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     

Interdiffusion of Calcium in Soda-Lime-Silica Glass at 880° to 1308°C

The interdiffusion of calcium in soda-lime-silica glass under the action of a concentration gradient was studied. Pairs of glass blocks differing by 2.9 mole % in CaO content were fused together to form diffusion couples and were held at 880° to 1308°C. The couples were allowed to cool to room temperature and the diffusion which had taken place was measured by optical interferometry and by an electron microprobe. The diffusion coefficients varied from 4.4 × 10⁻¹⁰cm²/sec at 880°C to 8.0 × 10⁻⁸ cm²/sec at 1308°C. The activation energy was 42,000 cal/mole. It is concluded that oxygen diffuses simultaneously with the calcium, maintaining the electrical neutrality of the glass.     

Thermal Expansion of Polycrystalline Ti3SiC2 in the 25°–1400°C Temperature Range

We measured the volume thermal expansion of Ti₃SiC₂ from 25° to 1400°C using high-temperature X-ray diffraction using a resistive heated cell. A piece of molybdenum foil with a 250 μm hole contained the sample material (Ti₃SiC₂+Pt). Thermal expansion of the polycrystalline sample was measured under a constant argon flow to prevent oxidation of Ti₃SiC₂ and the molybdenum heater. From the lattice parameters of platinum (internal standard), we calculated the temperature by using thermal expansion data published in the literature. The molar volume change of Ti₃SiC₂ as a function of temperature in °C is given by: V _M (cm³/mol)=43.20 (2)+9.0 (5) × 10⁻⁴ T +1.8(4) × 10⁻⁷ T ². The temperature variation of the volumetric thermal expansion coefficient is given by: α_v (°C⁻¹)=2.095 (1) × 10⁻⁵+7.700 (1) × 10⁻⁹ T . Furthermore, the results indicate that the thermal expansion anisotropy of Ti₃SiC₂ is quite mild in accordance with previous work.     

Extrinsic OH− Absorption in Transparent Polycrystalline Lanthana-Doped Yttria

Transparent lanthana-doped yttria fabricated by transient solid second-phase sintering under wet hydrogen typically has a broad absorption band with a peak at 3.08 μm. The absorption band shift observed in samples treated in wet deuterium indicated that the 3.08-μm absorption was due to OH⁻ ions. The diffusion rates of hydrogen defects in lanthana-doped yttria were determined in the temperature range from 1000° to 1400°C. The changes in the concentrations of OH⁻ ions upon anneals were determined by measuring infrared absorbance at 3.08 μm. The diffusion coefficient is 1.3 × 10⁻⁷, 9.9 × 10⁻⁷, and 4.1 × 10⁻⁶ cm²/s at 1000°, 1200°, and 1400°C, respectively, with an activation energy of 140 kJ/mol. Annealing in a controlled oxygen partial-pressure environment can remove the OH⁻ absorption band and bring the total absorption in the 3- to 5-μm range closer to the intrinsic values.     

Sintering of Zinc Sulfide

The mechanisms of the sintering of ZnS were determined by measurement of the rate of growth of the necks between polycrystalline spheres. In vacuum (10⁻⁶ mm Hg) at temperatures higher than 600° C the mechanism of sintering is that of volume diffusion with coefficient D_v, = 1.06 × 10⁻² exp (-26,400/RT); below 600°C surface diffusion predominates, with coefficient D₃, = 9.14 × 10_-3 exp (-5700/RT). In Zn vapor (10⁻² mm Hg) between 550° and 650°C the predominating mechanism of sintering is surf ace diffusion, D₃, = 7.06 × 10⁻² exp (-8600/RT). It has been found that in an argon atmosphere the diffusion coefficient is much lower.     

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.     

