Advances in Materials Science and Engineering

Volume 2013 (2013), Article ID 740625, 6 pages

http://dx.doi.org/10.1155/2013/740625

Research Article

Influence of Eu³⁺-Doped on Phase Transition Kinetics of Pseudoboehmite

Fuliang Zhu and Yanshuang Meng

School of Material Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China

Received 4 May 2013; Revised 17 August 2013; Accepted 1 September 2013

Academic Editor: Dachamir Hotza

Copyright © 2013 Fuliang Zhu and Yanshuang Meng. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The influence of Eu³⁺-doped on phase transition kinetics of pseudoboehmite has not been reported in the literature. Through dropping Eu(NO₃)₃ into pseudoboehmite colloidal solution, pseudoboehmite xerogel was produced using spray pyrolysis. The influence of Eu³⁺-doped on the mechanism of pseudoboehmite phase transition kinetics has been calculated and analyzed by TG/DSC, XRD, and Kissinger equation. Part of Eu³⁺ ion formed compound EuAl₁₂O₁₉, which existed between α-Al₂O₃ grains. Bulk diffusion of Al³⁺ was prevented from compound EuAl₁₂O₁₉. Therefore, phase transition kinetics rate of θ-Al₂O₃ → α-Al₂O₃ was slowed down, causing an increase of phase transition activation energy and elevation of phase transition temperature.

1. Introduction

Pseudoboehmite (γ-AlOOH) is a crystal imperfection boehmite. Pseudoboehmite experienced a variety of intermediate phases in its phase change process, and ultimately formed a stable α-Al₂O₃ [1–4]. Phase transitions of Alumina were studied by many researchers [5–16], and the results were shown in Figure 1. When pseudoboehmite was sintered in the range of 673 K to 973 K, product γ-Al₂O₃ was widely applied as a catalyst, catalyst carrier, and adsorbent et al. Nano α-Al₂O₃, which is produced by sintering pseudoboehmite at 1473 K, was proverbially utilized as a paint additive, top-grade ceramic, petrochemical efficient catalyst, submicron/nanoabrasive and polishing materials, cosmetic filling materials, and inorganic membrane materials. Investigation of pseudoboehmite phase transition kinetics on its appliance had a vital significance, therefore it has gotten wide attentions [17, 18].

Figure 1: Phase transition process of pseudoboehmite may have happened.

According to reports in the literature [19–21], phase transition temperatures of γ-Al₂O₃ → θ-Al₂O₃ and θ-Al₂O₃ → α-Al₂O₃ can be changed by adding the metal ion salts or metal oxide. Adjunctions of La₂O₃, B₂O₃, CaO, and Y₂O₃ and salts of Ba, Sr, and Ca can increase θ-Al₂O₃ → α-Al₂O₃ phase transition temperature. θ-Al₂O₃ → α-Al₂O₃ phase transition temperature can be decreased by adding a certain amount of CuO/Cu₂O, MgAl₂O₄, Fe₂O₃, V₂O₅, TiO₂, γ-Al₂O₃, and α-Al₂O₃.

The effect of Eu³⁺-doped on phase transition kinetics of the pseudoboehmite phase transition process has not been reported in the literature. Firstly, Eu(NO₃)₃ was dissolved in the pseudoboehmite colloidal solution, where Eu³⁺ ions are uniformly dispersed. And then, pseudoboehmite sol is dried to obtain the Eu³⁺ ion doped pseudoboehmite xerogel as a precursor. The effect of Eu³⁺ ion on phase, phase transition temperature, and phase transition kinetics in the pseudoboehmite phase change process was investigated.

2. Experimental

Pseudoboehmite powder was provided by the Aluminum Corporation of China Limited. All other reagents were analytical grade. Pseudoboehmite powder and redistilled water were mixed in a definite mass ratio, stirring to produce a suspension of solid content of 5%. The suspension was continuously stirred and concentration of 5 mol/L nitric acid solutions was added simultaneously. Pseudoboehmite colloid was obtained under the conditions of and stirring time = 3 h. Pseudoboehmite xerogel drying by SD-06 spray dryer was marked as Sp. Eu(NO₃)₃ with a molar ratio of Eu(NO₃)₃ : AlOOH = 0.02 : 1 was joined to pseudoboehmite sol. Eu(NO₃)₃ was fully dissolved in pseudoboehmite sol for stirring time of 1 h. Eu³⁺ doped pseudoboehmite xerogel (AlOOH : Eu xerosol) sample drying by spray dryer was denoted as Sp-E. Spay dying conditions were determined as follows: the colloid solution flow of 15 mL/min, the samples outlet temperature of 335 K, and hot air temperature of 423 K. Samples of Sp and Sp-E were calcined at different temperature in GSL1600X type tube furnace under air atmosphere with the heating rate of 10 K/min and holding time of 3 h.

