Direct-Current Conductivity of Y2O3 as a Function of Water Vapor Pressure

The dc conductivity of a sintered sample of Y₂O₃ containing an excess of lower-valent metal (Ml) impurities has been measured at 600° to 1100°C in air as a function of the water vapor pressure (0.4 to 3000 Pa). The logarithm of the p -type conductivity vs log P _H2o has a slope of – 1/2 in certain regions of F _H2o and temperature. This reflects a defect situation where interstitial protons from water vapor balance the excess of lower-valent cation impurities: [H_˙] = [M1_']. The activation energy for the p -type conductivity under these conditions is evaluated and interpreted in terms of the enthalpy needed to dissolve interstitial protons in yttria.     

Strength of Hot Isostatically Pressed Si3N4 After Heat Treatment in Ar-O2 and H2-H2O

The effects of heat treatment in Ar-O₂ and H₂-H₂O atmospheres on the flexural strength of hot isostatically pressed Si₃N₄ were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si₃N₄ was exposed at 1400°C to (1) H₂ with water vapor pressure ( P _H2O) greater than 1 × 10⁻⁴ MPa or (2) Ar with oxygen partial pressure ( P _O2) of between 7 × 10⁻⁶ and 1.5 × 10⁻⁵ MPa. However, the strength of the material was degraded when the P _H2O in H₂ was lower than 1 × 10⁻⁴ MPa, and essentially unaffected when the P _O2 in Ar was higher than 1.5 × 10⁻⁵ MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y₂Si₂O₇ layer formed during exposure.     

Measurement of Oxygen Diffusion in Dy2O3 and Gd2O3 by Studying High-Temperature Oxidation of the Metals

Studies of the oxidation of Gd and Dy at P _O2's from 10^−0.3 to 10^−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P ₀₂ semiconducting domain yielded oxygen diffusion coefficients in Dy₂O₃ in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd₂O₃ which are considerably below literature values for monoclinic single-crystal Gd₂O₃. At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P _O2, the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10⁻⁶ to 10⁻⁷Ω⁻¹ cm⁻¹.     

The Effect of Atmosphere on the Sintering of Fe3O4 and Fe2O3 Powders at Low Temperatures

The sintering kinetics of submicrometer Fe₃O₄ and Fe₂O₃ powders were investigated at 300° to 500°C. Using measurements of the rate of reduction of surface area, the coefficients of surface diffusion on the oxides are estimated for a range of oxygen partial pressures. The surface-diffusion coefficients appear to be independent of P _O2 for magnetite and only slightly dependent on P _O2 for hematite.     

Ionic Conductivity of Cubic Solid Solutions in the System CaO—Y2O3—ZrO2

The ionic conductivity of cubic solid solutions in the system CaO -Y₂O₃-ZrO₂ was examined. Particular Y₂O₃-ZrO₂ binary compositions were more conductive at elevated temperatures (>600°C) than either CaO-ZrO₂ binary or CaO-Y₂O₃-ZrO₂ ternary compositions. The higher ionic conductivity appears to be related to a lower activation energy rather than to the number of oxygen vacancies dictated by composition. Those compositions of highest conductivity lie close to the cubic-monoclinic solid-solution phase boundary. Conductivity-temperature data are presented that indicate a reversible order-disorder transition for Y₂O₃-ZrO₂ cubic solid solutions containing 20 and 25 mole % Y₂O₃. The transference number for the oxygen ion at 1000°C for Y₂O₂-ZrO₂ cubic solid solutions is greater than 0.99.     

Nonstoichiometry in Undoped BaTiO3

Equilibrium electrical conductivity data for large-grained, poly crystalline, undoped BaTiO₃, as a function of temperature, 750° to 1000°C, and oxygen partial pressure, 10⁻²⁰< P _O2<10⁻¹ MPa, were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The latter are invariably present in sufficient excess to control the defect concentrations through the compensating oxygen vacancies, except under the most severely reducing conditions. Singly ionized oxygen vacancies play no discernible role in the defect chemistry of BaTiO₃ within this experimental range. The derived accidental acceptor content has a slight temperature dependence which may reflect some small amount of defect association. Deviation of the conductivity minima from the ideal shape yields a small P _O2-independent conductivity contribution, which is tentatively identified as oxygen vacancy conduction.     

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.     

Defect Structure of Ta2O5

Electrical conductivity, thermoelectric power, and weight change were measured for polycrystalline Ta₂O₅ from 900° to 1400°C. The predominant ionic and electronic defects in this temperature range are oxygen vacancies and electrons. The oxygen-vacancy and electron mobilities are 8.1 × 10³exp (−1.8 eV/ k T) and ∼0.05 cm²/V-s, respectively. At O₂ partial pressures near 1 atm, the ionic-defect concentration is essentially fixed by the presence of lower-valence cation impurities, and the total electrical conductivity is predominantly ionic, whereas at low P _o2's the conductivity is electronic and proportional to P P _o2^−1/6.     

