Enhancing Thermal Interface Conductance to Graphene Using Ni-Pd Alloy Contacts

Shape distortion in sintering results from nonhomogeneous temperature activating a long-range mass transport

Sintering theory predicts no long-range mass transport or distortion for uniformly heated particles during particle coalescence. However, in sintering-based manufacturing processes, permanent part distortion is often observed. The driving forces and mechanisms leading to this phenomenon are not understood, and efforts to reduce distortion are largely limited to a trial-and-error approach. In this paper, we demonstrate that distortion during sintering results from mass-transport driven by nonhomogeneous temperature distribution. We then show that hitherto unknown mass transport mechanisms, working in the direction opposite to temperature gradient are the likely cause of distortion. The experimental setup, designed for this purpose, enables the quantification of distortion during sintering. Two possible mass transport mechanisms are defined, and the continuum model applicable to both is formulated. The model accurately predicts the transient and permanent distortion observed during experiments, including their size dependence. Methods to control distortion that can give rise to 4D printing are discussed.

Reducing the uncertainty caused by the laser spot radius in frequency-domain thermoreflectance measurements of thermal properties

In a frequency-domain thermoreflectance (FDTR) experiment, the phase lag between the surface temperature response and the applied heat flux is fit with an analytical solution to the heat diffusion equation to extract an unknown thermal property (e.g., thermal conductivity) of a test sample. A method is proposed to reduce the impact of uncertainty in the laser spot radius on the resulting uncertainty in the fitted property that is based on fitting to the quotient of the test sample phase and that of a reference sample. The reduction is proven analytically for a semi-infinite solid and was confirmed using numerical and real experiments on realistic samples. When the spot radius and its uncertainty are well known, the reference phase can be generated numerically. In this situation, FDTR experiments performed on Au-SiO₂-Si and PbS nanocrystal test samples demonstrate 32% and 82% reductions in the overall uncertainty in thermal conductivity. When the spot radius used in the test sample measurement is not well known, a real reference sample, measured under conditions that lead to the same unknown spot radius, is required. Although the real reference sample introduces its own uncertainties, the total uncertainty in the fitted thermal conductivity can still be reduced. A reference sample can also be used to reduce uncertainty due to other sources, such as the transducer properties. Because frequency-domain solutions to the heat diffusion equation are the basis for time-domain thermoreflectance (TDTR) analysis, the approach can be extended to TDTR experiments.

Signatures of Coherent Phonon Transport in Ultralow Thermal Conductivity Two-Dimensional Ruddlesden-Popper Phase Perovskites

An emerging class of methylammonium lead iodide (MAPbI₃)-based Ruddlesden-Popper (RP) phase perovskites, BA₂MA_n-1Pb_nI_3n+1 (n = 1-7), exhibit enhanced stability to environmental conditions relative to MAPbI₃, yet still degrade at elevated temperatures. We experimentally determine the thermal conductivities of these layered RP phases for n = 1-6, where n defines the number of repeated perovskite octahedra per layer. We measure thermal conductivities of 0.37 ± 0.13/0.12, 0.17 ± 0.08/0.07, 0.21 ± 0.05/0.04, and 0.19 ± 0.04/0.03 W/m·K in thin films of n = 1-4 and 0.08 ± 0.06/0.04, 0.06 ± 0.04/0.03, 0.06 ± 0.03/0.03, and 0.08 ± 0.07/0.04 W/m·K in single crystals of n = 3-6. With the exception of n = 1, these thermal conductivities are lower than the range of 0.34-0.50 W/m·K reported for single-crystal MAPbI₃. Reduced-order lattice dynamics modeling suggests that the initially decreasing trend of thermal conductivity in similarly oriented perovskites with increasing n may result from the transport properties of coherent phonons, emergent from the superstructure, that do not scatter at the interfaces of organic butylammonium chains and perovskite octahedra. Reduced group velocity of coherent phonons in n = 3-6, a consequence of band flattening in the phonon dispersion, is primarily responsible for their ultralow thermal conductivities. Similar effects on thermal conductivity have been experimentally demonstrated in deposited superlattices, but never in naturally defined materials such as RP phases. GIWAXS measurements reveal that higher n RP phase thin films are less orientationally controlled and therefore possess apparently elevated thermal conductivities relative to single crystals of the same n.