Secondary Ion Mass Spectroscopy Study of Oxygen-18 Tracer Diffusion in 2/1-Mullite Single Crystals

Oxygen ¹⁸O tracer diffusion in Czochralski-grown mullite single crystals is investigated along [010] and [001]. Oxygen diffusion coefficients range between ∼5 × 10⁻²⁰ m²/s (1250°C) and ∼9 × 10⁻¹⁸ m²/s (1525°C). The data does not show any significant anisotropy. The values of the activation enthalpy (4.5 eV) and of the activation entropy ((3.4 ± 1.6) k _B, where k _B is the Boltzmann constant) suggest that the atomic transport occurs via thermally activated vacancies.     

Piezoelectric Properties of Pb[Zr0.45Ti0.5-xLux(Mn1/3Sb2/3)0.05]O3 Ceramics

The piezoelectric properties of Pb[Zr_0.45Ti_{0.5- x} Lu _x (Mn_1/3-Sb_2/3)_0.05]O₃ ceramics, with 0 lessthan equal to x lessthan equal to 0.03, have been investigated. The partial substitution of Ti⁴⁺ with Lu³⁺ permitted improvement of the electromechanical coupling factor ( k _p), the dielectric constant (epsilon^T₃₃), and the piezoelectric constant ( d ₃₃), while the dielectric loss (tan delta) increased and the mechanical quality factor ( Q _m) decreased with an increase of x . A pertinent piezoelectric material for actuator applications was Pb[Zr_0.45Ti_0.48Lu_0.02(Mn_1/3Sb_2/3)_0.05]O₃, and the piezoelectric properties were k _p = (58.5 ± 0.5)%, epsilon^T₃₃ = 32 ± 25, d ₃₃ = (373 ± 6) 10^-12 C/N, Q _m = 714 ± 22, and tan delta = (0.98 ± 0.03)%.     

Grain-Boundary Diffusion of Strontium in (La,Ca)CrO3 Perovskite-Type Oxide by SIMS

The grain-boundary-diffusion coefficient ( D _gb) of strontium in La_0.9Ca_0.13CrO_3-δ was determined by secondary-ion mass spectrometry (SIMS), using two different measurement modes: depth profiling from the surface and line scanning on the fracture surface. The depth profiles that were sputtered by an O₂⁺-primary-ion beam gave two slopes of strontium concentration profiles, which corresponded to grain (bulk) and grain-boundary diffusion. The depth profiles were fitted to an appropriate equation that gave the grain- (bulk-) and grain-boundary-diffusion coefficients ( D _bulk= 6.5 × 10^-20 m²/s and D _gb= 1.6 × 10^-15 m²/s in air at 1273 K, respectively). Initially, to obtain the D _gb value via the SIMS line-scanning measurement, the fracture surface of La_0.9Ca_0.13CrO_3-delta was measured by a low-energy O₂⁺-primary-ion beam. The line-scanning measurement enabled us to successfully determine the strontium concentration profiles around the grain boundary. However, the D _gb value that was obtained via the line-scanning mode was 6.0 10⁻¹³ m²/s, which was a factor of 100 greater than that which was obtained by the depth-profile mode. Comparison between the depth-profile and line-scanning modes will require additional careful examination.     

Controlled Oxygen Partial Pressure Sintering of (Pb,La)(Zr,Ti)O3 Ceramics

Transparent PLZT(7/60/40) ceramics with large piezoelectric coefficients were obtained using a two-step sintering process with controlled oxygen partial pressure. Specifically, low-oxygen-pressure and low-temperature sintering were used in the first step, followed by a high-oxygen-pressure, high-temperature sintering cycle. High-density ceramics with small grain sizes of about 3 µm were prepared. As a result, k _p= 0.71, k ₃₃= 0.78, d ₃₃= 850 × 10^-12 C/N, and a transparency of 15% (λ= 610 nm, thickness of 1 mm) have been achieved; 20% improvement of d ₃₃ was gained compared to conventional processed PLZT ceramics ( d ₃₃= 710 × 10^-12 C/N).     

