Effect of composition and grain size on dielectric,ferroelectric and induced strain behavior of PLZT/ZrO2 composites

Lead lanthanum zirconate titanate ceramics (PLZT) are well known for their excellent dielectric, piezoelectric and ferroelectric properties. In this study, PLZT 9/70/30, 9/65/35 and 9/60/40 ceramics were prepared by vibro-milling mixed-oxide method. All compositions of powders were uniaxial pressed in pellets and sintered at the temperatures of 1200–1275 °C with various soaking times of 2, 4 and 6 h. The X-ray diffraction (XRD) patterns confirmed that all the PLZT samples had perovskite structure with ZrO₂ as a second phase and PLZT/ZrO₂ composite structure was formed. Dielectric behavior at the frequency of 1 kHz showed broad peak indicating relaxor ferroelectric behavior and the difference of the temperature at maximum dielectric at different frequencies increased when Zr:Ti ratio increased. Polarization with electric field (P-E loop) at room temperature showed that when Zr:Ti ratio increased, the coercive field decreased resulting from crystal structure change from tetragonal to rhombohedral. Induced strain with electric field depended on microstructure where the value of S_max/E_max tended to decrease with increasing grain size. It can be concluded that dielectric and ferroelectric behavior predominantly depended on composition of PLZT ceramics and induced strain behavior predominantly depended on grain size of PLZT ceramics.     

Evaluation of the La0.75Sr0.25Mn0.8Co0.2O3−δ system as cathode material for ITSOFCs with La9Sr1Si6O26.5 apatite as electrolyte

In the past years, a major interest has been devoted to decrease the working temperature of solid oxide fuel cells (SOFCs) down to about 700 °C.Apatite materials (La_10?xSr_xSi₆O_27?x/2) are attractive candidates for solid electrolytes, with a high ionic conductivity at 700 °C, a chemical and a dimensional stability for a pO₂ ranging from 10^?25 to 0.2 atm. A perovskite oxide (La_0.75Sr_0.25Mn_0.8Co_0.2O_3?δ) has been used as a cathode material.Symmetrical cathode/electrolyte/cathode cells were fabricated by stacking layers obtained by tape casting of apatite and perovskite powders and co-sintering at 1400 °C for 2 h in air.Impedance spectroscopy measurements were performed on these cells in order to determine the electrode resistance. It has been shown that the latter decreases with the porosity content of the cathode and with the use of a composite material (apatite/perovskite) instead of a simple perovskite.     

Synthesis of nanosized (Pb,Sr)TiO3 perovskite powders by coprecipitation processing

Nanosized perovskite lead strontium titanate (Pb_xSr_1−x)TiO₃ (PST) powders were successfully prepared by a simple coprecipitation method. Lead-strontium titanyl oxalate (PSTO) precursor was first synthesized at room temperature, and the precursor was then calcined at 600 °C for 1 h to produce the single phase perovskite PST powders. Characterization studies were carried out on the as-dried precursor and the calcined PST powders by various techniques. The results showed a strong dependence of the chemical composition of final PST powders on pH value in the coprecipitation reaction. PST powders with desired composition could be synthesized by adding 25 mol% excess Sr. PST particles were found to be spherical in nature with an average size of 10 nm.     

The effect of the calcination temperature of LaFeO3 precursors on the properties and catalytic activity of perovskite in methane oxidation

In the synthesis of perovskite-type LaFeO₃ oxides iron and lanthanum nitrates were used as a precursors. The nitrates were dissolved in water, evaporated, crushed and calcined in temperature range of 650–850?°C. The obtained perovskites were applied as an active layer on monolithic catalysts for the oxidation of methane. The increase in the calcination temperature of the perovskite precursors from 650° to 850°C results in a reduction in the surface area of the powders from 10.1 to 4.2?m²/g. XRD studies revealed that calcination at 800–850?°C caused the formation of an almost homogeneous LaFeO₃ perovskite phase. A decrease in the La/Fe surface ratio from 12 to 5.2 with the rise in calcination temperature from 650° to 800°C was detected by XPS. EDX results confirmed that at 750–850?°C, the La/Fe ratio in the perovskite layer is close to the stoichiometric and amount to 1.01–1.03. The highest activity in methane oxidation was achieved when the LaFeO₃ perovskite was calcined at 700?°C. A further slight increase in the activity was noticed after H₂ treatment. As the calcination temperature of the perovskites is increased, the catalyst activity decreases due to a reduction in the specific surface area, despite the more complete LaFeO₃ perovskite phase formation.     

