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University Lecturer

MS Indian Institute of Science
PhD Indian Institute of Science

Ferroelectric materials for cooling and energy applications

My research within the Device Materials Group aims at developing an efficient and clean solid-state cooling technology based on the electrocaloric (EC) effect associated with phase transitions in ferroelectric materials. My interest lies in the development and understanding of new EC materials and the subsequent design of prototype cooling devices. Additionally, the piezoelectric and pyroelectric properties of these EC materials are attractive for energy harvesting applications.

Electrocaloric refrigeration

Electrocaloric (EC) refrigeration is a candidate for solid-state cooling that exploits electric field driven temperature/entropy changes near phase transitions in ferroelectric materials. There has been renewed interest in the field of electrocalorics since the discovery of giant EC effects (~12 K) in ferroelectric thin films, prior to which EC temperature changes were restricted to a few Kelvin in bulk ferroelectrics. This enhancement arises as thin films possess order-of-magnitude larger breakdown fields than in the bulk. However, reports of large EC temperature changes in films have mostly been predicted from indirect thermodynamic analysis of electrical data. These predictions are difficult to confirm via conventional thermometry/calorimetry as films cannot pump significant heat due to low thermal mass. We are focusing on novel techniques to accurately measure the EC effect in thin films of promising materials, and on modeling EC heat flow, to provide a basis for developing optimized EC devices for cooling applications.

Energy harvesting

Energy harvesting for small power applications is a hot research topic due to its potential applications in powering portable electronics, wireless sensors and medical implants, to name a few. Fixed energy sources such as batteries and fuel cells are the most commonly used power sources, but have serious disadvantages such as the constant need for replacing and recharging due to their short lifetime, and their size which does not scale with the diminishing sizes of modern electronic devices. Powering devices from scavenged ambient energy from the environment is an attractive avenue for replacing or extending the lifetime of traditional power sources. Our research in this area entails scavenging energy from mechanical vibrations and temperature variations of ambient environments using the piezoelectric and pyroelectric properties respectively of ferroelectric thin films and multilayer capacitors.

Electrocaloric effect in a commercial multilayer capacitor (MLC): (a) MLC cross-sectional schematic with overlay obtained by optical microscopy after sawing and polishing, (b) Experimental set-up, comprising MLC and sample thermometer, mounted on a bespoke variable-temperature copper block with a reference thermometer, and (c) EC temperature changes in the MLC over time due to the application and subsequent removal of 300 kV cm−1
  • R.A. Whiter, V. Narayan & S. Kar-Narayan, "A scalable nanogenerator based on piezoelectric polymer nanowires with high energy conversion efficiency", Advanced Energy Materials 4, 1400519 (2014). 
  • X. Moya, S. Kar-Narayan and N.D. Mathur, "Caloric materials near ferroic phase transitions", Nature Materials 13, 439 (2014).
  • S. Kar-Narayan, S. Crossley, X. Moya, V. Kovacova, J. Abergel, A. Bontempi, N. Baier, E. Defay, N.D. Mathur, “Direct electrocaloric measurements of a multilayer capacitor using scanning thermal microscopy and infra-red imaging” Appl. Phys. Lett102, 032903 (2013)
  • E. Defay, S. Crossley, S. Kar-Narayan, X. Moya, N.D. Mathur, “The electrocaloric efficiency of ceramic and polymer films” Adv. Mater. 25, 3337 (2013)
  • S. Kar-Narayan & N.D. Mathur, “Direct and indirect electrocaloric measurements using multilayer capacitors” J. Phys. D: Appl. Phys43, 032002 (2010).