Introduction The synthesis of materials in the nanometer range is a subject of great interest due to their distinct chemical, magnetic and optical properties. Among the various preparationproceduresof nanocomposites, the sol–gel method offers some special advantages. It al - lows the control of the final properties of the material and represents an effective remedy to the tendency of nanopowders to aggregate, ensuring thehomoge - neous dispersion of some ultra-fine metal oxide parti - cles in the host matrix. Concerning the sol–gel iron-silica nanocompo - sites,the material consists of a SiO 2 matrix which contains one of the polymorphs of the iron(III) oxide: α-Fe 2 O 3 (hematite), γ -Fe 2 O 3 (maghemite), ε-Fe 2 O 3 , β-Fe 2 O 3 or amorphousFe 2 O 3 [1–8]. Hematite ( α-Fe 2 O 3 ), the antiferromagnetic hexagonal form, represents the most thermodynamically stable poly - morph of the iron(III) oxides, so it is easiest to be ob - tained.However, the moststudied polymorph re - mains maghemite ( γ -Fe 2 O 3 ), the ferromagnetic cubic form [5,6, 9, 10],due to its magnetic properties, which ensure many technological applications. Its ob - taining is difficult because of the γ - to α-Fe 2 O 3 transi - tion, which is not easy to be controlled. The explana - tion consists in the multitude of factors that can affect the stability of iron oxides. Therefore, the reported transition temperatures (T γ→α ) vary in the range 300–600°C [11],even higherthan 650°C [9]. ε-Fe 2 O 3 , the ferromagnetic orthorhombic form is a rarepolymorph, difficultto synthesizeas single- phase, but typically obtained as mixtures of ε- plus α- and/or γ -Fe 2 O 3 [7, 8]. Depending on the iron precur - sor used and on the process parameters of the sol–gel synthesis,besidesall thesepolymorphsof the iron(III) oxide, a series of iron oxyhydroxides, such as goethite: α-FeO(OH) and lepidocrocite: γ -FeO(OH) or fayalite (Fe 2 (SiO 4 ), can be obtained. The authors of the present paper have recently studied the obtaining of Fe 3 O 4 –SiO 2 nanocomposites via alkoxideand colloidalrouteof the sol–gel method [12]. They put in evidence the influence of the type of silica matrix on the structure, size and dis - tribution of the Fe 3 O 4 nanoparticles in these nano - composites with magnetic properties. The properties of the finalnanocomposite are mainly determined by the sol–gel parameters, refer - ring both to the chemical composition, and to the re - action conditions: the metaland silica precursors, their concentrations, the molar ratios between compo - nents, the nature of the catalyst, the pH and tempera - ture values. Another parameter which is very impor - tant for the final propertiesof the obtained 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 88 (2007) 1, 163–169 THERMAL BEHAVIOUR STUDY OF SOME SOL–GEL IRON-SILICA NANOCOMPOSITES Ana Br²ileanu 1* , M. R²ileanu 1 , M. Criêan 1 , D. Criêan 1 , R. BÐrjega 2 , V. E. Marinescu 3 , J. Madarász 4 and G. Pokol 4 1 Roumanian Academy, Institute of Physical Chemistry Ilie Murgulescu, 202 Splaiul Independen ïei, 060021 Bucharest, Roumania 2 National Institute for Lasers, Plasma and Radiation Physics, PO Box MG-36, 76900 Bucharest, Roumania 3 INCDIE ICPE-CA, 313 Spl. Unirii, 030138 Bucharest, Roumania 4 Budapest University of Technology and Economics, Institute of General and Analytical Chemistry, Szt. Gellért tér 4 1521 Budapest, Hungary Two series of nanocomposites from the Fe x O y –SiO 2 system, containing 20 mass% iron oxide were prepared by the alkoxide route the sol–gel method, in the absence and presence of catalyst. The silica gel has been obtained using tetraethoxysilane. T nitrate nonahydrate has been used as iron oxides source. The samples have been prepared in identical conditions, diffe the gelation times, induced by different surface of evaporation/volume (S/V) ratios of sol let to gelify. Thermal analysis established the thermal treatments conditions of the prepared samples and were correlated with X-ray diffraction, IR s and TEM results, in order to accomplish a complete structural characterization. The correlation between the structura of the Fe x O y –SiO 2 nanocomposites and different conditions of drying has been established. Keywords: iron-silica nanocomposites, sol–gel method, thermal analysis * Author for correspondence: abrail@icf.ro