Constant Heating Rate Sintering Research Articles

Time-resolved powder neutron diffraction study of the phase transformation sequence of kaolinite to mullite

Abstract Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370 °C. Neutron diffractometry under this non-isothermal conditions is suitable for studying high-temperature reaction kinetics and to identify short-lived species which otherwise might escape detection. Data collected from dynamic techniques (neutron diffraction, DTA, TGA and constant-heating rate sintering) were consistent with data gathered in static mode (conventional X-ray diffraction and TEM). The full process occurs in successive stages: (a) kaolinite dehydroxylation yielding metakaolinite in the ∼400–650 °C temperature range, (b) nucleation of mullite in the temperature range ∼980–992 to ∼1121 °C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the ∼983–1030 °C temperature interval; (c) growth of mullite crystals from ∼1136 °C, (d) high (or β) cristobalite crystallization at T > ∼1200 °C and (e) secondary mullite crystallization at T > ∼1300 °C. The calculated activation energy for the kaolinite dehydration was 115 kJ/mol; for the mullite nucleation was 278 kJ/mol and for the growth of mullite process was 87 kJ/mol; finally for cristobalite nucleation the calculated apparent activation energy was 481 kJ/mol.

A novel stearate melting method for synthesizing highly reactive YAG nanopowders

Abstract Ultrafine YAG powders were synthesized by a novel stearate melting method, in which yttrium stearate and aluminum tristearate, having similar physical and chemical properties, were co-melted and then calcined to produce fine YAG nanopowders. This method has the advantages of precise control of Y:Al ratio, homogeneous mixing of cations at atomic level, fine particle size, and good particle dispersion. The formation mechanism of the precursor and the YAG nanopowder was studied by means of XRD, FT-IR, TG–DTA, BET and FE-SEM. Pure YAG nanopowder can be obtained by calcining the co-melted precursor at a relatively low temperature (750 °C), much lower than those of the traditional solid-state reaction method and various wet chemical synthesis methods. The resultant YAG powders are well dispersed and have excellent sinterability. For the YAG powder calcined at 1000 °C, the green compact has the maximum shrinkage rate at about 1450–1550 °C and a total shrinkage of ∼16.70% during constant heating rate sintering. The compact can be sintered to 99.4% of the theoretical density at 1600 °C. The prepared YAG powder can be sintered into transparent ceramics at 1700 °C for 5 h by vacuum sintering.

Apparent activation energy for densification of α-Al2O3 powder at constant heating-rate sintering

The apparent activation energy for densification is a characteristic quantity that elucidates the fundamental diffusion mechanisms during the sintering process. Based on the Arrhenius theory, the activation energy for densification of α-Al2O3 at constant heating-rates sintering has been estimated. Sintering of α-Al2O3 powder has been executed by the way of a push rod type dilatometer. It is shown that the apparent activation energy does not have a single value but depends directly on the relative density. The apparent activation energy corresponding to lower relative density was higher than that corresponding to higher relative density. In addition, the value of the evaluated activation energy is different at the same density level when the Arrhenius plot involves different heating rates.

Prediction of densification during low heating rate sintering of microcrystalline alumina ceramics based on master sintering curve theory

The master sintering curve (MSC) approach is useful for analysing the shrinkage behaviour of ceramics. In this study, the shrinkage behaviour of α-Al2O3 of mean particle size 2·5 μm during constant heating rate sintering was evaluated. An MSC has been constructed using dilatometry data containing lower heating rates only with the help of a combined stage sintering model. The validity of the MSC has been verified from experimental data. A comparison between the predicted and the Archimedes densities showed good agreement, suggesting that it is possible to control shrinkage behaviour during processing using the MSC. The concept of the MSC has been used to evaluate the apparent activation energy for the above powder; a high value of 1148 kJ mol−1 was obtained.

Manufacturing of NiO/NiTiO 3 porous substrates and the role of zirconia impurities during sintering

Anode substrates based on Ni/TiO 2 cermets were fabricated for planar solid oxide fuel cells (SOFC) with the aim of improving the thermoelastic behavior of Ni/8 mol.% Y 2O 3-stabilized zirconia (8YSZ) substrates. This offers the possibility of adjusting the thermal expansion coefficient to any electrolyte membrane considered. NiO/NiTiO 3 specimens were fabricated from Coat-Mix® powders by rate-controlled and constant heating rate sintering. The heat treatments led to a significant change in activation energy and, especially at high sintering temperatures, in the morphology of the microstructures compared with NiO/8YSZ substrates. A TEM investigation revealed droplet-shaped impurities at the grain boundaries, which are regarded as the reason for liquid phase sintering of these substrates.

