Fabrication and topchemical transformation mechanism of PbTiO3 microplatelets

Two-dimensional platelets of the perovskite-structured PbTiO₃ were synthesized from a layer-structured PbBi₄Ti₄O₁₅ precursor, using a topochemical microcrystal route. The PbBi₄Ti₄O₁₅ precursor showed a high aspect ratio with an average size of ～7.74 μm and thickness of ～0.29 nm. Later, PbTiO₃ platelets coexisting with Pb₄Bi₄Ti₇O₂₄ mesophase were obtained, which had a more stable Aurivillius structure and was difficult to completely convert to the perovskite structure during topochemical microcrystal conversion. The reason that the Pb₄Bi₄Ti₇O₂₄ mesophase coexisted in PbTiO₃ platelets was deeply discussed. Both the crystal structure difference between the unit cell parameters for (Pb₄Bi₂Ti₇O₂₂)^2- and the thickness for the (Bi₂O₂)²⁺ layer, and the stability of (Bi₂O₂)²⁺ layer were used to analyze the existence of Pb₄Bi₄Ti₇O₂₄ phase. However, the result showed that the stability of (Bi₂O₂)²⁺ layer was the dominant reason, because the [OBi₄]/[OPb₄] tetrahedron formed in the (Bi₂O₂)²⁺ layer due to the similar chemical properties of Pb²⁺ and Bi³⁺. The work provided deep theoretical guidance to fabricate anisotropic perovskite platform-based topchemical microcrystal conversion mechanisms.     

Morphological evolution of PbTiO3 microstructures synthesized by topochemical microcrystal conversion

PbTiO₃ (PT) microstructures were fabricated by topochemical microcrystal conversion (TMC) and their morphological evolution was investigated with variation of synthesis temperature and relative molar ratio of reactants. Using molten salt synthesis, Aurivillius phase of PbBi₄Ti₄O₁₅ (PBiT) synthesized at 1050°C was prepared as precursor. Large plate-like PT microstructures with lengths of hundreds of micrometer were optimally obtained when the relative ratio (x) of (PbCO₃)₂·Pb(OH)₂ to PBiT in reactants was 6. In addition, relatively homogeneous PT platelets were achieved at the annealing temperature of 1050°C by TMC; these materials are probably suitable for use in templated grain growth. However, a Pb-deficient secondary phase of PbTi₃O₇ formed in cases of low x values of less than 4 and low annealing temperatures of less than 1050°C. The porous PT pellets, orthogonally assembled by numerous PT platelets, were achieved by increasing temperature to 1100°C. Further increase in temperature to 1150°C could lead to partial formation of denser PT blocks.     

Bandgap modulation and magnetic switching in PbTiO3 ferroelectrics by transition elements doping

Transition metal (TM=Fe, Ni and Mn) ions doped PbTiO₃ perovskite ferroelectric ceramics prepared by a solid state reaction method have been studied by means of structural characterizations, optical and magnetic measurements. All the samples have pure tetragonal perovskite structure, but exhibit different grain shapes and sizes with the introduction of TM ions and oxygen vacancies. The observed structural changes arise from internal lattice strain, which is estimated by Williamson–Hall (W–H) analysis model. Moreover, TM ions doping plays simultaneously an important role on the energy band structure and magnetic orderings. The energy gap of PbTi_0.95TM_0.05O_3−δ shows a drastic decrease compared to that of PbTiO₃. Furthermore, PbTi_0.95TM_0.05O_3-δ materials possess multiple magnetism switching, in which diamagnetic–ferromagnetic transition and ferromagnetic–paramagnetic transition occur. In particular, the Fe-doped PbTiO₃ ceramic presents a typical ferromagnetic hysteresis, originating from the effective exchange coupling interaction between oxygen vacancies and Fe 3d spins.     

Phase formation and characterization of high Curie temperature xBiYbO3–(1 − x)PbTiO3 piezoelectric ceramics

Novel perovskite xBiYbO₃–(1 ? x)PbTiO₃ (BYPT) crystalline solutions were prepared by conventional ceramic processing. The effect of BiYbO₃ on the microstructure and electrical properties was investigated, and the reaction mechanism of phase formation was discussed. The results show that the proper calcining temperature is 860 °C and the optimum sintering temperature is 1140 °C. It is difficult for BYPT ceramics to obtain phase-pure perovskite structures. Pyrochlore phase Yb₂Ti₂O₇ was formed by Yb₂O₃ and TiO₂ that decomposed from BiYbO₃–PbTiO₃ at high temperature. With increasing BiYbO₃ content, the pyrochlore phase Yb₂Ti₂O₇ increases, which resulted in the decrease of densities and piezoelectric constants. The dielectric constants were in the range of 100–700 at room temperature and tan δ less than 4%. BYPT ceramics with high Curie temperatures of >500 °C are obtained. 0.1BiYbO₃–0.9PbTiO₃ ceramic is a very promising material for high-temperature (up to 550 °C) piezoelectric applications for high T_c, stable d₃₃, and high thermal-depoling temperature.     