Oxygen Tracer Diffusion in Lanthanum-Doped Single-Crystal Strontium Titanate

Strontium titanate (SrTiO₃) is known as a good high-temperature resistive oxygen sensor material; its response time depends on oxygen bulk diffusion and surface exchange processes. In the present work, ¹⁸O diffusion has been investigated in lanthanum-doped SrTiO₃, single crystals in the temperature range 700° to 900°C by secondary ion mass spectrometry (SIMS). Oxygen tracer diffusivities between 2 × 10⁻¹⁵ and 1 × 10⁻¹³ cm²/s have been calculated from the SIMS results. Low surface enrichment of ¹⁸O compared to the ¹⁸O concentration in the gas atmosphere gives clear evidence for a surface exchange reaction.     

Electrical and Pyroelectric Properties of Highly (001)-Oriented (Pb0.76Ca0.24)TiO3 Thin Films Grown by a Sol–Gel Process

Highly (001)-oriented (Pb_0.76Ca_0.24)TiO₃ (PCT) thin films were grown on Pt/Ti/SiO₂/Si substrates using a sol–gel process. The PCT film with a highly (001) orientation showed well-saturated hysteresis loops at an applied field of 800 kV/cm, with remanent polarization ( P _r) and coercive electric field ( E _c) values of 23.6 μC/cm² and 225 kV/cm, respectively. At 100 kHz, the dielectric constant and dielectric loss values of these films were 117 and 0.010, respectively. The leakage-current density of the PCT film was 6.15 × 10⁻⁸A/cm² at 5 V. The pyroelectric coefficient ( p ) of the PCT film was measured using a dynamic technique. At room temperature, the p values and figures-of-merit ( F _D) of the PCT film were 185 μC/m²K and 1.79 × 10⁻⁵ Pa^−0.5, respectively.     

Deviation from Stoichiometry in Cr2O3 at High Oxygen Partial Pressures

The deviation from stoichiometry, δ, in Cr₂−δO₃ was measured by a tensivolumetric method in the high pO₂ range of ≊10⁴ to 10⁴ Pa at 1100°C. The value of δ, or chromium vacancy concentration, was≊9×10⁻⁵ mol/mol Cr₂O₃ in air for Cr₂O₃ with 99.999% purity. The chemical diffusion coefficient, DT, determined from equilibration data was ≊4.6× cm²·s⁻¹ at 1100°C for pO₂= 2.2 ×10¹ Pa. The self-diffusion coefficient of Cr ions was calculated from and δ and found to be≊1.6×10-17 cm²-s⁻¹, in good agreement with recently measured values.     

Enhanced Oxygen Diffusion at 1400°C in Deformed Single-Crystal Magnesium Oxide

The rate of oxygen difusion at 1400°C into pure, deformed, and scandium-doped MgO was measured using a secondary-ion mass spectrometer with ¹⁶O^- primary ions as sputtering agents to monitor the penetration of ¹⁸O into the sample from an ¹⁸O-enriched surface layer. In samples doped with scandium, oxygen diffusivity did not differ significantly from the value, 1.3 × 10-¹⁵ cm²/s, found for the pure sample. In deformed samples, the diffusion coefficient was 5.8 × 10^-15 cm²/s .     

Formation Kinetics of MoSi2 and Mo5Si3 by the Reactive Diffusive Siliciding of Molybdenum

The formation kinetics of products formed by the reaction between dense molybdenum and vapor-supplied silicon at an activity approximating that of solid silicon under open flowing gas conditions was studied at 1200°C. An outer MoSi₂ layer overlaid the much thinner Mo5Si3 that formed on the molybdenum. Both phases obeyed parabolic growth laws over a 22 h period, having parabolic rate constants of 6.8 × 10⁻¹⁰ cm²/s for the MoSi₂ and 1.3 × 10⁻¹³ cm²/s for the Mo₅Si₃ phases. These results were 2 orders of magnitude less than prior results, mostly obtained by another processing route. Possible explanations include enhanced growth rates from chemical contamination. Gross distortion and abnormal layer thicknesses at specimen edges and the 159% volume increase during siliciding suggest that the kinetics also are strain dependent.     

