Photo of a Carbon Nanotube
What are Carbon Nanotubes (CNTs)
Carbon nanotubes (CNTs) are nanoscale forms of carbon exhibiting exceptional physical and chemical properties. Discovered in 1991, these cylindrical structures are typically one atom thick and one dimensional. They have garnered significant scientific and technological interest due to their unique attributes.
CNTs exhibit:
- Superior mechanical strength: Tensile strength 100 times greater than steel
- Electrical Conductivity: Comparable or exceeding copper
- Thermal Conductivity: Similar to a diamond
They come in various forms including fiber, film, liquid dispersion, and powder. Their unique properties position them as an ideal for a wide range of industrial applications.
Where Carbon Nanotubes Are Used
Carbon nanotubes are currently used in multiple industrial and consumer applications. They have emerged as promising candidates for energy transition solutions.
CNTs in Batteries:
The integration of CNTs into lithium-ion batteries holds immense potential, offering faster charging times, higher energy densities, extended lifecycles, and improved safety.
According to Nature Nanotechnology, CNTs in supercapacitors allow for enhanced energy densities. CNT ropes can achieve gravimetric energy densities of up to 2.1 MJ-kg^-1, 3X the energy density of a lithium-ion battery.
CNTs in Fuel Cells:
CNTS can serve as efficient catalysts, bipolar plate, and electrode material, with an ability to replace noble metals which are expensive and difficult to extract. Studies have found coating bipolar plates improves corrosion in acidic environments, according to AIM Press.
CNTs in Hydrogen:
Carbon nanotubes have been proven to absorb hydrogen to high density providing a lightweight and stable solution for hydrogen storage solutions, according to AZONano.
CNTs in Solar Cells:
Solar cells can use CNTs for light absorbing and electron transporting materials in solar cells. They can also aid in thermal management of panels to remain cool.
CNTs in Biomedical:
CNTs also demonstrate potential in biomedical applications. Their biocompatibility and unique surface properties pave the way for biosensors with extremely high sensitivity and selectivity. Additionally, their strength and flexibility offer opportunities for innovative medical implants and drug delivery systems.
Other CNT applications:
Composite materials with carbon nanotubes embedded significantly improve properties in polymer materials such as: metals, rubbers, silicones, latexes, thermoset resins, and various thermoplastics. There is also the ability to 3D print with CNT embedded in the material. The list goes on with applications in fibers and fabrics, air and water filtration, structural applications, construction materials, and cables.
Application Requirements
Chemical Vapor Deposition (CVD) is the most widely used method to produce carbon nanotubes. During the CVD process, a substrate is prepared with a layer of metal catalyst particles (nickel, cobalt, iron). The metal particle size is chosen based on the desired nanotube diameter. The substrate is heated to approximately 700 °C. (See Figure 1)
Figure 1: How CNTs are grown. Image courtesy of OCSiAl.
A carbon containing gas decomposes on the catalyst surface with carbon atoms diffusing through forming precipitates at the catalyst edge. There are two growth mechanisms: 1) Tip growth where particles remain at the top of the growing nanotube. 2) Base growth where the catalyst stays attached to the substrate surface.
From the CVD chamber, the CNTs can be collected into a fiber spool. Further processing of fibers can create CNT film, powder, and liquid dispersions. When designing a CVD process for CNT production, key experimental variables include catalyst composition, gas flow rate, pressure, reaction temperature, residence time, additive types, and the process and precursor/carbon-containing gases below:
A process gas:
- Ammonia
- Hydrogren
- Nitrogen
And, a precursor/carbon-containing gas:
- Actetylene
- Ethylene
- Ethanol
- Methane
By adjusting any of these variables, the yield and quality of the CNT will vary. See figures 2 and 3 below.
Figure 2: SpringerOpen detailing CNT example experimental data: flow rate, time, and temperature cycles.
Figure 3: MDPI detailing experimental conditions for various samples.
