Effect of Sintering Atmosphere on Densification of MgO-Doped Al2O3

The densification behavior of Al₂O₃-MgO (0.1 wt%) has been studied in O₂ and N₂ atmospheres. Powder compacts have been sintered at 1600°C for 0.5 to 8 h. For some specimens the sintering atmosphere has been changed after 30 min of sintering. Irrespective of sintering atmosphere, sintered densities are approximately the same up to 99% relative density, implying that the capillary pressure effect dominates the atmosphere effect for most of the densification stage. During extended sintering treatment the density of specimens sintered in O₂ becomes higher than that in N₂. When the sintering atmosphere is changed from O₂ to N₂, enhanced densification results, and vice versa. Such an effect of sintering atmosphere is explained by the diffusiveness of gases entrapped in pores, as well as by oxygen potential differences inside and outside of the specimen. Differences in grain growth rate in various atmospheres are discussed on the basis of different densification rates.     

Effect of Grain Growth on Pore Coalescence During the Liquid-Phase Sintering of MgO-CaMgSiO4 Systems

During the liquid-phase sintering of MgO-CaMgSiO₄ systems in N₂ atmosphere, the total porosity and the average pore size increase while the number of pores decreases. The negligible permeability of entrapped nitrogen through the liquid matrix and the observed linear relationship between the number of pores and that of the MgO grains suggest that the pores coalesce as a consequence of grain growth during sintering. An analysis of the balance between the pressure of the entrapped N₂ gas and the capillary pressure shows that pore coalescence in turn causes the observed porosity increase. When Fe₂O₃ or Cr₂O₃ is added to MgO-CaMgSiO₄, the pore size and the total porosity become larger or smaller, because the grain growth is accelerated or retarded, respectively.     

Low-Temperature Pressureless Sintering of Sr- and Mg-Doped Lanthanum Gallate Ceramics by Sintering Atmosphere Control

The present investigation reports a new processing technique that can reduce the sintering temperature of Sr- and Mg-doped lanthanum gallate (LSGM), a good candidate material for the electrolyte of the solid oxide fuel cell (SOFC). When LSGM was sintered at 1623 K for 5 h in N₂ or O₂, the samples were densified over 98% relative density. In contrast, only 93% relative density was achieved after sintering in air, the conventional sintering atmosphere. As a result of better densification in N₂ or O₂, the electrical conductivity of the N₂-sintered and air-annealed or the O₂-sintered sample was higher than that of the air-sintered sample by 30%. This result shows the beneficial effect of N₂ or O₂ sintering of LSGM and provides a high possibility of a low-temperature preparation of an LSGM-based SOFC.     

Final Sintering of Cr2O3

The effect of oxygen activity on the sintering of high-purity Cr₂O₃ is shown. Theoretical density was approached at the equilibrium O₂ partial pressure needed to maintain the Cr₂O₃ phase ( P _o2=2×10⁻¹² atm). The presence of N₂ in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O₂ activity, correlated with the changing predominant volatile species in the Cr-O system.     

Effect of Iron Oxide on the Sintering Kinetics of Al2O3

Alumina powders with varying iron oxide contents were prepared by coprecipitation. The powders were spheroidized by passing them through an oxygen-acetylene flame. The spheres were sized, annealed, and sintered in air and in N₂ with 132 ppm O₂. Isothermal studies were combined with constant-rate-of-heating studies to identify the mechanism of sintering and to calculate the diffusion coefficients. The contribution of surface diffusion during initial-stage sintering of Fe-doped Al₂O₃ was estimated by combining shrinkage and neck-growth data. The effect of Ti on the sintering rate of Fe-doped Al₂O₃ was also studied. Both Fe²⁺ and Ti⁴⁺ ions enhanced the sintering rate of Al₂O₃. A defect model for corundum is proposed to explain the sintering data for transition-metal-ion-doped Al₂O₃.     

Rapid Calcination and Sintering of YBa2Cu3Ox Superconductor Powder Mixture in Inert Atmosphere

The rates of forming the superconducting YBa₂Cu₃O_x phase during the calcination of the Y₂O₃, BaCO₃, and CuO powder mixture at 790° and 850°C are considerably enhanced when an inert atmosphere of N₂ or He is used instead of O₂. Sintering in an inert atmosphere also produces higher density and larger grain size than in O₂. These results are consistent with the possibility of rapid atomic diffusion in tetragonal YBa₂Cu₃O_x due to either high oxygen vacancy concentration or expanded lattice in an inert atmosphere.     

