全文获取类型
收费全文 | 25500篇 |
免费 | 2183篇 |
国内免费 | 1163篇 |
专业分类
电工技术 | 1467篇 |
技术理论 | 2篇 |
综合类 | 1666篇 |
化学工业 | 4233篇 |
金属工艺 | 1483篇 |
机械仪表 | 1672篇 |
建筑科学 | 2280篇 |
矿业工程 | 891篇 |
能源动力 | 715篇 |
轻工业 | 1477篇 |
水利工程 | 437篇 |
石油天然气 | 1925篇 |
武器工业 | 167篇 |
无线电 | 2580篇 |
一般工业技术 | 2977篇 |
冶金工业 | 1261篇 |
原子能技术 | 296篇 |
自动化技术 | 3317篇 |
出版年
2024年 | 59篇 |
2023年 | 473篇 |
2022年 | 676篇 |
2021年 | 1058篇 |
2020年 | 835篇 |
2019年 | 693篇 |
2018年 | 758篇 |
2017年 | 855篇 |
2016年 | 735篇 |
2015年 | 980篇 |
2014年 | 1319篇 |
2013年 | 1485篇 |
2012年 | 1681篇 |
2011年 | 1716篇 |
2010年 | 1541篇 |
2009年 | 1411篇 |
2008年 | 1384篇 |
2007年 | 1317篇 |
2006年 | 1456篇 |
2005年 | 1307篇 |
2004年 | 824篇 |
2003年 | 716篇 |
2002年 | 642篇 |
2001年 | 629篇 |
2000年 | 619篇 |
1999年 | 745篇 |
1998年 | 557篇 |
1997年 | 470篇 |
1996年 | 433篇 |
1995年 | 374篇 |
1994年 | 294篇 |
1993年 | 159篇 |
1992年 | 153篇 |
1991年 | 130篇 |
1990年 | 103篇 |
1989年 | 81篇 |
1988年 | 56篇 |
1987年 | 33篇 |
1986年 | 30篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 6篇 |
1982年 | 11篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1951年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
寒区河道凌汛灾害河势“弯道效应”的量化评估十分重要。基于分形理论提出河道横断面-纵剖面-平面多维度河势分形维数计算方法及其物理机制,并探讨黄河内蒙古段不同维度河势演变分形特征及其与凌汛灾害的关联关系。结果表明,黄河内蒙古段不同维度河势均具有多尺度自相似分形特征,且具有多年记忆周期的长程相关性;冰坝(严重性冰塞)发生频次与河道主槽弯曲分形维数呈正相关指数型函数关系,与河相系数、深泓点高程和河段平均底坡分形维数负相关,与水深-面积分形维数正相关,总体表明冰坝灾害更易发生于主槽偏移摆动大、蜿蜒曲折、河湾发育程度高的宽浅型弯曲河道,研究成果可为凌汛期冰塞冰坝灾害易发河段诊断及预测提供重要理论依据。 相似文献
3.
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
4.
以具有轻质高强优异性能的蜻蜓翅脉结构为设计灵感,在分析翅脉网格结构抗冲击原理的基础上,设计了传统和仿生两类对比结构。采用熔融挤出3D打印机成功制备了具有不同结构的连续碳纤维增强聚乳酸复合材料试样,并对不同结构复合材料试样的拉伸性能和抗冲击性能进行了测试和对比分析。研究分析结果表明:由于拉伸力方向上的连续碳纤维含量相对较少,限制了仿生结构复合材料抗拉强度的提高,但仿生结构的平均抗拉强度为传统结构的1.18倍;当仿生结构复合材料试样受到冲击力时,其内部六边形结构的连接角度会发生变化,从而极大消耗冲击能量,同时具有六边形网格结构的连续碳纤维可以有效阻碍裂纹的扩展,因此仿生结构的平均冲击韧性可以达到传统结构的2.46倍;仿生蜻蜓翅脉结构可以显著提高增材制造复合材料的综合力学性能,且对于抗冲击性能的提高具体突出效果。连续碳纤维增强树脂基复合材料的有效可行的仿生蜻蜓翅脉结构设计和增材制造,可极大扩展其在高冲击载荷领域中的相应应用。 相似文献
5.
6.
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
7.
8.
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
9.
10.