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   3266篇
  免费   82篇
  国内免费   8篇
电工技术   24篇
综合类   16篇
化学工业   289篇
金属工艺   24篇
机械仪表   34篇
建筑科学   26篇
矿业工程   6篇
能源动力   29篇
轻工业   117篇
水利工程   10篇
石油天然气   10篇
无线电   76篇
一般工业技术   207篇
冶金工业   2337篇
原子能技术   7篇
自动化技术   144篇
  2024年   4篇
  2023年   9篇
  2022年   41篇
  2021年   53篇
  2020年   28篇
  2019年   15篇
  2018年   37篇
  2017年   29篇
  2016年   21篇
  2015年   24篇
  2014年   28篇
  2013年   48篇
  2012年   53篇
  2011年   74篇
  2010年   40篇
  2009年   58篇
  2008年   62篇
  2007年   39篇
  2006年   38篇
  2005年   38篇
  2004年   24篇
  2003年   38篇
  2002年   41篇
  2001年   26篇
  2000年   15篇
  1999年   94篇
  1998年   726篇
  1997年   421篇
  1996年   272篇
  1995年   159篇
  1994年   134篇
  1993年   166篇
  1992年   23篇
  1991年   22篇
  1990年   17篇
  1989年   20篇
  1988年   33篇
  1987年   31篇
  1986年   23篇
  1985年   23篇
  1983年   10篇
  1982年   13篇
  1981年   22篇
  1980年   28篇
  1979年   6篇
  1978年   8篇
  1977年   69篇
  1976年   120篇
  1975年   10篇
  1971年   4篇
排序方式: 共有3356条查询结果,搜索用时 15 毫秒
31.
32.
33.
OBJECTIVES: Syphilis in the United States is focally distributed, with high incidence rates in the South and in metropolitan areas nationwide. In this study an ecological analysis, using the county as the unit of analysis, was performed to generate hypotheses about community-level determinants of syphilis rates. METHODS: Bivariate rank correlations and multivariate, backward stepwise elimination linear regressions were performed. Mean annual incidence of primary- and secondary-stage syphilis in a county was the dependent variable, and county sociodemographic characteristics (from census data) were the independent variables. RESULTS: In the multivariate regression model, sociodemographic characteristics accounted for 71% of the variation in syphilis rates among counties. With other factors accounted for, the most highly correlated characteristics were percentage non-Hispanic Black population, county location in the South, percentage of the population that was urban, percentage Hispanic population, and percentage of births to women younger than 20 years. CONCLUSIONS: Most of the variation in syphilis rates among counties is accounted for by sociodemographic characteristics. Identification and remediation of modifiable health determinants for which these factors are markers are needed to improve the health status of these populations.  相似文献   
34.
The ultrastructural diagnosis of tumors requires a careful analysis that should be done in an orderly fashion. It requires precise planning from the time of specimen collection to the selection of the area to be examined. Pictures must be taken systematically and every micrograph should allow to answer whether the number of cells photographed is adequate; whether mitoses are present, what is the pattern of the tumor; what is the appearance of the cell membrane; whether the cells are joined by junctional complexes; whether free surfaces possess microvilli or cilia; what organelles are present and how they are distributed; whether there are secretory granules, melanosomes, or other cytoplasmic elements. Nuclear and nucleolar size and shape have to be taken into consideration. The composition of the interstitial extracellular matrix is important in certain types of tumors. Although these questions are not the only ones to be addressed, their use in a logical fashion is helpful when it concerns the ultrastructural diagnosis.  相似文献   
35.
36.
In human leukemias and lymphomas nonrandom chromosomal rearrangements cause changes in cell growth and/or survival in such a way as to promote malignancy. The detailed study of the biochemical and genetic pathways altered in human cancer requires the identification or development of models to allow the study and manipulation of cancer gene function. Recently, the breakpoint gene TCL1, involved in chromosome translocations observed mostly in mature T-cell proliferations and chronic lymphocytic leukemias (CLL), was isolated and characterized, and showed to be part of a new gene family of proteins involved in these tumors. The murine Tcl1 gene, is similar in sequence to the murine and human MTCP1 gene also involved in T cell leukemias. The murine Tcl1 gene was shown to reside on mouse chromosome 12 in a region syntenic to human chromosome 14. Furthermore, we show that the murine Tcl1 gene is expressed early in mouse embryonic development and demonstrates expression in fetal hematopoietic organs as well as in immature T and B cells. Characterization of the murine Tcl1 gene will help in developing a mouse model of CLL and would provide the best opportunity to study and decipher the role of TCL1 in malignant transformation.  相似文献   
37.
38.
Russo  P.  d'Ippolito  A.  Ferrarotti  A.  Ruggieri  M. 《Telecommunication Systems》1997,8(2-4):319-340
A satellite system based on the frequency scanning concept has been recently assessed for L‐band land mobile communications. The study – promototed by ESA/ESTEC for an European application mission – has reached an advanced electrical design phase. The selected configuration is particularly advantageous in terms of Carrier‐to‐Intermodulation ratio (C/I) performance, which constitutes a requirement of key‐importance in the envisaged user applications. The advantage in C/I is due to the proper choice of the on‐board transmitter configuration, which adopts power module pairs where the overall power demand is evenly distributed. The consequent improved C/I is then achievable at both transmitter and radiated far field level. The present paper aims a contribution to the design methodology of frequency scanning satellite systems, by joining two key‐aspects: in fact an analytical approach is proposed to demonstrate the C/I advantage achievable through frequency scanning satellite systems and the methodology is applied to a concrete example, the ESA/ESTEC system, whose feasibility has been fully proved in previous studies. The analysis is performed both at transmitter level and in the far field. The main results achieved are hereinafter presented. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
39.
Basal cell carcinoma (BCC) is the most common type of skin cancer and the incidence of BCC is expected to rise, with increased demand on dermatology resources. Little is known of the effect on people's lives of having skin cancer. The aim of this study was to quantify the handicap caused gy basal cell carcinomas before and after therapy. Forty-four patients (22 males, mean age = 65 yrs, range = 35-81 yrs) with 48 BCCs were recruited and 37 patients completed the study. Each patient completed the UK Sickness Impact Profile (UKSIP) and the Dermatology Life Quality Index (DLQI) at the initial visit to the dermatology clinic, 1 week after treatment and 3 months after treatment. Lesions had been present for a mean of 25 months (range = 1-240 months), their mean diameter was 9.6mm (range = 3-35mm) and the sites were head and neck (79%), trunk (17%) and limbs (4%). There was no relationship between the quality of life score and size of lesion. Overall the scores at presentation were very low, rising 1 week after treatment and falling to below the initial scores at 3 months (mean UKSIP 0.4%, 0.7%, 0.13%; mean DLQI 5.3% 8.7%, 1.2%). BCCs cause little handicap. This may explain the delay in seeking medical attention and should be considered in planning public health education about BCCs.  相似文献   
40.
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