E. Venkata Ramana

Enhanced Thermal Conductivity and Viscosity of Nanodiamond-Nickel Nanocomposite Nanofluids

We report a new type of magnetic nanofluids, which is based on a hybrid composite of nanodiamond and nickel (ND-Ni) nanoparticles. We prepared the nanoparticles by an in-situ method involving the dispersion of caboxylated nanodiamond (c-ND) nanoparticles in ethylene glycol (EG) followed by mixing of nickel chloride and, at the reaction temperature of 140°C, the use of sodium borohydrate as the reducing agent to form the ND-Ni nanoparticles. We performed their detailed surface and magnetic characterization by X-ray diffraction, micro-Raman, high-resolution transmission electron microscopy, and vibrating sample magnetometer. We prepared stable magnetic nanofluids by dispersing ND-Ni nanoparticles in a mixture of water and EG; we conducted measurements to determine the thermal conductivity and viscosity of the nanofluid with different nanoparticles loadings. The nanofluid for a 3.03% wt. of ND-Ni nanoparticles dispersed in water and EG exhibits a maximum thermal conductivity enhancement of 21% and 13%, respectively. For the same particle loading of 3.03% wt., the viscosity enhancement is 2-fold and 1.5-fold for water and EG nanofluids. This particular magnetic nanofluid, beyond its obvious usage in heat transfer equipment, may find potential applications in such diverse fields as optics and magnetic resonance imaging.

Ferroelectric and magnetic properties of Fe-doped BaTiO3 thin films grown by the pulsed laser deposition

Fe-doped BaTiO3 thin films were grown on (001) oriented SrTiO3 substrates using pulsed-laser deposition technique. These films had a single-phase character and good epitaxial relationship with the substrate. Polarization-electric field (P-E) hysteresis revealed a saturated polarization with a remnant polarization (Pr) of 13.5 μC/cm2 for 10 mol% Fe-doped BaTiO3 films. Further increase of composition resulted in the large leakage currents and reduction of polarization. The piezoelectric domain switching in the films was confirmed by local hysteresis using piezoelectric force microscopy measurements. The Fe-doped BaTiO3 thin films exhibited room temperature ferromagnetism, and the magnetization value increased with increasing Fe concentration. Our results demonstrated that the addition of Fe ≤10 mol% in BaTiO3 induce the ferromagnetism and the switchable ferroelectric state.

Effect of Fe-doping on the structure and magnetoelectric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 synthesized by a chemical route

B-site Fe-doped (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 was synthesized by a facile chemical route to study the effect of doping on its physical properties. Detailed analysis of X-ray diffraction and Raman spectroscopy data revealed an increased lattice strain and thereby deviation from the morphotropic phase boundary with the progressive doping of Fe from 1 to 5 mol%. Such structural changes have resulted in the weakening of the energy band gap as well as deterioration of the ferroelectric polar nature which was evidenced by a shift of tetragonal to cubic transitions towards room temperature and hard doping effects in ferroelectric hysteresis. The doped samples exhibited room temperature ferromagnetism. Combined Mossbauer and X-ray photoelectron spectroscopic studies suggest that oxygen vacancies and defect complexes induced by Fe doping play a major role in magnetic properties. Local piezoresponse measurements illustrated imprint characteristics of ferroelectric domains in undoped and doped samples at the nanoscale. Room temperature magnetoelectric (ME) measurements revealed that 1 mol% Fe doped sample, having higher ferroelectric polarization and moderate magnetization, exhibits a strong ME response with a coefficient of 12.8 mV cm−1 Oe−1. The present study on Fe-doping effects on the structure and related ME properties of this important lead-free material is useful to tailor multiferroic applications in electronics.

