1 实验方法
1.1 Rb纳米溶胶的制备
Rb纳米颗粒的制备采用固/液转变+超声分散的方法,如图1所示。首先,在N2手套箱中(O2和H2O浓度分别小于10 × 10-6和1 × 10-6),切20 g钾块置于200 mL甲苯溶液中,于90℃加热搅拌1.5 h直至钾块的新鲜暴露面不再氧化变色,说明甲苯中的O2和H2O已经被完全除去(步骤I)；接着,将装有金属Rb的安瓿瓶在50℃的加热台上加热至金属Rb熔化,迅速切割开安瓿瓶,取10 μL液态金属Rb和10 mL已去除O2和H2O的甲苯共同放入透明样品瓶中,盖好瓶盖,再利用石蜡密封住瓶口(步骤II)。
图1
图1
固/液转变+超声分散法制备Rb纳米溶胶的示意图
Fig.1
Schematic of the rubidium sol preparation via solid/liquid transformation and ultrasonic dispersion
然后,将密封好的样品瓶从手套箱中取出并放置在功率可调的KQ-400DE数控超声波清洗器水池中,于80℃水浴加热。样品瓶中的Rb再次熔化成液态并沉在瓶中的底部,以40 kHz的频率和400 W的功率超声5 min,在此过程中,可以观察到Rb液滴逐渐消失,溶液的颜色从无色变成蓝灰色(步骤III)。最后,将样品瓶从超声池中取出冷却,即获得了甲苯分散的Rb纳米溶胶(步骤IV)。类似地,通过调节超声功率至320 W和240 W,可获得系列胶体溶液。
1.2 表征
在UV-3600紫外-可见-近红外分光光谱仪上进行Rb纳米溶胶的吸收光谱测试,并将纯甲苯的光吸收谱线作为基线。在N2手套箱中,使用移液枪将制备的胶体溶液滴在深度为2 mm、直径为15 mm的玻璃槽内,加热使甲苯溶剂挥发后残留物质形成粉末,用聚酰亚胺胶带密封住粉末样品以隔绝O2和H2O,采用X'Pert X射线衍射仪(XRD)进行物相分析,所使用的X射线为CuKα。将胶体溶液滴在铜网上,干燥后采用JEM-2010透射电子显微镜(TEM)对样品进行微结构的表征。使用TEM上配备的IE250X-Max50能谱仪(EDS)进行元素分析。
1.3 燃烧特性的评估
将陶瓷舟放置于加热台上加热至一定温度(温度由红外测温计测量),取300 μL的金属Rb纳米溶胶滴入陶瓷舟内,利用摄像机记录胶体溶液的着火、燃烧过程。通过Adobe Premiere软件慢速播放并截取特定时间或状态下的照片来研究相关的燃烧行为。
2 实验结果与讨论
图2
图2
所制备的Rb溶胶的光学吸收谱、溶胶装在样品瓶中光学照片及稀释10倍后的Tyndall效应
Fig.2
Optical absorption spectrum of the as-prepared sol solution (Inset I shows the optical photograph of the as-prepared sol in sample bottle; inset II shows the Tyndall effect of the sol after it was diluted to one-tenth of the original)
通过观察胶体溶液的颜色(纳米Rb的团聚、氧化均会造成颜色变化),发现在室温下,这样的储存方式保存半个月后,溶胶颜色无明显变化(尽管底部似有沉淀物,但可通过超声重新分散),表明通过这种方法制备、保存的溶胶具有较好的稳定性。若将装有Rb溶胶的蜡封样品瓶再通过真空袋包装,并放置在-18℃的冷冻环境中,则可以存放3个月以上。这说明一个良好的存储方式需要隔绝O2、H2O和保持低的保存温度。
2.1 形貌与结构
图3
图3
所制备的Rb溶胶中产物的表征
Fig.3
Characterization of the products
(a) XRD spectrum (b) TEM image (c) size distribution
(d) EDS result (f—atomic fraction) (e) element mapping of an isolated nanoparticles (NPs)
2.2 尺寸的可控性
图4
图4
不同超声功率下获得的溶胶的光学吸收谱、溶胶中产物的TEM像及粒径统计
Fig.4
Optical absorption spectra under different ultrasonic powers (Insets show the photos of the corresponding sols) (a), TEM images (b, d) and size distributions (c, e) of sols obtained with ultrasonic powers of 320 W (b, c) and 240 W (d, e)
2.3 Rb纳米溶胶的形成
图5
图5
Rb纳米溶胶的形成过程示意图
Fig.5
Schematics of the formation of rubidium sol
(a) ultrasonic cavitation effect
(b) schematic of the formation of rubidium NPs in toluene (I: cavitation bubble collapse induced mutual sputtering of rubidium and toluene at their interface, which produces relatively large liquid rubidium particles; II: smaller rubidium droplets produced via repeated cavitation bubble collapse; III: rubidium droplets reach critical size to form sol)
在本实验的体系中,在超声波作用下,许多小气泡会在甲苯和Rb液滴中形成、长大进而崩灭。气泡崩灭瞬间将产生局部的高压、高热微流,使金属Rb液滴与甲苯溶液在界面处互相溅射,分离出许多较大的金属Rb液滴,并在表面张力的作用下转变为近球形,如图5b中的步骤I所示。随着超声空化过程不断进行,这些分离出来的较大的Rb液滴也会反复在空化效应的作用下形成更小的Rb液滴,并分散在甲苯中,如图5b中的步骤II所示。当小的Rb液滴的表面张力与超声造成的应力达到平衡时,液滴将维持一个稳定的尺寸[25,26]。冷却后,Rb液滴凝固为固态颗粒,形成甲苯分散的金属Rb纳米颗粒溶胶,如图5b中的步骤III所示。由于在固定频率的情况下,一定范围内随着超声波功率增大,空化作用增强[27],所以,最终的金属Rb纳米颗粒尺寸就越小(如图4所示)。
3 点火特性
图6
图6
将300 μL的甲苯/金属Rb纳米溶胶置于120℃的陶瓷舟中不同时间的点火特性
Fig.6
Photos at intervals of 0 s (a), 1.06 s (b), 1.25 s (c), 2.05 s (d), 4.19 s (e), and 4.75 s (f) after 300 μL sol was placed in the ceramic boat at 120oC
图7
图8
图8
在120℃陶瓷舟中的溶胶液膜的形态演变及引燃示意图
Fig.8
Evolution of liquid film morphology in the ceramic boat at 120oC (a) and schematic of the ignition principle (b)
由于N2手套箱的密闭性有限,这种方法制备的纳米Rb颗粒的表面将会不可避免地被氧化,而氧化生成的Rb的氧化物或RbOH的存在会减慢纳米Rb的氧化速率、降低产生的热量,从而会降低Rb颗粒的点火性能。如果能够完全隔绝H2O和O2,将能够制备出点火性能更好的Rb纳米颗粒。
4 结论
依据Rb的低熔点特点和超声的乳化效应,提出并建立了固/液转变+超声分散的Rb的纳米化策略,即通过将熔化的液态Rb置于惰性的甲苯介质中进行超声分散、进而冷却凝固实现了Rb的纳米化,获得了甲苯分散的金属Rb纳米溶胶。Rb纳米颗粒尺寸可通过调节超声功率进行控制。该方法具有良好的可控性,既便捷又高效,适合于批量制备Rb纳米溶胶。同时,这种方法也可用来制备其他低熔点活泼金属的纳米颗粒。由于金属Rb纳米颗粒易于氧化从而形成局部的高温点,它可以对有机物(甲苯等)进行快速的引燃。
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