Amit Mahajan

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.

The impact of lone-pair electrons on the lattice thermal conductivity of the thermoelectric compound CuSbS2

The discovery and design of compounds with intrinsically low thermal conductivity, especially compounds with a special bonding nature and stable crystal structure, is a new direction to broaden the scope of potential thermoelectric (TE) materials. This study revealed unambiguously the origin of the impact of the lone pair electrons on lattice thermal conductivity in Cu–Sb–S compounds by correlating the special bonding on the Sb site with the phonon dispersion spectrum and density of states. By substitution of Sb with the transition metal Fe and group IIIA element Ga without s2 electrons, lone-pair electrons on some of the Sb sites were removed, which created a scenario with opposite influences on lattice thermal conductivity from the loss of lone-pair electrons and gain of alloy scattering. We investigated the competition between the alloy phonon scattering and the extra phonon scattering mechanism linked to lone-pair electrons on trivalent Sb3+ sites in chalcostibite CuSbS2, which is a model system for benchmarking and quantifying the impact of lone-pair electrons on the lattice thermal conductivity of Cu–Sb–S compounds. A significant deviation from the classic alloy model was observed. Along with the impact of the lone-pair electrons on the bonding arrangement and crystal structure, the role of lone-pair electrons in the phonon transport of the TE compound CuSbS2 was well demonstrated and quantified. Two Sb-related quasi-single-frequency vibration modes behaving like localised Einstein harmonic oscillators were discovered and correlated with the bonding circumstance around Sb sites. These results give unequivocal evidence that the trivalent VA atom creates special bonding and vibration modes because of its nonbonding 5s lone-pair electrons.

Aluminium oxide nanoparticles induced morphological changes, cytotoxicity and oxidative stress in Chinook salmon (CHSE-214) cells

Aluminium oxide nanoparticles (Al2O3 NPs) are increasingly used in diverse applications that has raised concern about their safety. Recent studies suggested that Al2O3 NPs induced oxidative stress may be the cause of toxicity in algae, Ceriodaphnia dubia, Caenorhabditis elegans and Danio rerio. However, there is paucity on the toxicity of Al2O3 NPs on fish cell lines. The current study was aimed to investigate Al2O3 NPs induced cytotoxicity, oxidative stress and morphological abnormality of Chinnok salmon cells (CHSE‐214). A dose‐dependent decline in cell viability was observed in CHSE‐214 cells exposed to Al2O3 NPs. Oxidative stress induced by Al2O3 NPs in CHSE‐214 cells has resulted in the significant reduction of superoxide dismutase, catalase and glutathione in a dose‐dependent manner. However, a significant increase in glutathione sulfo‐transferase and lipid peroxidation was observed in CHSE‐214 cells exposed to Al2O3 NPs in a dose‐dependent manner. Significant morphological changes in CHSE‐214 cells were observed when exposed to Al2O3 NPs at 6, 12 and 24 h. The cells started to detach and appear spherical at 6 h followed by loss of cellular contents resulting in the shrinking of the cells. At 24 h, the cells started to disintegrate and resulted in cell death. Our data demonstrate that Al2O3 NPs induce cytotoxicity and oxidative stress in a dose‐dependent manner in CHSE‐214 cells. Thus, our current work may serve as a base‐line study for future evaluation of toxicity studies using CHSE‐214 cells. Copyright

Flash spark plasma sintering of magnesium silicide stannide with improved thermoelectric properties

Spark plasma sintering has become a routine method for the densification of thermoelectric (TE) materials. However, the impacts and details of direct Joule heating within TE materials have not been fully quantified and clarified. Here we investigated the feasibility of flash-sintering (high heating rate Joule heating) magnesium silicide stannide (MSS) using a spark plasma sintering furnace. A Mg2.1Si0.487Sn0.5Sb0.013 (MSS) green compact was sandwiched between two graphite punches without a die. Then a DC pulse voltage was applied between the punches and the current passed completely though the compact, without any of the current bypassing through a graphite die as occurs with a convectional SPS die–punch system. The direct heating was so efficient that a heating rate of ∼1000 °C was achieved and the sample was fully sintered in less than 45 s. Due to the high local Joule heating at the contacts of the particles, the MgO distribution pattern was modified and optimised, which broke the coated passivation layer on the MSS aggregates. The onset densification temperature was 170 to 350 °C lower than that in convectional SPS (750 °C). Importantly, it was possible to produce dense samples in a wide sintering window of ∼6 s, and the flash-sintering was controllable and repeatable. Flash sintering could open a new way for rapid densification of dense nanostructured and/or textured TE materials with low electrical resistivity by optimising the distribution or removal of the surface oxidation of the powder grains.

Room temperature magnetoelectric coupling in intrinsic multiferroic Aurivillius phase textured ceramics

Spark plasma sintering was employed in order to obtain textured Aurivillius phase ceramics that simultaneously exhibit ferroelectric and ferromagnetic properties at room temperature. The sintered multiferroics are layer-structured, nearly single-phase materials. Although a small amount of the secondary phase consisting of magnetic Co and Fe was detected by SEM/EDX, a majority of the observed ferromagnetic behaviour was attributed to the Aurivillius phase Bi4.25La0.75Ti3Fe0.5Co0.5O15 based on the observed magnetic anisotropy. The ferroelectric switching was demonstrated to exist in the Aurivillius phase ceramics by measuring the current peaks upon electric field reversal. Piezoresponse force microscopy at room temperature revelaed substantial changes of the ferroelectric domain structure when the Aurivillius phase material is subjected to an external magnetic field.

