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A combined study of the thermoluminescence and electron paramagnetic resonance of point defects in ZrO2:Er3+
Radiation Physics and Chemistry 172 (2020) 108767
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
A combined study of the thermoluminescence and electron paramagnetic resonance of point defects in ZrO2:Er3+
T
H.S. Lokesha, M.L. Chithambo∗ Department of Physics and Electronics, Rhodes University, Grahamstown, 6140, South Africa
ARTICLE INFO
ABSTRACT
Keywords: ZrO2:Er Thermoluminescence Tm–Tstop analysis Kinetic parameters EPR
This work describes thermoluminescence of beta irradiated zirconium oxide (ZrO2) doped with erbium. The sample was synthesized by the solution combustion method. Kinetic analysis of the thermoluminescence is reported and paramagnetic defects sensed by electron paramagnetic resonance (EPR) have been investigated. Using X-ray diffraction, the phase of the sample was determined to be monoclinic. Glow curves measured at 1 °C s−1 show two peaks at 50°C (peak 1) and at 112°C (peak 2). The dose response of peak 1 is sublinear within 51–411 Gy whereas that of peak 2 is linear between 51 and 155 Gy becoming sublinear thereafter up to 411 Gy. Peak 1 fades within 1800 s of irradiation whereas peak 2 fades to 59% of its initial intensity within 14,600 s of irradiation. Kinetic analysis of the peaks was done using the initial rise, whole glow peak and curve fitting methods. The order of kinetics of both peaks was determined to be first order. The activation energy of peaks 1 and 2 were found to be 0.67 ± 0.01 eV and 0.71 ± 0.02 eV respectively. The EPR spectrum of erbium doped ZrO2 reveals that there are two types of Zr3+ ions present in the sample as distinguished by their coordination in the material. The F+ type centres are generated during sample irradiation and these centres accountable for the TL peaks.
1. Introduction
Isasi-Marín et al., 2009). The TL intensity increases and peak positions change. On this, Hsieh et al. (Hsieh and Su, 1994), reported on UVinduced TL from undoped ZrO2 and ZrO2 doped with 1 mol% of Er. Undoped ZrO2 shows two peaks at 90 and 205°C whereas the doped sample has peaks at 85 and 205°C . Studies of undoped and Pr3+ doped ZrO2 exposed to beta radiation was carried out by Isasi-Marin et al. (Isasi-Marín et al., 2009). The glow curve for the undoped sample showed three peaks at 100, 132 and 185°C . When measurements were made on the doped sample (Pr3+ at 0.5 mol%), three peaks were also observed but at higher temperatures of 227, 310 and 370°C. In these examples, the dose response or kinetic analysis were not studied to any significant extent. Electron paramagnetic resonance (EPR) spectroscopy also used in this work, is a sensitive technique used to detect unpaired electrons in a material. EPR spectroscopy provides information about the electronic structure of paramagnetic defects such as transition metal ions and their charge state, charge state of oxygen vacancies and interstitials (Wright and Barklie, 2009). Qualitative analysis by EPR of characteristics of the nature of electron traps and their concentration depending on absorption dose is useful in radiation dosimetry applications. Induced paramagnetic defects in ZrO2 and ZrO2:Y (9.5 mol%) have been extensively investigated (Wright and Barklie, 2009; Polliotto et al., 2018; Gionco
Zirconium oxide (ZrO2), also otherwise referred to as zirconia, is a wide band-gap (5–7 eV) material which has been extensively used in industry for coatings, catalysis, and sensor applications (Manziuc et al., 2019; Hua et al., 2013; Sato et al., 2013; Abd El-Ghany and Sherief, 2016; Sinhamahapatra et al., 2016) because of its hardness, transparency, high refractive index, chemical stability, and high melting point. Zirconia ceramics stabilized by oxides and/or doped with rare earths (REs) have a wide range of interesting optical, mechanical and thermoluminescence (TL) features (Abd El-Ghany and Sherief, 2016; Chen, 2006; Rivera et al., 2007; Liu et al., 2011). Of the various REs available, Er3+ has been widely exploited in such work. Erbium doped ZrO2 shows strong green emission at low concentration of the dopant Er3+ ions (~less than 4 mol%) whereas strong red emission is seen for greater concentration of Er3+ ions (Soares et al., 2015; Jia et al., 2006). The thermoluminescence of ZrO2 exposed to UV light, gamma, beta or X-ray irradiation is intense (Rivera, 2011; Villa-Sánchez et al., 2014a; Salas et al., 2003; La and Su, 2000). One drawback of monoclinic undoped ZrO2 is that its signal fades within a few minutes of irradiation. Doping ZrO2 with RE ions affects its TL properties considerably (Tamrakar and Upadhyay, 2016; Villa-Sánchez et al., 2014b; ∗
Corresponding author. E-mail address: [email protected] (M.L. Chithambo).
https://doi.org/10.1016/j.radphyschem.2020.108767 Received 14 November 2019; Received in revised form 3 February 2020; Accepted 7 February 2020 Available online 10 February 2020 0969-806X/ © 2020 Elsevier Ltd. All rights reserved.
Radiation Physics and Chemistry 172 (2020) 108767
H.S. Lokesha and M.L. Chithambo
et al., 2013; Costantini et al., 2018). In this way, bulk and surface Zr3+ sites were identified in undoped ZrO2 and hole trapping centres found in ZrO2:Y (9.5 mol%). The aim of this work is to study the thermoluminescence of ZrO2:Er. We report kinetic analysis of the TL and describe dosimetric properties of ZrO2:Er. Intrinsic and radiation induced defects in ZrO2:Er have been analysed using EPR and attempts to compare the radiation induced EPR results and TL results have been made. 2. Experimental details 2.1. Synthesis Undoped and 1 mol% of erbium doped ZrO2 powder (henceforth identified as ZrO2:Er) were synthesized by the solution combustion technique. The procedure adopted to synthesize undoped ZrO2 was described elsewhere (Lokesha et al., 2015). To achieve ZrO2:Er, zirconium (IV) oxynitrate hydrate (99%), carbohydrazide (98%) and erbium nitrate pentahydrate (99.9%) were used as precursors. Stoichiometric amounts of chemicals were dissolved in 50 ml of double distilled water. The mixture was stirred until transparent then placed in a muffle furnace preheated to 350°C . Initially, the solution boiled, dehydrated followed by decomposition and effervescence. Finally a voluminous amount of ZrO2:Er powder was obtained. The powder was then ground into even more fine particles using an agate mortar and pestle. The sample was thereafter annealed at 850°C for 3 h in air to remove any carbonates in the sample.
Fig. 1. XRD pattern of ZrO2:Er. The inset shows a plot of β*cosθ/λ against (4sinθ)/λ.
data as follow from PDF # 86–1451. To estimate the effective strain and crystallite size from the XRD data, the Williamson Hall equation was used (Mote et al., 2012). This is expressed as
cos
=
1 4 sin + D
(1)
where D is crystallite size, β is the full width at half maximum (FWHM), θ is the Bragg angle, λ is the X-ray wavelength (1.5406 Å) and ε is the effective strain. A plot of ( cos )/ against (4 sin θ)/λ is shown in Fig. 1. The average crystallite size and effective strain were determined as 57 nm and 0.23% from the ‘y’ intercept and slope respectively.
