Big ideas often come in small packages
Today, though scientists are experimenting with a wide variety of nanoparticles and quantum dots, no reliable methods are available to manipulate them into regular patterns or integrate them with existing structures. As a result, these nano objects can only be used as randomly oriented dispersions on surfaces or in solids / liquids.
If we could directly build structures in three dimensions with a free choice of materials, it would drastically change the way we think about design and construction at the nanoscale. Thus, the concept of nanoscale-manipulation can unlock enormous opportunities in a wide spectrum of disciplines – quantum optics, biophysics, condensed matter physics, energy conversion, mainstream electronics, to name a few.
Gaurav Vats, a PhD scholar with the IITB-Monash Research Academy, recently won the Indian Science Congress Association’s ‘Young Scientist Award 2015-16 (Materials Science Category)’ for his work related to nanoscale manipulation (for details see ‘Giant pyroelectric energy harvesting and a negative electrocaloric effect in multilayered nanostructures’ alongside and http://pubs.rsc.org/is/content/articlehtml/2016/ee/c5ee03641k
Gaurav was part of a team of researchers (from the IITB-Monash Research Academy, NPL-India, University of Bath-UK and University of Puerto Rico-USA) that explored PbZr0.53Ti0.47O3/CoFe2O4 (PZT/CFO) layered nanostructures for both pyroelectric energy harvesting and ECE investigations.
The awards were presented on January 7 by the former Indian Prime Minister, Mr. H D Deve Gowda, at a glittering ceremony attended by eminent scientists, including Nobel laureates.
Prof Murali Sastry, CEO, IITB-Monash Research Academy, is justifiably delighted. “The Academy is yet another example of the strong internationalisation strategies of both IIT Bombay and Monash University. The ‘Academy’ is an exemplar of a vibrant joint-venture collaboration in India. It will combine the strengths of both institutions from two countries – diverse in size and population – to establish excellence in collaborative research.”
Little things, as Gaurav will testify, make big things happen.
Recent advancements in nanotechnology have significantly raised the demand for small scale refrigeration and energy conversion technologies. The performance of these technologies must be sufficiently high for heat extraction or recycling waste energy in integrated circuits, computer memories and medical equipment.
Unfortunately the conventional vapour-cycle technologies are unable to meet these demands as they involve bulky components, such as compressors. Therefore, these emerging demands have led researchers to consider novel energy conversion systems. Among these, the electrocaloric effect(ECE)- and pyroelectric effect-based mechanisms have been considered alternatives for these applications.
The electrocaloric effect is believed to have first been reported in 1930 and was later explored by US and Japan during World War II. Thereafter, it was not given much importance until it was observed near ferroelectric transitions of potassium dihydrogen phosphate in 1950. This gave birth to the debate about the possibility of ECE in hydrogen-bonded materials.
Later, in the 1960s, with the discovery of ceramic perovskites, the effect was detected at room temperature and above. Research escalated in this field after the work of Mischenko, et al 1 who reported a large ECE temperature change (|?T|) of 12 K at comparatively large values of applied electric field (|?E|=480 kVcm-1) in PbZr0.95Ti0.5O3 near its Curie temperature (495 K). The study was based on indirect measurements using Maxwell relations, which was first proposed by Thacher2.
This method soon earned popularity primarily because of the convenience offered in measuring the ECE in thin films. Prior to this approach the ‘giant’ ECE temperature changes were difficult to accurately measure in thin films as the measurements were prone to influence by probe-based techniques. However, the indirect method is subjected to a few limitations which can be studied in detail elsewhere3. At the same time, it is to be noted that the ECE works as a reverse pyroelectric effect, which produces an electric charge on exposure to a thermal change.
Pyroelectric energy conversion exploits the fundamental idea of generating an electric charge due to change in remnant and/or saturation polarization as a result of thermal fluctuations. The degree of energy conversion can be significantly enhanced by adopting an appropriate operation cycle. In this context, Mohammadi and Khodayari stressed the use of an Ericsson cycle.
In addition, there exists a well-known variant of the Ericsson cycle, termed the “Olsen cycle”4. The advantage of this cycle is that it operates under unipolar electric fields, rather than bipolar electric fields used in the conventional Ericsson cycle, and has a comparatively reduced hysteresis loss and enhanced energy conversion.
However, it is to be noted that the energy harvested using the Olsen cycle is not merely contributed by the pyroelectric effect but is also a result of the change in electrical energy storage capacity of the material with temperature. Interestingly, it has been reported that the Olsen cycle is capable of providing an energy density three orders in magnitude higher than that of obtained using the conventional pyroelectric effect. The claim has later been verified for many well-known compositions by Olsen, et al.
Moreover, recent studies in this direction also strongly support their claim and suggests that the Olsen cycle has particular advantages for pyroelectric based energy harvesting. Therefore, both the Olsen cycle and ECE are being extensively explored for ‘giant’ energy conversion applications.
Since both methods work on the same principle, but in opposite directions, the materials requirements in order to achieve enhanced ECE or pyroelectric energy harvesting are similar.
The team of researchers from IITB Monash Research Academy, NPL-India, University of Bath-UK and University of Puerto Rico-USA that I was part of, developed an understanding of these requirements and explored PbZr0.53Ti0.47O3/CoFe2O4 (PZT/CFO) layered nanostructures for both pyroelectric energy harvesting and ECE investigations.
It is found that unlike the conventional ECE, the effect in PZT/CFO MLNs is governed by a dynamic magneto-electric coupling (MEC) and can be tuned by the arrangement of the various ferroic layers. The ECE is investigated in alternate multilayers of PZT and CFO that consist of stacks of three (L3), five (L5) and nine (L9) alternating PZT and CFO layers.
Intriguingly, all configurations exhibit a negative ECE, calculated using Maxwell relations, which has a high magnitude in comparison with previously reported giant negative ECE (|?T|=6.2 K). The maximum ECE temperature change calculated in three (L3), five (L5) and nine (L9) layers is 52.3 K, 32.4 K and 25.0 K respectively. In addition, the maximum pyroelectric energy harvesting calculated for these layers using a modified Olsen cycle is four times higher than the highest reported pyroelectric energy density of 11549 kJm-3cycle-1.
This increase is attributed to the cumulative effect of multiple layers that induce an enhancement in the overall polarization, 1.5 times of lead zirconate titanate, and leads to abrupt polarization changes with temperature fluctuations. The study also shed light the thermodynamic processes involved in the ECE and it is concluded that the refrigeration obtained from reversed Olsen cycle is a combined effect of an isothermal entropy as well as adiabatic temperature change.”
Gaurav Vats, et al, Giant Pyroelectric Energy Harvesting and Negative Electrocaloric Effect in Multilayered Nanostructures. Energy and Environmental Science, 2016
- Mischenko, A., Zhang, Q., Scott, J., Whatmore, R. & Mathur, N. Giant electrocaloric effect in thin-film PbZr0. 95Ti0. 05O3. Science 311, 1270-1271 (2006).
- Thacher, P. Electrocaloric effects in some ferroelectric and antiferroelectric Pb (Zr, Ti) O3 compounds. Journal of applied physics 39, 1996-2002 (1968).
- Valant, M. Electrocaloric materials for future solid-state refrigeration technologies. Progress in Materials Science 57, 980-1009 (2012).
- Olsen, R. B., Bruno, D. A. & Briscoe, J. M. Pyroelectric conversion cycles. Journal of applied physics 58, 4709-4716 (1985).