Phase Relations in the SrO–Bi2O3–Fe2O3 System

The phase relations in the composition region SrFeO_{3 –} –Fe₂O₃–BiFeO₃ are studied in air by x-ray diffraction and electron microscopy. The 1000°C phase compatibility diagram is constructed. Sr_{1 – x} Bi _x FeO_{3 –} solid solutions are prepared in the range 0 < x 0.8. Their lattice parameter is found to vary nonlinearly with x. Two new phases were identified: (Sr,Bi)₃Fe₄O _y (tetragonal lattice, a= 3.907(2) Å, c= 27.30(2) Å) and Sr_0.6Bi_0.4FeO_{3 –} (tetragonal lattice,a = 5.555(2) Å, c= 11.848(5) Å).     

Coating of hydroxyapatite on various substrates via hydrothermal reactions of Ca(edta)2- and phosphate

Hydroxyapatite was coated on various substrates such as 12 mol % ceria-doped tetragonal zirconia (12Ce-TZP), 3 mol % yttria-doped tetragonal zirconia (3Y-TZP), alumina, monetite coated titanium (Ti/CaHPO₄) and calcium titanate coated titanium (Ti/CaTiO₃) via hydrothermal reactions of Ca(edta)^2- and 0.05 M NaH₂PO₄ at initial pH 6 and 160–200 °C for 0.5–6 h. Rod-like particles of hydroxyapatite precipitated to form film on the substrates above 160 °C. The morphology of the film changed significantly depending on the characteristics of substrate, i.e. hydroxyapatite entirely coated the surfaces of 12Ce-TZP, Ti/CaHPO₄ and Ti/CaTiO₃ plates, but sparsely deposited on 3Y-TZP and Al₂O₃ plates. Film thickness increased with time (ca. 20 and 90 m on 12Ce-TZP plates for 0.5 and 6 h, respectively, at pH 6 and 200 °C). The adhesive strength of the film for the substrate was in the order, 12Ce-TZP/hydroxyapatite(28 MPa) > Ti/CaTiO₃/hydroxyapatite (22 MPa) > Ti/CaHPO₄/hydroxyapatite (9 MPa). © 2001 Kluwer Academic Publishers     

Sintering and compensation effect of donor- and acceptor-codoped 3mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350 °C. The enhancement is accounted for by a liquid phase sintering mechanism. The addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol % at 1300–1600 °C results in the destabilization of tetragonal (t-) phase and the decrease of final density in 3 mol% Y₂O₃-TZP (tetragonal ZrO₂ polycrystals). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m-) ZrO₂ and a second phase of Ta₂Zr₆O₁₇. The decreasing in final density is attributed to the increase of m-ZrO₂ content. Complete destabilization of t-ZrO₂ to m-ZrO₂ in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor- and acceptor-codoping defect chemistry.     

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO,CaO, CuO)

The thermal expansion of superconducting Bi_1.6Pb_0.4Sr₂Ca₂Cu₃Ox (BiPbSrCaCuO) and its oxide components Bi₂O₃, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is =6.4×10^–6K^–1. The temperature dependences of L/L of pressed Bi₂O₃ reveals sharp changes of length on heating (T ₁=712°C), and on cooling (T ₂=637°C and T ₃=577°C), caused by the phase transition monoclinic-cubic (T ₁) and by reverse transitions via a metastable phase (T ₂ and T ₃). By thermal expansion measurements of melted Bi₂O₃ it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi₂O₃. The thermal expansion of red PbO reveals a sharp decrease in L/L, on heating (T ₁=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi₂O₃ (_m=3.6 × 10^–6 and _c=16.6×10^–6K^–1 for monoclinic and cubic Bi₂O₃ respectively), the melted Bi₂O₃ (_m=7.6×10^–6 and _c=11.5×10^–6K^–1), PbO (_t=9.4×10⁶ and _or=3.3×10^–6K^–1 for tetragonal and orthorhombic PbO respectively), CaO (=6.1×10^–6K^–1) and CuO (=4.3×10^–6K^–1) are presented.     

