全文获取类型
收费全文 | 7826篇 |
免费 | 489篇 |
国内免费 | 10篇 |
专业分类
电工技术 | 59篇 |
综合类 | 18篇 |
化学工业 | 1915篇 |
金属工艺 | 184篇 |
机械仪表 | 156篇 |
建筑科学 | 330篇 |
矿业工程 | 27篇 |
能源动力 | 233篇 |
轻工业 | 714篇 |
水利工程 | 112篇 |
石油天然气 | 21篇 |
无线电 | 459篇 |
一般工业技术 | 1690篇 |
冶金工业 | 1207篇 |
原子能技术 | 21篇 |
自动化技术 | 1179篇 |
出版年
2024年 | 12篇 |
2023年 | 64篇 |
2022年 | 139篇 |
2021年 | 208篇 |
2020年 | 168篇 |
2019年 | 201篇 |
2018年 | 220篇 |
2017年 | 208篇 |
2016年 | 244篇 |
2015年 | 187篇 |
2014年 | 314篇 |
2013年 | 563篇 |
2012年 | 467篇 |
2011年 | 674篇 |
2010年 | 453篇 |
2009年 | 423篇 |
2008年 | 475篇 |
2007年 | 438篇 |
2006年 | 366篇 |
2005年 | 326篇 |
2004年 | 267篇 |
2003年 | 251篇 |
2002年 | 229篇 |
2001年 | 114篇 |
2000年 | 102篇 |
1999年 | 101篇 |
1998年 | 91篇 |
1997年 | 91篇 |
1996年 | 76篇 |
1995年 | 84篇 |
1994年 | 81篇 |
1993年 | 56篇 |
1992年 | 67篇 |
1991年 | 27篇 |
1990年 | 31篇 |
1989年 | 42篇 |
1988年 | 42篇 |
1987年 | 40篇 |
1986年 | 35篇 |
1985年 | 54篇 |
1984年 | 45篇 |
1983年 | 41篇 |
1982年 | 40篇 |
1981年 | 25篇 |
1980年 | 28篇 |
1979年 | 20篇 |
1978年 | 21篇 |
1977年 | 18篇 |
1976年 | 17篇 |
1975年 | 11篇 |
排序方式: 共有8325条查询结果,搜索用时 0 毫秒
81.
In the paper, the characters and fields were introduced for utilizing the Series/Parallel Switch (SPS). The circuit principle of the SPS technology was analyzed in detailed, explained and illuminated. We understood clearly the difference between SPS and single-phase transformator by the figure and table. Finally, it summarized that the main characters of SPS. 相似文献
82.
Christopher A. Mullin 《Journal of chemical ecology》1988,14(10):1867-1888
Epoxide hydrolase catalyzes a simple hydrolysis of reactive cyclic ethers that may otherwise alkylate and impair critical proteins and nucleic acids required for life. Although much less studied than the cytochrome P-450 monooxygenases that produce epoxides, differences in subcellular, tissue, pH, substrate, and inhibitor specificities argue for at least three forms of insect epoxide hydrolase. Increasing numbers of epoxides are being identified as plant allelochemicals, antifeedants, and essential hormones or precursors for herbivorous arthropods, and in many cases an associated alkene to diol pathway of metabolism is found. A role for epoxide hydrolase in arthropod-plant interactions is strongly supported by species comparisons and by age-activity and induction studies. Two major limitations for study in biochemical ecology of epoxide hydrolase are the lack of an effective in vivo inhibitor and a range of commercially available radiolabeled substrates for the enzymes. 相似文献
83.
