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 Sintering Atmosphere on Isolated Pores During the Liquid-Phase Sintering of MgO-CaMgSiO4

The shrinkage behavior of isolated pores during the liquidphase sintering of MgO-CaMgSiO₄ at 1650°C in O₂ and N₂ atmospheres has been studied. When 90MgO- 10Ca MgSiO₄ specimens containing artificially produced large spherical pores are sintered in O₂, the liquid and grains flow into the pores as oxygen diffuses out. When sintered in N₂ the pores remain intact even after a long time, because the N₂ gas entrapped in them does not diffuse out. The effect of the sintering atmosphere has also been studied in a fine powder mixture of 80MgO 20CaMgSiO₄ composition. Changing the atmosphere from O₂ to N₂ during the sintering treatment reduces the porosity, probably because of the enhanced oxygen diffusion from the pores. The pores grow when the sintering atmosphere is changed from N₂ to O₂, probably because of oxygen diffusion into the pores from the specimen surface. The practical implication of these results is that changing the atmosphere from O₂ to air during the liquid-phase sintering of oxide ceramics can greatly reduce the porosity.     

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.     

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.     

Interfacial Reaction between Nickel Oxide and Lanthanum Gallate during Sintering and its Effect on Conductivity

The results of interdiffusion and reaction between NiO and strontium- and magnesium-doped lanthanum gallate (LSGM) during sintering were investigated. Electron microprobe analysis showed that the NiO content of LSGM sintered 4 h at 1350°C was slightly >2 wt% within 5 µm of the interface, which is equivalent to 7 mol% nickel on B cation sites. This concentration decreased rapidly to <0.25% at a distance of 15 µm. The interfacial reaction between NiO and LSGM, however, led to the formation of a LaSrGa(Ni)O_4-delta-type phase. A separate experiment involving conductivity measurements of nickel-doped LSGM indicated that the oxygenion conductivity did not decrease significantly if the nickel content was <5%-8%. At higher nickel contents, the conductivity increased in air because of hole conduction and markedly decreased under conditions of low oxygen pressure because of the formation of the LaSrGa(Ni)O_4-delta-type phase. The nickel ions in the LSGM electrolyte and in the reaction product, LaSrGa(Ni)O_4-delta, were stable as Ni²⁺ at reduced oxygen pressures and, therefore, did not cause electronic shorting.     

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.     

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.     

Titania as a Sintering Additive in Indium Oxide Ceramics

The influence of TiO₂ additives on the sintering behavior of In₂O₃ ceramics has been investigated. TiO₂ increases the densification rate, decreases the grain growth during the intermediate stage of sintering, and hinders the pore/boundary breakaway that can affect the final stage of sintering. For a given grain size, TiO₂ shifts the grain size/density trajectory toward higher densities. TiO₂ mainly acts by a second-phase mechanism, but it also may decrease the decomposition rate of In₂O₃.     

Sintering of Translucent Alumina in a Nitrogen–Hydrogen Gas Atmosphere

The sintering of alumina doped with magnesia-based additives to translucency previously has been possible only in atmospheres of hydrogen or oxygen or under vacuum. This paper reports on the sintering of alumina in N₂–H₂ atmospheres that contain as little as 2% H₂, where optical transparency that is equivalent to that of normal H₂ sintering is achieved. In an effort to explain these results, experiments that vary the content of H₂ and H₂O, as well as the changes in the dynamic atmosphere, have been conducted. The evidence indicates that the final-stage sintering involves a point-defect pair mechanism that includes hydrogen interstitials and nitrogen solutes.     

Effect of Oxygen-Gas Concentration in the Heat-Treatment Atmosphere and High-Voltage Pulse Application on the Electrical Properties of Carbon Fibers

Semiconductive carbon fibers suitable for infrared-ray sensor materials were prepared by final heat treatments at 600°-700°C in an atmosphere with a controlled oxygen-gas (O₂) concentration, and the influence of O₂ concentration on the electrical properties and surface composition/micro - structure was investigated. Resistivities at 20°C were almost equal and independent of the O₂ concentration of heat treatment in all cases, but the thermistor constant decreased as the O₂ concentration increased. Carbon fibers heat-treated in O₂-mixed nitrogen gas (N₂) exhibited higher surface oxygen concentrations than did those treated in pure N₂, and surface microstructures differed between the two fibers.     

Master Sintering Curve: A Practical Approach to Sintering

One of the ultimate objectives for sintering studies is to be able to predict densiflcation results under different thermal histories for a given processing method. It has been reported that the geometric parameters related to sintering often are functions only of density for a given powder and green-body process, provided that one diffusion mechanism dominates in the sintering process. Based on this report, the concept of a master sintering curve has been developed that characterizes the sintering behavior for a given powder and green-body process regardless of the heating profiles. The formulation and construction of the master sintering curve are given in this paper. A model experiment on sintering of alumina is used and analyzed to demonstrate this new concept. Examples of the master sintering curves obtained from other powder systems (ZnO, nickel, A1₂O₃(5 vol% TiO₂), and A1₂O₃(5 vol% ZrO₂)) are presented. When this new method is used, densification behavior can be predicted under arbitrary temperature-time excursions following a minimal set of preliminary experiments, and these predictions can be used in planning sintering strategies. Moreover, deviations from the assumption of a single mechanism can be observed readily.     

Enhanced Properties of Tin(IV) Oxide Based Materials by Field-Activated Sintering

The densification of SnO₂ (0.9 mol)–Sb₂O₃ (0.1 mol) solid solution without any additives was studied by conventional and field-activated sintering technique (FAST). FAST sintering achieved a relative density value of 92.4% at 1163 K for 10 min versus 61.3% in conventional sintering at 1273 K for 3 h. An abnormal reduction of the IR transmittance and a semiconductor defect structure with only one donor level in the SnO₂ energy gap were noticed in the FAST-sintered as compared with the conventionally sintered Sn_0.82Sb_0.18O₂ solid solution. A high charge carrier concentration (i.e., electronic conduction) was shown in the FAST-sintered sample by conductivity measurements and the negative values of the Seebeck coefficient.     

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.     

Pressureless Sintering of Dense Si3N4 and Si3N4/SiC Composites with Nitrate Additives

The effect of aluminum and yttrium nitrate additives on the densification of monolithic Si₃N₄ and a Si₃N₄/SiC composite by pressureless sintering was compared with that of oxide additives. The surfaces of Si₃N₄ particles milled with aluminum and yttrium nitrates, which were added as methanol solutions, were coated with a different layer containing Al and Y from that of Si₃N₄ particles milled with oxide additives. Monolithic Si₃N₄ could be sintered to 94% of theoretical density (TD) at 1500°C with nitrate additives. The sintering temperature was about 100°C lower than the case with oxide additives. After pressureless sintering at 1750°C for 2 h in N₂, the bulk density of a Si₃N₄/20 wt% SiC composite reached 95% TD with nitrate additives.     

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.     

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.     

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.