‘Wearable’ electronics such as smart-watches are fast picking up in popularity. Apart from making a fashion statement, wearable electronics can also help in monitoring health by measuring blood pressure, glucose levels and even alert care-givers if necessary.
Thus, it comes as no surprise that wearable, next-generation technology is being developed and improved at a rapid pace to meet growing expectations.
One of the polymer materials that is of particular interest in the case of wearable electronics is the polymer material, poly vinylidene fluoride (PVDF). This is because it exhibits excellent ferroelectric and piezoelectric characteristics.
PVDF is used all over the world for the development of efficient, flexible energy storage, and mechanical energy harvesting devices for next generation wearable electronics. In order to improve performance, secondary inorganic nanomaterials are added to the PVDF matrix.
However, even though the energy storage density is improved by the presence of space charges, the addition of these nanomaterials causes a reduction in the storage efficiency of the PVDF-based binary composite film. This causes a dilemma as the space charges are a necessity. What needs to be done, is to reduce the space charges without affecting the energy harvesting activity. Thus, researchers the world-over are trying to find a suitable filler nanomaterial to counteract this problem.
If the PVDF based composites are prepared without any filler addition, space charges can be reduced, but the polar phase formation will be interrupted due to the tendency of pristine PVDF to exhibit energetically more favourable, non-polar alpha phase in a normal condition.
Thus, even though the efficiency would be improved, the energy storage density will be low due to the low value of dielectric permittivity, and energy harvesting performance will also be low due to non-polar chain conformation and unavailability of space charges.
This finding has motivated researchers to search for a filler that has high permittivity, and can modify the filler surface and/or filler-polymer interface in order to achieve polar chain conformation along with reduced space charge.
In this study, Mr. Abhishek Sasmal, Ms. Payel Maiti, and Prof A. Arockiarajan from the Department of Applied Mechanics, Indian Institute of Technology, Madras, Chennai, India, Mr. Sourav Maity, and Ms. Shrabanee Sen from the Functional Materials and Devices Division, CSIR- Central Glass & Ceramic Research Institute, Kolkata, India, have used a different, novel approach to solve these problems.
Instead of using difficult surface/interface modification techniques, external plasma treatment on the PVDF membrane has been done, as better dipole orientation occurs which improves the piezoelectricity of the device.
Air-plasma discharging on PVDF-based binary composites was done in this study. The effect of this technique on the output energy storage and mechanical energy harvesting performance of the composite system was studied.
Another important aspect of this study was in selecting filler materials. As the air-plasma discharging technique was found to improve the piezoelectricity, magnetic fillers were the focus of this study. This was done so that magnetoelectricity would be improved, as PVDF-based magnetoelectric (ME) devices have recently drawn the attention of researchers, especially for antenna applications.
Bismuth ferrite (BiFeO3) was chosen as the filler material due to its excellent single phase room temperature multiferroicity, and piezoelectricity.
In this study, air-plasma discharging technique on PVDF-based composite film loaded with multiferroic BiFeO3 nanoparticles was studied. The use of this technique on developed film has helped in accumulating discharged charges on its surface, which in turn has oriented its piezoelectric dipoles in the thickness direction. This dipole orientation has helped in improving the piezoelectric characteristics of the composite film which in turn has induced the coupling of these dipoles with the magnetization of BiFeO3. Furthermore, the improved dipole alignment was able to suppress the leakage current of the system thereby increasing the capacitive energy storage efficiency along with storage density.
This kind of a plasma discharging technique has proved to be fruitful in simultaneously improving capacitive energy storage density and efficiency along with enhanced mechanical to electrical energy conversion efficiency of PVDF-based binary composite films.
Dr. Kadhiravan Shanmuganathan, from the Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune, India, has appreciated the work done by the authors thus: “This work by Prof. Arockiarajan et al. has shown that air-plasma discharging on PVDF-based composite films can be a facile way to realize simultaneous enhancement in energy storage density and energy storage efficiency without elaborate chemical modification of fillers. Also, the enhancement in magnetoelectric coupling achieved with a magnetic filler such as BiFeO3 has added a new dimension to the work. It is an exciting study offering new insights into the design of advanced mechanical energy harvesting devices given the flexibility and roll-to-roll processability of polymer based ferroelectric and magnetoelectric composites.”
Article by Akshay Anantharaman
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