**3.1. XRD**

Figure 1a shows the XRD of pure alumina produced by sol –gel without any dopants. A very well agreement with standard α-Al2O3 pattern is observed (Figure1a). On the other hand, doped samples show additional peaks related with new structures formed by the dopants and the alumina matrix, for example, in the case of Tb doped samples it was verified the Tb3Al5O12 crystalline structure (Figure 1b), and for Nd doped sample the AlNd structure (Figure 1c).

Sample doped with Er and Yb and calcinated at 1200 °C supplied a broad background, related to amorphous phase, and many other peaks associated to Yb2O3, Er2O3 and Yb3Al5O12. For the sample calcinated at 1600°C it was not observed the background, but the peaks of Yb2O3, Er2O3 with predominance of the Yb3Al5O12 (Figure 1d and e) were noted.

Figure 2 shows XRD patterns of alumina powder obtained by Pechini process. Applying crescent calcinations temperatures, we can obtain alumina in gamma and alpha phase (Figure 2a and 2b). The Mg addition promoted the formations of MgAl2O4 crystals (Figure 2c). When the doped sample is heated up to 1100 °C (Figure 2d), most of the material is converted to α-Al2O3, except for a few low intensity peaks related to the occurrence of magnesium spinel (MgAl2O4). This observation will be corroborated in the next section through TEM images.

## **3.2. Thermoluminescence**

TL glow curves of samples obtained with different calcinations temperatures, and detected in UV and VIS regions are shown in Figure 3a and 3b, respectively. It can be seen that calcinations at 1600 °C favored the increase of 190 °C TL dosimetric peak and diminution on intensity of high temperature peak simultaneously. The TL intensity in VIS region is higher than those found in the UV one.

180 Heat Treatment – Conventional and Novel Applications

diffractometer of Rigaku Corporation.

detected in the UV with a Schott U-340 optical filter.

TL/OSL reader Model DA - 20.

**3. Results and discussion** 

**3.1. XRD** 

structure (Figure 1c).

through TEM images.

**3.2. Thermoluminescence** 

The morphological characteristics of the samples were analyzed using a Philips CM200 TEM equipped with EDS operating at 160 keV, some Cu contamination from the sample holder can be observed in the all the EDS results, the samples were located at 400 mm from the source. The X-ray powder diffractions were recorded with the MiniFlex II model

TL and OSL measurements were performed in an oxygen-free nitrogen atmosphere using Daybreak Nuclear and Medical Systems Inc, model 1100-series TL/OSL reader and RISØ

TL was detected using the BG-39 (340-610 nm) optical filter and heating rate of 10 °C/s. OSL measurements were made using an array of blue (470 nm) LEDs for sample stimulation and

The irradiations were performed at RT in a 60Co source with dose rate of 28.7 Gy/h and with a beta source (90Sr/90Y) coupled to the RISØ TL/OSL reader, with dose rate of 0.08 Gy/s.

Figure 1a shows the XRD of pure alumina produced by sol –gel without any dopants. A very well agreement with standard α-Al2O3 pattern is observed (Figure1a). On the other hand, doped samples show additional peaks related with new structures formed by the dopants and the alumina matrix, for example, in the case of Tb doped samples it was verified the Tb3Al5O12 crystalline structure (Figure 1b), and for Nd doped sample the AlNd

Sample doped with Er and Yb and calcinated at 1200 °C supplied a broad background, related to amorphous phase, and many other peaks associated to Yb2O3, Er2O3 and Yb3Al5O12. For the sample calcinated at 1600°C it was not observed the background, but the peaks of Yb2O3, Er2O3 with predominance of the Yb3Al5O12 (Figure 1d and e) were noted.

Figure 2 shows XRD patterns of alumina powder obtained by Pechini process. Applying crescent calcinations temperatures, we can obtain alumina in gamma and alpha phase (Figure 2a and 2b). The Mg addition promoted the formations of MgAl2O4 crystals (Figure 2c). When the doped sample is heated up to 1100 °C (Figure 2d), most of the material is converted to α-Al2O3, except for a few low intensity peaks related to the occurrence of magnesium spinel (MgAl2O4). This observation will be corroborated in the next section

TL glow curves of samples obtained with different calcinations temperatures, and detected in UV and VIS regions are shown in Figure 3a and 3b, respectively. It can be seen that calcinations at 1600 °C favored the increase of 190 °C TL dosimetric peak and diminution on

**Figure 1.** XRD pattern of the α-Al2O3 samples, a) undoped sample, b) doped with Tb, c) doped with Nd, d) Er and Yb (1 and 2 mol%) doped and calcinated at 1200 °C and (e) Er and Yb (1 and 2 mol%) and calcinated at 1600 °C.

