Wetting in an Electronic Packaging Ceramic System: 1, Wetting of Tungsten by Glass in Controlled Oxygen Partial Pressure Atmospheres

The wetting of tungsten by a Cr₂O₃-colored, CaO-MgO-Al₂O₃-SiO₂ glass was found to be independent of temperature between 1300° and 1500°C, but strongly dependent on furnace atmosphere. Similar results using two gas buffer systems (CO/CO₂ and H₂/H₂O) established oxygen partial pressure, p₀₂ as the critical parameter. The contact angle decreased over a narrow p₀₂ range as the p₀₂ increased, with a stable contact angle existing in both lower and higher p₀₂ ranges. The solid-liquid interfacial energy, γ_SL, controlled the wetting behavior. An increase in the adsorbed oxygen layer at the solid-liquid interface resulted in a lower γ_SL and a lower contact angle. The equilibrium contact angles, established after 8-h isothermal holds, ranged from 50-55° at a p₀₂= 10^−15.5 atm to 30° at a p₀₂ = 10^−10.9 atm. Two different drop formation techniques were used to show that the temperature and atmospheric conditions at the time of solid-liquid interface formation affect the stable contact angles. The contact angle was higher when the solid-liquid interface was established at the test temperature (doser tube technique) than when the drop was formed in situ from a piece of glass placed on the substrate at room temperature (nondoser method). This contact angle difference was again attributed to a higher γ_SL from the doser method due to the presence of less adsorbed oxygen at the time of the creation of the solid-liquid interface.     

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.     

Oxygen Non-Stoichiometry and Thermal–Chemical Expansion of Ce0.8Y0.2O1.9−δ Electrolytes by Neutron Diffraction

A dense tubular solid electrolyte with the composition Ce_0.8Y_0.2O_1.9−δ (CY20) was prepared. In situ time-of-flight neutron powder diffraction (TOF-ND) was performed at 900°C in the oxygen partial pressure p O₂ range from 10⁻¹–10⁻¹⁸ atm, and TOF-ND data were analyzed by the Rietveld method. Diffraction data showed that the lattice parameter moderately increased with decreasing p O₂ in the range of p O₂>10⁻¹⁴ atm, while a dramatic expansion (∼0.6%) of the fluorite structure occurred at a p O₂ of 10⁻¹⁸ atm. By refining all reasonable structural parameters, an approximately linear relationship between lattice parameter and oxygen vacancy δ was observed, resulting in ɛ_c/δ=0.08 and corresponding to δ=0.10 at a p O₂ of 10⁻¹⁸ atm, all in agreement with the data published in the literature. The relative change in lattice parameter Δ a / a followed a −1/4 power relation with p O₂ in a low- p O₂ regime. As several (often strongly correlated) structural parameters can affect the intensities in ND profiles, care was taken to select refinement variables. It was found that O atom thermal factors for CY20 increased as the oxygen vacancy concentration and lattice expansion increased.     

Effect of Oxide Defect Structure on the Electrical Properties of ZrO2

The defect structure of monoclinic ZrO₂ was studied by measuring the transfer numbers and electrical conductivity as functions of O₂ pressure and temperature. The data suggest a defect structure of doubly ionized oxygen vacancies at low pressures, i.e. <10⁻¹⁹ atm, and singly ionized oxygen interstitials at pressures >10⁻⁹ atm. Zirconia is primarily an ionic conductor below #700°C and an electronic conductor at 700° to 1000°C for 10⁻²²≤P_o2≤1 atm.     

Structural Behavior of Oxygen Permeable SrFe0.2Co0.8Ox Ceramic Membranes with and Without pO2 Gradients