The key indicators of successful CNT production can be measured by:
Yield (g/h), production cost ($), graphitic crystallinity (graphite:graphitic defect ratio), diameter and length (μm/nm), alignment and orientation, defect density (defects/μm), purity (%), catalyst efficiency (CNT:catalyst mass), scalability (kg/m^2 of reactor area), and processability.
Figure 4: Carbon Trends depicting a visual representation and measurement of graphitic crystallinity
To initiate the growth of high-quality nanotubes, Brooks Instrument mass flow controllers (MFCs), known for exceptional gas flow control, enabled with digital communication protocols are a critical part of the process in managing and precisely controlling the long-term stability of multiple gas feeds.
Brooks Instrument mass flow controllers enable:
- Precise Flow Control: Enables accurate and consistent control of carbon-containing and process gases to maintain optimal conditions for nanotube synthesis.
- Real-Time Data Acquisition: Provides real-time insights on flow rates, temperature, totalized flow, and valve drive for continuous process monitoring and optimization.
- Improved Yield and Quality: Enhances nanotube growth rates and yield quality by ensuring precise gas flow and stable furnace conditions.
- Stable Flow Rates: Prevents non-uniform growth and defects by delivering a steady and repeatable gas flow across varying conditions. Brooks has proven long term stability with minimal drift in process accuracy over time.
- Gas Flexibility: Supports multiple gas pages for seamless switching between gases or mixtures without changing devices.
- Cost Savings: Reduces the number of SKUs and inventory requirements by enabling gas flexibility.
- Lab Scalability: Provides flow rates from lab experiments (<10 SCCM) to pilot processes (up to 2500 SLPM).
- Automation Integration: Supports protocols of Profibus, DeviceNet, EtherCAT, and EtherNet/IP for seamless integration into automation systems, aligning with Industry 4.0 trends.
- Diagnostics and Warnings : Real-time diagnostics simplify allow for predictive and prescriptive maintenance in real time as well as schedules based on historical device performance.
- Enhanced Safety: Multiple safety certifications (ISO, CE, UL, ATEX, IECEx, KOSHA).
Process Solution
A typical setup starts with Brooks Instrument SLA5800 Series thermal mass flow controllers with the EtherNet/IP or PROFINET interface, enabling real-time data communication between the gas flow controllers and PLC. SLA5800 Series digital mass flow controllers, known for exceptional precision and accuracy, measure and control multiple gas feeds into a furnace.
Mass flow controller accuracy is impacted by long-term stability of the instrument, which ensures more accurate gas flow control, reducing your total cost of ownership. Additionally, Brooks Instrument 122 Series mechanical pressure gauges provide reliable local pressure monitoring. With the inputs to the process in place, the growth of carbon nanotubes can begin.
Pictured here is a gas supply panel of a leading CNT rector tool consisting of four SLA5850 Series MFCs.
The high accuracy, long-term stability, and wide range of available communication protocols offered by SLA5800 mass flow controllers, coupled with Brooks Instrument 122 Series pressure gauges are critical instruments in this application setup. The results will speak for themselves - you will spend less time verifying and recalibrating your mass flow controllers while maximizing system uptime with consistent production.
Flow Scheme
Citations:
Figure 2: Ahn, J.H., Na, M., Koo, S. et al. Development of a fully automated desktop chemical vapor deposition system for programmable and controlled carbon nanotube growth. Micro and Nano Syst Lett 7, 11 (2019). https://doi.org/10.1186/s40486-019-0091-8
Figure 3: Raniszewski, G., & Pietrzak, Ł. (2021). Optimization of Mass Flow in the Synthesis of Ferromagnetic Carbon Nanotubes in Chemical Vapor Deposition System. Materials, 14(3), 612. https://doi.org/10.3390/ma14030612
Figure 4: Sownyak Mondal, Soumya Ghosh, Quantifying defects in graphene oxide structures, Carbon Trends, Volume 14, 2024, https://doi.org/10.1016/j.cartre.2024.100323.
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