The effects of coupling, nonlocality, and nonlinearities
Noteworthy literature has shown that composite lead-free piezoelectric ceramic exhibits excellent piezoelectric properties. In addition to this, lead-free piezocomposites offer the promise of an environmentally-friendly alternative to lead-based materials for conversion of mechanical stimuli into electrical energy. As such, they have drawn worldwide attention in academia and research. Nonetheless, lead-free materials lag behind lead-based materials in terms of their performance. The performance issues can be typically addressed by tuning the mechanical electrical, and crystalline structural properties of the matrix and the piezoelectric inclusions. In fact, various design pathways have been proposed. More so, noteworthy efforts to both model and experimentally demonstrate these design pathways have also been reported. However, in the aspect of design, most of these efforts overlook the contributions due to several important physical effects.
Generally, lead-free piezocomposites are an ecofriendly route for sensing and harvesting energy from mechanical stimuli and it is important to develop accurate models which can capture essential physical processes underlying their performance. Unfortunately, current piezocomposite design heavily relies on the linear piezoelectric model which neglect nonlocal and nonlinear electro-elastic processes. To address this, Dr. Jagdish A. Krishnaswamy (Post-doctoral fellow) and Professor Roderick Melnik from the Wilfrid Laurier University in Canada, in collaboration with Dr. Federico C. Buroni, Dr. Luis Rodriguez-Tembleque and Professor Andres Saez at the University of Sevilla in Spain developed a new and more accurate modelling paradigm to determine the contributions from nonlocal flexoelectric and nonlinear electrostrictive effects towards the performance of lead-free piezocomposites. Their work is currently published in the research journal, Composite Structures.
In their approach, the research team first developed a fully coupled electro-elastic model, starting from free-energy considerations, which could simultaneously account for the linear-piezoelectric, nonlinear and nonlocal effects in a lead-free composite architecture. Next, they evaluated the contributions of each of these effects towards the electro-elastic response of a two-component piezocomposite consisting of a polymer matrix with embedded piezoelectric micro-sized inclusions. Overall, they evaluated the contribution of the effects in a three-component composite architecture consisting of a CNT-modified matrix with polycrystalline piezoelectric inclusions.
The authors reported that in the case of microscale randomly shaped piezoelectric inclusions which represent a practical scenario, the flexoelectric effect did not contribute appreciably towards the piezoelectric response. However, the nonlinear electrostrictive effects imparted significant strain-dependent responses. Further, in nano-modified composites, the team found out that the nonlinear electro-mechanical coupling can have different effects on the transverse and the longitudinal electro-elastic responses. In particular, the longitudinal electric field response, with the nonlinear contribution, was seen to be less sensitive to the polycrystalline structure of the piezoelectric inclusions.
As an important extension of this effort, the group recently applied the developed model to understand size-dependent flexoelectricity in geometrically anisotropic piezocomposite structures. Taking an example of a two-dimensional tapered structure, the group was able to demonstrate significant size-dependent enhancements in the piezoelectric response at small length scales. The best enhancements were observed at lower inclusion concentrations and with the inclusions positioned along the tapered surfaces rather than in the bulk of the composite. This serves as a demonstration that significantly high piezoelectric responses can be obtained at low inclusion concentrations by strategically tuning the shape of the composite structure and the size and spatial distribution of the inclusions.
In summary, the study developed a mathematical paradigm to model piezoelectric composites by taking into account important physical effects such as nonlocal flexoelectric and nonlinear electro-strictive effects. Generally, the observation reported provide critical insight into the nonlinear behavior of piezocomposites and emphasize the importance of developing advanced models to describe electro-elastic behavior. A further significant outcome led to obtaining insights into tuning the shape and size of the composite structure and the piezoelectric inclusions to maximize flexoelectric size-dependent enhancements at low inclusion concentrations to design superior piezoelectric materials. In a statement to Advances in Engineering, Dr. Jagdish A. Krishnaswamy, first author emphasized that their models can in fact act as a starting point for the design of efficient piezocomposites and directed experimental efforts to tap into these coupled electromechanical effects to improve piezoelectric performance.
- Jagdish A. Krishnaswamy, Federico C. Buroni, Roderick Melnik,Luis Rodriguez-Tembleque, Andres Saez. Advanced modeling of lead-free piezocomposites: The role of nonlocal and nonlinear effects. Composite Structures, volume 238 (2020) 111967.
- Jagdish A. Krishnaswamy, Luis Rodriguez-Tembleque, Roderick Melnik, Federico C. Buroni, Andres Saez. Size dependent electro-elastic enhancement in geometrically anisotropic lead-free piezocomposites, International Journal of Mechanical Sciences, Volume 182 (2020) 105745.