Thermodynamic assessment of BaO–Ln2O3 (Ln = La,Pr, Eu,Gd, Er) systems

Heat capacities and enthalpies of formation of BaGd₂O₄ were determined by high-temperature differential scanning calorimetry and high-temperature oxide melt solution calorimetry, respectively. Thermodynamic stability of BaLn₂O₄ compounds increases with decreasing Ln³⁺ ionic radius. Previously reported data on BaNd₂O₄ and BaSm₂O₄ corroborate this trend. Missing data for compounds in BaO–Ln₂O₃ (Ln = La, Pr, Eu, Er) systems were estimated from established relations, thermodynamic assessment was performed, and binary phase diagrams were calculated.     

Sintering behavior and microwave dielectric characteristics of BaO–Sm2O3–4TiO2 ceramics with B2O3 and BaB2O4 addition

We studied the low temperature sintering and the reaction in BaO–Sm₂O₃–4TiO₂ ceramics with boron-based additives for the application to microwave dielectric devices. The amount of the boride glasses of B₂O₃ and BaB₂O₄ was varied from 1 to 10 wt.% and the green compacts were sintered in the temperature range of 900–1200 °C for 2 h. When B₂O₃ was added, second phases of Sm₂Ti₂O₇, BaTi(BO₃)₂, Ba₂Ti₉O₂₀, and TiO₂ were formed, while BaB₂O₄ addition resulted in the formation of BaSm₂Ti₄O₁₂ single phase without second phases. On the basis of these results, it is regarded that the B₂O₃ is a reactive glass and the BaB₂O₄ is a non-reactive glass. The second-phase development, sintering behavior and microwave dielectric characteristics of BaO–Sm₂O₃–4TiO₂ ceramics were examined.     

Phase equilibria and stability of intermediate phases in the Sm2O3–BaO–Fe2O3 system

Phase equilibria in the ½ Sm₂O₃–BaO–½ Fe₂O₃ system were systematically studied at 1100°C in air. Two individual compounds: Sm_1.875Ba_3.125Fe₅O_15-δ and Ba₃SmFe₂O_7.5+δ have been detected in the studied system at 1100°C in air. One more complex oxide with double perovskite structure SmBaFe₂O_5+w has been obtained under reduced oxygen partial pressure. Thermodynamic stability of SmBaFe₂O_5+w and its thermal stability in metastable state in air were determined. The subsolidus phase diagram for the ½ Sm₂O₃ – BaO – ½ Fe₂O₃ system at 1100°C in air has been constructed.     

Thermodynamics of copper‐manganese and copper‐iron spinel solid solutions

High‐temperature oxide melt solution calorimetry has been performed to investigate the energetics of spinel solid solutions in the Mn₃O₄‐CuMn₂O₄ and Fe₃O₄‐CuFe₂O₄ systems. The spinel solid solutions were synthesized by a ceramic route and calcined at appropriate temperatures to obtain single phase samples. The drop solution enthalpies of the solid solutions in the Mn₃O₄‐CuMn₂O₄ system are the same within experimental error as the enthalpies of drop solution of mechanical mixtures of the end‐members, indicating a zero heat of mixing, i.e., ideal mixing in terms of enthalpy. In Fe₃O₄‐CuFe₂O₄, the drop solution enthalpy of the solid solutions shows a positive deviation from those of the mechanical mixture of the end‐members, suggesting negative mixing enthalpy. The formation enthalpies of the spinel solid solutions from their constituent oxides plus oxygen and from the elements were also determined.     

BaO−B2O3 system and its mysterious member Ba3B2O6

The first systematic study of the BaO–B₂O₃ system and barium orthoborate Ba₃B₂O₆ (3BaO·B₂O₃) was reported in 1949. Thereafter, the system was repeatedly refined but the structure of Ba₃B₂O₆ compound has not been adequately studied yet. In our study we have, for the first time, obtained the crystalline samples of Ba₃B₂O₆. The solved structure (Pbam, a = 13.5923(4) Å, b = 13.6702(4) Å, c = 14.8894(3) Å) belongs to the class of ‘anti‐zeolite’ borates with a pseudotetragonal [Ba₁₂(BO₃)₆]⁶⁺ cation pattern which contains channels along the c axis filled with anionic clusters. The Ba₃B₂O₆ compound may be regarded as a fluorine‐free end‐member of the Ba₃(BO₃)_2–xF_3x solid solution. The BaO–B₂O₃ phase diagram presented in our study is based on our research and literature data.     

