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Thermoluminescence of monoclinic ZrO2: Kinetic analysis and dosimetric features
Journal Pre-proof Thermoluminescence of monoclinic ZrO2: Kinetic analysis and dosimetric features H.S. Lokesha, M.L. Chithambo, S. Chikwembani PII:
S0022-2313(19)31526-1
DOI:
https://doi.org/10.1016/j.jlumin.2019.116864
Reference:
LUMIN 116864
To appear in:
Journal of Luminescence
Received Date: 1 August 2019 Revised Date:
22 October 2019
Accepted Date: 1 November 2019
Please cite this article as: H.S. Lokesha, M.L. Chithambo, S. Chikwembani, Thermoluminescence of monoclinic ZrO2: Kinetic analysis and dosimetric features, Journal of Luminescence (2019), doi: https:// doi.org/10.1016/j.jlumin.2019.116864. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Thermoluminescence of monoclinic ZrO2: Kinetic analysis and dosimetric features H.S. Lokeshaa, M.L. Chithamboa*, S. Chikwembanib aDepartment
of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa bDepartment of Chemical & Physical Sciences, Walter Sisulu University, Mthatha 5117, South Africa *Corresponding author email: [email protected]
Abstract Thermoluminescence (TL) of beta irradiated monoclinic-phase zirconium oxide (ZrO2) is reported. The sample produces intense emission, a feature that facilitated study of kinetic and dosimetric characteristics. Glow curves measured at 1 ℃ s-1 show two prominent peaks at 44 and 112℃. A secondary peak is observed at 240℃. For convenience the peaks are henceforth referred to as peak 1, peak 2 and peak 3. Peak 3 at 240℃ is ill-defined and of particularly low intensity and was thus not studied any further. Repetitive measurements on the same aliquot confirm that ZrO2 exhibits excellent repeatability. The dose response of peak 1 at 44℃ peak is linear for doses from 1 to 31 Gy. In comparison, the dose response of peak 2 at 112℃ peak is linear between 10 and 21 Gy and then becomes supralinear thereafter up to 31 Gy. Peak 1 fades completely within 1800 s of irradiation whereas the intensity of peak 2 drops down to 59% of its initial value within 10800 s of irradiation. Regarding kinetic analysis, the Tm- Tstop procedure confirms that there are in fact five peaks in the glow curve of ZrO2. Peak 2 follows first order kinetics whereas peak 1 is subject to non-first order kinetics. The intensity of both peaks 1 and 2 decrease with heating rate. For peak 2, this shows that it is affected by thermal quenching. The corresponding value of activation energy for the thermal quenching was calculated as 1.20±0.08 eV. Key words: ZrO2, Thermoluminescence; Dose response; Fading; Kinetic parameters
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1.
Introduction Zirconium oxide (ZrO2), commonly known as zirconia, is an innovative material with
a wide variety of applications including as a clinical implant due to its non-toxicity [1], for thermal barrier- [2] or optical coating [3], as a sensor or for energy conversion and storage [4]. In particular, ZrO2 is considered to be an excellent host material in the field of photonics based applications because of its wide band gap (5−7 eV), good transparency, high refractive index [5] as well as good thermal and chemical stability [6]. The stretching frequency of ZrO2 matrix is about 470 cm-1 which is comparatively less than that in other host materials such as MgO, SrSO4, etc. This low phonon energy material allows the possibility of higher efficient luminescence of active ions being incorporated into the ZrO2 matrix [7]. ZrO2 mainly exhibits three crystallographic phases viz monoclinic, tetragonal and cubic. The monoclinic phase is stable at room temperature but transforms to tetragonal form with lattice contraction when the sample is heated to 1200℃. The tetragonal phase inverts to the cubic one with lattice expansion after heating to 2270℃ [6,8]. On cooling from 2270℃ to room temperature, the reverse phase transformation occurs. The phase transition from monoclinic to tetragonal is rather complex. There is a change in coordination of Zr atoms from seven to eightfold symmetry. However, the high temperature (tetragonal and cubic) phases can be stabilized by adding dopants with lower valence for example Mg2+, Ca2+ or Y3+ [9,10]. The presence of oxygen vacancies stabilizes the tetragonal or cubic phases in ZrO2. It has been observed that some synthesis methods
2
such as the inert gas condensation technique [11], solvothermal [12] or sol-gel methods [13,14] produce a mixed (monoclinic and tetragonal) phase of ZrO2. Several popular TLD materials such as LiF:Mg,Ti, LiF:Mg,Cu,P, CaF2, Li2B4O7, CaSO4:Dy and CaF2:Dy are microcrystalline in size. However, with the use of tiny particles comes the problem of high intensity and saturation at high doses. This is because the surface to volume ratio is greater in nano-sized particles than in micro-size alternatives. The search for new materials with high thermoluminescence yield for possible use in radiation dosimetry is an enduring research problem. On this aspect, Rivera [15] reported the TL properties of undoped ZrO2 and ZrO2 doped with rare earth (RE) ions to demonstrate its prospects for dosimetry. Previous measurement of TL from UV irradiated monoclinic ZrO2 showed two main glow peaks at 70 and 130℃ [16].Interestingly, an additional peak is observed at 400℃ in UV irradiated tetragonal phase ZrO2 [14]. For mixed phase material (monoclinic and tetragonal) a glow peak is observed at 418℃ along with peaks at 70 and 130℃ if the sample is UV irradiated. Villa-Sanchez et al. [17] reported TL induced by UV and gamma radiation. Their glow curve had peaks at 65, 132 and 244℃ following UV irradiation material and at 144 and 262℃ after gamma irradiation ZrO2. TL following ion implantation is yet another interesting approach. After implantation with 100 MeV Si7+ ions, the sample showed peaks at 145 and 229℃ [18]. These examples show that ZrO2 is a good material for dosimetric investigation. The monoclinic phase of ZrO2 sample is expected to be produce intense TL emission in comparison to that from tetragonal and cubic materials. Although previous studies of
3
ZrO2 under UV irradiation have showed its promise for dosimetry [14−17], there is very little else of such studies corresponding to beta irradiation. Indeed, the dose response and kinetic analysis of its TL in this case have not been studied to any significant extent. Previous reports on ZrO2 e.g. [19] show that the TL emission band is a wide one centred at 500 nm. On the other hand, the photoluminescence emission band lies between 470 and 500 nm in monoclinic ZrO2. The nature of the emission band has been accounted for in two ways. Firstly, the PL emission is attributed to the recombination of trapped electrons at F centers and holes in the valence band [20,21]. Excitation with energy exceeding the zirconia band gap produces electron-hole pairs. Some electrons are then trapped at F+ centers thereby creating F centre. Recombination of the holes with F centers creates an excited F+ centre. The light emission observed in ZrO2 between 470-500 nm is due to transitions at the excited F+ centre. In another point of view, the PL emission at ~500 nm is assigned to titanium ions (Ti3+) corresponding to the 3d1(eg) → 3d1(t2g) transition. The Ti ions are nominally present in ZrO2 sample in trace concentration [22–24]. A broad PL emission band with maxima at 480 nm in ZrO2 is due to complex defects associated with oxygen vacancies and Ti3+ ions [25]. The aim of their work is to study the thermoluminescence of pure monoclinic phase ZrO2. We report kinetic analysis and some of its dosimetric properties. 2.
Experimental details The ZrO2 powder used in this work is commercially available material of 99% purity
(Sigma-Aldrich, India). The sample, in powder form, was annealed at 850℃ for three hours in air to deplete its carbon content. The X-ray diffraction (XRD) pattern of the sample was 4
measured by an advanced X–ray diffractometer (Bruker D8 AXS) 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° at an increment of 0.02°. The operating current and voltage were maintained as 40 mA and 40 kV respectively. An aliquot ZrO2 of mass 0.05 g was used for TL measurements. The sample was irradiated as required at room temperature by a 90Sr/90Y beta source at a nominal dose rate of 0.1028 Gy/s. TL was measured on a RISØ TL/OSL DA20 Luminescence Reader. 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 five 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. 3.