Identification of BaO. A12O3.5TiO2 in the Ternary System BaO. A12O3-TiO2

A new identification and indexing for the phase BaAl₂Ti₅O₁₄ were accomplished using an X-ray diffraction technique. The new lattice parameters for the tetragonal lattice structure are: a₀=9.990 × 10^-10 m and c₀=12.264 × 10^-10 m, with a corresponding volume 1.224 × 10^-27 m³. The data provided by the Joint Committee on Powder Diffraction Standards are inconsistent both in lattice parameter values and Miller indices. The X-ray powder diffraction pattern of BaAl₂Ti₅O₁₄ was indexed using the LSUCR (least-squares unit cell refinement) computer program.     

Thermal, Mechanical, and Chemical Properties of Sintered Xenotime-Type RPO4 (R = Y, Er, Yb, or Lu)

Xenotime-type RPO₄ (R = Y, Er, Yb, or Lu) powder was dry-pressed into disks and bars. The disks and bars could be sintered to a relative density of greaterthan equal to98% in air without cracking at 1300° (R = Yb or Lu) or 1500°C (R = Y or Er), depending on the grain size. The linear thermal expansion coefficient (at 1000°C), thermal conductivity (at 20°C), and bending strength (at 20°C) of the xenotime-type RPO₄ ceramics were 6.2 10^-6/°C, 12.02 W(mK)^-1, and 95 ± 29 MPa for R = Y; 6.0 10^-6/°C, 12.01 W(mK)^-1, and 100 ± 21 MPa for R = Er; 6.0 10^-6/°C, 11.71 W(mK)^-1, and 135 ± 34 MPa for R = Yb; and 6.2 10^-6/°C, 11.97 W(mK)^-1, and 155 ± 25 MPa for R = Lu. The xenotime-type RPO₄ ceramics did not react with SiO₂, TiO₂, Al₂O₃, ZrO₂, or ZrSiO₄, even at 1600°C for 3 h in air, and were stable in aqueous solutions of HCl, H₂SO₄, HNO₃, NaOH, and NH₄OH at 20°C.     

Solubilities of Magnesia, Titania, and Magnesium Titanate in Aluminum Oxide

On heat treatment in air the solubility of MgO or TiO₂, in Al₂₃ is too small to detect by lattice parameter shifts. The solubility of MgTiO₃ in Al₂O₃ in air increased to the measured values, expressed as atomic fractions Mg:A1or Ti:A1of0.82 × lo-², 1.43 × 10^-2, and 1.75 × 10^-2 at 1250°, 1650°, and 1850°C, respectively. In 1 atm hydrogen the TiO₂ solubility expressed as the atomic fraction Ti:A1 is 0.55 × lo^-2, 0.75 × 10W², 1.15 × 10^-2, and 1.50 × 10^p2 at 1400°, 1500°, 1600°, and 1700°C, respectively. The increased solubility in H₂ was attributed to reduction of the titanium ion. The solubility of MgO in A1₂O₃ in vacuum (0.3μ) expressed as the atomic fraction Mg:A1 was measured as 1.10 × loW⁴, 3.00 × 10"⁴, 6.80 × 10–⁴, and 1.40 × 10^-3 at 1530°, 1630°, 1730°, and 183O°C, respectively. These contents did not cause an observable change in lattice parameter, but a slight change was observed when MgO was dissolved in A1₂O₃ in a hydrogen atmosphere.     

Creep Behavior of Uranium Carbide-Based Alloys

Compression creep tests were performed on fully dense specimens of UC_1.01, UC_1.05, UC_1.01.+ 4 wt% W, and U_0.9Zr_0.1C_1.01+ 4 wt% W. Steady-state creep rates were measured from 1400° to 1800°C in a vacuum of 1.33 × 10^-3 N/m² (1 × 10^-5 torr) at stresses of 4.55 to 69.0 MN/m² (660 to 10,000 psi). The data for UC_1.01 could best be fit by an expression of the form ɛ= 1773σ^6.024 exp (106.5/RT) , where σ is the steady-state creep rate (h^-l), σ is the applied stress (MN/m²), and the creep activation energy is given in kcal/mol. The stress dependence for creep of UC_1.05 decreased with decreasing temperature because of second-phase precipitation; therefore, a unique creep activation energy could not be established for this U/C ratio. At all temperatures, the creep strength of UC_1.05 exceeded that of UC_1.01. For example, at 1700 ° C steady-state creep rates for UC_1.05 are ∼1/4 those for UC_1.01, but at 1400°C the creep rates are ∼ 3 orders of magnitude less. At 1700°C, creep rates for UC alloys are ∼4 orders of magnitude lower than those for unalloyed UC_1.01.     

