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Papers by Derek Sinclair

Advanced Engineering Materials, Sep 13, 2022

field is generated across the sensor (output). PTs have several advantages over their magnetic co... more field is generated across the sensor (output). PTs have several advantages over their magnetic counterparts including high power density and high efficiency. They are also free from stray electromagnetic fields and related emission and sensitivity problems. [4-6] PTs can also adopt various structural shapes, operating modes, voltage ratios, and operating frequencies, allowing them to fulfill a range of design requirements. Wide bandgap semiconductor materials and devices (i.e., SiC and GaN) are now achieving commercial success since their higher operating frequency and lower losses translate to a reduction in passive component size and thereby an increase in power density. [7] Wide bandgap devices can also operate at higher temperatures than Si-based devices and so engineers are investigating the opportunities this makes available, but are restricted due to the lack of high temperature capable passive components. [8,9] There are several potential applications of high-temperature power converters such as downhole (oil and geothermal wells) for power logging and monitoring tools at temperatures of up to 250 and 350°C, respectively. In electric and conventional vehicles, they can be used for powering the motors and sensors at up to 300°C. In the aerospace industry, they can be used for the control circuitry, motor and braking controls, and for simplifying the electrical design at temperatures of up to 350°C. [10,11] Pb(Zr 1Àx Ti x)O 3 (PZT)-based ceramics have been the most successful piezoelectrics since their development in the 1950s. [1,12-14] The most common PZT ceramics lie near the morphotropic phase boundary (MPB), between tetragonal (PbTiO 3) and rhombohedral (PbZrO 3) phases, where PZT exhibits anomalously high dielectric and piezoelectric properties. These exceptional properties have driven a wide range of applications such that PZT is ubiquitous throughout modern life. However, depending on the application, PZT-based bulk ceramics are limited in their operating temperature to %50-100°C below their Curie temperature (T C ¼ 386°C for undoped MPB PZT), which means a realistic upper temperature of operation for PZT-based devices is around 200°C. [15] In 2001, Eitel et al. reported a ceramic solid solution system (1Àx)BiScO 3 ÀxPbTiO 3 (BSPT) with an MPB around x ¼ 0.64 between the tetragonal (PbTiO 3) and rhombohedral (BiScO 3) phase regions. [16-19] Compared to PZT, BSPT ceramics have a lower concentration of PbO, a comparable piezoelectric constant,

Nanoscale, 2011

Scanning Probe Microscopy with conductive tips has been used to image and study the dielectric pr... more Scanning Probe Microscopy with conductive tips has been used to image and study the dielectric properties of giant permittivity CaCu 3 Ti 4 O 12 ceramics at the nanoscale. Since measurements are generally carried out on sections of a sample, particular attention has been devoted to possible artefacts due to surface imperfections, such as substantial surface roughness and/or contamination that can result in controversial interpretation, particularly at nanometric spatial dimensions. A reliable surface investigation has been carried out after the definition of both the physical and geometrical unbiased criteria to avoid any artefacts due to surface roughness and/or anomalous tip-sample contact variations. The presence of insulating grain boundaries and the measurement of a depletion layer at the grain-grain boundary interfaces unambiguously demonstrate the relevance of the Internal Barrier Layer Capacitor effect, among all the proposed physical mechanisms, to explain the giant dielectric behaviour. Such imaging provided a clear correlation between the macroscopic dielectric properties and the nanometric structure at the interfaces. Moreover, the ''general criteria'' for reliable nanoelectrical characterization as well as the related measurement resolution have been defined.

Journal of the American Ceramic Society, Oct 24, 2015

This is a repository copy of The Influence of La Doping and Heterogeneity on the Thermoelectric P... more This is a repository copy of The Influence of La Doping and Heterogeneity on the Thermoelectric Properties of Sr3Ti2O7 Ceramics.

Advanced Functional Materials, Jun 4, 2020

This is a repository copy of Displacive order-disorder behavior and intrinsic clustering of latti... more This is a repository copy of Displacive order-disorder behavior and intrinsic clustering of lattice distortions in bi-substituted NaNbO3.