Sol–gel synthesis of La0.6Sr0.4CoO3−x and Sm0.5Sr0.5CoO3−x cathode nanopowders for solid oxide fuel cells

Nano-powders of La_0.6Sr_0.4CoO_3?x (LSC) and Sm_0.5Sr_0.5CoO_3?x (SSC) compositions, which are being investigated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) with La(Sr)Ga(Mg)O_3?x (LSGM) as the electrolyte, were synthesized by low-temperature sol–gel method using metal nitrates and citric acid. Thermal decomposition of the citrate gels was followed by simultaneous DSC/TGA methods. Development of phases in the gels, on heat treatments at various temperatures, was monitored by X-ray diffraction. Sol–gel powders calcined at 550–1000 °C consisted of a number of phases. Single perovskite phase La_0.6Sr_0.4CoO_3?x or Sm_0.5Sr_0.5CoO_3?x powders were obtained at 1200 °C and 1300 °C, respectively. Morphological analysis of the powders calcined at various temperatures was done by scanning electron microscopy. The average crystallite size of the powders was ～15 nm after 700 °C calcinations and slowly increased to 70–100 nm after heat treatments at 1300–1400 °C.     

Direct synthesis of PMN samples by spray-drying

The processing and characterisation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) materials, obtained either by spray-drying the solution of the precursors or by the conventional “columbite” method, were investigated and the morphological and micro-structural characteristics were compared. The acid solution of ammonium-peroxo-niobium complex, magnesium and lead nitrates was spray-dried and the precursor powder obtained was calcined at different temperatures ranging from 350 to 900 °C. The morphologies and the XRD patterns of the powders were compared. The calcined powders exhibited a pyrochlore phase above 400 °C converting into an almost pure perovskite phase at 800 °C. The powder calcined at 350, 500 and 800 °C were sintered at different temperatures, ranging from 950 to 1150 °C, always resulting in a pure perovskite PMN material. The XRD patterns of as-fired surfaces of samples sintered at 950 and 1050 °C showed an unwanted PbO phase together with the main PMN, nevertheless this secondary phase is not present in the ground surfaces. The high reactivity of sprayed powder is reflected in the formation and densification of pure perovskite PMN material with a faster process as regards the conventional one; in particular samples of about 96% theoretical density were obtained starting from the amorphous powder calcined at low temperature (350 °C) through a reaction sintering process. Furthermore, due to the better flowability of the spray-dried powder, the cold consolidation process is highly improved and no binder addition to powder is necessary.     

The characterization of mixed titanate Ba1−xSrxTiO3 phase formation from oxalate coprecipitated precursor

Mixed titanates Ba_1−xSr_xTiO₃ were synthesized via calcination of oxalate coprecipitated precursors. On heating there were three thermal event occurred: T<250°C corresponds to the evaporation of trapped water and dehydration, T=250°C–450°C corresponds to the decomposition of oxalate and the formation of an intermediate phase where the composition is close to [Ba_1−xSr_x]₂Ti₂O₅.CO₃ and T=600°C–700°C corresponds to the carbonate decomposition and the formation of Ba_1−xSr_xTiO₃ phase. Powders calcined at T=700°C for 2 h are single phase, have grain size ranges of 0.2–2 μm and elongated morphology. Rietveld refinements of the XRD data showed single phase of perovskite structure in which their tetragonality decreased with increasing concentration of Sr²⁺ incorporated in Ba²⁺ site. The transition temperature showed strong correlation with the tetragonality.     

