Structural Evolution and Electrical Properties of Sc2O3-Stabilized ZrO2 Aged at 850°C in Air and Wet-Forming Gas Ambients

X-ray diffraction (XRD) and electron microscopy investigations have been performed on Sc₂O₃-stabilized ZrO₂ as-sintered and after aging in air or in wet-forming gas at 850°C for 1000 h. Some tetragonal to monoclinic transformation had occurred in the near-surface regions of 4 mol% Sc₂O₃ samples after aging; the phase transition was more severe for samples aged in the forming gas ambient. A decrease of ∼20% in electrical conductivity accompanied the aging. In 6 mol% Sc₂O₃ samples, although no cubic to tetragonal transformation was detected, both the electrical conductivity and the activation energy for ionic conductivity decreased significantly during aging. Ten mole percent Sc₂O₃ samples did not show appreciable change in electrical conductivity due to aging, although some near-surface cubic to rhombohedral transformation did occur. Sharpening of the (400)_t XRD peak of Sc₂O₃-stabilzed tetragonal ZrO₂ accompanies the change(s) in the electrical conductivity.     

High-Temperature Phase Relations in the Sc2O3-Ta2O5 System

The phase relations for the Sc₂O₃-Ta₂O₅ system in the composition range of 50-100 mol% Sc₂O₃ have been studied by using solid-state reactions at 1350°, 1500°, or 1700°C and by using thermal analyses up to the melting temperatures. The Sc_5.5Ta_1.5O₁₂ phase, defect-fluorite-type cubic phase (F-phase, space group Fm 3 m ), ScTaO₄, and Sc₂O₃ were found in the system. The Sc_5.5Ta_1.5O₁₂ phase formed in 78 mol% Sc₂O₃ at <1700°C and seemed to melt incongruently. The F-phase formed in ∼75 mol% Sc₂O₃ and decomposed to Sc_5.5Ta_1.5O₁₂ and ScTaO₄ at <1700°C. The F-phase melted congruently at 2344°± 2°C in 80 mol% Sc₂O₃. The eutectic point seemed to exist at ∼2300°C in 90 mol% Sc₂O₃. A phase diagram that includes the four above-described phases has been proposed, instead of the previous diagram in which those phases were not identified.     

Improvement in Mechanical Properties of Powder-Processed MoSi2 by the Addition of Sc2O3 and Y2O3

Additions of 1-20 mol% Sc₂O₃ or Y₂O₃ to MoSi₂ eliminate glassy SiO₂, which improves mechanical properties at both ambient and high temperatures. In particular, only 1 mol% ScO₃ additions dramatically enhance three-point bending strength from 521 to 1081 MPa. Vickers hardness, Young's modulus, fracture toughness, and high-temperature strength are also improved by this low level of additive. The improvement of mechanical properties is attributed to the formation of crystalline silicates: Sc₂Si₂O₇, Y₂Si₂O₇, Y₂SiO₅, and Y₄Si₃O₁₂, which are analyzed by XRD, SEM-EDS, and TEM-EDS methods.     

Microstructure, Conductivity, and NO2 Sensing Characteristics of α-Al2O3-Doped (8 mol% Sc2O3)ZrO2 Composite Solid Electrolyte

α-Al₂O₃-doped (8 mol % Sc₂O₃)ZrO₂ composite solid electrolyte has been investigated in the fabrication of solid-state ceramic gas sensors. The microstructure and electrical conductivity of the composite solid electrolyte have been measured over a range of temperature from 240°C to 596°C. The composite solid electrolyte has been found to exhibit a higher conductivity compared with the commonly used (8 mol% Y₂O₃)ZrO₂ at temperatures above ∼448°C. The sensing characteristics for NO₂ detection have been studied in the temperature range of 500–650°C at the low concentration from 10 to 30 ppm and at high concentration from 100 to 500 ppm of NO₂. The NO₂ sensor was found to respond reproducibly and rapidly to the variations of NO₂, concentration, indicating that the composite solid electrolyte has promising application as a solid electrolyte for on-board exhaust gas monitoring.     

High Electrical Conductivity and High Fracture Strength of Sc2O3-Doped Zirconia Ceramics with Submicrometer Grains

The microstructure, crystal phase, electrical conductivity, and mechanical strength of less than 7-mol%-Sc₂O₃-doped zirconia ceramics fabricated by comparatively low-temperature sintering at 1200–1300°C for 1 h were investigated. Zirconia ceramics having a uniform microstructure (grain size < 0.5 μm) stabilized with 6 mol% Sc₂O₃ showed high electrical conductivity (0.15 S/cm at 1000°C) and high fracture strength (660 MPa). With the increase of Sc₂O₃ content from 3.5 to 7 mol%, the grain size, fracture strength, and electrical conductivity at 1000°C changed from 0.2 to 0.5 μm, 970 to 440 MPa, and 0.07 to >0.2 S/cm, respectively. Sc₂O₃-doped zirconia polycrystals with high fracture strength and high electrical conductivity are promising candidates for the electrolyte material of solid oxide fuel cells.     

