AC - conductivity studies on Y1-xBixCrO3 solid solution

YCrO3 and the substitution of Bi+3(6s2) in the Y-site were synthesized by the sol gel process. The conductivity measurements using dielectric spectroscopy reveals that conductivity increases with increase of composition. Dielectric loss were used to understand the nature of conducting species and it reveals that chromates exhibits the usual reciprocal behaviour of charge carriers to motion of oxygen vacancies as we increase the Bi composition and temperature. Impedance and microstructural studies revealed that conductivity of the chromates was dominated by the grain contribution

Effect of crystal structure and cationic order on phonon modes across ferroelectric phase transformation in Pb(Fe0.5-xScxNb0.5)O3 bulk ceramics

Pb(Fe0.5-xScxNb0.5)O3 [(PFSN) (0 ≤ x ≤ 0.5)] multiferroic relaxors were synthesized and the temperature dependence of phonon modes across ferroelectric to paraelectric transition was studied. With varying Sc content from x = 0 to 0.25 the structure remains monoclinic and with further addition (x = 0.3 - 0.5) the structure transforms into rhombohedral symmetry. Structural refinement studies showed that the change in crystal structure from monoclinic to rhombohedral symmetry involves a volume increment of 34-36%. Associated changes in the tolerance factor (1.024 ≤ t ≤ 0.976) and bond angles were observed. Structure assisted B′-B″ cation ordering was confirmed through the superlattice reflections in selected area electron diffraction (SAED) pattern of Pb(Sc0.5Nb0.5)O3 (x = 0.5). Cation ordering is also evident from the evolution of Pb-O phonon mode in Raman spectra of compositions with rhombohedral symmetry (x ≥ 0.3). The high temperature Raman scattering studies show that the B-localized mode [F1u, ∼250 cm−1] and BO6 octahedral rotational mode [F1g, ∼200 cm−1], both originating from polar nano regions (PNRs) behave like coupled phonon modes in rhombohedral symmetry. However, in monoclinic symmetry they behave independently across the transition. Softening of B localized mode across the transition followed by the hardening for all compositions confirms the diffusive nature of the ferroelectric transformation. The presence of correlation between the B localized and BO6 rotational modes introduces a weak relaxor feature for systems with rhombohedral symmetry in PFSN ceramics, which was confirmed from the macroscopic dielectric studies

Enhanced magnetization and improved insulating character in Eu substituted BiFeO3

The polycrystalline Bi1-xEuxFeO 3 (x=0, 0.05, and 0.1) ceramics were synthesized by conventional solid-state route. X-ray diffraction studies and Raman measurements revealed that the compounds crystallized in rhombohedral structure with R3c space group. In addition, a nominal percentage of orthorhombic phase was observed in 10 mol.% Eu substituted BiFeO3. Appearance of weak ferromagnetism and significant increase in Néel temperature TN in the substituted compounds were discussed on the basis of structural distortions. Enhanced remnant magnetization of 75 memu/g and large coercive field of 6.4kÖe were observed in 10 mol.% Eu substituted BiFeO3. Equivalent circuit model was employed to estimate the grain and grain boundary contributions towards the electrical parameters such as resistance and conductivity. Non-Debye type of relaxation was confirmed from impedance and electric modulus studies. The obtained frequency variation of ac conductivity at different temperature obeys Jonscher's power law and is consistent with the correlated barrier hopping model. Temperature variations of ac conductivity explained that electronic hopping, oxygen vacancies movement, and/or creation of defects contribute to conduction in all the compounds

Structural and dielectric studies of ferroelectric Nd2Ti2O7 for high temperature pyro sensor application

High temperature pyroelectric sensing technology is important in materials processing as well as in automotive and aerospace engineering. In aerospace it can be used as Exhaust Gas Temperature Sensor (EGTS) for hot chamber of jet engine as EGTS measures engine’s health and other sensing application. The primary requirement from the material perspective is it must be stable in high temperature range (600 to 1000° C). Ferroelectric material with high Tc (>1000°C) can be used in temperature sensing technology due to its pyroelectric properties. Neodymium Titanate (Nd2Ti2O7) is a promising candidate for this application as it has curie temperature of ~ 1480°C. NTO is a member of layered perovskite ferroelectric material. High Tc of NTO is due to the presence of dominant 180° ferroelectric domains and ferroelectric to paraelectric phase transition contains displacement of the whole sub lattice of TiO6 octahedron. In this thesis, we report the studies on synthesis of material, FullProf refinement and high temperature dielectric and pyroelectric properties of Neodymium Titanate material. Synthesis of NTO material is done by solid state reaction method. Neodymium oxide and titanium oxide is taken as primary materials in 1:2 molar ratios and the mixture is grinded with mortar and pestle for 4 hours. Phase purity is obtained by calcination of powder sample at 1250°C for 4 hours. XRD studies confirm the phase purity of neodymium titanate material. After ball milling powder is pressed to make pellet. Green pellets were sintered at 1400°C for 5 hours with a heating and cooling rates of 5°C/min. Detailed study of structural refinement was carried out by FullProf Suite software with CIF file taken from COD data base. SEM images of pellet shows densely packing of particles. Finally, dielectric property study is done by impedance analyzer instrument and furnace that goes up to 800°C. First experiment is done with applied voltage 0.8 V and data is taken during cooling from 750°C-50°C which shows the variation of dielectric constant and dielectric loss with temperature and frequency. Good curve is obtained with increasing temperature for higher frequency like 100kHz or 1kHz. For higher frequency almost no variation of dielectric viii constant and dielectric constant with temperature and that is expected as temperature range is low. Low dielectric loss for higher frequencies will be good for application. For temperature sensing application it is very important to show pyroelectricity. NTO is a ferroelectric material, so it’s must have pyroelectric property. To know its temperature sensitivity pyrocurrent measurement is done. Pellets connected with platinum wires using silver paste kept inside the furnace and wires connected to source meter unit (Keithley-2401). Current vs time data is taken with temperature ramping between 600°C to 500°C which shows exponentially increasing or decreasing current w.r.t to temperature ramping. This is called pyrocurrent. This is the evidence of pyroelectricity in NTO material for temperature sensing application

