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   2833篇
  免费   15篇
  国内免费   2篇
电工技术   5篇
综合类   1篇
化学工业   96篇
金属工艺   7篇
机械仪表   12篇
建筑科学   21篇
矿业工程   2篇
能源动力   9篇
轻工业   78篇
水利工程   5篇
石油天然气   5篇
武器工业   1篇
无线电   76篇
一般工业技术   86篇
冶金工业   2367篇
自动化技术   79篇
  2021年   11篇
  2020年   6篇
  2019年   6篇
  2018年   13篇
  2016年   6篇
  2015年   6篇
  2014年   8篇
  2013年   27篇
  2012年   18篇
  2011年   23篇
  2010年   10篇
  2009年   17篇
  2008年   25篇
  2007年   21篇
  2006年   14篇
  2005年   19篇
  2004年   23篇
  2003年   23篇
  2002年   20篇
  2001年   15篇
  2000年   11篇
  1999年   79篇
  1998年   633篇
  1997年   398篇
  1996年   265篇
  1995年   155篇
  1994年   111篇
  1993年   153篇
  1992年   29篇
  1991年   47篇
  1990年   49篇
  1989年   44篇
  1988年   38篇
  1987年   33篇
  1986年   32篇
  1985年   38篇
  1984年   12篇
  1983年   12篇
  1982年   17篇
  1981年   32篇
  1980年   24篇
  1979年   14篇
  1978年   12篇
  1977年   85篇
  1976年   168篇
  1975年   11篇
  1974年   4篇
  1973年   10篇
  1969年   3篇
  1967年   3篇
排序方式: 共有2850条查询结果,搜索用时 0 毫秒
61.
OBJECTIVE: To determine prostate specific antigen density (PSAD) in a risk population without evidence of prostatic cancer, and to assess the long-term usefulness of PSAD as a parameter for determining the need for a prostatic biopsy in patients with a normal digital rectal examination (DRE) and transrectal ultrasound (TRUS). METHODS: The records of 582 patients referred to the clinic between February, 1992 and February, 1994 were studied retrospectively. All these patients with lower urinary tract symptoms (LUTS) were evaluated based on the following parameters: digital rectal examination, serum PSA levels, prostate volume measured using transrectal ultrasound and PSAD. Prostatic biopsy was performed on 431 patients who had a serum PSA level greater than 4.0 ng/mL. A total of 299 patients (69.3%) had PSA levels between 4.0 and 10.0 ng/mL and represented the target population. The study had two parts, in the first one cancer was diagnosed just by one biopsy and in part II, the patients with negative biopsy in part I were followed for a two-year period and required 2 or 3 biopsies for diagnosis. Of the total of patients who had a negative prostate biopsy in part I of the study, 269 were followed for a period of two years with repeated prostate biopsies. RESULTS: Overall prostate cancer was detected in 22/299 (13.9%) patients, 6/105 (5.7%) with PSAD up to 0.15 and 16/194 (8.2%) with PSAD over 0.15 (p = 0.569). CONCLUSION: PSAD is a useful indicator in decreasing the number of negative biopsies in patients with benign prostatic hyperplasia. However, in a long-term follow-up the PSAD (cutoff level 0.15) was unable to predict which patients had a positive biopsy. According to our results, 5.6% of patients with prostate cancer will be missed using the PSAD criteria.  相似文献   
62.
Epiglottic disruption is an uncommon injury usually associated with significant supraglottic trauma. This injury may be overlooked because of the difficulty in examining the larynx or other associated severe injuries. We present two cases of clinically unsuspected epiglottic disruption that were first seen on MR images of the neck.  相似文献   
63.
64.
Short RNA species that encompass the psi domain of the retroviral genome spontaneously form dimers in vitro, and the retroviral nucleocapsid protein activates this dimerization in vitro. Addition of gag RNA sequences downstream of the 3' end of the psi domain decreases the level of spontaneous dimerization. Here, we report the effects of RNA length on dimerization in vitro, studied with RNA fragments from Moloney murine leukaemia virus that contain the psi domain and all or part of the gag sequence. Extension of the RNA leads to progressive inhibition of the in vitro dimerization process. Sequences located downstream of the 3' end of the psi domain seem to stabilize the monomeric structures. This stabilization participates in dimerization of the RNA sequences involved in the recognition of two RNA molecules. We studied the ability of nucleocapsid protein 10 to promote dimerization of such long RNA fragments, and found that the protein greatly enhances their dimerization in vitro. We propose that nucleocapsid protein 10 stimulates the overall dimerization process by reduction of the energy barrier that must be overcome to allow dimer formation. Our results show that dimerization of RNA form Moloney murine leukaemia virus in vitro is enhanced by nucleocapsid protein 10. This finding is in agreement with the involvement of the nucleocapsid protein in RNA dimerization in vivo.  相似文献   
65.
66.
67.
68.
Beginning at wk 5 of lactation, 136 cows (34 per treatment) were supplemented daily for 38 wk with 0, 10.3, 20.6, or 41.2 mg of recombinantly derived bST monomer. Cows were obtained from University of Kentucky, University of Minnesota, University of Pennsylvania, and The Ohio State University. Nine cows (4 at 0 mg/d, 1 at 10.3 mg/d, 1 at 20.6 mg/d, and 3 at 41.2 mg/d) did not complete the experiment because of health problems. Data from these cows were included in the reproduction and health databases but not in the production database. Cows supplemented with bST produced more milk, consumed more feed, had lower rates of BW gain, and had improved efficiencies of milk production (conversion of feed and NEL to milk). Additional increases in productivity were modest at 20.6 and 41.2 mg/d versus productivity at 10.3 mg/d of bST. Concentrations of fat, protein, and TS in milk were unaffected. At 10.3 mg/d, bST did not adversely affect reproduction or health.  相似文献   
69.
PURPOSE: To determine trends in mammography in the United States. MATERIALS AND METHODS: A sample of mammographic facilities was selected for each year of the Nationwide Evaluation of X-ray Trends. The same protocol was followed for the 1985, 1988, and 1992 surveys. Data were collected with use of the same imaging phantom for all three surveys and also with a different phantom in the 1988 and 1992 surveys. RESULTS: Of the 356 facilities surveyed in 1992, 59% claimed to be in compliance with the Health Care Financing Administration (HCFA) mammography requirements, 42% were accredited by the American College of Radiology (ACR), and 23% did not hold credentials from either the HCFA or the ACR. Since 1985, there has been a 34% improvement in acceptable phantom image quality score and a 20% decrease in the mean glandular dose. CONCLUSION: Mammography as practiced today is essentially a screen-film technique. Mammographic phantom image quality has improved considerably. The overall mean glandular dose has decreased primarily because of the elimination of xeroradiography.  相似文献   
70.
We have measured the Fourier transform spectrum of natural OCS from 3700 to 4800 cm-1 with a near Doppler resolution and a line-position accuracy between 4 and 8 x 10(-5) cm-1. For the normal isotopic species, 37 vibrational transitions have been analyzed for both frequencies and intensities. We also report 15 bands of OC34S, eight bands of O13CS, nine bands of OC33S, and two bands of 18OCS. Important effective Herman-Wallis terms are explained on the basis of eigenvectors. A comparison of different line-pointing programs is also presented. Copyright 1998 Academic Press.  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