Investigation of Sm2O3 additive on mechanical and electrical properties of Li2O‐ stabilized β″‐alumina electrolyte

Li₂O‐ stabilized β″‐alumina was synthesized by the double zeta process. The effect of Sm₂O₃ additive as the sintering aid, on microstructure, mechanical and electrical properties of Li₂O‐ stabilized β″‐alumina ceramics was studied by means of X‐ray diffraction, field emission scanning electron microscope, biaxial flexure test and ionic conductivity measurement. The results indicated that both the fracture strength and the ionic conductivity of the sample containing 0.2 wt% Sm₂O₃ improved approximately 52% and 54%, respectively, that can be attributed to its higher density, higher amount of β″‐Al₂O₃ phase and more uniform microstructure.     

Microstructure Evolution and Cation Interdiffusion in Thin Sm2O3 Films on CeO2 Substrates

Samaria (Sm₂O₃) thin films with a thickness of 180 nm were deposited on polycrystalline CeO₂ substrates by pulsed layer deposition to study phase formation and bulk cation interdiffusion in the Ce_1?xSm_xO_2?x/2 system after annealing at temperatures between 987°C and 1266°C. Transmission electron microscopy combined with electron diffraction and analytical techniques was applied for phase determination. The cubic fluorite and cubic bixbyite phases were observed at low and intermediate Sm concentrations. The monoclinic phase occurs only at very high Sm concentrations due to the low Ce‐solubility in Sm₂O₃. Furthermore, a cubic phase with I2₁3 structure was observed at higher Sm concentrations. Cation interdiffusion coefficients were derived from Sm concentration profiles across the Sm₂O₃/CeO₂ interface using the diffusion–couple solution of the diffusion equation. The temperature dependence of interdiffusion coefficients is well described by an Arrhenius‐type relation, which yields an activation enthalpy of 2.26 eV/atom for bulk cation interdiffusion.     

Phase evolution mechanism of non‐oxide bonded Al–Al2O3–MgO–ZrO2 composites at 1873 K in flowing nitrogen

In flowing nitrogen, non‐oxides such as Al₄O₄C, Al₂OC, Zr₂Al₃C₄, and MgAlON bonded Al₂O₃‐based composites were successfully prepared by a gaseous phase mass transfer pathway using aluminum, zirconia, alumina, and magnesia as raw materials at 1873 K, after an Al–AlN core‐shell structure was formed at 853 K. Resin bonded Al–Al₂O₃–MgO–ZrO₂ composites after sintering were characterized and analyzed by X‐ray diffraction (XRD), scanning electron microscope (SEM) and, energy dispersive spectrometer (EDS), and the influence of the MgO content on the sintered composites was studied. The results show that after sintering, the phase composition of the Al–Al₂O₃–ZrO₂ composite is Al₂O₃, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄, while the phase composition of the Al–Al₂O₃–ZrO₂ composite with the addition of MgO 6 wt% and MgO 12 wt% is Al₂O₃, MgAlON, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄ as well as Al₂O₃, MgAlON, Al₂OC, and Zr₂Al₃C₄, respectively. The addition of MgO changed the phase composition and distribution for the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites after sintering. When the added MgO content is equal to or more than 12 wt%, the Al₄O₄C in the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites is unable to exist in a stable phase.     

Phase equilibria study and thermodynamic description of the BaO‐CaO‐Al2O3 system

A combined experimental investigation and thermodynamic assessment was performed for the BaO‐CaO‐Al₂O₃ system. By using a high‐temperature equilibration/quenching technique and scanning electron microscopy, electron probe microanalysis, and X‐ray powder diffraction analysis, the phase equilibria at 1500°C and phase stability of BaCa₂Al₈O₁₅ phase were determined. An extensive literature survey was conducted for the experimental and thermodynamic modeling data of the BaO‐CaO‐Al₂O₃ system. According to the literature data and the present measurements, a thermodynamic assessment was made in order to obtain a set of self‐consistent thermodynamic parameters to describe the BaO‐CaO‐Al₂O₃ system. Based on the thermodynamic parameters acquired in this work, isothermal sections at 1100°C, 1250°C, 1400°C, 1475°C, and 1500°C and the BaO·Al₂O₃‐CaO·Al₂O₃ and BaO·6Al₂O₃‐CaO·6Al₂O₃ joints were calculated and compared with the available experimental data.     