Melanie T. Schaal Ashley Y. Metcalf Joseph H. Montoya J. Paul Wilkinson Carol C. Stork Christopher T. Williams John R. Monnier 《Catalysis Today》2007,123(1-4):142-150
Electroless deposition has been used to prepare Cu–Pd/SiO2 bimetallic catalysts wherein initial Cu coverages are limited only to the pre-existing Pd surface. Cu loading on the Pd surface can be systematically varied by modification of deposition kinetic parameters. In this case deposition time was used as the kinetic variable for the preparation of a series of Cu–Pd catalysts. These materials have been characterized using atomic absorption, CO chemisorption, and FT-IR (adsorption of CO), and then evaluated for the hydrogenation of 3,4-epoxy-1-butene, a functionalized olefin having many potential reaction pathways. Catalyst performance and characterization results suggest that Cu is not distributed in a monodisperse manner on the Pd surface, indicating the existence of autocatalytic deposition of Cu on Cu sites. The FT-IR results suggest that although CO adsorption on all sites is suppressed by Cu addition, initial Cu deposition occurs more readily on certain sites. The bimetallic Cu–Pd sites that are formed exhibit unusually high activity for EpB conversion and formation of unsaturated alcohols and aldehydes. This bimetallic effect on catalyst activity and selectivity is best explained, not by the existence of either ligand or ensemble effects, but rather by the bifunctional nature of the Cu–Pd sites present on the surface of these catalysts. 相似文献
84.
The National Aeronautics and Space Administration's (NASA's) PS304 coating is a plasma spray deposited tribological coating with feedstock composed of NiCr, Cr2O3, Ag and BaF2-CaF2 powders. The effects of rounded BaF2-CaF2 particles on the gravity-fed flow characteristics of PS304 feedstock have been investigated. The BaF2-CaF2 powder was fabricated by water atomization using four sets of process parameters. Each of these powders was then characterized by microscopy and classified by screening to obtain 45-106 μm particles and added incrementally from 0-10 wt.% to the other constituents of the PS304 feedstock, namely nichrome, chromia and silver powders. The relationship between feedstock flow rate, measured with the Hall flowmeter, and concentration of fluorides was found to be linear in each case. The slopes of the lines were between those of the linear relationships previously reported using angular and spherical fluorides and were closer to the relationship predicted using the rule of mixtures. The results offer a fluoride fabrication technique potentially more cost-effective than gas atomization processes or traditional comminution processes. 相似文献
85.
The polymorph method, which provides phase analysis from a small number of integrated intensities in a powder diffraction scan, is adapted for the determination of monoclinic zirconia in a mixture with cubic, tetragonal. and orthorhombic zirconias and the γ-phase (Mg2 Zr5 O12 ). Such a mixture is representative of Mg-PSZ after subeutectoid aging. The quantitative determination of the monoclinic depends in principle on a knowledge of the relative amounts of the other phases present in the mixture. It is demonstrated, however, that without this knowledge, even in complex mixtures, the traditional polymorph method analysis gives an acceptable estimate of the monoclinic fraction in the sample. 相似文献
86.
We put forward effective methods of increasing the tensile strain of cementitious composites with 2% PVA fiber and high fly ash content. The test results show that curing condition has a significantly effect on the tensile performance. It is approved that the specimens incorporated appropriate volume fraction rubber powder and lightweight aggregate greatly increase the tensile strain of composites at medium-term age, but indefinitely at long-term age. To a certain extent, EVA can limitedly enhance the tensi... 相似文献
87.
The distribution of plasmalogenase for the hydrolysis of 1-alkenyl-2-acyl-sn-glycero-3-phosphoethanolamine (plasmenylethanolamine) in the subcellular fractions of guinea pig tissues was examined. Plasmalogenase
activity was found in high abundance in the cytosolic fractions of the brain and the heart. Assessment of microsomal marker
enzyme activities in the cytosolic fraction revealed that plasmalogenase activity in the cytosol was not due to microsomal
contaminations. The characteristics of the cytosolic plasmalogenase were very similar to the microsomal enzyme with respect
to the pH profile of the reaction, the presence of divalent cations and Km values for plasmenylethanolamine. However, the cytosolic enzyme was slightly less stable at 55°C than the microsomal enzyme.
Cytosolic enzyme activity was eluted as a broad peak in Sepharose 6B chromatography with an average molecular weight of 250,000.
Our results demonstrate that most of brain plasmalogenase activity is soluble which makes the brain cytosol an excellent source
to initiate the purification of this enzyme. 相似文献
88.
