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A new computational analysis of tilt behaviour in perovskites is presented. This includes the development of a computational program – PALAMEDES – to extract tilt angles and the tilt phase from molecular dynamics simulations. The results are used to generate simulated selected-area electron and neutron diffraction patterns which are compared with experimental patterns for CaTiO3. The simulations not only reproduced all symmetrically allowed superlattice reflections associated with tilt but also showed local correlations that give rise to symmetrically forbidden reflections and the kinematic origin of diffuse scattering.

The TiO2 polymorph has a dramatic impact on the electrical conductivity and conduction mechanisms of nominally stoichiometric NBT ceramics prepared by solid-state reaction.

Low levels of impurities can have a dramatic influence on the electrical properties of metal oxides. Here, we use a combination of impedance spectroscopy and Seebeck coefficient measurements to show the defect chemistry and p-type conduction mechanism in undoped LaFeO3(LF) ceramics is significantly influenced by impurities in Fe2O3 reagents. A low but significant concentration of impurities associated with 99.9 % Fe2O3 reagent is sufficient to compete against the intrinsic Schottky disorder responsible for the p-type behaviour normally observed for undoped LF ceramics. This produces electrically heterogeneous grain (bulk) responses that are strongly dependent on the processing conditions. To achieve electrically homogeneous bulk responses requires adjustment of the oxygen content by annealing in a reducing atmosphere (n-type) or via doping with a significant level of acceptors (mixed ionic/ p-type electronic). These results illustrate the importance of using high purity reagents when studying the electrical properties of ferrite-based perovskites.

Lead-free bismuth sodium titanate piezoceramics were processed by a solid-state route with a novel precursor approach. By carefully controlling the processing parameters and sintering atmospheres, translucent ceramics with pore-free, homogeneous microstructures with exceptionally low dielectric loss at elevated temperatures. The pore-free microstructure can influence the operating life of a dielectric device positively, since pores and other microstructural defects are usually responsible for material failures within devices. Synchrotron and surface X-ray diffraction experiments revealed a clear dependence of the crystal structure on sintering parameters and defect chemistry. Microstructural analysis showed a dependency of a secondary phase on the sintering atmosphere. The grain size could be adjusted using a shortened grinding process instead of using the common methods like increasing calcining or sintering temperature.

0.9Ba(Ti1−xMgx)O3−x-0.1NaNbO3 (BTNN-100xMg) solid solutions are investigated with a view to developing Bi, Pb, and rare earth free, high voltage multilayer ceramic capacitors. Mg doping on the B-site significantly reduced the electronic conductivity and resulted in ceramics that could withstand a pulsed unipolar field of >300 kV/cm (Emax) to give a recoverable energy density of 3.4 J/cm3 at 82.6% efficiency for x = 0.01. The high Emax is accompanied by a high dielectric permittivity (ε′ ∼ 1700 at room temperature) with temperature-stable dielectric permittivity of Δε/ε298K ≤ ±15% and loss tangent tan δ < 0.02 from 116 to 378 K, corresponding to an X6R designation in the Electronic Industry Alliance codes.

The compositional limits of a previously reported (J. Am. Ceram. Soc., 61, 5‐8. (1978)) but relatively unstudied sodium‐bismuth titanate pyrochlore solid solution are revised and their electrical properties presented. The pyrochlore solid solution we report forms via a different mechanism to that originally reported and occurs in a different location within the Na2O‐Bi2O3‐TiO2 ternary system. In both cases, relatively large amounts of vacancies are required on the A‐sites and on the oxygen sites, similar to that reported for undoped ‘Bi2Ti2O7’ pyrochlore. In contrast to ‘Bi2Ti2O7’, this ternary pyrochlore solid solution can be prepared and ceramics sintered using conventional solid‐state methods; however, the processing requires several challenges to be overcome to obtain dense ceramics. This cubic pyrochlore series has low electrical conductivity (and does not exhibit any evidence of oxide‐ion conduction) and exhibits relaxor ferroelectric behavior with a broad permittivity maximum of ~100 near room temperature. Variable temperature neutron diffraction data do not provide any conclusive evidence for a phase transition in the pyrochlore solid solution between ~4 and 873 K.

Existing equivalent circuit extraction techniques are inaccurate for piezoelectric transformers (PTs) with low-input capacitance or high damping. A new method is presented, offering improved accuracy in both damping resistance and resonant frequency extraction compared with state-of-the-art methods. Effectiveness is evaluated on two sample PTs, with the proposed method achieving up to 84% decrease in error compared with previous methods.

Micro‐contact impedance spectroscopy (MCIS) is potentially a powerful tool for the exploration of resistive surface layers on top of a conductive bulk or substrate material. MCIS employs micro‐contacts in contrast to conventional IS where macroscopic electrodes are used. To extract the conductivity of each region accurately using MCIS requires the data to be corrected for geometry. Using finite element modeling on a system where the resistivity of the surface layer is at least a factor of ten greater than the bulk/substrate, we show how current flows through the two layers using two typical micro‐contact configurations. This allows us to establish if and what is the most accurate and reliable method for extracting conductivity values for both regions. For a top circular micro‐contact and a full bottom counter electrode, the surface layer conductivity (σs) can be accurately extracted using a spreading resistance equation if the thickness is ~10 times the micro‐contact radius; however, bulk conductivity (σb) values can not be accurately determined. If the contact radius is 10 times the thickness of the resistive surface, a geometrical factor using the micro‐contact area provides accurate σs values. In this case, a spreading resistance equation also provides a good approximation for σb. For two top circular micro‐contacts on thin resistive surface layers, the MCIS response from the surface layer is independent of the contact separation; however, the bulk response is dependent on the contact separation and at small separations contact interference occurs. As a consequence, there is not a single ideal experimental setup that works; to obtain accurate σs and σb values the micro‐contact radius, surface layer thickness and the contact separation must all be considered together. Here we provide scenarios where accurate σs and σb values can be obtained that highlight the importance of experimental design and where appropriate equations can be employed for thin and thick resistive surface layers.

