Michael J. Reece

THE CONTRIBUTION OF ELECTRICAL CONDUCTIVITY, DIELECTRIC PERMITTIVITY AND DOMAIN SWITCHING IN FERROELECTRIC HYSTERESIS LOOPS

Triangular voltage waveform was employed to distinguish the contributions of dielectric permittivity, electric conductivity and domain switching in current-electric field curves. At the same time, it is shown how those contributions can affect the shape of the electric displacement — electric field loops (D–E loops). The effects of frequency, temperature and microstructure (point defects, grain size and texture) on the ferroelectric properties of several ferroelectric compositions is reported, including: BaTiO3; lead zirconate titanate (PZT); lead-free Na0.5K0.5NbO3; perovskite-like layer structured A2B2O7 with super high Curie point (Tc); Aurivillius phase ferroelectric Bi3.15Nd0.5Ti3O12; and multiferroic Bi0.89La0.05Tb0.06FeO3. This systematic study provides an instructive outline in the measurement of ferroelectric properties and the analysis and interpretation of experimental data.

Review of graphene–ceramic matrix composites

Abstract Graphene has remarkable mechanical properties, which makes it potentially a good reinforcement in ceramic composites. It also has unique electrical and thermal properties, which makes it an attractive filler for producing multifunctional ceramics for a wide range of applications. In the past few years, relatively little attention has been focused on graphene ceramic matrix composites (GCMC) in comparison to polymer composites. This review gives a comprehensive overview on the state of the art of GCMC, including materials synthesis, densification and characterisation. The published literature allows us to define the critical steps for processing GCMC, and identify its influence on the multifunctional and mechanical properties of the composites. Finally, the potential future applications and current research trends in GCMC are presented.

Thermal depoling of high Curie point Aurivillius phase ferroelectric ceramics

The thermal depoling behavior of several different Aurivillius phase ferroelectric ceramics has been studied. This includes two-layer (CaBi2Nb2O9,Ca0.9Ba0.1Bi2Nb2O9,Bi3NbTiO9,Bi3Nb1.2Ti0.8O9), three-layer (Bi4Ti3O12), and four-layer [CaBi4Ti4O15,Ca0.94(Na,Ce)0.03Bi4Ti4O15] compounds. All of them have a high Curie point (Tc⩾675°C). The orthorhombic structured materials show good resistance to thermal depoling up to temperatures close to their Curie points. However, Bi4Ti3O12, which has a monoclinic structure, shows a significant reduction in d33 well before its Curie point. The monoclinic distortion produces more non-180° ferroelectric domain structures, and it is the thermal instability of these that accounts for their thermal depoling behaviour. Excess Nb doping of Bi3NbTiO9 produces a significant reduction in its resistance to thermal depoling, suggesting that the doping produces a lowering of the crystallographic symmetry.

CYCLIC FATIGUE OF CERAMICS

The fatigue behaviour of alumina, zirconia-toughened alumina (ZTA) and tetragonal zirconia (TZP) have been investigated using three different techniques. Direct push-pull testing has been used to generate both static and cyclic fatigue data. The results clearly show that all the materials studied are susceptible to both static and cyclic fatigue, and that the times to failure under cyclic loading are considerably shorter than under static loads. The fatigue failure origins have been identified and the influence of surface condition on fatigue life has been assessed. The slow propagation of cracks subject to cyclic tensile and compressive loads has been studied using compact tension specimens and tapered double cantilever beam specimens. These investigations have confirmed the existence of cyclic fatigue effects in coarse-grained alumina and have shown the crack increment per cycle (da/dN) to have a power-law dependence on the peak stress intensity factor. A technique, based on repeated indentation, has been used to investigate the propagation of sub-surface cracks subjected to cyclic loading in both fine-grained alumina and ZTA. The results of the investigation suggest that compressive or closure loads on the crack faces are factors which affect the cyclic fatigue crack growth in ceramics. Based on those observations, an explanation is proposed for the mechanical cyclic fatigue effects in the ceramics investigated.

Physical, Mechanical, and Structural Properties of Highly Efficient Nanostructured n- and p-Silicides for Practical Thermoelectric Applications

Cost-effective highly efficient nanostructured n-type Mg2Si1−xSnx and p-type higher manganese silicide (HMS) compositions were prepared for the development of practical waste heat generators for automotive and marine thermoelectric applications, in the frame of the European Commission (EC)-funded PowerDriver project. The physical, mechanical, and structural properties were fully characterized as part of a database-generation exercise required for the thermoelectric converter design. A combination of high maximal ZT values of ∼0.6 and ∼1.1 for the HMS and Mg2Si1−xSnx compositions, respectively, and adequate mechanical properties was obtained.

Dielectric relaxation, lattice dynamics and polarization mechanisms in Bi0.5Na0.5TiO3-based lead-free ceramics

In 0.95[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-0.05CaTiO3 ceramics, the temperature TS (dielectric permittivity shoulder at about 125 °C) represents a transition between two different thermally activated dielectric relaxation processes. Below TS, the approximately linear decrease of the permittivity with the logarithm of frequency was attributed to the presence of a dominant ferroelectric phase. Above TS, the permittivity shows a more complicated dependence of the frequency and Raman modes indicate a sudden increase in the spatial disorder of the material, which is ascribed to the presence of a nonpolar phase and to a loss of interaction between polar regions. From 30 to 150 °C, an increase in the maximum polarization with increasing temperature was related to three possible mechanisms: polarization extension favoured by the simultaneous presence of polar and non-polar phases; the occurrence of electric field-induced transitions from weakly polar relaxor to ferroelectric polar phase; and the enhanced polarizabili...

