Low-Temperature Synthesis of Lead Tantalate Pyrochlore Solid Solutions Pb1.5+x(Ta2−yPby)O7−δ (0.0 < x < 0.5; 0.0 < y < 0.6)

Synthesis of lead tantalate pyrochlores by the reaction of PbO and Ta₂O₅ in various molar ratios between 1.5 and 4.0 at low temperatures (500–650°C) has been carried out. It has been shown that when PbO is reacted with Ta₂O₅ in various molar ratios at low temperatures, metastable lead tantalate pyrochlore solid solutions having cubic symmetry are formed. X-ray diffraction data give evidence for an increase in the a lattice parameter with increasing PbO content and partial occupancy of Pb in the Ta sites leading to the formula Pb_{1.5+ x} ²⁺(Ta_{2− y} Pb _y ⁴⁺)O_7−δ (0.0 < x < 0.5; 0.0 < y < 0.6) for this phase. It has also been found that the intermediate members of the solid solutions are metastable and tend to segregate into Pb-deficient and Pb-excess compositions leading to significant compositional inhomogeneity for the intermediate members.     

Formation and Transformation of Cubic Lead Niobate Pyrochlore Solid Solutions

Formation of lead niobate pyrochlores by the reaction of PbO with Nb₂O₅ in various molar ratios and at various temperatures has been investigated. It has been shown that for a PbO/Nb₂O₅ molar ratio between 1.5 and 3.0, a pyrochlore solid solution having cubic symmetry is formed at low temperatures (∼500-600°C). X-ray diffraction data show evidence for the existence of compositional inhomogeneity for this phase which contains part of the Pb in the Nb site leading to the formula Pb_{1.5+ x} (Nb_{2- y} Pb _y )O_7-delta (0.0 < x < 0.5; 0.0 < y < 0.5). A phase-pure Pb-deficient pyrochlore (Pb_1.5Nb₂O_6.5) and Pb-excess pyrochlores (Pb_2.5Nb₂-O₈, Pb₃Nb₂O₈) are formed by the decomposition of this cubic solid solution and subsequent reaction at high temperatures. These results indicate that the mechanism of the structural transformation from the low-temperature (LT) cubic solid solution to the corresponding high-temperature (HT) phases is reconstructive.     

Preparation of Metal Ruthenates by Spray Pyrolysis

Submicrometer crystalline metal ruthenate powders with perovskite structure, MRuO₃ (M = Sr, La), and pyrochlore structure, M₂Ru₂O_{7- x} (0.5 < x < 1; M = Bi, Pb, Y, Eu, Gd, Tb, Dy, Ho, Er, Tm), were prepared by spray pyrolysis using metal nitrates and glycolates under an oxygen-gas atmosphere at temperatures up to 1100°C. Submicrometer-sized solid single crystals (SrRuO₃), submicrometer-sized hollow spheres consisting of nanocrystallites (pyrochlore rare-earth ruthenates, Bi₂Ru₂O₇, and Pb₂Ru₂O_6.5 below 1000°C), and nanometer-sized particles (Pb_2.31Ru_1.69O_6.5 and Bi-Pb-O above 1000°C) were observed. Particle formation proceeded by intraparticle reaction and intraparticle reaction followed by evaporation of volatile metal oxides to form metal oxide vapors followed by condensation and reaction to form particles. The former was observed for systems where no volatile metal oxides were formed, whereas the latter occurred for the Pb-Ru-O and Bi-Ru-O systems, where volatile metal oxides, such as Bi₂O, PbO, and RuO _x could occur. Particle morphology depended strongly on precursor properties. Submicrometer-sized single-crystal SrRuO₃ particles could be formed from the metal nitrates but not from Sr(NO₃)₂ and ruthenium glycolate, which gave hollow polycrystalline particles. In general, crystallite size could be controlled by varying precursor properties and reactor temperature, with higher temperatures giving larger crystallite sizes.     

