全文获取类型
收费全文 | 317069篇 |
免费 | 5620篇 |
国内免费 | 890篇 |
专业分类
电工技术 | 5469篇 |
综合类 | 374篇 |
化学工业 | 48221篇 |
金属工艺 | 11135篇 |
机械仪表 | 9453篇 |
建筑科学 | 7724篇 |
矿业工程 | 1184篇 |
能源动力 | 7735篇 |
轻工业 | 32093篇 |
水利工程 | 3062篇 |
石油天然气 | 4136篇 |
武器工业 | 72篇 |
无线电 | 36808篇 |
一般工业技术 | 57705篇 |
冶金工业 | 62836篇 |
原子能技术 | 5661篇 |
自动化技术 | 29911篇 |
出版年
2022年 | 2108篇 |
2021年 | 3431篇 |
2020年 | 2617篇 |
2019年 | 3263篇 |
2018年 | 5282篇 |
2017年 | 5303篇 |
2016年 | 5656篇 |
2015年 | 3807篇 |
2014年 | 5911篇 |
2013年 | 15094篇 |
2012年 | 9456篇 |
2011年 | 11911篇 |
2010年 | 9379篇 |
2009年 | 10142篇 |
2008年 | 10865篇 |
2007年 | 10568篇 |
2006年 | 9285篇 |
2005年 | 8374篇 |
2004年 | 8036篇 |
2003年 | 7763篇 |
2002年 | 7322篇 |
2001年 | 7154篇 |
2000年 | 6821篇 |
1999年 | 7186篇 |
1998年 | 19512篇 |
1997年 | 13528篇 |
1996年 | 10279篇 |
1995年 | 7641篇 |
1994年 | 6706篇 |
1993年 | 6671篇 |
1992年 | 4834篇 |
1991年 | 4508篇 |
1990年 | 4306篇 |
1989年 | 4069篇 |
1988年 | 3902篇 |
1987年 | 3432篇 |
1986年 | 3298篇 |
1985年 | 3870篇 |
1984年 | 3528篇 |
1983年 | 3159篇 |
1982年 | 3015篇 |
1981年 | 3129篇 |
1980年 | 2961篇 |
1979年 | 2723篇 |
1978年 | 2582篇 |
1977年 | 3350篇 |
1976年 | 4425篇 |
1975年 | 2389篇 |
1974年 | 2366篇 |
1973年 | 2384篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Five flow improvers have been synthesized to study rheological properties of Nada crude oil (Gujarat, India). Anhydride copolymers were prepared making use of the copolymerization of acrylates of different alkyls with maleic anhydride and the Poly (n-alkyl acrylates-co-N-hexadecylmaleimide) were prepared by the reaction of copolymer with hexadecylamine. The additives were purified and characterized by FTIR, GPC. The prepared polymeric additives shows dual function both as wax dispersants and flow improvers and all of them acts as good pour point depressants. Yield stress and the viscosity of the crude oil at different temperatures and concentrations of additives were evaluated by zero friction advanced rheometer AR-500 of TA instrument. Comparison of morphologies and structures of wax crystals or aggregates in waxy crude oils beneficiated with and without a PPD was also done by micro photographic studies which show the modification in wax crystal morphology due to additives. 相似文献
982.
David P. Cormode Jason J. Davis Paul D. Beer 《Journal of Inorganic and Organometallic Polymers and Materials》2008,18(1):32-40
Disulfide and dithiocarbamate functionalized porphyrins have been synthesized and used as protecting ligands for gold nanoparticle
formation either via ligand substitution reactions or by direct synthesis. These nanoparticles have been shown to recognize
anions via changes in the absorbance spectrum of the surface adsorbed porphyrin moieties. Association constants, derived from
quantitative titrations, indicate a remarkable surface enhancement effect where the surface bound porphyrins bind anions much
more strongly than the free receptor in solution. 相似文献
983.
L. B. Pavlovich N. N. Nazarov V. P. Dolgopolov A. V. Kalinina T. A. Bulis D. V. Bal’tser V. P. Konstantinov 《Coke and Chemistry》2008,51(7):277-282
The use of biochemically purified water in place of industrial-grade water for the exhaust-gas scrubbers in the drying department of the coal-enrichment shop and for irrigation of the cyclone washers in the coke shops at OAO ZSMK is considered. It is found that biochemically purified water may be used for 100% of the water needs in coke production. Using biochemically purified water in the dust-trapping equipment, the coaland coke-dust content of the atmospheric emissions may be reduced by 24.5%. Additional purification of wastewater by adsorption on coke and coal dust is possible. 相似文献
984.
