# Effect of Sintering Temperature on Structural and Morphological Properties of Europium (III) Oxide Doped Willemite

Authors: Nur Fatin Syamimi; Khamirul Amin Matori; Way Foong Lim; Sidek Abdul Aziz; Mohd Hafiz Mohd Zaid
Journal: Journal of Spectroscopy
 (2014)

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## Abstract

Willemite- (Zn2SiO4-) based glass ceramics doped with various amounts of europium oxide (Eu2O3) were prepared by solid state melting and quenching method. Effect of sintering temperature (600–1000°C) on structural and morphological properties of the doped samples was investigated. Phase composition, phase evolution, functional groups, and microstructure analysis were, respectively, characterized using X-ray diffractometer (XRD), fourier transform infrared spectroscopy, field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray. XRD analysis detected the presence of rhombohedral crystalline phase in the doped samples sintered at different temperatures. FE-SEM and bulk density results confirmed that doping of the willemite with Eu2O3 effectively enhanced densification. The microstructural analysis of the doped samples showed that the average grain size increased with the increase of sintering temperature.


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## Body

## 1. Introduction

During the past few years, rare earth materials have been giving satisfactory accomplishment in the ceramic industry [1, 2]. To date, only small percentage of the rare earth materials have been consumed in the production of ceramic materials. However, the function of the rare earth materials remains unclear and is important to be further explored [3–5]. Rare earth materials have been also doped with inorganic phosphor (willemite) based glass ceramics for potential applications in photonic devices, next generations flat panel displays (FPD) [6–9], optical amplifiers [10], and phosphors [11]. The reason for using the willemite-based glass ceramics as the host materials for doping is the high quantum efficiency, super chemical and thermal stability, water resistance, and low cost. By doping the rare earth elements into the willemite-based glass ceramics, the characteristics of white light under UV excitation [12] could be studied.Thus far rare earth doped willemite-based glass ceramics have been widely used as the activators in various phosphors. Among these, europium oxide- (Eu2O3-) doped compounds, which emit intense red light, have been broadly used as the phosphor materials due to the occurrence of electronic transition among the 4fn inner shell energy levels [13–15]. Zinc silicate has an increasing attention for its potential as a phosphor host and glass ceramic because of its high temperature stability as a ceramic pigment [16, 17]. There has been also a broad interest towards its luminescence, crystallography, and application as industrial materials. Many efforts have been made by researchers to produce pure willemite and willemite doped with various rare earth materials. However, there is a lack of work on studies of willemite that are obtained from waste materials.In this paper, we report the synthesis of Eu2O3 doped willemite-based glass ceramic using waste materials that were prepared with different weight percentages (1–3 wt%) of Eu3+ using solid state melting and quenching technique. Bulk density measurement using Archimedes principle, X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), and field emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray (EDX) was used to investigate the effects of sintering temperature and the role played by the Eu3+ ion on the structural and morphological properties of the doped willemite-based glass ceramic samples. To the best of our knowledge, Eu2O3 doped willemite-based glass ceramic using waste material has not been reported yet, which was another motivation for this research.


