全文获取类型
收费全文 | 21351篇 |
免费 | 1617篇 |
国内免费 | 41篇 |
专业分类
电工技术 | 136篇 |
综合类 | 116篇 |
化学工业 | 5860篇 |
金属工艺 | 458篇 |
机械仪表 | 377篇 |
建筑科学 | 737篇 |
矿业工程 | 59篇 |
能源动力 | 655篇 |
轻工业 | 5065篇 |
水利工程 | 201篇 |
石油天然气 | 171篇 |
武器工业 | 1篇 |
无线电 | 937篇 |
一般工业技术 | 3301篇 |
冶金工业 | 1986篇 |
原子能技术 | 115篇 |
自动化技术 | 2834篇 |
出版年
2024年 | 46篇 |
2023年 | 208篇 |
2022年 | 700篇 |
2021年 | 907篇 |
2020年 | 567篇 |
2019年 | 645篇 |
2018年 | 811篇 |
2017年 | 833篇 |
2016年 | 860篇 |
2015年 | 655篇 |
2014年 | 971篇 |
2013年 | 1793篇 |
2012年 | 1444篇 |
2011年 | 1570篇 |
2010年 | 1192篇 |
2009年 | 1126篇 |
2008年 | 1027篇 |
2007年 | 906篇 |
2006年 | 733篇 |
2005年 | 559篇 |
2004年 | 502篇 |
2003年 | 526篇 |
2002年 | 436篇 |
2001年 | 351篇 |
2000年 | 284篇 |
1999年 | 284篇 |
1998年 | 780篇 |
1997年 | 508篇 |
1996年 | 360篇 |
1995年 | 245篇 |
1994年 | 202篇 |
1993年 | 148篇 |
1992年 | 79篇 |
1991年 | 57篇 |
1990年 | 49篇 |
1989年 | 54篇 |
1988年 | 63篇 |
1987年 | 50篇 |
1986年 | 33篇 |
1985年 | 55篇 |
1984年 | 44篇 |
1983年 | 43篇 |
1982年 | 37篇 |
1981年 | 25篇 |
1980年 | 42篇 |
1979年 | 21篇 |
1978年 | 20篇 |
1977年 | 30篇 |
1976年 | 54篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
K Sugiura M Inaba H Hisha K Borisov EE Sardi?a RA Good S Ikehara 《Canadian Metallurgical Quarterly》1997,15(6):461-468
The objectives of this study were 1) to evaluate interleukin-8 concentrations in cervical secretions in predicting preterm delivery, microbial invasion of the amniotic cavity and histologic chorioamnionitis in patients with preterm labor and intact membranes and 2) to compare the diagnostic value of interleukin-8 with fetal fibronectin determinations in predicting preterm delivery, microbial invasion of the amniotic cavity and histologic chorioamnionitis in patients with preterm labor and intact membranes. Interleukin-8 and fetal fibronectin were assayed in cervical secretions in 106 patients with singleton pregnancies and intact membranes admitted for preterm labor. Amniotic fluid obtained by amniocentesis was cultured and placentas (No = 43) analyzed for the presence of chorioamnionitis. The prevalence of pregnancies delivered preterm was 46.2% (49/106) and 15.09% (16/106) of amniotic fluid cultures were positive. Interleukin-8 levels in cervical secretions were significantly increased in patients who delivered preterm (p < or = 0.0001), in presence of positive amniotic fluid culture (p = 0.0016) and histological chorioamnionitis (p = 0.008) than in patients with negative findings. Receiver-operator characteristics curve analysis showed that an interleukin-8 value > 450 pg/ml is comparable to that of a fetal fibronectin value > 50 ng/ml in predicting preterm delivery (p = 0.247). Among patients who delivered preterm interleukin-8 concentrations > 860 pg/ml predicted a positive amniotic fluid culture with a sensitivity of 81.2% and a specificity 66.6%. Further, in patients who delivered preterm and had a negative amniotic fluid culture, IL-8 concentrations > 480 pg/ml predicted histological chorioamnionitis with a sensitivity 78.5% and specificity 61.5%. A positive fetal fibronectin > 50 ng/ml was not predictve of either a positive amniotic fluid culture or the presence of histological chorioamnionitis. In conclusion, increased concentrations of interleukin-8 and fetal fibronectin are associated with impending delivery and their diagnostic value seems comparable. However, interleukin-8 concentrations identify patients at risk of a positive amniotic fluid culture and the presence of histological chorioamnionitis. Measurement of interleukin-8 in cervical secretion is a non-invasive method to identify patients at risk for both preterm delivery and intrauterine infection. 相似文献
123.