X-ray diffraction The crystallinity of the ZrO2 sample was analyzed by XRD. Figure. 1 shows the XRD
pattern of ZrO2. The pattern shows prominent diffraction peaks at 28.14° and 31.42° corresponding to (−111) and (111) planes which are characteristic of a monoclinic structure with space group P21/C (PDF # 86-1451). All other less intense diffraction peaks are ascribed to different (hkl) planes of a monoclinic phase of ZrO2. The crystallite size and microstrain were calculated from XRD data using the Williamson – Hall relation [26], given by: βθ λ
= +
ε θ
(1)
λ
5
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 microstrain present in the samples. The W – H plot is linear as shown in the inset to Fig. 1. The crystallite size is estimated from inverse of the y-intercept whereas the slope of the plot gives the microstrain. The average crystallite size and microstrain were hence determined as 71 nm and 0.95% respectively. 4.
Glow curve characteristics The sample produced natural TL. Its glow curve is compared with that from an
irradiated (10 Gy) sample in Fig. 2. The natural TL (solid circles) shows two peaks one at 58℃ and the other at 126℃. The natural TL intensity is quite weak and for ease of comparison with that from the irradiated material, it has been scaled up 30. The glow curve from the irradiated sample (open circles) shows two prominent peaks at 44℃ (peak 1) and at 112℃ (peak 2). There is a discernible but ill-defined peak at 240℃ as shown in the inset to Fig. 2. The sample produces intense phosphorescence following irradiation. This is the time-dependent decrease of signal evident prior to start of heating at 20℃ (open circles). The natural TL peaks are not reproduced under beta irradiation. 4.1
Repeatability
The position of peaks 1 and 2 were properly reproducible at 43.8 ± 0.63℃ and 112.8 ± 1.03℃ respectively in a set of ten repetitive measurements corresponding to a dose of 10 Gy. 4.2
Qualitative assessment of the glow curve using thermal cleaning
6
In order to determine the total number of peaks associated with a particular glow curve, the thermal cleaning technique described elsewhere [27] was used. The sample, freshly irradiated each time, was heated beyond the maximum of a particular peak and then reheated to measure the remnant glow curve. The aim was to enable any concealed or unclear peaks to appear properly. Fig. 3 shows a glow curve measured immediately after irradiation together with others recorded after preheating to 74, 150, 201 and 290℃. It is observed that apart from peaks at 44℃, 112℃ and 240℃ peaks, there is an additional peak at 154℃. The results of thermal cleaning therefore suggest that the glow curve of ZrO2 studied has at least four glow peaks. 5.
Kinetic Analysis
5.1
Order of kinetics
The order of kinetics was first determined using the dependence of peak position on dose and by use of the Tm-Tstop procedure, a partial heating method. 5.1.1
Assessment of order of kinetics using the dependence of peak position on dose In order to investigate the influence of dose on peak temperature (Tm), glow curves
corresponding to doses between 1 - 31 Gy were measured. The dependence of Tm on dose is shown in Fig. 4. The position of peak 1 changes from 42 to 46℃ when the dose is increased from 1 to 31 Gy. On the other hand, the position of peak 2 is independent of dose. The behaviour of peak 2 with dose is consistent with first order kinetics [28]. On the other hand, the position of a second order peak decreases with dose. At this stage of the discussion, we can only say that peak 1 follows non-first order kinetics. 5.1.2
Qualitative assessment of glow peaks using Tm-Tstop analysis 7
The partial heating method, so called Tm-Tstop, was used to determine the order of kinetics of peaks. The same method was used to decide whether each peak is indeed single or whether it may be a combination of several components. In this method, a sample irradiated to 10 Gy was heated to a temperature Tstop of 26℃ corresponding to a point on the low temperature part of a peak. After cooling, the sample was reheated to 500℃ to obtain the complete glow curve and the peak position Tm of the remaining peak noted. This procedure was repeated several times, re-irradiating the same sample to 10 Gy each time and Tstop increased in steps of 1℃ up to 200℃. Fig. 5 shows a plot Tm against Tstop. There are five peaks associated with the glow curve. These are labelled as 1, 1a, 2, 2a, and 3.. The position of peak 2 and 3 is in each case independent of Tstop. In comparison, the positions of peaks 1, 1a and 2a increase with Tstop. Peaks 2 and 3 are hence not affected by change in concentration of trapped charge and are thus first order peaks. On the other hand, the change for peaks 1, 1a and 2a means that either each peak consists of closely overlapping components or that they are of second order kinetics. In general, for second order kinetics or for closely overlapping peaks, the peak position of Tm increases with Tstop temperature. 6.
Evaluation of kinetic parameters Kinetic parameters associated with the glow peaks comprising of the activation
energy, frequency factor and order of kinetics were evaluated using the initial rise (IR) method, whole glow peak (WGP) technique, curve fitting (GCF) and variable heating rate (VHR) procedure. The basis of these methods is discussed elsewhere [29]. 6.1
Initial rise method
8
This method is applicable to the rising part of an isolated glow peak. According to theory, a semi-logarithmic plot of TL intensity against 1/kT (where k is Boltzmann's constant and T is temperature) is expected to be linear with slope equal to the activation energy [29]. Fig. 6 shows a plot of ln(TL intensity) against 1/kT for peak 2 corresponding to a dose of 10 Gy. The activation energy for this dose for peak 1 was found as 0.60 ± 0.01 eV. 6.2
Whole glow curve method
In this method, the area under a glow peak is related to order of kinetics b as, ln
= ln −
(2)
where s1 (cm3(b−1) s−1) is the effective frequency factor. If a plot of ln(TLexp/b) versus 1/kT for a particular value of b is linear, the E and s values are evaluated from the slope and yaxis intercept respectively. Fig. 7 shows results of analysis on peaks 1 (at 44℃) and 2 (at 112℃) for a dose of 10 Gy. The best fit is found to be for b=0.9 and b=1.1 for peaks 1 and 2 respectively for which R2 = 0.995 and 0.998 respectively. The activation energy (E) of peaks 1 and 2 for a dose of 10 Gy was 0.60 ± 0.01 eV and 0.700 ± 0.003 eV respectively. 6.3
Peak shape method As a another assessment, the activation energy of the glow peaks were calculated
using the equation Eα = C α #
$ % α
& − bα (2kT, )
(3)
where for a particular peak, α stands for τ, δ and ω. These correspond to the half- width on the rising side (τ), the half width at the fall-off side (δ) and full-width (ω) at half maximum respectively; cα and bα are constants [29]. For a dose of 10 Gy, the average activation
9
energy of Eτ, Eδ and Eω were found to be 0.64 ± 0.06 eV for peak 1 and 0.75 ± 0.01 eV for peak 2. The geometrical factors μ = δ/ω were calculated as μ = 0.46 and 0.43 for peak 1 and 2 respectively. The value of μ for peak 2 is consistent with first order kinetics in agreement with the whole glow peak method. 6.4
Curve fitting method TL glow curves were deconvolved using a glow curve deconvolution method using
the Kitis general order relation [30,31] given by,
I(T) = I, b /
0$ ×[(b $
− 1)(1 − Δ)
4
$
4 exp
0$ + Z, ]0/ $
(4)
where Δ = 2kT/E, Δ, = 2kT, /E, Z, = 1 + (b − 1)Δ, , Im is peak maximum intensity, Tm is peak temperature, E is activation energy, s is frequency factor and b is order of kinetics. 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) [30] given as, FOM =
∑ ⃒ 0@AB ⃒ ∑ @AB
(5)
here TLexp and TLfit represent the experimental data and value from the fitting function respectively. Fig. 8 shows the deconvolved glow curves of ZrO2 for a dose of 10 Gy. The residuals plot is included to show goodness of fit. TL glow curves of ZrO2 (26-210℃) are best fitted as a minimum of four peaks. The residuals fluctuating about zero, signify a good fit. For a dose of 10 Gy, the kinetic parameters were determined as 0.69 eV for peak 1 and 0.74 eV for peak 2 respectively. The FOM is less than 1.3%. This shows that the values from fitting are acceptable. The frequency factor of the peaks were calculated by using the general order equation, s=
$ 4 D$
exp
$
(6) 10
where β is the heating rate and k is Boltzmann’s constant. The frequency factors of peaks 1 and 2 were evaluated to be of the order of 109 and 108 s-1 respectively for the dose of 10 Gy. 6.5
Variable heating rate method A further method used for kinetic analysis was the variable heating rate method
[29]. According to theory, for a constant value of frequency factor, the maximum temperature Tm of the peak is dependent on the heating rate β as, ln
$ 4
=
$
+ ln
(7)
where all parameters in are as previously defined. A plot of ln(T2m/β) versus 1/kTm is expected to be linear with slope E and an intercept is equal to ln(E/sk). Fig. 9 shows plots of ln(T2m/β) versus 1/kTm for peaks 1 and 2. The activation energy of peaks 1 and 2 were found to be 0.75±0.01 and 0.91±0.02 eV respectively. The frequency factor of these peaks evaluated as 7.38×109 s-1 and 7.18×1010 s-1 respectively. 6.5.1
The influence of heating rate on TL intensity Fig. 10 shows a plot of TL intensity (in counts/℃) against heating rate for a sample
irradiated to 10 Gy. The peak intensity decreases with heating rate is a manner suggestive of thermal quenching. The thermoluminescence intensity EF corresponding to the lowest heating rate is related to the subsequent quenched ones EG measured at higher heating as EG =
HI
(8)
O PQR
JK LMN0
where W is the activation energy of thermal quenching, S = TUVWX where UVWX is the radiative lifetime at absolute zero of temperature and T is the frequency factor applicable to the non-radiative process [32].