2.2. Characterization The phase of the sample was determined by powder X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer. Measurements were made at room temperature by use of Cu-Kα (1.5406 Å) radiation. The XRD pattern was recorded in the range of Bragg angle 20–70°. EPR measurements were carried out using a Freiberg Instruments Miniscope EPR spectrometer (Model 5500) at room temperature and operating at the X-band frequency with 100 kHz modulation frequency and microwave power of 1 mW. Diphenyl Picryl Hydrazyl (DPPH) was used to calibrate the g-factors of defect centres. The sample used for EPR was irradiated to 100 Gy using a60Co gamma source at a nominal dose rate of 1.512 Gy/s. TL was measured on a RISØ TL/OSL DA-20 Luminescence Reader. ZrO2 and ZrO2:Er samples in powder form with mass 0.04 g and 0.02 g respectively were used for TL measurements. The TL intensity was corrected for sample mass. The sample was irradiated by a90Sr/90Y beta source at a nominal dose rate of 0.1028 Gy/s. The luminescence was detected by an EMI 9235QB photomultiplier tube (PMT) through a combination of a BG3 and a BG39 filter (overall transmission band 350–450 nm FWHM). TL measurements were carried out at 1 °C s−1 in a nitrogen atmosphere. All measurements were recorded four times to obtain the best estimate of a particular value as the average result and the margin of error as the standard deviation of the set.
4. Glow curve characteristics 4.1. General features of the glow-curve Fig. 2 shows several glow curves. The natural TL from ZrO2:Er (solid circles) is compared with a glow curve measured after irradiation to 205 Gy (open circles). Both measurements were made at 1 °C s−1 and have been corrected for background. There is no well-defined peak in the natural TL. On other hand, the glow curve from the irradiated sample has two peaks at 50°C (peak 1) and at 112°C (peak 2) and
3. X-ray diffraction The XRD result of the sample is shown in Fig. 1. The pattern shows prominent diffraction peaks at 28.14° and 31.42° corresponding to (1 11) and (111) planes. These peaks are characteristic of the monoclinic phase and are consistent with the Powder diffraction file (PDF) # 86–1451 indicated in Fig. 1 for comparison. Other less intense diffraction peaks are ascribed to different (hkl) planes in the monoclinic phase of ZrO2. Structural parameters such as lattice parameters and cell volume were calculated from XRD data using the unit cell program (Holland and Redfern, 1997). The lattice parameters were found to be a = 5.141 Å, b = 5.211 Å, and c = 5.299 Å and the cell volume was 3 evaluated as (140.19 ± 0.01) Å . These values match well with standard
Fig. 2. A natural TL glow curve (solid circles) shown together with that of ZrO2:Er after irradiation to 205 Gy (open circles). A TL glow curve of undoped ZrO2 (solid triangles) is shown for comparison. 2
Radiation Physics and Chemistry 172 (2020) 108767
H.S. Lokesha and M.L. Chithambo
possibly a third one at 264°C . The analysis of TL presented in this work was carried out on peaks 1 and 2. For comparison, a glow curve of undoped ZrO2 irradiated to a much lower dose of 40 Gy is included in Fig. 2. This glow curve, also recorded at 1°C s−1, shows two obvious peaks at 54 and 120°C. The glow curve structure in ZrO2 is known to depend on the morphology of the sample (Villa-Sánchez et al., 2014a). In our case, the crystalline phase of both undoped and doped sample is the same, monoclinic. The only change noted was the crystallite size decreasing from 54 to 57 nm as determined from XRD data of undoped and doped samples respectively. Therefore, by comparing the intensity of an undoped sample and the 1 mol% doped one, we deduce that the decrease of TL intensity in ZrO2:Er sample may be due to change in particle size and possibly Er ion concentration quenching. 4.1.1. Repeatability The reproducibility of peaks 1 and 2 was examined in TL measurements made consecutively on a sample irradiated to 205 Gy each time. The positions of peaks 1 and 2 were found to be reproducible at 49.3 ± 1.0°C and 111.5 ± 0.9°C respectively in a set of eight repetitive measurements.
Fig. 4. The Tm-Tstop plot. Here, peak 1 and 2 are main peaks, and peaks 3 and 4 are secondary peaks.
4.1.2. Qualitative assessment of the glow curve using thermal cleaning In order to estimate the number of peaks in a glow curve, the thermal cleaning technique discussed elsewhere (McKeever, 1985) was used. A sample freshly irradiated to 205 Gy each time was heated to beyond the maximum of a particular peak and after cooling reheated to 500°C to measure the remaining glow curve so as to reveal any remaining peaks. Fig. 3 shows a glow curve measured immediately after irradiation together with those after partial heating to 72, 150, 235 and 290°C. The results show that apart from peaks at 50°C and 112°C noted in Fig. 2, there are two additional ones at 172 and 270°C. There are thus four glow peaks in the glow curve of ZrO2:Er. In comparison, Lai and Su (1999) reported three peaks within 30–300°C also in ZrO2:Er.
against Tstop. The results of the Tm-Tstop analysis shows there are at least four peaks associated with the glow curve. These peaks are labelled as 1, 2, 3 and 4. This result matches well with that from thermal cleaning. The position of peak 1 remains constant when Tstop changes from 22 to 40°C . Similarly, the position of peak 2 is independent of Tstop in two neighbouring regions at 114 and 118°C as clarified in the inset to Fig. 4. Beyond preheating to 112°C, the position of peak 2 increases with Tstop. This behaviour indicates that the apparently single peak 2 either consists of multiple first order components or that this peak follows second order kinetics. The order of kinetics of peak 2 is clarified later. The results of Tm-Tstop analysis demonstrate that peaks 1, 3 and 4 follow first order kinetics.
4.1.3. Qualitative assessment of the glow peaks using Tm-Tstop analysis As an alternative method to determine the number of peaks in the glow curve and their order of kinetics, the Tm-Tstop method was used. Here, a sample irradiated to 205 Gy was first partially heated to 22°C (a Tstop temperature) and after cooling, the sample was reheated to obtain the complete glow curve. This procedure was repeated several times on the same sample, freshly irradiated each time and Tstop increased in steps of 2°C from 22 to 200°C. For each Tstop, the position of the remaining peaks in the glow curve were noted. Fig. 4 shows a plot of Tm
4.2. Influence of irradiation on thermoluminescence properties 4.2.1. Effect of dose on peak temperature The order of kinetics of peaks 1 and 2 was further determined using the dependence of Tm on dose. Fig. 5 shows Tm against dose between 51 and 411 Gy. The position of peak 1 changes from 48.5 ± 1 to 51.0 ± 0.5°C when the dose changes from 51 to 411 Gy. In the same dose range, the position of peak 2 changes from 113.5 ± 1 to
Fig. 3. Glow curves of 205 Gy irradiated ZrO2:Er recorded without preheat (i) after preheating to 72°C and to (ii) 150°C (iii) 235°C (iv) and 290°C (v).
Fig. 5. The dependence of peak temperature Tm on irradiation dose. 3
Radiation Physics and Chemistry 172 (2020) 108767
H.S. Lokesha and M.L. Chithambo
Fig. 6. Change of glow peak intensity with dose. The inset shows glow curves of ZrO2:Er corresponding to different irradiation doses.
Fig. 7. A plot of ln (TLexp/n b) against 1/kT for peak 2 for different values of the order of kinetics b.
109.0 ± 0.5°C. Fig. 5 shows that both peaks 1 and 2 go through regions where their peak maximum is independent of dose. For peak 1, Tm is stable at 48.5 ± 1°C between 51 and 102 Gy and at 51.0 ± 0.5°C between 205 and 411 Gy. For peak 2, Tm is stable at 113.5 ± 1, 111 ± 1 and 109.0 ± 0.5°C for doses within 51–102, 154–308 and 359–411 Gy respectively. The results of Fig. 5 imply that peaks 1 and 2, which appear as single, in fact consist of multiple first-order peaks which individually become prominent at specific irradiation doses as concluded in similar cases previously (Chithambo, 2017; Thomas et al., 2019). Based on results of the dependence of Tm on dose, peaks 1 and 2 follow first order kinetics.
