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

Authors: Fuliang Zhu; Yanshuang Meng
Journal: Advances in Materials Science and Engineering
 (2013)

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## 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.


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## Body

## 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 ofpH=2 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 Figure4. 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:
(1)ln(ΦTp2)=lnA-ln(EaR)-EaRTp,
where Φ denotes heating rate (K/min), Tp is peak temperature (K), R is ideal gas constant [8.314 J/(mol·K)], Ea is phase transition activation energy (kJ/mol), and A is preexponential factor (min−).DSC curves of samples at different heating ratesΦ were carried out. Peak temperature Tp (K) was acquired. The relation between ln(Φ/Tp2) and 1/Tp was plotted. Putting the slope and intercept of the straight line into (1), reaction activation energy Ea and the nucleation rate v can be solved. DSC curves of Sp and Sp-E samples at different heating rates (Φ=5 K/min, 10 K/min, 15 K/min, 20 K/min) were measured. According to (1), phase transition activation energy Ea and preexponential factor A of γ-Al2O3 → θ-Al2O3 and θ-Al2O3 → α-Al2O3 phase transition can be calculated.DSC curves ofθ-Al2O3 → α-Al2O3 phase transition process of pseudoboehmite xerogel Sp samples were shown in Figure 6. Peak temperatures Tp (K) were listed in Table 1. When -ln(Φ/Tp2) was plotted against 1/Tp as shown in Figure 7. Fitting experimental data in Figure 7 to a straight line, -ln(Φ/Tp2)=-53.3153+98874.8(1/Tp) was gotten. The linear coefficient of r=0.9998 was included in Table 1. According to Ea/R=98874.8 and ln(Ea/R)-lnA=-53.3153, phase transition activation energy Ea and pre-exponential factor A of θ-Al2O3 → α-Al2O3 were calculated for Ea=822.0 kJ/mol and A=1.14×1028, respectively. The consequences were also listed in Table 1.Table 1
Kinetics parameters of theθ-Al2O3 to α-Al2O3 phase transition.
Sample
Slope,Ea/R
Intercept,ln(Ea/R)-lnA
Linear coefficient,r
E
a, kJ/mol
A, 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 × 1036Figure 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 ofθ-Al2O3 → α-Al2O3 phase transition of Eu3+ doped pseudoboehmite xerogel Sp-E samples were described in Figure 8. The relations between -ln(Φ/Tp2) and 1/Tp were plotted as showed in Figure 9. The data in Figure 9 have done a linear fitting, and -ln(Φ/Tp2)=-72.3094+127874.8(1/Tp) equation was obtained. Linear coefficient r was 0.9990. Phase transition activation energy Ea and pre-exponential factor A of θ-Al2O3 → α-Al2O3 were calculated by equations Ea/R=127874.8 and ln(Ea/R)-lnA=-72.3094 for Ea=1063.1 kJ/mol and A=3.24×1036. 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 ofθ-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) experiencedγ-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 ofθ-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.



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