Journal of The Australian Ceramic Society Volume 52[2], 2016, 87 – 91 87

Crystallization and sintering behavior of Glass-ceramic powder synthesized by Sol-Gel Process Ashley Thomas and J. Bera*

Department of Ceramic Engineering, National Institute of Technology, Rourkela, India Email: jbera@nitrkl.ac.in Available Online at: www.austceram.com/ACS-Journal Abstract The glass-ceramics in the system SiO2-CaO-Na2O-P2O5, well known as an excellent biomaterial for bone tissue engineering was synthesized by aqueous sol-gel method. The thermal decomposition behavior of the gel was studied using thermo-gravimetric analysis and differential scanning calorimetry. The crystallization behavior of glass was characterized by X-ray diffraction phase analysis, Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The results revealed that the crystallization occurred at a temperature of 700°C and the glass-ceramics was composed of primary phases; combeite (Na2Ca2Si3O9) and devitrite (Na2Ca3Si6O16). Microstructural analysis revealed that there were simultaneous sintering and crystallization processes occurring during heat treatment. Sintering kinetics of the glass-ceramics powder was also investigated. Keywords: Glass-Ceramics, Sol-Gel synthesis, Sintering, Crystallization behavior INTRODUCTION Bioactive glasses and glass-ceramics are a class of biomaterial widely used in bone tissue engineering [1, 2]. They have the ability to interact with living tissue and form bond with the bone [3, 4] and are widely used in orthopedic applications to promote bone regrowth in small, non-load bearing structures [5]. Bioactive glasses such as 45S5 have been used successfully in non-load bearing applications for many decades [5, 6]. In spite of numerous advantages, bioglasses are brittle materials and not suitable for load bearing applications [7-9]. One approach to improving the mechanical properties is the use of glass–ceramics such as Cerabone® [2, 10]. These materials show improved mechanical properties while maintaining their bioactivity [2, 10, 11]. Recently a glass-ceramics in the system SiO2-CaO-Na2O-P2O5 has been synthesized, which showed greater flexural strength than cortical bone, lower elastic modulus compared to Cerabone® and a very high bioactivity that was comparable to that of 45S5 [11].

There are two methods by which glass-ceramics are processed; the traditional melt quenching method and the sol-gel method [7]. In the traditional melt quenching process, the mixture of the oxide precursors are melted together at high temperatures (usually above 1300 °C) followed by the quenching of the molten liquid to form the glass. The glass is further annealed at a high temperature to obtain the glass-ceramics [7, 12]. On the other hand, sol-gel

technique is a chemical method in which the precursors undergo polymeric reactions in sol to form the gel. This gel is further dried and calcined to form the glass-ceramic [12, 13]. In comparison to the melt quenching technique, the sol-gel method has various advantages. In this process, the temperature required is much lower and that reduces the cost involved in the synthesis. Also, the glass-ceramics prepared by sol-gel process has nano-porosity and greater surface area which improves the bio-activity and bio-dissolution of the material. Better control of composition and homogeneity is another advantage of this method [6, 12]. During the processing of the glass-ceramics biomaterial, various thermal treatments are involved13, which lead to the crystallization of glass and sintering of particles. It is important to know the crystallization behavior and the sintering kinetics of the material. The crystallization behavior helps in understanding the structural transformation that occurs during the heat treatment and the phase that can be achieved in the final body. A proper densification process can be designed by studying the sintering kinetics. Also, the interaction between the sintering and crystallization can be understood [14, 15] from the kinetics study. Recently a glass-ceramics in the system SiO2-CaO-Na2O-P2O5 has been synthesized which showed

