Hydrothermal Oxidation of Hf Metal Chips in the Preparation of Monoclinic HfO2 Powders

Fine monoclinic HfO₂ powders were prepared from Hf metal chips by reaction with high-temperature high-pressure water. Dependence of the reaction on temperature-pressure-time was examined in both closed and open systems. Hydrothermal oxidation of Hf proceeded through three stages: the surface oxidation of Hf metal chips with H₂O, the reaction of the Hf with released H₂ to form hydrides, and the succeeding oxidation of the hydrides with H₂O to form HfO₂. Most of the fine HfO₂ powders were, however, considered to be formed by the latter two reactions after the cracking and pulverizing of metal chips associated with hydriding. The difference in the reaction rates between the closed and open systems was explained by taking into account the fugacity relations of H₂ and H₂O in the respective systems.     

Monoclinic Crystal Structures of ZrO2 and HfO2 Refined from X-ray Powder Diffraction Data

The crystal structures of ZrO₂ and HfO₂ (P2₁/c) were refined in detail from X-ray powder diffraction data. The precision obtained for the atom positions for both compounds is comparable to that of previous single-crystal work.     

Hydrothermal Preparation of Ultrafme Monoclinic ZrO2 Powder

Ultrafine powder of single-phase manoclinic ZrO₂ was prepared by hydrothermal treatments of amorphous hydrated zirconia with 8 wt% KF solution under 100 MPa at 200° to 500°C for 24 h. The process yielded well-crystallized particles 16 nm and 22 nm in size at 200° and 500°C, respectively .     

The System HfO2-TiO2

The system HfO₂-TiO₂ was studied in the 0 to 50 mol% TiO₂ region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO₂ was found at 1750°± 20°C. The definite compound HfTiO₄ melts incongruently at 1980°± 10°C, 53 mol% TiO₂. A metatectic at 2300°± 20°C, 35 mol% TiO₂ was observed. The eutectoid decomposition of HfO_2,ss) ( T ) → HfO_2,ss ( M ) + HfTiO_34,ssss occurred at 1570°± 20°C and 22.5 mol% TiO₂. The maximum solubility of TiO₂ in HfO_2,ss,( M ) is 10 mol% at 1570°± 20°C and in HfO_2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO₂-rich side and in the 10 to 30 mol% TiO₂ range a second monoclinic phase M of HfO₂( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO₂ region, which are based on thermal analysis techniques.     

Pressure-Induced Phase Transformation of HfO2

The pressure dependence of the Raman spectra of HfO₂ was measured by a micro-Raman technique using a singlecrystal specimen in the pressure range from 0 to 10 GPa at room temperature. The symmetry assignment of Raman bands of the monoclinic phase was experimentally accomplished from the polarization measurements for the single crystal. With increased pressure, a phase transformation for the monoclinic phase took place at 4.3 ± 0.3 GPa. Nineteen Raman bands were observed for the high-pressure phase. The spectral structure of the Raman bands for the high-pressure phase was similar with those reported previously for ZrO₂. The space group for the high-pressure phase of HfO₂ was determined as Pbcm , which was the same as that of the high-pressure phase for ZrO₂ on the basis of the number and the spectral structure of the Raman bands.     

Electrical Properties and Defect Structure of HfO2

The defect structure of high-purity, polycrystalline HfO₂ was investigated by measuring the oxygen partial pressure dependence of the electrical conductivity and the sample weight. From 1000° to 1500°C and above oxygen partial pressures of 10 ⁻⁶, the conductivity is electronic and proportional to p o₂^1/5. The predominant defect is completely ionized hafnium vacancies. At lower oxygen partial pressures a broad shallow minimum in the lower temperature conductivity isotherms indicates the presence of an oxygen pressure independent source of electronic charge carriers. By combining the weight change and conductivity data, mobility values were found to vary from 1.6 × 10⁻³ to 3 × 10⁻⁴ cm²/V-sec. The activation energies for the hole mobilities were calculated to be 0.2 ev above 1300° C and 0.7 ev below this temperature.     

Vibrational Spectra of HfO2–ZrO2 Solid Solutions

Vibrational spectra were measured and analyzed for HfO₂–ZrO₂ solid solutions. Some Raman bands in the high–wave–number region shift almost linearly with changes in ZrO₂ concentration between the pure end–members; their band locations can be used to determine ZrO₂ concentrations in annealed solid solutions. The Raman bands of pure ZrO₂ and HfO₂ can be correlated using the band shifts of the solid solutions. Induced stress causes band shifts for the solid solutions.     

Characteristic Spherulitic Microstructure in Partial-Melt-Processed YBa2Cu3Ox/HfO2

The microstructure of partial-melt-processed YBa₂Cu₃O _x /HfO₂ has been studied by transmission electron microscopy. A characteristic spherulitic microstructure is formed in the system. A model for the growth mechanism has been proposed. The critical heterogeneous nucleation of the YBa₂Cu₃O _x phase appears to occur from the melt in an epitaxially controlled manner on CuO particles. Subsequent growth of YBa₂Cu₃O _x platelets from the nucleus region is repeatedly interrupted by the nucleation of hafnium-rich phases in the liquid at the solid/liquid interface in a manner that again appears to be epitaxially controlled and that promotes the splay of the c orientation of the YBaCuO grain.     

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.     

Synthesis of Transparent Nd-doped HfO2-Y2O3 Ceramics Using HIP

A Nd-doped HfO₂-Y₂O₃ ceramic having excellent transmittance was synthesized by HIPing, using high-purity powders (>99.99 wt%) of Y₂O₃, Nd₂O₃, and HfO₂. The mixed powder compacts of these powders were sintered at 1650°C for 1 h under vacuum and HIPed at 1700°C for 3 h under 196 MPa of Ar. The specimen after HIPing consisted of uniform grains measuring about 30 μm and having pore-free structure. The optical transmittance of 1 at.% Nd-doped 2.6 mol% HfO₂-Y₂O₃ ceramics ranging between visible and infrared wavelength was almost equivalent or superior to that of a Nd:Y₂O₃ single crystal grown by the Verneuil method.     

