Strained Polymer Thermal Conductivity Enhancement Counteracted by Additional Off-Axis Strain

Enhancing thermal transport of epoxy composites with vertically aligned graphene in situ grown on the thermal interface

The escalating demands for miniaturization, integration, and portability in electronic devices have underscored the criticality of efficient heat dissipation. The utilization of high-performance thermal interface materials (TIMs) to fill the gaps between contacting surfaces holds significant potential for enhancing heat transfer efficiency. Herein, we successfully enhance the thermal properties of the epoxy composite TIM by integrating in situ grown vertically aligned graphene on the metal surface using radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD). To investigate the effect of vertical graphene on epoxy, the sandwich structure of copper/vertical graphene-epoxy/copper (Cu/VG-EP/Cu) is fabricated by incorporating epoxy resin. The experimental results demonstrate that the thermal conductivity of VG-EP reaches 2.06 W m-1 K-1 and achieves an impressive 1215% maximum enhancement. Furthermore, the numerical simulation findings show that vertical graphene consistent with the temperature gradient exhibits the highest heat transfer efficiency. This work presents an in-depth study of vertically aligned graphene within the epoxy resin, highlighting the advantages of vertically aligned fillers and offering novel perspectives for the advancement of TIMs.

An instrumentation guide to measuring thermal conductivity using frequency domain thermoreflectance (FDTR)

Frequency Domain Thermoreflectance (FDTR) is a versatile technique used to measure the thermal properties of thin films, multilayer stacks, and interfaces that govern the performance and thermal management in semiconductor microelectronics. Reliable thermal property measurements at these length scales (≈10 nm to ≈10 μm), where the physics of thermal transport and phonon scattering at interfaces both grow in complexity, are increasingly relevant as electronic components continue to shrink. While FDTR is a promising technique, FDTR instruments are generally home-built; they can be difficult to construct, align, and maintain, especially for the novice. Our goal here is to provide a practical resource beyond theory that increases the accessibility, replicability, and widespread adoption of FDTR instrumentation. We provide a detailed account of unpublished insights and institutional knowledge that are critical for obtaining accurate and repeatable measurements of thermal properties using FDTR. We discuss component selection and placement, alignment procedures, data collection parameters, common challenges, and our efforts to increase measurement automation. In FDTR, the unknown thermal properties are fit by minimizing the error between the phase lag at each frequency and the multilayer diffusive thermal model solution. For data fitting and uncertainty analysis, we compare common numerical integration methods, and we compare multiple approaches for fitting and uncertainty analysis, including Monte Carlo simulation, to demonstrate their reliability and relative speed. The instrument is validated with substrates of known thermal properties over a wide range of isotropic thermal conductivities, including Borofloat silica, quartz, sapphire, and silicon.

Measurements of Thermal Resistance Across Buried Interfaces with Frequency-Domain Thermoreflectance and Microscale Confinement

Confined geometries are used to increase measurement sensitivity to thermal boundary resistance at buried SiO2 interfaces with frequency-domain thermoreflectance (FDTR). We show that radial confinement of the transducer film and additional underlying material layers prevents heat from spreading and increases the thermal penetration depth of the thermal wave. Parametric analyses are performed with finite element methods and used to examine the extent to which the thermal penetration depth increases as a function of a material's effective thermal resistance and the degree of material confinement relative to the pump beam diameter. To our surprise, results suggest that the measurement technique is not always the most sensitive to the largest thermal resistor in a multilayer material. We also find that increasing the degree to which a material is confined improves measurement sensitivity to the thermal resistance across material interfaces that are buried 10s of μm to mm below the surface. These results are used to design experimental measurements of etched, 200 nm thick SiO2 films deposited on Al2O3 substrates, and offer an opportunity for thermal scientists and engineers to characterize the thermal resistance across a broader range of material interfaces within electronic device architectures that have historically been difficult to access via experiment.

Nanoscale Meniscus Dynamics in Evaporating Thin Films: Insights from Molecular Dynamics Simulations

Evaporation studies are focused on unraveling heat transfer and flow dynamics near the solid-liquid-vapor contact line, particularly focusing on the meniscus, which encompasses the nonevaporating adsorbed layer, thin-film, and bulk meniscus regions. Continuum models assume that there are no evaporating adsorbed layers due to the strong intermolecular forces. However, recent molecular dynamics (MD) simulations have unveiled the significant role of adsorbed layers in thin-film evaporation. Leveraging a recently published energy-based interface detection method, the current study presents nonequilibrium MD simulation results for thin-film evaporation from a phase-change-driven nanopump using liquid argon confined between parallel platinum plates. Notably, unlike the transient simulations often encountered in the literature, the simulation system achieves a statistically steady transport. In this context, we showcase the shapes of the evaporating menisci for two distinct channel heights, 8 and 16 nm, and elucidate the underlying flow physics through velocity vectors and temperature contours. This comprehensive investigation advances our understanding of thin-film evaporation and its mechanisms, offering insights that span from nanoscale phenomena to broader thermal management applications.

