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   3160篇
  免费   92篇
  国内免费   18篇
系统科学   118篇
丛书文集   3篇
教育与普及   2篇
理论与方法论   26篇
现状及发展   1002篇
研究方法   34篇
综合类   2074篇
自然研究   11篇
  2024年   21篇
  2023年   27篇
  2022年   9篇
  2020年   14篇
  2017年   11篇
  2016年   29篇
  2015年   56篇
  2014年   28篇
  2013年   22篇
  2012年   144篇
  2011年   162篇
  2009年   6篇
  2007年   10篇
  2006年   17篇
  2005年   199篇
  2004年   506篇
  2003年   467篇
  2002年   142篇
  2001年   76篇
  2000年   138篇
  1999年   75篇
  1992年   37篇
  1991年   25篇
  1990年   35篇
  1989年   34篇
  1988年   32篇
  1987年   16篇
  1986年   26篇
  1985年   35篇
  1984年   26篇
  1983年   24篇
  1982年   15篇
  1981年   11篇
  1980年   16篇
  1979年   61篇
  1978年   39篇
  1977年   30篇
  1976年   29篇
  1975年   38篇
  1974年   56篇
  1973年   49篇
  1972年   50篇
  1971年   66篇
  1970年   54篇
  1969年   68篇
  1968年   59篇
  1967年   41篇
  1966年   55篇
  1965年   41篇
  1964年   11篇
排序方式: 共有3270条查询结果,搜索用时 15 毫秒
31.
The existence of sulfur in the petroleum would erode the transport lines and poison the chemical catalysts. In addition, it is the main cause of air pollution and the for-mation of acid rain. At present, hydrodesulfurization (HDS) is the commonly used technology for fuel desulfu-rization. HDS has the disadvantages of high cost of ma-nipulation and equipment investment. Furthermore, HDS is difficult to remove the sulfur from condensed thio-phenes such as dibenzothiophene (DBT). These forms…  相似文献   
32.
汽车离合器传动片是离合器传递载荷的重要零件 ,承受复杂载荷 ,而汽车离合器传动片设计方法的研究却不多 笔者分析了汽车离合器传动片工作时的各种受力情况及其变形量的大小 ,给出了传动片设计计算的双端固定梁的力学模型和进行强度、稳定性校核准则 ,并进行了应力和变形试验 ,试验结果与理论计算基本吻合  相似文献   
33.
0 IntroductionXiantaoHanjiangRiverHighwayBridgelocatesonnorth westsuburbofXiantaocity ,crossesovertheHanjiangRiv er.Thecable stayedbridgeisasingle towersystemwiththespansof (5 0 + 82 + 180 )m .Thenon symmetricalstructureisusedheresothatthemainpiercankeepoffthemainchannel.Thesectionofthegirderisnotconstantinorderthatthedeadloadcanbeinbalancealthoughthespansaredifferent.Thedeckwidthis 2 3m (notincludingthewidthofthecablelane) ,laterallayoutisthatofdouble directionfour lanewithoutsidewalk ,b…  相似文献   
34.
During the past several years, the research and development of InP material has made great progress due to serving as the substrate for most optoelectronic devices operating at the communications wavelength of 1.31 and 1.55 μm. At present, InP has become an important semiconductor material together with Si and GaAs. When compared to GaAs, InP has higher electron velocity, higher radiation hardness and better heat-conducting property. The advantage of InP crystal material allows higher frequency operation and lower power requirements. Therefore, InP is widely being used for the manufacture of microwave devices, high-frequency devices and optoelectronic integrated circuits (OEICs) which are indispensable for wireless technology, satellite communications[1—3]. Although n-type and p-type InP can meet actual needs, semi-insulating InP substrates remain to be improved due to their poor uniformity and consistency. For this reason, several possible approaches have been reported to the preparation of SI InP by wafer annealing under different conditions[4—9].  相似文献   
35.
In the present work, it was studied that nanoosized NiO electrodes doping with C and Co were prepared by electrochemical deposition in an ethanol solution containing nickel nitrate and cobalt nitrate and sintering processes. The capacitance performance of such electrodes was also studied. The results reveal that samples obtained by electrochemical deposition first on a porous nickel cathode in an ethanol solution contain 0.5 mol/L nitrates, in which Ni(NO3)2 and Co(NO3)2 were mixed in a molar proportion of 8 : 2. After sintering at 300℃ for 2 h, they exhibited the best performance. XRD analysis illustrates that the essential phase of the product is NiO doping with certain content of C and Co. The particles of product imaged a spherical morphology with sizes of 20---30 nm in diameter, and with an average crystal size of 4--5 nm. The capacitance measurement of the simulating C/NiO capacitors indicates that a single electrode can reach as high as 410.0 F/g, and still maintain the value of 323 F/g after 30 recycles showed as a stabilized value with increasing recycles, when it was discharged at a current of 5 mA.  相似文献   
36.
