Microstructure and Dielectric Properties of a LaAlO3–TiO2 Diffusion Couple

Near-field scanning microwave microscopy was applied to investigate the dielectric properties and microstructure in a polycrystalline LaAlO₃–TiO₂ diffusion couple, which included three regions containing different phases and microstructures. Relatively low (La₂Ti₄Al₁₈O₃₈), high (α-La_2/3TiO₃), and intermediate (La₄Ti₉O₂₄) dielectric constant phases were distinguished at the inter-diffusion interface in optical, backscattered electron scanning electron microscopy, and scanning microwave microscopy (SMM) images. The relative ranking of dielectric constants based on SMM examination was as follows: TiO₂>α-La_2/3TiO₃>La₄Ti₉O₂₄>LaAlO₃>La₂Ti₄Al₁₈O₃₈. La_2/3TiO₃ and LaAlO₃ will form solid solutions in the LaAlO₃-rich region. The reaction paths leading to phase development are discussed.     

X-ray Diffraction and Microstructural Investigation of the Al2O3-La2O3-TiO2 System

The ternary phase diagram of Al₂O₃-La₂O₃-TiO₂ at 1400°C was determined with 12 compatibility triangles. Al₂O₃ stabilizes the A-site-deficient La_2/3TiO₃ perovskite structure. According to XRD and microstructural investigations, the solid solution extends along the La_2/3TiO₃-LaAlO₃ tie line from at least 4 mol% LaAlO₃ to pure LaAlO₃. With increasing LaAlO₃ content, the stabilized La_2/3TiO₃ structure changes from orthorombic via tetragonal to cubic.     

Microstructural Characteristics of Strontium Magnesium Niobate

Microstructural studies were conducted on the domain boundaries in Sr(Mg_1/3Nb_2/3)O₃ (SMN) complex perovskite compound using X-ray diffractometry and transmission electron microscopy. Both the 1:2 chemical ordering of B-site cations and the tilting of oxygen octahedra were involved in SMN. SMN had a 1:2 ordered monoclinic unit cell, which was distorted by the antiphase tilting of oxygen octahedra. Two types of domain boundaries were found: the antiphase boundaries (APBs), which are not confined crystallographically, and the ferroelastic domain boundaries, which were parallel to the crystallographic planes. SMN had the superlattice reflections of type ±⅙[111] and ±½[111] in the electron diffraction patterns, which cannot be indexed in terms of the 1:2 ordered trigonal phase with only a hexagonal unit cell. The presence of the ferroelastic domains that contained both the 1:2 ordered and the antiphase tilting had been verified by a high-resolution transmission electron microscopy lattice image. The structure of SMN was well explained by a model proposed by other researchers. The formation of the 1:2 ordered domains preceded the ferroelastic domains. Normally, the growth of the ferroelastic domain is not affected by APBs, but it is interrupted by them when the driving force for growth is insufficient, resulting in the stoppage of the domains at APBs.     

Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

Thermal Expansion Behavior of Titanium-Doped La(Sr)CrO3 Solid Oxide Fuel Cell Interconnects

La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10⁻⁶/°C in air, 10.5 × 10⁻⁶/°C under a He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 10.9 × 10⁻⁶/°C in a H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁹ atm at 1000°C). La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect.     

Defect Structure and Electrical Properties of La1−ySryFe1−xAlxO3−δ

La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were studied as potential materials for solid-oxide fuel cell (SOFC) cathodes. The phase relations in the LaFeO₃–SrFeO_3−δ–LaAlO₃ system were investigated by X-ray powder diffraction analysis. The defect structure of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites was investigated by Mössbauer spectroscopy and weight-loss analysis. Relations between the nonstoichiometry and the conductivity of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were investigated. The incorporation of aluminum ( x ) into LaFe_{1− x} Al_xO₃ was found to have no influence on the defect structure but to decrease the conductivity. The incorporation of strontium ( y ) into La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ promotes the formation of anion vacancies and Fe⁴⁺ that lead to higher conductivity.     