182 Heat Treatment – Conventional and Novel Applications

Effects of Heat Treatments on the Thermoluminescence and Optically Stimulated

Luminescence of Nanostructured Aluminate Doped with Rare-Earth and Semi-Metal Chemical Element 183

**Figure 3.** TL glow curves of pure alumina samples obtained by sol-gel process, study of effects of calcination temperatures and time on the TL response. a) UV emission with different calcinations temperatures; b) VIS emission with different calcinations temperatures; c) UV emission with different

Where F center loses an electron after absorption of high energy radiation and become F+ center. On thermal stimulation, the recombination of the electron and the F+ center produces an excited F center (F\*), which decays into its ground state (3P transition state → 3S) with

Therefore, following our results the long set point time (~ 8 h) favored the electron traps formations, and in the case of UV emission we have the great rate of formation of hole traps

Figure 4 shows TL glow curves of pure alumina obtained with different heating and cooling rates. In all cases the slow rate of 3 °C/min supplied the best result, confirming that longer calcinations time can promote a better diffusion of the defects and ions and also eliminates

TL glow curves of the samples doped with Er and Yb are shown in the Figure 5a and 5b. Samples calcinated at 1200 °C supplied one peak at high temperature (Figure 5a), which did not increase proportionally to the dose and another peak at low temperature region with very low intensity. After calcinations at 1600 °C, two prominent peaks at 224 °C and 442 °C

the internal tension forces in the crystalline lattice, which can homogenize the crystal.

set-point times and d) VIS emission with different set-point times.

the emission of photons at 410-420 nm.

after 4 hours of calcinations.

**Figure 2.** XRD patterns from Al2O3 samples obtained from Pechini routine. a) shows undoped samples annealed at 600 and 900 °C and standard patterns found for γ-Al2O3 and α-Al2O3. b) shows XRD from undoped sample annealed at 1100 °C and the standard α-Al2O3. Magnesium doped sample annealed at 900 °C is shown in (c) with the standard pattern of magnesium spinel (MgAl2O4). After annealing at 1100 °C, magnesium doped sample produced the XRD shown in (d).

Figure 3c and 3d show results of TL responses in UV and VIS regions with set point times varying of 30 min, 1, 2, 4 and 8 h. In the UV region, the thermal treatment of 4 h promoted a high increment of the 190 °C peak while in VIS region, the time was of 8 h. It is known from literature that the emission mechanism of these two luminescence regions is different. In the case of UV emission, the responsible is the F+ center according the mechanism:

$$\text{^{1}F} + \text{h}^{+} \rightarrow \text{F}^{+} \rightarrow \text{F}^{+} + \text{h} \text{v}\_{325\text{nm}} \text{ (stimulated process, TL/OSL)} \tag{l)}$$

Where the recombination of the F center with a hole (h+) generates an excited F+\*, which decays into the ground state (transition 1B → 1A) emitting a photon at 325 nm. Therefore, the calcinations at 1600 °C stimulated an increase of F centers concentration.

In the other case of VIS emission, it is believed that the luminescence occurs as follows [31, 32]:

$$\text{IF} \rightarrow \text{F}^+ + \text{e}^- \text{ (irradiation process)}\tag{2}$$

$$\text{F}^+ + \text{e}^- \rightarrow \text{F}^\* \rightarrow \text{F} + \text{h}\nu\_{\text{410-420nm}} \text{ (heating process, TL)}\tag{3}$$

182 Heat Treatment – Conventional and Novel Applications

**Figure 2.** XRD patterns from Al2O3 samples obtained from Pechini routine. a) shows undoped samples annealed at 600 and 900 °C and standard patterns found for γ-Al2O3 and α-Al2O3. b) shows XRD from undoped sample annealed at 1100 °C and the standard α-Al2O3. Magnesium doped sample annealed at 900 °C is shown in (c) with the standard pattern of magnesium spinel (MgAl2O4). After annealing at

Figure 3c and 3d show results of TL responses in UV and VIS regions with set point times varying of 30 min, 1, 2, 4 and 8 h. In the UV region, the thermal treatment of 4 h promoted a high increment of the 190 °C peak while in VIS region, the time was of 8 h. It is known from literature that the emission mechanism of these two luminescence regions is different. In the

Where the recombination of the F center with a hole (h+) generates an excited F+\*, which decays into the ground state (transition 1B → 1A) emitting a photon at 325 nm. Therefore,

In the other case of VIS emission, it is believed that the luminescence occurs as follows [31,

→ F� + h����������������������e��� ������� (1)

F→F� + e�������������������e��� (2)

F� + e� → F<sup>∗</sup> → F + hν�����������he����������e��� ��� (3)

case of UV emission, the responsible is the F+ center according the mechanism:

the calcinations at 1600 °C stimulated an increase of F centers concentration.

1100 °C, magnesium doped sample produced the XRD shown in (d).