Interactions between Ruthenia-Based Resistors and Cordierite–Glass Substrates in Low-Temperature Co-fired Ceramics

Low-temperature co-fired ceramics (LTCCs) that are composed of a RuO₂-based resistor and a cordierite–glass substrate have been sintered at temperatures of 850° and 900°C. The microstructure of the resistor/substrate interface has been investigated using scanning and transmission electron microscopy, and its correlation to the overall resistance has been discussed. X-ray diffractometry has revealed that lead ruthenate pyrochlore (Pb₂Ru₂O_6.5) in peak-fired thick-film resistors (TFRs) disappears and the co-fired samples contain only RuO₂ in the resistor film when sintered at 900°C. The overall resistance of the LTCC resistors is increased by a factor of ∼3 when temperature is increased from 850°C to 900°C. The cordierite–glass composition of the initial substrate reacts with glass in the resistor film. The greatly extended layer of the resistor/substrate interface that contains the conductor particles is either broad or diffuse, which contrasts the abrupt interface that often is observed in conventional TFRs. This layer contains predominantly faceted platelike crystals of anorthite, in addition to other phases (such as diopside, sapphirine, and cristobalite) that apparently crystallize during co-firing as vitrification and chemical reactions between glass compositions of the substrate and the resistor occur. The increase in the resistance of the LTCC resistors is attributed to the interruption of the conducting path by platelike anorthite crystals that are produced in the resistor/substrate interface when subjected to co-firing.     

Structural and Dielectric Characterization of Nanocrystalline (Ba, Pb)ZrO3 Developed by Reverse Micellar Synthesis

Nanocrystalline zirconates of barium and lead have been synthesized using a modified reverse micellar route (avoiding alkoxides). The entire solid solution of Ba_{1− x} Pb _x ZrO₃ (0≤x≤1) has been synthesized for the first time. Powder X-ray diffraction studies show the monophasic nature of the powders after heating at 800°C except minor impurities of ZrO₂ (2%–3%) at a higher lead content ( x =0.50 and 0.75). The oxides crystallize in the cubic structure till x =0.25; for higher values, they crystallize in the orthorhombic structure. The particle size obtained from X-ray line-broadening studies and transmission electron microscopic studies is found to be in the range of 20–60 nm for all the oxides obtained after heating at 800^oC. The grain size of the solid solution of Ba_{1− x} Pb _x ZrO₃ (0≤ x ≤1) was found to increase with the lead content. The dielectric constant of the solids corresponding to Ba_{1− x} Pb _x ZrO₃ (0≤ x ≤1) was found to be a maximum at x =0.50. Note that the cubic to orthorhombic transition is also observed between x =0.25 and 0.5. Dielectric properties with respect to variation in frequency and temperature are reported for these nanocrystalline oxides for the first time.     

Effect of Rare-Earth Additives on Electromechanical Properties of Modified Lead Titanate Ceramics

The influence of rare-earth additives, such as La, Nd, Sm, and Gd, and poling conditions on the electromechanical properties of (Pb_{1−3 x /2}Ln _x )(Ti_0.98Mn_0.02)O₃ compositions, x = 0.04–0.12, were investigated. The type and amount of additive were found to affect the lattice anisotropy, dielectric constant, and electromechanical properties. A large electromechanical anisotropy ( k _t / k _p ) could be obtained in 10 mol% Sm-modified and 8 mol% Gd-modified lead titanate ceramics, and seemed to correlate to a low Poisson's ratio.     

Microstructure and Dielectric Properties of Lead Magnesium Niobate-Pyrochlore Diphasic Mixtures

The microstructure and dielectric properties of lead magnesium niobate (PMN) ceramics containing the determental second phase, pyrochlore, have been studied. The pyrochlore phase within PMN was found to be present as isolated grains in the microstructure with a composition of Pb₂(Mg_0.25Nb_1.75)O_6.625 and a preferred octahedral morphology. The amount of pyrochlore second phase can be easily controlled by changing the Mg/Nb ratio on the B-site of the perovskite structure, and a series of diphasic mixtures from near-pure PMN to near-pure pyrochlore were obtained. Its effect on the permittivity follows the isolated Wiener's mixture rule up to a pyrochlore content of 18 vol%.     

