Fabrication and Crystallization of ZnO-SLS Glass Derived Willemite Glass-Ceramics as a Potential Material for Optics Applications

Willemite glass-ceramics were successfully derived from conventional melt-quench ZnO-SLS precursor glass by an isothermal heat treatment process. The effect of heat treatment temperatures on the physical properties was investigated by Archimedes principle and linear shrinkage. The generation of willemite crystal phase and morphology with increase in heat treatment temperature was examined by X-ray diffraction (XRD), Fourier transform infrared (FTIR), and field emission scanning electron microscopy (FESEM) techniques. X-ray diffraction revealed that the metastable β-Zn 2 SiO 4 and thermodynamically stable zinc orthosilicate αZn 2 SiO 4 phases can be observed at temperatures above 700C.The experimental results indicated that the density and shrinkage of the glass-ceramic vary with increasing the sintering temperature. FTIR studies showed that the structure of glass-ceramic consists of SiO 2 and ZnO 4 units and exhibits the structural evolution of willemite glass-ceramics. The characteristic of strong vibrational bands can be related to the [SiO 4 ]4− tetrahedron corresponding to reference spectra of willemite.


Introduction
Recently, manufacturing industries played an important role in economic growth.However, this rapid industrialization generates enormous amount of waste such as glass waste which necessitates different attitudes toward solid waste management [1].Therefore, converting these solid wastes into more valuable and environment friendly products is a new focus across the world.This is in line with the increasing demand for limited natural resources; hence waste recovery provides alternative resources which reduces dependency on natural resource such as sand-silica for production of glasses [2] and glass-ceramics [3,4].Among silica-rich wastes, soda lime silica (SLS) glass from bottle banks has attracted much attention.SLS glass from urban waste consists mainly of silicon (SiO 2 ), sodium (Na 2 O), and calcium oxides (CaO).Thanks to its potential of low temperature viscous flow sintering, SLS glass can be considered as a good candidate for total, or partial, replacement of the natural fluxes [5][6][7].Glassceramics possess many favorable properties including low density, low thermal conductivity, high surface area, virtuous thermal shock resistance, and great specific strength [8].
Glass-ceramic containing willemite (Zn 2 SiO 4 ) has been known, for many years, as materials for electronic applications and has been identified as a suitable host matrix for many rare earth and transition metal dopant ions for efficient luminescence [9][10][11][12][13][14]. Willemite is one of the zinc ore minerals having the phenakite structure.In Zn 2 SiO 4 , all the atoms occupy general position and are composed of tetrahedral framework where zinc and silicon are positioned in three different fourfold crystallographic sites: two slightly different zinc sites Zn 1 (⟨Zn-O⟩ 1.950 Å) and Zn 2 (⟨Zn-O⟩ 1.961 Å), and Si (⟨Si-O⟩ 1.635 Å), thus resulting in rhombohedra symmetry with lattice parameters  =  ∼ 13.948 Å and  ∼ 9.315 Å [15].This kind of rigid lattice, with only noncentrosymmetric cationic sites, gives the chance to get special optical properties.
Willemite or zinc silicate (Zn 2 SiO 4 ) is known as an ideal host material for phosphor and optoelectronic material due to its large band gap of 5.5 eV, chemical stability, and transparency in the UV-visible range [16].Willemite also has attractive features such as much larger exciting binding energies which make it suitable for applications in coating, cathode ray tubes (CRT), plasma display panels (PDPs), laser crystals, upconversion luminescent materials, electroluminescent devices, and catalyst [17][18][19][20][21][22].Most of research has only been focused on a thermal dynamically stable zinc orthosilicate (-Zn 2 SiO 4 ) with the different micro-or nanostructures produced by solid-state reaction, radio frequency magnetron sputtering, spray pyrolysis, solgel, polymer precursor, and hydrothermal methods [23][24][25][26][27][28].Producing of willemite phosphor without incorporating expensive material and high temperature mechanism is of interest of researchers so that low energy processes might be developed.However, many valuable results have been kept within industry because willemite is a highly important technological material.
In the present work, the preparation of novel ZnO-SLS glass by conventional melt-quench technique and willemite glass-ceramics by isothermal controlled crystallization of precursor glasses is reported.The main advantage of ZnO-SLS based glass systems compared to normal zinc-sodalime-silica glass systems is that the melting and processing temperature will decrease substantially.This will reduce the cost of production of this glass-ceramic.In this context, the major purposes of this work are to fabricate and characterize willemite glass-ceramics from waste materials as a potential material for use in optics application.