F+h� → F�<sup>∗</sup>

32]:

**Figure 3.** TL glow curves of pure alumina samples obtained by sol-gel process, study of effects of calcination temperatures and time on the TL response. a) UV emission with different calcinations temperatures; b) VIS emission with different calcinations temperatures; c) UV emission with different set-point times and d) VIS emission with different set-point times.

Where F center loses an electron after absorption of high energy radiation and become F+ center. On thermal stimulation, the recombination of the electron and the F+ center produces an excited F center (F\*), which decays into its ground state (3P transition state → 3S) with the emission of photons at 410-420 nm.

Therefore, following our results the long set point time (~ 8 h) favored the electron traps formations, and in the case of UV emission we have the great rate of formation of hole traps after 4 hours of calcinations.

Figure 4 shows TL glow curves of pure alumina obtained with different heating and cooling rates. In all cases the slow rate of 3 °C/min supplied the best result, confirming that longer calcinations time can promote a better diffusion of the defects and ions and also eliminates the internal tension forces in the crystalline lattice, which can homogenize the crystal.

TL glow curves of the samples doped with Er and Yb are shown in the Figure 5a and 5b. Samples calcinated at 1200 °C supplied one peak at high temperature (Figure 5a), which did not increase proportionally to the dose and another peak at low temperature region with very low intensity. After calcinations at 1600 °C, two prominent peaks at 224 °C and 442 °C were observed (Figure 5b). For the sample doped with Er (1 mol %) and Yb (2 mol %), the peak temperature changed to 203 °C and an increment about 1.4 time in the TL intensity was observed. In all the samples, the TL response of the high temperature peak is not proportional to the dose. For samples doped with Tb (2.5 mol%) and Nd (2.5 mol%) an increased in UV intensity of the 190 °C peak was also noted, the first one increased 3.5 times and for Nd was 2.5 times, when compared to undoped one (Figure 5c).

Effects of Heat Treatments on the Thermoluminescence and Optically Stimulated

Luminescence of Nanostructured Aluminate Doped with Rare-Earth and Semi-Metal Chemical Element 185

the trapping centers (more stable TL signal). High temperature treatments can destroy as well as create trapping and recombination centers, and that explains why some TL peaks

**Figure 5.** TL glow curve of α-Al2O3 obtained by sol gel: a) doped with Er, Yb (1, 2 mol%), calcinated at 1200 °C and irradiated with γ-rays, b) doped with Er, Yb (1, 2 mol%), calcinated at 1600 °C and irradiated with γ-rays and c) TL glow curve UV emission of sample doped with Nd (2.5 %) (black open

**Figure 6.** TL glow curves from Al2O3 (a and b) and Al2O3:Mg (c and d) samples obtained from Pechini routine and annealed at 1100, 1350 and 1600 °C. Measurements were taken in both visible (a and c) and

UV (b and d) spectra.

circle) and Tb (2.5 %) (red triangle), both calcinated at 1600 °C and irradiated with 10 Gy.

may disappear, whilst others may rise or increase.

**Figure 4.** TL glow curve of pure alumina samples obtained by sol-gel process a) UV emission with different heating rates b) VIS emission with different heating rates; c) UV emission with different cooling rates d) VIS emission with

Figure 6 shows TL glow curves of pure and Mg doped samples obtained by Pechini process, the curves are slightly different from those obtained by sol-gel. High intensities for 190 °C TL peak from pure samples were obtained at 1100 oC for VIS region (Figure 6a) and at 1350 °C for UV one (Figure 6b). On sample doped with Mg high intensities were detected in both cases UV and VIS with calcinations at 1600 °C, the same value found in sol-gel samples.

Figure 6 shows TL glow curves for samples produced via Pechini method. As the calcination temperature increases, different observations can be made, depending on the composition and the measurement spectra. In Figure 6a, showing TL emission of undoped sample in the visible region, all the peaks intensities decrease for higher temperatures, but mainly low temperature (90 °C) and high temperature (410 °C) peaks. In this case, the best sample would be the one calcinated at 1350 °C, due to its high intensity and low competition among the trapping centers (more stable TL signal). High temperature treatments can destroy as well as create trapping and recombination centers, and that explains why some TL peaks may disappear, whilst others may rise or increase.