Mechanical Activation-Assisted Synthesis of Pb(Fe2/3W1/3)O3

Perovskite Pb(Fe_2/3W_1/3)O₃ (PFW) was prepared via a mechanical activation-assisted synthesis route from mixed oxides of PbO, Fe₂O₃, and WO₃. The mechanically activated oxide mixture, which exhibited a specific area of >10 m²/g, underwent phase conversion from nanocrystalline lead tungstate (PbWO₄) and pyrochlore (Pb₂FeWO_6.5) phases on sintering to yield perovskite PFW, although the formation of perovskite phase was not triggered by mechanical activation. When heated to 700°C, >98% perovskite phase was formed in the mechanically activated oxide mixture. The perovskite phase was sintered to a density of ∼99% of theoretical density at 870°C for 2 h. The sintered PFW exhibited a dielectric constant of 9800 at 10 kHz, which was ∼30% higher than that of the PFW derived from the oxide mixture that was not subjected to mechanical activation.     

Dielectric Properties of Pb0.97La0.02(Zr0.87−xSnxTi0.13)O3 Thin Films with Compositions near the Morphotropic Phase Boundary

Pb_0.97La_0.02(Zr_{0.87− x} Sn _x Ti_0.13)O₃ (PLZST, x =0.27, 0.17, 0.07)) thin films with the compositions in ferroelectric rhombohedral (FE_R) region, near the morphotropic phase boundary (MPB), were deposited on the Pt-electroded silicon (PtSi) substrates by the sol–gel process. The phase structure and surface morphology of PLZST thin films were analyzed by XRD and SEM, respectively. The dc electric field and temperature-dependent dielectric properties of the PLZST thin films were investigated in detail. The results indicated that the dielectric constant, remnant polarization, and the Curie temperature ( T _c) of PLZST films were elevated with the decrease of Sn content. Hence, the larger dielectric tunability (τ) was obtained for PLZST thin films with x =0.07, and the maximum τ value was 78.1%.     

Perovskite Phase Formation in the Relaxor System [Pb(Fe1/2Nb1/2)O3]1-x–[Pb(Zn1/3Nb2/3)O3]x

Phase formation and dielectric properties of the compositions in the system [Pb(Fe_1/2Nb_1/2)O₃]₁_ _x –[Pb(Zn_1/3Nb_2/3)O₃] _x were investigated as possible materials for multilayer ceramic capacitors. The formation of the phase with perovskite structure and dielectric properties of ceramics at room temperature in the entire composition range are presented. The undesirable pyrochlore phase can be suppressed up to x = 0.6 by adopting calcination of B-site oxides, followed by reaction with PbO. Compositions in the single-phase range can be sintered at less than 1000°C.     

Solubility of Ruthenium Dioxide in Lead Borosilicate Glasses

The solubility of RuO₂ in a lead borosilicate glass was measured in the range of 700° to 1000°C. The effect of dissolved alumina in the glass was studied in the same range using 2%, 4%, 6%, and 10% dissolved alumina. The solubility showed an exponential dependence on temperature, and the dissolved alumina decreased the solubility at any given temperature; however, the solubility was essentially the same in all of the alumina-containing glasses. The Kelvin equation was used to deduce the concentration of RuO₂ in glass as a function of the curvature of the RuO₂ particles, and the dependence was found to be quite small. The implications of the results in processing thick-film resistors using RuO₂ as the conductive phase and these glasses as insulating phases on an alumina substrate are discussed.     

Electrical Conductivity of RuO2–Borosilicate Glasses: Effect of the Synthesis Route

Although glass–RuO₂ composites are well known for their particular electrical properties, the reasons for their very low percolation thresholds are still subject to debate. In this paper, a detailed study of the influence of various experimental parameters (temperature, RuO₂ content, stirring, etc.) on the electrical conductivity and, in particular, on the percolation threshold in borosilicate glass–RuO₂ composites is presented. This percolation threshold is shown to increase by a factor of two (from 0.6 to 1.2 vol%) when stirring is applied during synthesis and by more than a factor of three (>2.1 vol%) when a sol–gel route is used. Besides, the study of various synthesis temperatures reveals that the electronic part of the electrical conductivity is highly correlated to Ru solubility in the glass matrix. It can be concluded from these various experiments that both the presence of dissolved ruthenium in the glass matrix and the possibility of RuO₂ particles to rearrange in the melt in order to form kind of a network are necessary for a low percolation threshold.     

