全文获取类型
收费全文 | 28909篇 |
免费 | 2593篇 |
国内免费 | 1349篇 |
学科分类
工业技术 | 32851篇 |
出版年
2024年 | 67篇 |
2023年 | 539篇 |
2022年 | 780篇 |
2021年 | 1217篇 |
2020年 | 1032篇 |
2019年 | 865篇 |
2018年 | 910篇 |
2017年 | 1030篇 |
2016年 | 922篇 |
2015年 | 1183篇 |
2014年 | 1588篇 |
2013年 | 1829篇 |
2012年 | 2012篇 |
2011年 | 2024篇 |
2010年 | 1795篇 |
2009年 | 1637篇 |
2008年 | 1603篇 |
2007年 | 1539篇 |
2006年 | 1532篇 |
2005年 | 1420篇 |
2004年 | 861篇 |
2003年 | 760篇 |
2002年 | 745篇 |
2001年 | 678篇 |
2000年 | 564篇 |
1999年 | 699篇 |
1998年 | 573篇 |
1997年 | 467篇 |
1996年 | 415篇 |
1995年 | 358篇 |
1994年 | 278篇 |
1993年 | 173篇 |
1992年 | 153篇 |
1991年 | 139篇 |
1990年 | 103篇 |
1989年 | 86篇 |
1988年 | 61篇 |
1987年 | 35篇 |
1986年 | 42篇 |
1985年 | 19篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 21篇 |
1981年 | 12篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1978年 | 5篇 |
1976年 | 5篇 |
1970年 | 3篇 |
1969年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Yujie Chen Yiping Song Zhen Zhang Yali Chen Qiliang Deng Shuo Wang 《Advanced functional materials》2021,31(41):2104885
Various products, including foods and pharmaceuticals, are sensitive to temperature fluctuations. Thus, temperature monitoring during production, transportation, and storage is critical. Facile indicators are required to monitor temperature conditions via color changes in real time. This study aimed to prepare and apply thiol-functionalized covalent organic frameworks (COFs) as a novel indicator for monitoring thermal history and temperature abuse. The COFs underwent obvious color changes from bright yellow to purple after exposure to different temperatures for varying durations. The reaction kinetics are analyzed under isothermal conditions, which reveal that the order of reaction rates is k−20°C < k4°C < k20°C < k35°C < k55°C. The activation energy (Ea) of the COFs is calculated using the Arrhenius equation as 50.71 kJ moL−1. The COFs are capable of sensitive color changes and offer a broad temperature tracking range, thereby demonstrating their application potential for the monitoring of temperature and time exposure history during production, transportation, and storage. This excellent performance thermal history indicator also shows promise for expanding the application field of COFs. 相似文献
3.
4.
Yuanming Xie Tianda Yu Chaojun Deng Xuefei Hu 《Journal of Nuclear Science and Technology》2020,57(9):1074-1090
ABSTRACT In the lower chamber of pressurized water reactor (PWR), the flow distribution device is the core module to distribute coolant into the core. It has complex structure and numerous design parameters. Therefore, it has important theoretical and practical significance to optimize the device. The mesh independence verification, turbulence model selection, and data processing all can influence the numerical simulation results of the lower chamber, in order to research the influence, a numerical simulation method based on the original model of CNP1000 reactor lower chamber is proposed in this paper. In the method, an optimization design method of flow distribution device is established based on surrogate model. The main design variables and optimization objectives are determined based on the device’s structure and function characteristics. And then it respectively adopts Kriging algorithm and multi-objective genetic algorithm to establish a surrogate model of flow distribution device and optimize it globally. Finally, the optimal design variables are obtained. Compared with the device’s performance before optimization, the after optimization has smaller total pressure loss and more uniform flow. The effectiveness and practicability of proposed optimization design method can be verified. 相似文献
5.
6.
采用改性木质素(MZS)作为成核剂,生物质纤维(BF)作为增强剂,通过双螺杆挤出机制备了生物降解的聚乳酸(PLA)/BF/MZS材料。采用差式扫量热仪(DSC)、电子万能试验机和扫描电子显微镜(SEM)分析了BF和MZS对PLA材料性能的影响。结果表明,BF和MZS有效提高了PLA的结晶能力和力学性能。当BF和MZS的含量分别为15%和1%时,PLA材料结晶度提高至67.1%,在50℃/min降温速率下仍具有较高的结晶能力。105℃等温结晶时,15%BF和1%MZS的PLA材料半结晶时间降低至9.0 s,比纯PLA缩短了72.2%。当PLA含有3%BF和1%MZS时,拉伸强度和冲击强度分别为70.1 MPa和7.4 kJ/m^2,比纯PLA分别提高了7.8%和10.4%,根据SEM显示,当BF含量为3%时,在PLA材料中分布较均匀。 相似文献
7.
Huizhu Cai Bingbing Chen Xiao Zhang Yuchen Deng Dequan Xiao Ding Ma Chuan Shi 《Nano Research》2021,14(1):122-130
Over recent years,catalytic materials of Fe-N-C species have been recognized being active for oxygen reduction reaction(ORR).However,the identification of active site remains challenging as it generally involves a pyrolysis process and mixed components being obtained.Herein Fe3C/C and Fe2N/C samples were synthesized by temperature programmed reduction of Fe precursors in 15%CH4/H2and pure NH3,respectively.By acid leaching of Fe2N/C sample,only single sites of FeN4species were presented,providing an ideal model for identification of catalytic functions of the single sites of FeN4in ORR.A correlation was conducted between the concentration of FeⅡN4in low spin state by Mossbauer spectra and the kinetic current density at 0.8 V in alkaline media,and such a structure-performance correlation assures the catalytic roles of low spin FeⅡN4 species as highly active sites for the ORR. 相似文献
8.
9.
10.