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   93695篇
  免费   1760篇
  国内免费   426篇
电工技术   919篇
综合类   2335篇
化学工业   13951篇
金属工艺   5102篇
机械仪表   3419篇
建筑科学   2407篇
矿业工程   572篇
能源动力   1488篇
轻工业   4560篇
水利工程   1302篇
石油天然气   347篇
武器工业   1篇
无线电   10802篇
一般工业技术   18638篇
冶金工业   3294篇
原子能技术   364篇
自动化技术   26380篇
  2023年   153篇
  2022年   230篇
  2021年   395篇
  2020年   265篇
  2019年   270篇
  2018年   14704篇
  2017年   13642篇
  2016年   10347篇
  2015年   996篇
  2014年   740篇
  2013年   908篇
  2012年   3849篇
  2011年   10259篇
  2010年   8876篇
  2009年   6133篇
  2008年   7247篇
  2007年   8199篇
  2006年   537篇
  2005年   1521篇
  2004年   1451篇
  2003年   1430篇
  2002年   735篇
  2001年   285篇
  2000年   364篇
  1999年   228篇
  1998年   317篇
  1997年   185篇
  1996年   172篇
  1995年   87篇
  1994年   99篇
  1993年   74篇
  1992年   69篇
  1991年   66篇
  1990年   39篇
  1989年   39篇
  1988年   35篇
  1987年   24篇
  1969年   29篇
  1968年   45篇
  1967年   35篇
  1966年   48篇
  1965年   44篇
  1963年   28篇
  1960年   31篇
  1959年   38篇
  1958年   41篇
  1957年   36篇
  1956年   34篇
  1955年   64篇
  1954年   68篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
It is experimentally ascertained that light stimulates the negative magnetoresistance observed in a high electric field in silicon doped with boron and manganese. The optimum conditions (the electric field, temperature, illumination, and resistivity of the material) for observation of the largest magnitude of negative magnetoresistance in (Si:B):Mn are determined. The dependence of the negative magnetoresistance on the concentration of compensating impurity is established.  相似文献   
42.
Nanocrystalline LaFeO3 is prepared by the dehydration of coprecipitated lanthanum and iron(III) hydroxides. It is shown that the behavior of the samples during heating and the size distribution of LaFeO3 nanocrystals can be considerably different depending on the scheme used for coprecipitation of lanthanum and iron hydroxides; independently of the method employed for coprecipitation of the initial compounds, sintering of the samples at 950°C leads to the formation of lanthanum orthoferrite crystals up to 100 nm in size.  相似文献   
43.
This study focuses on the relationship between photo-catalytic performance and optical property over Si-incorporated TiO2. The Si-incorporated TiO2 particles exhibited a pure structure of anatase having a particle size of less than 20 nm and surface area of more than 190 m2/g. The absorbance did not shift to a higher wavelength in spite of the incorporation of the Si ions, but the intensity of the photoluminescence (PL) curve was the smallest in the case of the 2.0 mol% Si-TiO2, which was related to the recombination between the excited electrons and holes. Based on these results, the photodecomposition of methyl orange in the liquid reaction was enhanced over the 2.0 mol% Si-incorporated TiO2 compared with that over pure TiO2: Methyl orange at 10.0 ppm was completely decomposed after 100 min when 1.0 g of the 2.0 mol% Si-incorporated TiO2 was used.  相似文献   
44.
Functionalized polyanilines containing biphenyl, terphenyl, carbazole, anthracene, and 4-n-hexylphenyl moieties were synthesized though the reaction of polyaniline in emeraldine base form with sodium salt of corresponding vinylketoaromatics with quantitative yields. Polymers were characterized with Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and thermogravimetry. The emission characteristics of these polymers in N-methyl-2-pyrrolidone solution were examined. The functionalized polyanilines exhibited an intense green emission.  相似文献   
45.
The effects of key parameters on the preparation and regeneration of protoplast from the β-carotene-producing fungus Blakeslea trispora were discussed in this paper, including the combination of various enzymes, mycelial age, digesting time and temperature, pH value, osmotic stabilizers, pretreatment, culture medium and culture method. Under the condition of mixed enzymes in osmotic stabilizer (0.6 M NaCl) combined with 2% lysozyme, 3% cellulase and 3% snailase, the highest protoplast yield, as high as 7.48×106 protoplasts/mL, was obtained when mycelial age was 60 h at pH 5.0–6.0 with digesting for 14–16 h at 28 °C. After purification of the obtained protoplasts, they were regenerated in PDA regenerative medium using bilayer plate culture method. To validate the usability of the protoplasts, a novel plasmid with green fluorescent protein (GFP) was used in transformation for easy visual observation. The results showed that the protoplasts prepared by the optimized method were active and applicable in further gene manipulation experiments. This work was presented at 13 th YABEC symposium held at Seoul, Korea, October 20–22, 2007.  相似文献   
46.
When a horizontal homogeneous solid is melted from below, convection can be induced in a thermally unstable melt layer. In this study the onset of buoyancy-driven convection during time-dependent melting is investigated by using similarly transformed disturbance equations. The critical Rayleigh numbers based on the melt-layer thickness are found numerically for various conditions. For small superheats, the present predictions approach the well known results of classical Rayleigh-Bénard problems, that is, critical Rayleigh numbers are located between 1,296 and 1,708, regardless of the Prandtl number. However, for high superheats the critical Rayleigh number increases with an increase in phase change rate but with decrease in Prandtl number.  相似文献   
47.

