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   252885篇
  免费   4132篇
  国内免费   1044篇
电工技术   4335篇
综合类   203篇
化学工业   37292篇
金属工艺   8252篇
机械仪表   7278篇
建筑科学   6832篇
矿业工程   541篇
能源动力   7264篇
轻工业   28719篇
水利工程   2129篇
石油天然气   1852篇
武器工业   5篇
无线电   33463篇
一般工业技术   46357篇
冶金工业   46868篇
原子能技术   3376篇
自动化技术   23295篇
  2021年   1807篇
  2020年   1355篇
  2019年   1685篇
  2018年   2585篇
  2017年   2605篇
  2016年   2894篇
  2015年   2179篇
  2014年   3669篇
  2013年   12001篇
  2012年   6792篇
  2011年   9711篇
  2010年   7475篇
  2009年   8425篇
  2008年   8951篇
  2007年   8971篇
  2006年   8308篇
  2005年   7518篇
  2004年   7239篇
  2003年   7117篇
  2002年   6623篇
  2001年   7042篇
  2000年   6345篇
  1999年   6844篇
  1998年   16462篇
  1997年   11424篇
  1996年   8616篇
  1995年   6577篇
  1994年   5746篇
  1993年   5608篇
  1992年   4010篇
  1991年   3783篇
  1990年   3566篇
  1989年   3389篇
  1988年   3293篇
  1987年   2606篇
  1986年   2501篇
  1985年   3164篇
  1984年   2805篇
  1983年   2569篇
  1982年   2357篇
  1981年   2400篇
  1980年   2208篇
  1979年   2059篇
  1978年   1883篇
  1977年   2188篇
  1976年   2753篇
  1975年   1597篇
  1974年   1498篇
  1973年   1564篇
  1972年   1151篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
171.
This review examines, by means of a literature search, the needs of parents who have a critically ill child on adult intensive therapy units. These needs are compared with the needs of relatives of adult patients in ITUs. Whether nurses trained in adult nursing have the necessary skills, knowledge and attitudes to care for the parents is also discussed. Recommendations for ensuring care of the parent point to addressing skills gaps in adult-trained nurses and to reappraising visiting policies.  相似文献   
172.
The major histocompatibility complex (MHC) is an immunologically important group of genes that appears to be under natural as well as sexual selection. Several hypotheses suggest that certain MHC-allele combinations (usually heterozygous ones) are superior under selective pressure by pathogens. This could influence mate choice in a way that preferences function to create MHC-heterozygous offspring, or that they function to create specific allele combinations that are beneficial under the current environmental conditions through their complementary or epistatic effects. To test these hypotheses, we asked 121 men and women to score the odours of six T-shirts, worn by two women and four men. Their scorings of pleasantness correlated negatively with the degree of MHC similarity between smeller and T-shirt-wearer in men and women who were not using the contraceptive pill (but not in Pill-users). Depending on the T-shirt-wearer, the amount of variance in the scorings of odour pleasantness that was explained by the degree of MHC similarity (r2) varied between nearly 0 and 23%. There was no apparent effect of gender in this correlation: the highest r2 was actually reached with one of the male odours sniffed by male smellers. Men and women who were reminded of their own mate/ex-mate when sniffing a T-shirt had significantly fewer MHC-alleles in common with this T-shirt-wearer than expected by chance. This suggests that the MHC or linked genes influence human mate choice. We found no significant effect when we tested for an influence of the MHC on odour preferences after the degree of similarity between T-shirt-wearer and smeller was statistically controlled for. This suggests that in our study populations the MHC influences body odour preferences mainly, if not exclusively, by the degree of similarity or dissimilarity. The observed preferences would increase heterozygosity in the progeny. They do not seem to aim for more specific MHC combinations.  相似文献   
173.
Color image retrieval based on hidden Markov models   总被引:1,自引:0,他引:1  
In this correspondence, a new approach to retrieving images from a color image database is proposed. Each image in the database is represented by a two-dimensional pseudo-hidden Markov model (2-D PHMM), which characterizes the chromatic and spatial information about the image. In addition, a flexible pictorial querying method is used, by which users can paint the rough content of the desired images in a query picture. Image matching is achieved by comparing the query picture with each 2-D PHMM in the database. Experimental results show that the proposed approach is indeed effective.  相似文献   
174.
