Characterization of Carbon Nanotube Forest Interfaces Using Time Domain Thermoreflectance

Abstract

Forests comprised of nominally vertically aligned carbon nanotubes (CNTs) are excellent candidates for thermal interface materials (TIMs) due to their theoretically predicted outstanding thermal and mechanical properties. Unfortunately, due to challenges in the synthesis and characterization of these materials reports of the thermal conductivity and thermal contact resistance of CNT forests have varied widely and typically fallen far short of theoretical predictions. In particular, the micro- and nano-length scales characteristic of the heat transfer in CNT forests pose significant challenges and may lead to misreported results. Here we examine the ability of a popular and well-established thermal metrology technique, time-domain thermoreflectance (TDTR), to resolve the properties of CNT forest TIMs. The characteristic heating frequencies of TDTR (1–10 MHz) are used to probe heat transfer at length scales spanning ∼0.1–1 μm, applicable for measuring the contact resistance between the CNT forest free tips and an opposing substrate. We identify the range of CNT forest-opposing substrate interface resistances that can be resolved with TDTR, and simultaneously demonstrate the effectiveness of several processes developed to reduce the resistance of these interfaces. The limitations of characterizing CNT forests with TDTR are discussed in terms of uncertainty and sensitivity to parameters of interest.

Probing Growth-Induced Anisotropic Thermal Transport in High-Quality CVD Diamond Membranes by Multifrequency and Multiple-Spot-Size Time-Domain Thermoreflectance

“…The anisotropic heat conduction in CVD diamond has attracted great attention because it affects the ability of extracting heat through conduction. ,− However, almost all experimental measurements have been focused on two-dimensional (2D) anisotropy of thermal conduction. ,− Three-dimensional anisotropy has not been reported before experimentally due to difficulties in thermal measurements. TDTR is a popular noncontact optical pump and probe thermal characterization method used to measure thermal properties of both bulk and nanostructured materials. − A modulated pump beam heats a sample periodically, and a probe beam measures the temperature of the sample surface through a change in reflectance (i.e., thermoreflectance). The probe beam delay time is controlled by a mechanical stage, which is used to create a temperature decay curve from 0.1 to 5 ns.…”

Near Room-Temperature Synthesis of Vertical Graphene Nanowalls on Dielectrics

“…In this work, the excellent contact at the interfaces improves the thermal contact resistance between the VGNs and substrates. By utilizing a time-domain reflectance (TDTR) method (Figure S15 and Table S1), the thermal contact resistance between the VGNs and substrates was measured to be 3.39 × 10 –9 m 2 ·K·W –1 , which was orders of magnitude lower than those reported in previous reports, as shown in Figure f. ,− The thermal conductivity of the VGNs was also measured to be 5.16 W·m –1 ·K –1 , which was much higher than that of the commonly used commercial TCTs. According to the classic thermal conductive theory, the thermal contact resistance is mainly determined by the contact area (Figure S16).…”

Tunable anisotropic thermal transport in super-aligned carbon nanotube films

“…The biggest challenge for this sandwiched sample configuration to work is the weak bonding between the CNT/Al interface through the van der Waals force, resulting in an ultra-low thermal conductance (G) that could make the measurements insensitive to the thermal conductivity of the CNT samples. 3,[20][21][22] We find that the sensitivity to the cross-plane thermal conductivity could be lower than 0.05 when G=1.0 MW/(m 2 K), as shown in Figure S2a. Here the sensitivity coefficient is defined as , where is the parameter that we are interested in and / is the detected signal.…”