184 Heat Treatment – Conventional and Novel Applications

cooling rates d) VIS emission with

were observed (Figure 5b). For the sample doped with Er (1 mol %) and Yb (2 mol %), the peak temperature changed to 203 °C and an increment about 1.4 time in the TL intensity was observed. In all the samples, the TL response of the high temperature peak is not proportional to the dose. For samples doped with Tb (2.5 mol%) and Nd (2.5 mol%) an increased in UV intensity of the 190 °C peak was also noted, the first one increased 3.5 times

**Figure 4.** TL glow curve of pure alumina samples obtained by sol-gel process a) UV emission with different heating rates b) VIS emission with different heating rates; c) UV emission with different

Figure 6 shows TL glow curves of pure and Mg doped samples obtained by Pechini process, the curves are slightly different from those obtained by sol-gel. High intensities for 190 °C TL peak from pure samples were obtained at 1100 oC for VIS region (Figure 6a) and at 1350 °C for UV one (Figure 6b). On sample doped with Mg high intensities were detected in both cases UV and VIS with calcinations at 1600 °C, the same value found in sol-gel samples.

Figure 6 shows TL glow curves for samples produced via Pechini method. As the calcination temperature increases, different observations can be made, depending on the composition and the measurement spectra. In Figure 6a, showing TL emission of undoped sample in the visible region, all the peaks intensities decrease for higher temperatures, but mainly low temperature (90 °C) and high temperature (410 °C) peaks. In this case, the best sample would be the one calcinated at 1350 °C, due to its high intensity and low competition among

and for Nd was 2.5 times, when compared to undoped one (Figure 5c).

**Figure 5.** TL glow curve of α-Al2O3 obtained by sol gel: a) doped with Er, Yb (1, 2 mol%), calcinated at 1200 °C and irradiated with γ-rays, b) doped with Er, Yb (1, 2 mol%), calcinated at 1600 °C and irradiated with γ-rays and c) TL glow curve UV emission of sample doped with Nd (2.5 %) (black open circle) and Tb (2.5 %) (red triangle), both calcinated at 1600 °C and irradiated with 10 Gy.

**Figure 6.** TL glow curves from Al2O3 (a and b) and Al2O3:Mg (c and d) samples obtained from Pechini routine and annealed at 1100, 1350 and 1600 °C. Measurements were taken in both visible (a and c) and UV (b and d) spectra.

Figure 6b is the TL emission of undoped sample in the UV region, which indicates that high temperature peaks tend to fade when the sample is calcinated at higher temperatures. Once again, the sample calcinated at 1350 °C showed the best glow curve. For the undoped sample, calcination at 1600 °C seems to increase the competition, which decreases the overall intensity. It is not likely that the high temperature is damaging the material, since the melting point is still too far away (around 2050 °C). Also, the high temperature may be causing the crystallites to grow, decreasing the surface area exposed to the incoming radiation, thus changing the trapping dynamics at some level.

Effects of Heat Treatments on the Thermoluminescence and Optically Stimulated

Luminescence of Nanostructured Aluminate Doped with Rare-Earth and Semi-Metal Chemical Element 187

**Figure 7.** TL spectra of Alumina, a) undoped sample, b) Tb doped sample, c) Er and Yb doped sample,

In the case of Al2O3:Er:Yb sample, the band is centered at 528 nm (448-589 nm) and the emission mechanism can be related to these rare-earth elements. It is well known that Er3+ has high efficiency for the infrared to visible light conversion and cooperative sensitization properties. The 4I11/2 (Er3+) level and 2F5/2 (Yb3+) state are very closely matched in energy, thus the exposition from the 0.9 to 1.1 µm range will excite both Er3+ and Yb3+ ions. The visible emission can occurs because the Yb3+ transfers the excitation energy to Er3+ and the final state of this process is the population of the 4F7/2 state and nonradiative relaxation to the 4S3/2 level, from which green photon (547 nm) is emitted. The excitation routes for red emission at 660 nm are not clear yet; in this case the Yb3+ transfer energy to Er3+ and the red emission

For Nd doped sample, it was noted emission at 396 nm (364-460 nm). It is known that Nd3+ can emit in UV region and the possible transitions related to these emissions are: 4D5/2 4D3/2

As seen in TL emission curves showed previously, the response of OSL signal also increased for samples calcinated at high temperatures, for both samples obtained by sol-gel (Figure 8a)

d) Nd doped sample.

occurs in the 4F9/2→4I15/2 transition [34].

4P3/2→ 4I9/2; 4D5/2 4D3/2 4P3/2→4I11/2 [35].

**3.4. Optically stimulated luminescence** 

The incorporation of magnesium atoms in the crystalline lattice made a great deal on the TL response of the samples. In the first place, both visible and UV emissions (Figures 6c and 6d, respectively) increased with the increasing of the calcination temperature, which was not observed for undoped samples. Secondly, the high temperature peak (355 °C) of the visible emission had its intensity increased by a factor of 3; a minor difference on the relation between the main dosimetric peak and the high temperature one was also observed for the UV emission.

It is important to observe that only samples calcinated above 1100 °C exhibited some appreciable TL emission; this means that α-Al2O3 acts as a better ionizing radiation sensor than other phases (δ and γ). For samples calcinated below that temperature (600 and 900 °C), most of the trapping and recombination centers may not yet be active.

