Effect of addition of Cr 2 O 3 on the properties of reaction sintered MgO–Al 2 O 3 spinels Ritwik Sarkar*, Samir Kumar Das, Goutam Banerjee Refractory Division, Central Glass and Ceramic Research Institute, Calcutta, India Received 2 May 2001; received in revised form 7 August 2001; accepted 15 August 2001 Abstract Effect of addition of Cr 2 O 3 up to 4 wt.% was studied on three different spinel compositions with MgO:Al 2 O 3 molar ratios 2:1, 1:1 and 1:2. Attritor mill was used to reduce the particle size of the starting materials and a single stage sintering technique was employed in the temperature range of 1550–1650  C for all the compositions. Final sintered products were then characterised in terms of den- sification and shrinkage studies, phase analysis, strength evaluation both at the ambient temperature and at 1300  C, strength reten- tion capacity after different cycles of thermal shock at 1000  C, quantitative elemental analysis and microstructural studies. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Spinels; MgAl 2 O 4 ; Sintering; Mechanical properties; Thermal shock 1. Introduction Magnesium aluminate spinel is a potential refractory material with excellent high temperature mechanical, thermal and chemical properties. Development of mag- nesium aluminate spinel bodies and method of fabricating spinel refractories were available from about a century ago; 1 and phase diagram of MgO–Al 2 O 3 system was established in 1916, 2 which has not modified essentially. But the use of such materials was limited due to higher cost of production and magnesium chromite bodies were the substitutes. But now, as the refractory practice has changed to high quality items to cope up the changing critical operating parameters and less down time than practiced earlier, magnesium aluminate spinel is gaining its ground in a hard competitive market. Again it has the additional advantage of environment friendliness that attracts the users and discards the use of magnesium chromite refractories due to toxic nature of the chrome bearing compounds. 3 Spinel in its major applications, 4 in burning and transi- tion zones of cement rotary kilns and bottom and side- walls of steel teeming ladles, is used as a major component in magnesia rich or alumina rich matrix respectively. Again another important application is the checker work of the glass tank furnace regenerators where pure spinel products are important. Hence all the stoichio- metric, magnesia excess and alumina excess spinel compo- sitions are important from the application point of view. Formation of magnesium aluminate from its con- stituent oxides is a counter diffusion process of Al +3 and Mg +2 ions 5 and accompanies a volume expansion of 5%. 6 The expansion does not allow the material to densify in a single firing and thus a double stage sinter- ing process was essential for obtaining dense spinel bodies. But utilising the technique of reaction sintering by the reduction of particle size, 7,8 well-densified mag- nesium aluminate spinels are obtained in a single stage sintering process from oxide reactants 9 11 through attritor milling. Effects of different additives on the development of magnesium aluminate spinel were studied by a number of workers. Addition of salt vapours were reported 12 to increase the amount of spinel formation and that of alkali fluorides to increase the crystal growth of the formed spi- nel which can hinder the sintering process. Kostic and others 13 reported that fluorine ions (from AlF 3 or CaF 2 ) enhance the solid-state reaction synthesis of magnesium aluminate spinel by increasing the cation vacancy con- centration when fluorine was incorporated in the lattice by replacing oxygen ions. Addition of LiF to make a liquid phase sintering was reported 14,15 to improve the densification characteristics of magnesium aluminates. Better sintered properties were also reported for the 0955-2219/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(01)00446-0 Journal of the European Ceramic Society 22 (2002) 1243–1250 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Tel.: +91-33-483-8084; fax: +91-473- 0957. E-mail address: ritwiksarkar@rediffmail.com (R. Sarkar).