Evaggelia D. Tzika Vassiliki Papadimitriou Theodore G. Sotiroudis Aristotelis Xenakis 《European Journal of Lipid Science and Technology》2008,110(2):149-157
The autoxidation at alkaline pH and enzymatic oxidation by mushroom tyrosinase of oleuropein, the dominant biophenol present in the fruits and leaves of Olea europea, was followed by both electron paramagnetic resonance (EPR) and absorption spectroscopy. For comparison, the same oxidation processes were applied to 4‐methylcatechol, a simple polyphenol present in olive mill wastewaters. EPR spectra of stable o‐semiquinone radicals produced during autoxidation at pH 12 and short‐lived o‐semiquinone free radicals produced during autoxidation at pH 9.0 or tyrosinase action and stabilized by chelation with a diamagnetic metal ion (Mg2+) were recorded for both polyphenols, and the corresponding hyperfine splitting constants were determined. The UV‐Vis spectral characteristics of the oxidation of polyphenols were highly dependent on the type of polyphenol, oxidant type and the pH of the reaction. The kinetic behavior of tyrosinase in the presence of oleuropein and 4‐methylcatechol was followed by recording spectral changes at 400 nm (absorption maximum) over time. The tysosinase activity with oleuropein showed a pronounced pH optimum at pH 6.5 and a minor one around pH 8. From the data analysis of the initial rate at pH 6.5, the kinetic parameters Km = 0.34 ± 0.03 mM and Vmax = 0.029 ± 0.002 ΔA400 min–1 were determined for oleuropein. 相似文献
985.
S. Toita D. Kang K. Kobayashi H. Kawamoto K. Kojima M. Tachibana 《Diamond and Related Materials》2008,17(7-10):1389-1393
DNA-wrapped single-wall carbon nanotubes (DNA-SWNT hybrids) prepared from different diameter HiPco- and Arc-SWNTs were investigated by atomic force microscopy. The mean diameter of DNA-HiPco-SWNT hybrids is 1.94 nm that is consistent with one HiPco-SWNT (~ 0.9 nm) wrapped by DNA (~ 1 nm). On the other hand, the mean diameter of DNA-Arc-SWNT hybrids is 3.74 nm that can correspond to one Arc-SWNT (~ 1.4 nm) wrapped by several layers of DNA. It is suggested that the DNA-wrapping mechanism for large diameter Arc-SWNTs is different from that for small diameter HiPco-SWNTs. 相似文献
986.
María Esther Diupotex-Chong Néstor J Cazzaniga Alejandra Hernández-Santoyo José Miguel Betancourt-Rule 《Biocell》2004,28(3):279-285
Mitotic chromosomes of the freshwater snail Pomacea patula catemacensis (Baker 1922) were analyzed on gill tissue of specimens from the type locality (Lake Catemaco, Mexico). The diploid number of chromosomes is 2n = 26, including nine metacentric and four submetacentric pairs; therefore, the fundamental number is FN = 52, No sex chromosomes could be identified. The same chromosome number and morphology were already reported for P. flagellata, i.e., the other species of the genus living in Mexico. The basic haploid number for family Ampullariidae was reported to be n = 14 in the literature; so, its reduction to n = 13 is probably an apomorphy of the Mexican Pomacea snails. Lanistes bolteni, from Egypt, also shows n = 13, but its karyotype is much more asymmetrical, and seems to have evolved independently from P. flagellata and P. patula catemacensis. The nominotypical subspecies, P. patula patula (Reeve 1856), is a poorly known taxon, whose original locality is unknown. A taxonomical account is presented here, and a Mexican origin postulated as the most parsimonious hypothesis. 相似文献
987.
Qualitative methods are available for risk management, but better practice would use quantitative risk management based on expected losses and related metrics. Measuring the success of information security investments is best accomplished by measuring reductions in expected loss. 相似文献
988.
989.
Self-stabilizing somersaults 总被引:1,自引:0,他引:1
Mombaur K.D. Bock H.G. Schloder J.P. Longman R.W. 《Robotics, IEEE Transactions on》2005,21(6):1148-1157
We investigate the open-loop stability of a planar biped robot performing a periodic motion of forward somersaults with alternating single-leg contacts. The robot has a trunk and two actuated telescopic legs with point feet which are coupled to the trunk by actuated hinges. There is compliance and damping in the hip and in the legs. The concept of open-loop control implies that all actuators of the system receive predetermined inputs that are never altered by any feedback interference. Only with the right choice of model parameters and actuator inputs is it possible to create such self-stabilizing motions exploiting the natural stability properties of the system. These unknowns have been determined using special-purpose stability-optimization methods. The resulting motion is not only stable, but also a more efficient form of forward motion than running for the investigated robot. 相似文献
990.
Imperfection in information can be considered a crosscutting concern that manifests itself in diverse kinds of imprecision, uncertainty, or inconsistency in the data models of a software system. The extension of existing programming and querying interfaces for the different aspects of information imperfection requires a proper modularization of the different concerns of numerical imprecision handling, so that the extensions do not interfere with existing programming practices and do not obscure the original design. Aspect‐oriented design (AOD) enables such a form of nonintrusive extensions to be added to existing software libraries, clearly separating fuzziness or other imperfections in data as a differentiated concern that can be considered from the early phases of development. In this article, a general framework for aspect‐based extension of data models and fuzzy databases is described, and some design and implementation issues of such AOD‐based extensions on OJB database libraries are described as a case study. © 2006 Wiley Periodicals, Inc. Int J Int Syst 21: 1199–1216, 2006. 相似文献