Effect of Sb 2 O 3 Doping and Temperature Treatment on Optical Energy Band Gap Properties in Zn-Bi-Ti-O Varistor Ceramics

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1 Effect of Sb 2 Doping and Temperature Treatment on Optical Energy Band Gap Properties in Zn-Bi-Ti-O Varistor Ceramics Mohd Sabri Mohd Ghazali a*, Wan Rafizah Wan Abdullah b, Azmi Zakaria c, Halimah Mohamed Kamari c, Mohd Hafiz Mohd Zaid c, Muhamad Azman Zulkifli a and Zahid Rizwan d a School of Fundamental Science, Universiti Malaysia Terengganu, Kuala Terengganu, Terengganu, Malaysia b School of Ocean Engineering, Universiti Malaysia Terengganu, Kuala Terengganu, Terengganu, Malaysia c Department of Physics, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia d Department of Applied Sciences, Convener Purchase, National Textile University, Faisalabad (37610) Pakistan mohdsabri@umt.edu.my Abstract The optical energy band-gap (E g ) is an important feature of semiconductor which determines their applications in optoelectronics. It is necessary to investigate the electronic states of ceramic based ZnO vasristor and effect of doped impurities at different concentration. E g of the ceramic (99-x) mol% ZnO mol% Bi mol% TiO 2 + xsb 2 where x = 0, 0.2, 0.4, 0.6 and 0.8 mol%, were determined using UV-Vis spectrophotometer. The samples was prepared via solid-state route and sintered at the sintering temperature from 1110, 1140 and for 45 and 90 min in open air. At no doping of Sb 2, the values of E g are ± 0.001, ± ev for 45 and 90 min sintering time; respectively. E g was increased to ± and ± ev at 0.2 for 45 and 90 min sintering time; respectively. XRD analysis indicates that there is hexagonal ZnO and few small peaks of intergranular layers of secondary phases of pyrochlore phases, Zn 2 Bi 3 Sb 3 O 14 and Bi 4 Ti 3 O 12, and spinel phases, Zn 7 Sb 2 O 12 and Zn 2 TiO 4. The relative density of the sintered ceramics decreased with the increase of Sb 2 concentration for 45 min sintering time, however, its trend increased for 90 min sintering time and the average grain size decreased with the increase of sintering temperatures. The variation of sintering temperatures and XRD findings are correlated with the UV-Vis spectrophotometer results of based ZnO varistor doped with Sb 2 due to the formation of interface states. Keywords: Energy band gap, Optical properties, Sb 2, Sintering, ZnO varistors 2016(2): eissn

2 1.0 Introduction Zinc oxide (ZnO) varistors are polycrystalline ceramics that consist of ZnO as the base and a couple of dopants as an unique feature of grains and grains boundaries is created in the ceramics during sintering [1, 2]. The varistors are useful for protecting a variety of electrical equipment s against over voltage and they manage to operate efficiently without damage. Nowadays, rapid developments of micro-electronic technology and large-scale integrated circuits have encouraged the production of this device for lowvoltage applications in automobile and small semiconductor electronics application [3, 4]. They are fabricated with different types of dopants such as Bi 2, TiO 2, MnO 2, Co 3 O 4 and CaMn [5-11]. Its unique grain boundary feature is responsible for nonlinear current-voltage (I-V) characteristics of the device [2, 12] and thus, is used to protect electrical surges. I-V studies have been extensively investigated for ZnO based varistor by previous researchers for many years [7, 13] and it is necessary to investigate the electronic states of ceramic ZnO and the effect of doped impurities at different processing conditions. The characterization of the absorption spectrum in semiconductors leads to the determination of the optical band-gap energy [14, 15]. In this study, the investigation regarding the optical band gap energy and relationship with X-ray diffraction (XRD) findings of ZnO-Bi 2 -TiO 2 doped Sb 2 with different sintering temperatures of 1110, 1140 and for 45 and 90 min in air. 2.0 Methodology All oxides precursors of 99.9% purity (Alfa Aesar, USA) were used. The composition consists of (98- x) mol% ZnO mol% Bi mol% TiO 2 + x Sb 2 where x = 0, 0.2, 0.4, 0.6 and 0.8 mol%. The powder was ball milled for 24 h in deionized water. The slurry was dried at 70 C using hot plate and continuously magnetically stirred to avoid the sedimentation of the heavy particle and pre-sintered at 800 C for two hours in open air with heating and cooling rate of 6 Cmin -1. The pre-sintered mixture was pulverized using an agate mortar/pestle and after 1.75 wt.% Polyvinyl Alcohol binder addition, dried, grinded and granulated by sieving 75 micron mesh screen. The mixture was then pressed into discs of 10 mm in diameter and 1 mm in thickness, each at a pressure of 2 ton/m 2. Finally, the discs were sintered at 1110, 1140 and 1170 C in open air for 45 and 90 min sintering time at heating and cooling rate of 2.66 C min -1. The disk from each sample was ground for optical and XRD characterizations. The crystalline phases were identified by an XRD (PANalytical X Pert Pro PW3040/60, Philips) with CuK α radiation and the data were analyzed, using X Pert High Score software. According to Keskenler and co-researcher [16], structural disordering can be obtained by Urbach s Rule for the optical absorption (exponential tail) [17]. The density was measured by the geometrical method taking the average of 10 discs [18, 19]. For the microstructure analysis, each of the disk samples was thermally etched at 150 C in a tube furnace. The microstructure morphology of sintered discs was examined by Variable Pressure Scanning Electron Microscopy (VPSEM, Leo 1455). The average grain size (d) was determined by lineal intercept method [20], given by Equation 1: d = 1.56 L/MN (1) where L is the random line length on the micrograph, M is the magnification of the micrograph and N is the number of the grain boundaries intercepted by lines. The UV-Vis spectrophotometer was used to measure the optical band-gap energy of the ceramics. The transmission signal was measured for the wavelength from 200 to 800 nm and then converted to absorption signal for further evaluation [21]. It was 2016(2): eissn