5.2. Whole glow curve method In this method, the area n under a glow peak is related to the order of kinetics b as
ln
+1
= ln
sl
E kT
(3)
3(b−1) −1
where s (m s ) is the effective frequency factor for general order kinetics. For a specific value of b, a plot of ln (TLexp/n b) versus 1/kT (where K is in Kelvin as for other methods used) is linear. The values of E and s are evaluated from the slope and y-axis intercept respectively. Fig. 7 shows the results of analysis of peak 2 for a dose of 205 Gy. The best straight line for the fitting is found to be for b = 1.2 for which R2 = 0.99. The corresponding activation energy and frequency factor were estimated as 0.66 ± 0.01 eV and 1.5 × 106 s−1 respectively. The value of b is consistent with first order kinetics.
Fig. 6 shows peak intensity against beta dose from 51 to 411 Gy. Selected glow curves corresponding to different doses are shown in the inset to Fig. 6. The intensity increases with dose. The dose response was analysed using the superlinearity index g(D),
D y (D)/D y (D)/D
nb l
4.3. Effect of dose on thermoluminescence intensity
g(D) =
TL exp
5.3. Glow curve deconvolution
(2)
The kinetic parameters of the glow peaks were also analysed by curve fitting using the equation
In the above relation, y(D) is the analytical function describing the behaviour of dose response, y’(D) and y’’(D) are each the first and second derivative of y(D). A value of g(D) = 1 denotes a linear response, g(D) > 1 refers to supralinearity and g(D) < 1 represents sublinearity (Thomas et al., 2019). Using such analysis, the dose response of peaks 1 and 2 is concluded to be effectively sublinear (see plot of g(D) against dose in the supplementary submission).
I(T) = Im b b
b
1
E T Tm × (b 1)(1 kT Tm
)
T2 E T Tm exp + Zm kT Tm T2m
b b 1
(5) where I(T) is the temperature dependence of the TL, = 2kT/E, 1) m , Tm is the position of the peak whose m = 2kTm /E , Z m = 1 + (b peak intensity is Im, E is activation energy and b is the order of kinetics (Pagonis et al., 2006). The software package Microsoft Excel with the solver utility was used for curve fitting. The goodness of fit was tested by the figure of merit (FOM) (Afouxenidis et al., 2012), given as
5. Kinetic analysis Peaks 1 and 2 were analysed for kinetic parameters using the initial rise, whole glow peak and curve fitting methods. Multiple methods were used as a means to corroborate findings. The details of the methods have been discussed elsewhere and will not be repeated here (Pagonis et al., 2006).
FOM =
TL exp
TL fit
TL fit
(6)
where TLexp and TLfit represent the experimental data and values from the fitting function respectively. A fit is acceptable if the figure of merit (FOM) is less than 3.5%. Fig. 8 shows the deconvolved glow curve and a plot of residuals of the fit. The residuals of the fit, fluctuating about zero, signify a good fit. TL glow curve of ZrO2:Er (between 22 and 320°C) is best described as a minimum of four peaks. The number of peaks used for deconvolution agree well with experimentally
5.1. Initial rise method The activation energy of peak 2 was determined using the initial rise method as 0.71 ± 0.02 eV. The corresponding plot is available as supplementary information. 4
Radiation Physics and Chemistry 172 (2020) 108767
H.S. Lokesha and M.L. Chithambo
Fig. 8. A deconvolved glow curve of ZrO2:Er irradiated to 205 Gy. The residuals of the fit confirms that the fit is acceptable.
Fig. 10. The ESR spectra of ZrO2:Er sample (a), after gamma irradiated to 100 Gy (b) and after TL measurement (20–500°C ) followed by gamma irradiation (c).
determined conclusions from thermal cleaning and the Tm-Tstop procedure. The FOM for the glow curve fit is less than 2.7%. This denotes that the experimental data and fitted function match properly. For a dose of 205 Gy, the activation energy and order of kinetics were determined as 0.66 ± 0.01 eV, 0.74 ± 0.02 eV and 1.4 ± 0.05, 1.3 ± 0.01 respectively. These values agree well with those calculated from the initial rise method. The frequency factor of the peaks was calculated by using the general order equation,
s=
E 1 exp kT2m Z m
E kTm
storage time up to 14,600 s. The peak intensities are normalized by dividing by their respective intensity measured immediately after irradiation. The intensity of peak 1 fades to background level within 1800 s whereas the intensity of peak 2 drops to 23% of its initial intensity after 1800 s. The overall decrease of intensity of peak 2 after 14,600 s is 59%. Further, the intensity of peak 3 (at 172°C ) and peak 4 (at 260°C) decreased by 23% and 2% of the initial intensity after 14,600 s respectively. Also, the position Tm of all four peaks is independent of storage time.
(7)
7. Electron paramagnetic resonance
where β is the heating rate and k is Boltzmann's constant. The frequency factors of peaks 1 and 2 were estimated to be 2.1 × 109 and 3.4 × 108 s−1 respectively.
EPR spectroscopy was used to identify any intrinsic or radiation induced colour centres in ZrO2:Er. EPR results of pristine and gamma irradiated ZrO2:Er are compared as a way to identify radiation induced defects. The information is then linked to TL results. Fig. 10 shows the EPR spectra of ZrO2:Er. The spectra consists of one recorded from pristine sample (a), after gamma irradiation to 100 Gy (b) and that after TL measurement followed by gamma irradiation (c). The EPR spectra were recorded at room temperature. The EPR signal is not a flat line as one would expect in the case of perfect stoichiometry. The ZrO2:Er sample shows three distinct weak signals (referred to as centres I, II and III). These signals are related to the presence of intrinsic defects in the material (Gionco et al., 2013). In spectrum (a), centre I resonates at g⊥ = 1.975 and g∥ = 1.958. This signal was earlier assigned to Zr3+ sites in the bulk (Polliotto et al., 2018). Centres II and III have the same perpendicular value (g⊥ = 1.977) but differ in their parallel line components g∥ = 1.922 and g∥ = 1.905 respectively. Centres II and III corresponding to surface Zr3+ ions situated in the disordered environment of the surface (Gionco et al., 2013). Oxygen vacancies in ZrO2:Er form owing to the substitution of Zr4+ ions by Er3+ ions. However no EPR lines were observed in the ZrO2:Er sample in the studied magnetic field range. Spectrum (b) measured after gamma irradiation to 100 Gy shows two defect centres labelled as centre IV and centre V. Gamma radiation induces the generation of many electron–hole pairs which can end up at electron trap or hole centres. Centre IV with g values g1 = 2.022, g2 = 2.015 and g3 = 2.006 has a rhombic g-tensor ( g1 g2 g3 ). Polliotto et al. (2018) reported EPR results of UV irradiated ZrO2 and identified UV induced hole trapping centres (O centres) at a rhombic symmetry site (with g1 = 2.022, g2 = 2.015, and g3 = 2.004) (Polliotto et al., 2018). Also, O centres were noted in ZrO2:Y (9.5 mol%) under X-ray irradiation
6. Fading The change of intensity between irradiation (to 205 Gy) and measurement was studied. Fig. 9 shows the variation of peak intensity with
Fig. 9. The change of TL peak intensity with delay between irradiation and measurement for peaks 1 through 4 as shown. 5
Radiation Physics and Chemistry 172 (2020) 108767
H.S. Lokesha and M.L. Chithambo
(g1 = 2.019, g2 = 2.012, and g3 = 2.004) and under 1.5 MeV electron irradiation (g1 = 2.014, g2 = 2.010, and g3 = 2.007) (Orera et al., 1990; Costantini et al., 2013). Therefore, centre IV can be unambiguously assigned to O centres. Centre V appears as a broad signal centered at g = 1.99. The g value of centre V is rather close to the freeelectron value (2.0023) related to F+ type centres. F+ type centres (g = 1.974, 1.997 assigned to F+2 centres) and T-centres (g = 1.89) were also observed in ZrO2:Y (9.5 mol%) under electron and swift heavy ion irradiation (Costantini and Beuneu, 2011). In the present study, no T-centre is observed and centre V is tentatively assigned to F+2 centre. Furthermore, spectrum (c) shows that the signals corresponding to different centres (I–V) decrease to that of the unirradiated ZrO2:Er level. Therefore, the EPR analysis confirm that O centres and F+ type centres are generated during gamma irradiation.
