Nitridation of SiO 2 –B 2 O 3 aerogels K. Szaniawska, M. Gładkowski, L. Wicikowski, L. Murawski * Faculty of Applied Physics and Mathematics, Gdan ´sk University of Technology, Narutowicza 11/12, 80-952 Gdan ´sk, Poland article info Article history: Available online 4 August 2008 PACS: 61.43.j 81.05.Rm 78.30j Keywords: Nitride glasses Aerogels abstract SiO 2 –B 2 O 3 aerogels have been prepared by drying wet gels at a supercritical condition for ethanol in an autoclave. Aerogels have been nitrided for 6 h in flowing ammonia at the temperature of 1200 °C. It has been found that the amount of nitrogen incorporated in these aerogels always exceeds 20 wt%. This is a much higher value compared with the amount of nitrogen incorporated in a pure silica aerogel nitrided at the same conditions. The specific surface area of SiO 2 –B 2 O 3 aerogels has been between 312 and 359 m 2 /g. After nitridation some shrinkage of aerogels has been observed and the surface area decreases about 20%. In FTIR spectra of SiO 2 –B 2 O 3 aerogels a typical bands for SiO 2 are observed. After nitridation a shift and broadening of 1100 cm 1 band to lower wavenumbers indicates that Si–N and B–N bonds are formed in nitrided aerogels. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Aerogels, prepared by sol–gel processing and supercritical extraction, exhibit many interesting properties including high transparency, low refractive index, high surface area and high porosity [1]. A large specific surface area and very high porosity exceeding 95% are very useful properties of aerogels for their struc- tural modifications. We have found that more than 20 wt% of nitro- gen can be incorporated in silica aerogels during long time (>10 h) heat treatment in ammonia (ammonolysis) at 1200 °C. In addition to that, some of the nitrided aerogels have been densified to mono- lithic oxynitride glasses containing 8.4–13 wt% of nitrogen [2]. In this work, we present the results of nitridation of SiO 2 –B 2 O 3 aero- gels. The purpose of these investigations has been to find a way of gel preparation leading to a synthesis of nitrided aerogel contain- ing the highest amount of nitrogen. To the best of our knowledge there are no reports about the ammonolysis of SiO 2 –B 2 O 3 aerogels. 2. Experimental Twelve samples of SiO 2 –B 2 O 3 gels containing 20–40-mol% of B were prepared by hydrolysis and polycondensation of tetraetoxysi- lane (TEOS) and trimethyl borate (TMOB) in an alcohol solution. A two-step hydrolysis process was performed. Initially the TEOS dis- solved in ethanol was partly hydrolyzed with 0.01 M HCl. After 24 h NH 4 OH and TMOB were added. Different gelling conditions were performed using different amounts and time of NH 4 OH add- ing (before or after adding TMOB). The compositions of initial solu- tions and details of preparation are shown in Table 1. The gels were dried under supercritical conditions for ethanol (243 °C, 65 bar) by applying inert gas (N 2 ) pressure in an autoclave. The initial pressure was 80 bar of N 2 and then the samples were heated at 50 °C/h up to 300 °C when the pressure reached 240 bar. After 1 h the vessel was decompressed at 80 bar/h and cooled down. Silica aerogels were exposed to reaction with ammonia in a sil- ica glass tube furnace under NH 3 gas flow (100 ml/min) and heated at 1200 °C for 6 h. The details of the ammonolysis procedure are shown in Fig. 1. Aerogels were investigated by the Fourier transform infrared spectroscopy (FTIR). The FTIR spectra were recorded with the KBr disk method using an IFS66 BRUKER spectrophotometer. The specific surface area of aerogels was measured at 77 K by the single-point BET method using the nitrogen adsorption tech- nique. All samples were heated to 200 °C under flowing nitrogen for 2 h prior to analysis in order to remove any volatiles from the surface. The specific surface areas of the samples were measured using Areameter II (Strohlein). The statistical error in determining the specific surface area was within 1%. The nitrogen content of nitrided aerogels and glasses was deter- mined chemically using the method of Guyader et al. [3]. In this method ammonia is removed from the sample by molten KOH according to the reaction: xN 3 þ 3xKOH ! xNH 3 þ 3x=2K 2 O þ 3x=2O 2 For this purpose, a pulverized specimen was placed in a silica tube furnace and melted with KOH in a stream of nitrogen. The 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.06.072 * Corresponding author. E-mail address: lemur@mif.pg.gda.pl (L. Murawski). Journal of Non-Crystalline Solids 354 (2008) 4481–4483 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol