Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint From sub-microsized MgAl 2 O 4 powder to MgAl 2 O 4 -Si 3 N 4 nanocomposite powder by spray drying of ultrasonicated suspensions M.R. Loghman-Estarki a, ⁎ , H. Sheikh a, ⁎ , E. Mohammad sharifi a , M.H. Babaee b , M. Kharraziha b , A. Alhaji a a Department of Materials Engineering, Malek Ashtar University of Technology, Shahin Shahr, Isfahan 83157-13115, Iran b Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran ARTICLE INFO Keywords: Ultrasonic irradiation Nanocomposite Granules ABSTRACT The prime objective of this work is an investigation on the effect of ultrasonic processing and suspension formulation including the amount of the binder and the dispersant agent on the morphology, circularity factor, and flowability of the resulting granules. In order to do so, the MgAl 2 O 4 -2.5vol%Si 3 N 4 suspension was firstly prepared with ultrasonic irradiation of MgAl 2 O 4 -Si 3 N 4 nanoparticles in distilled water with the assistance of a dispersant agent (Dolapix CE 64) and PVA (poly vinyl alcohol) binder. Then, the nanocomposite granules were produced by spray drying of the suspension. The characterizations of the initial powders and the obtained granules were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) analyses. The results showed that the ultrasonic treatment can break the initial agglomerated spinel powders into a fine distribution of particle size. Also, spray drying of the ultrasonic-treated suspension can produce granules with desirable morphology and flowability. 1. Introduction Magnesium aluminate (MgAl 2 O 4 ) with spinel structure has a high melting point, high chemical inertness, good thermal stability, and excellent mechanical properties. Due to these properties, this material has application in electronic devices, optic industries, IR windows, solar heat absorber. MgAl 2 O 4 spinel is processed at high temperatures to produce a dense product; hence a large grain size is obtained during sintering processes. Therefore, the drawback of the spinel ceramic is the low toughness and thermal shock resistance [1–7]. One approach to solve this problem is strengthening of MgAl 2 O 4 spinel using Si 3 N 4 nanoparticles [8,9]. Wu et al. [9] fabricated the MgAl 2 O 4 –Si 3 N 4 ceramics via pressure-less sintering by using α-Si 3 N 4 , α-Al 2 O 3 , and MgO as starting materials. This group showed that the composite with 30 wt%MgAl 2 O 4 sintered at 1620 °C has the best physical properties, excellent thermal shock resistance, suitable oxidation resistance, and good solar absorbance. Moreover, Gledhill et al. [8] successfully produced MgAl 2 O 4 -2.5vol%Si 3 N 4 ceramic by the hot press technique. Their purpose was an increase in mechanical properties of MgAl 2 O 4 spinel. The reason for using a low amount of nano-dispersoids (amorphous Si 3 N 4 nanoparticles) was to keep IR-visible transparency of the bulk nanocomposite. The nanocomposite had more than 70% transparency at infrared range (3–4.5 μm). Furthermore, the results showed that after heat treatment at 1000 °C for 4 h and then cooling in the air, the tensile strength and toughness of nanocomposite were increased 29% and 85%, respectively. It is noteworthy that the optical transparency in the infrared range remained constant after the heat treatment. The formation of a compressive layer on the surface of nanocomposite is due to surface oxidation of Si 3 N 4 nanoparticles [8]. However, this group only reported that the powder of nanocomposite was prepared by spray drying method without any description about the details of the experimental procedure. To produce this nanocomposite by spray drying method, a stable suspension with a suitable pH is needed to be prepared. This suspen- sion should be stable at least 1 h without any sedimentation. Thus, many factors such as controlling the pH, formulation, and stability of suspension are very effective to produce nanostructured granules. The effect of ammonium polyacrylate (referred as Dolapix CE64) on the sustainability of magnesium aluminate spinel was investigated by Kadosh et al. [10]. They added 5 wt% of this dispersant agent. This addition caused a shift in spinel isoelectric point from the pH of 11.8 to 3.3. They showed that the spinel suspension in an alkaline pH (the zeta potential of -60 mV) was very stable as compared with an acidic medium (-7 mV zeta potential) [10,11]. In Albano's research [12], the effects of pH on the stability of ternary suspension (Si 3 N 4 + Al 2 O 3 + Y 2 O 3 ) in the presence of ammonium polyacrylate was examined. http://dx.doi.org/10.1016/j.ceramint.2017.06.165 Received 22 April 2017; Received in revised form 18 June 2017; Accepted 26 June 2017 ⁎ Corresponding authors. E-mail addresses: loghman57@gmail.com (M.R. Loghman-Estarki), sheikh_scientific@yahoo.com (H. Sheikh). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: Sheikh, H., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.06.165