Electrical Conductivity in the ZrO2-Rich Region of Several M2O3—ZrO2 Systems

The electrical conductivity of M₂O₃-ZrO₂ compositions containing 6 to 24 mole % M₂O₃, where M represents La, Sm, Y, Yb, or Sc, was examined. Only Sm₂O₃, Y₂O₃, and Yb₂O₃ formed cubic solid solutions with ZrO₂ over most of this substitutional range. Scandia forms a wide cubic solid solution region with ZrO₂ at temperatures above 130°C whereas the cubic solid solution region at room temperature is narrow (6 to 8 mole % Sc₂O₃). Lanthana additions to ZrO₂produced no fluorite-type cubic solid solutions within the compositional range investigated. Generally, the electrical conductivity of these cubic solid solutions increased as the size of the substituted cation decreased and the electrical conductivity for each binary system attained a maximum at about 10 to 12 mole % M₂O₃.     

The System Sc2O3-WO3

Phase relations in the system Sc₂O₃-WO₃ were characterized. Two stable binary compounds were, found. The 1:3 compound, SC₂(WO₄)₃, melts congruently at 1640°±10°C and forms a simple eutectic with WO₃ at ∼90 mol% WO₃ and 1309°+10°C. The 3 : 1 compound, Sc₆WO₁₂, forms a simple eutectic with the 1:3 compound at -69 mol% WO₂, and 1580°+10°C. The melting temperature of SC₆WO₁₂ was >1600°C.     

Electrical Conduction in β-Bi2O3 Doped with Sb2O3

Electrical conduction in tetragonal β-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, ionic transference number, and Seebeck coefficient. The β-Bi₂O₃ doped with 1 to 10 mol% Sb₂O₃ was stable up to 600°C and showed an oxygen ionic and electronic mixed conduction, where the electron conduction was predominant at low oxygen pressures. The oxygen-ion conductivity showed a maximum at 4 mol% Sb₂O₃, whereas the activation energy for the ionic conduction remained unchanged for 4 to 10 mol% Sb₂O₃-doped specimens. These results were interpreted in terms of the oxygen vacancy concentration and the distortion of the tetragonal structure. The electron conductivity and its oxygen pressure dependence decreased with increasing Sb₂O₃ content. The fact that Sb⁵⁺ is partially reduced by excess electrons in heavily doped β specimens at low oxygen pressures is explained.     

Ionic Conductivity of the Fluorite-Type Hafnia–R2O3 Solid Solutions

The ionic conductivity of the hafnia-scandia, hafnia-yttria, and hafnia-rare earth solid solutions with high dopant concentrations of 8, 10, and 14 mol% was measured in air at 600° to 1050°C. Impedance spectroscopy was used to obtain lattice conductivity. A majority of the investigated samples exhibited linear Arrhenius plots of the lattice conductivity as a function of temperature. For all investigated dopant concentrations the ionic conductivity was shown to decrease as the dopant radius increased. The activation enthalpy for conduction was found to increase with dopant ionic radius. The fact that the highest ionic conductivity among 14-mol%-doped systems was obtained with HfO₂─Sc₂O₃ suggested that the radius ratio approach should be used to predict the electrical conductivity behavior of HfO₂─R₂O₃ systems. A qualitative model based on the Kilner's lattice parameter map does not seem to apply to these systems. For the three systems HfO₂─Yb₂O₃, HfO₂─Y₂O₃, and Hf₂O₃─Sm₂O₃ a conductivity maximum was observed near the dopant concentration of 10 mol%. Deep vacancy trapping is responsible for the decrease in the ionic conductivity at high dopant concentrations. Formation of microdomains of an ordered compound cannot explain the obtained results. A comparison between the ionic conductivities of doped HfO₂ and ZrO₂ systems indicated that the ionic conductivities of HfO₂ systems are 1.5 to 2.2 times lower than the ionic conductivities of ZrO₂ systems.     