Stable and Metastable Phase Equilibria in the Bi2O3–BiPO4 System

The stable and metastable phase equilibria in the Bi₂O₃–P₂O₅ system were studied in the range 0–50 mol % P₂O₅ by differential thermal analysis and x-ray diffraction. The results were used to construct the corresponding phase diagrams. In the equilibrium state, the system contains one sillenite phase with Bi₂O₃ : P₂O₅ = 12 : 1 and four other bismuth phosphates (2 : 1, 3 : 1, 5 : 1, and 1 : 1). In a metastable state, resulting from solidification of metastable melts, there exist * solid solutions (0–25 mol % P₂O₅) based on the high-temperature form of Bi₂O₃. At lower temperatures, the * phase transforms eutectoidally into the metastable phase, which has the same structural basis as the high-temperature solid solutions. At room temperature, the phase exists in a narrow composition range around 15 mol % P₂O₅. At lower P₂O₅ contents, the * phase decomposes at 868 K by a monotectoid reaction to form a mixture of the metastable and phases. The phases 3Bi₂O₃ · P₂O₅ () and 2Bi₂O₃ · P₂O₅ (), melting incongruently at 1193 and 1223 K, respectively, appear in both the equilibrium and metastable phase diagrams.     

Preparation of Bi2Sr2CaCu2O8 + x-Matrix Composites Containing Fine Strontium Calcium Indate Inclusions via Glass Crystallization

The crystallization of glass with a composition of Bi₂Sr₂CaCu₂O_{8 + x} + 0.25Sr_0.6Ca_0.4In₂O₄was studied in air between 400 and 800°C. Below 700°C, crystalline phases were formed in the following sequence: (Sr,Ca)_0.9Bi_1.1O_2.55, Cu₂O, Bi-2201, (Sr,Ca)In₂O₄, (Sr,Ca)₃Bi₂O₆, and Bi-2212. Above 700°C, the predominant phases were Bi-2212 and (Sr,Ca)In₂O₄. The introduction of In into Bi–Sr–Ca–Cu–O was shown to reduce the glass-forming capability of this system, without suppressing Bi-2212 formation. The Bi₂Sr₂CaCu₂O_{8 + x} + 0.25Sr_0.6Ca_0.4In₂O₄composites prepared by annealing the precursor glass contained 0.2- to 0.4-m inclusions and possessed enhanced superconducting properties.     

Processing and properties of Y-TZP/Al2O3 composites

The processing and property measurement of Y-TZP/Al₂O₃ ceramic-ceramic composites was investigated. The wet chemical synthesis route was adopted for the preparation of 3Y-TZP matrix dispersed with Al₂O₃ in three different volume fractions. Characterization of the resultant powders was carried out and their densification behaviour was studied by sintering in air in the temperature range 1200–1600 °C. The role of alumina as grain-growth inhibitor for Y-TZP, and the mechanical response of these ultrafine-grain ceramic composites in terms of K_lc characteristics, have been discussed.     

Nanopatterning of diamond films with composite oxide mask of metal octylates in electron beam lithography

The fabrication of diamond nanopatterns by electron cyclotron resonance (ECR) oxygen plasma with a composite metal octylate mask was investigated using electron beam lithography technology. A high etching selectivity of 14 was obtained with Bi₄Ti₃O₁₂ octylate film as a mask under the plasma-etching conditions of microwave power of 300 W and oxygen gas flow rate of 3 sccm. The metal naphthenates and metal octylates exhibited negative exposure characteristics. The sensitivity of metal naphthenates (1.2×10^–3 C cm^–2) was ten times lower than that of polymethyl methacrylate (PMMA) resist, while that of octylates (8.0×10^–5 C cm^–2) was in good agreement with that of PMMA resist (6.0×10^–5 C cm^–2). The resulting minimum chemical vapor deposited (CVD) diamond line-width of 100 nm with a height of approximately 1 m was fabricated with a Bi₄Ti₃O₁₂ octylate mask.     

