全文获取类型
收费全文 | 111652篇 |
免费 | 11053篇 |
国内免费 | 7520篇 |
专业分类
电工技术 | 9045篇 |
技术理论 | 3篇 |
综合类 | 9866篇 |
化学工业 | 15492篇 |
金属工艺 | 7119篇 |
机械仪表 | 7806篇 |
建筑科学 | 8468篇 |
矿业工程 | 3697篇 |
能源动力 | 2752篇 |
轻工业 | 9992篇 |
水利工程 | 3009篇 |
石油天然气 | 5896篇 |
武器工业 | 1266篇 |
无线电 | 12240篇 |
一般工业技术 | 11360篇 |
冶金工业 | 4666篇 |
原子能技术 | 1873篇 |
自动化技术 | 15675篇 |
出版年
2024年 | 505篇 |
2023年 | 1800篇 |
2022年 | 3688篇 |
2021年 | 4955篇 |
2020年 | 3786篇 |
2019年 | 2787篇 |
2018年 | 3086篇 |
2017年 | 3435篇 |
2016年 | 3048篇 |
2015年 | 4581篇 |
2014年 | 5869篇 |
2013年 | 6932篇 |
2012年 | 8211篇 |
2011年 | 8969篇 |
2010年 | 8169篇 |
2009年 | 7864篇 |
2008年 | 7865篇 |
2007年 | 7626篇 |
2006年 | 7035篇 |
2005年 | 5588篇 |
2004年 | 4034篇 |
2003年 | 3163篇 |
2002年 | 2945篇 |
2001年 | 2700篇 |
2000年 | 2254篇 |
1999年 | 1911篇 |
1998年 | 1380篇 |
1997年 | 1183篇 |
1996年 | 1046篇 |
1995年 | 881篇 |
1994年 | 701篇 |
1993年 | 501篇 |
1992年 | 435篇 |
1991年 | 287篇 |
1990年 | 228篇 |
1989年 | 197篇 |
1988年 | 130篇 |
1987年 | 88篇 |
1986年 | 70篇 |
1985年 | 48篇 |
1984年 | 42篇 |
1983年 | 38篇 |
1982年 | 33篇 |
1981年 | 33篇 |
1980年 | 30篇 |
1979年 | 18篇 |
1965年 | 3篇 |
1959年 | 16篇 |
1951年 | 11篇 |
1940年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
以醛、酮为原料经羟醛缩合反应合成了具有一类含查尔酮结构的光敏性二胺:1-(3-氨基苯基)-3-(4-氨基苯基)-2-丙烯-1-酮与对-二(间氨基肉桂酰)苯.通过优化实验得到最佳的合成工艺为:醛、酮在温度为0℃下,以乙醇为溶剂,氢氧化钠为催化剂,进行羟醛缩合反应3h.收率可达到86%,经液相色谱分析其纯度分别为99.92%,99.91%,紫外可见最大吸收波长分别为349nm、361nm.所得方法操作简便,收率较高,原料易得.并用元素分析、核磁共振、红外光谱等手段对产物的结构进行了表征. 相似文献
994.
Shangru Zhai Ye Zhang Xi'e Shi Dong Wu Yuhan Sun Yongkui Shan Mingyuan He 《Catalysis Letters》2004,93(3-4):225-229
A facile two-step sol–gel method has been developed for the preparation of mesostructured aluminosilicate nano-particles (Si/Al = 7.5) with wormhole-like but uniform pore structures and highly catalytic activities towards bulky hydrocarbon cracking. It is desirable that, as this synthesis takes place in the absence of alkali cations, the mesoporous nano-particulate material is directly obtained in the acid form, making a subsequent ion-exchange and calcination treatment superfluous. 相似文献
995.
996.
997.
