Elastic Properties of 0.98Li2O-1.0Al2O3-nSiO2 Glasses and Keatite-Phase Glass-Ceramics

Bulk physical properties such as elastic moduli, thermal expansions, and moduli of rupture were measured for a series of 0.98Li₂O-1.0ALO₃- n SiO₂ glasses and the corresponding keatite solid-solution-phase glass-ceramics. The SiO₂ content ranged from n =4 to 12. The magnitude of the elastic properties of the glasses changed monotonically with increasing SiO₂ content. The properties of keatite-phase glass-ceramics depended almost linearly on SiO₂ content, but their behavior differed qualitatively from that of the glasses.     

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.     

Pressure and Temperature Dependence of the Elastic Moduli of Na2O-TiO2-SiO2 Glasses

Ultrasonic interferometry was used to measure elastic-wave velocities and moduli in six Na₂O-TiO₂-SiO₂ glasses; three glasses contained 20 mol% TiO₂ and three 25 mol% TiO₂. The elastic moduli and their pressure derivatives varied systematically with the SiO₂/Na₂O molar ratio of the glasses. In the group of glasses which contained 20 mol% TiO₂, dK/dP ( K =bulk modulus) decreased linearly from 4.85 to 2.59 as the SiO₂/Na₂O ratio increased; in the other group, dK/dP decreased from 4.00 to 3.05. Similarly, dμ/dP (μ=shear modulus) decreased with the SiO₂/Na₂O ratio, but somewhat non-linearly. The extrinsic and intrinsic contributions to the temperature dependencies of the elastic moduli are evaluated in light of the measured pressure dependencies of these moduli.     

Elastic and Anelastic Response of Polycrystalline UO2-PuO2

A sonic resonance technique was used to investigate the room-temperature elastic and anelastic properties of physically mixed U_0.8PU_0.2O₂ as a function of density, stoichiometry, and cation homogeneity. The effect of porosity on the elastic moduli was linear and is described by E =2102.7 (1–2.03 P )± 13.5 Kbars for the Young's modulus, G =823.5(1–2.05 P )± 9.1 kbars for the shear modulus, and B = 1584.8(1–1.89 P )± 59.1 kbars for the bulk modulus, where P is the volume fraction porosity. Poisson's ratio was 0.28 and was not a function of porosity. The Debye temperature of U_0.8Pu_0.2O₂ computed from the Young's and shear moduli for theoretically dense specimens was 379°K. Variation of the O/M ratio from 1.968 to 2.006 produced no significant change in either the damping capacity or the elastic moduli of single-phase 80%UO₂-20% PuO₂ solid solutions. An approximate 24% decrease of the room-temperature Young's and shear moduli and an approximate increase by a factor of 14 in the internal friction were observed with gross modifications of plutonium cation homogeneity. Preliminary results suggest that internal friction measurements might be used to assay the homogeneity of UO₂-PuO₂ solid solutions.     

Hot-Pressing and Mechanical Properties of Al2O3 with an Mo-Dispersed Phase

Compositions of alumina with a molybdenum dispersed phase were investigated in the 0 to 5 vol% Mo range. These compositions were also prepared with a 0.5 wt% MgO addition. All specimens were fabricated by hot-pressing, and near theoretical densities were achieved. Specimens were characterized by metallographic and X-ray diffraction analyses, and microhardness, elastic moduli, tensile strength, and fracture energy were determined. Results revealed that Mo additions did not affect grain growth; in contrast, MgO additions significantly inhibited grain growth. However, Mo additions did reduce the elastic moduli and microhardness but did not measurably affect the tensile strength. Tensile strength was dependent on grain size and fitted the G^−1/3 relation. The fracture energy of Al₂O₃+5% Mo was 50% greater than that of Al₂O₃. Specimens were successfully hot-pressed with a micro-structure graded from that of Al₂O₃ to that of the 5% Mo composition.     

Elastic Properties of Al2O3 and Si3N4 Matrix Composites with SiC Whisker Reinforcement

A study of the elastic moduli of Al₂O₃ and Si₃N₄ ceramics reinforced with 0 to 25 wt% SiC whiskers has been performed. The Young's moduli, shear moduli, and longitudinal modulus are compared with calculated predictions for aligned fiber composites by Hill and Hashin and Rosen, and for fibers randomly oriented in three dimensions by Christensen and Waal. The measured values are in excellent quantitative agreement with those derived for the random orientation of the SiC whiskers.     