The effect of Bi2O3 on the microstructure and electrical conductivity of ZrO2-Y2O3 ceramics

ZrO₂-Y₂O₃ ceramics with varying Bi₂O₃ contents were prepared and their microstructures and electrical conductivities investigated. The phase stability of cubic fluorite zirconia was disturbed by the introduction of Bi₂O₃ and tetragonal or monoclinic second phases appeared. The effect of the second phases on the intragrain and the grain boundary conductivities was investigated in the 300–550 C range using complex plane analysis in the frequency range of 5 Hz to 13 MHz. It showed that conductivity data could readily be interpreted in terms of possible physical models and electrical equivalent circuits. Tetragonal phases had a small positive influence on the intragrain conductivity. The grain 9boundary resistivity could be diminished by discrete monoclinic second phases which offered more conductive intergranular contacts.     

Single Crystals of p-Type Solid Solutions between Bismuth and Antimony Chalcogenides for Cooling to T < 150 K

Sb₂Te₃–Bi₂Te₃ crystals (25–60 mol % Bi₂Te₃) doped with Bi₂Se₃ and excess Te were studied with the aim of identifying the optimal compositions for the p-legs of low-temperature coolers. The crystals were grown by the floating-crucible technique. Their transport properties were studied in the range 100–400 K. By measuring axial thermopower profiles, it was shown that increasing the Bi₂Te₃ and Bi₂Se₃ contents of the crystals has an adverse effect on their homogeneity. Crystals were prepared with a carrier concentration in the range (1–5) × 10¹⁹ cm^–3 and a thermoelectric power above 200 V/K at room temperature and the highest thermoelectric figure of merit at temperatures below 200 K. The maximum temperature drops and thermoelectric figures of merit were calculated for low-temperature stages of magnetothermoelectric coolers with hot-junction temperatures of 200 and 170 K and Bi–Sb n-legs.     

Electrical and microstructural properties of Bi2−x Sb x Te and Bi2−x Sb x Te2 foils obtained by the ultrarapid quenching process

This paper is concerned with the feasibility and reproducibility of the ultrarapid quenching process used to fabricate Bi_2–x Sb _x Te and Bi_2–x Sb _x Te₂ alloys for thermoelectric applications. Microstructural properties of the materials, obtained in the shape of foils, were studied concerning the phase analysis, cell parameters, texture, and microstructure observations. The Bi_2–x Sb _x Te alloys were found to have the (2 0 3) texture. The (2 0 4) texture, with an additional (1 1 0) component for x values greater than 0.4, was predominant for Bi_2–x Sb _x Te₂ foils. The electrical properties of these materials were then characterized by measuring the Seebeck coefficient, Hall coefficient, and electrical resistivity. It was found that Bi_2–x Sb _x Te foils changed from n- to p-type for an x-value of about 1.2. A maximum Seebeck coefficient, ||, of 36×10^–6 V K^–1 was measured for Bi₂Te. In the case of Bi_2–x Sb _x Te₂ foils, the change from n- to p-type was observed for an x value of about 1. A maximum Seebeck coefficient, ||, of 32×10^–6 V K^–1, was measured for Bi_1.4Sb_0.6Te₂. Measurements of the temperature-dependent electrical resistivity, Hall and Seebeck coefficients of the foils were carried out and the analysis revealed a semi-metallic behavior.     

Fabrication of CdO–Bi2O3 Ceramics

The phase formation in the CdO-rich part (10 wt % Bi₂O₃) of the CdO–Bi₂O₃ system was studied by x-ray diffraction, IR spectroscopy, electron microscopy, and other techniques. The results indicate that the grinding of the starting powder mixtures in a planetary mill leads to an inhomogeneous Bi₂O₃ distribution and a high defect density in the CdO and Bi₂O₃ particles. Sintering in the range 910–990 K gives rise to the formation of the _h phase, whose content depends strongly on the mixture composition and sintering conditions. At sintering temperatures above its melting point, the _h phase acts as a binder of CdO grains.     

