The Effects of Na2O and SiO2 on Liquid Phase Sintering of Bayer Al2O3

To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na₂O and 603 ppm SiO₂ and heated at 1525°C up to 8 h. Na₂O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al₂O₃, Na₂O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO₂ concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20?400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na₂O/SiO₂ ratios, densification of Bayer Al₂O₃ at 1525°C is controlled by diffusion of Al³⁺ through the grain‐boundary liquid, whereas for high Na₂O/SiO₂ ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al₂O₃ or diffusion of Al³⁺. Increasing Na₂O in SiO₂‐doped samples increases diffusion of Al³⁺ and Al₂O₃ solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al₂O₃ densification can be manipulated by adjusting the Na₂O to SiO₂ ratio.     

Rationalizing the role of magnesia and titania on sintering of α-alumina

The rationalization of selection of sintering additives for α-alumina was investigated using two oxides (MgO and TiO₂) to discern their individual roles. Using both dynamic heating study in a thermomechanical analyzer and static heat treatment, the precise role of each oxide was established. Grain growth trajectory of different doped samples sintered at 1700 °C revealed that MgO neither significantly affected densification nor facilitated grain growth upto 1700 °C. MgO reacted with alumina to form spinel prior to the densification process. Thus it could not generate further extrinsic defects in corundum lattice during sintering, which usually facilitate densification. In contrast, TiO₂ significantly enhanced the densification and promoted grain growth in α-alumina. At 1700 °C, the average grain size of titania doped samples were 7.7x larger than undoped ones and 10x larger than magnesia dopes samples. The sintered grains developed higher aspect ratio when TiO₂ was used which may be ascribed to preferred growth of the 012 and 024 planes of corundum. The nearly perfect junction of grain boundaries meeting at ~120° indicates absence of liquid phase and that the entire sintering process most probably took place in solid state for both MgO and TiO₂ doped samples.     

Low-temperature sintering of Al2O3 ceramics doped with 4CuO-TiO2-2Nb2O5 composite oxide sintering aid

Dense alumina ceramics doped with 5 wt% 4CuO-TiO₂-2Nb₂O₅ composite sintering aids were obtained at low sintering temperatures of 950∼975 °C. The ceramic sintered at optimal condition shows good microwave dielectric properties (ε_r = 12.7, Q × f = 7400 GHz), high thermal conductivity (18.4 W/m K) and high bending strength (320 MPa). TEM and EDS analysis revealed that amorphous Cu-Ti-Nb-O interfacial films with nanometer thickness formed at the grain boundaries, which could provide paths of mass transportation for densification. Al³⁺ ions may be involved in mass transportation through substitution by Ti³⁺ and Ti⁴⁺ ions near the grain boundary during the sintering process. The accumulation of copper ions at the trigeminal grain boundary was observed. The migration and reaction of copper ions in grain boundaries may also play an important role in promoting mass transportation and low-temperature densification of alumina ceramics.     

The influence of additives on microstrucutre of sub-micron alumina ceramics prepared by two-stage sintering

For various systems two-stage sintering has been reported as a successful way of suppressing the grain growth in the final stage of densification of polycrystalline ceramics. Our previous results on two-stage sintering of high purity submicrometre polycrystalline alumina indicate limited efficiency of the process with respect to suppression of grain growth. The present work deals with the influence of deliberate additions of various metal oxides (500 ppm of MgO, Y₂O₃ or ZrO₂) whose grain growth retarding effect in conventional sintering has been well documented, on two-stage sintering of submicrometre alumina ceramics. The addition of MgO was observed to enhance densification. Addition of yttria and zirconia impaired densification, but addition of all three dopants resulted in suppression of the grain growth and microstructure refinement in comparison to undoped alumina.     