Co‐synthesis of Sm0.5Sr0.5CoO3‐Sm0.2Ce0.8O1.9 Composite Cathode with Enhanced Electrochemical Property for Intermediate Temperature SOFCs

A 70 wt.% Sm_0.5Sr_0.5CoO₃ – 30 wt.% Sm_0.2Ce_0.8O_1.9 (SSC–SDC73) composite cathode was co‐synthesized by a facile one‐step sol–gel method, which showed lower polarization resistance and overpotential than those of physically mixed SSC–SDC73 cathode. The polarization resistance of co‐synthesized SSC–SDC73 cathode at 800 °C was as low as 0.03 Ω cm² in air. Scanning electron microscopy (SEM) images showed that the enhanced electrochemical property was mainly attributed to the smaller grains and good dispersion of SSC and SDC phases within the composite cathode, leading to an increase in three‐phase boundary length. The dependence of polarization resistance with oxygen partial pressure indicated that the rate‐limiting step for oxygen reduction reaction was the dissociation of molecular oxygen to atomic oxygen process. An anode supported fuel cell with a co‐synthesized SSC–SDC73 cathode exhibited a peak power density of 924 mW cm⁻² at 800 °C. Our results suggested that co‐synthesized composite was a promising cathode for intermediate temperature solid oxide fuel cells (IT‐SOFCs).     

Low‐firing and temperature stable microwave dielectric ceramics: Ba2LnV3O11 (Ln = Nd,Sm)

Low‐firing and temperature stable microwave dielectric ceramics of Ba₂LnV₃O₁₁ (Ln = Nd, Sm) were prepared by solid‐state reaction. X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the phase purity, crystal structure, sintering behavior, and microstructure. The XRD patterns indicated that Ba₂LnV₃O₁₁ (Ln = Nd, Sm) ceramics belong to monoclinic crystal system with P2₁/c space group in the whole sintering temperature range (800°C ‐900°C). Both ceramics could be well densified at 880°C for 4 hours with relative densities higher than 96%. The Ba₂LnV₃O₁₁ (Ln = Nd, Sm) samples sintered at 880°C for 4 hours exhibited excellent microwave dielectric properties: ε_r = 12.05, Q × f = 23 010 GHz, τ_f = ?7.7 ppm/°C, and ε_r = 12.19, Q × f = 27 120 GHz, τ_f = ?16.2 ppm/°C, respectively. Besides, Ba₂LnV₃O₁₁ (Ln = Nd, Sm) ceramics could be well co‐fired with the silver electrode at 880°C.     

Structure and property of multicomponent germanate glass containing Y2O3

Abstract

CaO–BaO–Al₂O₃–SiO₂–GeO₂ glasses doped with 0·00–16·67 wt-% Y₂O₃ have been prepared by conventional melt quenching method. The influence of Y₂O₃ addition on the properties and structure of the glasses has been investigated. The results show that, with the introducing of Y₂O₃, the density, glass transition temperature and thermal expansion coefficient of the glass increase but the chemical durability declines. In addition, compared with the glass without Y₂O₃, the bend strength of the glass including 4·76 wt-% Y₂O₃ increased from 54 to 110 Mpa. The possible mechanism is that yttria acts as a network former in the structure and makes the island shape network unit repolymerisation by forming Ge–O–Y bond. The absorption strength caused by hydroxyl vibration decreased up to completely disappearance as the Y₂O₃ content increased continuously. The introducing of yttria in the composition causes the conversion from four coordination to higher coordination germanium, which decreases non-bridge oxygen (NBO) and weakens hydroxyl characteristic absorption.     

MW dielectrics with perovskite-like structure based on Sm-containing systems

The material formation conditions in the systems BaO–Sm₂O₃–nTiO₂ (n=4–5) and (Ba_1-xCa_x)O– Sm₂O₃–4·5TiO₂ (where x=0–0·1), and the electrophysical properties of materials were investigated. It was shown that in samarium-containing specimens there are temperature anomalies of permittivity, which make it possible to control the dielectric characteristics of microwave materials.     

