Enhancing Thermal Transport in Silicone Composites via Bridging Liquid Metal Fillers with Reactive Metal Co-Fillers and Matrix Viscosity Tuning.

Polymer matrix composites containing room temperature liquid metal (LM) microdroplets offer a unique set of thermo-mechanical characteristics that makes them attractive candidates for high performance thermal interface materials. However, to achieve the desired level of the composite thermal conductivity, effective bridging of such fillers into interconnected percolation networks needs to be induced. Thermal percolation of the LM microdroplets requires two physical barriers to be overcome. First, the LM microdroplets must directly contact each other through the polymer matrix. Second, the native oxide shell on the LM microdroplet must also be ruptured. In this work, we demonstrate that both physical barriers can be penetrated to induce ample bridging of the LM microdroplets and thereby achieve higher thermal conductivity composites. We accomplish this through a synergistic combination of solid silver and LM fillers, tuning of the silicone oil "matrix" viscosity, and sample compression. We selected silver as the solid additive because it rapidly alloys with gallium to form microscale needles that could act as additional paths that aid in connecting the LM droplets. We systematically explore the impact of the composition (filler type, volume fraction, and matrix oil viscosity) and applied pressure on the thermal conductivity and multiscale structure of these composites. We reveal the microscopic mechanism underlying the macroscopic experimental trends and also identify an optimal composition of the multiphase Ag-LM-Silicone oil composite for thermal applications. The identified design knobs offer path for developing tunable LM-based polymer composites for microelectronics cooling, biomedical applications, and flexible electronics.

Gallium oxide-stabilized oil in liquid metal emulsions.

Gallium based liquid metals (LM) have prospective biomedical, stretchable electronics, soft robotics, and energy storage applications, and are being widely adopted as thermal interface materials. The danger of gallium corroding most metals used in microelectronics requires the cumbersome addition of "barrier" layers or LM break-up into droplets within an inert matrix such as silicone oil. Such LM-in-oil emulsions are stabilized by native oxide on the droplets but have decreased thermal performance. Here we show that mixing of the silicone oil into an LM-air foam yields emulsions with inverted phases. We investigate the stability of these oil-in-LM emulsions through a range of processing times and oil viscosities, and characterize the impact of these parameters on the materials' structure and thermal property relationships. We demonstrate that the emulsion with 40 vol% of 10 cSt silicone oil provides a unique thermal management material with a 10 W m-1 K-1 thermal conductivity and an exterior lubricant thin film that completely prevents corrosion of contacting aluminum.

Oxide-mediated mechanisms of gallium foam generation and stabilization during shear mixing in air.

Foaming of gallium-based liquid metals improves their processability and-seemingly in contrast to processing of other metal foams-can be achieved through shear-mixing in air without addition of solid microparticles. Resolving this discrepancy, systematic processing-structure-property characterization demonstrates that many crumpled oxide particles are generated prior to air bubble accumulation.

Pressure-Activated Thermal Transport via Oxide Shell Rupture in Liquid Metal Capsule Beds.

Liquid metal (LM)-based thermal interface materials (TIMs) have the potential to dissipate high heat loads in modern electronics and often consist of LM microcapsules embedded in a polymer matrix. The shells of these microcapsules consist of a thin LM oxide that forms spontaneously. Unfortunately, these oxide shells degrade heat transfer between LM capsules. Thus, rupturing these oxide shells to release their LM and effectively bridge the microcapsules is critical for achieving the full potential of LM-based TIMs. While this process has been studied from an electrical perspective, such results do not fully translate to thermal applications because electrical transport requires only a single percolation path. In this work, we introduce a novel method to study the rupture mechanics of beds composed solely of LM capsules. Specifically, by measuring the electrical and thermal resistances of capsule beds during compression, we can distinguish between the pressure at which capsule rupture initiates and the pressure at which widespread capsule rupture occurs. These pressures significantly differ, and we find that the pressure for widespread rupture corresponds to a peak in thermal conductivity during compression; hence, this pressure is more relevant to LM thermal applications. Next, we quantify the rupture pressure dependence on LM capsule age, size distribution, and oxide shell chemical treatment. Our results show that large freshly prepared capsules yield higher thermal conductivities and rupture more easily. We also show that chemically treating the oxide shell further facilitates rupture and increases thermal conductivity. We achieve a thermal conductivity of 16 W m-1 K-1 at a pressure below 0.2 MPa for capsules treated with dodecanethiol and hydrochloric acid. Importantly, this pressure is within the acceptable range for TIM applications.