Structural analysis and electrode performance of Ce doped SrMnO3 synthesised by EDTA citrate complexing process

Abstract

Sr_1?xCe_xMnO₃ (SCM, 0·1≤x≤0·4) powders were synthesised by an ethylenediaminetetraacetic acid citrate complexing process, and their properties were investigated. The synthesised Sr_1?xCe_xMnO₃ powders showed a pure perovskite phase, whereas the composition with x?=?0·4 had second phases. The unit cell volumes increased with increasing Ce content because substituted Ce ions formed some Mn³⁺ ions, which have a larger ionic radius than Mn⁴⁺. The electrical conductivity improved with increasing Ce content up to x?=?0·3 (291 S cm^?1 at 750°C), revealing a double exchange interaction. Although the electrical conductivity was increased by doping Ce ions, the polarisation resistance increased due to the increase in lattice distortion with doping Ce content. The substitution of Ce ions for Sr in SrMnO₃ led to the formation of larger Mn³⁺ ions than Mn⁴⁺ ions and lattice distortion, which would affect the electrical and oxygen ion conductivity.     

Preparation and characterization of nanometer-sized (Pb1−x,Bax)TiO3 powders using acetylacetone as a chelating agent in a non-aqueous sol-gel process

Nanometer-sized lead barium titanate (Pb_1−xBa_xTiO₃, PB_xT) powders were prepared by a non-aqueous sol-gel process using lead acetate, barium acetate, and titanium isopropoxide as precursors and ethylene glycol as the solvent. In this procedure, Ti-isopropoxide was chelated with acetylacetone. The samples were characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis/differential thermal analysis (TGA/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific surface area analysis. The results indicate that perovskite PB_xT phases were obtained by heat treatment at 450 °C for 5 h, and a pure perovskite was examined at 600 °C. The average particle sizes of perovskite PB_xT powders calcined at 600 °C were approximately about 40-80 nm, and BET analysis showed that the surface areas of the powders obtained at 600 °C were approximately 6-16 m²/g. In addition, the phase transition from the tetragonal ferroelectric phase to the cubic paraelectric phase occurred in a range of approximately 0.6 < x < 0.8.     

Preparation of spherical nanoparticles of LaAlO3 via the reverse microemulsion process

Spherical LaAlO₃ nanoparticles in a reverse microemulsion consisting of solution (water phase), Tween-80 and Span-80 (surfactant), n-butanol (cosurfactant, and cyclohexane (oil phase) were prepared. Precursor powders and calcined powders were characterized by differential thermal analysis (DTA), thermogravimetry analysis (TG), X-ray diffraction (XRD) and transmission electron microscopy (TEM). A pure perovskite LaAlO₃ formed when the precursor hydroxides calcined at 800 °C for 2 h. The particle size was about 50 nm and the shape of the monodisperse particles is spherical. The reverse microemulsion process can dramatically lower the crystallization temperature of LaAlO₃ about 700 °C than the classical solid-state reaction method.     

Effects of Preparation Conditions on the Structural and Optical Properties of Spark Plasma-Sintered PLZT (8/65/35) Ceramics

Transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics with high density were fabricated using spark plasma sintering (SPS), a recently developed hot-pressing method. A wet–dry combination method was used to prepare the fine PLZT powders. The average grain size of the PLZT ceramics was less than 1 μm, because of a relatively low sintering temperature and a very short sintering time. The transmittance of PLZT ceramics increased with an increase of calcination temperature up to 700°C and then it slightly decreased with further increase of calcination temperature. The transmittance strongly depended on the SPS temperature and heat-treatment temperature. The pellet sintered at 900°C for 10 min and heat treated at 800°C for 1 h with a thickness of 0.5 mm showed a transmittance of 31% at a wavelength of 700 nm. The relationships between the transmittance and the microstructure were investigated.     

Influence of low external magnetic field on electric field induced strain behavior of 9/70/30, 9/65/35 and 9/60/40 PLZT ceramics

In this work, Pb_0.91La_0.09(Zr_xTi_1−x)_0.9775O₃ PLZT ceramics, where x =0.70, 0.65 and 0.60, were prepared by a solid-state mixed-oxide technique. Structural information was investigated by X-ray diffraction method. Electric field induced polarization and strain behavior of the PLZT ceramics under an influence of static magnetic field were measured by a Sawyer-Tower circuit in conjunction with Michelson interferometer at various static magnetic fields (0–30.7 mT). The results showed that the imposed magnetic field affected the polarization and induced strain behaviors in PLZT ceramics More interestingly, the imposed magnetic field showed more significant influence on the PLZT ceramic composition near morphotropic phase boundary (MPB), i.e. PLZT 9/65/35. The spontaneous polarization of 180° domain switching was seen to be suppressed by the applied magnetic field.     