Thermal Expansion Properties of Lanthanum-Substituted Lead Titanate Ceramics

The Pb_{1− x} La _x TiO₃ powders were prepared by a sol–gel route in the composition range from x =0.0 to 0.3 in 0.05 increments. The lattice parameters of Pb_{1− x} La _x TiO₃ ( x =0.0, 0.05, 0.10, 0.15, 0.20, 0.30) were determined by a high temperature X-ray diffraction, and the intrinsic thermal expansion coefficients were obtained in the temperature range from room temperature to 900°C. The Pb_{1− x} La _x TiO₃ compounds ( x =0.10, 0.15, 0.20) in the tetragonal system exhibit low thermal expansion behaviors. As the La content is increased, the phase transition in the Pb_{1− x} La _x TiO₃ changes continuously from conventional to diffuse phase transition.     

Mechanism of Formation of Perovskite Phase and Dielectric Properties of Pb(Zn,Mg)1/3Nb2/3O3 Ceramics Prepared by Columbite Precursor Routes

The mechanism of formation of the perovskite phase and the dielectric properties of Pb(Zn,Mg)_1/3Nb_2/3O₃ (PZMN) ceramics were examined using two different types of columbite precursors, (Mg,Zn)Nb₂O₆ (MZN) and MgNb₂O₆+ ZnNb₂O₆ (MN + ZN). The formation of perovskite phase in the PbO + MN + ZN system is characterized by an initial rapid formation of Mg-rich perovskite phase, followed by a sluggish formation of Zn-rich perovskite phase. On the other hand, due to the formation of pyrochlore phase of mixed divalent cations Pb_{2– x} (Zn,Mg)_yNb_2−yO_{7− x −3y/2}, the pyrochlore/perovskite transformation in the PbO + MZN system proceeded uniformly with a spatial homogeneity. Further analysis suggested that the formation of perovskite phase is a diffusion-controlled process. The degree of diffuseness of the rhombohedral/cubic phase transition (DPT) is higher in the PbO + MN + ZN system than in the PbO + MZN specimen for T > T _max (temperature of the dielectric permittivity maximum), indicating a broadened compositional distribution of the B-site cations in the PbO + MN + ZN system.     

A Soft Chemistry Route for the Synthesis of Nanostructured Pb2Ru2O6.5 with a Controlled Stoichiometry

A new chemical route to synthesize nanostructured lead ruthenium pyrochlore (Pb₂Ru₂O_6.5) powders was developed. The synthesis was performed starting from a metal nitrate aqueous solution and N , N , N ', N ' tetramethylethylendiamine (TMEDA). The amine, which is a mild basic chelating agent, was able to control the simultaneous precipitation of lead and ruthenium oxo/hydroxides. For the sake of comparison, Pb₂Ru₂O_6.5 powders were prepared using the more conventional polymeric precursor method. The new method was effective for the synthesis of nanostructured Pb₂Ru₂O_6.5 powders already stable at 400°C and up to 1000°C.     

Fabrication and Electrical Properties of Mn–Ni–Co–Cu–Si Oxides Negative Temperature Coefficient Thermistors

The microstructure and electrical properties of Mn–Ni–Co–Cu–Si oxides negative temperature coefficient (NTC) thermistors were studied. The as-sintered (Mn_1.62Ni_0.72Co_{0.57− x} Cu _x Si_0.09)O₄ (0≤ x ≤0.12) and (Mn_1.2Ni_0.78Co_{0.87− x} Cu_0.15Si _x )O₄ (0≤ x ≤ 0.15) ceramics showed the solid solutions of Mn–Ni–Co–Cu–Si oxides with a cubic spinel structure. The addition of SiO₂ led to an increase in the temperature coefficient of resistivity. This demonstrates that the SiO₂ addition is desirable for developing highly sensitive NTC thermistors. In addition, the resistivity and the temperature coefficient of resistivity for (Mn_1.62Ni_0.72Co_{0.57− x} Cu _x Si_0.09)O₄ and (Mn_1.2Ni_0.78Co_{0.87− x} Cu_0.15Si _x )O₄ NTC thermistors were controlled by changing the composition.     

