Question 15

Question 1

q15a)

You have produced two silica bulk xerogels doped by 0.2 and 16.0 mol% of Sn, respectively.
Please describe the procedure you would follow in order to transform the xerogels into densified SiO2 glasses or glassceramics.
Please also list all the phenomena occurring during the process.

Answer 1

• Heat treatment: at elevated temperature Sn and Si switch positions until enough Sn is together that it can nucleate and form a stable phase (SnO2, if enough Sn exist)
• densification of the xerogel into a dense ceramic at higher temperatures

Steps: softening (Si-O-Si bonds start to move), nucleation + growth (switch places), segregation (Sn crystallites form), densification (decrease porosity)

how to change number and size of nanocrystals:
Dopant concentration, densification temperature, densification atmosphere

Question 2

q15b)

Explain in which terms the two samples are different after full densification and how you would be able to distinguish one from the other

Answer 2

0.2 %mol: homogeneous distribution of Sn in silica matrix 16%mol: presence of SnO2 crystallites

• Nanocrystals are formed when Sn > 0.5% can see it in XRD if there are other crystalline phases, SEM/EDX for elemental distribution
• SnO2 crystals have a higher refractive index than SiO2 → measure with ellipsometry
• Color → Sn < 0.5% is transparent and doesn‘t have any color while with 16% Sn, the color is dark orange (because introduce SnO2 → new energy levels → colors (UV-Vis, eyes)