Rapid preparation process, structure and thermal stability of lanthanum doped alumina aerogels with a high specific surface area

Abstract

In this study, we developed a new and rapid preparation method of alumina aerogels based on the sol–gel method and supercritical drying technique. In the developed method, the prepared wet gels were sealed to directly conduct supercritical drying without aging or solvent exchange which is required in conventional supercritical drying and ambient pressure drying techniques, which greatly shortened the preparation process. Lanthanum doped alumina aerogels with a high specific surface area (SSA) were prepared firstly according to the developed method and then its structure and thermal stability at high temperature were investigated. Investigation results proved that the incorporation of lanthanum resulted in the formation of LaAl₁₁O₁₈ on the surface of alumina particles, which distinctly delayed the transformation of the α-phase, and improved the thermal stability. Under the optimum atomic ratio of La/Al = 0.05, lanthanum doped alumina aerogels possessed high SSA and pore volume, while excessive lanthanum would result in the decreases in SSA and pore volume. Lanthanum doped alumina aerogels still had a high SSA and pore volume at 1000 °C and was not transformed into α-phase until the temperature rose to 1300 °C, exhibiting the excellent thermal stability. However, its SSA drastically decreased due to the collapse of pore structure.

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1. Introduction

Aerogels are solid nanoporous materials with a three-dimensional network structure, and possess a number of unique physical properties: such as high SSA, high porosity, and low density and the materials have been applied in high-energy particle detectors, thermal and acoustic insulations, catalysts, catalyst supports, and so on.^1–7 In some applications, aerogels are operated at a temperature above 1000 °C, such as an electrically heated catalyst system of an automobile exhaust gas converter, novel thermoelectric generators, and diesel engines.^8–10 An alumina aerogel has relatively enhanced thermal stability compared with silicon and other aerogels, which is more applicable to high temperatures, but it will experience severe shrinkage and lose its extraordinary properties at 1000 °C.^11,12 Therefore, it is necessary to improve the thermal stability of alumina aerogels and increase the SSA at high temperature.

In many studies, the incorporation of many cations, such as La³⁺, Ba²⁺, Zr⁴⁺, Ce⁴⁺, and Si⁴⁺, showed positive inhibition effects on the sintering and the α-phase transformation.^8,13–20 Among these cations, La³⁺ appears to be one of the most effective and widely used ions.^8,20,21 In addition, controlling alumina particle morphology and reducing the bulk density can improve thermal stability of alumina aerogels and increase SSA.^22,23 Herein, previous studies, solution–sol–gel method and ambient pressure drying technique were adopted to obtain homogeneous particle size and low density, but SSA and thermal stability should be improved further. X. Chen et al. used γ-Al₂O₃ and lanthanum nitrate to prepared lanthanum modified alumina powder which has a SSA of 98.5 m² g⁻¹ at 1000 °C.⁸ N. Al-Yassir et al. prepared yttrium oxide doped alumina aerogels with the sol–gel method and supercritical drying technique, and the SSA of alumina aerogels reached 145 m² g⁻¹ at 1000 °C and 11 m² g⁻¹ at 1200 °C.²³ However, thermal stability and SSA should be further improved. Moreover, synthesis period was too long in previous studied. Q. Sun et al. synthesized lanthanum doped ordered mesoporous alumina with high SSA according the sol–gel method and ambient pressure drying, but the synthesis period was longer than 53 h.²⁴

To solve these problems, we made the two efforts to improve thermal stability of alumina and increase the SSA in the study. Firstly, inspired by the preparation method of Bono et al.,^25,26 we developed a new and rapid method based on the sol–gel method and supercritical drying technique. In the developed method, the prepared wet gels were sealed and to directly conduct supercritical drying without aging or solvent exchange which was required in conventional supercritical drying and ambient pressure drying techniques, thus shortening the preparation period and reducing solvent consumption. Secondly, we prepared lanthanum doped alumina aerogels by incorporating La³⁺ and further improved the SSA and thermal stability of alumina. Moreover, we explored the pore structure, phase transformation and morphology at the high temperature and discussed the improvement mechanism of the thermal stability and SSA.

2. Experimental

2.1. Synthesis

The used precursors were aluminum tri-sec-butoxide (ASB, Strem chemicals, 98%) and lanthanum(III) chloride hydrate (Sinopharm chemicals, AR). The aerogel samples were prepared as follows. Firstly, lanthanum chloride hydrate was dissolved in the solution of absolute ethanol and deionized water. Subsequently, the adopted preparation process of sol and gel were the same to the method by J. F. Poco.²⁷ It should be noted that a hydrothermal synthesis reactor was used to replace J. F. Poco's mold to seal gels, thus the solvent in the wet gels could not leak at room temperature. However, the solvent could leak through the gap between the lid and bottom of the hydrothermal synthesis reactor when the solvent was in the supercritical state.