Synthesis of Pb(Mg1/3Nb2/3)O3 in Excess Lead Oxide by Mechanical Activation

Twenty hours of mechanical activation of mixed oxides at room temperature led to the formation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) in excess PbO. The crystallinity of the activation-derived perovskite PMN phase was further established when the activated PMN–PbO phase mixture was subjected to calcination at 800°C. Pyrochlores, such as Pb₃Nb₄O₁₃ and Pb₂Nb₂O₇, were not observed as transitional phases on mechanical activation and subsequent calcination, although 50% excess PbO was deliberately added. The perovskite PMN phase was recovered by washing off excess PbO using acetic acid solution at room temperature. It was sintered to a relative density of 98.9% of theoretical at 1200°C for 1 h and the sintered PMN exhibited a dielectric constant of ∼14 000 at 100 Hz and a Curie temperature of −11°C.     

Dielectric-State Analysis in Solid-Solution Pb(Yb1/2Ta1/2)O3–Pb(Lu1/2Nb1/2)O3

The structure and temperature dependence of complex lead perovskite dielectrics were investigated for the system (1 − x )Pb(Yb_1/2Ta_1/2)O₃– x Pb(Lu_1/2Nb_1/2)O₃. Superlattice reflections for the compositions 0.8 < x < 1.0 were observed by X-ray diffractometry, and the temperature-composition dielectric-state diagram was determined. In the present study, the disordered middle composition, with 0.2 < x < 0.8, showed a diffuse paraelectric–ferroelectric phase transition, whereas the ordered end compositions, with 0 ≤ x < 0.2 and 0.8 < x ≤ 1.0, revealed successive sharp paraelectric–antiferroelectric and weak antiferroelectric–ferroelectric phase transitions. The dielectric state was confirmed by examining the variation of polarization ( P ) with electric field ( E ).     

Fatigue-Free Lead Zirconate Titanate (Pb(Zr,Ti)O3)-Based Capacitors Co-modified by Lanthanum and Lithium for Nonvolatile Memories

Pb_0.98(La_{1− x} Li _x )_0.02(Zr_0.55Ti_0.45)O₃(PLLZT with 0.1 ≤ x ≤ 0.7) thin films were sol-gel-grown on Pt(111)/Ti/SiO₂/Si substrates, employing a thin lead zirconate titanate (PZT) template layer. Films annealed at >550°C showed a highly (111)-oriented preferential growth. Typical values of the switchable remanent polarization (2 P _r) and the coercive field ( E _c) of the PLLZT/PZT/Pt film capacitor for x = 0.3 were 50 μC/cm² and 39 kV/cm, respectively, at 5 V. All the PLLZT/PZT/Pt capacitors (for 0.1 ≤ x ≤ 0.7) exhibited fatigue-free behavior up to 6.5 × 10¹⁰ switching cycles, a quite stable charge retention profile with time, and high 2 P _rvalues, all which assure their suitability for nonvolatile ferroelectric memories.     

Thermodynamic Properties of RuO2

The free energy change for the reaction RuO₂( s )+4Cu( s ) = 2Cu₂O( s )+Ru( s ) was determined from 600° to 1000°C from emf measurements on a solid oxide galvanic cell using a stabilized ZrO₂ electrolyte. The cell was designed to minimize the reduction of RuO₂ by the gas phase. The results were used to develop an equation for the standard molar free energy of formation of RuO₂:
The standard molar enthalpy and entropy of formation of RuO₂ at 298°K were calculated to be −72,430 ±200 cal/mol and –40.44±0.2 eu, respectively, using the available heat capacity data. The absolute entropy of RuO₂ at 298°K was calculated to be 15.46±0.2 eu.