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   13665篇
  免费   339篇
  国内免费   21篇
电工技术   191篇
综合类   12篇
化学工业   3015篇
金属工艺   224篇
机械仪表   283篇
建筑科学   643篇
矿业工程   97篇
能源动力   376篇
轻工业   966篇
水利工程   130篇
石油天然气   162篇
武器工业   1篇
无线电   828篇
一般工业技术   2003篇
冶金工业   3309篇
原子能技术   138篇
自动化技术   1647篇
  2022年   116篇
  2021年   168篇
  2020年   144篇
  2019年   159篇
  2018年   179篇
  2017年   178篇
  2016年   203篇
  2015年   165篇
  2014年   225篇
  2013年   805篇
  2012年   442篇
  2011年   617篇
  2010年   398篇
  2009年   434篇
  2008年   560篇
  2007年   546篇
  2006年   536篇
  2005年   444篇
  2004年   360篇
  2003年   357篇
  2002年   370篇
  2001年   235篇
  2000年   229篇
  1999年   247篇
  1998年   212篇
  1997年   228篇
  1996年   237篇
  1995年   240篇
  1994年   243篇
  1993年   290篇
  1992年   245篇
  1991年   184篇
  1990年   201篇
  1989年   214篇
  1988年   169篇
  1987年   214篇
  1986年   203篇
  1985年   247篇
  1984年   223篇
  1983年   201篇
  1982年   180篇
  1981年   189篇
  1980年   161篇
  1979年   179篇
  1978年   180篇
  1977年   136篇
  1976年   142篇
  1975年   172篇
  1974年   133篇
  1973年   132篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
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   
12.
Vipin K. Garg  William R. Nes 《Lipids》1985,20(12):876-883
While seeds ofCucurbita maxima contain both Δ5- and Δ7-sterols, the former, which have been described earlier, now have been found to disappear during germination. This suggests that a function exists for the Δ5-compounds only in the early part of the life cycle ofC. maxima, unlike most of the other higher plants studied. In contrast to the Δ5-sterols, the level of Δ7-sterols increased during germination as well as during seedling development and maturation. The period of transition between germination and seedling development appeared to be of special importance in terms of sterol changes. This period represented a surge of sterol biosynthesis with an ontogenetic shift in sterol composition from approximately equal amounts of 24α- and 24β-ethyl stereochemistry to a predominance of the former. The sterol composition of the mature plants included only about 5% of the 24β-ethylsterols. The configurational relationships were demonstrated by high resolution1H-NMR. The sterols of the mature plants were: 25(27)-dehydrochondrillasterol, 24β-ethyl-25(27)-dehydrolathosterol, avenasterol, spinasterol, 22-dihydrospinasterol and 24ξ-methyllathosterol. Based on the changes which occurred in the relative amounts of the Δ7-sterols, it did not appear that the Δ5-components were being converted to their Δ7-analogs. A portion of this work was presented at the meeting of the American Oil Chemists' Society in May, 1985 in Philadelphia.  相似文献   
13.
With the goal of improving process efficiency and reducing risk, this article describes a pollution-prevention analysis technique that can be applied to most secondary steel heat-treating operations involving quenching. Costs, risk assessments, and pollution-prevention options are discussed, as are the effects of quenching on two subsequent processes—parts cleaning and tempering. In contrast to many process comparisons, the issues of worker exposure, material safety and toxicity, liability, waste reduction, reclamation, and disposal are emphasized.  相似文献   
14.
15.
A new type of radiative cooling system is described. The radiator makes use of nonimaging optics and evacuated-tube technology to radiate significant amounts of heat to the 3 K environment of outer space. The nonimaging optics are used to direct the radiation overhead, through the most transparent part of the 8–13 μm atmospheric window. The required optical concentration is small, and relatively simple mirror geometries are possible. The evacuated-tube technology is used to attain storage temperatures at or below the freezing point, even when dewpoint temperatures are well above the freezing point.  相似文献   
16.
The dependence of the cetane number of hydrotreated light cycle oil on chemical composition has been investigated. Cetane number was related to hydrogen content and aromaticity. The latter was determined by high performance liquid chromatography and also by carbon-13 n.m.r. spectroscopy. Specific correlations were developed in each case and a regression coefficient of 0.99 and a standard error of 1.25 were obtained for the best case. The usefulness of predicting cetane number from fuel composition is discussed.  相似文献   
17.
18.
MKT1 is required for m aintenance of K2 above 30°C in strains with the L-A-HN variant of the L-A double-stranded RNA virus of Saccharomyces cerevisiae. We report that MKT1 encodes a 92 979 Da protein with serine-rich regions and the retroviral protease signature, DTG, but with no substantial homology to proteins presently in the databases. This sequence is available from GenBank under Accession Number U09129.  相似文献   
19.
Examined the effects of traditional Asian cultural norms, family roles, and values upon the psychodynamic formulation of a 25-yr-old Asian American man. The main tenets of control-mastery theory (CMT; J. Weiss et al [1986]), a cognitive approach to psychoanalytic psychotherapy, are presented, and its plan formulation method is discussed. The need for a more comprehensive consideration of Asian culture, Asian-American identity formation, and acculturative stress in clinical case formulations is presented. Theory modifications are discussed that should help to reconcile differences between the Western values inherent in CMT and traditional Asian approaches to psychotherapy and mental health. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
20.
72 patients engaged in residential drug abuse treatment who also had cognitive impairment were randomly assigned to 1 of 4 groups. One group of patients received 2 hrs of computer-assisted cognitive rehabilitation per week over a 6-mo period; a 2nd group received 2 hrs of progressive muscle relaxation per week over a 6-mo period; a 3rd group was taught typing on a computer; and a 4th group received no treatment beyond that provided by the program. All patients were tested with a neuropsychological test battery at admission and at monthly intervals thereafter for 6 mo. Results show that Ss in the cognitive rehabilitation group demonstrated a faster rate of cognitive recovery during the first 2 mo of treatment and had more efficient cognitive functioning over the first 4 mo of residence. These patients were also rated as more "appropriately participatory" in the treatment program by the clinical staff. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  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