The influence of Ti-site acceptor-doping (Mg2+, Zn2+, Sc3+, Ga3+ and Al3+) on the electrical conductivity and conduction mechanism of a nominally Bi-excess sodium bismuth titanate perovskite, Na0.5Bi0.51TiO3.015 (NB0.51T), is reported. Low levels of acceptor-type dopants can introduce appreciable levels of oxide-ion conductivity into NB0.51T, i.e., 0.5% Mg-doping for Ti4+ can enhance the bulk conductivity of NB0.51T by more than 3 orders of magnitude with the oxide-ion transport number going from 0.9 at 600 °C. The intriguing electrical behaviour in acceptor-doped NB0.51T dielectrics is a synergistic effect based on the defect chemistry and ceramic microstructure in these materials. NB0.51T ceramics with extremely low levels of doping show an inhomogeneous microstructure with randomly distributed large grains embedded in a small grained matrix. This can be considered as a two-phase composite with large grains as a conductive phase and small grains as an insulating phase based on an empirical conductivity – grain size relationship. Variation in the fraction of the conductive, large grained phase with increasing doping levels agrees with the oxide-ion transport number. This electrical two-phase model is supported by finite element modelling. This study reveals the significance of ceramic microstructure on the electrical conduction behaviour of these materials and can provide a guideline for selecting suitable doping strategies to meet the electrical property requirements of NBT-based ceramics for different applications.

The critical role of electrical homogeneity in optimising electric-field breakdown strength (BDS) and energy storage in high energy density (0.7 − x)BiFeO3–0.3BaTiO3–xBi(Li0.5Nb0.5)O3 (BF–BT–xBLN) lead-free capacitors is demonstrated. The high BDS for bulk ceramics and multilayers (dielectric layer thickness ∼ 8 μm) of ∼260 and ∼950 kV cm−1, respectively, gives rise to record-performance of recoverable energy density, Wrec = 13.8 J cm−3 and efficiency, η = 81%. Under an electric field of 400 kV cm−1, multilayers are temperature stable up to 100 °C, frequency independent in the range 10−2 to 102 Hz, have low strain (

A nominal Bi-excess starting composition of sodium bismuth titanate, Na0.5Bi0.51TiO3.015 (NB0.51T) produces dielectric ceramics that exhibit mixed n-type and oxide-ion conductivity with an ionic transport number of ∼0.1 at ≥600 °C. The bulk electrical conductivity, σb, of NB0.51T ceramics under a dc bias field of ≤100 V cm−1 has been investigated by impedance spectroscopy. Over the temperature range ∼550 to 750 °C, σb increases by up to one order of magnitude under the dc bias and returns to its initial value on removal of the bias. The enhancement of conductivity is dependent on temperature, atmosphere, dc bias field and the electrode materials. A maximum conductivity enhancement of >2000% is achieved at 600 °C in nitrogen using Pt electrodes. This is in contrast to that observed for other n-type perovskite titanates and oxygen-deficient rutile where σb is suppressed under a dc bias. This ‘unusual’ non-ohmic behaviour is attributed mainly to the influence of highly mobile oxygen vacancies in NB0.51T. The field-enhanced σb is best described to be a consequence of increased pumping of oxygen from the cathode to the anode, in which the electrode reactions play an important role. In addition, dissociation of defect clusters may also contribute to the enhanced σb under a dc bias. The high, fast and reversible response to the dc bias voltage may expand the potential application of NBT-based materials to memory devices.

The rising cost of energy and concerns about the environmental impact of manufacturing processes have necessitated the need for more efficient and sustainable manufacturing. The ceramic industry is an energy intensive industrial sector and consequently the potential to improve energy efficiency is huge, particularly through the introduction of modern sintering technologies. Although several energy efficient sintering processes have been developed, there is no comprehensive techno-economic analysis which compares and contrasts these techniques. This paper presents a critical review and analysis of a number of sintering techniques and compares them with the recently developed cold sintering process (CSP), including mode of operation, sintering mechanism, typical heating rates, duration of sintering, energy consumption profile and energy saving potential, limitations, key challenges for further development and current research efforts. By using a figure of merit, pounds per tonne of CO2 saved (£/tCO2-eq), which links initial capital investment with energy savings, within a framework derived from ranking principles such as marginal abatement cost curves and Pareto optimisation, we have demonstrated that under the scenarios considered for 3 separate functional oxides ZnO, PZT and BaTiO3, CSP is the most economically attractive sintering option, indicating lower capital costs and best return on investment as well as considerable energy and emission savings. Although the current work establishes the viability of CSP as a competitive and sustainable alternative to other sintering techniques, the transition from laboratory to industry of CSP will require hugely different facilities and instrumentation as well as relevant property/performance validation to realise its full potential.