Elastic Properties of Polycrystalline Monoclinic Gd2O3

The elastic properties of polycrystalline monoclinic Gd₂O₃ were determined by the sonic-resonance method. Volume-fraction porosity varied from 0.025 to 0.367 and temperature from room temperature to 1400°C. The Young's and shear moduli are linear functions of volume-fraction porosity, but the rate of their decrease with increasing porosity is less than that expected. The moduli decreased more rapidly than expected with increasing temperature. The Debye temperature is 362°K. With increasing temperature, the first Grueneisen constant, γ, decreases, whereas the second Grueneisen constant, δ, increases.     

Self-Diffusion of Er and Hf in Polycrystalline Er2O3-Stabilized HfO2

Tracer self-diffusion coefficients for Er and Hf were measured in polycrystalline Er₂O₃-stabilized HfO₂ (fluorite phase) at compositions between 10 and 40 mol% Er₂O₃ and temperatures between 1700° and 2033°C. In some instances, grain-boundary coefficients were also determined. The Hf volume coefficients were generally slightly smaller than Er coefficients, but neither showed a strong composition dependence, a finding most likely attributable to extensive clustering of charged defects which apparently rendered a large proportion of the "notional" defects essentially immobile. The activation energies for volume coefficients were very large, ranging from 630 to 760 kJ-mol⁻¹, and generally decreased with increasing Er₂O₃ content.     

Ordered Fluorite-Related Phases in the Systems ZrO2(HfO2)-MgO-Nb2O5 (Ta2O5)

New phases in the ternary systems ZrO₂ (HfO₂)-MgO-Nb₂O₅ (Ta₂O_s) were identified. These M₇O₁₂ compositions have fluorite superstructures isomorphous with the rhombohedral Y₆UO₁₂ type.     

Condensed Phase Relations in the Systems ZrO2-WO2-WO3 and HfO2-WO2-WO3

Phase relations for the systems ZrO₂–WO₂–WO₃ and HfO₂–WO₂–WO₃ from 1000° to 1700° C were determined by the quenching technique using sealed sample containers. In the system ZrO₂–WO₃, 1:2 compound, ZrW₂O₈ forms, having a cubic structure with a= 9.159 A. The ZrW₂O₈ melts incongruently at 1257°± 3°C to ZrO₂ and liquid and has a lower limit of stability at 1105°C, below which ZrO₂ and WO₃ coexist in equilibrium. One eutectic and one peritectic were established: at 1231°± 3°C and 74 mole % WO₃, and at 1257°± 3°C and 71 mole % WO₃, respectively. Along the join ZrO₂–WO₂, no compound formed. Two invariant points were determined: ZrO₂, WO₂, W, and liquid are in equilibrium at 1430°± 5°C and 76 mole % WO₂, whereas WO₂, W₁₈O₄₉, W, and liquid coexist at 1530°± 5°C and 89 mole % WO₂- Equilibrium relations in the system ZrO₂–WO₂–WO₃ were investigated at four temperatures. At 1200°C, a cubic phase with composition near W₂₀O₅₈ was found; it exists in equilibrium with ZrO₂, W₁₈O₄₉, W₂₀O₅₈, and WO₃. As the temperature increases, the liquid formed along the ZrO₂–WO₃ join extends into the ternary system, crosses the join ZrO₂–W₂₀O₅₈ at 1300°C, and crosses the join ZrO₂–W₁₈O₄₉ at 1400°C. The cubic phase can take more zirconium into its solid solution at 1300° than at 1200°C. At 1500°C, the system can no longer be treated as a simple ternary oxide system because of the presence of metallic tungsten, and equilibrium relations are presented on the basis of the system ZrO₂–W–WO₃. Phase equilibrium relations in the systems HfO₂–WO₃, HfO₂–WO₂, and HfO₂–WO₂–WO₃ in the temperature ranges studied are much like those in the corresponding zirconium system.     

Self-Diffusion of Er in Pure and HfO2-Doped Polycrystalline Er2O3

Using a tracer sectioning technique, the self-diffusion of Er in pure and HfO₂-doped polycrystalline Er₂O₃ was measured at 1614° to 1900°C. Up to ≊ 10 mol% HfO₂ dopant level, the Er self-diffusion coefficients followed a relation based on cation vacancies as the principal mobile defects present and available for cation diffusion. Above 10 mol% HfO₂, deviation from this relation occurred, apparently due to clustering of cation vacancies and oxygen interstitials around the dopant hafnium ions. The activation energy for the self-diffusion of Er in pure Er₂O₃ was 82.2 kcal/mol and increased with the HfO₂ dopant level present.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Sol–Gel Synthesis of High-k HfO2 Thin Films

HfO₂ films were prepared using alkoxy-derived precursor solutions. The effects of the chemical composition of precursor solutions on the microstructure development were investigated for HfO₂ films on Si substrates. The microstructure distinguished developed in the HfO₂ films prepared using the precursor solutions with and without diethanolamine. This result is considered to be due to the difference in the progress of organic decomposition and the behavior of nucleation and grain growth. The flatness and refractive index of the HfO₂ films were improved using diethanolamine-added solution. The refractive index and the dielectric constant of the HfO₂ film prepared at 400°C using a diethanolamine-added solution were about 1.85 and 17, respectively. A similar microstructure developed in the HfO₂ films on polyimide films. Much flat and uniform HfO₂ films are expected for application to integrated optical devices.