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greater flexural strength than cortical bone, lower elastic modulus compared to Cerabone® and a very high bioactivity that was comparable to that of 45S5[11]. This was commercially named as “Biosilicate” [16] and synthesized through the melt quenching method. Because of the various advantages of sol gel method mentioned above, in this study, Biosilicate glass ceramic was synthesized through the sol gel process and a comprehensive analysis on the crystallization behavior and sintering kinetics was carried out. METHODS AND PROCEDURES The sol-gel based glass-ceramic was prepared using the composition; SiO2 - 49.04, Na2O - 23.49, CaO -26.05, and P2O5 - 1.7 in molar proportion that reported earlier [11]. The following precursors; tetraethyl orthosilicate (TEOS) and triethyl phosphate (TEP), both from Alfa Aesar, sodium nitrate (MERCK) and calcium nitrate tetrahydrate (Lobs Chemie) were used for the synthesis. The sol-gel process used in this study has been described earlier by Chen et al [17]. Briefly, the molar ratios of precursor TEOS, TEP, NaNO3 and Ca (NO3)2 were designed according to the molar ratio of SiO2, Na2O, CaO, and P2O5 as stated above. The precursors were added into 0.25 molar nitric acid solution with constant stirring in the sequence; TEOS, TEP, NaNO3 and Ca (NO3)2 respectively. Also, each precursor was added only after the sol became clear. The final sol was kept overnight for gelling. The resulting gel was aged at 60°C for three days to stabilize the network structure of gel. Differential scanning calorimetry and thermogravimetric analysis (DSC-TGA) of the gel was performed using Netzsch instrument to study the thermal decomposition behavior. Based on the DSC-TGA results, the gel was further calcined at different temperatures.

The calcined powder was characterized using X-Ray Diffraction (XRD) using a Rigaku diffractometer with CuKα radiation analysis to assess the crystalline phase formation and structural transformations that occur during high-temperature treatment. FTIR measurements were carried out using a spectrometer in the range 550 to 2000 cm -1

for samples calcined at different temperatures (60, 700 and 1000°C). Microstructure of the samples was observed using field emission gun scanning electron microscope (FESEM) (Nova Nano-SEM/ FEI). The sintering behavior of the glass-ceramics powder compacts was studied using dilatometer (Netzsch Instrument) with a heating rate of 10°C/min up to 1000°C. The pressed glass-ceramic compacts were sintered at three different temperatures; 950, 1000 and 1050 °C and with three different dwelling time; 12, 60 and 240 minutes respectively to study the effect of temperature and time on the densification. The above three sintering temperatures were selected

based on the dilatometric shrinkage curve. After the heat treatments, the geometrical densities of the samples were measured. The activation energy for the sintering process was evaluated by using the Arrhenius equation. RESULTS AND DISCUSSION Fig. 1 shows the DSC-TGA curves of the gel powder. There was a weight loss of about 26% from room temperature to 192°C, which corresponds to a huge endothermic peak at 128°C. The weight loss was due to the evaporation of physically absorbed water. There was continuous slow weight loss from 200 to 550°C, which corresponds to the removal of water due to the condensation reaction of gel network [6]. The endothermic peak at 273°C was due to the removal of that water. Another rapid weight loss was observed in the range of 550 to 800°C mainly due to the decomposition of nitrates salts. There are two endothermic peaks at 697 and 775°C. The peak at 697°C is due to the decomposition of sodium nitrate as reported by others [17]. The 775°C peak was due to the decomposition of other nitrogen-containing compounds [6]. Hence, the sol-gel powders should be calcined at temperatures higher than 700°C for the complete decomposition of the gel.

Fig. 1: DSC-TGA curves of sol-gel derived gel powder. Fig. 2 shows the FTIR spectra of sol-gel derived glass-ceramic powder after drying, calcination and sintering. In dry gel powder, the peak observed at 1639 cm-1 corresponds to the deformation mode of the H-O-H bond of the water molecule [17]. The peaks at 1300 and 821 cm-1 correspond to the crystalline NaNO3 in dried powder [17, 18]. The bands at 960 and 1040 cm-1 may be due to the silicate absorption band [17]. These bands are very prominent in the FTIR patterns of calcined and sintered materials (Fig. 2 (b) and (c)). The 700°C calcined powder and sintered materials have no absorption band for H20 and NaNO3. Only a few bands are prominent at 1019 and 928 cm-1 corresponding to silica network structure [6, 17].

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The peak at 615 cm-1 corresponds to the deformation modes of the ‘P-O’ bond in the crystalline phosphate phase of glass-ceramics [17, 18].