3 Intensity (a.u.) assumed that the fundamental absorption edge of the ZnO based varistor ceramics is due to the direct allowed transition [22]. The optical band-gap energy is given by Equation 2 [23]: (Ahυ) 2 = C(hυ-E g ) (2) where A is the optical absorption coefficient, C is the constant independent of photon energy (hυ), and E g is the direct allowed optical energy band-gap. From the plot of (Ahυ) 2 versus hυ, Fig. 6, the value of E g is obtained by using Origin Pro 8.0 software within the linear fitted regions at (Ahυ) 2 = Result and Discussion 3.1 Crystalline Phase, Density and Microstructure The XRD analysis, Figure 1, reveals diffraction peaks which belong to two phases observed in XRD patterns are ZnO (ICSD code: ) and secondary phases of pyrochlore phases, Zn 2 Bi 3 Sb 3 O 14 (ref. code: ) and Bi 4 Ti 3 O 12 (ref. code: ), and spinel phases, Zn 2 Sb 2 O 12 (ref. code: ) and Zn 2 TiO 4 (ref. code: ). The pyrochlore phases were observed at low concentration of Sb 2 while spinel phases existed at high concentration and theses take part in the control of grain growth [5, 24]. ZnO Bi 4 Ti 3 O 12 Zn 2 TiO 4 Zn 7 Sb 2 O 12 Bi 3 Sb 3 Zn 2 O mol% 0.6 mol% 0.4 mol% 0.2 mol% (degree) Figure 1 XRD patterns ZnO based varistor at 1140 C for 45min sintering time at different Sb 2 dopant concentration The relative density of the ceramics, Figure 2, decreases from 94.8 to 91.0 % with the increase of Sb 2 for 45 min sintering time but its trend increases from 93.6 to 95.2 % for 90 min sintering time. It is expected that many pores was created at lower sintering time at high Sb 2 doping level which indicates 2016(2): eissn

4 Relative density (%) Relative density (%) insufficient time to eliminate the porosity. Further prolonged sintering time make the elimination of porosity process occur and hence increase the density. The average grain size decreases from 32.2 to 15.2 μm, 31.2 to 14.2 μm with the increase of sintering temperature for 45 and 90 min; respectively, Fig. 3. This is due to the secondary phase, Zn 7 Sb 2 O 12, becomes affixed within the ZnO grains, there by impeding material transport at higher doping level of Sb 2. Average grain size increases with the increase of sintering time at all doping level of the Sb 2 indicating that the material transport is enhanced with the increase of sintering time [25] Figure 2 Relative density of 45min (top) and 90min (bottom) 2016(2): eissn

