Properties of high nitrogen content mixed alkali earth oxynitride glasses (AE x Ca 1Àx ) 1.2(1) SiO 1.9(1) N 0.86(6) , AE = Mg, Sr, Ba Ali Sharafat, Jekabs Grins, Saeid Esmaeilzadeh * Department of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden article info Article history: Received 23 October 2008 Received in revised form 15 April 2009 Available online 28 May 2009 PACS: 61.05.a 62.20.Qp 64.70.ph 78.20.Ci Keywords: Oxynitride glass Hardness Refractive index abstract Mixed alkali earth element containing high nitrogen content oxynitride glasses (Ca 1Àx AE x ) 1.2(1) - SiO 1.9(1) N 0.86(6) , with AE = Mg, Sr, Ba, x 6 0.30 for Mg and x 6 0.46 for Sr and Ba, and nominally constant (Ca/AE):Si:O:N ratios were prepared in order to investigate the compositional dependencies of physical properties on alkali earth element composition. The glasses were prepared by melting mixtures of AEH 2 , CaH 2 , SiO 2 and Si 3 N 4 powders in nitrogen atmosphere at 1600–1700 °C and characterized by X- ray powder diffraction and scanning and transmission electron microscopy. Cation and anion glass com- positions were determined by respectively energy dispersive X-ray analysis and combustion analysis. The determined physical properties were density, glass transition temperature, Vickers hardness, and refrac- tive index. The physical properties were found to vary linearly with the degree of substitution of Ca by the AE elements. The density of the glasses increases substantially upon substitution by Sr and Ba, up to 3.99 g/cm 3 . Glass transition temperatures are found to be higher for Mg and Sr substituted glasses, ca. 900 °C, in comparison with Ba substituted glasses, ca. 850 °C. The hardness increases upon substitution by Mg, up to 12.2 GPa at x = 0.46, and decreases upon substitution by Sr and Ba. The refractive index increases upon substitution by Sr and Ba, up to 1.97 for Ba at x = 0.46, and decreases upon substitution by Mg. The transparency of the glasses was found to increase upon increasing substitution by Mg and completely transparent glasses were obtained for x = 0.24. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Oxynitride silicate glasses are high performance glasses with nitrogen atoms incorporated in the tetrahedral oxide glass net- work. Historically, the main interest in them originates from their use in densifying silicon nitride based ceramics. Densification of the latter is very difficult, due to the strong covalent Si–N bonds, without the use of additives. The additives, usually oxides, react with the surface silica and some of the nitride to form an oxynit- ride liquid that promotes densification and, at the same time, also the phase transformation from a to b Si 3 N 4 type phases. The den- sification proceeds via a liquid phase sintering mechanism [1–3]. Recently, CaH 2 has been utilized as an additive for densification of Ca–Si–Al–O–N ceramics [4] and also as a starting material for the preparation of high nitrogen content Ca–Si–O–N glasses [5]. The CaH 2 decomposes at comparatively low temperatures and forms nitride phases by reaction with the nitrogen atmosphere. By the occurrence of similar reactions, the degree to which nitro- gen can be incorporated in rare earth (RE) silicate oxynitride RE–Si–O–N [6] glasses has been found to increase when using RE metal precursors. Nitrogen contents up to 70 e/o were achieved in the La–Si–O–N system by using this synthesis route [7]. The way in which the composition of the oxynitride glasses determines their physical properties is of both fundamental inter- est and of importance for applications. For oxynitride glasses with RE elements, studies [8–12] have shown that the substitution of lanthanide elements for yttrium modifies the glass viscosity and elastic and thermal properties. Reported results for La–Er modified Si–Al–O–N glasses [13], indicate that changes in molar volume (MV), hardness and glass transition temperature (T g ) are controlled by the cation field strength (CFS) of the RE modifier or an effective CFS in the case of a mixed modifier glass. Similarly, Ohashi et al. [11] found that the Young’s modulus and T g increase with an in- crease of the RE element CFS and Ramesh et al. [12] observed a decrease in thermal expansion coefficient and an increase in hard- ness with increasing CFS of the RE element. The oxidation resis- tance of RE–Si–Al–O–N glasses is, furthermore, found to increase with decreasing radius of the RE ion [10]. It is, however, clear, as shown by e.g. the study by Becher et al. [14], that the Young’s mod- ulus and microhardness of RE–Si–Al–O–N glasses (RE = La, Nd, Gd, Y, Lu) not only depend on the type of RE element but also on its amount and the amount of nitrogen. Fewer studies have been made on oxynitride glasses with alkali earth (AE) elements. For AE–Si–Al–O–N glasses [15] with AE = Mg, 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.04.036 * Corresponding author. Tel.: +46 8 161258; fax: +46 8 152187. E-mail address: zaida@inorg.su.se (S. Esmaeilzadeh). Journal of Non-Crystalline Solids 355 (2009) 1259–1263 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol