Detection of single-walled carbon nanotube paste

We heat a dried paste sample (10–20 mg) from 25°C to 950°C at 10°C/min under air. The weight loss steps are analysed:

• Below 200°C – Residual solvent and moisture
• 200–500°C – Polymer/surfactant decomposition and amorphous carbon oxidation
• 550–750°C – SWCNT combustion (crystalline carbon)
• Residue at 950°C – Inorganic impurities (metal catalysts: Fe, Co, Ni, Mo; or ash from stabilisers)

High‑quality SWCNT paste should have SWCNT content > 70% (excluding solvent). Low SWCNT content (< 40%) with high residue (> 10%) indicates poor purification.

3. Scanning Electron Microscopy (SEM) – Morphology and Bundle Size – ISO 19716

We prepare a dried specimen and image it at magnifications of 10,000× to 100,000×. We measure:

• Average bundle diameter (nm) – Individual SWCNT are 0.8–2 nm, but bundles range from 5–50 nm. Excessive bundling (> 50 nm) indicates poor dispersion.
• Presence of amorphous carbon or metal nanoparticles – Identified by EDS (energy‑dispersive X‑ray spectroscopy).
• Network homogeneity – Uniform coverage without large aggregates is essential for conductive films.

For printed electronics, SWCNT bundles should be ≤ 20 nm to achieve smooth, highly conductive films.

4. Electrical Resistivity (Sheet Resistance) – ASTM D4496 / Four‑Point Probe

We deposit a uniform SWCNT paste film (by spin coating, bar coating, or screen printing) onto an insulating substrate (glass or PET). After drying (e.g., 120°C for 10 minutes), we measure sheet resistance (Ω/□) using a four‑point probe. For a 1 µm thick dry film, typical sheet resistance ranges:

• High‑quality (well‑dispersed) SWCNT: 100–500 Ω/□
• Moderate quality: 500–2000 Ω/□
• Poor (aggregated) > 5000 Ω/□

We also measure the resistivity after bending (flexible electronics) to check adhesion and network integrity.

5. Rheological Properties (Viscosity and Thixotropy) – ASTM D2196

For paste printability, we measure viscosity at varying shear rates (0.1–1000 s⁻¹) using a rotational rheometer with a cone‑plate or parallel‑plate geometry. Key parameters:

• Low‑shear viscosity (at 1 s⁻¹) – Indicates paste stability (too low causes sedimentation; too high causes poor flow).
• High‑shear viscosity (at 1000 s⁻¹) – Determines printability (inkjet requires < 20 cP; screen printing requires 1000–10,000 cP).
• Yield stress (Pa) – Ensures the paste does not run after deposition.
• Thixotropy (recovery after shear) – Critical for sharp feature resolution.

We report viscosity as a function of shear rate and provide recommendations for specific printing methods.

6. Particle Size Distribution – Laser Diffraction or Dynamic Light Scattering (DLS)

We dilute the paste to a concentration suitable for DLS or laser diffraction (e.g., 0.01% w/v). The d10, d50, and d90 particle sizes are reported. For SWCNT pastes, typical d50 is 0.5–5 µm (agglomerated bundles). Larger d50 (> 20 µm) indicates poor dispersion and will clog print nozzles.

7. Surfactant and Dispersant Quantification – UV‑Vis or HPLC

For pastes containing stabilising surfactants (e.g., SDS, Triton X‑100), we extract and quantify the surfactant content using UV‑Vis spectrophotometry (specific adsorption peaks) or HPLC. High surfactant residues (> 30% of solid content) can degrade conductivity and compatibility with matrices. Acceptable surfactant content is typically 5–15% of dry weight.

8. Zeta Potential – Dispersion Stability – ASTM E2868

We dilute the paste and measure the zeta potential (mV) using electrophoretic light scattering. Stable dispersions require |zeta| > 30 mV. Low zeta potential (< |20|) indicates risk of agglomeration and sedimentation.