Tunable polarization components and electric field induced crystallization in polyvinylidenefluoride (PVDF); a piezo polymer

Polyvinylidenefluoride (PVDF) a semicrystalline pieozoelectric polymer was synthesized with varying process conditions and its ferroelectric domain orientations were studied using piezoresponse force microscope (PFM). PVDF thin films fabricated using tape casting technique with precursor solutions of varying viscosities reveal that the polarization components transform from a dominant planar component to an out-of-plane polarization components with increase in viscosity. Interestingly the planar components possessed a head to head or tail to tail kind of paired domains separated by a distance of ~ 380-400nm. The electrostatic energies computed by numerically solving the electrostatic equilibrium equation for the electrically inhomogeneous system are in good correlation with the experiments. On increment of electric field, the domains were observed to grow in size and shape which indicates amorphous to crystalline transformation in the case of PVDF. Such transformation was evident from x-ray diffraction studies performed in-situ in the presence of an applied electric field

Tunable polarization components and electric field induced crystallization in polyvinylidenefluoride: A piezo polymer

Polyvinylidenefluoride (PVDF) a semicrystalline pieozoelectric polymer was synthesized with varying process conditions and its ferroelectric domain orientations were studied using piezoresponse force microscope (PFM). PVDF thin films fabricated using tape casting technique with precursor solutions of varying viscosities reveal that the polarization components transform from a dominant planar to an out‐of‐plane configuration with increase in viscosity. Interestingly the planar components possessed a head to head or tail to tail kind of paired domains separated by a distance of ~ 380‐400 nm. Electrostatic energy minimization of an electrically inhomogeneous system containing similar domain arrangements as the experiments shows that the head to head and tail to tail arrangements with a minimum separation distance are more favorable than head to tail arrangements of domains. With increment of applied field, the domains grew in size and shape indicating amorphous to crystalline transformation of PVDF films. Such transformation was evident from X‐ray diffraction studies performed in‐situ in the presence of an applied electric field

Microstructural influence on ferroelectric domain pattern and piezoelectric properties of Na 0.5 Bi 0.5 TiO 3 thin films

Sodium bismuth titanate (Na0.5Bi0.5TiO3: NBT) thin films were fabricated under various growth conditions (substrate temperature from 400 °C to 650 °C and oxygen pressures from 50 to 200 mTorr) by using pulsed laser deposition technique. The films grown at low partial pressures ( direction and at higher partial pressures (> 100 mTorr) exhibited polycrystalline nature. The microstructures were tuned from coarse faceted grains to fine spherical grains by varying the ambient pressures and the growth temperatures. The ferroelectric domain studies reveal that in case of fine spherical grains, the domain pattern was dominated by the surface morphological features and in the case of coarse faceted grain structure, domain features were independent of its morphology. Fast Fourier Transform (FFT) spectrum analysis of the domain patterns confirmed that only highly oriented films possessed periodic domain pattern and the periodicity is in the range of 140–240 nm. Further, the estimated piezocoefficient value (d33) increased from 16 to 31 pm/V with increasing the oxygen partial pressures (50–200 mTorr) and substrate temperatures (400–650 °C). The leakage current density measurements confirm that films grown at low partial pressures possess relatively larger leakage current density at room temperature

Microstructural influence on piezoresponse and leakage current behavior of Na0.5Bi0.5TiO3 Thin Films

Sodium bismuth titanate (Na0.5Bi0.5TiO3: NBT) a lead free piezoelectric; exhibits promising features such that it could be an alternate to lead based piezoelectrics. In this work, we report the microstructural influence on piezoelectric and leakage current behavior of NBT thin films grown by pulsed laser ablation (PLD). Various microstructural features like coarse faceted grains and fine spherical grains was achieved by effective optimization of substrate temperature and oxygen partial pressures. The studies reveals that, leakage current of NBT thin films were dominated by interface limited modified Schottky emission type of conduction. The piezoelectric domain studies reveal that for NBT thin films with fine spherical grain the domain pattern was highly dominated by the morphology and in the case of coarse faceted grains the domains were relatively large and the domains were extending beyond the grain boundaries

Texture and Microstructural Influence on Piezoelectric Properties of Na0.5Bi0.5TiO3 Thin Films: A Lead Free Piezoelectric Material

