Sintering of a nano-crystalline metastable alumina (original) (raw)
Related papers
International Journal of Applied Ceramic Technology, 2009
Deagglomeration of a nanocrystalline transition alumina performed using different techniques was first demonstrated to be active in the achievement of a better powder compaction ability under uniaxial pressing and consequently in the development of a highly dense and homogeneous microstructure during pressureless sintering. A major effect, however, was associated to the heating rate chosen during the densification cycle. In fact, the influence of different heating rates (10°C/min or 1°C/min) on phase and microstructural evolution during sintering was investigated in depth on the above best green bodies. A low-rate thermal cycle leads to a significant reduction of the α-Al2O3 crystallization temperature and promotes a more effective particle rearrangement during phase transformation. As a consequence, in the low-rate treated material, it was possible to avoid the development of a vermicular structure as usually expected during the densification of a transition alumina and to yield a more homogenously fired microstructure.
Effect of initial particle packing on the sintering of nanostructured transition alumina
Journal of The European Ceramic Society, 2008
The effect of forming method (cold isostatic pressing and slip casting) on particle packing and the consequent effects on densification, phase transformation and microstructural evolution were evaluated during sintering of a transition alumina powder (Nanotek ® , particle size of 47 nm, ␦ and ␥ phases). It is well known that the transformation of transition alumina towards the stable ␣ phase has a critical influence on the sintering behaviour. Therefore, correlation between microstructural evolution and shrinkage in compacts was established using dilatometry, Scanning Electron Microscopy and X-Ray Diffraction. By the two mentioned forming methods, green bodies with the same density were processed, in order to investigate only particle packing homogeneity and its effect on phase transition and sintering. For the same initial green density, the samples prepared by slip casting present a better homogeneity of particle packing, due to an optimal dispersion of particles in the slurry. This initial microstructure feature improves the particles rearrangement during the transition to ␣-alumina and hence enhances the transformation to the thermodynamic stable ␣-phase and the densification step.
Effect of particle packing on the sintering of transition alumina
The effect of forming (cold isostatic pressing and slip casting) on particle packing and the consequent effects on densification, phase transformation and microstructural evolution was evaluated during isothermal sintering of a transition alumina powder (Nanotek®, particle size of 47 nm, δ and γ phases). It is well known that the transformation of transition alumina towards the stable α phase has a critical influence on the sintering behaviour. Therefore, correlation between microstructural evolution and shrinkage in compacts was established using dilatometry, scanning electron microscopy and X-Ray diffraction. Green samples were obtained via pressing or slip casting. For the same initial green density, the samples prepared by slip casting present a more homogeneous pore distribution, and a better homogeneity of particle packing, due to an optimal dispersion of particles in the slurry. This initial microstructure feature improves the particles rearrangement during the transition to α-alumina and hence enhances the transformation to the thermodynamique stable α phase and the densification step. It leads to significant improvement in final material by modifying at the same time the microstructure evolution and the densification rate.
Densification of nanostructured alumina assisted by rapid nucleation of α-alumina
Materials Letters, 1998
We have shown that it is possible to achieve dense nanostructured alumina, with density q98% of the theoretical maximum at sintering temperature as low as 13008C without any help of additives or a-seeding. The drastic reduction in sintering temperature from 16008C to 13008C is possible by rapid nucleation of the stable a-alumina phase. It is achieved by introducing the compacted pellet of bayrite fine particles derived by the sol-gel process, in a furnace at 12508C, the temperature high enough for the rapid transformation to a-alumina. Since the nucleation rate is higher than the growth rate of particles, large and extensive pore network which usually develops during the sequential transformation to a-Al O could 2 3 be avoided. This newly developed cost effective simple process of sintering through rapid nucleation will help to fulfill the demand of highly sintered nanostructured alumina for various industrial applications.
Ceramics International, 2011
An ultra-fine alumina powder was doped with yttrium or zirconium chloride to produce Al 2 O 3 -5 vol.%ZrO 2 (AZ-5) and Al 2 O 3 -5 vol.%YAG (AY-5) composite powders. Composite samples and pure alumina, used as a reference, were submitted to dilatometric analyses up to 1500-1550 8C at 2, 5 and 10 8C/min for supplying the data required for the modeling of their sintering behaviour. The best fit for the three samples was obtained by applying an Avrami-Erofeev nucleation and growth model (An) and a subsequent power law reaction (AnFn). The evolution of the activation energy with densification is given for the three samples, as well as the prediction of their best sintering conditions. Finally, densification mechanism for the immiscible (AZ-5) and miscible (AY-5) systems are hypothesized and correlated with their final microstructures. #
Colloidal processing and sintering of nanosized transition aluminas
Powder Technology, 2005
The dispersion of nanosized gamma aluminas with high specific surfaces areas (100 m 2 /g) and primary particle sizes around 20 nm, using polyacrylic acid has been investigated. The effect of pH and polymer concentration showed that the highest density green bodies were produced using high polymer concentrations (6% wt) and pH of 6. Interparticle potential calculations have been made and help explain the underlying dispersion mechanism at least on a qualitative level. The dispersions were then used to slip cast green bodies followed by drying and sintering. The types of gamma alumina powder have been investigated, the pure gamma alumina, doped with MgO and also with the addition of alpha alumina seeds. The high degree of agglomeration of the gamma alumina powders led to very low densities (60%) even the alpha seeded alumina reached only 85 % theoretical density. Attrition milling with zirconia media improves both green density and sintered densities significantly with all powders showing sintered densities > 97%. Mictrostructural analysis on polished and etched surfaces show, however, that the grain sizes are well above 1 micron over 50 times greater than the initial gamma alumina primary particles. A two step sintering cycle was investigated with the Mg doped powder and average grain sizes around 580 nm were achieved.
Journal of the European Ceramic Society, 2012
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, Y 2 O 3 or ZrO 2 ) whose grain growth retarding effect in conventional sintering has been well documented, on twostage 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.
Ultra-fine grained Al 2 O 3 was fabricated by in-situ spark plasma sintering (SPS) process directly from amorphous powders. During in-situ sintering, phase transformation from amorphous to stable α-phase was completed by 1100 1C. High relative density over 99% of in-situ sintered Al 2 O 3 was obtained in the sintering condition of 1400 1C under 65 MPa pressure without holding time. The grain size of in-situ sintered Al 2 O 3 body was much finer ($ 400 nm) than that of Al 2 O 3 sintered from the crystalline α-Al 2 O 3 powders. For in-situ sintered Al 2 O 3 from amorphous powders, we observed a characteristic microstructural feature of highly elongated grains in the ultra-fine grained matrix due to abnormal grain growth. Moreover, the properties of abnormally grown grains were controllable. Fracture toughness of in-situ sintered Al 2 O 3 with the elongated grains was significantly enhanced due to the self-reinforcing effect via the crack deflection and bridging phenomena.