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   9736篇
  免费   560篇
  国内免费   19篇
电工技术   128篇
综合类   8篇
化学工业   2058篇
金属工艺   200篇
机械仪表   175篇
建筑科学   457篇
矿业工程   10篇
能源动力   359篇
轻工业   828篇
水利工程   69篇
石油天然气   25篇
无线电   821篇
一般工业技术   1991篇
冶金工业   1458篇
原子能技术   75篇
自动化技术   1653篇
  2023年   113篇
  2022年   144篇
  2021年   312篇
  2020年   214篇
  2019年   232篇
  2018年   274篇
  2017年   243篇
  2016年   294篇
  2015年   264篇
  2014年   374篇
  2013年   596篇
  2012年   576篇
  2011年   716篇
  2010年   539篇
  2009年   512篇
  2008年   509篇
  2007年   456篇
  2006年   406篇
  2005年   326篇
  2004年   305篇
  2003年   236篇
  2002年   230篇
  2001年   114篇
  2000年   139篇
  1999年   140篇
  1998年   259篇
  1997年   166篇
  1996年   139篇
  1995年   108篇
  1994年   105篇
  1993年   90篇
  1992年   62篇
  1991年   55篇
  1990年   66篇
  1989年   71篇
  1988年   51篇
  1987年   65篇
  1986年   43篇
  1985年   56篇
  1984年   58篇
  1983年   47篇
  1982年   48篇
  1981年   51篇
  1980年   33篇
  1979年   52篇
  1978年   38篇
  1977年   46篇
  1976年   58篇
  1974年   32篇
  1973年   29篇
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
51.
A family of energy/economic/environmental (E3) models is presented as a mechanism for analysing national policy issues. The family consists of discrete models which are designed to be run in an integrated manner. The outputs of certain models provide the inputs to the next. This structure allows the analyst to readily incorporate an understanding of regional factors such as local energy prices, concerns over air quality, water availability, or attitudes towards construction of new energy facilities, into national assessments of energy policies. This paper reviews the analytic framework within which energy policy issues are currently addressed. The initial family of E3 models is described with the emphasis on the data linkages and feedback which are provided when these models are run sequentially. The ongoing MITRE research programme with the E3 family of models is presented and plans and opportunities for future work are outlined.  相似文献   
52.
Thermodynamic analysis of solar photovoltaic cell systems   总被引:1,自引:0,他引:1  
The thermodynamic characteristics of solar photovoltaic (PV) cells are investigated from a perspective based on exergy. A new efficiency is developed that is useful in studying PV performance and possible improvements. Exergy analysis is applied to a PV system and its components, and exergy flows, losses and efficiencies are evaluated. Energy efficiency is seen to vary between 7% and 12% during the day. In contrast, exergy efficiencies, which incorporate the second law of thermodynamics and account for solar irradiation exergy values, are lower for electricity generation using the considered PV system, ranging from 2% to 8%. Values of “fill factors” are determined for the system and observed to be similar to values of exergy efficiency.  相似文献   
53.
This paper describes how various characterisation methods can be used to investigate the physicochemical and spectral properties of gold films in contact with glass or ceramic. The interface between the metal and substrate has been given special attention. The examples chosen to illustrate this include: liquid gold films (decorated and fired) or inlayed gold layers on ceramic wares and “sandwich” gold foils between two glass sheets for mosaic application. Characterisation methods include: microanalytical scanning electron microscopy, X-ray diffraction, ion beam analyses (PIXE: particle induced X-ray emission; RBS: Rutherford back-scattering spectrometry), photo-spectrometry and colorimetry. The adhesion mechanism and colour behaviour of the coated objects is discussed.  相似文献   
54.
Infection by human immunodeficiency virus type 1 (HIV-1) is often complicated by a variety of neurological abnormalities. The most common clinical syndrome, termed acquired immunodeficiency syndrome (AIDS) dementia complex, presents as a subcortical dementia with cognitive, motor, and behavioral disturbances and is unique to HIV-1 infection. The pathogenesis of this syndrome is poorly understood but is believed to involve interactions among virally infected macrophages/microglia, astrocytes, and neurons. In this study, we show that exposure of primary rat and human astrocytes to heat-activated HIV-1 virions, or to eukaryotically expressed HIV-1 and HIV-2 envelope glycoproteins (gp120) stimulates amiloride-sensitive Na+/H+ antiport, potassium conductance, and glutamate efflux. These effects are blocked specifically by amiloride, an inhibitor of Na+/H+ antiport and by the selective removal of gp120 with immobilized monoclonal antibody. As a result of modulation of astrocytic function by gp120, the ensuing neuronal depolarization and glutamate exposure could activate both voltage-gated and N-methyl-D-aspartate-regulated Ca2+ channels, leading to increases in intraneuronal Ca2+ and neuronal death. These findings implicate the astrocyte directly in the pathogenesis of AIDS dementia complex.  相似文献   
55.
