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   2596篇
  免费   61篇
  国内免费   2篇
电工技术   17篇
综合类   1篇
化学工业   248篇
金属工艺   25篇
机械仪表   31篇
建筑科学   131篇
矿业工程   3篇
能源动力   63篇
轻工业   331篇
水利工程   28篇
石油天然气   5篇
无线电   138篇
一般工业技术   191篇
冶金工业   1224篇
原子能技术   8篇
自动化技术   215篇
  2022年   7篇
  2021年   20篇
  2020年   9篇
  2019年   19篇
  2018年   28篇
  2017年   30篇
  2016年   36篇
  2015年   25篇
  2014年   36篇
  2013年   80篇
  2012年   64篇
  2011年   86篇
  2010年   77篇
  2009年   42篇
  2008年   91篇
  2007年   72篇
  2006年   67篇
  2005年   83篇
  2004年   60篇
  2003年   57篇
  2002年   38篇
  2001年   38篇
  2000年   48篇
  1999年   70篇
  1998年   401篇
  1997年   249篇
  1996年   150篇
  1995年   86篇
  1994年   90篇
  1993年   69篇
  1992年   29篇
  1991年   33篇
  1990年   24篇
  1989年   25篇
  1988年   26篇
  1987年   15篇
  1986年   16篇
  1985年   22篇
  1984年   14篇
  1983年   9篇
  1982年   18篇
  1981年   15篇
  1980年   14篇
  1979年   10篇
  1978年   7篇
  1977年   35篇
  1976年   66篇
  1975年   7篇
  1974年   7篇
  1973年   11篇
排序方式: 共有2659条查询结果,搜索用时 828 毫秒
31.
We report the clinical and pathologic findings of two patients with sporadic visceral myopathy. Both presented with chronic intestinal pseudo-obstruction that necessitated colectomy. Microscopically, typical changes of primary visceral myopathy were present, including variable fibrous replacement of the muscularis externa and vacuolar degeneration of myocytes. In addition, the muscle cells contained cytoplasmic inclusions that have only been recently reported in visceral myopathy. These inclusions were numerous and easily visible in routine hematoxylin-eosin-stained sections but greatly enhanced by periodic acid-Schiff staining. They were reactive immunohistochemically at their periphery and were seen to be myofibrils at various stages of degeneration on electron microscopy. Inclusions were present in both muscularis externae and muscularis mucosae and were identified in mucosal biopsy specimens, providing a means of diagnosing this type of myopathic change on endoscopic biopsies.  相似文献   
32.
A test chamber has been developed in order to provide a small and simple emission testing facility capable of testing construction products in a climate where the important climatic parameters such as temperature, ventilation rate and air velocity can be varied independently around typical indoor values. The test chamber CLIMPAQ is made of panes of window glass. Other main surface materials are stainless steel and eloxated aluminium. The chamber has a volume of 50.9 litres and is designed to meet the requirements for quantifying air pollution. In this investigation human subjects acted as air pollution judges, and chemical characterization of the air pollution was carried out. Carpet, linoleum, wall paint and seal- ant were tested simultaneously in the CLIMPAQ and in four other chambers ranging from a full-scale chamber of 28 m3 to a field and laboratory emission cell of 3.5· 10?5m3. Product ranking is the same in all chambers for the sensory measurements. Emission rates based on sensory measurements differ for all products less than 100 % except for tests in a 3-litre chamber where emission rates were higher. Chemical measurements differ up to approximately 10 times for the same product in different chambers. Deviations appear to be the result of different environmental parameters in the various chambers. Low air concentrations or high specific ventilation rates seem to increase emissions, while differences in air velocities and sink properties may also be the cause of differences in emission rates.  相似文献   
33.
In a double-blind, double-dummy, randomized, multi-centre study, the effects of bendroflumethiazide vs. enalapril on blood pressure, glycaemic control, lipoprotein concentrations and albuminuria were compared in non-proteinuric, hypertensive type 2 diabetic patients; they were treated for 20 weeks with either bendroflumethiazide 2.5-5.0 mg (n = 59) or enalapril 10-20 mg (n = 55). Age, fasting plasma glucose, HbA1c and BMI were similar in the groups. Systolic and diastolic blood pressure were reduced in both groups. Bendroflumethiazide was accompanied by minor but significant elevations in fasting plasma glucose and serum C-peptide. HbA1c was increased during both treatments. Lipoproteins and urinary albumin/creatinine ratio were stable. Bendroflumethiazide caused a decrease in serum potassium and an increase in serum urate. No significant correlations were observed between the decline in blood pressure and changes in the metabolic risk factors. Baseline levels of age, sex, BMI, blood pressure or urinary albumin/creatinine ratio were not related to changes in blood pressure, metabolic parameters or urinary albumin/creatinine ratio.  相似文献   
34.