## 2. Experimental Procedure

Eu2O3 doped willemite-based glass ceramic samples were prepared by solid state melting and quenching technique. High purity zinc oxide (ZnO) (99.99%, Aldrich) powders and soda lime silica (SLS) glass waste powders were mixed together to produce the willemite-based glass ceramic. The SLS glass waste was chosen from the same brand from soya bean bottles. In order to produce the SLS glass powders, the SLS glass was crushed into powder form. The powders were blended in a ball-mill uniformly for 24 h to produce a mixture of powders containing 50 wt% ZnO and 50 wt% SLS glass. The mixture was melted at 1400°C for 3 h and then quenched in water to form glass frits. The glass frits were dried in air for 1 day and were then crushed into powders to the size of 45 µm using a specific sieve. After that, the powders were sintered at 900°C for 4 h to produce willemite. In doping process, Eu2O3 powder ranging from 1 to 3 wt% was doped into the willemite. In order to obtain mixed powders, the milling process was carried out for 24 h. The mixed powders were uniaxially pressed into pellets of 13 mm in diameter and 2 mm in thickness with an applied load of 3.5 tons for 2 min using polyvinyl alcohol (PVA) as the binders. The soft green pellets were then sintered at 600, 700, 800, 900, and 1000°C for 4 h in the air using a conventional furnace. Archimedes principle with water as the fluid medium was used to measure the bulk density of the samples.Structural changes of the compact samples were studied by XRD modeled Philips X’PERT MPD using a filtered Cu-Kα radiation (λ = 0.1542 nm) in a wide range of 2θ (10° < 2θ < 90°). The average crystallite size of all the samples was determined from full-width half-maximum (FWHM) of the reflection of major intensity peak in the XRD patterns using Debye Scherrer’s formula:
(1)
D
=
0.94
λ
β
cos
⁡
θ
,
where λ is the wavelength of the incident X-ray; β the full-width at half-maxima; and θ Bragg’s diffraction angle. Functional groups of the samples were measured in the range of 360–1000 cm−1 using a Perkin-Elmer 1752X spectrophotometer (Waltham, MA) using KBr disc method. Morphological observations were performed using a Fe-SEM FEI NOVA NanoSEM 230 machine equipped with EDX and the grain size was measured using mean linear intercept method.


## 3. Results and Discussion

### 3.1. Density and Linear Shrinkage

Figure1 shows the bulk density of Eu2O3 doped willemite with different weight percentage (wt%) of Eu3+ sintered from 600 to 1000°C for 4 h. The graph revealed an increase in the bulk density with increasing of the sintering temperature, with the highest density of 3.37 g/cm3 for the samples sintered at 1000°C for 4 h. It is mainly due to the decrease in total fractional porosity of the sample with the increase in sintering temperature. This value was compared favourably with the theoretical density of 3.90 g/cm3 [18]. Hence the bulk density achieved using the solid state melting and quenching method was found to be 86.41% of the average theoretical density. The result of obtaining the high bulk density value without any sintering aid is interesting. Figure 2 shows the results of linear shrinkage of the pellets according to sintering temperature. There was a significant increase with the increasing in the sintering temperature.Figure 1
Bulk density of Eu2O3 doped willemite-based glass ceramic as a function of sintering temperature.Figure 2
A variation of sintering temperature with linear shrinkage of Eu2O3 doped willemite-based glass ceramic.


### 3.2. X-Ray Diffraction (XRD)

The XRD patterns of willemite and Eu2O3 doped willemite sintered in the temperature range from 600°C to 1000°C for 4 h are presented in Figures 3 and 4. The observed peaks in these diffractograms were indexed to the rhombohedral willemite structure [19, 20]. All peaks in the doped samples were shifted towards higher 2θ compared to the undoped samples. In common the shift towards higher 2θ values shows a slight reduction in thed-spacing values. This was to be expected since Zn2+ (0.74 Å) ionic radius differs slightly from the Eu3+ (0.947 Å) ionic radii [21]. The peak shifting might also be credited to the lattice mismatching, lattice distortion, strain of the crystal, and finite size effect. By referring to the XRD patterns, it was observed that crystal structure of the samples did not change and the peak difference was not much with the increase in the weight percentage of Eu3+. By applying Scherrer’s formula and considering FWHM of the diffraction peaks, average crystallite size of the pure willemite and Eu2O3 doped willemite samples sintered at various temperatures was calculated. Table 1 shows the value of crystallite size of each sample. Higher sintering temperature would increase atomic mobility and this would cause grain growth, which results in a better crystallinity. The Eu2O3 doped willemite samples show a variation of size, ranging from 44.30 to 53.14 nm.Table 1
Average crystallite size of willemite and 3 wt% Eu2O3 doped willemite-based glass ceramics.
Samples
Crystallite size (nm)
Willemite
53.14
600°C
44.30
700°C
53.14
800°C
53.14
900°C
44.31
1000°C
53.14Figure 3
XRD patterns of willemite and 3 wt% Eu2O3 doped willemite samples sintered at various temperatures for 4 h.Figure 4
XRD patterns of various percentages of Eu2O3 doped willemite samples sintered at 1000°C for 4 h.


