Applications in Medicine of Piezoelectric Mini Cantilever Beam
Curator: Danut Dragoi, PhD
Piezoelectric materials are now finding applications in a wide variety of environmental conditions and medicine. Such materials are capable of converting mechanical energy into electrical energy. Indeed, when subjected to mechanical stress become electrically charged at their surface and vice versa. In their paper titled “Analytical Modeling of a Piezoelectric Bimorph Beam”, the researchers from Mechanical Engineering Department, Faculty of Technology Sciences, University Constantine 1, Algeria and Departement de Mécanique Appliquée, ENSMM, France, see the link in here, focused on a simple analytical model based on Euler–Bernoulli beam theory with the following assumptions:
- (a) the piezoelectric layer thickness in comparison to the length of the beam is very thin and
- (b) the electrical field between the upper surface and lower surface of the piezoelectric layer is uniform.
They have applied this model to study its static responses and predict the ambient deformations into usable electrical energy from a cantilever piezoelectric beam.
The piezoelectricity of well known materials, such as Pb[ZrxTi1-x]O3 (0≤x≤1) is due to the asymmetry of central atom that creates a local electrical dipole whose amplitude is direct proportional to the displacement of Ti/Zr+4 ions from the center of the crystallographic unit cell. These materials do not possess any piezoelectric properties owing to the random orientations of the ferroelectric domains in the ceramics before poling. During poling, which is an electric field applied on the ferroelectric ceramic sample during the fabrication to force the domains to be oriented or preferred oriented in the direction of polling of the electric field. After poling, the electric field is removed and a remnant polarization and remnant strain are maintained in the sample, so a preferred orientation of the domains exist and the sample exhibits piezoelectricity. We can imagine that a single crystal will have 100% orientation in the direction we like to be, but the processing cost may be prohibited in this way.
The work presented in the paper concerns the problems of characteristic phenomena of piezoelectricity. The attention is focused on the different deformations effect by voltage generation of the piezoelectric beam. The relation between the voltage imposed and the curvature is analytically derived which is used to explain the effect of voltage generation as a function of the curvature of the beam. Figure 4 from the paper, shows the deflection family of curves as a function of asymmetry of the four electrodes on the mini piezo beam, where a and b on Figure 4 are the position of the gap between electrodes on two parallel faces of the beam relative to the middle section plan that runs parallel to the longest side of the beam.
The dimensions of the piezo cantilever beam are 10 mm x 1 mm x 0.200 mm. We notice the higher asymmetry (high values for a and b parameters) the better, as the slope on lines on Figure 4 are coupled with the beam deflection, the parameter of interest. We remark the case when a=b=0, no asymmetry, the deflection is zero as expected. Also we notice that the device has two piezo materials glued together. The need for two piezo materials is due to the fact that we have four asymmetric electrodes that produce four asymmetric polarizations that induce the necessary curvature of the beam and ultimately the deflection. In order to reduce the mechanical influence of the electrodes on deflections, the electrodes were made extremely thin, about 0.5 microns thickness. The electrodes were glued to the piezoelectric cantilever beam using epoxy. Analytical data showed that the proposed model simulations are in good agreement with the FE results. A detailed analysis of piezoelectric cantilever bi-morph is made on a dissertation thesis, see link in here.
Pushing for deflection parameter higher on piezoelectric devices is now related not only with energy harvesting in industry, but also with medical devices like BioMEMS where short life batteries for powering the electronic microcircuits have a major inconvenient of recharging once they are depleted of energy, and also they have to be replaced after a not so high number of charging cycles.The usage of human body movement is a viable approach for using piezoelectric cantilever beam to power implantable medical devices as well as other microbot and BioMEMS devices. The predictive models presented are very promising and show the trend towards a highly efficient device that will replace the actual batteries in many applications.
American Journal of Mechanical Engineering, 2016, Vol. 4, No. 1, 7-10, Available online at http://pubs.sciepub.com/ajme/4/1/2 © Science and Education Publishing
ANALYTICAL MODELING AND DESIGN OPTIMIZATION OF PIEZOELECTRIC BIMORPH ENERGY HARVESTER by LONG ZHANG