Magnetoelectric studies on CoFe2O4/0.5(BaTi0.8Zr0.2O3)-0.5(Ba0.7Ca0.3TiO3) lead-free bilayer thin films derived by the chemical solution deposition

Lead-free multiferroic bilayer thin films were fabricated on (111)Pt/Si substrate via a simple sol-gel chemical solution deposition, by altering the position of piezoelectric (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 (BCTZO) and ferromagnetic CoFe2O4 (CFO). Single layer BCTZO experiences the out-of-plane compressive stress, while this layer is under tensile strain in both the bilayers. The microstructural study confirms the formation of bilayers with expected chemical composition composed of multiple well-developed crystallites having no crystallographic dependencies. Thin films of BCTZO and CFO/BCTZO exhibited saturated ferroelectric hysteresis loops at room temperature with a Pr of 7.2 and 5.6 μC/cm2. The magnetic field induced shift in phonon vibrations coupled with direct magnetoelectric (ME) measurements demonstrated a stress-mediated coupling mechanism in the bilayers. We found a superior ME coefficient (105 MV/cm Oe) and dielectric tunability (∼52%) for CFO/BCTZO bilayer compared to the BCTZO/CFO bilayer, which demonstrates that the modification of strain state in bilayers is useful for the desired ME coupling. The BCTZO having piezoelectricity on par with that of lead-based ones can be useful to tailor lead-free ME applications.

High dielectric constant and capacitance in ultrasmall (2.5 nm) SrHfO3 perovskite nanoparticles produced in a low temperature non-aqueous sol–gel route

Strontium hafnium oxide (SrHfO3) has great potential as a high-k gate dielectric material, for use in memories, capacitors, CMOS and MOSFETs. We report for the first time the dielectric properties (relative permittivity and capacitance) of SrHfO3 nanoparticles (NPs), as opposed to thin films or sintered bulk ceramics. These monodisperse, ultra-small, perovskite-type SrHfO3 nanocrystals were synthesised through a non-aqueous sol–gel process under solvothermal conditions (at only 220 °C) using benzyl alcohol as a solvent, and with no other capping agents or surfactants. Advanced X-ray diffraction methods (whole powder pattern modelling, WPPM), CS-corrected high-resolution scanning transmission electron microscopy (HRSTEM), dielectric spectroscopy, and optical (UV-vis, Raman) and photoluminescent spectroscopy were used to fully characterise the NPs. These SrHfO3 NPs are the smallest reported and highly monodisperse, with a mean diameter of 2.5 nm, a mode of 2.0 nm and a small size distribution. The formation mechanism of the NPs was determined using NMR and GC-MS analysis of the species involved. Our SrHfO3 nanoparticles had a dielectric constant of 17, which is on par with literature data for bulk and thin film samples, and they also had a relatively large capacitance of 9.5 nF cm−2. As such, they would be suitable for applications as gate dielectrics for capacitors and in metal-oxide semiconductor field-effect transistor (MOSFET) technology.

Effect of samarium and vanadium co-doping on structure, ferroelectric and photocatalytic properties of bismuth titanate

We performed a systematic analysis of the structure, ferroelectric and photocatalytic activities of Sm and V co-doped Bi4Ti3O12 (SBVT) ceramics. The formation of 3-layered Aurivillius structure and chemically induced strain of SBVT were analyzed by SEM and HRTEM studies. From the results of structural refinements, temperature dependent XRD and Raman spectra we found that SBVT has a distorted orthorhombic structure and transforms to tetragonal in the temperature range of 475–600 °C. Raman modes of Bi sites experience the phase transition earlier compared to torsional bending modes of BO6 octahedra. From the results of polarization and piezoelectric strain measurements, we found that SBVT exhibits superior ferroelectric characteristics as well as improved S33 (0.02 × 10−3) compared to other rare-earth doped Bi4Ti3O12 materials reported in the literature. SBVT exhibited fatigue endurance up to 108 switching cycles. Such a behavior is attributed to the structural distortions due to the smaller ionic radius of Sm3+ and reduced oxygen vacancies. Local piezoresponse measurements demonstrated imprint characteristics of ferroelectric domains. Furthermore, we assessed the photocatalytic (PC) activity of SBVT – for the first time – in the gas–solid phase, monitoring the degradation of nitrogen oxides (i.e. NO + NO2). Results demonstrated that SBVT was photocatalytically active in the solar spectrum, being able to degrade NOx at ppb level concentration, and exhibited a very stable PC activity (five consecutive PC runs), proving itself suitable for reuse.

