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   4544篇
  免费   29篇
电工技术   30篇
综合类   1篇
化学工业   262篇
金属工艺   20篇
机械仪表   38篇
建筑科学   81篇
矿业工程   5篇
能源动力   37篇
轻工业   118篇
水利工程   25篇
石油天然气   8篇
无线电   287篇
一般工业技术   304篇
冶金工业   3099篇
原子能技术   25篇
自动化技术   233篇
  2020年   19篇
  2019年   18篇
  2018年   16篇
  2017年   12篇
  2016年   25篇
  2015年   19篇
  2014年   22篇
  2013年   81篇
  2012年   49篇
  2011年   50篇
  2010年   46篇
  2009年   48篇
  2008年   66篇
  2007年   48篇
  2006年   74篇
  2005年   62篇
  2004年   35篇
  2003年   50篇
  2002年   45篇
  2001年   55篇
  2000年   43篇
  1999年   121篇
  1998年   984篇
  1997年   560篇
  1996年   369篇
  1995年   221篇
  1994年   192篇
  1993年   211篇
  1992年   48篇
  1991年   47篇
  1990年   65篇
  1989年   55篇
  1988年   66篇
  1987年   56篇
  1986年   67篇
  1985年   50篇
  1984年   30篇
  1983年   37篇
  1982年   26篇
  1981年   28篇
  1980年   25篇
  1979年   24篇
  1978年   26篇
  1977年   64篇
  1976年   159篇
  1975年   20篇
  1974年   19篇
  1973年   22篇
  1972年   11篇
  1971年   15篇
排序方式: 共有4573条查询结果,搜索用时 15 毫秒
91.
The tropical forest cover has varied greatly over the last few decades. The rapid advance of agricultural crops and illegal clearings in natural areas has resulted in the conversion of the majority of the world’s forest into desolated patches. Although rates of deforestation have decreased compared to previous years, forest loss still remains a crucial concern. Latest studies conducted on a global scale identified the Latin American continent as one of the regions exhibiting the highest rates of deforestation in the world. The dynamics of forests over the past 40 years has attracted numerous remote-sensing-based studies to monitor forest loss, analyse patterns, and understand the drivers of land conversion. This review article provides a comprehensive overview of the remote-sensing-based studies of tropical forest dynamics in Latin America. Following an introduction with respect to global forest mapping products, a general outline of tropical forest ecoregions and drivers of deforestation in Latin America is provided. Subsequently, a review and categorization of the existing studies is presented, where focus is laid on selected sensors and data analysis methodologies apply. Furthermore, a case study for the whole of Paraguay is presented; Paraguay is a region which contains highly diverse ecosystems that have been ravaged as a result of deforestation over the past 40 years. The main results, challenges, and future needs are discussed.  相似文献   
92.
Summary Concentrates of natural linoleic acid (linoleic acid content, 85–95%) have been prepared in 50–72% yields from corn oil fatty acids by preferential precipitation of the saturated and monounsaturated fatty acids at room temperature as their urea complexes. By a similar procedure, concentrates of natural linolenic acid (linolenic acid content, 87–89%) have been prepared in 55–61% yields from perilla oil fatty acids by preferential precipitation of the saturated, monounsaturated, and diunsaturated fatty acids. Although concentrates of natural linolenic acid containing only 66–70% linolenic acid were obtained from linseed oil fatty acids, yields were 87–90%. A levelling-off effect has been observed in the use of the preferential precipitation technique in raising the purity of concentrates of linoleic and linolenic acid. This parallels the experience in the purification of these acids by low-temperature crystallization. The preceding papers in this series are references 12 and 13. Presented at the Fall Meeting of the American Oil Chemists' Society, Cincinnati, O., Oct. 20–22, 1952. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture.  相似文献   
93.
94.
Compositionally and structurally varied copolymers all containing n-octadecyl acrylate were prepared and evaluated as viscosity index improvers in a common base oil under conditions of low shear. Systems evaluated over a range of copolymer and blend composition were: copolymers of n-octadecyl acrylate with, respectively, methyl methacrylate, 2-ethylhexyl acrylate, and n-dodecyl acrylate; and homopolymers of poly(n-octadecyl acrylate), prepared with a wide range of molecular weights. Properties were compared with those of blends of commercial methacrylate copolymers (acryloids) which had been freed of their entraining liquid. Mixtures of base oil with copolymers of n-octadecyl acrylate and methyl methacrylate, compared at fixed SAE viscosities, were the most efficient of all blends studied. They had the smallest rate of change of viscosity with temperature (as measured by their ASTM slopes), particularly in the composition region of incipient polymer precipitation at room temperature. Efficiency of certain of these composition was somewhat greater than that of the acryloids. A parameter that related concentration and weight-average molecular weight was used to correlate all of the data for ASTM slope and viscosity. Empirical relations developed by using this parameter enabled rheological data to be estimated that agree within 6% of experimental values for the case of thermodynamically good base oil solvents. These data demonstrated the relatively small contributions of copolymer structure to viscosity index improvement.  相似文献   
95.
