Synthesis and Characterization of Monophase Cao-TiO 2 -SiO 2 (Sphene) Based Glass-Ceramics

: Sphene based glass-ceramics (CaTiSiO 5 ), an excellent candidate for a host lattice of ceramic materials and for nuclear waste immobilization, has been prepared from a powder mixture of CaCO 3 , TiO 2 and SiO 2 using vibro-milling for homogenization. Starting powders were melted at 1400 °C for 2 h, cooled to room temperature, grounded again, then crystallized by thermal treatment yielding a sphene glass-ceramic. The evolution of the phase composition during thermal treatment was investigated by X-ray powder diffraction (XRPD), FT-IR, Raman and thermal analyses (TG-DTA). Pure synthetic single phase sphene was formed at 800 °C for 4 h, even it is very hard to obtain monophase powder at such low temperature. Powder morphology was analyzed by scanning electron microscopy (SEM).


Introduction
Glass-ceramics can be used for various applications, such as thermal, chemical, biological and dielectric ones. These kinds of materials offer great possibilities as we can control their properties, including strength, resistance to abrasion and coefficient of thermal expansion [1]. Another advantage is the simple fabrication process in combination with a lower production cost [2][3][4][5]. The synthesis of the parent glass is an important step in preparing the final glass-ceramic material because the precursors and their percentage in the glass composition manage the precipitation of the crystalline phases. The results of this process can provide glass-ceramic with the desired properties.
Beside the biomaterials field, the glass-ceramics can be used as nuclear waste storage. They are significantly more durable than borosilicate glasses in a wide variety of leachates at neutral or alkaline pH values [6]. In previous research, the Canadian Nuclear Fuel Waste Management program has considered the possibility of waste storage with glass-ceramics containing crystalline titanite embedded in an aluminosilicate glass [7,8]. It can also be used for the stabilization of waste sludge [9,10] and other waste material [11,12].
Sphene or titanite (CaTiSiO 5 or CaTiO(SiO 4 )) belongs to the nesosilicate family of minerals. It crystallizes in monoclinic symmetry in two space groups: A2/a and P2 1 /a [13]. Sphene is a phase well known for its excellent containment capacity and long-term behavior (high chemical durability and self-radiation resistance). It has good thermal stability and it is an excellent candidate as a host material [14], as well as biomedical engineering (coatings on Ti-6Al-4V) [15]. Furthermore, it can be used for nuclear waste disposal [16], luminescent materials [17] and pigments [18][19][20][21] because of the ability to incorporate many elements into its crystal lattice.
When preparing the glass-ceramics at the laboratory, the crystallization of the parent glass is carried in two-phase via thermal treatment: nucleation and growth. In this paper we present the evolution of the crystallization, followed by scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR), Raman spectroscopy and X-ray powder diffraction (XRPD), with the temperature of the crystal growth thermal treatment, in the range 650-1250 °C. The formation process from glass to the final glass-ceramic product is discussed for different temperature treatments.

Powder preparation and synthesis
Reactants used in the synthesis were commercial powders: TiO 2 (Lab. Art. 808 E. Merck), SiO 2 (ASP-K-amorphous, Prahovo) and CaCO 3 (pro analysi, 11490, Kemika, Zagreb). Sample was prepared from stoichiometric amounts of powders and weighed 5 grams. The powder mixtures were homogenized in the vibratory mill (Fritsch Puloerisette Analysette Laborette, type 09 003, no. 155, 380 volt). Detailed description of the synthesis procedures for sample can be found in the original paper [28]. Samples were ground for 30 min in air atmosphere prior to melting at 1400 °C in a platinum crucible for 2 h. After melting, sample was poured in water and grinded for glass homogenization before further thermal treatment. The glass was transformed into glass-ceramics by annealing in an open-air atmosphere in furnace. Calcination of powders was carried out at different temperatures from 600 to 1250 °C in at a heating rate of 10 °C/min and a soaking period of 4 h in alumina crucibles.