One reason for the high luminescence of α-Al2O3:C comes from the theory of point defects. It is known that the synthesis of carbon doped alumina is done in a highly reductive atmosphere of carbon ions, resulting into a great production of oxygen vacancies in the crystalline lattice. If these vacancies are occupied by two electrons we have the formation of the neutral F center. Otherwise, if the vacancy is occupied by only one electron, we have the formation of F+ center. In the latter situation, the presence of charge compensation is demanded for the F+ center formation; in the case of the α-Al2O3:C it is C2+ impurity, which replaces Al3+ ion in the crystalline lattice. In our case we suppose that the Mg is acting as C impurity.

## **3.3. Thermoluminescence spectra**

TL spectra of pure and doped with Tb, Er-Yb, and Nd alumina are shown in Figures 7a) to 7d) respectively. On visible region from 360 to 600 nm luminescence bands due to rare-earth elements are observed on doped samples. However all the samples pure and doped showed an intense luminescence band between 650 and 800 nm not identified yet in the literature. However, from fluorescence results, this band is usually associated to Cr3+ impurity incorporated in raw materials used for alumina production [33].

When the dopant are incorporated, other bands are detected related to the rare-earth elements (Figure 7b, 7c, and 7d). The Tb doped sample shows, in addition for the first at 694 nm, another broad band at 428 nm, with 187 nm of width, due to the transitions of Tb3+. The results are in agreements with the transitions of 5D3 and 5D47Fj (j=1-6).

Effects of Heat Treatments on the Thermoluminescence and Optically Stimulated Luminescence of Nanostructured Aluminate Doped with Rare-Earth and Semi-Metal Chemical Element 187

**Figure 7.** TL spectra of Alumina, a) undoped sample, b) Tb doped sample, c) Er and Yb doped sample, d) Nd doped sample.

In the case of Al2O3:Er:Yb sample, the band is centered at 528 nm (448-589 nm) and the emission mechanism can be related to these rare-earth elements. It is well known that Er3+ has high efficiency for the infrared to visible light conversion and cooperative sensitization properties. The 4I11/2 (Er3+) level and 2F5/2 (Yb3+) state are very closely matched in energy, thus the exposition from the 0.9 to 1.1 µm range will excite both Er3+ and Yb3+ ions. The visible emission can occurs because the Yb3+ transfers the excitation energy to Er3+ and the final state of this process is the population of the 4F7/2 state and nonradiative relaxation to the 4S3/2 level, from which green photon (547 nm) is emitted. The excitation routes for red emission at 660 nm are not clear yet; in this case the Yb3+ transfer energy to Er3+ and the red emission occurs in the 4F9/2→4I15/2 transition [34].

For Nd doped sample, it was noted emission at 396 nm (364-460 nm). It is known that Nd3+ can emit in UV region and the possible transitions related to these emissions are: 4D5/2 4D3/2 4P3/2→ 4I9/2; 4D5/2 4D3/2 4P3/2→4I11/2 [35].

## **3.4. Optically stimulated luminescence**

186 Heat Treatment – Conventional and Novel Applications

UV emission.

impurity.

**3.3. Thermoluminescence spectra** 

radiation, thus changing the trapping dynamics at some level.

most of the trapping and recombination centers may not yet be active.

incorporated in raw materials used for alumina production [33].

results are in agreements with the transitions of 5D3 and 5D47Fj (j=1-6).

Figure 6b is the TL emission of undoped sample in the UV region, which indicates that high temperature peaks tend to fade when the sample is calcinated at higher temperatures. Once again, the sample calcinated at 1350 °C showed the best glow curve. For the undoped sample, calcination at 1600 °C seems to increase the competition, which decreases the overall intensity. It is not likely that the high temperature is damaging the material, since the melting point is still too far away (around 2050 °C). Also, the high temperature may be causing the crystallites to grow, decreasing the surface area exposed to the incoming

The incorporation of magnesium atoms in the crystalline lattice made a great deal on the TL response of the samples. In the first place, both visible and UV emissions (Figures 6c and 6d, respectively) increased with the increasing of the calcination temperature, which was not observed for undoped samples. Secondly, the high temperature peak (355 °C) of the visible emission had its intensity increased by a factor of 3; a minor difference on the relation between the main dosimetric peak and the high temperature one was also observed for the

It is important to observe that only samples calcinated above 1100 °C exhibited some appreciable TL emission; this means that α-Al2O3 acts as a better ionizing radiation sensor than other phases (δ and γ). For samples calcinated below that temperature (600 and 900 °C),