Oxidation Behavior of SiCN–ZrO2 Fiber Prepared from Alkoxide-Modified Silazane

The oxidation behavior of SiCN–ZrO₂ fibers and SiCN at 1350°C are compared. The as-measured parabolic rate constants for the two materials are nearly the same (15–20 × 10⁻¹⁸ m²/s). However, after implementing a correction for the difference in the compositions, the rate constant is 13.2 × 10⁻¹⁸ m²/s for the fiber, and 29.4 × 10⁻¹⁸ m²/s for SiCN. The lower oxidation rate of the fiber is ascribed to the lower carbon content in the fiber material.     

Ion-Probe Measurement of Oxygen Self-Diffusion in Single-Crystal Al2O3

Anion self-diffusion coefficients normal to (1102) were obtained for single-crystal Al₂O₃ in a 1.3 × 10 ³ N/m² (10⁻⁵ torr) vacuum at 1585° to 1840°C. Tracer was supplied from an initial 650 to 1300 A Al₂¹⁸O₃ layer produced by the oxidation of vapor-deposited Al metal films in an ¹⁸O₂ atmosphere at 520°C. Concentration gradients extended over depths of 3000 to 5000 A and were measured by mass spectrometry of material sputtered from the samples with a beam of Ar⁺ ions. Crystals which had not been preannealed to remove surface damage displayed enhanced diffusion. Diffusion coefficients from preannealed crystals may be described by D₀ =6.4×10⁵cm²/s, with an activation energy of 188 ± 7 kcal/mol. The diffusion is interpreted as an extrinsic vacancy mechanism.     

Grain Boundary Grooving Studies of Yttrium Aluminum Garnet (YAG) Bicrystals

Grain boundary grooving experiments were conducted with Σ5 (210) twist boundaries in Y₃Al₅O₁₂ (YAG) with the goal of extracting information on diffusion in YAG. Planar boundaries oriented 90° to the surface were annealed in air at various times and temperatures. Atomic force microscopy was used to characterize the subsequent grooves. The Mullins approach leads to the following expression for the diffusion coefficient: D (m²/s) = 3.9 × 10⁻¹⁰ exp[−330 ± 75 (kJ/mol)/ RT ]. The relatively low activation energy agrees well with earlier oxygen tracer diffusion measurements on YAG, suggesting that oxygen is the limiting diffusing species in boundary grooving of YAG.     

Vapor Pressure, Enthalpy, Evaporation Coefficient, and Enthalpy of Activation of the Beryllium Nitride (Be3N2) Decomposition Reaction

Beryllium nitride (Be₃N₂) vaporizes congruently in the range 1640° to 1960°K by the reaction Be, N₂( c ) = 3Be( g ) + N₂( g ). The equilibrium nitrogen partial pressure, in atmospheres, at the composition for congruent sublimation is given by the expression log P _N2= [(–1.952 ± 0.038) × 10⁴] T ⁻¹+ (6.509 ± 0.207). The measured enthalpy of decomposition (370 ± 5 kcal at 298° K) yields an enthalpy of formation for Be₃N₂( c ) of –136 ± 6 kcal/mole. The upper limit to the evaporation coefficient at 1600° to 2000°K can be set as 10^–4 by comparison of equilibrium data to Langmuir data obtained with a sample of 18% porosity. The apparent enthalpy of activation for the reaction is 409 ± 7 kcal/mole at 1800°K for the porous Langmuir specimen. An expression is developed to predict the temperature dependence of the reduced apparent pressures in Knudsen studies of substances with low evaporation coefficients in terms of the enthalpy of activation. The variation in temperature dependence of the Langmuir measurements and Knudsen measurements with three different-sized orifices is consistent with predictions from this expression.     

Fabrication and Mechanical Properties of Titanium Boride Ceramics

Dense TiB ceramics (99.6% of theoretical) with a grain size of ∼5 µm have been fabricated by reaction hot pressing of TiB₂ and titanium for 2 h at 1900°C and 28.5 MPa. The TiB ceramics exhibit a fracture toughness ( K_IC ) of 4.5 MPa·m^1/2 and a bending strength (sigma_b) of 360 MPa. Electrical resistivity (rho) is 3.4 × 10^-7 Omega·m at room temperature.     