Study on the Physical and Electrical Properties of Bi2 ? δZnδSr2Ca2Cu3O10 + y Glass-Ceramic Superconductors

We have fabricated a series of glass-ceramic (Bi_{2 –} Zn) Sr₂Ca₂Cu₃O_{10 + y} , where = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, and investigated the effect of Zn ions on the glass formation, crystallization, thermal, electrical, and on the magnetic properties of the BSCCO-2223 superconductor system. The structural symmetry was found to be tetragonal in all the substitution levels. The best electrical performance was obtained from the = 0 sample, the T _c and T _zero was obtained at 110 K and 107 K, respectively. The J _c values of the samples were determined using the magnetization hysteresis and Bean's model. The crystallization kinetics were investigated using nonisothermal models of Augis–Bennett. The calculated activation energy, E _a, of the system was found to be in the range of 258–336 kJ/mol.     

On the nano-size intergrowth of Sr4Fe6O13±δ and (Sr1?xLax)FeO3?δ phases in the mixed conductor Sr3.6La0.4Fe6Oz

The microstructures of mixed-conducting Sr₄Fe₆O_13±, and Sr_3.6La_0.4Fe₆O _z (nominal composition) have been studied by electron microscopy. The lanthanum free material shows a microstructure containing mainly Sr₄Fe₆O_13±, with some 5 vol% Sr_1–y La _y Fe₁₂O₁₉embedded as micron-size inclusions. The lanthanum-containing material revealed a significantly different microstructure consisting of 20 vol% micron-size Sr_1–y La _y Fe₁₂O₁₉embedded in 40 vol% (Sr_0.85La_0.15)FeO_3–tetragonal perovskite, and 40 vol% of plate-shaped nano-scale intergrowths between (Sr_1–x La _x )FeO_3–and Sr₄Fe₆O_13±phases. Domains with dimensions of 20–50 nm are observed in the tetragonal perovskite when viewed along its fourfold axis. From compositional analysis it is concluded that there is little or no solubility of La in the Sr₄Fe₆O_13±phase. The observed microstructure is important input in explaining the significant effect of La addition on the transport properties of Sr₄Fe₆O_13±materials.     

Preparation of tetragonal ZrO2-Gd2O3 powders

ZrO₂-Gd₂O₃ alloys containing 2,3,5 and 8 mol.% Gd₂O₃ have been prepared by mixed oxide (MO), hybrid sol-gel (SG), and co-precipitation (CP) routes. No tetragonal (t) phase is retained in the MO method, while 100% t phase is obtained in the calcined CP samples; the SG method leads to only partial stabilization of the t phase. Washing of the CP powders with propan-2-ol leads to unagglomerated powders with increased specific surface area (145 versus 89 m²g^–1) and sintered density (98% versus 79%). Cubic and t phase also appear on sintering the samples with >2 mol.% Gd₂O₃.     

Mechanochemical Synthesis and Thermal Behavior of Metastable Mixed Oxides in the CaO–Sb2O3–Bi2O3 System

The phase composition of crystalline mechanochemical synthesis products in the CaO–Sb₂O₃–Bi₂O₃ system was determined. Of the known phases in this system, only three could be prepared mechanochemically: Ca₂Sb₂O₅, CaSb₂O₄, and CaBiO_2.5 (fcc). A new metastable phase, "-Bi₂O₃, with an orthorhombic structure close to that of the high-temperature, fluorite phase -Bi₂O₃, was obtained by mechanical processing at 30°C. A number of new metastable fluorite solid solutions of binary and ternary oxides were obtained as single-phase powders by mechanochemical synthesis. The mechanochemical yield of primary crystalline products was shown to be several times higher than that of secondary products. A broad composition range was revealed in which perovskite and fluorite phases are in mechanochemical equilibrium. The composition dependence of the lattice parameter of the metastable fluorite phase Bi_{2 – x} Sb _x O₃ was found to be the opposite of the one predicted by Vegard's law. Metastable mixed oxides undergo phase transformations during heating (starting at 280°C in the case of the ternary perovskite phase). Bi_{2 – x} Ca _x O_{3 – 0.5x} fluorite solid solutions experience a transformation at 400°C, accompanied by oxygen loss. During heating in air, Sb₂O₃-containing fluorite phases partially stabilize owing to oxidation but, nevertheless, undergo structural transformations above 480°C. The transformation of Sb_{2 – x} Ca _x O_{3 – 0.5x} metastable fluorite solid solutions near 500°C in air is accompanied by the formation of needle-like Sb₂O₃ crystals. A mechanism is proposed for the extremely rapid growth of such crystals: extrusion of the Sb₂O₃ resulting from fluorite decomposition in agglomerates through triple junctions of aggregates and through cracks in the surface layer of agglomerates.     