One reason for the high luminescence of α-Al2O3:C comes from the theory of point defects. It is known that the synthesis of carbon doped alumina is done in a highly reductive atmosphere of carbon ions, resulting into a great production of oxygen vacancies in the crystalline lattice. If these vacancies are occupied by two electrons we have the formation of the neutral F center. Otherwise, if the vacancy is occupied by only one electron, we have the formation of F+ center. In the latter situation, the presence of charge compensation is demanded for the F+ center formation; in the case of the α-Al2O3:C it is C2+ impurity, which replaces Al3+ ion in the crystalline lattice. In our case we suppose that the Mg is acting as C

TL spectra of pure and doped with Tb, Er-Yb, and Nd alumina are shown in Figures 7a) to 7d) respectively. On visible region from 360 to 600 nm luminescence bands due to rare-earth elements are observed on doped samples. However all the samples pure and doped showed an intense luminescence band between 650 and 800 nm not identified yet in the literature. However, from fluorescence results, this band is usually associated to Cr3+ impurity

When the dopant are incorporated, other bands are detected related to the rare-earth elements (Figure 7b, 7c, and 7d). The Tb doped sample shows, in addition for the first at 694 nm, another broad band at 428 nm, with 187 nm of width, due to the transitions of Tb3+. The

As seen in TL emission curves showed previously, the response of OSL signal also increased for samples calcinated at high temperatures, for both samples obtained by sol-gel (Figure 8a) and Pechini process (Figure 8b). Figure 8c and 8d show an example of OSL increment about 3 times, after calcination at 1600 °C.

OSL decays can be fitted by exponential functions [36], depending on the number of the traps involved in the process, for example for two traps we have:

$$\mathcal{I}\_{\rm OSL} = \mathcal{I}\_1 \exp\left(-\frac{\mathfrak{t}}{\mathfrak{r}\_1}\right) + \mathcal{I}\_2 \exp\left(-\frac{\mathfrak{t}}{\mathfrak{r}\_2}\right) \tag{4}$$

Effects of Heat Treatments on the Thermoluminescence and Optically Stimulated

Luminescence of Nanostructured Aluminate Doped with Rare-Earth and Semi-Metal Chemical Element 189

**Figure 8.** OSL curves of alumina, a) undoped samples obtained by sol-gel with different calcinations

temperatures, c) Er, Yb doped samples calcinated at 1200 °C and irradiated with gamma-radiations, d) Er, Yb doped samples calcinated at 1600 °C and irradiated with gamma-radiations and e) Nd and Tb

According to TEM images, it was verified that all the doped samples present nanocrystals formations on the surface of alumina grains (Figure 9a). In Tb doped sample, it was verified the presence of Tb3Al5O12, the chemical structure was determined by electron diffraction and EDS results (Figure 9b). Nanocrystals of AlNd are easily observed with its well developed

temperatures, b) undoped samples obtained by Pechini process with different calcinations

doped samples calcinated at 1600°C.

**3.5. TEM** 

Where IOSL is the total OSL intensity, I1 and I2 are the initial intensities of exponentially decaying from faster and slower components of the shinedown curve; τ1 and τ2 the respective decay constants.


**Table 1.** Values for decay constants obtained from theoretical fitting of the OSL curves of alumina (equation 4).

Table 1 shows the decay constants obtained for the studied samples. All the OSL decay curves can be fitted by second order exponential function, except for Er and Yb doped sample, which followed first order decay. For samples obtained by Pechini process the high temperatures calcinations diminished the constants values, the fast component value for both pure and Mg doped samples heated at 1600 °C supplied almost same constants decays. In the case of samples made by sol-gel, the Tb doped sample has constants similar to those determined by pure alumina. However, the incorporation of Nd, Er and Yb changed the constants values. The OSL curves of the samples obtained by sol-gel supplied greatest decay constants. Yang et al. [37] showed some OSL fitting results of α-Al2O3: C excited by green light and detected in the UV, and they found that τ1 vary from 4.6 to 11.3 s-1 and τ2 from 24.1 to 30.1 s-1. They attributed the variations to different concentrations of F and F+ centers in the sample. Nevertheless, our results indicated that τ values are constant and do not depend of dose values delivered to the samples, the results for τ1 are similar to those found in the literature, however τ2 are relatively higher.

**Figure 8.** OSL curves of alumina, a) undoped samples obtained by sol-gel with different calcinations temperatures, b) undoped samples obtained by Pechini process with different calcinations temperatures, c) Er, Yb doped samples calcinated at 1200 °C and irradiated with gamma-radiations, d) Er, Yb doped samples calcinated at 1600 °C and irradiated with gamma-radiations and e) Nd and Tb doped samples calcinated at 1600°C.

#### **3.5. TEM**

188 Heat Treatment – Conventional and Novel Applications

3 times, after calcination at 1600 °C.

respective decay constants.

Sample – Pechini Process

Sample Sol Gel Process

literature, however τ2 are relatively higher.