Phase Equilibria and Ionic Conductivity in the System ZrO2−Yb2O3−Y2O3

The phase equilibria in the zirconia-rich part of the system ZrO₂−Yb₂O₃−Y₂O₃ were determined at 1200°, 1400°, and 1650°C. The stabilizing effects of Yb₂O₃ and Y₂O₃ were found to be quite similar with <10 mol% of either being necessary to fully stabilize the cubic fluorite-structure phase at 1200°C. The two binary ordered phases, Zr₃Yb₄O₁₂ and Zr₃Y₄O₁₂, are completely miscible at 1200°C. These were the only binary or ternary phases detected. The ionic conductivities of ternary specimens in this system were measured using the complex impedance analysis technique. For a given level of total dopant, the substitution of Yb₂O₃ for Y₂O₃ gives only minor increases in specimen conductivity.     

Effect of Heat Treatment on the Microstructure and Properties of Dense AIN Sintered with Y2O3 Additions

The secondary phase constitution in two sintered AIN ceramics (1.8% and 4.2% Y₂O₃ additions) was studied as a function of heat treatment temperatures between 1750° and 1900°C under pure nitrogen atmosphere. The effect of the phase constitution on the physical properties, such as density, thermal conductivity ( K ), and lattice constants, and on the mechanical properties in three-point bending, was also investigated. Y₃Al₅O₁₂ was found to getter dissolved oxygen from the AIN lattice below 1850°C, but evaporated at 1850°C and above. Y₄Al₂O₉ appeared to sublimate below 1850°C in the atmosphere used in this study. Depending on the secondary phase constitution, heat treatment affected thermal conductivity favorably or adversely. Occasionally, samples with similar lattice oxygen contents were found to have different thermal conductivities, suggesting that factors besides dissolved oxygen can also influence K . Lattice parameter measurements indicated that, within the small range of lattice oxygen concentrations in the AIN samples studied, the c-axis was more sensitive than the a -axis to oxygen content.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Compressive Creep and Oxidation Resistance of an Si3N4 Material Fabricated in the System Si3N4-Si2N2O-Y2Si2O7

The compressive creep behavior and oxidation resistance of an Si₃N₄/Y₂Si₂O₇ material (0.85Si₃N₄+0.10SiO₂+0.05Y₂O₃) were determined at 1400°C. Creep re sistance was superior to that of other Si₃N₄ materials and was significantly in creased by a preoxidation treatment (1600°C /120 h). An apparent parabolic rate constant of 4.2 × 10⁻¹¹ kg²·m-⁴·s⁻¹ indicates excellent oxidation resistance.     

Phase Relations in the System MgO-V2O3-VO2 at 1200°C

Phase relations in the quasi-ternary system MgO-V₂O₃-VO₂ at 1200°C were studied using the quenching technique under controlled O₂ atmospheres. A new phase of a type z VO _y Mg_{2− x} V_{1+ x} O₄ (0< x <1, y ≥1.5, z >0) was found with a compositional region along the MgV₂O₄-Mg₂VO₄ join. Equilibrium P _{O 2} observed for Mg_{2− x} V_{1+ x} O₄ is quite different from that for V _n O_{2 n -1} with an equal ratio of V³⁺/V⁴⁺, corresponding to the V³⁺ stabilities in two types of compounds. Thus, the phase relations in the ternary system were constructed on a conventional triaxial diagram as a function of P _O2.     

High-Temperature Phase Relations in the System Y2O3-Ta2O5

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid-state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phases (W2 phase, space group C222₁), fluorite-type cubic phases (F phase, space group Fm3m )and another orthorhombic phase (O phase, space group Cmmm )are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.     

Defect Structure and Electrical Conductivity of ThO2-Y2O3 Solid Solutions

Compositions in the system ThO₂-YO_1.5 were coprecipitated as oxalates and converted to oxides. Disks were pressed and sintered in oxygen at 1400° to 2200°C. Densities of the sintered disks were 96 to 98% of theoretical. Solid solutions with the fluorite-type structure were formed up to 20 to 25 mole % YO_1.5 at 1400°C and up to 45 to 50 mole % YO_1.5 at 2200°C. Density data showed that these solid solutions correspond to Th_{1— x} Y _x O_{2—0.5 x} , having a complete cation sub-lattice filled by Th⁴⁺ and Y³⁺ ions, and vacancies in the anion sublattice. The observed increase in electrical conductivity with increase in YO_1.5content is consistent with charge transport by oxygen ions through a vacancy mechanism. Approximately 7 mole % ThO₂ is soluble in YO_1.5 at 2200°C. Density results indicate an anion interstitial structure for the Y₂O₃ phase. Transference number measurements indicate that the electrical conductivities are only partly due to ions.     