Si segregation and its role in reaching transparent YAG

It is well‐known that doping YAG with Si dramatically affects densification and grain growth, and as a result Si is commonly added to YAG as a sintering additive to achieve full density and transparency. In recent studies, the influence of Si was explored, but segregation of Si to grain boundaries in YAG was not detected. The present article contradicts previous findings by revealing an excess of Si at grain boundaries in YAG. The findings were corroborated using atom probe tomography and energy‐dispersive spectroscopy in a scanning transmission electron microscope. To the best of the author's knowledge, this is the first time Si segregation was discovered in SiO₂‐doped YAG, and the first time a dopant concentration profile at grain boundaries in a ceramic material was characterized by atom probe tomography. Finally, a change is proposed for the method to calculate grain‐boundary mobility in the presence of a solute (Lucke and Detert), showing a good correspondence to the experimental results. These results can change the view on solute distribution in ceramic grain boundaries from theoretical and characterization aspects.     

Effect of silica doping on the densification and grain growth in zinc oxide

The ability of silica (SiO₂) in controlling the densification and grain growth behavior of nano crystalline zinc oxide (ZnO) has been systematically studied. It has been observed that SiO₂ acts as a sintering inhibitor in the ZnO–SiO₂ system up to 4 wt.% limiting value beyond which densification behavior of the system remains almost unchanged, especially above 1100 °C. The addition of SiO₂ to ZnO retards grain growth which in turn results a finer ultimate grain size as compared to the undoped ZnO. However, stabilization in grain size occurs at ≥4 wt.% SiO₂ addition. It has been observed that SiO₂ incorporation changes the grain growth mechanism up to 4 wt.% addition, beyond which no remarkable changes was noticed. The grain growth (n) shows distinctly different slopes as a function of sintering time for the SiO₂ doped ZnO systems than undoped ZnO. The different slopes tend to indicate that different diffusion mechanisms and probably the formation of a secondary phase (Zn–Si–O) at the grain boundary control the densification and grain growth. The thermal expansion coefficient of the system has been found to decrease substantially beyond 4 wt.% SiO₂ addition to ZnO.     

Comparison of the Microwave and Conventional Sintering of Alumina: Effect of MgO Doping and Particle Size

The effects of MgO doping and specific surface area of powder on microwave sintering behavior of α‐Al₂O₃ were investigated. A comparative study was simultaneously achieved in conventional and microwave heating with an identical thermal process. The experimental results show that both MgO and particle size have significant influence on microwave enhancement in the densification of the alumina samples. It is found that an amount of MgO surrounding the solubility limit in Al₂O₃ or leading to second phase precipitation of MgAl₂O₄ spinel induces more significant microwave enhancement. A significant microwave gain in densification is also observed while powder has a high specific surface area. These results indicate that the enhancements during microwave sintering processes are associated with the formation of lattice defect and with the increase in concentration of grain‐boundary region.     

Effects of Ca-, Mg- and Si-doping on microstructures of alumina–zirconia composites

The aim of this work was to obtain zirconia toughened alumina composites with different microstructures, using a simple process (powder mixing and natural sintering). Adjusting the amount of zirconia directly controls the size and localization of zirconia grains and the size of the alumina grains. Doping the composite with CaO, MgO and SiO₂ allows further control of the microstructures. The influence of the thermal treatments is also investigated. The composites exhibit different structures (nano/nano-, micro/nano- and micro-composites) with zirconia and alumina grains as small as 100 and 200 nm, respectively, and with the proportion of intragranular zirconia grains varying between 0% and 90%. Zirconia plays a major role on grain size distributions as compared to CaO and MgO, whose role is almost negligible. The use of SiO₂ leads to micro/nano composites with intragranular zirconia particles. The influence of these different additions is related to adjustments of the grain boundaries mobility.     

Effects of Magnesium Oxide on Grain-Boundary Segregation of Calcium During Sintering of Alumina

The advantage of certain amounts of MgO addition in alumina sintering has been realized, and it is common practice. In an attempt to understand the role of MgO in the presence of CaO in commercial-grade alumina, grain-boundary segregation of Ca was investigated by scanning Auger electron microprobe (SAM) using an ultrapure alumina after controlled doping of CaO and/or MgO. The commercial-grade alumina, which usually contains a small amount of CaO, is difficult to sinter to high density. The pure alumina composition (<99.999%) gives "clean" boundaries when it is sintered under "clean" conditions. As the powder was doped with 100 ppm of CaO and sintered at 1300° to 1500°C, all of the grain boundaries were enriched by Ca as observed by others. However, it was also discovered that some of the grain boundaries are enriched by an exceptionally high concentration of Ca. Such a large variation of Ca contents depending on the grain-boundary facets disappeared when samples were codoped with small amounts of MgO. The results suggest that MgO plays a beneficial role in controlling the anisotropic segregation of Ca to various interfaces including grain boundaries and pore surfaces during sintering of alumina. MgO thus enhances chemical homogeneity of commercial-grade alumina.     

