Glasses- Colour and Colloids

Question 1

What is in white glass?

Answer 1

Typically 1-5wt% nom-absorbing precipitated crystals. Examples are CaF2, BaF2, NaF, SnO2. These are transparent particles that scatter incident light.

Question 2

Key factors for scattering light in white glass

Answer 2

Difference in refractive index between glass and crystal.
Particle size.
Number and density of particles.

Question 3

Effect of size of particles for white glass

Answer 3

Optimum size about half the wavelength of light in the glass. Smaller particles scatter less so give more transparent product. Larger particles mean too few boundaries for efficient scattering giving a translucent, non-opaque product.

Question 4

Making white glasses

Answer 4

In most white glasses the crystalline phase is initially in solution in the melt but precipitates on cooling giving many small crystals. High nucleation rate and low growth rate just above Tg.

Question 5

Liquid-liquid phase separation

Answer 5

In selected glasses (particularly borosilicates) at temperatures over Tg separation into two distinct liquid phases occurs naturally

Question 6

Examples of dopants and their colours

Answer 6

Au- pink/hint of purple
Cu- deep red
Ag- yellow/orange
Se- bright red
CdS/CdSe- yellow/orange/red

Question 7

Particle concentrations needed for colouring

Answer 7

Glasses are strongly coloured even at low particle concentrations (<10ppm). So can use flashing (thin layer on clear substrate). For metals absorption excites surface electron resonance- allowed transitions which are very intense

Question 8

What happens if particles grow too large?

Answer 8

Grow to 50nm or more. Strongly coloured glasses are produced which also partially scatter light. They become translucent with different colours in transmission/reflection.

Question 9

Silver staining

Answer 9

For stained glass windows. Involves Ag+ diffusing in from a salt bath (e.g molten AgCl) in exchange for Na+. The Ag+ ions are reduced to Ag by redox reaction with Fe2+ or Sn2+ in the glass at the process temperature (450C). The Ag atoms agglomerate to form nanoparticles of silver metal giving a strong yellow (absorbs blue).

Question 10

Crystal growth

Answer 10

Crystals usually derived from phase soluble in melt at high T but precipitates on cooling as melt supersaturates. Precipitate on very fine scale (