Low temperature synthesis of Zircon by sol-gel process
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P. Leture, M. Prassas¹ , A. Lecomte, A. Dauger

E.N.S.C.L, 47 a 73 bId Albert Thomas, 87065 LIMOGES Cedex
¹Corning S.A., 7 bis avenue de Valvins, 77210 AVON Cedex

Zircon film on top of an Alumina substrate

Manufacturing of ultra-fine powders has become of great interest  in ceramic technologies. Sol-gel techniques are mostly used in order to have high purity and very reactive powders. Zircon phase which has an excellent chemical durability and refractoriness is not easily obtained at low temperatures even starting with stoechiometric compositions. The aim of this work is to provide a recipe which allows, zircon (ZrSiO4) powder to be synthesized at temperature below 1100°C.

Sol-Gel preparation

It is well known that, the reactivities of TEOS and Zr(OPr)₄ are very different when mixed to water-alcohol solution. Zr(OPr)₄ particularly hydrolyze much faster than TEOS leading to an independent ZrO2 network within the silica network. In order to lower the reactivity of zirconium alkoxides in respect to TEOS and obtain a random network constituted of interlinked ZrO2 and SiO2 tetrahedron, the Zirconium precursor was chelated with acetylacetone ¹. In addition,  TEOS has been pre-hydrolyzed at 70°c for 1 hour before adding the chelated Zr(OPr)4  to the reaction vessel.  Different  transition elements were also used as dopants and their impact on facilitating Zircon crystallization evaluated by XRD.    

Figure 1 shows the generic process used to manufacture the gel whose composition was calculated so that it was stoechiometric to zircon. This procedure was adopted after investigating separately the reactivities of both silica and zirconia sols.

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 R_w = 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 R_w = 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.  
Gelation occurs at a Porod constant of 2 indicating an opened ramified structure.

Zirconia precursor reactivity and gel formation

Zirconia sol was synthesized according to the figure 1 procedure with 
R_w = 10, R_a = 0,66 and Zr(OPr)4 = 0,25mo1/l. 

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.
Fig.6 shows the evolution of SAXS intensity of Zirconia sol and gel during a period of 24 hrs. All curves present intensity maxima which shift to smaller angles with reaction time.
This behaviour is  an indication of a medium distance order which length is proportional to the inverse of the scattering vector value at the maximum of the scattered intensity. The gyration radius appears lower than the silica one and is equal to 7 nm after 24 hrs.

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 R_Cu= [Cu]/ [Zr] =0.0166 shows smaller size particles and even more open structure than undoped ones (Porod = 1.5) 

Zircon sols

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

Fig.7 shows the evolution of SAXS intensity versus the scattering vector. All the curves show a maximum similar to the pure 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 R_Cu = 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 R_Cu 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 R_Cu 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

Conclusions

Zircon composition has been  synthesized by  hydrolysis and polycondensation of TEOS and zirconium propylate.  In order to lower the reactivity of zirconium alkoxide in respect to TEOS and obtain a random network constituted of interlinked ZrO2 and SiO2 tetrahedron  predisposed to facilitated crystallization at low temperature, the Zirconium precursor was chelated with acetylacetone. The structural evolution of sols and gels was followed by Small Angle X-Ray Scattering.
Even with this precaution however, where a mixture at the atomic scale is expected, zircon phase becomes predominant only at temperature higher than 1400°C.  It was found that the adjunction of  Ag, Cu, Co, Cr, Fe, Mn, Ni, V or Zn ions as dopants into initial system favors crystallization of zircon between 1000°C and 1200°C for an atomic ratio dopant/(Zr+Si) equal to 1,5.  Both  Cu and Zn ions were found  most efficient additives for zircon crystallization, which in this case takes place below 1050°C.