首页 | 官方网站   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   257976篇
  免费   3867篇
  国内免费   1056篇
工业技术   262899篇
  2021年   1887篇
  2020年   1387篇
  2019年   1706篇
  2018年   2629篇
  2017年   2649篇
  2016年   2932篇
  2015年   2226篇
  2014年   3730篇
  2013年   12138篇
  2012年   6921篇
  2011年   9884篇
  2010年   7596篇
  2009年   8571篇
  2008年   9101篇
  2007年   9129篇
  2006年   8470篇
  2005年   7654篇
  2004年   7369篇
  2003年   7248篇
  2002年   6746篇
  2001年   7170篇
  2000年   6450篇
  1999年   6975篇
  1998年   17006篇
  1997年   11761篇
  1996年   8864篇
  1995年   6744篇
  1994年   5904篇
  1993年   5724篇
  1992年   4082篇
  1991年   3867篇
  1990年   3633篇
  1989年   3445篇
  1988年   3346篇
  1987年   2643篇
  1986年   2552篇
  1985年   3234篇
  1984年   2849篇
  1983年   2612篇
  1982年   2385篇
  1981年   2439篇
  1980年   2253篇
  1979年   2078篇
  1978年   1902篇
  1977年   2227篇
  1976年   2775篇
  1975年   1615篇
  1974年   1504篇
  1973年   1580篇
  1972年   1155篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The pattern synthesis techinque described allows the power pattern to be optimized in a very useful sense over the desired operating band of the antenna array. This is an improvement over previous techniques involving constraints on such artificial parameters as source norm and superdirectivity ratio, which do not, in fact, ensure a useful pattern bandwidth. The procedure described provides minimum mean square error over an arbitrary number of discrete frequencies  相似文献   
992.
From 1978 to 1992, 121 cases of postinfarction left ventricular aneurysm (99 males, 22 females, mean age 60 years) were operated on. The authors insist on a high rate of clinical arhythmogenicity (31.4%) and associated mechanical complications (21%). 76% of patients were in functional NYHA class III or IV. Resection was performed in 90% of patients, plication in 10%. 58% underwent coronary artery bypass grafting (1.7 graft/patient), 16% encircling ventriculotomy, 8% mitral valve replacement and 13% closure of ventricular septal defect. Operative mortality was 14.9% (10% when other mechanical complications where excluded). 5-year survival is 67.9%. Late cardiac deaths are as follow: left ventricular failure (1.8% A/P), Sudden death (1.4% A/P), Myocardial infarction (0.6% A/P). 82% of survivals are in functional NYHA class I or II. Only functional class NYHA III or IV is predictive of late death. We conclude that postinfarction left ventricular aneurysm remains a high risk complication especially when associated with other mechanical complications. When arhythmogenicity is present we suggest rhythmologic surgery and in all cases, complete revascularization.  相似文献   
993.
A design-for-testability scheme for detecting CMOS analog faults was reported by Favalli et al. (see ibid., vol.25, no.5, p.1239-46, 1990). The authors propose two alternative designs, one for small circuits and another for large circuits, which require significantly less area overhead (about 1/4 to 1/3) than that of Favalli's design. With the proposed modification in the first design, the untestable problem, which occurred in Favalli's design, can be alleviated. Furthermore, the proposed schemes are also fit to be implemented in VLSI circuits  相似文献   
994.
This paper discusses the need for interface management within construction, with particular reference to building facade interfaces. It categorizes the three different types of interface management—physical, contractual, and organizational. The implication of interface management in construction contracting is discussed; it is explained how the U.K. procurement trends have changed, affecting the interface management within projects. U.K. cladding procurement is very fragmented, and the paper maps the development of a typical cladding system showing the complexities of interface management. Finally, the paper discusses CladdISS, a U.K. government funded research project to develop a standardized strategy for the design and management of window and cladding interfaces. It explains the principles of CladdISS with reference to process maps, action plans, management strategy, bibliography, advice on standards, materials, maintenance, joints, movement, and tolerances. This will enable strategies to be developed to avoid the endemic interface problems occurring on-site. It will also guide the management of the interfaces throughout the project.  相似文献   
995.
996.
A 10-year-old German shepherd dog presented with a periodontal 10 mm interproximal defect between the left mandibular fourth premolar and first molar teeth. Bone graft removed from the caudoventral portion of the ipsilateral hemimandible was placed in the defect as a component of the periodontal treatment plan. The use of bone graft likely contributed to periodontal pocket reduction.  相似文献   
997.