Somatic hypermutation of the immunoglobulin variable genes during germinal reactions might permit the expansion of B-cell clones with unwanted (e.g. autoreactive) specificity. Here, Ernst Lindhout and colleagues propose three antigen-specific checkpoints that ensure the appropriate antigen specificity of activated B cells is maintained by regulating the activation, selection and further differentiation of B cells.  相似文献   
175.
BACKGROUND: The establishment of the nasopharyngeal flora was followed in Costa Rican children from birth to 1 year of age. METHODS: Nasopharyngeal cultures were obtained at 1 (n = 413), 3 (n = 393), 6 (n = 376) and 12 months (n = 356) of age from children representative of the population in the Puriscal district. Weekly cultures were obtained from a subcohort of these children (n = 101). Mother-infant diads (n = 95) and preschool children (n = 208) attending day-care centers were also studied. RESULTS: The estimated proportion of colonized children in the population differed markedly depending on the frequency of culture. Quarterly cultures showed a slow increase in carrier rates from 3.9% for Haemophilus influenzae, 3.1% for Streptococcus pneumoniae and 6.5% for Moraxella catarrhalis at 1 month of age to 10.1% carrying H. influenzae and 19.4% carrying S. pneumoniae by the end of the first year. By quarterly culture the proportion of children colonized at least once was 36% for S. pneumoniae, 26% for H. influenzae and 28% for M. catarrhalis. In contrast weekly sampling showed that 95 to 100% of the children were colonized at least once during the first year of life with H. influenzae, S. pneumoniae or M. catarrhalis. Nasopharyngeal carriage of H. influenzae, S. pneumoniae and M. catarrhalis was low in the mothers, and very few mother-infant pairs carried identical bacteria at the same time. In contrast carrier rates were high in the siblings attending day care (H. influenzae 27.9%, S. pneumoniae 39.4%, both organisms 26.6%). Infants with siblings had significantly higher bacterial carriage at all ages than infants without siblings. CONCLUSIONS: Quarterly nasopharyngeal cultures showed that Costa Rican infants acquire their nasopharyngeal flora at a rate comparable with that for infants in developed countries and that siblings are an important source of the bacteria. Weekly samplings showed that virtually all children were colonized at least once during the first year of life.  相似文献   
176.
177.
178.
Within the realm of oncology nursing, research has been an integral part in its development as a specialty practice. Yet despite the growing volume of published nursing research studies, little is known about how nurses working in oncology care settings perceive research. Therefore, the purposes of this study were to examine clinical oncology nurses' perceptions of research and to determine factors influencing their perceptions. Two hundred and eighty-three registered nurses providing cancer care to patients in 40 health care agencies across northern Ontario participated in the survey. Data were collected using a questionnaire developed by Alcock and colleagues (1990) which addressed nurses' perceived value of research, their role, interest and experience in research as well as the research climate of the agency. The findings showed that respondents valued nursing research and perceived a research role for staff nurses. However, the respondents did not perceive strong administrative or collegial support for nurses' involvement in research activities. In addition, the study results indicated that the clinical oncology nurses' perceptions of research were influenced by educational preparation.  相似文献   
179.
180.
Reviews the book, Constructing the subject: Historical origins of psychological research by Kurt Danziger (1990). Kurt Danziger's Constructing the Subject: Historical Origins of Psychological Research is a book of singular importance because it provides such a penetrating analysis, and does so in a manner that is cause for considerable reflection. In brief, Danziger provides a history lesson that not only situates the names and the projects of experimental psychology in the first part of this century, but also aims to clarify the project of knowledge generation both past and present. Indeed, shades of Quine, Kuhn, and Hesse permeate this book in a way that demands psychologists examine their own investigative practices and logics of justification. Through Wundt, through Galton, through Ebbinghaus and others, Danziger illuminates the development of experimental psychology along with the historical and philosophical vicissitudes that have given rise to numerous psychological knowledge claims. If it is true that we must understand our history in order to understand our present, then Danziger's book should be required reading in all research laboratories. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  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