全文获取类型
收费全文 | 88248篇 |
免费 | 6954篇 |
国内免费 | 3417篇 |
专业分类
电工技术 | 4904篇 |
技术理论 | 5篇 |
综合类 | 5685篇 |
化学工业 | 15494篇 |
金属工艺 | 5372篇 |
机械仪表 | 5479篇 |
建筑科学 | 7757篇 |
矿业工程 | 2542篇 |
能源动力 | 2373篇 |
轻工业 | 5345篇 |
水利工程 | 1548篇 |
石油天然气 | 5166篇 |
武器工业 | 683篇 |
无线电 | 9262篇 |
一般工业技术 | 10532篇 |
冶金工业 | 4366篇 |
原子能技术 | 1064篇 |
自动化技术 | 11042篇 |
出版年
2024年 | 271篇 |
2023年 | 1406篇 |
2022年 | 2192篇 |
2021年 | 3269篇 |
2020年 | 2556篇 |
2019年 | 2129篇 |
2018年 | 2482篇 |
2017年 | 2823篇 |
2016年 | 2411篇 |
2015年 | 3237篇 |
2014年 | 4094篇 |
2013年 | 5032篇 |
2012年 | 5546篇 |
2011年 | 5924篇 |
2010年 | 5031篇 |
2009年 | 4965篇 |
2008年 | 4758篇 |
2007年 | 4575篇 |
2006年 | 4881篇 |
2005年 | 4336篇 |
2004年 | 2894篇 |
2003年 | 2494篇 |
2002年 | 2294篇 |
2001年 | 2023篇 |
2000年 | 2299篇 |
1999年 | 2676篇 |
1998年 | 2178篇 |
1997年 | 1762篇 |
1996年 | 1810篇 |
1995年 | 1472篇 |
1994年 | 1198篇 |
1993年 | 849篇 |
1992年 | 689篇 |
1991年 | 545篇 |
1990年 | 390篇 |
1989年 | 324篇 |
1988年 | 264篇 |
1987年 | 176篇 |
1986年 | 107篇 |
1985年 | 79篇 |
1984年 | 50篇 |
1983年 | 29篇 |
1982年 | 39篇 |
1981年 | 28篇 |
1980年 | 22篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1965年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
单向Cf/SiC复合材料的弯曲疲劳性能 总被引:2,自引:1,他引:2
对单向Cf/SiC复合材料进行了三点弯曲疲劳性能测试,得到了复合材料的应力-寿命曲线(S-N曲线),并对其进行线性拟合,得到疲劳最大应力与复合材料疲劳寿命的关系;考察了疲劳过程中刚度下降和疲劳裂纹产生情况。结果表明在疲劳过程中复合材料的弯曲模量有3个变化阶段:首先在疲劳加载初期,弯曲模量的下降速度及幅度都较大;其次在弯曲模量下降到原始弯曲模量的85%(133GPa)后,其变化方式没有明显的规律可循,有时甚至可能上升;最后复合材料发生疲劳断裂时,模量将发生突变。显微结构分析表明:基体横向裂纹群的产生是疲劳断裂的独有特征。它的产生是由于基体SiC的断裂应变小于碳纤维的断裂应变,基体首先开裂并导致应力重新分布的结果。 相似文献
992.
Zhou D Lagoja IM Rozenski J Busson R Van Aerschot A Herdewijn P 《Chembiochem : a European journal of chemical biology》2005,6(12):2298-2304
Oligonucleotides that contain up to three aminopropyl nucleoside analogues have been synthesized. Dimers of aminopropyl adenine and thymidine were prepared and used as building blocks by applying phosphoramidite chemistry. Both R and S isomers of the aminopropyl nucleosides were used. This incorporation led to a reduction of thermal stability of double-stranded DNA. Furthermore, the (R)-adenine analogue, which yielded (S)-APNA, can be considered as a candidate for universal base pairing. 相似文献
993.
Chunnuan Ji Rongjun Qu Cheng Bo Cao Huo Chen Qiang Xu Changmei Sun Chunhua Wang Huafang Yuan Jun Zhou 《应用聚合物科学杂志》2006,102(6):6054-6059
A novel chelating resin containing sulfoxide and diethylene glycol, poly{4‐vinylbenzyl‐[2‐2‐(hydroxyethyl)ethoxyl]sulfoxide} (PVESO) was synthesized using chloromethylated polystyrene (PS‐Cl) as material. Its structure was characterized by elemental analysis, infrared spectra, and scanning electron microscopy (SEM). The adsorption capacities of the resin for Hg2+, Ag+, Cu2+, Zn2+, and Pb2+ at various pH values were determined. The maximum adsorption capacities for Hg2+ and Ag+ were 1.56 and 0.75 mmol g?1 respectively. The resin had high selectivity for Hg2+ and Ag+ over the pH range 1.0–7.0. The adsorption capacities for Hg2+ and Ag+ under competitive condition were also determined by batch experiment method. In addition, the adsorption kinetics of the resin towards Hg2+ at different temperatures was also investigated. The results showed that the adsorption rate was governed by film diffusion at 20°C and 25°C, by particle diffusion at 30°C and 35°C. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 6054–6059, 2006 相似文献
994.