Microstructure and High-Temperature Corrosion Behavior of a Cr–Al–C Composite

A Cr–Al–C composite was successfully synthesized by a hot-pressing method using Cr, Al, and graphite as starting materials. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses revealed that the composite contained Cr₂AlC, AlCr₂, Al₈Cr₅, and Cr₇C₃. The orientation relationships and atomic-scale interfacial microstructures among Cr₂AlC, AlCr₂, and Al₈Cr₅ are presented. This composite displays both excellent high-temperature oxidation resistance in air and hot-corrosion resistance against molten Na₂SO₄ salt. The parabolic rate constants for the oxidation in air at 1000°, 1100°, and 1200°C are 3.0 × 10⁻¹², 6.2 × 10⁻¹¹, and 6.2 × 10⁻¹⁰ kg² (m⁴·s)⁻¹, respectively, while the linear weight gain rates for the hot corrosion of Na₂SO₄-coated samples at 900° and 1000°C are, respectively, 1.2 × 10⁻³ and 4.4 × 10⁻³ mg (cm²·h)⁻¹. The mechanism of the excellent high-temperature corrosion resistance can be attributed to the formation of a protectively alumina-rich scale.     

Oxidation Kinetics of Single-Crystal SrTiO3

The oxidation kinetics were determined for single-crystal SrTiO₃ by measuring the time and temperature dependence of the weight gain of reduced crystals. The oxidation can be described as a diffusion-controlled process. The calculated diffusion coefficients between 850° and 1460°C are represented by D = 0.33 exp (-22.5 ± 5.0 kcal/ RT ) cm²/sec. Directly measured oxygen ion diffusion coefficients in the same temperature interval reported earlier are interpreted as being extrinsic and can be represented by D = 5.2 × 10⁻⁶ exp (-26.1 ± 5.0 kcal/ RT ) cm²/sec, where the activation energy is for mobility only. Assuming that the calculated diffusion coefficients are for vacancy diffusion and the two activation energies are equivalent within experimental error, a vacancy concentration (fraction of vacant lattice sites), [O□], fixed by impurities in the fully oxidized crystal is calculated to be 1.6 × 10⁻⁵ by virtue of the relation between the oxygen self-diffusion coefficient, D_02-, and the oxygen vacancy diffusion coefficient, D_o□ ; D _o2-= [O□] D _o□ where the oxygen ion concentration [O^2-] is taken as unity.     

Internal Reduction of an Iron-Doped Magnesium Aluminosilicate Melt

Optical and electron microscopies are used to analyze the mechanism and kinetics of internal reduction of an Fe²⁺-doped magnesium aluminosilicate melt. Melt samples are heated to temperatures in the range of 1300°–1400°C under a flowing gas mixture of CO/CO₂, which corresponds to a p _O2 range of 1 × 10⁻¹³–4 × 10⁻¹³ atm. The melt experiences an internal reaction in which a dispersion of nanometer-scale iron-metal precipitates forms at an internal interface. The metal precipitates show no signs of coarsening within the samples; however, the crystals at the surface (which formed in the initial part of the reaction) do grow via vapor phase transport. The overall reaction is characterized by parabolic kinetics, which is indicative of chemical diffusion being the rate-limiting step. The diffusion of network-modifier divalent cations—particularly Mg²⁺ cations—is demonstrated to be the rate-limiting factor, and its diffusion coefficient is calculated to be ∼1 × 10⁻⁶ cm²/s within the temperature range of the experiments.     

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.     

Oxygen Self-Diffusion in Lithium-Doped Single-Crystal Magnesium Oxide

Oxygen self-diffusion coefficients for a single-crystal MgO doped with 400 ppm Li were determined at 848° to 1300°C by isotope exchange technique using ¹⁸O as the tracer. The diffusion coefficient increased with increasing temperature in the lower temperature range, as described by D = 7.8 × 10⁻⁴ exp[−279 (kJ/mol)/RT] cm²/s, which was interpreted to be an extrinsic diffusion due to the oxygen vacancies introduced by substitutional Li ions. The oxygen diffusivity tended to decrease above 1050°C, presumably becauuse of evaporation of Li₂O.     

