Energy Conversion and Sensing with Multi-Functional Nanomaterials

Junqiao Wu (UCB) and Ramamoorthy Ramesh (UCB)

Nanoscale sensing of temperature, airflow, convection and sound is also at the heart of COINS’ research missions. Vibrational and near-room temperature thermal energy harvesting is a promising source to power nanoscale sensors and mobile devices. We are working toward ultrasensitive, nanoscale sensing and energy conversion with a unique class of multi-functional nanomaterials.

These materials are based on vanadium dioxide (VO2), which shows a metal-insulator phase transition at 68oC coupled with a structural transition with a large spontaneous strain. Benefiting from the superior mechanical strength of nanomaterials, we have mapped and explored the phase diagram of VO2 over a range of strain ten times wider than previously attained. When a VO2 nanobeam is coupled with an inactive layer (Cr) forming a bimorph, it bends to large curvature in response to a small temperature difference, and oscillates at high frequencies in response to a weak wind flow. These device responses are derived from the electronic-structural phase transition and resultant domain dynamics in the microstructures, and demonstrate new concepts of sensing and energy harvesting using multi-functional nanomaterials. Watch the device in action here.

This work was performed under the auspices of the National Science Foundation by University of California Berkeley under Grant No. 0832819.

Left: Phase diagram of VO2 is mapped and explored over a range of strains one order of magnitude wider than previously attained. Right: large-curvature change in a Cr-coated VO2 microbeam in response to a small temperature variation, promising ultra-sensitive sensing and thermal/electro-mechanical energy conversion. An array of coherent M1 and R phase domains can be seen in the inset, which is critical for the high sensitivity.

Thermal Energy Scavenging

Unlike electrical resistivity, which can vary by more than 10^12 from insulators to metals, thermal conductivity varies by less than 10^4 from the best thermal conductors to the best thermal insulators. In addition, unlike typical field-effect transistors, which can change on-off resistances by more than 10^6, no devices have been shown to exhibit tunable thermal conductance. This lack of variability and tunability of phonon transport in materials is the main obstacle for heat management and further processing of phonons as information carriers, which in turn is crucial to accurately understand device performance. Zettl and Majumdar demonstrated that the thermal conductance of an individual multiwall carbon nanotube (MWCNT) can be controllably and reversibly tuned by sliding the outer shells with respect to the inner core. The thermal conductance drops to 15% of the original value after extending the length of the MWCNT by 190nm. By controlling the thermal conductance in these materials, we set the stage to develop novel thermoelectrics made from nanoscale building blocks, with substantially higher figures of merit than present-day materials.

Enhancing Thermoelectric Performance

Approximately 90 per cent of the world’s power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30–40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT above 1 since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT above 1, the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20–300nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50nm exhibit 100-fold reduction in thermal conductivity, yielding ZT of 0.6 at room temperature. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity withoutmuch affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays showpromise as high-performance, scalable thermoelectric materials.