Measuring the thermal conductivity of hydrogels with a bidirectional 3ω method

Area of Science:

• Materials Science
• Thermal Engineering
• Polymer Science

Background:

• Hydrogels are versatile water-absorbing polymers used in various fields.
• Encapsulating phase change materials in hydrogels for thermal energy storage is a growing area.
• Accurate measurement of hydrogel thermal conductivity is essential for optimizing these applications.

Purpose of the Study:

• To implement and validate the bidirectional 3ω technique for measuring the thermal conductivity of small hydrogel volumes.
• To investigate the influence of moisture content on hydrogel thermal conductivity.
• To provide a reliable method for characterizing soft materials for thermal storage.

Main Methods:

• Utilized a bidirectional 3ω technique optimized for small sample volumes (∼20 μl).
• Employed frequency-domain 3-D modeling to simulate sensor designs and minimize errors from substrate and insulation.
• Verified the setup using water and measured thermal conductivity of polyacrylamide and PAMPS hydrogels.

Main Results:

• Achieved accurate thermal conductivity measurements without fitting additional parameters.
• Identified optimal frequency ranges (∼1-20 Hz) for reduced measurement error.
• Swollen hydrogels showed thermal conductivity close to water (0.6 W m⁻¹ K⁻¹); neat hydrogels measured at 0.43 W m⁻¹ K⁻¹ (polyacrylamide) and 0.42 W m⁻¹ K⁻¹ (PAMPS).

Conclusions:

• The bidirectional 3ω method is a viable technique for measuring hydrogel thermal conductivity, especially for small sample sizes.
• The method provides reliable data crucial for developing advanced hydrogel-based thermal energy storage systems.
• The ±7% error, primarily from sensor calibration, is consistent with established 3ω methods.