Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint A comparison of the eﬀect of nanostructured MgCr 2 O 4 and FeCr 2 O 4 additions on the microstructure and mechanical properties of direct-bonded magnesia-chrome refractories Najmeh Lotﬁan a , AmirAbbas Nourbakhsh a,* , Seyed Nezamoddin Mirsattari b , Ali Saberi c , KennethJ.D. Mackenzie d a - Department of Materials Science and Engineering, Shahreza Branch, Islamic Azad University, Shahreza, Iran b - Department of Chemistry, Shahreza Branch, Islamic Azad University, Shahreza, Iran c - Research Department of the Arvin Dirgodaz Vije Company, Isfahan Science and Technology Town (ISTT), Isfahan, Iran d - MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, New Zealand ARTICLE INFO Keywords: Mag-chrome Spinel MgCr 2 O 4 FeCr 2 O 4 ABSTRACT The eﬀect on the microstructure and mechanical properties of direct-bonded magnesia-chrome refractories of additions of nanostructured MgCr 2 O 4 and FeCr 2 O 4 is reported. The nanostructured additives, synthesized by the citrate-nitrate route and calcined at several diﬀerent temperatures, were characterized by XRD, BET and TEM. Additions of 0.5 and 1 wt % of these nanostructured oxides were made to magnesia-chrome refractories and calcined at 1650 O C in a shuttle kiln. Their microstructures were analyzed by SEM/EDX and their physical and mechanical properties (permanent linear change (PLC), bulk density, apparent porosity, cold crushing strength (CCS) and hot modulus of rupture (HMOR) were determined according to the respective DIN standards. The addition of the nanostructured oxides to the magnesia-chrome refractories facilitated the formation of secondary spinels, inﬂuencing the physical and mechanical properties. FeCr 2 O 4 additions increased the size of the sec- ondary spinel due to liquid phase formation in the presence of magnetite impurities in the FeCr 2 O 4 nano-powder. The addition of nano-sized MgCr 2 O 4 and FeCr 2 O 4 to the base formulation of the refractory increased the CCS from 67.4 MPa to 82.8 MPa and 81.0 MPa respectively, while nano-sized MgCr 2 O 4 increased the HMOR value from 5.48 MPa to 5.91 MPa and nano-sized FeCr 2 O 4 increased the HMOR from 5.48 MPa to 5.72 MPa. This smaller increase than that obtained with FeCr 2 O 4 additions is attributed to liquid phase formation in the pre- sence of magnetite, as observed by XRD. 1. Introduction Due to their high thermal stability, low thermal expansion, high thermal shock resistance and inertness to molten metals and slags, magnesia-chrome refractories have been widely used in a number of applications such as rotary cement kilns, non-ferrous copper converter furnaces and secondary metallurgical processing, e.g. argon oxygen decarburization (AOD), vacuum oxygen decarburization (VOD), Ruhrstahl-Heraeus (RH) degassing, etc. [1]. To enhance direct inter- granular bonding in magnesia-chrome refractories, the SiO 2 content of the chromite must be less than 3 wt%, but this increases the sintering temperature, increasing the production costs [2]. In recent years much research has been devoted to decreasing the sintering temperature while maintaining the physical properties. One important means of decreasing the sintering temperature is to decrease the dihedral angle between the magnesia and magnesia-chromite grains. This can be achieved by adding other oxides to improve the direct bonding between the grains [3]. Thus, Zhao [4] investigated the eﬀect of nanosized Fe 2 O 3 on the sintering and mechanical performance of magnesia-chrome re- fractories and reported the addition of a small amount resulted in a decrease of 150 °C in the sintering temperature and improvement in the mechanical performance due to improved direct grain bonding [4]. Azhari also reported enhanced direct bonding in the presence of na- nosized iron oxide and attributed this to a decrease in the viscosity of the liquid silicate phase, facilitating solid state sintering and the for- mation of magnesioferite spinel at a lower temperature [1]. Yu [5] reported improved densiﬁcation of magnesia-chrome refractories by the addition of alumina and titania. Bavand [6] showed the addition of https://doi.org/10.1016/j.ceramint.2019.09.028 Received 23 July 2019; Received in revised form 3 September 2019; Accepted 3 September 2019 * Corresponding author. E-mail address: Anourbakhs@yahoo.com (A. Nourbakhsh). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2019 Published by Elsevier Ltd. Please cite this article as: Najmeh Lotfian, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.028