Then the hydrothermal synthesis reactor was placed in an autoclave and to conduct the supercritical drying process with ethanol as the solvent medium. To attain the critical conditions of ethanol solvent as early as possible, we added N₂ into the autoclave by increasing the pressure according to the Clausius–Clapeyron equation (eqn (1)).²⁸

here, P₁ and P₂ are the pressures at two temperatures T₁ and T₂, ΔH_vap is the enthalpy of vaporization, and R is the gas constant (8.3145 J (mol K)⁻¹). According to the Clausius–Clapeyron equation, the boiling point will increase to 218 °C for ethanol at 4 MPa (enthalpy of vaporization is 38.95 kJ mol⁻¹). Hence, the heat of vaporization of ethanol indicates that ethanol is very close to its critical point (T_c = 243 °C and P_c = 6.3 MPa). In addition, a small amount of absolute ethanol (about 200 mL in a 2 L autoclave) was added before increasing temperature.

Finally, the autoclave was rapidly heated to 260 °C according to the rate of 6 °C min⁻¹. When the pressure reached 12 MPa, ethanol in the supercritical state was removed. After cooling, alumina aerogel samples were obtained. The schematic representation of preparation process of lanthanum doped alumina aerogels is shown in Fig. 1, wherein Fig. 1a shows the process flow chart, Fig. 1b exhibits the difference between the proposed and conventional supercritical drying process. In the conventional route, the solvent in the wet gels is firstly converted from liquid into gas and then into the supercritical state, while the gas phase exerts the capillary pressure, which affects the structure and surface area of alumina aerogels. In the proposed method, the solvent can be converted from liquid directly into the supercritical state without conversion into the gas phase and then removed. Thus, the influence of the capillary pressure can be neglected. Fig. 1c exhibits that the pressure–temperature curves in the rapid supercritical drying process of the proposed method. The rapid supercritical drying period excluding final cooling time was in 3 h.

Fig. 1 Schematic representation of synthesis process of lanthanum doped alumina aerogels, (a) the process flow chart, (b) the phase diagram of ethanol in supercritical drying process, (c) the pressure–temperature curve with respect to time in supercritical drying process.

Four alumina aerogel samples doped with lanthanum were respectively prepared according to different atomic ratios of (La/Al = 0.025; 0.05; 0.1; 0.2). Lanthanum concentrations are listed in Table 1. In the paper, alumina aerogels doped with different lanthanum contents was noted as xLa/Al, where x was the atomic ratio of La/Al. For comparison, undoped sample was prepared according to the same method except that no dopant was added. Before characterization, different samples were respectively heated from room temperature to 1000 °C, 1200 °C, and 1300 °C within 210 min, 300 min, and 400 min and then maintained for 60 min in air.

Table 1 SSA and pore width of different samples

Loading amounts			200 °C		1000 °C		1200 °C		Resi. Cl^f (wt%)
La/Al^a	wt^b%	Conc.^c	SSA^d	PW^e	SSA^d	PW^e	SSA^d	PW^e
a La/Al atomic ratio.b La concentrations estimated with wt%.c La concentrations (atom per nm^2) estimated with the surface area of samples calcined at 200 °C.d BET surface area (m^2 g^−1).e BJH adsorption average pore width (nm).f Residual amount of Cl estimated according to XRF spectrum of non-heat-treat samples. In addition, the residual amount of Cl is zero for all lanthanum doped samples after calcined at 1000 °C and 1200 °C, according to XRF spectrum.
0			757	9.5	162	17.1	67	28.2
0.025	6.3	0.35	774	9.8	170	19.6	63	28.1	0.724
0.05	11.7	0.63	801	12.4	202	17.0	58	23.5	2.019
0.1	20.6	1.63	549	13.4	136	18.3	39	27.9	0
0.2	33.2	3.28	436	11.8	97	22.7	27	47.9	0

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2.2. Characterization

Nitrogen sorption experiments were conducted at 77.3 K using Micrometrics ASAP 2020 HD88. Before sorption measurements, the samples were degassed at 200 °C for 5 h in a vacuum. The SSA was calculated according to Brunauer–Emmett–Teller (BET) method and the pore size distributions were obtained with the adsorption data according to Barrett–Joyner–Halenda (BJH) method. All the samples were degassed at 200 °C for 2 h in a vacuum before test.

