Front heating–back sensing in photothermal measurement of cross-plane thermal conductivity: Experimental and theoretical insights

This work presents a modified photothermal (PT) technique to improve the sensitivity in thermophysical property measurements, particularly for materials with a high thermal conductivity and a greater thickness. The modification uses a front heating–back sensing (FHBS) configuration, where a laser irradiates the sample’s front surface, while thermal radiation is detected from the back surface. Compared to the previously developed front heating–front sensing (FHFS) PT technique, the FHBS yields weaker signals due to heat penetration through the sample, requiring a sufficiently powerful laser source. The FHBS method is applied to microthick graphene paper and fused silica (SiO2) samples, yielding out-of-plane thermal conductivities of 4.91 and 1.75 W m−1 K−1, respectively. These results agree well with FHFS PT results and literature values, validating the approach. The slight deviation for the SiO2 sample is attributed to experimental uncertainty in the FHBS configuration, especially in thickness measurement since heat must penetrate deeper in FHBS, making thickness more critical. The FHBS configuration is also tested on a microthick sapphire sample. Although due to sapphire’s high thermal conductivity and the limited laser power in our setup, the measurement does not yield reliable data, and the technique remains theoretically feasible with the use of a stronger laser source. This work highlights the importance of selecting an appropriate PT configuration based on the sample’s thermal properties and experimental limitations to ensure high accuracy. Moreover, the FHBS PT technique avoids the extreme surface temperature rise often required in the laser flash method, thereby reducing the risk of sample damage and improving suitability for microscale materials.