Oxide-Mediated Formation of Chemically Stable Tungsten-Liquid Metal Mixtures for Enhanced Thermal Interfaces.

Modern microelectronics and emerging technologies such as wearable devices and soft robotics require conformable and thermally conductive thermal interface materials to improve their performance and longevity. Gallium-based liquid metals (LMs) are promising candidates for these applications yet are limited by their moderate thermal conductivity, difficulty in surface-spreading, and pump-out issues. Incorporation of metallic particles into the LM can address these problems, but observed alloying processes shift the LM melting point and lead to undesirable formation of additional surface roughness. Here, these problems are addressed by introducing a mixture of tungsten microparticles dispersed within a LM matrix (LM-W) that exhibits two- to threefold enhanced thermal conductivity (62 ± 2.28 W m-1 K-1 for gallium and 57 ± 2.08 W m-1 K-1 for EGaInSn at a 40% filler volume mixing ratio) and liquid-to-paste transition for better surface application. It is shown that the formation of a nanometer-scale LM oxide in oxygen-rich environments allows highly nonwetting tungsten particles to mix into LMs. Using in situ imaging and particle dipping experimentation within a focused ion beam and scanning electron microscopy system, the oxide-assisted mechanism behind this wetting process is revealed. Furthermore, since tungsten does not undergo room-temperature alloying with gallium, it is shown that LM-W remains a chemically stable mixture.

Angle resolved vibrational properties of anisotropic transition metal trichalcogenide nanosheets.

Layered transition metal trichalcogenides (TMTCs) are a new class of anisotropic two-dimensional materials that exhibit quasi-1D behavior. This property stems from their unique highly anisotropic crystal structure where vastly different material properties can be attained from different crystal directions. Here, we employ density functional theory predictions, atomic force microscopy, and angle-resolved Raman spectroscopy to investigate their fundamental vibrational properties which differ significantly from other 2D systems and to establish a method in identifying anisotropy direction of different types of TMTCs. We find that the intensity of certain Raman peaks of TiS3, ZrS3, and HfS3 have strong polarization dependence in such a way that intensity is at its maximum when the polarization direction is parallel to the anisotropic b-axis. This allows us to readily identify the Raman peaks that are representative of the vibrations along the b-axis direction. Interestingly, similar angle resolved studies on the novel TiNbS3 TMTC alloy reveal that determination of anisotropy/crystalline direction is rather difficult possibly due to loss of anisotropy by randomization distribution of quasi-1D MX6 chains by the presence of defects which are commonly found in 2D alloys and also due to the complex Raman tensor of TMTC alloys. Overall, the experimental and theoretical results establish non-destructive methods used to identify the direction of anisotropy in TMTCs and reveal their vibrational characteristics which are necessary to gain insight into potential applications that utilize direction dependent thermal response, optical polarization, and linear dichroism.

Domain Architectures and Grain Boundaries in Chemical Vapor Deposited Highly Anisotropic ReS2 Monolayer Films.

Recent studies have shown that vapor phase synthesis of structurally isotropic two-dimensional (2D) MoS2 and WS2 produces well-defined domains with clean grain boundaries (GBs). This is anticipated to be vastly different for 2D anisotropic materials like ReS2 mainly due to large anisotropy in interfacial energy imposed by its distorted 1T crystal structure and formation of signature Re-chains along [010] b-axis direction. Here, we provide first insight on domain architecture on chemical vapor deposited (CVD) ReS2 domains using high-resolution scanning transmission electron microscopy, angle-resolved nano-Raman spectroscopy, reflectivity, and atomic force microscopy measurements. Results provide ways to achieve crystalline anisotropy in CVD ReS2, establish domain architecture of high symmetry ReS2 flakes, and determine Re-chain orientation within subdomains. Results also provide a first atomic resolution look at ReS2 GBs, and surprisingly we find that cluster and vacancy defects, formed by collusion of Re-chains at the GBs, dramatically impact the crystal structure by changing the Re-chain direction and rotating Re-chains 180° along their b-axis. Overall results not only shed first light on domain architecture and structure of anisotropic 2D systems but also allow one to attain much desired crystalline anisotropy in CVD grown ReS2 for the first time for tangible applications in photonics and optoelectronics where direction-dependent dichroic and linearly polarized material properties are required.