Journal of materials chemistry. A, Materials for energy and sustainability, 2016

An in-house finite element modelling package is used to simulate the electrical response of core-... more An in-house finite element modelling package is used to simulate the electrical response of core-shell microstructures of BaTiO 3-based (BT) dielectric materials as a design strategy for multilayer ceramic capacitors. By combining this method with experimental material properties, both the temperature coefficient of capacitance (TCC) and the volume ratio of undoped BT ceramic (core phase) to chemically doped BT ceramic (shell phase) to optimise temperature stability can be predicted. This is a faster, more reliable and efficient design strategy than current techniques which are based on iterative experimental protocols. This methodology is illustrated using rare earth-free NaNbO 3-doped BT (NNBT) ceramics as an example for which we predict and experimentally confirm an optimised volume fraction ratio of 0.66 undoped BT core to 0.34 of a 2.5% NNBT shell.

Journal of the American Ceramic Society, Jun 5, 2018

The relationship between the piezoelectric properties and the structure/microstructure for 0.05Bi... more 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<x<0.70, characterised by the coexistence of phases. The temperature dependence of relative permittivity, polarisation-electric field hysteresis loops, bipolar strain-electric field curves revealed that BBFT transformed from relaxor-like to ferroelectric behaviour with an increase in x, consistent with changes in the phase assemblage and domain structure. The largest strain was 0.41 % for x = 0.63 at 10kV/mm. The largest effective piezoelectric coefficient (d33 *) was 544 pm/V for x = 0.63 at 5 kV/mm but the largest Berlincourt d33 (148 pC/N) was obtained for x = 0.70. We propose that d33 * is optimised at the point of crossover from relaxor to ferroelectric which facilitates a macroscopic field induced transition to a ferroelectric state but that d33 is optimised in the ferroelectric, rhombohedral phase. Unipolar strain was measured as a function of temperature for x = 0.63 with strains of 0.30% achieved at 175 o C, accompanied by a significant decrease in hysteresis with respect to room temperature measurements. The potential for BBFT compositions to be used as high strain actuators is demonstrated by the fabrication of a prototype multilayer which achieved 3 m displacement at 150 o C.

Journal of materials chemistry. A, Materials for energy and sustainability, 2022

Bulk conductivity (σb) values of nominally stoichiometric Na0.5Bi0.5TiO3 (NBT) prepared by solid-... more Bulk conductivity (σb) values of nominally stoichiometric Na0.5Bi0.5TiO3 (NBT) prepared by solid-state reaction collated from literature show random variation between 10-6-10-3 S cm-1 (at 600 °C). This makes it challenging to obtain reliable and reproducible performances of NBT-based devices, especially as the underlying reason(s) for this variance are not fully understood. Here we report the dramatic impact of the TiO2 reagent, in particular, the polymorphic form of TiO2 on the electrical conductivity and conduction mechanism of NBT. Based on our solid-state processing route, NBT ceramics prepared by rutile TiO2 are ionically conductive, and those prepared by anatase TiO2 are insulating. The dramatic difference in electrical properties of NBT prepared using rutile and anatase TiO2 is related to the NBT formation process: the intermediate phase Bi12TiO20 is more stable during formation of NBT in the case of anatase TiO2, which reduces the volatility of Bi2O3 during solidstate reaction. These results give plausible explanations for the large variation of σb reported in the literature and highlight the importance of selecting an appropriate TiO2 reagent when targeting controllable σb in NBT-based ceramics. For ionconducting applications (such as in intermediate-temperature solid oxide fuel cells, IT-SOFCs), rutile TiO2 should be used, and for dielectric applications (such as in multilayer ceramic capacitors, MLCC) anatase TiO2 should be used.

Materials Today: Proceedings, 2017

The thermoelectric properties of Sr1-xLax/2Smx/2TiO3-(0.05 ≤ x ≤ 0.30) ceramics have been investi... more The thermoelectric properties of Sr1-xLax/2Smx/2TiO3-(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 () of x ≤ 0.15 displayed metallic behaviour with < 1000 S/cm, whereas x ≥ 0.20 were semiconducting with < 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 W/K 2 .m (at 573 K) and overall x = 0.15 had the highest dimensionless figure of merit (ZT) of 0.24 (at 875 K).

