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   247414篇
  免费   3036篇
  国内免费   434篇
电工技术   4365篇
综合类   259篇
化学工业   41126篇
金属工艺   9483篇
机械仪表   7579篇
建筑科学   5714篇
矿业工程   1414篇
能源动力   5395篇
轻工业   25709篇
水利工程   2530篇
石油天然气   5127篇
武器工业   32篇
无线电   25435篇
一般工业技术   45752篇
冶金工业   45610篇
原子能技术   5996篇
自动化技术   19358篇
  2021年   2391篇
  2019年   2226篇
  2018年   3746篇
  2017年   3698篇
  2016年   3945篇
  2015年   2460篇
  2014年   4193篇
  2013年   10722篇
  2012年   6565篇
  2011年   8803篇
  2010年   6897篇
  2009年   7595篇
  2008年   7902篇
  2007年   8001篇
  2006年   7124篇
  2005年   6221篇
  2004年   5947篇
  2003年   5749篇
  2002年   5798篇
  2001年   5629篇
  2000年   5457篇
  1999年   5357篇
  1998年   12614篇
  1997年   8945篇
  1996年   6902篇
  1995年   5362篇
  1994年   4762篇
  1993年   4649篇
  1992年   3680篇
  1991年   3403篇
  1990年   3603篇
  1989年   3488篇
  1988年   3314篇
  1987年   2903篇
  1986年   2940篇
  1985年   3384篇
  1984年   3226篇
  1983年   2898篇
  1982年   2749篇
  1981年   2827篇
  1980年   2725篇
  1979年   2631篇
  1978年   2703篇
  1977年   2951篇
  1976年   3813篇
  1975年   2372篇
  1974年   2244篇
  1973年   2380篇
  1972年   1972篇
  1971年   1859篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
This article presents data from a population-based, random-digit dialing telephone survey of 1228 employed adults in Washington State, conducted 1989 through 1990. Eighty-one percent of men and 91% of women reported work-site smoking restrictions. Employees in work sites with no-smoking policies were less likely to be current smokers; men in work sites with policies restricting smoking smoked fewer cigarettes on both workdays and nonworkdays. Forty-eight percent of male and 53% of female smokers reported reduced smoking as a result of work-site policy. Work-site smoking policies, intended to protect against smoke exposure, may also reduce employee smoking.  相似文献   
992.
We injected botulinum toxin to treat hemifacial spasm, and investigated the effects and the patient's impression of this treatment. Average duration of improvement lasted about 3.5 months in both the initial treatment group and the recurrent group. However the patients in the recurrent group received fewer units of botulinum toxin than those in the initial treatment group. Except for local paralysis that disappeared within a month, there were very few complications. Most patient were satisfied with this treatment. We conclude that the treatment of hemifacial spasm with botulinum toxin is both simple and useful.  相似文献   
993.
To determine possible relationships between DNA hypomethylation and chromosome instability, human lymphoblastoid cell lines from different genetic constitutions were studied with regard to 1) uncoiling and rearrangements, which preferentially affect the heterochromatic segments of chromosomes 1 and 16; 2) the methylation status of the tandemly repetitive sequences (classical satellite and alphoid DNAs) from chromosomes 1 and 16, and of the L1Hs interspersed repetitive sequences. The methylation status largely varied from cell line to cell line, but for a given cell line, the degree of methylation was similar for all the repetitive DNAs studied. Two cell lines, one obtained from a Fanconi anemia patient and the other from an ataxia telangiectasia patient were found to be heavily hypomethylated. The heterochromatic segments of their chromosomes 1 and 16 were more frequently elongated and rearranged than those from other cell lines, which were found to be less hypomethylated. Thus, in these lymphoblastoid cell lines, alterations characterized by uncoiling and rearrangements of heterochromatic segments from chromosomes 1 and 16 seem to correlate with the hypomethylation of their repetitive DNAs. Two-color in situ hybridizations demonstrated that these elongations and rearrangements involved only classical satellite-DNA-containing heterochromatin. This specificity may be related to the excess of breakages affecting the chromosomes carrying these structures in a variety of pathological conditions.  相似文献   
994.
Following a brief introduction to the applications for wavelength conversion and the different available conversion techniques, the paper gives an in depth analysis of cross gain and cross phase wavelength conversion in semiconductor optical amplifiers. The influence of saturation filtering on the bandwidth of the converters is explained and conditions for conversion at 20 Gb/s or more are identified. The cross gain modulation scheme shows extinction ratio degradation for conversion to longer wavelengths. This can be overcome using cross phase modulation in semiconductor optical amplifiers that are integrated into interferometric structures. The first results for monolithic integrated interferometric wavelength converters are reviewed, and the quality of the converted signals is demonstrated by transmission of 10 Gb/s converted signals over 60 km of nondispersion shifted single mode fiber  相似文献   
995.
996.
997.
Special anhydrides, such as hexahydrophthalic, methyl-hexahydrophthalic, nadic, methylnadic and methyl-tetrahydroxyphthalic, have been polycondensed with glycols such as propylene and di-propylene, to obtain new formulations for unsaturated polyesters. The properties of these resins, such as tensile behaviour and ultraviolet resistance, neat or mixed with anti-shrinkage additives have been examined and compared to standard polyester resins. Particular attention was paid to the study of the influence of low-profile agents on the resulting phase structures of cured systems. Moreover, the role played by the different phases on the fibre/matrix addition in composite materials reinforced with coated glass fibres was also investigated.  相似文献   
998.
A method of reacquiring a previous phase lock-point using the pulsed phase-locked loop (PPLL) ultrasonic system in situations where the measurement is interrupted such as by removing and recoupling the transducer is developed. Operation of the PPLL is mathematically analyzed in the time domain rather than in the conventional frequency domain, providing a method of characterizing multiple lock-points. The general lock-point reacquisition method that follows from the model relies on measurement over a frequency range that spans several lock-points and is demonstrated to work well for interrupted measurements on threaded fasteners. The method is analyzed for sensitivity to signal noise, and an equation is derived, expressing the number of lock-points over which to measure for successful lock-point reacquisition as a function of signal noise level. Experimental verification on a glass block shows that theoretical values obtained with this model are in good agreement with measurements. Automation of the method presented has been demonstrated, showing that it offers a practical, objective approach to making interrupted load measurements, thereby greatly widening the range of applications of the PPLL  相似文献   
999.
The lack of uniform and traceable measurement of pulsed high currents (1-200 kA) utilized in the resistance welding process has long been a problem in the welding community. A testing facility utilizing equipment and knowledge gained in the Department of Energy programs is being made available to the American welding industry to solve these problems. Problems in making these measurements traceable to national standards and proposed solutions are presented. Test equipment and methods are described that will be used to evaluate current-sensing coils and weld-current monitors. Comparison of positional errors for various current-sensing coils is presented, including a recently developed, machinable Rogowski coil  相似文献   
1000.
Modelling the effect of the dynamic behaviour of a system on its PSA study leads, in a Markovian framework, to a development at first order of the Chapman-Kolmogorov equation, whose solutions are the probability densities of the problem. Because of its size, there is no hope of solving directly these equations in realistic circumstances. We present in this paper a biased simulation giving the marginals and compare different ways of speeding up the integration of the equations of the dynamics.  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