Polymorphism of Bismuth Sesquioxide. II. Effect of Oxide Additions on the Polymorphism of Bi2O3

The effect of small oxide additions on the polymorphism of Bi₂O₃ was studied by means of high-temperature x-ray diffractometry. Solidus and occasional liquidus temperatures were approximated, so that tentative partial phase diagrams for 33 oxide additions were constructed. Only the monoclinic and the cubic forms of Bi₂O3 were found to be stable. Other phases, frequently reported by previous investigators, such as tetragonal and body-centered cubic (b.c.c.), were shown to form metastably from cooled liquid or cubic. An impure b.c.c. phase of distinct but variable composition appeared in systems of ZnO, PbO, B₂O₃, Al₂O₃, Ga₂O₃, Fe₂O₃, SiO₂, GeO₂, TiO₂, and P₂O₅. The impure b.c.c. phase in the systems with SiO₂, GeO₂, and TiO₂ melted congruently about 100 °C above the m.p. of Bi₂O₃. The impure b.c.c. phase was formed metastably in systems with Rb₂O, NiO, MnO, CdO, V₂O₅, and Nb₂O₅; the conditions of formation were dependent on composition, preparation, and heating schedules. The impure b.c.c. phases, both stable and metastable, had smaller unit cell dimensions than that of pure Bi₂O₃.     

Acoustic emission study of phase relations in low-Y2O3 portion of ZrO2-Y2O3 system

The (metastable) tetragonal phase in 3–4 mol% Y₂O₃-ZrO₂ alloys undergoes a transition to the monoclinic form in the 200–300 °C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating. The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic tetragonal transition in ZrO₂-1 mol% Y₂O₃ alloy at 850–750 °C and the same phase change in 2, 3, 4 and 6 mol% Y₂O₃ compositions at the eutectoid temperature of about 560 °C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol% Y₂O₃ compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1–12 mol% Y₂O₃ region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction.     

Effect of additives on densification and deformation of tetragonal zirconia

The effect of additives (Bi₂O₃, Fe₂O₃) on densification and creep rates of tetragonal ZrO₂-Y₂O₃ has been investigated. In Bi₂O₃-doped Y-TZP, a reactive liquid forms at temperatures above 800–900C, which leads to a strong enhancement of densification for concentrations of 1–2 mol % Bi₂O₃. However, during cooling from the processing temperature a strong, undesirable transformation of the tetragonal to the monoclinic phase occurs. The addition of 0.6–1.2 mol % FeO_3/2 promotes densification without destabilizing the tetragonal phase. A concentration of 1.2 mol %, however, induces discontinuous grain growth, while this is not the case for 0.6 mol %. Creep rates of Y-TZP were enhanced by a factor of 4–6 by adding 0.6 mol % FeO_3/2.     

Combustion synthesis and properties of nanostructured ceria-zirconia solid solutions

Nanostructured ceria-zirconia solid solutions (Ce_{1 − x}Zr_xO₂, x = 0 to 0.9) have been synthesized by a single step solution combustion process using cerous nitrate, zirconyl nitrate and oxalyl dihydrazide (ODH) / carbohydrazide (CH). The as-synthesized powders show extensive XRD line broadening and the crystallite sizes calculated from the XRD line broadening are in the nanometer range (6–11 nm). The combustion derived ceria zirconia solid solutions have high surface area in the range of 36–120 m²/g. Calcination of Ce_{1 −x}Zr_xO₂ at 1350 °C showed three distinct solid solution regions: single phase cubic (x ≤ 0.2), biphasic cubic-tetragonal (0.2 < x 0&#x030C;.8) and tetragonal (x > 0.8). When x ≥ 0.9, the metastable tetragonal phase formed transforms to monoclinic phase on cooling after calcination above 1100 °C. The homogeneity of Ce_{1 − x}Zr_xO₂ has been confirmed by EDAX analysis. The Temperature Programmed Reduction (TPR) measurement of Ce_0.5Zr_0.5O₂ was carried out with H₂ and the TPR profile showed two water formation peaks corresponding to the utilization of surface and bulk oxygen.     

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.     

