Amorphous Mg–Fe silicates from microwave-dried sol–gels - Multi-scale structure, mid-IR spectroscopy and thermal crystallisation

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Volume 624 (April 2019)

A&A, 624 (2019) A136

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Table 1.

Lengths and dimensions derived from the standard SAXS plots for the precursor gel and MgSiO₃ silicate obtained by drying the precursor gel by microwave, in air and in vacuum.

Sample	Guinier plot	Porod plot	Zimm plot	Kratky plot
Precursor gel	Region G1: R_g∼ 10 Å low-q upturn indicates aggregation	Region P1: α ∼ 1.37 (mass fractal) r₀ ∼ 6.6 Å, L_m ∼ 40 Å	Region Z1: R_g ∼ 16 Å Region Z2: R_g ∼ 68 Å	Strong narrow K2 peak indicates highly globular large-scale structure, weaker broad K1 feature indicates less-defined structure at shorter scales

Microwave-dried	Region G1: R_g ∼ 3.3 Å low-q upturn indicates aggregation	Region P1: α ∼ 2.5 (mass fractal), r₀ ∼ 7 Å, L_m ∼ 36 Å Region P2: α∼ 2.72 (mass fractal), r₀ ∼ 23 Å, L_m ∼ 140 Å region P3: α ∼ 2.18 (mass fractal), r₀∼ 96 Å (L_m> experimental range)	Region Z1: R_g ∼ 2.8 Å Region Z2: R_g ∼ 32 Å	K3 and K2 indicate large-scale globular structures, K1 indicates structure at shorter scale

Air-dried	Region G1: R_g ∼ 3.3 Å low-q upturn indicates aggregation	Region P1: α ∼ 2.32 (mass fractal), r₀ ∼ 6.4 Å, L_m ∼ 32.3 Å Region P2: α ∼ 2.43 (mass fractal), r₀ ∼ 30 Å, L_m ∼ 214 Å	Region Z1: R_g ∼ 2.8 Å Region Z2: R_g ∼ 38 Å	Features similar to microwave-dried sample, indicating similar large-scale structure

Vacuum-dried	Region G1: R_g ∼ 2.8 Å low-q upturn indicates aggregation	Region P1: α ∼ 3.6 (surface fractal – uniform core, rough surface), r₀ ∼ 13.4 Å, L_m ∼ 173 Å	Region Z1: R_g ∼ 3.96 Å Region Z2: R_g ∼ 20 Å	Variation in scattering contrast more uniform and similar to precursor gel phase

Notes. See Figs. 4, 6 and 7 for locations of regions G1, P1, Z1 etc. and Appendix A.2 for definitions of the Porod exponent α, radius of gyration R_g, monomer radius r₀ and aggregate radius L_m.

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