Effect of Sintering on Crystallization and Structural Properties of Soda Lime Silica Glass

The effect of sintering temperatures on crystallization and structural of the soda lime silica (SLS) glass was reported. Elemental weight composition of the SLS glass powder was identified through Energy dispersive X-ray fluorescence (EDXRF) analysis while the thermal behavior of the glass was determined using Differential thermal analysis (DTA) technique. Archimedes’ method and direct geometric measurement were respectively used to determine bulk density and linear shrinkage of the glass samples. Crystallisation behavior of the samples was investigated by X-ray diffraction (XRD) analysis and chemical bonds present in the samples were measured using Fourier Transform Infrared (FTIR) spectroscopy. Results showed an increase in the density and linear shrinkage of the samples as a function of the sintering temperature. The XRD analysis revealed the formation of α-quartz (SiO2) and a minor amount of devitrite phases in the samples and these were further verified through the detection of chemical bonds by FTIR after sintering at 800oC. The properties of the glassceramics can be explained on the basis of crystal chemistry which indicated that the alkali ions formed as carriers in the random network structure and can be recommended for the manufacture of glass fiber or toughened glass-ceramic insulators.


Introduction
Nowadays, many countries are having difficulties on disposing solid wastes from industries due to limited landfill sites for the solid wastes disposal.A number of methodologies of treatment and recycling the waste materials have been developed to minimize the harmful effects caused by the landfill disposal solid waste [1][2][3].Among the materials being recycled today, glass is one of the most difficult materials to be recycled.It is a solid material with a mixture of inorganic compounds, which has been widely used for optics, sensor detection, and solid-state lasers [4][5][6].
Basically, three important compounds present in the glasses include the network formers, modifiers and intermediate [7].In general, network formers are oxide compounds that participated in the formation of the amorphous glassy network.Amongst the primary glass formers, which have been used in the commercial glasses are the SiO 2 , B 2 O 3 and P 2 O 5 which form continuous three-dimensional networks, according to Zachariasen's theory and capable of forming glassy network independently.
Besides, other glass formers such as GeO 2 , Bi 2 O 3 , As 2 O 3 , Sb 2 O 3 , TeO 2 , Al 2 O 3 , Ga 2 O 3 , V 2 O 5 , As 2 S 3 and GeS 2 are also used due to the capability of their cations to form polyhedral [8].Unlike the network formers, network modifiers do not participate directly but instead modified the glassy network formation.The presence of the network modifiers in the glasses will improve specific properties of the glasses.The cations which have a tendency to be a network modifier are Li, Na, K, Cs, Rb, Be, Mg, Ca, Ba, Sr, Zn, Cd, Hg, Ga, Sn, and Pb [9].Intermediate species fall somewhere between network formers and modifiers; although they cannot form a glass on their own, intermediate species may substitute as a network former in the glassy state.Some oxides classified as intermediates can act either as network modifiers or formers.For instance, Al 2 O 3 is not originally functioned as a glass former.However, the Al 3+ cations can replace Si 4+ cations in the network (tetrahedral coordination) or can act as a modifier in octahedral coordination.
Among the conventional glasses, soda lime silica (SLS) glasses have attracted much attention because of their good glass forming nature.When compared to several other conventional glass systems, the SLS glasses offer fine optical and mechanical properties, such as good chemical stability, high UV transparency, low thermal expansion coefficient, leading to strong thermal resistance, low nonlinear refractive index, high surface damage threshold, large tensile fracture strength and good durability [10][11][12][13].Differing from most of the commercially made glasses that are composed primarily of silica (70.980 wt.%), the SLS glass consists of SiO 2 , Na 2 O, CaO, MgO and Al 2 O 3 in amorphous with the main framework attributed to the SiO 4 (tetrahedral of silica) [14].SiO 2 itself has a very small thermal expansion, high resistance to thermal shock and chemically inert [15].However, due to highly cost and a high melting point (1723ºC), network modifiers such as alkali/alkaline earth oxides were added into the SiO 2 glass to break the bridging oxygen bonds in the Si-O-Si bonds.With this, the melting temperature of the glass was reduced.Therefore, the SLS glasses have improved properties, which include high stability against crystallization, high viscosity at the liquidus temperature permitting glass forming, high potential for fiber drawing and compositional control of index of refraction and coefficient of thermal expansion, making them very important in the glass industry [16][17][18].
Generally, the possibility to control physical properties of the glasses, such as refractive index and density, using a proper variation of glass composition would suggest the feasibility of a chemical control of the materials according to the needs of respective applications.Thus far, the majority of the work was carried out with an attempt to understand the properties of the SLS glasses and to improve the glass quality [19][20][21][22].The Little approach was found regarding crystallization of the SLS glasses with respective structure and properties.
Crystallization of the SLS glasses would produce SLS-based glass-ceramics; likewise, most glass-ceramics are produced through the sintering process on glasses.Sintering is a process of heating the material at high temperature, above the crystallization temperature, (T c ), and below melting temperature (T m ), in order to achieve particular chemical reaction or crystallization for the material improvement and also due to the particles in the glass obtained sufficient energy at this temperature range to break the chemical bonds and enable them to align into lattice structure [23].As the temperature of the material increases, the average kinetic energy of the material particles would increase.The kinetic energy would cause the particles to have a higher amplitude of vibration and induce particles diffusion [24].The particles would tend to realign themselves into a periodic arrangement.The resulting glassceramics have both amorphous and at least one crystalline phase in it.For example, opal glass is a glass-ceramic which has an opalescent appearance due to the formation of small crystallites in the glass [25].
In recent years, many researchers have explored in transforming waste materials into glass-ceramics products meant for various applications [26][27][28].Yang et al. have studied the properties of SiO 2 -Na 2 O-Al 2 O 3 -CaO glass-ceramics prepared from coal gangue [29].The phases detected in the glass-ceramics were gehlenite (Ca 2 Al 2 SiO 7 ) and nepheline (NaAlSiO 4 ).Characterization showed that the glass-ceramics have a potential use for construction.On the other hand, Toya et al. prepared a series of glass-ceramics from wastes (Kira) of silica sand and kaolin clay upon firing and studied the effects of firing on the phase transformation, macroscopic appearance, thermal expansion, hardness and four-point bending strengths of the glass-ceramics [30].The study demonstrated that the glass-ceramics could be considered for building materials, ceramic tiles and advanced ceramic material.Several researches were undergone on the SiO 2 -Na 2 O-CaO glass-ceramics in dental applications.Wang et al. studied the crystallization, microstructure, vicker microhardness and thermal expansion of SiO 2 -Na 2 O-Al 2 O 3 glass-ceramics for dental porcelain [31].Hamzawy and El-Meliegy however studied the crystallization, microstructure and thermal expansion of SiO 2 -Na 2 O-Al 2 O 3 -CaO glass-ceramics for dental crown construction [32].
In this work, the SLS glass with a composition of SiO 2 -Na 2 O-CaO-MgO-Al 2 O 3 was used to prepare the glass-ceramic via sintering at different temperatures in order to study the effects of sintering temperatures on crystallization and properties of the glass and glass ceramics.