High-Temperature Creep and Kinetic Demixing in (Co,Mg)O

The creep behavior of fine-grained (Co_0.5Mg_0.5)O and (Co_0.25Mg_0.75)O has been characterized as part of an investigation of kinetic demixing in solid-solution oxides due to a nonhydrostatic stress. (i) For low stresses and small grain sizes, the dominant deformation mechanism for both compositions is diffusional creep limited by the transport of oxygen along grain boundaries. The oxygen grain-boundary diffusivity, D _o ^b is independent of oxygen partial pressure. The values of ω D _o ^b , where ω is the grain-boundary width, that have been determined from the steady-state diffusional creep rates are given by ω D _o ^b =4.7×10⁻⁸ exp[-230 (kJ/mol)/ RT ] (cm³/s) for (Co_0.5Mg_0.5)O in the range 950° to 1200°C and ω D _o ^b =7.4 × 10⁻⁸ exp[-263 (kJ/mol)/ RT ] (cm³/s) for (Co_0.25Mg_0.75)O in the range 1100° to 1250°C. Since oxygen diffusion controls the rate of diffusional creep, kinetic demixing is not observed in deformed samples of either composition. (ii) For high stresses and large grain sizes, the dominant deformation mechanism in both cases is dislocation-climb-controlled creep, where the rate of dislocation climb is controlled by oxygen lattice diffusion. Based on the positive dependence of creep rate on oxygen partial pressure, it is concluded that oxygen diffuses through the lattice by an interstitial mechanism.     

Pore Drag and Pore-Boundary Separation in Alumina

Microdesigned interfacial pore structures were used to study pore drag and pore-boundary separation in Al₂O₃. This approach allows the creation of pore arrays containing pores of controlled size and spacing at well-defined singlecrystal seed/polycrystalline matrix interfaces, and enables experimental determination of the peak pore velocity. From the peak pore velocity, values of the surface diffusion coefficient pertinent to sintering can be extracted. At 1600°C, the surface diffusion coefficient is ∼1 × 10⁻⁷ cm²/s for undoped Al₂O₃ and ∼4 × 10⁻⁷ cm²/s for MgO-doped Al₂O₃. The values appear to be insensitive to the seed orientation for the two seed orientations studied. The results suggest a strong influence of pore spacing on the separation condition in undoped Al₂O₃, and a diminished influence in MgO-doped Al₂O₃. Quantitative agreement between theoretically predicted and experimentally observed separation/attachment conditions was obtained.     

Electrical Properties and Defect Structure of HfO2

The defect structure of high-purity, polycrystalline HfO₂ was investigated by measuring the oxygen partial pressure dependence of the electrical conductivity and the sample weight. From 1000° to 1500°C and above oxygen partial pressures of 10 ⁻⁶, the conductivity is electronic and proportional to p o₂^1/5. The predominant defect is completely ionized hafnium vacancies. At lower oxygen partial pressures a broad shallow minimum in the lower temperature conductivity isotherms indicates the presence of an oxygen pressure independent source of electronic charge carriers. By combining the weight change and conductivity data, mobility values were found to vary from 1.6 × 10⁻³ to 3 × 10⁻⁴ cm²/V-sec. The activation energies for the hole mobilities were calculated to be 0.2 ev above 1300° C and 0.7 ev below this temperature.     

Initial Sintering Kinetics of Hyperstoichiometric Uranium Dioxide

The initial sintering kinetics of uranium dioxide powder compacts were determined as a function of temperature and stoichiometry (800°< T <1050°C, 2.03−6 exp (−55,000/ RT ) cm²/s when O/U=2.08. The diffusion coefficient was approximately proportional to the square of the excess O present, with some deviation at higher O/U values. This behavior can be explained by assuming that the U vacancy concentration is a function of the O activity.