Phase-Diagram Studies in the La2O3–SrO–CaO–Mn3O4 System at 1200°C in Air

The phase equilibria of the La₂O₃–SrO–CaO–Mn₃O₄ system in air at 1200°C has been studied. Under these conditions, eight univariant four-phase equilbria were observed. Quaternary phases, as well as liquid phases, were not observed. Perovskite-structure phases LaMnO₃, SrMnO₃, and CaMnO₃ did not form complete solid solutions within the system.     

Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3–Cr2O3 and V2O3–Cr2O3 Systems

The phase diagrams in the Al₂O₃–Cr₂O₃ and V₂O₃–Cr₂O₃ systems have been assessed by thermodynamic modeling with existing data from the literature. While the regular and subregular solution models were used in the Al₂O₃–Cr₂O₃ system to represent the Gibbs free energies of the liquid and solid phases, respectively, the regular solution model was applied to both phases in the V₂O₃–Cr₂O₃ system. By using the liquidus, solidus, and/or miscibility gap data, the interaction parameters of the liquid and solid phases were optimized through a multiple linear regression method. The phase diagrams calculated from these models are in good agreement with experimental data. Also, the solid miscibility gap and chemical spinodal in the V₂O₃–Cr₂O₃ system were estimated.     

Sintering Characteristics in BaTiO3–Nb2O5–Co3O4 Ternary System: I, Electrical Properties and Microstructure

The influences of the Nb/Co ratio on electrical properties, densification behavior, and microstructural evolution were investigated on ceramics in the ternary system BaTiO₃-Nb₂O₅-Co₃O₄. Temperature-stable dielectrics were obtained using either a large amount of Nb + Co or a large Nb/Co ratio. The sintering characteristics and electrical properties were studied for the niobium-rich composition (Nb/Co = 3.00; Comp.N) and the cobalt-rich composition (Nb/Co = 1.67; Comp.C) with the same Nb + Co amount of 2 at.%. The temperature characteristic of the dielectric constant was flat, irrespective of the firing temperature, for Comp.N, whereas it was dependent largely on the firing temperature for Comp.C. The grains did not grow in Comp.N but grew in Comp.C. The reaction of Nb₂O₅ and Co₃O₄ with BaTiO₃ yielded secondary phases: Ba₆Ti₁₇O₄₀ phase for Comp.N, and a barium-poor, titanium-rich, and cobalt-rich phase for Comp.C. These secondary phases formed a liquid phase during firing. Comp.N contained a larger amount of the secondary phase than Comp.C. It was concluded that the liquid phase contributed little to densification and microstructural evolution in the system BaTiO₃-Nb₂O₅-Co₃O₄.     

Thermochemical Analysis of the Silicon Carbide-Alumina Reaction with Reference to Liquid-Phase Sintering of Silicon Carbide

The stabilities of different phases in the Si-Al-C-O system are calculated from thermodynamic considerations with the objective of identifying the liquid phases formed during sintering of SiC in the presence of Al₂O₃. It is shown that a liquid phase can form at the sintering temperatures by the reaction of SiC with Al₂O₃. Depending on the carbon activity, the liquid can be either of the following: Al₂O₃+ Al₄C₃, SiC + Al₄C₃, or molten aluminum. The stability of the aluminosilicate melts that can form by the reaction of Al₂O₃ with the surface silica layer on SiC powders is also evaluated. Several factors that influence liquid-phase sintering, such as the solubility of SiC in the melts and the generation of gases during sintering, are discussed. The results of the thermodynamic analysis are compared with the observed sintering behavior for SiC.     

Synthesis and Characterization of Aurivillius Phases in the Bi–Ag–Ti–O System

New Aurivillius phases in the Bi–Ag–Ti–O system were investigated by means of a solid-state reaction and X-ray diffraction. We found that the oxygen partial pressure has a significant influence on the synthesis of the Aurivillius phases. The mixed-layer Aurivillius phase Ag_0.5Bi_8.5Ti₇O₂₇ was observed after firing in an O₂ flow, but a single-phase material is difficult to obtain. A single-phase compound of the four-layer Aurivillius phase Ag_0.5Bi_4.5Ti₄O₁₅ was obtained on firing in an oxygen partial pressure of 10 bar (1 × 10⁶ Pa). The dielectric properties (at 1 MHz) of the Ag_0.5Bi_4.5Ti₄O₁₅ compound were as follows: T _max=687°C, ɛ _r =166 (∼20°C), and tan δ=0.004 (∼20°C).     

Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.     

Thermal Expansion of Uranium Pyrophosphate and of Ceramic Bodies in the System UO2–UP2O7

During an investigation of methods of predicting the thermal expansion of pure, single-phase ceramics, uranium pyrophosphate (UP₂O₇) was synthesized and the thermal-expansion properties were measured by the X-ray diffraction and dilatometer methods. The basis for the selection of UP₂O₇ is discussed. Cubic UP₂O₇ expands with increasing temperature up to 400°C. Above this temperature the material contracts so that the specimen returns to the room-temperature length at about 1000°C. Intermediate phases in the system UO₂–UP₂O₇ were prepared and the thermal-expansion properties were measured. Ceramic bodies in the system UO₂–UP₂O₇ were prepared. The thermal-expansion properties of these bodies are discussed in relation to the properties of the individual phases.     

Thermodynamic Assessment of the Lithium-Borate System

The Li₂O–B₂O₃ quasi-binary system is assessed. A two-sublattice ionic solution model, (Li¹⁺) _P (O²⁻, BO₃³⁻, B₄O₇²⁻, B₃O_4.5) _Q , is adopted to describe the liquid phase. All solid phases are treated as stoichiometric compounds. A set of parameters consistent with most of the available experimental data on phase diagram and thermodynamic properties is obtained by using the CALPHAD technique.     

Phase Relations in the System Na2O-TiO2-SiO2

Liǵuid us and subsolidus phase relations were studied using the quenching technique. The system contains four ternary compounds stable to liquidus temperatures: Na₂TiSi₄O₁₁, Na₂TiSi₂O₇, Na₂TiSiO₅, and Na₂Ti₂Si₂O₉. Six eutectics, eight peritecties, and eight thermal maxima were located and a region of liquid immiscibility was delineated. X-ray powder data are given for the stable and metastable crystalline phases. Glass-transition temperatures were determined by thermal analysis. The relation between physical properties of melts and glasses and the configuration of the liquidus is discussed.     

The System La2O3— TiO2; Phase Equilibria and Electrical Properties

Phase relations at liquidus temperatures in the system La₂O₃-TiO₂ were studied in air. The existence of two previously unreported compounds, La₂O₃. TiO₂ and 2La₂O₃-TiO₂, is postulated on the basis of X-ray and microscopic examination of crystalline samples, and in the case of La₂O₃.-TiO₂, on a maximum in the liquidus curve at that composition. Quenched liquids of the primary-phase field of rutile were found to be semiconducting. This property was related to oxygen loss from both liquid and crystalline phases and is discussed in the light of weight loss experiments, microscopic examination of quenched samples, and information obtained from the literature. Dielectric constant and loss factor of the compounds La₂O₃-TiO₂, La₂O₃-2TiO₂, and 2La₂O₃-9TiO₂ are reported at 1 Mc over the temperature range 25° to 500°C.     

The System Al-O

Previous data by other writers who infer the existence of aluminum suboxides in the solid state are critically discussed and alternatively interpreted. High-temperature X-ray diffraction patterns previously attributed to crystalline Al₂O and AlO are identified as those of Al₄C₃ and AlTaO₂, respectively. The melting curve observed by Baur and Brunner, suggesting the existence of a compound of the approximate composition Al₂O₉, corresponds to the pseudobinary system Al₂O₃−Al₄C₃. The T-x melting curve of the system Al₂O₃−Al was determined. There is no indication of the existence of crystalline aluminum suboxide, the only condensed phases being α-Al₂O₃ and Al. The liquidus indicates demixing of liquid Al and molten Al₂O₃. There is very limited intersolubility, and the addition of Al does not lower the melting point of Al₂O₃ measurably. With these facts and some additional information from the literature about the gaseous species, the P-T-x diagram of the system Al-O was constructed.     