RSC Advances, 2013

To date, all existing literature on the so called 'high permittivity' perovskite oxide CaCu 3 Ti ... more To date, all existing literature on the so called 'high permittivity' perovskite oxide CaCu 3 Ti 4 O 12 (CCTO) in the form of ceramics, single crystals and thin films show the grains (bulk) to exhibit semiconductivity with room temperature, RT, resistivity of y10 100 V cm. Here we show that CCTO grains can be highly resistive with RT resistivity >1 GV cm when CCTO ceramics are processed at lower temperature (700 uC). With increasing processing temperature, the semiconducting CCTO phase commonly reported in the literature emerges from grain cores and grows at the expense of the insulating phase. For sintering temperatures of y1000 1100 uC, the grains are dominated by the semiconducting phase and the insulating phase exists only as a thin layer grain shell/grain boundary region. This electrical microstructure results in the formation of the so called Internal Barrier Layer Capacitance (IBLC) or Maxwell Wagner mechanism that produces the commonly reported high effective permittivity at radio frequencies in dense ceramics. The relationship between Cu loss at elevated processing temperatures and the transformation of the grain resistivity from an insulating to semiconducting state with increasing processing temperature is also discussed.

Journal of The European Ceramic Society, Sep 1, 2018

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

Journal of Applied Physics, May 1, 2012

Density functional theory (DFT) calculations have been used to study the nature of intrinsic defe... more Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both monoand di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of vacancies in the system are presented for numerous temperatures. Comparisons are drawn with the cubic polymorph as well as with previous potential-based simulations and experimental results.

Chemistry of Materials, Dec 22, 2016

The development of oxides with high figure of merit, ZT, at modest temperatures (∼300−500°C) is d... more The development of oxides with high figure of merit, ZT, at modest temperatures (∼300−500°C) is desirable for ceramic-based thermoelectric generator technology. Although ZT is a compound metric with contributions from thermal conductivity (κ), Seebeck coefficient (S), and electrical conductivity (σ), it has been empirically demonstrated that the key to developing thermoelectric n-type oxides is to optimize σ of the ceramic to ∼1000 S/cm at the operating temperature. Titanate-based perovskites are a popular choice for the development of n-type oxide ceramics; however, the levels of σ required cannot be achieved without control of the ceramic quality, significant reduction of the ceramic in low P(O 2) atmosphere (e.g., N 2 /5%H 2), and the use of specific dopants and dopant mechanisms, which allow the egress of oxygen homogeneously from the lattice. Here, we discuss the processing protocols to fabricate reliable, reproducible ceramic oxides and schemes for inducing high levels of σ, thereby optimizing the power factor (PF = σS 2) and ZT. The problems associated with measuring κ, σ, and S to achieve reproducible and accurate values of ZT are discussed, as are future directions which should enable further optimization. Finally, we comment on how these protocols may be applied to other systems and structures.

Chemistry of Materials, Oct 29, 2014

Classical computer simulations are performed on the whole solids solution range of Ba 1−x Ca x Ti... more Classical computer simulations are performed on the whole solids solution range of Ba 1−x Ca x TiO 3 (BCT) and Ba 1−x Sr x TiO 3 (BST). The enthalpies and volumes of mixing are produced and a full local structural analysis is performed. The simulations demonstrate that large degrees of disorder form in the BCT solid solution which leads to distortions in the TiO 6 octahedra. Comparing the positions of Sr in BST and Ca in BCT, the position of the Sr cation is largely central within the dodecahedra while the position of the Ca is significantly off-centre in many configurations. The relaxation is associated with a shift towards an eight coordinate site compared to a twelve coordinate cation. An empirical model is fitted for predicting the Curie Temperature of the solid solution based on the local structure which shows excellent agreement with experimental values.

Journal of materials chemistry. A, Materials for energy and sustainability, 2019

Acta Materialia, Apr 1, 2021

We demonstrate a fast, efficient combinatorial method for the optimisation of materials for multi... more 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(U, T, S and R) 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.

Acta Materialia, Apr 1, 2021

We present an aberration corrected scanning transmission electron microscopy (ac-STEM) analysis o... more We present an aberration corrected scanning transmission electron microscopy (ac-STEM) analysis of perovskite (LaFeO 3) and pyrochlore (Yb 2 Ti 2 O 7 and Pr 2 Zr 2 O 7) 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 invisibleor difficult to observe due to limited sensitivityin x-ray and neutron diffraction measurements, are the origin of the complex magnetic ground state of pyrochlore oxides. In addition, we present a method by which light atoms can be resolved in the quantitative ADF-STEM images. Using this method, we resolved oxygen atoms in perovskite and pyrochlore oxides and propose this method to be suitable for other materials containing both light and heavy elements.

