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Carbon nanomaterials are considered to be next-generation materials to help design electronics with desirable properties for high-temperature applications. Their prominent features can be evaluated through already established analysis techniques, such as thermogravimetry (TGA) and differential scanning calorimetry (DSC).
Properties of Carbon Nanomaterials
Carbon has four electrons in the outer electron layer, making a valence of four, through which it can form either single, double, or triple covalent bonds with other elements.
The ability to create a long and branching chain of carbon compounds results in many valuable allotropes, such as diamond, carbon nanotubes, nanofibers, and graphene. The functional properties of carbon make a diamond transparent, hard and an excellent thermal conductor, whereas graphite is an excellent electrical conductor.
Carbon nanomaterials with a crucial dimension range from 1-100 nm provide unique physical and chemical properties, reinforced by their structural geometry and chemical bonds. In nano size, the carbon material’s surface is highly accessible to the electrolyte, promoting an enhanced reaction to generate high stability and good conductivity. Their desirable properties endorse carbon allotropes to improve mechanical, electrical, thermal properties in various applications.
Techniques for Thermal Analysis
To exploit the versatile properties of nanomaterials, they are desired to be pure and thermally stable. The National Institute of Standards carried out sufficient characterization of nanomaterials and commented on the importance of scaling instrumentation to a level of microgram-sized samples of nanoparticles to accurately measure their purity (Mansfield & Quinn, 2011).
The nanomaterials’ thermal performance can be monitored by thermal analysis techniques, such as TGA and DSC, prominently established to determine the physical or chemical properties of a material at different temperatures during heating, cooling, or at a constant temperature.
The DSC measures the flow of heat into and out of a sample as a function of temperature or time, which could change the material's mass (Tomoda, et al., 2020).
The TGA determines the change in the sample mass, charges, size, and other properties during heating at a given temperature. This analysis is recommended to perform before DSC measurement to avoid any inconsistency due to DSC-prone damage to the sample.
For the analysis, the sample is heated using a specific temperature program in the atmosphere, controlled by introducing vacuum, inert or reactive gases.
With the difference in energy of the sample temperature to the reference, the amount of excess heat released or absorbed can be determined, which assists in the understanding of the thermodynamic changes, such as heat capacity, enthalpy, and entropy, and kinetic data, including activation energies of the material.
Thermal Investigation of Carbon Nanomaterials
Carbon nanotubes (CNTs), graphene, and carbon fibers are the commonly researched carbon nanomaterials for thermal analysis.
The composition of nanomaterials is usually analyzed at the initial stage to understand different parameters before they can be deployed into other systems or materials.
Heating a nanomaterial sample and comparing the oxidation temperature transitions to those of the pure components is the conventional method to determine this analysis.
CNTs are considered to be a good material for TGA analysis as it is one of the fastest methods to measure relative percentages of single-walled (SWCNTs) versus multi-walled CNTs (MWCNTs) in a sample (Bužarovska, Stefov, MNajdoski, & Bogoeva-Gaceva, 2015).
The researchers investigated the TGA of 7 mg MWCNTs obtained by catalytic pyrolysis of waste low-density polyethylene under air atmosphere in the temperature range between 30 ºC and 900 ºC at heating rates of 2-20 ºC per minute and adopted curve fitting method to quantify the complex oxidation behavior of the material.
The results demonstrated that competitive oxidation/combustion processes greatly influence the derivative thermogravimetry curves. By heating the CNTs powder that consists of amorphous carbon, structured CNTs, and metal catalyst particles in an oxidizing environment, the decomposition temperature can be observed for each component, which can help understand the purity of CNTs.
G. Bannov, V. Popov, and Kurmashov compared TGA and DSC data on the reduction of graphene oxide during heating and understood the thermal behavior.
Another team (Hsieh, Chou, Lin, Hsieh, & Shu, 2010) purified MWCNTs with nitric acid (HNO3) to enhance its activation energy. They used DSC and TGA spectrometers to analyze the thermal stability of MWCNTs. The DSC operated at the heating rate of 0.25 to 2.0 °C/min, which demonstrated the increase in activation energy and exothermic onset temperature with increasing HNO3 concentration.
The TGA results showed that both as-grown and modified MWCNTs’ decomposition temperatures were higher than 500 °C in the air. The researchers have already advanced their experiments by linking TGA with Fourier transform infrared (FTIR) to identify the gas phase product of MWCNTs to be CO2.
Future of the Thermal Analysis Techniques
TGA and DSC are long-established thermal analysis techniques assisting the research and engineering of high-temperature applications.
The techniques could be enhanced further by linking them to spectroscopic techniques, such as Raman and mass spectrometry, to acquire an advanced understanding of decomposition products and improve their sensitivity.
References and Further Reading
Bužarovska, A., Stefov, V., MNajdoski, e., & Bogoeva-Gaceva, G. (2015). Thermal analysis of multi-walled carbon nanotubes material obtained by catalytic pyrolysis of polyethylene. Macedonian Journal of Chemistry and Chemical Engineering. doi:10.20450/mjcce.2015.620
G. Bannov, A., V. Popov, M., & Kurmashov, P. (2020). Thermal analysis of carbon nanomaterials: advantages and problems of interpretation. Journal of Thermal Analysis and Calorimetry volume. doi:10.1007/s10973-020-09647-2
Hsieh, Y.-C., Chou, Y.-C., Lin, C.-P., Hsieh, T.-F., & Shu, C.-M. (2010). Thermal Analysis of Multi-walled Carbon Nanotubes by Kissinger’s Corrected Kinetic Equation. Aerosol and Air Quality Research, 10(3), 212-218. doi:10.4209/aaqr.2009.08.0053
Mansfield, E., & Quinn, T. P. (2011). Microscale thermogravimetric device analyzes nanoparticle purity and coatings. Retrieved from SPIE: https://spie.org/news/3528-microscale-thermogravimetric-device-analyzes-nanoparticle-purity-and-coatings?SSO=1 (Accessed on 10 March, 2021)
Tomoda, B. T., Yassue-Cordeiro, P. H., Ernesto, J. V., Lopes, P. S., Péres, L. O., Silva, C. F., & Moraes, M. A. (2020). Biopolymer Membranes and Films. doi:10.1016/B978-0-12-818134-8.00003-1
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