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Fig. 11 show a plot of ln[(Iuq/Iq)-1] versus 1/kTm for peak 2. From the slope and intercept of the plot, values of W and C were found to be 1.20±0.08 eV and 3.87×1014 respectively. For comparison, the values of W and C were also calculated using the peak area and found as W = 1.050±0.003 eV and C = 3.89×1012 respectively. The activation energy of thermal quenching calculated using the peak area and peak intensity are comparable. To our knowledge this is the first time these values have been reported for ZrO2. Incidentally, values of the activation energy determined using the initial rise may sometimes require correction for thermal quenching [26]. We determined the difference between the corrected and uncorrected value as 1.69×10-4 eV which is negligible and so the original result is reported as is. 7.
Dosimetric features
7.1
Dose response The dose response of peak 1 and peak 2 was studied for doses from 1 to 31 Gy. Fig.
12 shows the peak intensity against beta dose. The dose response was analysed using the relation, \( )⁄ ^ )⁄
f(D) = [\(
(9)
where y(D) is the TL intensity at a dose D and y (D1) is TL intensity at a low dose D1. A value of f(D) = 1 denotes a linear dose response, f(D) > 1 refers to supralinearity and f(D) < 1 represents sublinearity. For peak 1, f(D) ~1 over the dose range 1-31 Gy. This denotes that the intensity of peak 1 increases linearly with beta dose. In case of peak 2, a linear dose
12
response occurs between 6 and 20 Gy. Beyond this dose f(D) > 1, thus the intensity of peak 2 is supralinear up to 31 Gy. 8.
Fading To study fading, TL glow curves of ZrO2 were recorded at different intervals of time
between irradiation and readout. Fig. 13 shows the fading of peaks 1 and 2 for TL corresponding to 10 Gy. The intensity of peak 1 decreased rapidly with delay in measurement and fades completely by 1800 s. The intensity of peak 2 decreased by 59 % over 10800 s. The intensity of each of peaks 1 and 2 decreased to half maximum by 104 s and 2000 s respectively. These values are also found by fitting the experimental data with the empirical function, I(t) = A + I0 exp(−t/τ)
(10)
where the constant A represents the off-set, I0 is the initial intensity and τ is a constant. The dependence of peak temperature Tm with storage time is shown in the inset to Fig. 13. The position of peak 2 is independent of storage time. The position of peak 1 shifts towards higher temperature with increase in increased delay between irradiation and measurement. This may indicate the appearance of a new overlapping peak when the previous peak fades as deduced in other similar work [33]. 9.
Discussion Previous studies on TL have provided some key information about the mechanisms
leading to TL emission between 400-650 nm and between 480-500 nm in pure ZrO2 [19,22]. Both PL and TL emission occurs at similar bands in pure ZrO2. In addition to the
13
main peak at 480-500 nm, peaks of lower intensity have been observed at 385 nm [34], 440 nm [19], 456 and 516 nm [35] in pure ZrO2. An explanation for the 490 nm emission was given in the introduction. The emission band observed at lower wavelength side (<460 nm) is due to singly ionized oxygen-vacancy defects (F+ centers). Petrik et al. [21] reported that the association of F+ centers with one or two Y3+ ions in yttria- stabilized cubic ZrO2 would result in a red shift of the emission band of F+ centers. Therefore we deduce that the emission response at higher wavelength (>460 nm) in pure ZrO2 is related to singly ionized oxygen vacancy cluster defects [34,36] and/or relaxation from different excited state levels at an F+ center or different forms of this centre. The TL kinetic parameters evaluated by various methods are consistent except VHR method which overestimates parameters. Chernov et al., [37] reported that a glow curve measured at 2 ℃ s-1 had peaks at 67 and 142℃ after beta irradiation. The corresponding activation energy of the peaks were estimated as 0.65 and 0.78 eV respectively. Ahmed Kadari et al. [38] reported that the activation energy estimated by curve fitting for peaks at 67 and 140℃ were 0.55 and 0.79 eV respectively. Furthermore, they concluded that these peaks follow non-first order kinetics. Our present kinetic analysis in ZrO2 shows values close to those in the literature. However, the order of kinetics is first order for peak 2. As confirmed by various tests. In the present study, the TL glow curve (22-180℃) corresponds to four electron traps at different energy depth. These are at 42℃ (peak 1), 57℃ (peak 1a), 111℃ (peak 2) and 138℃ (peak 2a). The activation energy of thermal quenching for peak 2 was found to be W=1.20±0.08 eV. This value is comparable with that estimated using isothermal TL decay and the heating rate method by Nikiforov et al. [22]. We suggest that the thermal 14
quenching of luminescence in ZrO2 is caused by thermal ionization from an excited state of the F+ center [39]. 10.
Conclusion The TL characteristics of beta irradiated monoclinic ZrO2 have been investigated.
The glow curve recorded at 1 °C s-1 from ZrO2 sample consists of two prominent peaks at 44 and 112℃ as well as a weaker intensity peak at 240℃. Kinetic analysis using various methods shows that peaks 2 follows first order kinetics whereas peak 1 is subject to nonfirst order kinetics. The activation energy of peaks 1 and 2 were determined as 0.64±0.06 eV and 0.75±0.01 eV in measurements corresponding to irradiation to 10 Gy. Peak 2 is affected by thermal quenching. The activation energy for thermal quenching was calculated as 1.20 ± 0.08 eV. The dose response and fading of the peaks were studied. The kinetic properties suggest that both peaks are made up of more than one component. Acknowledgments H S Lokesha acknowledges with thanks the Rhodes University Post-Doctoral Research Fellowship. We also appreciate financial support from Rhodes University and National Research Foundation of South Africa.