Author statement This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. Acknowledgments H S Lokesha acknowledges with thanks Rhodes University PostDoctoral Research Fellowship (PDF No. s1900093). We also appreciate the financial support from Rhodes University and National Research Foundation of South Africa. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.radphyschem.2020.108767.
8. Discussion
References
The presented EPR spectrum of the ZrO2:Er sample shows the presence of bulk and surface Zr3+ sites at room temperature. Orera et al. (1990) reported that the PL emission band in stabilized zirconia depends on Zr3+ ions coordinated environment. An absorption band at 480 nm and a PL band at 375 nm were related to hepta-coordinated and hexa-coordinated Zr3+ ions respectively. Centres II and III correspond to surface Zr3+ sites that could result from the different coordinative environments. Trapped holes released from the O centre recombine with an electron at the F+ centre (Singh et al., 2012). The value of g for centre V, resonating at g = 1.99, is smaller than that of the free-electron value (g = 2.0023). This centre has been attributed to F+2 centre, that is, an aggregate of a singly ionized oxygen vacancy (Costantini and Beuneu, 2011; Singh et al., 2012). Concerning TL, as Fig. 2 shows, glow peaks were observed at 50 and 112°C in ZrO2:Er after beta irradiation. This finding is somewhat similar to that seen in the TL of erbium doped ZrO2 under UV and X-ray irradiation (Lai and Su, 1999; Hristov et al., 2010). Therefore, the identified defect centres tentatively correlate to the TL peaks based on the assumption that the ionization due to beta and gamma irradiation produce similar types of electron-hole pairs. Going by the results of EPR signals, we assume that the O centre and F+ type centres are associated with the TL peaks. In particular, we propose that the peaks at 50°C and 112°C are associated with transitions at F+ type centres probably F+ and F+2 centre acting as electron traps. Regarding kinetic analysis, kinetic parameters calculated by various methods are consistent. Hristov et al. (Singh et al., 2012) reported that TL of ZrO2:Er (at 1 mol%) co-doped with Li shows peaks at 65, 105 and 150°C after UV irradiation (Hristov et al., 2010). The corresponding activation energies of the peaks were found as 0.52, 0.96 and 1.03 eV respectively and determined to follow non first order kinetics. Our kinetic analysis in ZrO2:Er shows values close to those in the literature but it is reasonable to assume that these values are influenced by the dopant. However, the order of kinetics is first order for both peak 1 and peak 2 as confirmed by various tests.
Abd El-Ghany, O.S., Sherief, A.H., 2016. Zirconia based ceramics, some clinical and biological aspects: Review. Future Dental Journal 2, 55–64. Afouxenidis, D., Polymeris, G.S., Tsirliganis, N.C., Kitis, G., 2012. Computerised curve deconvolution of TL/OSL curves using a popular spreadsheet program. Radiat. Protect. Dosim. 149, 363–370. Chen, L.B., 2006. Yttria-stabilized zirconia thermal barrier coating- A review. Surf. Rev. Lett. 13, 535–544. Chithambo, M.L., 2017. Thermoluminescence of the main peak in SrAl2O4: Eu2+, Dy3+: spectral and kinetics features of secondary emission detected in the ultra-violet region. Radiat. Meas. 96, 29–41. Costantini, J.-M., Beuneu, F., 2011. Point defects induced in yttria-stabilized zirconia by electron and swift heavy ion irradiations. J. Phys. Condens. Matter: An Institute of Physics Journal 23, 115902. Costantini, J.-M., Beuneu, F., Weber, W.J., 2013. Radiation damage in cubic-stabilized zirconia. J. Nucl. Mater. 440, 508–514. Costantini, J.-M., Touati, N., Binet, L., Lelong, G., Guillaumet, M., Beuneu, F., 2018. Colour centre recovery in yttria-stabilised zirconia: photo-induced versus thermal processes. Phil. Mag. 98, 1241–1255. Gionco, C., Paganini, M.C., Giamello, E., Burgess, R., Di Valentin, C., Pacchioni, G., 2013. Paramagnetic defects in polycrystalline zirconia: An EPR and DFT study. Chem. Mater. 25, 2243–2253. Holland, T.J.B., Redfern, S.A.T., 1997. Unit cell refinement from powder diffraction data: the use of regression diagnostics. Mineral. Mag. 61, 65–77. Hristov, H., Arhangelova, N., Velev, V., Penev, I., Bello, M., Moschini, G., Uzunov, N., 2010. UV light-induced thermoluminescence of Er+Li doped ZrO2. J. Phys. Conf. 253 012025. Hsieh, W.-C., Su, C., 1994. UV induced thermoluminescence in ZrO2 doped by Er2O3. J. Phys. Appl. Phys. 27, 1763–1768. Hua, J., Liu, Z., Wang, Y., Zeng, Y., Zheng, X., Wang, H., 2013. In situ microstructure characterization of ZrO2 coating at high temperature. Prog. Org. Coating 76, 1792–1797. Isasi-Marín, J., Pérez-Estébanez, M., Díaz-Guerra, C., Castillo, J.F., Correcher, V., CuervoRodríguez, M.R., 2009. Structural, magnetic and luminescent characteristics of Pr3+doped ZrO2 powders synthesized by a sol–gel method. J. Phys. Appl. Phys. 42 075418. Jia, R., Yang, W., Bai, Y., Li, T., 2006. Upconversion photoluminescence of ZrO2:Er3+ nanocrystals synthesized by using butadinol as high boiling point solvent. Opt. Mater. 28, 246–249. La, L.-J., Su, C.-S., 2000. Thermoluminescence in ZnO doped ZrO2 induced by 2.5–9 keV X-ray synchrotron radiation. J. Appl. Phys. 87, 236–240. Lai, L.-J., Su, C.-S., 1999. Thermoluminescence of ZrO2 doped with Er2O3 induced by ultraviolet and x-ray radiation. J. Appl. Phys. 85, 8362. Liu, L., Wang, Y., Zhang, X., Yang, K., Bai, Y., Huang, C., Song, Y., 2011. Optical thermometry through green and red upconversion emissions in Er3+/Yb3+/Li+:ZrO2 nanocrystals. Optic Commun. 284, 1876–1879. Lokesha, H.S., Nagabhushana, K.R., Singh, F., 2015. Luminescence properties of 100 MeV swift Si7+ ions irradiated nanocrystalline zirconium oxide. J. Alloys Compd. 647, 921–926. Manziuc, M., Gasparik, C., Negucioiu, M., Constantiniuc, M., Burde, A., Vlas, I., Dudea, D., 2019. Optical properties of translucent zirconia. A review of the literature 3, 45–51. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press. Mote, V., Purushotham, Y., Dole, B., 2012. Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles. Journal of Theoretical and Applied Physics 6, 6. Orera, V.M., Merino, R.I., Chen, Y., Cases, R., Alonso, P.J., 1990. Intrinsic electron and hole defects in stabilized zirconia single crystals. Phys. Rev. B 42, 9782–9789. Pagonis, V., Kitis, G., Furetta, C., 2006. Numerical and Practical Exercises in Thermoluminescence. Springer. Polliotto, V., Livraghi, S., Giamello, E., 2018. Electron magnetic resonance as a tool to monitor charge separation and reactivity in photocatalytic materials. Res. Chem.
