Effects of annealing temperature and atmosphere

The annealing treatment at 1350 ^oC under oxygen atmosphere is confirmed to be helpful for increasing the Cr⁴⁺ concentration in the crystal fiber. In this section, we discuss the optimization of annealing crystal fibers with different temperatures and atmospheres to study the reaction of charge compensation between Cr³⁺ and Cr⁴⁺ ions.

The oxidation state of the samples subjected to reduction in nitrogen annealing atmosphere and oxidation in annealing oxygen atmosphere. After annealing at temperature from 100 ^oC to 1500 ^oC, the reduction and oxidation annealing processes can change the ratio of Cr⁴⁺/Cr³⁺. The Cr³⁺ and Cr⁴⁺ fluorescence intensities are measured to understand their relation. In Fig. 4.18, it shows the Cr⁴⁺ fluorescence intensity with several annealing temperatures under nitrogen and oxygen atmospheres.

The Cr⁴⁺ fluorescence intensity is slightly increased when annealing temperature increases from 100 ^oC to 700 ^oC in both nitrogen and oxygen atmospheres. When the annealing temperature becomes higher than 700^oC, the Cr⁴⁺ fluorescence intensity is dramatically decreased under nitrogen atmosphere, but it increases under oxygen atmosphere. It is indicated that the threshold temperature of reduction and oxidation reactions is 700^oC. In Fig. 4.19, the annealing processes in different temperatures and atmospheres were illustrated. The dash line shows that the Cr⁴⁺ fluorescence intensity is increased by improvement of crystal quality. After temperature higher than 700^oC, the change of Cr⁴⁺ fluorescence intensity was affected by its concentration due to the redox reactions.

0 200 400 600 800 1000 1200 1400 1600 0.0

0.2 0.4 0.6 0.8 1.0

Cr

4+

fluor escence intensity ( a.u.)

Annealing temperature (

^o

C) N

₂

O

₂

Fig. 4.18 The Cr⁴⁺ fluorescence intensity with different annealing temperatures under nitrogen and oxygen atmospheres with 4 hours annealing time.

(O

₂

)

(N

₂

)

600 900 1500

25 300 1200

Annealing temperature (

^oC)

Cr

4+

fl u o rescence intensity (a.u .)

(O

₂

)

(N

₂

)

600 900 1500

25 300 1200

Annealing temperature (

^oC)

Cr

4+

fl u o rescence intensity (a.u .)

Fig. 4.19 The simplified Cr⁴⁺ fluorescence intensity diagram with different annealing temperatures under nitrogen and oxygen atmospheres.

In Fig. 4.20, it shows the Cr³⁺ fluorescence intensity with different annealing temperature under nitrogen and oxygen atmospheres. The Cr³⁺ fluorescence intensity is increasing with annealing temperature from 100 ^oC to 700 ^oC in both nitrogen and oxygen atmospheres. When the annealing temperature gets higher than 700^oC, both of Cr³⁺ fluorescence intensities are still increasing under nitrogen and oxygen atmospheres. Therefore, the threshold temperature is also about 700^oC. In contrary to the Cr⁴⁺ fluorescence intensity, the Cr³⁺ fluorescence intensity under nitrogen atmosphere is stronger than that under oxygen atmosphere. That is due to more Cr³⁺

ions transfer to Cr⁴⁺ ions under oxygen atmosphere. When annealing temperature higher than 700^oC under oxygen atmosphere, the Cr³⁺ concentration should be decreased with increasing annealing temperature. However, the Cr³⁺ fluorescence intensity is linearly increased with annealing temperature. The main reason is also as a result of the improvement of the crystal quality, which results in the increase of emission cross section. In Fig. 4.21, the annealing processes in different temperatures and atmospheres were also illustrated. The dash line shows increased Cr³⁺

0 200 400 600 800 1000 1200 1400 1600 0.0

0.2 0.4 0.6 0.8 1.0

Cr

3+

fluorescence intensity (a.u.)

Annealing temperature (

^o

C) N ₂

O ₂

Fig. 4.20 The Cr³⁺ fluorescence intensity with different annealing temperatures under nitrogen and oxygen atmospheres with 4 hours annealing time.

(N

₂

)

(O

₂

)

Cr

3+

fl uorescence int ensity (a.u. )

600 900 1500

25 300 1200

Annealing temperature (

^oC)

(N

₂

)

(O

₂

)

Cr

3+

fl uorescence int ensity (a.u. )

600 900 1500

25 300 1200

Annealing temperature (

^oC)

Fig. 4.21 The simplified Cr³⁺ fluorescence intensity diagram with different annealing temperatures under nitrogen and oxygen atmospheres.