Characterization
The thermal stability of samples was investigated by non-isothermal thermogravimetric analysis (TG) and DTA analysis using a SETARAM SETSYS Evolution-1750 instrument. The measurements were conducted at a heating rate of 10°C /min in a dynamic air atmosphere (flow rate 16 cm3/min) in the temperature range from 30 to 1250 °C Fourier transform infrared spectroscopy (FTIR) was performed in the absorbance mode using a BOMEM Michelson Series MB FTIR spectrometer set to give undeformed spectra. The resolution was 4 cm −1 in the 400-2000 cm −1 analyzed range. The spectra were obtained at room temperature from KBr pressed pellets prepared by mixing 1.5 mg of a sample with 150 mg of KBr.
All of the samples were characterized at room temperature by X-ray powder diffraction (XRPD) using Ultima IV Rigaku diffractometer equipped with Cu K α1,2 radiation using a generator voltage (40.0 kV) and a generator current (40.0 mA). The range of 10 -90 °2θ was used for all powders in a continuous scan mode with a scanning step size of 0.02 ° and at a scan rate of 2 °/min. Phase analysis was done by using the PDXL2 software (version 2.0.3.0) [23], with reference to the patterns of the International Centre for Diffraction Data database (ICDD) [29], version 2012.
The average crystallite size (D) was calculated on the basis of the full-width at halfmaximum intensity (FWHM) of the main reflections by applying Scherrer's formula: where K is a Scherrer's constant (~0.9), λ is the wavelengths of the X-ray used, θ is diffraction angle and β is corrected half-width for instrumental broadening given as β = (β mβ s ) where β m and β s are observed half-width and half-width of the standard monoclinic sphene sample, respectively.
Internal lattice strain (Δd/d) of calcined samples was estimated from the Williamson-Hall plots, using following equation [30]: where β total represents full-width half-maximum of the characteristic XRPD peak and Δd is the difference of the d spacing corresponding to a typical peak. The strain of nanocrystals, Δd/d, can be estimated from the slope of function β·cosθ vs. sinθ whereas crystallite size, D, can be estimated from the y-intercept.
Micro-Raman scattering measurements were performed at room temperature using a Jobin-Yvon T64000 triple spectrometer system equipped with a liquid-nitrogen cooled CCD detector. The λ=514.5 nm line of an Ar + /Kr + mixed laser was used as an excitation source.
Microstructure and grain size were investigated using Field Emission Scanning Electron Microscopy (FESEM), performed on a JEOL-5200F Scanning electron microanalyzer.

