Influence of molecular weight on dielectric properties and piezoelectric constant of poly(vinylidene fluoride) membranes obtained by electrospinning

A significant influence of the molecular weight on the dielectric properties and piezoelectric constant of poly(vinylidene fluoride) (PVDF) membranes obtained by electrospinning was demonstrated. Electrochemical impedance spectroscopy and d33 meter were used to evaluate dielectric properties and piezoelectric constant respectively. The presence of the β-phase was determined by Fourier transform infrared spectroscopy (FTIR) and X-Ray diffraction (XRD). The membranes with the lowest molecular weight (180,000 g/mol) possessed the best dielectric properties. They also had the highest piezoelectric constant (21 pC/N) and dielectric constant (2.9 at 50 Hz) as well as the highest β-phase content (80.25%).

Theoretical Predictions on the Effective Piezoelectric Coefficients of Wurtzite LaN compound from DFT and DFPT Calculations

To study the structural, elastic, piezoelectric and dielectric properties of wurtzite (LaN), we have used the ab initio total-energy pseudo-potential plane-wave method (PP-PW) based on density functional theory (DFT) and density functional perturbation theory (DFPT), which performed with Generalized gradient approximation (GGA) as well as local density approximation (LDA). The calculations of piezoelectric constants by mean of linear-response calculations second derivative with respect to a strain and a modification of the electric field. The compound shows a large effective piezoelectric constant (d33) being in the order of 66.96, and 77.260 pc/N with (GGA), and (LDA) calculations, respectively. The dielectric properties and electromechanical coupling coefficients have been calculated. Finally, our piezoelectric findings indicate that the LaN compound has a high effective piezoelectric constant value (d33) which allows it to be in direct competitor of zinc oxide ZnO and aluminum nitride AIN. We hope that our findings provide guidelines for the experimental realization and further research of high-performance materials appropriate for applications in different fields of study, such as biomedical engineering.

DEPENDENCE OF PHYSICAL PROPERTIES OF PIEZOELECTRIC ALUMINUM-SCANDIUM NITRIDE ON SCANDIUM CONCENTRATION

In this work the physical properties of the piezoelectric aluminum-scandium nitride (ASN) solid solution as a function of scandium concentration were studied using the density functional theory and experimental methods. The phase transition from the wurtzite phase to the rock salt phase at a Sc concentration of 43% was shown. The barriers of transformation from the wurtzite phase to the rock salt phase for various Sc concentrations were obtained. The behavior of the ASN piezoelectric constant d33 calculated by the piezoelectric constants e33, e31, and e15 shows a sharp increase with increasing Sc concentration compared to aluminum nitride AlN. The relationship between the increase in the piezoelectric response of ASN and the softening of the lattice, accompanied by a decrease in the main elastic constants C11, C33, C44 and C66, as well as a decrease in the c/a ratio with increasing Sc concentration, is shown. ASN films with a predominance of the crystal orientation (00·2) were obtained experimentally by magnetron sputtering. The structural properties of the films were studied by X-ray diffraction analysis. A comparison of the experimentally obtained dependence of the c/a ratio on the Sc concentration with the theoretical values showed a good correspondence. Studies of the physical properties of ASN thin films were performed using microwave multi-overtone composite resonators on diamond substrates with a longitudinal bulk acoustic wave (BAW) as the operating mode in the range of 0.5 – 20 GHz. The frequency dependences of the Q-factor of BAW-resonators with different ASN films were obtained, and the frequency dependences of the square of the modulus of the form factor as |m|2 were calculated. The dependences of the elastic constant С33 and the piezoelectric constant e33 for the ASN films with different Sc concentrations were calculated. The calculated and measured values of these constants are agreed within the experimental error.

Significant Improvement in The Piezoelectric Properties and Electromechanical Coupling Factors of Wurtzite AlN Compound Under High Pressures

This work describes a theoretical study of the pressure effect in structural, elastic, piezoelectric and dielectric properties as well as electromechanical coupling factors of wurtzite AlN, obtained by ab-initio calculations using pseudo-potential plane waves (PP-PW) that combine the density functional theory (DFT) and density functional perturbation theory (DFPT). The results of the calculation indicate that the parameters of AlN crystal cells and the volume of AlN crystallin crystal cells decrease notably with increasing pressures from 0 to 40 GPa. Due to an increase in the value of the direct piezoelectric constant ( Tand a decrease in the value of the elastic constant (h), there is a significant improvement in the value of the converse piezoelectric constant (i). The improvement in the piezoelectric value leads to a higher value in electromechanical coupling coefficient. Our results agree well with previous theoretical and experimental research. We hope that our results will provide guidelines for the realistic application as well as further research of high-performance compounds appropriate for applications in a multitude of fields of study, such as biomedical engineering.

Transverse piezoelectric constant of aluminium nitride films deposited on aluminium substrate by reactive magnetron sputtering

In view of high temperature applications, c-axis oriented aluminium nitride films on aluminium substrate were produced by magnetron sputtering at low pressure (0.3 and 0.5 Pa) and different values of nitrogen concentration. XRD data show the highest intensity of (002) diffraction peak with nitrogen concentration of 0.4, and the peak value decreases when the nitrogen concentration moves away from 0.4. The transverse piezoelectric constant (absolute value) was determined for all conditions, the highest values observed with nitrogen concentration of 0.4 (in agreement with XRD data) and 0.8, with a slight preference for 0.4. These new experimental data and the presence of the two peaks of similar amplitude on the estimated transverse piezoelectric constant are useful information for the identification of good practical operative conditions for AlN films sputtered on aluminium, basic structure for the development of high temperature piezoelectric transducers.

The Effects of Additive Manufacturing and Electric Poling Techniques on PVdF Thin Films: Towards 3D Printed Functional Materials

Mechanical flexibility, faster processing, lower fabrication cost and biocompatibility enable poly (vinylidene fluoride) (PVdF) to have a wide range of applications. This work investigated the use of a piezoelectric polymeric material, PVdF, in combination with 3D printing, to explore new strategies for the fabrication of smart materials with embedded functions, namely sensing. The motivation behind this research was to design and fabricate PVdF thin films that will be used to build pressure sensors with applications in active intelligent structures. In this work, 3D printed PVdF thin films with thickness values in the range of 250 to 350 μm were poled under high direct current electrical fields, which were varied from 0.4 to 12 MV/m and temperatures from 80 to 140 °C. Copper electrodes were applied, forming a standard capacitor layered structure, to facilitate poling and to collect piezoelectric output voltage. The poling process enabled the piezoelectric crystalline phase transition of printed PVdF films to transfer from the non-active a α-phase to the piezoelectric active β-phase and rearranged the dipole alignments of the β-phase. The efficiency of poling was evaluated through the piezoelectric constant calculated from measured calibration curves. These calibration curves demonstrated the PVdF sensing device have a positive linear correlation between mechanical input and voltage output. We found that a peak value in piezoelectric constant correlated with poling voltages and temperatures. The highest piezoelectric constant achieved through contact poling was 32.29 pC/N poled at 750 V and 120 °C, and temperature was deemed the most important factors to influence piezoelectric constant. We believe that the present work demonstrates a path towards fully 3D printed smart, functional materials.