The Effect of Mechanical Elongation on the Thermal Conductivity of Amorphous and Semicrystalline Thermoplastic Polyimides: Atomistic Simulations

Over the past few decades, the enhancement of polymer thermal conductivity has attracted considerable attention in the scientific community due to its potential for the development of new thermal interface materials (TIM) for both electronic and electrical devices. The mechanical elongation of polymers may be considered as an appropriate tool for the improvement of heat transport through polymers without the necessary addition of nanofillers. Polyimides (PIs) in particular have some of the best thermal, dielectric, and mechanical properties, as well as radiation and chemical resistance. They can therefore be used as polymer binders in TIM without compromising their dielectric properties. In the present study, the effects of uniaxial deformation on the thermal conductivity of thermoplastic PIs were examined for the first time using atomistic computer simulations. We believe that this approach will be important for the development of thermal interface materials based on thermoplastic PIs with improved thermal conductivity properties. Current research has focused on the analysis of three thermoplastic PIs: two semicrystalline, namely BPDA-P3 and R-BAPB; and one amorphous, ULTEM^TM. To evaluate the impact of uniaxial deformation on the thermal conductivity, samples of these PIs were deformed up to 200% at a temperature of 600 K, slightly above the melting temperatures of BPDA-P3 and R-BAPB. The thermal conductivity coefficients of these PIs increased in the glassy state and above the glass transition point. Notably, some improvement in the thermal conductivity of the amorphous polyimide ULTEM^TM was achieved. Our study demonstrates that the thermal conductivity coefficient is anisotropic in different directions with respect to the deformation axis and shows a significant increase in both semicrystalline and amorphous PIs in the direction parallel to the deformation. Both types of structural ordering (self-ordering of semicrystalline PI and mechanical elongation) led to the same significant increase in thermal conductivity coefficient.

Unwrapping a full temporal cycle in time domain thermoreflectance for enhanced measurement sensitivity in thermally insulating materials

Time delayed pump-probe measurement techniques, such as Time Domain Thermoreflectance (TDTR), have opened up a wealth of opportunities for metrology at ultra-fast timescales and nanometer length scales. For nanoscale thermal transport measurements, typical thermal lifetimes used to measure thermal conductivity and thermal boundary conductance span from sub-picosecond to ∼6 nanoseconds. In this work, we demonstrate a simple rearrangement and validation of a configuration that allows access to the entire 12.5 ns time delay available in the standard pulse train. By reconfiguring a traditional TDTR system so that the pump and probe arrive concurrently when the delay stage reaches its midpoint, followed by unwrapping the temporal scan, we obtain a dataset that is bounded only by the oscillator repetition rate. Sensitivity analysis along with conducted measurements shows that great increases in measurement sensitivity are available with this approach, particularly for thin films with low thermal conductivities.

State-of-the-Art, Opportunities, and Challenges in Bottom-up Synthesis of Polymers with High Thermal Conductivity

In contrast to metals, polymers are predominantly thermal and electrical insulators. With their unparalleled advantages such as light weight, turning polymer insulators into heat conductors with metal-like thermal conductivity is...

Sequence-Engineering Polyethylene-Polypropylene Copolymers with High Thermal Conductivity Using a Molecular-Dynamics-Based Genetic Algorithm

Polymer sequence engineering is emerging as a potential tool to modulate material properties. Here, we employ a combination of a genetic algorithm (GA) and atomistic molecular dynamics (MD) simulation to design polyethylene-polypropylene (PE-PP) copolymers with the aim of identifying a specific sequence with high thermal conductivity. PE-PP copolymers with various sequences at the same monomer ratio are found to have a broad distribution of thermal conductivities. This indicates that the monomer sequence has a crucial effect on thermal energy transport of the copolymers. A non-periodic and non-intuitive optimal sequence is indeed identified by the GA, which gives the highest thermal conductivity compared with any regular block copolymers, for example, diblock, triblock, and hexablock. In comparison to the bulk density, chain conformations, and vibrational density of states, the monomer sequence has the strongest impact on the efficiency of thermal energy transport via inter- and intra-molecular interactions. Our work highlights polymer sequence engineering as a promising approach for tuning the thermal conductivity of copolymers, and it provides an example application of integrating atomistic MD modeling with the GA for computational material design.

Thermal Imaging of Block Copolymers with Sub-10 nm Resolution

Thermal silicon probes have demonstrated their potential to investigate the thermal properties of various materials at high resolution. However, a thorough assessment of the achievable resolution is missing. Here, we present a probe-based thermal-imaging technique capable of providing sub-10 nm lateral resolution at a sub-10 ms pixel rate. We demonstrate the resolution by resolving microphase-separated PS-b-PMMA block copolymers that self-assemble in 11 to 19 nm half-period lamellar structures. We resolve an asymmetry in the heat flux signal at submolecular dimensions and assess the ratio of heat flux into both polymers in various geometries. These observations are quantitatively compared with coarse-grained molecular simulations of energy transport that reveal an enhancement of transport along the macromolecular backbone and a Kapitza resistance at the internal interfaces of the self-assembled structure. This comparison discloses a tip-sample contact radius of a ≈ 4 nm and identifies combinations of enhanced intramolecular transport and Kapitza resistance.