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   283篇
  免费   0篇
电工技术   2篇
化学工业   25篇
金属工艺   7篇
机械仪表   5篇
建筑科学   5篇
能源动力   2篇
轻工业   12篇
水利工程   2篇
无线电   29篇
一般工业技术   15篇
冶金工业   160篇
原子能技术   1篇
自动化技术   18篇
  2020年   3篇
  2013年   2篇
  2012年   1篇
  2011年   3篇
  2009年   4篇
  2008年   6篇
  2007年   8篇
  2006年   4篇
  2005年   2篇
  2004年   3篇
  2003年   3篇
  2002年   2篇
  2001年   3篇
  2000年   4篇
  1999年   10篇
  1998年   47篇
  1997年   15篇
  1996年   17篇
  1995年   5篇
  1994年   12篇
  1993年   8篇
  1992年   5篇
  1991年   3篇
  1990年   3篇
  1989年   10篇
  1988年   6篇
  1987年   9篇
  1986年   9篇
  1985年   8篇
  1984年   7篇
  1983年   2篇
  1982年   2篇
  1981年   6篇
  1980年   3篇
  1979年   4篇
  1978年   9篇
  1977年   5篇
  1976年   13篇
  1975年   1篇
  1974年   1篇
  1972年   1篇
  1971年   1篇
  1970年   1篇
  1968年   1篇
  1962年   1篇
  1961年   1篇
  1957年   1篇
  1956年   3篇
  1955年   2篇
  1954年   1篇
排序方式: 共有283条查询结果,搜索用时 78 毫秒
11.
We present a simple distributed algorithm that resolves store-and-forward deadlocks in data communication networks. The basic idea of the algorithm is to detect cycles of nodes that may cause store-and-forward deadlocks, and to rotate packets along these cycles. The algorithm uses a fixed amount of storage in each node for its execution, and, under reasonable assumptions upon the routing and packet handling, it ensures that packets that enter the network arrive at their destinations in finite time.Part of this work was done while this author was on sabbatical leave with IBM, Thomas J. Watson Research Center, Yorktown Heights, NY 10598, USA.  相似文献   
12.
In the early days of the dispute resolution movement—the ‘80s and ‘90s—the concern was often expressed that we were developing a system of “second‐class justice” for those who couldn't afford the courts. The wealthy, of course, would continue to have access to the “first‐class justice” that the public courts provided.  相似文献   
13.
Data from several clinical samples and the Minnesota Multiphasic Personality Inventory–2 standardization group are presented to familiarize the reader with response patterns of different groups on a new validity scale designed to assist in the identification of exaggeration or fabrication of psychological disturbance. Sensitivity-specificity analyses are included along with suggestions for use of the F(p) Scale with other validity scales. Cautions about setting single cutoff scores are also discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
14.
OBJECTIVE: To evaluate the histopathologic changes that occur in human small intestine or time when preserved in Viaspan organ preservation solution. DESIGN: Short segments of human small intestine were placed in standard organ preservation solution (Viaspan) and stored in conditions that mimic the clinical situation associated with clinical organ procurement, preservation, and transplantation. The intestinal segments were removed at sequential time points and placed in 10% formalin. Specimens underwent histopathologic examination to determine time-related changes. SPECIMENS: Short intestinal segments were obtained from seven multiorgan cadaver donors. Specimens were obtained in a way that exactly mimicked small intestinal organ retrieval. RESULTS: Small intestinal histology remained normal for the first 6 hours. After 6 hours, vacuolar separation began to occur between the epithelium and the basement membrane in the upper half of the villi. After 9 hours of cold preservation, epithelial detachment extended deep into the crypts with occasional shedding of cells and villi. CONCLUSIONS: Currently used small intestinal preservation using Viaspan results in considerable histopathologic changes in human jejunum after 9 hours of cold storage. The histopathologic pattern appears normal for the first 6 hours and suggests that preservation times should be limited to this time period when possible.  相似文献   
15.
