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   8415篇
  免费   635篇
  国内免费   12篇
电工技术   125篇
综合类   10篇
化学工业   2418篇
金属工艺   91篇
机械仪表   185篇
建筑科学   293篇
矿业工程   11篇
能源动力   374篇
轻工业   1529篇
水利工程   44篇
石油天然气   45篇
无线电   580篇
一般工业技术   1323篇
冶金工业   237篇
原子能技术   75篇
自动化技术   1722篇
  2024年   10篇
  2023年   99篇
  2022年   388篇
  2021年   409篇
  2020年   256篇
  2019年   266篇
  2018年   296篇
  2017年   327篇
  2016年   378篇
  2015年   266篇
  2014年   427篇
  2013年   679篇
  2012年   576篇
  2011年   689篇
  2010年   535篇
  2009年   530篇
  2008年   497篇
  2007年   418篇
  2006年   334篇
  2005年   239篇
  2004年   208篇
  2003年   182篇
  2002年   201篇
  2001年   107篇
  2000年   80篇
  1999年   82篇
  1998年   91篇
  1997年   49篇
  1996年   49篇
  1995年   50篇
  1994年   36篇
  1993年   37篇
  1992年   28篇
  1991年   14篇
  1990年   18篇
  1989年   20篇
  1988年   15篇
  1987年   10篇
  1986年   16篇
  1985年   23篇
  1984年   20篇
  1983年   7篇
  1982年   20篇
  1981年   13篇
  1980年   12篇
  1979年   11篇
  1978年   11篇
  1977年   10篇
  1975年   6篇
  1973年   5篇
排序方式: 共有9062条查询结果,搜索用时 21 毫秒
111.
The extent to which chloride ion incorporated in portland cement concrete as calcium chloride accelerator at the usual treatment levels remains dissolved in the pore solution was investigated. This was examined by direct analysis of pore solutions expressed from cement pastes. The chloride ion concentration of the pore solution remains high during the first day of hydration and only gradually declines. It appears that appreciable concentrations of chloride ion likely remain in solution indefinitely .  相似文献   
112.
The human plasma high density lipoproteins (HDL) are a heterogeneous ensemble of five proteins associated with both neutral and polar lipids. The sequences of all five proteins are known. ApoA-I and apoA-II are the major protein components; apoC-I, apoC-II and apoC-III are the minor protein components. All these apoproteins spontaneously recombine with phospholipids to give stable lipid-protein complexes and freely exchange between the two major HDL subclasses, HDL2 and HDL3. In addition, ApoC-I, apoC-II, and apoC-III exchange between HDL and very low density lipoproteins. Furthermore, certain HDL apoproteins are activators for plasma enzymes that are important in lipid metabolism. ApoA-I and apoC-I activate lecithin/cholesterol acyltransferase; apoC-II is an activator of lipoprotein lipase. The regions of apoC-I and apoC-II that are involved in the activation of these enzymes have been localized with synthetic peptides. Studies of synthetic and native fragments of apoA-II, apoC-I, apoC-II, and apoC-III as well as model lipid-binding peptides have identified specific regions with structural features common to lipid-binding proteins. These special properties, which include helical potential, sequences with a critical amphipathic length, and high hydrophobicity of the nonpolar side of the amphipathic helix, are the determinants of HDL structure and metabolism.  相似文献   
113.
Solvent free oxidation of benzyl alcohol was investigated in the absence of a base using Au catalysts prepared by sol immobilization on titania and carbon supports. Comparison between the Au supported catalysts revealed that activity and distribution of products was dependent on the nature of support and heat treatment. Specifically, heat pre-treatment of the Au catalysts has a beneficial effect in terms of activity, but is detrimental in terms of selectivity to the benzaldehyde. We conclude that sol immobilization is a suitable technique for preparing gold catalysts with small particle size and narrow particle size distributions and very high activity and selectivity for benzyl alcohol oxidation.  相似文献   
114.