J Caballero Güeto R Arana G Calle FJ Caballero Güeto E García del Río M Sancho C Pi?ero 《Canadian Metallurgical Quarterly》1997,50(12):906-908
Propionibacterium acnes is often considered to be a contaminant but it has also been found to be the principal pathogen in serious infections. P. acnes is a rare cause of infective endocarditis. It has been suggested that aortic root abscesses are caused by bacteria that are particularly virulent. The strongest risk factor for serious infections by this bacteria is the presence of foreign bodies. A case is presented in which endocarditis of a native aortic valve caused by P. acnes was associated with an aortic root abscess. Transesophageal echocardiography is particularly helpful in the diagnosis of this severe complication of infective endocarditis. The literature for P. acnes endocarditis is reviewed. 相似文献
124.
An overview of FDA medical device regulation as it relates to deep brain stimulation devices. 总被引:1,自引:0,他引:1
Carlos Pe?a Kristen Bowsher Ann Costello Robert De Luca Sara Doll Khan Li Marie Schroeder Theodore Stevens 《IEEE transactions on neural systems and rehabilitation engineering》2007,15(3):421-424
The United States Food and Drug Administration (FDA) is charged with assuring the safety and effectiveness of a variety of medical products and the FDA's Center for Devices and Radiological Health is responsible for premarket and postmarket regulation of medical devices. In this paper, we review--from device classification and clinical studies to the final marketing application--FDA's premarket requirements and postmarket requirements as they relate to deep brain stimulation devices. 相似文献
125.
The effects of cooking temperature, time and temperature of brine-salting, and temperature of ripening on the behavior of Enterobacteriaceae in Manchego cheese were studied. Fifty lots of cheese from raw ewe's milk manufactured and ripened under different conditions were investigated throughout a 60-day ripening period. Differences in pH values due to temperature-dependent whey retention accounted for the effect of cooking temperature on coliform and fecal coliform counts. Temperature of brine-salting had no effect on Enterobacteriaceae counts, but a significant effect of salting time on Enterobacteriaceae and fecal coliform counts was detected. Ripening temperature was the manufacturing variable with greatest influence (P<.01) on Enterobacteriaceae and coliform counts during the whole curing period. 相似文献
126.
P Sánchez García 《Canadian Metallurgical Quarterly》1997,114(1):157-63; discussion 164-7
127.
Modelling the rate of secondary succession after farmland abandonment in a Mediterranean mountain area 总被引:1,自引:0,他引:1
Secondary succession after farmland abandonment has become a common process in north Mediterranean countries, especially in mountain areas. In this paper, a methodology is tested which combines Markov chains and logistic multivariate regression to model secondary succession after farmland abandonment in environments where abiotic constraints play a major role, like mountain areas. In such landscapes, a decay in the succession rate with time is usually found, as the best locations are progressively occupied. This is frequently addressed using non-stationary Markov chains. Here, we test if the combination of logistic multivariate regression with Markov chains, however, allows for spatially distributed transitions probabilities based on abiotic factors and therefore, it is able to reproduce the preferential colonization of the most favourable locations. The model is tested in the Ijuez Valley in the Spanish Pyrenees, which underwent generalized land abandonment during the 1950s. Results confirm a substantial improvement in the prediction success of the Markov-logistic model when compared to the standard Markov chain approach. As a result, the decay in the succession rate can be successfully modelled. The specific results for our study area are discussed further in an ecological context. The methodology proposed is applicable to any landscape where vegetation dynamics are constrained by environmental factors. However, the inclusion of land use as an explanatory factor would be necessary in human-managed landscapes. 相似文献
128.
The first Brazilian uranium mine and mill are located on the Poços de Caldas plateau, in the Central State of Minas Gerais. The pre-operational environmental survey was carried out over a period of two years by the Brazilian Nuclear Energy Commission (CNEN), through the Instituto de Radioproteção e Dosimetria (IRD). The selection of the materials to be monitored, the sampling points and the radionuclides to be analysed were based on critical parameters taking into account the meteorological and hydrological characteristics of the site as well as land use. The results obtained are sufficient to characterize the environmental background of the area, and will allow an assessment of the environmental impact due to the operation of the facility and an evaluation of the adequacy of the effluent emmission control measures. In relation to their contribution to the population dose, the results indicated that 226Ra and 210Pb will be the important radionuclides. 相似文献
129.
We describe a patient with aspergillosis of the maxillary antrum, and systemic affectation (anemia, anorexia, fever). Through a Caldwell-Luck approach, the maxillary antrum was opened and an irregular mass removed, postoperatively the patient has done well, disappearing in two months the ORL and systemic symptoms. 相似文献
130.