temperature synthesis of Zircon by sol-gel process
Zircon film on top of an Alumina substrate
Silica precursor reactivity and gel formation
It was followed by transmission IR spectroscopy and SAXS. The sol characteristics were (TEOS) = 0,5 mol/l and the hydrolysis ratio Rw = 10
Figures 2 and 3 respectively show the IR spectra evolution of the TEOS sol during hydrolysis and condensation at 40°C. Fig.2 shows that water and TEOS ( 1170cm-1 band) concentrations are falling down till an ageing of 2.5 hrs.
Then as shown on Fig.3, the water concentration increases with a concomitant increases of the band around 1220 cm-1 characteristic of the Si-0-Si formation 2.
Fig.4 gives the number of water molecules that have reacted with one TEOS molecule as calculated from the area under the 1655cm-1 band and the Beer s law in the water-ethanol system. TEOS is nearly fully hydrolyzed after 2.5 h before condensation occurs either at 40°C or at 70°C for Rw = 10.
Fig.5 shows the evolution of SAXS curves of the sol and gel (prepared at 70°C)
versus ageing time. For reaction time less than 1 day, although IR spectroscopy
shows that polycondensation is occurring, the SAXS intensity is very weak. The
calculated gyration radius increases continuously up to the gel point (10 days)
and reach a value close to 10 nm.
Zirconia precursor reactivity and gel formation
Zirconia sol was synthesized according to the figure 1 procedure with
Due to a rapid reaction kinetic Zirconia sol has been followed only by SAXS
and for period of time much smaller than the silica sol.
The Porod constant is invariable during gelation at 1.6 indicating a more open structure for ZrO2 in respect to silica (=2).
Although not shown here Zirconia sols doped with Copper salts at RCu= [Cu]/ [Zr] =0.0166 shows smaller size particles and even more open structure than undoped ones (Porod = 1.5)
Stoechiometric Zircon sols have been prepared according to the procedure shown in figure 1 with the following characteristics taking into account the results obtained on pure silica, undoped and doped Zirconia sols
The Porod constant is equal to 2 as it is the case for the pure silica sol and gel The Guinier radius reach rapidly (30 min) 4 nm and evolves slightly then up to 5.5 nm for a reaction time of 11 hrs. (figure 7b)
The above trends indicate that under the present experimental conditions
1. Nanosize ZrO2 species are formed immediately and their size doesn't evolves significantly with reaction time.
2. Cu dopant decreases the size of the final ZrO2 species and make the Zirconia network more open.
3. Silica seems to condense around ZrO2 particles up to the gel point (11hrs) and leads to an homogeneous at the nanosize level network of SiO2 and ZrO2.
This is confirmed by XRD. Indeed without Copper as shown in figure 8 up to 1300°C the main crystalline phase is tetragonal ZrO2. Zircon appears only at 1500°C whereas this temperature drops at 900°C with the presence of copper at RCu = 3.32 as shown in figure 9.
Since the Copper decreases the Zirconia particle size and affects the way the SiO2-ZrO2 systems crystallize, it is expected that ZrSiO4 formation should be dependent on Cu concentration. This dependence has been confirmed by DTA for Zircon gels with different ratio RCu as shown in figure 10.
The first high temperature exothermic peak around 900°C is due to the crystallization of t-ZrO2 and the second one (1300-935°C) due to Zircon.
As one can clearly see in Fig.10 without Copper the only crystalline phase is tetragonal ZrO2 and appears around 900°C. By increasing the concentration of Copper t-ZrO2 peak shift to lower temperature and ultimately disappears at RCu above 6.6. At the same time the Zircon peak shift from 1500°C (not shown here) without Copper to 935°C at R= 6.65.
A variety of other dopants were used in order to lower the crystallization temperature of Zircon and they rank as follows:
Decreasing order of efficiency on Zircon crystallization