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   3480篇
  免费   23篇
  国内免费   3篇
电工技术   20篇
化学工业   532篇
金属工艺   62篇
机械仪表   71篇
建筑科学   91篇
矿业工程   7篇
能源动力   179篇
轻工业   390篇
水利工程   7篇
石油天然气   16篇
无线电   71篇
一般工业技术   392篇
冶金工业   1343篇
原子能技术   10篇
自动化技术   315篇
  2024年   71篇
  2023年   34篇
  2022年   48篇
  2021年   92篇
  2020年   71篇
  2019年   61篇
  2018年   122篇
  2017年   106篇
  2016年   116篇
  2015年   82篇
  2014年   119篇
  2013年   301篇
  2012年   96篇
  2011年   124篇
  2010年   149篇
  2009年   113篇
  2008年   100篇
  2007年   98篇
  2006年   44篇
  2005年   29篇
  2004年   31篇
  2003年   16篇
  2002年   22篇
  2001年   9篇
  2000年   11篇
  1999年   40篇
  1998年   369篇
  1997年   234篇
  1996年   148篇
  1995年   79篇
  1994年   84篇
  1993年   79篇
  1992年   16篇
  1991年   25篇
  1990年   23篇
  1989年   24篇
  1988年   16篇
  1987年   35篇
  1986年   26篇
  1985年   23篇
  1984年   9篇
  1983年   7篇
  1982年   18篇
  1981年   20篇
  1980年   22篇
  1978年   6篇
  1977年   39篇
  1976年   82篇
  1975年   5篇
  1974年   4篇
排序方式: 共有3506条查询结果,搜索用时 0 毫秒
1.
The authors report the case of a woman admitted to hospital for minor trauma of the left hip and who presented major ST segment elevation on the second day suggesting an acute anterior wall myocardial infarction at the same time as a cerebrovascular accident. Complementary investigations and follow-up excluded the diagnosis of myocardial infarction with normalisation of the ECG on the 5th day. ECG changes during cerebrovascular accidents may be very variable ranging from extrasystoles to ST elevation mimicking myocardial infarction. They carry a poor prognosis with an increased risk of sudden death necessitating continuous ECG monitoring until the ECG reverts to normal. The cardiac involvement is not ischaemic but due to disease of the insular cortex of the brain which induces myocytolysis (centered around the intra-cardiac nerve endings) due to the sudden liberation of catecholamines. These lesions may be treated by propranolol or phentolamine.  相似文献   
2.
A new, powerful method of analysis, involving the combined use of finite integral transform and finite element techniques, is presented for the solution of time dependent heat flow systems composed of many one-dimensional elements connected through the nodes. This method leads to an eigenvalue problem which is not of the conventional Sturm-Liouville type. A procedure for the determination of the eigenvalues is described. The solution obtained is in the form of an infinite series and contains quasi-steady and transient terms. The general solution obtained can be applied in the mathematical modelling of many engineering applications such as the determination of the penetration of the daily temperature cycle into buildings, the analysis of heat transfer in array of extended surfaces in compact heat exchangers, and many others.  相似文献   
3.
Belonging to the Annonaceae family, marolo (Annona coriaceae) is a native species of the Brazilian "cerrado" región (Minas Gerais, Goiás and Distrito Federal) and can be found in South American tropical zones. Its fruits are highly consumed by local people and commercialized in markets or street stalls. There is, however, a tendency for the extinction of marolo due to deforestation and the large scale plantation of monocultures instead of native plants. The literature still offers no data on the chemical composition of the proximate composition and vitamin C, A and tannin contents were carried out on the yellow marolo pulp as well as the determination of the physico-chemical characteristics of the seed oil. Five batches of fruit from the Alfenas region--south of Minas Gerais State--were analysed in this work and their average composition were: humidity 77%, total sugar 15%, reducing sugar 11%, crude protein 1%, lipids 3%, fiber 5% and fixed mineral residue 1%. The contents of vitamin C and A were 8.2 mg/100g and 117.5 RE/100g, respectively, and the tannin content was 245 mg/100g. The results showed high fiber and lipid contents of marolo pulp in comparison with many other tropical fruit pulps. The vitamin C contents were equivalent to those found in avocado, pineapple and watermelon, while the vitamin A contents were equivalent to papaya, peach, guava and several other tropical fruits. Marolo seed contains 45% of oil on a dry basis. Its composition and physico-chemical characteristics showed the possibility of producing a good quality oil, with great potential for the fine oil market. However the presence of alkaloids in the oil needs to be further studied. Their elimination could be done by refining or extraction in a continuous press. The results exalt the high quality of marolo pulp, showing that the preservation of native species should be stimulated.  相似文献   
4.
