Relaxation of Libyan desert glass: Evidence for negative viscosity–pressure dependence in silica?

Journal of Non-Crystalline Solids 355 (2009) 1666–1668

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Relaxation of Libyan desert glass: Evidence for negative viscosity–pressure dependence in silica? S. Krolikowski, S. Brungs, L. Wondraczek * Department of Materials Science, Chair of Glass and Ceramics, University of Erlangen-Nuremberg, Martensstrasse 5, 91052 Erlangen, Germany

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Article history: Received 10 March 2009 Received in revised form 8 June 2009 Available online 7 July 2009 PACS: 61.43.Fs 64.70.Pf 62.50.+p Keywords: Silica Pressure effects Calorimetry Enthalpy relaxation Stress relaxation

a b s t r a c t Libyan desert glass (LDG) with silica content >99 mol% was examined to obtain evidence for negative viscosity–pressure dependence in silica. Calorimetric scanning experiments under ambient pressure revealed a shift of 22 K in glass transition temperature (Tg) from pristine to relaxed LDG, respectively. While the endothermic overshot in the isobaric heat capacity at Tg remains practically unaffected, the shift occurs due to a decrease in the onset of relaxation. Because in silicate glasses, caloric and kinetic glass transition are strongly coupled, this finding indicates that kinetic freezing of LDG originally occurred at lower temperature than it does in glasses of equivalent composition under normal conditions. Considering the most probable origin of LDG – a meteoritic impact – and assuming that at least some compression is preserved in natural LDG samples, this observation is interpreted as evidence for decreasing viscosity with increasing pressure, and is related to decreasing Si–O–Si bond angle in the pressure-regime below 1 GPa. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The dependence of viscosity in inorganic (mineralic) melts, g, on pressure, p, is of central interest for the understanding of geophysical processes [1]. In this context, decreasing viscosity with increasing pressure is certainly the most intriguing as well as the most highly disputed phenomenon. It has now been observed experimentally in a variety of liquids such as certain aluminosilicate melts and water (e.g. [2–5]). As of today, however, one may say that only for the case of water, a certain level of understanding has been reached [3,6–8] – attributing negative dg/dp to negative thermal expansion of the liquid phase. Like water [9,10], liquid and vitreous silica are materials that, although chemically simple, are full of mysteries that are still largely unexplained [11–15]. Due to a very high melting temperature and low fragility (and high viscosity at the melting temperature [16], respectively), many of even the basic properties are relatively difficult to access with today’s experimental means. Continuously discussed among these is the high pressure behavior of silica, both in vitreous and in liquid form. The dependence of viscosity on pressure within the glass forming region is presently unknown, although, based * Corresponding author. Tel.: +49 0 9131 85 27553. E-mail address: [email protected] (L. Wondraczek). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.06.009

on molecular dynamics (MD) simulations, some suggestions have been made that it might be negative (e.g. [17]). As of today, direct data on dg/dp in the glass transition region of silica is not available. However, at least one of the experimental problems, generating sufficiently large and homogeneous glasses under pressure, may be overcome by utilizing a natural material: Libyan desert glass [18] (LDG). LDG is a silica-rich glass (SiO2 > 97 mol%), originating from Egypt and the Libyan desert, where it can be found in relatively large quantities and slab sizes. Its geological origin is still debated in detail, but it is now widely accepted that it formed by relatively slow cooling from high temperature and high pressure, most probably as a result of a meteor impact [19,20]. Although it is a natural material, highly homogeneous samples, even with SiO2 > 99 mol%, can be found. Such a sample was examined in this study to explore relaxation of frozen-in compression and to obtain evidence for negative dg/dp within the glass forming region of silica, based on the assumption that at least a pressure of 100 MPa (for this pressure regime, Mitra [17] predicted a decrease in the Si–O–Si-angle of a few degrees) is retained in the glass. Then, a relaxation experiment under ambient pressure would reveal the viscosity that was pertinent to the original pressure-of-freezing. Thus, for negative dg/dp, it would monitor a shift in the region of glass transition to lower temperature (memory effect [21]).

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2. Experimental The LDG slab that was used for this study was provided by Frischat and originates from a field trip to Egypt by Weeks in 1981. From this slab, small disks of 0.5 mm thickness and 35 mg mass were cut. The chemical composition of each sample was verified by energy-dispersive X-ray spectroscopy (EDX, Table 1). Thermophysical relaxation was studied by differential scanning calorimetry (heat-flux DSC, Netzsch DSC 404 F1, nitrogen atmosphere), employing a highly-shielded sample holder for heat flow measurements and calculation of isobaric heat capacity for an average molar mass of (60.5 ± 0.2) g/mol (Cp) with an approximate accuracy of ±2 % (sapphire reference) at temperatures up to 1400 °C. Samples were polished (silicon carbide, isopropanol) on each side to guarantee efficient heat transfer from the crucible (platinum) bottom into the sample. The isobaric heat capacity of pristine LDG was recorded as a function of temperature for temperatures up to 1200 °C at a heating rate of 20 K/min. For each sample, a second scan was performed after cooling at a rate between 5 K/min and 20 K/min inside the DSC furnace. No mass loss was detected after the DSC experiments. The complete procedure of calibration, equilibration and scanning is described in detail elsewhere [22–24]. In principle, the first scans of all samples should be equivalent within the accuracy of Cp analyses (see above) and compositional variations (Table 1). Comparison of the first scans thus enables a consistency and reproducibility test of the undertaken experiments (Fig. 1). From the DSC data, apparent fictive temperature TfA [25] (pristine samples) and fictive temperature Tf (second scans), respectively, were extracted, using Moynihan’s enthalpy matching method [26]. The ¼ C pl  C pg was deisobaric configurational heat capacity DC conf p rived by fitting experimental data below Tg to the Maier–Kelley polynomial [27], C p ðTÞ ¼ a þ bT þ cT 2 , and extrapolating to Tf. Data are given in Table 1.

