Titanium Science
Titanium is another of those materials that is both complex and not yet fully understood.
Its most common form is TiO2, with a valency of +4. But it is also found with valencies of 3 (Ti2O3), 2 (TiO) and -1.
Is it a glass former?
One would think that TiO2 is a glass former, as this is the same form as SiO2. But there's more to it than that in deciding what is a glass former, and Ti fails on a number of the rules people have come up with to determine glass formers:
- Goldschmidt says that the ratio of diameter of the metal ion to the oxygen ion must be in the range of 0.2 to 0.4, to allow tetrahedra to form, whereas for Ti4+ it is 0.6, increasing to 0.8 for Ti3+
- Stanworth says that the electronegativity must be in the range of 1.90 - 2.20, whereas Ti is 1.54
- Sun gives the strength of the oxygen bond as the criterion, with bond strengths >80kcal/mol being glass formers, <60 network formers, and 60-80 as intermediates. For titanium, the bond strength is 73. This was extended by Rawson and then Boubata to include the melting point and the specific heat capacity, but titanium still fails the test
Although it cannot form a glass on its own, when mixed with something like potassium a glass will be formed, even though neither potassium nor titanium are glass formers.
Adding Ti
For additions of up to 10% molar of TiO2, the Ti replaces Si in the glaze matrix. Above tht (at least to 12%, the limit of the experiments), TiO5 polyhedra are formed. This increases the melting point, stiffness and hardness, but makes the glaze more brittle and prone to cracking.
In a borosilicate glaze, in at least some instances, adding Ti significantly hardens and toughens the glaze.
Colouring
Non-crystaline TiO2 is pretty well transparent. Whilst about 8-10% by weight can be dissolved into the glaze at high temperature, on cooling this drops to 5%. The anatase form of TiO2 is an excellent opacifier, except that at above about 850°C it changes to rutile. Rutile crystals then start growing, and they absorb UV light very close to the visible band, giving a cream colour.
Small amounts of Ti (<2% molar) can have strong effects on the colour. This is most noticeable in reduction and in some boron and phosphor glazes, where the TiO2 reduces to Ti2O3, giving violet-purple-blue colours.
Titanium can have a strong effect on the behaviour of other colourants. In the case of iron, it is mostly present as Fe3+, taking the role of an almost colourless network modifier, with a small amount acting as a glass former, giving the amber yellow colour. Adding titanium enables more of the iron to act as a glass former, deepening the colour.
It has the same effect on copper, moving the Cu2+ from network modifier to glass former, and shifting the colour from blue to green or brown. And on uranium, moving U6+ from modifier to former, the uranyl changing to uranate, and creating strong yellow colours.
When used with small amounts of manganese, the colour shifts from a weak yellow to a strong amber, or orange if cerium is also present.
Crystals
Small amounts of Ti (3 - 5% by weight from one source, upwards of 2.5% molar from another) will act as a seed, encouraging other materials to crystallise. Note that in many cases the crystals would normally just form on the surface, whereas in the presence of a seed the crystals form throughout the thickness of the glaze.
Adding 10% or more will result in the formation of long, thin rutile crystals.
Titanium
Titanium States
Titanium is typically present in multiple forms, with coordination numbers of 4 (TiO2), 5 and 6 (TiO3).
The tetrahedral TiO2, and takes part in the glaze matrix as a glass former, in exactly the same way as silica - though the larger atomic radius and weaker bonds means its addition weakens the network and reduces viscosity.
The other forms of Ti act as network modifiers.
The five fold form is probably a square based pyramid of O, with 4 Ti-O bonds at the base and one shorter Ti=O bond to the apex, the titanyl bond (there is also a possibility that it is a trigonal bipyramid). The titanyl bond is non-bridging, so this form is a network modifier.
"Silicate Glasses"
At low concentrations with alkali network modifiers (up to about 6% molar), it is present mainly as TiO2. Above this, the amount of five and six fold forms increase.
Alkaline Earth Silicate Glasses
Similar behaviour occurs, but the average coordination number is higher at 4.8 - 5.8, indicating greater proportions of the higher coordinate number forms. TiO2 dominates at low levels of alkali earth, but drops to below 10% as the alkali earth increases. Little no sixfold coordinated titanium is found.
