全文获取类型
收费全文 | 554384篇 |
免费 | 5705篇 |
国内免费 | 1925篇 |
专业分类
电工技术 | 8800篇 |
综合类 | 2909篇 |
化学工业 | 82686篇 |
金属工艺 | 27866篇 |
机械仪表 | 19275篇 |
建筑科学 | 12032篇 |
矿业工程 | 5158篇 |
能源动力 | 11135篇 |
轻工业 | 33533篇 |
水利工程 | 7642篇 |
石油天然气 | 16354篇 |
武器工业 | 68篇 |
无线电 | 57902篇 |
一般工业技术 | 120760篇 |
冶金工业 | 78428篇 |
原子能技术 | 15847篇 |
自动化技术 | 61619篇 |
出版年
2021年 | 4365篇 |
2019年 | 4218篇 |
2018年 | 21615篇 |
2017年 | 20826篇 |
2016年 | 18142篇 |
2015年 | 5513篇 |
2014年 | 8697篇 |
2013年 | 20220篇 |
2012年 | 15753篇 |
2011年 | 25765篇 |
2010年 | 21223篇 |
2009年 | 20472篇 |
2008年 | 22006篇 |
2007年 | 22678篇 |
2006年 | 12965篇 |
2005年 | 12779篇 |
2004年 | 12563篇 |
2003年 | 12334篇 |
2002年 | 11367篇 |
2001年 | 10898篇 |
2000年 | 10579篇 |
1999年 | 10207篇 |
1998年 | 22086篇 |
1997年 | 16279篇 |
1996年 | 12492篇 |
1995年 | 9811篇 |
1994年 | 8807篇 |
1993年 | 8963篇 |
1992年 | 6974篇 |
1991年 | 6917篇 |
1990年 | 6808篇 |
1989年 | 6704篇 |
1988年 | 6347篇 |
1987年 | 5916篇 |
1986年 | 5839篇 |
1985年 | 6344篇 |
1984年 | 5961篇 |
1983年 | 5669篇 |
1982年 | 5271篇 |
1981年 | 5423篇 |
1980年 | 5402篇 |
1979年 | 5354篇 |
1978年 | 5554篇 |
1977年 | 5881篇 |
1976年 | 7168篇 |
1975年 | 4915篇 |
1974年 | 4969篇 |
1973年 | 5014篇 |
1972年 | 4351篇 |
1971年 | 3945篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The technique of analytical pyrolysis has been used to characterize 40 similar mid-rank western Kentucky coals of widely differing ASTM Gieseler plasticity. Certain pyrolysis/g.c. variables were shown to correlate well with both ASTM Gieseler and isothermal plasticity. Highly plastic coals were shown to exhibit certain characteristic peaks in the 450 °C pyrogram which were absent in the pyrogram of the non-plastic coals. Two coals, representing the two extremes in plasticity, were selected for further study. After extraction with solvents such as DMF the characteristic peaks were absent in the 450 °C pyrogram of the extraction residue of the highly plastic coal. These peaks were also shown to decrease with increasing severity of air oxidation of the plastic coal. This technique appears to be a useful tool for the analysis of mid-rank plastic coals and supports the view that the substances contained in the bitumen fraction of these coals are involved in the development of the plastic state. 相似文献
992.
Paul I. Dalven James R. Hildebrandt Abraham Shamir Anthony J. Laccetti Leonard T. Hodgins Harry P. Gregor 《应用聚合物科学杂志》1985,30(3):1113-1132
Various high molecular weight copolymers of acrylonitrile and a vinyl comonomer containing an aryl amine, a pyridine, or an aliphatic hydroxyl group were synthesized via slurry polymerization techniques so as to contain from 1 to 15 mol % functional comonomer. The comonomer content was quantitated by ultraviolet absorbance, base titration of acid polymer salts, and/or relative chemical reactivity with trichloro-s-triazine. Thin films were cast from copolymer solutions, coagulated into unsupported ultrafiltration membrances, and characterized with respect to both water permeability and pore size distribution. Analysis by size exclusion chromatography of the membrane permeate of a pool of dextran fractions yielded a continuous distribution curve for membrane pore size over the range 1.5 to 70 nm. The ultrafiltration membranes were used for protein immobilization after appropriate chemical activation. The three distinct types of functional copolymers gave comparable results for α-chymotrypsin, with protein weight loadings of 6–12% and 40–65% retention of enzymatic specific activity. 相似文献
993.
