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   540066篇
  免费   7546篇
  国内免费   2482篇
电工技术   11069篇
技术理论   5篇
综合类   1572篇
化学工业   80725篇
金属工艺   24867篇
机械仪表   19327篇
建筑科学   13113篇
矿业工程   3239篇
能源动力   11790篇
轻工业   44499篇
水利工程   5986篇
石油天然气   10117篇
武器工业   311篇
无线电   62132篇
一般工业技术   106350篇
冶金工业   92919篇
原子能技术   10668篇
自动化技术   51405篇
  2021年   4154篇
  2019年   3786篇
  2018年   13076篇
  2017年   13561篇
  2016年   10647篇
  2015年   5192篇
  2014年   7974篇
  2013年   21122篇
  2012年   15169篇
  2011年   24273篇
  2010年   20512篇
  2009年   20837篇
  2008年   22015篇
  2007年   23790篇
  2006年   15044篇
  2005年   16216篇
  2004年   13965篇
  2003年   13183篇
  2002年   12085篇
  2001年   11675篇
  2000年   11132篇
  1999年   11154篇
  1998年   25303篇
  1997年   18435篇
  1996年   14399篇
  1995年   11040篇
  1994年   9983篇
  1993年   9808篇
  1992年   7771篇
  1991年   7238篇
  1990年   7188篇
  1989年   6879篇
  1988年   6553篇
  1987年   5740篇
  1986年   5639篇
  1985年   6546篇
  1984年   6189篇
  1983年   5666篇
  1982年   5151篇
  1981年   5450篇
  1980年   5066篇
  1979年   5088篇
  1978年   4863篇
  1977年   5420篇
  1976年   6864篇
  1975年   4404篇
  1974年   4257篇
  1973年   4301篇
  1972年   3554篇
  1971年   3280篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Background: Despite increases in ecstasy (MDMA) use in the United States, little is known about characteristics linked with recent-onset ecstasy use, especially psychiatric symptoms and deviant behaviors. Aims: To test whether individuals with high levels of other drug use are more likely to be recent-onset ecstasy users; to test whether psychiatric symptoms in adults are associated with recent-onset ecstasy use; to explore the association between recent-onset ecstasy use and concomitant deviant behaviors in adolescents and adults. Methods: Data from the 2001 National Survey on Drug Use and Health. Findings: Recent-onset ecstasy use was significantly more likely to occur among adolescents and adults (18-34 years old) who engaged in deviant behaviors during the past year as compared with those who did not engage in deviant behaviors during the past year. Higher levels of deviancy indicated a higher likelihood of being a recent-onset ecstasy user, and associations were strongest with nonviolent deviant behaviors such as selling illegal drugs and stealing. Associations between deviant behaviors and recent-onset ecstasy use were similar in strength to associations between deviant behaviors and recent-onset cocaine and marijuana use, respectively. Adults who had past-year psychiatric symptoms (both depressive and panic symptoms) were twice as likely to be recent-onset ecstasy users as compared with those without past-year psychiatric symptoms. Greater levels of drug involvement increased the odds of being a recent-onset ecstasy user. Conclusion: Recent-onset ecstasy use seems to be associated with a range of other behavioral problems and may reflect one aspect of a larger problem behavior syndrome. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
102.
In this paper we investigated the BER performance of DS-CDMA using various chip-waveforms, which include three time-limited chip-waveforms and two band-limited chip-waveforms. Closed-form formulae were derived for evaluating the achievable bit-error rate performance with the aid of the standard Gaussian approximation, when communicating over a Nakagami-m channel. The time-limited waveforms impose a low implementational complexity, since they maybe over sampled and read from a look-up table. However, they are outperformed by the frequency-domain raised-cosine waveform as well as the optimum waveform specifically designed by Cho and Lehnert for achieving the lowest possible bit error rate  相似文献   
103.
The solutions to a differential equation for the problem of heating of a cylinder in a medium with a fixed temperature are used to obtain equations for estimating the change in the hydrogen concentration in forgings from data on the hydrogen concentration at the cylinder axis or on the average bulk hydrogen concentration. Similar equations are also obtained for forgings having square cross sections. The calculation demonstrates that, for the case of a twofold decrease in the hydrogen concentration in a forging, the calculated times of annealing of a cylindrical forging differ by three times.  相似文献   
104.
