Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, 61-614 Poznań, Poland and Centre for Advanced Technologies, Adam Mickiewicz University, Umultowska 89c, 61-614 Poznań, Poland.
Université de Strasbourg, CNRS, ISIS, 8 allée Gaspard Monge, 67000 Strasbourg, France.
Nanoscale. 2018 Dec 28;10(48):23182-23190. doi: 10.1039/c8nr08364a. Epub 2018 Dec 5.
Phyllosilicates are layered materials possessing unique thermal properties, commonly exploited in their multilayered crystalline form as refractory insulators and heating elements. A more versatile use of such materials, however, would require their existence in the form of inks and dispersions ready to be patterned. Within this framework, the liquid-phase exfoliation of low-cost, low-purity materials such as bulk multiphasic minerals and powders represents an economically advantageous approach for the production of 2D nano-sized objects with a defined composition, size and morphology. Here, ultrasound-assisted exfoliation and shear-mixing of a multi-phasic vermiculite in mild acidic aqueous solutions were employed to successfully obtain dispersions of mono- and few-layer thick clay nanosheets. The exfoliated materials were thoroughly investigated through granulometry, X-Ray Diffraction (XRD), specific surface area measurements and AFM imaging. Despite the fact that the lateral size and the thickness distribution of exfoliated flakes obtained with the two approaches appear similar, the ultrasound-assisted exfoliation process yielded a larger amount of mono- and bi-layer thick flakes as well as materials with a higher specific surface area. XRD analysis revealed that the use of ultrasound waves in an acidic environment results in the complete exfoliation of the vermiculite layer, whereas the use of shear forces under the same conditions results in the exfoliation of hydrobiotite and mica crystalline phases. Thermal conductivity measurements provided clear evidence on how the structural changes - arising from the exfoliation process - have a direct impact on the properties of the exfoliated clay. Remarkably, compared to the raw material, the thermal conductivity of the exfoliated material decreases by 25%, reaching the ultra-low thermal conductivity regime (<0.1 W m K). Our approach may enable in the future the generation of patterns of thermal insulators onto different surfaces by applying vermiculite nanosheets in the form of dispersions and printable inks.
层状硅酸盐是具有独特热性能的层状材料，通常以其多层晶体形式作为耐火绝缘体和加热元件加以利用。然而，要更灵活地使用这些材料，就需要将其制成可用于图案化的油墨和分散体形式。在这种情况下，使用低成本、低纯度的多相块状矿物和粉末等材料进行液相剥离，代表了一种具有经济优势的方法，可用于生产具有特定组成、尺寸和形态的二维纳米级物体。在这里，采用超声辅助剥离和剪切混合的方法，对多相蛭石在温和酸性水溶液中的进行处理，成功地获得了单层和少数层厚的粘土纳米片分散体。通过粒度分析、X 射线衍射（XRD）、比表面积测量和原子力显微镜成像对剥离材料进行了全面研究。尽管两种方法得到的剥离材料的横向尺寸和厚度分布相似，但超声辅助剥离过程产生了更多的单层和双层厚的薄片以及比表面积更高的材料。XRD 分析表明，在酸性环境中使用超声波会导致蛭石层完全剥离，而在相同条件下使用剪切力会导致水云母和云母晶相的剥离。热导率测量提供了明确的证据，证明了结构变化（源于剥离过程）如何直接影响剥离粘土的性质。值得注意的是，与原材料相比，剥离材料的热导率降低了 25%，达到超低热导率范围（<0.1 W m K）。我们的方法将来可能通过将蛭石纳米片以分散体和可印刷油墨的形式应用于不同表面来生成热绝缘体图案。
