Fused Silica

Question 1

What is fused silica?

Answer 1

Glass forming oxides (SiO2 and B2O3) with high covalent character

Question 2

Why do fused silica liquids have relatively high viscosity?

Answer 2

more cohesion among SiO4(4-) tetrahedra in liquid state than liquids that have ionic or metallic bonding in solid state

Question 3

What is the equation for viscosity? Units?

Answer 3

n = shearing stress (F/A) / strain rate (velocity/plate separation distance)

Pa*s

Question 4

What effect does high viscocity have on fused silica liquid?

Answer 4

greater resistance to atomic rearrangements required to solidify into a crytalline array with coolin

Question 5

What happens to viscosity upon cooling?

Answer 5

viscosity becomes infinite

glass behaves like elastic solid

SiO4(4-) go from glass–> amorphous –> vitreous

Question 6

What does x-ray diffraction show for fused silica?

Answer 6

amorphous hump in place of diffraction peaks

Question 7

What bond angles are in fused silica?

Answer 7

bond angles in SiO4(4-) remain strictly 109.47

BETWEEN TETRAHEDRA, Si4+~O2-~Si4+ bond angle varies between 120-180, forming random network of tetrahedra

Question 8

What are the properties of fused silica?

Answer 8

high near-infrared transmittance for fiber optics

high UV transmittance for semiconductors

low CTE, high thermal shock resistance

Question 9

What are the requirements for fused silica to be formed and shaped?

Answer 9

extreme tempuratures (>2000C) for low viscosity

Question 10

What are applications for fused silica? What temperatures are required for melting for these applications?

Answer 10

beverage containers, windows, light bulbs

lower temperatures like 1400C are adequate