Wetting in an Electronic Packaging Ceramic System: II, Wetting of Alumina by a Silicate Glass Melt under Controlled pO2 Conditions

The wetting of polycrystalline alumina by a colored, calciamagnesia aluminosilicae glass was found to be dependent on temperatutre between 1300° and 1500°C, but independent of gas atmosphere effects. Neither the oxygen partial pressure, oveer the range of 10^-6 to 10^-10 Pa, the gas buffer system (Co/CO₂ or H₂/H₂O), nor pre-equilibration of the substrate surface with the atmosphere at tge exoperimental temperature before solid-liuid interface formation affected the stable contact angle. An initial drop in contact angle the stable contact angle. An initial drop in contact angle occurring within the first hour is attributed to repaid dissolution of alumina and the formation of a stable glass/alumina interface. The contact angle after an 8-h isothermal hold decreased from 1300° to 1500°C. The solid-liquid interfacial energy, μM_S1, controls the wetting behavior. Changes in μM_s1 are attributed to he breakup of the silica network as temperature increases.     

Effect of Oxygen on the Reaction Between Copper and Sapphire

The effect of oxygen potential on the wetting behavior and interfacial energy between Cu and sapphire was studied using the sessile drop technique in a CO-CO₂ atmosphere. A linear relation was found between γ_SL and log p O₂ (atm) from 10⁻¹⁶ to 10⁻⁵. Beyond 10⁻⁵ atm γ_SL approached a constant value asymptotically. A barrier surface layer was proposed to explain this change. The Gibbs adsorption equation was used to evaluate the characteristics of the interfaces. Formation of a Cu₂O film at the liquid-vapor interface and a CuAlO₂ film at the solid-liquid interface is suggested. The work of adhesion reached a maximum at ∼ 0.01 at.% oxygen, corresponding to p O₂∼ 10⁻⁹atm. Measurements of the basal radius as a function of oxygen content were used to evaluate the role of oxygen in promoting spreading. Spreading on sapphire is directly proportional to the logarithm of oxygen present in the molten Cu drops.     

Influence of Oxygen Partial Pressure and Oxygen Content on the Wettability in the Copper–Oxygen–Alumina System

The influence of oxygen on the wetting behavior of copper on single-crystal Al₂O₃ has been studied. By controlling the oxygen partial pressure ( p _O2) and oxygen content in the copper simultaneously, contact angle can be varied between 125° and 22°. Evaluation of the Gibbs adsorption equation for the liquid/solid interface at 1300°C suggests that adsorption of a Cu-O complex at that interface plays a key role in promoting wetting. Formation of CuAlO₂ and dissolution of Al₂O₃ in the melt also influence the contact angle, especially in the range of p _O2 > 10⁻⁵ bar (1 Pa).     

Effect of Carbon and Water on Wetting and Reactions of B2O3-Containing Glasses on Platinum

Sessile-drop experiments were conducted to analyze the wetting characteristics of molten B₂O₃ and B₂O₃-containing glasses on Pt. In atmospheres free of carbonaceous gases, the contact angle for all glasses on Pt was small and invariant under ambient pressures of 10^-4 torr to 1 atm for the range 700° to 1000°C. Adsorption of carbon at the metal surface decreased the solid/vapor surface energy and increased the contact angle. Carbon was removed by oxidation at a specific ambient pressure, resulting in a reduction in the contact angle. Temperature-pressure data are reported for Pt. Absorption of water vapor by the glasses resulted in a reaction at the solid/liquid interface, spreading of the glass, and adherence. The possible reactions that can occur are given.     

Thermal Expansion Behavior of Titanium-Doped La(Sr)CrO3 Solid Oxide Fuel Cell Interconnects

La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10⁻⁶/°C in air, 10.5 × 10⁻⁶/°C under a He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 10.9 × 10⁻⁶/°C in a H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁹ atm at 1000°C). La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect.     

Relationship between the Mesoporous Intermediate Layer Structure and the Gas Permeation Property of an Amorphous Silica Membrane Synthesized by Counter Diffusion Chemical Vapor Deposition

An amorphous silica membrane with an excellent hydrogen/nitrogen (H₂/N₂) permselectivity of >10 000 and a He/H₂ permselectivity of 11 was successfully synthesized on a γ-alumina (γ-Al₂O₃)-coated α-alumina (α-Al₂O₃) porous support by counter diffusion chemical vapor deposition using tetramethylorthosilicate and oxygen at 873 K. An amorphous silica membrane possessed a high H₂ permeance of >1.0 × 10⁻⁷ mol·(m²·s·Pa)⁻¹ at ≥773 K. The dominant permeation mechanism for He and H₂ at 373–873 K was activated diffusion. On the other hand, that for CO₂, Ar, and N₂ at 373–673 K was a viscous flow. At ≥673 K, that for CO₂, Ar, and N₂ was activated diffusion. H₂ permselectivity was markedly affected by the permeation temperature, thickness, and pore size of a γ-Al₂O₃ mesoporous intermediate layer.     