Influence of Copper(II) Oxide Additions to Zinc Niobate Microwave Ceramics on Sintering Temperature and Dielectric Properties

The effect of CuO additions on the firing temperature of ZnNb₂O₆ ceramics was investigated using dilatometry, transmission electron microscopy, and X-ray diffractometry. A 5 wt% CuO addition to ZnNb₂O₆ ceramics significantly lowered the firing temperature from 1150° to ∼900°C. The presence of a CuO-rich intergranular phase in the specimen was observed and was evidence of the formation of a liquid phase during sintering. The composition of the liquid phase was (ZnCu₂)Nb₂O₈. In particular, the low-fired ZnNb₂O₆ ceramics had good microwave dielectric characteristics— Q × f = 59 500, ɛ_r= 22.1, τ_f=–66 ppm/^oC. These properties were correlated with the formation of a second phase, (ZnCu₂)Nb₂O₈.     

Phase Relations and Ordering in the Systems MgO-Y2O3-ZrO2 and CaO-MgO-ZrO2

The phase relations in the systems MgO-Y₂O₃-ZrO₂ and CaO-MgO-ZrO₂ were established at 1220° and 1420°C. The system MgO-Y₂O₃-ZrO₂ possesses a much-larger cubic ZrO₂ solid solution phase field than the system CaO-MgO-ZrO₂ at both temperatures. The ordered δ phase (Zr₃Y₄O₁₂) was found to be stable in the system ZrO₂-Y₂O₃ at 1220°C. Two ordered phases φ₁ (CaZr₄O₉) and φ₂ (Ca₆Zr₁₉O₄₄) were stable at 1220°C in the system ZrO₂-CaO. At 1420°C no ordered phase appears in either system, in agreement with the previously determined temperature limits of the stability for the δ, φ₁, and φ₂ phases. The existence of the compound Mg₃Y_zO₆ could not be confirmed.     

Phase Relations in the System Fe2O3–Fe3O4–YFeO3 in Air

Measurements were made of temperature and ternary composition for coexisting liquid and crystalline phases on the air isobar in the system Fe₂O₃-Fe₃O₄-YFeO₃ with particular regard to the stability range and compositional limits of yttrium iron garnet. Phase equilibrium relations were determined by conventional quenching techniques combined with measurements of loss in weight at the reaction temperature to locate true ternary compositions. The intersection of the air isobar with the ternary univariant boundary curve for coexisting magnetite, garnet, and liquid phases results in a eutectic-type situation at the composition Y_0.27Fe_1.73 O_2.87 and 1469°± 2°C. A similar intersection of the isobar with the boundary curve for coexisting garnet, orthoferrite, and liquid phases gives rise to a peritectic-type reaction at 1555° 3°C. and Y_0.44Fe_1.56 O_2.89 The yttrium iron garnet crystallizing from liquids within these temperature and composition limits contains up to 0.5 mole % iron oxide in excess of the stoichiometric formula in terms of the starting composition 37.5 mole % Y₂O₃, 62.5 mole % Fe₂O₃. At 1470° C. the garnet phase in equilibrium with oxide liquid contains 2 mole % FeO in solid solution. The small solubility of excess of iron oxide and partial reduction of the garnet phase in air are unavoidable during equilibrium growth from the melt.     

Phase Equilibrium Studies in the System Iron Oxide- A12O3 in Air and at 1 Atm. O2 Pressure

Phase equilibrium data, obtained by using the quenching technique, are presented for the system iron oxide-A1₂O₃ in air and at 1 atm. O₂ pressure in the temperature interval 1085° to 1725°C. Stability ranges of the various phases are delineated, and approximate compositions of crystalline and liquid phases are determined. Special attention is focused on the phase Fe₂O₃·A1₂O₃(ss) and its stability relationships.     

