Thermal resistance of CNT forests for thermal interface materials: Transient thermal response and structure function analysis

We systematically investigated the thermal resistances in carbon nanotube (CNT) forests employed as solid-state thermal interface materials (TIMs) between a heat source and a metal heat sink. Utilizing transient thermal response measurements and structure function analysis, we determined the contributions of interfacial and intrinsic resistances, highlighting the necessity of understanding the detailed CNT forest structure to achieve good thermal performance. For short CNT forests (≲400 μm), the total thermal resistance is predominantly governed by the interfacial resistance (0.6–0.7 cm 2 K/W), with the intrinsic resistance of the forest being relatively minor (∼0.1 cm 2 K/W). However, for longer forests (≳400 μm), a pronounced density decay effect significantly elevates the total thermal resistance, reaching 2–5 cm 2 K/W for ∼1 mm lengths, demonstrating how structural changes degrade performance. High-temperature annealing (2200 °C) reduced the thermal resistance due to improved CNT crystallinity. Our dynamic density decay model quantitatively reproduced the increase in thermal resistance with forest length, yielding an intrinsic thermal conductivity of ∼160 W/mK for as-grown CNTs, which drastically increased to ∼450 W/mK after annealing. This work provides critical insights for CNT-based solid-state TIMs, suggesting the importance of tailoring CNT forest structurers such as CNT length, density, and crystallinity.