Densification and Thermal Conductivity of AIN Doped with Y2O3, CaO, and Li2O

Small amounts of Li₂O result in sintering in the AIN-Y₂O₃-CaO and AIN-CaO systems at firing temperatures <1600°C. The effect is ascribed to reduction of the liquidus temperature. Furthermore, Li₂O is removed by volatization at temperatures from 1300° to 1600°C, and its content decreases several ppm from the initial 0.3 wt%. Li₂O-doped AIN specimens containing Y₂O₃ and CaO additives are well densified by firing at 1600°C for 6 h, and their thermal conductivity is 135 W^.m^−1.K⁻¹.The effect of Li₂O addition on sintering and thermal conductivity also is discussed through thermo-dynamic considerations.     

Electrical Properties and Defect Structure of Y2O3

Guarded measurements of the electrical conductivity of high-purity, polycrystalline Y₂O₃ in thermodynamic equilibrium with the gas phase were made under controlled temperature and oxygen partial pressure conditions. Data are presented as isobars from 1200° to 1600°C, and as isotherms from oxygen partial pressures of 10⁻¹ to 10⁻¹⁷ atm. The ionic contribution to the total conductivity, determined by the blocking electrode polarization technique, was less than 1% over the entire range of temperatures and oxygen partial pressures studied. Yttria is shown to be an amphoteric semiconductor with the region of predominant hole conduction shifting to higher pressures at higher temperatures. In the region of p -type conduction, the conductivity is represented by the expression σ= 1.3 × 10³ p O₂^3/16 exp (-1.94/kT). The observed pressure dependence is attributed to the predominance of fully ionized yttrium vacancies. Yttria is shown to be a mixed conductor below 900°C.     

Characterization of Photoluminescent (Y1–xEux)2O3 Thin Films Prepared by Metallorganic Chemical Vapor Deposition

The purpose of this study was to identify and correlate the microstructural and luminescence properties of europium-doped Y₂O₃ (Y_{1– x} Eu _x )₂O₃ thin films deposited by metallorganic chemical vapor deposition (MOCVD), as a function of deposition time and temperature. The influence of deposition parameters on the crystallite size and microstructural morphology were examined, as well as the influence of these parameters on the photoluminescence emission spectra. (Y_{1– x} Eu _x )₂O₃ thin films were deposited onto (111) silicon and (001) sapphire substrates by MOCVD. The films were grown by reacting yttrium and europium tris(2,2,6,6-tetramethyl–3,5-heptanedionate) precursors with an oxygen atmosphere at low pressures (5 torr (1.7 × 10³ Pa)) and low substrate temperatures (500°–700°C). The films deposited at 500°C were smooth and composed of nanocrystalline regions of cubic Y₂O₃, grown in a textured [100] or [110] orientation to the substrate surface. Films deposited at 600°C developed, with increasing deposition time, from a flat, nanocrystalline morphology into a platelike growth morphology with [111] orientation. Monoclinic (Y_{1– x} Eu _x )₂O₃ was observed in the photoluminescence emission spectra for all deposition temperatures. The increase in photoluminescence emission intensity with increasing postdeposition annealing temperature was attributed to the surface/grain boundary area-reduction effect.     

Phase Relations in the System ZrO2-Y2O3 at Low Y2O3 Contents

Subsolidus phase relations in the low-Y₂O₃ portion of the system ZrO_2-Y₂O₃ were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples. Approximately 1.5% Y₂O₃ is soluble in monoclinic ZrO₂, a two-phase monoclinic solid solution plus cubic solid solution region exists to ∼7.5% Y₂O₃ below ∼500°C, and a two-phase tetragonal solid solution plus cubic solid solution exists from ∼1.5 to 7.5% Y₂O₃ from ∼500° to ∼1600°C. At higher Y₂O₃ compositions, cubic ZrO₂ solid solution occurs.     

Phase Transition of Rare-Earth Aluminates (RE4Al2O9) and Rare-Earth Gallates (RE4Ga2O9)

Lattice parameters of RE₄Al₂O₉ (RE = Y, Sin, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) prepared at 1600–1800°C and those of RE₄Ga₂O₉ (RE = La, Pr, Nd, Sm, Eu, and Gd) prepared at 1400–1600°C were refined by Rietveld analysis for the X-ray powder diffraction patterns. The parameters increased linearly with the ionic radius of the trivalent rare-earth elements ( r _RE). High-temperature differential calorimetry and dilatometry revealed that both RE₄Al₂O, and RE₄Ga₂O, have reversible phase transitions with volume shrinkages of 0.5–0.7% on heating and thermal hystereses. The transition temperatures (7_tr) decreased from 1300°C (Yb) to 1044°C (Sm) for RE₄A1₂O₉, except for Y₄Al₂O₉ ( T_tr = 1377°C), and from 1417°C (Gd) to 1271°C (La) for RE₄Ga₂O, with increasing ionic radius of the rare-earth elements. These transition temperatures were plotted on a curve against the ionic radius ratio of Al³⁺ or Gd³⁺ and RE³⁺ ( r _A1Ga/r_RE) except for Y₄Al₂O₉.