X-ray diffraction (XRD) was carried out on a D8 Advance X-ray diffractometer using Cu/Kα-1 radiation with a Ni filter. The power patterns were recorded in the step scanning mode from 15° to 80° (2θ) at a scanning speed of 1° min⁻¹ with a step size of 0.02°. Fourier transform infrared (FTIR) spectra were obtained on KBr pellets with a Nexus 870 infrared spectrometer. Structural and morphological investigations of various samples were analyzed by an ultra-high resolution scanning electron microscope (HRSEM, SU8020) and transmission electron microscope (TEM, Tecnai-G2-F30). The ingredients of the samples were analyzed by energy dispersive X-ray spectroscopy (EDS) and X-ray fluorescence (XRF) spectroscopy with a Magix PW2403 analyzer (PANalytical, Inc.).

3. Results

3.1. SSA and pore volume

Fig. 2a–c shows the cumulative pore volume of undoped and doped samples. Before heat treatment, all samples exhibit high pore volume. The cumulative pore volumes of 0.025 and 0.05La/Al samples are respectively 8% and 38% higher than that of undoped samples; the cumulative pore volumes of 0.1 and 0.2La/Al are 3% and 32% lower than that of undoped samples. The pore volume of undoped sample sharply increases when the pore width gradually decreases from ca. 30 nm. For lanthanum doped samples, observable pores appear when pore width reaches ca. 140 nm; when pore width decreases, the pore volume gradually increases and the sample shows the wider pore size distribution (Fig. 2a). The mean value of pore size for undoped samples is 9.5 nm, and the mean value of pore size for lanthanum doped samples was in the range of 9.8–13.4 nm (Table 1).

Fig. 2 Cumulative pore volumes of different samples (a) non-heat-treated, (b) heat-treated at 1000 °C, and (c) heat-treated at 1200 °C, and (d) SSAs of various samples with different lanthanum contents at different temperatures.

The pore volume of all samples decreased after calcination at 1000 °C. The pore volume of undoped sample shapely decreased to 0.73 cm³ g⁻¹ and the mean pore size of increased to 17.1 nm; the pore volumes of 0.025, 0.05, 0.1, and 0.2La/Al samples were 0.93, 0.89, 0.69, and 0.57 cm³ g⁻¹, respectively. The results indicated that heating treatment obviously reduced the pore volume. The pore sizes of all samples showed the broader distribution (Fig. 2b) and the mean value of pore size was in the range of 17.0–22.7 nm (Table 1). After calcination at 1200 °C, the pore volume of undoped sample sharply decreased to 0.47 cm³ g⁻¹, the pore volume of lanthanum doped samples decreased to 0.36–0.21 cm³ g⁻¹; the mean value was in the range of 23.5–47.9 nm (see Fig. 2c and Table 1).

Fig. 2d shows the SSAs of undoped and doped samples at different heat-treated temperatures. The SSAs of undoped sample and 0.05La/Al are respectively 757 m² g⁻¹ and 801 m² g⁻¹. After heat treatment at 1000 °C, the SSAs of undoped sample and 0.05La/Al are respectively 162 m² g⁻¹ and 202 m² g⁻¹. The SSAs of the two samples are larger than previously reported values,^8,20,23,29 indicating that the effect of supercritical drying technique on improving SSA is significant. In addition, the incorporation of lanthanum has a positive effect on the SSA when the content of lanthanum is low (0.025 and 0.05La/Al), but the high content of lanthanum (0.1 and 0.2La/Al) will result in the gradual decrease in the SSA. After heat treatment at 1200 °C, when the content of lanthanum is no higher than 0.05La/Al, the effect of lanthanum loading on the SSA is decreased, but the SSA is greatly decreased. Therefore, the optimum content of lanthanum should be selected and 0.05La/Al allows the largest SSA.

3.2. XRD observation

Fig. 3a shows the XRD patterns of undoped and doped samples before heat treatment. The XRD patterns of undoped and 0.05La/Al samples show the same characteristic peaks of pseudoboehmite (AlOOH).³⁰ However, the XRD patterns of 0.1La/Al and 0.2La/Al (curves 3 and 4) have no obvious diffraction peak, indicating the amorphous structure, which was possibly resulted from excessive lanthanum covering the surface of alumina particles and inhibiting nanoparticle crystals.