Experimental
SLS glass was prepared using a conventional melt quenching technique.The SLS glass waste powder was ground and sieved to obtain a fine powder of size ≤ 63 µm.The SLS glass powder was subsequently melted in an alumina crucible at a temperature of 1300ºC for 2 h until achieving a homogenous bubble-free liquid.The melt was then poured into a water tank and the glass produced was optically clear with no inclusion shown.The glass frit was then ground in a vibratory mill jar to produce fine glass powder (≤ 63 µm).After an addition of 1.75 wt.% of Polyvinyl Alcohol (PVA) binder, the powder was pressed into discs of 10 mm in diameter and 2 mm in thickness, each at a pressure of 5 tons for 10 min for subsequent analysis.
Chemical compositions of the sintered pellets and the starting powders were measured by energy dispersive X-ray fluorescence (EDXRF) spectrometer (EDX720/800HS/900HS) and thermal behavior of the glass was determined using differential thermal analysis (DTA) technique.The DTA scans were carried out on glass powders with a particle size of 45-100 μm, at 10°C/min heating rate and using a Diamond Pyris TG/DTA (Perkin Elmer) with Al 2 O 3 powder as a reference material in a dynamic pure nitrogen atmosphere at a flow rate of 50 cm 3 /min at the temperature range between 50 to 1000ºC at 10ºC/min.
The bulk density of glass and sintered sample are measured by the standard principle of Archimedes using a sensitive micro-balance with acetone liquid as the immersion fluid.The sample was first weighed in air, W air , and then in an immersion acetone, W ac , with the following density: ρ ac = 0.789 g/cm 3 .The weighing process was performed with an electronic balance.The density of the sample was then calculated using the following relationship (Equation (1)): ρ = W air ρ ac / (W air − W ac ) (1) where the estimated error was ±0.001 g/cm 3 .
Shrinkage during sintering is related to the rate of energy dissipation.In order to study the effect of sintering temperature on glass ceramic shrinkage, the linear shrinkage, LS (%), of sintered samples has been determined by means of (Equation ( 2)): ) where L i is the symbol of green body dimension (ceramic pellet) before it has been sintered while L f is the dry dimension of the sintered ceramic samples.
The amorphous/crystalline nature of the samples was measured using X-ray diffraction (XRD) using Philips X-ray diffractometer with Cu-Kα radiation in the 2θ range from 10° to 70° using 0.02° steps.Fourier transform infrared (FTIR) spectroscopy absorption spectral measurements were carried out on powdered glass samples at room temperature in the region 280-4000 cm -1 using Perkin Elmer Spectrum 100 spectrometer with Universal attenuated total reflectance (ATR) accessory.

