CBET Award Achievements Notable Accomplishments from CBET Awards

Evaluating Refrigeration Potential of Magnetic Nanostructures Hariharan Srikanth – University of South Florida

Outline: Local cooling and/or heating have become important in an ever-shrinking world of electronic devices fabricated using Micro-Electro-Mechanical-Systems (MEMS) technology. New and innovative methods to achieve this have to be explored. While magnetic refrigeration itself has been around for several decades, conventional materials have limitations in terms of the operating temperature. Unfortunately, some of the most promising candidate materials for magnetic refrigeration cannot be easily synthesized or processed in thin film or nanostructured forms. An attractive alternative is to consider chemically synthesized assemblies of magnetic nanoparticles. This project is aimed at conducting a systematic assessment of the potential and viability of using ferromagnetic and ferrimagnetic nanoparticles for refrigeration. Methodology: Magnetic refrigeration potential can be deduced from measuring the entropy change as an external magnetic field is cycled up and down from zero to fields up to 3 Tesla. Cooling is possible in the demagnetizing cycle when the magnetic moments undergo a change from a saturated ordered state to a disordered state. This causes absorption of heat energy from the surrounding medium in which the magnetic particles are embedded thus resulting in cooling of the lattice. This phenomenon described as the magneto-caloric effect (MCE) is employed by the PI and his team in this project to study the entropy change in various configurations of nanoparticles synthesized in the PI’s laboratory. Results: Systematic MCE experiments done by the PI’s group on nanoparticles have yielded important results that point towards certain benefits of using nanoparticle-based materials for magnetic refrigeration. The entropy change itself is low compared to some of the “giant-MCE” bulk materials that are currently known. However, the size dispersion of the nanoparticles and broad variation in the blocking temperature resulted in maintaining the entropy change constant over a broad temperature range. This is promising for applications where operating temperatures in the 100K to 300K range are required. Small changes in inter-particle separation have a significant effect on the MCE and the PI’s group is currently exploring this in chemically assembled nanoparticle arrays with different surfactant coatings on the nanoparticles. A future goal is to deposit nanoparticle arrays on a high thermal conductive substrate and directly measure the temperature change under field cycling by attaching a thermocouple to such a device.

Transmission Electron Microscope image of chemically synthesized polydisperse cobalt ferrite nanoparticles and the entropy change as a function of temperature for various external magnetic field cycles. Credit:  Hariharan Srikanth – University of South Florida

Scientific Uniqueness: In conventional bulk and thin film ferromagnets, the cooling capacity is maximum around the magnetic ordering temperature (also known as the Curie temperature). The Curie temperature is material-dependent and is not easy to tune for applications that require a broad range of operating temperatures. The innovative idea behind this project is to take advantage of a versatile order-disorder transition temperature (also known as the blocking transition) in nanosize magnetic particles that separates the phases where the magnetic moments are fluctuating (superparamagnet) and frozen (blocked state). This blocking transition can be selectively tuned in nanoparticles by controlling the particle size and inter-particle separation, thus leading to a broader range in terms of operating temperature. Impact on Industry and/or Society: The project has a broad impact from a scientific perspective as well as in training students in the cutting-edge field of nanotechnology. Students in PI’s group have developed hands-on expertise in all aspects of the project. This covers a wide range of experimental skills from chemical synthesis of nanoparticles, electronic instrumentation, cryogenics, analysis of results and presentation in peer-reviewed journals and conferences. LabView interface and software for analysis of the MCE were developed completely in-house by the students. From a global perspective, refrigeration generally involves gas compression techniques that use ozone-depleting gases. Magnetic refrigeration is environmentally friendly and provides an alternative method for cooling/heating. This work is notable because it is an attempt to search for new materials based on nanostructures that could be seamlessly integrated into the technology of magnetic refrigeration to achieve superior performance for on-chip spot cooling applications. The work addresses both the fundamental scientific aspects of magnetic nanostructures as well as the targeted application of building a prototype cooling device based on magnetic entropy change associated with external field cycle. Advancement of S&E in this area would result in new environmentally friendly refrigeration methods. This work involves multidisciplinary research. The PI is a physicist training graduate students seeking degrees in Applied Physics. Research is multidisciplinary as its components include chemical synthesis of nanoparticles, characterizing the physical properties using various analytical instruments, device fabrication and engineering. This involves Applied Physics, Materials Science and Chemical Engineering.

Program Officer:		Judy Raper
NSF Award Number:		0408933
Award Title:		Magnetocaloric Effect in Nanoparticle Assemblies for Refrigeration Applications
PI Names:		Hariharan Srikanth
Institution Name:		University of South Florida
Program Element:		1415

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This Nugget was Updated on 20 November 2006.