The Next Solid State Revolution (T. Sands)

Solid-state energy conversion devices, as a class, offer improved reliability, smaller size and reduced environmental impact as compared to established technologies such as incandescent lighting and compressor-based refrigeration.

We are all very familiar with the impact that the silicon integrated circuit has had on society; Perhaps less obvious is our daily reliance on III-V compound semiconductor optoelectronics for fiber optic communications and optical information storage. What's next for solid state? Several nascent solid-state technologies, born in the 1950's, are now poised to uproot such stalwarts as the light bulb and the compressor-based refrigerator. These replacement technologies offer the potential for dramatic savings in energy, space and maintenance costs. More intriguing, though, are the many new applications that may become practical with the solid-state format; energy scavenging for distributed sensor networks, electrical power generation from waste heat in automobile exhaust, and permanent embedded architectural lighting are just a few that spring to mind. At the heart of these envisioned devices are new composite materials, engineered at the nanoscale to outperform traditional bulk and thin-film materials. The Heterogeneous Integration Research Group headed by Tim Sands, Purdue's Basil S. Turner Professor of Engineering, is designing and creating these new nanocomposites with a focus on synthesis, assembly and integration processes that may be scaled up for manufacturing.

Nanocomposite materials for solid-state energy conversion devices

The Promise: Solid-state energy conversion devices have long held the potential to revolutionize lighting, cooling, energy production, and energy conservation. As a class, these devices offer improved reliability, smaller size and reduced environmental impact as compared to established technologies such as incandescent lighting and compressor-based refrigeration.

Device Energy Conversion Function Application
Light-emittingDiode (LED) Electrical to optical Lighting
Photovoltaic cell Optical to electrical Energy production
Thermoelectric refrigerator Electrical to thermal (cooling) Refrigeration
Thermoelectric generator Thermal to electrical Waste heat recovery

The Challenge: Although solid-state has proven its value in space applications where reliability and launch weight are critical, terrestrial solid-state energy conversion devices have largely been relegated to niche applications such as traffic lights (LEDs), spot cooling of electronics (thermoelectric refrigerators), and remote powering (photovoltaics). The principal obstacles to widespread implementation are efficiency and manufacturing cost. In each of the solid-state technologies highlighted above, an improvement in efficiency by about a factor of two, combined with the reduction in manufacturing costs that could be reasonably expected with mass production, would be sufficient to either replace an existing technology or enable widespread implementation of a new energy conversion application. Everyone is familiar with the promise of solar power, but there is equivalent potential in solid-state lighting. Sandia Laboratories estimates that replacement of incandescent and fluorescent lighting with solid state lighting over the next 20 years could reduce energy consumption for lighting by 29%. Similarly, improved thermoelectric generators recovering waste heat from vehicular exhaust and converting it to electrical power could improve fuel efficiency by 10% (DoE).

The Opportunity: Efficiency improvements by a factor of two or three must start from breakthroughs in materials. The greatest opportunity resides in the engineering of nanocomposites designed to exploit the unique properties of materials that are structured on the scale of the wavelengths and scattering lengths of electrons, photons (quanta of light), and phonons (quanta of lattice vibrations). As in the familiar structural composites that combine two very different classes of materials resulting in performance that exceeds the performance of either constituent alone, recent theory and experiments demonstrate that functional nanocomposites have the potential to outperform conventional bulk and thin-film materials in energy conversion efficiency. To realize this potential at the device level with assembly and integration methods that are amenable to low-cost manufacturing remains a formidable challenge. The Heterogeneous Integration Research Group is addressing long-term scientific and technical obstacles to realizing the promise of nanocomposites in energy conversion devices. Some of the group's current projects are highlighted below:

Nitride metal-semiconductor multilayers for solid-state thermionic energy conversion: The Heterogeneous Integration Research Group is part of the Thermionic Energy Conversion (TEC) Center, a Multidisciplinary University Research Initiative (MURI) funded by the Office of Naval Research (ONR) and directed by Professor Ali Shakouri (UC Santa Cruz). TEC MURI participants from UCSC, Harvard, UCSB, UC Berkeley, MIT, NCSU and Purdue are working together to explore new concepts in thermionic direct thermal to electrical energy conversion for defense applications, including the Navy's all-electric vessel concept. At Purdue, MSE graduate student, Vijay Rawat and Professor Sands are investigating nitride metal-semiconductor multilayer composites for electron energy filtering at high temperatures. These heterostructures, such as the TiN/GaN multilayer shown above, are prepared by reactive pulsed laser deposition from elemental metal targets in an ammonia process ambient.

Toward a monolithic phosphor-free white LED:

The Heterogeneous Integration Research Group was recently funded by the NSF to exploit the elastic strain relief in nanorod heterostructures in extending the spectral range of (In,Ga)N LEDs from uv-blue-green toward amber and red. Today's (In,Ga)N LEDs are familiar as the green traffic lights with the intense color and pixilated appearance. Bright blue (In,Ga)N LEDs are also manufactured in high volume, and are widely employed in cell phones. These blue LEDs combined with a yellow phosphor are also used in the first generation of white LEDs. The Purdue effort is focused on understanding the relationship between strain and InN incorporation in "quantum disks" grown within (In,Ga)N nanorods, with the ultimate aim of demonstrating emission of light across the full visible spectrum from a single nanostructured heterostructure, without phosphor downconversion. Graduate students Parijat Deb (MSE), Sangho Kim (ECE) and HoGyoung Kim (Physics) are working together to investigate the epitaxial growth of (In,Ga)N nanorod arrays using OMVPE and a custom-built halide vapor phase epitaxy (HVPE) reactor.

Thermoelectric conversion of exhaust heat to electrical power in automobiles: Recent advances in bulk and thick-film nanocomposites have shown that materials engineering at the nanoscale can result in marked improvements in energy conversion efficiency. As the thermoelectric materials improve, the ohmic contact metallizations must improve as well. In November 2004, the Sands group joined a DoE-funded team of industrial researchers led by BSST LLC in a project aimed at demonstrating the potential of thermoelectric conversion of exhaust heat to electrical power in automobiles. MSE graduate student David Ewoldt is studying ohmic contacts to novel nanocomposite thermoelectric materials in an effort to reduce the contact resistance while maintaining thermomechanical integrity at elevated operating temperatures.

Porous anodic alumina as a template for integrated nanowire composites:

Several projects in the Sands group are focused on developing porous anodic alumina (PAA) as a platform for nanowire integration for applications in energy conversion, nanoelectronics, molecular electronics, nanomagnetics and sensing. PAA-based nanocomposites can be produced by electrochemical means, allowing for the fabrication of self-organized high-aspect-ratio nanostructures at low cost. Combined with top-down lithographic patterning, these PAA-based materials can be integrated into a microsystem format. MSE graduate students Manuel DaSilva and Kalapi Biswas are developing processes for fabricating uniform patterned nanowire arrays with lateral and vertical composition modulation. These same templates are being used to create frameworks for vertical single-walled carbon nanotube devices in a project with postdoctoral fellow Dr. Placidus Amama and the group of Professor Tim Fisher (ME) as part of the NASA-funded Institute for Nanoelectronics and Computing.