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Fine powders synthesized via sol-gel route were employed to fabricate Sr(2-x)(Na0.5Bi0.5)(x)Bi4Ti5O18 (SNBT, where x = 0, 0.1, 0.25, 0.3, 0.4, and 0.5) ceramics. The composition (x)-dependent structural changes associated with SNBT ceramics were analyzed using X-ray powder diffraction, transmission electron microscopy, and Raman spectroscopic techniques. Average grain size analyses carried out on the SNBT ceramics by scanning electron microscopy revealed an important role played by the dopants in inhibiting the grain growth. Dielectric constants and the Curie temperature of the ceramics were found to decrease and increase, respectively, with increase in x. The increase in Curie temperature with increase in x was attributed to the decrease in the tolerance factor. The specific composition (x = 0.3) of the SNBT ceramics exhibited improved piezo- and ferroelectric properties associated with a higher Curie temperature (569 K). The piezoelectric coefficient (d(33)) and the planar electromechanical coupling coefficient (k(p)) of SNBT(x = 0.3) were enhanced by 25% and 42%, respectively, as compared to the undoped ceramics.

Polymer (poly(vinylidene fluoride) (PVDF)) nanocrystal composites based on lanthanum nickelate (percolative oxide) and barium titanate were fabricated to obtain material systems with a high dielectric constant and low loss to be used for high-charge-storage applications. Lanthanum nickelate (LaNiO3) nanocrystallites were synthesized from a simple citrate-assisted sol-gel route that yielded agglomerated crystallites of an average size of 120 nm. The defective nature of the lanthanum nickelate nanocrystals was revealed by the transmission electron microscopy studies. Hot-pressing method was executed to fabricate the LaNiO3/PVDF nanocrystal composites, and their dielectric characteristics showed a low percolation threshold in the region of fLN (volume fraction of lanthanum nickelate) = 0.10. The percolative conductive filler-polymer nanocrystal composite at the percolation threshold exhibited a dielectric constant (er) and loss ( D) of 55 and 0.263, respectively, at 10 kHz; the dielectric constant obtained was more than 5 times that of host matrix PVDF. To further improve upon the obtained dielectric properties from the two-phase composites, a high-dielectric-constant material, barium titanate (BaTiO3) nanocrystals, with an average size of 100 nm, was embedded in the polymer matrix as the third phase. The dielectric properties of the three-phase nanocrystal composites were measured as a function of the volume fraction of lanthanum nickelate (which was limited within the percolation threshold), and a dielectric constant as high as 90 and the associated loss of 0.13 at 10 kHz were achieved from fLN = 0.09 and fBT = 0.20. The obtained dielectric constant from this system is 9 times more than that of PVDF and 3 times that of a two-phase barium titanate/PVDF composite, which proves to be a promising material for charge-storage applications.

Strontium bismuth titanate (Sr2Bi4Ti5O18) powders comprising crystallites of average sizes in the range of 94–1400 nm were prepared via citrate-assisted sol-gel route. With an increase in the average crystallite size there was a change in the lattice parameters and shift in the Raman vibration modes. Second harmonic signal (532 nm) intensity of the Sr2Bi4Ti5O18 powders increased with the increase in the average crystallite size and the maximum intensity obtained in the reflection mode was 1.4 times as high as that of the powdered KH2PO4. Piezo Force Microscopic analyses carried out on isolated crystallite of size 74 nm, established a single domain nature with the coercive field as high as 347 kV/cm. There was a systematic increase in the d33 value with an increase in the size of the isolated crystallites and a high piezoelectric coefficient of ∼27 pm/V was obtained from an isolated crystallite of size 480 nm.

Fine powders comprising nano-crystallites of Sr2Bi4Ti5O18 (SBT) were synthesized via citrate-assisted sol–gel route. Different ceramics of SBT with different grain sizes (93 nm–1.42 μm) were fabricated by varying sintering temperatures and durations. The usage of nano-crystalline powders for fabricating ceramics facilitated lower sintering temperatures. The grain growth in these powder compacts was found to be controlled by the grain boundary curvature mechanism, associated with the activation energy of 181.9 kJ/mol. Interestingly, with a decrease in grain size there was an increase in structural distortion which resulted in a shift of Curie temperature (phase transition) toward higher temperatures than that of conventional bulk ceramics. Extended Landau phenomenological theory for the spherical ferroelectric particles was invoked to explain experimentally observed size-dependent phase transition temperature, and the critical size for SBT is predicted to be 11.3 nm. Grain size-dependent dielectric, ferroelectric and piezoelectric properties of the SBT ceramics were studied and the samples comprising average grain size of 645 nm exhibited superior physical properties that include remnant polarization (2P r) = 16.4 μC cm−2, coercive field (E c) = 38 kV cm−1, piezoelectric coefficient (d 33) = 22 pC N−1 and planar electromechanical coupling coefficient (k p) = 14.8 %.