The Li2O–Nb2O5–TiO2 Composite Microwave Dielectric Ceramics with Adjustable Permittivities

A group of new y M-phase/(1− y ) Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ composite ceramics with adjustable permittivities for low-temperature co-fired ceramic applications was initially investigated in the study. The 0.5 M-phase/0.5 Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ ( x =0.01, 0.02, 0.04, 0.06, 0.081) composite ceramics were first investigated to find the appropriate "Li₂TiO₃ss" composition ( x value). The best dielectric properties of ɛ_r=40.1, Q × f values up to 9318 GHz, τ_f=25 ppm/°C, were obtained for the ceramics composites at x =0.02. Based on the good dielectric properties, the suitable "Li₂TiO₃ss" composition with x =0.02 was mixed with the Li_1.0Nb_0.6Ti_0.5O₃ powder as the ratio of y "M-phase"/(1− y ) "Li₂TiO₃ss" ( y =0.2, 0.4, 0.5, 0.6, 0.8). By adjusting the y values, the group of composite ceramics could exhibit largely are adjustable permittivities varying from ∼20 to ∼60, while Q × f and τ_f values relatively good. Nevertheless, in this study, because there are interactions between the M-phase and Li₂TiO₃ss during sintering process, their microwave dielectric properties could not be predicted precisely by the empirical model.     

Subsolidus Phase Equilibria in the System Na2O-Bi2O3-TiO2 at 1000°C

Subsolidus phase relations in the system Na₂O-Bi₂O₃-TiO₂ at 1000°C were investigated by solid-state reaction techniques and X-ray diffraction methods. Five ternary compounds were observed in the system: Na_0.5Bi_4.5Ti₄O₁₅; Na_0.5Bi_0.5TiO₃; a cubic pyrochlore solid solution composed of xNa₂O.25Bi₂O₃.(75−;x) TiO₂ where x is 2.5 to 3.75; a new compound Na_0.5Bi_8.5Ti₇O₂₇ indexed with the orthorhombic cell of a = 5.45, b = 5.42, and c = 36.8 Å; and an unidentified phase with the probable composition NaBiTi₆O₁₄.     

Structure and Electrical Properties of A-site-Deficient Perovskite Compounds in the La2/3TiO3-La1/3NbO3 System

The structure and electrical properties of an A-site-deficient perovskite compound found in the La_2/3TiO₃-La_1/3NbO₃ system were investigated. The composition of the perovskite compound seemed to be very close to La_0.633(Ti_0.90Nb_0.10)O₃. X-ray diffraction analysis revealed a superstructure with a doubled c -axis parameter, resulting from an ordered arrangement of the A-site cation vacancies. Impedance measurements on the compound showed that La_0.633(Ti_0.90Nb_0.10)O₃ had high ionic conductivity at relatively low temperature (<770 K) and increased electronic conduction at high temperature (>770 K). The bulk ionic conductivity was comparable with that of La_0.683(Ti_0.95Al_0.05)O₃, which has the highest ionic conductivity among the La-(Ti,Al)-O perovskite compounds.     

Photoluminescence Enhancement of (La0.95Eu0.05)2Ti2O7 Nanophosphors via Li+ Doping

Li⁺ ions have been successfully doped into the La sites of (La_0.95Eu_0.05)₂Ti₂O₇ nanocrystals through a facile citric acid sol–gel method. The doping concentration of Li⁺ ions can be as high as 15 mol%. Photoluminescence (PL) performances of the obtained samples have been investigated. The results showed that a doping with small number of Li⁺ ions improves the PL intensity of the synthesized La₂Ti₂O₇:Eu³⁺ nanophosphors. The highest emission intensity was observed using the formula of (La_0.92Eu_0.05Li_0.03)₂Ti₂O₇, whose brightness was increased by almost 20% in comparison with that of (La_0.95Eu_0.05)₂Ti₂O₇.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

