Nanoscale Transition Metal Pnictides: Materials by Design

Wayne State University, Detroit MI

Investigators

Abstract

NON-TECHNICAL SUMMARY With support from the National Science Foundation, Division of Materials Research, the development of new materials and knowledge for the design of cooling devices will be achieved. Air conditioning and refrigeration is a part of daily life, with air conditioning consuming 5%, and commercial refrigeration up to 20%, of energy output. The conventional process of refrigeration (gas compression and expansion) has limited efficiency, and the refrigerants are often ozone-depleting and/or Greenhouse gases, raising concerns whenever inevitable leaks arise. A more energy efficient (by up to 50%) and environmentally friendly process is magnetic refrigeration, where application of a magnetic field to a magnetocaloric (MC) material results in heat emission, and removal of that field results in heat absorption, effectively acting as a heat pump. Traditional MC materials are based on the rare-earth metal gadolinium, which suffers from poor efficiency near room temperature and is prohibitively expensive. In this NSF-DMR supported project a rational approach will be exploited for the development of MC materials based on nanoparticles comprising earth-abundant elements. The effect of nanoparticle composition and size, as well as interactions between particles on magnetic properties will studied en route to developing inexpensive and efficient devices for magnetic refrigeration. Over the course of the project, graduate and undergraduate students will develop critical thinking and technical skills, as well as hands-on experience with cutting-edge techniques, for developing the next generation of advanced technologies. The project will also introduce Detroit-area middle and high school girls, many of whom are underrepresented minorities, to materials science through the GO-GIRL (Gaining Options-Girls Investigate Real Life) Material Girls outreach project. TECHNICAL SUMMARY With support from the National Science Foundation, Division of Materials Research, the principal investigator will produce functional transition metal pnictide (pnicogen = Group 15 element) nanoparticles and assemblies, focusing on materials for magnetic refrigeration (MR). Transition metal pnictides comprise a large but relatively underexplored class of materials, despite having properties that span the energy landscape. These materials are being investigated in thermoelectric devices (waste heat conversion), batteries (energy storage), catalysis (fuel processing), and magnetic refrigeration (climate control). Many of the envisioned advances in these technologies presume the ability to control the physical dimensions of the materials on the nanoscale, a significant challenge. Moreover, confinement to nanoscale dimensions impacts properties in sometimes unexpected ways (intraparticle effects), as does integration/assembly (interparticle effects). These issues have hampered the development of transition metal pnictide nanoparticles to address key technological problems. MR is based on the absorption and release of energy during a magnetic transition (the magnetocaloric effect), and is 50% more efficient than traditional gas compression/expansion systems and more environmentally friendly, as it does not exploit ozone-depleting or greenhouse gases. Heretofore inaccessible but promising nanoscale phases for MR will be produced by exploiting ion-exchange routes and direct syntheses, targeting Fe-doped Mn pnictides (Aim 1). On a parallel track, anion (P, Sb) doped MnAs nanoparticles will be synthesized and detailed structure-property-size correlations established (Aim 2). This information will be used to narrow the parameter space for production of new materials in Aim 1 so as to target the most promising phases and sizes. The final parallel investigation (Aim 3) will focus on integration and assessment of interparticle interactions in known phases, subsequently moving to new phases as materials from Aims 1 and 2 become available.