Elastic Modulus and Fracture Toughness of Ternary PbO-ZnO-B2O3 Glasses

The elastic moduli and fracture toughnesses of a series of PbO-ZnO-B₂O₃ glasses were measured for different PbO/ZnO ratios and for B₂O₃ contents from 30 to 70 mol%. Substituting ZnO for PbO increased both the elastic modulus and fracture toughness at all B₂O₃ levels with the fracture toughness being related to the elastic modulus. Structural effects on these properties are discussed.     

Microcracking of Eu2O3=Ta2O5 Bodies

Europium sesquioxide is of interest as a control material for reactors because it has several large-cross-section isotopes which form successively on neutron capture. Pure monoclinic Eu₂O₃ tends to grow large grains during sintering, causing microcracks to develop and leading to anomalous mechanical properties. The effect of doping the Eu₂O₃ with the grain growth suppressant Ta₂O₃ was determined using studies of the elastic properties and internal friction by the sonic resonance technique. Compositions with <3 at.% Ta cation substitution had anomalously low room temperature elastic moduli values and showed a hysteresis on thermal cycling. These specimens also had high internal frictions. The finer-grained specimens which resulted when the dopant level exceeded 3 at.% Ta did not show these anomalous characteristics.     

Room-Temperature KIc Values for Single-Crystal and Polycrystalline MgAl2O4

The K_lc values for (100), (110), and (111) single-crystal MgAl₂O₄ as well as those for polycrystalline MgAl₂O₄ with transgranular and intergranular fractures are presented and discussed on the basis of their elastic moduli. It is observed that the single-crystal toughnesses are directly related to the elastic modulus. Polycrystalline and single-crystal toughnesses are comparable; however, the intergranular fracture has the lowest toughness.     

Elastic Moduli and Refractive Indices of Aluminosilicate Glasses Containing Y2O3, La2O3, and TiO2

Glasses in the system Al₂O₃-Y₂O₃-SiO₂, containing TiO₂ and La₂O₃, were investigated. Glasses of high refractive index and elastic modulus were developed. The observed Young's and shear moduli of these glasses show good agreement with theoretical values. Agreement was also found between the observed and calculated values of refractive index when the Appen's empirical coefficients were used.     

Phase Distribution and Residual Stresses in Melt-Grown Al2O3-ZrO2(Y2O3) Eutectics

Al₂O₃-ZrO₂ eutectics containing 0 to 12.2 mol% Y₂O₃ (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. Processing variables were chosen to obtain homogeneous, colony-free, interpenetrating microstructure for all of the compositional range, optimum from the viewpoint of mechanical properties. The amount of cubic, tetragonal, or monoclinic zirconia phases was determined using a combination of Raman and X-ray diffraction techniques. Monoclinic zirconia was present up to concentrations of 3 mol% Y₂O₃, while the amount of tetragonal zirconia gradually increased with yttria content up to 3 mol%. Cubic zirconia was the only phase detected when the yttria content reached 12 mol%. The residual stresses in alumina were measured using the shift of the ruby R lines. Compressive stresses were isotropic when measured in the samples containing tetragonal and cubic zirconia, while higher tensile, anisotropic stresses were found when monoclinic zirconia was present. They were partially relieved in the eutectic sample without yttria. These results were compared with a thermoelastic analysis based on the self-consistent model.     

Composition Dependence of Elastic Moduli in Na2O-TiO2-SiO2 Glasses

Measurements of the elastic moduli for Na₂O-TiO₂-SiO₂, glasses located in 2 adjacent primary phase fields are quantitatively represented by equations in terms of linear mole fractions. Evaluation of the data showed that these linear relations are unique for each group of glasses studied.     

Ionic Mobilities and Association Energies from an Analysis of Electrical Impedance of ZrO2–Y2O3 Alloys

The dc conductivities of ZrO₂–Y₂O₃ ceramic alloys (in the range 3–12 mol% of Y₂O₃) have been obtained from ac impedance measurements at temperatures between 250° and 370°C. The Almond–West ac conductivity model has been applied to evaluate hopping rates in this system. The migration enthalpies were evaluated and shown to increase with yttria concentration, but all values determined were shown to be lower than the corresponding activation enthalpies for conductivity. The association enthalpies thus calculated were shown to be very small in 3 mol% Y₂O₃–ZrO₃ and to increase with yttria concentration until the yttria contents were high enough to form fully stabilized cubic zirconia. For these samples the association enthalpies are about 0.19 eV, and no longer sensitive to yttria content. The low hopping rate at high yttria concentration might be attributed to low entropy in the system, which might be attributed to the formation of vacancy clusters and/or an ordering of the structure.     