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in nanometer-sized ZnO particles, Journal of Theoretical and Applied Physics. 6 (2012) 6. [27] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge University Press, 1985. [28] J.M. Kalita, M.L. Chithambo, Thermoluminescence of α-Al2O3:C, Mg: Kinetic analysis of the main glow peak, Journal of Luminescence. 182 (2017) 177–182. [29] V. Pagonis, G. Kitis, C. thermoluminescence, 2006.
Furetta,
Numerical
and
practical
exercises
in
[30] D. Afouxenidis, G.S. Polymeris, N.C. Tsirliganis, G. Kitis, Computerised curve deconvolution of TL/OSL curves using a popular spreadsheet program, Radiation Protection Dosimetry. 149 (2012) 363–370. [31] G. Kitis, J.M. Gomez-Ros, J.W.N. Tuyn, Thermoluminescence glow-curve deconvolution functions for first, second and general orders of kinetics, Journal of Physics D: Applied Physics. 31 (1998) 2636–2641. [32] M.L. Chithambo, The analysis of time-resolved optically stimulated luminescence : II . Computersimulations and experimental results, Journal of Physics D: Applied Physics. 40 (2007) 1880–1889. [33] M.L. Chithambo, Thermoluminescence of the main peak in SrAl2O4 : Eu2+, Dy3+ : Spectral and kinetics features of secondary emission detected in the ultra- violet region, Radiation Measurements. 96 (2017) 29–41. [34] Z. Wang, B. Yang, Z. Fu, W. Dong, Y. Yang, W. LiU, UV – blue photoluminescence from ZrO2 nanopowders prepared via glycine nitrate process, Applied Physics A, 81 (2005) 691–694. [35] V. Chauhan, T. Gupta, N. Koratkar, R. Kumar, Studies of the electronic excitation modifications induced by SHI of Au ions in RF sputtered ZrO2 thin films, Materials Science in Semiconductor Processing. 88 (2018) 262–272. [36] S. Ashraf, M. Irfan, D. Kim, J.H. Jang, W.T. Han, Y.D. Jho, Optical influence of annealing in nano- and submicron-scale ZrO2 powders, Ceramics International. 40 (2014) 8513–8518. [37] V. Chernov, A. Belykh, R. Meléndrez, M. Barboza-Flores, Beta radiation induced thermoluminescence in pure ZrO2 prepared by sol-gel, Journal of Non-Crystalline Solids. 352 (2006) 2543–2547. [38] A. Kadari, D. Kadri, Modeling of the thermoluminescence mechanisms in ZrO2, Applied Radiation and Isotopes. 82 (2013) 49–54. [39] S.V Nikiforov, V.S. Kortov, D.L. Savushkin, A.S. Vokhmintsev, I.A. Weinstein, Thermal 18
quenching of luminescence in nanostructured monoclinic zirconium dioxide, Radiation Measurements, 106 (2017) 155–160.
19
Figure Captions Fig. 1. XRD pattern of ZrO2 sample annealed at 850℃ for three hours in air. The inset shows a plot of β*cosθ/λ against 4sinθ/λ. Fig. 2. A natural TL glow curve (solid circles) compared with one measured after irradiation to 10 Gy. Fig. 3. Glow curves of ZrO2 recorded after a dose of 10 Gy but without preheating (i), after preheating to 74℃ (ii)150℃ (iii) 205℃ (iv) and 290℃ (v) respectively. Fig. 4. The variation of peak temperature Tm with irradiation dose 1 - 31 Gy. Fig. 5. The Tm-Tstop plot. Here, peak 1 and 2 are main peaks, and peaks 1a, 2a and 3 are secondary peaks. Fig. 6. A plot of ln(TL intensity) against 1/kT for peak 2 for a dose of 10 Gy. Fig. 7. The plot of ln(TLexp/b) against 1/kT for peak 1 and 2 for different values of the order of kinetics b. Fig. 8. A deconvolved glow curve of ZrO2 irradiated to 10 Gy. The residuals of the fit shows that the fit is acceptable. Fig. 9. Plots of ln (T2m/β) versus 1/kTm used in the variable heating rate method. Fig. 10. The change in intensity of peak 2 with heating rate corresponding to a dose of 10 Gy. Fig. 11. A plot of ln[(Iuq/Iq)-1] versus 1/kTm for peak 2. Fig. 12. The variation peak intensity with dose from 1-31 Gy for peaks 1 and 2. Fig. 13. The variation of intensity with storage time. The inset shows dependence of Tm with storage time.
20
Figure 1 400
(111) (111)
ZrO2
β∗cos θ/λ
300
0.0036 0.0027 0.0018 0.0009 0.6
0.8
1.0
Intensity (a.u.)
200
1.2
1.4
4∗sinθ/λ
100
0 1000 500 0 20
30
40
2θ (degree)
50
60
70
Figure 2
5x10
5
Natural TL TL following 10 Gy irradiation 5
3
4x10
3x10
5 3
x30 2x10
5
TL Intensity (a.u.)
TL Intensity (a.u.)
4x10
3x10
o
240 C 3
2x10
3
1x10
1x10
5
200
220
240
260
280
300
o
Temperature ( C)
0 100
200
300 o
Temperature ( C) 21
400
500
Figure 3 5
Without any preheat
3.5x10
o
after preheating to 74 C 5
o
3.0x10
after preheating to 150 C o
after preheating to 205 C 5
o
after preheating to 290 C 3
6.0x10
5
2.0x10
3
5
TL Intensity (a.u.)
TL Intensity (a.u.)
2.5x10
1.5x10
5
1.0x10
4
4.0x10
3
2.0x10
0.0
5.0x10
100
150
200
250
300
o
Temperature ( C)
0.0 100
200
300
o
400
Temperature ( C)
Figure 4 120
115
110
o
Tm ( C)
peak 2 105
peak 1
45
40
0
5
10
15
Dose (Gy) 22
20
25
30
Figure 5 270
3 240 210 180
o
Tm ( C)
2a 150
2
120 90
1a
60
1
30 20
40
60
80
100
120
140
160
180
200
o
Tstop ( C)
Figure 6
10.2
peak 2
ln (TL intensity)
9.9
9.6
9.3
9.0
8.7 33.5
34.0
34.5
35.0
35.5 -1
1/kT (eV )
23
36.0
36.5
Figure 7 3
peak 1
b=0.8 0.9 1 1.1 1.2
0 -3
b
ln (TLexp/n )
-6 -9 35
3
36
37
38
39
40
peak 2
0
b=0.8 0.9 1 1.1 1.2
-3 -6 -9 28
29
30
31
32
33
34
35
36
-1
1/kT (eV )
TL Intensity (a.u.)
Residuals
Figure 8 40 0 -40
3.2x10
5
2.4x10
5
1.6x10
5
8.0x10
4
Experimental data Deconvoluted peaks Fitted
0.0 30
60
90
120 o
Temperature ( C)
24
150
180
Figure 9
14.0
peak 1
13.6 13.2 12.8
34.4
2
ln (Tm/β)
12.4 34.8
35.2
35.6
36.0
36.4
12.0 11.6
peak 2
11.2 10.8 10.4 28.4
28.8
29.2
29.6
30.0
-1
1/kTm (eV )
Figure 10 5
1x10
5
o
TL intensity (counts/ C)
1x10
4
9x10
4
8x10
4
7x10
1
2
3
4 o
Heating rate ( C/s)
25
5
Figure 11
-0.8
ln {(Iuq/Iq)-1}
-1.2
-1.6
-2.0
-2.4
28.5
28.8
29.1
29.4 -1
1/kTm (eV )
Figure 12
10
6
TL Intensity (a.u.)
peak 1 peak 2
10
5
10
4
1
10
Dose (Gy)
26
29.7
30.0
Figure 13 1.2
125 120 115
peak 2
110 105
o
Tm ( C)
Normalized TL intensity (a.u.)