5 Average grain size (m) Average grain size (m) Figure 3 Average grain size of 45min (top) and 90min (bottom) The SEM micrographs, Figure 4 and EDX analysis (Figure 5) show that Zn 7 Sb 2 O 12 is segregated [25] in the grain boundaries and was observed very clearly at higher doping levels of Sb 2. The Bi was detected at the grain boundaries and at the triple point junctions. EDX analysis at the grain boundaries and at the surface of the grains show the presence of Ti but some patches of Ti ions have been seen on the surface of the grains and near the triple point junctions. This explains that the Ti ions substituted in ZnO lattice, Fig. 5. The pattern of grains is uniform but several small grains are also present in the ceramics. Bi 2 makes potential barrier at boundaries by becoming itself a layer of inter-granular material and supplying ions to the ZnO grain boundaries. Segregation of Bi ion at the grain boundary is said to be the source of the nonohmic behavior of the ZnO varistors ceramic [26] Bi 2 acts as grain enhancer, produces the interface state and nonlinearity effects [27]. The presence Ti indicates that the Ti ions are substituted in the Zn lattice as the ionic radii of Ti ion (0.68 Å) is smaller than that of Zn (0.74 Å). TiO 2 commonly use as the grain growth enhancer to achieve large grain growth [26, 28, 29]. However, the addition of Sb 2 starting from 0.2 mol% clearly shows that a great action of grain growth retarded and the function of TiO 2 dopant seemly become negligible. It is believe that the suitable amount of Sb 2 to be doped in the recipe is less than 0.1 mol% to enhance the stability of the electrical performance. 2016(2): eissn

6 3.2 Optical Band Gap Energy As it is shown in the Figure 6, the values of E g of the ceramic combination ZnO Bi TiO 2 + x Sb 2 at 1140 C are ± 0.001, ± ev for 45 and 90 min sintering time, respectively, at 0 mol% of Sb 2. A proper interpretation related to the decrement of E g is due to the growth of interface states due to presence of 0.5 mol% Bi 2 at the grain boundaries and triple point junctions in the ceramic ZnO due to the segregation of Bi 2, in the absence of Sb 2. Furthermore, the addition of 0.5 mol% TiO 2 in the varistor ceramics will lower the E g value. E g cannot be reduced further with the doping concentration of Sb 2. However, the values of E g are increased to ± and ± ev at 0.2 for 45 and 90 min sintering time, respectively. The slight increase in the E g was possibly due to the decrease in the interface states that may be due to the development of the secondary phases composed from Sb 2. The value of E g for the samples is about constant at 1140 and 1170 C for 45 min sintering time, after 0.2. This indicates that the build of interface states is limited and seem impossible as the saturation level has been reached [25]. Prolonged heat treatment, the E g values slightly decrease with the increase of dopant concentration, might be related to growth of interface states at the particle surfaces and at the grain boundaries. Figure 4 SEM micrographs of sintered varistor ceramics: from left (45min) ad right (90min) from up to below is 0.2mol&, 0.4mol%, 0.6mol% and 0.8mol% of Sb 2 dopant at 1140 C sintering temperature 2016(2): eissn

7 E g (ev) E g (ev) Figure 5 EDX micrographs and spectrum of varistor ceramics Figure 6 Band gap at 45min (top) and 90min (bottom) 2016(2): eissn

8 Steepness factor (a.u.) Steepness factor A (a.u.) According to optical absorption spectra from transformation of transmission spectra, steepness factor σ A and σ B can be determined from their slope [25]. It is observed that steepness factor σ A, Figure 7, increases at the initial doping of Sb 2 indicating the decrease of structural disordering in the varistor ceramics. Thus, this structural ordering gave elimination of interface states and as a consequence, the E g value increases. Steepness factor σ A is slightly increase or about constant after further addition of Sb 2 dopant. As a result, the E g value is constant. The steepness factor σ B, Fig. 7, increases with the sintering temperatures indicating the decrease in the thermal energy of displacement [23], in consequence, decrease the structural disordering at high sintering temperature with slightly decrement of E g as compared to pure ZnO min min min 90 min Figure 7 Variation of σ A (top) and σ B (bottom) with mol% of Sb 2 at 1140 C 4.0 Conclusion It is observed that steepness factor σ A increases with the doping level of Sb 2 indicating the decrease of structural disordering in the varistor ceramics. Thus, this structural ordering gave elimination of interface states and as a consequence, the E g value increases. The steepness factor σ B, increases with the sintering temperatures indicating the decrease in the thermal energy of displacement, in consequence, decrease the structural disordering at high sintering temperature with slightly decrement of E g as compared to pure ZnO ceramics. 2016(2): eissn

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