Wetting of Ceramic Oxides by Molten Metals Under Ultrahigh Vacuum

Contact angles, surface free energies, and work of adhesion were determined by a sessile drop technique for the wetting of MgO, Al₂O₃, and SiO₂ by In, Ga, and Sn at 10^-10 torr. The surface free energies of In, Ga, and Sn were 540, 632, and 573 ergs/cm² (±5%), respectively, at their melting points. Works of adhesion and equilibrium contact angles for wetting of MgO by In are 172 ergs/cm² and 133° by Ga, 356 ergs/cm² and 116° by Sn, 278 ergs/cm² and 121°. For wetting of Al₂O₃ by In, they are 237 ergs/cm² and 124° by Ga, 226 ergs/cm² and 130° by Sn, 257 ergs/cm² and 123°. For wetting of SiO₂ by In, they are 208 ergs/cm² and 128° by Ga, 260 ergs/cm² and 126° by Sn, 252 ergs/cm² and 124°.     

Critical Surface Energies of Al2O3 and Graphite

Based on the wetting properties of a -Al₂O₃, and pyrolytic graphite by liquid metals, the critical surface energy for spreading was determined and compared with the surface energy of the ceramics. The surface energy γ-₈ (ergs/cm²) of the (0001) surface of a -Al₂O₃ is estimated to be 892–0.12 T (°C), that of the c plane of pyrolytic graphite 1139–0.13 T (°C), and that of the a plane of pyrolytic graphite 1300–0.17 T (°C). Also, the liquid-solid interfacial energies were calculated and found compatible with the values reported in the literature. The surface energy of liquid Ag or liquid Cu is not significantly affected by carbon.     

Surface Energy of Uranium Dioxide

Using the method of interfacial angle equilibrium in the system UO₂-Ni, a surface energy of 754±150 ergs/cm² was determined for UO_2.01 at 1500°C in high-purity Ar. The absolute interfacial energies in the system UO₂-Ni at 1500°C were also determined.     

Wetting of AlN and TiC by Liquid Ag and Liquid Cu

The wetting of AlN and TiC by liquid Ag and liquid Cu was investigated by the sessile drop technique at 10⁻⁶ torr or less. An empirical relation was established between the cosine of the contact angle and the temperature or surface tension of the liquid drop. The critical surface tension for spreading and its physical significance are discussed. A method for estimating the surface energies of ceramics is proposed. The surface tension of TiC is estimated to be 1242±158 dynes/cm and that of AlN 990±110 dynes/cm. The surface tension of liquid Ag followed the equation γ _Lv (Ag) (dynes/cm) = 1092–0.14T (°C) and that of liquid Cu γ _LV (Cu) (dynes/cm) = 1462–0.27T (°C).     

Reaction and Diffusion in Silver-Arsenic Chalcogenide Glass Systems

The diffusion of Ag from the metal or Ag₂Se in amorphous As₂S₃ and As₂Se₃ at 175°C is accompanied by the reduction of As from a valence of 3+ to 2+ or 2+ to 1 + to maintain charge neutrality in the glass. Only Ag⁺ diffuses at this temperature; all other ions are essentially immobile. An amorphous reaction-product phase is formed in the diffusion zone with a composition range of 28.6 to 44.4 at % Ag. The lower limit corresponds to all As cations of 2+ valence (equivalent to amorphous Ag₂As₂S₃); the upper limit, the maximum solubility of Ag in these glasses, corresponds to all As cations of 1 + valence (equivalent to amorphous Ag₁As₂S₃). The diffusivity of Ag in these glasses at 175°C for concentrations of 10 to 35 at.% Ag is
Sulfide 4× 10⁻¹⁴ exp[(+0.23±0.01)(at.% Ag)]cm²/s
Selenide 2' 10⁻¹¹ exp[(+0.14±0.01)(at.% Ag)]cm²/s     

Durability and Properties in Alkali Aluminofluorophosphate Glasses

Glass-forming and properties of glasses in the systems AI-Li-Na-K-P-O-F are presented. Useful durability, which is defined as a weight less in deionized water at 95°C of <1 mg/cm².h, limits the range of properties available. These boundaries are represented by T_gs of 305°±25°C, density of 2.63±0.02 g/cm³, and refractive indices of 1.46±0.01.     

