全文获取类型
收费全文 | 99157篇 |
免费 | 7433篇 |
国内免费 | 6214篇 |
专业分类
电工技术 | 8016篇 |
技术理论 | 2篇 |
综合类 | 6870篇 |
化学工业 | 20558篇 |
金属工艺 | 6192篇 |
机械仪表 | 7622篇 |
建筑科学 | 3692篇 |
矿业工程 | 1423篇 |
能源动力 | 3447篇 |
轻工业 | 6573篇 |
水利工程 | 645篇 |
石油天然气 | 4410篇 |
武器工业 | 892篇 |
无线电 | 12055篇 |
一般工业技术 | 10528篇 |
冶金工业 | 2478篇 |
原子能技术 | 1138篇 |
自动化技术 | 16263篇 |
出版年
2024年 | 282篇 |
2023年 | 1287篇 |
2022年 | 2239篇 |
2021年 | 2653篇 |
2020年 | 2352篇 |
2019年 | 2353篇 |
2018年 | 2206篇 |
2017年 | 2845篇 |
2016年 | 3107篇 |
2015年 | 3221篇 |
2014年 | 4598篇 |
2013年 | 5389篇 |
2012年 | 5711篇 |
2011年 | 6846篇 |
2010年 | 5670篇 |
2009年 | 6527篇 |
2008年 | 6319篇 |
2007年 | 6914篇 |
2006年 | 6651篇 |
2005年 | 5483篇 |
2004年 | 4793篇 |
2003年 | 4585篇 |
2002年 | 3780篇 |
2001年 | 3008篇 |
2000年 | 2656篇 |
1999年 | 2098篇 |
1998年 | 1520篇 |
1997年 | 1201篇 |
1996年 | 1124篇 |
1995年 | 1130篇 |
1994年 | 1002篇 |
1993年 | 833篇 |
1992年 | 635篇 |
1991年 | 388篇 |
1990年 | 247篇 |
1989年 | 247篇 |
1988年 | 168篇 |
1987年 | 114篇 |
1986年 | 112篇 |
1985年 | 85篇 |
1984年 | 72篇 |
1983年 | 47篇 |
1982年 | 50篇 |
1981年 | 56篇 |
1980年 | 28篇 |
1979年 | 23篇 |
1978年 | 23篇 |
1977年 | 20篇 |
1976年 | 26篇 |
1975年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
在常压、无溶剂条件下,以聚乙二醇(PEG)为相转移剂,钾盐为催化剂,碳酸乙烯酯(EC)和硫醇(RSH)为原料合成了β-羟乙基烷基硫醚(RSCH2CH2OH).系统考察了PEG对不同钾盐在RSH的β-羟乙基化反应中的固/液相转移作用以及EC用量对RSCH2CH2OH选择性的影响.通过GC和GC-MS计算与测定RSH转化率、RSCH2CH2OH选择性和副产物.结果表明,PEG本身没有催化活性,与单独加入0.5%(以RSH物质的量为基准,下同)K2CO3相比,同时加入1.0%PEG和0.5%K2CO3可显著提高RSH转化率和反应速率.PEG相对分子质量<1000时,PEG对K2CO3催化活性的强化作用随着PEG链长度的增长而增强.RSCH2CH2OH的选择性随着EC与RSH物质的量比(≥1.02)的增大而降低,副产物为乙烯基烷基硫醚和乙二醇.无溶剂条件下,PEG的加入能有效打破钾盐催化剂与液相反应物之间的相界面限制,增强钾盐催化活性,缩短反应时间,提高产品收率. 相似文献
952.
953.
为了实现滇池流域截污效果的最大化,基于前期建立的东岸排水管网SWMM模型,结合该区域1995-2016年间降雨资料,研究环湖截污干渠的错峰调蓄技术。根据雨水干渠液位高度执行不同控制模式:当水位低于6.76 m时执行典型污染物浓度阈值控制模式;当水位高于6.76 m时执行典型污染物阈值控制的同时执行液位-污染物通量控制模式。采用SS和TN作为干渠截流的典型污染物控制指标,其控制浓度阈值分别取12和5 mg/L。模拟重现期为0.5~1 a降雨时SS、TN浓度-负荷通量调蓄方案下干渠负荷收集情况,结果表明两种调蓄方案均能有效提高雨水干渠的负荷收集率,提高污水干渠出口浓度,降低雨水径流对污水厂的高水量和低浓度冲击负荷。随着重现期增大,干渠对污染物的高效和最大化收集效果越明显。但是TN调控下污染负荷收集效果优于SS,因此选择TN作为干渠截流的最优典型污染物控制指标。 相似文献
954.
