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   161046篇
  免费   4104篇
  国内免费   2406篇
化学   91742篇
晶体学   2542篇
力学   6786篇
综合类   166篇
数学   15173篇
物理学   51147篇
  2022年   1244篇
  2021年   1511篇
  2020年   1844篇
  2019年   1974篇
  2018年   2160篇
  2017年   2121篇
  2016年   3308篇
  2015年   2328篇
  2014年   3206篇
  2013年   7067篇
  2012年   6119篇
  2011年   7192篇
  2010年   5103篇
  2009年   5060篇
  2008年   6677篇
  2007年   6351篇
  2006年   6100篇
  2005年   5607篇
  2004年   5020篇
  2003年   4690篇
  2002年   4692篇
  2001年   6159篇
  2000年   4348篇
  1999年   3399篇
  1998年   2303篇
  1997年   2376篇
  1996年   2209篇
  1995年   2042篇
  1994年   1993篇
  1993年   1797篇
  1992年   2296篇
  1991年   2385篇
  1990年   2208篇
  1989年   2139篇
  1988年   2109篇
  1987年   2106篇
  1986年   1935篇
  1985年   2393篇
  1984年   2409篇
  1983年   1916篇
  1982年   1915篇
  1981年   1753篇
  1980年   1738篇
  1979年   2028篇
  1978年   2100篇
  1977年   2046篇
  1976年   1975篇
  1975年   1859篇
  1974年   1842篇
  1973年   1874篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
993.
Russian Journal of Electrochemistry -  相似文献   
994.
995.
996.
Mother-to-child transmission (MTCT) of HIV is the most significant source of HIV infection in children below the age of 15 years. In 2000 alone, about 600 000 new infections occurred, the vast majority from mothers living in developing countries who were not aware of their HIV-positive status. To date, at least 4.3 million children have died from AIDS. In this paper, we discuss the development of an operational model that can be used to evaluate intervention options for the prevention of MTCT of HIV. The problem was approached by defining suitable pregnancy risk groups, prevention options and the required model outputs to evaluate different intervention strategies. The method chosen to approach the problem was a discrete-event, three-phase simulation, built in Visual Basic, with a stochastic semi-Markov structure. The developed model takes individual pregnant women through each stage of their pregnancy, labour and birth. Different intervention strategies may be modelled at any time, including short-course antiretroviral drugs and cessation of breastfeeding. The model is demonstrated with data from Botswana, which has one of the highest HIV-infection rates in the world.  相似文献   
997.
We study computable trees with distinguished initial subtree (briefly, I-trees). It is proved that all I-trees of infinite height are computably categorical, and moreover, they all have effectively infinite computable dimension.  相似文献   
998.
Microspheres of Li2TiO3 were fabricated by a classical, inorganic sol-gel process from commercially available TiCl4. Elaborated process consists of the following main steps: (1) dissolving of TiCl4 in concentrated aqueous HCl and addition of LiOH; (2) formation of sol emulsion in 2-ethylhexanol-1 containing the surfactant SPAN-80 (EH); (3) gelation of emulsion drops by extraction of water with partially dehydrated EH; (4) impregnation of gel to Li:Ti molar ratio MR = 2; (5) thermal treatment at 1200°C in order to receive chloride free product. This temperature can be significantly lowered (to 750°C) by dechlorination starting solution TiCl4 by chemical treatment of the with nitric acid to form of nitrate-stabilized titania sols. Tritium release from sol-gel made Li2TiO3 microspheres were found very close to that observed for other traditional materials, however for the first sample process starts slightly earlier.  相似文献   
999.
Using the full-potential linearized augmented plane wave (FP-LAPW) method, we have studied the effect of chemistry on the average intercalation voltage (AIV) caused by the Na ions intercalating into transition metal oxides. The effect of transition metal was systematically studied by varying M=Co, Ni and Mn in NaMO2 and fixing the α-NaFeO2 layered structure. The effect of the guest atoms into the host material is discussed in terms of the structural and electronic properties. Comparatively to Li intercalation, a significant electron transfer towards transition metal was found. This observation suggests that the transition metal contribute to the AIV determination and confirms the common assumption that intercalated electron reduces M4+ to M3+.  相似文献   
1000.
 A validation procedure based on the ISO/IEC 17025 standard was used to demonstrate the long-term stability of a calibration process and to assess the measurement uncertainty of a standard test method for optical emission vacuum spectrometric analysis of carbon and low-alloy steel (ASTM E 415–99a). The validation was used to provide documented evidence that the selected method fulfils the requirements and that the method is ”fit for purpose”. A test for drift was applied to determine statistically whether the analytical results vary systematically with time. The accuracy and traceability of the optimised method were tested by an analysis of closely matched matrix certified reference materials (CRMs). The measurement uncertainty estimations took account of the precision study, the bias and its uncertainty, and the qualification of uncertainties not considered in the overall performance studies. Received: 2 November 2002 Accepted: 2 January 2003 Acknowledgement The author expresses gratitude to Dr. Aleš Fajgelj for helpful discussions during the 3rd Central European Conference on Reference Materials and Measurements. Presented at CERMM-3, Central European Reference Materials and Measurements Conference: The function of reference materials in the measurement process, May 30–June 1, 2002, Rogaška Slatina, Slovenia Correspondence to T. Drglin  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