5.
How to design the pilot tones that are used in channel estimation has a significant effect on the estimation performance. To achieve good performance in least square (LS) algorithm, we propose the artificial bee colony (ABC) algorithm for optimizing the placement of pilot tones in MIMO–OFDM systems. We also derive the upper bound of mean square error of LS estimation with the help of Gerschgorin disc theorem for fitness function of ABC algorithm. The results show that designing pilot tones using the ABC algorithm outperforms other considered placement strategies in terms of high system performance and low computational complexity.  相似文献   
6.
In a wireless multihop broadcasting scenario, a number of relay nodes may cooperate the source node in order to improve the capacity of the network. However, the imposition of total energy and maximum hop constraints to this system in a practical setting. In this paper, we study an ad-hoc network with infinitely many nodes and analytically find the number and positions of rebroadcasting relay nodes to achieve the optimal broadcast capacity. The interference due to multiple transmissions in the same geographical area is taken into account. According to the results of this theoretical model, we propose two heuristics, one distributed and one centralized, as suboptimal but practical solutions to the relay selection problem in wireless multihop broadcasting. We discuss the broadcast capacity performances and CSI (channel state information) requirements of these algorithms. The results illustrate that the benefits of peer-assisted broadcasting are more pronounced in the centralized relay selection algorithm when compared to the fully randomized and distributed selection under a realistic system model.  相似文献   
7.
An exergy-based thermoeconomic optimization application is applied to a subcooled and superheated vapor compression refrigeration system. The advantage of using the exergy method of thermoeconomic optimization is that various elements of the system—i.e., condenser, evaporator, subcooling and superheating heat exchangers—can be optimized on their own. The application consists of determining the optimum heat exchanger areas with the corresponding optimum subcooling and superheating temperatures. A cost function is specified for the optimum conditions. All calculations are made for three refrigerants: R22, R134a, and R407c. Thermodynamic properties of refrigerants are formulated using the Artificial Neural Network methodology.  相似文献   
8.
Irradiation combined with a conventional cooking procedure was applied to meatball and the effects on bacterial load and inoculatedAeromonas hydrophila were determined. Meatball samples were irradiated by using a60Co source at the dose levels of 0, 0.30,0.75,1.50,2.50 kGy and cold stored at 4±1°C for 7 days. Bacterial load and the count ofA. hydrophila decreased when the irradiation dose level increased. A minimum inhibition effect was found at the dose of 0.30 kGy. Irradiation in combination with a conventional cooking procedure was found to be more effective in reducingA. hydrophila and the bacterial load in meatball. This study indicated that a dose of 0.75 kGy was sufficient to destroy approximately 104 cfu/g ofA. hydrophila in meatball.  相似文献   
9.
The susceptibility to cold cracking in steel welds can be assessed by using carbon equivalent formulae based on the composition of steel being welded. In this study, carbon equivalent formulae available in literature have been tested for their sufficiency on what they can measure in weld metal properties by performing calculations using compositions of structural shielded metal arc welding (SMAW) steel weld metals. A set of correlation data was produced to demonstrate the level of relationship between the microstructural and mechanical properties of weld metals and carbon equivalent values. It is shown that carbon equivalent formulae are designed to measure certain mechanical performance of the steel weld metals as well as their hardness rather than microstructural content. A new formula is also proposed to predict weldability by including Ti content of the steel weld metal.  相似文献   
10.
The anticonvulsant activity of furanocoumarins, coumarin mixture and the essential oil obtained from the fruits of Heracleum crenatifolium was examined against maximal electroshock (MES)-induced seizures in mice. Bergapten showed significant anticonvulsant activity. The furanocoumarins isolated from the fruits of the plant were identified using thin-layer chromatography, melting points and spectroscopic methods (IR, MS, 1H NMR) as isobergapten (1), pimpinellin (2), bergapten (3), isopimpinellin (4), sphondin (5) and byak-angelicol (6). The essential oil content of the fruits were found as 5.5%. Twenty-two compounds representing 99.3% of the essential oil obtained from the fruits of H. crenatifolium were determined and the major components were identified as octanol and octyl acetate (3.1% and 88.4% respectively) by GC and GC–MS.  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