Glasses- Solution Colours Flashcards
Why are glasses intrinsically transparent?
They are homogeneous and lack grain boundaries.
The chemical bonds in their structures don’t offer mechanisms (electronic energy levels, vibrational states) which can absorb photons with energies in the near UV/visible/near IR.
What does optical absorption involve?
Usually involve transition metal or rare earth ions. Involves electronic transitions between ground and excited states for partially filled 3d or 4f levels. The transition energies are in the UV/visible/near IR.
What does colour depend on?
Oxidation state. So depends on melting atmosphere and balance of oxidising/reducing agents in batch. Also depends on the elements used. Coordination number on cation
Dependence of colour on temperature
Temperature causes small shifts in absorption peak positions/size. Need to specify the colour of an artefact at its operating temperature. Large temperature changes can even cause major colour change If coordination changes are involved.
Amber glass
Involves cation-anion pairs. Tetrahedral unit O2-(3), Fe3+, S2-. Fe3+ in oxidised state, S2- in reduced state so colour control difficult. Absorb strongly in UV and blue end of visible spectrum, so can prevent degradation of contents by light.
Formation of off-flavours in beer
UV and blue photons cause the molecule isohumolone to break in two. Sulfur or sulfides arise from proteins and amino acids in beer. These can react with one of the radicals produced to form thiols which have a strong taste. Amber glass absorbs the photons responsible for the initial breakdown of isohumolone.
Use of Fe2+
Strong absorption at 1100nm so is useful in heat absorbing glasses (for solar control) but bad in covers for solar cells as it will harvest visible and near IR photons.
Decolourising effect of iron impurities
Iron is common contaminant of raw materials and can give glasses a green tint because Fe2+ absorbs red and Fe3+ absorbs blue.
Reducing effect of decolourisation from iron impurities
Chemical decolourising: oxidising agents in batch which convert Fe2+ to Fe3+ which is a weaker colouring state.
Physical decolourisers: Co and Se, these absorb but at different wavelengths to Fe3+, net effect is grey which is less noticeable than a specific tint.
Overall effect may be to enhance appearance of product.
Radiation conductivity in glasses
Glass is opaque at long wavelengths (room temp) so heat transfer is by conduction. At furnace temperatures radiation is at shorter wavelength and glass is partially transparent so there is a different mechanism for heat transfer - radiation conductivity. IR transmission is affected by transition metals. Need to add radiation conductivity to normal thermal conductivity term (small for glasses) to get effective thermal conductivity used for modelling of hot melting and forming processes.
Trend for effective thermal conductivity with temperature
Effective thermal conductivity rises with temperature like exponential curve with temperature for clear glass. Rises less so for green glass. True thermal conductivity doesn’t rise very much at all. All because of rise in radiation conductivity.
Formula for radiation conductivity
λR=(16n^2σT^3)/3α n is refractive index σ is Stefan-Boltzmann constant α is absorptivity (grey body approximation) T is temperature
Dependence of heat transfer on Fe concentration in batch
Radiation conductivity decreases like 1/x graph and is almost flat at 0.1wt% Fe. Manufacturers deliberately dope batch with this level of iron to ensure consistency.
