Sinha Group

Large-scale nanowire arrays (left, with Ferreira group) for thermoelectric waste heat harvesting (right) waste heat harvesting

Engineered surface roughness in nanowires (left) controls phonon transmission through multiple scattering(right) thermoelectric materials physics

Calorimetry of sub-20 nm polymers (left) and conversion time in carbonation of lime nanoparticles (right). thermal energy storage

Silicon inverse opal (left, with Braun group) exhibit coherent phonon grain boundary scattering (right). nanoscale phononics

Simulated fields in metallic core-dielectric shell cylinder (left) and soft-lithography of sub-wavelength structures (right). thermal plasmonics

Transport across transfer-printed interfaces (with Cahill/Rogers) and percolation of thermal conductivity (right). fundamental phonon transport



Brief descriptions of selected past and current topics follow. Our current emphasis is on thermoelectric energy conversion using nanostructured silicon, thermal storage for vehicular and concentrated solar applications, interface engineering for enhanced heat exchange across solid-solid and solid-liquid interfaces, transfer-printing, soft lithography and near-field radiative transfer.


Waste Heat Harvesting using Nanostructured Thermoelectrics


Nearly 60% of the total energy consumption in the US is wasted as heat. A significant fraction of this consists of heat sources at too low a temperature (< 100 C) to viably extract useful work using fluid machines. Thermoelectric devices are solid-state heat engines that directly convert a temperature gradient into an electrostatic potential and can act as a battery running on waste heat sources. A key technical challenge involves finding a scalable thermoelectric material with a high efficiency of conversion. Typical conversion efficiencies of thermoelectrics are only a few % of the Carnot limit thus rendering them unattractive as heat engines. Further, the best materials involve rare earth elements and are not scalable. Our effort is to improve the efficiency of thermoelectrics based on scalable materials such as silicon, using nanostructures such as roughened nanowires to tailor electron and phonon transport for optimal conversion efficiency.


Thermal Energy Storage


While electrochemical storage has attracted much attention recently, the world's energy usage continues to be predominantly thermal centric. High density thermal energy storage has a key role in improving the efficiency of thermal energy consumption and conversion in applications such as concentrated solar power and electric vehicles. Our effort here is to increase the volumetric and gravimetric storage density using new nanomaterials and heat transfer engineering.


Nanofabrication


We collaboratively develop novel fabrication techniques for nanostructures of specific interest to transport physics and engineering. Current examples are large-scale, high-density arrayed silicon nanowires using metal-assisted chemical etching, soft lithography and transfer-printing.


Thermal Engineering of Transfer-Printed Interfaces


We recently reported the first measurement of thermal conductance across transfer-printed interfaces. Transfer-printing relies on kinetically controlled adhesion to enable large-scale assembly of devices on diverse substrates. Our investigation is focused on understanding the role of contact mechanics at the nanoscale in promoting thermal transport. We have also investigated the possibility of using such interfaces in electrostatically controlled thermal switches.


Cooling Handheld Devices Using Phase Transitions


Phase change materials (PCMs) store thermal energy through a phase transition. PCMs offer a fanless thermal management solution for handling the power spikes in handheld electronics. However, available PCMs fall short in meeting the space constraints in handheld applications. Our objective is to overcome this barrier by boosting the enthalpy of storage per unit volume using nanostructures. We have designed and fabricated a micro-DSC (differential scanning calorimetry) using MEMs technology to measure storage capacities in nanometer scale film systems.


Percolation of Thermal Conductivity in Fluoropolymers


We have recently investigated heat conduction across polymer layers less than 20 nm thick using time domain thermoreflectance (TDTR) setup at the Materials Research Lab to observe the percolation of thermal conductivity. While the percolation of thermal resistance is common in many composites, the percolation of an intrinsic material property is rare. Our work establishes a link between the percolation of rigidity and the percolation of thermal conductivity for the very first time.


Near-Terahertz Frequency Surfonics


The Terahertz frequency regime (0.1-10 THz) can potentially enable unprecedented applications in the areas of communication and sensing. Our interest in this area is in extending surface acoustic technology to the frequency regime of surface acoustic phonons (surfons). Specifically, we are investigating the generation of near-THz surface acoustic waves that can potentially be used not only as ultrasensitive SAW sensors but also to study ultrafast dynamic processes in material systems. We employ picosecond ultrasonic techniques for generation and detection of such high frequency acoustic waves.