(equation 4).

and Pechini process (Figure 8b). Figure 8c and 8d show an example of OSL increment about

OSL decays can be fitted by exponential functions [36], depending on the number of the

��

Where IOSL is the total OSL intensity, I1 and I2 are the initial intensities of exponentially decaying from faster and slower components of the shinedown curve; τ1 and τ2 the

Al2O3 1100 3.05±0.03 13.4±0.3 Al2O3 1350 2.415±0.005 11.11±0.04 Al2O3 1600 2.27±0.05 11.5 ± 0.2 Al2O3: Mg 1100 2.49±0.09 13.8±0.6 Al2O3: Mg 1350 2.6±0.1 12.6±0.9 Al2O3: Mg 1600 2.22±0.05 11.3±0.3

Al2O3 1600 9.0±2.0 41±11 Al2O3 :Yb:Er 1600 22.5±1.0 -0- Al2O3: Nd 1600 13.3±0.8 54±11 Al2O3:Tb 1600 8.7±2.2 45±6.0

**Table 1.** Values for decay constants obtained from theoretical fitting of the OSL curves of alumina

Table 1 shows the decay constants obtained for the studied samples. All the OSL decay curves can be fitted by second order exponential function, except for Er and Yb doped sample, which followed first order decay. For samples obtained by Pechini process the high temperatures calcinations diminished the constants values, the fast component value for both pure and Mg doped samples heated at 1600 °C supplied almost same constants decays. In the case of samples made by sol-gel, the Tb doped sample has constants similar to those determined by pure alumina. However, the incorporation of Nd, Er and Yb changed the constants values. The OSL curves of the samples obtained by sol-gel supplied greatest decay constants. Yang et al. [37] showed some OSL fitting results of α-Al2O3: C excited by green light and detected in the UV, and they found that τ1 vary from 4.6 to 11.3 s-1 and τ2 from 24.1 to 30.1 s-1. They attributed the variations to different concentrations of F and F+ centers in the sample. Nevertheless, our results indicated that τ values are constant and do not depend of dose values delivered to the samples, the results for τ1 are similar to those found in the

��I���� �� �

��

τ1 (s-1)

� (4)

τ2 (s-1)

traps involved in the process, for example for two traps we have:

I��� = I���� �� �

Calcination temperature (°C)

> According to TEM images, it was verified that all the doped samples present nanocrystals formations on the surface of alumina grains (Figure 9a). In Tb doped sample, it was verified the presence of Tb3Al5O12, the chemical structure was determined by electron diffraction and EDS results (Figure 9b). Nanocrystals of AlNd are easily observed with its well developed

faces (Figure 10a), in most of the case, the nanocrystals average size is about 200 nm, these aluminates composition was also verified by the EDS (Figure 10b).

Effects of Heat Treatments on the Thermoluminescence and Optically Stimulated

Luminescence of Nanostructured Aluminate Doped with Rare-Earth and Semi-Metal Chemical Element 191

Figure 11a) shows an example of TEM images obtained for α-Al2O3 doped with 1mol% of Er and 2mol% of Yb, and calcinated at 1200 °C. EDS analysis show the presence of Er and Yb dopants in the α-Al2O3. Electron diffraction analysis identified the crystal as Yb2O3, however by XRD analysis showed plus two new composition in the samples doped and attributed to Er2O3 and Yb3Al5O12 (Figure 1e). The results of TEM analysis give the following average nanocrystals diameters D = (36±2) nm for the sample calcinated at 1200 °C and D=(182±8) nm for sample calcinated at 1600 °C (Figures 9 d), these results and the homogeneity in the crystalline size suggests that the nanocrystal powder growth is depending on thermal treatment temperature. The growth and cluster formations of the nanocrystals are the consequence of the reduction of grain boundary area and therefore the total energy of the

In the case of Mg doped alumina there is a formation of Mg spinel (magnesium aluminate) nanocrystals dispersed on the surface of alumina grains, with size about 40 nm (Figure 12). It is considered that the high temperature calcination makes magnesium atoms to diffuse to the surface of the clusters, creating a thin layer of magnesium spinel, due to the high local

**Figure 11.** Results of TEM, EDS and Electron diffraction analysis obtained for for α-Al2O3 doped with 1 mol% of *Er* and 2 mol% of *Yb*, and calcinated at 1200oC and 1600°C, (*a*) TEM images sample calcinated

at 1200oC, (*b*) TEM images sample calcinated at 1600°C.

system.

concentration of the dopant.

**Figure 9.** TEM image from Tb doped Al2O3 and electron diffraction showing the presence of nanocrystals of Tb3Al5O12. b) EDS results with Tb peak.