1.0
0.8
60 55
peak 1
50 45
0.6
40 35 30 100
1000
10000
Storage time (s)
0.4
0.2
0.0 0
2000
4000
6000
8000
10000
Storage time (s)
Table 1. Kinetic parameters determined using various methods. The superscripts a, b, c
corresponding to the peak shape method refer to Eτ, Eδ and Eω respectively.
b
s (s-1)
E (eV)
Methods
reference Peak 1 Peak 2
Peak 1
IR WGP
VHR
Peak 1
Peak 2
0.60±0.01 0.9
1.1
PS
CF
Peak 2
1.1
1.02
Sec. 6.1
0.60±0.01
0.70±0.003 1.15×109
1.58×107
0.70±0.01a
0.77±0.01a
0.58±0.01b
0.74±0.01b
0.64±0.01c
0.76±0.01c
0.69±0.01
0.74±0.01
9.32×109
3.04×108
Sec. 6.4
0.75±0.01
0.91±0.01
7.38×109 7.18×1010
Sec. 6.5
Sec. 6.2 Sec. 6.3
IR-Initial rise, WGP-Whole glow peak, PS-Peak shape, CF-Curve fitting and VHR-Variable heating rate
27
Highlights •
TL of nanocrystalline pure monoclinic ZrO2 phase is reported.
•
TL characteristics and dosimetric features of beta irradiated ZrO2 have been investigated.
•
Kinetic analysis of the main peaks (44℃ and 111℃) using various methods are discussed.
Conflict of Interest and Authorship Conformation Form
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
S0022-2313(19)31526-1
DOI:
https://doi.org/10.1016/j.jlumin.2019.116864
Reference:
LUMIN 116864
To appear in:
Journal of Luminescence
Received Date: 1 August 2019 Revised Date:
22 October 2019
Accepted Date: 1 November 2019
Please cite this article as: H.S. Lokesha, M.L. Chithambo, S. Chikwembani, Thermoluminescence of monoclinic ZrO2: Kinetic analysis and dosimetric features, Journal of Luminescence (2019), doi: https:// doi.org/10.1016/j.jlumin.2019.116864. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Thermoluminescence of monoclinic ZrO2: Kinetic analysis and dosimetric features H.S. Lokeshaa, M.L. Chithamboa*, S. Chikwembanib aDepartment
of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa bDepartment of Chemical & Physical Sciences, Walter Sisulu University, Mthatha 5117, South Africa *Corresponding author email: [email protected]
Abstract Thermoluminescence (TL) of beta irradiated monoclinic-phase zirconium oxide (ZrO2) is reported. The sample produces intense emission, a feature that facilitated study of kinetic and dosimetric characteristics. Glow curves measured at 1 ℃ s-1 show two prominent peaks at 44 and 112℃. A secondary peak is observed at 240℃. For convenience the peaks are henceforth referred to as peak 1, peak 2 and peak 3. Peak 3 at 240℃ is ill-defined and of particularly low intensity and was thus not studied any further. Repetitive measurements on the same aliquot confirm that ZrO2 exhibits excellent repeatability. The dose response of peak 1 at 44℃ peak is linear for doses from 1 to 31 Gy. In comparison, the dose response of peak 2 at 112℃ peak is linear between 10 and 21 Gy and then becomes supralinear thereafter up to 31 Gy. Peak 1 fades completely within 1800 s of irradiation whereas the intensity of peak 2 drops down to 59% of its initial value within 10800 s of irradiation. Regarding kinetic analysis, the Tm- Tstop procedure confirms that there are in fact five peaks in the glow curve of ZrO2. Peak 2 follows first order kinetics whereas peak 1 is subject to non-first order kinetics. The intensity of both peaks 1 and 2 decrease with heating rate. For peak 2, this shows that it is affected by thermal quenching. The corresponding value of activation energy for the thermal quenching was calculated as 1.20±0.08 eV. Key words: ZrO2, Thermoluminescence; Dose response; Fading; Kinetic parameters
1
1.
Introduction Zirconium oxide (ZrO2), commonly known as zirconia, is an innovative material with
a wide variety of applications including as a clinical implant due to its non-toxicity [1], for thermal barrier- [2] or optical coating [3], as a sensor or for energy conversion and storage [4]. In particular, ZrO2 is considered to be an excellent host material in the field of photonics based applications because of its wide band gap (5−7 eV), good transparency, high refractive index [5] as well as good thermal and chemical stability [6]. The stretching frequency of ZrO2 matrix is about 470 cm-1 which is comparatively less than that in other host materials such as MgO, SrSO4, etc. This low phonon energy material allows the possibility of higher efficient luminescence of active ions being incorporated into the ZrO2 matrix [7]. ZrO2 mainly exhibits three crystallographic phases viz monoclinic, tetragonal and cubic. The monoclinic phase is stable at room temperature but transforms to tetragonal form with lattice contraction when the sample is heated to 1200℃. The tetragonal phase inverts to the cubic one with lattice expansion after heating to 2270℃ [6,8]. On cooling from 2270℃ to room temperature, the reverse phase transformation occurs. The phase transition from monoclinic to tetragonal is rather complex. There is a change in coordination of Zr atoms from seven to eightfold symmetry. However, the high temperature (tetragonal and cubic) phases can be stabilized by adding dopants with lower valence for example Mg2+, Ca2+ or Y3+ [9,10]. The presence of oxygen vacancies stabilizes the tetragonal or cubic phases in ZrO2. It has been observed that some synthesis methods
2
such as the inert gas condensation technique [11], solvothermal [12] or sol-gel methods [13,14] produce a mixed (monoclinic and tetragonal) phase of ZrO2. Several popular TLD materials such as LiF:Mg,Ti, LiF:Mg,Cu,P, CaF2, Li2B4O7, CaSO4:Dy and CaF2:Dy are microcrystalline in size. However, with the use of tiny particles comes the problem of high intensity and saturation at high doses. This is because the surface to volume ratio is greater in nano-sized particles than in micro-size alternatives. The search for new materials with high thermoluminescence yield for possible use in radiation dosimetry is an enduring research problem. On this aspect, Rivera [15] reported the TL properties of undoped ZrO2 and ZrO2 doped with rare earth (RE) ions to demonstrate its prospects for dosimetry. Previous measurement of TL from UV irradiated monoclinic ZrO2 showed two main glow peaks at 70 and 130℃ [16].Interestingly, an additional peak is observed at 400℃ in UV irradiated tetragonal phase ZrO2 [14]. For mixed phase material (monoclinic and tetragonal) a glow peak is observed at 418℃ along with peaks at 70 and 130℃ if the sample is UV irradiated. Villa-Sanchez et al. [17] reported TL induced by UV and gamma radiation. Their glow curve had peaks at 65, 132 and 244℃ following UV irradiation material and at 144 and 262℃ after gamma irradiation ZrO2. TL following ion implantation is yet another interesting approach. After implantation with 100 MeV Si7+ ions, the sample showed peaks at 145 and 229℃ [18]. These examples show that ZrO2 is a good material for dosimetric investigation. The monoclinic phase of ZrO2 sample is expected to be produce intense TL emission in comparison to that from tetragonal and cubic materials. Although previous studies of
3
ZrO2 under UV irradiation have showed its promise for dosimetry [14−17], there is very little else of such studies corresponding to beta irradiation. Indeed, the dose response and kinetic analysis of its TL in this case have not been studied to any significant extent. Previous reports on ZrO2 e.g. [19] show that the TL emission band is a wide one centred at 500 nm. On the other hand, the photoluminescence emission band lies between 470 and 500 nm in monoclinic ZrO2. The nature of the emission band has been accounted for in two ways. Firstly, the PL emission is attributed to the recombination of trapped electrons at F centers and holes in the valence band [20,21]. Excitation with energy exceeding the zirconia band gap produces electron-hole pairs. Some electrons are then trapped at F+ centers thereby creating F centre. Recombination of the holes with F centers creates an excited F+ centre. The light emission observed in ZrO2 between 470-500 nm is due to transitions at the excited F+ centre. In another point of view, the PL emission at ~500 nm is assigned to titanium ions (Ti3+) corresponding to the 3d1(eg) → 3d1(t2g) transition. The Ti ions are nominally present in ZrO2 sample in trace concentration [22–24]. A broad PL emission band with maxima at 480 nm in ZrO2 is due to complex defects associated with oxygen vacancies and Ti3+ ions [25]. The aim of their work is to study the thermoluminescence of pure monoclinic phase ZrO2. We report kinetic analysis and some of its dosimetric properties. 2.