XRD result of the sample was carried out on a D/max-2500/PC type XRD diffractometry (Rigaku, Japan). TG/DSC analysis of the sample was performed on a STA 449C type thermal analysis (Netcsch, Germany). Sample weighed about 15 mg into the platinum crucible under N₂ atmosphere with nitrogen flow of 15 mL/min, heating rate of 10 K/min, the detection temperature in the range of 313 K to 1773 K, and the temperature error of ±0.1 K.

3. Results and Discussions

Thermal decomposition curves of Eu³⁺ doped (Sp-E) and undoped (Sp) pseudoboehmite xerogel samples under optimum conditions from the ambient temperature to 1773 K in nitrogen atmosphere are shown in Figures 2 and 3, respectively. In Figure 2, there was an endothermic peak appeared at 395 K accompanied with 4% weight loss. The reason was that the pseudoboehmite contained part of the interlayer water. When temperature was 395 K, pseudoboehmite xerogel samples (Sp) lost interlayer water by changing of (AlOOH)·nH₂O (n = 0.080~0.602) into AlOOH. As can be seen from Figure 2, endothermic effect occurred from 493 K, endothermic peak was existed at 665 K, and endothermic effect was finished at 723 K. Combined with TG curve in Figure 2, 23% weight loss between 493~723 K was considered for decomposition of AlOOH·nH₂O, and phase transition of AlOOH → γ-Al₂O₃ was finished. Phase transition temperature of AlOOH → γ-Al₂O₃ was about 773 K. Two exothermic peaks at 1155 K and 1497 K were thought to be phase transition temperatures of γ-Al₂O₃ → θ-Al₂O₃ and θ-Al₂O₃ → α-Al₂O₃, respectively. Similar results were obtained in the literature [22]. The exothermic peak formed at 1669 K, which owing to the reduction of surface energy, was considered to be the crystal growth of α-Al₂O₃.

Figure 2: TG and DSC curves of Sp.

Figure 3: TG and DSC curves of Sp-E.

TGA curve of Eu³⁺-doped pseudoboehmite Sp-E specimen was shown in Figure 3. Endothermic peak occurred at 395 K was the removal of physically adsorbed water from the sample Sp-E. Endothermic peak at 663 K was the loss of crystal water in Sp-E sample. Eu³⁺ ion in the form of substitution doping replaced Al³⁺ ion into the lattices of γ-Al₂O₃ or θ-Al₂O₃. Substitution doping of Eu³⁺ ion was an endothermic reaction. There is no apparent phase transition endothermic peak of AlOOH → γ-Al₂O₃ in the temperature range of 773 K to 1173 K. Therefore, AlOOH → γ-Al₂O₃ phase transition temperature of Sp-E sample cannot be confirmed according to the DSC curve in Figure 3. Exothermic peaks appeared at 1327 K and 1510 K were correspondingly considered for the γ-Al₂O₃ → θ-Al₂O₃ and θ-Al₂O₃ → α-Al₂O₃  phase transition temperatures, respectively.

XRD results of Pseudoboehmite xerogel Sp samples sintered at different temperature were shown in Figure 4. Figure 4 shows the main phase was cubic structure of γ-Al₂O₃ (PDF#10-0425) at 873 K. γ-Al₂O₃ with a small amount of monoclinic structure of θ-Al₂O₃ (PDF#04-0877) was obtained after calcinations at 1173 K. θ-Al₂O₃ with a small amount of rhombohedral structure of α-Al₂O₃  (PDF#10-0173) was gained at 1373 K. After calcination at 1573 K, sample completely converted to α-Al₂O₃ phase. Simultaneously, Figure 4 further indicated that phase transition temperatures of γ-Al₂O₃ → θ-Al₂O₃ and θ-Al₂O₃ → α-Al₂O₃ were existed at 1155 K and 1497 K, respectively. Figure 4 also displayed that diffraction peaks of γ-Al₂O₃ and θ-Al₂O₃ significantly broadened and diffraction peak of α-Al₂O₃ sharpened, indicating that both γ-Al₂O₃ and θ-Al₂O₃ were nanograins with a lower crystallinity degree and α-Al₂O₃ has a higher crystallinity degree.

