Tim P. Comyn

Temperature stability of ([Na0.5K0.5NbO3]0.93–[LiTaO3]0.07) lead-free piezoelectric ceramics

A polymorphic phase transition (PPT) is often engineered into lead-free materials to generate high piezoelectric activity at room temperature, limiting their temperature stability. We report [Na0.5K0.5NbO3]0.93–[LiTaO3]0.07 tetragonal ceramics with favorable properties over a broad temperature range due to a high Curie temperature at 447 °C and PPT at −15 °C involving a transition to a monoclinic rather than low temperature orthorhombic phase. Piezoelectric k31 and d31 coefficients varied from 0.19 to 0.14 and −53 to −33 pC/N, respectively, over the range of −15 to 300 °C. Strain-electric field loops provided strains of ∼0.2% and a high-field d33 of 205 pm/V.

Large remanent polarization in ferroelectric BiFeO3–PbTiO3 thin films on Pt∕Si substrates

Bismuth ferrite–lead titanate thin films in the region of the morphotropic phase boundary, with compositions of (1−x)BiFeO3–xPbTiO3 (0.3<x<0.5), were prepared by pulsed laser deposition on Pt∕Si substrates. X-ray diffraction confirmed the formation of pure perovskite phase at a substrate temperature of 565°C under 75mTorr of oxygen. The films exhibit remanent polarizations with 2Pr up to 100μCcm−2 for a field amplitude of 820kVcm−1 and switchable polarization up to 80μCcm−2.

Leakage mechanisms in bismuth ferrite-lead titanate thin films on Pt∕Si substrates

(1−x)BiFeO3–xPbTiO3 (0.5<x<0.3) thin films have been reported to exhibit high remanent polarizations but device integration is hindered by the presence of leakage currents. An insight into the nature of leakage mechanisms in the films is presented. Films with x=0.4 and 0.5 exhibit lower leakage currents as compared to x=0.3 films. At applied fields above 190kVcm−1, in the region of the coercive field of these films, x=0.4 and 0.5 exhibit a Poole–Frenkel mechanism while films with x=0.3 exhibit a combination of space charge limited current and the Schottky effect.

Electrocaloric enhancement near the morphotropic phase boundary in lead-free NBT-KBT ceramics

The electrocaloric effects (ECEs) of the morphotropic phase boundary (MPB) composition 0.82(Na0.5Bi0.5)TiO3-0.18(K0.5Bi0.5)TiO3 (NBT-18KBT) are studied by direct measurements. The maximum ECE ΔTmax = 0.73 K is measured at 160 °C under 22 kV/cm. This corresponds to an ECE responsivity (ΔT/ΔE) of 0.33 × 10−6 K m/V, which is comparable with the best reported values for lead-free ceramics. A comparison between the direct and indirect ECE measurements shows significant discrepancies. The direct measurement of both positive and negative electrocaloric effect confirms the presence of numerous polar phases near the MPB of NBT-based materials and highlights their potential for solid-state cooling based on high field-induced entropy changes.

Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

Crystalline [Fe(bppSMe)2][BF4]2 (1; bppSMe = 4-(methylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine) undergoes an abrupt spin-crossover (SCO) event at 265±5 K. The crystals also undergo a separate phase transition near 205 K, involving a contraction of the unit-cell a axis to one-third of its original value (high-temperature phase 1; Pbcn, Z = 12; low-temperature phase 2; Pbcn, Z = 4). The SCO-active phase 1 contains two unique molecular environments, one of which appears to undergo SCO more gradually than the other. In contrast, powder samples of 1 retain phase 1 between 140-300 K, although their SCO behaviour is essentially identical to the single crystals. The compounds [Fe(bppBr)2][BF4]2 (2; bppBr = 4-bromo-2,6-di(pyrazol-1-yl)pyridine) and [Fe(bppI)2][BF4]2 (3; bppI = 4-iodo-2,6-di(pyrazol-1-yl)-pyridine) exhibit more gradual SCO near room temperature, and adopt phase 2 in both spin states. Comparison of 1-3 reveals that the more cooperative spin transition in 1, and its separate crystallographic phase transition, can both be attributed to an intermolecular steric interaction involving the methylsulfanyl substituents. All three compounds exhibit the light-induced excited-spin-state trapping (LIESST) effect with T(LIESST = 70-80 K), but show complicated LIESST relaxation kinetics involving both weakly cooperative (exponential) and strongly cooperative (sigmoidal) components.