Objective: To compare the impact of network support and different types of perceived functional support on all-cause mortality or nonfatal reinfarction for patients with a recent acute myocardial infarction (AMI). Design: Participants were recruited from the Enhancing Recovery in Coronary Heart Disease (ENRICHD) trial; 2,481 AMI patients with depression or low social support were randomized to a cognitive-behavioral intervention or to a usual care control group. Data collection for certain measures of social support was limited: 2,466 participants completed the ENRICHD Social Support Inventory; 2,457 completed the Perceived Social Support Scale; 1,296 completed the Social Network Questionnaire; and 707 completed the Interpersonal Support and Evaluation List, Tangible Support subscale. Patients also completed the Beck Depression Inventory and were followed for up to 4.5 years. Main Outcome Measure: Time to death or nonfatal reinfarction. Results: Over the follow-up period, 599 patients (24%) died or had a nonfatal AMI. Survival models controlling age, sex, race, socioeconomic status, smoking, antidepressant use, and a composite measure of increased risk revealed that higher levels of perceived social support were associated with improved outcome for patients without elevated depression but not for patients with high levels of depression. Neither perceived tangible support nor network support were associated with more frequent adverse events. Conclusion: AMI patients should be assessed for multiple dimensions of perceived functional support and depression to identify those at increased psychosocial risk who may benefit from treatment. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
16.
Risk factors for Kaposi's sarcoma in HIV-positive subjects in Uganda   总被引:1,自引:0,他引:1  
BACKGROUND: Kaposi's sarcoma (KS) is associated epidemiologically with HIV infection and with human herpesvirus 8 (HHV-8 or KSHV). Both KS and HIV infection are common in Uganda. We conducted a case-control study of 458 HIV-seropositive. Ugandan adults with KS and 568 HIV-seropositive subjects without KS to examine risk factors for HIV-associated KS. METHODS: We recruited newly diagnosed adult KS cases from five hospitals in Kampala, Uganda and controls from a large referral clinic for HIV infection at Mulago Hospital. All cases and controls were counselled and tested for HIV and answered an interviewer-administered questionnaire about their home, socio-economic conditions, lifestyle and sexual behaviour before they became ill. Only HIV-seropositive subjects were included in the analysis. RESULTS: There were 295 males and 163 females with KS and 227 male and 341 female controls. Age distribution was similar but there was a higher proportion of cases (45%) than controls (29%) residing in rural regions of Uganda. KS cases were more likely than controls to have a higher level of education (X2 for trend, 4.8; P = 0.03), to have occupations associated with affluence [chi 2 for heterogeneity, 17.3 on 5 degrees of freedom (df); P = 0.004] and to come from larger settlements [adjusted odds ratio (OR) for settlements of > 1000 versus 10-99 houses, 1.8; 95% confidence interval (CI), 1.1-3.0]. Cases were more likely than controls to have high household income (chi 2 for trend, 32.6; P < 0.001) and other markers of urban or rural wealth such as owning several cows (chi 2 for trend, 9.5; P = 0.002). Cases were more likely to travel away from home (adjusted OR, 1.6; 95% CI, 1.1-2.3) and more likely to have spent increasing time in contact with water (chi 2 for trend, 12.3; P < 0.001). Few indices of sexual behaviour were related to risk of KS, including reported number of sexual partners. Cases were more likely than controls to be married to one rather than several spouses (adjusted OR, 1.6; 95% CI, 1.2-2.2) and to have reported a history of sexually transmitted diseases (STD) (adjusted OR, 1.6; 95% CI, 1.2-2.3). CONCLUSIONS: Among HIV-infected subjects, KS cases are characterized by better education and greater affluence, compared with controls. Urban address, travel away from home, exposure to water, monogamous marriage and self-reported STD were also more frequent among KS cases than controls. The higher socio-economic status of persons with HIV and KS may be a marker for enhanced exposure to a possibly sexually transmitted agent, or for a delayed exposure to a childhood infection. The risk posed by exposure to water among KS cases requires further study.  相似文献   
17.
