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In-situ polarized ultra-thin PVDF film based flexible piezoelectric nanogenerators

Name: Vaibhav Khurana

Department: MEMS

Program: Ph D. (3rd year)

Name of supervisor: Prof. Dipti Gupta

Topic of research: In-situ polarized ultra-thin PVDF film based flexible piezoelectric nanogenerators.

 

The self-powered devices (SPDs) have been investigated, over the years, to reduce the dependency of various electronic devices ranging from smart sensors to thin-film transistors. The prefix ‘self-powered’ coveys that the devices do not need an external power supply, thus reduces the complexity of the device architecture added with reduced consumption of input energy. Notably, the devices based minimum energy input can be effectively integrated with self-powering sub-systems so to make the entire system to run at minimum energy expenditure. Our study step in towards self-powered systems by formulating an easy way to develop an efficient energy harvesting device which can be further used in powering applications.

Piezoelectric nanomaterials open a huge room for generating useful electrical energy output via mechanical actuation including vibrational energy, anthropogenic activities, wind energy, ocean energy etc. There are series of organic and inorganic materials which can be directly implemented for the piezoelectric action. High-performance piezoelectric nanogenerators (PNGs) have been realised using various inorganic materials such as zinc oxide (ZnO), barium titanate (BTO), lead zirconate titanate etc. in the recent past; but their brittle nature offers limited usage in alternating loads. However, in terms of self-powered systems to extricate energy via several degrees of motion PNGs require more flexibility. In addition to this, polymers such as polyvinylidene fluoride (PVDF), polyamides (PA), cellulose and its derivatives, polylactic acids etc. are some well-known piezoelectric materials studied that offers outstanding flexibility. Among these, PVDF has been studied extensively owing to its polar structure due to the presence of H-F dipoles across the C-C chain and their polarization in an all trans state (β phase); providing a net dipole moment to the matrix which is often achieved via mechanical stretching or stress induction in the chain. 

Since PVDF has a low piezoelectric strain coefficient (d33) (property describing the charge generation via mechanical actuation) inherently; it is often blended with inorganic perovskites in order to enhance its β phase. In addition to this, techniques such as electrospinning, mechanical stretching, solution casting etc. have been adopted. Usually, in order to achieve efficient polarization across C-C chain in PVDF a high electric filed is applied that aligns the dipoles along the direction of field termed as ‘poling’ process. Also, enhancing the polarization improves the electrical performance of PNGs. In addition, few recent studies on PNGs have also reported avoidance of electrical poling step through the usage of 2D nanomaterials such as n-graphene, SnO2 nanosheets etc. to fabricate self-poled or self-oriented dipole devices, however, they had to construct a multilayer structure in order to observe piezoelectric phenomenon. 

In this work, we have addressed these issues and have presented an ultra-thin, compact, self-poled, and a robust filler free-PVDF based piezoelectric nanogenerator with high flexibility along with enhanced piezoelectric performance. In order to create self-poled PVDF film, we have made use of the quenching process, which has not been much explored in fabricating PNGs. Oka and Koizumi had extensively studied the quenching mechanism in 1985 and had provided a clear picture of the formation of β phase form of PVDF based on nucleation and growth theories and found that the nucleation of β crystallites are found to be predominant under low temperature quenching. Other studies reported the phase inversion in PVDF via quenching and the formation of β phase fraction up to 90% under certain optimum conditions which were based on mechanical stretching, choice of solvents, spin coating speed and temperature of the quenching bath. 

We have taken the results of above-mentioned studies into consideration and have targeted the growth of β phase in PVDF films via rapid quenching at sub-zero temperature and selecting a high boiling point polar solvent. These films were then incorporated into a PNG device which had single active layer providing a thin device which is self-poled and paves the way towards a small area, robust energy harvester. The novelty of our work thus lies in the formation of in-situ beta phase stabilization in PVDF matrix via quenching process and further using the same quenched film to fabricate self-poled and robust piezoelectric nanogenerators that have demonstrated promising applications in energy harvesting and enhanced power output via human body motion.

This device envisages the use of polar solvents and quenching process to obtain the β phase in PVDF, thereby eradicating the electrical poling process and making the fabrication a two-step simple process. The PVDF films obtained a maximum d33 value of 20.97 pm/V, as calculated via PFM. The device was able to produce a maximum of 19.2V peak to peak voltage upon human finger pressing. The nanogenerator develops current of the order of > 0.7μA along with cyclic pressing lights numerical displays on LCDs. Further, the rectified output of the device charges up the capacitors quickly, thus showing high promise in energy storage applications. In summary, we are able to demonstrate a thin, flexible and small size nanogenerator which is capable to fulfil requirements in the field of energy harvesting and storage requiring less space and power for execution.

Figure: The image representing the fabrication process of the device along-with voltage response on actuation via 96kPa and image depicting numeric display on LCD on mechanical compression of the device.