Thermogravimetric Analysis of Single-Wall Carbon Nanotube Slurries: Principles, Methods, and Applica

Thermogravimetric analysis (TGA) of single-wall carbon nanotube (SWCNT) slurries delivers quantitative data on solid content, dispersant loading, carbon purity, and residual metal catalysts within a single experiment. As SWCNT conductive agents become essential additives in lithium-ion battery electrodes, conductive inks, and functional coatings, TGA has become a fast and reliable quality-control technique aligned with ASTM and ISO methods. This article explains the analytical principle, sample types, measurable parameters, test procedures, and typical industrial applications of TGA for SWCNT slurries.

Technical Overview

Single-wall carbon nanotube (SWCNT) slurries are stable dispersions of single-wall carbon nanotubes in aqueous or organic media, typically formulated with surfactants, polymeric dispersants, or binders. They are widely used as high-performance conductive additives in lithium-ion battery electrodes, conductive inks, antistatic coatings, and heating films. The slurry form improves handling safety and dispersion uniformity compared with dry nanotube powders, but it also introduces a multi-component composition that must be tightly controlled.

Thermogravimetric analysis (TGA) is a thermal characterization technique that continuously records the mass of a sample while it is heated under a controlled atmosphere and temperature program. Because each component of an SWCNT slurry evaporates, decomposes, or combusts within a characteristic temperature window, a single TGA run can resolve moisture, solvent, dispersant, carbon phases, and inorganic residue. This makes TGA one of the most information-dense quality-control tools available for nanotube slurry manufacturers and downstream battery material users.

Measurement Principle

A small aliquot of the slurry is placed in an open crucible on a high-precision microbalance inside the TGA furnace. The temperature is ramped at a defined rate, typically 5 to 10 degrees Celsius per minute, first under an inert nitrogen purge and then under air or oxygen. The instrument records the percentage mass loss curve and its first derivative (DTG), and each mass-loss step is assigned to a specific component or decomposition event.

Relevant Standards

• ASTM E1131 — Standard Test Method for Compositional Analysis by Thermogravimetry, the core method for multi-step compositional profiling of slurries.
• ISO 11358 — Plastics thermogravimetry of polymers, frequently referenced for degradation behavior of dispersants and binders in inert atmospheres.
• ASTM E1582 and ASTM E2040 — temperature and mass calibration practices that ensure traceable, reproducible results.
• GB/T 6425 — general terminology and requirements for thermal analysis, commonly applied in Chinese-language technical documentation.

Testing Samples

Water-Based SWCNT Conductive Agent Slurries

Aqueous slurries are the most common commercial form, with typical solid contents ranging from 0.1 to 6 percent by weight. Water and hygroscopic co-solvents evaporate below approximately 120 degrees Celsius, while the surfactant or dispersant package decomposes between roughly 200 and 500 degrees Celsius under nitrogen.

Organic Solvent-Based SWCNT Slurries

N-methyl-2-pyrrolidone (NMP), ethanol, isopropanol, and dimethylacetamide based slurries are used for solvent-processable electrode and ink formulations. Their lower-boiling solvents require careful low-temperature drying steps to prevent spattering and mass-loss artifacts.

Dried SWCNT Powders Recovered from Slurries

When slurry matrix effects interfere, a portion of the slurry is vacuum-dried at low temperature and the recovered powder is analyzed. This approach sharpens the distinction between amorphous carbon, graphitic carbon, and nanotube combustion steps and improves ash determination.

Electrode Coatings and Conductive Films Containing SWCNT

Coated foils, free-standing films, and printed traces containing SWCNT slurry can be analyzed to verify residual solvent, binder content, and inorganic residue in finished parts, supporting process validation and failure analysis.

Testing Items

TGA testing of SWCNT slurries typically reports the following quantitative parameters:

• Non-volatile solid content — the mass percentage remaining after complete removal of water and solvent.
• Moisture and residual solvent content — the first mass-loss step, usually below 150 degrees Celsius.
• Dispersant, surfactant, and binder content — decomposition steps between approximately 200 and 550 degrees Celsius under nitrogen.
• Thermal stability in inert atmosphere — onset temperature of decomposition and the temperature at 5 percent mass loss.
• Oxidation onset temperature and peak combustion temperature — indicators of nanotube structural quality and graphitization level.
• Amorphous and graphitic carbon content — intermediate combustion steps that burn at lower temperatures than highly crystalline SWCNTs.
• Total carbon nanotube content — the total combustible mass in the oxidative segment.
• Ash content and metal catalyst residue — the non-combustible remainder, reflecting residual iron, cobalt, nickel, or molybdenum catalyst and its oxides.

Results are normally reported as percentage by mass of the original sample, accompanied by the full TG and DTG curves for traceability and batch comparison.

