cations. The as‐prepared manganese oxide–aluminum oxide nanocomposite is X‐ray amorphous whereas heat‐treatment gives rise to the crystallization of an α‐MnO2 phase at 600 °C and Mn3O4/Mn2O3 and γ‐Al2O3 phases at 800 °C. Electron microscopy and N2 adsorption‐desorption‐isotherm analysis clearly demonstrate that the as‐prepared nanocomposite is composed of a porous assembly of monodisperse primary particles with a size of ~20 nm and a surface area of >410 m2 g?1. Of particular interest is that the small particle size of the as‐prepared nanocomposite is well‐maintained up to 600 °C, a result of the prevention of the growth of manganate grains through nanoscale mixing with alumina grains. The calcined nanocomposite shows very‐high catalytic activity for the oxidation of cyclohexene with an extremely high conversion efficiency of >95% within 15 min. The present results show that the improvement of the crystallinity without significant crystal growth is very crucial for optimizing the catalytic activity of manganese oxide nanocrystals.  相似文献   
994.
Recently, several papers reported problems in data exchange using industry foundation classes (IFC). However, most comparisons were made based on a visual check, a manual count, and observation of properties that were selectively chosen. This study proposes a set of metrics for quantifying the similarities and differences between IFC files. The proposed metrics include the similarity rate, the matching rate, the globally unique identifier (GUID) preservation rate, the missing rate, and the addition rate. A long-term goal of this study is to develop a set of metrics for quantifying the information exchange rate between two IFC files. Automated identification of modified information versus newly generated information is an unsolved challenge. The proposed metrics were used in analyzing 88 IFC files generated from different systems to demonstrate the potential use of the proposed metrics.  相似文献   
995.
Cognitive–behavioral therapies (CBTs) can be effective treatments for posttraumatic stress disorder (PTSD) but their effectiveness is limited by high rates of premature dropout. Few studies have compared pretreatment characteristics of treatment completers and dropouts, and only one has examined these factors in Operations Iraqi Freedom and Enduring Freedom (OIF/OEF) Veterans. This study analyzed archival clinical data from 117 OEF/OIF Veterans evaluated and treated through a Veterans Affairs PTSD clinic. High numbers dropped out of treatment (68%). Treatment dropouts (n = 79) and completers (n = 38) differed significantly on Minnesota Multiphasic Personality Inventory-2 (MMPI-2) scales, PTSD symptom severity, and age. Regression analyses identified one MMPI-2 scale, TRT (negative treatment indicators), and age as unique but modest predictors of dropout. (PsycINFO Database Record (c) 2011 APA, all rights reserved)  相似文献   
996.
In this paper, effects of variables concerning slurry properties and drying conditions on the particle size and morphology of feedstock were discussed. The results indicated that, the increase of powder loading increased D 50 and volume content but reduced recovery rate slightly. Other variables, such as inlet temperature, showed a maximum on the desired property. The particle size of resulting feedstock increased with the rise of feeding rate of slurry, however, decreased with the increase of atomization pressure based on the premise that atomization pressure was enough to force slurry to form the liquid membrane. For preparation of spherical feedstock, optimized variables were as follows: 60 wt.% powder in slurry, 2 wt.% PVA in slurry, inlet temperature of 60°C, atomization pressure of 13 × 103 KPa and feeding rate of 12 ml min?1. The volume content of feedstocks with 10–45 μm was above 65% and for D 50 it was 15·42 μm. In addition, according to the heat and mass transfer equation, the drying mechanism was established, which was well in agreement with the experimental results.  相似文献   
997.
The identification and production of recombinant morphogens and growth factors that play key roles in tissue regeneration have generated much enthusiasm and numerous clinical trials, but the results of many of these trials have been largely disappointing. Interestingly, the trials that have shown benefit all contain a common denominator, the presence of a material carrier, suggesting strongly that spatio-temporal control over the location and bioactivity of factors after introduction into the body is crucial to achieve tangible therapeutic effect. Sophisticated materials systems that regulate the biological presentation of growth factors represent an attractive new generation of therapeutic agents for the treatment of a wide variety of diseases. This review provides an overview of growth factor delivery in tissue engineering. Certain fundamental issues and design strategies relevant to the material carriers that are being actively pursued to address specific technical objectives are discussed. Recent progress highlights the importance of materials science and engineering in growth factor delivery approaches to regenerative medicine.  相似文献   
998.
This study used in vitro and in vivo experimental models to investigate the anti-inflammatory potential of the butanol fraction of red bean ethanol extract (BF-RBEE), which contains the biologically active molecule catechin-7-β-d-glucopyranoside. Treatment with BF-RBEE inhibited nitric oxide production in lipopolysaccharide (LPS)-stimulated macrophages through suppression of extracellular signalregulated kinase and IκBα activation. In an in vivo murine sepsis model, oral administration of BF-RBEE improved mouse survival. Specifically, the survival rate of mice injected with LPS was 0 and 40% in ICR and BALB/c mice, respectively, whereas the survival of mice co-treated with BF-RBEE was 100% in both mouse types. This increase in survival with the BF-RBEE administration was correlated with decreased tumor-necrosis factor-α and interferon-γ and increased interleukin-10. Oral administration of BF-RBEE also inhibited total and ovalbumin-specific IgE production in experimental mice. These results suggest the possible usefulness of red beans in the treatment of inflammatory diseases.  相似文献   
999.
1000.
Dielectric properties of iron doped CaCu3Ti4O12 (CCTO), viz. CaCu3Ti3.9Fe0.1O12 (CCTFO) prepared by a novel semi-wet route have been investigated. X-ray diffraction of powder sintered at 900 °C show formation of single phase solid solution. Energy dispersive X-ray spectroscopy (EDX) confirmed the presence of CuO rich phase at grain boundaries of CCTFO. Nature of dielectric relaxation observed above room temperature is studied using complex plane impedance analysis and modulus spectroscopy. It has been found that out of the two relaxations reported earlier above room temperature, one occurring at lower temperature is due to grainboundaries interfacial polarization.  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