Synthesis and characterization of a pyrochlore solid solution in the Na2O‐Bi2O3‐TiO2 system

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

Design of a bilayer ceramic capacitor with low temperature coefficient of capacitance.

We show how a simple bilayer system that combines a layer of undoped BaTiO3 (BT) with a second layer of Ba0.975Na0.025Ti0.975Nb0.025O3 (2.5NNBT) can be used to improve the temperature coefficient of capacitance (TCC) of BaTiO3-based materials for capacitor applications. The bilayer system emulates the volume ratio between a conventional core and shell phase microstructure allowing a simple resource efficient approach to optimise the system for low TCC. Optimisation was achieved with a volume ratio of 0.67 2.5NNBT with 0.33 BT and results in a TCC of ±6% over the temperature range ∼25 to 125 °C whilst maintaining a permittivity of εr ∼ 3000 and low dielectric loss

Finite element modeling of resistive surface layers by micro‐contact impedance spectroscopy

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 analysis of impedance spectra for core–shell microstructures : why a multiformalism approach is essential

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 is confirmed by multiformalism impedance analysis, which reveals features in the data that are not apparent using a single formalism. Finally, the conductivity and permittivity values for both the shell and core are extracted from the simulated spectra using all formalisms and compared to the original input values. The accuracy of the extracted values often depends on the impedance formalism used. It is concluded that impedance spectroscopy data must be analyzed using multiple formalisms when considering core–shell microstructures

Structural, dielectric, and conduction behaviour of A-site deficient SrxNa1−2xNbO3 ceramics

The crystallographic and electrical properties of A-site deficient SrxNa1−2xNbO3 (x = 0.00, 0.05, 0.10, 0.15, 0.20, and 0.25) perovskites have been studied by X-ray diffraction, scanning electron microscopy, dielectric spectroscopy, impedance spectroscopy and polarisation against electric field loops. Despite complexities in the microstructural evolution due to the formation of a NaNb3O8 liquid phase, the crystallographic and dielectric properties show much clearer trends. The introduction of A-site vacancies shows an almost linear expansion of the pseudo-cubic unit cell volume of ∼0.05 Å3 per at%, stabilising the ferroelectric P21ma Q-phase. Compositions x > 0.10 show increasingly relaxor-like behaviour in the dielectric properties and impedance spectroscopy shows increasing conductivity and tan δ values associated with predominantly electronic conduction from x = 0.05 to 0.25. Measurements in N2 at 600 °C show progressively n-type behaviour with increasing levels of A-site vacancies due to the increased tendency to lose under-bonded O2− ions and therefore partially reduce some Nb5+ to Nb4+ ions. Although it is not possible to fully establish the electrical properties of our undoped NaNbO3 ceramics, we propose they contain mixed ionic–electronic behaviour with the clear presence of a weak n-type dependent grain boundary contribution to the total conductivity

Controlling mixed conductivity in Na 1/2 Bi 1/2 TiO 3 using A-site non-stoichiometry and Nb-donor doping

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

Origin of improved tunability and loss in N2 annealed barium strontium titanate films

Barium strontium titanate (BSTO) thin films were deposited on Pt(111) by high throughput evaporative physical vapor deposition and then annealed at 650 °C for 30 min under N2 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 Ti3+ 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 N2 concentration during annealing

Using metadynamics to obtain the free energy landscape for cation diffusion in functional ceramics : dopant distribution control in rare earth-doped BaTiO3

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)

Alternative explanation for the relaxor ferroelectric behavior in FeTiNbO6 rutile ceramics: The influence of electrode contacts

The radio-frequency permittivity-temperature profiles of acceptor-donor codoped TiO2-based rutiles commonly produce higher than expected values (>103 at 300 K) often with conflicting interpretations. A combination of dielectric spectroscopy (DS) and impedance spectroscopy (IS) with different electrode materials is used to reinvestigate the electrical properties of Fe3+-Nb5+-doped rutile in the form of FeTiNbO6 ceramics that show permittivity-temperature characteristics that are consistent with relaxor ferroelectrics (RFE). IS results reveal semiconducting grains with an activation energy of ∼0.16eV, and relative permittivity of similar magnitude and temperature dependence to undoped TiO2(<250). Reducing the work function of the electrode material by replacing Au with InGa has a dramatic effect on the IS and DS data. We propose the apparent RFE behavior observed by DS and previously attributed to the formation of nanoclustering of the cations is an extrinsic effect primarily associated with the development of Schottky barriers between the semiconducting ceramics and Au contacts

Equivalent circuit parameter extraction of low-capacitance high-damping PTs

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