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   241410篇
  免费   2632篇
  国内免费   427篇
电工技术   4292篇
综合类   257篇
化学工业   39412篇
金属工艺   9383篇
机械仪表   7398篇
建筑科学   5463篇
矿业工程   1391篇
能源动力   5162篇
轻工业   24826篇
水利工程   2477篇
石油天然气   5092篇
武器工业   32篇
无线电   25048篇
一般工业技术   44793篇
冶金工业   45389篇
原子能技术   5950篇
自动化技术   18104篇
  2021年   2108篇
  2019年   2017篇
  2018年   3490篇
  2017年   3426篇
  2016年   3689篇
  2015年   2236篇
  2014年   3901篇
  2013年   10189篇
  2012年   6152篇
  2011年   8316篇
  2010年   6560篇
  2009年   7239篇
  2008年   7589篇
  2007年   7721篇
  2006年   6891篇
  2005年   6051篇
  2004年   5820篇
  2003年   5600篇
  2002年   5676篇
  2001年   5563篇
  2000年   5385篇
  1999年   5303篇
  1998年   12593篇
  1997年   8912篇
  1996年   6860篇
  1995年   5336篇
  1994年   4739篇
  1993年   4626篇
  1992年   3669篇
  1991年   3389篇
  1990年   3594篇
  1989年   3473篇
  1988年   3303篇
  1987年   2895篇
  1986年   2932篇
  1985年   3373篇
  1984年   3219篇
  1983年   2892篇
  1982年   2738篇
  1981年   2822篇
  1980年   2721篇
  1979年   2625篇
  1978年   2684篇
  1977年   2948篇
  1976年   3808篇
  1975年   2368篇
  1974年   2242篇
  1973年   2376篇
  1972年   1970篇
  1971年   1858篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
961.
Pb1–x Ca x [(Co0.5W0.5)0·05Ti0.95]O3 ceramics with x = 0.24, 0.30 and 0.35, are prepared by a solid state reaction of oxides. Deviations from nominal chemical compositions and formation of segregated phases different from modified lead titanate perovskites, are studied. Pyro- and non-pyroelectric currents excited in the material by a thermal wave are investigated and related to mobility of electrical charges in the materials.  相似文献   
962.
963.
By the methods of thermal-desorption spectrometry and nuclear reactions, we study the processes of trapping, thermal desorption, and evolution of the profiles of distribution of deuterium ionically implanted with an energy of 12 keV into Kh18N10T steel at room temperature. The evolution of the microstructure of steel under the conditions of irradiation of samples with D2+ ions up to doses of 1 · 1018 D/m2−2 · 1022 D/m2 is studied by the method of electron microscopy. The regularities of changes in the mechanical properties of steel after irradiation are determined. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 117–121, September–October, 2007.  相似文献   
964.
This paper deals with an economic comparison between costs and incomes in small wastewater treatment plants where the anaerobic co-digestion process of sludge and biowaste with energy recovery is operated. Plants in the size range 1,000-30,000 persons equivalent (pe) were considered in the study: typical costs, comprehensive of capital and operating costs, were in the range euro69-105 per person per year depending on the plant size: the smaller the size the higher the specific cost. The incomes deriving from taxes and fees for wastewater treatment are generally in the range euro36-54 per person per year and can only partially cover costs in small wastewater treatment plants. However, the co-treatment of biowaste and the use of produced energy for extra credits (green certificates) determine a clear improvement in the possible revenues from the plant. These were calculated to be euro23-25 per person per year; as a consequence the costs and incomes can be considered comparable for wastewater treatment plants (WWTPs) with size larger than 10,000 pe. Therefore, anaerobic co-digestion of biowaste and sludge can also be considered a sustainable solution for small wastewater treatment plants in rural areas where several different kinds of biowaste are available to enhance biogas production in anaerobic reactors.  相似文献   
965.
966.
Existing NASA supported scientific databases are usually developed and managed by a team of database administrators whose main concern is the efficiency of the databases in terms of normalization and data search constructs. The populating of the database is usually done in a manual fashion by row and column as the data become available, and the data dictionary is usually defined by the same team (at times with little input from the end science user). This process is tedious, error prone and self-limiting in terms of what can be described in a relational Data Base Management System (DBMS). The next generation Earth remote sensing platforms [i.e., Earth Observing System (EOS)] will be capable of generating data at a rate of over 300 Megabits per second from a suite of instruments designed for different applications. What is needed is an innovative approach that creates object-oriented data-bases that segment, characterize, and catalog, and are manageable in a domain-specific context, and whose contents are available interactively and in near-real-time to the user community. This paper describes work in progress that utilizes an artificial neural net approach to characterize satellite imagery of undefined objects into high-level data objects. The characterized data is then dynamically allocated to an object-oriented database where it can be reviewed and accessed by a user. The definition, development, and evolution of the overall data system model are steps in the creation of an application-driven knowledge-based scientific information system.  相似文献   
967.
968.
Shikimate dehydrogenase (E.C.1.1.1.25), extracted from heart-of-palm, was purified by precipitation with ammonium sulphate and elution from columns of Sephadex G-25, and DEAE-Sephadex A-50. The enzyme was unstable and it was not possible to maintain the activity by the addition of cysteine and 2-mercaptoethanol to the buffer. The heart-of-palm contained two isoenzymes which were separated by polyacrylamide gel electrophoresis. The enzyme was inhibited by metal ions and p-CMB. The p-CMB inhibition was reversed by cysteine. The shikimate dehydrogenase was competitively inhibited by protocatechuic acid (Ki 7.5 × 10?4M), and Km values of 0.021 mM and 0.011 mM for shikimic acid and NADP+, respectively, were obtained at pH 9.5 in Tris-HCl-glycine buffer. The molecular weight determination was performed on Sephadex G-100 and the MW was estimated to be 50,000 daltons.  相似文献   
969.
A method of calculating the temperature resistance coefficient and its temperature dependence for resistive materials in the approximation of a matrix-statistical structure is outlined.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 3, pp. 477–481, September, 1985.  相似文献   
970.
Cookies enriched with 0, 5, 10, 15, 20, and 25% full-fat sweet lupine flour (FFSL) were evaluated by a sensory panel using the rank of preference and paired comparison tests. Cookies with 0, 5, and 10% FFSL were preferred while those containing 20 and 25% FFSL were rejected (p≤0.01). Studies conducted with school children showed similar acceptability for 0 and 10% FFSL-containing cookies which was different (p = 0.05) from those containing 20% FFSL. Fortification of the basic formula with 10% FFSL was recommended on the basis of acceptability.  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