Nan Yao Guoxing Xiong Shishan Sheng Mingyuan He Weishen Yang Xinhe Bao 《Catalysis Letters》2002,78(1-4):37-41
A novel sol–gel process is used to synthesize mesoporous silica–alumina catalysts with controlled pore size distribution without using templates or pore-regulating agents. The nitrogen sorption analysis shows that the synthesized materials have high specific surface area in the range 587–692 m2/g and similar mesoporous distribution within 2–11 nm. Ultrasound is applied to prepare precursor silica–alumina sol with narrow particle diameter distribution. By calculation, it is found that ultrasonic treatment is able to provide 22.94 kJ/mol energy, which is just in accordance with the weak bond's bonding energy. Based on the calculation result, it is proposed that the role of ultrasonic treatment followed by acid peptization is to provide energy to break down the weak bonds. After gelation, the regular and close packing of similar-sized sol particles is capable of formation of controlled-sized interstices which are the precursors of mesopores. Solid-state 27Al MAS NMR and TEM are used to characterize the samples' structure and particle morphology. They show that most of the aluminum is located in the tetrahedral position in the present materials. According to TEM results, it is shown that the synthesized materials have spherical particles with size of about 15 nm. 相似文献
998.
The curing behavior of synthesized phenol–urea–formaldehyde (PUF) resol resins with various formaldehyde/urea/phenol ratios was studied with differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The results indicated that the synthesis parameters, including the urea content, formaldehyde/phenol ratio, and pH value, had a combined effect on the curing behavior. The pH value played an important role in affecting the shape of the DSC curing curves, the activation energy, and the reaction rate constant. Depending on the pH value, one or two peaks could appear in the DSC curve. The activation energy was lower when pH was below 11. The reaction rate constant increased with an increase in the pH value at both low and high temperatures. The urea content and formaldehyde/phenol ratio had no significant influence on the activation energy and rate constant. DMA showed that both the gel point and tan δ peak temperature (Ttanδ) had the lowest values in the mid‐pH range for the PUF resins. A different trend was observed for the phenol–formaldehyde resin without the urea component. Instead, the gel point and Ttanδ decreased monotonically with an increase in the pH value. For the PUF resins, a high urea content or a low formaldehyde/phenol ratio resulted in a high gel point. The effect of the urea content on Ttanδ was bigger than that on the gel point because of the reversible reaction associated with the urea component. Too much formaldehyde could lead to more reversible reactions and a higher Ttanδ value. The effects of the synthesis conditions on the rigidity of the cured network were complex for the PUF resins. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1368–1375, 2005 相似文献
999.
Copolythiophenes (Co‐PTs), poly(3‐hexylthiophene‐co‐3‐thiophene carboxylic acid) (P3HT‐TCa), poly(3‐hexyloxylthiophene‐co‐3‐thiophene carboxylic acid) (P3HOT‐TCa), and poly(3‐phenylthiophene‐co‐3‐thiophene carboxylic acid) (P3PhT‐TCa), were synthesized by chemical oxidized polymerization to investigate the effect of copolymerization on the properties of polythiophenes (PTs). Gel permeation chromatography showed that the molecular weight (MW) of Co‐PT was lower than that of homopolythiophene. Fourier transform infrared (FTIR) spectra indicated that the copolymerization was successful between the monomers. The λmax of Co‐PTs gave a “blue shift” in ultraviolet‐visible (UV‐VIS) spectra. Photoluminescence (PL) spectra showed that the PL intensity of Co‐PT became weaker than that of homopolythiophene and the disappearance of PL had been observed in P3HOT‐TCa. The thermal stability of Co‐PT was influenced by the carboxyl for its low decomposition temperature. Furthermore, the copolymerization between multi‐wall carbon nanotube containing thiophene ring (MWNT‐Th) and 3‐hexyloxylthiphene could also take place successfully. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 相似文献
1000.
广东南华水泥有限公司原煤均化库因生产需要进行了改造,相应地,原煤均化库布料小车的电气控制回路也进行了改造。在改造过程中,该厂充分利用原控制电路和改进后的电路元器件,只增加了若干干簧管和控制开关,稍作改动便达到了控制目的,既科学可靠,又经济实用。 相似文献