Results and Discussion
The results of thermal analysis of sample (as-prepared glass) after melting at 1400 °C are presented in Figure 1. As TG curve indicates, there is no obvious mass loss. At low temperature (below 300 °C), small exotermic peaks corresponding to volatiles appear larger, presumable due to the use of coarse powder [31]. As shown in DTA curves, there is a small endothermic peak attributed to the glass transition temperature range. Its minimum that starts at 760 °C refers to the glass transition temperature (T g ). The temperature at 886 °C belongs to sharp exothermic peak, due to the amorphous-crystalline transformation. The maximum temperature belongs to the crystallization peak temperature (T p ).  The changes in the X-ray pattern due to annealing are presented in Figure 2. The pattern of glass sample and sample obtained at 650 °C corresponds to amorphous materials; no crystalline phases were detected. According to the results of TG/DTA, the glass starts to crystallize around 760 ºС. At 800 ºС X-ray powder diffraction results indicated that there was a significant change in the sample, and glass recrystallized to form sphene (CaTiSiO 5 ). All of the diffraction peaks belonging to sphene were observed. On further increasing the temperature up to 1250 °C, the intensity of sphene reflections increased, due to better crystallization. In addition, the peaks moved to slightly higher scattering angles on annealing, while the lattice volume decreases [32,33]. The height of the strongest peak, the (200) reflection, is plotted against the annealing temperature in Figure 3, and the changes in lattice volume are similarly plotted in Figure 4. Sphene is a principle crystalline phase above 800 °C.
The main reflections in these patterns are observed at 2θ of 17, 27, 29, and 34 °, which are typical for the sphene structure. All the structure information was taken from American Mineralogist Crystal Data Structure Base (AMCDSB) [34]. Pure synthetic single phase sphene was formed at 800 °C for 4 h, even it is very hard to obtain monophase powder at such low temperature.  The values of crystallite size and internal strain of samples after melting at 1400 °С and calcined at different temperatures for 4 h are presented in Table I. The average crystallite size increases with an increase in calcination temperature (up to 1100 °С) because of accelerated diffusion at higher temperatures, with decreasing the lattice parameters. At 1250 °С, due to close temperature range of melting point, crystallite size starts to decrease. Furthermore, the internal strain of samples calcined at different temperature which was estimated from the slope of Williamson -Hall plots is presented in Figure 5. Just after the crystalization, there is no evident strain. To confirm X-ray powder diffraction results, FT-IR measurements were done. FT-IR spectra of samples sintered at different temperatures are shown in Figure 6. From 800 °C up to 1250 °C, vibrations centered at about: 895 cm -1 , 870 cm -1 , 694 cm -1 , 561 cm -1 , 468 cm -1 and 424 cm -1 correspond to vibration of sphene and they are in good agreement with published data [35][36][37][38]. The spectra are dominated by the IR band near 870 cm -1 which is attributed mainly to SiO 4 stretching modes. The broad band near 694 cm -1 is associated with TiO 6 octahedral stretching modes, polarized along the crystallographic a-axis and it is related to the crystal quality [39]. Vibration band around 1636 cm -1 due to the asymmetric stretching mode of CO 3 -2 were also detected [40].
The Raman spectra collected from samples are shown in Figure 7. The characteristic bands of sphene that occur in Raman spectra are centered around 167, 258, 473, 548 and 608 cm -1 [38,43]. All peaks shift to higher wavenumbers with increasing temperature.
Regarding amorphous systems, in Ti-Si-O frameworks the Ti 4+ cations can occur as 6-, 5-and 4-coordinated [48,49], and penta-and tetra-coordinated Ti 4+ can be found in heavily metamict sphene [50][51][52]. When decreasing Ti coordination, the Ti-O bond strength increases. As a result, in a disordered framework, the Ti-O bond stretching mode would move to higher wavenumbers as compared to the Ti-O bond stretching mode having only TiO 6 octahedra.
The SEM micrographs of glass-ceramics obtained at various temperatures are shown in Figure 8. Samples were crushed in mortal prior the measurents. For glass obtained at 650 °С, particles with irregular shapes were observed, as shown in Figure 6(b). After being calcined at higher temperatures (800-1250 °С), similar anhedral shape is seen (Figure 8 (c-f)). Particles have a smooth fracture surface with no obvious cracks or faults on the surface.

Conclusion
Glass and glass-ceramics in the CaO-TiO 2 -SiO 2 system have been successfully synthesized. From DTA curves, glass transition temperature (T g ) starts at 760 °C and temperature at 886 °C belongs to the crystallization peak temperature (T p ). X-ray powder diffraction results indicated that at 800 ºС glass recrystallize to form sphene (CaTiSiO 5 ), and the peaks moved to slightly higher scattering angles on annealing, while the lattice volume decreases. Pure synthetic single phase sphene was formed at 800 °C for 4 h, despite being difficult to obtain monophase powder at such low temperature. The effect of temperature increase in FT-IR and Raman measurements is seen as a decrease in band width and an increase in band intensity. All peaks shift to higher wavenumbers with increasing temperature, according to the Raman. SEM images showed anhedral shaped particles.
For crystal growth temperature (T c ) ranging from 800 to 1250 °C, sphene is the only crystalline phase. Thus there is a wide range of temperature for the preparation of monophase sphene-based glass-ceramics that can be designed as durable waste forms for immobilization.