### 3.3. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectra of samples sintered at different temperatures are shown in Figures5 and 6. The series of all detected infrared absorption bands are summarized in Table 2. These measurements were made to make a comparison between the experimental data that was gained in the present work with the previous research [22–25]. FTIR spectra of the glass-ceramic matrix, Zn2SiO4:Eu2O3, consisted of eight wide and strong absorption bands placed at 459 cm−1, 576 cm−1, 615 cm−1, 697 cm−1, 865 cm−1, 901 cm−1, 932 cm−1, and 978 cm−1. It has been known that metal oxide vibrations would occur at wavenumber below 1000 cm−1. The peak in the lower wavenumber at 459 cm−1 was attributed to the symmetric stretching of SiO4 group and Eu2O3 [26]. The band position located at 576 cm−1 was due to the Si-O bending vibration modes [27]. The peak at 615 cm−1 was assigned to the asymmetric stretching vibration ZnO4 group. The bands that were present at 697, 865, 901, 932, and 978 cm−1 were corresponded to the symmetric stretching vibrations of the Si-O bonding. It was found that as the sintering temperature increased from 600 to 1000°C, there was an increase in the intensity of the IR bands.Table 2
FTIR transmission features and respective assignment for Eu2O3 doped willemite sintered at various temperatures and different weight percentages.
Wavenumber(cm−1)
Assignment
400–500
Eu2O3 vibration
459
Si–O symmetric stretching vibration in SiO4 units
576
Zn–O symmetric stretching vibration in ZnO4 units
615
Zn–O asymmetric stretching vibration in ZnO4 units
697
Si–O bond vibration
865
Si–O symmetric stretching vibration
901–932–978
Si–O asymmetric stretching vibrationFigure 5
FTIR spectra of pure willemite and Eu2O3 doped willemite samples sintered at various temperatures and doped with different weight percentages of Eu3+.Figure 6
FTIR spectra of various percentages of Eu2O3 doped willemite samples sintered at 1000°C for 4 h.


### 3.4. Field Emission Scanning Electron Microscopy (FE-SEM)

Figure7 depicts the FE-SEM images of Eu2O3 doped willemite-based glass ceramics. The particle size was seen to increase after an increase in the sintering temperature. In lower temperature range (600–800°C) the crystallized grains were aggregated and were irregular in shape. When the sintering temperature was increased to 900°C, surface morphology of the sample became granular and it showed an initial stage of homogeneous distribution in shape, as shown in Figure 7(d). After increasing the temperature to 1000°C, the grains that were rectangular-like clear were present. The grain size was increased during sintering as a result of grain growth. A grain size distribution graph of the samples is presented in Figure 8 while Table 3 shows the variation in the grain size of the samples. These results confirmed the increase of grain size along with sintering. The presence of Eu2O3 in the willemite matrix was confirmed by Fe-SEM-EDX analysis as shown in Figure 9. The EDX spectra showed clearly the peaks of Zn, O, Si, Al, and Eu. The detection of Au peak in the sample was due to sputtering of the sample’s surface with gold for conduction to provide clearer viewing. The Al peak was due to the utilization of SLS glass from the waste material.Table 3
Grain size of 3 wt% Eu2O3 doped willemite-based glass ceramics sintered at different temperatures for 4 h.
Samples (°C)
Grain size (nm)
6
00
374.3
700
489.3
800
534.5
900
862.9
1000
963.4FE-SEM micrographs of 3 wt% Eu2O3 doped willemite samples sintered at (a) 600°C, (b) 700°C, (c) 800°C, (d) 900°C, and (e) 1000°C for 4 h.
(a)
(b)
(c)
(d)
(e)Grain size distribution of 3 wt% Eu2O3 doped willemite samples sintered at (a) 600°C, (b) 700°C, (c) 800°C, (d) 900°C, and (e) 1000°C for 4 h.
(a)
(b)
(c)
(d)
(e)Figure 9
Microstructures of 3 wt% Eu2O3 doped willemite samples sintered at 1000°C for 4 h where the EDX analysis was performed.


