DON26TZ01-NV013 — Flexible Printed Thermoelectric Cooling Film

Award Maximum: $300,000 | Period of Performance: 6 months | Phase Type: Phase I

OBJECTIVE: Develop a low-cost and lightweight thermoelectric cooling film that could be used to cool the warfighter (small scale) or surfaces on military platforms (larger scale) using printed organic semiconductors. The flexible cooling films should have a bending radius of less than one inch to easily wrap around pipes, wrists, and ankles, and be able to conform to complex curvatures on larger surfaces.

DESCRIPTION: Thermoelectric cooling devices based on narrow bandgap semiconductors such as bismuth telluride are commercially available. They are solid state devices and thus do not have the large footprint and moving parts associated with vapor compression refrigeration systems; however, they operate with lower efficiency. They are well-suited for cooling small flat surfaces where one is more concerned with the form factor than efficiency. For many practical applications, these square ceramic tile thermoelectric devices are heavy and too rigid, and do not offer conformal contact to curved surfaces.

Over the past fifteen years, a lot of progress has been made on organic thermoelectric materials. Though the thermoelectric figure of merit (ZT) has not caught up to that of bismuth telluride and other inorganic materials, the potential to make low-cost, lightweight, and flexible devices has opened a new application space for thermoelectric cooling where flexibility and large-area conformal contact are prioritized over efficiency. For instance, lightweight headbands and wristbands only need to remove a small amount of heat to provide significant cooling sensation to the user. Likewise, there are diffuse, large surface area applications with similar cooling needs.

The conducting polymer Poly(3,4-ethylenedioxythiophene) [PEDOT] was identified as a strong candidate for the p-type leg in the p-n device, but device performance has been limited by the lack of suitable n-type materials. The organic electronics community has long wrestled with n-type materials due to potential oxidation of the electron carriers. A number of inherently stable and high performing n-type polymers have recently been developed that should complement the available p-type materials and enable significantly improved thermoelectric cooling device performance. New device designs obtainable with simple fabrication must be developed to take advantage of the anisotropic thermal conductance and charge transport in these materials, which is typically maximized in-plane and along the polymer molecular backbones, such that measured thin film behaviors successfully translate into device performance. A number of design and fabrication strategies have been demonstrated but much more innovation is possible. It is an appropriate time to develop lightweight, flexible thermoelectric cooling devices for these niche applications.

This STTR topic is for low-cost, lightweight, and flexible thermoelectrics for personal cooling as well as for large area applications.

The flexible cooling films should have a bending radius of less than one inch to easily wrap around pipes, wrists, and ankles, and be able to conform to complex curvatures on larger surfaces. The stated applications are near-ambient temperatures though the conjugated polymers should be able to handle temperatures up to 200°C. Composite approaches that are appropriate are welcome. This topic is not soliciting a fabric-based solution.

PHASE I: Select n and p type materials. Demonstrate that the selected n and p type materials can be processed (and doped) into films with reasonable Seebeck coefficient, thermal conductivity, and electrical conductivity in device relevant materials planes. Measure these properties in device relevant planes. Prepare a simple thermoelectric device and characterize performance. Model this simple device structure and compare with achieved performance. Model novel device geometries that could be manufactured with low cost processing approaches for both the personal cooling (small area, 4 kelvin gradient) and surface cooling (large area, 20 kelvin gradient) applications. Describe material and processing advances that would be accomplished in Phase II to enable these devices.

PHASE II: Optimize materials properties and device designs for personal and surface cooling applications. Model new designs as necessary. Make prototypes for both applications and start work towards commercially relevant device fabrication processes. In year two, prepare larger devices (10 square inches) by using commercially relevant processing methods and fully characterize device performance including achievable temperature gradients and efficiency. Model power requirements for both applications. Compare performance against commercial thermal electrics for ambient cooling applications. Prepare the cost analysis and business case for the two products.

PHASE III DUAL USE APPLICATIONS: Support transition to Navy use.

Both wearable thermoelectric coolers and large area films should have commercial and military applications. For wearable devices, performance metrics (power requirements) should be known for the Phase II prototype and there would be a lot of work to optimize the product for skin contact. In the longer term, some level of elasticity to the substrates would enable better contact and comfort. For the surface cooling film, a modestly stretchable substrate would also enable better contact with the large, curved surfaces of military platforms. An adhesive backing would be needed for large area applications.

In addition to directly developing products to keep warfighters cooler, industry that provides clothing and accessories to construction workers, law enforcement, and agricultural workers could develop appropriate products. Larger area devices could cool and heat automobile seats and outdoor surfaces.