The application of carbon nanotubes in the energy field
CNTs play multiple key roles in the energy field. Its high electrical conductivity and large specific surface area make it an ideal electrode conductive additive and active material carrier for lithium-ion batteries and supercapacitors, which can significantly increase the charge and discharge rate and cycle life. Especially in silicon-carbon anode materials, its extremely high aspect ratio and mechanical strength can be woven into a strong and tough conductive network framework like "nano-steel bars", tightly wrapping or embedding silicon nanoparticles within it. This structure not only provides a high-speed electronic conduction path, but more importantly, it effectively binds silicon particles, buffers their drastic volume changes during repeated charging and discharging processes, and prevents particle powdering, shedding, and the collapse of the conductive network.
Structural classification and preparation of carbon nanotubes
1. Single-walled carbon nanotubes (SWCNTs)
It is made by curling single-layer graphene, with a diameter usually ranging from 0.4 to 3 nanometers. Its electronic properties (metallic or semiconductor) depend on the curly chiral Angle ((n, m) index).
2. Multi-walled carbon nanotubes (MWCNTs)
Composed of multiple (2 to dozens of) coaxially nested graphene cylinders with an interlayer spacing of approximately 0.34 nm and a wide diameter range (2–100 nm).
3. Carbon Nanotube Arrays (CNT Arrays)
Referring to an assembly of CNTs grown vertically or horizontally in an oriented manner on a substrate via specific methods (primarily Chemical Vapor Deposition, CVD), which can be either Single-Walled Carbon Nanotube (SWCNT) arrays or Multi-Walled Carbon Nanotube (MWCNT) arrays.
The dispersion problem of carbon nanotubes
1. Strong vander Waals forces and high specific surface area
Carbon nanotubes (CNTs), especially the slender single-walled carbon nanotubes (SWCNTs), possess an enormous specific surface area. Strong van der Waals forces exist between the tube walls, leading to their high tendency to agglomerate and entangle, forming intractable bundled or networked aggregates. Such agglomeration severely impairs the unique nano-effects of CNTs (e.g., high electrical/thermal conductivity, mechanical strength, and quantum effects), resulting in significant degradation or even complete loss of their performance.
2. Chemical inertness
The perfect carbon nanotube (CNT) walls are composed of sp²-hybridized carbon atoms, featuring high chemical stability and inertness. Lacking sufficient active functional groups (e.g., -OH, -COOH) on their surface, CNTs struggle to form strong interactions with solvents or matrix materials, resulting in poor compatibility.
3. High aspect ratio
The huge aspect ratio of CNTs makes their resistance to movement in fluids large, and they are prone to entanglement and knotting under shear force, increasing the difficulty of uniform dispersion.
Dispersion methods and their limitations
1. Physical dispersion (ultrasonic, grinding, shearing
The most commonly used initial method is to temporarily disrupt the aggregates by applying high-energy inputs (ultrasonic cavitation, high-speed shearing, ball milling impact). However, there are significant drawbacks: the effect is short-lived, and excessive energy can easily damage the CNT structure (causing defects and shortening the length); it is difficult to completely dissociate tightly bound SWCNT bundles; and once the energy input is stopped, the aggregates are prone to re-aggregate again. Ultrasonic treatment requires particularly careful control of power and time.
2. Chemical dispersion (Surface modification)
Covalent modification: Introducing functional groups (such as carboxylation, amination, fluorination, etc.) through covalent bonding on the CNT tube wall. The advantages are that it can be firmly modified, has good dispersion stability, and can also improve compatibility with the matrix. However, the fatal drawback is that it will severely damage the sp² carbon network structure and impair the intrinsic excellent electrical, optical and mechanical properties of CNT.
Non-covalent modification: CNT surfaces are coated using surfactants (such as SDS, SDBS, CTAB, Triton X series), high molecular polymers (such as polyvinylpyrrolidone PVP, polystyrene sulfonate PSSS, DNA), or π-π conjugated molecules (such as pyrene derivatives) through weak interactions such as physical adsorption, hydrophobic interaction, and π-π stacking. The advantage is that it can maximize the protection of the intrinsic properties of CNTs. However, the disadvantages are also obvious: the dispersion stability is greatly affected by the environment (pH, temperature, ionic strength); the residual surfactants/polymer may interfere with subsequent applications (such as electrical properties); for extremely compact SWCNT bundles, the disentanglement efficiency is limited.
The revolutionary role of Yocell high-pressure microfluidization homogenizer technology in the dispersion of carbon nanotubes
In the face of the numerous limitations of traditional dispersion methods, the Yocell high-pressure microfluidization homogenization technology stands out and becomes a powerful tool for solving the dispersion problem of CNTs (especially SWCNTs). Its working principle is as follows: The CNT suspension that has been preliminarily wetted or pre-dispersed is driven by ultra-high pressure, and flows through the internally precisely designed micro-channels (such as Z-shaped, Y-shaped intercommunicating cavities) at an extremely high speed. During this process, the fluid will undergo multiple physical effects.
Mechanism of action
1. Ultra-high shear force
When the fluid moves at high speed (at supersonic speed) within extremely narrow microchannels, a significant velocity gradient and inter-layer shear force are generated, which can strongly dislodge and tear the CNT aggregates.
2. Intense cavitation effect
The extremely high-speed flow causes a sudden drop in local pressure, causing the liquid to vaporize and generating a large number of tiny bubbles (vacuum bubbles). These bubbles collapse violently in the high-pressure area in an instant, releasing intense shock waves and micro jets. This micro jet impact can efficiently "explode" the most stubborn CNT bundles, especially SWCNT bundles.
3. High-frequency collisions and turbulence
In the complex microchannel structure, fluids will generate intense turbulence, and high-frequency and high-speed collisions will occur between CNT particles and between particles and cavity walls, further promoting deagglomeration.
4. Synergistic effect
The powerful physical effects such as shearing, cavitation, collision and turbulence occur simultaneously within the micro-jet chamber and reinforce each other, forming a synergistic dispersion mechanism. This mechanism is highly effective in unraveling the CNT aggregates (especially SWCNT bundles) that are bound by strong van der Waals forces.
Core advantage
1. Outstanding deagglomeration ability
It can efficiently dissociate tight SWCNT beams that are difficult to handle by traditional methods (such as ultrasound), achieving a smaller tube bundle and a state closer to single-strand dispersion, significantly improving the dispersion quality.
2. High-concentration dispersion
The extremely high-speed flow causes a sudden drop in local pressure, causing the liquid to vaporize and generating a large number of tiny bubbles (vacuum bubbles). These bubbles collapse violently in the high-pressure area in an instant, releasing intense shock waves and micro jets. This micro jet impact can efficiently "explode" the most stubborn CNT bundles, especially SWCNT bundles.
3. High-frequency collisions and turbulence
In the complex microchannel structure, the fluid will form intense turbulence. Frequent and high-speed collisions occur between CNT particles and between particles and the cavity walls, further promoting the disintegration of agglomerations.
4. Synergistic effect
The powerful physical effects such as shearing, cavitation, collision and turbulence occur simultaneously within the micro-jet chamber and reinforce each other, forming a synergistic dispersion mechanism. This mechanism is highly effective in unraveling the CNT aggregates (especially SWCNT bundles) that are bound by strong van der Waals forces.