9. Conclusion The TL characteristics of beta irradiated monoclinic ZrO2:Er and defects analysis have been investigated. Glow curve show two peaks at 50 and 112°C. Analysis of dose response of both peaks is sublinear within 51–411 Gy. Kinetic analysis of the peaks 1 and 2 verified using various methods, both the peaks follows first order kinetics. The activation energy of peaks 1 and 2 corresponding to irradiation dose 205 Gy, were determined as 0.67 ± 0.01 eV and 0.71 ± 0.02 eV respectively. The frequency factor of peak 1 and 2 were estimated in the order of 109 and 108 s-1 respectively. EPR analysis confirm that the F+ and F+2 centres are associated with the TL possibly as electron traps. 6
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H.S. Lokesha and M.L. Chithambo Intermed. 44, 3905–3921. Rivera, T., 2011. Synthesis and Thermoluminescent Characterization of Ceramics Materials 2011. In: Sikalidis, C. (Ed.), Advances In Ceramics - Synthesis and Characterization, Processing and Specific Applications. InTech, pp. 127–164. Rivera, T., Furetta, C., Azorín, J., Barrera, M., Soto, A.M., 2007. Thermoluminescence (TL) of europium-doped ZrO2 obtained by sol-gel method. Radiat. Eff. Defect Solid 162, 379–383. Salas, P., De la Rosa-Cruz, E., Diaz-Torres, L.A., Castaño, V.M., Meléndrez, R., BarbozaFlores, M., 2003. Monoclinic ZrO2 as a broad spectral response thermoluminescence UV dosemeter. Radiat. Meas. 37, 187–190. Sato, A.G., Volanti, D.P., Meira, D.M., Damyanova, S., Longo, E., Bueno, J.M.C., 2013. Effect of the ZrO2 phase on the structure and behavior of supported Cu catalysts for ethanol conversion. J. Catal. 307, 1–17. Singh, V., Watanabe, S., Gundu Rao, T.K., Al-Shamery, K., Haase, M., Jho, Y.-D., 2012. Synthesis, characterisation, luminescence and defect centres in solution combustion synthesised CaZrO3:Tb3+ phosphor. J. Lumin. 132, 2036–2042. Sinhamahapatra, A., Jeon, J., Kang, J., Han, B., Yu, J., 2016. Oxygen-deficient zirconia
(ZrO2−x): A new material for solar light absorption. Sci. Rep. 1–8. Soares, M.R.N., Holz, T., Oliveira, F., Costa, F.M., Monteiro, T., 2015. Tunable green to red ZrO2:Er nanophosphors. RSC Adv. 5, 20138–20147. Tamrakar, R.K., Upadhyay, K., 2016. Effect of dysprosium concentration on thermoluminescence behavior of ZrO2:Eu3+ phosphor. Optik 127, 3602–3604. Thomas, S., Kalita, J.M., Chithambo, M.L., Ntwaeaborwa, O.M., 2019. The influence of dopants on thermoluminescence of Sr2MgSi2O7. J. Lumin. 208, 104–107. Villa-Sánchez, G., Mendoza-Anaya, D., Eufemia Fernández-García, M., Escobar-Alarcón, L., Olea-Mejía, O., González-Martínez, P.R., 2014a. Co nanoparticle effects on the thermoluminescent signal induced by UV and gamma radiation in ZrO2 powders. Opt. Mater. 36, 1219–1226. Villa-Sánchez, G., Mendoza-Anaya, D., Mondragón-Galicia, G., Pérez-Hernández, R., Olea-Mejía, O., González-Martínez, P.R., 2014b. Thermoluminescence response induced by UV radiation in Eu-doped zirconia nanopowders. Radiat. Phys. Chem. 97, 118–125. Wright, S., Barklie, R.C., 2009. Electron paramagnetic resonance characterization of defects in monoclinic HfO2 and ZrO2 powders. J. Appl. Phys. 106, 103917.
7
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A combined study of the thermoluminescence and electron paramagnetic resonance of point defects in ZrO2:Er3+
T
H.S. Lokesha, M.L. Chithambo∗ Department of Physics and Electronics, Rhodes University, Grahamstown, 6140, South Africa
ARTICLE INFO
ABSTRACT
Keywords: ZrO2:Er Thermoluminescence Tm–Tstop analysis Kinetic parameters EPR
This work describes thermoluminescence of beta irradiated zirconium oxide (ZrO2) doped with erbium. The sample was synthesized by the solution combustion method. Kinetic analysis of the thermoluminescence is reported and paramagnetic defects sensed by electron paramagnetic resonance (EPR) have been investigated. Using X-ray diffraction, the phase of the sample was determined to be monoclinic. Glow curves measured at 1 °C s−1 show two peaks at 50°C (peak 1) and at 112°C (peak 2). The dose response of peak 1 is sublinear within 51–411 Gy whereas that of peak 2 is linear between 51 and 155 Gy becoming sublinear thereafter up to 411 Gy. Peak 1 fades within 1800 s of irradiation whereas peak 2 fades to 59% of its initial intensity within 14,600 s of irradiation. Kinetic analysis of the peaks was done using the initial rise, whole glow peak and curve fitting methods. The order of kinetics of both peaks was determined to be first order. The activation energy of peaks 1 and 2 were found to be 0.67 ± 0.01 eV and 0.71 ± 0.02 eV respectively. The EPR spectrum of erbium doped ZrO2 reveals that there are two types of Zr3+ ions present in the sample as distinguished by their coordination in the material. The F+ type centres are generated during sample irradiation and these centres accountable for the TL peaks.