A single compound with sex pheromone activity was isolated from the female soybean cyst nematode,Heterodera glycines, by a sequence of four high-performance liquid chromatographic steps and identified as vanillic acid by a combination of ultraviolet spectroscopy and chromatography. The structure was confirmed by gas chromatography-mass spectrometry. Both attractancy and coiling behavior in male soybean cyst nematode were elicited by authentic vanillic acid.  相似文献   
18.
Little is known about the effects of depression on adherence to medical treatment regimens in older patients with chronic medical illnesses. Poor adherence may explain the increased risk of medical morbidity and mortality found in depressed medical patients. Ten of 55 patients over the age of 64 with coronary artery disease met the criteria for major depression from the Diagnostic and Statistical Manual of Mental Disorders (3rd ed., rev., American Psychiatric Association, 1987). All patients were prescribed a twice-per-day regimen of low dose aspirin to reduce their risk for myocardial infarction. Medication adherence was assessed for 3 weeks by an unobtrusive electronic monitoring device. Depressed patients adhered to the regimen on 45% of days, but nondepressed patients, on 69% (p?  相似文献   
19.
Ozone is an important air pollutant that affects lung function. In the U.S., the EPA has reduced the allowable O(3) concentrations several times over the last few decades. This puts greater emphasis on understanding the interannual variability and the contributions to surface O(3) from all sources. We have examined O(3) data from 11 rural CASTNET sites in the western US for the period 1995-2009. The 11 surface sites show a similar seasonal cycle and generally a good correlation in the deseasonalized monthly means, indicating that there are large scale influences on O(3) that operate across the entire western US. These sites also show a good correlation between site elevation and annual mean O(3), indicating a significant contribution from the free troposphere. We examined the number of exceedance days for each site, defined as a day when the Maximum Daily 8-h Average (MDA8) exceeds a threshold value. Over this time period, more than half of these sites exceeded an MDA8 threshold of 70 ppbv at least 4 times per year, and all sites exceeded a threshold value of 65 ppbv at least 4 times per year. The transition to lower threshold values increases substantially the number of exceedance days, especially during spring, reflecting the fact that background O(3) peaks during spring. We next examined the correlation between surface O(3) and free tropospheric O(3) in the same region, as measured by routine balloon launches from Boulder, CO. Using ozone measured by the balloon sensor in the range of 3-6 km above sea level we find statistically significant correlations between surface and free tropospheric O(3) in spring and summer months using both monthly means, daily MDA8 values, and the number of surface exceedance days. We suggest that during spring this correlation reflects variations in the flux of O(3) transport from the free troposphere to the surface. In summer, free tropospheric and surface concentrations of O(3) and the number of exceedance days are all significantly correlated with emissions from biomass burning in the western US. This indicates that wildfires significantly increase the number of exceedance days across the western U.S.  相似文献   
20.
A new method for designing signals with a given time-bandwidth product, such that all the signals in the set have a flat amplitude spectrum and have low cross-correlation function values is introduced. It is shown that these signals lie on a signal parameter space ellipse. For a given duration and bandwidth, it is possible to trade off the cardinality of the set for better cross-correlation properties between remaining members of the set. Proof that the cross-correlation between these signals is bounded from above by a factor proportional to the square-root of inverse distance between these signals along one of the axes of the ellipse is given. It is also shown that the cross-correlation between two signals that are furthest apart is bounded by the square-root of inverse time-bandwidth product of the signals in the set. An example design is then given and a comparison of this signal set to some well-known signal sets illustrates that it performs better when the metric is maximum cross-correlation value. The proposed signal set also has the advantage that it can be designed for any time-bandwidth product instead of discrete values  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