Introduction

Multiwall carbon nanotubes (MWCNTs) are quasi-one-dimensional tubular carbon nanomaterials rolled up or coaxial nested by three or more graphene sheets. The production of carbon nanotubes (CNT) generally results in significant amounts of carbon impurities (carbon material content not in the form of CNT, including amorphous carbon and trace amounts of other types of structured carbon), which influence the physical and chemical properties of the nanomaterial. Therefore, the measurement of carbon impurities content in MWCNT samples is highly desirable for the determination of their purity.

Several methods have been reported to characterize carbon impurities in MWCNT samples, including transmission electron microscopy (TEM), temperature programmed oxidation (TPO) and thermogravimetric analysis (TGA), etc., among which TGA can provide quantitative results. This technique makes use of the fact that MWCNTs are more stable than the majority of carbon impurities, so carbon impurities less stable than MWCNTs will react firstly with carbon dioxide in carbon dioxide atmosphere. The oxidation of carbon impurities with carbon dioxide is an endothermal process, which prevents overheating in certain areas and restrains the reaction of MWCNTs at the same time. Therefore, the separation between the oxidation of carbon impurities and those of MWCNTs is enhanced, allowing the amount of carbon impurities less stable than MWCNTs to be calculated from the mass loss in thermogravimetric analysis.

Principle

Thermogravimetric analysis measures the change in mass of a material as a function of temperature. In order to accomplish this, TGA requires the precise measurements of mass, temperature and temperature change. The change in mass of a material relates to change in composition and structure of the material. Observed mass changes with temperature increases may result from the removal of absorbed moisture, solvent residues, chemically bound moieties and/or the thermal or oxidative decomposition of product. The experiments are carried out in an inert or oxidising atmosphere. The recorded mass change as a function of temperature is a thermogravimetric (TG) curve. Mass change and the extent of these changes of a material in a TG curve are indicators of the thermal stability of the material. Derivative thermogravimetric (DTG) curve is a display of the first derivative of thermogravimetry data with respect to temperature or time.

The method specified in this document is based on different reactivity of MWCNTs and carbon impurities under carbon dioxide (CO2) atmosphere during heating. Carbon dioxide works as a mild oxidant to first oxidize carbon impurities less stable than MWCNTs. Moreover, the reaction between carbon impurities and CO2 absorbs heat from environment, which prevents local overheating, and thus enhances the separation of carbon impurities and MWCNTs. The amount of carbon impurities in MWCNT samples can be calculated from the mass loss in thermogravimetric analyser. 

Thermogravimetric analyser should consist of a furnace, which is capable of heating from room temperature to 1 000 °C or above. Heating rate during experiment should be controlled by temperature programme set in software.The linear heating rate should be controllable in the range from 1 °C min-1 to 50 °C min-1. The balance sensitivity should be at least 1 μg, and the temperature controller sensitivity less than or equal to 0,01 °C. A crucible should be used as a sample container. The crucible is generally made of alumina, platinum, quartz or other materials, which does not change or react under the measurement conditions.

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