Initial Coarsening and Microstructural Evolution of Fast‐Fired and MgO‐Doped Al2O3

The effect of an initial coarsening step (50‐200 h at 800°C) on the subsequent densification and microstructural evolution of high–quality compacts of undoped and MgO–doped Al2O3 has been investigated during fast–firing (5 min at 1750°C) and during constant–heating–rate sintering (4°C/min to 1450°C). In constant–heating–rate sintering of both the undoped and MgO–doped Al2O3, a refinement of the microstructure has been achieved for the compact subjected to the coarsening step. A combination of the coarsening step and MgO doping produces the most significant refinement of the microstructure. In fast–firing of the MgO–doped Al2O3, the coarsening step produces a measurable increase in the density and a small refinement of the grain size, when compared with similar compacts fast–fired conventionally (i.e., without the coarsening step). This result indicates that the accepted view of the deleterious role of coarsening in the sintering of real powder compacts must be reexamined. Although extensive coarsening after the onset of densification must be reduced for the achievement of high density, limited coarsening prior to densification is beneficial for subsequent sintering.

Densification and crystallization of glass powder compacts during constant heating rate sintering

Heating microscopy was used to study the interaction between the processes of densification and crystallization of glass powder compacts under constant heating rate sintering conditions without the application of external loads. For barium magnesium aluminosilicate (BMAS) glass powder compacts sintered between 800–1100 °C, it has been shown that the relative rates of crystallization and densification can be controlled by changing the heating rate. Samples sintered at a high heating rate of 15 K min −1 could be fully densified in the amorphous state, delaying the onset of crystallization. In the samples sintered at a low heating rate of 1 K min −1, the onset of crystallization coincided with the termination of densification at ∼ 1000 °C. Since the experiments were performed without an application of external loads, the results are applicable for the manufacturing of dense BMAS glass-ceramics via a pressureless sintering route.

Microstructural development of ZnO varistor during reactive liquid phase sintering

The microstructural evolution, grain growth and densification for the varistor systems ZnO-Bi2O3 (ZB), ZnO-Bi2O3-Sb2O3 (ZBS), ZnO-Bi2O3-Sb2O3-MnO-Cr2O3-CoO (ZBSCCM) were studied using constant heating rate sintering, scanning electron microscopy (SEM) andin situ phase formation measurement by high temperature X-ray diffraction (HT-XRD). The results showed that the densifying process is controlled by the formation and decomposition of the Zn2Bi3Sb3O14 pyrochlore (PY) phase for the ZBS and ZBSCCM systems. The addition of transition metals (ZBSCCM system) alters the formation and decomposition reaction temperatures of the pyrochlore phase and the morphology of the Zn7Sb2O12 spinel phase. Thus, the spinel grains act as inclusions and decrease the ZnO grain growth rate. Spinel grain growth kinetics in the ZBSCCM system showed ann value of 2.6, and SEM and HT-XRD results indicate two grain growth mechanisms based on coalescence and Ostwald ripening.

Modeling of pressureless constant heating rate sintering of ceramic matrix composites

A viscoelastic finite-element model is developed for analyzing the pressureless constant-heating-rate sintering of particulate-or whisker-reinforced ceramic-matrix composites. The sintering potential is assumed to be a function of heating rate, temperature and relative density and is obtained from experimental data on the densification of an identical unconstrained (unreinforced) matrix sample. The reinforcement is modelled as a spherical or ellipsoidal unsinterable inclusion and the matrix as a compressible viscoelastic material. The results show that inclusions result in reduced sinterability and cause a heterogeneous density distribution, the heterogeneity of density, mechanical properties and residual stresses being affected by the shape of the inclusions. The initial homogeneity (or lack of it) of the green compact has been also shown to affect the final density.

Theoretical model for the analysis of a ceramic thin film sintering on a non-sintering substrate

A viscoelastic finite element model is developed to simulate the constant heating rate sintering of a ceramic thin film on a non-sintering substrate. Compared with free sintering, the constrained sintering exhibits much lower densification rates. Away from the film edge, the coating is uniformly stressed and the density and other mechanical properties tend to be homogeneously distributed. The highest density is achieved at the free corner, which also has the highest elastic modulus. In contrast, the constrained corner of the film suffers the highest thermal residual stress and has the lowest density, which indicates that the edge of the coating is most likely to debond from the substrate.