Results and discussions
The EDXRF results shown in Tab.I. revealed that the SLS glass powder mainly consists of 69.5% of SiO 2 , which is consistent with the values reported by Aboud et al. and Strnad et al. [33][34].Besides, the SLS glass also consists of sodium (12.5%), calcium (11.3%), aluminium (2.8%), magnesium (2.0%) and potassium (1.5%) in weight percentage.These elements are present in the oxide form in the SLS glass powder.DTA analysis was carried out at 50-1000ºC on the SLS glass powder before the sintering process.The DTA curve of the SLS glass powder is shown in Fig. 1.An endothermic peak found at ~573ºC represents glass transition temperature (T g ) of the SLS glass, which, caused by an increase in heat capacity as a result of transformation of the glass structure.Ozawa assumed that at a constant heating rate, a broad exothermic effect would indicate a sluggish crystallization propensity, which would then lower the crystallization rate or surface crystallization characteristics [35].In contrast, a sharp exotherm signified higher crystallization ability; a higher crystallization velocity or a bulk crystallization process.In this work, it was observed from the DTA data that there existed a broad exothermic crystallization peaks in the range of 770ºC to 840ºC, which revealed the occurrence of glass crystallization temperature (T c ) at this point.This finding signified that the SLS glass was rather poor in performing effective bulk crystallization.Bulk density and linear shrinkage of the sintered samples was determined according to Archimedes' method (Eq.( 1) and Eq. ( 2)) are shown in Fig. 2 and Fig. 3, respectively.The density was frequently measured in order to understand the molecular packing inside the material.According to Fig. 2, the density of the samples was increased from 2.520 g/cm 3 to 2.701 g/cm 3 with an increase in the sintering temperature.It was mainly due to a decrease in total fractional porosity of the sample with the increase of sintering temperature.In general, density would increase and porosity would decrease monotonically with depth.This has been expected for the differential pressures would usually increase with depth.As the sintering temperature was increased, the grains would shift to reach more dense packing.Consequently, more forces would be imposed on the grain contacts.As the bulk density was correlated with the linear shrinkage, and this it was anticipated to increase with the increase of sintering temperature.