Unleashing the Full Sustainable Potential of Thick Films of Lead-Free Potassium Sodium Niobate (K0.5Na0.5NbO3) by Aqueous Electrophoretic Deposition

A current challenge for the fabrication of functional oxide-based devices is related with the need of environmental and sustainable materials and processes. By considering both lead-free ferroelectrics of potassium sodium niobate (K0.5Na0.5NbO3, KNN) and aqueous-based electrophoretic deposition here we demonstrate that an eco-friendly aqueous solution-based process can be used to produce KNN thick coatings with improved electromechanical performance. KNN thick films on platinum substrates with thickness varying between 10 and 15 μm have a dielectric permittivity of 495, dielectric losses of 0.08 at 1 MHz, and a piezoelectric coefficient d33 of ∼70 pC/N. At TC these films display a relative permittivity of 2166 and loss tangent of 0.11 at 1 MHz. A comparison of the physical properties between these films and their bulk ceramics counterparts demonstrates the impact of the aqueous-based electrophoretic deposition (EPD) technique for the preparation of lead-free ferroelectric thick films. This opens the door to the possible development of high-performance, lead-free piezoelectric thick films by a sustainable low-cost process, expanding the applicability of lead-free piezoelectrics.

Unveiling the Role of CNTs in the Phase Formation of One-Dimensional Ferroelectrics.

Carbon nanotubes (CNTs) have the potential to act as templates or bottom electrodes for three-dimensional (3D) capacitor arrays, which utilize one-dimensional (1D) ferroelectric nanostructures to increase the memory size and density. However, growing a ferroelectric on the surface of CNTs is nontrivial. Here, we demonstrate that multiwalled (MW) CNTs decrease the time and temperature for the formation of lead zirconium titanate Pb(Zr1-xTix)O3 (PZT) by ∼100 °C commensurate with a decrease in activation energy from 68 ± 15 to 27 ± 2 kJ/mol. As a consequence, monophasic PZT was obtained at 575 °C for MWCNTs/PZT, but for pure PZT, traces of pyrochlore were still present at 650 °C, where the PZT phase formed due to homogeneous nucleation. The piezoelectric nature of MWCNTs/PZT synthesized at 500 °C for 1 h was proven. Although further work is required to prove the concept of 3D capacitor arrays, our result suggests that it is feasible to utilize MWCNTs as templates/electrodes for the formation of 1D PZT nanoferroelectrics.

Bi3.25La0.75Ti2.5Nb0.25(Fe0.5Co0.5)0.25O12, a single phase room temperature multiferroic

Multiferroics (MFs) have attracted great research interest due to the coexistence of ferroelectric and magnetic ordering, as well as magnetoelectric (ME) coupling. At present, there is a very limited number of single-phase MFs known and these are still far from practical applications. In single-phase MFs, the simultaneous presence of electric and magnetic dipoles does not guarantee strong (ME) coupling, as the microscopic mechanisms of ferroelectricity and magnetism are quite different and do not intrinsically interact with each other. Here we show that in the ceramic system, Bi3.25La0.75Ti3−2xNbx(Fe0.5Co0.5)xO12, the x = 0.25 composition is ferroelectrically and ferromagnetically active at room temperature. A single-phase structure is supported by XRD, SEM/EDX and neutron diffraction data. Clear ME couplings were observed in this single-phase material at room temperature, where the magnetic iron and cobalt ions contribute to ferroelectric polarization and magnetic moment simultaneously. The results of structural, electrical and magnetic measurements are supported by first principle calculations. This material represents one of the first truly single phase bulk ceramics that exhibits multiferroic behaviour at room temperature and its discovery will help to guide the design of room temperature single-phase MFs with strong ME coupling for sensors and solid-state memory applications.

Titanium Dioxide Engineered for Near-dispersionless High Terahertz Permittivity and Ultra-low-loss.

Realising engineering ceramics to serve as substrate materials in high-performance terahertz(THz) that are low-cost, have low dielectric loss and near-dispersionless broadband, high permittivity, is exceedingly demanding. Such substrates are deployed in, for example, integrated circuits for synthesizing and converting nonplanar and 3D structures into planar forms. The Rutile form of titanium dioxide (TiO2) has been widely accepted as commercially economical candidate substrate that meets demands for both low-loss and high permittivities at sub-THz bands. However, the relationship between its mechanisms of dielectric response to the microstructure have never been systematically investigated in order to engineer ultra-low dielectric-loss and high value, dispersionless permittivities. Here we show TiO2 THz dielectrics with high permittivity (ca. 102.30) and ultra-low loss (ca. 0.0042). These were prepared by insight gleaned from a broad use of materials characterisation methods to successfully engineer porosities, second phase, crystallography shear-planes and oxygen vacancies during sintering. The dielectric loss achieved here is not only with negligible dispersion over 0.2–0.8 THz, but also has the lowest value measured for known high-permittivity dielectrics. We expect the insight afforded by this study will underpin the development of subwavelength-scale, planar integrated circuits, compact high Q-resonators and broadband, slow-light devices in the THz band.

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.