Experimental Procedure
High purity zinc oxide (ZnO) (99.99%,Aldrich) powders and soda lime silica (SLS) waste glass powders are used as a raw material.Homogeneous mixtures of batches (∼100 g) with empirical formula of (ZnO) 0.5 (SLS) 0.5 are obtained by milling process and further melting at 1400 ∘ C for 2 h.The glass frits are obtained by quenching of the melt in the cold water.Then, the frits are dried and then milled in a highspeed porcelain mill, resulting in powders with the mean particle size of 45 m.Next, the mixtures are uniaxial pressed with an applied load of 3.5 tons for 15 min using polyvinyl alcohol (PVA) as the binder to obtain the pellet samples.The dimension of pellets was 10 mm in diameter and 2 mm thick.The pellets were subjected to heat treatment process in the range of 600-1000 ∘ C for 2 h in air at a slow heating rate of 10 ∘ C/min.Differential thermal calorimetry of precursor glass powder was carried out on glass powders with particle size ranging from 45 to 100 m, using a Diamond Pyris TG/DTA (Perkin Elmer) with Al 2 O 3 powder as reference material in a dynamic pure nitrogen atmosphere at a flow rate of 50 cm 3 /min at the temperature range between 400 and 900 ∘ C  1).The linear shrinkage of the sample is then calculated using the following equation: The densities () of the samples are measured in room temperature by the standard Archimedes method and using acetone as the immersion fluid.Firstly, the samples are weighed in air,  air , and then measured in an immersion liquid (acetone)  ac , with density  ac = 0.789 g cm −3 .The densities of the samples are then calculated using the following equation: where the estimated error is ±0.001 g cm −3 .The evolution of crystalline phases of the sintered sample at different temperatures was determined by X-ray diffraction analysis (Philips X-ray diffractometer with Cu-K radiation) and Fourier-transform infrared spectra (FTIR) recorded with a Perkin Elmer Spectrum 100 series.Field emission scanning electron microscopy (FE-SEM, FEI NOVA NanoSEM 230) was used for microstructure observation.

