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   510033篇
  免费   6508篇
  国内免费   1307篇
电工技术   8260篇
综合类   303篇
化学工业   79950篇
金属工艺   23271篇
机械仪表   17397篇
建筑科学   11329篇
矿业工程   4339篇
能源动力   11045篇
轻工业   40567篇
水利工程   6314篇
石油天然气   14864篇
武器工业   37篇
无线电   52380篇
一般工业技术   109554篇
冶金工业   81786篇
原子能技术   14303篇
自动化技术   42149篇
  2021年   4980篇
  2020年   3965篇
  2019年   4869篇
  2018年   8806篇
  2017年   9037篇
  2016年   9719篇
  2015年   5869篇
  2014年   9580篇
  2013年   23189篇
  2012年   14945篇
  2011年   19159篇
  2010年   15328篇
  2009年   16887篇
  2008年   17369篇
  2007年   17036篇
  2006年   14612篇
  2005年   13096篇
  2004年   12703篇
  2003年   12411篇
  2002年   11729篇
  2001年   11831篇
  2000年   11104篇
  1999年   10918篇
  1998年   24094篇
  1997年   17527篇
  1996年   13211篇
  1995年   10248篇
  1994年   9138篇
  1993年   9263篇
  1992年   7149篇
  1991年   7070篇
  1990年   6966篇
  1989年   6685篇
  1988年   6519篇
  1987年   5814篇
  1986年   5623篇
  1985年   6422篇
  1984年   6007篇
  1983年   5659篇
  1982年   5317篇
  1981年   5426篇
  1980年   5296篇
  1979年   5261篇
  1978年   5344篇
  1977年   5746篇
  1976年   7023篇
  1975年   4758篇
  1974年   4760篇
  1973年   4869篇
  1972年   4109篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
The basic methods of verifying continuous automatic belt weighers are described. A comparative analysis of these methods on the basis of experimental studies is made and ways of implementing the results in industry are recommended.  相似文献   
22.
The proliferation of both online and bricks and mortar outlet stores underscores the observation that secondary markets are readily accessible to retailers of short-life-cycle products. These secondary markets provide recourse channels for retailers to sell excess inventory of out-of-favor items at reduced prices when overstocking occurs in a primary market. We study the problem of determining when a retailer should terminate its primary selling season by selling remaining inventory on a secondary market. The retailer has a single opportunity to procure prior to a primary selling season consisting of multiple periods. Demand in each period is random, but correlated. At the end of each period, any remaining inventory incurs a holding cost. Then, based upon the current level of inventory and the cumulative demand-to-date, the retailer decides either to terminate the primary selling season by selling all or part of the remaining inventory on a secondary market, or to extend the current primary selling season by another period. We develop structural properties of the optimal policy for determining when to terminate the primary selling season, and we develop corresponding implications for procurement.  相似文献   
23.
Behavior of Magnesium‐Alloys for Automotive Applications under Mechanical and Environmental Loading: Influence of Passivating Films and Mechanisms of Local Breakdown To assure an efficient design of components under cyclic loading, all available data concerning fatigue have to be observed. Therefore the influences of manufacturing on the material condition, the mechanical loads and environmental effects have to be analysed. Magnesium‐alloys are of special interest for lightweight applications because of their excellent strength‐density ratio. The corrosion resistance of magnesium‐alloys depends on the same factors that are critical to other metals. The alloys have a good stability to atmospheric exposure and a good resistance to attack by alkali, chromic and hydrofluoric acids. However, because of the electrochemical activity of magnesium, the relative importance of some factors is greatly amplified. The nature and composition of passive films formed on magnesium‐alloys depend on the prevailing conditions, viz. alloy‐composition, passivation potential, pH, electrolyte composition and temperature. Passive films may be damaged by local breakdown. Because of this, magnesium‐alloys suffer a degradation of their properties when exposed to an aqueous environment. The main topic of the present investigations is the verification of mechanisms of the local breakdown of the protecting film. At least two mechanisms are possible for this localization: mechanical breakdown by slip steps and electrochemical breakdown (for e.g. by the effects of chloride ions). Corrosion and passivation of different high purity alloys have been studied in different solutions (neutral, alkaline with specific anions and cations) using electrochemical techniques. The diecasted alloys were tested as produced and machined. The results clarified that depending on alloy/material and surface condition/corrosion environment different mechanisms for electrochemical breakdown of the protecting films are possible. Hence fatigue life under environmental loading is influenced by surface and testing conditions.  相似文献   
24.
