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   4503篇
  免费   74篇
  国内免费   9篇
电工技术   177篇
综合类   7篇
化学工业   778篇
金属工艺   81篇
机械仪表   67篇
建筑科学   71篇
能源动力   106篇
轻工业   275篇
水利工程   20篇
石油天然气   10篇
无线电   527篇
一般工业技术   707篇
冶金工业   1331篇
原子能技术   127篇
自动化技术   302篇
  2023年   17篇
  2022年   49篇
  2021年   80篇
  2020年   33篇
  2019年   43篇
  2018年   38篇
  2017年   34篇
  2016年   67篇
  2015年   29篇
  2014年   81篇
  2013年   157篇
  2012年   110篇
  2011年   162篇
  2010年   113篇
  2009年   130篇
  2008年   137篇
  2007年   117篇
  2006年   117篇
  2005年   102篇
  2004年   108篇
  2003年   113篇
  2002年   95篇
  2001年   123篇
  2000年   92篇
  1999年   131篇
  1998年   550篇
  1997年   336篇
  1996年   242篇
  1995年   129篇
  1994年   135篇
  1993年   112篇
  1992年   63篇
  1991年   55篇
  1990年   55篇
  1989年   53篇
  1988年   53篇
  1987年   50篇
  1986年   45篇
  1985年   46篇
  1984年   35篇
  1983年   37篇
  1982年   40篇
  1981年   44篇
  1980年   23篇
  1979年   28篇
  1978年   19篇
  1977年   27篇
  1976年   53篇
  1975年   13篇
  1973年   15篇
排序方式: 共有4586条查询结果,搜索用时 15 毫秒
51.
The Phased-Array L-Band SAR (PALSAR) aboard the Advanced Land Observing Satellite (ALOS) is capable of globally acquiring fully polarimetric data. In order to confirm the ability of L-band polarimetric synthetic aperture radar (SAR) to investigate sea ice before the ALOS launch, we conducted a field experiment using an airborne Polarimetric and Interferometric SAR (Pi-SAR) in the Sea of Okhotsk in 1999. This paper presents the analyzed results of data acquired in that experiment. The extracted polarimetric parameters of several ice types suggested that polarimetric coherences and phase differences between right-right (RR) and left-left (LL) are good candidates for discriminating ice types. The polarimetric anisotropy as well as the beta angle of the first eigenvector calculated in the polarimetric decomposition procedure are alternative parameters that are sensitive to ice type differences. Due to the low depolarization characteristics of open water, it could be discriminated from sea ice by scattering entropy in all incidence angle ranges. From the relation between ice thickness and the polarimetric parameters, we found that backscattering coefficients and vertical (VV) to horizontal (HH) backscattering ratio are highly correlated with ice thickness. Since the ratio is sensitive to ice surface dielectric constants, a simple simulation using the integral equation method surface model was conducted by using the physical parameters of typical sea ice. A two-dimensional ice thickness map was derived from an empirical relation between the VV-to-HH backscattering ratio and ice thickness.  相似文献   
52.
High-performance carbon counter electrode for dye-sensitized solar cells   总被引:1,自引:0,他引:1  
Here, we reported that a new carbon electrode prepared with an activated carbon was superior to a Pt sputtered electrode as the counter electrode of dye-sensitized solar cells. The photovoltaic performance was largely influenced by the roughness factor of carbon electrode. The open-circuit voltage increased by about 60 mV using the carbon counter electrode compared to the Pt counter electrode because of positive shift of the formal potential for I3/I couple.  相似文献   
53.
Advanced glycosylation end products (AGEs) accumulate on long-lived extracellular matrix proteins and have been implicated in the micro- and macrovascular complications of diabetes mellitus. Within the arterial wall, AGE-modified proteins increase vascular permeability, inactivate nitric oxide activity, and induce the release of growth-promoting cytokines. Recently developed anti-AGE antibodies were used in an immunohistochemical analysis of coronary arteries obtained from type II diabetic and nondiabetic patients. High levels of AGE reactivity were observed within the atherosclerotic plaque present in vessels from selected patients with diabetes. Considered together with the pathological effects of AGEs on vascular wall homeostasis, these data support the role of advanced glycosylation in the rapidly progressive atherosclerosis associated with diabetes mellitus.  相似文献   
54.
