全文获取类型
收费全文 | 2856篇 |
免费 | 122篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 29篇 |
综合类 | 2篇 |
化学工业 | 793篇 |
金属工艺 | 60篇 |
机械仪表 | 62篇 |
建筑科学 | 92篇 |
矿业工程 | 4篇 |
能源动力 | 93篇 |
轻工业 | 171篇 |
水利工程 | 15篇 |
石油天然气 | 7篇 |
无线电 | 271篇 |
一般工业技术 | 586篇 |
冶金工业 | 400篇 |
原子能技术 | 12篇 |
自动化技术 | 382篇 |
出版年
2023年 | 23篇 |
2022年 | 51篇 |
2021年 | 84篇 |
2020年 | 61篇 |
2019年 | 65篇 |
2018年 | 72篇 |
2017年 | 57篇 |
2016年 | 86篇 |
2015年 | 57篇 |
2014年 | 72篇 |
2013年 | 156篇 |
2012年 | 143篇 |
2011年 | 158篇 |
2010年 | 130篇 |
2009年 | 107篇 |
2008年 | 145篇 |
2007年 | 131篇 |
2006年 | 105篇 |
2005年 | 95篇 |
2004年 | 93篇 |
2003年 | 67篇 |
2002年 | 72篇 |
2001年 | 57篇 |
2000年 | 57篇 |
1999年 | 42篇 |
1998年 | 77篇 |
1997年 | 53篇 |
1996年 | 51篇 |
1995年 | 50篇 |
1994年 | 40篇 |
1993年 | 39篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 28篇 |
1989年 | 24篇 |
1988年 | 22篇 |
1987年 | 24篇 |
1986年 | 21篇 |
1985年 | 19篇 |
1984年 | 21篇 |
1983年 | 27篇 |
1982年 | 17篇 |
1981年 | 21篇 |
1979年 | 13篇 |
1978年 | 24篇 |
1977年 | 21篇 |
1976年 | 22篇 |
1975年 | 17篇 |
1974年 | 17篇 |
1973年 | 15篇 |
排序方式: 共有2979条查询结果,搜索用时 15 毫秒
31.
The aqueous eluate from fruits ofAmmi majus (Bishop's weed, Umbelliferae) remarkably inhibited germination of adjacent seeds ofAnastatica hierochuntica, lettuce, or tomato but had no effect on intact fruits ofAmmi. Similar inhibition was found in dark or in light, except that seeds ofA. hierochuntica were significantly more inhibited in the dark than in the light. Xanthotoxin was isolated, identified, and found to account for about a sixth of the inhibitory activity of the eluate. After fruits ofAmmi were submerged in a large volume of water for 4 days, the fruits still exuded enough inhibitors to prevent germination ofA. hierochuntica, lettuce, or tomato. Data support also the proposal that the phytotoxins are compartmentalized between the inner and the outer fruit envelopes. The inner layer excludes inhibitors from the embryo and autotoxicity is thus avoided, whereas the outer one ensures a gradual liberation of the phytotoxic compounds. This, as well as the high reactivity of the eluate, the high densities ofAmmi fruits in nature, and their relatively limited annual germination, suggest chemical inhibition of neighboring plant species other thanAmmi. Hence, in addition to their chemical protection against predators of either lower or higher organisms, furanocoumarins in fruits ofAmmi majus may contribute to its success as a weed. 相似文献
32.
The e.m.f. of a concentration cell for SO x (x=2,3)-O2 incorporating Nasicon as the main solid electrolyte has been measured in the temperature range 720 to 1080 K. The cell arrangement can be represented as, $$Pt, O'_2 + SO'_2 + SO'_3 \left| {Na_2 SO_4 \left\| {\left. {Nasicon} \right\|} \right.} \right.\left. {Na_2 SO_4 } \right|SO''_3 + SO''_2 + O''_2 , Pt$$ The Na2SO4 acts both as an auxiliary electrode, converting chemical potentials of SO x and O2 to equivalent sodium potentials, and as an electrolyte. The presence of Na2SO4 provides partial protection of Nasicon from chemical reaction with gas mixtures containing SO x . The open circuit e.m.f. of the cell is in close agreement with values given by the Nernst equation. For certain fixed inlet gas compositions of SO2+O2, the e.m.f. varies non-linearly with temperature. The intrinsic response time of the cell to step changes in gas composition is estimated to vary from ~2.0 ksec at 723K to ~ 0.2 ksec at 1077K. The cell functions well for large differences in partial pressures of SO3(p″SO 3/p′SO 3≈104) at the electrodes. 相似文献
33.