Advanced Functional Materials, Jul 23, 2019

The impedance response of a core-shell microstructure with 80% core volume fraction has been simu... more The impedance response of a core-shell microstructure with 80% core volume fraction has been simulated using finite element modeling and compared to two equivalent circuits for a wide range of shell permittivity and conductivity values. Different equivalent circuits, corresponding to different variants of the well-known brick layer model, are applicable for different combinations of material properties in the microstructure. When the shell has a similar conductivity or permittivity to the core, adding a parallel pathway increases the accuracy of the fit by ~±10%. When both the conductivity and permittivity values of the core and shell regions are different the series circuit is a better fit. This was confirmed by multiformalism impedance analysis, which revealed features in the data that were not apparent using a single formalism. Finally, the conductivity and permittivity values for both the shell and core were extracted from the simulated spectra using all formalisms and compared to the original input values. The accuracy of the extracted values often depended on the impedance formalism used. We conclude that impedance spectroscopy data must be analyzed using multiple formalisms when considering core-shell microstructures.

Advanced Functional Materials, Mar 6, 2013

The energetics of La-doping in BaTiO3 are reported for both (electronic) donor-doping with the cr... more The energetics of La-doping in BaTiO3 are reported for both (electronic) donor-doping with the creation of Ti 3+ cations and ionic doping with the creation of Ti vacancies. Our experiments (for samples prepared in air) and simulations demonstrate that ionic doping is the preferred mechanism for all concentrations of La-doping. The apparent disagreement with electrical conduction of these ionic doped samples is explained by subsequent oxygen-loss which leads to the creation of Ti 3+ cations. Our simulations show that oxygen-loss is much more favourable in the ionic-doped system than pure BaTiO3 due to the unique local structure created around the defect site. These findings resolve the so-called 'donor-doping' anomaly in BaTiO3 and explain the source of semiconductivity in positive temperature coefficient of resistance (PTCR) BaTiO3 thermistors.

Journal of Materials Chemistry C, 2016

Precise control of electronic and/or ionic conductivity in electroceramics is crucial to achieve ... more 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 Na 1/2 Bi 1/2 TiO 3 (NBT) can be tuned into a novel oxide-ion conductor with an oxide-ion transport number (t ion) 4 0.9 by creating bismuth and oxygen vacancies. A small Bi-excess in the nominal starting composition (Na 0.50 Bi 0.50+x TiO 3+3x/2 , x = 0.01) or Nb-donor doping (Na 0.50 Bi 0.50 Ti 1Ày Nb y O 3+y/2 , 0.005 r y r 0.030) can reduce significantly the electrical conductivity to create dielectric behaviour by filling oxygen vacancies and suppressing oxide ion conduction (t ion r 0.10). Here we show a further increase in the starting Bi-excess content (0.02 r x r 0.10) reintroduces significant levels of oxide-ion conductivity and increases t ion B 0.4-0.6 to create mixed ionic/electronic behaviour. The switch from insulating to mixed conducting behaviour for x 4 0.01 is linked to the presence of Bi-rich secondary phases and we discuss possible explanations for this effect. Mixed conducting behaviour with t ion B 0.5-0.6 can also be achieved with lower levels of Nb-doping (y B 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 t ion 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.

Journal of Materials Chemistry C, 2018

The activation energy associated with bulk electrical conduction in functional materials is an im... more 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 of the characteristic frequency fmax obtained from the large D M-logf spectroscopic plots against T-1 which gives an activation energy Ea(fmax). We report an analysis of Ea 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 logfmax T-1 relationship and Ea(fmax). Results show the logfmax 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 nonlinear logfmax T-1 plots in the vicinity of Tm. In cases where data are obtained either well above or below Tm, linear logfmax 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 should be used. 0 is a pre-exponential factor, kB is the Boltzmann constant and T is the absolute temperature. Impedance data can also be presented in complex electric modulus (M*), Y

