Study of The Effect of TiO2 Addition On The Crystalline Phases, Structure and Chemical Durability of a Nuclear Glass Ceramic

This study focused on the effect of TiO 2 addition on the crystallines phases’ formation, structure and chemical durability of a nuclear glass ceramic constituted by an aluminosilicate glass in the system: SiO 2 -Al 2 O 3 -CaO-MgO-ZrO 2 -TiO 2 . The materials with four contents of TiO 2 , ranging from 4.11 to 7.11 wt.%, are synthesized by a discontinuous method,. For the whole of materials, X-ray diffraction analysis allow identifying an aluminosilicate belonging to pyroxenes silicates family as a main phase, powelite and calzirtite. Both SEM and DTAanalyses confirmed these results. The materials FTIR analysis reveals the glass ceramics complex chemical composition. MCC1 and MCC2 tests, performed on selected glass ceramic materials, indicate that the materials with 4.11 and 5.11 wt.% TiO 2 are the most durable against Si, Al, Mg and Ce elements release, in MCC2 test; The results make conclusions valuable on the selection of such glass ceramics as candidate for the disposal of high-level waste.


Introduction
The storage and disposal of radioactive waste (RW) is a difficult task. It necessitates meticulous control. One of the RW disposal principles is to follow the waste through all stages of its life: packaging, storage, disposal, and monitoring until its radioactivity is comparable to natural background radiation [1]. A multitude of sequestration materials have been studied, among glasses, minerals, and glass-ceramic matrices.
The main works dealing with this last kind of materials concern the sequestration of the whole of radionuclides present in a liquid waste solution [2,3].
Nevertheless, because of both their high chemical durability and high ability to incorporate specific radionuclides, several ceramic matrices such as zirconates, titanates and phosphates have been proposed for the immobilization of long-lived radionuclides such as minor actinides [4-6].
The ceramics elaboration processes are difficult to implement, and many glass-ceramics are also under study. They consist of crystals homogeneously distributed in a glass matrix, and exhibit increased performances. Both glass technology and the interesting confinement properties of crystalline ceramic phases will support research on glass-ceramics (noted: GC) matrices for RW sequestration. It have been demonstrated that different nucleating agents added to a chosen parent glass has an important role on the crystallization processes and on the physico-mechanical properties of a given GC [7,8]. The effect of many nucleating agents such as ZrO2 and TiO2 on the crystallization, microstructure and durability properties of the prepared GC was reported in the literature [9][10][11].
Despite the significance of TiO2's role in the structure of silicate glasses, few studies have been conducted to date [12]. TiO2 is known to be quite soluble in silicate melts. It decreases the melt glass viscosity [7]. TiO2 addition influences the crystalline phase nature and the microstructure formed in the GC materials, and their physico-mechanical properties [13,14].
Zhou et al. [15] noted that TiO2 is often used to facilitate nucleation in silicate glass-ceramics systems. The effect of TiO2 content on the crystalline phases, structure, and aqueous dissolution rate of iron phosphate based glass ceramic waste forms was studied in depth by Fu Wang et al. [16].
In this study, we have synthesized a TiO2-rich GC by a discontinuous method, consisting in a double melting at 1350 °C, followed by both a crystallization and crystal-growth, treatments at 564 and 1010 °C, respectively. In order to study the influence of the effect of TiO2 addition The materials Archimedes density is measured by pycnometer with water as a wetting liquid.
The XRD analysis is carried out using a PANALytical X'Pert Pro diffractometer equipped with a copper anticathode, with Kα1 = 1.5418Å. The analyses are performed with a voltage of 40 kV and a current intensity of 40 mA, for 2θ ranging between 3 and 80 °. The ceramics phase identification is performed using a Philips X'Pert High Score Plus software, version 4.1 [19].
where V: is the leaching mixture total volume (cm 3 ), S is the initial sample surface effectively in contact with the leachate (cm 2 ), and Fi is the i element weight fraction of the GC materials pellet. S(cm 2 ) is assumed to be constant during the tests, and Ci, the initial elemental concentrations in the leachate negligible.
Each i element mean normalized dissolution rate RLi (g/cm 2 d -1 ) was deduced from the mathematical relation number (2). The Origin 8.5 Graph software was used for mathematical calculations.
where: ∆t: is the test duration in days (d).

