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   1242篇
  免费   14篇
  国内免费   12篇
综合类   5篇
化学工业   23篇
金属工艺   10篇
机械仪表   8篇
建筑科学   12篇
矿业工程   2篇
能源动力   4篇
轻工业   12篇
水利工程   1篇
石油天然气   8篇
无线电   16篇
一般工业技术   9篇
冶金工业   1134篇
原子能技术   7篇
自动化技术   17篇
  2022年   3篇
  2021年   2篇
  2020年   4篇
  2017年   3篇
  2016年   1篇
  2015年   5篇
  2014年   3篇
  2013年   4篇
  2012年   14篇
  2011年   13篇
  2010年   18篇
  2009年   10篇
  2008年   6篇
  2007年   10篇
  2006年   6篇
  2005年   9篇
  2004年   2篇
  2003年   7篇
  2002年   8篇
  2001年   8篇
  2000年   1篇
  1999年   31篇
  1998年   312篇
  1997年   209篇
  1996年   130篇
  1995年   65篇
  1994年   71篇
  1993年   73篇
  1992年   9篇
  1991年   12篇
  1990年   13篇
  1989年   7篇
  1988年   17篇
  1987年   12篇
  1986年   10篇
  1985年   8篇
  1983年   3篇
  1982年   3篇
  1981年   17篇
  1980年   19篇
  1979年   1篇
  1978年   4篇
  1977年   30篇
  1976年   70篇
  1975年   1篇
  1955年   4篇
排序方式: 共有1268条查询结果,搜索用时 15 毫秒
61.
The 1991 census for England and Wales provides a substantial amount of data on demography, ethnicity, housing tenure, employment status, and other social factors for geographical areas ranging in size from enumeration districts upwards. Many in the health service and in the academic community are making use of the data in the 1991 census. However, users of census data need to be aware of the problems and limitations of these data, which include the format of the data, data modification and suppression, sampling error, and underenumeration. An important innovation of the 1991 census was that the census form included a question on the postcode of respondents; this allowed the Office of Population Censuses and Surveys to produce a postcode-enumeration district look up table which overcomes many of the problems previously encountered in trying to assign postcodes to enumeration districts. The new look up table also includes the grid reference of postcodes, and this will improve the geographical referencing of census data.  相似文献   
62.
63.
OBJECTIVE: To identify patient- and admission-related risk factors for a medically inappropriate admission to a department of internal medicine. METHODS: Cross-sectional study of a systematic sample of 500 admissions to the department of internal medicine of an urban teaching hospital. The appropriateness of each admission and reasons for inappropriate admissions were assessed using the Appropriateness Evaluation Protocol. Risk factors included the time (day of week and holidays) and manner (through emergency department or direct admission) of admission, patient age and sex, health status of patient and spouse, living arrangements, formal home care services, and informal support from family or friends. RESULTS: Overall, 76 (15.2%) hospital admissions were rated as medically inappropriate by the Appropriateness Evaluation Protocol. In multivariate analysis, the likelihood of an inappropriate admission was increased by better physical functioning of the patient (odds ratio [OR], 1.5; 95% confidence interval [CI], 1.1-2.1 [for 1 SD in Physical Functioning scores]), lower mental health status of the patient's spouse (OR, 2.6; 95% CI, 1.3-5.6), receipt of informal help from family or friends (OR, 3.3; 95% CI, 1.5-7.2), and hospitalization by one's physician (OR, 3.6; 95% CI, 1.7-7.5). Receiving formal adult home care was not associated with inappropriateness of hospitalization. CONCLUSIONS: Inappropriate admissions to internal medicine wards are determined by a mix of factors, including the patient's health and social environment. In addition, the private practitioners' discretionary ability to hospitalize their patients directly may also favor medically inappropriate admissions.  相似文献   
64.
65.
The cytotoxicity induced by the transient expression of functional N-methyl-D-aspartate (NMDA) receptors has been examined with the use of a luciferase reporter assay in Chinese hamster ovary cells. Various NMDA receptor antagonists, in a dose-dependent manner, prevented a loss of luciferase activity 24 to 48 hr post-transfection of either the NR1/NR2A or NR1/ NR2B subunit receptor configurations, likely correlating to the time required to express functionally these receptors. Both glutamate and NMDA were potently cytotoxic to transfected cells previously protected by antagonists. The novel ifenprodil analog (1S,2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol (CP101,606-27) protected cells expressing NR1/NR2B, but not those cells expressing either NR1/NR2A or, putatively, NR1/NR2A/NR2B. Decreased cytotoxicity was observed when a mutated NR1 subunit (N616R) with reduced Ca++ permeability was used in coexpression studies with NR2A or NR2B. In contrast to our results with NR1/NR2A or NR1/NR2B, cells expressing NR1/NR2C did not perish. Our studies suggest that expression of functional NMDA receptors in non-neuronal cells leads to a form of excitotoxicity similar to that observed in mammalian neurons in vitro.  相似文献   
66.