Advanced Engineering Materials, Sep 13, 2022

field is generated across the sensor (output). PTs have several advantages over their magnetic co... more field is generated across the sensor (output). PTs have several advantages over their magnetic counterparts including high power density and high efficiency. They are also free from stray electromagnetic fields and related emission and sensitivity problems. [4-6] PTs can also adopt various structural shapes, operating modes, voltage ratios, and operating frequencies, allowing them to fulfill a range of design requirements. Wide bandgap semiconductor materials and devices (i.e., SiC and GaN) are now achieving commercial success since their higher operating frequency and lower losses translate to a reduction in passive component size and thereby an increase in power density. [7] Wide bandgap devices can also operate at higher temperatures than Si-based devices and so engineers are investigating the opportunities this makes available, but are restricted due to the lack of high temperature capable passive components. [8,9] There are several potential applications of high-temperature power converters such as downhole (oil and geothermal wells) for power logging and monitoring tools at temperatures of up to 250 and 350°C, respectively. In electric and conventional vehicles, they can be used for powering the motors and sensors at up to 300°C. In the aerospace industry, they can be used for the control circuitry, motor and braking controls, and for simplifying the electrical design at temperatures of up to 350°C. [10,11] Pb(Zr 1Àx Ti x)O 3 (PZT)-based ceramics have been the most successful piezoelectrics since their development in the 1950s. [1,12-14] The most common PZT ceramics lie near the morphotropic phase boundary (MPB), between tetragonal (PbTiO 3) and rhombohedral (PbZrO 3) phases, where PZT exhibits anomalously high dielectric and piezoelectric properties. These exceptional properties have driven a wide range of applications such that PZT is ubiquitous throughout modern life. However, depending on the application, PZT-based bulk ceramics are limited in their operating temperature to %50-100°C below their Curie temperature (T C ¼ 386°C for undoped MPB PZT), which means a realistic upper temperature of operation for PZT-based devices is around 200°C. [15] In 2001, Eitel et al. reported a ceramic solid solution system (1Àx)BiScO 3 ÀxPbTiO 3 (BSPT) with an MPB around x ¼ 0.64 between the tetragonal (PbTiO 3) and rhombohedral (BiScO 3) phase regions. [16-19] Compared to PZT, BSPT ceramics have a lower concentration of PbO, a comparable piezoelectric constant,

Nanoscale, 2011

Scanning Probe Microscopy with conductive tips has been used to image and study the dielectric pr... more Scanning Probe Microscopy with conductive tips has been used to image and study the dielectric properties of giant permittivity CaCu 3 Ti 4 O 12 ceramics at the nanoscale. Since measurements are generally carried out on sections of a sample, particular attention has been devoted to possible artefacts due to surface imperfections, such as substantial surface roughness and/or contamination that can result in controversial interpretation, particularly at nanometric spatial dimensions. A reliable surface investigation has been carried out after the definition of both the physical and geometrical unbiased criteria to avoid any artefacts due to surface roughness and/or anomalous tip-sample contact variations. The presence of insulating grain boundaries and the measurement of a depletion layer at the grain-grain boundary interfaces unambiguously demonstrate the relevance of the Internal Barrier Layer Capacitor effect, among all the proposed physical mechanisms, to explain the giant dielectric behaviour. Such imaging provided a clear correlation between the macroscopic dielectric properties and the nanometric structure at the interfaces. Moreover, the ''general criteria'' for reliable nanoelectrical characterization as well as the related measurement resolution have been defined.

Journal of the American Ceramic Society, Oct 24, 2015

This is a repository copy of The Influence of La Doping and Heterogeneity on the Thermoelectric P... more This is a repository copy of The Influence of La Doping and Heterogeneity on the Thermoelectric Properties of Sr3Ti2O7 Ceramics.

Advanced Functional Materials, Jun 4, 2020

This is a repository copy of Displacive order-disorder behavior and intrinsic clustering of latti... more This is a repository copy of Displacive order-disorder behavior and intrinsic clustering of lattice distortions in bi-substituted NaNbO3.