Structural and thermoelectric properties of hydrothermal-processed Ag-doped Fe2O3

Fe_2-xAg_xO₃ (0?≤?x?≤?0.04) nanopowders with various Ag contents were synthesized at different hydrothermal reaction temperatures (150?°C and 180?°C). Their structural properties were fully investigated through an X-ray diffraction, a Fourier transform infrared spectroscopy, and an X-ray photoelectron spectroscopy. The hydrothermal reaction temperature, time, and Ag content remarkably affected the morphological characteristics and crystal structure of the synthesized powders. The Fe_2-xAg_xO₃ (0?≤?x?≤?0.04) powders synthesized at 150?°C for 6?h and the Fe_2-xAg_xO₃ (0.02?≤?x?≤?0.04) powders synthesized at 180?°C for 12?h formed the orthorhombic α-FeOOH phase with a rod-like morphology, whereas the Fe_2-xAg_xO₃ (0?≤?x?≤?0.01) powders synthesized at 180?°C for 12?h formed the rhombohedral α-Fe₂O₃ phase with a spherical-like morphology. The Fe_1.98Ag_0.02O₃ fabricated by utilizing Fe_1.98Ag_0.02O₃ powders synthesized at 180?°C showed the largest power factor (0.64?×10^?5 Wm^?1 K^?2) and dimensionless figure-of-merit (0.0036) at 800?°C.     

Ionic and electronic transport in calcium-substituted LaAlO3 perovskites prepared via mechanochemical route

The present work explores mechanosynthesis of lanthanum aluminate-based perovskite ceramics and corresponding effects on ionic-electronic transport properties. La_1-xCa_xAlO_3-δ (x = 0.05–0.20) nanopowders were prepared via one-step high-energy mechanochemical processing. Sintering at 1450 °C yielded dense ceramics with submicron grains. As-prepared powders and sintered ceramics were characterized by XRPD, XPS and SEM. Electrochemical studies showed that partial oxygen-ionic conductivity in prepared La_1-xCa_xAlO_3-δ increases with calcium content up to 10 at.% in the lanthanum sublattice and then levels off at ∼6 × 10⁻³ S/cm at 900 °C. La_1-xCa_xAlO_3-δ ceramics are mixed conductors under oxidizing conditions and ionic conductors with negligible contribution of electronic transport in reducing atmospheres. Oxygen-ionic contribution to the total conductivity is 20–68% at 900 °C in air and increases with Ca content, with temperature and with reducing p(O₂). Impedance spectroscopy results showed however that electrical properties of mechanosynthesized La_1-xCa_xAlO_3-δ ceramics below ∼800 °C are determined by prevailing grain boundary contribution to the total resistivity.     

Synthesis and thermal stability of (Co,Ni)O solid solutions

(Co,Ni)O solid solutions are considered as promising protective materials of O₂-evolving anodes for Al production. In this context, two solid-state synthesis methods, namely high-energy ball milling (HEBM) and calcination, have been evaluated for the synthesis of (Co,Ni₎O solid solutions. In all cases, Co_xNi_1−xO solid solutions can be formed over the whole composition range. However, undesired WC contaminant is observed using the HEBM method due to the erosion of the milling tools. Their thermal stability in air has been analyzed by thermogravimetric analyzes (TGA) complemented by X-ray diffraction (XRD) analyses. It is shown that Co_xNi_1−xO solid solutions are stable at 1000°C over the whole composition range whereas they are only stable for x ≤ 46 and x ≤ 22 at 800°C and 700°C, respectively. For higher Co contents, the formation of Co₃O₄ is observed. This is a relevant information for their future use for Al production, which can be done at different temperatures (~700-1000°C) depending of the electrolyte composition.     

