Feature: Thermal management
Figure 1: WAXS data for PGA acquired using a temperature-controlled capillary stage, showing the forward and reverse phase changes at (a, b) 80°C and 70°C (fast transition) and (c, d) -150°C and -130°C (slow transition). Data was collected over a period of 2s at temperature intervals of 10°C
for this purpose because they do not aff ect the morphology of the substrate. One such material is graphene, which has the desirable property of being highly impermeable to liquids, gases and chemicals. However, it has high electronic conductivity, which could create a galvanic cell if placed in contact with a conductor, potentially causing degradation over time. T erefore, hexagonal boron nitride (h-BN), which has the same permeability as graphene and does not form a galvanic cell, has been studied as an alternative. A study carried out at the Micro and Nanotechnology
Department at the Technical University of Denmark, Copenhagen, compared the protective properties of graphene and h-BN under two oxidation environments using Raman spectroscopy. This technique uses a laser beam to study the vibrational modes of molecules, to provide a material ‘fingerprint’, enabling compound identity to be determined. The study generated layers of the materials on copper through chemical vapour deposition, and examined their
34 November 2022
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properties under two oxidation environments, one short- term and one long-term. The results from the short-term study (Figure 2) indicated graphene to be an effective barrier. However, between 150°C and 300°C, h-BN was less effective, believed to be due to the higher density of grain boundaries and wrinkles, which are known to induce faster oxidation of the copper substrate. Another observation was that above 300°C, oxidation of the
graphene coating increased, as measured by the increase in Raman intensity of the CuO and Cu2
to be larger than for h-BN. Results from the long-term study (Figure 3) showed the
barrier properties of graphene were effective only for short periods. After being held for nine hours at 50°C, the oxidation of graphene resulted in an increase in copper oxide peaks, which were attributed to the formation of a galvanic cell. However, the performance of h-BN was much better, with near-uniform signals for the duration of the study.
O peaks, which were seen
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