Advances and applications of electrohydrodynamics   总被引:1,自引:1,他引:1  
Electrohydrodynamics (EHD) has been applied in many areas, such as EHD enhanced heat transfer, EHD pump, electrospray mass spectrometry, electrospray nanotechnology, etc. The basic theories underlying these applications and a brief review of the EHD advances are presented in this paper. As electrospray is an important branch of EHD,the applications of electrospray are particularly discussed.Finally, this paper gives some suggestions for the future research and development.  相似文献   
37.
The effects of lanthanum at different concentrations on the related photosynthetic activities of Hill reaction, Mg^2+-ATPase and Ca^2+-ATPase in spinach chloroplast were studied. Experimental results showed that lanthanum can increase all the activities at suitable concentration (15-30 mg· L^-1), however, it behaves toxically on them when over used (60 mg. L^-1). To get an improved understanding of the mechanism of lanthanum effects on the photosynthesis of spinach, the different subcomponents in the chloroplast of the cultured spinach were isolated, and the content of lanthanum in each subcomponent was determined by ICP-MS. The results obtained indicated that among these different subcomponents, about 90% out of the total chloroplast lanthanum was located in photosystem Ⅱ (PS Ⅱ) while there was little lanthanum in photosystem Ⅰ (PS Ⅰ). Moreover, size exclusion high performance liquid chromatography (SE-HPLC) coupled with online UV and ICP-MS detections was novelly used for locating lanthanum binding sites in PS Ⅱ proteins for the first time. It was found that lanthanum has two binding sites in PS Ⅱ: La associates with chlorophyll together with magnesium in PS Ⅱ by partly replacing magnesium and also shares the common binding sites of PS Ⅱ proteins together with the inorganic cofactors of calcium and manganese, influencing the process of photosynthesis.  相似文献   
38.
An angiosperm compression flora is found in Palaeogene from Lanzhou Basin and the cuticular analysis of Populus davidiana Dode in the flora is carefully made. Furthermore, the fossil cuticles are compared with the epidermal structures of extant Populus leaves growing in different environments, i.e. moist, semimoist, and semiarid to arid climatic regions. The present experiments indicate that mature leaves of P. davidiana show leaf size from big to small, leaf cuticles from thick to thin and anticlinal walls of epidermal cells from faintness to clarity along with the increase of lattudes of the plant distributions, the climatic variation from moist to arid, the annual precipitation from more to less and the annual mean temperature from high to low. The fossil P. davidiana differs from the specimens collected from Shandan in semiarid to arid climatic regions but closely resembles the Wushan leaves in a semi-moist climatic area in a lot of features. In a word, the new research may reflect that the flora lives in a semi-moist climatic environment. The present discovery of compression of Paleogene Populus davidiana is of great significance to studying vegetation types, climatic and environmental changes during the primal uplifting of the Qinghai-Tibet Plateau.  相似文献   
39.
Removal of NOx(DeNOx, NOx is the total of NO and NO2) from flue gas by radical injection has been investigated .the discharge characteristics were examined and the steady streamer corona was acquired hy adjusting the nozzle gases properly. It was Found that an increase in the voltage resulted in a decrease in the NO concentration and the concentration of the NO2 increased at tow voltages but decreased as the voltage rose to a certain level. The DeNOx efficiency increased as the applied voltage rose and reached a maximum of 70% when the voltage approached the breakdown voltage. The hypothetical mechanism of NOx removal suggested that the radicals formed in the discharge process converted the NO and NO2 into acidic species. The Monte Carlo method was used to calculate the rate coefficients and the productivity of the radicals, and then the concentrations of both NO and NO2 and the DeNOx effielencies were calculated with chemical kinetics. The calculated DeNOx efficiencies were coalparable with the experimental DeNOx efficiencies at low voltages, but were lower at high voltages.  相似文献   
40.
主要讨论利息税的政策属性.政府对利息税作用的预期并就进一步优化利息税政策提出变20%的统一比例税率为超额累进税率;将利息税收入用于政府购买支出;将利息税政策改变为弹性税收政策等建议.  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