A new indirect fluorescent-antibody (IFA) assay with antigen produced in vitro in the human promyelocytic leukemia cell line HL60 was used to identify the first recognized case of human granulocytic ehrlichiosis in Rhode Island. This IFA assay was used to detect granulocytic ehrlichiae in white-footed mice and in a dog inhabiting the area surrounding the patient's residence. Host-seeking Ixodes scapularis ticks found in the same habitat also were infected. I. scapularis ticks collected from other locations were fed on dogs and New Zealand White rabbits to assess the competency of these species as hosts of granulocytotropic Ehrlichia. Tick-induced infections of dogs were confirmed by serologic testing, tissue culture isolation, and PCR amplification, whereas several rabbits seroconverted but were PCR and culture negative. PCR amplification of the 16S rRNA gene and DNA sequencing of the PCR products or culture isolation was used to confirm granulocytic Ehrlichia infections in humans, dogs, white-footed mice, and ticks.  相似文献   
998.
BACKGROUND AND PURPOSE: Studies on risk factors for stroke have been less intensive than those for coronary disease. Only a few studies have addressed the question of the role of heredity in the occurrence of stroke. We analyzed whether a positive parental history of cardiovascular disease predicts the risk of stroke independently from other risk factors and whether the role of parental history varies by age and stroke subtypes. METHODS: This study was a prospective follow-up of 14371 middle-aged men and women. A positive parental history of cardiovascular disease was defined as either stroke or coronary disease before the age of 60 years. The end point of the follow-up was an incident case of stroke. Multivariate analyses were performed with the Cox proportional hazards model. RESULTS: The risk ratio of stroke after multifactorial adjustment (age, smoking, blood pressure, cholesterol, diabetes, and education) associated with a positive parental history of stroke was 1.89 (P = .004) in men and 1.80 (P = .007) in women. The association between parental history of stroke and the risk of stroke was stronger among subjects aged 25 to 49 years than among older subjects. Parental history of coronary disease was not associated with the risk of stroke in men, but in women it had a borderline significant association with the risk of ischemic stroke. CONCLUSIONS: A positive parental history of stroke predicted the risk of stroke independently from the other risk factors.  相似文献   
999.
In mouse Peyer's patches (PP), dendritic cells (DC) are localized in T cell areas as NLDC145+ CD11c+ cells, and in the dome and corona region of the follicle as NLDC145- CD11c+ cells, respectively, suggesting the presence of two different DC populations with distinct roles in antigen uptake, processing, and presentation. However, it is not clear how this relates to DC maturation. In this report, we demonstrate that freshly-isolated CD11c+ DC have the properties of immature DC since they endocytose soluble antigens, phagocytose particulate material such as latex beads, synthetize major histocompatibility complex (MHC) class II and invariant chain, but, at the same time, display low stimulatory activity for resting T cells, as shown in mixed-lymphocyte reaction and oxidative mitogenesis assays. When cultured for 24 h in the presence of the cytokines granulocyte-macrophage colony-stimulating factor and tumor necrosis factor or anti-CD40, the cells undergo dramatic phenotypic and functional changes characteristic of DC maturation. After 24 h stimulation in vitro, CD11c+ cells lose the ability to take up proteins such as ovalbumin, and in parallel with this decline, the biosynthesis of MHC class II and invariant chain is dramatically down-regulated or eliminated. On the other hand cells treated in vitro exhibit on the cell surface higher levels of MHC class II, of co-stimulatory molecules (CD80, CD86), of adhesion molecules (CD44, intercellular adhesion molecule-1), and acquire expression of the interdigitating DC surface marker NLDC145. Concomitantly, the ability to stimulate naive T cells drastically increased after in vitro treatment with both stimuli. Taken together, our results indicate that the majority of DC in the PP are immature in terms of their antigen-uptake capacity. These sentinel antigen presenting cells are strategically positioned at the dome region of PP, where antigens are transcytosed via the M cells from the gut lumen. A second population of mature interdigitating NLDC145+ CD11c+ DC stimulates naive unprimed T cells in interfollicular areas by up-regulation of surface ligands and accessory signals.  相似文献   
1000.
In the last decade the 'comet assay' or 'single cell gel electrophoresis assay' has been established as a sensitive method for the detection of DNA damage and the measurement of its recovery. The results published in the literature have often been obtained with different methods for comet structure measurement. In most cases these data are not comparable with each other. Even when using similar systems for the analysis, it is difficult to obtain matching data. This presentation will describe some technical aspects of our measurement equipment and evaluation software. It focuses on necessary experimental conditions to minimize errors in obtaining such data. The software developed here allows the rapid analysis of the microscopic samples (< 2 s per image). The image analysis was designed with respect to the morphological shapes of comet cells, which were investigated with a confocal laser microscope. The system is built with standard components which are commercially available. As a measure of the amount of DNA damage the ratio of fluorescence intensity was used inside the comet tail and the fluorescence intensity of the comet head. Other parameters such as DNA content, comet area, head radius, tail length and tail moment are also determined. The reproducibility of the system has been evaluated in several experiments over a period of 5 years.  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-23

京公网安备 11010802026262号