This paper describes frequency control techniques for cold start and channel recognition functions in coherent multichannel local area networks. A Z-cut quartz etalon can be used to stabilize the optical transmitter frequencies. The capture range and the influence of humidity on a Fabry-Perot etalon are discussed. The frequency deviation of the optical transmitter was estimated to be less than ±1 GHz in the long term. A double AFC loop has been proposed for local LD frequency control. It showed a wide capture range of ±100 GHz. Coherent FDM receiver switching to the desired channel was demonstrated over a range from 0 to 40°C in an experiment  相似文献   
55.
56.
57.
BACKGROUND: The relationship between echosonographic patterns of patients with cirrhosis who are antihepatitis C virus (HCV)-positive, the DNA synthesis of hepatocytes, and the risk for HCC were studied. METHODS: Thirty-eight patients with anti-C-100 antibody-positive and Child's grade A posthepatitic cirrhosis were studied. DNA synthesis activity was measured by a bromodeoxyuridine (BrdU, a thymidine analogue)-labeling index (LI), using the BrdU-anti-BrdU in vitro method, and the patients were followed prospectively by frequent liver ultrasonography for 3 years. The ultrasound patterns were classified into fine, coarse, and coarse-nodular (CN) patterns, and the reproducibility of the classification in practical use also was confirmed. RESULTS: Of the 21 patients with high DNA synthesizing cirrhosis (BrdU LI > or = 1.5%), 10 (48%) showed coarse-nodular, 5 (24%) coarse, and 6 (29%) fine pattern in ultrasonography. Conversely, of the 17 patients with low DNA synthesizing LC (BrdU LI < 1.5%), only 1 (6%) showed coarse-nodular, 2 (12%) coarse, and 14 (82%) fine pattern. A significant relationship was found between the two groups of BrdU LI and ultrasound imaging patterns (P < 0.05). The incidence of CN pattern was significantly higher (P < 0.01) in the high DNA synthesizing group than in low DNA synthesizing group. Of the 11 patients with CN pattern by ultrasound imaging, 10 (91%) were in the high DNA synthesizing group, and 9 (82%) developed HCC during the follow-up period, compared with 3 of 7 (43%) with coarse, and only one of 20 (5%) with fine pattern developed HCC. The incidence of HCC was significantly higher (P < 0.01) in patients with a CN cirrhosis pattern than in those with a fine pattern. CONCLUSIONS: In patients with cirrhosis who are anti-HCV-positive, the CN pattern by ultrasound imaging indicates increased DNA synthesis of hepatocytes and a high risk for developing HCC.  相似文献   
58.
STATEMENT OF PROBLEM: Improved dental stone has been widely used for producing dies for the fabrication of restorations with the lost-wax technique. Improved dental stone is used for several reasons, but it is selected most often because of its reasonable cost, ease of use, and ability to produce consistent results. PURPOSE: This study evaluated the ability of an epoxy resin die material and a type IV dental stone to dimensionally reproduce a custom-fabricated metal die. MATERIAL AND METHODS: Dies were fabricated and measurements were made from three reference lines. Measurements were repeated three times for the master die and for the specimen dies. A mean measurement and percent relative change for each dimension was calculated. RESULTS: A significant difference in the relative change for die height was found between the groups studied (p < 0.003). CONCLUSIONS: This epoxy die system will provide a degree of dimensional accuracy comparable to gypsum when used with addition silicone impression material.  相似文献   
59.
60.
A secondary medical area is defined as a medical administrative area under the Medical Care Law by which every prefectural government is expected to provide general medical care supplies such as beds for inpatients. This study evaluated the distribution in medical supplies among the secondary medical areas in Japan and the correlation between medical supply and demand. The numbers of hospital inpatients (residence of patients), physicians, and general beds excluding those for mental disorders and tuberculosis were analysed in relation to the population in all secondary medical areas in Japan. Those data were obtained from surveys conducted by the Ministry of Health and Welfare. The main results were as follows; 1) The distribution of physicians and general hospital beds among the secondary medical areas was not equal. 2) Even adjusted for the sex and age distribution, the highest hospital inpatients/population ratio among the secondary medical areas was four times as large as that of the lowest. 3) With multiple regression models, two independent variables (normal hospital beds/population ratio and aged 65 and over/total population ratio) accounted about 60% of the variance in hospital inpatients/population ratio. 4) The differences in medical supplies among the secondary medical areas may cause the differences in hospital inpatients/population ratio.  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