A portable 3 ω setup for thermal measurements

The 3ω method has recently been proposed for in situ and operando measurements, for example, lithium concentration detection in lithium-ion pouch cells [e.g., Zeng et al., Joule 5, 2195-2210 (2021)] and human health monitoring [e.g., Qiu et al., Int. J. Heat Mass Transfer. 163, 120550 (2020)]. Yet, the bulky and expensive instruments hinder these proposals from moving toward practical applications. Here, we develop a portable 3ω setup that drives the measurements by a data acquisition card and detects the weak 3ω voltage signal through a homemade digital phase-sensitive (lock-in) detector. This portable setup is at least an order of magnitude lighter, smaller, and cheaper than its conventional laboratory counterpart. We validate this setup by measuring the thermal properties of a bulk SiO2 sample and a SiO2-film-on-Si-substrate sample. We conduct synthetic experiments to demonstrate the feasibility of this setup for in situ and operando lithium concentration detection in lithium-ion pouch cells.