International Journal of Inorganic Materials 3 (2001) 693–698 Characterisation of stoichiometric sol–gel mullite by fourier transform infrared spectroscopy * P. Padmaja, G.M. Anilkumar, P. Mukundan, G. Aruldhas, K.G.K. Warrier Ceramics Division, Regional Research Laboratory ( CSIR), Kerala, Trivandrum 695019, India Received 27 February 2001; accepted 14 August 2001 Abstract Fourier Transform Infrared Spectroscopic investigation has been carried out to study the structural changes and coordination of Al and Si in the mullite system as a function of temperature (60–14008C). The FTIR spectral patterns of the gel and the gel calcined at 2008C show bands corresponding to Al–O–Al and Si–O–Si vibrations. At higher temperatures hydroxyl groups and organics present have been 21 removed. The absence of fine structure in the region 400–900 cm in the temperature range 400–10008C is interpreted as due to the existence of nanostructured alumina, silica and alumina–silica agglomeration in the amorphous state. Analysis of the spectra indicates the presence of both octahedral and tetrahedral coordination at 10008C, the temperature around which spinel is formed. The shifting of the asymmetric stretching frequencies of the Si–O–Si and Al–O–Si networks to higher wave numbers indicate the formation of mullite at 12008C. Crystalline nature of the product is indicated by the sharpening of spectral peaks. The spectra also indicates the completion of mullite formation at 12508C.  2001 Elsevier Science Ltd. All rights reserved. 1. Introduction Several techniques such as XRD, high-resolution elec- 27 29 tron microscopy, XANES, Al and Si MAS NMR Mullite has been identified as an important high tem- spectroscopy, IR etc., have been reported for the structural perature ceramic due to its remarkable physical and characterisation of mullite [12–23]. Infrared measurements chemical properties like low thermal expansion and con- of heat treated 3:2 mullite broadly indicates that the ductivity, good thermal and chemical stability, low creep intensity variation of the absorption band in the spectral 21 rate, reasonable toughness and strength, thermal shock range 1100–1200 cm provides a useful hint for identify- resistance, infrared transparency etc. [1–5]. As mullite is ing the formation of mullite [16,17,20,22]. the only stable crystalline phase formed at atmospheric Though several basic studies have been done on mullite pressure in the alumina silica system, its synthesis and systems, a systematic study is lacking regarding structural characterisation have become the object of considerable aspects in various stages of mullite formation in diphasic interest [6,7]. Among the various methods developed, sol– colloidal gels. In the present investigation, FTIR spec- gel method provides maximum homogeneity of alumina troscopy is used to study the structural changes including and silica species and results in the formation of mullite at coordination during heat treatment of sol–gel mullite relatively lower temperatures compared to the other con- precursor. ventional methods [8]. The mullite formation is reported to be dependent on the type of precursor gel as well as the structural changes occurring during heat treatment [9]. 2. Experimental procedure Based on the precursor level mixing the sol–gel processing of mullite results in two types of gels. The molecular level To synthesise stoichiometric mullite (3Al O 2SiO ), 2 3 2 mixing results in diphasic gels while atomic level mixing boehmite (AlOOH) is used as the precursor for alumina of the precursors results in a monophasic gel [10,11]. and tetraethoxysilane (TEOS, Si(OC H ) ) as the pre- 2 5 4 cursor for silica. Boehmite sol was prepared by precipi- tating boehmite by addition of dilute (25%) ammonia *Corresponding author. Tel.: 191-471-490-674; fax: 191-471-491- solution (Qualigens Chemicals) drop by drop at pH 8 at 712. E-mail address: warrier@csrrltrd.ren.nic.in (K.G.K. Warrier). approximately 908C to an aqueous solution of aluminium 1466-6049 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049(01)00189-1