Fully dense aluminas, prepared by hot-pressing, were found to swell during annealing at 1600°C in air, but not during annealing in a reducing atmosphere (po2 = 10-7 Pa). The reaction followed the relation p - po = -K log t, where po and p are the initial and final densities, respectively, t is the time, and AT is a constant. The rate of swelling was enhanced by MgO solute. The reduction in density resulted from the nucleation and growth of grain-boundary pores. Pore formation was attributed to the reaction of carbon and sulfur impurities at the boundaries with oxygen, which had diffused down the grain boundaries from the ambient, to form CO/CO2 and SO2 gas at high pressures. Preliminary results indicate that this reaction can be avoided by preannealing powders in flowing oxygen prior to hot-pressing. The consequences of internal gas-forming reactions to other processes such as high-temperature creep and sintering are also discussed. 相似文献
994.
Enthalpies of segregation for isovalent impurities in magnesium and calcium oxide as a function of surface concentration were calculated by using an atomistic computer simulation method. We have considered Be2+ , Mg2+ , Ca2+ , Ba2+ , and Ni2+ , segregating to both (001) and (110) faces. The results obtained can be extrapolated to predict the behavior of other impurities including Mn2+ , Fe2+ , and Co2+ , We find, for example, that Fe2+ , Mn2+ , Ca2+ , Sr2+ , and Ba2+ will concentrate at the (001) surface of MgO, while Ni2+ will be depleted. The enthalpy of segregation is found to vary substantially with coverage particularly for the larger impurities. The enthalpy becomes less negative with increasing impurity concentration due to the increasing lattice strain until the surface is nearly saturated. Then additional stabilization is obtained by restructuring of the surface layer. We predict reconstructed surfaces for both the (001) and (110) faces, which contain a high concentration of a larger impurity ion. The enthalpy of segregation shows a maximum at around 50% surface coverage implying a bimodal surface distribution of segregant. The influence of segregation on surface energy suggests two unusual effects. The (001) surface energy of the impure crystal becomes negative for surface concentrations of impurity greater than 10% Ba2+ or 75% Sr2+ in MgO. This implies a thermodynamic barrier to sintering. At high coverages of Ba2+ in MgO the (110) surface becomes more stable than the (001) face suggesting that facetting may occur. 相似文献
995.
996.
Renaudier J. Brenot R. Dagens B. Lelarge F. Rousseau B. Poingt F. Legouezigou O. Pommereau F. Accard A. Gallion P. Duan G.-H. 《Electronics letters》2005,41(18):1007-1008
Self-pulsation at 45 GHz repetition frequency has been demonstrated in 1.5 /spl mu/m monolithic single-section quantum dot Fabry-Perot semiconductor lasers without saturable absorber. The mode-beating exhibits a narrow linewidth below 100 kHz, demonstrating high phase correlation between these modes. Such modelocked lasers open ways to low timing-jitter components for clock recovery or millimetre-wave generation in wireless transmission applications. 相似文献
997.
The kinetics of electronic processes in vidicon phototargets based on semiconductor-insulator structures with a narrow-gap semiconductor is considered taking into account charge drain in the insulator layer and relaxation of the nonequilibrium depletion region in the semiconductor layer. The integration time, threshold sensitivity, and resolution at various intensities of incident radiation are estimated. 相似文献
998.
999.
This paper considers the integration of alternative technical systems (TS) belonging to a wider class of competing TS, into a supersystem. The paper introduces the notion of alternative TS, shows the peculiarities of their incorporation into a supersystem, reveals the mechanisms used for such incorporation, and shows areas of application for the suggested method. 相似文献
1000.