The polyamide-6/attapulgite nanocomposites were prepared via an in situ polymerization route with attapulgites pre-modified with cetyltrimethylammonium bromide (CTAB) and toluene-2,4-diisocyanate (TDI). Morphology observation showed that the exfoliated attapulgite fibers were well dispersed in the polyamide-6 matrix on a nanometer scale and formed a percolation network structure. The rheological behaviors of such polymer/fibrous clay nanocompostie samples were investigated by an ARES rheometer with parallel plate geometry. The storage moduli (G′), loss moduli (G″), and dynamic viscosities of these samples increased monotonically with attapulgite content at low frequencies. The presence of attapulgites caused these nanocomposite melts to have solid-like behaviors and slower relaxation. This behavior can be explained in terms of the development of a grafting-percolated fibrous-silicate network structure. Monte Carlo simulations were performed to determine the critical threshold for attapulgites fibers in 3D. The calculated critical threshold from simulations fitted the results of our rheological experiments very well. 相似文献
955.
The nano-CeO2/ZnO catalysts were prepared using a novel combination of homogeneous precipitation with micro-emulsion for oxidative coupling of methane with CO2 as an oxidant. The prepared catalysts were compared with those prepared using the conventional impregnation. The catalysts prepared in two ways were characterized with FTIR, TEM, XRD and CO2-TPD. The effects of the reaction temperature, the amount of ZnO doped in the catalysts and the average size were investigated. The experimental investigation demonstrated that methane conversion over the nano-CeO2/ZnO catalysts prepared by the combined technique was higher than that obtained over catalysts prepared by the conventional impregnation. A better low-temperature activity has also been achieved over the nanocatalysts. There was no clear trend between the average size of nano-CeO2/ZnO catalysts and their catalytic performance but methane conversion increased with increasing fractal dimension of nanocatalysts. 相似文献
956.
957.
尼龙/聚丙烯合金的性能与制备 总被引:4,自引:0,他引:4
介绍国外各种尼龙/聚丙烯合金的性能和制备工艺。聚合物合金的相容性原理,以及PA/PP合金适用的各种相容剂。 相似文献
958.
分解炉是烧成系统的关键设备,尤其是对于燃无烟煤的分解炉,既要保证无烟煤粉在炉内的充分燃烧.又要利于系统的操作控制。基于分解炉煤粉燃烧机理,并通过分析影响炉内煤粉燃烧的各种因素及依据所用无烟煤的热失重分析结果,中材国际南京水泥设计研究院开发设计出了用于华润贵港5000砌工程的喷旋管道式在线分解炉,并与高效、低阻、防堵的旋风预热系统组成了性能优异的预分解系统。从实际运行情况看,该预分解系统易于操作控制,且对物料适应性强、阻力损耗小。 相似文献
959.
The Fe/ZrO2 catalyst (1% Fe by weight) shows a strong adsorption capacity toward the nitric oxide (at room temperature the ratio NOFe is ca. 0.5) as a consequence of the formation of a highly dispersed iron phase after reduction at 500–773 K. Nitric oxide is adsorbed mainly as nitrosyl species on the reduced surface where the Fe2+ sites are prevailing, but it is easily oxidised by oxygen forming nitrito and nitrato species adsorbed on the support. However, in the presence of a reducing gas such as hydrogen, carbon monoxide, propane and ammonia at 473–573 K the Fe-nitrosyl species react producing nitrogen, nitrous oxide, carbon dioxide and water, as detected by FTIR and mass spectrometers. The results show that nitric oxide reduction is more facile with hydrogen containing molecules than with CO, probably due the co-operation of spillover effects. Experiments carried out with the same gases in the presence of oxygen show, however, a reduced dissociative activity of the surface iron sites toward the species NOχ formed by NO oxidation and therefore the reactivity is shifted to higher temperatures. 相似文献
960.
Systems of Pd supported on various La2O3-modified -Al2O3 and CeO2–Al2O3 catalysts were tested for catalytic methanol decomposition and characterized by means of XRD, BET, TPR, H2-chemisorption and CO–FTIR. The addition of lanthanum significantly improved the selectivity of CO and H2 for all the catalysts but showed a different influence on the catalytic activity in two systems. Methanol conversion decreased on La2O3-modified Pd/-Al2O3 catalysts, in line with the reduction of Pd dispersion, while the addition of La2O3 improved the dispersion of Pd and reinforced Pd–CeO2 interaction for La2O3-modified Pd/CeO2–Al2O3 catalysts, which resulted in a high production rate of CO and H2. Thus, a synergistic effect between CeO2 and La2O3 was observed on -Al2O3-supported Pd catalyst for methanol decomposition. 相似文献