Alkaline Silicate Glasses
In sodium silicate glass, there is a mix of 4 and 5 fold Ti up to 14.3% TiO2 by weight, above which only 5 fold Ti is found. The same happens with potassium silicate glass though with more 4 and less 5 fold Ti. Generally, alkaline network modifiers result in more 4 fold Ti and less 5 or 6 fold, whereas it is the other way round for alkaline earths.
Note that the Na will join with the Al in preference to the Ti. Whilst Al with join with one Na, each Ti will join with 1.2 - 1.5Na (decreasing with increasing temperature); another study for K found that one Na linked to 0.7K, though the test procedures weren't identical).
At temperatures above about 750°C, viscosity decreases with increasing Ti, though at lower temperatures the reverse was found to be the case.
Alkaline Earth Alumino-Silicate Glasses
There is no less than 60% of the Ti in 5 fold form, and the rest in 4 fold. Less than 20% is 6 fold.
One study shows that if Ti < 2.8% (molar), or possibly in reduction, a mix of TiO and Ti2O3, and possibly Ti, is formed and these act as network modifiers. In this case, Ti2O3 is the strongest network modifier, and decreases viscosity of the melt, whereas the other increase viscosity.
Alkaline Alumino-Silicate Glasses
The Na will bond preferentially with Al over Ti.
With 5% molar TiO2, about 60 - 75% of the Ti is in 5 fold form, and the rest in 4 fold.
It is likely that there is phase separation, with one phase of Si and Ti, and the other of Si and Al (with both having balancing Na for the Ti and Al as needed). Where there is a deficit of Na, this may lead to Anatase (a phase of TiO2) precipitating out of the Si/Ti phase where there is high Ti, transforming to rutile as the temperature drops.
In another melt of borosilicate glass (Si, B, Na and Ti) it was found that the Ti just took up a tetrahedral structure, without the complexities given above. Whether this is due to the presence of boron or the absence of aluminium isn't clear.
Anatase (a phase of TiO2) may precipitate out, transforming to rutile as the temperature drops.
Cordierite (2MgO•2Al2O3•5SiO2) with Ti(2MgO•2Al2O3•5SiO2•TiO2)
Although in this melt there is enough Mg to charge balance al of the Al, and so we would expect it all to be in the form of 4 fold tetrahedra, in practice measurements show than in all Mg and Ca glasses there exists some 5 and 6 fold Al - this is largely because the alkali earths are less good at charge balancing than the alkalis. The base melt was 9% five fold and 2% 6 fold, but on adding the Ti the 5 fold increased to 15%, and in other tests the higher the Ti, the higher the proportion of 5 and 6 fold Al. This is because the Ti can compete with the Al for the charge balancing alkali earths, so less Al is charge balanced. The presence of 5 and 6 fold Al acts as a network modifier, distorting and weakening the matrix, and reducing melt viscosity. Also, looking at the Mg, its coordination number goes from 5.1 to 4.7; the Mg is taking places in the network that would be taken up by the Al, so there is less Mg to act as a charge balancer on the Al, and so the Al has to take up higher coordination numbers.
About 50% of the Ti is in 5 fold form, and 25% each in 4 and 6 fold; this differs from glasses without Al, where only a little Ti is in 4 fold form, and none in 6 fold.
Bonds formed are as follows:
Ti-Ti polyhedra: Connected at corners. No edge bonding as would occur in a TiO2 crystal.
Ti polyhedra and 4-fold Al: Connected at corners
Ti polyhedra and 5-fold Al: Connected along an edge: with 4, 5 or 6 O on the Ti, and 5 on the Al, 2 adjacent O are shared to the same Ti and Al - these act as the seeds for crystallisation
Generally, high coordination number Al is found next to Ti.
Crystal Formation
It is generally known that the presence of titanium acts as a seed to promote the growth of crystals in the glaze. The way this works is that the titanium doesn't actually grow crystals, but the structure of the Ti and 5 fold Al is close enough to that of a crystal (due to the shred edge) that substances that can form crystals will start growing off it