Fig. 3 X-ray diffraction patterns of samples with different lanthanum contents at different heat treatment temperatures ((a) non-heat-treated, (b) heat-treated at 1000 °C, (c) heat-treated at 1200 °C, (d) heat-treated at 1300 °C, (♦) AlOOH, (∇) γ-Al₂O₃, (⊗) θ-Al₂O₃, (⋄) α-Al₂O₃, (♥) LaAl₁₁O₁₈).

As shown in Fig. 3b, when the temperature rises to 1000 °C, the XRD patterns of all samples exhibit the characteristic peaks of γ-phase alumina (PDF no. 10-425), which indicate that all samples undergo phase transition from pseudoboehmite (AlOOH) to γ-Al₂O₃ in the stage. All the diffraction peaks show the high degree of broadness due to the formation of nanocrystals.

After calcination at 1200 °C, the XRD pattern of undoped sample (Fig. 3c) shows the sharper diffraction peaks indicating an enhanced crystallinity and the main crystalline phase is θ-phase (PDF no. 86-1410), followed by a small amount of α-phase (PDF no. 10-173). The XRD patterns of 0.025, 0.05 and 0.1La/Al samples show the presence of θ-Al₂O₃, but the difference is that a small amount of hexagonal LaAl₁₁O₁₈ (PDF no. 33-0699) is detected. Moreover, with the increase in lanthanum content, the indications of LaAl₁₁O₁₈ become more clear and stronger. The result is good agreement with the previous result by Alphonse et al.²⁰ The previously reported formations of LaAlO₃ and La₂O₃ were not found in the study.^8,31 The difference might be related to the content of lanthanum. Besides, α-phase was not observed in the patterns of lanthanum doped samples, thus proving the improvement of the thermal stability.

When the calcining temperature is further increased to 1300 °C, the content of α-phase obviously increases in the pattern of undoped samples (Fig. 3d). However, in the patterns of lanthanum doped samples, except α-Al₂O₃, only θ-Al₂O₃ and LaAl₁₁O₁₈, are observed, indicating that the formation of LaAl₁₁O₁₈ restrains the transformation of α-phase.

3.3. FTIR analysis

Fourier-transmittance infrared (FTIR) spectroscopy was employed to identify the functional groups of undoped and lanthanum doped samples at different temperatures. Undoped and 0.05La/Al samples were selected to perform FTIR analysis. In the spectra of undoped samples (see Fig. 4a), the main absorption peaks are at 3440 cm⁻¹ (–OH stretching), 1637 cm⁻¹ (H–O–H bending vibration), 2973 cm⁻¹, 2925 cm⁻¹ (C–H stretching of CH₃ groups), 1076 cm⁻¹ (symmetric AlO–H deformation in the pseudoboehmite structure), 1116 cm⁻¹ (asymmetric AlO–H deformation in the pseudoboehmite structure),^32–34 891 cm⁻¹, 786 cm⁻¹ (deformation of interlayer AlO–H groups in pseudoboehmite), 632 cm⁻¹ (octahedral Al–O stretching), and 499 cm⁻¹ (skeletal modes of Al–O layers).^35,36 These peaks indicate a structural characteristic of pseudoboehmite in undoped sample. For comparison, the spectrum of 0.05La/Al (Fig. 3b, non-heat-treated) is observed. The corresponding characteristic peaks are nearly accordance with those of undoped sample, but the relative peak intensity decreases due to lanthanum doping.

Fig. 4 The FTIR spectra of undoped samples and 0.05La/Al at different temperatures ((a) undoped samples, (b) 0.05La/Al).

After the samples are calcined at 1000 °C, some significant changes appear at 1200–400 cm⁻¹ in the spectrum of undoped samples. The characteristic peaks of pseudoboehmite disappear and a broad band emerges at 1000–400 cm⁻¹ and can be attributed to the characteristic absorption band of nano-sized Al₂O₃.³⁷ The curve of undoped samples possesses a dual-peak (570 cm⁻¹ and 822 cm⁻¹) in the range from 400 cm⁻¹ to 1000 cm⁻¹, which is characteristic peak of nano-γ-Al₂O₃, and the curve of 0.05La/Al also exhibits the same characteristic peak of nano-γ-Al₂O₃. The most prominent change after calcination at 1200 °C is the characteristic peak of θ-Al₂O₃, a triple-peak (856 cm⁻¹, 754 cm⁻¹, and 576 cm⁻¹) in 1000–400 cm⁻¹ for undoped sample, which proves that γ-Al₂O₃ has been converted into θ-Al₂O₃ in this stage. The curve of 0.05La/Al shows a weak triple-peak, which may be related to the low content of θ-Al₂O₃. The results prove that lanthanum retards the phase transition from γ-Al₂O₃ to θ-Al₂O₃. When the temperature increases to 1300 °C, the curve of undoped sample shows the peak at 456 cm⁻¹, which is assigned to α-Al₂O₃. Moreover, the characteristic peaks of θ-Al₂O₃ are not obvious, indicating that the undoped sample is in the transition state between θ-Al₂O₃ and α-Al₂O₃. However, in the spectra of 0.05La/Al, a triple-peak is observed in 1000–400 cm⁻¹, indicating the presence of θ-Al₂O₃. The analysis results of FTIR are basically consistent with the results of XRD.