Journal of materials chemistry. A, Materials for energy and sustainability, 2016

An in-house finite element modelling package is used to simulate the electrical response of core-... more An in-house finite element modelling package is used to simulate the electrical response of core-shell microstructures of BaTiO 3-based (BT) dielectric materials as a design strategy for multilayer ceramic capacitors. By combining this method with experimental material properties, both the temperature coefficient of capacitance (TCC) and the volume ratio of undoped BT ceramic (core phase) to chemically doped BT ceramic (shell phase) to optimise temperature stability can be predicted. This is a faster, more reliable and efficient design strategy than current techniques which are based on iterative experimental protocols. This methodology is illustrated using rare earth-free NaNbO 3-doped BT (NNBT) ceramics as an example for which we predict and experimentally confirm an optimised volume fraction ratio of 0.66 undoped BT core to 0.34 of a 2.5% NNBT shell.

Journal of the American Ceramic Society, Jun 5, 2018

The relationship between the piezoelectric properties and the structure/microstructure for 0.05Bi... more 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<x<0.70, characterised by the coexistence of phases. The temperature dependence of relative permittivity, polarisation-electric field hysteresis loops, bipolar strain-electric field curves revealed that BBFT transformed from relaxor-like to ferroelectric behaviour with an increase in x, consistent with changes in the phase assemblage and domain structure. The largest strain was 0.41 % for x = 0.63 at 10kV/mm. The largest effective piezoelectric coefficient (d33 *) was 544 pm/V for x = 0.63 at 5 kV/mm but the largest Berlincourt d33 (148 pC/N) was obtained for x = 0.70. We propose that d33 * is optimised at the point of crossover from relaxor to ferroelectric which facilitates a macroscopic field induced transition to a ferroelectric state but that d33 is optimised in the ferroelectric, rhombohedral phase. Unipolar strain was measured as a function of temperature for x = 0.63 with strains of 0.30% achieved at 175 o C, accompanied by a significant decrease in hysteresis with respect to room temperature measurements. The potential for BBFT compositions to be used as high strain actuators is demonstrated by the fabrication of a prototype multilayer which achieved 3 m displacement at 150 o C.

Journal of materials chemistry. A, Materials for energy and sustainability, 2022

Bulk conductivity (σb) values of nominally stoichiometric Na0.5Bi0.5TiO3 (NBT) prepared by solid-... more Bulk conductivity (σb) values of nominally stoichiometric Na0.5Bi0.5TiO3 (NBT) prepared by solid-state reaction collated from literature show random variation between 10-6-10-3 S cm-1 (at 600 °C). This makes it challenging to obtain reliable and reproducible performances of NBT-based devices, especially as the underlying reason(s) for this variance are not fully understood. Here we report the dramatic impact of the TiO2 reagent, in particular, the polymorphic form of TiO2 on the electrical conductivity and conduction mechanism of NBT. Based on our solid-state processing route, NBT ceramics prepared by rutile TiO2 are ionically conductive, and those prepared by anatase TiO2 are insulating. The dramatic difference in electrical properties of NBT prepared using rutile and anatase TiO2 is related to the NBT formation process: the intermediate phase Bi12TiO20 is more stable during formation of NBT in the case of anatase TiO2, which reduces the volatility of Bi2O3 during solidstate reaction. These results give plausible explanations for the large variation of σb reported in the literature and highlight the importance of selecting an appropriate TiO2 reagent when targeting controllable σb in NBT-based ceramics. For ionconducting applications (such as in intermediate-temperature solid oxide fuel cells, IT-SOFCs), rutile TiO2 should be used, and for dielectric applications (such as in multilayer ceramic capacitors, MLCC) anatase TiO2 should be used.

Materials Today: Proceedings, 2017

The thermoelectric properties of Sr1-xLax/2Smx/2TiO3-(0.05 ≤ x ≤ 0.30) ceramics have been investi... more The thermoelectric properties of Sr1-xLax/2Smx/2TiO3-(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 () of x ≤ 0.15 displayed metallic behaviour with < 1000 S/cm, whereas x ≥ 0.20 were semiconducting with < 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 W/K 2 .m (at 573 K) and overall x = 0.15 had the highest dimensionless figure of merit (ZT) of 0.24 (at 875 K).