The oxygen partial pressure ( p O₂)-dependent structural behaviors of two dense tubular ceramic membranes in composition SrFe_0.2Co_0.8O _x with cubic perovskite structure have been investigated by high-temperature neutron powder diffraction: one in "static" mode and one in simulated-operation mode in which one side of the membrane was exposed to air and the other side to reducing gases with variable p O₂ levels. Rietveld analysis on data collected for the membrane without p O₂ gradients showed that the perovskite is stable in p O₂ down to ∼10⁻¹² atm, and at ∼10⁻¹⁴ atm it starts to decompose into a three-phase mixture containing layered intergrowth Ruddlesden–Popper phases Sr _{n +1}(Fe,Co) _n O _x with n =2 and 3, along with CoO with rocksalt structure. Similar phase evolution was observed when insufficient air flowed on the air side of the membrane exposed to a p O₂ gradient. The data support a nonlinear model of oxygen content in perovskite across the membrane thickness, corresponding to a p O₂ profile that is shallow inside and steep near the reducing side surface. Gas compositions measured with mass spectrometry indicated that oxygen is permeated from the air side to the reducing side of the membrane. The oxygen permeation fluxes at 900°C were estimated to be 0.4–0.9 sccm/cm² for the ∼1 mm thick membrane containing perovskite, depending upon p O₂ gradient.     

Use of Stabilized ZrO2 to Measure O2 Permeation

The rate of permeation of CaO-stabilized ZrO₂ (CSZ) by O₂ gas was measured from 640° to 1200°C with the CSZ tubing used simultaneously as the sample and the O₂ pressure detector. The apparent permeation rate depended significantly on the O₂ pressure at the low-pressure side. The rate measured by this method was orders of magnitude smaller than that measured under steady-state conditions, except when the O₂ partial pressure was high (>10⁻⁴ atm), in which case the agreement was good. The difference between steady-state permeability and non-steady-state permeability is related to the deviation in stoichiometry in a sample or detector. The transient response (measured under variable pressure difference) may be very different from steady-state permeation (measured under constant differential pressures across the membranes). To apply CSZ to typical O₂ gas permeability measurements, the O₂ pressure must be kept above ∼ 10^−3.5 atm. In this range, the permeability of CSZ may be regarded as a temperature-dependent material property which is governed by the electron-hole mobility. At lower O₂ pressures the permeation rate is a more complex function of the pressure difference and level.     

Electrical Conduction in Single-Crystal and Polycrystalline Al2O3 at High Temperatures

The electrical conductivity and ion/electron transference numbers in Al₃O₃ were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O₂ partial pressure over single-crystal Al₂O₃ at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O₂ (1 atm) the conductivity above 1400°C was σ≃3×10³ exp (–80 kcal/ RT ) Ω⁻¹ cm⁻¹, which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al₂O₃ and MgO-doped Al₂O₃ showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O₂ partial pressure (below 10⁻¹⁰ atm).     

Electrical Conductivity of Coal Slag

The electrical conductivity of natural and synthetic slags (containing 14 to 36 wt% Fe) was measured from 1200 to 1700 K at O₂ pressures from 1 to 2×10⁻⁶ atm. The conductivity is relatively high (∼10⁻²Ω⁻¹ cm⁻¹ at 1700 K) and stems from the transfer of electrons between Fe²⁺ and Fe³⁺ ions. Anomalies in the conductivity around 1600 K are the result of devitrification of the glass samples.     

Gravimetric Determination of Defect Concentrations in NiO

Thermogravimetric measurements were made on NiO from 800° to 1100°C over the oxygen pressure range 10⁻¹ to 10⁻⁴ atm. On the basis of complementary conductivity measurements showing a P (O₂)^1/5 oxygen pressure dependence, it is proposed that the predominant defects are described by an electroneutrality condition involving doubly ionized metal vacancies, impurities, and electron holes, 2[ V"_M ]=[ F_M ]+ p. It is shown that for this defect model, the weight change relative to a low oxygen pressure reference weight is a measure of the effective vacancy concentration, defined as [ V"_M ]_eff≡[ V"_M ]-≡[ F_M ], and therefore has the same oxygen pressure dependence as the electron hole concentration followed in the conductivity measurements. The expression [ V"_M ]_eff=0.168 P (O₂)^1/5 exp (−0.86±0.15/ kT ) is derived to express the effective vacancy concentration in NiO. The probable effective impurity content of the specimens used is calculated.     

