pISSN 0374-4914 eISSN 2289-0041

## Research Paper

New Physics: Sae Mulli 2016; 66: 1077-1081

Published online September 30, 2016 https://doi.org/10.3938/NPSM.66.1077

## Measurement of the Thermal Diffusivity of Transparent Solids by Using the Periodic Thermal Wave Method

Bokwan CHOEN, Hyun HWANGBO, Jaeran LEE, Sok Won KIM*

Department of Physics, University of Ulsan, Ulsan 44610, Korea

Correspondence to:sokkim@ulsan.ac.kr

Received: February 17, 2016; Revised: July 11, 2016; Accepted: July 11, 2016

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

In this study, the thermal diffusivities of three kinds of transparent solids were measured using the thermal wave method when a gradient in the refractive index was present due to a temperature difference. Two heaters were attached to the solid samples, and the temperature gradually increased with a period of 25 s, to within 1 $^\circ$C by using the system's power supply at fixed temperatures of 25 $^\circ$C, 40 $^\circ$C, 55 $^\circ$C, 70 $^\circ$C, and 85 $^\circ$C. During the temperature increase, when a laser beam was passed through the sample, the beam was periodically deflected due to the gradient of the refractive index caused by the periodic heating. The amount of laser-beam deflection was measured using a photodetector, and the thermal diffusivity was calculated using the phase difference between two sine waves, one being the temperature variation of sample and the other being the laser beam's deflection. The experimental results at 25 $^\circ$C for solids such as BK7, Quartz, and Pyrex were (0.55 $\pm$ 0.012) $\times$ 10$^{-7}$ m$^2$s$^{-1}$, (0.72 $\pm$ 0.018) $\times$ 10$^{-7}$ m$^2$s$^{-1}$, and (0.62 $\pm$ 0.017) $\times$ 10$^{-7}$ m$^2$s$^{-1}$, respectively, and agree with the literature values to within $\pm$10.90%. Also, as the temperature was increased, the thermal diffusivity of and the difference in the thermal diffusivities between the samples increased extensively.

Keywords: Periodic thermal wave method, Thermal diffusivity, Gradient of refractive index, Laser beam deflection