Effect of TiO 2 on phase evolution and microstructure of MgAl 2 O 4 spinel in different atmospheres R. Naghizadeh, H.R. Rezaie * , F. Golestani-Fard Department of Metallurgical and Materials Engineering, Iran University of Science and Technology (IUST), Iran Received 21 December 2009; received in revised form 6 July 2010; accepted 30 August 2010 Available online 29 September 2010 Abstract The effect of TiO 2 on the formation and microstructure of magnesium aluminate spinel (MgAl 2 O 4 ) at 1600 8C in air and reducing conditions were investigated. Under reducing conditions, stoichiometric MgAl 2 O 4 spinel shifted toward alumina-rich types owing to volatilization of MgO, resulting in an increase in the porosity of fired samples. Addition of graphite to mixtures of MgO and Al 2 O 3 intensified the reducing conditions and accelerated the formation of non-stoichiometric MgAl 2 O 4 . For TiO 2 -containing samples on addition of MgAl 2 O 4 , magnesium aluminum titanium oxide (Mg x Al 2(1x) Ti (1+x) O 5 , x = 0.2 or 0.3) was detected as a minor phase. Under reducing conditions, XRD peak shifts were smaller for TiO 2 - containing samples than for samples without TiO 2 owing to the formation of a solid solution of TiO 2 in MgAl 2 O 4 and establishment of alumina-rich spinel, which have opposite effects on increasing the lattice parameter. In bauxite-containing samples, MgAl 2 O 4 spinel, corundum, magnesium orthotitanate spinel (Mg 2 TiO 4 ) and amorphous phases were identified. Mg 2 TiO 4 spinel formed a complete solid solution with MgAl 2 O 4 spinel but Mg 2 TiO 4 remained as a distinct phase owing to the heterogeneous microstructure of bauxite-containing samples. Also dense microstructure established in air fired TiO 2 containing samples. The results are discussed with emphasis on the application and design of alumina–magnesiacarbon refractory materials, which are used in the steel industry. # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: D. TiO 2 ; MgAl 2 O 4 ; Bauxite; Alumina-rich spinel; Mg 2 TiO 4 1. Introduction Magnesium aluminate spinel is an important refractory material because of its excellent properties such as high melting point (2105 8C), low thermal expansion, high thermal spalling and corrosion resistance [1]. Dense spinel can be prepared by reaction of magnesium and aluminum compounds, generally in a two-stage firing process, with spinel formation at 1400 8C and densification at 1700 8C [2,3]. This approach is used because of the high volume expansion (5%) that occurs during spinel formation that prevents densification of the product. Therefore, additives such as TiO 2 , ZnO and CaF 2 were proposed for the formation and densification of magnesium aluminate spinel [1,4]. TiO 2 improved the density of spinel ceramics and had no influence on the phase constituents up to 2 wt.% [4]. A decrease in grain size and an increase in the roundness of spinel grains are the other effects of TiO 2 in spinel [4,5]. All studies were carried out in an ambient atmosphere and little information has been reported on the effect of the atmosphere. Although the effect of TiO 2 addition has been investigated, the influence of TiO 2 -bearing additives such as bauxite is not really understood. A new generation of materials called alumina–magnesia– carbon (AMC) refractory materials has become of interest for application in ladle furnaces in the steel industry. The presence of carbon in this type of material creates a reducing environment in the interior parts of linings. This can cause volatilization of Mg and variations in spinel chemistry [6]. In the present study the stability of MgAl 2 O 4 in the presence of TiO 2 at 1600 8C in air and a reducing atmosphere is reported. 2. Experimental procedures The raw materials were calcined alumina (Fiberona, India, >99.3% purity, d 50 =2 mm), titanium oxide (Merck, >99% purity, d 50 =3 mm), magnesium oxide (Merck, >98% purity, d 50 =4 mm) calcined bauxite (Bigglar, Iran, d < 73 mm) and www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 37 (2011) 349–354 * Corresponding author. Tel.: +98 91 21025394; fax: +98 21 77240480. E-mail address: hrezaie@iust.ac.ir (H.R. Rezaie). 0272-8842/$36.00 # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2010.09.016