1. Introduction
Isasi-Marín et al., 2009). The TL intensity increases and peak positions change. On this, Hsieh et al. (Hsieh and Su, 1994), reported on UVinduced TL from undoped ZrO2 and ZrO2 doped with 1 mol% of Er. Undoped ZrO2 shows two peaks at 90 and 205°C whereas the doped sample has peaks at 85 and 205°C . Studies of undoped and Pr3+ doped ZrO2 exposed to beta radiation was carried out by Isasi-Marin et al. (Isasi-Marín et al., 2009). The glow curve for the undoped sample showed three peaks at 100, 132 and 185°C . When measurements were made on the doped sample (Pr3+ at 0.5 mol%), three peaks were also observed but at higher temperatures of 227, 310 and 370°C. In these examples, the dose response or kinetic analysis were not studied to any significant extent. Electron paramagnetic resonance (EPR) spectroscopy also used in this work, is a sensitive technique used to detect unpaired electrons in a material. EPR spectroscopy provides information about the electronic structure of paramagnetic defects such as transition metal ions and their charge state, charge state of oxygen vacancies and interstitials (Wright and Barklie, 2009). Qualitative analysis by EPR of characteristics of the nature of electron traps and their concentration depending on absorption dose is useful in radiation dosimetry applications. Induced paramagnetic defects in ZrO2 and ZrO2:Y (9.5 mol%) have been extensively investigated (Wright and Barklie, 2009; Polliotto et al., 2018; Gionco
Zirconium oxide (ZrO2), also otherwise referred to as zirconia, is a wide band-gap (5–7 eV) material which has been extensively used in industry for coatings, catalysis, and sensor applications (Manziuc et al., 2019; Hua et al., 2013; Sato et al., 2013; Abd El-Ghany and Sherief, 2016; Sinhamahapatra et al., 2016) because of its hardness, transparency, high refractive index, chemical stability, and high melting point. Zirconia ceramics stabilized by oxides and/or doped with rare earths (REs) have a wide range of interesting optical, mechanical and thermoluminescence (TL) features (Abd El-Ghany and Sherief, 2016; Chen, 2006; Rivera et al., 2007; Liu et al., 2011). Of the various REs available, Er3+ has been widely exploited in such work. Erbium doped ZrO2 shows strong green emission at low concentration of the dopant Er3+ ions (~less than 4 mol%) whereas strong red emission is seen for greater concentration of Er3+ ions (Soares et al., 2015; Jia et al., 2006). The thermoluminescence of ZrO2 exposed to UV light, gamma, beta or X-ray irradiation is intense (Rivera, 2011; Villa-Sánchez et al., 2014a; Salas et al., 2003; La and Su, 2000). One drawback of monoclinic undoped ZrO2 is that its signal fades within a few minutes of irradiation. Doping ZrO2 with RE ions affects its TL properties considerably (Tamrakar and Upadhyay, 2016; Villa-Sánchez et al., 2014b; ∗
Corresponding author. E-mail address: [email protected] (M.L. Chithambo).
https://doi.org/10.1016/j.radphyschem.2020.108767 Received 14 November 2019; Received in revised form 3 February 2020; Accepted 7 February 2020 Available online 10 February 2020 0969-806X/ © 2020 Elsevier Ltd. All rights reserved.
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et al., 2013; Costantini et al., 2018). In this way, bulk and surface Zr3+ sites were identified in undoped ZrO2 and hole trapping centres found in ZrO2:Y (9.5 mol%). The aim of this work is to study the thermoluminescence of ZrO2:Er. We report kinetic analysis of the TL and describe dosimetric properties of ZrO2:Er. Intrinsic and radiation induced defects in ZrO2:Er have been analysed using EPR and attempts to compare the radiation induced EPR results and TL results have been made. 2. Experimental details 2.1. Synthesis Undoped and 1 mol% of erbium doped ZrO2 powder (henceforth identified as ZrO2:Er) were synthesized by the solution combustion technique. The procedure adopted to synthesize undoped ZrO2 was described elsewhere (Lokesha et al., 2015). To achieve ZrO2:Er, zirconium (IV) oxynitrate hydrate (99%), carbohydrazide (98%) and erbium nitrate pentahydrate (99.9%) were used as precursors. Stoichiometric amounts of chemicals were dissolved in 50 ml of double distilled water. The mixture was stirred until transparent then placed in a muffle furnace preheated to 350°C . Initially, the solution boiled, dehydrated followed by decomposition and effervescence. Finally a voluminous amount of ZrO2:Er powder was obtained. The powder was then ground into even more fine particles using an agate mortar and pestle. The sample was thereafter annealed at 850°C for 3 h in air to remove any carbonates in the sample.
Fig. 1. XRD pattern of ZrO2:Er. The inset shows a plot of β*cosθ/λ against (4sinθ)/λ.
data as follow from PDF # 86–1451. To estimate the effective strain and crystallite size from the XRD data, the Williamson Hall equation was used (Mote et al., 2012). This is expressed as
cos
=
1 4 sin + D
(1)
where D is crystallite size, β is the full width at half maximum (FWHM), θ is the Bragg angle, λ is the X-ray wavelength (1.5406 Å) and ε is the effective strain. A plot of ( cos )/ against (4 sin θ)/λ is shown in Fig. 1. The average crystallite size and effective strain were determined as 57 nm and 0.23% from the ‘y’ intercept and slope respectively.
2.2. Characterization The phase of the sample was determined by powder X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer. Measurements were made at room temperature by use of Cu-Kα (1.5406 Å) radiation. The XRD pattern was recorded in the range of Bragg angle 20–70°. EPR measurements were carried out using a Freiberg Instruments Miniscope EPR spectrometer (Model 5500) at room temperature and operating at the X-band frequency with 100 kHz modulation frequency and microwave power of 1 mW. Diphenyl Picryl Hydrazyl (DPPH) was used to calibrate the g-factors of defect centres. The sample used for EPR was irradiated to 100 Gy using a60Co gamma source at a nominal dose rate of 1.512 Gy/s. TL was measured on a RISØ TL/OSL DA-20 Luminescence Reader. ZrO2 and ZrO2:Er samples in powder form with mass 0.04 g and 0.02 g respectively were used for TL measurements. The TL intensity was corrected for sample mass. The sample was irradiated by a90Sr/90Y beta source at a nominal dose rate of 0.1028 Gy/s. The luminescence was detected by an EMI 9235QB photomultiplier tube (PMT) through a combination of a BG3 and a BG39 filter (overall transmission band 350–450 nm FWHM). TL measurements were carried out at 1 °C s−1 in a nitrogen atmosphere. All measurements were recorded four times to obtain the best estimate of a particular value as the average result and the margin of error as the standard deviation of the set.
4. Glow curve characteristics 4.1. General features of the glow-curve Fig. 2 shows several glow curves. The natural TL from ZrO2:Er (solid circles) is compared with a glow curve measured after irradiation to 205 Gy (open circles). Both measurements were made at 1 °C s−1 and have been corrected for background. There is no well-defined peak in the natural TL. On other hand, the glow curve from the irradiated sample has two peaks at 50°C (peak 1) and at 112°C (peak 2) and
3. X-ray diffraction The XRD result of the sample is shown in Fig. 1. The pattern shows prominent diffraction peaks at 28.14° and 31.42° corresponding to (1 11) and (111) planes. These peaks are characteristic of the monoclinic phase and are consistent with the Powder diffraction file (PDF) # 86–1451 indicated in Fig. 1 for comparison. Other less intense diffraction peaks are ascribed to different (hkl) planes in the monoclinic phase of ZrO2. Structural parameters such as lattice parameters and cell volume were calculated from XRD data using the unit cell program (Holland and Redfern, 1997). The lattice parameters were found to be a = 5.141 Å, b = 5.211 Å, and c = 5.299 Å and the cell volume was 3 evaluated as (140.19 ± 0.01) Å . These values match well with standard
Fig. 2. A natural TL glow curve (solid circles) shown together with that of ZrO2:Er after irradiation to 205 Gy (open circles). A TL glow curve of undoped ZrO2 (solid triangles) is shown for comparison. 2
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possibly a third one at 264°C . The analysis of TL presented in this work was carried out on peaks 1 and 2. For comparison, a glow curve of undoped ZrO2 irradiated to a much lower dose of 40 Gy is included in Fig. 2. This glow curve, also recorded at 1°C s−1, shows two obvious peaks at 54 and 120°C. The glow curve structure in ZrO2 is known to depend on the morphology of the sample (Villa-Sánchez et al., 2014a). In our case, the crystalline phase of both undoped and doped sample is the same, monoclinic. The only change noted was the crystallite size decreasing from 54 to 57 nm as determined from XRD data of undoped and doped samples respectively. Therefore, by comparing the intensity of an undoped sample and the 1 mol% doped one, we deduce that the decrease of TL intensity in ZrO2:Er sample may be due to change in particle size and possibly Er ion concentration quenching. 4.1.1. Repeatability The reproducibility of peaks 1 and 2 was examined in TL measurements made consecutively on a sample irradiated to 205 Gy each time. The positions of peaks 1 and 2 were found to be reproducible at 49.3 ± 1.0°C and 111.5 ± 0.9°C respectively in a set of eight repetitive measurements.
