全文获取类型
收费全文 | 36203篇 |
免费 | 1871篇 |
国内免费 | 33篇 |
专业分类
电工技术 | 255篇 |
综合类 | 20篇 |
化学工业 | 7314篇 |
金属工艺 | 670篇 |
机械仪表 | 730篇 |
建筑科学 | 979篇 |
矿业工程 | 75篇 |
能源动力 | 699篇 |
轻工业 | 5818篇 |
水利工程 | 332篇 |
石油天然气 | 128篇 |
武器工业 | 5篇 |
无线电 | 1337篇 |
一般工业技术 | 5837篇 |
冶金工业 | 8920篇 |
原子能技术 | 135篇 |
自动化技术 | 4853篇 |
出版年
2024年 | 81篇 |
2023年 | 315篇 |
2022年 | 427篇 |
2021年 | 902篇 |
2020年 | 733篇 |
2019年 | 852篇 |
2018年 | 1405篇 |
2017年 | 1372篇 |
2016年 | 1438篇 |
2015年 | 1114篇 |
2014年 | 1323篇 |
2013年 | 2707篇 |
2012年 | 2046篇 |
2011年 | 1926篇 |
2010年 | 1551篇 |
2009年 | 1401篇 |
2008年 | 1351篇 |
2007年 | 1274篇 |
2006年 | 882篇 |
2005年 | 733篇 |
2004年 | 704篇 |
2003年 | 619篇 |
2002年 | 614篇 |
2001年 | 448篇 |
2000年 | 420篇 |
1999年 | 487篇 |
1998年 | 2658篇 |
1997年 | 1786篇 |
1996年 | 1143篇 |
1995年 | 685篇 |
1994年 | 522篇 |
1993年 | 619篇 |
1992年 | 208篇 |
1991年 | 213篇 |
1990年 | 165篇 |
1989年 | 170篇 |
1988年 | 172篇 |
1987年 | 147篇 |
1986年 | 127篇 |
1985年 | 148篇 |
1984年 | 123篇 |
1983年 | 96篇 |
1982年 | 134篇 |
1981年 | 155篇 |
1980年 | 163篇 |
1979年 | 79篇 |
1978年 | 86篇 |
1977年 | 322篇 |
1976年 | 699篇 |
1973年 | 63篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Laura Gosalbo Mireia Barrot Gemma Fabriás Gemma Arsequell Francisco Camps 《Lipids》1993,28(12):1125-1130
To develop a synthesis of tritiated cyclopropene fatty acids (CPFA), compounds that should prove useful for affinity labeling
of desaturases in insect pheromone biosynthetic studies, a series of novel, selectively deuterated CPFA analogues was prepared
and characterized. In methyl [16-2H]12,13-methylene-12-hexadecenoate, the incorporation of deuterium was achieved by treatment of the corresponding ω-chloro
derivative with sodium borodeuteride in dimethylsulfoxide at 70°C for 24 h (67% yield) following conventional procedures.
Alkylation of the tetrahydropyranyl derivative of 13-tridecynol in the presence of lithium diisopropylamide in tetrahydrofuran
at −20°C with 1-chloro-3-iodopropane in hexamethylphosphoramide, followed by Jones oxidation of the crude product, yielded
16-chloro-12-hexadecynoic acid (54%), which was esterified to the corresponding methyl ester by treatment with potassium carbonate
and methyl iodide in dimethylformamide. Treatment of this acetylenic ester with ethyldiazoacetate in the presence of activated
copper-bronze as catalyst followed by hydrolysis in KOH solution at room temperature yielded 16-chloro-12,13-(carboxymethylene)-12-hexadecenoic
acid. This diacid was treated with excess oxalyl chloride to give the corresponding diacyl chloride, which was decarbonylated
in a diethyl ether solution with zinc chloride, and the cyclopropenium ions thus formed were added at −40°C to a methanolic
sodium hydroxide solution of sodium borohydride to give methyl 16-chloro-12,13-methylene-12-hexadecenoate. Analogous procedures
were followed to prepare methyl [17-2H]10,11-methylene-10-hexadecenoate, methyl [17-2H]11,12-methylene-11-hexadecenoate and methyl [17-2H]12,13-methylene-12-hexadecenoate from the corresponding diacids using sodium borodeuteride in the reduction of the cyclopropenium
ions. Alternatively, methyl [2,2,3,3-2H4]hexadecynoate, prepared by reaction of methyl 2,11-hexadecadiynoate with magnesium in deuterated methanol at room temperature,
was submitted to the above cyclopropenylation and reductive decarbonylation sequence to give methyl [2,2,3,3,17-2H5]-11,12-methylene-11-hexadecenoate. In summary, complementary methods for the selective incorporation of one to five deuterium
atoms into cyclopropene fatty acids, at different sites, in moderate to high yields have been developed. The methods should
easily be applicable to the preparation of the corresponding tritiated analogues. 相似文献
82.
Four-year microplot tests were performed to study the utilization by wheat, in the presence and absence of the nitrification inhibitor N-Serve, of15N labeled urea spread in fall. The percentages of fertilizer nitrogen taken up by winter wheat (grains and straw) were 18 to 37 percent when urea was spread in fall, 33 to 45 percent when urea plus N-Serve was applied in fall, and 36 to 49 percent when urea was spread in spring. In the 0–30 cm layer of soil there were found, for the treatments listed above, 15 to 23 percent, 24 to 45 percent, and 15 to 47 percent of urea nitrogen after the harvesting of winter wheat. Application of urea plus N-Serve in fall and of urea alone in spring resulted in similar amounts of fertilizer nitrogen being taken up by spring wheat to those taken up by winter wheat. Of the urea nitrogen applied in fall, 20 to 28 percent and 47 to 50 percent were not recovered from the plants and 0–30 cm soil layer with and without additional N-Serve treatment, respectively. The utilization by winter wheat of urea nitrogen spread, with no additional N-Serve, in fall from mid-October onward was considerably lower in those cases where, after fertilizing, weather conditions were such as to favour both nitrification and leaching. On heavy and loamy soil such influences of weather were offset by application of N-Serve. 相似文献
83.