Background  

Excess body fat is a major risk factor for disease primarily due to its endocrine activity. In recent years several criteria have been introduced to evaluate this factor. Nevertheless, treatment need is currently assessed only on the basis of an individual's Body Mass Index (BMI), calculated as body weight (in kg) divided by height in m2. The aim of our study was to determine whether application of the BMI, compared to adiposity-based criteria, results in underestimation of the number of subjects needing lifestyle intervention.  相似文献   
48.

Background  

Bioelectrical Impedance (BIA) derived phase angle is increasingly being used as an objective indicator of nutritional status in advanced cancer. Subjective Global Assessment (SGA) is a subjective method of nutritional status. The objective of this study was to investigate the association between BIA derived phase angle and SGA in advanced colorectal cancer.  相似文献   
49.
Cloning of observables, unlike standard cloning of states, aims at copying the information encoded in the statistics of a class of observables rather then on quantum states themselves. In such a process the emphasis is on the quantum operation (evolution plus measurement) necessary to retrieve the original information. We analyze, for qubit systems, the cloning of a class generated by two noncommuting observables, elucidating the relationship between such a process and joint measurements. This helps in establishing an optimality criterion for cloning of observables. We see that, even if the cloning machine is designed to act on the whole class generated by two noncommuting observables, the same optimal performances of a joint measurement can be attained. Finally, the connection with state dependent cloning is enlightened.  相似文献   
50.
Zusammenfassung Dieser Artikel beschreibt einige Erfahrungen und typische Problemstellungen mit Textmining in der Medizin und gibt einen Einblick in aktuelle und zukünftige Herausforderungen in Forschung & Entwicklung. Interessant ist n?mlich, dass auch im ,,Multimedia-Zeitalter“ die meiste Information immer noch als ,,Text“ vorliegt. Mithilfe von statistischen und linguistischen Verfahren wird mit sogenannter ,,Textmining-Software“ versucht, aus Freitexten Information ,,heraus zu schürfen“ (deshalb ,,Textmining“). Allerdings ist es damit noch nicht genug. Der n?chste Schritt besteht darin, die Information sowohl nutzbar als auch brauchbar zu machen. Die jeweiligen End-Benutzerinnen und End-Benutzer müssen in die Lage versetzt werden, auf der Basis der gewonnenen Information deren Wissen zu erweitern. In unserem konkreten Fall sollen damit Entscheidungen im Rahmen ?rztlichen Handelns unterstützt werden. Probleml?sungen in diesem Bereich erfordern eine holistische Sicht- und Herangehensweise. Daher wird es immer wichtiger, Erkenntnisse aus Informatik und Psychologie zusammenflie?en zu lassen und auf systemischer Ebene technologisch umzusetzen.  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