Effects of La2O3/ZnO Additives on Microstructure And Microwave Dielectric Properties of Zr0.8Sn0.2TiO4 Ceramics

Dielectric ceramics of Zr_0.8Sn_0.2TiO₄ containing La₂O₃ and ZnO as sintering aids were prepared and investigated for microstructure and microwave dielectric properties. Low-level doping with La₂O₃ and ZnO (up to 0.30 wt%) is good for densification and dielectric properties. These additives do not affect the dielectric constant and the temperature coefficient. Dielectric losses increase significantly at additive levels higher than 0.15 wt%. The combined additives La₂O₃ and ZnO act as grain growth enhancers. With 0.15 wt% additives, a ceramic having a dielectric constant, a quality factor, and a temperature coefficient of frequency at 4.2 GHz of 37.6, 12 800, and –2.9 ppm/°C, respectively, was obtained. The quality factor was considerably improved by prolonged sintering.     

Extended Defects in ZnO Ceramics Containing Bi4Ti3O12 Additive

Extended defects in ZnO ceramics containing, 6 wt% Bi₄Ti₃O₁₂ were studied by analytical electron microscopy. Apart from basal plane condensation stacking faults, which are also present in as-received ZnO, extended defects related to the presence of Bi₄Ti₃O₁₂ were observed. In samples sintered at 900°C they lie in the basal or in the prismatic     planes and they quite often form closed loops, whereas they form serpentine-shaped boundaries in samples sintered at 1200°C. Evidence is given that they are inversion boundaries. Their TEM image characteristics, as well as the unambiguous presence of Ti at the boundaries, suggest that they are formed due to the presence of 2-D coherent precipitates of Ti-rich (possibly Zn₂TiO₄-type spinel) phase.     

Solid-State Synthesis of 70 nm Li4Ti5O12 Particles by Mechanically Activating Intermediates with Amino Acids

A novel synthetic method of fine particles of highly pure Li₄Ti₅O₁₂ with the smallest possible particle size was proposed. A stoichiometric mixture of CH₃COOLi·2H₂O and anatase fine particles of ca. 50 nm was calcined at 773 K to obtain an intermediate, comprising Li₂TiO₃ and unreacted titania. The intermediate was then intensively milled by a planetary ball mill for 1 h. After adding an amino acid, glycine, alanine (ALA), or l -phenylalanine (PHE), the mixture was further milled under a milder condition for another 3 h and subjected to a second calcination at 973 K for 1 h to obtain Li₄Ti₅O₁₂ with a phase purity above 97%. When we used ALA or PHE as an additive, the microscopically determined average particle size of Li₄Ti₅O₁₂ was 70±10 nm. The mechanisms and significance of the present two-step calcination with mechanical activation and addition of an amino acid to the intermediate were discussed in terms of the topotactic nature of transformation between the two layer-structured species: Li₂TiO₃ and Li₄Ti₅O₁₂.     

Dielectric and Ferroelectric Properties of Nanocrystalline Bi2Ti2O7 Prepared by a Metallorganic Decomposition Method

Nanopowders of Bi₂Ti₂O₇ were synthesized by a metallorganic decomposition (MOD) technique. Pure Bi₂Ti₂O₇ nanocrystals formed after annealing at 550°C for 5 min. X-ray patterns show that Bi₂₀TiO₃₂ is a metastable phase during Bi₂Ti₂O₇ formation. It was found that there were two peaks in the curves of the dielectric response as a function of temperature for pressed nanocrystalline Bi₂Ti₂O₇ samples. The Curie temperature decreases with decrease of grain size whereas the ferroelectric-ferroelectric phase transition temperature increases. The hysteresis loops observed also suggest that Bi₂Ti₂O₇ might belong to a ferroelectric material.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Structure–Property Relations in xBaTiO3–(1−x)La(Mg1/2Ti1/2)O3 Solid Solutions