Effect of Processing Parameters on Ferroelectric Properties of 0.5(Ba,Ca)TiO3-0.5Ba(Zr,Ti)O3:Bulk, Thin Films and Fibers

We studied the effect of processing conditions on ferroelectric properties of lead-free ferroelectric composition Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCTZO) having high piezoelectric properties. BCTZO samples were fabricated successfully by conventional solid state (bulk), sol-gel chemical solution deposition (film) and laser floating zone (LFZ, fiber crystals) methods. X-ray diffraction (XRD) analysis indicated the formation of single-phase for bulk, fiber and thin film samples. We observed soft ferroelectric nature with a remanent polarization of 10.5 μC/cm2 for fibers with 5 mm/h, comparable to the reports on bulk. The observation of piezoresponse hysteresis for bulk, film and fibers confirmed the switchable ferroelectric nature at nanoscale. Our studies demonstrated that by controlling the growth rate, ferroelectric properties of fibers by LFZ can be tailored.

Direct fabrication of BiFeO3 thin films on polyimide substrates for flexible electronics

One of the major challenges for the integration of functional oxides as ferroelectrics into flexible electronics is the reduction of their processing temperature below that which causes the degradation of plastic substrates. With this aim, particular attention has been given to the low-temperature processing of oxide films by chemical solution deposition methods. In this work, lead-free multifunctional and multiferroic BiFeO3 (BFO) thin films were fabricated for the first time at a temperature as low as 300 °C directly on flexible polyimide substrates by our own proprietary solution-based Seeded Photosensitive Precursor Method. Despite this exceptionally low thermal budget, a remanent polarization, Pr, of 2.8 μC cm−2 was obtained for these BFO films, with a coercive field, Ec, of 380 kV cm−1. In addition, and of significant relevance, the films exhibited a room temperature ferromagnetic response, showing for the first time the multiferroic behaviour at room temperature of BFO films prepared at 300 °C. The ferroelectric, piezoelectric and ferromagnetic functionalities demonstrated for these films revealed their potential for applications in microelectronic devices as well as their feasibility for being used in flexible electronics. The results shown here are a proof of concept of the Seeded Photosensitive Precursor Method for the successful integration of lead-free ferroelectric thin films with flexible plastic substrates and have a broad impact in terms of the extended use of functional oxide thin films processed with low thermal schedules.

Electrical and Pyroelectric Measurements on Charge Imbalanced Sr2Bi2Nb3O12 Sol-Gel Ceramic

Oxygen-deficient and charge- imbalanced polycrystalline materials of three-layered Auruvillius compound, namely Sr2Bi2Nb3O12 (SBN), was synthesized by Sol-Gel method. X-ray diffraction analysis was made on SBN and lattice parameters were calculated. Electrical impedance and Pyroelectric measurements were performed on SBN. The broad peak observed in the dielectric permitivity data indicates that the transition is diffused phase transition (DPT) and the modified Curie-Weiss law was used to describe the DPT of SBN. The sharp intensity peak observed at 176 and 219 cm−1 in Raman spectra is an indicative of Sr-O-Sr and Sr-O-Bi bonds and its competitive interactions. The peaks of FTIR spectra appeared at 610 cm−1 confirmed the same. The nature of difffused type was attributed to compositional heterogenity and the results were corroborated with the log-log spectroscopic plots of impedance,modulus and permitivity for better understanding of the disorder ferroelectric behavior of the sample. The broad semicircle behavior observed in the complex impedance aswell as modulus plot, associated with grain and grain boundary, confirm the same.

Improved magnetoelectric effect in modified (Na 0.5 Bi 0.5 )TiO 3 -BaTiO 3 -CoFe 2 O 4 lead-free multiferroic composites

Particulate composites of (Na_0.5Bi_0.5TiO₃)-BaTiO₃(NBBT)+(Co_0.6Zn_0.4)(Fe_1.7Mn_0.3)O₄ (CZFMO) were synthesized by the conventional ceramic method to study ferroelectric, magnetic and magnetoelectric properties. Ferroelectric switching has been seen in all the ferroelectric and composite ceramics. Remanent polarization of 34 and 26μC/cm² was obtained for the pure and 85 mol% NBT-BT ceramics, while a larger leakage current was observed for the remaining composites. The magnetoelectric coefficient (α_ME) of 8 mV/cm-Oe was observed in the composition with 65 mol% ferroelectric and the coupling seems to be lower for higher ferroelectric content ceramics. A change of ~10% in saturated magnetization was observed in composites of 65 mol% NBBT, which is an indication of the converse magnetoelectric effect. The results indicate that these lead-free composites are promising and can be tunable for the low-field magnetoelectric applications.