Structural,capacitive and resistive characteristics of (Pb0.6Bi0.2Sm0.2)(Fe0.4Ti0.6)O3

Samarium substituted BiFeO₃–PbTiO₃ ceramic compound of (Pb_0.6 Sm_0.2 Bi_0.2)(Fe_0.4Ti_0.6)O₃ has been fabricated by mixed oxide solid state reaction method. The crystallographic structure from XRD study, distribution of grains from SEM micrograph, dielectric behavior, conductivity spectrum, impedance along with electric modulus spectroscopy have been illustrated. The experimental results corroborate the impact of samarium substitution in BiFeO₃–PbTiO₃ entailing reduced grain size, higher dielectric response with reasonably dielectric loss and enrichment in capacitive behavior with negative temperature coefficient of resistance. The temperature dependent conductivity spectra exhibit Arrhenius behavior, whereas the frequency dependent conductivity spectra follow the Jonscher universal power law. The basic correlated barrier hopping (CBH) model governs the charge transport mechanism in the fabricated compound. The exploration reveals the enriched dielectric and electrical behavior that endorse the samarium substituted material as a potential ceramic entity for designing electronic devices such as capacitors and ferroelectric accessories.     

Enhanced electrical and photocurrent characteristics of sol-gel derived Ni-doped PbTiO3 thin films

Partial substitution of group 10 metal for titanium is predicted theoretically to be one of the most effective ways to decrease the band gap of PbTiO₃-based ferroelectric photovoltaic materials. It is therefore of interest to experimentally investigate their ferroelectric and photovoltaic properties. In this work, we focus on the electrical and photocurrent properties of Ni-doped PbTiO₃ thin films prepared via a sol-gel route. The nickel incorporation does not modify the crystalline structure of PbTiO₃ thin film, but it can increase the dielectric constant, ferroelectric polarization and photocurrent, and simultaneously decrease the band gap. The maximum remnant polarization (P_r) of 58.1 μC/cm² is observed in PbTi_0.8Ni_0.2O₃ thin film, and its photocurrent density is improved to be approximately one order larger than that of PbTiO₃ thin film and simultaneously exhibits the polarization-dependent switching characteristic, which may be a promising choice for ferroelectric photovoltaic applications.     

Synthesis,structural evolution,and dielectric properties of a new perovskite solid solution (Pb0.5Sr0.5)(Zr0.5Ti0.5)O3–PbTiO3

To explore lead-reduced dielectric materials in the SrTiO₃–PbTiO₃–PbZrO₃ ternary system, a novel solid solution between relaxor ferroelectric (Pb_0.5Sr_0.5)(Zr_0.5Ti_0.5)O₃ and ferroelectric PbTiO₃, namely (1 − x)(Pb_0.5Sr_0.5) (Zr_0.5Ti_0.5)O₃–xPbTiO₃ (lead–strontium–zirconate–titanate [PSZT]–PT), has been synthesized in the perovskite structure by high-temperature solid-state reaction method in the form of ceramics. The crystal structure and phase symmetry of the materials synthesized were analyzed and resolved based on X-ray powder diffraction (XRD) data through both the Pawley and Rietveld refinements. The results of the structural refinements indicate that at low PT-concentration end of the solid solution system, for example, x = 0.05, the PSZT–PT solid solution exhibits a cubic structural symmetry (with the space group Pm-3m). As the PT concentration (x) increases, the structure of (1 − x)PSZT–xPT gradually transforms from the cubic to a tetragonal (P4mm) phase. In the composition range of x = 0.10–0.25, a mixture of the cubic and tetragonal phases was identified. As the concentration of PT increases, the proportion of the tetragonal phase increases at the expense of the cubic phase. For a composition of x > 0.25, a pure tetragonal phase is observed. The dielectric properties of the materials were studied by measuring the permittivity as a function of temperature at various frequencies. For the composition of x = 0.05, the temperature dependence of dielectric constant shows typical relaxor behavior. For x = 0.35, the dielectric peaks indicate a normal ferroelectric phase transition. Overall, a structural transformation from a central-symmetric, nonpolar cubic phase to a non-centrosymmetric, polar tetragonal phase is induced by the substitution of PT for PSZT in the pseudo-binary solid solution of (1 − x)PSZT–xPT, which also reveals an interesting relaxor to ferroelectric crossover phenomenon.     