The dielectric properties of dense ceramics of the "twinned" 8H-hexagonal perovskite Ba8Nb4Ti3O24 are reported. Single-phase powders were obtained from the mixed-oxide route at 1325°C and ceramics (>92% of the theoretical X-ray density) by sintering in air or flowing O2 at 1400°–1450°C. The ceramics are dc insulators with a band gap >3.4 eV that resonate at microwave frequencies with relative permittivity, ɛr∼44–48, quality factor, Q × f r∼21 000–23 500 GHz (at f r∼5.5 GHz) and temperature coefficient of resonant frequency, TC f,∼+115 ppm/K.  相似文献   
115.
Spinel ferrite MnFe2O4 nanoparticles were synthesized by means of a nanocasting technique using a low-cost mesoporous silica gel as a hard template. The magnetic nanoparticles, of <10 nm diameter and with a surface area of around 100 m2/g, were tested as a heterogeneous Fenton catalyst for the decomposition of hydrogen peroxide under neutral and basic conditions. This catalyst shows a much higher activity than previous heterogeneous catalysts reported in the literature, which is mainly ascribed to its small particle size. Furthermore, the magnetic catalyst can be easily separated from the reaction medium by means of an external magnetic field. The effects of residual silica and the purity of the catalyst (hematite formation) on catalytic activity have been studied and correlated. The results obtained show this catalyst to be a suitable candidate for the removal of pollutants in wastewaters by means of the Fenton heterogeneous reaction.  相似文献   
116.
Despite of its general use in industry, particle sedimentation is still a not well understood unit operation. Hydrodynamics is complex in essence, mainly because the possible volumes are depending on the operating conditions, which in turn has consequences on the stability of the equipment in unsteady state conditions.  相似文献   
117.
Automatic prediction tools play a key role in enabling the application of non-functional requirements analysis, to simplify the selection and the assembly of components for component-based software systems, and in reducing the need for strong mathematical skills for software designers. By exploiting the paradigm of Model-Driven Engineering (MDE), it is possible to automatically transform design models into analytical models, thus enabling formal property verification. MDE is the core paradigm of the KlaperSuite framework presented in this paper, which exploits the KLAPER pivot language to fill the gap between design and analysis of component-based systems for reliability properties. KlaperSuite is a family of tools empowering designers with the ability to capture and analyze quality of service views of their systems, by building a one-click bridge towards a number of established verification instruments. In this article, we concentrate on the reliability-prediction capabilities of KlaperSuite and we evaluate them with respect to several case studies from literature and industry.  相似文献   
118.
X‐ray photoelectron spectroscopy (XPS) is a quantitative surface analysis technique used to identify the elemental composition, empirical formula, chemical state, and electronic state of an element. The kinetic energy of the electrons escaping from the material surface irradiated by an x‐ray beam produces a spectrum. XPS identifies chemical species and quantifies their content and the interactions between surface species. It is minimally destructive and is sensitive to a depth between 1–10nm. The elemental sensitivity is in the order of 0.1 atomic %. It requires ultra high vacuum ( Pa) in the analysis chamber and measurement time varies from minutes to hours per sample depending on the analyte. XPS dates back 50 years ago. New spectrometers, detectors, and variable size photon beams, reduce analysis time and increase spatial resolution. An XPS bibliometric map of the 10 000 articles indexed by Web of Science[1] identifies five research clusters: (i) nanoparticles, thin films, and surfaces; (ii) catalysis, oxidation, reduction, stability, and oxides; (iii) nanocomposites, graphene, graphite, and electro‐chemistry; (iv) photocatalysis, water, visible light, and ; and (v) adsorption, aqueous solutions, and waste water.  相似文献   
119.
Three new dihydro--agarofuran sesquiterpenes from two species ofMaytenus were isolated and their structures were elucidated by means of1H and13C NMR studies. The differences and similarities noted in the chemical content of the dihydro--agarofuran sesquiterpenes from the fourMaytenus species from Chile are in line with the taxonomic characterization of these species; their geographical distribution is also given.  相似文献   
120.
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