Nanocrystalline powders of Ba1-xMgxZr0.1Ti0.9O3 (x = 0.025-0.1) were synthesized via citrate assisted sol-gel method. Interestingly, the one with x = 0.05 in the system Ba1-xMgxZr0.1Ti0.9O3 exhibited fairly good piezoelectric response aside from the other physical properties. The phase and structural confirmation of synthesized powder was established by X-ray powder diffraction (XRD) and Raman Spectroscopic techniques. Two distinct Raman bands i.e., 303 and 723 cm(-1) characteristic of tetragonal phase were observed. Thermogravimetric analysis (TGA) was performed to evaluate the phase decomposition of the as-synthesized Ba0.95Mg0.05Zr0.1Ti0.9O3 sample as a function of temperature. The average crystallite size associated with Ba0.95Mg0.05Zr0.1Ti0.9O3 was calculated using Scherrer formula based on the XRD data and was found to be 25 nm. However, Scanning and Transmission Electron Microscopy studies revealed the average crystallite size to be in the range of 30-40 nm, respectively. Kubelka-Munk function was employed to determine the optical band gap of these nanocrystallites. A piezoelectric response of 26 pm/V was observed for Ba0.95Mg0.05Zr0.1Ti0.9O3 nanocrystal by Piezoresponse Force Microscopy (PFM) technique. Photoluminescence (PL) study carried out on these nanocrystals exhibited a blue emission (470 nm) at room temperature.

Ceramics of Bi1-xPrxFeO3 (x = 0-0.1) were fabricated using the nanocrystalline powders obtained via sol-gel route. X-ray powder diffraction studies confirmed that these belonged to rhombohedral perovskite structure associated with R3c space group. The dielectric properties of the ceramic samples as a function of frequency (100 Hz-10 MHz) and temperature (30 degrees C-250 degrees C) were studied. The dielectric constant increased while the loss decreased with the increase of Pr content. Dielectric dispersion in these samples was found to be poly dispersive Debye type relaxation as confirmed by invoking Cole-Cole relation. Impedance spectroscopy was employed to determine the electrical parameters associated with the grain and grain boundaries. Grain and grain boundary resistances were found to decrease with the increase of temperature for all the samples under study. The activation energies for the dielectric relaxation were evaluated by electric modulus spectra and these increase with the increase of Pr dopant level. The frequency dependent conductivity at various temperatures demonstrated the involvement of correlated barrier hopping conduction mechanism. The electrical conduction in these ceramics was ascribed to long and short range migration of oxygen ion vacancies as demonstrated by temperature dependent ac conductivity studies.

Polymer nanocrystal composites were fabricated by embedding polyvinylidene fluoride (PVDF) with K0.5Na0.5NbO3 (KNN) nanocrystallites of different volume fraction using the hot-pressing technique. For comparison, PVDF–KNN microcrystal composites of the same compositions were also fabricated which facilitated the studies of the crystallite size (wide range) effect on the dielectric and piezoelectric properties. The structural, morphological, dielectric, and piezoelectric properties of these nano and micro crystal composites were investigated. The incorporation of KNN fillers in PVDF at both nanometer and micron scales above 10 vol% resulted in the formation of polar β-form of PVDF. The room temperature dielectric constant as high as 3273 at 100 Hz was obtained for the PVDF comprising 40 vol% KNN nanocrystallites due to dipole–dipole interactions (as the presence of β-PVDF is prominent), whereas it was only 236 for the PVDF containing the same amount (40 vol%) of micron-sized crystallites of KNN at the same frequency. Various theoretical models were employed to predict the dielectric constants of the PVDF–KNN nano and micro crystal composites. The PVDF comprising 70 vol% micron-sized crystallites of KNN exhibited a d 33 value of 35 pC/N, while the nanocrystal composites of PVDF–KNN did not exhibit any piezoelectric response perhaps due to the unrelieved internal stress within each grain, besides the fact that they have less domain walls.