Fig. 2: FTIR spectra of sol-gel derived glass-ceramic; (a) dried gel, (b) gel calcined at 700°C and (c) glass-ceramics sintered at 1000°C. Based on the DSC-TGA data, the glass-ceramics samples were fired at temperatures greater that 700°C. Fig. 3 shows the XRD patterns of glass-ceramics powders those were heat treated at different temperatures. In 700 and 950°C heat treated samples, an equal amount of devitrite (Na2Ca3Si6O16) (PDF No. 77-0386) and combeite (Na2Ca2Si3O9) (PDF No. 75-1686) crystalline phases were observed. Crystalline phases were estimated through RIR method using X’Pert HighScore software. In 1000 and 1050°C heat treated samples, about 70% devitrite and 30 % combeite were observed. It is reported that the crystalline combeite phase formation in the glassy matrix improves the mechanical strength of the glass-ceramics [17]. In addition, combeite phase does not inhibit the bioactivity [11, 18]. Hence, the formation of the combeite phase in the glass-ceramics is an essential requirement for scaffold material. Fig. 4 shows the FESEM images of sintered specimen surfaces. The figure shows a typical porous structure of the glass-ceramics derived through the sol-gel process. High porosity found may be due to the formation of gaseous by-products during the heat treatment of gel. From the images, it is clearly evident that greater amount of crystalline phases appeared with an increase in sintering temperature. In 1050°C sintered glass-ceramics, the formation of fat cubical crystals along with small needle-like crystals was observed. The pore size also increased with the increase in sintering temperature, which is due to the

simultaneous growth of grain and pores. The pore growth at higher sintering temperature is due to the coarsening effect.

Fig. 3: X-Ray diffraction patterns of sol-gel derived powders after calcinations at (a) 700°C and after sintering at (b) 950, (c) 1000 and (d) 1050°C. Fig. 5 shows the dilatometric curve depicting the relative variation of sample length dL/L0

with the temperature up to the temperature 1000oC. There was a small shrinkage up to about 2000C, which may be due to the evaporation of physically absorbed water from the glass-ceramics. Since the glass-ceramics has high porosity it can readily absorb moisture during the processing of powder compacts. There was a slight shrinkage at about 550oC and then the major densification shrinkage starts at about 850oC. Similar shrinkage behavior was also reported earlier for the 45S5 bioglass [14, 19]. The contraction near 535oC is due to the glass transition as reported earlier [19]. The second major shrinkage after 850oC is associated with the densification due to the viscous flow of glass. The glass-ceramics compacts were also fired at three different temperatures with three different times of holding. The inset of Fig. 5 shows the plot of bulk density against sintering time for each sintering temperatures. Based on this plot, the time (t) required to obtain a density common to the three isotherms was calculated for each sintering temperature (T). The activation energy for the densification process was calculated using the Arrhenius plot of ln (1/t) versus 1/T as shown in the small inset in Fig. 5. The activation energy for the densification process was 170 kJ/mol.

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Fig. 4: FESEM image of glass-ceramics compacts after sintering at (a) 950, (b) 1000 and (c) 1050°C. CONCLUSION Biosilicate glass-ceramics in the system SiO2-CaO-Na2O-P2O5 was synthesized through sol-gel process. The thermal decomposition behavior investigated using DSC-TGA revealed that the gel should be calcined at a temperature greater than 700oC for its complete decomposition. FTIR analysis showed the separation of phosphate based glass-ceramics from silica rich one. XRD phase analysis results indicated the crystallization of combeite and devitrite phases during the heat treatment of gel at 700oC. Microstructural analysis showed the porous structure in the glass-ceramics. The pore size, as well as grain size increased with the increase in the sintering temperature.

Dilatometric investigation revealed that there was a small shrinkage starting at 5350C due to the glass transition in the system. The major densification occurred after 850oC due to the viscous glass sintering mechanism. The activation energy for the sintering process was 170 kJ/mol. Finally, it may be concluded that a porous glass-ceramics suitable for bone tissue engineering scaffold may be synthesized through the sol-gel process.

Fig. 5: Dilatometric curve of glass-ceramics powder compact obtained with a heating rate of 10oC per min. The inset figure shows the variation of bulk density of glass-ceramics compacts with sintering time at three different sintering temperatures. Small inset shows the Arrhenius plot for activation energy calculation ACKNOWLEDGEMENTS This research was financially supported by the National Institute of Technology, Rourkela, India. REFERENCES 1. L.L. Hench, The story of Bioglass, J. Mater.

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