Thermodynamics and Phase Equilibria in the Ca─Cu─O System

Phase equilibria in the CaO─CuO─Cu system were determined at 1173 K from the results of X-ray diffraction measurements using specimens annealed in the oxygen partial pressure range from P O₂= 1 to 10⁻⁸ atm. Electromotive force (emf) measurements using ZrO₂ solid electrolyte cells were carried out in the ternary phase equilibria. Gibbs free energies for the chemical reactions were summarized by equations with linear temperature dependence, and the standard free energy of formation for Ca₂CuO₃ was derived. The stability conditions of the oxides are displayed in the p – T – x diagram, and the possible phase equilibria with the liquid are evaluated.     

Phase Equilibria and Physical Properties of Oxygen-Deficient Zirconia and Thoria

The compositions of anion-deficient zirconia and thoria in equilibrium with O₂ were measured from 1 to 10⁻⁶ atm and 1400° to 1900°C; for ZrO_{2- x} (po₂ in atm, and T in °K), log x∼−0.890-[(0.400×10⁴)/ T ]-[(log p )/6]; for ThO_{2- x} , log x∼−1.870-[(0.340×10⁴)/ T ]-[(log p )/6]. The ZrO_{2- x} -Zr boundary was located at x=0.014 at 1800°C; thoria was single-phase over the entire range. Consistent results were obtained when O₂/inert gas mixtures were used, but use of H₂/H₂O and CO/CO₂ at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic-tetragonal phase change of ZrO₂ and the lattice thermal expansion, room-temperature Young's modulus, and strength properties of ZrO₂ and ThO₂ bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased slightly with departure from stoichiometry.     

Constraint of Oxygen Fugacity During Field-Assisted Sintering: TiO2 as a Test Case

Field-assisted sintering exposes samples in a graphite die to reducing conditions. Using TiO₂ as a test case, this work shows that internal redox equlibria in the sample, rather than the graphite–CO–O₂ equilibrium, appear to control the oxygen fugacity. Samples sintered at 1160°C for 20 min are homogeneous in oxygen content and have an average composition of TiO_1.983±0.001. The oxygen fugacity during these sintering experiments is calculated to be about 10⁻¹⁶ atm, which is higher than the value obtained from thermodynamic equilibrium of graphite–CO–O₂ at the given temperature. The oxygen fugacity is similar to that for the quasi-two-phase region, or hysteresis loop, representing the coexistence of reduced rutile with random crystallographic shear (CS) planes and the first ordered CS phase.     

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.     

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.     

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⁺.     

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.     

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.     

Transference Numbers for In-Plane Carrier Conduction in Thin Film Nanostructured Gadolinia-Doped Ceria Under Varying Oxygen Partial Pressure

We demonstrate a modified Hebb–Wagner approach to quantitatively estimate transference numbers for carrier conduction in thin film oxide conductors using blocking electrodes in an in-plane geometry. We report ionic transference numbers, t _i, for gadolinia-doped ceria (GDC) thin films, a model mixed ion–electron conductor, at 973K and oxygen partial pressure ranging from 0.21 atm down to approximately 10⁻²² atm. Our results indicate that GDC reaches the electrolytic regime ( t _i=0.5) at an oxygen partial pressure of 5 × 10⁻¹⁹ atm at 973K. This approach may be useful for understanding carrier transport mechanisms in low-dimensional oxide heterostructures with specific relevance to nanostructured energy materials.     

Electrical Properties and Defect Structure of Y2O3

Guarded measurements of the electrical conductivity of high-purity, polycrystalline Y₂O₃ in thermodynamic equilibrium with the gas phase were made under controlled temperature and oxygen partial pressure conditions. Data are presented as isobars from 1200° to 1600°C, and as isotherms from oxygen partial pressures of 10⁻¹ to 10⁻¹⁷ atm. The ionic contribution to the total conductivity, determined by the blocking electrode polarization technique, was less than 1% over the entire range of temperatures and oxygen partial pressures studied. Yttria is shown to be an amphoteric semiconductor with the region of predominant hole conduction shifting to higher pressures at higher temperatures. In the region of p -type conduction, the conductivity is represented by the expression σ= 1.3 × 10³ p O₂^3/16 exp (-1.94/kT). The observed pressure dependence is attributed to the predominance of fully ionized yttrium vacancies. Yttria is shown to be a mixed conductor below 900°C.