Subsolidus Phase Relations of the BaO–Y2O3–MnO2 System in Air

The subsolidus phase relations of the BaO–Y₂O₃–MnO₂ system have been investigated in air. There are eight binary compounds, a new ternary compound and 11 three‐phase regions in this system. The ternary compound with the BaO:YO_1.5:MnO₂ molar ratio of 8:2:5 was indexed by a rhombohedral lattice with a = 5.7929 (6) and c = 28.586 (4) Å. The synthesized compounds were determined to be stoichiometric except for the Ba_1?xY_xMnO₃ phase (x ≤ 0.01).     

Surface Properties of CaO (or BaO)–La2O3–MgO Catalysts and Their Performance in Oxidative Coupling of Methane

CaO–La₂O₃–MgO and BaO–La₂O₃–MgO catalysts with different compositions have been studied for their bulk and surface properties (viz. crystal phases, surface area, acidity/acid strength distribution, basicity/base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) at different processing conditions (reaction temperature, 700–850°C; CH₄/O₂ ratio in feed, 3·0, 4·0 and 8·0 and GHSV, 102000 and 204000 cm³ g⁻¹ h⁻¹). The surface acidity and strong basicity of La₂O₃–MgO are found to be increased due to the addition of a third component (CaO or BaO), depending upon its concentration in the catalyst. The addition of CaO or BaO to La₂O₃–MgO OCM catalyst causes a significant improvement in its performance. Both the CaO- and BaO-containing catalysts show a high activity and selectivity at 800°C, whereas, the activity and selectivity of BaO-containing catalysts at 700°C is lower than that of CaO-containing catalysts. © 1997 SCI.     

Preparation and characterization of Pb(Lu1/2Nb1/2)O3–Pb(In1/2Nb1/2)O3–PbTiO3 ternary ferroelectric ceramics with high phase transition temperatures

To explore new relaxor‐PbTiO₃ systems for high‐power and high‐temperature electromechanical applications, a ternary ferroelectric ceramic system of Pb(Lu_1/2Nb_1/2)O₃–Pb(In_1/2Nb_1/2)O₃–PbTiO₃ (PLN–PIN–PT) have been investigated. The phase structure, dielectric, piezoelectric, and ferroelectric properties of the as‐prepared PLN–PIN–PT ceramics near the morphotropic phase boundary (MPB) were characterized. A high rhombohedral‐tetragonal phase transition temperature T_R‐T of 165°C and a high Curie temperature T_C of 345°C, together with a good piezoelectric coefficient d₃₃ of 420 pC/N, were obtained in 0.38PLN–0.20PIN–0.42PT ceramics. Furthermore, for (0.8?x)PLN–0.2PIN–xPT ceramics, the temperature‐dependent piezoelectric coefficients, coercive fields and electric‐field‐induced strains were further studied. At 175°C, their coercive fields were found to be above 9.5 kV/cm, which is higher than that of PMN–PT and soft P5H ceramics at room temperature, indicating PLN–PIN–PT ceramics to be one of the promising candidates in piezoelectric applications under high‐driven fields. The results presented here could benefit the development of relaxor‐PbTiO₃ with enhanced phase transition temperatures and coercive fields.     

White‐emitting phosphor Ba2B2O5:Ce3+, Tb3+, Sm3+: Luminescence,energy transfer,and thermal stability

A series of Ba₂B₂O₅: RE (RE=Ce³⁺/Tb³⁺/Sm³⁺) phosphors were synthesized using high‐temperature solid‐state reaction. The X‐ray diffraction (XRD), luminescent properties, and decay lifetimes are utilized to characterize the properties of the phosphors. The obtained phosphors can emit blue, green, and orange‐red light when single‐doped Ce³⁺, Tb³⁺, and Sm³⁺. The energy can transfer from Ce³⁺ to Tb³⁺ and Tb³⁺ to Sm³⁺ in Ba₂B₂O₅, but not from Ce³⁺ to Sm³⁺ in Ce³⁺ and Sm³⁺ codoped in Ba₂B₂O₅. However, the energy can transfer from Ce³⁺ to Sm³⁺ through the bridge role of Tb³⁺. We obtain white emission based on energy transfer of Ce³⁺→Tb³⁺→Sm³⁺ ions. These results reveal that Ce³⁺/Tb³⁺/Sm³⁺ can interact with each other in Ba₂B₂O₅, and Ba₂B₂O₅ may be a potential candidate host for white‐light‐emitting phosphors.     

