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   181948篇
  免费   2450篇
  国内免费   709篇
电工技术   3372篇
综合类   102篇
化学工业   29231篇
金属工艺   7881篇
机械仪表   5291篇
建筑科学   4599篇
矿业工程   889篇
能源动力   4858篇
轻工业   17191篇
水利工程   1748篇
石油天然气   3149篇
武器工业   2篇
无线电   20205篇
一般工业技术   34019篇
冶金工业   33586篇
原子能技术   4314篇
自动化技术   14670篇
  2021年   1570篇
  2019年   1452篇
  2018年   2293篇
  2017年   2299篇
  2016年   2410篇
  2015年   1733篇
  2014年   2919篇
  2013年   8182篇
  2012年   4854篇
  2011年   6708篇
  2010年   5317篇
  2009年   6152篇
  2008年   6075篇
  2007年   6043篇
  2006年   5183篇
  2005年   4801篇
  2004年   4579篇
  2003年   4249篇
  2002年   4188篇
  2001年   4187篇
  2000年   3985篇
  1999年   4107篇
  1998年   10497篇
  1997年   7500篇
  1996年   5732篇
  1995年   4330篇
  1994年   3662篇
  1993年   3611篇
  1992年   2621篇
  1991年   2555篇
  1990年   2431篇
  1989年   2456篇
  1988年   2389篇
  1987年   2139篇
  1986年   2085篇
  1985年   2386篇
  1984年   2204篇
  1983年   2039篇
  1982年   1891篇
  1981年   1952篇
  1980年   1808篇
  1979年   1819篇
  1978年   1786篇
  1977年   2121篇
  1976年   2704篇
  1975年   1560篇
  1974年   1550篇
  1973年   1610篇
  1972年   1350篇
  1971年   1262篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The utility of heart-rate variability (HRV) to index important aspects of neural control of the heart has made it one of the most valuable tools for researchers interested in health and behavior. However, most of the literature and extant data on the central origins of HRV have been derived from animal models. As yet little is known about the central nervous system (CNS) origins and concomitants in humans. In the following we will briefly detail a set of neural structures that have been associated with important CNS functions including cardiac regulation and emotional regulation. We will briefly summarize recent data using pharmacological, neuroimaging, and psychophysiological techniques that have examined CNS concomitants of HRV especially during emotion. Based on these and other data will we report the results of a computational model that examined the interactions of several neural structures and their effect on emotion-related HRV.  相似文献   
992.
Limitations of several anisotropic yield criteria are discussed. Hill's 1948 criterion overestimates the variation of yield strength with direction, while the specializations of Hill's 1979 theory assume planar isotropy. Shear stress terms cannot simply be included in either the author's 1979 criterion or in Hill's 1979 general criteria.It is suggested that the author's criterion be modified so that it is expressed in terms of principal stresses rather than referring stresses to the symmetry axes.  相似文献   
993.
994.
This paper comes to fill a gap in watermarking theory, analyzing the exact performance of the scalar Costa scheme (SCS) facing additive Gaussian attacks when the usual approximation of high-resolution quantization is not valid, thus taking into account the host statistics. The analysis is focused on the assessment of the probability of error, showing new results, an although it is valid in a general scenario, its practical interest is increased when SCS is used in conjunction with the so-called spread-transform. The accomplished reformulation of the problem also permits to show that the achievable rate of SCS is never worse than that of classical spread-spectrum-based methods, as it was thought so far, and allows to establish interesting links with spread spectrum and the Improved Spread Spectrum method.  相似文献   
995.
996.
997.
A variational principle for the magnetoelastic stability problem of superconductors is constructed. Independently, a pair of integral equations is derived, from which the initial and the perturbed field can be computed. The integral equations are solved for in-plane buckling of a slender pair of concentric tori, and out-of-plane buckling of a slender pair of equal coaxial tori. By using the variational principle, it is shown that both cases can become unstable when the currents on the two tori are equally directed, and the pertinent buckling values are calculated. The thus obtained buckling values are compared with the results of an alternative, mathematically less rigorous, method. A good correspondence between the two methods is found (at least as long as the two tori are not too near).  相似文献   
998.
The indentation hardness of four glass-bonded ceramics has been examined as a function of load and temperature, It has been observed that the relative hardness values may interchange depending on the temperature and load of testing. For each material there are two regimes of temperature dependence. A has been possible to qualitatively correlate the transition temperature with the composition of the glassy binder phase.  相似文献   
999.
Summary The synthesis and thermal degradability of poly (DL-lactide) were investigated. Key factors affecting the polymer molecular weight were found to be monomer recrystallization, initiator concentration and the vacuum level during drying/sealing of the polymerization reaction ampoule. It was found that poly (DL-lactide) is thermally unstable above its melting temperature. Monomer recrystallization, polymer precipitation and a low initiator content of the polymer significantly inhibited the rate and extent of thermal degradation.  相似文献   
1000.
This study examined the predictive efficacy of 10 rationally derived, archivally coded variables for assessing reoffense risk among extrafamilial child molesters. Follow-up data on 111 child molesters who were discharged from the Massachusetts Treatment Center between 1960 and 1984 were used. Degree of sexual preoccupation with children, paraphilias, and number of prior sexual offenses predicted sexual recidivism. Juvenile and adult antisocial behavior, paraphilias, and low amount of contact with children predicted nonsexual victim-involved and violent recidivism. Area under the receiver-operating characteristic curves in all three analyses indicated discrimination that was substantially better than chance, ranging from .73 to .79. (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