Research on Dispersion Process of Carbon Nanotubes

Carbon nanotubes were dispersed using a sand mill. The impact of milling time and the amount of dispersant on the dispersion of multi-walled carbon nanotubes was explored. The experiments showed that the sand mill had a significant shearing effect on carbon nanotubes, reducing their aspect ratio. With longer milling times, the length of carbon nanotubes decreased, reaching 1 μm after 4 hours. Aqueous dispersants significantly reduced the system's viscosity and improved the dispersion of carbon nanotubes. The optimal dispersion effect was achieved when the mass ratio of carbon nanotubes to dispersants was 5:4.

IntroductionCarbon nanotubes have few surface defects and lack active functional groups, making them insoluble in water and various solvents. Additionally, carbon nanotubes exhibit strong van der Waals forces between them and have a high aspect ratio, making them prone to aggregation or entanglement. To fully harness the excellent properties of carbon nanotubes and apply them widely, achieving uniform and stable dispersion of carbon nanotubes is a critical challenge. Due to their excellent structure and properties, carbon nanotubes have become a hot topic in the field of nanomaterials for improving composite material performance. The dispersibility of carbon nanotubes in the matrix significantly influences their functionality. The sand milling process is known for its high shear force and efficiency, making it an effective dispersion technique for nanomaterials.

Experimental1.1 MaterialsCarbon nanotubes with a purity greater than 97%, impurities less than 3%, a diameter of 40-60 nm, and a length less than 5 μm were obtained from Shenzhen Jinko Special Materials Co., Ltd. The dispersant used was BYK-190 from ALTANA Group's BYK Chemie.

1.2 Ball MillingA Mini-Easy sand mill with specifications was used, obtained from Beijing Ruichi Tuowei Technology Co., Ltd. Zirconia balls with a diameter of 2 mm and a density of 6 g/cm3 were used, supplied by Nikkato Co., Ltd. Japan. The milling chamber had a volume of 400 mL, with over half of the volume occupied by zirconia beads. Carbon nanotubes and 10 g of BYK were added to water in the sand mill, and ball milling was performed at 2000 rpm for different time intervals.

1.3 Testing MethodThe morphological characteristics of carbon nanotubes were observed using a Hitachi Hitchis-3400 scanning electron microscope. A carbon nanotube dispersion sample was taken with a brush, applied once to non-conductive, water-absorbent paper, and allowed to dry. The point resistance values for a defined length were measured five times, and the average resistance per unit length was calculated.

Results and Analysis2.1 Impact of Milling Time on Dispersion EffectThe effect of ball milling time on carbon nanotubes is shown in the electron microscope images. After 30 minutes of milling in the sand mill, carbon nanotubes still appeared as long rods, with lengths exceeding 5 μm. After 1 hour of milling, the length reduced to a range between 1 μm and 5 μm. After 2 hours of milling, the lengths of carbon nanotubes were all below 3 μm. After 3 hours of milling, all carbon nanotube lengths were below 1 μm. After 4 hours and 5 hours of milling, the carbon nanotubes were broken and aggregated, as observed in the electron microscopy images. In summary, as the milling time in the sand mill increased, the length of carbon nanotubes became progressively shorter, with a significant dispersion effect achieved after 4 hours, with all carbon nanotube lengths below 1 μm.

2.2 Influence of Dispersant Amount on Carbon Nanotube PerformanceThe electron microscopy images revealed that as the ball milling time increased, the dispersion effect of carbon nanotubes improved, and the corresponding sample resistance values became smaller. When 12.5 g of BYK was added, the electrical conductivity of carbon nanotubes decreased, indicating that an excessive amount of BYK led to a deterioration in the performance of carbon nanotubes. In contrast, the other two groups showed a consistent trend of increasing electrical conductivity from the beginning of milling to the end, confirming the significantly improved dispersion effect of carbon nanotubes after 4 hours of ball milling.

Conclusion(1) The sand mill dispersion process had a significant shearing effect on carbon nanotubes, reducing their aspect ratio.(2) The dispersion effect of carbon nanotubes was closely related to the ball milling time in the sand mill, and longer milling times resulted in significantly improved dispersion.(3) Aqueous dispersant BYK-190 significantly reduced the system's viscosity and improved the dispersion of carbon nanotubes. The optimal dispersion effect was achieved with a mass ratio of carbon nanotubes to dispersant of 5:4, without affecting the performance of carbon nanotubes.