Interdiffusion and Association Phenomena in the System NiO-Al2O3

The rate of formation of NiAl₂O₄ by reaction between single crystals of NiO and Al₂O₃ can be described by k = 1.1 × 10⁴ exp (−108,000 ± 5,000/ RT ) cm²/s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity-vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al³⁺ ions diffuse as [Al_Ni· V_Ni]'pairs. The diffusion coefficient of pairs, D_p , obeys the equation 6.6 × 10⁻² exp (−54,000 ± 3,000/ RT ) cm²/s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al³⁺ dissolved in the spinel.     

Crystallization and Properties of Fe2O3—CaO—SiO2 Glasses

Nucleation and crystal growth rates and properties were studied in a two-stage heat treatment process for Fe₂O₃-CaO-SiO₂ glasses. Glass transition (T_g) and crystallization temperatures (T _c ) for the glasses lay between about 612.0° and 710.0°C, and 858.5° and 905.0°C, respectively, and magnetite was the main crystal phase. For a glass of 40Fe₂O₃. 20CaO·40SiO₂ (in wt%) the maximum nucleation rate was (68.6 ± 7) × 10⁶/mm³·s at 700°C, and the maximum crystal growth rate was 9.0 nm/min^1/2 at 1000°C. The mean crystal size of the magnetite increased from 30 to 140 nm with variation of nucleation and crystal growth conditions. The glass showed the maxima in saturation magnetization and coercive force, 212.1 × Wb/m² and 30.8 × 10³ A/m, when heat-treated for 4 h at 1000°C and 1050°C, respectively. The variation of the saturation magnetization could be quantitatively interpreted well in terms of the volume fraction of the magnetite, whereas that of the coercive forces could be explained only qualitatively in terms of the particle size of the magnetite. Hysteresis losses showed the maximum value of 1493 W/m³ when heat-treated at 1000°C for 4 h prenucleated at 700°C for 60 min, and increased linearly with increasing heat treatment time under a magnetic field up to 800 × 10³ A/m.     

Electrochemical Liquid Deposition of Ceria

Electrochemical liquid deposition (ELD) has been used to grow fully dense films of ceria on impervious gadolinia-doped ceria substrates of variable thickness. The process is similar to electrochemical vapor deposition (EVD) except that a liquid is used instead of a vapor to provide precursor material to the growing film. ELD film growth data have been analyzed in a manner similar to that for EVD demonstrated previously. Experimental results verify that the pertinent rate equation is obeyed phenomenologically. Rate constants for the film (CeO₂) and the substrate (l-l-mol%-Gd₂O₃-doped CeO₂) at 1200°C have been determined as K_f = 33.1 × 10⁻¹² m²/s and K^s = 51.6 × 10⁻¹² m²/s, respectively.     

Oxygen Self-Diffusion in Single-Crystal Mn-Zn Ferrite

Self-diffusion coefficients for the oxygen ion in single-crystal Mn-Zn ferrite were determined by the gas-solid isotope exchange technique. The oxygen volume diffusion coefficients can be expressed as D =6.70 × 10⁻⁴ exp (-330 (kJ /mol) /RT)m₂/s (>1350°C), D=3.94 × 10⁻¹⁰ exp (−137 (kJ/mol)/RT)m²/s (1100° to 1350°C), and D=7.82 × 10⁴ exp (−507 (kJ/mol)/RT)m²/s (<1100°C).     

Influence of Distributed Particle Size on the Determination of the Parabolic Rate Constant for Oxidation by the Powder Method

The established analysis for the study of oxidation using powder specimens is based on the assumption of monosized particles. The experiments, however, are conducted on powders with a distributed particle size. Here we present a statistical approach for the calculation of the rate constant for oxidation. The results of the analysis are applied to new data on oxidation studies of dense powders of silicon carbonitride amorphous ceramics. The monosized model requires a wide range of values for the rate constant to fit the short term and the long-term data, leading to considerable ambiguity in the estimate of the parabolic rate constant, k _p, for oxidation. In contrast the statistical model fits over the entire range of data, yielding a much more reliable value for k _p. For example, the monosized approach gave a value in the range 19.7 × 10⁻¹⁸ < k _p < 2.7 × 10⁻¹⁸ m²/s. In contrast, the statistical model yields a specific value of 4.5 × 10⁻¹⁸ m²/s.