3.4. SEM and TEM observations

We observed the morphology of undoped and 0.05La/Al samples by SEM and TEM. The significant microstructural difference was found. Though both undoped and 0.05La/Al samples had the interpenetrating and homogeneous structures which were composed of nanoparticles (Fig. 5a and b), the undoped sample exhibited fibriform nanoparticles and 0.05La/Al samples showed flake nanoparticles with the bigger size because lanthanum iron attached to the alumina surface. In the TEM images, we also observed vimineous fibriform nanoparticles with the size of about 20 nm (Fig. 6a) and column nanoparticles with the size of about 40 nm (Fig. 6b). The much weaker diffraction patterns (insets in Fig. 6a and b) suggest a lower degree of crystallinity. Therefore, the addition of lanthanum significantly changes the morphology.

Fig. 5 SEM images of undoped and 0.05La/Al samples at different heat-treated temperatures ((a) undoped; (b) 0.05La/Al; (c) undoped, 1000 °C; (d) 0.05La/Al, 1000 °C; (e) undoped, 1200 °C; (f) 0.05La/Al, 1200 °C; (g) undoped, 1300 °C; (h) 0.05La/Al, 1300 °C).

Fig. 6 TEM images and diffraction patterns (inset) of undoped and 0.05La/Al samples ((a) undoped; (b) 0.05La/Al; (c) undoped, 1200 °C; (d) 0.05La/Al, 1200 °C; (e) undoped, 1300 °C; (f) 0.05La/Al, 1300 °C).

After heat treatment at 1000 °C, undoped sample and 0.05La/Al sample underwent the phase transition of γ-Al₂O₃ (as revealed by XRD and FTIR analysis). For undoped sample, the initial structure was preserved and the shape of nanoparticles was transformed into strip objects; the nanoparticle length increased to 30–50 nm; the nanoparticle width increased to about 10 nm (Fig. 5c). For 0.05La/Al, initial shape and structure are preserved, but the flake size became bigger after heat treatment (Fig. 5d).

When the temperature increased to 1200 °C, nanoparticles formed many columns with the diameter of about 10 nm and the length of about 30–60 nm and the initial network structure collapses due to reunion and sintering of alumina particles (Fig. 5e). However, the morphology of 0.05La/Al showed the obvious evidence of nanoparticle sintering and reunion, and still presented the partial network skeleton structure though severe collapse of network structure (Fig. 5f). The image of TEM showed that the column nanoparticle size of 0.05La/Al was smaller than that of undoped sample (Fig. 6c and d). The diffraction pattern indicated that undoped sample and 0.05La/Al samples were polycrystalline, but the crystalline degree of 0.05La/Al was poor (insets in Fig. 6c and d).

When the temperature further increased to 1300 °C, excess sintering and reunion enlarges nanoparticles and changes the nanoparticle shape into the round shape (Fig. 5g and h). The larger nanoparticles with the size of about 200 nm were observed in Fig. 6e, whereas the nanoparticle size of 0.05La/Al samples was 100 nm. The results indicated that the incorporation of lanthanum restrained excess growth of nanoparticles (Fig. 6f). The diffraction pattern revealed the presence of polycrystalline in undoped and 0.05La/Al samples as well as the high crystalline degree (insets in Fig. 6e and f).

3.5. Effect and residual amount of Cl

X-ray fluorescence (XRF) spectrum analysis was performed with a Magix PW2403 (PANalytical, Inc.) to determine the residual amount of Cl in lanthanum doped samples (Table 1). The 0.025 and 0.05La/Al samples still contained a small amount of residual Cl, because all LaCl₃ was not transformed into La₂O₃, and the amount of the reactive lanthanum was less than the nominal value. However, 0.1 and 0.2La/Al samples and all lanthanum doped samples calcined at 1000 °C and 1200 °C, the residual amount of Cl was zero, indicating all LaCl₃ had been transformed to La₂O₃. Therefore, the residual Cl only affects initial SSAs of samples.