Experimental details The ZrO2 powder used in this work is commercially available material of 99% purity
(Sigma-Aldrich, India). The sample, in powder form, was annealed at 850℃ for three hours in air to deplete its carbon content. The X-ray diffraction (XRD) pattern of the sample was 4
measured by an advanced X–ray diffractometer (Bruker D8 AXS) 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° at an increment of 0.02°. The operating current and voltage were maintained as 40 mA and 40 kV respectively. An aliquot ZrO2 of mass 0.05 g was used for TL measurements. The sample was irradiated as required at room temperature by a 90Sr/90Y beta source at a nominal dose rate of 0.1028 Gy/s. TL was measured on a RISØ TL/OSL DA20 Luminescence Reader. 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 five 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. 3.
X-ray diffraction The crystallinity of the ZrO2 sample was analyzed by XRD. Figure. 1 shows the XRD
pattern of ZrO2. The pattern shows prominent diffraction peaks at 28.14° and 31.42° corresponding to (−111) and (111) planes which are characteristic of a monoclinic structure with space group P21/C (PDF # 86-1451). All other less intense diffraction peaks are ascribed to different (hkl) planes of a monoclinic phase of ZrO2. The crystallite size and microstrain were calculated from XRD data using the Williamson – Hall relation [26], given by: βθ λ
= +
ε θ
(1)
λ
5
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 microstrain present in the samples. The W – H plot is linear as shown in the inset to Fig. 1. The crystallite size is estimated from inverse of the y-intercept whereas the slope of the plot gives the microstrain. The average crystallite size and microstrain were hence determined as 71 nm and 0.95% respectively. 4.
Glow curve characteristics The sample produced natural TL. Its glow curve is compared with that from an
irradiated (10 Gy) sample in Fig. 2. The natural TL (solid circles) shows two peaks one at 58℃ and the other at 126℃. The natural TL intensity is quite weak and for ease of comparison with that from the irradiated material, it has been scaled up 30. The glow curve from the irradiated sample (open circles) shows two prominent peaks at 44℃ (peak 1) and at 112℃ (peak 2). There is a discernible but ill-defined peak at 240℃ as shown in the inset to Fig. 2. The sample produces intense phosphorescence following irradiation. This is the time-dependent decrease of signal evident prior to start of heating at 20℃ (open circles). The natural TL peaks are not reproduced under beta irradiation. 4.1
Repeatability
The position of peaks 1 and 2 were properly reproducible at 43.8 ± 0.63℃ and 112.8 ± 1.03℃ respectively in a set of ten repetitive measurements corresponding to a dose of 10 Gy. 4.2
Qualitative assessment of the glow curve using thermal cleaning
6
In order to determine the total number of peaks associated with a particular glow curve, the thermal cleaning technique described elsewhere [27] was used. The sample, freshly irradiated each time, was heated beyond the maximum of a particular peak and then reheated to measure the remnant glow curve. The aim was to enable any concealed or unclear peaks to appear properly. Fig. 3 shows a glow curve measured immediately after irradiation together with others recorded after preheating to 74, 150, 201 and 290℃. It is observed that apart from peaks at 44℃, 112℃ and 240℃ peaks, there is an additional peak at 154℃. The results of thermal cleaning therefore suggest that the glow curve of ZrO2 studied has at least four glow peaks. 5.
Kinetic Analysis
5.1
Order of kinetics
The order of kinetics was first determined using the dependence of peak position on dose and by use of the Tm-Tstop procedure, a partial heating method. 5.1.1
Assessment of order of kinetics using the dependence of peak position on dose In order to investigate the influence of dose on peak temperature (Tm), glow curves
corresponding to doses between 1 - 31 Gy were measured. The dependence of Tm on dose is shown in Fig. 4. The position of peak 1 changes from 42 to 46℃ when the dose is increased from 1 to 31 Gy. On the other hand, the position of peak 2 is independent of dose. The behaviour of peak 2 with dose is consistent with first order kinetics [28]. On the other hand, the position of a second order peak decreases with dose. At this stage of the discussion, we can only say that peak 1 follows non-first order kinetics. 5.1.2
Qualitative assessment of glow peaks using Tm-Tstop analysis 7
The partial heating method, so called Tm-Tstop, was used to determine the order of kinetics of peaks. The same method was used to decide whether each peak is indeed single or whether it may be a combination of several components. In this method, a sample irradiated to 10 Gy was heated to a temperature Tstop of 26℃ corresponding to a point on the low temperature part of a peak. After cooling, the sample was reheated to 500℃ to obtain the complete glow curve and the peak position Tm of the remaining peak noted. This procedure was repeated several times, re-irradiating the same sample to 10 Gy each time and Tstop increased in steps of 1℃ up to 200℃. Fig. 5 shows a plot Tm against Tstop. There are five peaks associated with the glow curve. These are labelled as 1, 1a, 2, 2a, and 3.. The position of peak 2 and 3 is in each case independent of Tstop. In comparison, the positions of peaks 1, 1a and 2a increase with Tstop. Peaks 2 and 3 are hence not affected by change in concentration of trapped charge and are thus first order peaks. On the other hand, the change for peaks 1, 1a and 2a means that either each peak consists of closely overlapping components or that they are of second order kinetics. In general, for second order kinetics or for closely overlapping peaks, the peak position of Tm increases with Tstop temperature. 6.