Enhanced energy-storage performance of 0.94NBT-0.06BT thin films induced by a Pb0.8La0.1Ca0.1Ti0.975O3 seed layer

0.94(Na_0.5Bi_0.5TiO₃)-0.06BaTiO₃ ferroelectric thin films with and without the Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer were deposited on platinum-buffered silicon substrates by using Sol–Gel process. The influence of Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer and annealing temperatures on the microstructures, ferroelectric properties and energy-storage performances of the as-prepared films were investigated in details. The low annealing temperature and Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer could improve the values of electric break-down field strength and P_max-P_r, which play a vital role for high recoverable energy-storage density. Owing to the high electric break-down field strength value of 3310 kV/cm, a large recoverable energy density of W=17.2 J/cm³ and a high energy efficiency of η=74.3% were obtained for the 0.94(Na_0.5Bi_0.5TiO₃)-0.06BaTiO₃ thin film with the Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer, which was annealed at 450 °C.     

Ferroelectric thin films obtained by pulsed laser deposition

We review our significant results concerning pulsed laser deposition (PLD) of some ferroelectric compounds: (i) lead magnesium niobate Pb(Mg_1/3Nb_2/3)O₃ (PMN); (ii) lead magnesium niobate–lead titanate Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT), with variable PT contents; (iii) La-doped lead zirconate titanate (Pb_1 − xLa_x)(Zr_0.65Ti_0.33)O₃ (PLZT); and (iv) Nb-doped lead zirconate titanate Pb_0.988(Zr_0.52Ti_0.48)_0.976Nb_0.024O₃ (PNZT). A parametric study has been performed in order to evidence the influence of the deposition parameters (laser wavelength, laser fluence, oxygen pressure, substrate type and temperature, RF power discharge addition, etc.) on the film properties and to identify the best growing conditions. Techniques including atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), secondary ions mass spectroscopy (SIMS), transmission electron microscopy (TEM), electrical and ferroelectric hysteresis measurements have been used for layer characterization.     

Effects of the starting materials and mechanochemical activation on the properties of solid-state reacted Li4Ti5O12 for lithium ion batteries

Li₄Ti₅O₁₂ was synthesized by a solid-state reaction between Li₂CO₃ and TiO₂ for applications in lithium ion batteries. The effects of the TiO₂ phase and mechanochemical activation on the Li₄Ti₅O₁₂ particles as well as the corresponding electrochemical properties were investigated. Rutile TiO₂ was more desirable in acquiring high purity Li₄Ti₅O₁₂ than anatase due to the anatase to rutile phase transformation, which was found to be more rigid in the solid-state reaction than the intact rutile phase. Mechanochemical activation of the starting materials was effective in decreasing the reaction temperature and particle size as well as increasing the Li₄Ti₅O₁₂ content. The specific capacity depended significantly on the Li₄Ti₅O₁₂ content, whereas the rate capability improved with decreasing particle size due to the enhanced contact area and reduced diffusion path. Overall, a 200 nm-sized Li₄Ti₅O₁₂ powder with a specific capacity of 165 mAh/g could be synthesized by optimizing the milling method and starting materials.     

Compositional‐driven multiferroic properties in samarium substituted LaFeO3‐PbTiO3 solid solutions

Samarium‐substituted LaFeO₃‐PbTiO₃ solid solutions Pb_1?xLa_x/2Sm_x/2Ti_1?xFe_xO₃ were synthesized within the solid solubility limit 0.10≤x≤0.30. The structural analysis reveals the phase transition from tetragonal to cubic with the variation in composition. Refined lattice and structural parameters vary gradually with composition, and the structural data correlate very well with the microstructural, Raman, and dielectric data. Magnetic ordering in the solid solutions continuously increases with the increase in Fe content. Electric polarization hysteresis loops reveal enhancement in remnant polarization with addition of La/Sm.     