Elimination of large Pores During Gas-Pressure Sintering of β'-Sialon

The effect of N₂-gas pressure on the liquid filling of large pores (20 to 120 μm in diameter) is studied in sintered β'-sialon ( z = 1). The initially sintered sialon with large pores is sintered again and infitrated by a liquid (41A1₂O₃-41Y₂O₃-14Si₃Y₄-4AIN (wt%)) at 17800°C for various times under 0.1-, 0.3-, and 0.5-MPa (1-, 3-, and 5-atm) N₂. The liquid fills large interconnected pores; the size of the pores filled with liquid increases with N₂-gas pressure and time. In some liquid pockets, gas bubbles are formed and subsequently disappear during prolonged sintering treatment. The liquid-filling be havior with sintering pressure and time is explained by gas pressure in the pore and thermal decomposition of the material. The benefit of gas-pressure sintering for the elimination of large pores is assessed.     

Preparation of the High-Quality Superconductor La1Ba2Cu3Oy

The relationship between the preparation procedure and superconducting properties of La₁Ba₂Cu₃O_y was studied. A series of samples was sintered in an N₂-gas atmosphere for various lengths of time ranging from 1 to 40 h, followed by a fixed annealing procedure in O₂. It was found that the shorter the sintering period, the higher was the oxygen content. The samples sintered for a period of less than 15 h contained excess oxygen compared with La₁Ba₂Cu₃O₇ and exhibited poor superconducting properties. The sample sintered for 40 h had an oxygen content y equal to 6.95, and had excellent superconducting properties. The mechanism for preparing high-quality La₁Ba₂Cu₃O_y is discussed.     

Initial Sintering of Rutile

The initial shrinkage of powder compacts of rutile was measured in air at 700° to 1130°C. The shrinkage behavior agrees well with a model based on grain-boundary diffusion. The apparent activation energy for the shrinkage rate is 76.9 ± 2.5 kcal/mole. Changes in ambient atmosphere (O₂, N₂, vacuum) had no effect on the initial sintering kinetics.     

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.     

Sintering of UN as a Function of Temperature and N2 Pressure

The sintering of uranium mononitride (UN) depends on temperature and the N₂ pressure maintained over the nitride during heat treatment. At a given temperature, an N₂ pressure that maintained the UN in the single-phase region slightly above the phase boundary where the reaction UN→U+½N₂(g) occurred was most effective in accelerating the sintering of single-phase UN. For example, specimens sintered at 1600°C under N₂ pressures of either 1140 or 1.7XlO⁻⁴ torr had essentially identical compositions, but the density of the former was 10.78 g/cm³ (75% of theoretical), whereas that of the latter was 12.20 g/cm³ (85% of theoretical). Results were similar at temperatures up to 2100°C. The X-ray lattice constant of UN sintered at reduced N₂ pressures was slightly larger than that of UN sintered in 1140 torr of N₂. The observed constants ranged from 4.88904 to 4.88991 Å; the combined O+C content varied from 400 to 900 ppm.     

Effects of Sintering Atmosphere on Densification Behavior and Piezoelectric Properties of Pb(Ni1/3Nb2/3)O3–PbZrO3 Ceramics

Densification of polycrystalline Pb(Ni_1/3Nb_2/3)O₃–PbTiO₃–PbZrO₃ (PNN–PT–PZ) specimens was enhanced as the partial pressure of O₂ in the sintering atmosphere was increased. This observation was attributed to the increase in the internal pressure of a closed pore due to the thermal decomposition of PbO at a low partial pressure of O₂. The relative dielectric permittivity (ε_r), d ₃₃, k _p, and grain size of sintered specimens were also increased as the partial pressure of O₂ in the sintering atmosphere was increased. The observed dependence of piezoelectric properties on the partial pressure of O₂ was discussed in terms of the enhanced formation of the A-site vacancy ( V "_Pb) or the suppression of the B-site defect ( V ¨_O) as the oxygen potential increased.     

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

The electrical properties of a series of CaCu₃Ti₄O₁₂ ceramics prepared by the mixed oxide route and sintered at 1115°C in air for 1–24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu₃Ti₄O₁₂ ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N₂ at 800°–1000°C but can be recovered to the original value by heat treatment in O₂ at 1000°C. The bulk resistivity decreases from ∼80 to 30 Ω·cm with increasing sintering time but is independent of heat treatment in N₂ or O₂ at 800°–1000°C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu₃Ti₄O₁₂ at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.     

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.     

H2O-Catalyzed Sintering of ∼2-nm-Cross- Section Particles of MgO

MgO produced by vacuum decomposition of Mg(OH) ₂ is formed as aggregates containing both open and closed pores. The surface area and the volume of the open pores, as measured by N₂ adsorption, were found to be 300 m²/g and 0.14 cm³/g, respectively. Through calorimetric and TEM studies, reported elsewhere, the initial surface area and volume of the closed pores are shown to be 550 ± 150 m²/g and 0.19 cm³/g, respectively. The samples were sintered in the presence of water at 823 K. The elimination of the closed pores and coarsening of the open pores dominate early stage sintering under the conditions studied. From the above observations, it is argued that the sintering behavior is governed primarily, not by diffusion, but by a surface step. It is postulated that the nature of the surface step is the formation of new ion layers by random fluctuations in closed pores and in smaller open pores for which edge effects are thermodynamically favorable.     

