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   330422篇
  免费   4415篇
  国内免费   1869篇
电工技术   6992篇
技术理论   5篇
综合类   1248篇
化学工业   50519篇
金属工艺   17253篇
机械仪表   13037篇
建筑科学   7826篇
矿业工程   2349篇
能源动力   6250篇
轻工业   26856篇
水利工程   3922篇
石油天然气   7223篇
武器工业   243篇
无线电   34442篇
一般工业技术   67453篇
冶金工业   52532篇
原子能技术   6603篇
自动化技术   31953篇
  2021年   2474篇
  2019年   2118篇
  2018年   11494篇
  2017年   11992篇
  2016年   8206篇
  2015年   3309篇
  2014年   4665篇
  2013年   11407篇
  2012年   10191篇
  2011年   18025篇
  2010年   15654篇
  2009年   15061篇
  2008年   15659篇
  2007年   17671篇
  2006年   8499篇
  2005年   10791篇
  2004年   8535篇
  2003年   7787篇
  2002年   6836篇
  2001年   6381篇
  2000年   6041篇
  1999年   5894篇
  1998年   12860篇
  1997年   9538篇
  1996年   7290篇
  1995年   5534篇
  1994年   5110篇
  1993年   4976篇
  1992年   3959篇
  1991年   3775篇
  1990年   3824篇
  1989年   3772篇
  1988年   3534篇
  1987年   3019篇
  1986年   3050篇
  1985年   3410篇
  1984年   3321篇
  1983年   3075篇
  1982年   2699篇
  1981年   2902篇
  1980年   2643篇
  1979年   2839篇
  1978年   2737篇
  1977年   2841篇
  1976年   3693篇
  1975年   2457篇
  1974年   2290篇
  1973年   2320篇
  1972年   1982篇
  1971年   1785篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
This paper describe a freshman course in engineering design that stresses the open-ended and ill-structured nature of design in a project-based context. The projects are chosen in part for their social context and utility. The projects are supplemented by lectures on design methodology and other topics related to engineering practice. Among the deliverables required of the students are proposals, progress reports, and written and oral presentations—the latter to an external design jury as part of a design competition.  相似文献   
92.
93.
PURPOSE: We investigated Sertoli cells in testicular biopsies with carcinoma in situ (CIS) in respect to cytokeratin expression to elucidate the status of Sertoli cell differentiation adjacent to CIS in human testes. Cytokeratin 18 intermediate filaments indicate a state of undifferentiation usually observed in Sertoli cells of prepubertal testes. MATERIALS AND METHODS: 29 testicular biopsies presenting CIS were investigated by means of immunohistochemistry, using a polyclonal antibody against placental-alkaline phosphatase to detect CIS cells and a monoclonal antibody against human cytokeratin 18 to show expression of cytokeratin 18 intermediate filaments in Sertoli cells. RESULTS: All tubules bearing CIS showed positive cytokeratin expression of Sertoli cells if tubules were devoid of normal germ cells. However, a total of 13 specimen revealed CIS cells together with normal germ cells. In the presence of CIS cells together with round or elongated spermatids, adjacent Sertoli cells did not express cytokeratin immunoreactivity. In the case of combined presence of CIS and spermatogonia and primary spermatocytes, Sertoli cells could be found either immunopositive or immunonegative, and were positive in tubules with CIS and spermatogonia only. CONCLUSIONS: Sertoli cells associated with CIS cells undergo a process of dedifferentiation, seen by the re-expression of cytokeratin intermediate filaments. We suggest that this dedifferentiation results in a loss of Sertoli cell function and leads to a cessation of spermatogenic activity.  相似文献   
94.
An implicit assumption of several causal reasoning models is that readers adopt the goals of a narrative's protagonist during text comprehension. In apparent violation of this assumption, readers participating in Experiment 1 of the present study drew inferences relevant to a protagonist's goal even when that goal was already satisfied from the perspective of the protagonist. In Experiments 2 and 3, participants were explicitly asked to view the text situation from the point of view of the protagonist. In this case, the goals of the reader and the protagonist should be the same. In these experiments, participants focused on the goals of the protagonist only when those goals had not been satisfied from the perspective of the protagonist. These results are discussed in terms of reader- and character-based perspectives and in terms of text characteristics that cue perspective taking. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
95.
96.
The biotransformation of oxazepam by Bifidobacterium bifidum was studied. The major metabolite was purified by chromatographic methods and found to be desmethyldiazepam using NMR, IR and other physicochemical data.  相似文献   
97.
98.
We report the results of the first national survey of psychologists in private practice regarding their participation in peer consultation groups. The sample (71% return) was drawn from 800 randomly selected psychologists listed in the National Register of Health Service Providers in Psychology. We found that 23% of the sample currently belonged to peer consultation groups, and 24% had belonged in the past. Of those not currently in groups, 61% expressed the desire to belong if one were available. There were virtually no significant demographic differences between current members and nonmembers. We examined the following group characteristics: formation, length of existence, size, membership, leadership, theoretical orientation, range of experience, time and place, content, and group process. Groups tended to be small, informal, and leaderless; however, we found great variation among groups on all dimensions. Findings also showed a high degree of satisfaction with membership. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
99.
100.
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