Reaction Sintering of Zinc Ferrite during Constant Rates of Heating

The reaction sintering of equimolar quantities of zinc oxide and ferric oxide was investigated under conditions of constant rates of heating (1–10°C/min from room temperature to 1350°C) and the data were compared with those for a calcined, single-phase zinc ferrite powder. For the heating rate of 1°C/min, the densifications of the reaction-sintered sample and the calcined sample were approximately the same. However, as the heating rate increased, the density at any temperature increased slightly for the reaction-sintered sample but decreased slightly for the calcined powder. The factors responsible for this slight difference in sintering between the reaction-sintered sample and the calcined sample are discussed. For the constant heating rates used, the reaction was completed prior to any significant densification. Relative densities of >95% were obtained for both the reaction-sintered sample and the calcined sample under identical sintering conditions (1–10°C/min to 1350°C). Reaction sintering in a steep temperature gradient produced a nearly fully dense body prior to complete reaction; a composite microstructure consisting of fine zinc oxide grains in a matrix of zinc ferrite was obtained.     

Alumina Volatility in Water Vapor at Elevated Temperatures

The volatility of alumina in high-temperature water vapor was determined by a weight loss technique. Sapphire coupons were exposed at temperatures between 1250° and 1500°C, water partial pressures between 0.15 and 0.68 atm in oxygen, a total pressure of 1 atm, and flowing gas velocities of 4.4 cm/s. The water vapor pressure dependence of sapphire volatility was consistent with Al(OH)₃( g ) formation. The enthalpy of reaction to form Al(OH)₃( g ) from sapphire and water vapor was determined to be 210 ± 20 kJ/mol, comparing favorably to other studies. Microstructural examination of tested sapphire coupons revealed surface rearrangement consistent with a volatilization process.     

Activity-Composition Relations in Orthosilicate and Metasilicate Solid Solutions in the System MnO-CoO-SiO2

Activity-composition relations in Mn-Co orthosilicate and metasilicate solid solutions were determined at 1200° to 1250°C by equilibrating selected phase assemblages with metallic cobalt and a gas phase of known oxygen potentials at a total pressure of 1 atm. The orthosilicate solid solution shows a slight negative departure from ideality, whereas the metasilicate solid solution is ideal. The stability data derived for the Mn₂SiO₄ and MnSiO₃ end-members are in good agreement with those previously determined from the system MnO-"FeO"-SiO₂. The free-energy change for the reaction 2CoSiO₃= Co₂-SiO₄+ SiO₂ was determined to be -3.8 kcal at 1250°C.     

Oxidation of Si3N4 in the Range 1300° to 1500°C

The isothermal oxidation behavior of commercial hot-pressed Si₃N₄ was evaluated for temperatures from 1300° to 1500°C. Multiphase scales were formed, consisting mainly of α-cristobalite and enstatite. A large increase in reaction rate above 1450°C is believed to be caused by melting in the scale and the consequent increase in the rate of oxygen transport. No oxygen pressure dependence was observed at 1400°C over the oxygen pressure range 10^-9 atm to 600 torr. However, a small decrease in the kinetics was observed when measurements were made in reduced total pressures of oxygen as compared to O₂/N₂mixtures at a constant total pressure.     

Low-Temperature Sintering and Microwave Dielectric Properties of Li2MgSiO4 Ceramics

Development of a low-temperature sintered dielectric material derived from Li₂MgSiO₄ (LMS) for low-temperature cofired ceramic (LTCC) application is discussed in this paper. The LMS ceramics were prepared by the solid-state ceramic route. The calcination and sintering temperatures of LMS were optimized at 850°C/4 h and 1250°C/2 h, respectively, for the best density and dielectric properties. The crystal structure and microstructure of the ceramic were studied by the X-ray diffraction and scanning electron microscopic methods. The microwave dielectric properties of the ceramic were measured by the cavity perturbation method. The LMS sintered at 1250°C/2 h had ɛ_r=5.1 and tan δ=5.2 × 10⁻⁴ at 8 GHz. The sintering temperature of LMS is lowered from 1250°C/2 h to 850°C/2 h by the addition of both lithium borosilicate (LBS) and lithium magnesium zinc borosilicate (LMZBS) glasses. LMS mixed with 1 wt% LBS sintered at 925°C/2 h had ɛ_r=5.5 and tan δ=7 × 10⁻⁵ at 8 GHz. Two weight percent LMZBS mixed with LMS sintered at 875°C/2 h had ɛ_r=5.9 and tan δ=6.7 × 10⁻⁵ at 8 GHz.     