## 3.1. Density and Linear Shrinkage

Figure1 shows the bulk density of Eu2O3 doped willemite with different weight percentage (wt%) of Eu3+ sintered from 600 to 1000°C for 4 h. The graph revealed an increase in the bulk density with increasing of the sintering temperature, with the highest density of 3.37 g/cm3 for the samples sintered at 1000°C for 4 h. It is mainly due to the decrease in total fractional porosity of the sample with the increase in sintering temperature. This value was compared favourably with the theoretical density of 3.90 g/cm3 [18]. Hence the bulk density achieved using the solid state melting and quenching method was found to be 86.41% of the average theoretical density. The result of obtaining the high bulk density value without any sintering aid is interesting. Figure 2 shows the results of linear shrinkage of the pellets according to sintering temperature. There was a significant increase with the increasing in the sintering temperature.Figure 1
Bulk density of Eu2O3 doped willemite-based glass ceramic as a function of sintering temperature.Figure 2
A variation of sintering temperature with linear shrinkage of Eu2O3 doped willemite-based glass ceramic.


## 3.2. X-Ray Diffraction (XRD)

The XRD patterns of willemite and Eu2O3 doped willemite sintered in the temperature range from 600°C to 1000°C for 4 h are presented in Figures 3 and 4. The observed peaks in these diffractograms were indexed to the rhombohedral willemite structure [19, 20]. All peaks in the doped samples were shifted towards higher 2θ compared to the undoped samples. In common the shift towards higher 2θ values shows a slight reduction in thed-spacing values. This was to be expected since Zn2+ (0.74 Å) ionic radius differs slightly from the Eu3+ (0.947 Å) ionic radii [21]. The peak shifting might also be credited to the lattice mismatching, lattice distortion, strain of the crystal, and finite size effect. By referring to the XRD patterns, it was observed that crystal structure of the samples did not change and the peak difference was not much with the increase in the weight percentage of Eu3+. By applying Scherrer’s formula and considering FWHM of the diffraction peaks, average crystallite size of the pure willemite and Eu2O3 doped willemite samples sintered at various temperatures was calculated. Table 1 shows the value of crystallite size of each sample. Higher sintering temperature would increase atomic mobility and this would cause grain growth, which results in a better crystallinity. The Eu2O3 doped willemite samples show a variation of size, ranging from 44.30 to 53.14 nm.Table 1
Average crystallite size of willemite and 3 wt% Eu2O3 doped willemite-based glass ceramics.
Samples
Crystallite size (nm)
Willemite
53.14
600°C
44.30
700°C
53.14
800°C
53.14
900°C
44.31
1000°C
53.14Figure 3
XRD patterns of willemite and 3 wt% Eu2O3 doped willemite samples sintered at various temperatures for 4 h.Figure 4
XRD patterns of various percentages of Eu2O3 doped willemite samples sintered at 1000°C for 4 h.