An experimental study has been made of temperature profiles incident within a zeolite adsorbent particle in the course of adsorption of n-heptan. Using thermocouple wires 0.1 mm in diameter it has been possible to measure simultaneously the temperatures of the surface and in the centre. It was observed that temperature differences between center of the pellet and ambient stagnant gas may be more than 20°C. The results of this work indicate that the overheating of an adsorbent particle is a very rapid process which is followed by a slow cooling of the hot particle. Experimental data are compared with predictions calculated from the theoretical model. 相似文献
84.
Polymer Bulletin - Solutions of syndiotactic (s) poly (methyl methacrylate) (PMMA) and samples of solid s-PMMA prepared from these solutions were studied by NMR and infrared (IR) spectroscopy and... 相似文献
85.
86.
87.
Short syntheses of (Z)-7-dodecen-1-yl acetate, (Z)-7-tetradecen-1-yl acetate, (Z)-9-dodecen-1-yl acetate, and (Z)-9-tetradecen-1-yl acetate from 7-hydroxyheptanal and 9-oxononanoic acid precursors obtained by oxidative cleavage of easily available aleuritic acid are reported. The key step in these syntheses is a stereoselective Wittig reaction between aldehyde and alkyl-phosphonium salt. Wittig-Horner type reaction of 7-hydroxyheptanal and diethyl cyanomethylphosphonate gave the ,-unsaturated nitrile derivative which after protection of the hydroxyl group was reduced to the corresponding aldehyde. Wittig reaction of the latter, followed by acetylation, completed the synthesis of (E,Z)-7,9-dodecadien-1-yl acetate, the sex pheromone of the European grapevine mothLobesia botrana Schiff. 相似文献
88.
A.?Patrascioiu J.?M.?Fernández-Pradas A.?Palla-Papavlu J.?L.?Morenza P.?SerraEmail author 《Microfluidics and nanofluidics》2014,16(1-2):55-63
Laser-based techniques provide excellent means for liquid microprinting, with several advantages over other more conventional printing techniques, such as being nozzle-free (as opposed to inkjet, for instance) or requiring minimal engineering of the liquid properties in the pre-printing stage. In such techniques, the transfer is usually mediated by liquid jets that contact a receiver substrate placed nearby the liquid source, leading to the deposition of a small droplet. The main cause of jetting lies in a laser-generated bubble produced inside the liquid, whose dynamics dictates the evolution of liquid ejection. However, the detailed relationship between the bubble and the jet is not completely understood, as the studies carried out so far have been mostly focused on the jetting dynamics taking place above the liquid free-surface, without access to the liquid interior and therefore to the behavior of the bubble. In this work, we analyze through time-resolved imaging the film-free laser printing of an aqueous solution by simultaneously visualizing both the bubble evolution and the liquid ejection dynamics, thus making possible the correlation between the two phenomena. We find that the pulsating behavior of the bubble leads to successive jetting events with different jet morphologies arising from the particular geometries that the bubble acquires during its evolution. Finally, we find good agreement between our results and those from studies analyzing the dynamics of cavitation bubbles near the free-surface of a liquid through numerical solution of the fluid dynamics equations. 相似文献
89.
The homogeneous decomposition of ozone in the presence of a Co(II) catalyst has been investigated in aqueous solution. Under the conditions investigated (Co(II) concentration: 0.0 – 2.0?ppm, pH: 1.6- 8.4, O3 concentration: 5 10?5 – 2.0 104?M) the process can be assumed to follow pseudo first order kinetics with respect to ozone. Cobalt concentration dependency also obeys first order kinetics although it may be considered to reach a steady state concentration. pH exerts a positive influence on the decomposition rate from 1.6 to 7.1, the process leveling off at pH 8.4. An Arrhenius analysis of the temperature effect gave a moderate activation energy of the global reaction (E=10917?cal mol?1). A more detailed radical mechanism than a simple pseudo first order reaction can be postulated for simulation purposes. By numerical optimization of some unknown kinetic constants the influence of several operating variables can be adequately predicted. 相似文献
90.
S. Lucas E. Gonzlez M.P. Calvo C. Palencia E. Alonso M.J. Cocero 《The Journal of Supercritical Fluids》2007,40(3):462-469
Supercritical impregnation of Radiata pine with ethyl acetate and decanal using CO2 as carrier solvent has been studied at pilot plant scale. Radiata pine is one of the most common wood species that is originally from Australia and is widely grown in Spain and Portugal and ethyl acetate and decanal were selected as organic compounds.Some experiences were carried out to obtain the optimal operating conditions for the supercritical impregnation process. Experiments were conducted at pressures of 7–15 MPa, temperatures of 35–50 °C and solvent flow rate between 1.5 and 3.5 kg/h. The results of this study have indicated that the treatment gives much better preservative penetration and retention operating with low pressures (7.5 MPa), low temperatures (close to 35 °C) and moderate CO2 flow rate (3.5 kg/h) in the selected operating range. Moreover, a simple mathematical model of two adjustable parameters (external mass transfer coefficient and partition coefficient) has demonstrated to fit the experimental impregnation curves with reasonable accuracy (average absolute deviation, 3–10%). 相似文献