**Figure 10.** a) TEM image from Nd doped Al2O3, b) EDS results with Nd peak.

Figure 11a) shows an example of TEM images obtained for α-Al2O3 doped with 1mol% of Er and 2mol% of Yb, and calcinated at 1200 °C. EDS analysis show the presence of Er and Yb dopants in the α-Al2O3. Electron diffraction analysis identified the crystal as Yb2O3, however by XRD analysis showed plus two new composition in the samples doped and attributed to Er2O3 and Yb3Al5O12 (Figure 1e). The results of TEM analysis give the following average nanocrystals diameters D = (36±2) nm for the sample calcinated at 1200 °C and D=(182±8) nm for sample calcinated at 1600 °C (Figures 9 d), these results and the homogeneity in the crystalline size suggests that the nanocrystal powder growth is depending on thermal treatment temperature. The growth and cluster formations of the nanocrystals are the consequence of the reduction of grain boundary area and therefore the total energy of the system.

190 Heat Treatment – Conventional and Novel Applications

faces (Figure 10a), in most of the case, the nanocrystals average size is about 200 nm, these

**Figure 9.** TEM image from Tb doped Al2O3 and electron diffraction showing the presence of

**Figure 10.** a) TEM image from Nd doped Al2O3, b) EDS results with Nd peak.

nanocrystals of Tb3Al5O12. b) EDS results with Tb peak.

aluminates composition was also verified by the EDS (Figure 10b).

In the case of Mg doped alumina there is a formation of Mg spinel (magnesium aluminate) nanocrystals dispersed on the surface of alumina grains, with size about 40 nm (Figure 12). It is considered that the high temperature calcination makes magnesium atoms to diffuse to the surface of the clusters, creating a thin layer of magnesium spinel, due to the high local concentration of the dopant.

**Figure 11.** Results of TEM, EDS and Electron diffraction analysis obtained for for α-Al2O3 doped with 1 mol% of *Er* and 2 mol% of *Yb*, and calcinated at 1200oC and 1600°C, (*a*) TEM images sample calcinated at 1200oC, (*b*) TEM images sample calcinated at 1600°C.

Effects of Heat Treatments on the Thermoluminescence and Optically Stimulated

Luminescence of Nanostructured Aluminate Doped with Rare-Earth and Semi-Metal Chemical Element 193

stabilization, improving the luminescence response in the visible spectra, causing the main peak to increase 5 times in comparison with the undoped sample. It is believed that the occurrence of the nanometric spinel layer created an interface between both materials

OSL shinedown curves, supplied by undoped samples calcinated to 1200 and 1600 °C, could be fitted by second for all the samples except to α-Al2O3:Yb, Er, which was fitted by first order exponential decay. TL intensity of 190 °C peak and OSL responses with the dose increased linear for low doses region, from 80 to 1000 mGy, and the minimum dose detected value was 5 mGy obtained for TL (UV) and 350 µGy for OSL α-Al2O3 + Tb3Al5O12.

In summary, calcination conditions are of great importance for materials production that are being used as radiation sensors, once it greatly influences the stabilization of intrinsic defects, diffusion of dopants and the occurrence of new phases, due to the incorporation of dopants alongside the matrix, and others. These new phases also seem to play an important role in the luminescence emissions, due to the creation of new trapping and recombination centers, producing materials with unique properties that can be exploited to obtain better

, Alexandre Ventieri, José Francisco Sousa Bitencourt,

Katia Alessandra Gonçalves, Juan Carlos Ramirez Mittani, René Rojas Rocca

*Universidade Federal de São Paulo, Universidade de São Paulo, Centro Estadual de Educação* 

The authors wish to thanks to FAPESP, CAPES and CNPq for financial support.

l'Influence des Radiations Infra-rouge. Comptes Rendus 96: 1853-1856.

[2] Becquerel AH (1883) Maxima et Minima d'excitinction de al Phosphorescence sous

[3] Randall JT, Wilkins MHF (1945) Phosphorescence and Electron Traps. I. The study of

[4] McKeever SWS (1985) Thermoluminescence of Solids. Cambridge University Press. 8 p. [5] Cameron JR, Suntharalingam N, Kenney GN (1968) Thermoluminescent Dosimetry.

[1] Aitken MJ (1985) Thermoluminescence Dating. Academic Press. 3 p.

Trap Distributions. Proc. Royal Soc. Lond. 184: 366-389.

University of Wisconsin Press.

(Al2O3/MgAl2O4) with high concentration of defects.

dosimeters.

**Author details** 

Sonia Hatsue Tatumi \*

and Shiva do Valle Camargo

*Tecnológica Paula Souza, Brazil* 

**Acknowledgement** 

**5. References** 

Corresponding Author

 \*

**Figure 12.** TEM image from magnesium doped Al2O3. It is considered that the high temperature annealing makes magnesium atoms to diffuse to the surface of the clusters, creating a thin layer of magnesium spinel.
