Original Research PaperA facile route for producing spherical granules comprising water reactive aluminium nitride added composite powders
Graphical abstract
Introduction
It is well known that, for producing ceramic products by uniaxial or isostatic pressing like cold isostatic pressing (CIP-ing), use of spherical granules (consisting of primary particles, additives and binders) instead of powders mixture is preferred for achieving superior properties in the green as well as in the sintered compacts [1], [2], [3]. During the uniaxial compaction of powder in metallic die or CIP-ing of powder inside a flexible rubber bag, the flow of powder for uniformly filling the die-cavity or rubber mould is inhibited due to high interparticle friction between irregular shaped fine particles especially when such particles are submicrometer or nanometer sized. As a result, non-uniform density distribution and presence of hard agglomerates often exist in the green compact, which, in turn, form inclusions and deteriorate the properties of sintered compact [4]. Therefore, for producing ceramic component, it is of paramount importance to start with spherical granules to ensure: (i) free flow of starting feed material and their uniform distribution inside die cavity, (ii) improvement in the density, homogeneity and in the dimensional stability in green as well as in sintered product [2], [3].
Ready-to-press (RTP) spherical granules can be produced from ceramic suspension which is prepared by dispersing the powders in a suitable liquid medium (aqueous or non-aqueous) with the help of an appropriate dispersant of optimized quantity. Thereafter, the suspension is atomized into liquid droplets by spraying through a nozzle at a suitable gas pressure followed by drying in a chamber of spray-drying (SD) system or in a freeze-drier of spray-freeze-drying (SFD) system [5], [6].
The necessary raw materials such as silicon nitride (Si3N4), aluminium nitride (AlN), aluminium oxide (Al2O3), and yttrium oxide (Y2O3) were selected to produce granules for the development of SiAlON based ceramic component [7], [8], [9]. Among the afore-mentioned powders, AlN is highly reactive with water and form aluminium hydroxide (Al(OH)3) gel and ammonia (NH3) gas [10], [11], [12]. Thus, the actual composition is found to be deviated from the stoichiometric composition. The treatment of AlN powder using phosphoric acid as reported in literature, does not provide water resistant capability for long time in aqueous medium and under vigorous milling conditions [13]. Generally, a phosphate coating (aluminium di-hydrogen phosphate based) is formed on the surface of AlN particles to make the raw AlN powder water resistant (WR) for processing in aqueous suspension. However, the formation of WR coating on AlN particles requires an additional process which is very tedious [10], [11], [12]. Moreover, such WR coating is also not stable for a long time in water medium [12].
In the past, the research was carried out on the granulation of Si3N4 powder using SD or SFD technique where Al2O3 and Y2O3 were added as sintering activators [14], [15], [16]. There are few scientific reports highlighting the processing conditions for the formation of granules consisting of Si3N4, AlN, Al2O3 and Y2O3 powders for SiAlON formation, by spray drying of aqueous slurry [17], [18]. However, it is not clear from these studies, whether the AlN powder was used in raw or WR coated form for processing in aqueous medium [17], [18].
The SD or SFD techniques are commercially exploited and commonly used granulation technique in research laboratories or in industries [5], [6]. The spray drying unit (a thermal process) is equipped with drying arrangement for evaporation of liquid medium from liquid droplets (formed after atomization) either in counter-current, co-current or mixed flow direction of drying gas in drying chamber (temperature ≈ 120–180 °C) of SD unit [5].
In the case of SFD technique, organic mediums (except a few organic compounds like cyclohexane, benzene) cannot be used for making suspension for producing granules. The freezing of liquid droplets due to their very low melting point (m.p.) (for example, m.p. of ethyle alcohol ≈ −114 °C, for acetone ≈ − 94.7 °C) makes freeze drying operation extremely difficult for such cases [19], [20].
In order to overcome the afore-said difficulties, a simple and cost-effective process is developed in the present study for producing spherical, RTP granules consisting of water reactive powder like AlN along with other nitride and oxide powders for the formation of SiAlON products, and the mechanism of the formation of granules is described elaborately. In the present approach, organic solvent like acetone was used as dispersing medium which is not reactive to AlN and other powders used in this study.
The major novelty of this work stands on the single-step production of granules (comprising mixture of Si3N4, Al2O3, Y2O3, and highly water reactive AlN Powders) upon their rapid drying through instantaneous evaporation of acetone at ambient temperature (i.e. ≈ 27 °C) without the requirement of any additional drying set-up with the help of an indigenously developed simple spray granulation set-up. Following this approach, it was possible to avoid the process for applying water resistant coating on AlN, and, also, expensive spray dryer is not required for granulation purpose.
Here, authors are trying to show that how acetone is effectively utilized to produce granules comprising water sensitive powder like aluminium nitride (AlN) along with other nitride and oxide powders for feedstock preparation to produce components of SiAlON ceramic. Acetone is not reactive with AlN and other powders.
The granules thus produced were used for making complex shaped components with the help of cold iso-static pressing (CIP-ing). It should be noted that the current study is mainly focussed on the development of spherical, RTP, free flowing granules comprising the nitride powders including AlN. Further, the granule characteristics and properties of sintered SiAlON have been also discussed briefly.
Section snippets
Raw materials
Commercially available Si3N4 powder (Grade: SicoNide P95H, D50 ≈ 0.9 µm, Vesta Ceramics, Sweden), AlN powder (Grade : AT, D50 ≈ 7–11 µm, ABCR GmbH, Germany), Al2O3 powder (Grade: D50 ≈ 0.5 µm, Grade : CT 3000LS SG, ALMATIS, India), Y2O3 powder (Grade: B, D50 ≈ 0.9–1.7 µm, ABCR GmbH, Germany) were used as starting raw materials in this study. Yttrium oxide acts as sintering additive and do not take part in β-SiAlON formation. The particle size related information of various powders is in
Characterization techniques
The morphology of granules, fracture surface of green compacts, and microstructure of sintered sample was examined using secondary electron imaging in scanning electron microscope (SEM-SE) equipped with thermionic electron sources (SEM, Model: S3400N, HITACHI, Japan). Elemental analysis and elemental mapping of SFD granules (SLN-M) were carried out with the help of energy dispersive spectroscopy (EDS) facility attached to the SEM system. The sphericity (ψ) of granules was determined from SEM
Drying of granules
Once atomization of ceramic suspension is occurred, the surface area-to-volume ratio of the liquid droplets increases significantly in comparison to that of the bulk suspension [5]. Generally, the resultant increase in surface area -to-volume ratio upon atomization is equivalent to (6/d), where ‘d’ is the diameter of the liquid droplet [5]. Of course, the surface area increase depends on the droplet size. After atomization of suspension, the formed liquid droplets travel from nozzle exit to the
Summary
In this study, main emphasis has been given to develop a simple process to produce free-flowable, spherical, RTP grade granules comprising highly water reactive powder like AlN added composite powder. Based on the experimental results obtained from this study, the following major points can be summarized:
- (i)
The rapid acetone evaporation from the liquid droplets leads to produce RTP granules in a single step and without additional drying set-up.
- (ii)
The sphericity of granules increases with the decrease
Acknowledgements
The authors would like to thank Director-ARCI for his kind permission to publish this work.
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