Sintering and microstructural study of mullite prepared from kaolinite and reactive alumina: Effect of MgO and TiO2

Mullite ceramic was prepared using kaolinite and synthesized alumina (combustion route) by solid-state interaction process. The influence of TiO₂ and MgO additives in phase formation, microstructural evolution, densification, and mechanical strengthening was evaluated in this work. TiO₂ and MgO were used as sintering additives. According to the stoichiometric composition of mullite (3Al₂O₃·2SiO₂), the raw materials, ie kaolinite, synthesized alumina, and different wt% of additives were wet mixed, dried, and uniaxially pressed followed by sintering at different temperature. 1600°C sintered samples from each batch exhibit enhanced properties. The 1 wt% TiO₂ addition shows bulk density up to 2.96 g/cm³ with a maximum strength of 156.3 MPa. The addition of MgO up to 1 wt% favored the growth of mullite by obtaining a density and strength matching with the batch containing 1 wt% TiO₂. These additives have shown a positive effect on mullite phase formation by reducing the temperature for complete mullitization by 100°C. Both additives promote sintering by liquid phase formation. However, the grain growth, compact microstructure, and larger elongated mullite crystals in MgO containing batch enhance its hardness properties.     

Nanoscale Microstructural and Chemical Analysis of SiO2–Zn1−xAlxO Nanocomposites: Towards a Better Understanding of Si and Al Substitution in ZnO

In this study, we report on the microstructure of SiO₂‐coated Al‐doped ZnO nanoparticles densified by spark plasma sintering(SPS), using a multiscale approach. Our observations show that it is possible to successfully prepare dense pellets while keeping the nanostructure with well‐defined Si‐rich grain boundaries. Although a very limited partial solubility of Si in the ZnO matrix has been observed, Si is mostly concentrated at the grain boundaries. More surprisingly, we evidenced some areas with nanoscale inhomogeneity of the Al concentration, which can locally strongly exceed the average composition of the matrix. It could explain the apparent discrepancy observed in the literature between the simultaneous presence of ZnAl₂O₄ in Al‐doped ZnO, which should be the signature of the doping level exceeding the solubility limit, and the concentration of carriers that still depends on the nominal Al concentration in ZnO even in the presence of ZnAl₂O₄.     

Influence of sintering additives on densification kinetics and high‐temperature strength in γ‐Y2Si2O7 ceramics

Pressureless sintering of pure γ‐Y₂Si₂O₇ powders that had been synthesized by a solid‐liquid reaction method using Y₂O₃ and SiO₂ powders with Li₂O, MgO, and Al₂O₃ additives was reported. The sintering kinetics of γ‐Y₂Si₂O₇ powders was analyzed to track details of densification evolution. Apparent activation energies of the densification of γ‐Y₂Si₂O₇ powders were reported for the first time, which was 57.1, 96.6, and 100.2 kJ/mol for the powders with Li₂O, MgO, and Al₂O₃ additives, respectively, indicating that Li₂O could promote the densification behavior effectively. The flexural strengths as a function of temperature for the γ‐Y₂Si₂O₇ ceramics with different additives were also investigated. The degradation of high‐temperature flexural strength was mainly ascribed to the softening of grain‐boundary glassy phase. γ‐Y₂Si₂O₇ specimens fabricated using the powders with MgO or Al₂O₃ additives exhibited better high‐temperature mechanical properties.     