The Chelyabinsk Metallurgical Combine has periodically used Kachkanar agglomerate to make conversion pig iron since 1972. Use of this material reached a peak in 1998, when it comprised an average of 27.1% of the charge. In some months, the amount of this material charged into the furnace reached highs of 45.6 and 74.1% on certain furnaces. The coolers on the iron notches of furnaces 4 and 5 burned during that year, the quality of the smelting products and the condition of the ladles rapidly deteriorated, and the smelting rate declined. No Kachkanar agglomerate was used in 2000–2001, but it was again added to the charge in February 2002. The amount used was smaller than previously, averaging 10.7% for the shop as a whole. It was determined that using a charge with 15% Kachkanar agglomerate has both positive and negative consequences. N. N. Gorshkov, A. V. Denisov, and V. Kh. Barinov also participated in this study. __________ Translated from Metallurg, No. 3, pp. 43–45, March, 2006.  相似文献   
105.
While the motion of twist boundaries can be readily studied by atomistic simulations with molecular dynamics (MD) under the action of an elastic driving force, the approach fails for tilt boundaries. This is due to the interaction of the elastic stress with the grain boundary (GB) structure, which causes plastic strain by GB sliding. A novel concept, the orientation correlated driving force, is introduced to circumvent this problem. It is shown that this concept can be successfully applied to the study of the migration of tilt boundaries. The migration behavior of several twist and tilt GBs was investigated. The transition from low-to high-angle boundaries can be captured, and a structural transition of tilt boundaries was found at high temperatures, which also affected the migration behavior. The results compare well with experimental results of the motion high-angle boundaries, but for low-angle boundaries, the agreement is poor. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden.  相似文献   
106.
In this study, feasibility of membrane separation for the removal of indigenous noroviruses (NVs) is evaluated. The indigenous NV gene was never detected from ultrafiltration (UF) permeates of sewage sludge and treated wastewater. Indigenous NV gene was also not detected from permeates of sewage sludge and treated wastewater by microfiltration (MF) with a pore size of 0.1 microm (MF0.1). Even though the pore size of MF (0.1 microm) was much larger than the diameter of virus particle (approximately 30-40nm), more than 4-log10 reduction value (LRV) at maximum was achieved by membrane separation with MF0.1. NV genes were often detected from permeates of sewage sludge and treated wastewater by MF with a pore size of 0.45 microm (MF0.45), although the maximum log10 reduction values were more than 3.59 for sewage sludge and more than 2.90 for treated wastewater. It is important to verify factors determining the removal efficiency of viruses with MF membranes.  相似文献   
107.
At the present time, organic solid wastes from industries and agricultural activities are considered to be promising substrates for biogas production via anaerobic digestion. Moreover solids stabilisation is required before reutilization or disposal. Slaughterhouses are among the most important industries in Uruguay and produce 150,000 tons of ruminal content (RC) and 30,000 tons of blood per year. In order to determine the influence of the solids and blood contents, the ammonia inhibition and the inoculum adaptation co-digestion batch tests were performed. A set of experiences with TS concentration of 2.5%, 5% and 7.5% and different ratios of RC/blood were carried out using an inoculum from an UASB reactor. In all experiences fast blood hydrolisation was observed. A higher methane production was detected in the experiences with higher TS content. However, the fraction of solids degradation was lower in these experiences. A plateau in the biogas production was found. The free ammonia level, which was above the reported inhibitory levels, could explain this behaviour. After the inhibition period the biogas production restarted probably due to the biomass acclimatisation to the ammonia. In order to determine the inoculum adaptation a new experiment was performed. The inoculum used was the sludge coming from the first set of experiences. Based upon batch tests a 3.5 m3 pilot reactor was designed and started up. Ammonia inhibition was avoided by the start-up strategy and in two weeks the biogas production was 3.5 m3/d. The VS stabilisation with a solid retention time of 20 days was of 43%. The pilot reactor working at steady state had a TS concentration of 3-4% with a ratio of RC/blood of 10:1 at the entrance.  相似文献   
108.
Paving-and roofing-grade asphalts were obtained at LUKOIL-Permnefteorgsintez Ltd. by mixing peroxidized asphalt and a number of high-boiling unoxidized petroleum products in certain ratios. The asphalts obtained have improved performance characteristics: high plasticity and low brittleness temperature. __________ Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 2, pp. 26–28, May–June, 2006.  相似文献   
109.
就软交换网络和WCDMAR4网络互通的关键技术进行了分析和探讨,并借此提出了对互通节点的技术要求。  相似文献   
110.
Co-browsing is an activity in which a group of users navigate their way through of a set of Web pages together for a shared purpose. Effective co-browsing among users with different device capabilities requires a shared understanding of those Web pages. This paper demonstrates the concept of shared viewpoints (SVPs), and personal viewpoints (PVPs), for co-browsing, before detailing a framework for implementing these concepts. Finally, the effectiveness of the framework is presented through a perceptual experiment.  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