Light intensity gradients and light exposure time gradients were combined to produce contours of constant dose on a sample substrate. These polymerized samples were subsequently analyzed using high-throughput Fourier transform infrared spectroscopy to measure conversion as a function of both gradients. Three (meth)acrylate monomers were analyzed over light doses ranging from 0 mJ/cm2 to 920 mJ/cm2, demonstrating that in thin films, higher light intensities at a constant light dose produce higher conversion due to a decreased oxygen inhibition time and larger thermal excursions. At a light dose of 75 mJ/cm2, the conversion of 2-ethylhexyl acrylate increases from 40 ± 2% at a light intensity of 0.9 mW/cm2 to 59 ± 3% at 7.2 mW/cm2. The two acrylate monomers exhibited rapid photopolymerization up to a specific conversion, after which additional radiation dose produced only marginal increases in overall conversion. For hexanediol diacrylate, a light dose of 300 mJ/cm2 was the minimum amount required to reach the maximum conversion over the entire range analyzed. For the dimethacrylate system, a similar effect was seen, with a reduced oxygen inhibition time and conversion above 70% showing a similar conversion at a constant light dose of 500 mJ/cm2. In all three systems, dose contours were used to determine a range of light intensities at which a statistically similar conversion would occur for a specified light dose. 相似文献
89.
Alaa S. Abd-El-Aziz T. Christopher Corkery Erin K. Todd Tarek H. Afifi Guozhang Ma 《Journal of Inorganic and Organometallic Polymers》2003,13(3):113-130
The synthesis of an aromatic ether complex of cyclopentadienyliron containing two terminal aldehyde groups was achieved via metal-mediated nucleophilic aromatic substitution reactions. This dialdehyde monomer was subsequently reacted with a variety of aliphatic and aromatic diamines to produce the corresponding soluble cationic organoiron polyether-imines. These cationic organometallic polymers were characterized using IR, 1H, and 13C NMR, viscosity and thermogravimetric analysis. Viscosity measurements showed that these polymers exhibited polyelectrolyte effects in DMSO solutions. Thermogravimetric analysis showed that decoordination of the iron moieties occurred at about 300°C for polymers with aliphatic spacers in their backbones, while the cyclopentadienyliron moieties were cleaved from the polymers with aromatic spacers in their backbones at about 200°C. Photolytic demetallation of the organoiron polymers resulted in the removal of the pendent cyclopentadienyliron moieties and allowed for the isolation of their organic analogs. While the organoiron polymers were soluble in polar organic solvents, the corresponding organic polymers exhibited very limited solubilities or were insoluble. The organic polymers had glass transition temperatures between 101 and 120°C 相似文献
90.
Darryl P. Butt Klaus S. Lackner Christopher H. Wendt Samuel D. Conzone Harriet Kung Yung-Cheng Lu Julie K. Bremser 《Journal of the American Ceramic Society》1996,79(7):1892-1898
The kinetics of simultaneous dehydroxylation and carbonation of precipitated Mg(OH)2 were studied using isothermal and nonisothermal thermogravimetric analyses. Specimens were analyzed using X-ray diffraction, transmission electron microscopy, and through measurements of the volume of carbon dioxide evolved in a subsequent reaction with hydrochloric acid. From 275° to 475°C, the kinetics of isothermal dehydroxylation in helium were best fit to a contracting-sphere model, yielding an activation energy of 146 kJ/mol, which was greater than values reported in the literature for isothermal dehydroxylation under vacuum (53–126 kJ/mol). The carbonation kinetics were complicated by the fact that dehydroxylation occurred simultaneously. The overall kinetics also could be fit to a contracting-sphere model, yielding a net activation energy of 304 kJ/mol. The most rapid carbonation kinetics occurred near 375°C. At this temperature, Mg(OH)2 underwent rapid dehydroxylation and subsequent phase transformation, whereas thermodynamics favored the formation of carbonate. During carbonation, MgCO3 precipitated on the surface of disrupted Mg(OH)2 crystals acting as a kinetic barrier to both the outward diffusion of H2 O and the inward diffusion of CO2 . 相似文献