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   1648247篇
  免费   26968篇
  国内免费   7122篇
电工技术   34968篇
综合类   6626篇
化学工业   278478篇
金属工艺   66110篇
机械仪表   46611篇
建筑科学   49429篇
矿业工程   11689篇
能源动力   50599篇
轻工业   131224篇
水利工程   16709篇
石油天然气   37900篇
武器工业   158篇
无线电   200427篇
一般工业技术   306710篇
冶金工业   233700篇
原子能技术   34354篇
自动化技术   176645篇
  2021年   15813篇
  2020年   12137篇
  2019年   14881篇
  2018年   18006篇
  2017年   17360篇
  2016年   22780篇
  2015年   17846篇
  2014年   29129篇
  2013年   88670篇
  2012年   39522篇
  2011年   54207篇
  2010年   45405篇
  2009年   52788篇
  2008年   49841篇
  2007年   47617篇
  2006年   48132篇
  2005年   43149篇
  2004年   44918篇
  2003年   44585篇
  2002年   43160篇
  2001年   40592篇
  2000年   38399篇
  1999年   38721篇
  1998年   66912篇
  1997年   52625篇
  1996年   44079篇
  1995年   36012篇
  1994年   33147篇
  1993年   32867篇
  1992年   27547篇
  1991年   24795篇
  1990年   24976篇
  1989年   24109篇
  1988年   22601篇
  1987年   20782篇
  1986年   20218篇
  1985年   23556篇
  1984年   23129篇
  1983年   21051篇
  1982年   19835篇
  1981年   20009篇
  1980年   18641篇
  1979年   19044篇
  1978年   18276篇
  1977年   19267篇
  1976年   22353篇
  1975年   16350篇
  1974年   15809篇
  1973年   15927篇
  1972年   13397篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Life-history characteristics of female threespine stickleback (Gasterosteus aculeatus) were examined in 12 populations, 11 freshwater and one anadromous, within the Cook Inlet region of Alaska. Because this area has been deglaciated during the last 20 000 years, the freshwater populations are recently derived, probably independently, from the local marine or anadromous stickleback. Freshwater threespine stickleback have undergone considerable morphological evolution within this region, apparently in response to environmental factors including predatory regimes and environmental productivity. Our freshwater study populations were selected to sample this range of morphological variation in order to determine whether life-history traits and morphologies have followed similar evolutionary trajectories. Freshwater populations could be categorized generally into one of three ecomorphotypes: those inhabiting relatively productive lakes having one or more piscivorous fishes present, and in which the stickleback exhibit a fully developed pelvic girdle; those inhabiting low-calcium lakes that lack piscivorous fishes, and in which the pelvic structures are incomplete; those living in streams with piscivorous fishes, in which the stickleback have fully developed pelvic girdles. The anadromous population constituted a fourth ecomorphotype that lives in marine waters, and is robustly armored. The freshwater populations showed considerable variation in all life-history traits assessed, and this variation generally corresponded to our ecomorphological classifications. Nevertheless, within each ecomorphotype there was sufficient variation to suggest that morphological and life-history traits may not always respond in the same manner in response to the same selective regime.Copyright 1998 The Linnean Society of London  相似文献   
992.
The Frizzled genes encode receptors for WNTs, secreted glycoproteins implicated in development as well as in carcinogenesis. In this paper, we report molecular cloning of Hfz6, the human homologue of Mfz6. Nucleotide sequence analysis showed that the Hfz6 gene encodes the 706 amino-acid protein with seven transmembrane domains, a cystein-rich domain in the N-terminal extracellular region, two N-linked glycosylation sites, and two cystein residues in the second and third extracellular loops. Hfz6 mRNA 4.4-kb in size was detected in various normal adult and fetal tissues, and a larger amount of Hfz6 mRNA was detected in both fetal lung and fetal kidney. The Hfz6 gene has been mapped to human chromosome 8q22.3-q23.1. In conclusion, we have cloned Hfz6, which encodes a seven-transmembrane receptor with the cystein-rich domain in the N-terminal extracellular region, but without the Ser/Thr-X-Val motif in the C-terminus.  相似文献   
993.
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   
994.
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   
995.
996.
Vaccination with peptides that induce a specific immune response is a potential prophylactic or therapeutic strategy against viral infections and tumors. Because of the extensive polymorphism of the HLA loci, synthetic peptide vaccines must consist of a cocktail of peptides that bind specifically to different HLA molecules. Such cocktails should be optimized for the target population as each population has its specific HLA gene frequencies. To achieve maximum population coverage with a minimum number of peptides, information is needed on the ranking of the most frequent HLA phenotypes. We introduce the minimal phenotype panel, which is the smallest combination of HLA antigens selected so that the proportion of individuals in a population that express at least one of the antigens in the panel exceeds a desired minimum value. We developed a method for assembling minimal phenotype panels based on known HLA class I gene frequencies. We give an example based on a set of 2446 well-defined HLA-typed, random, healthy, unrelated, Dutch Caucasoid individuals. In addition, we discuss the possibility of assembling minimal phenotype panels based on two-locus haplotypes, which enables the assembly of phenotype panels from the antigens of both loci.  相似文献   
997.
998.
999.
本文概述了离子注入过程中污染产生的原因和防止污染的措施,特别强调了对微粒污染和金属污染的防护以满足ULSI加工对离子注入的要求。  相似文献   
1000.
This paper discusses the problem of violence and its expression upon mortality due to external causes. A few indicators are offered, which have been worked upon it to emphasise the importance of the theme. In a general way, the study demonstrates violent death has had its magnitude increased along the years, not only throughout Latin America but also in Brazil and in Santa Catarina.  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