Phase stabilization and structural studies of nanocrystalline La<Superscript>2</Superscript>O<Superscript>3</Superscript>-ZrO<Superscript>2</Superscript>

La²O³ doped nanocrystalline zirconia (ZrO²) has been prepared by chemical co-precipitation method for various dopant concentrations, varying from 3 to 30 mol%. Structural phases have been characterized by X-ray diffraction technique. All the as-synthesized samples were found to be in monoclinic phase. Annealing of the samples at different temperatures from 400 to 1000∘C stabilized ZrO² either partially or fully the tetragonal/cubic phases. When they were annealed at 1200∘C, the monoclinic phase appeared again with a new cubic pyrochlore structured La²Zr²O⁷ at the expense of stabilized tetragonal phase. Formability of the tetragonal/cubic phase has been influenced by the dopant concentration and the annealing temperature. Sample with 8 mol% La²O³ has been stabilized completely in tetragonal/cubic phase after annealing at 900∘C for 1 h. Smallness of the grain in these nanocrystalline materials may also have assisted in the formation of La²O³-ZrO² solid solution.     

Heats of Solution,Transition, and Formation of Three Crystalline Forms of Metaboric Acid

The three crystalline forms of metaboric acid HBO₂ were prepared, purified, and analyzed. Heats of solution in water or of reaction with sodium hydroxide solution were compared with those of orthoboric acid H₃BO₃(c). The best values for the heats of transition at 25 °C are: (c,I) to (c,II), 2.33±0.23 kcal/mole; (c,II) to (c,III), 1.30±0.05 kcal/mole; (c,I) to (c,III), 3.63±0.24 kcal/mole. The following heats of formation at 25 °C were derived: −192.77 ± 0.35 kcal/mole for the cubic HBO₂(c,I), −190.43 ±0.34 kcal/mole for the monoclinic HBO₂ (c,II), and −189.13 ± 0.34 kcal/mole for the orthorhombic HBO₂(c,III).     

A structural study of metastable tetragonal zirconia in an Al2O3-ZrO2-SiO2-Na2O glass ceramic system

The structural and microstructural properties (crystalline system at the beginning of crystallization, lattice disorder and crystallite size) of metastable zirconia have been studied by an X-ray line broadening analysis using simplified methods based on suitably assumed functions describing the diffraction profiles. Metastable tetragonal zirconia has been crystallized at 970, 1000 and 1050° C, respectively, starting from an Al₂O₃-ZrO₂-SiO₂Na₂O glassy system with a chemical composition very close to that of well known electromelted refractory materials. In the present work we have definitely shown the presence, inside the crystallized zirconia phase, of internal microstrains having values ranging approximately between 2 and 4×10^–3. Moreover, we have confirmed the peculiar smallness in size of precipitated zirconia crystallites ( 200 Å). Therefore, in the present system, the stabilization of the tetragonal form of ZrO₂ with respect to the stable monoclinic one can be explained in terms of a contribution to the amount of free energy due to strain energy, in addition to the previously hypothesized surface energy. The observed strong line broadening for some samples treated at lower temperatures (970 and 1000° C) gives rise to an apparent cubic lattice pattern; but the asymmetry of each apparent single line masks unequivocally a tetragonal doublet. This latter conclusion disagrees with some hypotheses on the existence of a cubic metastable form of ZrO₂ which could originate at the beginning of zirconia crystallization.     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.     

Phase Equilibrium Relations in the Sc2O3-Ga2O3 System

The phase equilibrium diagram was determined for the Sc₂O₃-Ga₂O₃ system. A quenching furnace, wound with 60 percent Pt—40 percent Rh wire, was employed for experiments conducted at temperatures up to 1,800 °C. An induction furnace, having an iridium crucible susceptor, was used to obtain higher temperatures. Temperatures in the quenching furnace were measured with both an optical pyrometer and a 95 percent Pt—5 percent Rh versus 80 percent Pt—20 percent Rh thermocouple. The melting point of Ga₂O₃ was determined as 1,795 ±15 °C. Experiments at temperatures as high as 2,405 °C failed to melt Sc₂O₃. Two intermediate binary phases, a compound believed to be 6Sc₂O₃·5Ga₂O₃ and a solid solution occur in the system. The solid solution phase appears as a single phase in the region roughly defined by the compositional limits of 55 to 73 mole percent Ga₂O₃ at the solidus. The 6:5 compound, stable only at high temperatures, melts incongruently at 1,770 ±15 °C and decomposes below 1,700 ±15 °C. The compound appears to have orthorhombic symmetry with a=13.85 A, b= 9.80 A, and c=9.58 A. The indicated uncertainties in the melting points are a conservative estimate of the overall inaccuracies.     

