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   355篇
  免费   6篇
电工技术   1篇
化学工业   23篇
金属工艺   4篇
机械仪表   24篇
建筑科学   13篇
能源动力   4篇
轻工业   7篇
石油天然气   2篇
无线电   23篇
一般工业技术   30篇
冶金工业   196篇
原子能技术   3篇
自动化技术   31篇
  2019年   2篇
  2018年   4篇
  2017年   5篇
  2016年   3篇
  2014年   2篇
  2013年   10篇
  2012年   5篇
  2011年   19篇
  2010年   12篇
  2009年   14篇
  2008年   12篇
  2007年   19篇
  2006年   5篇
  2005年   5篇
  2004年   10篇
  2003年   11篇
  2002年   8篇
  2001年   8篇
  2000年   9篇
  1999年   6篇
  1998年   28篇
  1997年   22篇
  1996年   13篇
  1995年   9篇
  1994年   6篇
  1993年   11篇
  1992年   8篇
  1991年   7篇
  1990年   10篇
  1989年   10篇
  1988年   5篇
  1986年   3篇
  1985年   4篇
  1984年   4篇
  1983年   2篇
  1982年   2篇
  1981年   2篇
  1980年   2篇
  1979年   3篇
  1978年   2篇
  1977年   7篇
  1976年   8篇
  1975年   6篇
  1973年   3篇
  1971年   2篇
  1970年   3篇
  1965年   2篇
  1960年   1篇
  1957年   1篇
  1956年   1篇
排序方式: 共有361条查询结果,搜索用时 12 毫秒
71.
72.
STUDY DESIGN: This study was intended to identify normal degenerative morphologic evolution in the bony foramen in asymptomatic subjects by decade in comparison with symptomatic subjects of like decades. OBJECTIVES: To determine normal degenerative changes in the cervical spine caused by the aging process that predispose a person to foraminal stenosis and radiculopathy. SUMMARY OF BACKGROUND DATA: Cervical radiculopathy is a common problem caused by degenerative changes as people age. The characteristics of the foramen that result in stenosis are not known. METHODS: Five to six symptomatic and asymptomatic people in each decade volunteered for magnetic resonance imaging. Lordosis, disc heights, and ratio of spinal cord diameter to spinal canal diameter were measured at C4-C5, C5-C6, and C6-C7 from sagittal magnetic resonance images. Foraminal heights, widths, and areas were measured at the isthmus of the same foramen from oblique images. RESULTS: Foraminal heights, widths, and areas were larger in asymptomatic patients than in symptomatic patients. Morphologic analysis showed that inferior facet hypertrophy tended to decrease the width of the foramen in aging people. Disc heights, lordosis, and ratio of spinal cord diameter to spinal canal diameter showed no significant differences. CONCLUSIONS: Foraminal height affects overall foraminal area but tends to change little with age. Width also affects overall area and not only decreases in older people but also significantly affects the available area for the exiting nerve root.  相似文献   
73.
We report a series of picture naming experiments in which target pictures were primed by briefly presented masked words. Experiment 1 demonstrates that the prior presentation of the same word prime (e.g., rose-ROSE) facilitates picture naming independently of the target's name frequency. In Experiment 2, primes that were homophones of picture targets (e.g., rows-ROSE) also produced facilitatory effects compared with unrelated controls, but priming was significantly larger for targets with low-frequency names relative to targets with high-frequency names. In Experiment 3, primes that were higher frequency homophones of picture targets produced facilitatory effects compared with identical primes. These results are discussed in relation to different accounts of the effects of masked priming in current models of picture naming.  相似文献   
74.
Extracellular adenosine triphosphate (ATP) and adenosine diphosphate (ADP) activate multiple types of P2-nucleotide receptors expressed in platelets or leukocytes. Electrophysiological and biochemical studies have indicated expression of the P2X1 receptor, an ATP-gated cation channel, in human and rat platelets, rat basophilic leukemia (RBL) cells, and phorbol myristate acetate (PMA)-differentiated HL-60 myeloid cells. Although these findings suggest that P2X1 receptors are present in both blood leukocytes and blood platelets, the relative levels of P2X1 receptor expression and function in human blood leukocytes and platelets have not been directly characterized. On the basis of both immunoblot analysis and functional assays of P2X1 receptor-mediated ionic fluxes, we report that there is significant expression of P2X1 receptors in human platelets, but not in neutrophils, monocytes, or blood lymphocytes. Thus, unlike platelets and myeloid progenitor cell lines, fully differentiated human blood leukocytes do not express functionally significant numbers of P2X1 receptors, suggesting the downregulation of P2X1 receptor gene expression during the differentiation of phagocytic leukocytes. By contrast, P2X1 receptor expression is strongly maintained during megakaryocytic differentiation and platelet release. Immunoblot analysis indicated that the platelet P2X1 receptor migrates as an approximately 60-kD protein during SDS-electrophoresis under reducing or nonreducing conditions. Treatment of platelet membranes with endoglycosidase-F causes the P2X1 receptor band to migrate as a 46-kD protein, verifying the highly glycosylated nature of the mature receptor protein. Additional studies of nucleotide-induced changes in Ca2+ influx/mobilization demonstrated that the platelet P2X1 receptors are pharmacologically distinct from the well-characterized ADP receptors of these cells. This finding suggests a unique role for these ATP-gated ion channels during hemostasis or thrombosis.  相似文献   
75.
