Synthesis and characterisation of piezoelectric composites for sensing and actuation applications

Eltouby, Pakinam (2023) Synthesis and characterisation of piezoelectric composites for sensing and actuation applications. Doctoral thesis, Northumbria University.

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Abstract

The need for high-performance dielectric ceramic-polymer composites is growing day after day. These composites are characterised by their functional properties and find their applications in various fields of current modern technology. Amongst the various factors influencing the performance of these composites, the morphology of the incorporated ceramic fillers within the polymeric matrix has proven to have a significant impact. Achieving the desired morphology is highly dependent on the synthesis process employed. This work has therefore implemented the response surface methodology approach (RSM) within the widely employed molten salt synthesis (MSS) framework to systematically explore the intricate parameter space associated with MSS and to develop predictive mathematical models that can capture the morphology of future molten salt synthesised two-dimensional (plate-like) particles. With bismuth sodium titanate (BNT) as a case study, this work has focused primarily on the optimisation of two-dimensional plate-like particles’ production, intended for subsequent incorporation in a polymeric matrix (Epoxy) to attain high performance dielectric composites. The main and combined interaction effects of the various molten salt synthesis parameters, i.e., synthesis temperature, holding time and heating rate, on the morphology of the resulting powder such as the area, thickness, Feret's diameter, particle size, and aspect ratio of particles, have been investigated. The models for the aspect ratio and particle size were found to be insignificant in this study, with P-values of more than 0.05. Three predictive models depicting the area, thickness, and Feret's diameter of the synthesised powder particles as a function of the affecting synthesis parameters were developed, optimised, and validated. Using the optimised synthesis conditions, optimally synthesised large BNT plate-like particles with an average area of approximately 157 μm2, an average platelet thickness of approximately 1.5 μm, and an average Feret's diameter of approximately 17 μm were synthesised using the NBiT precursor particles. Since the alignment of the filler particles within the matrix is also amongst the various factors influencing the performance of dielectric composites, this study has further explored a mechanical alignment approach to align optimally synthesised plate-like particles into a chain-like quasi 1-3 structure, offering a cost-effective and straightforward alternative approach to existing alignment methods and anticipating resulting in improved dielectric constant and energy storage properties of the aligned composites. In comparison to composites made with non-optimally synthesised BNT particles, composites made with optimally synthesised particles showed superior degrees of alignment with observable chain-like structure similar to that achieved in dielectrophoretically aligned composites. The dielectric, piezoelectric, ferroelectric and energy storage properties of the developed BNT-Epoxy composites were also investigated throughout this work. A remarkable dielectric constant of 52 was achieved at a 35% BNT volume content in the composite, surpassing values reported in literature for other two-phasic similar composites. The piezoelectric performance seemed to be affected by the poling direction, in which composites poled parallel to the applied electric field exhibited greater piezoelectric charge coefficients, d33 values, compared to those poled in a perpendicular direction, with a maximum value of 4.5 pC/N at 35% BNT. The piezoelectric voltage coefficient, g33, increased linearly with BNT volume content, peaking at 3.5 mV.m/N at 35% BNT. At this volume content, a remarkably high discharge energy density of 3.7 J/cm3 was achieved under a maximum breakdown electric field of 1267 kV/cm, which up to our knowledge also surpasses the value of other two-phasic similar systems. The herein developed models, along with the results of the studied multi-objective optimisation provide means to customise and regulate the size and shape of the particles according to specific application requirements. The developed BNT-Epoxy composites with improved dielectric and energy storage properties compared to state-of-the-art, hold promise for diverse dielectric applications, including capacitors.

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