Review of flash sintering: materials, mechanisms and modelling

ABSTRACT Flash sintering (FS) is an energy efficient sintering technique involving electrical Joule heating, which allows very rapid densification (<60 s) of particulate materials. Since the first publication on flash-sintered zirconia (3YSZ) in 2010, it has been intensively researched and applied to a wide range of materials. Going back more than a century ago, we have found a close similarity between FS of oxides and Nernst glowers developed in 1897. This review provides a comprehensive overview of FS and is based on a literature survey consisting of 88 papers and seven patents. It correlates processing parameters (i.e. electric field magnitude, current density, waveforms (AC, DC) and frequency, furnace temperature, electrode materials/configuration, externally applied pressure and sintering atmosphere) with microstructures and densification mechanisms. Theorised mechanisms driving the rapid densification are substantiated by modelling work, advanced in situ analysis techniques and by established theories applied to electric current assisted/activated sintering techniques. The possibility of applying FS to a wider range of materials and its implementation in industrial scale processes are discussed. GRAPHICAL ABSTRACT Abbreviations: ECAS: Electric Current Assisted/Assisted Sintering; FS: flash sintering; SPS: spark plasma sintering; FSPS: flash spark plasma sintering; HIP: hot isostatic press; HP: hot press; TF: furnace temperature; TOnset: onset temperature; E: electrical field; TS: sample temperature; TSoft: softening temperature for glasses; Tm: melting temperature; PTC or NTC: positive or negative temperature coefficient of electrical resistance; IS: impedance spectroscopy; OES: optical emission spectroscopy; AES: atomic emission spectroscopy; DBS: Dog-bone shape, L: length, W: width, T: thickness, D: diameter, H: height, R: rectangular, CS: cross-section; P: particle size; C: crystallite size; 3YSZ: 3 mol-% yttria-stabilised zirconia; I: ion, H: hole, V: vacancy, E: electron, P: proton

Structural and chemical stability of multiwall carbon nanotubes in sintered ceramic nanocomposite

Abstract Abstract The structural and chemical stability of multiwall carbon nanotubes (MWNTs) in ceramic nanocomposites prepared by spark plasma sintering was studied. High resolution electron microscopy, X-ray diffraction and Raman spectroscopy were used to evaluate any degradation of the MWNTs. They were found to be well preserved in alumina after sintering up to 1900°C/100 MPa/3 min. In boron carbide, structural degradation of MWNTs started from ∼1600°C when sintered for 20 min. Multiwall carbon nanotubes maintained their high aspect ratio and fibrous nature even after being sintered in boron carbide at 2000°C for 20 min. However, no Raman vibrations of MWNTs were observed for nanocomposites processed at temperatures <2000°C, which indicates that they were severely degraded. Structural preservation of MWNTs in ceramic nanocomposites depends on the ceramic matrix, sintering temperature and dwell time. Multiwall carbon nanotubes were not preserved for matrices that require high sintering temperatures (>1600°C) and longer processing times (>13 min).

Reversibility in electric field-induced transitions and energy storage properties of bismuth-based perovskite ceramics

The effects of temperature and electric field-induced structural modifications on the energy storage properties of 0.95[0.94Bi0.5Na0.5TiO3–0.06BaTiO3]–0.05K0.5Na0.5NbO3 (BNT–BT–5KNN) ceramics were investigated. X-ray diffraction performed on unpoled and poled ceramics in the temperature range 25–500 °C suggested an increment in the rhombohedral phase intensity peaks and in the tetragonal distortion after electrical poling. The rhombohedral phase content reduced with increasing temperature in both unpoled and poled ceramics. In the unpoled ceramic, the rhombohedral phase eventually disappeared, while it survived in the poled specimen up to 500 °C. The stabilization of the rhombohedral ferroelectric phase by dc poling produced remarkable differences in the temperature dependence of permittivity, loss, current–polarization–electric field loops and energy density. As a consequence of a reversible transition induced by an alternating electric field, competitive energy densities (0.39–0.51 J cm−3 in the range 25–175 °C) with those of lead-based and lead-free bulk ceramics recently developed was obtained, indicating bismuth-based perovskites as potential lead-free systems for energy storage applications.

Thermal activation of ferroelectric switching

By applying the theory of thermally activated nucleation to the switching of ferroelectric domains, a method is developed to experimentally obtain the value of both the activation enthalpy, ΔH, and activation volume, V*, for the thermally activated process involved in ferroelectric switching. The method was applied to the switching of a soft lead zirconate titanate and values of ΔH=(0.16±0.02) eV and V*=(1.62±0.16)×10−25 m3 were obtained at the coercive field. These values imply that the energy, ΔU, required for the formation of switching nuclei is mainly supplied by the work done by the electric field. A comparison of these values with those obtained from theoretical considerations suggests that the switching is achieved by the sideways expansion of nuclei formed at the domain boundaries in the form of low amplitude and long wavelength fluctuations of the domain walls.