Carbon nanotubes (CNTs) are allotropic forms of carbon with cylindrical nanostructures. As a result of their geometric structures, these materials exhibit unique mechanical, thermal and electrical properties. CNTs are synthesized by several different methods, including pulsed laser vaporization, arc discharge, high pressure disproportionation of carbon monoxide and chemical vapour deposition (CVD). These processes typically yield a heterogeneous mixture of CNTs and impurities, often requiring post-synthesis purification. Commonly observed impurities include other forms of carbon [e.g. fullerenes, amorphous carbon, graphitic carbon, single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) outside the desired size or chirality range], as well as residual metallic catalyst nanoparticles. Purification can be accomplished using gaseous, chemical or thermal oxidation processes.

Thermogravimetric analysis (TGA) measures changes in the mass of a material as a function of temperature and time, which provides an indication of the reaction kinetics associated with structural decomposition, oxidation, pyrolysis, corrosion, moisture adsorption/desorption and gas evolution. By examining the reaction kinetics for a given sample, the relative fraction of different constituents present can be either quantitatively or qualitatively determined.

TGA is one of a number of analytical techniques that can be used to assess impurity levels in samples containing CNTs. For CNT-containing samples, TGA is typically used to quantify the level of non-volatile impurities present (e.g. metal catalyst particles). TGA is also used to assess thermal stability of a given sample, providing an indication of the type(s) of carbon materials present. Recent advances in TGA instrumentation enable better resolution during analysis. However, TGA alone is not specific enough to conclusively quantify the relative fractions of carbonaceous products within the material. Therefore, the information obtained from TGA should be used to supplement information gathered from other analytical techniques in order to achieve an overall assessment of the composition of a CNT sample.

Scope

This document gives guidelines for the characterization of carbon nanotube (CNT)-containing samples by thermogravimetric analysis (TGA), performed in either an inert or oxidizing environment. Guidance is provided on the purity assessment of the CNT samples through a quantitative measure of the types of carbon species present as well as the non-carbon impurities (e.g. metal catalyst particles) within the material. 

In addition, this technique provides a qualitative assessment of the thermal stability and homogeneity of the CNT-containing sample. Additional characterization techniques are required to confirm the presence of specific types of CNT and to verify the composition of the metallic impurities present

Measurement

At the basic functional level, TGA measures the change in mass of a material as a function of temperature as it is heated at a specified rate. In order to accomplish this, TGA requires the precise measurements of mass, temperature and temperature change. The change in mass of a material is related to the composition of the material and its thermal reactivity with the atmospheric conditions of the measurement. TGA analysis of CNT samples is most commonly performed in an oxidizing atmosphere, but can also be done with inert, reducing or other atmospheric conditions to probe different thermal reaction kinetics. Mass loss relative to an increase in temperature can result from the removal of absorbed moisture, solvent residues, chemically bound moieties and/or the thermal or oxidative decomposition of product.

TGA alone cannot identify the volatile materials released during heating; analytical techniques such as mass spectrometry (MS), gas chromatography (GC) and Fourier-transform infrared spectroscopy (FTIR) can be combined with TGA in order to identify volatile materials by coupling the appropriate instrumentation to the exhaust. Similarly, with respect to CNT-containing materials, TGA cannot by itself identify the different carbon forms present within the material. What it does do is determine the temperature at which the carbon species oxidize, which can be indicative of the identity of the species, as well as provide a quantitative measure of the non-volatile component.

When a CNT-containing sample is subjected to elevated temperatures in the presence of air, the carbon species present will oxidize into gaseous compounds such as CO or CO2. The residue comprises nonvolatile materials, which for the most part are metal impurities.

Exothermic and endothermic reactions

Some CNT-containing samples have been observed to undergo combustive reactions during TGA analysis, resulting in rapid burning of material, possibly catalysed by residual metals in the sample. Such events are distinguished by a difference in temperature between the sample and the reference, but also a rapid change in sample mass with little or no change in reference temperature.

Sample pan selection

Sample pan size and type will vary depending on the instrument being used and the temperature range of interest. There is no restriction on the sample pan size so long as it is compatible with the instrument and it is capable of accommodating the required amount of CNT sample with minimal compaction and the references within for a discussion of sample compaction). Aluminium, ceramic or platinum pans may be used depending on the experimental temperature range. Aluminium pans are the least likely to catalytically oxidize the CNT material, but they do not cover as wide of a temperature range. Ceramic and platinum pans are more likely to have adsorbed contaminants that can lead to inconsistent or even erroneous data. Table 1 provides guidance to some of the available pans. Standard manufacturer recommended procedures should be followed to remove any residual material from previous samples prior to conditioning. To remove adsorbed moisture, it is recommended that the pans be conditioned by heating to at least 300 °C in an air environment prior to introduction of the CNT sample. If the pans are not used immediately after conditioning, they should be stored in a dry box or desiccator until loading to prevent the reintroduction of moisture.

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