Heat-loss modified Angstrom method for simultaneous measurements of thermal diffusivity and conductivity of graphite sheets: The origins of heat loss in Angstrom method

摘要

Angstrom method is a steady-state measurement for thermal diffusivity a using ac heating. Since thermal conductivity λ is a better-known quantity, measured diffusivity a is sometimes transformed into X based on relation λ = C_vα using recorded or DSC measured C_v However, Angstrom method itself is principally possible to extend to specific heat measurements, yet the accuracy is not promising due to the complexity of heat loss. Here we present a modified method for simultaneous measurements of thermal diffusivity and thermal conductivity with high accuracy by taking heat loss into account. A linear heat loss term m~2T is introduced into the diffusion equation and the thermal conductivity λ can be directly measured instead of specific heat The measured thermal properties of commercial graphite sheets agree well with their nominal value. The origins of m~2 are also discussed, m~2 can be divided into amplitude independent and dependent part. From the basics of radiation and convection, the first-order radiation and convection comprise the amplitude independent part, while the dependent part includes higher order (dominated by second-order) radiation. Although the amplitude independent part agrees well with the extrapolated value of m~2 at zero amplitude, the second-order radiation cannot fully cover the measured amplitude dependent part This discrepancy is further explained by floating temperature baseline variation due to residual heat during heat oscillation.