Using Java-based tools in multimedia collaborative environments accessed over the Internet can increase an application's client base. Most operating systems support Java, and its "compile once-run everywhere" architecture is easy to maintain and update. The Java-based tools presented here let users share Internet resources, including resources originally designed for single use.  相似文献   
25.
The effect of crystal orientation on the photogeneration of free charge carriers was studied for C60 single crystals in a weak magnetic field. The photoconductivity sharply depends on the orientation of magnetic field with respect to the crystallographic directions, showing a 5–8% increase for seven axes of the C60 crystal.  相似文献   
26.
V. G. Deibuk 《Semiconductors》2003,37(10):1151-1155
The miscibility gaps and the critical temperatures of spinodal decomposition of ternary semiconducting Ga-In-Sb, Ga-In-P, and In-As-Sb systems are calculated by taking into account the deformation energy and the effect of plastic relaxation caused by the misfit dislocations. It is shown that taking into consideration elastic energy narrows the ranges of spinodal decomposition and lowers its critical temperature. The introduction of the phenomenological parameter into Matthews-Blakeslee formula makes it possible to reach a satisfactory agreement between theoretically calculated values of critical thickness of epitaxial films and the experimental data.  相似文献   
27.
The design of a sylphon bellows sensor and the basic circuits of an LC-generator and of a microprocessor unit are presented. An analytical pressure–frequency conversion function and a special method of adjusting the sensor ensure an error of less than 0.05%. The dynamic range is up to 105. The instruments developed cover the ranges 103, 104, and 105 Pa.  相似文献   
28.
The application of barrier discharges at atmospheric pressure in air expands on the market of plasma technology, because it is an ecological and cost‐effective alternative to other processes of surface treatment. These plasmas usually consist of a multitude of spatially and temporally localized filaments, whose distribution should be as even as possible for homogeneous treatment. This holds especially for the plasma treatment of sensitive goods such as wool or other textiles. In equipment for continuous pass of material the barrier arrangements often consist of a system cylinder – cylinder or cylinder – plane, whereby the gap width changes locally. Space distribution and intensity of filaments has been investigated by means of short‐time photography and spatially resolved measurement of current distribution and energy distribution derived from it. The local dependency found can be explained by means of a capacitive equivalent circuit.  相似文献   
29.
Amphiphilic magnetic microspheres ranging in diameter from 5 to 100 µm were prepared by dispersion copolymerization of styrene and poly(ethylene oxide) vinylbenzyl (PEO‐VB) macromonomer (MPEO) in the presence of Fe3O4 magnetic fluid. The effects of various polymerization parameters on the average particle size were systematically investigated. The average particle size was found to increase with increasing styrene concentration and initiator concentration. It also increased with decreasing stabilizer concentration and molecular weight of MPEO. The content of the hydroxyl groups localized in the microspheres ranged from 0.01 to 0.2 mmol g?1. © 2003 Society of Chemical Industry  相似文献   
30.
Ho  Y. S.  Chiu  C. H.  Tseng  T. M.  Chiu  W. T. 《Scientometrics》2003,57(3):369-376
Honour Index (HoI), a method to evaluate research performance within different research fields, was derived from the impact factor (IF). It can be used to rate and compare different categories of journals. HoI was used in this study to determine the scientific productivity of stem cell research in the Asian Four Dragons (Hong Kong, Singapore, South Korea and Taiwan) from 1981 to 2001. The methodology applied in this study represents a synthesis of universal indicator studies and bibliometric analyses of subfields at the micro-level. We discuss several comparisons, and conclude the developmental trend in stem cell research for two decades. This revised version was published online in August 2006 with corrections to the Cover Date.  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