Among the lead free piezoelectric compounds Sodium Bismuth Titanate (Na0.5Bi0.5TiO3: NBT) show promising features that could replace the existing lead based piezoelectric materials in various applications. In this work, we report the effect of substrate temperature (400-650 °C), oxygen partial pressures (50-200mTorr) on the crystallographic orientations and microstructural evolution of NBT thin films grown on (111) Pt/TiO2/SiO2/Si (100) substrate by pulsed laser deposition (PLD) technique. The films grown at low partial pressures exhibits a preferred orientation along the direction and at high pressures they exhibit polycrystalline nature. The variation of the growth temperature and oxygen partial pressures are allowed the microstructural features tuning from coarse faceted grains to fine spherical grains. The ferroelectric domain studies reveal that in case of fine spherical grain structure, microstructural features dominate the domain distribution and in case of coarse faceted grain structure, domain features are independent of its morphology. A control over the crystallographic orientations facilitate the tunability over the polarization vector components, which allows us to attain the dominating planar piezoresponse in case of highly oriented () samples. Further, in the case of polycrystalline films there are certain regions at which the ferroelectric domains are extending beyond the grain boundaries. Such an extension could plausibly arise due to the close crystallographic relation between the adjacent grains. The estimated grain boundary angles between the planes (-211), (-111) and (-311), (-111) are showing that there exist some of the planes that forms a low angle grain boundary (≤15°) with polarization components which are favoring the domain walls to cross over the adjacent grain boundaries in NBT polycrystalline films. Moreover from the empirical observation of surface roughness, when the surface roughness is larger than the average roughness (~ 2nm), the grain boundary acts as a physical boundary for the domain. Thus, the surface roughness also play a crucial role on defining the domain pattern by introducing a physical boundary for the coherent interaction of the polarization across grains. Fast Fourier Transform (FFT) spectrum analysis of the domain patterns confirmed that only highly oriented films possess periodic domain distribution. Moreover, the nano scale piezocoefficient values (d33) are increased from 16±0.4 to 30±0.46 pm/V with increasing the oxygen partial pressures and growth temperatures. Further the temperature dependent leakage current studies are performed for NBT thin films with different microstructures. For faceted grain structure, Schottky emission is dominated in the temperature range from 30°C to 120°C. Further in the temperature range 130-200 °C Ohmic conduction is dominated. In case of spherical grain structure 30-120 °C Poole-Frenkel emission and from 130-200 °C, Ohmic conductions are dominated. More over the highly oriented film with needle shaped grain structure exhibits Schottky emission in the temperature of 30-140 °C and from 150-200 °C, ohmic conductions was dominated. Further, we also performed the variation in the leakage currents by changing the oxygen partial pressures for a given temperature. The leakage current density under an applied electric field for the films grown at 650 ºC with various oxygen partial pressures. At a given field as the oxygen partial pressure increases, the leakage current density decreases. This is an indicative of the role of oxygen vacancies on leakage currents in NBT thin films. Hence, the optimized oxygen partial pressures could result in improved electrical properties in NBT thin films. Further, the microstructure influence on leakage currents studies reveal that coarse faceted columnar structure exhibit less leakage currents than the spherical grain structure. In addition, the phase transition studies indicates the plausible antiferroelectric transition at 120 °C and paraelectric transition in the range of 350-370 °C. In addition, the epitaxial NBT thin films are fabricated on LNO coated STO single crystal substrates with (100) (110) and (111) crystallographic orientations by pulsed laser deposition technique. The in- plane and out-of-lattice parameters and epitaxial strains are calculated from the reciprocal space map studies. Further, the FE-SEM studies shows that the surface energies of the substrate orientations play a crucial role in deciding the morphology of the respective film. We have observed a smooth 2D growth morphology in case of (100) orientation whereas (110) and (111) acquired 3D growth morphology. The ellipsometry studies reveal that the onset of the absorption is highly depends on the planar densities of the respective orientations. For all the three orientations, the calculated absorption coefficient values are in the range of 3.1-3.5 eV. The PFM studies reveals that the OP phase contrast is weak in case of (100) orientation and it is strong in case of (111) orientation. Further the IPPhase is strong in case of (100) than the (111) orientation. This is explained based on the polarization components that are participating in deriving the different types of domains in all three orientations. The single point piezoresponse measurements confirms that (111) oriented film is pocessing a large piezoelectric coefficient (d33) value of 63 pm/V compared with the other two orientations. The obtained d33 values for (100) and (110) are 42 pm/V and 51 pm/V respectively. This suggest that NBT (111) oriented film can be a potential candidate for replacing the lead based compounds for device applications

A phase-field study of domain dynamics in ferroelectric BCT-BZT system

We present a thermodynamically consistent phase-field model describing the free energy of perovskite-based BCT-BZT solid solution containing an intermediate morphotropic phase boundaries. The Landau coefficients are derived as functions of composition of zirconium. The electrostrictive and elastic constants are appropriately chosen from experimental findings. The resulting Landau free energy is constructed to describe the stable polarization states as a function of composition. The evolution of the polarization order parameters at a particular composition is described by a set of time-dependent Ginzburg-Landau (TDGL) equations. Additionally, we solve Poisson's equation and mechanical equilibrium equation to account for the ferroelectric/ferroelastic interactions. We have performed two dimensional and three-dimensional simulations with appropriate electrical boundary conditions to study the effect of external electric field on domain dynamics in BCT-BZT system at the equimolar composition