A combination of X-ray diffraction, analytical-electron microscopy, differential scanning calorimetry, impedance spectroscopy and electromotive force measurements (for oxide-ion transport number measurements, tion) are used to report on the influence of a small amount of a continuous Bi2O3-rich phase along the grain boundaries in sample composition x = 0.4 (BS0.4BT) of the high temperature dielectric solid solution series, x(BiScO3)-(1-x)(BaTiO3). Its presence produces a dramatic change in conductivity of ~ two orders of magnitude and a switch in tion over the range ~ 600 - 800 °C that is not observed for other ceramics with lower BiScO3 content. Below ~ 700 °C the grain boundaries in BS0.4BT act as electrically blocking layers and dominate the impedance of the ceramics. In contrast, at > 800 °C the grain boundaries become highly conductive due to a polymorphic phase transition to, and melting of δ-Bi2O3 which results in the current percolating along the grain boundaries and therefore avoiding the grains. The value of tion increases from ~ 0.13 at ~ 600 °C to near unity at ~ 800 °C for BS0.4BT, consistent with oxide ion conduction due to the presence of liquid Bi2O3 at grain boundary regions. This behaviour was reproduced by adding a small excess of 3 wt% Bi2O3 into x = 0.3 (BS0.3BT) samples to induce a Bi2O3-rich grain boundary phase, not otherwise present in this composition.

The electrical behaviour of the interface between the ceramic and electrode layers in multi layer ceramic capacitors has been studied using finite element modelling. Interface models were produced with varying amplitudes of roughness based upon analysis of micrographs both captured in-house and from the literature. The impedance responses, direct current electric field and current density distributions of the different interfaces were compared. Increasing the root-mean-squared amplitude roughness from 0 to 0.16 μm increased the maximum field strength by over a factor of four. The electric field distribution showed that fluctuations in the increase of field strength were due to local interface morphology. Sharp intrusions of the electrode into the ceramic layer resulted in particularly large field enhancements and should be avoided to reduce the likelihood of device breakdown.

Ceramics-ceramic composites in series (1-x)Li2MoO4-xBaFe12O19 (LMO-BF12, 0.00 ≤ x ≤ 0.15) have been cold sintered at 120 °C and their structure and properties characterized. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed that compositions were dual phase and had a dense microstructure. Composites in the xBF12-(1-x)LMO (0.0 ≤ x ≤ 0.15) series resonated at MW frequencies (∼6 GHz) with 5.6≤er ≤ 5.8 and Qf = 16,000–22,000 GHz, despite the black colour of compositions with x > 0. The permeability of the composites was measured in the X band (∼8 GHz) and showed an increase from 0.94 (x = 0.05) to 1.02 (x = 0.15). Finite element modelling revealed that the volume fraction of BF12 dictates the conductivity of the material, with a percolation threshold at 10 vol% BF12 but changes in er as a function of x were readily explained using a series mixing model. In summary, these composites are considered suitable for the fabrication of dual mode or enhanced bandwidth microstrip patch antennas.

Globally, the issue of climate change due to greenhouse gas (GHG) emissions is now broadly acknowledged as one of the major challenges facing humankind that requires urgent attention. Accordingly, considerable efforts on clean energy technologies and policy recommendations have been developed to address this challenge. Solid oxide fuel cells (SOFCs) have been touted to play a role in achieving a reduction in global GHG emissions, offering numerous advantages including higher efficiencies and reduced emissions, over other conventional methods of energy generation. The increasing recognition and emphasis on fuel cells as a representative power generation system of the future has raised concerns over their environmental profile. Extensive research regarding the environmental profile of current structures of SOFCs can be found in the literature, but none consider the use of new materials to achieve lower environmental impacts. This research fills the gap and presents a comparison of the environmental profile of three SOFC structures: a commercially available structure, and two intermediate temperature structures, one using erbia-stabilised bismuth oxide electrolytes and a proposed structure using strontium-doped sodium bismuth titanate electrolytes. Using a functional unit of kg/100 kW of power output for each of the SOFC structures (excluding the interconnects), within a hybrid life cycle analysis framework, the environmental hotspots across the supply chains of each SOFC type are identified, quantified and ranked. The results show the use of these novel material combinations leads to a reduction in embodied materials and toxicological impact but higher electrical energy consumption during fabrication, in comparison to commercial SOFCs. The findings support the move to reduce the operating temperatures of SOFCs using these novel material architectures, which leads to an overall reduction in environmental impact due to the lower operational energy requirement of the chosen material constituents.

