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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.

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.

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.

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 formation of the internal barrier layer capacitor (IBLC) structure in CaCu3Ti4O12 (CCTO) ceramics was found to be facilitated by the ceramic heat treatment. Electrically insulating grain boundary (GB) and semi-conducting grain interior areas were characterized by impedance spectroscopy to monitor the evolution of the IBLC structure with increasing sintering temperature TS (975–1100 °C). The intrinsic bulk and GB permittivity increased by factors of ≈2 and 300, respectively and the bulk resistivity decreased by a factor of ≈103. These trends were accompanied by increased Cu segregation from the CCTO ceramics as detected by scanning electron microscopy and quantitative energy dispersive analysis of X-rays. The chemical changes due to possible Cu-loss in CCTO ceramics with increasing TS are small and beyond the detection limits of X-ray absorption spectroscopy near Cu and Ti K-edges and Raman Spectroscopy.

Undoped-BaTiO3 powder prepared from the mixed oxide route (0 h sample) has been attrition milled using yttria-stabilized ZrO2 milling media for 0.5–3 h prior to formation of dense ceramics. Both isovalent (Zr4+ → Ti4+) and aliovalent (Y3+ → Ba2+) dopant contamination effects have been observed. Systematic changes occur in the lattice parameters, polymorphic phase transition temperatures and dielectric properties below the Curie temperature for 0–2 h attrition milled powders and these are attributed primarily to an ion-size effect associated with Zr-doping on the Ti-site due to milling media contamination. The bulk conductivity of attrition milled samples at high temperature decreases by ∼2–3 orders of magnitude compared to 0 h samples and the activation energy associated with bulk conduction increases from ∼1 eV for 0 and 0.5 h samples to 1.4 eV for 3 h samples. The change in bulk conductivity and conduction mechanism for the attrition milled samples is attributed to aliovalent Y3+ dopant contamination from the milling media.

La0.4Ba0.6Ti0.6RE0.4O3 (where RE = Y, Yb) powders were synthesised by the mixed oxide route. According to laboratory X-ray diffractometry both compositions are single-phase and can be indexed on the Pm (3) over barm. space group; however, electron diffraction suggests the occurrence of anti-phase octahedral tilting or 1: 1 ordering. Ceramics fired at 1600 degrees C for 4 h exhibit an average grain size of similar to 20 mu m. Bright field TEM images suggest the occurrence of chemical inhomogeneity in both compositions. The dc conductivities and Raman spectra are similar for both compositions; however, La0.4Ba0.6Ti0.6Y0.4O3 ceramics exhibit epsilon(r) similar to 57, quality factor (Qf) similar to 750 GHz and temperature coefficient of resonance (TCF) similar to 12 ppm/K, whereas La0.4Ba0.6Ti0.6Yb0.4O3 ceramics exhibit epsilon(r) similar to 65, Qf similar to 4500 GHz and TCF similar to 1 ppm/K.

A review is given of the origins of high permittivity in two groups of materials, La-doped BaTiO3 and a new barrier layer capacitor material, CaCu3Ti4O12. Factors that influence permittivity include: dopant, doping mechanism, processing conditions and grain size. La-doped BaTiO3 has high permittivity due to its ferroelectric nature at low temperatures and a novel doping mechanism: A-site substitution linked to the creation of B-site vacancies for charge compensation. Permittivities of 25,000 have been achieved, which can be increased further to ∼36,000 by additional doping with Zr. The value of impedance spectroscopy to characterize materials that have heterogeneous electrical microstructures is illustrated with the example of CaCu3Ti4O12; the high permittivity is not a bulk effect, as widely stated in the literature, but is a thin layer effect typical of a barrier layer capacitor. By attention to processing conditions to achieve large grain sizes, effective permittivities as high as 300,000 have been obtained.