Synthesis and Low-Temperature Sintering of Gd-doped CeO2 Ceramic Materials Obtained by a Coprecipitation Process

Well-dispersed ceria–gadolinia oxide powders were obtained from thoroughly isopropanol-washed coprecipitated hydroxides and oxalates, followed by a controlled drying at low temperature and calcining at 550°C. The characteristics of the calcined powders and the microstructure of the green compacts were found to be of great importance in the sintering behavior. Green bodies with high agglomerate sizes need higher sintering temperatures for attaining a final density >99% D _th, while those having soft agglomerates with lower sizes were almost fully densified at a sintering temperature as low as 1250°C. The densification process was studied by isothermal and constant heating rate dilatometry, and microstructural development by scanning electron microscopy. By controlling the processing variables, it was possible to obtain this low-temperature, nearly fully dense (better than 99%) sample with a homogeneous microstructure.     

Phase Equilibrium Studies in the System Iron Oxide-Al2O3-Cr2O3

Subsolidus phase relations in the system iron oride-Al₂O₂-Cr₂O₃ in air and at 1 atm. O₂ pressure have been studied in the. temperature interval 1250° to 1500°C. At temperatures below 1318° C. only sesquioxides with hexagonal corundum structure are present as equilibrium phases. In the temperature interval 1318° to 1410°C. in air and 1318° to 1495° C. at 1 atm. O₂, pressure the monoclinic phase Fe₂O₃. Al₂O₃ with some Cr₂O₃ in solid solution is present in the phase assemblage of certain mixtures. At temperatures above 1380°C. in air and above 1445°C. at 1 atm. O₂ pressure a complex spinel solid solution is one of the phases present in appropriate composition areas of the system. X-ray data relating d- spacing to composition of solid solution phases are given.     

Diffusion of Oxygen in Fused Silica

Oxygen diffusion in fused silica was measured over the range 850° to 1250°C by means of heterogeneous isotope exchange. In this temperature range and at 1 atm oxygen pressure the diffusion coefficients can be represented by the relation: D = 2 × 10⁻⁹ exp (-29 kcal/ RT). The oxygen diffusion coefficients are almost directly proportional to the oxygen pressure which suggests that the oxygen is diffusing in a molecular form.     

Solid-State Sintering of YBa2Cu3Ox in Different Oxygen Partial Pressures

The effect of oxygen partial pressure, ranging from 0.001 to 1.0 atm, and temperature, in the range of 930°–955°C, on the solid-state sintering kinetics of the superconducting ceramic YBa₂Cu₃O_7-δ has been studied. Isothermal compaction rates between 930° and 955°C reached a maximum at some critical PO₂ (PO₂^CRIT), with decreasing rates both above and below this oxygen partial pressure. This behavior was not observed for YBa₂Cu₃O_7-δ sintered at 960°C, when a liquid phase is present. The activation energy for sintering above PO₂^CRIT has been estimated to be ∼190 kJ/mol, whereas below PO₂^CRIT it was found to be ∼130 kJ/mol. The oxygen ion diffusion was considered to be the rate-determining step above PO₂^CRIT, while it is hypothesized that lattice strain caused by the formation of oxygen ion vacancies below PO₂^CRIT affected the rate of sintering.     

Preparation of High-Density Si3N4 by a Gas-Pressure Sintering Process

Si₃N₄ compacts, containing ≅7 wt% of both BeSiN₂ and SiO₂ as densification aids, can be reproducibly sintered to relative densities >99% by a gas-pressure sintering process. Nearly all densification takes place via liquid-phase sintering of transformed β-Si₃N₄ grains at T =1800° to 2000°C. Compacts with high density are produced by first sintering to the closed-pore stage (≅92% relative density) in 2.1 MPa (20 atm) of N₂ pressure at 2000°C and then increasing the N₂ pressure to 7.1 MPa (70 atm) where rapid densification proceeds at T = 1800° to 2000°C. The experimental density results are interpreted in terms of theoretical arguments concerning the growth (coalescence) of gas-filled pores and gas solubility effects. Complex chemical reactions apparently occur at high temperatures and are probably responsible for incomplete understanding of some of the experimental data.     

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.     

Effect of sintering temperature on the characteristics of ceramic hollow spheres produced by sacrificial template technique

Ceramic hollow spheres were produced by a sacrificial template technique with subsequent sintering under temperatures ranging from 1100 °C to 1250 °C. The effect of the sintering temperature on the structure of the ceramic hollow spheres was investigated by optical and scanning electron microscopy, and a gas adsorption method. The results show that the structure of the ceramic hollow spheres can be controlled, with the retention of the hollow spherical shape, by variation of the sintering temperature. Increase of the sintering temperature from 1100 °C to 1250 °C decreased the outer diameter of the ceramic hollow spheres by 14 percent, the shell thickness by 18 percent, and the void area ratio of the shell surface by 9.2 times; both of the specific surface area and the total pore volume of ceramic hollow spheres decreased by 60 percent.     

