Thermal Percolation in Well-Defined Nanocomposite Thin Films.

Thermal percolation in polymer nanocomposites─the rapid increase in thermal transport due to the formation of networks among fillers─is the subject of great interest in thermal management ranging from general utility in multifunctional nanocomposites to high-conductivity applications such as thermal interface materials. However, It remains a challenging subject encompassing both experimental and modeling hurdles. Successful reports of thermal percolation are exclusively found in high-aspect-ratio, conductive fillers such as graphene, albeit at filler loadings significantly higher than the electrical percolation threshold. This anomaly was attributed to the lower filler-matrix thermal conductivity contrast ratio kf/km ∼104 compared to electrical conductivity ∼1012-1016. In a randomly dispersed composite, the effect of a low contrast ratio is further accentuated by uncertainties in the morphology of the percolating network and presence of other phases such as disconnected aggregates and colloidal dispersions. Thus, the general properties of percolating networks are convoluted as they lack a defined structure. In contrast, a prototypical system with controllable nanofiller placement enables the elucidation of structure-property relations such as filler size, loading, and assembly. Using self-assembled nanocomposites with a controlled 1,2,3-dimension nanoparticle (NP) arrangement, we demonstrate that thermal percolation can be achieved in spite of using spherical, nonconductive fillers (kf/km ∼60) at a low volume fraction (9 vol %). We observe that the effects of volume fraction, interfacial thermal resistance, and filler conductivity on thermal conductivity depart from effective medium approximations. Most notably, contrast ratio plays a minor role in thermal percolation above kf/km ∼60─a common range for semiconducting nanoparticles/polymer ratios. Our findings bring new perspectives and insights to thermal percolation in nanocomposites, where the limits in contrast ratio, interfacial thermal conductance, and filler size are established.

Remarkably Weak Anisotropy in Thermal Conductivity of Two-Dimensional Hybrid Perovskite Butylammonium Lead Iodide Crystals.

Two-dimensional (2D) hybrid organic-inorganic perovskites consisting of alternating organic and inorganic layers are a new class of layered structures. They have attracted increasing interest for photovoltaic, optoelectronic, and thermoelectric applications, where knowing their thermal transport properties is critical. We carry out both experimental and computational studies on thermal transport properties of 2D butylammonium lead iodide crystals and find their thermal conductivity is ultralow (below 0.3 W m-1 K-1) with very weak anisotropy (around 1.5) among layered crystals. Further analysis reveals that the unique structure with the preferential alignment of organic chains and complicated energy landscape leads to moderately smaller phonon lifetimes in the out-of-plane direction and comparable phonon group velocities in in-plane and out-of-plane directions. These new findings may guide the future design of novel hybrid materials with desired thermal conductivity for various applications.

Demonstration of a cost-effective single-pixel UV camera for flame chemiluminescence imaging.

Combustion is the dominant form of energy conversion for a span of power systems such as engines and power plant boilers. It is an extremely complicated process which produces a huge number of intermediate products that usually feature non-uniform spatial distributions. Among those intermediate products, free radicals emitting in the UV band are of special interest because they contain abundant useful information of the target flame. Imaging methods such as planar laser-induced fluorescence or chemiluminescence imaging with UV cameras are of paramount importance to resolve the non-uniformities for a better understanding of combustion. However, a major limitation of UV cameras is that they are usually expensive, especially when multiple cameras are needed, such as in a tomographic system. In this work, we report the attempt of flame imaging with a cost-effective single-pixel UV camera which enables 2D spatial resolution of a single-pixel detector through light modulation to overcome this limitation. Meanwhile, numerical studies were conducted to systematically assess the performance of a few representative reconstruction algorithms. The impact of important factors such as sampling ratio and measurement matrix were also investigated. A validation as well as a demonstrative experiment was also conducted to reconstruct a laminar diffusion flame. The high similarity between the reconstruction and the image taken by a CMOS camera proves the feasibility of flame imaging with a single-pixel camera.