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   482962篇
  免费   5607篇
  国内免费   1757篇
电工技术   8289篇
综合类   2627篇
化学工业   70462篇
金属工艺   20952篇
机械仪表   16415篇
建筑科学   10466篇
矿业工程   2696篇
能源动力   12605篇
轻工业   31816篇
水利工程   5453篇
石油天然气   8236篇
武器工业   17篇
无线电   58023篇
一般工业技术   97905篇
冶金工业   77364篇
原子能技术   9395篇
自动化技术   57605篇
  2022年   2566篇
  2021年   3904篇
  2020年   2954篇
  2019年   3814篇
  2018年   20234篇
  2017年   19174篇
  2016年   16430篇
  2015年   4889篇
  2014年   7336篇
  2013年   20206篇
  2012年   14336篇
  2011年   24152篇
  2010年   19969篇
  2009年   18966篇
  2008年   20480篇
  2007年   21187篇
  2006年   12523篇
  2005年   12225篇
  2004年   11523篇
  2003年   11501篇
  2002年   10327篇
  2001年   9821篇
  2000年   9299篇
  1999年   9571篇
  1998年   24372篇
  1997年   16734篇
  1996年   12835篇
  1995年   9525篇
  1994年   8374篇
  1993年   8387篇
  1992年   6068篇
  1991年   5717篇
  1990年   5679篇
  1989年   5318篇
  1988年   5053篇
  1987年   4420篇
  1986年   4299篇
  1985年   4815篇
  1984年   4421篇
  1983年   3995篇
  1982年   3656篇
  1981年   3730篇
  1980年   3429篇
  1979年   3327篇
  1978年   3334篇
  1977年   3752篇
  1976年   4850篇
  1975年   2851篇
  1974年   2663篇
  1973年   2730篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
The purpose of this paper is to evaluate two methods of assessing the productivity and quality impact of Computer Aided Software Engineering (CASE) and Fourth Generation Language (4GL) technologies: (1) by the retrospective method; and (2) the cross-sectional method. Both methods involve the use of questionnaire surveys. Developers' perceptions depend on the context in which they are expressed and this includes expectations about the effectiveness of a given software product. Consequently, it is generally not reliable to base inferences about the relative merits of CASE and 4GLs on a cross-sectional comparison of two separate samples of users. The retrospective method that requires each respondent to directly compare different products is shown to be more reliable. However, there may be scope to employ cross-sectional comparisons of the findings from different samples where both sets of respondents use the same reference point for their judgements, and where numerical rather than verbal rating scales are used to measure perceptions.  相似文献   
52.
53.
54.
Malignant bone pain: pathophysiology and treatment   总被引:1,自引:0,他引:1  
2-acetyl aminofluorene (AAF) reacts in acidic conditions with nitrous fume yielding N-nitroso-AAF (N-NO-AAF), as previously described, that exerts more toxic and mutagenic effects than its parental compound. In this study, the effect of sodium nitrite (NaNO2) on the tumorigenicity of AAF in rats fed with AAF and NaNO2 was observed. Wistar rats were divided into five groups: group I served as control; group II were treated with NaNO2 (0.3%); group III was given 0.02% AAF alone; groups IV and V received both AAF and NaNO2 (0.2 and 0.3% respectively) in their diet for 12 weeks. At the end of the experiment, all rats in groups III, IV and V developed early stage phenomena of hepatocellular carcinoma, including hepatomegaly with variable-sized foci and neoplastic nodules. Severe damage was observed in the rats treated with AAF and NaNO2. Feeding of AAF (0.02%) for 3 months elevated the levels of c-Fos, c-Jun and c-Myc proteins in the rat livers. The AAF-induced c-Jun, c-Fos and c-Myc expressions were significantly magnified (P < 0.001) by NaNO2. These data confirmed that the strengthening of AAF-induced hepatocarcinogenesis by NaNO2 should be associated with its enhancing effect on the AAF-induced increases in the expressions of c-Jun, c-Fos and c-Myc.  相似文献   
55.
56.
57.
58.
Translated from Kibernetika i Sistemnyi Analiz, No. 3, pp. 172–176, May–June, 1994  相似文献   
59.
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   
60.
A novel class of narrow-band tunable wavelength filters is proposed and evaluated. Wavelength selectivity of the proposed filters Is derived from the finite time response of an optical nonlinearity. The nonlinearity is gain saturation in semiconductor optical amplifier structures. The filters are shown to have very narrow passbands tunable over the entire semiconductor gain bandwidth. The key to filter implementation is a device configuration in which the wave-mixing products can be isolated from the amplified inputs. Three integrated optics compatible configurations are considered and shown to have high filter throughputs 34 to 180% and subangstrom bandwidths  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