A Study of Commercial Nanoparticulate γ‐Al2O3 Catalyst Supports

This study investigates a range of commercially available γ‐Al2O3 powders by using a combination of integrated experimental techniques. These included general measurements of powder properties by using helium density, BET surface area, and scanning electron microscopy (SEM) analyses. In addition, dynamic light scattering and zeta potential measurements were used to investigate nanoparticle dispersions. Bulk crystal structures were analysed by using comparative X‐ray and neutron powder diffraction (XRD and NPD) analyses. Conventional transmission electron microscopy (TEM) was used to determine particle morphology, particle size, composition, and structure. Aberration‐corrected TEM was used to investigate the crystallinity of nanoparticles including the existence of any surface reconstruction on commonly observed facetted, cubeoctahedral γ‐Al2O3 nanoparticles. From the observation of peak splittings in diffraction data, we favour a description of the γ‐Al2O3 structure based on a distortion of the conventionally accepted face‐centred cubic (Fd

Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics

\bar 3

Dielectric and piezoelectric properties in the lead-free system Na 0.5 K 0.5 NbO 3 ¿BiScO 3 ¿LiTaO 3

m) structure into a tetragonal I41/amd structure. Distinct differences between TEM, XRD, and NPD data indicate the presence of some cation disorder within a rigid close‐packed oxygen framework. The Rietveld refinement of the NPD data suggests a high level of microstrain of 1.2 %. An improvement to the model is achieved by reducing the aluminium content in the unit cell, which is commensurate with the migration of aluminium ions to the surface and some degree of nonstoichiometry in the particle core. Aberration‐corrected TEM imaging and exit wave reconstruction confirm previous evidence for the presence of enhanced surface contrast at {1 1 1} surface facets, which we associate with the presence of excess cation termination. In addition, these {1 1 1} facets are observed to be heavily stepped. These results may have important implications for the thermal stability of metal catalyst nanoparticles on these high‐surface area supports; the migration of aluminium ions to the surface provides clear evidence of why these materials perform so well as catalyst supports.

High temperature neutron diffraction studies of 0.9BiFeO3―0.1PbTiO3

Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest for sensors and memory applications. Layered bismuth titanates with an Aurivillius structure, (BiFeO<inf>3</inf>)nBi<inf>4</inf>Ti<inf>3</inf>O<inf>12</inf>, can possess ferroelectric and ferromagnetic order parameters simultaneously. It has recently been demonstrated that one such example, Bi<inf>5</inf>Fe<inf>0.5</inf>Co<inf>0.5</inf>Ti<inf>3</inf>O<inf>15</inf>, where n = 1 with half the Fe³⁺ sites substituted by Co³⁺ ions, exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report the fabrication of highly-oriented polycrystalline ceramics of this material, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is a secondary phase within the ceramic matrix which is rich in cobalt and iron, hence this secondary phase could contribute in the main phase ferromagnetic property. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. TEM was used to identify the crystallographic structure of the secondary phase, which was shown to be cobalt ferrite, CoFe<inf>2</inf>O<inf>4</inf>. It is inferred from the data that the resultant ferromagnetic response identified using VSM measurements was due to the presence of the minor secondary phase. The Remanent magnetization at room temperature was M<inf>r</inf> ≈ 76 memu/g which dropped down to almost zero (M<inf>r</inf> ≈ 0.8 memu/g) at 460 °C, far lower than the anticipated for CoFe<inf>2</inf>O<inf>4</inf>.

Maltose and pectin assisted sol–gel production of Ce0.8Gd0.2O1.9 solid electrolyte nanopowders for solid oxide fuel cells

Phase relations, dielectric and piezoelectric properties are reported for the ternary system 98%[(1 - x) (Na_0.5K_0.5NbO₃)-x(LiTaO₃)]-2%[BiScO₃] for compositions x-10 mol% LiTaO₃. The phase content at room-temperature changed from mixed phase, monoclinic + tetragonal, for unmodified 98%(Na_0.5K_0.5NbO₃)-2%(BiScO₃), to tetragonal phase for compositions >;2 mol% LiTaO₃. Curie peaks at 360 to 370°C were observed for all compositions, but peaks became diffuse at x-3 mol%, and two dielectric peaks, at 370 and 470°C, were observed for 5 mol% LiTaO₃. Phase segregation, and finite size affects associated with the core-shell structure, account for the occurrence of two dielectric peaks in 5 mol% LiTaO₃, and diffuse dielectric behavior. The value of d₃₃ piezoelectric charge coefficient increased from ~160 pC/N for 0 mol% LiTaO₃ to 205 to 214 pC/N for 1 to 2 mol% LiTaO3 solid solutions, before falling sharply at 3 mol% LiTaO₃. TEM-EDX analysis revealed core-shell grain structures with segregation of Bi, Sc, and Ta in the outer ~100-nm shell of the 5 mol% LT sample.