Ultrahigh discharge energy density (Wdis = 10.5 J cm−3) and efficiency (η = 87%) have been obtained in doped BiFeO3–BaTiO3 ceramic multilayers by achieving an electrically rather than chemically homogeneous microstructure. Back scattered scanning and transmission electron microscopy combined with energy dispersive X-ray spectroscopy mapping of (0.7 − x)BiFeO3–0.3BaTiO3–xNd(Zn0.5Zr0.5)O3 (0.05 ≤ x ≤ 0.10) ceramics revealed a core–shell grain structure which switched from a bright to dark contrast as x increased. Compositions with x = 0.08 were at the point of cross over between these two manifestations of core–shell contrast. Dielectric measurements together with the absence of macrodomains in diffraction contrast TEM images suggested that compositions with x = 0.08 exhibited relaxor behaviour within both the core and shell regions. Impedance spectroscopy demonstrated that, despite being chemical dissimilar, the grains were electrically homogeneous and insulating with little evidence of conductive cores. Multilayers of x = 0.08 had enhanced breakdown strength, EBDS > 700 kV cm−1 and a slim hysteresis loop which resulted in large Wdis and high η which were temperature stable to

Various two terminal electrode geometry configurations are commonly employed to probe the electrical properties of materials with the key consideration being how current flows through the sample. Here, finite element modeling is used to simulate the dc electrical response of an electrically homogeneous sample (based on single crystal SrTiO3) using two terminal electrode geometries based on full surface, top‐bottom macro‐contacts as is commonly used when characterizing bulk ceramics or large single crystals and top‐bottom and top‐top micro‐contacts that are used to characterize thin films and local intra‐ and inter‐granular regions in ceramics. Well‐known equations for macro‐ and micro‐contacts are used to calculate the conductivity of the sample and are compared to the intrinsic values to determine their accuracy. A geometric factor returns accurate bulk conductivity values when there is homogeneous current flow whereas the spreading resistance equation gives the most accurate conductivity values for heterogeneous current flow. When micro‐contacts are used, the response is dominated by a small region of high current density in the vicinity of the contact, providing local electrical properties. Interference can occur when regions of high current density overlap, providing a less resistive route for current flow, thus reducing the applicability of the spreading resistance equation. For top‐top micro‐contacts at small separations, the conductivity is overestimated. The accuracy of the spreading resistance equation increases as the contact separation increases and is within 10% error when they are separated by eight times the micro‐contact radius. Convergence of the error to values lower than 10% becomes increasingly difficult and requires excessively large (and experimentally challenging) separation distances. For example, to obtain a result with an error below 5% requires separations in excess of 28 times the micro‐contact radius. Confinement occurs when the sample size limits the ability of the current to spread out from a micro‐contact, thus increasing the resistance. As the sample shape and dimensions can limit current flow, a geometric factor can sometimes be used to determine accurate conductivity values. In some cases, interference can counter‐balance confinement to yield fortuitously accurate conductivity values.

The demand for capacitors exhibiting low sensitivity towards temperature changes and high power peaks has increased significantly. Recently, Na0.5Bi0.5TiO3 (NBT) based ceramics became excellent candidates for such extreme temperature capacitors. The dielectric loss of these materials is, however, difficult to control because of the complex defect chemistry of NBT based ceramics. Therefore, it is the limiting factor for high temperature applications. In this work, we present a strategy to increase the upper temperature limit for low dielectric loss. The addition of BiAlO3 to Na0.5Bi0.5TiO3-BaTiO3-CaZrO3 reduces the loss and sensitivity towards Bi evaporation during synthesis. For unmodified samples, the relative permittivity (er = 581, at 1 kHz) varies less than 15 %, while the dielectric loss stays below 0.02 between -68 and 368 °C. With the addition of BiAlO3, the temperature range of low loss extends from -68 to 391 °C at even higher permittivity (er = 628, at 1 kHz).

Physical chemistry, chemical physics : PCCP 23(19), 11327-11333 (2021). doi:10.1039/d1cp01120k, Published by RSC Publ., Cambridge

Barium strontium titanate (BSTO) thin films were deposited on Pt(111) by high throughput evaporative physical vapor deposition and then annealed at 650 \ifmmode^\circ\else\textdegree\fi{}C for 30 min under ${\mathrm{N}}_{2}$ atmosphere. Using advanced transmission electron microscopy, energy-dispersive x-ray spectroscopy and electron energy-loss spectroscopy, we directly show that not only does N substitute for O in the BSTO lattice but that it also compensates for $\mathrm{T}{\mathrm{i}}^{3+}$ ions, suppressing conductivity, thereby reducing dielectric loss and enhancing dielectric tunability. However, this effect is negated near the film edge where we speculate that exposed Pt acts as a reservoir of adsorbed/absorbed O and alters the local ${\mathrm{N}}_{2}$ concentration during annealing.

A crystallochemical framework is proposed based on electronegativity difference (en) and\ud tolerance factor (t) to optimise the BiMeO3 dopants and therefore the piezoelectric and\ud electrostrictive response in BaTiO3-BiFeO3 based ceramics. Compositions in the series\ud 0.05Bi(Me)O3-0.25BaTiO3-0.7BiFeO3 (BMe-BT-BF, Me: Y, Sc1/2Y1/2, Mg2/3Nb1/3, Sc,\ud Zn2/3Nb1/3, Zn1/2Ti1/2, Ga, and Al) were fabricated using solid state synthesis and furnace\ud cooled. Scanning electron microscopy and X-ray diffraction revealed that only\ud Bi(Mg2/3Nb1/3)O3 and BiScO3 dopants, which lie in a narrow range of en vs. t, form\ud homogeneous ceramics, free from secondary phases reflected in their superior\ud piezoelectric coefficients (d33 ~145 pC/N). All other BiMeO3 additions exhibited either\ud secondary phases (Y) and/or promoted a two-phase perovskite matrix (Zn, Ga and Al).\ud The promising initial properties of BiScO3 doped compositions prompted further studies\ud on 0.05BiScO3-(0.95-x)BaTiO3-(x)BiFeO3 (BS-BT-BF, x = 0.55, 0.60, 0.625, 0.65, and\ud 0.70) ceramics. As x increased the structure changed from predominantly pseudocubic to\ud rhombohedral, resulting in a transition from a relaxor-like to ferroelectric response. The\ud largest d33\ud * (465 pm/V) was achieved for x = 0.625 under 5 kV/mm at the crossover from\ud relaxor to ferroelectric behaviour. BS-BT-BF with x = 0.625 showed >0.3% strain under\ud 6 kV/mm up to 175ºC, demonstrating its potential for actuator applications.