The glass transition temperatures (Tg's) of several polystyrenes and styrene–alkyl methacrylate copolymers and terpolymers were measured using thermomechanical analysis (TMA) and differential scanning calorimetry (DSC). The polymers studied had number-average molecular weights from 3000 to 250,000 g/mole. The results indicate that the composition dependence of the Tg's for the copolymers and terpolymers can be satisfactorily described by a general Fox equation. In general, the measured Tg's of the copolymer and terpolymer samples depend more on the steric effects of the constituent pendent groups than on their molecular weights. The chain flexibility rather than the size of the pendent group is the determining factor in the glass transition properties of the styrene polymers. 相似文献
34.
The rheological behavior of ethylene propylene diene monomer (EPDM) compounds containing ground EPDM waste (W‐EPDM) of known composition was studied by using a Monsanto processability tester in a temperature range of 90–110°C and a shear rate range of 306.7–1533.24 s?1. It is found that the shear viscosity decreases slightly with increasing W‐EPDM loading because of wall slip that results from the migration of lubricants from the W‐EPDM. The addition of W‐EPDM to raw EPDM results in a decreased die swell at all temperatures and shear rates. SEM photomicrographs of the EPDM extrudate surface show improved surface smoothness and reduced extrudate distortion when EPDM is blended with W‐EPDM. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2204–2215, 2003 相似文献
35.
The ablation rate of frozen mixtures of water or wax with finely divided solids subjected to a turbulent axisymmetric water jet was investigated both experimentally and theoretically. The range of the water jet Reynolds number was 8700–29300 and the jet temperature varied between 22 and 60°C. The solids used were very fine kaolinite clay, titanium oxide and aluminium powder. The type of solids and their concentration were found to affect the ablation rate of ice-solids and wax-solids mixtures. This is mainly due to changes in the physical properties of the melt layer of the water-solids or wax-solids mixtures which forms between the impinging jet and the frozen mixtures. 相似文献
36.
37.
Rudolf H. Scheffrahn Ruei-Ching Hsu Nan-Yao Su Jacob B. Huffman Sharon L. Midland James J. Sims 《Journal of chemical ecology》1988,14(3):765-776
The heartwood of bald cypress,Taxodium distichum (L.) Rich., resisted feeding attack by the Formosan subterranean termite,Coptotermes formosanus Shiraki. Hexane-extracted heartwood, however, was consumed at > 12 times the amount of sound heartwood eaten. A bioassay usingT. distichum sapwood as a feeding substrate was employed to assess the antitermitic activity of successive hexane, acetone, and methanol extracts of heartwood shavings and isolates derived from the active hexane extract. Two fractions, eluted from the crude hexane extract by liquid chromatography, significantly reduced termite feeding compared to the parent extract, while a third fraction was less active than the original hexane extract. Each fraction contained one major component. All three components were structurally related diterpenes. The two most active heartwood constituents were identified by GC-MS and NMR as ferruginol and manool, while the third and least active, but most prevalent, compound in heartwood was identified as nezukol. Results of bioassays suggest that these allelochemicals act principally as feeding deterrents with accompanying termite mortality due to starvation. 相似文献
38.
Analysis of the hornet’s hemolymph revealed the presence of C16 and C18 fatty acids (70%), which were accompanied by minor quantities (ranging from 0.1% to 0.6%) of the following acids: C10∶0, C11∶0, C12∶0, C13∶0, C14∶0, C15∶0, C16∶0, and C17∶0. The hemolymph of the queen larvae contained more C18∶1 than the hemolymph of the worker larvae, and the percentage of C16∶1 was higher in the fat body and the midgut than in the hemolymph. The significance of these results is discussed. 相似文献
39.
40.
Phase relations in the systems SrO-Y2O3-CuO-O2 and CaO-Y2O3-CuO-O2 at 1173 K were estab-lished by equilibrating different compositions in flowing oxygen gas at a pressure of 1.01 × 105 Pa. The quenched samples were examined by optical microscopy, X-ray diffraction (XRD), energy dis-persive analysis of X-rays
(EDAX), and electron spin resonance (ESR). In the system SrO-Y2O3-CuO-O2, except for the limited substitution of Y3+ for Sr2+ ions in the ternary oxide Sr14 Cu24O41, no new quaternary phase was found to be stable. The compositions corresponding to the solid solution Sr14-xYxCu24O41 and the compound SrCuO2+δ lie above the plane containing SrO, Y2O3, and CuO, displaced towards the oxygen apex. However, in the system CaO-Y2O3-CuO-O2 at 1173 K, all the condensed phases lie on the plane containing CaO, Y2O3, and CuO, and a new quaternary oxide YCa2Cu3O6.5 is present. The quaternary phase has a composition that lies at the center of the non-stoichiometric field of the analogous
phase YBa2Cu3O7-δ in the BaO-Y2O3-CuO-O2 system. The com-pound YCa2Cu3O6.5 has the tetragonal structure and does not become superconducting at low temperature. Surprisingly, phase relations in the
three systems CaO-Y2O3-CuO-O2, SrO-Y2O3-CuO-O2, and BaO-Y2O3-CuO-O2 are found to be quite different. 相似文献