全文获取类型
收费全文 | 15393篇 |
免费 | 2984篇 |
国内免费 | 3166篇 |
专业分类
化学 | 6860篇 |
晶体学 | 298篇 |
力学 | 744篇 |
综合类 | 276篇 |
数学 | 1328篇 |
物理学 | 4180篇 |
无线电 | 7857篇 |
出版年
2024年 | 95篇 |
2023年 | 301篇 |
2022年 | 654篇 |
2021年 | 635篇 |
2020年 | 576篇 |
2019年 | 555篇 |
2018年 | 504篇 |
2017年 | 697篇 |
2016年 | 563篇 |
2015年 | 795篇 |
2014年 | 950篇 |
2013年 | 1169篇 |
2012年 | 1291篇 |
2011年 | 1297篇 |
2010年 | 1271篇 |
2009年 | 1342篇 |
2008年 | 1344篇 |
2007年 | 1298篇 |
2006年 | 1183篇 |
2005年 | 926篇 |
2004年 | 617篇 |
2003年 | 461篇 |
2002年 | 462篇 |
2001年 | 519篇 |
2000年 | 527篇 |
1999年 | 293篇 |
1998年 | 143篇 |
1997年 | 130篇 |
1996年 | 108篇 |
1995年 | 99篇 |
1994年 | 103篇 |
1993年 | 93篇 |
1992年 | 85篇 |
1991年 | 50篇 |
1990年 | 75篇 |
1989年 | 54篇 |
1988年 | 45篇 |
1987年 | 38篇 |
1986年 | 31篇 |
1985年 | 27篇 |
1984年 | 26篇 |
1983年 | 26篇 |
1982年 | 14篇 |
1981年 | 15篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1965年 | 4篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
Using renewable green hydrogen and carbon dioxide (CO2) to produce methanol is one of the fundamental ways to reduce CO2 emissions in the future, and research and development related to catalysts for efficient and stable methanol synthesis is one of the key factors in determining the entire synthesis process. Metal nanoparticles stabilized on a support are frequently employed to catalyze the methanol synthesis reaction. Metal-support interactions (MSIs) in these supported catalysts can play a significant role in catalysis. Tuning the MSI is an effective strategy to modulate the activity, selectivity, and stability of heterogeneous catalysts. Numerous studies have been conducted on this topic; however, a systematic understanding of the role of various strengths of MSI is lacking. Herein, three Cu/ZnO-SiO2 catalysts with different strengths of MSI, namely, normal precipitation Cu/ZnO-SiO2 (Nor-CZS), co-precipitation Cu/ZnO-SiO2 (Co-CZS), and reverse precipitation Cu/ZnO-SiO2 (Re-CZS), were successfully prepared to determine the role of such interactions in the hydrogenation of CO2 to methanol. The results of temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) characterization illustrated that the MSI of the catalysts was considerably affected by the precipitation sequence. Fourier transform infrared reflection spectroscopy (FT-IR) results indicated that the Cu species existed as CuO in all cases and that copper phyllosilicate was absent (except for strong Cu-SiO2 interaction). Transmission electron microscopy (TEM), X-ray diffraction (XRD), and N2O chemical titration results revealed that strong interactions between the Cu and Zn species would promote the dispersion of Cu species, thereby leading to a higher CO2 conversion rate and improved catalytic stability. As expected, the Re-CZS catalyst exhibited the highest activity with 12.4% CO2 conversion, followed by the Co-CZS catalyst (12.1%), and the Nor-CZS catalyst (9.8%). After the same reaction time, the normalized CO2 conversion of the three catalysts decreased in the following order: Re-CZS (75%) > Co-CZS (70%) > Nor-CZS (65%). Notably, the methanol selectivity of the Re-CZS catalyst was found to level off after a prolonged period, in contrast to that of Co-CZS and Nor-CZS. Investigation of the structural evolution of the catalyst with time on stream revealed that the high methanol selectivity of the catalyst was caused by the reconstruction of the catalyst, which was induced by the strong MSI between the Cu and Zn species, and the migration of ZnO onto Cu species, which caused an enlargement of the Cu/ZnO interface. This work offers an alternative strategy for the rational and optimized design of efficient catalysts. 相似文献
83.
反相乳液聚合制备壳聚糖接枝共聚物及应用 总被引:1,自引:0,他引:1
以壳聚糖、丙烯酰胺、甲基丙烯酰氧乙基三甲基氯化铵为原料,硝酸铈铵为引发剂,Span-20为乳化剂,通过反相乳液聚合技术,合成壳聚糖阳离子接枝共聚物。分析讨论了乳化剂用量、引发剂浓度、油水体积比、单体配比、反应时间、反应温度对共聚物接枝率的影响,并采用正交试验方法对合成条件进行了优化。研究了其对重金属离子Cu2 、Cd2 、Zn2 的吸附性能。结果表明,在Cu2 、Cd2 、Zn2 的混合离子体系中,该共聚物对Cu2 、Zn2 有选择性吸附。 相似文献
84.
85.
FTIR-ATR has been used for understanding the interaction between bacteria and surfaces in the adsorption progress. 相似文献
86.
本文用数学期望方法研究了非线型共缩聚物的重均分子量,并推导出重均分子量表达式,将3种特例的理论曲线与实验结果相比较,两者基本相符。 相似文献
87.
锂离子电池正极材料LiMn2O4的合成与晶体结构(英) 总被引:2,自引:0,他引:2
Spinel LiMn2O4 powders were prepared using two-step synthesis method consisting of solid-state reaction method and citrate modified sol-gel method. The effects of the calcination temperature and the Li/Mn ratio of raw materials were studied on the physicochemical and electrochemical properties of the spinel LiMn2O4 powders, such as crystallinity, lattice constant and density. The title compound was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Polycrystalline LiMn2O4 powers calcined at 750 ℃ were found to be composed of very uniformly-sized microcrystal with an average particle size of 300 nm. The improvement in electrochemical properties was mainly attributed to the process of re-grinding by absolute alcohol. 相似文献
88.
微波辐照下无皂阳离子聚(St-MMA)高分子纳米粒子的制备和表征 总被引:10,自引:1,他引:10
讨论了微波辐照下 ,以丙酮 水为分散介质 ,利用阳离子型自由基引发剂偶氮二异丁基脒盐酸盐(AIBA)引发苯乙烯 (St)和甲基丙烯酸甲酯 (MMA)共聚 ,合成出表面带正电荷的P(St MMA)共聚物纳米粒子 ,考察了丙酮用量、单体和引发剂浓度对纳米粒子粒径、粒径分布和乳液稳定性的影响 .结果表明 ,丙酮 水的体积比由 0增加到 1 2 6∶1时 ,粒子的平均水化半径从 12 2 2 1nm降低到 2 4 6 8nm ,粒径分布变宽 ,乳液抗电解质稳定性逐渐增强 ;增加引发剂和共聚单体MMA的浓度 ,粒子的水化半径逐渐减小 ,粒径分散系数增大 . 相似文献
89.
90.