这个综合学校包括一个主建筑,一个体育馆和各种户外设施,朝向一片处在前军事基地上的住宅开发区。整个主建筑沿纵向被一个由顶部采光的回廊从中间分开,由此可以进入两侧的教室,回廊局部变宽的部分构成一个公共空间。这个构想与将城市设计理论运用于建筑内部设计的概念相符合:街道、广场和建筑被看成了建筑内部空间部分。虽然这个建筑从外部看来是个大体块,但内部空间却设计流畅,通道有的通向楼层的各个角落,也有的依着倾斜的地势因势而建。由墙面反射的散射光线加强了内部空间的肃穆和厚重感。在具有宗教感的整体风格下,整个学校的设计和装饰也显得古香古色,如一部宽阔的观光楼梯、两个塔形和壁炉形的建筑构成元素等。不同历史风格建筑的叠加给这个校区带来一种复杂的味道,堪比阿尔多·罗西(Aldo Rossi)的设计或泰辛(Tessin)新理性主义的代表作。  相似文献   
56.
Routine inspection is the most common form of highway bridge inspection to satisfy the requirements of the National Bridge Inspection Standards. The accuracy and reliability of documentation generated during these inspections are critical to the allocation of Department of Transportation construction, maintenance, and rehabilitation resources. Routine inspections are typically completed using only the visual inspection technique and rely heavily on subjective assessments made by bridge inspectors. In light of this, and given the fact that visual inspection may have other limitations that influence its reliability, the Federal Highway Administration initiated an investigation to examine the reliability of visual inspection as it is currently applied to bridges in the United States. This paper will summarize results from this study related to the accuracy and reliability of routine inspection documentation. A number of important conclusions were developed from the experimental study. Generally, it was found that all structural condition documentation is collected with significant variability. Specifically, 95% of primary element condition ratings for individual bridge components will vary within two rating points of the average and only 68% will vary within one point. Documentation generally collected to support condition ratings also has significant variability as exemplified by the number and types of field notes and photographs taken by inspectors. With respect to the use of element-level inspections, it was found that element usage was generally consistent with the Commonly Recognized Element Guide. However, there is significant variability in the condition state assignments of those elements and in some cases the condition states are not applied correctly to particular elements.  相似文献   
57.
Abstract. A sufficient condition for the invertibility of univariate periodic moving-average models has been given by Cipra and Ghysels and Hall. We show that this condition is not a necessary one, and provide a necessary and sufficient condition for the general m -variate, d -periodical moving-average MA( q ) case.  相似文献   
58.
Conducted prolonged tests of male Long-Evans rats' penile responses outside the context of copulation (ex copula) to facilitate analysis of these penile components of copulatory behavior. Penile erections and flips were evoked for 1 hr following penile sheath retraction (SR), and Ss were retested for 30 min after a rest interval of 5 min to 4 hr. The number of penile responses declined sharply over the course of 1 hr. Following a 5-min rest, there was little likelihood of additional responses. Within 2 hrs, some measures of penile response potential approached their original high asymptotes: other measures were still depressed after the 4-hr rest. In control tests, the penile sheath was unretracted (SU) during the 1st 60 min, and hence few or no penile responses occurred. This condition caused no reduction in the number of erections and flips in the subsequent 30-min SR test; hence, the reduction in response to potential during and after SR tests was due to the responses displayed, not to the conditions of restraint. Ejaculation occurred frequently in the SU condition but rarely in SR tests. These results suggest that SR may normally inhibit ejaculation in ex copula tests and during copulation. (41 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
59.
The performance standards for flexible PVC compounds continue to become more demanding as manufacturers pursue more value-added applications traditionally dominated by rubbers and thermoplastic elastomers. Blending of PVC with other polymers has proven to be a viable approach in developing higher performance flexible PVC-based materials. This paper will review some of the typical approaches to producing PVC-based elastomers, and show how these standard flexible PVC blends compare to a newly developed family of PVC alloys. The advantages of these materials will be discussed, and some examples of commercial materials and their application will also be reviewed.  相似文献   
60.
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