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.     

Investigation on the phase stability,sintering and thermal conductivity of Sc2O3–Y2O3–ZrO2 for thermal barrier coating application

Rare earth oxides co-doped zirconia have been developed for application in thermal barrier coating systems to promote the performance and durability of gas turbines. 8 mol%Sc₂O₃, 0.6 mol%Y₂O₃–stabilized ZrO₂ (ScYSZ) powder was synthesized by chemical co-precipitation method. The phase stability, sintering resistance and thermo-physical properties of ScYSZ and 8 wt%Y₂O₃ stabilized ZrO₂ (8YSZ) were investigated. The results indicated that both ScYSZ and 8YSZ show single tetragonal phase before heat treatment. After heat treating at 1500 °C for 300 h, ScYSZ exhibits excellent phase stability with 100% metastable tetragonal (t′) phase, whereas the content of monoclinic phase in 8YSZ reached 49.4 mol%. ScYSZ also exhibits higher sintering resistance and lower thermal conductivity than 8YSZ. ScYSZ can be considered to be explored as candidate material for TBC application.     

Phase Equilibrium Relations in the Binary System Barium Oxide-Niobium Pentoxide

A large portion of the phase equilibrium diagram for the binary system barium oxide-niobium pentoxide has been constructed from observations of fusion characteristics and X-ray diffraction data. In the system five binary compounds were observed with BaO: Nb₂O₅ ratios of 5:2, 1:1, 6:7, 3:5, and 1:3 and a 6:1 compound was postulated. The 1:1 compound was found to melt congruently at 1,455 °C and have only one stable polymorph, although a second metastable polymorph can also be prepared. The 5:2 compound melts congruently at 1,542 °C; the 6:7, 3:5, and 1:3 phases melt incongruently at 1,330, 1,290, and 1,315 °C, respectively. The phase relations of the 6:1 compound could not be determined due to the reaction between this phase and platinum metal. No 2:1 compound was observed in this system.     

UV Raman spectroscopic study on the surface phase of ZrO2 modified with Nd2O3

The Nd₂O₃ modified ZrO₂ was synthesized using two methods of co-precipitation (Nd-ZrO₂) and wet impregnation (Nd/ZrO₂). The surface and bulk crystalline phases of Nd₂O₃ modified ZrO₂ were investigated by using UV Raman spectroscopy, visible Raman spectroscopy, and X-ray diffraction (XRD). It is observed that the tetragonal phase in the surface region of Nd-ZrO₂ was not effectively stabilized by Nd₂O₃, as Nd₂O₃ is mainly present in the bulk of Nd-ZrO₂. However, in Nd/ZrO₂, it is found that with the impregnation of 0.5 mol% Nd₂O₃ on ZrO₂, the surface tetragonal phase of Nd/ZrO₂ can be stabilized even after calcination at 700 °C. The UV Raman results indicate that a disordered structure, or intermediate structure, which is involved in the transition from the tetragonal to the cubic phase, is formed at the surface region of Nd/ZrO₂. The formation of the aforementioned intermediate structure inhibits the phase transition from tetragonal to monoclinic in the surface region of Nd/ZrO₂. Furthermore, it is observed that the mixed tetragonal and monoclinic phases in the surface region of ZrO₂ which has been impregnated with Nd₂O₃ can also be stabilized after calcination at 700 °C. This work provides a simple method for controlling the surface phase of ZrO₂ at high temperatures.     

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.     

