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   716篇
  免费   9篇
  国内免费   2篇
电工技术   5篇
化学工业   44篇
金属工艺   21篇
机械仪表   11篇
建筑科学   23篇
矿业工程   3篇
能源动力   21篇
轻工业   56篇
水利工程   4篇
无线电   73篇
一般工业技术   108篇
冶金工业   279篇
原子能技术   3篇
自动化技术   76篇
  2021年   5篇
  2019年   10篇
  2018年   5篇
  2017年   5篇
  2016年   9篇
  2014年   7篇
  2013年   65篇
  2012年   14篇
  2011年   17篇
  2010年   23篇
  2009年   21篇
  2008年   23篇
  2007年   27篇
  2006年   19篇
  2005年   16篇
  2004年   12篇
  2003年   10篇
  2002年   15篇
  2001年   8篇
  2000年   11篇
  1999年   18篇
  1998年   59篇
  1997年   38篇
  1996年   30篇
  1995年   20篇
  1994年   12篇
  1993年   18篇
  1992年   12篇
  1991年   13篇
  1990年   12篇
  1989年   14篇
  1988年   8篇
  1987年   6篇
  1986年   10篇
  1985年   5篇
  1984年   4篇
  1983年   5篇
  1982年   9篇
  1981年   11篇
  1980年   9篇
  1979年   9篇
  1977年   8篇
  1976年   17篇
  1975年   7篇
  1974年   6篇
  1973年   6篇
  1971年   4篇
  1969年   4篇
  1968年   4篇
  1967年   7篇
排序方式: 共有727条查询结果,搜索用时 15 毫秒
51.
Research on work-related stress has tended to focus on males and to neglect gender as a variable; often, findings from studies of men are incorrectly generalized to women. The failure to "build women in" to conceptual models has impaired our understanding of both work and family role stressors. This article focuses on assumptions, gaps, and biases in the literature; the home, for example, has been viewed as a stress-free sanctuary, whereas workplace stress has been overemphasized and seen as particularly dangerous for women. To better understand the costs and benefits of employment and of multiple roles for women, and the stressfulness of family roles, more attention to the qualitative aspects of roles is needed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
52.
Models approaching consumer expectations of their water supplier from a risk perspective suggest that consumers primarily and overwhelmingly want safe drinking water supply. In this study consumer preferences in the water sector are investigated in two contrasting case studies: Cyprus, where there have been significant quantity and continuity of supply issues, and Riga, where there have been water quality issues. While water quality is undoubtedly the main priority of water consumers in Riga, in Cyprus consumers indicate that they prioritise a more reliable service even though many are sufficiently dissatisfied with water quality that they do not drink the tap water. The analysis of consumer attitudes in the two case studies suggests that when water supply is unreliable, reliability takes precedence; once it is reliable quality issues come to the fore.  相似文献   
53.

Background

Many studies have illustrated that ambient air pollution negatively impacts on health. However, little evidence is available for the effects of air pollution on cardiovascular mortality (CVM) in Tianjin, China. Also, no study has examined which strata length for the time-stratified case-crossover analysis gives estimates that most closely match the estimates from time series analysis.

Objectives

The purpose of this study was to estimate the effects of air pollutants on CVM in Tianjin, China, and compare time-stratified case-crossover and time series analyses.

Method

A time-stratified case-crossover and generalized additive model (time series) were applied to examine the impact of air pollution on CVM from 2005 to 2007. Four time-stratified case-crossover analyses were used by varying the stratum length (Calendar month, 28, 21 or 14 days). Jackknifing was used to compare the methods. Residual analysis was used to check whether the models fitted well.

Results

Both case-crossover and time series analyses show that air pollutants (PM10, SO2 and NO2) were positively associated with CVM. The estimates from the time-stratified case-crossover varied greatly with changing strata length. The estimates from the time series analyses varied slightly with changing degrees of freedom per year for time. The residuals from the time series analyses had less autocorrelation than those from the case-crossover analyses indicating a better fit.

Conclusion

Air pollution was associated with an increased risk of CVM in Tianjin, China. Time series analyses performed better than the time-stratified case-crossover analyses in terms of residual checking.  相似文献   
54.

Background

A number of epidemiological studies have examined the adverse effect of air pollution on mortality and morbidity. Also, several studies have investigated the associations between air pollution and specific-cause diseases including arrhythmia, myocardial infarction, and heart failure. However, little is known about the relationship between air pollution and the onset of hypertension.

Objective

To explore the risk effect of particulate matter air pollution on the emergency hospital visits (EHVs) for hypertension in Beijing, China.

Methods

We gathered data on daily EHVs for hypertension, fine particulate matter less than 2.5 μm in aerodynamic diameter (PM2.5), particulate matter less than 10 μm in aerodynamic diameter (PM10), sulfur dioxide, and nitrogen dioxide in Beijing, China during 2007. A time-stratified case-crossover design with distributed lag model was used to evaluate associations between ambient air pollutants and hypertension. Daily mean temperature and relative humidity were controlled in all models.