On the modeling of the co2‐catalyzed sintering of calcium oxide

A comprehensive mathematical model for the CO₂‐catalyzed sintering of CaO is proposed. It takes into account the mechanisms of surface diffusion and grain boundary diffusion, catalyzed by CO₂ chemisorption and dissolution, respectively. In addition, the model proposed here considers the change in pore size distribution during sintering, grain growth, and the densification by lattice diffusion, which is the intrinsic sintering mechanism of the CaO. Model predictions are validated using experimental data on the sintering of two CaO samples, one of them derived from pure CaCO₃ and the other from limestone. It is found that impurities in limestone‐derived CaO do not significantly affect the CO₂ dissolution or chemisorption processes; however, they strongly increase the rate of sintering by lattice diffusion. It is also established that low temperatures and CO₂ partial pressures promote the coarsening by surface diffusion, whereas high temperatures and CO₂ partial pressures favor densification. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3286–3296, 2017     

Microstructure Development of PZT Ceramics by Doping with Small Amounts of Al2O3, SiO2, and Fe2O3

Lead zirconate titanate (PZT) ceramics are used in a wide range of applications as sensors and actuators. Typically, they are formulated by the mixed oxide route, using several mixing and milling steps. Due to wear, these processes introduce impurities into the ceramic mass, which on their part can strongly influence densification behaviour and final properties of the PZT. In this study, the effect of such impurities, such as iron oxide, alumina, and silica, on the sintering behaviour and microstructure development of PZT ceramic is evaluated. A commercial Nb‐doped PZT powder was used and doped by adding Al₂O₃, SiO₂, and Fe₂O₃ with an amount of up to 0.001 mol%. Bulk samples were prepared and sintered in air. The mass loss, density, and grain size were correlated by regression analysis using the doping elements and levels, respectively. Due to the complex interactions between the oxides and the ceramic properties, the experiments were performed with the design‐of‐experiment method (DoE). The results showed a significant influence of these low amounts of doping levels on the microstructure development. Moreover, it was shown that doping after calcination affects the microstructure in a similar way to doping before calcination. Thus, a possibility to compensate concentration variations in the calcined ceramic mass is demonstrated, to homogenize the chemical composition and the final microstructure of the sintered PZT ceramic in the manufacturing processes.     

Enhancement of reaction-sintering of alumina-excess magnesium aluminate spinel in presence of titania

Alumina-excess magnesium aluminate spinel finds use in different high temperature applications including steel ladles. Alumina-excess spinel was prepared by solid oxide reaction using magnesia (MgO=10?wt%) and calcined alumina (Al₂O₃ = 90?wt%), in the sintering temperature range of 1500–1700?°C. The role of titania on the densification, spinelisation, evolution of microstructure and phase assemblage was investigated in this MgO-Al₂O₃ system. Titania addition increased the rate of densification 20x compared to undoped composition at 1500?°C under dynamic heating condition. However, under static firing, the beneficial effect of titania on densification could only be discerned at lower temperatures. The microstructure of titania doped sintered alumina-excess spinel compacts contain magnesium aluminium titanate phase in the grain boundary of corundum and spinel grains. The beneficial effect of titania on densification is attributed to magnesium aluminium titanate phase (Mg_xAl_2(1-x)Ti_(1+x)O₅) development and also by incorporation of Ti⁴⁺ into the spinel structure.     

The influence of MgO,Y2O3 and ZrO2 additions on densification and grain growth of submicrometre alumina sintered by SPS and HIP

Spark plasma sintering followed by hot isostatic pressing was applied for preparation of polycrystalline alumina with submicron grain size. The effect of additives known to influence both densification and grain growth of alumina, such as MgO, ZrO₂ and Y₂O₃ on microstructure development was studied. In the reference undoped alumina the SPS resulted in some microstructure refinement in comparison to conventionally sintered materials. Relative density >99% was achieved at temperatures >1200 °C, but high temperatures led to rapid grain growth. Addition of 500 ppm of MgO, ZrO₂ and Y₂O₃ led, under the same sintering conditions, to microstructure refinement, but inhibited densification. Doped materials with mean grain size <400 nm were prepared, but the relative density did not exceed 97.9%. Subsequent hot isostatic pressing (HIP) at 1200 and 1250 °C led to quick attainment of full density followed by rapid grain growth. The temperature of 1250 °C was required for complete densification of Y₂O₃ and ZrO₂-doped polycrystalline alumina by HIP (relative density >99.8%), and resulted in fully dense opaque materials with mean grain size<500 nm.     

