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   875篇
  免费   33篇
电工技术   7篇
综合类   7篇
化学工业   175篇
金属工艺   79篇
机械仪表   14篇
建筑科学   56篇
矿业工程   1篇
能源动力   32篇
轻工业   75篇
水利工程   2篇
无线电   70篇
一般工业技术   164篇
冶金工业   102篇
原子能技术   5篇
自动化技术   119篇
  2022年   4篇
  2021年   9篇
  2020年   14篇
  2019年   5篇
  2018年   16篇
  2017年   13篇
  2016年   17篇
  2015年   22篇
  2014年   23篇
  2013年   50篇
  2012年   50篇
  2011年   64篇
  2010年   46篇
  2009年   54篇
  2008年   58篇
  2007年   40篇
  2006年   42篇
  2005年   34篇
  2004年   34篇
  2003年   22篇
  2002年   19篇
  2001年   15篇
  2000年   24篇
  1999年   18篇
  1998年   21篇
  1997年   16篇
  1996年   16篇
  1995年   13篇
  1994年   10篇
  1993年   12篇
  1992年   7篇
  1990年   5篇
  1989年   9篇
  1988年   6篇
  1987年   6篇
  1986年   5篇
  1985年   9篇
  1984年   5篇
  1983年   7篇
  1982年   6篇
  1981年   7篇
  1980年   8篇
  1978年   5篇
  1977年   10篇
  1975年   3篇
  1974年   4篇
  1973年   3篇
  1970年   2篇
  1969年   2篇
  1955年   4篇
排序方式: 共有908条查询结果,搜索用时 15 毫秒
41.
Biological aging research is expected to reveal modifiable molecular mechanisms that can be harnessed to slow or possibly reverse unhealthy trajectories. However, there is first an urgent need to define consensus molecular markers of healthy and unhealthy aging. Established aging hallmarks are all linked to metabolism, and a ‘rewired’ metabolic circuitry has been shown to accelerate or delay biological aging. To identify metabolic signatures distinguishing healthy from unhealthy aging trajectories, we performed nontargeted metabolomics on skeletal muscles from 2-month-old and 21-month-old mice, and after dietary and lifestyle interventions known to impact biological aging. We hypothesized that common metabolic signatures would highlight specific pathways and processes promoting healthy aging, while revealing the molecular underpinnings of unhealthy aging. Here, we report 50 metabolites that commonly distinguished aging trajectories in all cohorts, including 18 commonly reduced under unhealthy aging and 32 increased. We stratified these metabolites according to known relationships with various aging hallmarks and found the greatest associations with oxidative stress and nutrient sensing. Collectively, our data suggest interventions aimed at maintaining skeletal muscle arginine and lysine may be useful therapeutic strategies to minimize biological aging and maintain skeletal muscle health, function, and regenerative capacity in old age.  相似文献   
42.
The aim of this study was exploration of the genetic background of conjunctival melanoma (CM) and correlation with recurrent and metastatic disease. Twenty-eight CM from the Rotterdam Ocular Melanoma Study group were collected and DNA was isolated from the formalin-fixed paraffin embedded tissue. Targeted next-generation sequencing was performed using a panel covering GNAQ, GNA11, EIF1AX, BAP1, BRAF, NRAS, c-KIT, PTEN, SF3B1, and TERT genes. Recurrences and metastasis were present in eight (29%) and nine (32%) CM cases, respectively. TERT promoter mutations were most common (54%), but BRAF (46%), NRAS (21%), BAP1 (18%), PTEN (14%), c-KIT (7%), and SF3B1 (4%) mutations were also observed. No mutations in GNAQ, GNA11, and EIF1AX were found. None of the mutations was significantly associated with recurrent disease. Presence of a TERT promoter mutation was associated with metastatic disease (p-value = 0.008). Based on our molecular findings, CM comprises a separate entity within melanoma, although there are overlapping molecular features with uveal melanoma, such as the presence of BAP1 and SF3B1 mutations. This warrants careful interpretation of molecular data, in the light of clinical findings. About three quarter of CM contain drug-targetable mutations, and TERT promoter mutations are correlated to metastatic disease in CM.  相似文献   
43.
The validity of traffic intensity near the home as an estimate for the personal long-term exposure to traffic-related air pollution in an adult population was tested. Personal and near-home outdoor exposure to PM2.5, soot, NO, NO2, and NOx was monitored four to five times during 48 h periods in older adults. A group of 23 participants lived in high traffic intensity streets (>10000 vehicles/(24 h)), and 22 lived in low traffic intensity streets. The relation between average personal exposure and traffic intensity at the residential address was explored by taking indoor sources into account. Large differences in the measured outdoor concentrations between locations in high traffic and low traffic intensity streets were found for soot (68%), NO (127%), and NOx (35%). Differences were smaller for PM2.5 (14%) and NO2 (22%). Slightly elevated ratios were found for personal exposure to soot (1.15; 95% confidence interval (CI), 1.01-1.30)when comparing adults living in high traffic intensity streets with adults living in low traffic intensity streets. For NO, increased personal exposure (1.16) was seen for the same comparison, but this difference failed to reach statistical significance (CI, 0.80-1.66). Traffic intensity on the street of residence predicted personal exposure to soot but not to PM2.5 or nitrogen oxides. Traffic intensity may not correlate well to personal exposure and accordingly substantial misclassification of exposure may occur when traffic intensity is used as an exposure indicator in epidemiological studies. Time spent in traffic and spending time outdoors were associated with increased personal exposure of soot and PM2.5, but not NOx.  相似文献   
44.