Mechanical and solution properties, melting transitions, torsional stiffness temperatures, Tf, and selected modulus-temperature curves are presented for copolymers of the N-n-alkylacrylamides with vinylidene chloride. Copolymers were prepared at 60°C across the range of compositions, using as comonomers N-n-butyl-, octyl-, dodecyl- and oleyl-acrylamide, which have amorphous side-chains, and N-n-octadecyl acrylamide and n-octadecyl acrylate whose side-chains are crystalline. The mechanical properties reflected the effect of the decline in backbone crystallinity and the simultaneous development of strong intermolecular interactions in the amorphous stage. Copolymers were stiff or showed brittle failure across the compositional range except when intermolecular forces were reduced (with n-octadrcyl acrylate) and side-chain crystallization eliminated (with N-oleylacrylamide). These systems and the n-dodecylacrylamide copolymers had yield strengths less than brittle strengths and substantial elongations. Backbone crystallinity was eliminated at about 15 mole % amide and side-chain crystallinity vanished at less than 10 mole % of the amide in the N-n-octadecylacrylamide series. No depression in side-chain melting point occurred with dilution by segments of vinylidene chloride. Over-all decline in the flex-temperature was the normal monotonic function of composition except that values increased in magnitude at high vinylidene chloride contents, the effect presumably being caused by the presence of crystallinity. An empirical equation was developed which permitted the calculation of Tf for any N-n-alkylacrylamide composition with any number of carbon atoms in the side-chain, above 3.  相似文献   
96.
An undergraduate computer engineering laboratory that supports a one-quarter computer arithmetic course is described. This course is required for the computer engineering degree and is taken as a technical option by many students in electrical engineering and computer science. The features of this state-of-the-art laboratory include a local area network of computers, remote access via the campus network, electronic mail, online documentation, and automatic program submittal, grading, and plagiarism detection  相似文献   
97.
We describe how transform techniques may be applied to the design of arrayed-waveguide gratings (AWG's). A hybrid Fourier-Fresnel transform model indicates that a flat AWG router passband over 30% of the device free-spectral range (FSR) is feasible. Minimized passband ripple is achieved by applying a subparabolic phase-profile (phase-exponent P=1.82) to the Fourier plane of the AWG. Simulations of a complex-apodized AWG with passband 3 dB width increased from 1 to 12 nm, with a near-trapezoidal amplitude response, and band-edge group delay of 70 ps reduced to 10 ps are presented for a device with a FSR of 40 nm  相似文献   
98.
Cry Woolf     
The Woolf Reforms, recently instituted to make civil litigation more user friendly, targeted, amongst other things, the investigator’s best friend, the Anton Piller Order (AP). The Order was targeted directly, by having its name changed to the Search Order, and indirectly, in that Woolf aimed to make claimants expend more effort in finding solutions before resorting to the “nuclear option” of the AP.  相似文献   
99.
To make media resources a prime citizen on the Web, we have to go beyond simply replicating digital media files. The Web is based on hyperlinks between Web resources, and that includes hyperlinking out of resources (e.g., from a word or an image within a Web page) as well as hyperlinking into resources (e.g., fragment URIs into Web pages). To turn video and audio into hypervideo and hyperaudio, we need to enable hyperlinking into and out of them. The W3C Media Fragments Working Group is taking on the challenge to further embrace W3C??s mission to lead the World Wide Web to its full potential by developing a Media Fragment protocol and guidelines that ensure the long-term growth of the Web. The major contribution of this paper is the introduction of Media Fragments as a media-format independent, standard means of addressing media resources using URIs. Moreover, we explain how the HTTP protocol can be used and extended to serve Media Fragments and what the impact is for current Web-enabled media formats.  相似文献   
100.
The α-phase of the Al−Mn−Si ternary system has been precipitated from four liquid solutions (2.8 wt% Mn−3.7 wt% Si, 4.5 wt% Mn−7 wt% Si, 7.7 wt% Mn−10 wt% Si and 8.7 wt% Mn−12.6 wt% Si) and equilibrated slightly over the melting point of the (secondary) solid solution. Quenching into water enabled the precipitates to be separated from the solid solution and the co-nodal tie-lines for the α-phase and liquid to be determined. The silicon content varied from 9.8 to 11.8 wt%, while the manganese content, 29.6±0.6 wt%, did not vary significantly. The lattice parameter of the cubic unit cell decreased from 1.267 6(2) nm to 1.265 10(4) nm with increasing silicon. Neutron powder diffraction showed silicon to constitute approximately 50% of atoms on the icosahedral aluminium sublattices and 70% of a six-fold aluminium site in the interstices between the almost spherical, 54-atom clusters. This decrease in unit cell volume is interpreted as evidence for substitution of the silicon atoms on the six-fold sublattice.  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