Results and Discussion
Energy dispersive X-ray fluorescence (EDXRF) is a nondestructive analytical technique used to characterize the chemical composition of SLS glass waste and ZnO-SLS glass samples.Chemical compositions present in the glass samples were analyzed by EDXRF as shown in Table 1.From the table, the multicompounds like SiO 2 , CaO, Na 2 O, Al 2 O 3 , MgO, ZnO, and other minor elements were detected in the samples.The increase of ZnO content to the SLS glass has resulted in a percentage reduction of other elements in the glass samples.
Differential thermal analysis (DTA) was recorded for the precursor glass powder to determine   and crystallization Exo.
Endo. temperatures.The DTA of the precursor glass is shown in Figure 1.  estimated from this curve is found to be 520 ∘ C. Besides, from the figure, two exothermic peaks at 710 ∘ C ( 1 ) and 800 ∘ C ( 2 ) have been detected.The onset of first crystallization peak ( 1 ) is at 710 ∘ C and this peak ( 1 ) is corresponding to the -willemite (-Zn 2 SiO 4 ) crystallization.The crystallization peak which is peaking at 800 ∘ C ( 2 ) is due to the formation of -willemite (-Zn 2 SiO 4 ).This fact is confirmed by XRD and is presented in the XRD section.
Table 2 shows the variations of density and linear shrinkage of glass at various heat treatment temperatures.It can be seen that the density of the sample increases with increasing the heat treatment temperature from 2.84 to 3.96 g/cm 3 .It is mainly due to the structure compactness of the glass-ceramic sample with the increasing of heat treatment temperature.Generally, the increasing in pressure (temperature) on a material will decrease the volume of the material and thus increases its density [29].Further, as heat treatment temperature increases, grains shift to obtain more dense packing.As it is expected, at 600 ∘ C the shrinkage is too small and arguably there is no change.As heat treatment temperature increases up to 1000 ∘ C, a slight increase in the shrinkage of the sample is observed which can be attributed to the effect of alloying element factors such as the diffusion coefficients, thermal expansion coefficients, and the volume change during the phase transformations upon cooling.Besides, increases in the density of the sample have also resulted in the changes of the cross-link density due to the formation of willemite crystal.It can be observed that the density of the sample increases when sintered; this may be attributed to the decrease in the bond length due to the formation of ZnO 4 octahedral groups  [30].Solid oxides reveal mixed ionic and covalent bonding.
The ionic bond fraction decreases as the cation size and coordination number decrease while the oxidation state of the ion increases.This results in a decrease in bond length between Zn 2+ and O 2− ion and formation of a close-packed structure, consequently increasing the density of the glassceramics.
The XRD pattern of the glass and glass-ceramics is shown in Figure 2. No diffraction peak appears for the precursor glass sample heat treated below 600 ∘ C, which indicates that these samples are amorphous in nature.Crystallization starts at temperatures higher than 600 ∘ C. The diffraction peak of the samples treated for different temperature above 700 ∘ C for 2 h is indexed and it resembled to some extent willemite (-Zn 2 SiO 4 ) phase (JCPDS file 37-1485).By increasing the heat treatment temperature to 700 ∘ C, the -willemite (-Zn 2 SiO 4 ) and minor tridymite (SiO 2 ) phases with JCPDS number of 14-0653 and 75-0638 are formed.Regarding thermodynamically metastable phases, two crystallographic variations are known and each is referred to as -phase Zn 2 SiO 4 .The crystal system is considered to be orthorhombic.The phase Zn 2 SiO 4 was probably the same as that produced by melting and annealing ZnO and SiO 2 in which the Si/Zn molar ratio was 1.7 more than the stoichiometric value of 0.5 [31].By further increasing the heat treatment temperature to 800 ∘ C, the -Zn 2 SiO 4 transforms to -willemite (-Zn 2 SiO 4 ) and minor akermanite (Ca 2 MgSi 2 O 7 ) phase with JCPDS number of 37-1485 and 76-0841.Akermanite forms solid solutions, being the end member of melilite, which belongs to the group of sorosilicates [32].Akermanite usually occurs in igneous and metamorphic rocks and meteorites, but also in the slag of blast furnaces which can be considered as an evidence of their high chemical stability at elevated temperatures.According to Figure 2, akermanite is the minor crystalline phase after glass sample was treated at 700 ∘ C. The intensity of akermanite X-ray peaks decreased when the temperature of the heat treatment increased.These results indicate that the amount of CaO and MgO may play a key role in the yield of akermanite phase in the investigated system.Hence, a deep understanding of the role of CaO and MgO has to be resolved in future studies.Increasing the temperature to 1000 ∘ C for 2 hours resulted in better crystallinity in the glassceramic sample.
The -phase is a nesosilicate or orthosilicate that consists of isolated SiO 4 4− tetrahedrons and ZnO 4 6− tetrahedrons [11].The twelve major diffraction peaks at 2 = 22.07 ∘ , 25.54 ∘ , 31.53 ∘ , 34.00 ∘ , 38.82 ∘ , 48.93 ∘ , 45.01 ∘ , 46.97 ∘ , 48.93 ∘ , 57.60 ∘ , 59.52 ∘ , 65.63 ∘ , 68.67 ∘ , and 70.37 ∘ corresponding planes are (300), ( 220), ( 113), ( 410), ( 223), (600), (333), ( 710), ( 006), ( 713), (633), and (416) and can be indexed using standard diffraction pattern of -Zn 2 SiO 4 crystal phase (Figure 2).It is found that advancement of heat treatment process leads to an increase in diffraction peak intensity.By evaluating the diffraction peaks, it is also observed that the diffraction peaks become sharper and full width at half maxima (FWHM) value is decreased with progression of heat treatment process.The broader crystal peaks are caused due to smaller crystallite sizes and sharp peaks are due to the formation of comparatively larger crystallites.At higher temperature of heat treatment, diffusion of ions in the sample is increased; hence the crystals growth rate is accelerated and larger size crystals result in the glass structure.Judging by the relative intensities of the XRD peaks of these two -Zn 2 SiO 4 and -Zn 2 SiO 4 polymorphs, it can be concluded that -Zn 2 SiO 4 crystallized first at the lower temperatures and then gradually converted to thermodynamically stable -Zn 2 SiO 4 by increasing the heat treatment temperature.According to Figure 2, -Zn 2 SiO 4 is a single crystalline phase after heat treatment of the glasses at 900 ∘ C. Heat treatment process above 900 ∘ C leads to better crystallinity of -Zn 2 SiO 4 phase, as the sole zinc orthosilicate.
FESEM analysis is performed to study the size, shape, and morphology of glass-ceramic.The microstructure observation of the sample revealed well developed willemite crystals in Figures 4(a)-4(e).At temperatures 600 ∘ C and 700 ∘ C, the crystal grains are aggregated and irregular in shape.When the sintering temperature was increased from 800 to 1000 ∘ C, the morphology of the glass-ceramic became granular and initial stage of homogeneous distribution in rhombohedral shaped crystal.From the micrographs, it is clearly observed that the glassy matrices of the treated sample are phase separated with incipient precipitation of defined willemite crystals with increase in heat treatment temperature.The phase separated grains are irregular in shape and distributed uniformly throughout the bulk glass matrix.The grain size is increased by increasing the heat treatment temperature as a result of crystal growth.With increase in heat treatment temperature, these grains are grown with distinct features and composed of few smaller crystals together in the glass matrix.This phase separation followed by crystallization mechanism is a well-known phenomenon for synthesis of glass-ceramics from precursor glass [35].