Figure 4: XRD patterns of Sp which were heated under different temperature.

XRD results of Eu³⁺-doped pseudoboehmite Sp-E samples were shown in Figure 5. It indicated that the main phase were cubic structure of γ-Al₂O₃ (PDF#10-0425) when Sp-E samples were sintered at 873 K and 1173 K. Therefore, γ-Al₂O₃ and θ-Al₂O₃ (PDF#04-0877) were obtained when Sp-E was calcined at 1173 K and 1373 K, respectively. It further explained that 1327 K in Figure 3 was γ-Al₂O₃ → θ-Al₂O₃ phase transition temperature. The principal phase was rhombohedral structure of α-Al₂O₃ (PDF#10-0173) with a small amount of EuAl₁₂O₁₉ compound. In the literature [10, 15, 20], formation of six aluminate (BaO·6Al₂O₃) by doping of BaO can prevent the Al³⁺ bulk diffusion and make the transition state structure of γ-Al₂O₃ more stable. Introductions of Ca and Sr can slow down the sintering rate of γ-Al₂O₃ and phase transformation kinetic rate α-Al₂O₃, improving the thermal stability of γ-Al₂O₃. This paper argues that phase transition temperature of θ-Al₂O₃ → α-Al₂O₃ was increased by the introduction of Eu³⁺ ion for two reasons. On one hand, ion diffusion was prevented from formation of EuAl₁₂O₁₉ compound existed between α-Al₂O₃ grains, resulting in a higher phase transition temperature of θ-Al₂O₃ → α-Al₂O₃. On the other hand, Eu³⁺ ions partly substitute Al³⁺ ion into the θ-Al₂O₃ lattice. This may hinder the ion migration rate in the process of θ-Al₂O₃ → α-Al₂O₃ phase transition, elevating the Phase-change resistance. Mohanty and Ram also got the similar results [23].

Figure 5: XRD patterns of Sp-E which were heated under different temperature.

Mechanism of Eu³⁺-doped on θ-Al₂O₃ → α-Al₂O₃ phase transition of pseudoboehmite was similar as the literature [10, 15, 20], where Ba²⁺, Ca²⁺, and Sr²⁺ ions can increase θ-Al₂O₃ → α-Al₂O₃ phase transition temperature of pseudoboehmite. Eu³⁺-doped can raise θ-Al₂O₃ → α-Al₂O₃ phase transition temperature of pseudoboehmite to 1510 K. γ-Al₂O₃ → θ-Al₂O₃ and θ-Al₂O₃ → α-Al₂O₃ phase transition temperatures of pseudoboehmite through Eu³⁺ doping was elevated by 172 K and 13 K, respectively.

Phase transition kinetics parameters can be calculated through many equations [16]. Owing to the simplicity and accuracy of Kissinger equation, it is widely used. Kissinger equation describes the following:

where denotes heating rate (K/min), is peak temperature (K), is ideal gas constant [8.314 J/(mol·K)], is phase transition activation energy (kJ/mol), and is preexponential factor (min⁻).

DSC curves of samples at different heating rates were carried out. Peak temperature (K) was acquired. The relation between and was plotted. Putting the slope and intercept of the straight line into (1), reaction activation energy and the nucleation rate can be solved. DSC curves of Sp and Sp-E samples at different heating rates ( K/min, 10 K/min, 15 K/min, 20 K/min) were measured. According to (1), phase transition activation energy and preexponential factor of γ-Al₂O₃ → θ-Al₂O₃ and θ-Al₂O₃ → α-Al₂O₃ phase transition can be calculated.

DSC curves of θ-Al₂O₃ → α-Al₂O₃ phase transition process of pseudoboehmite xerogel Sp samples were shown in Figure 6. Peak temperatures (K) were listed in Table 1. When was plotted against as shown in Figure 7. Fitting experimental data in Figure 7 to a straight line, was gotten. The linear coefficient of was included in Table 1. According to and , phase transition activation energy and pre-exponential factor of θ-Al₂O₃ → α-Al₂O₃ were calculated for  kJ/mol and , respectively. The consequences were also listed in Table 1.