Influence of Oxygen Activity on the Sintering of MgCr2O4

The sintering behavior of MgCr₂O₄ powder compacts was investigated as a function of temperature, time, and oxygen activity. The results show that MgCr₂O₄ cannot be densified to >70% of theoretical density at temperatures up to 1700°C if the oxygen activity exceeds 10⁻⁶ atm. The oxygen activity must be decreased to <10⁻¹⁰ atm before densities exceeding 90% of theoretical can be achieved. Weight loss and X-ray data indicated that maximum density occurred at an oxygen activity just above that where MgCr₂O₄ becomes unstable.     

Control of the Thermal Expansion of Strontium-Doped Lanthanum Chromite Perovskites by B-site Doping for High-Temperature Solid Oxide Fuel Cell Separators

The thermal expansion of La_0.9Sr_0.1Cr_{1- x} M _x O₃(M = Mg, Al, Ti, Mn, Fe, Co, Ni; 0 ≤ x ≤ 0.1) perovskites has been studied in oxidizing and reducing atmospheres in the temperature range from 50° to 1000°C. Cobalt doping of La_0.9Sr_0.1CrO₃was an effective way of increasing the average linear thermal expansion coefficient (TEC), whereas titanium doping showed a negative effect. No effect on the TECs was observed for the B-site dopants in perovskites with the remaining dopants. Linear thermal expansion behavior was observed in the La_0.9Sr_0.1Cr_{1- x} M _x O₃ perovskites with doping of ≥1 mol% aluminum or 10 mol% cobalt. TECs of La_0.9Sr_0.1Cr_0.96Co_0.02Al_0.02O₃ were 10.5 × 10⁻⁶/°C in air, 10.7 × 10⁻⁶/°C under He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 11.8 × 10⁻⁶/°C in H₂ atmosphere.     

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.     

Interface Reactions Between Metals and Ceramics: IV, Wetting of Sapphire by Liquid Copper-Oxygen Alloys

The wettability of sapphire single crystals by liquid copper which contained oxygen added as cupric oxide was investigated using the sessile drop technique in vacuum at 1230°C. Additions of cupric oxide to copper, varying from 1 to 72% of copper weight, resulted in rapid chemical reaction at the solid-liquid interface with a significant reduction of the contact angle, the final value being dependent on the oxygen in the system. In all cases the interfacial product was CuAlO₂. A linear relation between the fourth power of the basal radius of the molten drop and the amount of oxygen present was observed. The initial stage of the reaction could be explained by the formation of a Cu₂O layer at the interface, followed by reaction between Cu₂O and Al₂O₃ to form CuAlO₂.     

Effects of Titanium Oxide on Equilibria Among Refractory Phases in the System CaO-MgO-Iron Oxide

The role of titanium oxide in some important refractory systems was elucidated by studying selected equilibria in the system CaO-MgO-iron oxide-titanium oxide at O₂ pressures of 0.21 atm (air) and 10⁻⁹ atm and under the extreme reducing conditions imposed by the presence of metallic Ti in contact with the oxide phases. Solidus relations were determined for the system CaO-MgO-TiO₂ in air; 6 composition triangles were delineated, within each of which 3 crystalline phases coexist in equilibrium with liquid at a constant solidus temperature. The solidus temperatures range from 1407° to 1670°C. There is also a composition area within which MgO coexists with a Ca₄Ti₃O₁₀-Ca₃Ti₂O₇ solid solution, with solidus temperatures varying continuously from 1659° to 1670°C. Studies of reactions between MgO and titanium oxide in contact with metallic Ti in a closed system indicate that the mutual solubility between MgO and TiO at 1400°C is very small. Addition of 5 wt% TiO₂ to the system CaO-MgO-iron oxide at 1500°C in air and in 10⁻⁹ atm O₂ decreases the amount of iron oxide which can be absorbed by a CaO-MgO body without formation of a liquid phase; hence, titanium oxide has a strong deleterious effect on the refractoriness of such bodies.     

Surface Orientation and Wetting Phenomena in Si/α-Alumina System at 1723 K

Wetting phenomena and the effect of alumina surface orientation on the wettability in Si/α-Al₂O₃ system were studied by an improved sessile drop method using     ,     , C(0001) faces of single crystals and polycrystals at 1723 K in a reducing Ar–3% H₂ atmosphere. The contact angles show a vibration behavior for all the single crystals but to a less extent for the polycrystals. The extent of the vibration correlates not only with the reaction intensity but also with the stability of the Si droplet on the alumina surfaces. The interfacial reaction leads to the formation of a series of reaction rings, which is more serious at the single crystal surfaces. More importantly, the wettability is dependent on the alumina surface orientation, with the intrinsic contact angles being about 98±2°, 101±1°, 69±1°, and 98±2°, respectively, for the     ,     , C(0001) and polycrystal α-Al₂O₃ substrates. The much smaller contact angle for molten Si on the C(0001) surface is explained by the favorable reduction in the Si/α-Al₂O₃ interfacial free energy by the terminated and enriched aluminum atoms at the reconstructed     surface. The importance of the aluminum presence at the Si/α-Al₂O₃ interface to the wettability of this system was further demonstrated by a substantial improvement in the wettability of the     α-Al₂O₃ substrates by Si–Al alloys.     