Phase Equilibria and Crystal Chemistry in Portions of the System SrO-CaO-Bi2O3-CuO,Part IV— The System CaO-Bi2O3-CuO

New data are presented on the phase equilibria and crystal chemistry of the binary systems CaO-Bi₂O₃ and CaO-CuO and the ternary CaO-Bi₂O₃-CuO. Symmetry data and unit cell dimensions based on single crystal and powder x-ray diffraction measurements are reported for several of the binary CaO-Bi₂O₃ phases, including corrected compositions for Ca₄Bi₆O₁₃ and Ca₂Bi₂O₅. The ternary system contains no new ternary phases which can be formed in air at ~700–900 °C.     

Solid-state reaction,microstructure and phase relations in the ZrO2-rich region of the ZrO2-Yb2O3 system

Phase relations below 1700°C in the ZrO₂-rich region of the zirconia-ytterbia system have been established using thermal expansion, room-temperature X-ray diffraction, precision lattice parameter measurements and microscopic observations. The solubility limits of ytterbia in both monoclinic and tetragonal zirconia were determined. A eutectoid reaction, tetragonal zirconia solid solution monoclinic + cubic zirconia solid solutions at 400 ± 20°C and 2.4 mol % ytterbia was found. The left-hand boundary of the cubic zirconia solid-solution field was redetermined between room temperature and about 1700°C. Long-range ordering was present at 40 mol % ytterbia and the formation of an ordered phase, Zr₃Yb₄O₁₂, isostructural with M₇O₁₂-type compounds was found. Its thermal stability was established between room temperature and 1630 ± 10°C, in which it decomposes into cubic zirconia solid solution by an order-disorder reaction.     

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.     

Phase equilibrium relations in the binary system Bi2O3-ZnO

Phase equilibria in the binary system Bi₂O₃-ZnO were studied by quenching technique. Heat-treated compositions were subjected to X-ray diffraction for phase identification, and differential thermal analysis, optical and scanning electron microscopy were used to determine the solid-liquid equilibria occurring in this system. The data thus obtained revealed that incorporation of a small amount of ZnO to the high-temperature face-centered cubic lattice of Bi₂O₃ leads to the formation of a body-centered cubic solid solution (-Bi₂O₃), which extends up to a composition of 2.2 mol% ZnO at a temperature near 750°C. On cooling, the -Bi₂O₃ solid solution undergoes a eutectoid transformation at a temperature of 710°C to yield the low-temperature monoclinic polymorph of Bi₂O₃ (-Bi₂O₃) and Bi₃₈ZnO₅₈. The eutectoid occurs at a composition of 1.8 mol% ZnO. The compound Bi₃₈ZnO₅₈ has a crystal structure analogous to the body-centered cubic -Bi₂O₃ solid solution and melts incongruently at a temperature near 753 ± 2°C to yield -Bi₂O₃ and liquid. A binary eutectic occurs between Bi₃₈ZnO₅₈ and ZnO at a composition near 25 ± 1.0 mol% ZnO with a melting temperature of 738 ±2°C. Based on the data obtained in this study, a revised phase diagram of the binary system Bi₂O₃-ZnO is proposed.     

Phase Equilibria in Systems Involving the Rare Earth Oxides. Part III. The Eu2O3?In2O3 System

The equilibrium phase diagram was determined for the Eu₂O₃−In₂O₃ system. An induction furnace, having an iridium crucible as the heating element (susceptor), was used to establish the solidus and liquidus curves. The 1:1 composition melts congruently at 1745 ± 10 °C. Melting point relations suggest that the 1:1 composition is a compound with solid solution extending both to 31 mole percent In₂O₃ and 71 mole percent In₂O₃. The compound is pseudohexagonal with a_H = 3.69 A and c_H = 12.38 A. Isostructural phases also occur in the 1:1 mixtures of both Gd₂O₃ and Dy₂O₃ with In₂O₃. The melting points of Eu₂O₃ and In₂O₃ were determined to be 2,240 ± 10 °C and 1910 ± 10 °C respectively. A eutectic occurs in the Eu₂O₃−In₂O₃ system at 1,730 °C and about 73 mole percent In₂O₃. The indicated uncertainties in the melting points are conservative estimates of the overall inaccuracies of temperature measurement.