Surface Diffusivity under High-Pressure Gas

The sintering behavior of TiO₂ under high-pressure gas (100 MPa) was studied with respect to densification rate, decreasing rate of specific surface area, and narrowing rate of pore size distribution. At the low sintering temperature (800°C) used in this study, the densification rate under high- pressure gas was similar to that under low (i.e., 1 atm) pressure. The specific surface area decreased and the pore size distribution narrowed at a faster rate under high- pressure gas than under low-pressure gas. These results imply that high-pressure gas enhances surface diffusion.     

Effect of Green Density on Densification and Creep During Sintering

The effect of green density on both the densification rate and the creep rate was measured simultaneously during sintering by loading dilatometry. The experiments were performed on zinc oxide powder compacts with five different green densities covering a range of 0.39 to 0.73 of theoretical. The samples were heated at a constant rate of 4°C/min up to 1100°C in air. The densification rate at any temperature increases significantly with decreasing green density. The data for the densification rate and creep rate as a function of density show two quite distinct regimes of behavior; the rates were strongly dependent on density below 0.80, while above this value they were weakly dependent on density. The ratio of the densification rate to the creep rate was almost independent of temperature but increased almost linearly with increasing green density. The representation of the data in terms of models for sintering and creep is discussed.     

Phase Transitions and Transient Liquid-Phase Sintering in Calcium-Substituted Lanthanum Chromite

This paper investigates sintering and phase transitions of La_0.7CaxCrO₃(0.25≥x≥0.35), a material useful as electrical interconnections in solid oxide fuel cells. Heating of the quenched, metastable single-phase chromite resulted in exsolution of CaCrO₄ due to Ca solubility limitations below 1200°C. A transient liquid phase formed between 850° and 1000°C as the CaCrO₄ melted, causing partial densification in materials having 0.25 < x < 0.30. A slight increase in Ca content induced an additional liquid-phase sintering event, likely due to melting of Ca₃(CrO₄)₂, which facilitated near-complete densification by 1250°C. After enhancing sintering, the secondary phases redissolved into the chromite.     

Effect of PO2 on Bulk and Grain Boundary Resistance of n-Type BaTiO3 at Cryogenic Temperatures

Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO₃ polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P _O2 as low as 10⁻¹⁵ atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO₃ sintered in reducing atmosphere and then annealed at P _O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO₃ is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.     

Oxidation of Thin Sheet Reaction-Sintered Silicon Nitride

Oxidation of reaction-sintered silicon nitride was studied in damp air. The formation of "passive" silica films was investigated at 1 atm and 700 to 1100°C and some limited work on weight loss behavior was performed in vacuo of 10⁻⁸ to 10⁻⁵ atm at 1050 to 1200°C. Passive behavior was dominated by reaction in the pore network. Oxidation was extensive at 900 to 1000° but slight at 700 to 800°C. At 1100°C a protective skin limited reaction. Weight loss in vacuo was slight at 1050°C. The vacuum pressure required to suppress the weight loss increased from 4 to 5 × 10⁻⁷ atm at 1050° to 1.5 to 2.5 × 10⁻⁵ atm at 1200°C.     

Enhanced Sintering of Yttrium-Doped Barium Zirconate by Addition of ZnO

The influence of transition metal oxides additives, especially zinc oxide, on the densification and electrical properties of doped barium zirconate have been examined. With the use of zinc oxide as a sintering aid, BaZr_0.85Y_0.15O_3–δ was readily sintered to above 93% of theoretical density at 1300°C. Scanning electron microscopic investigations showed Zn accumulation in the intergranular regions. Thermogravimetric analysis of the material under flowing CO₂ showed ZnO-modified barium zirconate to exhibit excellent chemical stability. The conductivity, as measured by A.C. impedance spectroscopy under H₂O saturated nitrogen, was slightly lower than that of unmodified barium zirconate. Electromotive force measurements under fuel cell conditions revealed the total ionic transport number to be ∼0.9 at 600°C. The combination of electrical and chemical properties and good sinterability render ZnO-modified barium zirconate an excellent candidate for reduced temperature solid oxide fuel cell applications.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

Evaporation of Silver during Cofiring with Barium Titanate

Silver and Ag-Pd alloy are cofired with BaTiO₃-based dielectrics during the manufacturing of capacitors. The diffusion of silver into BaTiO₃ during sintering is very slow. However, the vapor pressure of silver is high at temperatures greater than the melting point of silver (960°C). The effect of the evaporation of silver during the sintering with BaTiO₃ at 1200°, 1250°, and 1290°C is investigated in the present study. The silver vapor can transport through the pore channel of the dielectrics to a distance of a few hundred micrometers. The melting point of Ag-Pd alloy is higher than that of silver; therefore, the transportation of silver vapor from Ag-Pd alloy is hardly observed at temperatures >1200°C.