Controlling heat conduction in disordered granular systems is critical to the design of functional materials for energy conversion and storage applications such as granular-like electrodes in solid-state lithium-ion battery electrodes, suspensions in flow-batteries, and densely packed nanoparticles in thermoelectric devices. External mechanical stresses can induce irreversible microstructural rearrangements in these amorphous granular composites. Often, such stresses occur during material manufacturing or processing, as well as during field operation, and a detailed understanding of the microstructural response to stress also allows for the possibility of developing application-specific tailored composites. In this work, we examine the fundamentals of heat conduction in granular particle beds subjected to mechanical shearing flow, in order to understand and manipulate thermal transport through irreversible rearrangement of the microstructure.

     A device is fabricated to introduce Taylor-Couette shear flow in granular material that are packed between rough rotating concentric cylinders. In addition to inducing shear flows, a thermal gradient is applied across the sheared granular material through a heater embedded in the inner rotating cylinder and a water cooled/heated manifold on the outer stationary cylinder. Infrared (IR) microscopy monitors the spatio-temporal varying temperature fields at the surface of the granular material as it is simultaneously heated and sheared. The high resolution and sensitivity of the IR microscope enables a spatial and temperature resolution of 1.8 micron and 0.1 K, respectively. As a result, the temperature of individual microscale grains, as well as the temperature drops at grain-grain contacts that are due to contact resistances, are easily extracted from the IR image.

     Specifically, in this study, we measure the effects of grain shape, grain size dispersity, and external stress on the effective thermal conductivity of packed granular materials. In particular, the thermal images from IR microscopy highlight the dominant pathways for heat conduction in granular materials with large microstructural heterogeneity. Previous simulation work has identified the role of contact topology in thermal conduction through granular materials [1]. The efficacy of effective medium approximations in the prediction of effective thermal conductivity of dense packings of tetrahedra was explained on the basis of a lack of face-to-face tetrahedral contacts. Additionally, past experiment [2] and simulation [3] work has also shown that the aspect ratio of hexagonal bismuth telluride nanoplatelets, that are compacted into thermoelectric pellets, plays a dominant role in effective thermal conduction by impacting the net scattering rate of phonons at grain boundaries. The present study aims to directly identify the mechanisms for suppression/enhancement of heat conduction in disordered granular packings. The two-dimensional temperature maps illustrate how heat conduction pathways change as a result of external mechanical stresses induced by Taylor-Couette flows.

     Lastly, the identification and rigorous analysis of key microstructural characteristics of disordered granular packings and its correlation with effective thermal conductivity will enable predictive capabilities in calculating effective heat conduction in a wide range of granular systems. An important metric in this regard is the granular fabric tensor that can be extracted from optical images of granular material and correlated with the thermal images from infrared microscopy and effective thermal conductivity results. This study lays the foundation for calculating the temporal evolution of fabric tensor and correlating it with the temporal evolution of the effective thermal conductivity.

     [1] K. C. Smith and T. S. Fisher, J. Heat Transfer 135, 081301 (2013). [2] R. J. Mehta, Y. Zhang, C. Karthik, B. Singh, R. W. Siegel, T. Borca-Tasciuc, and G. Ramanath, Nat. Mater. 11, 233 (2012). [3] I. Srivastava and T. S. Fisher, 32nd International Thermal Conductivity Conference, Purdue University, USA (2014).