Elastic Moduli of Transparent Yttria

The elastic moduli of yttria (Y₂O₃) samples that were made from powders with various particle morphologies were studied by means of ultrasonic measurements. The soundwave velocities in the longitudinal and transverse modes were measured. The elastic moduli were calculated from the sound velocities and density. For the high-purity, high-density (>5000 kg/m³) Y₂O₃ that was prepared in the present study, the average density and elastic moduli (and their standard deviations) were as follows: density (ρ) of 5020 ± 18 kg/m³, Young's modulus ( E ) of 179.8 ± 4.8 GPa, shear modulus ( G ) of 69.2 ± 2.0 GPa, bulk modulus ( B ) of 148.9 ± 3.0 GPa, and Poisson's ratio (ν) of 0.299 ± 0.004. The average longitudinal and transverse soundwave velocities ( V _l and V _t, respectively) were 6931 ± 65 and 3712 ± 49 m/s, respectively. The elastic moduli of lanthana-strengthened yttria (LSY) were ∼6% lower than those of high-purity Y₂O₃, and the nu value for LSY was ∼0.304. It has been argued that soundwave velocity is better than density, in regard to predicting the elastic moduli of fully dense and slightly porous materials. A linear equation that describes the change of the elastic moduli with soundwave velocity alone has been suggested. This equation was applicable to a relative elastic moduli range of 0.75–1.02.     

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.     

Enhanced Mechanical Properties of Machinable LaPO4/Al2O3 Composites by Spark Plasma Sintering

LaPO₄/Al₂O₃ composites were fabricated by spark plasma sintering. The effects of LaPO₄ contents on the mechanical properties of the composites were investigated. The bending strength and fracture toughness can reach the maximum value of 568.2±30 MPa and 4.8±0.5 MPa·m^1/2 for the composite with 16.4 vol% LaPO₄ addition, respectively. The elastic moduli and hardness of the composites decreased with increasing LaPO₄ content. Furthermore, the experimental results show that the composites can be machined by a tungsten carbide drill as the LaPO₄ volume fraction is higher than 34.4 vol%.     

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.     

Elastic Constants of Single Crystal Calcium Molybdate (CaMoI4)

The seven elastic compliances and seven elastic constants of single crystal calcium molybdate were determined by a resonance method. The compliance s₁₆, which is zero for higher symmetry tetragonal crystals, need not be zero for crystals of lower tetragonal symmetry and contributes significantly to the orientation dependence of the elastic moduli of calcium molybdate. The values of the compliances and their standard errors (based on 41 measurements) in units of 10⁻¹¹m²/N are s₁₁= 0.974 + 0.005, s₃₃= 0.958 ± 0.004, s₄₄= 2.720 ± 0.009, s₆₆= 2.471 ± 0.022, s₁₂=−0.380 ± 0.010, s₁₃=−0.230 ± 0.009, and s₁₆= 0.433 ± 0.010.     

Pressureless Sintering of α-Si3N4 Porous Ceramics Using a H3PO4 Pore-Forming Agent

A new method for preparing high bending strength porous silicon nitride (Si₃N₄) ceramics with controlled porosity has been developed by using pressureless sintering techniques and phosphoric acid (H₃PO₄) as the pore-forming agent. The fabrication process is described in detail and the sintering mechanism of porous ceramics is analyzed by the X-ray diffraction method and thermal analysis. The microstructure and mechanical properties of the porous Si₃N₄ ceramics are investigated, as a function of the content of H₃PO₄. The resultant high porous Si₃N₄ ceramics sintered at 1000°–1200°C show a fine porous structure and a relative high bending strength. The porous structure is caused mainly by the volatilization of the H₃PO₄ and by the continous reaction of SiP₂O₇ binder, which could bond on to the Si₃N₄ grains. Porous Si₃N₄ ceramics with a porosity of 42%–63%, the bending strength of 50–120 MPa are obtained.     

Characterization and Sintering of a Porous Glass-Ceramic in the System Na2O-B2O3-Sc2O3

Clear glasses which included droplet-like microphases were produced when SiO₂ in sodium borosilicate glasses was replaced by Sc₂O₃. Phase separation and/or crystallization occurred after heat treatment. The porous skeleton of leached glasses consisted of hexagonal ScBO₃. The specific surface areas and pore radii are comparable to those of porous SiO₂ glass. The sintering temperature of porous Sc-based material is higher than that of porous SiO₂. Alumina contamination influenced the structure of the porous material.