Evaluation of kinetic parameters Kinetic parameters associated with the glow peaks comprising of the activation
energy, frequency factor and order of kinetics were evaluated using the initial rise (IR) method, whole glow peak (WGP) technique, curve fitting (GCF) and variable heating rate (VHR) procedure. The basis of these methods is discussed elsewhere [29]. 6.1
Initial rise method
8
This method is applicable to the rising part of an isolated glow peak. According to theory, a semi-logarithmic plot of TL intensity against 1/kT (where k is Boltzmann's constant and T is temperature) is expected to be linear with slope equal to the activation energy [29]. Fig. 6 shows a plot of ln(TL intensity) against 1/kT for peak 2 corresponding to a dose of 10 Gy. The activation energy for this dose for peak 1 was found as 0.60 ± 0.01 eV. 6.2
Whole glow curve method
In this method, the area under a glow peak is related to order of kinetics b as, ln
= ln −
(2)
where s1 (cm3(b−1) s−1) is the effective frequency factor. If a plot of ln(TLexp/b) versus 1/kT for a particular value of b is linear, the E and s values are evaluated from the slope and yaxis intercept respectively. Fig. 7 shows results of analysis on peaks 1 (at 44℃) and 2 (at 112℃) for a dose of 10 Gy. The best fit is found to be for b=0.9 and b=1.1 for peaks 1 and 2 respectively for which R2 = 0.995 and 0.998 respectively. The activation energy (E) of peaks 1 and 2 for a dose of 10 Gy was 0.60 ± 0.01 eV and 0.700 ± 0.003 eV respectively. 6.3
Peak shape method As a another assessment, the activation energy of the glow peaks were calculated
using the equation Eα = C α #
$ % α
& − bα (2kT, )
(3)
where for a particular peak, α stands for τ, δ and ω. These correspond to the half- width on the rising side (τ), the half width at the fall-off side (δ) and full-width (ω) at half maximum respectively; cα and bα are constants [29]. For a dose of 10 Gy, the average activation
9
energy of Eτ, Eδ and Eω were found to be 0.64 ± 0.06 eV for peak 1 and 0.75 ± 0.01 eV for peak 2. The geometrical factors μ = δ/ω were calculated as μ = 0.46 and 0.43 for peak 1 and 2 respectively. The value of μ for peak 2 is consistent with first order kinetics in agreement with the whole glow peak method. 6.4
Curve fitting method TL glow curves were deconvolved using a glow curve deconvolution method using
the Kitis general order relation [30,31] given by,
I(T) = I, b /
0$ ×[(b $
− 1)(1 − Δ)
4
$
4 exp
0$ + Z, ]0/ $
(4)
where Δ = 2kT/E, Δ, = 2kT, /E, Z, = 1 + (b − 1)Δ, , Im is peak maximum intensity, Tm is peak temperature, E is activation energy, s is frequency factor and b is order of kinetics. 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) [30] given as, FOM =
∑ ⃒ 0@AB ⃒ ∑ @AB
(5)
here TLexp and TLfit represent the experimental data and value from the fitting function respectively. Fig. 8 shows the deconvolved glow curves of ZrO2 for a dose of 10 Gy. The residuals plot is included to show goodness of fit. TL glow curves of ZrO2 (26-210℃) are best fitted as a minimum of four peaks. The residuals fluctuating about zero, signify a good fit. For a dose of 10 Gy, the kinetic parameters were determined as 0.69 eV for peak 1 and 0.74 eV for peak 2 respectively. The FOM is less than 1.3%. This shows that the values from fitting are acceptable. The frequency factor of the peaks were calculated by using the general order equation, s=
$ 4 D$
exp
$
(6) 10
where β is the heating rate and k is Boltzmann’s constant. The frequency factors of peaks 1 and 2 were evaluated to be of the order of 109 and 108 s-1 respectively for the dose of 10 Gy. 6.5
Variable heating rate method A further method used for kinetic analysis was the variable heating rate method
[29]. According to theory, for a constant value of frequency factor, the maximum temperature Tm of the peak is dependent on the heating rate β as, ln
$ 4
=
$
+ ln
(7)
where all parameters in are as previously defined. A plot of ln(T2m/β) versus 1/kTm is expected to be linear with slope E and an intercept is equal to ln(E/sk). Fig. 9 shows plots of ln(T2m/β) versus 1/kTm for peaks 1 and 2. The activation energy of peaks 1 and 2 were found to be 0.75±0.01 and 0.91±0.02 eV respectively. The frequency factor of these peaks evaluated as 7.38×109 s-1 and 7.18×1010 s-1 respectively. 6.5.1
The influence of heating rate on TL intensity Fig. 10 shows a plot of TL intensity (in counts/℃) against heating rate for a sample
irradiated to 10 Gy. The peak intensity decreases with heating rate is a manner suggestive of thermal quenching. The thermoluminescence intensity EF corresponding to the lowest heating rate is related to the subsequent quenched ones EG measured at higher heating as EG =
HI
(8)
O PQR
JK LMN0
where W is the activation energy of thermal quenching, S = TUVWX where UVWX is the radiative lifetime at absolute zero of temperature and T is the frequency factor applicable to the non-radiative process [32].
11
Fig. 11 show a plot of ln[(Iuq/Iq)-1] versus 1/kTm for peak 2. From the slope and intercept of the plot, values of W and C were found to be 1.20±0.08 eV and 3.87×1014 respectively. For comparison, the values of W and C were also calculated using the peak area and found as W = 1.050±0.003 eV and C = 3.89×1012 respectively. The activation energy of thermal quenching calculated using the peak area and peak intensity are comparable. To our knowledge this is the first time these values have been reported for ZrO2. Incidentally, values of the activation energy determined using the initial rise may sometimes require correction for thermal quenching [26]. We determined the difference between the corrected and uncorrected value as 1.69×10-4 eV which is negligible and so the original result is reported as is. 7.
Dosimetric features
7.1
Dose response The dose response of peak 1 and peak 2 was studied for doses from 1 to 31 Gy. Fig.
12 shows the peak intensity against beta dose. The dose response was analysed using the relation, \( )⁄ ^ )⁄
f(D) = [\(
(9)
where y(D) is the TL intensity at a dose D and y (D1) is TL intensity at a low dose D1. A value of f(D) = 1 denotes a linear dose response, f(D) > 1 refers to supralinearity and f(D) < 1 represents sublinearity. For peak 1, f(D) ~1 over the dose range 1-31 Gy. This denotes that the intensity of peak 1 increases linearly with beta dose. In case of peak 2, a linear dose
12
response occurs between 6 and 20 Gy. Beyond this dose f(D) > 1, thus the intensity of peak 2 is supralinear up to 31 Gy. 8.
Fading To study fading, TL glow curves of ZrO2 were recorded at different intervals of time
between irradiation and readout. Fig. 13 shows the fading of peaks 1 and 2 for TL corresponding to 10 Gy. The intensity of peak 1 decreased rapidly with delay in measurement and fades completely by 1800 s. The intensity of peak 2 decreased by 59 % over 10800 s. The intensity of each of peaks 1 and 2 decreased to half maximum by 104 s and 2000 s respectively. These values are also found by fitting the experimental data with the empirical function, I(t) = A + I0 exp(−t/τ)
(10)
where the constant A represents the off-set, I0 is the initial intensity and τ is a constant. The dependence of peak temperature Tm with storage time is shown in the inset to Fig. 13. The position of peak 2 is independent of storage time. The position of peak 1 shifts towards higher temperature with increase in increased delay between irradiation and measurement. This may indicate the appearance of a new overlapping peak when the previous peak fades as deduced in other similar work [33]. 9.
Discussion Previous studies on TL have provided some key information about the mechanisms
leading to TL emission between 400-650 nm and between 480-500 nm in pure ZrO2 [19,22]. Both PL and TL emission occurs at similar bands in pure ZrO2. In addition to the
13
main peak at 480-500 nm, peaks of lower intensity have been observed at 385 nm [34], 440 nm [19], 456 and 516 nm [35] in pure ZrO2. An explanation for the 490 nm emission was given in the introduction. The emission band observed at lower wavelength side (<460 nm) is due to singly ionized oxygen-vacancy defects (F+ centers). Petrik et al. [21] reported that the association of F+ centers with one or two Y3+ ions in yttria- stabilized cubic ZrO2 would result in a red shift of the emission band of F+ centers. Therefore we deduce that the emission response at higher wavelength (>460 nm) in pure ZrO2 is related to singly ionized oxygen vacancy cluster defects [34,36] and/or relaxation from different excited state levels at an F+ center or different forms of this centre. The TL kinetic parameters evaluated by various methods are consistent except VHR method which overestimates parameters. Chernov et al., [37] reported that a glow curve measured at 2 ℃ s-1 had peaks at 67 and 142℃ after beta irradiation. The corresponding activation energy of the peaks were estimated as 0.65 and 0.78 eV respectively. Ahmed Kadari et al. [38] reported that the activation energy estimated by curve fitting for peaks at 67 and 140℃ were 0.55 and 0.79 eV respectively. Furthermore, they concluded that these peaks follow non-first order kinetics. Our present kinetic analysis in ZrO2 shows values close to those in the literature. However, the order of kinetics is first order for peak 2. As confirmed by various tests. In the present study, the TL glow curve (22-180℃) corresponds to four electron traps at different energy depth. These are at 42℃ (peak 1), 57℃ (peak 1a), 111℃ (peak 2) and 138℃ (peak 2a). The activation energy of thermal quenching for peak 2 was found to be W=1.20±0.08 eV. This value is comparable with that estimated using isothermal TL decay and the heating rate method by Nikiforov et al. [22]. We suggest that the thermal 14
quenching of luminescence in ZrO2 is caused by thermal ionization from an excited state of the F+ center [39]. 10.