Sintering of LaCoO3 based ceramics

The densification, microstructure and phase evolution of near stoichiometric, Co-excess and Co-deficient perovskite La_1−xM_xCoO_3−δ (M=Ca, Sr; x=0, 0.2) powders have been investigated by electron microscopy and powder X-ray diffraction. Sub-micron powders were prepared from nitrate precursors using the glycin-nitrate and the EDTA methods. The sintering temperature was observed to decrease with Ca or Sr substitution. Dense materials with grain size in the order of 3–5 μm have been obtained at 1200°C for near stoichiometric powders. Considerable grain growth was observed at higher sintering temperatures. The presence of other crystalline phases in addition to the perovskite due to Co-excess/-deficiency considerably affects the microstructure and acts as grain growth inhibitors by grain boundary pinning. The volume fraction of secondary phases is particularly large in the case of Co-deficient LaCoO₃ due to the formation of La₄Co₃O₁₀. In non-stoichiometric La_0.8Ca_0.2CoO₃, a liquid phase consisting mainly of CaO and CoO was observed at 1400°C causing exaggerated grain growth. Considerable pore coarsening was observed in Co-excess La_0.8Ca_0.2CoO₃ at 1350°C. The present investigation demonstrates the importance of controlling the stoichiometry of LaCoO₃ based ceramics in order to obtain dense materials with well defined microstructure.     

Synthesis and applications of composite cathode based on Sm0.5Sr0.5Co3−δ for solid oxide fuel cell

Cobaltite based perovskites, such as Sm_0.5Sr_0.5Co_3?δ (SSC), are attractive solid oxide fuel cell (SOFC) cathodes due to their high electrochemical activity and electrical conductivity. To obtain higher fuel cell performance with smaller particles, nano-sized SSC powders were synthesized by a complex method with/without carbon black, HB170. However, during synthesis, carbon black reacted with Sr, and unfortunately formed SrCO₃. To obtain pure perovskite SSC, a calcination temperature of 900 °C is needed. At 680 °C, an SOFC with SSC (calcined at 700 °C and synthesized without HB170) exhibited a higher fuel cell performance, of 0.68W·cm^?2, than that with SSCHB (calcined at 900 °C and synthesized with HB170), of 0.58W·cm^?2. Adding GDC for composite cathode is more effective in SSCHB porous cathodes than in SSC porous cathodes. At 680 °C, the composite cathode of SSCHB6-GDC4 exhibited the highest maximum power density of 0.72W·cm^?2 which results from the combined effects of lowered charge transfer polarization and mass transfer polarization. To obtain higher fuel cell performance, optimum composition and processes are necessary.     

Enhancing piezoelectricity of PLZT ceramics by bismuth stearate coating via water-soluble defatted powder injection molding

Lead zirconate titanate-based piezoelectric ceramics (Pb_0·91La_0·06(Zr_0·58Ti_0.42)_0.975Nb_0·02O₃, PLZT) were prepared by water-soluble defatted powder injection molding using bismuth stearate as surfactant. The effects of bismuth stearate surfactant amount on the feed viscosity, water degreasing rate and electrical properties were studied. The results showed that the bismuth stearate coating reduces agglomeration between the PLZT powders and decreases the viscosity of the feed. During sintering, bismuth stearate is decomposed into bismuth oxide, which acts as sintering aid to increase the density of the sintered bismuth stearate-coated PLZT ceramics. Bi³⁺ dissolves into the perovskite crystal lattice, which increases the tetragonality factor and the content of tetragonal phase, and improves the dielectric, ferroelectric and piezoelectric properties of the bismuth stearate-coated PLZT ceramics. This study provides a novel modification method for the production of injection molded ceramics using a water-soluble binder system.     

Preparation by sol–gel and solid state reaction methods and properties investigation of double perovskite Sr2FeMoO6

Double perovskite Sr₂FeMoO₆ was prepared by two ways consisting in sol–gel technique and solid-state reaction method. The resulting powders from gel and mixed oxides precursors showed microstructures consisting of very fine grains (0.5–0.8 μm) and a crystalline perovskite structure. The structural and microstructural properties of the double perovskite Sr₂FeMoO₆ powders as-prepared and ceramics were compared. Tetragonal Sr₂FeMoO₆ pellets were prepared from the two powders by spark plasma sintering at: 1000, 1100 and 1200 °C and then annealing at 1200 °C, 2 h in 5%H₂/Ar. The pellets presented different magnetic characteristics. The saturation magnetization of the samples prepared by sol–gel is close to those prepared by conventional synthesis method.