Cation grain‐boundary diffusivity in SiO2‐ and MgO‐doped Al2O3

Cation grain‐boundary diffusion in undoped and aliovalent‐doped Al₂O₃ is characterized using Cr₂O₃ as a chemical tracer. The compositional depth profiles measured by secondary ion mass spectrometry are fitted to the Whipple‐LeClaire model. The results indicate that cation grain‐boundary diffusivity is insensitive to MgO and SiO₂ dopants between 1100°C and 1300°C.     

Effect of niobium doping on the densification and grain growth in alumina

The effect of niobium doping on the densification and grain growth of nano-sized α-Al₂O₃ powders during sintering has been investigated. The dopant concentration added ranged from 0.1 to 0.5 mol%. It was observed that addition of niobium oxide could improve the densification of the pure alumina with a lower sintering temperature, a shorter sintering time. The effect is strengthened by increasing the amount of dopant. It also demonstrated that niobium dopant significantly promotes the grain growth of alumina during sintering and the grain size of alumina increases with increasing the amount of dopant in the added range.     

Segregation-controlled densification and grain growth in rare earth-doped Y2O3

Cation doping of Y₂O₃ is an established approach for tailoring densification and grain growth during sintering. However, the segregation of doped cations to the grain boundary and their impact on processing are still not completely understood. Segregation can be driven by electrostatic effects due to charge mismatch with the host lattice or elastic effects induced by ion size mismatch. While segregation is caused by thermodynamics, it impacts diffusion and the kinetics of grain boundaries during densification and microstructure evolution. In this study, we utilize two isovalent dopants (La³⁺ and Gd³⁺), that is we focus on the elastic component of segregation. We investigate the densification as well as the grain growth kinetics of both doped and undoped Y₂O₃ during field-assisted sintering/spark plasma sintering (FAST/SPS). While Gd³⁺ is showing no significant effect on densification, La³⁺ resulted in a strongly reduced sintering activity. Furthermore, the analysis of the grain growth behavior during sintering and on predensified samples revealed a decrease in the grain growth coefficient, with La³⁺ having the strongest impact. The structure and chemistry at the grain boundary were observed by aberration-corrected TEM. While no structural change was caused by doping, the chemical analysis showed a strong segregation of La³⁺ to the grain boundary, which could not be observed for Gd³⁺. The results indicate that segregated La³⁺ causes a drastic decrease in grain boundary migration rates through solute drag as well as much slower sintering kinetics, likely caused by a decrease in the grain boundary self-diffusion due to segregation. This study further underlines the importance of the elastic contribution to cation segregation and establishes a clear relationship to grain growth and sintering kinetics, which are both decreased by segregation.     

Tape casting and characterizations of MgO ceramics

We report a high density MgO ceramic substrate produced by the tape casting technology. The tape casting formulation and process produced a uniform tape free of cracking. Y₂O₃ and SiO₂ were used as the sintering aid for the pressureless sintering of the green tape. X-ray diffraction phase identification indicates that MgO is the main phase, while both Y₂O₃ and SiO₂ sintering aids react with MgO to form MgY₄Si₃O₁₃ as the second phase. Liquid phase sintering occurs in the temperature range from 1030°C to ~1500°C, which is confirmed by the simultaneous Thermal Gravitation Analysis/Differential Scanning Calorimeter (TGA/DSC) and the percent linear shrinkage and densification. A 96.5% theoretical density was achieved by presureless sintering at 1650°C for 2 hours, which was further increased to a fully dense structure using hot-isostatic-pressing(HIP) at 1650°C and 207 MPa in argon. Scanning electron microscopy (SEM) and energy dispersive(EDS) spectroscopic analysis on the HIP’ed sample show that MgY₄Si₃O₁₃is located at the MgO grain boundary and the sample has a fully dense structure. The refractive indices and extinction coefficient were measured on the HIP’ed sample along with thermal properties and dielectric properties. Thermal diffusivity and heat capacity were measured to calculate the thermal conductivity.