We measured the levels of interferon alpha (IFN alpha) in the sera of Thai children hospitalized with dengue hemorrhagic fever (DHF) or dengue fever (DF) to examine the role of IFN alpha in dengue virus infections of humans. The percentage of patients who had detectable levels of IFN alpha (> or = 3 U/ml) was higher in patients with DHF (80%, P < 0.001) and in patients with DF (60%, P < 0.001) than in healthy Thai children (7%). The levels of IFN alpha were higher in patients with DHF and in patients with DF on the first few days after the onset of fever than in healthy Thai children. The average levels of IFN alpha in patients with DHF were high two days before defervescence, decreasing gradually until the day of defervescence. There was a subset of patients with DHF who had increasing levels of IFN alpha after defervescence. However, the levels of IFN alpha in patients with DF were not high after fever subsided. The levels of IFN alpha were not different among children with DHF grades 1, 2 and 3. Among patients with DHF, T lymphocytes were activated to a higher degree in high IFN alpha producers than in low IFN alpha producers. These results indicate that similarly high levels of IFN alpha are produced in vivo during the acute stages of DHF and DF, and that high levels of IFN alpha remain after fever subsides in some patients with DHF, but not in patients with DF.  相似文献   
67.
恶性肿瘤是一种严重威胁人类健康的常见病和多发病,是引起死亡的主要原因之一。近年来,抗肿瘤药物的靶向性研究越来越受到人们的重视。发展抗肿瘤药物的新型靶向载体,增加药物靶向性,提高生物利用度,降低毒副作用,是目前备受关注的课题。根据近年来文献,综述了一些新型的抗肿瘤药物载体,包括靶向的脂质体载药系统、纳米粒载药系统、微粒载药系统、胶束载药系统等,并分别总结了其颗粒大小、载药类型、载药模式及治疗效果等,为肿瘤的靶向给药研究提供思路。  相似文献   
68.
范志新  孙以材 《电子器件》2001,24(4):386-389
应用与晶体结构和掺杂上关的最佳掺杂含量的理论表达式,对氧化铁气敏纳米晶粉体中锡掺杂最佳含量进行理论计算。定量计算的结果与实验数据相符合。该理论经适当的修改和解释后也适用于某些其他电子薄膜材料的最佳掺杂含量问题。  相似文献   
69.
70.
OBJECTIVE: To determine the effect of continuing medical education (CME) with and without a quality assurance component (CME+QA) on physician practices in the prevention of venous thromboembolism. METHODS: A communitywide study was performed in 15 short-stay hospitals in central Massachusetts. The study population included 3158 patients in acute-care hospitals with multiple risk factors for venous thromboembolism. Study hospitals were randomly assigned to one of two educational strategies or to a control group that received no intervention. RESULTS: The proportion of patients at high risk for venous thromboembolism who received effective methods of prophylaxis increased significantly from 29% in 1986 to 52% in 1989 (P < .001). This increase was seen in all study groups: control hospitals, 40% to 51% (P < .001); CME hospitals, 21% to 49% (P < .0001); and CME+QA hospitals, 27% to 55% (P < .0001). The increase in prophylaxis use from 1986 to 1989 was significantly greater among patients cared for in hospitals whose physicians participated in a formal CME program (an increase of 28%) than in control hospitals (an increase of 11%) (P < .001). There was no significant difference in the use of prophylaxis in hospitals whose physicians received CME+QA interventions compared with hospitals whose physicians received CME interventions alone (identical increases of 28%). CONCLUSION: A formal CME program significantly increased the frequency with which physicians prescribed prophylaxis for venous thromboembolism. We believe the key factor in our CME interventions that motivated clinicians to change their practices was the provision of hospital-specific data demonstrating a compelling need for improvement. Despite the substantial investment by hospitals in QA, traditional QA intervention appeared to provide no additional benefit. Even after extensive CME/QA interventions, prophylaxis for venous thromboembolism remained underutilized, suggesting the need to develop new approaches to changing clinical practice.  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