Fig. 4. The Tm-Tstop plot. Here, peak 1 and 2 are main peaks, and peaks 3 and 4 are secondary peaks.
4.1.2. Qualitative assessment of the glow curve using thermal cleaning In order to estimate the number of peaks in a glow curve, the thermal cleaning technique discussed elsewhere (McKeever, 1985) was used. A sample freshly irradiated to 205 Gy each time was heated to beyond the maximum of a particular peak and after cooling reheated to 500°C to measure the remaining glow curve so as to reveal any remaining peaks. Fig. 3 shows a glow curve measured immediately after irradiation together with those after partial heating to 72, 150, 235 and 290°C. The results show that apart from peaks at 50°C and 112°C noted in Fig. 2, there are two additional ones at 172 and 270°C. There are thus four glow peaks in the glow curve of ZrO2:Er. In comparison, Lai and Su (1999) reported three peaks within 30–300°C also in ZrO2:Er.
against Tstop. The results of the Tm-Tstop analysis shows there are at least four peaks associated with the glow curve. These peaks are labelled as 1, 2, 3 and 4. This result matches well with that from thermal cleaning. The position of peak 1 remains constant when Tstop changes from 22 to 40°C . Similarly, the position of peak 2 is independent of Tstop in two neighbouring regions at 114 and 118°C as clarified in the inset to Fig. 4. Beyond preheating to 112°C, the position of peak 2 increases with Tstop. This behaviour indicates that the apparently single peak 2 either consists of multiple first order components or that this peak follows second order kinetics. The order of kinetics of peak 2 is clarified later. The results of Tm-Tstop analysis demonstrate that peaks 1, 3 and 4 follow first order kinetics.
4.1.3. Qualitative assessment of the glow peaks using Tm-Tstop analysis As an alternative method to determine the number of peaks in the glow curve and their order of kinetics, the Tm-Tstop method was used. Here, a sample irradiated to 205 Gy was first partially heated to 22°C (a Tstop temperature) and after cooling, the sample was reheated to obtain the complete glow curve. This procedure was repeated several times on the same sample, freshly irradiated each time and Tstop increased in steps of 2°C from 22 to 200°C. For each Tstop, the position of the remaining peaks in the glow curve were noted. Fig. 4 shows a plot of Tm
4.2. Influence of irradiation on thermoluminescence properties 4.2.1. Effect of dose on peak temperature The order of kinetics of peaks 1 and 2 was further determined using the dependence of Tm on dose. Fig. 5 shows Tm against dose between 51 and 411 Gy. The position of peak 1 changes from 48.5 ± 1 to 51.0 ± 0.5°C when the dose changes from 51 to 411 Gy. In the same dose range, the position of peak 2 changes from 113.5 ± 1 to
Fig. 3. Glow curves of 205 Gy irradiated ZrO2:Er recorded without preheat (i) after preheating to 72°C and to (ii) 150°C (iii) 235°C (iv) and 290°C (v).
Fig. 5. The dependence of peak temperature Tm on irradiation dose. 3
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H.S. Lokesha and M.L. Chithambo
Fig. 6. Change of glow peak intensity with dose. The inset shows glow curves of ZrO2:Er corresponding to different irradiation doses.
Fig. 7. A plot of ln (TLexp/n b) against 1/kT for peak 2 for different values of the order of kinetics b.
109.0 ± 0.5°C. Fig. 5 shows that both peaks 1 and 2 go through regions where their peak maximum is independent of dose. For peak 1, Tm is stable at 48.5 ± 1°C between 51 and 102 Gy and at 51.0 ± 0.5°C between 205 and 411 Gy. For peak 2, Tm is stable at 113.5 ± 1, 111 ± 1 and 109.0 ± 0.5°C for doses within 51–102, 154–308 and 359–411 Gy respectively. The results of Fig. 5 imply that peaks 1 and 2, which appear as single, in fact consist of multiple first-order peaks which individually become prominent at specific irradiation doses as concluded in similar cases previously (Chithambo, 2017; Thomas et al., 2019). Based on results of the dependence of Tm on dose, peaks 1 and 2 follow first order kinetics.
5.2. Whole glow curve method In this method, the area n under a glow peak is related to the order of kinetics b as
ln
+1
= ln
sl
E kT
(3)
3(b−1) −1
where s (m s ) is the effective frequency factor for general order kinetics. For a specific value of b, a plot of ln (TLexp/n b) versus 1/kT (where K is in Kelvin as for other methods used) is linear. The values of E and s are evaluated from the slope and y-axis intercept respectively. Fig. 7 shows the results of analysis of peak 2 for a dose of 205 Gy. The best straight line for the fitting is found to be for b = 1.2 for which R2 = 0.99. The corresponding activation energy and frequency factor were estimated as 0.66 ± 0.01 eV and 1.5 × 106 s−1 respectively. The value of b is consistent with first order kinetics.
Fig. 6 shows peak intensity against beta dose from 51 to 411 Gy. Selected glow curves corresponding to different doses are shown in the inset to Fig. 6. The intensity increases with dose. The dose response was analysed using the superlinearity index g(D),
D y (D)/D y (D)/D
nb l
4.3. Effect of dose on thermoluminescence intensity
g(D) =
TL exp
5.3. Glow curve deconvolution
(2)
The kinetic parameters of the glow peaks were also analysed by curve fitting using the equation
In the above relation, y(D) is the analytical function describing the behaviour of dose response, y’(D) and y’’(D) are each the first and second derivative of y(D). A value of g(D) = 1 denotes a linear response, g(D) > 1 refers to supralinearity and g(D) < 1 represents sublinearity (Thomas et al., 2019). Using such analysis, the dose response of peaks 1 and 2 is concluded to be effectively sublinear (see plot of g(D) against dose in the supplementary submission).
I(T) = Im b b
b
1
E T Tm × (b 1)(1 kT Tm
)
T2 E T Tm exp + Zm kT Tm T2m
b b 1
(5) where I(T) is the temperature dependence of the TL, = 2kT/E, 1) m , Tm is the position of the peak whose m = 2kTm /E , Z m = 1 + (b peak intensity is Im, E is activation energy and b is the order of kinetics (Pagonis et al., 2006). The software package Microsoft Excel with the solver utility was used for curve fitting. The goodness of fit was tested by the figure of merit (FOM) (Afouxenidis et al., 2012), given as
5. Kinetic analysis Peaks 1 and 2 were analysed for kinetic parameters using the initial rise, whole glow peak and curve fitting methods. Multiple methods were used as a means to corroborate findings. The details of the methods have been discussed elsewhere and will not be repeated here (Pagonis et al., 2006).
FOM =
TL exp
TL fit
TL fit
(6)
where TLexp and TLfit represent the experimental data and values from the fitting function respectively. A fit is acceptable if the figure of merit (FOM) is less than 3.5%. Fig. 8 shows the deconvolved glow curve and a plot of residuals of the fit. The residuals of the fit, fluctuating about zero, signify a good fit. TL glow curve of ZrO2:Er (between 22 and 320°C) is best described as a minimum of four peaks. The number of peaks used for deconvolution agree well with experimentally
5.1. Initial rise method The activation energy of peak 2 was determined using the initial rise method as 0.71 ± 0.02 eV. The corresponding plot is available as supplementary information. 4
Radiation Physics and Chemistry 172 (2020) 108767
H.S. Lokesha and M.L. Chithambo
Fig. 8. A deconvolved glow curve of ZrO2:Er irradiated to 205 Gy. The residuals of the fit confirms that the fit is acceptable.