The relationship between the piezoelectric properties and the structure/microstructure for 0.05Bi(Mg2/3Nb1/3)O3-(0.95-x)BaTiO3-xBiFeO3 (BBFT, x = 0.55, 0.60, 0.63, 0.65, 0.70, and 0.75) ceramics has been investigated. Scanning electron microscopy revealed a homogeneous microstructure for x < 0.75 but there was evidence of a core-shell cation distribution for x = 0.75 which could be suppressed in part through quenching from the sintering temperature. X-ray diffraction (XRD) suggested a gradual structural transition from pseudocubic to rhombohedral for 0.63

The activation energy associated with bulk electrical conduction in functional materials is an important quantity which is often determined by impedance spectroscopy using an Arrhenius-type equation. This is achieved by linear fitting of bulk conductivity obtained from complex (Z*) impedance plots versus T−1 which gives an activation energy Ea(σ) or by linear fitting of the characteristic frequency fmax obtained from the large Debye peak in M′′–log f spectroscopic plots against T−1 which gives an activation energy Ea(fmax). We report an analysis of Ea(σ) and Ea(fmax) values for some typical non-ferroelectric and ferroelectric materials and employ numerical simulations to investigate combinations of different conductivity–temperature and permittivity–temperature profiles on the log fmax–T−1 relationship and Ea(fmax). Results show the log fmax–T−1 relationship and Ea(fmax) are strongly dependent on the permittivity–temperature profile and the temperature range measured relative to Tm (temperature of the permittivity maximum). Ferroelectric materials with a sharp permittivity peak can result in non-linear log fmax–T−1 plots in the vicinity of Tm. In cases where data are obtained either well above or below Tm, linear log fmax–T−1 plots can be obtained but overestimate or underestimate the activation energy for conduction, respectively. It is therefore not recommended to use Ea(fmax) to obtain the activation energy for bulk conduction in ferroelectric materials, instead Ea(σ) should be used.

The electrical properties of (Na0.5Bi0.5TiO3)1−x(BiScO3)x (NBT-BS, 0.00 ≤ x ≤ 0.25) solid solutions are established by ac impedance spectroscopy and electromotive force transport number measurements. The bulk conductivity decreases with increasing BS incorporation but the oxide-ion transport number remains high (>0.85) over a wide compositional range 0.00 ≤ x ≤ 0.15 and drops to ∼0.7 for x ≥ 0.20. NBT-BS solid solutions can only present either predominant oxide-ion conduction or mixed ionic-electronic conduction behaviour, indicating that oxide-ion conduction cannot be fully eliminated by incorporation of BS. This is in contrast from our previous study where incorporation of ∼7% BiAlO3 (BA) can fully suppress the oxide-ion conduction in NBT. The conductivity–composition relationships of NBT-BS solid solutions are attributed to a competing effect from lattice expansion, which enlarges the channel for oxygen ion migration, with trapping between B-site acceptor ions, , and oxygen vacancies, , which decreases oxygen ion migration. Comparisons between NBT-BS, NBT-BA and NBT-BiGaO3 (BG) solid solutions suggest that small acceptor ions on the B-site are more effective in trapping oxygen vacancies and consequently more effective to suppress the oxide-ion conduction and thus reduce dielectric loss at elevated temperatures.

Organometallic perovskite materials based on MAPbI3 achieve photovoltaic efficiencies as high as 22% for solar cells; however, the long-term stability of these perovskite materials is still a hurdle for applications. Here, we report the air and charge induced instabilities of MAPbI3 perovskite materials with different local stoichiometry using Secondary Electron Hyperspectral Imaging (SEHI). We find that individual grains which do not have the ideal ABX3 stoichiometry degrade faster than stoichiometric grains. We also observe that the degradation pathways depend on the local stoichiometry. Non-stoichiometric grains (with excess MAI) degrade from the centre of the grain and further degradation moves towards the grain boundary, whereas in stoichiometric grains degradation sets in at the grain boundary. We further use deliberately high doses of electron beam exposure to highlight the presence of local non-stoichiometry in device cross-sections of bias degraded devices which pinpoints different onset locations of degradation, depending on the local stoichiometry. These results, therefore, show that precise control of local stoichiometry is required to improve the stability of MAPbI3 and thus the lifetime of perovskite solar cells.

