Structural studies on RO-MgO-Al 2 O 3-SiO 2 (R= Ca, Sr or Ba) glassy systems by density measurements

J O U R N A L OF M A T E R I A L S S C I E N C E L E T T E R S 13 (1994) 1 8 0 - 1 8 2

Structural studies on R O - M g O - A I 2 O a - S i 0 2 (R = Ca, Sr or Ba) glassy systems by density measurements L. BARBIERI, C. LEONELLI, T. MANFREDINI, C. SILIGARDI

Department of Chemistry, Faculty of Engineering, University of Modena, Via Campi 183, 41100 Modena, Italy

Since density responds to variations in glass composition much more sensitively than any other physical property of glass, density measurements are used routinely as a sensitive check method in technological practice according to Knight [1] and Duff [2]. Moreover, by elaborating density measurements it is possible to investigate the structure of the glassy network to a certain extent. In general, the density is a measurement of the degree of structural compactness and it is possible to draw important information about the modifications of the geometrical configuration of the glassy network, the co-ordination change of the former ions, the variation of dimensions of the interstitial holes, etc. In the alkalineearth silicoaluminates the density increases with the percentage of the oxide added. Actually, the effect of the entrance and the coupling of the alkalineearth ions in the interstitial holes is more important than the opening of the network caused by the breakage of the Si-O-Si bonds by the new oxygen ions added as alkaline-earth oxides. The final result is an increase in density correlated with a slight increase in volume and a consequence densification. With regard to this last effect, two parameters are very representative: the volume occupied by 1 mol glass (VM) and the molar volume of oxygen (Vo), the volume of glass which contains i mol oxygen [3]. The physical property of density (d) is defined as mass per unit volume (m/v); some mathematical approaches to density are very important because they consider each single oxide that forms the glassy structure. In particular, the methods of Appen and Huggins [4] are mathematical models that are based on the idea of the additivity in terms of "partial densities" of the oxides present in a glass. Appen was the first to base the additive calculations for properties resulting from the composition of glass on the foundation of thermodynamics. The Appen equation for density is d = ~ him i

where d is the glass density, 6i is the density of the single i oxide bound in glass and mi is the amount of oxide i expressed as mol %. The partial quantities 6~ are constants evaluated for some specific glassy systems and are valid only over the range 0-rn i which is specified for each generic oxide i. It is important to note that among the properties that can be determined in this way, only density is calculated additively (without corrections) from the amount of every single oxide in wt %. 180

The Huggins method, considered less accurate than that of Appen [4], is another additive method based on the principle of "atomic fractions". The atomic fraction is defined as the number of atoms (or gram-atoms) of element M related to 1 atom (or gram-atom) oxygen in a given glass. The atomic fraction is designated by the symbol NM. This means that according to the given composition of glass, each oxide MmOn has its own atomic fraction NM. The atomic fraction No for oxygen is then always equal to unity. The Huggins equation is d = 1 / ~ VMfM

where fM = pM/100, PM representing the weight percentage and the M index refers to the electropositive element in the MmO n oxide. VM is a tabulated value dependent on the atomic fraction of the silicon expressed as Nsi = Si/O. From density measurements it is possible to define the volume occupied for 1 mol glass (VM) and the molar volume of oxygen (11o) [3]. VM is defined as

vM = E xiM,/o where xi are Mi are, respectively, the molar fraction and the molecular weight of each component i, and p is the experimental density of the glass. The second parameter, Vo, is defined as

Vo = E x,M,/ Ex,o, where Oi represents the number of oxygen atoms present in each single oxide forming the glass. It is important to note that this parameter is more indicative than the first because it is a structural index that considers both the packing and the deformation of the oxygen ions. The purpose of this work was to compare experimental density measurements on different glassy systems with the two mathematical models and to gain information about the structures of the systems. Furthermore, the volume occupied for 1 mol glass (VM) and the molar volume of oxygen (Vo) were calculated for glassy systems containing BaO and SrO, obtained by substituting Ba 2+ and Sr2+ cations in place of Ca 2+ starting from the C a O - M g O A1203-SiO 2 quaternary system. A correlation between VM or 17o and the field strength of the cations was found. 0261-8028 © 1994 Chapman & Hall

The glassy systems were R O - M g O - A 1 2 0 3 - S i O 2 (R = c a ' ; , S r or Ba), obtained by the traditional method of-me!ring in an electric kiln. The raw materials, 'reh~ent-grade oxides and carbonates of high quality, were added in different ratios in order gradually to substitute BaO or SrO for CaO (1, 2, 5, 10, 15, 20 and 2 5 m o 1 % ) . The compositions are listed in Table I. The glasses were quenched on an iron mould to obtain bars 3 - 4 cm long suitable for the density measurements. It is important to note that the glasses had a satisfactorily high degree of homogeneity for this study. The density measurements were obtained by immersion pycnometry with distilled water as immersion fluid. Five replications were performed for each glass composition, which yielded confidence intervals of approximately 1%. Hydration was not observed. The experimental data were compared with those calculated using the mathematical approach (Fig. 1).

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The glass density was increased by increasing the amount of alkaline-earth oxide being substituted for CaO. This effect confirms experimentally the network modification previously described. Moreover, the general experimental trend supports the Appen model as being the most reliable and up-to-date in the literature, since it considers both the atomic weight and the co-ordination number of the cations. The increase in density is generally associated with an increase in volume and a consequent densification. This relationship is more apparent when shown by the two parameters VM and Vo. In the plots of VM and Vo versus calcium substitution (Fig. 2), as suggested by Scholze [5], the difference between Ba > and Sr 2+ is evident. The larger volume contraction recorded for Sr 2+, in particular when the added cation is more than 10-15 mol %, must be associated to its higher field strength (Z/aZsr2+ • 0.32; Z/a2BaZ+ 0.27 [6], where Z is the valence of the cation and a is the internuclear distance). In this way, when the strontium ions are added in the form of SrO into the interstitial holes they cause a slight opening (less than with BaO) of the network and therefore an increase in VM and Vo. The SrO, also, reorganizes the structure more than the BaO, causing an increase in density. AVM and AVo (%) were also evaluated (Fig. 3). It appears more evident than in Fig. 2 that the substitution of Ba 2+ and Sr 2+ ions in the place of Ca 2+ ions modifies the network in different ways. In each composition Ba 2+ ion enlarges the network and the percentage volume changes were more evident than those with Sr :+ ions. As far as this last cation is =

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CaO

MgO

A1203

SiO2

0 1 2 5 10 15 20 25.031

25.031 24.031 23.031 20.031 15.031 10.031 5.031 0

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