Calculation and Interpretation of Surface Free Energy of Wetting of E-Glass by Vapors

The surface free energy of wetting of E-glass by water and benzene vapors was calculated from Bangham and Razouk's free-energy equation. The method of Fu and Bartell for determining specific surface area was used to analyze the microstructure. The surface free energies, which were computed from adsorption isotherms obtained at temperatures from 18° to 20°C, had values from -235 to -254 ergs/cm² and -71.1 to -72.4 ergs/cm² for water and benzene adsorption, respectively, depending on temperature, surface geometry, and sample treatment. The lower affinity of E-glass for benzene, which resulted in a smaller work of adhesion, suggests that active adhesion of hydrocarbons is impossible in the presence of bulk water. Microstructural analysis indicated that the water-modified E-glass surface contains micropores 23 Å in diameter.     

Annealing of Irradiation-Induced Thermal Conductivity Changes in ThO2-1.3 wt% UO2

The thermal conductivities of sintered pellets of ThO₂-1.3 wt% U0₂ were measured at 60°C before and after irradiation. The irradiation temperature was below 156°C, and the exposures varied from 3.1 × 10¹⁴ to 4.7 × lo^L7 fissions/cm³. Each fission fragment damaged a region of 2.2 × 10^-16 cm³ with the reduction in conductivity saturating by about 10¹⁷ fissions/cm³. Samples having exposures from 10¹⁵ to 10¹⁶ fissions/cm³ were annealed isothermally at 651 °C or isochronally from 300° to 1200° C to study the annealing of damage. Most of the annealing occurred between 500° and 900°C. The width of this interval plus the slow isothermal annealing suggest that the damage is annealed by a number of single order processes with a spectrum of activation energies from 1.8 to 3.9 eV or, less probably, by a high order process with an activation energy of 3.55 ± 0.4 eV.     

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.     

Vacuum Vaporization in the System MgO-Cr2O3

The vaporization of the system MgO-Cr₂O₃ was studied in a vacuum of 10⁻⁵ torr (10⁻³N/m²) at 1500° to 1700°C using the Langmuir and Knudsen methods. It was found that the phases in the system vaporize nearly congruently and the logarithm of the vaporization coefficient, α, of MgCr₂O₄ increases linearly with increasing reciprocal temperature. Alpha tends to unity at a temperature near the melting point (2525±23°C). The additivity rule can be applied to the Langmuir vaporization rates on the basis of the surface area ratios of the phases in the 2-phase system MgO-Cr₂O₃. The enthalpies of vaporization of MgCr₂O₄ were 695.0 and 549.2 kcaVmol for activated and equilibrium processes, respectively.     

Sodium Diffusion in SiO2 Glass

Diffusion of the radioactive tracer ²²Na in a commercial SiO₂ glass was investigated from 170° to 1000°C. The temperature dependence curve had discontinuities at about 573° and 250°C. The resulting Arrhenius equations are D = 3.44 × 10² exp(-21.1 kcal/RT) cm²/sec between 1000° and 573°C, D = 0.398 exp(-25.8 kcal/RT) cm²/sec between 573° and 250°C, and D = 2.13 exp(-28.3 kcal/RT) cm²/sec between 250° and 170°C. The two anomalies are discussed in terms of "quartz-like" and "cristobalite-like" precrystalline elements in the structure of the glass. Comparison of the Na diffusion in SiO, glass with that in soda-silica and soda-lime-silica glasses shows that SiO₂ glass occupies a boundary position with respect to these systems. A possible diffusion mechanism is discussed.     

Creep of Polycrystalline MgO-FeO-Fe2O3 Solid Solutions

Steady-state creep experiments were performed on hot-pressed polycrystalline MgO doped with Fe. Dead-load 4-point bend creep tests were conducted at stresses of 26 to 270 kg/cm², at temperatures of 1250° to 1450°C, in O₂ partial pressures of 1 to 10⁻⁹ atm, on specimens with grain sizes of 10 to 65 μm. Viscous steady-state creep was always observed when the grain size was stable. Experiments at variable P _O2's and temperatures were used to identify regimes of high (117 ± 10 kcal/mol) and low (81 ± 5 kcal/mol) activation energy. In the latter, creep rates were nearly independent of Fe dopant concentration and P _O2, whereas in the former creep rates were enhanced by increasing P _O2's and Fe dopant levels. The high- and low-activation-energy regimes were interpreted as diffusional creep controlled primarily by Mg lattice diffusion and O grain-boundary diffusion, respectively.     

Kinetics of Precipitation and Measurements of Strain in the System MgO-Al2O3-Cr2O3

A quantitative X-ray technique for measuring precipitation strains has not been previously applied in metallic or oxide systems. The Warren-Averbach analysis of strain was used to determine the buildup of elastic strain energy in the spinel crystalline solution matrix (gross composition = 60 mol% MgAl₂O₂+ 40 mol% Cr₂O₃) during the isothermal (1135°C) precipitation of a metastable (coherent) monoclinic phase. The elastic strain energy of the spinel crystalline solution matrix increased to a maximum of about 3.1 × 10⁷ ergs/cm³ for a reaction time of 8 h. There was a marked decrease in the elastic strain energy during the initial precipitation of the equilibrium corundum crystalline solution with the composition (Al³⁺_0.72 Cr³⁺_0.25)O₃. An overall diffusion activation energy for precipitation of the mono-clinic phase was approximately 86 kcal/mol.     

Stress Relaxation Modulus of a Commercial Glass

The stress relaxation modulus in compression of a container glass was investigated over a wide range of strain, time, modulus, and temperature. The glass is a linear viscoelastic liquid up to 2% strain, and the modulus is a smooth function of time, with no pseudorubbery plateau apparent down to a modulus of 10° dynes/cm². The data cover 4 decades in time and a range of almost 100°C above the glass transition, T ₀ =536°C. Within experimental error, changes in temperature simply shift the modulus-vs-time curve along the time axis without altering its shape. This behavior implies that the same mechanism controls both the bulk and shear spectra. The shift factors fit the WLF equation well with values for the parameters c₁ and c₂ of 16.7±0.2 and 345±5°C, respectively. Data from the literature for silicate glasses agree with these parameters.     

Grain-Boundary Diffusion of Cr in Pure and Y-Doped Alumina

The diffusive transport of chromium in both pure and Y-doped fine-grained alumina has been investigated over the temperature range 1250°–1650°C. From a quantitative assessment of the chromium diffusion profile in alumina, as obtained from electron microprobe analysis, it was found that yttrium doping retards cation diffusion in the grain-boundary regime by over an order of magnitude. The Arrhenius equations for the undoped and Y-doped samples were determined to be: δ D _b=(4.77±0.24) × 10⁻⁷ exp (−264.78±47.68 (kJ/mol)/RT)(cm³/s) and δD_b=(6.87±0.18) × 10⁻⁸ exp (−284.91±42.57 (kJ/mol)/RT)(cm³/s), respectively. Finally, to elucidate the mechanism for this retardation, the impact of yttrium doping on diffusion activation energies and prefactors was examined.     

Wetting of Ceramics by Liquid Aluminum

The wettability of AlN, TiB₂, TiN, and TiC by liquid aluminum was investigated by the sessile drop technique at 2×10^-7 torr or less. The contact angle decreased linearly with increasing temperature in all cases. Variation of the contact angle with temperature is correlated with the surface tension of the drop, and the Zisman-type relation is established. The critical surface tensions (Rγ_c) of AlN, TiB₂, TiN, and TiC are 664, 700, 713, and 725 dynes/cm, respectively. These values are correlated with the melting point of the ceramics. The Zisman relation is discussed and applied to calculation of the work of adhesion. The surface tension of liquid aluminum varies linearly with temperature: γ_Lv (dynes/cm) = 948–0.202 T (°K).     

2223 Phase Formation in Bi(Pb)─Sr─C─a─Cu─O: II, The Role of Temperature—Reaction Mechanism

The formation mechanism, the temperature range for the growth, and the thermal stability of the 2223 phase in Bi(Pb)─Sr─Ca─Cu─O have been investigated using DTA/TG, XRD, SEM/EDAX, TEM, EPMA, four-probe resistance and acsusceptibility measurement. The formation of the 2223 phase was found to follow a dissolution–precipitation process. A 2212 phase first reacts with the liquid phase formed via an incongruent melting of the Ca₂PbO₄ phase, and a dissolution of CaO and CuO takes place. The 2201 phase, which has the largest negative free energy, is then precipitated from the melt; the nucleation and growth of the 2223 phase are subsequently developed by the reaction between the 2201 phase precipitates and ions of Ca²⁺ and Cu²⁺ present in the liquid phase. The 2223 phase is formed at temperatures in the range 827°C T < 856°C. The optimum temperature T _f for the formation of 2223 phase is 845°± 5°C. The 2223 phase is thermodynamically unstable at temperatures above 856°C.     

Ammonium Iodate—A Potentially Useful Nonlinear Optical Material

The space group of NH₄IO₃ was determined, from precession photographs and observations of symmetry-related characteristics, to be Pna 2₁. The cell constants are a ₀= 6.3740 ± 0.0005, b ₀= 6.4115±0.0005, and c ₀=9.1706±0.0005 Å. The crystal is piezoelectric along all three axes and pyroelectric with dP/dT ⋍(3±1)×10⁻⁹ C/cm²°C along the c ₀ axis at room temperature. At 85°C a first-order transition to a nonpolar piezoelectric phase is observed.     

Diffusion of Sputtered Vanadium, Nickel, and Silver in Lead Magnesium Niobate Ceramic

The sputtering of nickel-vanadium (Ni-V) and silver (Ag) to form thin films on lead magnesium niobate (PMN) ceramic substrates has been studied. An empirical equation was derived which correlated the average Ni-V film thickness with three sputtering variables: input power, argon pressure, and sputtering time. The films were dense, smooth, and feature less. Upon annealing at 800°C in argon, both Ni and V diffused into PMN; but reverse diffusion from the ceramic into the metal film was not observed. Vanadium diffused at a faster rate than Ni, leaving pores in the film. The diffusion coefficient of Ni in PMN was determined to be 4.1 × 10⁻¹² cm²/s, whereas the diffusion coefficient of V was determined to be between 3.5 × 10⁻⁹ and 2.9 × 10⁻¹¹ cm²/s at 800°C. Sputtered Ag was also dense, smooth, and featureless. However, the diffusion of Ag at 800°C in PMN was much slower than that of either Ni or V.     

Groove Angles and Surface Mass Transport in Polycrystalline Alumina

Grain-boundary grooving was studied on polished surfaces of polycrystalline alumina, after annealing, in air, under vacuum, and in argon atmospheres in the temperature range 1273 to 1736 K. The groove angles, measured by optical interferometry, showed no significant change with experimental conditions, giving a mean value of 138.57°± 0.08° and a ratio of the grain-boundary energy to surface energy of 0.707 ± 0.001. From the experiments it was determined that surface diffusion was the dominant mechanism for the mass transport. The surface diffusion coefficient, D s(cm²/s) = 0.48 exp(−256(kJ/mol)/ RT ), was practically independent of the furnace atmosphere, and, in the low-temperature region examined, depended on the structural damage in the near-surface region which occurred during mechanical polishing.