## 3.3. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectra of samples sintered at different temperatures are shown in Figures5 and 6. The series of all detected infrared absorption bands are summarized in Table 2. These measurements were made to make a comparison between the experimental data that was gained in the present work with the previous research [22–25]. FTIR spectra of the glass-ceramic matrix, Zn2SiO4:Eu2O3, consisted of eight wide and strong absorption bands placed at 459 cm−1, 576 cm−1, 615 cm−1, 697 cm−1, 865 cm−1, 901 cm−1, 932 cm−1, and 978 cm−1. It has been known that metal oxide vibrations would occur at wavenumber below 1000 cm−1. The peak in the lower wavenumber at 459 cm−1 was attributed to the symmetric stretching of SiO4 group and Eu2O3 [26]. The band position located at 576 cm−1 was due to the Si-O bending vibration modes [27]. The peak at 615 cm−1 was assigned to the asymmetric stretching vibration ZnO4 group. The bands that were present at 697, 865, 901, 932, and 978 cm−1 were corresponded to the symmetric stretching vibrations of the Si-O bonding. It was found that as the sintering temperature increased from 600 to 1000°C, there was an increase in the intensity of the IR bands.Table 2
FTIR transmission features and respective assignment for Eu2O3 doped willemite sintered at various temperatures and different weight percentages.
Wavenumber(cm−1)
Assignment
400–500
Eu2O3 vibration
459
Si–O symmetric stretching vibration in SiO4 units
576
Zn–O symmetric stretching vibration in ZnO4 units
615
Zn–O asymmetric stretching vibration in ZnO4 units
697
Si–O bond vibration
865
Si–O symmetric stretching vibration
901–932–978
Si–O asymmetric stretching vibrationFigure 5
FTIR spectra of pure willemite and Eu2O3 doped willemite samples sintered at various temperatures and doped with different weight percentages of Eu3+.Figure 6
FTIR spectra of various percentages of Eu2O3 doped willemite samples sintered at 1000°C for 4 h.


## 3.4. Field Emission Scanning Electron Microscopy (FE-SEM)

Figure7 depicts the FE-SEM images of Eu2O3 doped willemite-based glass ceramics. The particle size was seen to increase after an increase in the sintering temperature. In lower temperature range (600–800°C) the crystallized grains were aggregated and were irregular in shape. When the sintering temperature was increased to 900°C, surface morphology of the sample became granular and it showed an initial stage of homogeneous distribution in shape, as shown in Figure 7(d). After increasing the temperature to 1000°C, the grains that were rectangular-like clear were present. The grain size was increased during sintering as a result of grain growth. A grain size distribution graph of the samples is presented in Figure 8 while Table 3 shows the variation in the grain size of the samples. These results confirmed the increase of grain size along with sintering. The presence of Eu2O3 in the willemite matrix was confirmed by Fe-SEM-EDX analysis as shown in Figure 9. The EDX spectra showed clearly the peaks of Zn, O, Si, Al, and Eu. The detection of Au peak in the sample was due to sputtering of the sample’s surface with gold for conduction to provide clearer viewing. The Al peak was due to the utilization of SLS glass from the waste material.Table 3
Grain size of 3 wt% Eu2O3 doped willemite-based glass ceramics sintered at different temperatures for 4 h.
Samples (°C)
Grain size (nm)
6
00
374.3
700
489.3
800
534.5
900
862.9
1000
963.4FE-SEM micrographs of 3 wt% Eu2O3 doped willemite samples sintered at (a) 600°C, (b) 700°C, (c) 800°C, (d) 900°C, and (e) 1000°C for 4 h.
(a)
(b)
(c)
(d)
(e)Grain size distribution of 3 wt% Eu2O3 doped willemite samples sintered at (a) 600°C, (b) 700°C, (c) 800°C, (d) 900°C, and (e) 1000°C for 4 h.
(a)
(b)
(c)
(d)
(e)Figure 9
Microstructures of 3 wt% Eu2O3 doped willemite samples sintered at 1000°C for 4 h where the EDX analysis was performed.


## 4. Conclusion

In this study, the Eu2O3 doped willemite-based glass ceramic samples have been successfully prepared using the solid state melting and quenching technique. Bulk density of the samples was increased with the increase in the sintering temperatures and doping of various weight percentages of the Eu3+. The XRD spectra indicated the presence of rhombohedral phase of willemite compounds in the pure and doped willemite samples sintered at 1000°C for 4 h. The FE-SEM micrographs showed an increase in the grain size from 374 to 963 nm when the sintering temperature was increased. The chemical composition of the samples was also successfully investigated using the FE-SEM-EDX analysis.



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*Source: 328931-2014-12-31.xml*