Boron-based metal-free catalysts for oxidative dehydrogenation of propane (ODHP) have drawn great attention in both academia and industry due to their impressive activity and olefin selectivity. Herein, the SiO2 and B2O3 sequentially coated honeycomb cordierite catalyst is designed by a two-step wash-coat method with different B2O3 loadings (0.1%-10%) and calcination temperatures (600, 700, 800 ℃). SiO2 obtained by TEOS hydrolysis acts as a media layer to bridge the cordierite substrate and boron oxide via abundant Si-OH groups. The welldeveloped straight channels of honeycomb cordierite make it possible to carry out the reactor under high gas hourly space velocity (GHSV) and the thin wash-coated B2O3 layer can effectively facilitate the pore diffusion on the catalyst. The prepared B2O3/SiO2@HC monolithic catalyst exhibits good catalytic performance at low boron oxide loading and achieves excellent propylene selectivity (86.0%), olefin selectivity (97.6%, propylene and ethylene) and negligible CO2 (0.1%) at 16.9% propane conversion under high GHSV of 345,600 ml·(g B2O3)-1·h-1, leading to a high propylene space time yield of 15.7 g C3H6·(g B2O3)-1·h-1 by suppressing the overoxidation. The obtained results strongly indicate that the boron-based monolithic catalyst can be properly fabricated to warrant the high activity and high throughput with its high gas/surface ratio and straight channels. 相似文献
995.
乙二醇合成的研究进展 总被引:13,自引:0,他引:13
介绍了环氧乙烷、乙二醇技术的进展,总结了国内外合成乙二技术研究、开发的最新成果:合成气直接合成法、甲醛电化学加氢二聚法、甲醇醚氧化偶联法、多元醇加氢裂解法等。 相似文献
996.
Yu-Hang Yuan Xing-Gui Zhou Wei Wu Yi-Ran Zhang Wei-Kang Yuan Lingai Luo 《Catalysis Today》2005,105(3-4):544-2
Gas phase propylene epoxidation on gold catalysts has attracted wide attention from industry and academia due to its high selectivity. However, it suffers from low propylene conversion and rapid catalyst deactivation. Experiments showed that propylene conversion could be increased by raising H2, O2, or C3H6 concentration in the feed, but the feed compositions were within the explosion limit. It was also shown that the activity of the used catalyst could be fully recovered, but the regeneration temperature was 280 °C, much higher than that for reaction. Therefore a microchannel reactor was devised to suppress explosion and was constructed with Fecralloy, to raise the temperature rapidly for catalyst regeneration by electric heating. In two minutes the temperature of the reactor could be raised from 50 to 300 °C. Catalysts were coated on the alloy belt by dip coating, and the performance of the reactor was evaluated under different operating conditions. Results showed that in the microreactor the overall reaction rate was controlled mainly by the intrinsic reaction rate, and also influenced by film diffusion to a certain extent. The deactivated catalyst was regenerated in the microchannel reactor and the activity was fully recovered. 相似文献
997.
998.
Poly(diglycidyl maleate‐co‐stearyl methacrylate) (P(DGMA‐co‐SMA)) with reactive epoxy groups was synthesized by reaction of poly(maleic anhydride‐co‐stearyl methacrylate) (P(MA‐co‐SMA)) and epichlorohydrin. The effect of precipitant on self‐assembly behaviors of the resultant copolymer was investigated. It was found that vesicles and nanotubule liked aggregates can be obtained through self‐assembly of P(DGMA‐co‐SMA) in THF solution using CH3CH2OH (EtOH) as precipitant while spheral aggregates can be obtained using H2O as precipitant. The mechanism of the self‐assembly behavior was discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 相似文献
999.
Hardware security has become more and more important in current information security architecture. Recently collected reports have shown that there may have been considerable hardware attacks prepared for possible military usage from all over the world. Due to the intrinsic difference from software security, hardware security has some special features and challenges. In order to guarantee hardware security, academia has proposed the concept of trusted integrated circuits, which aims at a secure circulation of IC design, manufacture and chip using. This paper reviews the main problems of trusted integrated circuits, and concludes four key domains of the trusted IC, namely the trusted IC design, trusted manufacture, trusted IP protection, and trusted chip authentication. The main challenges in those domains are also analyzed based on the current known techniques. Finally, the main limitations of the current techniques and possible future trends are discussed. 相似文献
1000.
掺锰BaTiO3 PTCR陶瓷的电子顺磁共振研究 总被引:2,自引:1,他引:2
利用电子顺磁共振(EPR)方法对Mn掺杂的BaTiO3 PTCR陶瓷在120-450K的温度范围内进行了研究,研究结果表明:高自旋Mn^2 离子的6重谱线非常明显,表征谱线位置的g因子为2.009 9,Mn^2 离子信号弱弱与BaTiO3晶体结构密切相关,Mn^2 离子信号的温度分布呈U型,即在三方相和立方相时,信号强度较强,而在四方相和斜方相时信号很弱,这是因为在四方相和斜方相中,由于BaTiO3晶体结构的扭曲和畸变,导致Mn^2 和Vo空位缔合生成Mn^2 -Vo复合缺陷,未成对电子的消失以及复合缺陷n^2 -Vo的生成最Mn^2 在四方相和斜方相(200-380K)消失的主因,实验中没有发现Mn^3 和Mn^4 的信号,而与实验的环境条件一致,掺Mn后样品Curie点的漂移可能是Mn进入晶格的一个判据,此外,由于试样的n(Ba)/n(Ti)=1,因而没有发现钡空位VBa及其它的复合缺陷(如:VBa-Mn^2 ,Mn^3 -Vo,La^3 -Mn^3 等),Mn^2 离子等电子陷阱的生长是Curie温度处电阻骤然增加的主要原因。 相似文献