Nanopowder Preparation and Dielectric Properties of a Bi2O3–Nb2O5 Binary System Prepared by the High-Energy Ball-Milling Method

The high-energy ball-milling (HEM) method was used to synthesize the compositions of BiNbO₄, Bi₅Nb₃O₁₅, and Bi₃NbO₇ in a Bi₂O₃–Nb₂O₅ binary system. Reagent Bi₂O₃ and Nb₂O₅ were chosen as the starting materials. The X-ray diffraction patterns of the three compositions milled for different times were studied. Only the cubic Bi₃NbO₇ phase, Nb₂O₅, and amorphous matters were observed in powders after being milled for 10 h. After heating at proper temperatures the amorphous matters disappeared and the proleptic phases of BiNbO₄ and Bi₅Nb₃O₁₅ could be obtained. The Scherrer formula was used to calculate the crystal size and the results of nanopowders are between 10 and 20 nm. The scanning electron microscopy photos of Bi₃NbO₇ powders showed drastic aggregation, and the particle size was about 100 nm. The dielectric properties of ceramics sintered from the nanopowders prepared by HEM at 100–1 MHz and the microwave region were measured. Bi₃NbO₇ ceramics showed a good microwave permittivity ɛ_r of about 80 and a Q × f of about 300 at 5 GHz. The triclinic phase of BiNbO₄ ceramics reached its best properties with ɛ_r=24 and Q × f =14 000 GHz at about 8 GHz.     

Microstructures of SrTiO3 Internal Boundary Layer Capacitors During and After Processing and Resultant Electrical Properties

The microstructure of strontium titanate internal boundary layer capacitors at various stages in their processing was studied by transmission electron microscopy of rapidly quenched and normally cooled samples. Compositions containing excess TiO₂, Al₂O₃, and SiO₂ have a completely wetting liquid phase at the sintering temperature; during cooling Ti_nO_{2 n −1}, Magneli phases precipitate at multiple grain junctions. Diffused metal oxides and flux (Bi₂O₃, PbO, CuO, and B₂O₃) rapidly penetrate as a liquid phase along boundaries in postsintering heat treatment. This liquid phase disappears during slow cooling.     

Phase Diagram Studies in the System Tl2O3–BaO–CaO–CuO–Ag

The phase relations within the system Tl₂O₃–BaO–CaO–CuO including Ag have been studied with emphasis on the high-temperature superconducting phases TlBa₂Ca₂Cu₃O_8.5 (1223 phase), Tl₂Ba₂Ca₂Cu₃O₁₀ (2223 phase), TlBa₂CaCu₂O_6.5 (1212 phase), and Tl₂Ba₂CaCu₂O₈ (2212 phase) at 890°C in an oxygen atmosphere. 1223 has been found to be in equilibrium with a liquid phase that is Tl poor. 2223 and 2212 exhibit varying Tl/(Ba + Ca) ratios. The three-phase field 1223 + 2223 + 2212 has been identified. The results of this study emphasize that multiphase samples can be prepared which consist of three superconducting phases, each exhibiting a critical temperature of 100 K or above.     

Phase Evolution and Microwave Dielectric Properties of Lanthanum Borate-Based Low-Temperature Co-Fired Ceramics Materials

A new potential low-temperature co-fired ceramics (LTCC) system based on a simple lanthanum borate (La₂O₃–B₂O₃) glass was investigated with regard to phase development and microwave dielectric properties as functions of alumina filler content less than 50 wt% and firing temperature up to 1050°C. Unexpected crystalline phases, such as La(BO₂)₃, LaAl_2.03(B₄O₁₀)O_0.54, LaBO₃, and Al₂₀B₄O₃₆, developed during the firing process are likely responsible for substantial resultant changes in physical and microwave dielectric properties. As a specific example, a high-quality factor of 785 at 15.8 GHz obtained for a composition containing 30 wt% alumina supports the hypothesis that the phase-dielectric property relation exists in this LTCC system. On a practical basis, two phases of LaAl_2.03(B₄O₁₀)O_0.54 and LaBO₃ must be important in determining the final dielectric performance by manipulating the ratios of glass and filler and by selecting a desirable temperature.