Insight for the construction of R-T phase boundary in KNN piezoceramics from the view of energy band structure and electron density

The crystal structures and associated energy band structures of classic lead-based PZT (PbZr_0.5Ti_0.5O₃) and lead-free KNN (K_0.5Na_0.5NbO₃) piezoelectric ceramics prepared by a solid-state method are systematically studied based on the First-principles calculations. For better understanding relations between energy band structures and the phase structures in the PbZr_0.5Ti_0.5O₃ and KNN, the components PT (PbTiO₃)/PZ (PbZrO₃) and KN (KNbO₃)/NN (NaNbO₃) are also calculated. The results suggest that the vital contribution to R-T morphotropic phase boundary (MPB) in PbZr_0.5Ti_0.5O₃ comes from the hybridization of A-site and B-site ions, which causes stretching and tilting of oxygen octahedron and therefore leads to the formation of R and T phases. The KNN behaves differently. Moreover, it's concluded that the band structure highly inherited from tetragonal PbTiO₃ plays an important role in the construction of the R-T phase boundary according to the comparison of PbZr_0.5Ti_0.5O₃ and KNN. In general, the work inspires an idea from the view of energy band structure to realize R-T phase coexistence as well as improved piezoelectric properties in KNN-based lead-free piezoceramics.     

Ultralow sintering temperature and piezoelectric properties of Bi(Zn1/2Ti1/2)O3−BiScO3−PbTiO3 for low-temperature co-firing applications

Bi(Zn_1/2Ti_1/2)O₃−BiScO₃−PbTiO₃ (BZT−BS−PT) high Curie temperature piezoelectric ceramics were synthesized by the conventional solid-state reaction method. Systematical investigations on the sintering, piezoelectric, and dielectric properties of the piezoceramics have been conducted. It was found that the sintering temperature could be remarkably depressed by varying the compositions in BZT−BS−PT systems. For composition of 11BZT−34BS−55PT ceramic, the sintering temperature is even lowered down to 750°C without any extra additions of sintering aids. Meanwhile, the ceramic sintered at this ultralow temperature presents dense microstructure with relative density up to 97%, as well as optimal properties of piezoelectric coefficient d₃₃ of 336 pC/N and Curie temperature of 415°C. The mechanism of low sintering temperature may be ascribed to the low melting point bismuth-based components in BZT−BS−PT solid solutions. Furthermore, 11BZT−34BS−55PT multilayer ceramics have been co-fired at 750°C with Ag internal electrodes. The dense structures, low cost, and optimal comprehensive properties of the co-fired multilayers illustrate obvious advantages of the ultralow sintering temperature in LTCC devices, implying promising applications of this Bi(Zn_1/2Ti_1/2)O₃−BiScO₃−PbTiO₃ high Curie temperature ternary system.     

Hydrothermal phase transformation of hematite to magnetite

Different phases of iron oxide were obtained by hydrothermal treatment of ferric solution at 200°C with the addition of either KOH, ethylenediamine (EDA), or KOH and EDA into the reaction system. As usually observed, the α-Fe₂O₃ hexagonal plates and hexagonal bipyramids were obtained for reaction with KOH and EDA, respectively. When both KOH and EDA were added into the reaction system, we observed an interesting phase transformation from α-Fe₂O₃ to Fe₃O₄ at low-temperature hydrothermal conditions. The phase transformation involves the formation of α-Fe₂O₃ hexagonal plates, the dissolution of the α-Fe₂O₃ hexagonal plates, the reduction of Fe³⁺ to Fe²⁺, and the nucleation and growth of new Fe₃O₄ polyhedral particles.     

Reaction between titanium dioxide and carbon in flowing helium stream

Abstract

The reaction between titanium dioxide and carbon in a flowing helium stream was studied by scanning electron microscopy and X-ray diffraction. Experimental results indicate that the order of phase transformations during the reaction was TiO₂(rutile) → Ti₄O₇ → Ti₃O₅ → Ti₂O₃ → TiO → TiC. A mechanism is proposed to explain the overall reaction. The reaction rate was found to increase with increasing sample thickness, reaction temperature, and initial bulk density and with decreasing helium flowrate, molar ratio of TiO₂/C, and grain size of titanium dioxide.     

Rapid microwave-assisted sol-gel synthesis and exceptional visible light photocatalytic activities of Bi12TiO20

Crystalline Bi₁₂TiO₂₀ and Bi₄Ti₃O₁₂ particles were selectively synthesized by rapid microwave-assisted sol-gel method. During the thermal decomposition process of the dried gel, microwave calcination played a key role in producing single phase Bi₁₂TiO₂₀. Our Bi₁₂TiO₂₀ demonstrated one of the highest visible-light photocatalytic activities for MO degradation among the reported bismuth titanate particles with various compositions. Single phase Bi₄Ti₃O₁₂ can also be prepared by either a conventional calcination at high temperature or a combined heat treatment of a conventional heating followed by microwave calcination. The photocatalytic reaction rate constant of the Bi₄Ti₃O₁₂ prepared by microwave calcination was three times higher than that of conventionally calcined Bi₄Ti₃O₁₂, further confirming the advantage of microwave calcination in preparation of highly photocatalytically active bismuth titanate.     

