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Design of Thermal Diffusivity Measurement of the Material
Keywords: thermal diffusivity, heat pipe, vapor chamber
This study develops a thermal diffusivity measurement instrument namely TDMI that can quickly and accurately measure the thermal diffusivity α of the materials such as graphite sheets, heat pipes and vapor chamber etc. the method is based on the Angstrom theory. Five kinds of metals, such as copper, tin, brass, aluminum alloy and lead, were selected as the calibration of the instrument. In where, one-dimensional and two-dimensional thermal diffusion experiments were also verification. The experimental values were compared with the standard values, the experimental data shows that the accuracy error of the instrument is within 10% for the results of one-dimensional and two-dimensional measurement. For the definition of one-dimensional and two-dimensional measurement is based on the aspect ratio of the test sample that is the ratio of thickness (δ) -to-width (W) (τ=δ/W). When τ is greater than 0.02, One-dimensional analysis is used; when τ is less than 0.02, two-dimensional analysis is used. Graphite sheet is widely used for the thermal spreading of the mobile phone. Graphite sheet is generally composed of a three-layer structure that is the bottom layer PET as a carrier, in the middle is graphite powder as a thermal transport and the upper layer a colloid used as a fixed graphite powder. In this study, two kinds of different thickness of graphite sheet were test. αE of the thinner one is 1.43 cm2/s while the thicker one αF is 1.58 cm2/s. Based on the calculation of homogenous model, the total equivalent spreading thermal conductivity of the thinner graphite sheet is ktot,E=250W/m.℃, while the thicker one is ktot,F=340W/m.℃. If K value of PET and adhesive are excluded, then the equivalent spreading thermal conductivity of the thinner graphite sheet KE is 450 W/m.℃, while the equivalent spreading thermal conductivity of the thicker graphite sheet kc is 510 W/m.℃. Two different heat pipe samples A and B were also tested. The results of TDMI experiments showed that the A heat pipe measurement results were αA=115cm2/s and A heat pipe measurement results were αB=167cm2/s. Calculated kHP, A,TDMI is 23700 W/m.℃ while kHP,B,TDMI is 40800 W/m.℃. At the same time, the performance of the same heat pipe was measured by the heat pipe performance test system HPPTS, the result shows that the maximum heat transport Qmax of heat pipe A is 20W, thermal resistance Rth,A is 1.17 ℃/W, the thermal conductivity kHP,A,HPPTS is calculated by Fourier heat conduction law be 10163 W/m.℃; the maximum heat transport Qmax of heat pipe B is 40W, thermal resistance Rth, B is 0.8 ℃/W, and the thermal conductivity KHP,B,HPPTS is 14928 W/m.℃. Compare the thermal resistance obtained from the HPPTS which is converted into thermal conductivity value kHP,A,HPPTS or kHP,B,HPPTS and the thermal diffusivity from the TDMI which is converted into thermal conductivity value kHP,A,TDMI or kHP,B,TDMI, the k value obtained from HPPTS is much lower than that of k value obtained from TDMI. That is because when thermal resistance of the heat pipe is converted into the thermal conductivity by the Fourier heat conduction law, heat pipe is assumed be a solid object, but in fact the heat pipe is heat exchanged by two-phase flow to achieve heat conduction. It is different from the Fourier heat conduction of solids. Therefore, the thermal conductivity K value obtained by HPPTS is definitely much lower than the K value obtained by TDMI.
Wei-Keng Lin,
T-Global Technology
Taoyuan City 330, Taoyuan Dist.

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