Ta2O5/Nb2O5 and Y2O3 Co-doped Zirconias for Thermal Barrier Coatings

Zirconia doped with 3.2–4.2 mol% (6–8 wt%) yttria (3–4YSZ) is currently the material of choice for thermal barrier coating topcoats. The present study examines the ZrO₂-Y₂O₃-Ta₂O₅/Nb₂O₅ systems for potential alternative chemistries that would overcome the limitations of the 3–4YSZ. A rationale for choosing specific compositions based on the effect of defect chemistry on the thermal conductivity and phase stability in zirconia-based systems is presented. The results show that it is possible to produce stable (for up to 200 h at 1000°–1500°C), single (tetragonal) or dual (tetragonal + cubic) phase chemistries that have thermal conductivity that is as low (1.8–2.8W/m K) as the 3–4YSZ, a wide range of elastic moduli (150–232 GPa), and a similar mean coefficient of thermal expansion at 1000°C. The chemistries can be plasma sprayed without change in composition or deleterious effects to phase stability. Preliminary burner rig testing results on one of the compositions are also presented.     

Phase Relationships in the ZrO2-Sc2O3 and ZrO2-In2O3 Systems

Zirconia-rich subsolidus phase relationships in the ZrO₂–Sc₂O₃ and ZrO₂–In₂O₃ systems were investigated. Phase inconsistencies in the ZrO₂–Sc₂O₃ system resulted from a diffusionless cubic-to-tetragonal ( t' ) phase transformation not being recognized in the past. Through three different measuring techniques, along with microstructural observations, the solubility limits of the tetragonal and cubic phases were determined.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Phase Diagram and Oxygen-Ion Conductivity in the Y2O3-Nb2O5 System

The phase equilibria in the Y₂O₃-Nb₂O₅ system have been studied at temperatures of 1500° and 1700°C in the compositional region of 0-50 mol% Nb₂O₅. The solubility limits of the C-type Y₂O₃ cubic phase and the YNbO₄ monoclinic phase are 2.5 (±1.0) mol% Nb₂O₅ and 0.2 (±0.4) mol% Y₂O₃, respectively, at 1700°C. The fluorite (F) single phase exists in the region of 20.1-27.7 mol% Nb₂O₅ at 1700°C, and in the region of 21.1-27.0 mol% Nb₂O₅ at 1500°C, respectively. Conductivity of the Y₂O₃- x mol% Nb₂O₅ system increases as the value of x increases, to a maximum at x = 20 in the compositional region of 0 ≤ x ≤ 20, as a result of the increase in the fraction of F phase. In the F single-phase region, the conductivity decreases in the region of 20-25 mol% Nb₂O₅, because of the decrease in the content of oxygen vacancies, whereas the conductivity at x = 27 is larger than that at x = 25. The conductivity decreases as the value of x increases in the region of 27.5 ≤ x ≤ 50, because of the decrease in the fraction of F. The 20 mol% Nb₂O₅ sample exhibits the highest conductivity and a very wide range of ionic domain, at least up to log p _O2=−20 (where p _O2 is given in units of atm), which indicates practical usefulness as an ionic conductor.     

Depolymerization of GeO2 and B2O3-GeO2 Network Glasses by Sb2O3

Binary Sb₂O₃-GeO₂ glasses containing 45 mol% Sb₂O₃ and ternary Sb₂O₃-B₂O₃-GeO₂ glasses containing 50 mol% GeO₂ were prepared. Their densities (volumes), refractive indices, and infrared spectra were determined, and their colors and high-temperature viscosities were estimated visually. Small amounts of Sb₂O₃ (∼10 mol%) appear to perturb neither the Ge-O-Ge network nor those B-O-Ge networks with small B/Ge ratios (∼0.2). The B-O-Ge networks with larger B/Ge ratios (∼1.0) depolymerize in the presence of even less Sb₂O₃. Amounts of Sb₂O₃ >10 mol% appear to depolymerize the Ge-O-Ge and Ge-O-B networks progressively, possibly with the formation of chains. A structurally sensitive ir isofrequency contour technique developed for ternary glass systems was applied successfully to these Sb₂O₃-B₂O₃-GeO₂ glasses. These contours can thus readily detect significant network depolymerization in the absence of the usual network modifiers.     

Transmission Electron Microscopy of Annealed ZrO2+8 Mol% Sc2O3

The influence of heat treatment (800°C for 200 h) on the micro-structure of 8 mol% Sc₂O₃-ZrO₂ was investigated by XRD and TEM. The starting material was initially characterized and found to contain predominantly cubic-fluorite phase grains, with <5% of the grain containing the rhombohedral β phase. The β phase was positively identified by the analysis of electron diffraction pattern and by quantitative energy dispersive X-ray analysis. On aging, the relative amount of β phase was found to increase to about 15 to 20% by XRD measurement; this was confirmed by TEM observation. The orientation relation between the different variants of the β phase within a grain was determined to be (100)_r||(010)_r, and the geometrical arrangement of these variants within the grain was deduced using a 2¹/₂D imaging electron microscopy technique.     