Rapid low temperature synthesis of a titanate pyrochlore by molten salt mediated reaction

Ytterbium titanate pyrochlore, Yb₂Ti₂O₇, was prepared by molten salt mediated synthesis (MSS) from titanium oxide (TiO₂) and ytterbium oxide (Yb₂O₃) reagents. Potassium and sodium chloride mixtures were used as the molten salt medium and the effects of salt to reagent ratio, salt composition, synthesis temperature, reaction time, and TiO₂ particle size were explored. Synthesis temperatures and times required for formation of single phase Yb₂Ti₂O₇ were found to be lower than those required for solid state synthesis (SSS). Whereas MSS synthesis of single phase Yb₂Ti₂O₇ was achieved with micron-sized powders after a single reaction at 1200 °C for 1 h, SSS with micron-sized powders required an extended reaction time of 36 h at 1350 °C. Yb₂Ti₂O₇ micron-sized powder prepared by MSS showed similar particle size and morphology to that of the TiO₂ precursor demonstrating a template growth mechanism. However, the use of TiO₂ nano-sized powder changed the dominant synthesis mechanism from template growth to dissolution–precipitation and facilitated synthesis of near single phase Yb₂Ti₂O₇ at the remarkably low temperature of 700 °C in only 1 h. The potential application for lanthanide and actinide immobilisation from molten salt reprocessing wastes was demonstrated by preparation of Yb₂Ti₂O₇ by molten salt mediated synthesis from TiO₂ and ytterbium chloride (YbCl₃) reagents.     

Temperature-independent and enhanced dielectric properties for two-layer structure capacitors with 0.8Pb(Fe2/3W1/3)O3–0.2PbTiO3–MnO and 0.7Pb(Fe2/3W1/3)O3–0.3PbTiO3–MnO ceramics

This study fabricates capacitors with two-layer structure and different compositions, 0.8Pb(Fe_2/3W_1/3)O₃–0.2PbTiO₃–MnO and 0.7Pb(Fe_2/3W_1/3)O₃–0.3PbTiO₃–MnO, by using the conventional solid state oxide reaction method. By using the temperature compensation effect and adjusting the thickness ratio of the two layers with different compositions, the temperature–dielectric peak is enhanced and smoothed. The dielectric loss, space charge polarization and dc conduction are suppressed at the highest temperature region. Furthermore, the Maxwell–Wagner model is used to fit and explain the dielectric behaviors. This study also provides suggestions and discussion related to the effect of the interfacial region based on the experimental data and fitting results.     

Dielectric and ferroelectric properties of Ba0.8Sr0.2Ti1−5x/4NbxO3 ceramics

Ba_0.8Sr_0.2Ti_1−5x/4Nb_xO₃ ceramics, x = 0, 0.01, 0.05, 0.10, were fabricated by conventional solid-state reaction. With increasing niobium content the ferroelectric phase transition temperature decreases linearly, and the dispersivity of the transition increases. Niobium B-site decreases transition temperature more pronounced than Sr²⁺ at A-site. The heterovalent substitution of Nb⁵⁺ in low content causes local defect dipole, while more substitutions introduce disorder to disturb the long-range dipole correlation. Ba_0.8Sr_0.2Ti_1−0.5/4Nb_0.1O₃ ceramic shows weak ferroelectric loop at room temperature far from its transition temperature, 153 K.     

Microstructure and enhanced seebeck coefficient of textured Sr3Ti2O7 ceramics prepared by RTGG method

Textured Sr₃Ti₂O₇ ceramics (abbreviated as S3T) with grain orientation degree higher than 0.7 were successfully prepared by reactive-templated grain growth (RTGG) technique combined with tape casting process, using plate-like Sr₃Ti₂O₇ particles as templates. The phase transition, microstructure and thermoelectric properties were investigated. As the sintering temperature and soaking time increase, the density and texture fraction of ceramics increase. Textured Sr₃Ti₂O₇ ceramics were sintered in air and annealed in argon + graphene carbon (Ar+C) atmosphere. The results showed that the resistivity of textured Sr₃Ti₂O₇ ceramics annealed in Ar+C environment decreased significantly (seven orders of magnitude). After the annealing process, the texture fraction decreased, and Sr₃Ti₂O₇ phase on the surface of textured ceramics decomposed to SrTiO₃. The Seebeck coefficient was negative, and the absolute value of S increased with increasing temperature. Textured Sr₃Ti₂O₇ ceramics showed the maximum Seebeck coefficient (−399 μV/K) at 1073 K, which is higher than those of non-textured Sr₃Ti₂O₇-based materials.