3. Results With respect to composition, among the different types of silica, LDG is closest to type II/type III silica. Like type II, it most probably formed by melting of natural raw materials. The main differences lie in the significantly higher OH content of about 1400– 1800 ppm [28] (similar to type III silica), and the extent of metal oxide contamination, particularly Al2O3. Under ambient pressure, Tg of dry silica (type I) is 1203 °C [29]. Small additions of Al2O3 generally result in decreasing Tg, i.e. 1191 °C for 1Al2O3–99SiO2 and 1121 °C for 2Al2O3–98SiO2 [29]. More significant changes in Tg are caused by the water content, i.e. an additional decrease by 70 K may be expected for about 1500 ppm of OH [30]. As has been discussed previously, TfA is the temperature that corresponds to the fictive enthalpic equilibrium between pristine (compressed) glass and relaxed liquid [25]. It therefore reflects the contribution of compression to the total excess enthalpy of the considered glass [22], normally [22–25,31] visible in an increase of the endothermic Cp-overshoot in the region of glass transition with increasing difference between observation pressure and fictive pressure. In the present case, however, the overshoot

Fig. 1. DSC traces of LDG samples. Full lines: first scan (pristine sample), dashed lines: second scan after applying different cooling rates. Heating rate was 20 K/min for all samples. Inset: Zoom at the region of glass formation and the Tg overshoot during the second scan after cooling at the indicated rates.

changes only marginally from first to second scan (Fig. 1). Instead, a significant shift in the glass transition region is observed. Similar behavior was recently found for compressed aluminosilicate glasses with known negative dg/dp [21]. As has been discussed by Scherer [32], Narayanaswamy [33] and others, kinetic and caloric glass transition are strongly coupled for silicate melts. Therefore, Tg, the isochom temperature corresponding to a viscosity of 1012 P s, can be extracted with high accuracy from caloric data by standardized techniques [34]. In this sense, Tg20, the glass transition temperature that results for a heating rate of 20 K/min (note that by standard, Tg must be determined at heating and cooling rates of 10 K/min) was determined from the intersection of the asymptotes of Cp(T) of the glass and Cp(T) at the glass transition and taken as a measure of viscosity (Fig. 2). That is, at Tg20, viscosity is close to 1012 Pa s. Between first and second scan, a shift of 22 K ± 4 K occurs in Tg20, indicating that during original freezing, the viscosity of the examined LDG material was significantly lower that it is under ambient pressure at the same temperature. Upon reheating inside the DSC furnace, the structure that is pertinent to the frozen-in pressure gradually relaxes towards ambient pressure conditions. 4. Discussion The problem as to whether or not a high pressure structure is preserved in LDG was studied by Pratesi, Stebbins and co-workers [35], utilizing Al2O3 contamination in LDG as tracer for pressureinduced structural changes. High-resolution Al-27 MAS NMR revealed spectra that were consistent with data on Al-coordination in synthetic glasses prepared under ambient pressure [35,36]. On the other hand, significant changes in Al-coordination would become visible only if the frozen-in pressure would exceed several GPa [37]. Eventual viscosity-effects, however, could be expected already for preserved pressure as low as some 100 MPa [5,17,21]. The fact that here, relaxation occurs at lower temperature is interpreted as experimental evidence for negative dg/dp. As has been

Table 1 Composition, fictive temperatures and configurational heat capacity of examined glass samples. Values of Tf were determined by Moynihan’s enthalpy matching technique from DSC data: TfA (the apparent fictive temperature) from the first scan of pristine LDG, Tf2 from the second scan after cooling with the indicated cooling rate. DSC-heating rates were 20 K/min for each scan. Sample

SiO2 (mol%)

Al2O3 (mol%)

TfA (±10 K)

I DC conf (J mol1 K1) p

q (K/min)

Tf2 (±10 K)

I DC conf (J mol1 K1) p

LDG05 LDG10 LDG15 LDG20

99.02 99.25 99.45 98.66

0.98 0.75 0.55 1.34

1185 1183 1190 1165

3.3

5 10 15 20

1245 1260 1265 1280

3.7

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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

Fig. 2. Average of Cp(T) over four scans of pristine LDG (see Fig. 1, full line) and relaxed LDG (second scans), respectively. Dashed lines represent asymptotes as ðAÞ used for the determination of Tg20 by the intersection method. T g20 is the ðBÞ ‘relaxation onset’ of pristine LDG, T g20 that of relaxed LDG samples.

discussed previously, such an anomaly may have several origins. Suggested in early studies and experimentally confirmed for several aluminosilicate melts, high network polymerization may generally lead to negative dg/dp when compression may cause a breakdown of polymerization [4,17]. In the case of silica, MD simulation revealed the occurrence of coordination changes at very high pressures [38] as well as a small reduction of the Si–O–Si angle [17] at low and intermediate pressure (<1 GPa). Although most properties of vitreous silica are known to be highly sensitive to chemical impurities, in the present case, the principle rheological behaviour should not be affected by differences between LDG and pure SiO2. Rather, dg/dp is a function of free volume, network polymerization and coordination. There is no evidence that a total impurity amount of 1 mol% or less, mainly of Al2O3, would significantly alter these properties and their pressure dependence. Then, the present result indicates negative dg/dp for silica. Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft (DFG) under Grant no. WO 1220/3-1 is gratefully acknowledged. We further thank Professor G.H. Frischat for providing LDG samples.

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