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).     

Ractions of SiC with H2/H2O/Ar Mixtures at 1300°C

The reactions of a sintered α-SiC with 5% H₂/H₂O/Ar at 1300°C were studied. Thermomchemical modeling indicates that three reaction regions are expected, depending on the initial water vapor or equivalently oxygen content of the gas stream. A high oxygen content ( P (O₂) > 10⁻²² atm) leads to a SiO₂ formation. This generally forms as a protective film and limits consumption of the SiC (passive oxidation). An intermediate oxygen content (10⁻²² atm > P (O₂) > 10⁻²⁶ atm) leads to SiO and CO formation. These gaseous products can lead to rapid consumption of the SiC (active oxidation). Thermogravimetric studies in this intermediate region gave reaction rates which appear to be controlled by H₂O gas-phase transport to the sample and reacted microstructures showed extensive grain-boundary attack in this region. Finally, a very low oxygen content ( P (O₂) < 10⁻²⁶ atm) is thermochemically predicted to lead to selective removal of carbon and formation of free silicon. Experimentally low weight losses and iron silicides are observed in this region. The iron silicides are attributed to reaction of free silicon and iron impurities in the system.     

Effect of Hydrogen-Water Atmospheres on Corrosion and Flexural Strength of Sintered α-Silicon Carbide

Sintered α-SiC was exposed for 10 h to H₂ containing various partial pressures of H₂O ( P _H2O from 5×10⁻⁶ to 2×10⁻² atm; 1 atm≅10⁵ Pa) at 1300° and 1400°C. Weight loss, surface morphology, and room-temperature flexural strength were strongly dependent on P _H2O. The strength of the SiC was not significantly affected by exposure to dry H₂ at a P _H2O of 5×10⁻⁶ atm; and following exposure at P _H2O >5×10⁻³ atm, the strength was even higher than that of the as-received material. The increase in strength is thought to be the result of crack blunting associated with SiO₂ formation at crack tips. However, after exposure in an intermediate range of water vapor pressures (1×10⁻⁵< P _H2O <1×10⁻³ atm), significant decreases in strength were observed. At a P _H2O of about 1×10⁻⁴ atm, the flexural strength decreased approximately 30% and 50% after exposure at 1300° and 1400°C, respectively. The decrease in strength is attributed to surface defects caused by corrosion in the form of grain-boundary attack and the formation of pits. The rates of weight loss and microstructural changes on the exposed surfaces correlated well with the observed strength changes.     

Phase Diagram of a Nickel-Zinc Ferrite of Composition Ni0.685Zn0.177Fe2.138O4+γ

The oxygen content of Ni_0.685Zn_0.177Fe_2.138O_4+γ was determined gravimetrically at atmospheric pressure in varying Po₂ , 3.5 × 10⁻⁴ to 1.0 atm at 600° to 1450°C. The phase boundary associated with the precipitation of α-Fe₂O₃ was determined from the change in slope of γ vs T plots observed on heating. Metastability is particularly evident for curves observed on cooling. Isacompositional lines (0.002 < γ < 0.045) are shown on a plot of log PO₂ vs 1/T. An enthalpy of -21.6 kcal/mol is calculated for the oxidation of Fe²⁺.     

Point Defects in Optical Ceramics: High-Temperature Absorption Processes in Lanthana-Strengthened Yttria

The optical transmission of transparent polycrystaliine La₂O₃ strengthened Y₂O₃ was measured in both the near-ultraviolet and infrared regions at temperatures between 20° and 1400°C. The absorption remains low until about 900°C, but rises almost exponentially thereafter. The magnitude of this increase is a function of the oxygen partial pressure (Po₂) in the ambient atmosphere. The temperature-dependent absorption is lowest when the Po₂ is between 10⁻¹¹ and 10⁻⁸ atm (1 atm = 10⁵ Pa), representing the range in which the concentration of free carriers (conduction band electrons and valence band holes), generated by stoichiometty-related point defects (oxygen interstitiais and vacancies), is minimized. The temperature dependence is significantly greater in the uitraviolet than in the infrared, but the optimal Po ₂ range is the same. The absorption behavior can be described in terms of a simple phenomenological model involving carriers thermally liberated from defect states within the band gap of the material. Various aspects of the model and their experimental implications are discussed.     

Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.     

Thermogravimetric Study of the M-Co-O System: I, M = Ta and Nb at 1200°C

Phase equilibria in the Ta-Co-O and Nb-Co-O systems have been studied at 1200°C at oxygen partial pressures from 10^−0.68 to 10^−13.50 atm for the former and from 10^−0.68 to 10^−13.30 atm for the latter. In both systems, M₂CoO₆ and M₂Co₄O₉ are stable ternary compounds under the experimental conditions, and a new phase, Nb₅Co₂O₁₄, has been identified. The Ta-Co-O system is simple, whereas the Nb-Co-O system is somewhat more complicated because of the extra phase. The lattice constants of the ternary compounds have been determined and compared with previous values. The standard Gibbs energies of reactions have been determined using oxygen partial pressures in equilibrium with three solid phases.     

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°.