Reed canary grass (Phalaris arundinacea) is one of the possible raw materials for bioenergy production in northern Europe. Its cultivation is favoured because its high productivity and local origin. However, problems with the biomass quality for combustion have been reported. Usually delayed harvest in spring is suggested to improve the quality and decrease the moisture content of biomass. On the other hand, the feasibility of spring harvest depends on local climatic conditions and may cause yield losses. In current paper we studied reed canary grass fields in Estonia locating on different soil types and cultivated with various varieties. The influence of several fertilisation schemes on biomass yield was analysed. Our results indicated that production was higher on mineral soils than on the organic soil of abandoned peat extraction sites. Even different types of fertilisation did not increase the production on organic soils to the level comparable to those on mineral soils. Among studied varieties ‘Venture’ had the highest production. The highest yield per area was obtained late in the autumn (12.7 t d.w. per ha and 7.2 t d.w. per ha on mineral and organic soils, respectively). By spring the amount of biomass had decreased in all studied sites. Due to wet soil some of the fields remained unharvested, the others had high yield losses during practical harvesting. The chemical analyses did not reveal significant differences in the composition of biomass between late autumn and spring. Therefore we conclude that late autumn harvest should be preferred in local climatic conditions.  相似文献   
45.
To protect consumers from Salmonella infection acquired through the consumption of pork meat, it is necessary to eradicate Salmonella from pork. In order to achieve this, the whole pork production chain should be free from Salmonella, including the pigs at the farm. In epidemiological studies it was concluded that the use of fermented feed plays a significant role in the reduction of Salmonella prevalence in pig farms. However, the mechanism of Salmonella reduction in fermented feed is not known. A controlled feed fermentation was performed using a pure culture of Lactobacillus plantarum. pH reduction, organic acid profiles and bacterial counts were determined. In L plantarum‐fermented feed, lactic acid and acetic acid were produced and the pH dropped to a value below 4.0. Antimicrobial products (bacteriocins) could not be detected. The results showed that the produced lactic and acetic acid and the pH in the feed are responsible for Salmonella reduction in fermented feed. L plantarum did not show any other antimicrobial effect on Salmonella. © 2000 Society of Chemical Industry  相似文献   
46.
Abuse of market power by dominant generation firms is a growing concern in worldwide electricity markets. This paper argues that relying only on general competition rules—as is the case in most European countries—is insufficient and that complementary ex-ante regulation is needed. In particular, regulators should incentivize firms to sign contracts with retailers by regulating their risk exposure. In a simulation model we show that this type of regulation can significantly reduce the deadweight loss in the market, without imposing large costs on regulatees.  相似文献   
47.
In the context of future dynamic applications, systems will exhibit unpredictably varying platform resource requirements. To deal with this, they will not only need to be programmable in terms of instruction set processors, but also at least partial reconfigurability will be required. In this context, it is important for applications to optimally exploit the memory hierarchy under varying memory availability. This article presents a mapping strategy for wavelet-based applications: depending on the encountered conditions, it switches to different memory optimized instantations or localizations, permitting up to 51% energy gains in memory accesses. Systematic and parameterized mapping guidelines indicate which localization should be selected when, for varying algorithmic wavelet parameters. The results have been formalized and generalized to be applicable to more general wavelet-based applications.  相似文献   
48.
We introduce a simple and practical method for repairing inconsistent databases. Given a possibly inconsistent database, the idea is to properly represent the underlying problem, i.e., to describe the possible ways of restoring its consistency. We do so by what we call signed formulae, and show how the ‘signed theory’ that is obtained can be used by a variety of off-the-shelf computational models in order to compute the corresponding solutions, i.e., consistent repairs of the database. *This paper is a revised and extended version of [9].  相似文献   
49.
In order to investigate historical shifts in attitudes toward mental health and mental health services, two independent samples of older adults separated by a 14-year time interval were administered questionnaires. There were 91 in the 1977 sample (mean age 69.91 yrs) and 116 in the 1991 sample (mean age 71.94 yrs). Four newly created, internally consistent scales assessed multiple dimensions of their mental health attitudes (breadth of conceptions, bias, openness to help, range of problems). Analyses suggested that the younger cohorts of older adults held more positive attitudes toward mental health and mental health services than the older cohorts. These cohort differences remained when controlled for age, level of education, self-reported health, and income. These data indicate a positive cohort shift in attitudes toward mental health, a finding with numerous implications for the design and implementation of mental health services for future cohorts of older persons. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
50.
This paper reviews the recent progress in using single quantum systems, here mainly single fluorescent molecules, as local probes for nano-optical field distributions. We start by discussing the role of the absorption cross-section for the spatial resolution attainable in such experiments and its behaviour for different environmental conditions. It is shown that the spatial distribution of field components in a high-numerical aperture laser focus can be mapped with high precision using single fluorescent molecules embedded in a thin polymer film on glass. With this proof-of-principle experiment as a starting point, the possibility of mapping strongly confined and enhanced nano-optical fields close to material structures, e.g. sharp metal tips, is discussed. The mapping of the spatial distribution of the enhanced field at an etched gold tip using a single molecule is presented as an example. Energy transfer effects and quenching are identified as possible artefacts in this context. Finally, it is demonstrated that the local quenching at a sharp metal structure nevertheless can be exploited as a novel contrast mechanism for ultrahigh-resolution optical microscopy with single-molecule sensitivity.  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