The dissociative chemisorption of methane on a Ni(100) single crystal has been studied under thermal conditions as a function of pressure and temperature. The initial sticking coefficient was measured in the pressure range of 0.010–7.0 mbar at temperatures ranging from 375 to 500 K. A strong pressure dependence was observed, consistent with a direct dissociation mechanism under these thermal conditions. This was further confirmed by experiments where the gas at a low pressure was heated by a thermal finger facing the crystal surface. With the thermal finger at the same temperature as the surface, it was possible to ensure that the methane was fully equilibrated to the crystal and an activation energy of 59±1.5 kJ/mol was determined under isothermal conditions.  相似文献   
35.
36.
Personal exposure in a displacement ventilated room is examined. The stratified flow and the considerable concentration gradients necessitate an improvement of the widely used fully mixing compartmental approach. The exposure of a seated and a standing person in proportion to the stratification height is examined by means of full-scale measurements. A breathing thermal manikin is used to simulate a person. It is found that the flow in the boundary layer around a person is able to a great extent to entrain and transport air from below the breathing zone. In the case of non-passive, heated contaminant sources, this entrainment improves the indoor air quality. Measurements of exposure due to a passive contaminant source show a significant dependence on the flow field as well as on the contaminant source location. Poor system performance is found in the case of a passive contaminant released in the lower part of the room close to the occupant. A personal exposure model for displacement ventilated rooms is proposed. The model takes the influence of gradients and the human thermal boundary layer into account. Two new quantities describing the interaction between a person and the ventilation are defined.  相似文献   
37.
38.
Tooth loss diminishes oral function and quality of life, and national health targets aim to reduce population levels of tooth loss. OBJECTIVES: The purpose of this study was to determine tooth loss incidence and predictors of tooth loss among older adults in South Australia. METHODS: Data were obtained from a cohort study of a stratified random sample of community-dwelling dentate people aged 60+ years. Interviews and oral examinations were conducted among 911 individuals at baseline and among 693 of them (76.1%) 2 years later. Incidence rates and relative risks were calculated for population subgroups and multivariate logistic regression was used to construct risk prediction models. A method was developed to calculate 95% confidence intervals (95% CI) for relative risks (RR) from logistic regression models using a Taylor series approximation. RESULTS: Some 19.5% (95% CI = 15.4-23.6%) of people lost one or more teeth during the 2 years. Men, people with a recent extraction, people who brushed their teeth infrequently, smokers and people born outside Australia had significantly (P < 0.05) greater risk of tooth loss. Baseline clinical predictors of tooth loss included more missing teeth, retained roots, decayed root surfaces, periodontal pockets and periodontal recession. In a multivariate model that controlled for baseline clinical predictors, former smokers (RR = 2.55, 95% CI = 1.48-4.40) and current smokers (RR = 2.06, 95% CI = 0.92-4.62) had similarly elevated risks of tooth loss compared with non-smokers. CONCLUSIONS: The findings from this population suggest that a history of smoking contributes to tooth loss through mechanisms in addition to clinical disease processes alone.  相似文献   
39.
Abstract This study deals with the modeling of air pollution in apartments from laboratory measurements of source strengths, using formaldehyde and Total Volatile Organic Compounds (TVOCs) as model pollutants. The sources in two test apartments were grouped into two: building-related sources and occupant-related sources. The measured source strengths and ventilation rates were used for the prediction of concentrations expected in the apartments. These predictions were compared to measurements in the apartment over 12 months. The conclusions were that the model predictions based on emission rates measured in the laboratory can be used to predict the long-term concentration of the two model pollutants in the apartments. Considering the measured differences in ventilation between the apartments, an occupant emission rate of between 0.2 and 0.3 mg/h/kg body weight could be estimated. Based on previous suggested limits of acceptable exposures of humans to VOCs, an acceptable average emission rate of VOCs from building materials in general was estimated to be about 30 (μ/m2/h. The modeling showed that during the first 200 days, building materials dominated the emissions. After this, sources relating to the occupants dominated. On average about half of the VOC pollution originated from the building materials.  相似文献   
40.
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