全文获取类型
收费全文 | 31044篇 |
免费 | 3018篇 |
国内免费 | 1811篇 |
专业分类
电工技术 | 3384篇 |
综合类 | 3326篇 |
化学工业 | 4128篇 |
金属工艺 | 1743篇 |
机械仪表 | 1326篇 |
建筑科学 | 6515篇 |
矿业工程 | 847篇 |
能源动力 | 1234篇 |
轻工业 | 1019篇 |
水利工程 | 1050篇 |
石油天然气 | 856篇 |
武器工业 | 215篇 |
无线电 | 1876篇 |
一般工业技术 | 4062篇 |
冶金工业 | 2448篇 |
原子能技术 | 234篇 |
自动化技术 | 1610篇 |
出版年
2024年 | 127篇 |
2023年 | 448篇 |
2022年 | 717篇 |
2021年 | 904篇 |
2020年 | 980篇 |
2019年 | 948篇 |
2018年 | 969篇 |
2017年 | 1096篇 |
2016年 | 1165篇 |
2015年 | 1202篇 |
2014年 | 1817篇 |
2013年 | 2025篇 |
2012年 | 2069篇 |
2011年 | 2480篇 |
2010年 | 1759篇 |
2009年 | 1896篇 |
2008年 | 1945篇 |
2007年 | 2151篇 |
2006年 | 1969篇 |
2005年 | 1426篇 |
2004年 | 1180篇 |
2003年 | 998篇 |
2002年 | 894篇 |
2001年 | 705篇 |
2000年 | 605篇 |
1999年 | 539篇 |
1998年 | 445篇 |
1997年 | 419篇 |
1996年 | 327篇 |
1995年 | 299篇 |
1994年 | 234篇 |
1993年 | 191篇 |
1992年 | 173篇 |
1991年 | 111篇 |
1990年 | 80篇 |
1989年 | 98篇 |
1988年 | 54篇 |
1987年 | 59篇 |
1986年 | 24篇 |
1985年 | 45篇 |
1984年 | 32篇 |
1983年 | 37篇 |
1982年 | 40篇 |
1981年 | 16篇 |
1980年 | 18篇 |
1979年 | 24篇 |
1965年 | 14篇 |
1957年 | 13篇 |
1955年 | 16篇 |
1954年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
92.
The shear strength and aging characteristics of 63Sn–37Pb solder bumps were characterized with variation in solder ball and
UBM pad sizes. The shear strength increased with shorter effective crack size,a
effs
which was determined with the solder ball and pad sizes. The shear strength of the solder bumps on Au/Ni/Cu and Ni/Cu did
not change significantly with reflow time. Substantial decrease in the shear strength occurred for the solder bumps formed
on Au/Ni/Cu with aging treatment, and the shear strength after aging was also related to the bump shape which was determined
with the solder ball and pad sizes. 相似文献
93.
Copper wire, serving as a cost-saving alternative to gold wire, has been used in many high-end thermosonic ball bonding applications. In this paper, the bond shear force, bond shear strength, and the ball bond diameter are adopted to evaluate the bonding quality. It is concluded that the ef/~cient ultrasonic power is needed to soften the ball to form the copper bonds with high bonding strength. However, excessive ultrasonic power would serve as a fatigue loading to weaken the bonding. Excessive or less bonding force would cause cratering in the silicon. 相似文献
94.
95.
Polyurethanes were prepared from toluene diisocyanate (TDI), 1-4-butane diol (BDO) and polycaprolactone-based triols with varying molecular weights. Among each molecular weight triol-based urethane, hard segment content was varied from 20% to 70%. Differential scanning calorimetry, tensile testing, and Iosipescu shear testing were done on all the various urethanes prepared. Thermal characterization data revealed the dependence of phase separation on hard segment content as well as on the triol molecular weight. Tensile data and Iosipescu shear data further confirmed the observations made from the DSC data. The data further indicated that phase separation can greatly improve the modulus of cross-linked segmented urethanes. Adhesion of these urethanes to glass surface was evaluated using soda-lime float glass plate. Urethane samples were cast on the air side of the glass plates and adhesion was measured in shear mode. Adhesion data indicated that in addition to hard segment content, modulus, cross-link density, and molecular weight of the triols, phase separation seems to be a major factor in controlling adhesion. Surfaces of the failed adhesion samples were also analyzed and the failure mode was found to be cohesive, in varying degree, with the different urethane systems. 相似文献
96.
摘要:为解决常规聚合物无法满足超高温深井酸化压裂的技术要求,本文分别以丙烯酰胺(AM) 甲基丙烯酰氧乙基三甲基氯化铵(DMC)、AM DMC SMA(十八烷基甲基丙烯酸酯)为原料,通过自由基聚合制备出两种耐酸聚合物SY-1和SY-2,并对其结构进行表征及性能测试。结果表明:SY-2红外谱图中720 cm-1为—(CH2)n—的弯曲振动吸收峰,单体成功接枝到聚丙烯酰胺分子链上;热重分析表明,SY-1升温至600℃失重比为98%,SY-2失重比86%,热稳定性得到改善;质量分数为20%盐酸中,SY-2质量分数0.8%:180℃,170s-1流变性能测试,SY-2剪切终点黏度56mPa?s,90℃、120℃、150℃剪切1h,黏度为83.6mPa?s、43.5mPa?s、27.8mPa?s,剪切稳定性分别为85.66%、74.45%、52.01%; 180℃、3MPa条件下,静态酸岩反应速率为8.94?10-5(g/cm2?s),较常规SY-1反应速率1.69?10-4(g/cm2?s)减小一个量级。且与缓蚀剂、铁离子稳定剂配伍性良好,克服高温酸化压裂技术的应用瓶颈。 相似文献
97.