The thermoelectric properties of Sr1-xLax/2Smx/2TiO3-δ (0.05 ≤ x ≤ 0.30) ceramics have been investigated with compositions batched, synthesised by solid state reaction and sintered in 5% H2/N2 at 1500 °C for 6 hrs. All X-ray diffraction patterns were fully indexed according to a cubic perovskite phase. Scanning electron microscopy revealed homogeneous grain structure in the ceramics and confirmed relative density ≥ 89%. The electrical conductivity (σ) of x ≤ 0.15 displayed metallic behaviour with σ < 1000 S/cm, whereas x ≥ 0.20 were semiconducting with σ < 250 S/cm. The Seebeck coefficient of all compositions was negative indicating n-type behaviour. Within this series, x = 0.20 displayed the lowest thermal conductivity of ∼ 3 W/m.K (at 973 K), x = 0.10 displayed the highest power factor of 1400 μW/K2.m (at 573 K) and overall x = 0.15 had the highest dimensionless figure of merit (ZT) of 0.24 (at 875 K).

Novel synthetic routes to prepare functional oxides at lower temperatures are an increasingly important area of research. Many of these synthetic routes, however, use water as the solvent and rely on dissolution of the precursors, precluding their use with, for example, titanates. Here we present a low-cost solvent system as a means to rapidly create phase-pure ferroelectric barium titanate using a choline chloride-malonic acid deep eutectic solvent. This solvent is compatible with alkoxide precursors and allows for the rapid synthesis of nanoscale barium titanate powders at 950 °C. The phase and morphology were determined, along with investigation of the synthetic pathway, with the reaction proceeding via BaCl2 and TiO2 intermediates. The powders were also used to create sintered ceramics, which exhibit a permittivity maximum corresponding to a tetragonal-cubic transition at 112 °C, as opposed to the more conventional temperature of ∼120 °C. The lower-than-expected value for the ferro- to para-electric phase transition is likely due to undetectable levels of contaminants.

© 2019 The Royal Society of Chemistry. High electrostrain and breakdown strength (1 - x)BiFeO3-0.3BaTiO3-xNd(Li0.5Nb0.5)O3 (BF-BT-xNLN) ceramics were studied by in situ synchrotron X-ray diffraction (XRD) in combination with Rietveld refinement and conventional transmission electron microscopy. At zero field, compositions transformed from majority ferroelectric rhombohedral to pseudocubic as the NLN concentration increased, with 0.27% strain achieved at 60 kV cm-1 for x = 0.01. The large measured macroscopic strain was commensurate with peak shifting in XRD peak profiles, yielding 0.6% total strain at 150 kV cm-1. Strain anisotropy of ϵ200 > ϵ220 > ϵ111 was observed but despite the large applied field, no peak splitting was detected. We therefore concluded that the large electrostrain is not achieved through a conventional relaxor to field induced long-range ferroelectric transition. Instead, the data supports a model where local polar regions distort in the direction of the applied field within multiple local symmetries (pseudosymmetry) without long range correlation. We proposed that pseudosymmetry is maintained in BF-BT-xNLN even at high field (150 kV cm-1) due to the large ion radii mismatch and competing ionic/covalent bonding between Ba2+ and Bi3+ ions.

We present an aberration corrected scanning transmission electron microscopy (ac-STEM) analysis of the perovskite (LaFeO3) and pyrochlore (Yb2Ti2O7 and Pr2Zr2O7) oxides and demonstrate that both the shape and contrast of visible atomic columns in annular dark-field (ADF) images are sensitive to the presence of nearby atoms of low atomic number (e.g. oxygen). We show that point defects (e.g. oxygen vacancies), which are invisible - or difficult to observe due to limited sensitivity - in X-ray and neutron diffraction measurements, are the origin of the complex magnetic ground state of pyrochlore oxides. In addition, we present, for the first time, a method by which light atoms can be resolved in quantitative ADF-STEM images. Using this method, we resolved oxygen atoms in perovskite and pyrochlore oxides., Comment: 18 pages and 7 figures plus supplementary information

We demonstrate a fast, efficient combinatorial method for the optimisation of materials for multi-layer ceramic capacitors (MLCCs). Experimentally gathered permittivity-temperature profiles for nine compositions spanning a solid solution are used as input, and with series mixing rules, binary and ternary permittivity contour maps are calculated based on individual layer thicknesses. These are converted into Temperature Coefficient of Capacitance (TCC) contour maps and an algorithm is then used to identify material combinations and individual thicknesses suitable for various MLCC classifications. These facilitate targeted experimentation and allowed experimental verification of the methodology. The approach highlights that binary systems can achieve X9 (R, S T and U). classification but the addition of a third complimentary material can facilitate a tighter TCC classification (X9P) with a wider tolerance in layer thicknesses, providing a better strategy for mass production of MLCCs. The room temperature permittivity (ƐRT) for combinations with similar TCC values can also be evaluated to ensure adequate ƐRT is achieved for commercial applications.

