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   262517篇
  免费   4573篇
  国内免费   1285篇
电工技术   5095篇
综合类   282篇
化学工业   39827篇
金属工艺   11634篇
机械仪表   8268篇
建筑科学   6620篇
矿业工程   487篇
能源动力   7094篇
轻工业   25833篇
水利工程   1879篇
石油天然气   1350篇
武器工业   3篇
无线电   34281篇
一般工业技术   48892篇
冶金工业   51487篇
原子能技术   4104篇
自动化技术   21239篇
  2021年   2220篇
  2020年   1508篇
  2019年   1901篇
  2018年   2997篇
  2017年   2857篇
  2016年   3189篇
  2015年   2482篇
  2014年   4050篇
  2013年   12084篇
  2012年   6832篇
  2011年   9456篇
  2010年   7471篇
  2009年   8193篇
  2008年   8647篇
  2007年   8464篇
  2006年   7742篇
  2005年   7114篇
  2004年   6776篇
  2003年   6776篇
  2002年   6743篇
  2001年   6798篇
  2000年   6161篇
  1999年   6780篇
  1998年   17162篇
  1997年   11951篇
  1996年   9297篇
  1995年   6907篇
  1994年   6039篇
  1993年   5852篇
  1992年   4133篇
  1991年   4078篇
  1990年   3642篇
  1989年   3577篇
  1988年   3502篇
  1987年   2963篇
  1986年   2883篇
  1985年   3446篇
  1984年   3077篇
  1983年   2764篇
  1982年   2571篇
  1981年   2589篇
  1980年   2453篇
  1979年   2282篇
  1978年   2238篇
  1977年   2830篇
  1976年   4201篇
  1975年   1913篇
  1974年   1806篇
  1973年   1790篇
  1972年   1465篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
Point centromeres, found in some ascomycete yeasts such Saccharomyces cerevisiae, are very different in structure from the centromeres of other eukaryotes. They are tiny and nonrepetitive and contain only two short conserved sequence motifs. Until recently, point centromeres were thought to have a single evolutionary origin, in the budding yeast family Saccharomycetaceae. Most yeasts outside this family have centromeres that are many kilobases in size. Some have centromeres consisting of a large inverted repeat sequence, others have centromeric clusters of retrotransposons, and a third group including Candida albicans has centromeres with no conserved sequence features. It was recently reported that Scheffersomyces stipitis has point centromeres with a strongly conserved 125-bp core sequence, which is unexpected because Sstipitis is only distantly related to the known point-centromere species. We show here that the 125-bp core sequence is actually part of the long terminal repeat (LTR) of the Ty5-like retrotransposon Tps5, which forms a cluster in the centromeric region of each Sstipitis chromosome. Thus, the LTR of a centromere-associated retrotransposon confers centromere-like mitotic stability when cloned into a plasmid. The centromeric regions of Sstipitis contain three types of Tps5 element (Tps5a, Tps5b, and Tps5c) and a noncoding nonautonomous large retrotransposon derivative.  相似文献   
102.
The development of visualizing tools to monitor unsaturated moisture flow in cement-based materials is of great importance, as most degradation processes in cement-based materials are connected to and take place in the presence moisture. This paper investigates the ability of electrical capacitance tomography (ECT) to image two-dimensional (2D) unsaturated moisture flow in cement-based materials. In ECT, the electrical permittivity distribution within an object is reconstructed based on measured capacitances between electrodes attached on the object’s surface. In a series of experiments, mortar specimens with and without discrete cracks were imaged with ECT during a 2D moisture ingress. The results show that ECT is able to monitor the evolution of the moisture flow, and to approximate the shape and position of the moisture front. These findings indicate that ECT is a viable method for monitoring and visualizing 2D unsaturated moisture flow in cement-based materials in the presence and absence of discrete cracks.  相似文献   
103.
An agriglass composition containing different oxides acts as a slow release for macro and micro nutrients and was chosen to improve maize yield under most important abiotic stresses which affecting agriculture development; salinity and drought. A field experiment was performed in salt affected soil (EC =?7.5 dSm??1) by using different water deficit rates (I1 = 100, I2 = 85 and I3 = 70% of maize water requirements). Irrigation levels were located in main plots. Every main-plot divided into six sub-plots contained glassy fertilizer treatments [F1 = 55 kg fed?1 with 1/2 mm diameter of agriglass (fed. =?4200 m2), F2 = 55 kg fed?1 with 1 mm diameter, F3 = 80 kg fed?1 with 1/2 mm diameter, F4 = 80 kg fed?1 with 1 mm diameter, F5 = Recommendations of Ministry of Agriculture and F6 = control]. The experimental results demonstrated that, ears, straw, grains and biological yields increased with increasing both water and agriglass rates. Application of agriglass as a slow release fertilizer improved yield more than mineral fertilizer. Some growth parameters, water use efficiency (IWUE), macronutrients concentration and their relations were included. Other studies on residual effect of agriglass and the annual application rates to withstand salinity and drought stress by strategic crops are required.  相似文献   
104.
Sanad  M. H.  Marzook  F.  Saleh  G. M.  Farag  A. B.  Talaat  H. M. 《Radiochemistry》2019,61(4):478-482
Radiochemistry - Azathioprine, an antitumor agent, was labeled with 99mTc using stannous chloride dihydrate as a reducing agent. Factors such as the amounts of the reducing agent and substrate, pH,...  相似文献   
105.
Estimation of earth-fill dam height and dimension of spillway are strongly depend on accurate evaluation of dam overtopping reliability. Ideally, whole random and effective variables on overtopping should be considered for dam overtopping reliability assessment. However, taking into account all affecting random variables as well as overtopping reliability assessment using algorithms such as Monte Carlo (MC) could be excessively time consuming and impossible in some cases. In this study, new approach has been introduced as conditional reliability method and it has been tried to reduce the simulations time significantly by using combination of rainfall threshold theory, SUFI (Sequential Uncertainty Fitting) and MC methods. According two defined indexes, the obtain results shown, the relative error of new approach in computation of Jamishan dam overtopping reliability is less than 0.23%. Also, time saving in new method against to regular MC method is more than 87%. So, the new introduced method has more efficiency with acceptable accuracy for assessment of dam overtopping reliability. In addition, among hydrological and hydraulic uncertainties, hydrological uncertainties have more effect on overtopping reliability.  相似文献   
106.
107.
108.
109.
Metals and Materials International - This study investigated the influence of the initial grain size on the plastic deformation and tunnel defects that occurred from friction stir welding of...  相似文献   
110.
A facile sol–gel procedure has been developed for the synthesis of colloidal alumina nanocrystals. For the first time, optical characterization procedures were employed to study the quantum confinement effects in optical properties of the prepared Al2O3 sol. Accordingly, the hyperbolic band model was used to determine the optical band gap of colloidal alumina nanocrystals. X‐Ray diffraction pattern was used to study the crystallographic phase of the dried gel. Morphological characterization was performed using scanning electron microscopy (SEM). Inductively Coupled Plasma (ICP) emission spectroscopy was used to determination purity of the Al2O3 powder. High‐resolution TEM showed that the diameter of colloidal nanocrystals is about 10 nm. Photoluminescence spectroscopy demonstrated that quantum yields for colloidal nanocrystals are 68% with 300 nm excitation wavelength. The experimental observations confirm that highly stable alumina sol with strong UV emission was synthesized. The mentioned optical properties have not been reported before.  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