Table 1: Kinetics parameters of the θ-Al₂O₃ to α-Al₂O₃ phase transition.

Sample
Slope,
Intercept,
Linear coefficient,
, kJ/mol
, min⁻

Sp
98874.8
−53.3153
0.9998
822.0
1.14 × 10²⁸

Sp-E
127874.8
−72.3094
0.9990
1063.1
3.24 × 10³⁶

Figure 6: DSC curves for the pseudoboehmite xerogel Sp samples at different heating rates.

Figure 7: Kissinger plot for pseudoboehmite xerogel Sp samples at heating rates of 5, 10, 15, and 20 K/min.

DSC curves of θ-Al₂O₃ → -Al₂O₃ phase transition of Eu³⁺ doped pseudoboehmite xerogel Sp-E samples were described in Figure 8. The relations between and were plotted as showed in Figure 9. The data in Figure 9 have done a linear fitting, and equation was obtained. Linear coefficient was 0.9990. Phase transition activation energy and pre-exponential factor of θ-Al₂O₃ → α-Al₂O₃ were calculated by equations and for  kJ/mol and . All results were listed in Table 1.

Figure 8: DSC curves for the Eu³⁺ doped Pseudoboehmite xerogel Sp-E samples at different heating rates.

Figure 9: Kissinger plot for Eu³⁺ doped pseudoboehmite xerogel Sp samples at heating rates of 5, 10, 15, and 20 K/min.

Using pseudoboehmite xerogel as a precursor in this paper, phase transition activation energy of θ-Al₂O₃ → α-Al₂O₃ is 822.0 kJ/mol, which is higher than those of reports in the literature (557–850 kJ/mol) using γ-Al₂O₃ as a precursor and (522 kJ/mol) using boehmite as a precursor. It indicated that the precursor type had an apparent effect on phase transition activation energy of θ-Al₂O₃ → α-Al₂O₃. θ-Al₂O₃ → α-Al₂O₃ phase transition activation energy of Eu³⁺-doped pseudoboehmite xerogel was 1063.1 kJ/mol, which increased 241.1 kJ/mol than that of Eu³⁺ undoped pseudoboehmite xerogel (822.0 kJ/mol). Phase transition kinetics rate of θ-Al₂O₃ → α-Al₂O₃ was slowed down, resulting in an increase of phase transition activation energy and elevation of phase transition temperature.

4. Conclusions

Utilizing pseudoboehmite sol as a precursor, Eu³⁺ ion in the form of Eu(NO₃)₃ was dropped into sol. Eu³⁺-doped (Sp-E) and undoped (Sp) pseudoboehmite xerogel were prepared by spray drying method. Impact of Eu³⁺-doped on pseudoboehmite phase transition kinetics and microstructure has been investigated utilizing TG-DSC and XRD. Kissinger equation was proposed to calculate phase transition kinetics parameters of Sp-E and Sp samples. The main conclusions are as follows.

(1)Pseudoboehmite xerogel (Sp) experienced γ-Al₂O₃ → θ-Al₂O₃ and θ-Al₂O₃ → α-Al₂O₃ phase transition at 1155 K and 1497 K, respectively.(2)After doping with Eu³⁺ ion, phase temperatures of γ-Al₂O₃ → θ-Al₂O₃ and θ-Al₂O₃ → α-Al₂O₃ of Sp-E sample were 1327 K and 1510 K, respectively. Comparing with the results of Sp samples, phase transition temperatures of γ-Al₂O₃ → θ-Al₂O₃ and θ-Al₂O₃ → α-Al₂O₃ were increased by 172 K and 13 K, respectively.(3)Phase transition activation energy of θ-Al₂O₃ → α-Al₂O₃ for Sp-E and Sp samples was 1063.1 kJ/mol and 822.0 kJ/mol, respectively. Phase transition activation energy of θ-Al₂O₃ → α-Al₂O₃ was added by 241.1 kJ/mol as Eu³⁺ was doped to pseudoboehmite. EuAl₁₂O₁₉ compound occurred between α-Al₂O₃ grains can hinder diffusion of Al³⁺ ion. Therefore, θ-Al₂O₃ → α-Al₂O₃ phase transition rate was reduced, resulting in an increase of θ-Al₂O₃ → α-Al₂O₃ phase transition activation energy and elevation of phase transition temperature.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant nos. 51364024, 50364002) and Gansu Province Department of Education Fund (Grant no. 2013A-029).

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