Smart Materials Just Got Smarter

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How is it smart materials are getting smarter? That is what scientists are discovering.

Army and Naval scientists have demonstrated a new class of solid-state phase change material for thermal energy storage, with applications and immediate impact ranging from large-scale power generation to the suppression of thermal transients associated with high-energy laser directed energy applications.

Image Caption: Thermal energy storage figure of merit vs. transformation temperature for the r.t. Martensite and r.t. R-phase NiTi sample characterized herein (green stars), along with values for other solid-solid phase change materials and solid-liquid paraffin. The green band shows the potential range of FOM and and transformation temperature for NiTi-based alloys based on thermal conductivity, density and latent heat values from the literature. (U.S. Army graphic)

"In recent years, researchers and companies have attempted to develop smarter, more efficient and environmentally conscientious materials and energy systems," said Dr. Darin Sharar, principal investigator for the U.S. Army Combat Capabilities Development Command's Army Research Laboratory, the Army's corporate research laboratory also known as ARL. The manuscript he authored, "Shape memory alloys provide ultrahigh thermal energy storage figure of merit," was selected as April's featured article published in Applied Physics Letters.

With directed energy, next-generation systems provide unique thermal management challenges due to their high heat flux and short pulse duration, requiring thermal design with transient loads in mind, Sharar said. Architectures using phase change materials are quickly becoming the preferred thermal management solution because of their ability to absorb thermal energy with minimal temperature increase, ensuring the directed energy systems can operate within spec, Sharar explained. However, standard designs require engineering measures in the form of metallic fin structures to provide mechanical support, prevent liquid phase change materials leakage and enhance poor phase change materials thermal conductivity. Because of this, metallic fins account for ~1/2 to ~2/3 of the mass of state-of-the-art directed energy phase change materials heat exchangers, leaving as little as 1/3 for latent energy storage. This imposes significant design restrictions, hindering transition from the laboratory setting to real-world systems.

"The discovery of thermal energy storage via shape memory alloys provides an unprecedented two order of magnitude improvement in the cooling figure of merit, defined by the product of the material latent heat and thermal conductivity," Sharar said. "This opens a new paradigm of phase change material design, through which scientists can eliminate the need for heavy/large volume fin structures and fabricate thermal energy storage and heat transfer structures entirely out of metallic shape memory alloys."

He said this promises increased duty cycle for directed energy applications, along with significant size and weight savings, enabling more-capable, compact directed energy assets on smaller platforms.

The paper describing these seminal results was published by Applied Physics Letters, a highly-prestigious multi-disciplinary journal, as a featured article and was displayed prominently on the APL banner on April 12. The patent covering this concept received approval from ARL's Invention Evaluation Committee and a patent is being pursued. Sharar recently received funding to further-explore this research area, via an DIRA-ECI award.