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   3982篇
  免费   174篇
  国内免费   108篇
电工技术   68篇
综合类   149篇
化学工业   185篇
金属工艺   146篇
机械仪表   100篇
建筑科学   259篇
矿业工程   34篇
能源动力   21篇
轻工业   130篇
水利工程   49篇
石油天然气   44篇
武器工业   25篇
无线电   195篇
一般工业技术   352篇
冶金工业   2324篇
原子能技术   35篇
自动化技术   148篇
  2024年   3篇
  2023年   25篇
  2022年   70篇
  2021年   93篇
  2020年   60篇
  2019年   64篇
  2018年   55篇
  2017年   59篇
  2016年   51篇
  2015年   68篇
  2014年   76篇
  2013年   146篇
  2012年   109篇
  2011年   108篇
  2010年   104篇
  2009年   100篇
  2008年   84篇
  2007年   114篇
  2006年   119篇
  2005年   80篇
  2004年   56篇
  2003年   55篇
  2002年   58篇
  2001年   31篇
  2000年   41篇
  1999年   100篇
  1998年   686篇
  1997年   434篇
  1996年   269篇
  1995年   160篇
  1994年   137篇
  1993年   180篇
  1992年   31篇
  1991年   27篇
  1990年   31篇
  1989年   37篇
  1988年   24篇
  1987年   26篇
  1986年   18篇
  1985年   16篇
  1983年   5篇
  1982年   10篇
  1981年   11篇
  1980年   15篇
  1978年   6篇
  1977年   59篇
  1976年   135篇
  1975年   7篇
  1973年   2篇
  1955年   3篇
排序方式: 共有4264条查询结果,搜索用时 156 毫秒
11.
12.
BACKGROUND: This study examines the relationship between income, health insurance, and usual source of care characteristics and screening and management of hypertension. METHODS: This is a secondary analysis of data from the 1987 National Medical Expenditure Survey. Adult survey respondents constitute a sample representative of the total adult noninstitutionalized US population. Screening, follow-up care, and pharmacologic treatment for hypertension were examined among low income individuals, the uninsured, those without a usual source of care place, and those without a particular usual source of care physician. RESULTS: The uninsured, individuals without a usual source of care place, and those without a particular usual source of care physician received less screening, follow-up care, and pharmacologic treatment for hypertension. Income did not affect receipt of hypertensive care. CONCLUSIONS: Lack of health insurance and lack of a usual source of care are barriers to hypertensive care. Policies that increase access to health insurance or to usual source of care physicians may enable more individuals to attain control of hypertension.  相似文献   
13.
A novel strategy was utilised to assess the risk to health from exposure to polycyclic aromatic hydrocarbons (PAHs). Ambient monitoring was carried out by personal sampling. Urinary thioethers (UTh) and urinary 1-hydroxypyrene (1-HP) were utilised for biological monitoring. Urinary d-glucaric acid (UDGA) and sister chromatid exchange (SCE) in peripheral blood lymphocytes were used as biological effect markers. The population was categorised into exposed and control groups according to the external dose of PAHs. The excretion of 1-HP in the controls over the 3-day period showed a relatively stable baseline, while the exposed showed a significant increase over the same period of time. SCE frequency in the exposed population was significantly different from controls.  相似文献   
14.
本文着重对图象多分辨率技术进行分析,并提出了相应的系统结构成框架,同时还探讨了以图象多分辨率处理为核心,采用了 新的压缩编码的可能性。  相似文献   
15.
OBJECTIVE: The purpose of this study was to compare click-evoked otoacoustic emissions (COAEs) of subjects having similar auditory thresholds but different age ranges. It is well known that elevated hearing thresholds are common with increasing age and that deterioration of outer hair cells is often noted in cases of hearing loss due to increased age. It has also been reported that evoked otoacoustic emissions (EOAEs) decrease with increased age. However, there is still some question whether changes in EOAEs with aging are associated with the increased hearing threshold or whether there is some additional effect of aging that enfluences EOAEs. DESIGN: COAE input/output functions and synchronized spontaneous OAEs (SSOAEs) were measured in two groups of subjects having similar auditory thresholds, one ranging in age from 19 to 29 yr, the other ranging in age from 40 to 61 yr. Mixed-model ANOVAs were performed to determine whether there were any statistically significant differences in COAEs based on age group. RESULTS: There were no statistically significant differences in COAE level or COAE threshold between age groups. Significant differences in COAEs were found for subjects based on whether they had measurable SSOAEs, regardless of age. CONCLUSIONS: Age does not significantly reduce COAE level nor increase COAE threshold. Other factors, such as presence of SSOAEs and hearing loss, undoubtedly have more influence on COAEs than the factor of age.  相似文献   
16.
17.
Although the good appearance and biocompatibility of dental porcelains, failures are still of considerable concern because of the limited properties to all ceramic system. Physical properties that might be considered ideal include high strength, resistance to abrasion, and resistance to the hostile oral environment. Porcelain has been considered by many of its physical characteristic are similar to those of enamel. In 1983 a new modality of treatment, the etched porcelain restoration was introduced by Simonsen and Calamia. Numerous investigations have shown the strength of the etched porcelain bonded to composite resin and also the clinical success of this porcelain to be used as laminated veneers and etched inlays and onlays.  相似文献   
18.
19.
Through TCD test, observation on the blood flow dynamics change before and after treatment with 31 cases of the child cerebral atrophy was made. It is found that scalp therapy is able to speed up the blood flow of part artery, especially increase the even blood flow speed of MCA, ACA obviously (P < 0.05), which preliminary shows the mechanism of scalp therapy for child cerebral atrophy mean while. It is found that scalp therapy also has the function to restrain epilepsy.  相似文献   
20.
BACKGROUND AND PURPOSE: A rapid but transient expression of c-fos after cerebral ischemia has been extensively documented. However, the mechanism of this induction and whether induction of c-fos is neuroprotective or detrimental to the brain after ischemia is presently not clear. Fasting before transient cerebral ischemia has been shown to reduce delayed neuronal necrosis and infarct volume. The purpose of the present study was to examine the effect of preischemic fasting for 24 hours on the expression of c-fos after transient focal cerebral ischemia. METHODS: Focal cerebral ischemia was induced by temporary occlusion of the right middle cerebral artery and both common carotid arteries for 60 minutes. Male Long-Evans rats weighting 250 to 300 g were randomly divided into two groups: fed (control group) and food deprived for 24 hours (fasted group) before ischemic surgery. Infarct volumes were measured on the basis of triphenyltetrazolium chloride-delineated infarct areas, and plasma glucose levels were determined by the glucose oxidase method. Temporal and spatial expression of c-fos was assessed by Northern blot analysis, in situ hybridization, and immunohistochemistry. RESULTS: Fasting for 24 hours before 60 minutes of ischemia resulted in a 26.6% decrease in preischemic plasma glucose levels and a 74.5% reduction in infarct volumes in the fasted group compared with the control group. A rapid but transient induction of c-fos mRNA was observed in the ischemic cortex in control animals after 60 minutes of ischemia. Fasting not only prolonged but also enhanced the intensity of c-fos expression in the ischemic cortex. Regional c-fos expression was also different between these two groups. CONCLUSIONS: The results support the contention that c-fos expression may be compatible with its purported neuroprotective role in selected experimental paradigms. The signaling mechanisms underlying the effect of fasting and subsequent lowering of plasma glucose levels on postischemic c-fos expression remain to be explored.  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