For the detailed analysis of the relation between Cr³⁺ and Cr⁴⁺ ions, the basic reactions are shown as follows,

)

Cr⁴⁺ ions in tetrahedral site.

Since the doping of Ca²⁺ into YAG replacing the Y³⁺ in dodecahedral site may promote the Cr⁴⁺ concentration. If it is assumed that the no ions occupy interstitial lattice positions, doping with CaO may be formulated in Eq. (4.1). The oxygen vacancies ^•2

-V

O will accompany with the divalent co-dopant. The Eq. (4.2) proceeds due to diffusion motion of oxygen vacancies, which recombine with atmosphere of oxygen at the surface of the crystal fiber when the temperature is higher than 700^oC (for the opposite direction of Eq. (4.2), the vacancies are generated as a result of vaporization of oxygen from the crystal). A releasing positive charge is transported by the hole

P

^•to the crystal’s volume and is localized on a

Cr

_o³⁺ in the octahedral site, and a

Cr

_o⁴⁺ ion is formed, as shown in Eq. (4.3). For the opposite direction of the Eq.

(4.3), the positive charge is transported from a Cr⁴⁺ ion to the crystal’s surface and the opposite conversion takes place. Finally, in the Eq. (4.4), the Cr⁴⁺ ion in tetrahedral site (denoted as

Cr

_t⁴⁺) is formed in a process of cations exchange between

Cr

_o⁴⁺ and

Al

³_t⁺ during the thermal treatment. According to the Okhrimchuk’s work, it is found that the

Cr

⁴⁺/

Cr

⁴⁺ is a function of temperature [3.7]. The Eq. (4.4) leads to

The temperature dependence of reaction constant K0 is 0.087. The relative stabilizing energy of the reaction,

e

_CrAl, was found to be 0.11 eV. Therefore, it can be obtained that Cr_t⁴⁺ /Cr_o⁴⁺ =0.04 for a crystal thermally treated at 1700 K.

Since the total number of Cr ions is equal to the sum of Cr³⁺ in octahedral site and Cr⁴⁺ in both octahedral and tetrahedral sites, it can be expressed as

] The charge conservation requires,

] The concentrations of Ca and Cr ions were measured by EPMA method. The concentrations of Cr_o³⁺ and Cr_t⁴⁺ can be derived from the fluorescence measurement. Assuming that the Cr_o⁴⁺ is a function of Cr_t⁴⁺, the relation between Ca, Cr, Cr , _o³⁺ Cr_o⁴⁺, and Cr_t⁴⁺ ions are listed in Table 4.6. At 1500 ^oC annealing temperature, more than 40% of Cr ions are in 4+ oxidation state under oxygen atmosphere, while that is only 14.5% Cr ions are in 4+ oxidation state under nitrogen atmosphere. However, under both two annealing atmospheres, less than 3% of Ca ions become active in charge compensation to Cr⁴⁺ in tetrahedral site for NIR emission.

The Cr³⁺ ions are the majority of Cr ions both under oxygen and nitrogen atmospheres.

The charge compensation efficiency is listed in Table 4.7. It is found that the 37% Ca ions charge compensates to Cr⁴⁺ ions when annealing at 1500^oC under oxygen atmosphere. Under the reducing reaction in nitrogen atmosphere, only 13% of the Ca ions charge compensates the Cr⁴⁺ ions. It is found that the low charge compensation efficiency is due to the existence of oxygen vacancy in YAG matrix. In addition, by calculating the ratio of Cr_t⁴⁺/Cr_o⁴⁺, the temperature dependence of reaction constant K0 is estimated to be 0.365 and the relative stabilizing energy of the reaction,

e

_CrAl, is 0.278 eV under oxygen atmosphere.

Table 4.6 The relation between Cr, Cr ,

_o³⁺

Cr , and

_o⁴⁺

Cr ions.

_t⁴⁺

Annealing

treatment Ca/Cr Cr /Cr _o³⁺ Cr_o⁴⁺/Cr Cr_t⁴⁺/Cr Cr_t⁴⁺/Cr_o⁴⁺ 1500^oC (O2) 113.1% 57.9% 39.4% 2.8% 7.1%

1500^oC (N₂) 113.1% 85.2% 13.3% 1.2% 8.8%

Table 4.7 The relation between Ca, Cr ,

_o⁴⁺

Cr , and

_t⁴⁺

V

O^•2- ions.

Annealing treatment

+ 4

Cro /Ca Cr_t⁴⁺/Ca ^•2

-VO /Ca

1500^oC (O2) 34.8% 2.5% 62.7%

1500^oC (N₂) 11.8% 1.1% 87.1%

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