Precise control of electronic and/or ionic conductivity in electroceramics is crucial to achieve the desired functional properties as well as to improve manufacturing practices. We recently reported the conventional piezoelectric material Na1/2Bi1/2TiO3 (NBT) can be tuned into a novel oxide-ion conductor with an oxide-ion transport number (tion) > 0.9 by creating bismuth and oxygen vacancies. A small Bi-excess in the nominal starting composition (Na0.50Bi0.50+xTiO3+3x/2, x = 0.01) or Nb-donor doping (Na0.50Bi0.50Ti1−yNbyO3+y/2, 0.005 ≤ y ≤ 0.030) can reduce significantly the electrical conductivity to create dielectric behaviour by filling oxygen vacancies and suppressing oxide ion conduction (tion ≤ 0.10). Here we show a further increase in the starting Bi-excess content (0.02 ≤ x ≤ 0.10) reintroduces significant levels of oxide-ion conductivity and increases tion ∼ 0.4–0.6 to create mixed ionic/electronic behaviour. The switch from insulating to mixed conducting behaviour for x > 0.01 is linked to the presence of Bi-rich secondary phases and we discuss possible explanations for this effect. Mixed conducting behaviour with tion ∼ 0.5–0.6 can also be achieved with lower levels of Nb-doping (y ∼ 0.003) due to incomplete filling of oxygen vacancies without the presence of secondary phases. NBT can now be compositionally tailored to exhibit three types of electrical behaviour; Type I (oxide-ion conductor); Type II (mixed ionic-electronic conductor); Type III (insulator) and these results reveal an approach to fine-tune tion in NBT from near unity to zero. In addition to developing new oxide-ion and now mixed ionic/electronic NBT-based conductors, this flexibility in control of oxygen vacancies allows fine-tuning of both the dielectric/piezoelectric properties and design manufacturing practices for NBT-based multilayer piezoelectric devices.

Barium titanate is the dielectric material of choice in most multilayer ceramic capacitors (MLCCs) and thus in the production of ≈3 trillion devices every year, with an estimated global market of ≈$8330 million per year. Rare earth dopants are regularly used to reduce leakage currents and improve the MLCC lifetime. Simulations are used to investigate the ability of yttrium, dysprosium, and gadolinium to reduce leakage currents by trapping mobile oxygen defects. All the rare earths investigated trap oxygen vacancies, however, dopant pairs are more effective traps than isolated dopants. The number of trapping sites increases with the ion size of the dopant, suggesting that gadolinium should be more effective than dysprosium, which contradicts experimental data. Additional simulations on diffusion of rare earths through the lattice during sintering show that dysprosium diffuses significantly faster than the other rare earths considered. As a consequence, its greater ability to reduce oxygen migration is a combination of thermodynamics (a strong ability to trap oxygen vacancies) and kinetics (sufficient distribution of the rare earth in the lattice to intercept the migrating defects).

© 2018 The Author(s) The prohibition of lead in many electronic components and devices due to its toxicity has reinvigorated the race to develop substitutes for lead zirconate titanate (PZT) based mainly on the potassium sodium niobate (KNN) and sodium bismuth titanate (NBT). However, before successful transition from laboratory to market, critical environmental assessment of all aspects of their fabrication and development must be carried out in comparison with PZT. Given the recent findings that KNN is not intrinsically ‘greener' than PZT, there is a tendency to see NBT as the solution to achieving environmentally lead-free piezoelectrics competitive with PZT. The lower energy consumed by NBT during synthesis results in a lower overall environmental profile compared to both PZT and KNN. However, bismuth and its oxide are mainly the by-product of lead smelting and comparison between NBT and PZT indicates that the environmental profile of bismuth oxide surpasses that of lead oxide across several key indicators, especially climate change, due to additional processing and refining steps which pose extra challenges in metallurgical recovery. Furthermore, bismuth compares unfavourably with lead due to its higher energy cost of recycling. The fact that roughly 90–95% of bismuth is derived as a by-product of lead smelting also constitutes a major concern for future upscaling. As such, NBT and KNN do not offer absolute competitive edge from an environmental perspective in comparison to PZT. The findings in this work have global practical implications for future Restriction of Hazardous Substances (RoHS) legislation for piezoelectric materials and demonstrate the need for a holistic approach to the development of sustainable functional materials.

Combining the efforts of physicists, materials scientists, economists and resource-strategy researchers opens up an interdisciplinary route enabling the substitution of rare elements by more abundant ones, serving as a guideline for the development of novel materials.

The solid solution (K x Na 0.50-x )Bi 0.50 TiO 3 (KNBT) between Na 1/2 Bi 1/2 TiO 3 and K 1/2 Bi 1/2 TiO 3 (KBT) has been extensively researched as a candidate lead-free piezoelectric material because of its relatively high Curie temperature and good piezoelectric properties, especially near the morphotropic phase boundary (MPB) at x ∼ 0.10 (20 mol. % KBT). Here, we show that low levels of excess K 2 O in the starting compositions, i.e., (K y+0.03 Na 0.50-y )Bi 0.50 TiO 3.015 (y-series), can significantly change the conduction mechanism and electrical properties compared to a nominally stoichiometric KNBT series (K x Na 0.50-x )Bi 0.50 TiO 3 (x-series). Impedance spectroscopy measurements reveal significantly higher bulk conductivity (σ b ) values for y ≥ 0.10 samples [activation energy (E a ) ≤ 0.95 eV] compared to the corresponding x-series samples which possess bandgap type electronic conduction (E a ∼ 1.26-1.85 eV). The largest difference in electrical properties occurs close to the MPB composition (20 mol. % KBT) where y = 0.10 ceramics possess σ b (at 300 °C) that is 4 orders of magnitude higher than that of x = 0.10 and the oxide-ion transport number in the former is ∼0.70-0.75 compared to < 0.05 in the latter (between 600 and 800 °C). The effect of excess K 2 O can be rationalised on the basis of the (K + Na):Bi ratio in the starting composition prior to ceramic processing. This demonstrates the electrical properties of KNBT to be sensitive to low levels of A-site nonstoichiometry and indicates that excess K 2 O in KNBT starting compositions to compensate for volatilisation can lead to undesirable high dielectric loss and leakage currents at elevated temperatures.