Results

There were 1,491 EHVs for hypertension during the study period. In single pollutant models, an increase in 10 μg/m3 in PM2.5 and PM10 was associated with EHVs for hypertension with odds ratios (overall effect of five days) of 1.084 (95% confidence interval (CI): 1.028, 1.139) and 1.060% (95% CI: 1.020, 1.101), respectively.

Conclusion

Elevated levels of ambient particulate matters are associated with an increase in EHVs for hypertension in Beijing, China.  相似文献   
55.
56.
Solar cells utilizing thin-film polycrystalline silicon can achieve photovoltaic power conversion efficiencies greater than 19%. These high efficiencies are a result of light trapping and back surface passivation. Optimum silicon thickness, for devices employing such technology, has been determined to be 20 μm (Blakers et al., Appl. Phys. Lett. 60 (1992) 2572) to 35 μm (Rand et al., Proceedings of the IEEE Photovoltaic Specialist Conference, Orlando, FL, May 1990, p. 263). Low cost is achieved by minimizing the required amount of silicon feedstock per watt of power output. The use of an electrically insulating supporting substrate allows for monolithic, series connected sub-modules. A solar cell with a 20 μm thick polycrystalline Si-layer on a ceramic substrate, utilizing both light trapping and back-surface passivation, was fabricated and characterized. A short-circuit current of 25.8 mA/cm2 was measured and verified by the National Renewable Energy Laboratory (NREL).  相似文献   
57.
Cast-mono crystalline silicon wafers contain crystallographic defects, which can severely impact the electrical performance of solar cells. This paper demonstrates that applying hydrogenation processes at moderate temperatures to finished screen print cells can passivate dislocation clusters within the cast-mono crystalline silicon wafers far better than the hydrogenation received during standard commercial firing conditions. Efficiency enhancements of up to 2% absolute are demonstrated on wafers with high dislocation densities. The impact of illumination to manipulate the charge state of hydrogen during annealing is investigated and found to not be significant on the wafers used in this study. This finding is contrary to a previous study on similar wafers that concluded increased H or H0 from laser illumination was responsible for the further passivation of positively charged dangling bonds within the dislocation clusters.  相似文献   
58.
59.
This paper discusses a method for fault detection and isolation (FDI) in continuous dynamic systems. A key aspect of this approach is the coupling of a qualitative diagnosis engine and a monitoring system that computes symbolic feature values through a signal-to-symbol transformation on the continuously sampled measurement data. Signal analysis techniques with a sound statistical basis are employed to generate reliable symbolic data. The methodology is evaluated on the diagnosis of engineered faults in the cooling system of an automobile engine that has been instrumented with temperature and pressure sensors. Results show the interdependency between modeling for diagnosis and the feature extraction system  相似文献   
60.
OBJECTIVE: Use of the image-guided stereotactic brain biopsy has facilitated the diagnosis of previously inaccessible lesions with both safety and reliability. However, few studies have assessed the diagnostic yield of frozen section evaluation of the initial stereotactic target (FS-0). We describe our experience with 188 stereotactic brain biopsies in order to evaluate the diagnostic yield of FS-0. DESIGN: Retrospective study of 188 stereotactic brain biopsies from 185 patients. SETTING: Tertiary referral center with a high volume of neurosurgical cases including image-guided stereotactic brain biopsies. PATIENTS: One hundred eighty-five patients who underwent imaged-guided stereotactic brain biopsy over a 58-month period. RESULTS: The patients studied included 107 males and 78 females (mean age 48 years). Eleven (6%) biopsies were nondiagnostic. Diagnoses from FS-0 included a neoplastic condition in 96 (73%) of 131 cases and a nonneoplastic condition in 23 (50%) of 46 cases. In 119 (67%) of 177 cases, a diagnosis was reached at FS-0. A correct diagnosis was made on subsequent frozen section in 28 (16%) of cases, including 21 (16%) of 131 neoplasms and 7 (15%) of nonneoplastic conditions. In 15 (54%) of 28 cases, the correct diagnosis was made on the second frozen section; in 25 (89%) of 28, the correct diagnosis was made by the fourth frozen section. In 14 (11%) of 131 neoplastic cases, a sampling error relative to the lesion resulted in an inaccurate diagnosis at FS-0. A significant error in diagnosis occurred in three cases (1.7%). CONCLUSIONS: We conclude that (1) because 58 (33%) of 177 diagnosed cases in our series would have been potentially misdiagnosed if only one biopsy had been taken at the stereotactic target, frozen section evaluation or cytologic examination of material at the time of surgery should be performed routinely to ensure that adequate tissue has been obtained for purposes of diagnosis; (2) taking up to four biopsies increases the diagnostic yield (from 67% to 89% in this series); and (3) neoplastic lesions are more likely to be definitively diagnosed at FS-0 than non-neoplastic lesions.  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