4. Discussion

According to the analysis results of XRD and FTIR, undoped alumina aerogels could undergo continuous phase transition from pseudoboehmite to γ-phase, θ-phase, and α-phase with the temperature increase, while lanthanum doped alumina aerogels retarded the transformation of γ-phase, θ-phase and α-phase. The most remarkable evidence was that α-phase did not exist at 1300 °C. The evidence indicated that the thermal stability of alumina aerogels was obviously improved.

In previous studies,^20,29,38 alumina had some defects. Sintering started at surface defects corresponding to the most reactive sites where aluminium atom could not be fully coordinated, i.e., edges and corners of alumina particles. After doping lanthanum, lanthanum ions could be anchored on these reactive sites due to the role of strong Lewis acid sites, and prevent aluminum ions from entering these sites, thus retarding the sintering process and inhibiting phase transformation of γ-phase, θ-phase and α-phase.

After all nucleation sites were blocked, excessive lanthanum would result in the formation of LaAlO_x compounds, retard different phase transformations, and reduce the surface area of alumina aerogels. Under the low content of lanthanum, the La–Al mixed oxides on the reactive sites were in the two-dimensional dispersion phase and played a significant retarding role in the reduction of the surface area and the transformations of different alumina phases. Therefore, in order to improve surface area and thermal stability of alumina aerogels, an optimum content of lanthanum should be explored. Too low or excessive lanthanum is not conducive to the improvement of alumina aerogels.

Since the radius of lanthanum ions (ca. 1.032 nm) is larger than that of aluminum ions (ca. 0.051 nm), it is quite difficult for lanthanum to enter the interstice site of alumina lattice.^23,39 Stacey J. et al. mentioned that if La occupied octahedral Al sites inside alumina lattice, then the larger atomic radius of La resulted in the significant distortions which could be detected, and concluded that it was impossible for La to substitute Al inside the alumina lattice.²¹ Therefore, La–Al mixed oxides can be formed on the surface of alumina particles. Only in this way, it can reduce the contact among the alumina particles, retard sintering and phase transformation, and show a positive effect on increasing the SSA and improving the thermal stability. Otherwise, it has a negative effect.

The composition of the La–Al mixed oxides mainly depends on the content of lanthanum. At the high temperature above 1200 °C, when the content of lanthanum is low, it may be LaAl₁₁O₁₈, when the content of lanthanum is high, it may be LaAlO₃ or La₂O₃.⁸ However, the La–Al mixed oxides was not detected by XRD below 1200 °C because of the limitation of XRD technique.²³ Therefore, we investigated the elemental composition of undoped and 0.05La/Al samples by the EDS analysis and proved the presence of lanthanum in 0.05La/Al samples (Fig. 7a and b). In addition, LaAlO₃ and LaAl₁₁O₁₈ cannot be formed below 1200 °C.⁴⁰ At the temperature below 1200 °C, the La–Al mixed oxides as the two-dimensional dispersion phase were presented on the surface of alumina particles and resisted the sintering of alumina particles. In the study, LaAl₁₁O₁₈ was detected at 1200 °C and 1300 °C by XRD.

Fig. 7 EDS analysis of undoped and 0.05La/Al samples ((a) undoped samples, (b) 0.05La/Al).

5. Conclusions

We developed a new preparation method of alumina aerogels based on the sol–gel method and rapid supercritical drying technique. The developed method had several advantages over other synthesis methods. Firstly, it was faster than conventional method based on the ambient pressure drying technique. It costed only 5 h (containing preparation of the gels and supercritical drying process), other than dozens of hours. Secondly, it reduced the solvent consumption because solvent exchange and aging of wet gels required in conventional supercritical drying technique were avoided.

According to the proposed method, lanthanum doped alumina aerogels with high SSA were prepared. The positive effect of the incorporation of lanthanum on thermal stability and SSA could be attributed to the formation of LaAl₁₁O₁₈, which covered the surface of alumina particles and retarded the sintering and the transformation of α-phase of alumina aerogels. When La/Al atomic ratio was 0.05, lanthanum doped alumina aerogels possessed the highest SSA and the best thermal stability.

Acknowledgements

The authors would like to acknowledge the financial support from the National Basic Research Program of China (973 Program, Grant No. 2015CB057502).

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