RSC Advances, 2013

To date, all existing literature on the so called 'high permittivity' perovskite oxide CaCu 3 Ti ... more To date, all existing literature on the so called 'high permittivity' perovskite oxide CaCu 3 Ti 4 O 12 (CCTO) in the form of ceramics, single crystals and thin films show the grains (bulk) to exhibit semiconductivity with room temperature, RT, resistivity of y10 100 V cm. Here we show that CCTO grains can be highly resistive with RT resistivity >1 GV cm when CCTO ceramics are processed at lower temperature (700 uC). With increasing processing temperature, the semiconducting CCTO phase commonly reported in the literature emerges from grain cores and grows at the expense of the insulating phase. For sintering temperatures of y1000 1100 uC, the grains are dominated by the semiconducting phase and the insulating phase exists only as a thin layer grain shell/grain boundary region. This electrical microstructure results in the formation of the so called Internal Barrier Layer Capacitance (IBLC) or Maxwell Wagner mechanism that produces the commonly reported high effective permittivity at radio frequencies in dense ceramics. The relationship between Cu loss at elevated processing temperatures and the transformation of the grain resistivity from an insulating to semiconducting state with increasing processing temperature is also discussed.

Journal of The European Ceramic Society, Sep 1, 2018

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

Journal of Applied Physics, May 1, 2012

Density functional theory (DFT) calculations have been used to study the nature of intrinsic defe... more Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both monoand di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of vacancies in the system are presented for numerous temperatures. Comparisons are drawn with the cubic polymorph as well as with previous potential-based simulations and experimental results.

Chemistry of Materials, Dec 22, 2016

The development of oxides with high figure of merit, ZT, at modest temperatures (∼300−500°C) is d... more The development of oxides with high figure of merit, ZT, at modest temperatures (∼300−500°C) is desirable for ceramic-based thermoelectric generator technology. Although ZT is a compound metric with contributions from thermal conductivity (κ), Seebeck coefficient (S), and electrical conductivity (σ), it has been empirically demonstrated that the key to developing thermoelectric n-type oxides is to optimize σ of the ceramic to ∼1000 S/cm at the operating temperature. Titanate-based perovskites are a popular choice for the development of n-type oxide ceramics; however, the levels of σ required cannot be achieved without control of the ceramic quality, significant reduction of the ceramic in low P(O 2) atmosphere (e.g., N 2 /5%H 2), and the use of specific dopants and dopant mechanisms, which allow the egress of oxygen homogeneously from the lattice. Here, we discuss the processing protocols to fabricate reliable, reproducible ceramic oxides and schemes for inducing high levels of σ, thereby optimizing the power factor (PF = σS 2) and ZT. The problems associated with measuring κ, σ, and S to achieve reproducible and accurate values of ZT are discussed, as are future directions which should enable further optimization. Finally, we comment on how these protocols may be applied to other systems and structures.

Chemistry of Materials, Oct 29, 2014

Classical computer simulations are performed on the whole solids solution range of Ba 1−x Ca x Ti... more Classical computer simulations are performed on the whole solids solution range of Ba 1−x Ca x TiO 3 (BCT) and Ba 1−x Sr x TiO 3 (BST). The enthalpies and volumes of mixing are produced and a full local structural analysis is performed. The simulations demonstrate that large degrees of disorder form in the BCT solid solution which leads to distortions in the TiO 6 octahedra. Comparing the positions of Sr in BST and Ca in BCT, the position of the Sr cation is largely central within the dodecahedra while the position of the Ca is significantly off-centre in many configurations. The relaxation is associated with a shift towards an eight coordinate site compared to a twelve coordinate cation. An empirical model is fitted for predicting the Curie Temperature of the solid solution based on the local structure which shows excellent agreement with experimental values.

Journal of materials chemistry. A, Materials for energy and sustainability, 2019

Acta Materialia, Apr 1, 2021

We demonstrate a fast, efficient combinatorial method for the optimisation of materials for multi... more 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(U, T, S and R) 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.

Acta Materialia, Apr 1, 2021

We present an aberration corrected scanning transmission electron microscopy (ac-STEM) analysis o... more We present an aberration corrected scanning transmission electron microscopy (ac-STEM) analysis of perovskite (LaFeO 3) and pyrochlore (Yb 2 Ti 2 O 7 and Pr 2 Zr 2 O 7) 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 invisibleor difficult to observe due to limited sensitivityin x-ray and neutron diffraction measurements, are the origin of the complex magnetic ground state of pyrochlore oxides. In addition, we present a method by which light atoms can be resolved in the quantitative ADF-STEM images. Using this method, we resolved oxygen atoms in perovskite and pyrochlore oxides and propose this method to be suitable for other materials containing both light and heavy elements.

