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   643546篇
  免费   7583篇
  国内免费   1878篇
电工技术   12226篇
综合类   841篇
化学工业   96301篇
金属工艺   24180篇
机械仪表   19965篇
建筑科学   14874篇
矿业工程   2831篇
能源动力   17958篇
轻工业   52763篇
水利工程   6178篇
石油天然气   10455篇
武器工业   24篇
无线电   78712篇
一般工业技术   126320篇
冶金工业   121255篇
原子能技术   13064篇
自动化技术   55060篇
  2021年   5526篇
  2020年   4176篇
  2019年   5302篇
  2018年   8999篇
  2017年   8828篇
  2016年   9462篇
  2015年   6160篇
  2014年   10268篇
  2013年   30918篇
  2012年   16659篇
  2011年   22544篇
  2010年   17936篇
  2009年   20420篇
  2008年   21084篇
  2007年   20878篇
  2006年   18638篇
  2005年   16946篇
  2004年   16287篇
  2003年   16077篇
  2002年   15315篇
  2001年   15342篇
  2000年   14540篇
  1999年   15234篇
  1998年   37979篇
  1997年   26757篇
  1996年   20766篇
  1995年   15707篇
  1994年   14058篇
  1993年   13820篇
  1992年   10171篇
  1991年   9625篇
  1990年   9535篇
  1989年   9178篇
  1988年   8773篇
  1987年   7616篇
  1986年   7579篇
  1985年   8725篇
  1984年   7932篇
  1983年   7302篇
  1982年   6654篇
  1981年   6854篇
  1980年   6324篇
  1979年   6180篇
  1978年   6007篇
  1977年   7038篇
  1976年   9102篇
  1975年   5238篇
  1974年   4968篇
  1973年   5014篇
  1972年   4200篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
91.
Translated from Kibernetika i Sistemnyi Analiz, No. 1, pp. 24–33, January–February, 1994.  相似文献   
92.
93.
94.
Translated from Kibernetika i Sistemnyi Analiz, No. 3, pp. 172–176, May–June, 1994  相似文献   
95.
Variations in lipid profile and incidence of vascular events were evaluated. Group 1 had a 29.3% reduction of total and a 38.3% reduction of LDL cholesterol as against reductions of 9% and 9.6% respectively in group 2. No significant difference was observed between the two groups as far as the vascular events considered were concerned. Pravastatin is an effective and safe drug. The lack of influence on the evolution of vascular disease would appear to be correlated to the small number of subjects studied and the short follow-up.  相似文献   
96.
97.
98.
An extended logistic model with a varying asymptotic upper bound for long-range peak demand forecasting is described. The model has been applied to a typical fast growing system, the Saudi Consolidated Electric Company. The forecasts are compared with actual demands and with those obtained from classical forecasting methods. The model gave relatively accurate peak demand forecasts compared with other classical methods. The model with a single load observation is capable of producing several peak demand forecasts corresponding to different levels of maximum temperature and various levels of social activity. The forecasts produced by the model were also stable irrespective of the length of the ex-post simulation period  相似文献   
99.
The deletion of nine residues from the C-terminus of the bacterialchloramphenicol acetyltransferase (CAT) results in depositionof the mutant protein in cytoplasmic inclusion bodies and lossof chloramphenicol resistance in Escherichia coli. This foldingdefect is relieved by C-terminal fusion of the polypeptide withas few as two residues. Based on these observations, efficientpositive selection for the cloning of DNA fragments has beendemonstrated. The cloning vector encodes a C-terminally truncatedCAT protein. Restriction sites in front of the stop codon allowthe insertion of target DNA, resulting in the production ofproperly folded CAT fusion proteins and regained chloramphenicolresistance. The positive selection of recombinants is accomplishedby growth of transformants on chloramphenicol-containing agarplates. The method appears particularly convenient for the cloningof DNA fragments amplified by the PCR because minimal informationto restore CAT folding can be included in the primers. The cloningof random sequences shows that the folding defect can be relievedby fusion to a wide variety of peptides, providing great flexibilityto the positive selection system. This vector may also contributeto the determination of the role of the C-terminus in CAT folding.  相似文献   
100.
A criterion for determining the maximum spacing between magnetometers for measuring the magnetic field is derived. A two-dimensional (2-D) filter model is employed to determine the maximum spatial frequency component present in the magnetic field that is above the spectral noise level. This maximum frequency component is then sampled at a rate greater than twice per period as indicated by the Nyquist criterion, yielding the required magnetometer spacing. It is shown that the rule-of-thumb employed in current clinical biomagnetic array systems, that the spacing between the coils should be approximately equal to the depth of the source, is adequate when the signal-to-noise power ratio is less than 28.4 (14.5 dB). The analysis also quantitatively demonstrates that reducing the separation between the measurement and source planes has a greater effect on the resolution than decreasing the noise level by the same factor. This result is important for employing high Tc superconductor magnetometers that allow thinner thermal insulating layers at the cost of higher thermal noise  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