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   414篇
  免费   4篇
电工技术   7篇
化学工业   100篇
金属工艺   19篇
机械仪表   12篇
建筑科学   3篇
能源动力   13篇
轻工业   19篇
水利工程   1篇
无线电   28篇
一般工业技术   92篇
冶金工业   101篇
原子能技术   6篇
自动化技术   17篇
  2022年   2篇
  2020年   2篇
  2019年   5篇
  2018年   2篇
  2017年   6篇
  2016年   3篇
  2015年   3篇
  2014年   7篇
  2013年   15篇
  2012年   17篇
  2011年   34篇
  2010年   8篇
  2009年   15篇
  2008年   15篇
  2007年   9篇
  2006年   9篇
  2005年   11篇
  2004年   4篇
  2003年   6篇
  2002年   6篇
  2001年   10篇
  2000年   10篇
  1999年   8篇
  1998年   44篇
  1997年   28篇
  1996年   20篇
  1995年   12篇
  1994年   11篇
  1993年   12篇
  1992年   4篇
  1991年   10篇
  1990年   3篇
  1989年   9篇
  1988年   7篇
  1987年   3篇
  1986年   4篇
  1985年   6篇
  1984年   3篇
  1983年   6篇
  1981年   4篇
  1980年   4篇
  1979年   1篇
  1978年   6篇
  1977年   3篇
  1976年   3篇
  1975年   1篇
  1974年   2篇
  1968年   1篇
  1967年   2篇
  1963年   1篇
排序方式: 共有418条查询结果,搜索用时 15 毫秒
71.
Two Au fixed points filled using metal of different nominal purities in carbon crucibles have been developed at the National Research Council Canada (NRC). The primary motivation behind this project was to provide the means for direct thermocouple calibrations at the Au freezing point (\(1064.18~^\circ \hbox {C}\)). Using a Au fixed point filled with the metal of maximum available purity [99.9997 % pure according to glow discharge mass spectroscopy (GDMS)], multiple freezing plateaus were measured in a commercial high-temperature furnace. Four Pt/Pd thermocouples constructed and calibrated in-house were used to measure the freezing plateaus. From the calibration at Sn, Zn, Al and Ag fixed points, the linear deviation function from the NIST-IMGC reference function (IEC 62460:2008 Standard) was determined and extrapolated to the freezing temperature of Au. For all the Pt/Pd thermocouples used in this study, the measured EMF values agree with the extrapolated values within expanded uncertainty, thus substantiating the use of 99.9997 % pure Au fixed point cell for thermocouple calibrations at NRC. Using the Au fixed point filled with metal of lower purity (99.99 % pure according to GDMS), the effect of impurities on the Au freezing temperature measured with Pt/Pd thermocouple was further investigated.  相似文献   
72.
The influence of atmospheric humidity on dry separation of particles was experimentally investigated, making use of the difference in particle motion on an inclined wall, due to different geometrical characteristics. Using a tilted rotating cylinder with blades, glass beads with size and shape distributions were separated according to their geometrical characteristics under various levels of humidity. Fractional recovery of particles is discussed in relation to shape and size of particle and humidity. An angle of repose of the particles was also measured in order to determine the influence of adhesion force, which is closely related to humidity.Glass beads were separated predominantly based on their shape characteristics. Fractional recovery of particles changes strongly around a relative humidity of 60-70%, at which point the angle of repose also changes markedly. The influence of interaction between particles, which becomes significant for smaller particles, was found to reduce the fractional recovery and diminish the influence of humidity.  相似文献   
73.
The effect of the dispersion, ozone treatment and concentration of cup-stacked carbon nanotubes on mechanical, electrical and thermal properties of the epoxy/CSCNT nanocomposites were investigated. Ozone treatment of carbon fibers was found to increase the surface oxygen concentration, thereby causing the contact angle between water, epoxy resin and carbon fiber to be decreased. Thus, the tensile strength, modulus and the coefficient friction of carbon fiber reinforced epoxy resin were improved. Moreover, the dispersion of fibers in polymer was increased and the electrical resistivity was decreased with the addition of filler content. The dynamic mechanical behavior of the nanocomposite sheets was studied. The storage modulus of the polymer was increased by the incorporation of CSCNTs. But the glass transition temperature decreased with increasing fiber loading for the ozone treated fiber composites. The ozone treatment did affect the morphology, mechanical and physical properties of the CSCNT.  相似文献   
74.
We have examined how a growth interruption, caused by closing group-III sources, affects the crystalline quality of InGaN/GaN quantum-well (QW) structures grown by metalorganic vapor phase epitaxy. The QW samples were characterized by their photoluminescence (PL), and by atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy dispersive x-ray (EDX) microanalysis. The PL peak wavelength was strongly dependent on the duration of the growth interruption and on the number of QW layers. AFM measurements revealed that the size of the open hexagonally shaped pits in the QW structures increased dramatically as the interruption duration was lengthened. Through TEM and EDX microanalysis, we found that the formation of these hexahedronal pits, formed due to the growth interruption, causes a large fluctuation in the In composition, especially around the pits, and the presence of such pits in an underlying QW layer strongly affects the In incorporation into the upper QW layers, leading to significant growth-rate variation in an InGaN QW layer and red-shifting of the PL spectra when a multiple-QW structure is grown.  相似文献   
75.
This paper describes a center frequency tuning technique for a second intermediate-frequency (IF) bandpass filter. The utilization of the step signal response ensures the direct measurement of the center frequency. This tuning technique guarantees the accuracy of the design. A 450-kHz Gm-C bandpass filter tuned using this technique has been developed to integrate the IF module for personal digital cellular (PDC) handsets for use in 0.35-μm CMOS. The filter with the tuning scheme achieves a center frequency accuracy of ±0.5%  相似文献   
76.