Advanced Functional Materials, Jul 23, 2019

The impedance response of a core-shell microstructure with 80% core volume fraction has been simu... more The impedance response of a core-shell microstructure with 80% core volume fraction has been simulated using finite element modeling and compared to two equivalent circuits for a wide range of shell permittivity and conductivity values. Different equivalent circuits, corresponding to different variants of the well-known brick layer model, are applicable for different combinations of material properties in the microstructure. When the shell has a similar conductivity or permittivity to the core, adding a parallel pathway increases the accuracy of the fit by ~±10%. When both the conductivity and permittivity values of the core and shell regions are different the series circuit is a better fit. This was confirmed by multiformalism impedance analysis, which revealed features in the data that were not apparent using a single formalism. Finally, the conductivity and permittivity values for both the shell and core were extracted from the simulated spectra using all formalisms and compared to the original input values. The accuracy of the extracted values often depended on the impedance formalism used. We conclude that impedance spectroscopy data must be analyzed using multiple formalisms when considering core-shell microstructures.

Advanced Functional Materials, Mar 6, 2013

The energetics of La-doping in BaTiO3 are reported for both (electronic) donor-doping with the cr... more The energetics of La-doping in BaTiO3 are reported for both (electronic) donor-doping with the creation of Ti 3+ cations and ionic doping with the creation of Ti vacancies. Our experiments (for samples prepared in air) and simulations demonstrate that ionic doping is the preferred mechanism for all concentrations of La-doping. The apparent disagreement with electrical conduction of these ionic doped samples is explained by subsequent oxygen-loss which leads to the creation of Ti 3+ cations. Our simulations show that oxygen-loss is much more favourable in the ionic-doped system than pure BaTiO3 due to the unique local structure created around the defect site. These findings resolve the so-called 'donor-doping' anomaly in BaTiO3 and explain the source of semiconductivity in positive temperature coefficient of resistance (PTCR) BaTiO3 thermistors.

Journal of Materials Chemistry C, 2016

Precise control of electronic and/or ionic conductivity in electroceramics is crucial to achieve ... more 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 Na 1/2 Bi 1/2 TiO 3 (NBT) can be tuned into a novel oxide-ion conductor with an oxide-ion transport number (t ion) 4 0.9 by creating bismuth and oxygen vacancies. A small Bi-excess in the nominal starting composition (Na 0.50 Bi 0.50+x TiO 3+3x/2 , x = 0.01) or Nb-donor doping (Na 0.50 Bi 0.50 Ti 1Ày Nb y O 3+y/2 , 0.005 r y r 0.030) can reduce significantly the electrical conductivity to create dielectric behaviour by filling oxygen vacancies and suppressing oxide ion conduction (t ion r 0.10). Here we show a further increase in the starting Bi-excess content (0.02 r x r 0.10) reintroduces significant levels of oxide-ion conductivity and increases t ion B 0.4-0.6 to create mixed ionic/electronic behaviour. The switch from insulating to mixed conducting behaviour for x 4 0.01 is linked to the presence of Bi-rich secondary phases and we discuss possible explanations for this effect. Mixed conducting behaviour with t ion B 0.5-0.6 can also be achieved with lower levels of Nb-doping (y B 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 t ion 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.

Journal of Materials Chemistry C, 2018

The activation energy associated with bulk electrical conduction in functional materials is an im... more 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 of the characteristic frequency fmax obtained from the large D M-logf spectroscopic plots against T-1 which gives an activation energy Ea(fmax). We report an analysis of Ea 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 logfmax T-1 relationship and Ea(fmax). Results show the logfmax 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 nonlinear logfmax T-1 plots in the vicinity of Tm. In cases where data are obtained either well above or below Tm, linear logfmax 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 should be used. 0 is a pre-exponential factor, kB is the Boltzmann constant and T is the absolute temperature. Impedance data can also be presented in complex electric modulus (M*), Y