A scanning transmission electron microscope (STEM) technique was used to measure Mo concentrations at ferrite:austenite (α:γ) interfaces in an Fe-0.24 pct C-0.93 pct Mo alloy partially transformed at 650°C, 630°C, and 610°C. These concentrations were quite small at 650°C, which is just below the bay temperature of the time-temperature-transformation (TTT) curve for the initiation of ferrite formation. There were larger concentrations at 630°C, a temperature at which transformation stasis (incomplete transformation) occurred. Concentrations at 610°C were intermediate between the values observed at 650°C and 630°C. The average accumulation at the latter temperatures increased appreciably as a function of transformation time. After each heat treatment, there was considerable variation in Mo accumulation from one α:γ interface to another and, to a lesser extent, from one region to another along the same interface. These higher Mo concentrations were deduced to have developed largely through volume diffusion of Mo, mainly through ferrite, to interfaces whose ledgewise growth had been interrupted by growth stasis. (Mo2C precipitation at α:γ boundaries occurred only at the end of growth stasis.) It appears that only a very small amount of Mo segregation is needed, probably at specific interfacial sites, in order to produce growth cessation. Growth kinetics anomalies of this kind continue to provide the best evidence available for the operation of a coupled-solute drag effect. This article is based on a presentation given in the symposium “The Effect of Alloying Elements on the Gamma to Alpha Transformation in Steels,” October 6, 2002, at the TMS Fall Meeting in Columbus, Ohio, under the auspices of the McMaster Centre for Steel Research and the ASM-TMS Phase Transformations Committee.  相似文献   
76.
A central bias in spatial selection has been proposed to explain the decreasing search efficiency with increasing target eccentricity that results when distractors can occur closer to fixation than the target (J. M. Wolfe, P. O'Neill, & S. C. Bennett, 1998). The authors found evidence for such a bias using an odd-man-out variant of conjunction search. However, the bias was absent for the same displays when the identity of the odd-man-out target was known in advance. The authors propose that (a) top-down knowledge of a target feature supports grouping on this feature and (b) grouping links a peripheral target to central distractors expressing the same feature, increasing the attentional weighting afforded to the target and, consequently, facilitating its detection. The effects are independent of bottom-up priming effects occurring across trials. Thus, feature-based grouping can be driven top-down and can overrule the central bias in spatial selection. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
77.
Current theory assumes that individuals only use information from the immediate environment to perform relative arrival-time judgment tasks. This article presents a theoretical analysis of the memory requirements of this task. The authors present an analysis of the inputs to the memory system and the processes that map those inputs onto outputs. The analysis generates a set of predictions regarding the specificity of transfer and the role of context during learning. In 3 experiments, participants decided whether pairs of aircraft would violate a minimum separation standard or pass each other safely. Participants were presented with pairs of aircraft in which properties of the pair varied along 3 structural and 3 surface dimensions. Contexts were defined by the co-occurrence of specific values along stimulus dimensions and the use of a neutral label. The results suggest that transfer was limited by the dimensions that were varied in training and the context in which those dimensions were varied. The discussion focuses on the problems that complex tasks like relative judgment pose for associative learning mechanisms and the development of precise models of cognition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
78.
Evidence is presented for 2 modes of attention operating in simultanagnosia. The authors examined visual enumeration in a patient, GK, who has severe impairments in serially scanning across a scene and is unable to count the numbers of items in visual displays. However, GK's ability to judge the relative magnitude of 2 displays was consistently above chance, even when overall luminosity did not vary with the number of items present. In addition, several variables had a differential impact on GK's counting and magnitude estimation. Magnitude estimation but not counting was facilitated by using elements that grouped more easily and by presenting the elements in regular configurations. In contrast, counting was facilitated by placing the elements in different colors while magnitude estimation was disrupted. Also GK's performance on magnitude estimation tasks was disrupted by asking him to count the elements present. The data suggest that GK can process visual stimuli in either a focused or distributed attention mode. When in a focused attention mode, performance is limited by poor serial scanning of attention due to an impaired explicit representation of visual space. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
79.
Security is key to E-commerce (or E-business). Customers of E-commerce need the confidence that their online transactions are secure and that, if something goes wrong, they have some sort of redress for loss or damage. This growing need for assurance demands not only a set of security tools but also a legal framework in which to apply these tools. This includes the need to use cryptographic techniques and, in particular, ‘electronic signatures’ and related services allowing data authentication and certification. Electronic signatures enable someone receiving data over the Internet for example, to determine the origin of the data and to check that the data has not been altered.  相似文献   
80.
The role of perceptual grouping and the encoding of closure of local elements in the processing of hierarchical patterns was studied. Experiments 1 and 2 showed a global advantage over the local level for 2 tasks involving the discrimination of orientation and closure, but there was a local advantage for the closure discrimination task relative to the orientation discrimination task. Experiment 3 showed a local precedence effect for the closure discrimination task when local element grouping was weakened by embedding the stimuli from Experiment 1 in a background made up of cross patterns. Experiments 4A and 4B found that dissimilarity of closure between the local elements of hierarchical stimuli and the background figures could facilitate the grouping of closed local elements and enhanced the perception of global structure. Experiment 5 showed that the advantage for detecting the closure of local elements in hierarchical analysis also held under divided- and selective-attention conditions. Results are consistent with the idea that grouping between local elements takes place in parallel and competes with the computation of closure of local elements in determining the selection between global and local levels of hierarchical patterns for response. (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