Combined experimental and computational investigation of thermodynamics and phase equilibria in the CaO–TiO2 system

Phase relations in the CaO–TiO₂ system are of considerable interest in geology, metallurgy, and ceramics. Despite a number of studies of phase equilibria in the CaO–TiO₂ system, there are still some open questions regarding the stability of intermediate compounds. In this work, a series of specimens with different CaO:TiO₂ ratios were prepared by solid‐state reaction. The heat capacities of Ca₃Ti₂O₇ and Ca₄Ti₃O₁₀ from 300 to 1073 K were measured by differential scanning calorimetry and their formation enthalpies from the component oxides at 298 K were measured by high temperature oxide melt solution calorimetry. Using phase diagram information and thermodynamic data from the literature and the present measurements, thermodynamic optimization of the CaO–TiO₂ system was carried out by the CALPHAD technique. The phase diagram and the thermodynamic properties of the CaO–TiO₂ system were calculated using the obtained thermodynamic database, which clarify the stable and metastable phase equilibria of the system. The thermodynamic stability of the various compounds was discussed.     

Luminescence investigations of novel orange‐red fluorapatite KLaSr3(PO4)3F: Sm3+ phosphors with high thermal stability

Single‐phase KLaSr₃(PO₄)₃F: Sm³⁺ phosphors with fluorapatite structure were prepared via high‐temperature solid‐state method in air atmosphere for the first time. The X‐ray diffraction, scanning electron microscope, diffuse reflectance spectra, photoluminescence spectra, and temperature‐dependent emission spectra, as well as lifetimes were measured to characterize the as‐prepared phosphors. Phase results indicated that KLaSr₃(PO₄)₃F: Sm³⁺ belongs to hexagonal system with a space group of P‐6. Photoluminescence measurements showed the emission spectrum was composed of four sharp peaks at about 564, 602 (the strongest one), 646, and 702 nm, corresponding to the ⁴G_5/2‐⁶H_J (J=5/2, 7/2, 9/2, and 11/2) transitions of Sm³⁺ ions. The optimum doping concentration of Sm³⁺ ions was turned out to be 0.03 (mol), and the mechanism of energy transfer among Sm³⁺ ions was considered to be dipole‐dipole interaction by using Dexter's theory. In addition, the critical distance R_c for energy transfer among Sm³⁺ ions were calculated to be 9.97 Å according to Blasse concentration quenching method. The selected KLa_0.97Sr(PO₄)₃F: 0.03Sm³⁺ exhibited high thermal stability with an activation energy of 0.163 eV. Besides, the Commission International de l'Eclairage chromaticity coordinate of the phosphor were located in the orange‐reddish light region.     

Effect of temperature and stoichiometry on the long-range 1:2 cation order in BaZn1/3Ta2/3O3

We report on the compositional stability range, the degree of atomic order and Raman and optical spectra of the off-stoichiometric BaZn_1/3Ta_2/3O₃ (BZT) within the BaO–ZnO–Ta₂O₅ ternary diagram. Almost all off-stoichiometric BZT compositions equilibrated at 1200?°C show significant degree of the long-range 1:2 cation order ranging from 60% to 80%. Ceramics equilibrated at 1550?°C and annealed at 1450?°C show strong effect of composition on the 1:2 order. The regions where an 1:2 atomic order is robust to the deviation from stoichiometry include the off-stoichiometric compositions along the BZT–Ba₄Ta₂O₉, BZT–Ba₃Ta₂O₈, BZT–BaTa₂O₆, BZT–Ta₂O₅, BZT–ZnTa₂O₆ and BZT–Zn₄Ta₂O₉ (pseudo) tie lines. At the same time ceramics formulated along the BZT–BaO, BZT–ZnO, BZT–BaZnO₂, BZT–Ba₂ZnO₃ tie lines and BZT–‘Ba₃ZnO₄’ pseudo tie line show complete disorder. There is a very close correlation between the degree of the 1:2 order on one hand and the unit cell volume and lattice distortion on the other hand. The ordered BZT show contraction of the unit cell whereas disordered ceramics show expansion of the unit cell in the off-stoichiometric region. The pronounced signatures of the order-disorder phase transition in the Raman and optical spectra are discussed.