Fig. 10. The ESR spectra of ZrO2:Er sample (a), after gamma irradiated to 100 Gy (b) and after TL measurement (20–500°C ) followed by gamma irradiation (c).
determined conclusions from thermal cleaning and the Tm-Tstop procedure. The FOM for the glow curve fit is less than 2.7%. This denotes that the experimental data and fitted function match properly. For a dose of 205 Gy, the activation energy and order of kinetics were determined as 0.66 ± 0.01 eV, 0.74 ± 0.02 eV and 1.4 ± 0.05, 1.3 ± 0.01 respectively. These values agree well with those calculated from the initial rise method. The frequency factor of the peaks was calculated by using the general order equation,
s=
E 1 exp kT2m Z m
E kTm
storage time up to 14,600 s. The peak intensities are normalized by dividing by their respective intensity measured immediately after irradiation. The intensity of peak 1 fades to background level within 1800 s whereas the intensity of peak 2 drops to 23% of its initial intensity after 1800 s. The overall decrease of intensity of peak 2 after 14,600 s is 59%. Further, the intensity of peak 3 (at 172°C ) and peak 4 (at 260°C) decreased by 23% and 2% of the initial intensity after 14,600 s respectively. Also, the position Tm of all four peaks is independent of storage time.
(7)
7. Electron paramagnetic resonance
where β is the heating rate and k is Boltzmann's constant. The frequency factors of peaks 1 and 2 were estimated to be 2.1 × 109 and 3.4 × 108 s−1 respectively.
EPR spectroscopy was used to identify any intrinsic or radiation induced colour centres in ZrO2:Er. EPR results of pristine and gamma irradiated ZrO2:Er are compared as a way to identify radiation induced defects. The information is then linked to TL results. Fig. 10 shows the EPR spectra of ZrO2:Er. The spectra consists of one recorded from pristine sample (a), after gamma irradiation to 100 Gy (b) and that after TL measurement followed by gamma irradiation (c). The EPR spectra were recorded at room temperature. The EPR signal is not a flat line as one would expect in the case of perfect stoichiometry. The ZrO2:Er sample shows three distinct weak signals (referred to as centres I, II and III). These signals are related to the presence of intrinsic defects in the material (Gionco et al., 2013). In spectrum (a), centre I resonates at g⊥ = 1.975 and g∥ = 1.958. This signal was earlier assigned to Zr3+ sites in the bulk (Polliotto et al., 2018). Centres II and III have the same perpendicular value (g⊥ = 1.977) but differ in their parallel line components g∥ = 1.922 and g∥ = 1.905 respectively. Centres II and III corresponding to surface Zr3+ ions situated in the disordered environment of the surface (Gionco et al., 2013). Oxygen vacancies in ZrO2:Er form owing to the substitution of Zr4+ ions by Er3+ ions. However no EPR lines were observed in the ZrO2:Er sample in the studied magnetic field range. Spectrum (b) measured after gamma irradiation to 100 Gy shows two defect centres labelled as centre IV and centre V. Gamma radiation induces the generation of many electron–hole pairs which can end up at electron trap or hole centres. Centre IV with g values g1 = 2.022, g2 = 2.015 and g3 = 2.006 has a rhombic g-tensor ( g1 g2 g3 ). Polliotto et al. (2018) reported EPR results of UV irradiated ZrO2 and identified UV induced hole trapping centres (O centres) at a rhombic symmetry site (with g1 = 2.022, g2 = 2.015, and g3 = 2.004) (Polliotto et al., 2018). Also, O centres were noted in ZrO2:Y (9.5 mol%) under X-ray irradiation
6. Fading The change of intensity between irradiation (to 205 Gy) and measurement was studied. Fig. 9 shows the variation of peak intensity with
Fig. 9. The change of TL peak intensity with delay between irradiation and measurement for peaks 1 through 4 as shown. 5
Radiation Physics and Chemistry 172 (2020) 108767
H.S. Lokesha and M.L. Chithambo
(g1 = 2.019, g2 = 2.012, and g3 = 2.004) and under 1.5 MeV electron irradiation (g1 = 2.014, g2 = 2.010, and g3 = 2.007) (Orera et al., 1990; Costantini et al., 2013). Therefore, centre IV can be unambiguously assigned to O centres. Centre V appears as a broad signal centered at g = 1.99. The g value of centre V is rather close to the freeelectron value (2.0023) related to F+ type centres. F+ type centres (g = 1.974, 1.997 assigned to F+2 centres) and T-centres (g = 1.89) were also observed in ZrO2:Y (9.5 mol%) under electron and swift heavy ion irradiation (Costantini and Beuneu, 2011). In the present study, no T-centre is observed and centre V is tentatively assigned to F+2 centre. Furthermore, spectrum (c) shows that the signals corresponding to different centres (I–V) decrease to that of the unirradiated ZrO2:Er level. Therefore, the EPR analysis confirm that O centres and F+ type centres are generated during gamma irradiation.
Author statement This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. Acknowledgments H S Lokesha acknowledges with thanks Rhodes University PostDoctoral Research Fellowship (PDF No. s1900093). We also appreciate the financial support from Rhodes University and National Research Foundation of South Africa. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.radphyschem.2020.108767.
8. Discussion
References
The presented EPR spectrum of the ZrO2:Er sample shows the presence of bulk and surface Zr3+ sites at room temperature. Orera et al. (1990) reported that the PL emission band in stabilized zirconia depends on Zr3+ ions coordinated environment. An absorption band at 480 nm and a PL band at 375 nm were related to hepta-coordinated and hexa-coordinated Zr3+ ions respectively. Centres II and III correspond to surface Zr3+ sites that could result from the different coordinative environments. Trapped holes released from the O centre recombine with an electron at the F+ centre (Singh et al., 2012). The value of g for centre V, resonating at g = 1.99, is smaller than that of the free-electron value (g = 2.0023). This centre has been attributed to F+2 centre, that is, an aggregate of a singly ionized oxygen vacancy (Costantini and Beuneu, 2011; Singh et al., 2012). Concerning TL, as Fig. 2 shows, glow peaks were observed at 50 and 112°C in ZrO2:Er after beta irradiation. This finding is somewhat similar to that seen in the TL of erbium doped ZrO2 under UV and X-ray irradiation (Lai and Su, 1999; Hristov et al., 2010). Therefore, the identified defect centres tentatively correlate to the TL peaks based on the assumption that the ionization due to beta and gamma irradiation produce similar types of electron-hole pairs. Going by the results of EPR signals, we assume that the O centre and F+ type centres are associated with the TL peaks. In particular, we propose that the peaks at 50°C and 112°C are associated with transitions at F+ type centres probably F+ and F+2 centre acting as electron traps. Regarding kinetic analysis, kinetic parameters calculated by various methods are consistent. Hristov et al. (Singh et al., 2012) reported that TL of ZrO2:Er (at 1 mol%) co-doped with Li shows peaks at 65, 105 and 150°C after UV irradiation (Hristov et al., 2010). The corresponding activation energies of the peaks were found as 0.52, 0.96 and 1.03 eV respectively and determined to follow non first order kinetics. Our kinetic analysis in ZrO2:Er shows values close to those in the literature but it is reasonable to assume that these values are influenced by the dopant. However, the order of kinetics is first order for both peak 1 and peak 2 as confirmed by various tests.
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