Potassium sodium niobate (KNN) is a potential candidate to replace lead zirconate titanate in sensor and actuator applications but there are many fundamental science and materials processing issues to be understood before it can be used commercially, including the influence of composition and processing atmosphere on the conduction mechanisms and functional properties. Consequently, KNN pellets with different K/Na ratios were sintered to 95% relative density in air and N 2 using a conventional mixed oxide route. Oxygen vacancies (V O •• ) played a major role in the semi-conduction mechanism in low p(O 2 ) for all compositions. Impedance spectroscopy and thermo-power data confirmed KNN to be n-type in low p(O 2 ) in contradiction to previous reports of p-type behaviour. The best piezoelectric properties were observed for air- rather than N 2 -sintered samples with d 33 = 125 pC/N and k p = 0.38 obtained for K 0.51 Na 0.49 NbO 3 .

La-doping mechanisms and thermoelectric properties of Sr3Ti2O7 Ruddlesden-Popper (RP) ceramics sintered under air and flowing 5% H2 at 1773 K for 6 hours have been investigated. Changes in lattice parameters and conductivity revealed a limited interstitial anion mechanism (~1 at.%) based on La3+ + ½O2- → Sr2+ which resulted in insulating samples when processed in air. In contrast, electronic donor-doping (La3+ + e- → Sr2+) and oxygen loss [O2- → ½ O2 (g) + 2 e-] are the dominant mechanism(s) in 5% H2- sintered ceramics with a solution limit of ~5 at.%. The increased solubility limit is attributed to the formation of Ti3+ during reduction which compensates for the extra positive charge associated with La on the A-site and also to the occurrence of oxygen loss due to the reducing conditions. For 5% H2-sintered samples, an insulating surface layer formed associated with SrO volatilisation and oxygen up-take (during cooling) from the sintering. Unless removed, the insulating layer masked the conductive nature of the ceramics. In the bulk, significantly higher power factors were obtained for ceramics that 2 were phase mixtures containing highly conductive perovskite-based (Sr,La)TiO3- (ST). This highlights the superior power factor properties of reduced perovskite-type ST phases compared to reduced RP-type Sr3Ti2O7 and serves as a precaution for the need to identify low levels of highly conducting perovskite phases when exploring rare-earth doping mechanisms in RP-type phases.

In this work, American Iron and Steel Institute (AISI) O1 steel was pack borided in the temperature range of 1123–1273 K for treatment times between 2 and 8 hours. A kinetic model was proposed for estimating the boron diffusion coefficients through the Fe2B layers. As a result, the boron activation energy for the AISI O1 steel was estimated as 197·2 kJ mol− 1. This value of energy was compared to the literature data. In addtion, to extend the validity of the present model, two additional boriding conditions were done. The Fe2B layers grown on AISI O1 steel were characterised by use of the following experimental techniques: scanning electron microscopy, X-ray diffraction analysis and Daimler-Benz Rockwell-C indentation technique. Finally, the scratch and pin on disc tests for wear resistance were respectively performed using an LG Motion Ltd and a CSM tribometer under dry sliding conditions.

The volume fraction of core- and shell-regions is an important parameter in the control of temperature-dependent electrical properties of core–shell-microstructured electroceramics such as BaTiO3. Here, we highlight the potential unreliability of using capacitance ratios, obtained by simulating impedance spectra, to extract accurate volume fractions of the two regions. Two microstructures were simulated using a finite element approach: an approximation to a core–shell structure (the encased model) and a series-layer model (SLM). The impedance response of the microstructures was simulated for a range of input volume fractions. The volume fractions obtained from the simulation agreed with the input values for the SLM microstructure but differed for the encased model. Current density and electric field plots revealed that this discrepancy was caused by differences between the physical and electrical microstructures of the encased model. A stream trace analysis of current density demonstrated that the current follows the path of least resistance through the core, leaving regions of shell with lower current density. These differences are important when attempting to extract volume fractions from encased microstructures with small cores. In the present case, core volume fractions less than 0.7 produce differences in excess of 25%.

Recently, there has been considerable interest in the perovskite phase Na0.5Bi0.5TiO3 (NBT) as a promising lead-free piezoelectric material. Here we report low levels of Na nonstoichiometry (±2 atom % on the A-site) in the nominal starting composition of NBT ceramics can lead to dramatic changes in the magnitude of the bulk (grain) conductivity (σb) and the conduction mechanism(s). Nominal starting compositions with Na excess exhibit high levels of oxide-ion conduction with σb ∼ 2.2 mS cm–1 at 600 °C and an activation energy (Ea) < 1 eV whereas those with Na deficiency are dielectrics based on intrinsic electronic conduction across the band gap with σb ∼ 1.6 μS cm–1 at 600 °C and Ea ∼ 1.7 eV. Drying of reagents, especially Na2CO3, changes the starting stoichiometry slightly due to a small amount of adsorbed moisture in the raw materials but influences significantly the electrical properties. This demonstrates the bulk electrical properties of NBT to be highly sensitive to low levels of A-site nonstoichiom...