Conclusion
In the recent work, the thermal and crystallization of willemite glass-ceramic by controlled crystallization in the ZnO-SLS glass system are demonstrated.The results of XRD, FTIR, and FESEM show the evidence of the formation of willemite crystals in the ZnO-SLS glass matrix.It is found that the willemite phase is obtained from XRD pattern after the heat treatment process.The FESEM images evidence that, with progression of heat treatment, the glassy matrix initially is phase separated followed by formation of welldefined willemite phases.The appearance of Si-O-Zn vibration bands in FTIR reflectance spectra supports the formation of willemite phase.The FESEM micrographs evidence that the crystallites of willemite glass-ceramics are aggregated and irregular in shape.Therefore, this willemite glass-ceramic appears to be promising for use in the field of optics as a potential phosphors material.

Figure 2 :
Figure 2: X-ray diffraction of the sample at various heat treatment temperatures.

Figure 3 :
Figure 3: Fourier transforms infrared spectra of the sample at various heat treatment temperatures.

Table 1 :
Chemical composition of SLS glass waste and ZnO-SLS glass (wt.%).C/min to ascertain the glassy nature of the precursor glass and to find out   and crystallization peak temperature (  ).The percentage of linear shrinkage (LS) is determined by calculating the difference between the initial dimensions (  ) and the final dimensions ( ) of the sintered pellet.Then, the values are divided by initial dimension (  ) and multiplied with 100% (

Table 2 :
The density and linear shrinkage of the glass-ceramic sample at various heat treatment temperatures.