The microwave dielectric properties and microstructures of compounds in the solid solution series x BaTiO₃–(1− x )La(Mg_1/2Ti_1/2)O₃ (BTLMT) have been investigated. The structural phase transitions that occur as a function of x have been studied and are related to changes in the dielectric properties. For compounds where x ≤ 0.1, X-ray diffraction (XRD) showed evidence of 1:1 ordering between Mg and Ti cations. For x ≤ 0.3, XRD and electron diffraction revealed that compounds were tilted in both antiphase and in-phase. However, for 0.3 < x < 0.7, only antiphase tilting was present. The temperature coefficient of resonant frequency (τ_f) vs the relative permittivity (ɛ_r) was linear until x = 0.5 at which point in the solid solution the transition to a nontilted structure resulted in nonlinear behavior. τ_f values close to zero (−2 ppm/°C) were achieved at x = 0.5 (ɛ_r∼ 60), which had a quality factor ( Q · f _o) of 9600 GHz.     

Preparation of the High-Quality Superconductor La1Ba2Cu3Oy

The relationship between the preparation procedure and superconducting properties of La₁Ba₂Cu₃O_y was studied. A series of samples was sintered in an N₂-gas atmosphere for various lengths of time ranging from 1 to 40 h, followed by a fixed annealing procedure in O₂. It was found that the shorter the sintering period, the higher was the oxygen content. The samples sintered for a period of less than 15 h contained excess oxygen compared with La₁Ba₂Cu₃O₇ and exhibited poor superconducting properties. The sample sintered for 40 h had an oxygen content y equal to 6.95, and had excellent superconducting properties. The mechanism for preparing high-quality La₁Ba₂Cu₃O_y is discussed.     

Defect-Chemical Aspects in the Synthesis of Metallic Conductors La1–xBaxCuO3 (x= 0.0 to 0.5)

Compositions of La_{1- x} Ba _x CuO₃, where x ranges from 0.0 to 0.5, were fired in air, oxygen-enriched air, and oxygen. Studies show that BaO cannot make a solid solution with LaCuO₃ without changing the basic structure. The resulting phases, in all attempts, were the binary compounds La₂CuO₄, La_{1- x} Ba _x CuO_3-δ ( x = 0.2 to 0.5), or their mixtures. All samples showed metallic conductivity. Extra oxygen in the reaction atmosphere appeared to encourage the formation of the LaCuO₃-based phases of La_{1- x} Ba _x CuO_3-δ ( x = 0.2 or 0.5). We provide a defect-chemical and thermodynamical explanation for this observation.     

30 kbar Phase Equilibria of the La2O3–SrO–CuO System at 950°C

Phase equilibria of the La₂O₃–SrO–CuO system have been determined at 950°C at 30 kbar (3 GPa). Stable phases at the apexes of the ternary phase diagram are CuO, La₂O₃, and SrO. Stable intermediate phases are La₂, CuO₄ and La₂Cu₂O₅ in the LaO_1.5–CuO binary and Sr₂CuO₃, SrCuO₂, and Sr₁₄Cu₂₄O₄₁ in the CuO–SrO binary. The La_{2– x} Sr _x -CuO_4–δ solid solution is stable for 0.00 is ≤ x ≤ 1.29, the La_{2– x} Sr_{1+ x} Cu₂O_6+δ solid solution is stable for 0.03 ≤ x ≤0.20, the La_{2– x} Sr _x Cu₂O_5–δ solid solution is stable for 0.00 ≤ x ≤1.08, and the La _x Sr_{14– x} Cu₂₄O₄₁ solid solution is stable for 0.00 ≤ x ≤ 6.15. The 30 kbar phase diagram differs from the 1 atm (0.1 MPa) and 10 kbar (1 GPa) results principally in the absence of La_{1– x} Sr_{2+ x} Cu₂O_5.5+δ as a stable phase and the extended range of the La_{2– x} Sr _x Cu₂O_5–δ solid solution at 30 kbar.