Change in Oxygen Content of Y2O3-Nd2O3-Doped Silicon Nitride During Firing

The oxygen content of silicon nitride with 1 mol% Y₂O₃—Nd₂O₃ additive was measured after firing to determine the compositional change during gas-pressure sintering. Oxygen content decreases from 2.5 to 0.94 wt% during firing for 4 h at 1900°C and 10-MPa pressure in N₂. This decrease in oxygen results from the release of SiO gas generated by a thermaldecomposition reaction between Si₃N₄ and SiO₂. The resultant sintered silicon nitride material contains less than 1 wt% oxygen.     

Effects of Oxidation Curing and Sintering Additives on the Formation of Polymer-Derived Near-Stoichiometric Silicon Carbide Fibers

The effects of oxygen pick-up and sintering additives on the formation of silicon carbide (SiC) fibers from polyaluminocarbosilane are studied. It has been found that the strict control of oxygen pick up during the oxidation curing is essential to produce near-stoichiometric SiC fibers. When the molar ratio of oxygen to excess carbon in the pyrolyzed fibers (SiC _x O _y ) is slightly over 1 (O/C_Excess= y /( x −1)>1), the excess carbon is eliminated during the subsequent sintering as CO and CO₂ as a result of the decomposition of SiC _x O _y ; the remaining oxygen is removed as SiO and CO vapor, leaving near-stoichiometric SiC as the residue. However, with still increasing oxygen pick up, the final ceramic fibers become more porous and rich in silicon. The evolution of CO, CO₂, and SiO generates high porosity in the absence of a sintering additive, leading to low fiber density. The inter-connected and open porosity favors the formation of CO. In contrast, for the fibers containing aluminum (Al) or Al/B sintering additives, the pores are much smaller and essentially closed, favoring the formation of CO₂. Therefore, after sintering at 1800°C, the fibers without sintering additives contain excess silicon, while those with sintering additives are near stoichiometric. Al is beneficial to the densification but it alone cannot produce fibers of high density. When B is added in addition to Al, the fibers can be sintered to nearly full density.     

Sintering of Si3N4-Y2O3 Under Nitrogen Pressure

This study shows that the amount ofAl₂O₃ needed to form high density Si₃N₄-15Y₂-O₃ samples can be reduced by using high surface area Si₃N₄ powder and high N₂ overpressure (high sintering temperatures) during the sintering process. The reduction in AI₂O₃ content results in improved oxidation resistance of the sintered samples.     

Effects of Rice Straw on the Color and Microstructure of Bizen, a Traditional Japanese Stoneware, as a Function of Oxygen Partial Pressure

The effects of oxygen partial pressure during thermal treatment on the color and microstructure of Bizen, a traditional Japanese stoneware, were studied through model experiments using clay pellets covered lightly with rice straw as a coloring assistant. When heated in flowing nitrogen, the model pellet turned blackish owing to the formation of α-Fe particles coated with graphite. However, schreibersite (Fe₃P), which is also blackish, was formed specifically on the pellet surface in direct contact with the straw. The rice straw seems to have generated a strongly reducing atmosphere, strong enough for the metallization to α-Fe, and also to have provided phosphorus through contact. When oxygen content in the surrounding gas atmosphere was raised to N₂/O₂=99/1, the pellet surface turned yellowish brown because the main coloring material was Fe³⁺-containing mullite. At oxygen contents of N₂/O₂=98/2 or more, the formation of hematite (α-Fe₂O₃) pushed the color to deep red.     

Sinterability and Decomposition of Pb0.9175La0.055Zr0.975Ti0.025O3: Influence of Calcination and Sintering Temperature

The sinterability and decomposition of PLZT, (Pb,La)(Zr,Ti)O₃, depend on the temperature and soaking time of both the calcination and sintering temperature. They were determined from the density, linear shrinkage, weight loss, and appearance of extra phases. At moderate calcination temperatures and times, there is no escape of PbO from the PLZT. At calcination temperatures higher than 1050°C and soaking times above 3 h, PbO escapes, and ZrO₂ and La₂Zr₂O₇ can be detected. Even when sintered in a PbO-rich atmosphere, some PbO evaporates during sintering either from free PbO or from the PbO bound in the PLZT in regions in the outer surfaces of the sintered body. An aggressive depletion of PbO during sintering can result in a complete disappearance of the grain boundary phase, giving an intragranular fracture.