Tailoring the Band gap and magnetic properties by Bismuth substitution in Neodymium Chromite

The intrinsic distortions present in r are earth ortho chromites (RCrO 3 ) observed from lanthanum to lutetium (in R - site) can influence the magnetic properties like Neel Transition and weak ferromagnetic coupling . A n on - magnetic cation with similar ionic radius would be a suitable candidate to engineer the inherent distortions of particular orthochromite. In this study Bismuth (Bi 3+ ) with a 6s 2 lone pair was chosen to substitute in Neodymium (Nd 3+ ) site of NdCrO 3 (NCO) to tailor the int rinsic structural distortions. Th e variation of optical absorption edge evidently suggests that Bi ( 6 s 2 ) substituted in the m agnetic rare earth Nd +3 influences the Cr - O overlap integral. Th e interaction of Bi cation with oxygen bonds influences the structural distortions through Cr - O polyhedra which are evident from Raman scattering studies . The observed structural and magnetic properties of similar ionic radius of Bi 3+ in Nd 3+ reveals that intrinsic structural distortions are interrelated to enhanced weak ferromagnetic component and change in Neel and spin reorientation temperatures in our compou nds . In addition a reduction in the optical band gap of NCO from 3.1 eV to 2. 6 eV w as observed

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

Growth and Characterization of lead free Multiferroic Ba0.85Ca0.15Zr0.10Ti0.90O3 –CoFe2O4 Nano Composite Thin Films

Multiferroic materials simultaneously possess more than one ferroic orders. The most important multiferroic materials are those that show simultaneous ferroelectric and ferromagnetic properties which is called magneto-electric materials. Magneto-electric materials manifest a change in polarization under external magnetic field or change in magnetization under an electric field. Magnetoelectricity can be found both in single phase and composite materials. Single phase materials have both electric and magnetic orders and the magnetoelectric effect originate from the coupling of ferroelectric and magnetic dipoles. In a magnetoelectric composite structure, a coupling is induced via an interfacial elastic interaction between magnetostrictive and piezoelectric materials enabling the control of the magnetization by an electric field and vice versa. Magneto- electric thin film nano composites have drawn significant interest because of the easy incorporation in to the integrated magnetic and electric devices, such as sensors, microelectromechanical system and spintronics devices which cannot be realized in the bulk counterpart. This dissertation focused on the growth and characterization of lead free multiferroic Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) – CoFe2O4 (CFO) nano composite thin films deposited on Pt substrate through pulsed laser deposition. BCZT thin film was chosen as piezoelectric materials because of recent development in lead-free piezoelectric ceramics in which piezoelectric properties are quite comparable to that of lead based PZT ceramics which is non-environment friendly due the toxicity of lead. Similarly, CFO thin films were preferred because of its high magnetostriction value among the oxide ferromagnetic materials. First, the piezoelectric thin film was systematically optimized for different oxygen pressures and substrate temperatures. The effect of these process parameter on structure, microstructure, ferroelectric and mechanical properties has been studied. The piezoelectric domains and the ferroelectric switching of dipoles with applied electric field were also studied. From the above experiments the optimum growth conditions were realized as O2 Pressure – 13.3 Pa and Substrate temperature 700°C to achieve high quality BCZT thin films. In the same way, the effect of different oxygen pressures on structure, microstructure, magnetic and mechanical properties were studied for CFO film. In addition, magnetic annealing was carried out to increase the magnetostriction and decrease the strain sensitivity in-order to enhance the total magnetoelectric coupling in the nano composites. Enhanced magnetostrictive properties and uniaxial anisotropy has been observed for magnetic annealed film compared to the nonmagnetic annealed and as deposited films. By using the above optimized conditions, multilayers of BCZT/CFO/BCZT nano composites were deposited for different thickness ratios to obtain growth control morphology at the interface since there is a continuous change in the morphology texture and surface topography as a function of film thickness. The interface between the piezoelectric and ferromagnetic layer plays a key role in transferring the stress and to achieve high magnetoelectric coupling. In general, the polycrystalline ferroelectric thin film was grown with columnar grains and spinel ferrite with granular grains. This difference in the morphology at the interface reduces the coupling between the two phases. Here we propose a new structure in the polycrystalline 2-2 multilayered structure which stacks in a single continuous column like a superlattice structure which provides more flexibility for strain transfer and reduces the substrate clamping effect by altering the deposition conditions within the frame work of Structure Zone Model (SZM). Morphologically coherent continuous columnar structure was obtained for the nanocomposite having thickness ratio of BCZT (900nm)/CFO (180nm)/BCZT (900). The grains between the BCZT/CFO and CFO/BCZT interface grew in a grain-over grain pattern with same crystallographic orientation such that the films are in local epitaxy registry over a wide spread of different orientation at the interface. The effect of thickness on the structure, ferroelectric, magnetic and magneto-dielectric properties were studied along with microstructure and Raman studies at the interface. A high dielectric constant change of 21% obtained by an induced applied magnetic field attained in continuous columnar growth indicating the high strain coupling

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