由二氧化碳出发合成有机碳酸酯 总被引:4,自引:0,他引:4
对以二氧化碳为原料合成有机碳酸酯的环氧路线、酯交换和直接合成方法作了综合评述 ,认为直接合成是最佳的合成路线。碳酸酯的直接合成可在均相及多相催化体系中进行 ,负载金属、金属甲氧基化合物及碱均可作为催化剂。直接法研究的关键在于高性能催化剂的开发、由二氧化碳对金属 -氧键插入反应导致的催化循环的构筑及耦合反应的应用。 相似文献
98.
K HondaM Yoshimura R UchikadoT Kondo Tata N Rao D.A TrykA Fujishima M WatanabeK Yasui H Masuda 《Electrochimica acta》2002,47(27):4373-4385
Electrochemical characteristics for several redox systems at diamond films with highly ordered nanometer-scale cylindrical pores (‘nano-honeycombs’) were examined with cyclic voltammetry (CV) and electrochemical impedance measurements. The cyclic voltammetric anodic-cathodic peak separations for these nano-honeycomb electrodes were in the same range as those for polished polycrystalline diamond films, indicating that the involvement of the oxygen-terminated surface of the nano-pore walls, which should give rise to large peak separations for certain redox couples was only slight. Moreover, the peak currents in the CV were not enhanced to the extent expected on the basis of the roughness factors of the nano-honeycomb films. Ac impedance plots results indicated the existence of a concentration gradient of the reactant in the nano-pores, which is in agreement with theoretical predictions for charge transfer reactions in porous electrodes. The average concentration of the reactant (Fe2+/3+) inside the nano-pores was a factor of ca. 80 lower than that in the bulk electrolyte. The results of the impedance analysis also indicated an increase in the reaction resistances with decreasing pore diameters. 相似文献
99.
I. Hama T. Okamoto E. Hidai K. Yamada 《Journal of the American Oil Chemists' Society》1997,74(1):19-24
For the purpose of estimating the reaction mechanism of the direct ethoxylation of a fatty ester in the presence of an Al-Mg
composite oxide catalyst, a labeled fatty methyl ester C11H23CO18OCH3 containing 18O isotope was synthesized and directly ethoxylated. The product was evaluated by gas chromatography-mass spectrometry (GC-MS).
The GC-MS spectra showed that the 18O isotope label was present only in the methoxy group at the molecular end of the ethoxylated fatty methyl ester. This supports
the reaction mechanism of coordination anionic polymerization where the bond between the acyl and methoxy groups of the fatty
methyl ester molecule was broken, caused by the bifunctional effect of the acid-base active sites; an intermediate chemisorption
species was formed; and then ethylene oxide was addition-polymerized sequentially, in parallel. 相似文献
100.
H. Dodiuk A. Buchman S. Kenig M. Rotel J. Zahavi T. J. Reinhart 《The Journal of Adhesion》1993,41(1):93-112
An excimer laser may be used for preadhesion treatment of aluminum alloys. This method presents an alternative to the use of ecologically unfriendly chemicals involved in conventional anodizing pretreatments.
Experimental results indicate that preadhesion laser surface treatment significantly improved the shear strength of modified-epoxy bonded aluminum specimens compared with untreated and anodized substrates. The best results were obtained with laser energy of about 0.2 J/Pulse/cm2 where single lap shear strength was improved by 600-700% compared with that of untreated Al alloy, and by 40% compared with chromic acid anodizing pretreatment.
The mode of failure changed from adhesive to cohesive as the number of laser pulses increased during treatment. The latter phenomenon has been correlated with morphology changes as revealed by electron microscopy, and chemical modification as indicated by Auger and infrared spectroscopy.
It can be concluded that the excimer laser has potential as a precise, clean and simple preadhesion treatment of Al alloys. 相似文献
Experimental results indicate that preadhesion laser surface treatment significantly improved the shear strength of modified-epoxy bonded aluminum specimens compared with untreated and anodized substrates. The best results were obtained with laser energy of about 0.2 J/Pulse/cm2 where single lap shear strength was improved by 600-700% compared with that of untreated Al alloy, and by 40% compared with chromic acid anodizing pretreatment.
The mode of failure changed from adhesive to cohesive as the number of laser pulses increased during treatment. The latter phenomenon has been correlated with morphology changes as revealed by electron microscopy, and chemical modification as indicated by Auger and infrared spectroscopy.
It can be concluded that the excimer laser has potential as a precise, clean and simple preadhesion treatment of Al alloys. 相似文献