Two-way satellite time and frequency transfer (TWSTFT) is one of the main techniques used to compare atomic time scales over long distances. To both improve the precision of TWSTFT and decrease the satellite link fee, a new software-defined modem with dual pseudo-random noise (DPN) codes has been developed. In this paper, we demonstrate the first international DPN-based TWSTFT experiment over a period of 6 months. The results of DPN exhibit excellent performance, which is competitive with the Global Positioning System (GPS) precise point positioning (PPP) technique in the short-term and consistent with the conventional TWSTFT in the long-term. Time deviations of less than 75 ps are achieved for averaging times from 1 s to 1 d. Moreover, the DPN data has less diurnal variation than that of the conventional TWSTFT. Because the DPN-based system has advantages of higher precision and lower bandwidth cost, it is one of the most promising methods to improve international time-transfer links.  相似文献   
77.
In this study, the influence of the fluidized bed height on the float–sink of different sized spheres in a gas–solid fluidized bed was investigated. Fluidized beds with heights h = 200, 150, 100 and 50 mm were prepared using a cylindrical column of inner diameter = 290 mm and a mixture of zircon sand and iron powder as the fluidized medium. Float–sink experiments were carried out using density adjusted spheres of diameter Dsp = 40, 30, 20 and 10 mm. It was found that the float–sink performance at Dsp ?20 mm is not affected by the height of the bed, and the sharpness of separation (the density range where spheres neither float nor sink completely) is less than or equal to 200 kg/m3. In the case of Dsp = 10 mm, the sharpness of separation is a larger value (1100 kg/m3 at h = 200 mm), whereas it decreases with decreasing h and is 400 kg/m3 at h = 50 mm. The fluctuation of the surface height of the fluidized bed was visually recorded. The fluctuation is reduced by reducing h. The fluctuation vs. h correlates with the sharpness of separation at Dsp = 10 mm vs. h. These results indicate that the dry float–sink separation of smaller sized spheres is improved as the fluctuation of fluidized bed surface is decreased by reducing the fluidized bed height.  相似文献   
78.
Fatty acids are converted into energy via beta-oxidation. Although almost all natural occurring fatty acids are even-numbered, there are some odd-numbered fatty acids too. The details of the metabolism rate of odd-numbered fatty acids, however, are not clear. In the present study, we simultaneously administered a triacylglycerol containing four types of labeled even-numbered (palmitic acid and stearic acid) and odd-numbered (pentadecanoic acid and heptadecanoic acid) fatty acids to mice to compare the rates of their metabolism. The rates of metabolism were evaluated based on the accumulation of the labeled fatty acids in the small intestine epithelium, liver, and epididymal fat. Odd-numbered fatty acids accumulated mainly in the epididymal fat. In contrast, there was no accumulation of even-numbered fatty acids observed in the small intestine epithelium, liver, or epididymal fat. These results suggest that odd-numbered fatty acids might not be favorable substrates for beta-oxidation-related enzymes.  相似文献   
79.
In this study, the structure and properties of an organic–inorganic composite material prepared from nylon 6 doped with fine particles of silver iodide (AgI) were examined. The preparation of the composite involved the complexation of nylon 6 with polyiodide ions such as I and I by immersion in an iodine/potassium iodide (I2–KI) aqueous solution followed by reaction in a silver nitrate (AgNO3) aqueous solution; this resulted in the in situ formation of β-AgI fine particles within the nylon 6 matrix. The AgI content formed in the composite was dependent on the immersion temperatures of the I2–KI and AgNO3 solutions. Lower solution temperatures resulted in larger amounts of AgI in the composite. This method readily provided a composite with a high content of AgI in nylon 6 and a conductivity of approximately 10−5 Ω−1 cm−1. In a uniaxially oriented nylon 6 matrix, AgI particles precipitated with anisotropic shape, which was caused by the orientation of the precursor polyiodide ions. The structure of the oriented composite provided the anisotropic conductivity. Additionally, the composite exhibited high antibacterial properties. The procedure used in this study is considered a unique method for the preparation of organic–inorganic composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008  相似文献   
80.
In insect cell-baculovirus expression systems for recombinant protein production, it is sometimes necessary to supplement cultures with protease inhibitors to protect recombinant proteins against proteolysis. To date, however, there is no information available concerning protease activities in inhibitor-supplemented cultures. The aim of the present study was to investigate intracellular and extracellular protease activities in cultures of virus-infected Sf-9 insect cells which were supplemented with inhibitors against carboxyl and cysteine proteases produced during culture. Prior to the supplementation culture, the cell toxicity of several protease inhibitors was determined. As a result, pepstatin A (carboxyl protease inhibitor) and E64, cystatin, leupeptin, and antipain (cysteine protease inhibitors) tested in this study showed no apparent negative effects on the growth and viability of noninfected Sf-9 insect cells at low concentrations. In addition, E64 and pepstatin A could rapidly permeate virus-infected Sf-9 cells and inhibit the respective intracellular protease activities. A virus-infected culture with a multiplicity of infection of 1 was carried out with E64 and pepstatin A which were added to the culture medium at 2 d post-infection. As a result of inhibitor supplementation, the cellular activity for recombinant protein biosynthesis was reduced by 5-30%. However, a significant reduction in carboxyl and cysteine protease activities was observed not only in the medium but also intracellularly. This is the first study that directly demonstrates a reduction in extracellular and intracellular protease activities in protease inhibitor-supplemented cultures of virus-infected insect cells.  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