Materials morphologies and densities
The 7.11wt.% TiO2 content GC synthesis was not successful, this GC exhibits a very strong crystallization in the mass during melting.
For the other TiO2 contents in the GC, after the crystallization step, the transparent glassy samples color turns into an opaque dark gray color with brown reflections (Fig. 1).On the cross-sections of the glass-ceramics, one can observe with the naked eye a relatively clear difference in appearance between the surface and the bulk of the samples, which has a darker gray color in the materials bulk compared to the surface color.

Parent glass after crystallization
The density of the obtained GCs with different contents of TiO2 is measured by the Archimedes method. The results are given in Table 2.
The GC density varies randomly with the TiO2 content. In general, it is between 2.8913 and 2.9618 g/cm 3 , for GC' TiO2 contents ranging from 4.11 to 6.11 %. Overall, the results are close to other nuclear GC materials densities reported in the literature. GC with diopside (PNC 62) and diopside (PNC 718) mineral phases, synthesized in a TiO2 -rich borosilicate glass melted at Tf = 1185°C and 1120 ° C, have densities of 3.01 and 2.94 g/cm 3 , respectively. Other GC materials with sphenes minerals formed in their structure have a density between 2.78 and 2.95 g/cm 3 [22]. A. Quintas et al. [23] have synthesized a GC from a Lu-aluminoborosilicate glass, for high-level waste (HLW) sequestration, at a melting temperature of 1350 ° C and a crystallization step at Tc = 934 ° C. They report a density of 2.946 g / cm 3 . This value is close to our experimental values.

Microstructural characterization
The diffractograms of the studied GCs are gathered in fig. 2. The phases' identification of the GC at different TiO2 contents is given in table 3. Except for GC7, for the whole of materials, the main formed crystalline phase is an aluminosilicate, of the pyroxene family (Si2O6), which is known to be a containment barrier against radionuclide dissipation. Its crystal structure is orthorhombic or monoclinic. Its content in the GC is more than 70%. For 4.11 and 5.11 wt.% TiO2 GCs, the XRD semiquantitative analysis gave: 77 % of Mg0.6Al1.2Si1.8O6 pyroxene phase (JCPDS N°01-075-1568), 10 % of CaMoO4 (JCPDS N° 01-085-0585) and 13 % of Ca2Zr5Ti2O16 calzirtite phase (JCPDS N° 01-077-1131). The presence of these phases means that ions in the glass matrix are moving and reacting [25]. The formation of calzirtite, is favored by the increase in TiO2 content in the GC materials. It increases to 25 % for the 6.11 wt.% TiO2 content GC. One can conclude that for the whole of TiO2 contents in the materials, (4.11 to 6.11 wt.%), the GC microstructure contains self-irradiation highly resistant crystals, which are able to confine radioactive waste elements [26].
The leaching rate (RL) of Si, Al, Mg and Ce as a function of time, for each material doped with different contents of TiO2 are gathered on Fig. 6

Conclusion
In this study, we performed the synthesis and characterization of glass-ceramic matrices, based on an aluminosilicate glass, in the system SiO2-Al2O3-CaO-MgO-ZrO2-TiO2, with four contents of TiO2, ranging from 4.11 to 7.11 wt.%. These GC confinement matrices are intended to confine in their structure both fission products and lanthanides elements, from radioactive waste. The GC synthesis is carried out in several stages: a double fusion of the oxides mixture at 1350 °C, a nucleation at 564 °C, and a crystal growth at 1010 °C. The GC synthesis of the material with 7.11wt.% TiO2 content, was not successful. For the others materials, the obtained GC have Archimedes densities in the range of 2.891 -2.962 g/cm 3 .
Except for the GC with 7.11 wt.% TiO2, the XRD phase identification show that the main formed phases in the materials are pyroxene, powelite and calzirtite phases, which are known as self-irradiation resistant phases. The SEM confirmed the morphological aspect of these phases in the microstructures. The DTA analysis reveals the allotropic transformations of such phases. The FTIR analysis spectra are representative of the complex chemical composition of the studied glass-ceramics. MCC1