Testing Methods

Sample Preparation

The slurry container is shaken, rolled, or mechanically stirred to ensure homogeneity before sampling. A representative aliquot of approximately 5 to 20 milligrams is transferred into a pre-weighed open alumina or platinum crucible. Small sample masses are preferred for liquid slurries to avoid foaming, creep, or spattering during solvent evaporation. For low-conductivity aqueous slurries, a gentle pre-drying step at 40 to 60 degrees Celsius may be applied to concentrate the sample before the formal run.

Instrumentation and Calibration

A research-grade TGA with a horizontal or vertical dual-beam balance, purge gas switching capability, and a furnace rated to at least 900 degrees Celsius is used. Temperature calibration is performed with Curie point or melting point standards, and balance calibration with certified weights, following ASTM E1582 and ASTM E2040. Nitrogen and dry air or oxygen purge rates are typically set between 40 and 60 milliliters per minute to guarantee atmosphere purity during switching.

Typical Temperature Program

• Step 1: Equilibrate at 30 degrees Celsius, then ramp to 120 degrees Celsius at 10 degrees Celsius per minute under nitrogen and hold isothermally for 20 to 30 minutes to remove water and low-boiling solvents.
• Step 2: Ramp to 600 degrees Celsius at 10 degrees Celsius per minute under nitrogen to decompose dispersants, surfactants, and polymeric additives.
• Step 3: Switch the purge gas to air or oxygen, ramp to 850 degrees Celsius, and hold until a stable residue plateau is reached, burning all carbon phases.
• Step 4: Record the final residual mass as ash, which corresponds to metal catalyst oxides and inorganic impurities.

Data Interpretation

Each inflection on the TG curve is cross-checked against the corresponding DTG peak maximum. In the oxidative segment, amorphous carbon and carbon black generally combust between 350 and 450 degrees Celsius, while well-crystallized SWCNTs oxidize between approximately 450 and 600 degrees Celsius. Deconvolution of overlapping DTG peaks provides an estimate of the amorphous-to-nanotube carbon ratio, which is often complemented by Raman spectroscopy for structural confirmation. Ash is corrected to metal basis when catalyst composition is known, giving the true metal impurity level of the slurry.

Quality Assurance

Duplicate or triplicate runs are recommended for liquid slurries due to sampling heterogeneity. Certified reference materials, blank crucible runs, and control charts of a retention batch are used to monitor instrument drift. Tests performed at accredited third-party laboratories are typically executed under ISO/IEC 17025 aligned quality systems, ensuring that reported values are suitable for supplier qualification and incoming inspection decisions.

Applications

TGA of SWCNT slurries supports a broad range of industrial and research activities:

• Incoming inspection of SWCNT conductive agents at battery cell manufacturers, verifying solid content and catalyst residue against purchase specifications.
• Batch-to-batch consistency control at slurry producers, using solid content and compositional fingerprints as release criteria.
• Dispersion formulation development, quantifying how surfactant type and loading affect thermal decomposition behavior and residue.
• Purity verification of nanotube raw materials, estimating amorphous carbon and metal catalyst levels for reliability-critical applications.
• Electrode coating process validation, confirming complete solvent removal and correct binder fraction after drying and calendering.
• Failure analysis of conductive layers, correlating unexpected residue or volatile content with performance drift.
• Supporting documentation for material qualification dossiers submitted to automotive and electronics OEMs, often generated by accredited third-party laboratories.

FAQ

How much sample is required for a TGA test?

A single run consumes only 5 to 20 milligrams of slurry, but laboratories usually request 5 to 20 grams of the bulk slurry so that it can be homogenized and sub-sampled multiple times for replicate measurements and, if needed, complementary analysis.

Can TGA distinguish single-wall carbon nanotubes from amorphous carbon?

Partially. Well-crystallized SWCNTs oxidize at higher temperatures than amorphous carbon, so the DTG curve often shows separable combustion steps. However, when peaks overlap, the compositional split should be confirmed with Raman spectroscopy, TEM, or BET surface area analysis.

Why is the purge gas switched from nitrogen to air during the test?

Nitrogen prevents premature combustion so that solvents and organic additives can be quantified independently. Switching to air or oxygen at high temperature deliberately combusts all carbon species, allowing total carbon determination and leaving only inorganic ash on the balance.

What accuracy can be expected for solid content determination?

For a well-homogenized slurry analyzed in triplicate, the relative deviation of solid content is typically within 1 to 2 percent. Accuracy depends mainly on sample homogeneity, drying completeness, and avoidance of spattering during the low-temperature step.

How long does a typical analysis take?

A complete three-segment program including solvent hold, inert decomposition, and oxidative burn-off generally takes 2 to 3 hours per run. With replicates and report generation, results are commonly available within 3 to 5 working days.