Successful Strategies for Aqueous Colloidal Shapeforming of Nitride- Based Ceramics


Aqueous colloidal processing has been pursued in our lab along the most recent years as an alternative to alcoholic or other flammable and costly dispersion media. The advantages of aqueous processing are the healthier and more environmentally friend production at lower and more competitive costs, which enables to increase and diversify the applications for the nitride-based ceramics. However, nitride powders are susceptible to hydrolysis, what is particularly true in the case of aluminium nitride (AIN). Therefore, for a successful aqueous processing one must overcome the hydrolysis of powders' surface that degrade the nitrides by forming hydroxides and releasing ammonia gas bubbles in the suspension and increase the pH of the dispersing media. The gas bubbles trapped in the suspension and in the green bodies act like strength-degradation flaw populations, reducing the density and the general properties of the ultimate products. Other consequences of hydrolysis reactions include an increase of pH and the destabilization of the suspensions leading to structural and compositional inhomogeneities. Although oxidation and hydrolysis issues can be considered as minor problems in the case of Si3N4, and very successful case studies on aqueous colloidal processing of this material have been widely reported in the last decades, the ability to control the hydrolysis of AIN powders is generally still poor. The available know-how for protecting the surface of the AIN particles against hydrolysis and for dispersing them to obtain high concentrated suspensions is very scarce, hindering the implementation ofcolloidal processing of AlN-based materials. As far as we know, the most significant advances in this field all over the world have been made in our lab. In fact, we succeeded to prepare non-reactive and high stable and concentrated (50-vol.% solids) AlN-based suspensions (including the sintering aids), which could be used to consolidate homogeneous green bodies by slip casting, tape casting, or to granulate powders for dry pressing technologies. Furthermore, full dense AIN materials could be obtained by sintering homogeneous green bodies at 1750°C for 2 h. Such significant breakthroughs were only possible after conducting careful characterisation studies of the powders surface, finding suitable pre-treatments, selection of the dispersing and other processing additives, analysing the influence of several relevant factors, such as the deagglomeration conditions, aging time, and so on. The optimisation of the treatment methods of the powders' surface and of the processing steps was followed by exhaustive analysis of adsorption and evaluated by complementary techniques of UV, NMR and electrophoresis and of dispersion through rheology and particle size distribution.


Olhero SM, Oliveira MILL, Ferreira JMF

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