Oxide
Linear shrinkage of the sample was calculated using Eq. ( 2).The linear shrinkage curve is plotted in Fig. 3.The percentage of linear shrinkage was increased from 0 to 2.5 % with the increase of sintering temperature.It was deduced from the figure that at 400 and 500°C, the shrinkage was too small and therefore arguably there was no change due to the SLS glass transition temperature (T g ) at around 573°C.On the other hand, the different shrinkage observed could be attributed to the factors, such as the diffusion coefficients, thermal expansion coefficients, and the volume change of the phase transformations upon cooling [36].The phase transformation developed in the glass sample before and after sintering process at different temperatures was studied.The sample was sintered at 400-800ºC for 2 h and characterized by both the XRD and FTIR.The XRD results shown in Fig. 4 indicated that the SLS glass powder had a stable amorphous structure at a temperature below 700 ºC.After sintering at 800°C for 2 h, α-quartz (SiO 2 ) and a minor amount of devitrite (Ca 3 Na 2 Si 6 O 16 ) with JCPDS no. of 75-1252 and 77-0410 were detected, revealing the formation of crystals above the T g , attributed to a formation of the inhomogeneous microstructure.The presence of α-quartz and devitrite was further confirmed by the DTA analysis.Initially, the devitrite prevailed, followed by the bonding of Ca + and Na + ions in the structure of devitrite.Thereafter, the glass matrix became enriched with SiO 2 and the proportions were reversed with the formation of more α-quartz crystals.The crystals showed a reversible polymorphic transformation at a temperature around 573ºC, which made them distinguishable from other crystal forms.At T g , the ionic bonds between the Si 4+ and O 2-ions in the amorphous SiO 2 were dissociated by the energy of heat.These ions realigned to form a long-range order of αquartz and devitrite at a higher temperature.The energy of heat was evolved as the Si-O bonds were formed in the α-quartz structure.The crystallization temperature of the α-quartz was insignificant.It revealed that the SLS glass has a stable glassy structure and was capable to resist the formation of a long-range order of α-quartz.FTIR spectra of the SLS glass before and after sintering are shown in Fig. 5.The FTIR spectra of SLS before sintering displayed two very strong peaks at 297 and 434 cm -1 , a medium peak at 769 cm -1 , a strong peak at 947 cm -1 , as well as a weak peak at 1484 cm -1 .The peaks at ~520 cm -1 and a broad band at 550-280 cm -1 as well as the infrared absorption at 1484 cm -1 were due to the infrared absorption by the Ca-O bond.Besides, the bands at 434, 769 and 947 cm -1 were due to infrared absorption by the Si-O bond.The findings showed a good agreement with the spectra shown by Lee et al. [37].The band at 434 cm -1 was attributed to the Si-O the bending-rock mode.The band at 769 cm -1 was assigned to the Si-O symmetric stretching or bending mode whereas the band at 947 cm -1 was assigned to the Si-O stretching mode of non-bridging oxygen.As the SLS glass was sintered at 400-700ºC, the FTIR spectra at ~440, ~770 and ~950 cm -1 were almost similar to the spectra of the based SLS glass before sintering.However, the intensity of the infrared absorption was increased as the sintering temperature was increased.It was revealed that more Si-O bonds were formed in the SLS glass powder as the sintering temperature was increased.The disappearance of the peak at ~300 cm -1 and the lower infrared absorption at ~520 cm -1 for the spectra of SLS glass powder after sintering at 700ºC was due to the Ca-O bonds fission [38].The reappearance of the peak at ~300 cm -1 and the infrared absorption at ~520 cm -1 indicated that the Ca-O bonds were reformed as a formation of devitrite, as shown in the XRD result.After sintering of the glass sample at 800 ºC, the infrared absorption peaks at ~770 and ~950 cm -1 decreased.It was due to the Si-O bonds fission of the amorphous silica [39].The Si 4+ and O 2-ions were realigned in order to form α-quartz.It could be seen in the FTIR spectra that a weak peak appeared at 651 cm -1 was due to the bond vibration mode of the α-quartz.Hence, the FTIR spectra of the SLS glass after sintering at 800ºC was coherent with the XRD result, which revealed that the occurrence of nucleation of α-quartz and devitrite.

Conclusions
The pellets produced from the SLS glass waste were investigated to determine the effects of different sintering temperatures on the physical, thermal and phase transformation of the SLS glass-ceramic.EDXRF results showed the main composition of the SLS glass, corresponded by 69.5% of SiO 2 .DTA analyses of the SLS glass prove the occurrence of glass transition temperature, T g at ~573ºC and the existence of a broad exothermic crystallization (T c ) peaks in the range of 770 ºC.Increasing the sintering temperature had increased density and linear shrinkage of the sample.The XRD results indicated that after sintering at 800ºC for 2 h, the crystal formed in the sample was α-quartz (SiO 2 ) and devitrite (Ca 3 Na 2 Si 6 O 16 ).FTIR spectra showed that after sintering at 800ºC, the reappearance of the peak at ~300 cm -1 and the infrared absorption at ~520 cm -1 with a decrease in the infrared absorption peaks at ~770and ~950 cm -1 were observed.It was in conjunction with the re-formation of the Ca-O bonds and Si-O bonds fission of the amorphous silica whereby the Si 4+ and O 2-ions were realigned to form the crystallized α-quartz and devitrite.

Fig. 2 .
Fig. 2. The density of glass sample sintered at different temperatures.

Fig. 4 .
Fig. 4. X-ray diffraction pattern of SLS glass powder sintered at various temperatures for 2 h.