Sinterability of magnesium silicon nitride powder with yttrium oxide addition coated using the homogeneous precipitation method

The sinterability of magnesium silicon nitride (MgSiN₂) powder with yttrium oxide (Y₂O₃) addition was examined using the hot-pressing technique (31 MPa and N₂ atmosphere) at 1550°C for 90 min; the MgSiN₂ powder had been coated with 0–4 mass% of Y₂O₃ addition by a (urea-based) homogeneous precipitation method. Relative densities of the hot-pressed MgSiN₂ compacts (ceramics) with and without Y₂O₃ addition were 99.6% apart for the MgSiN₂ceramic with 4 mass% Y₂O₃ addition (98.4%). The thermal conductivities of the MgSiN₂ ceramics with 0–1 mass% Y₂O₃ addition were in the range of 20–21 W · m^–1 · K^–1 whilst the Vickers hardness was 19.7 GPa for the pure MgSiN₂ ceramic and decreased slightly with Y₂O₃ addition. Average fracture toughness values were in the range of 1.2–1.6 MPa · m^1/2 with significant trend being noted with regards to the ceramic containing 0.5 mass% of Y₂O₃. It was concluded that the use of homogeneous precipitation processing resulted in significant advantages regarding the densification, homogeneous microstructure, and fracture toughness despite the amount of Y₂O₃ addition being as low as 0.5 mass%.     

Preparation and photoconduction of rapidly quenched films in the Bi2O3-TiO2 system

The rapidly quenched films in the Bi₂O₃-TiO₂ system (0 to 60% TiO₂) were prepared using a twin-roller type apparatus. The films precipitated Bi₂O₃ solid solutions of different types in the composition ranges, with TiO₂ contents of 0 to 5, 7.5 to 10 and 12.5 to 40%, respectively. The first solid solution had a tetragonal structure of -form. The second, though also crystallized in the tetragonal structure, adopted a disordered modification of the -form. The third solid solution was -form (defect fluorite structure). The formation of amorphous phase commenced in the composition with 30% TiO₂, and the films became completely amorphous beyond 50% TiO₂. The quenched films showed a certain instability to decompose or transform into the different phase assemblage by annealing at higher temperatures (about 400 to 500° C, except 260° C for the pure Bi₂O₃ film). The quenched films were also characterized by a high photoconductivity. The photoconduction mechanism was suggested to be associated with a structural imperfection of Bi₂O₃ accompanied by a certain amount of pentavalent bismuth ion.     

Reactions of MnO2, Mn2O3, α-Bi2O3, and Bi12Ti1 – x Mn x O20with Supercritical Isopropanol

Heterogeneous reactions between supercritical isopropanol and metal oxides were studied for the first time. The results demonstrate that Bi₂O₃ is reduced by isopropanol to metallic bismuth, while MnO₂ and Mn₂O₃ are reduced to Mn₃O₄ (hausmannite). The possibility of oxygen extraction from nonstoichiometric oxides is demonstrated by the example of the reaction Bi₁₂Ti_{1 – x} Mn⁵⁺ _x O_{20 +} Bi₁₂Ti_1–x Mn²⁺ _x O_{20 –} .     

Microstructure and mechanical properties of 3Y-TZP/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites fabricated by liquid phase sintering

3Y-TZP/Al₂O₃ composites were pressureless sintered with the addition of TiO₂-MnO₂ and CaO-Al₂O₃-SiO₂ glass. The densification, microstructure and mechanical properties of the composites were investigated. It was found that the composites could be densified at a temperature as low as 1400^C by liquid phase sintering. The majority of the grain sizes for both Al₂O₃ and ZrO₂ were below 1 m because of the lower sintering temperature. A bending strength of 934 ± 28 MPa and fracture toughness of 7.82 ± 0.19 MPam^1/2 were obtained for 3Y-TZP/Al₂O₃ (20 vol%) composite. Transformation toughening is considered the responsible toughening mechanism.