JOURNAL OF CHEMISTRY Materials Effect of mechanochemical treatment on the synthesis of calcium dialuminate Jadambaa Temuujin, a Kenneth J. D. MacKenzie,* b Tsedev Jadambaa, c Banzar Namjildorj, d Budjav Olziiburen, e Mark E. Smith e and Paul Angerer f a Institute of Chemistry and Chemical Technology, Mongolian Academy of Sciences, Ulaanbaatar 51, Mongolia b Department of Materials, University of Oxford, Oxford, UK OX1 3PH. E-mail: kenneth.mackenzie@materials.ox.ac.uk c Department of Chemical Technology, Mongolian Technical University, Ulaanbaatar 46, Mongolia d Department of Building Materials, Corporation of Building and Architecture, Ulaanbaatar 38, Mongolia e Department of Physics, University of Warwick, Coventry, UK CV4 7AL f Institute of Materials, German Aerospace Center, Ko Èln, Germany Received 16th December 1999, Accepted 3rd February 2000 Calcium dialuminate (CaAl 4 O 7 ) powders were synthesised from mechanochemically treated mixtures of aluminium hydroxidezcalcium hydroxide and aluminium hydroxidezcalcium carbonate. On grinding, both mixtures produce X-ray amorphous precursor phases which show 27 Al MAS NMR resonances characteristic of Al in octahedral and tetrahedral sites, and a site identi®ed by a resonance at 37±39 ppm (possibly pentahedral Al). Although grinding does not completely destroy the carbonate XRD re¯ections in the carbonate mixture, both precursors show a high degree of homogeneity and behave similarly on heating, forming monophase CaAl 4 O 7 at v1050 ³C. By contrast, the same unground compositions form mixtures of a-alumina and various calcium aluminates (but not CaAl 4 O 7 ) on heating as high as 1250 ³C. Calcium dialuminate synthesised from the carbonate-containing mechanochemical precursor had a smaller particle size which may be advantageous for subsequent fabrication and sintering. Introduction Failure due to thermal shock constitutes a major problem encountered with many refractory ceramics. This behaviour is due to the large thermal expansion of the commonly used basic refractory oxides with high melting points, such as MgO and CaO. 1 However, Jonas et al. 1,2 and Criado and De Aza 3 have reported that calcium dialuminate (CaAl 4 O 7, mp~1750 ³C) shows very low thermal expansion and can be used for refractory purposes. Jonas et al. prepared CaAl 4 O 7 from CaCO 3 and Al 2 O 3 , ®ring the oxide mixture in three steps, at 1200, 1300 and 1450 ³C. Sunkowski and Sawkow 4 prepared calcium dialuminate in a single ®ring at 1500 ³C from technical raw materials but their product also contained phases other than the dialuminate. Although previous successful preparations of CaAl 4 O 7 required multiple high-temperature ®rings, it is possible to decrease the synthesis temperature of inorganic compounds by mechanochemical treatment of the reaction mixtures, especially if at least one component contains water or hydroxyl groups. 5±7 Such a synthesis route is described as a ``soft'' mechanochem- ical method. The aim of the present work was to determine the in¯uence of mechanochemical treatment on the formation of CaAl 4 O 7 precursors from mixtures of aluminium hydroxide with calcium hydroxide or calcium carbonate. The subsequent thermal crystallisation of these precursors was also investigated, and compared with the thermal reactions of unground mixtures of the same starting materials. Experimental The starting materials were analytical grade Al(OH) 3 (gibb- site), Ca(OH) 2 (portlandite) and CaCO 3 (calcite) (Reachim, Russia). X-Ray powder diffraction (XRD) showed the gibbsite and calcite to be pure, monophasic materials, but the portlandite showed evidence of a small amount of calcite resulting from atmospheric carbonation. Gibbsite±portlandite and gibbsite±calcite mixtures of molar composition equivalent to 1CaO : 2Al 2 O 3 were prepared, designated AH and AC, respectively. The mixtures were ground for 4 h using a Fritsch planetary mill (Pulverisette 5) with a rotation speed of 300 rpm. Both the pot and milling media were corundum and the weight ratio of balls to powder was 15 : 1. During the grinding the mill was stopped for 10 min every hour. After grinding, the mixtures were heated in air for 1 h at various temperatures. For comparison, a parallel set of heating experiments was carried out on the same mixtures which were not ground. The thermal behaviour of the unground and ground mixtures was examined by combined differential thermal analysis and thermogravimetry (DTA±TG) using a Rigaku Thermoplus TG 8120, and the ground precursors were characterised by XRD (Siemens D-5000 diffractometer using Cu-Ka radiation) and solid state 27 Al MAS NMR at a magnetic ®eld of 14.1 T using a Chemagnetics In®nity 600 MHz spectrometer and a 3.2 mm high-speed MAS probe in which the sample was spun at 18 kHz. The spectra were acquired at 156.374 MHz using a 15³ pulse of 0.5 ms and a recycle time of 1 s and were referenced by using the secondary standard of the AlO 6 resonance of DOI: 10.1039/a909888g J. Mater. Chem., 2000, 10, 1019±1023 1019 This journal is # The Royal Society of Chemistry 2000