Conclusion The TL characteristics of beta irradiated monoclinic ZrO2 have been investigated.
The glow curve recorded at 1 °C s-1 from ZrO2 sample consists of two prominent peaks at 44 and 112℃ as well as a weaker intensity peak at 240℃. Kinetic analysis using various methods shows that peaks 2 follows first order kinetics whereas peak 1 is subject to nonfirst order kinetics. The activation energy of peaks 1 and 2 were determined as 0.64±0.06 eV and 0.75±0.01 eV in measurements corresponding to irradiation to 10 Gy. Peak 2 is affected by thermal quenching. The activation energy for thermal quenching was calculated as 1.20 ± 0.08 eV. The dose response and fading of the peaks were studied. The kinetic properties suggest that both peaks are made up of more than one component. Acknowledgments H S Lokesha acknowledges with thanks the Rhodes University Post-Doctoral Research Fellowship. We also appreciate financial support from Rhodes University and National Research Foundation of South Africa.
15
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19
Figure Captions Fig. 1. XRD pattern of ZrO2 sample annealed at 850℃ for three hours in air. The inset shows a plot of β*cosθ/λ against 4sinθ/λ. Fig. 2. A natural TL glow curve (solid circles) compared with one measured after irradiation to 10 Gy. Fig. 3. Glow curves of ZrO2 recorded after a dose of 10 Gy but without preheating (i), after preheating to 74℃ (ii)150℃ (iii) 205℃ (iv) and 290℃ (v) respectively. Fig. 4. The variation of peak temperature Tm with irradiation dose 1 - 31 Gy. Fig. 5. The Tm-Tstop plot. Here, peak 1 and 2 are main peaks, and peaks 1a, 2a and 3 are secondary peaks. Fig. 6. A plot of ln(TL intensity) against 1/kT for peak 2 for a dose of 10 Gy. Fig. 7. The plot of ln(TLexp/b) against 1/kT for peak 1 and 2 for different values of the order of kinetics b. Fig. 8. A deconvolved glow curve of ZrO2 irradiated to 10 Gy. The residuals of the fit shows that the fit is acceptable. Fig. 9. Plots of ln (T2m/β) versus 1/kTm used in the variable heating rate method. Fig. 10. The change in intensity of peak 2 with heating rate corresponding to a dose of 10 Gy. Fig. 11. A plot of ln[(Iuq/Iq)-1] versus 1/kTm for peak 2. Fig. 12. The variation peak intensity with dose from 1-31 Gy for peaks 1 and 2. Fig. 13. The variation of intensity with storage time. The inset shows dependence of Tm with storage time.
20
Figure 1 400
(111) (111)
ZrO2
β∗cos θ/λ
300
0.0036 0.0027 0.0018 0.0009 0.6
0.8
1.0
Intensity (a.u.)
200
1.2
1.4
4∗sinθ/λ
100
0 1000 500 0 20
30
40
2θ (degree)
50
60
70
Figure 2
5x10
5
Natural TL TL following 10 Gy irradiation 5
3
4x10
3x10
5 3
x30 2x10
5
TL Intensity (a.u.)
TL Intensity (a.u.)
4x10
3x10
o
240 C 3
2x10
3
1x10
1x10
5
200
220
240
260
280
300
o
Temperature ( C)
0 100
200
300 o
Temperature ( C) 21
400
500
Figure 3 5
Without any preheat
3.5x10
o
after preheating to 74 C 5
o
3.0x10
after preheating to 150 C o
after preheating to 205 C 5
o
after preheating to 290 C 3
6.0x10
5
2.0x10
3
5
TL Intensity (a.u.)
TL Intensity (a.u.)
2.5x10
1.5x10
5
1.0x10
4
4.0x10
3
2.0x10
0.0
5.0x10
100
150
200
250
300
o
Temperature ( C)
0.0 100
200
300
o
400
Temperature ( C)
Figure 4 120
115
110
o
Tm ( C)
peak 2 105
peak 1
45
40
0
5
10
15
Dose (Gy) 22
20
25
30
Figure 5 270
3 240 210 180
o
Tm ( C)
2a 150
2
120 90
1a
60
1
30 20
40
60
80
100
120
140
160
180
200
o
Tstop ( C)
Figure 6
10.2
peak 2
ln (TL intensity)
9.9
9.6
9.3
9.0
8.7 33.5
34.0
34.5
35.0
35.5 -1
1/kT (eV )
23
36.0
36.5
Figure 7 3
peak 1
b=0.8 0.9 1 1.1 1.2
0 -3
b
ln (TLexp/n )
-6 -9 35
3
36
37
38
39
40
peak 2
0
b=0.8 0.9 1 1.1 1.2
-3 -6 -9 28
29
30
31
32
33
34
35
36
-1
1/kT (eV )
TL Intensity (a.u.)
Residuals
Figure 8 40 0 -40
3.2x10
5
2.4x10
5
1.6x10
5
8.0x10
4
Experimental data Deconvoluted peaks Fitted
0.0 30
60
90
120 o
Temperature ( C)
24
150
180
Figure 9
14.0
peak 1
13.6 13.2 12.8
34.4
2
ln (Tm/β)
12.4 34.8
35.2
35.6
36.0
36.4
12.0 11.6
peak 2
11.2 10.8 10.4 28.4
28.8
29.2
29.6
30.0
-1
1/kTm (eV )
Figure 10 5
1x10
5
o
TL intensity (counts/ C)
1x10
4
9x10
4
8x10
4
7x10
1
2
3
4 o
Heating rate ( C/s)
25
5
Figure 11
-0.8
ln {(Iuq/Iq)-1}
-1.2
-1.6
-2.0
-2.4
28.5
28.8
29.1
29.4 -1
1/kTm (eV )
Figure 12
10
6
TL Intensity (a.u.)
peak 1 peak 2
10
5
10
4
1
10
Dose (Gy)
26
29.7
30.0
Figure 13 1.2
125 120 115
peak 2
110 105
o
Tm ( C)
Normalized TL intensity (a.u.)
1.0
0.8
60 55
peak 1
50 45
0.6
40 35 30 100
1000
10000
Storage time (s)
0.4
0.2
0.0 0
2000
4000
6000
8000
10000
Storage time (s)
Table 1. Kinetic parameters determined using various methods. The superscripts a, b, c
corresponding to the peak shape method refer to Eτ, Eδ and Eω respectively.
b
s (s-1)
E (eV)
Methods
reference Peak 1 Peak 2
Peak 1
IR WGP
VHR
Peak 1
Peak 2
0.60±0.01 0.9
1.1
PS
CF
Peak 2
1.1
1.02
Sec. 6.1
0.60±0.01
0.70±0.003 1.15×109
1.58×107
0.70±0.01a
0.77±0.01a
0.58±0.01b
0.74±0.01b
0.64±0.01c
0.76±0.01c
0.69±0.01
0.74±0.01
9.32×109
3.04×108
Sec. 6.4
0.75±0.01
0.91±0.01
7.38×109 7.18×1010
Sec. 6.5
Sec. 6.2 Sec. 6.3
IR-Initial rise, WGP-Whole glow peak, PS-Peak shape, CF-Curve fitting and VHR-Variable heating rate
27
Highlights •
TL of nanocrystalline pure monoclinic ZrO2 phase is reported.
•
TL characteristics and dosimetric features of beta irradiated ZrO2 have been investigated.
•
Kinetic analysis of the main peaks (44℃ and 111℃) using various methods are discussed.
Conflict of Interest and Authorship Conformation Form
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.