Converting various powdery materials into micro-nanofibres by a non-destructive pathway

Published in Materials

The processing of zero-dimensional functional powders into usable multi-dimensional artificial materials has been a prerequisite for the use of materials throughout human history, starting with the early use of pottery made from clay powders. Over the past three decades, major breakthroughs have been made in the development of zero-dimensional functional powders, creating and accumulating marvellous nanoeffects and fine powder structure designs, triggering a boom in the nanoera. However, current processing techniques often result in the loss and deactivation of their remarkable nano-features when these fine and fragile particles are processed into macroscopic functional materials. If a nondestructive processing bridge connecting powders to macroscopic applications could be built for this process, well-designed powders could be completely transformed into highly engineered artificial materials, exhibiting unique functionality and unprecedented performance. This will greatly reduce the difficulty of manufacturing and processing of new nanomaterials and functional nanoproducts, revolutionising the status quo of macroscopic materials and leading to a new wave of technological breakthroughs in various fields.

To address these challenges, we have developed a universal and mild fibration technology for using diverse powdered materials to mass-produce guest-assembled micro-nanofibres (GAFs) maintaining the precise nanostructures and functions of the functional particles, which builds a state-of-the-art bridge between the primary particles to final macroscopic engineering applications (Figure  1)。

Figure 1 Illustration of the universal powder fibration technique. An optical photograph of the original powder and the produced GAFs shows that the fibres havethe colour of the original powders, and are appropriate for mass production. Scale bar, 5 cm.

The implementation of this technique relies on the topological deformation of 2D cellulose. In our initial studies, we found that the structure and morphology of the ZnO nanosheet guest was able to remain stable within the GAF, which piqued our interest. In order to clarify the forming regularity,  the surface of fragment  from the frozen precursor liquid was observed by in-situ SEM under vacuum condition, where  the contraction and rolling process of the intermediates were observed.  Theoretical studies and complementary experiments demonstrated that the self-shrinking force drives the two-dimensional cellulose and supported particles to pucker and roll into fibres (Figure 2), suggesting that this is mainly a physical process. Then, the strategy should also have broad applicability to various powdery materials.

Figure 2 Schematic mechanism of the fibration process for GAF. Illustration of the conversion mechanism from powder to GAF based on the deformation process of a 2D-cellulose isomer during water removal. TEM image on the left is 2D-cellulose.

To confirm the universality of this technology, we have collected all the micro and nanopowders available on the market and in collaborating laboratories within our capabilities, both purchased and synthesised. We have characterised and counted the size and physicochemical properties of these powders and also carried out experiments to synthesise and characterise topological fibres, a long and iterative process that has taken many years. Statistically, we have successfully converted more than sixty types of powders, including elements, compounds, organics and hybrids, into over 120 homogeneous micro/nanofibres (46 representative GAF samples in Figure 3). The results show that the fibration process is indeed very mild, and the original morphology and structural characteristics of the guests are completely preserved in the fibres, even for fragile and delicate nanostructures such as MnO2 nanoflowers, ZnO nanoblocks, and Fe-MOF nanospindles. This provides a structural basis for the perfect expression of the powder's properties. Also, we have tried using multi-component mixed powders as guests to assemble into topological fibres, and have shown that even a more complex guest powder mixed with ten different materials can still be transformed into a denary GAF with a highly uniform fibre shape and homogeneous element distribution.

Figure 3 Overview of the morphologies of GAFs. SEM images of 46 representative single-component GAFs. Elemental GAFs (highlighted in blue) including C, diamond, Si, Ti, Fe, W, Ni, Ag, Nb, Mo, Ta; compound GAFs (highlighted in orange) including oxides (MgO, Al2O3, SiO2, TiO2, Cr2O3, MnO2, NiO, CuO, ZnO, ZrO2, Gd2O3, In2O3, CeO2, Ho2O3, Sm2O3, WO3, Fe2O3, Sb2SnO5, LiFePO4, ZnFe2O4, HAP, BaTiO3), carbides (ZrC, TiC, SiC, WC), nitrides (Si3N4, TiN), phosphide (GePx) and sulfides (MoS2, WS2); organic GAFs (highlighted in green) including PPy, polystyrene ((C8H8)n), Zn MOF, Fe MOF. c, SEM images and elemental mappings of denary GAF. Scale bars, 5 μm.

In terms of GAF controllability studies, we first counted the diameter distribution and structural features of the above-mentioned topological fibres synthesised based on different powders. It observed that the vast majority of fibre diameters of topological fibres are higher than the minimum particle size of the powders used, which is understandable as the powders are rolled in the fibres. Secondly, we modulated the content of a powder in the fibres and observed that the greater the powder content, the greater the average diameter of the fibres. The maximum diameter of all fibre samples was less than 10 µm. Also, the fibration efficiency of the powders decreases as their diameter increases above 2 µm. Based on these experimental and statistical results, we believe that this fibration technique can well convert powders of less than 2 µm into micro- and nanofibres of less than 10 µm in diameter. In addition, to tailor powders to macro applications, a wide range of topological fibres have been customised to meet the needs of different fields by selecting powders with specific properties and structures. Our macromaterials constructed from such topological fibres, when compared with materials synthesised by other methods using the same powders, can better show the properties of the powder.

This technique builds a library of basic building blocks for macroscopic materials, which provides a rich materials platform and unlimited opportunities for fundamental research and technological applications. We believe that this fibration method, which is a simple and effective new processing technology for transforming powdery materials into the fibre, will play very important roles in a range of fields involving medical, environmental, protection, catalysis, energy-related, aerospace, photoelectric materials, food engineering and manufacture of daily necessities.

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Structural Materials
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Materials Characterization Technique
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