Hexagonal 6H-BaTiO 3 has been stabilised at room temperature according to the formula Ba(Ti 1−x M x )O 3-δ where M=Mg, Al, Cr, Mn, Fe, Co, Zn, Ga, Ni and In. Solid solution ranges are reported for samples prepared by the mixed oxide route at 1400 °C. Rietveld refinement of Neutron Diffraction data for BaTi 0.92 Ga 0.08 O 2.96 in the space group P6 3 /mmc shows the presence of oxygen vacancies in the hexagonally close packed layers between face sharing Ti 2 O 9 dimers. All materials are dielectric insulators at room temperature with permittivity values between ∼50–80. Several compositions resonate at microwave frequencies with quality factors (Q.f) between ∼1300–7700 GHz.

Impedance spectroscopy is used to estimate the bulk conductivity, σ b , of hexagonal (6H)-Ba(Ti 0.94 Ga 0.06 )O 2.97 ceramics in air, N 2 and 5%H 2 /95%N 2 between 400 and 1000 °C. Isothermal plots of log σ b vs log pO 2 in the temperature range between 700 and 1000 °C show the presence of a p–n transition with slopes of ∼−1/4 and +1/4 in the n- and p-type regions indicating that the conductivity obeys the ‘extrinsic’ model proposed by Smyth and co-workers for undoped and acceptor-doped cubic BaTiO 3 -based materials [ J. Am. Ceram. Soc . 64 (1981) 556; J. Am. Ceram. Soc . 65 (1982) 167]. The activation energy E a for oxidation in the p-type region to produce free holes is similar for 6H-Ba(Ti 0.94 Ga 0.06 )O 2.97 and cubic BaTiO 3 -based ceramics with an estimated value from σ b data of ∼0.8 eV. The band gap for 6H-Ba(Ti 0.94 Ga 0.06 )O 2.97 ceramics is ∼3.2 eV.

Impedance spectroscopy is used to deconvolute the dc conductivity ( σ ) of undoped BaTiO 3 ceramics (∼95% of the theoretical X-ray density) into bulk ( σ b ) and grain boundary ( σ gb ) components at two oxygen partial pressures, P O 2 ∼10 Pa (N 2 ) and ∼0.21 MPa (air). At 900°C, σ ∼ σ b in both atmospheres, however, at lower temperatures Z ∗ plots are dominated by the grain boundary component and σ ∼ σ gb . The temperature of the switch from σ ∼ σ b to σ ∼ σ gb is different in the two atmospheres and occurs at ∼850°C in air and ∼650°C in N 2 . Isothermal plots of log σ b vs log P O 2 in the temperature range 450–900°C show the expected oxygen partial pressure dependence with a gradient of +1/4. In contrast, σ gb is relatively insensitive to P O 2 and log σ gb vs log P O 2 plots have gradients σ vs log pO 2 plots have gradients σ is dominated by the grain boundary component. This may explain the wide range of gradients (∼1/4–1/9) reported in the literature for isothermal dc conductivity measurements on polycrystalline BaTiO 3 in the p-type regime.

The oxide-ion conducting phases La 0·95 Sr 0·05 GaO 3−δ (LSG), La 0·95 Sr 0·05 AlO 3−δ (LSA) and Y 0·95 Sr 0·05 AlO 3−δ (YSA) have been synthesised, either by solid state reaction or through a chemical route. Structural and microstructural characterisation was carried out using XRD and SEM/EDS techniques and a.c. impedance spectroscopy (250–1000°C) was employed to determine the electrical properties. Although low-temperature impedance results were strongly dependent on phase purity and microstructure, high temperature electrical conductivity data could be used to compare the electrical conductivities of LSG, LSA and YSA. Chemical modification from the well-known Sr-doped lanthanum gallate via replacement of different cations on the A and/or B site decreased the conductivity, increased the activation energies and decreased the ionic conductivities. From the results, there is no simple correlation between oxygen ion conductivity and geometric aspects related to unit cell parameters and cation radii. Instead, cation polarisability seems to play an extremely important role in designing improved oxygen ion conductors based on the perovskite structure.