The System HfO2-Y2O3-Er2O3

A mathematical model of the liquidus surface based on a reduced polynomial method was proposed for the system HfO₂-Y₂O₃-Er₂O₃. The results of calculations according to this model agree fairly well with the experimental data. Phase equilibria in the system HfO₂-Y₂O₃-Er₂O₃ were studied on melted (as-cast) and annealed samples using X-ray diffraction (at room and high temperatures) and micro-structural and petrographic analyses. The crystallization paths in the system HfO₂-Y₂O₃-Er₂O₃ were established. The system HfO₂-Y₂O₃-Er₂O₃ is characterized by the formation of extended solid solutions based on the fluorite-type (F) form of HfO₂ and cubic (C) and hexagonal (H) forms of Y₂O₃ and Er₂O₃. The boundary curves of these solid solutions have the minima at 2370°C (15. 5 mol% HfO₂, 49. 5 mol% Y₂O₃) and 2360°C (10. 5 mol% HfO₂, 45. 5 mol% Y₂O₃). No compounds were found to exist in the system investigated.     

Influence of TiO2 Solid Solution on the Thermal Property and Ionic Conductivity Of Partially Stabilized Zirconia

TiO₂(0–20 mol%)-3 mol% yttria-stabilized zirconia (3YSZ) ceramics were prepared by a solid-state reaction. With increasing TiO₂ content in 3YSZ, the structure of the main phase changed from a monoclinic, tetragonal, and cubic mixture to a tetragonal single phase. Increasing TiO₂ content in 3YSZ caused an increase in the average grain size of these ceramics. The thermal conductivity decreased from 4.1 to 2.1 at room temperature with an increase in the TiO₂ content. The specific heat of non-TiO₂-doped 3YSZ was slightly larger than all the doped TiO₂–3YSZ at room temperature. When the TiO₂ content was >8 mol% in 3YSZ, no abrupt expansion, shrinkage, or cracks were observed on heating and cooling these samples; thus, the thermal stability of 3YSZ was improved by TiO₂ solid solution. The ionic conductivity of the samples decreased with increasing TiO₂ solid solution.     

The Influence of Sb2O3 Addition on the Properties of Low-melting ZnO–P2O5 Glasses

The influence of 0–16 mol% Sb₂O₃ substitution for P₂O₅ on the properties of ZnO–P₂O₅ glasses has been investigated. It was shown that Sb₂O₃ could participate in the glass network and thermal stability of the glasses decreased with increasing Sb₂O₃ content. Glass transition temperature T _g, softening temperature T _s, and water durability all decreased firstly (up to 6 mol% Sb₂O₃ added) and then increased. Substitution of 12 mol% Sb₂O₃ led to a 16°C decrease in T _g and 30°C decrease in T _s, and weight loss of the glass was only 0.42 mg/cm², which is ∼11 times lower than that of the glass without Sb₂O₃ after immersion in deionized water at 90°C for 1 day. The glass containing 12 mol% Sb₂O₃ might be a substitute for Pb-based glasses in some applications.     

Electrical Properties of Cubic, Stabilized, Single ZrO2–Gd2O3 Crystals

Electrical properties of single-crystal, cubic ZrO₂ solid solutions containing 10.0, 12.5, and 15.0 mol% Gd₂O₃ were investigated. Electrical conductivities were measured by a two-terminal ac technique up to 831°C in air, and the observed data were compared with those reported for polycrystalline materials. Electrical conductivity and activation energy increased, and preexponential factor decreased with Gd₂O₃ content.     

The System K2O-BaO-SiO2

Phase equilibrium studies of compound formation and liquidus and solidus surfaces of the system K₂O-BaO-SiO₂ are presented. The system contains 3 ternary compounds: K₄BaSi₃O₉, K₈BaSi₁₀O₂₅, and K₂Ba₃Si₈O₂₀. Both high and low polymorphs of the third have fields on the ternary liquidus surface. Solid solution with SiO₂ depresses the high-low inversion from 1030°C at K₂Ba₃Si₈O₂₀ to 835°C at 70.2 mol% SiO₂. Data for 20 liquidus invariant points were found; 8 are thermal maxima and 12 are eutectics or peritectics. The isofracts of quenched glasses were determined.