J. Cent. South Univ. Technol. (2007)03-0330-06

DOI: 10.1007/s11771-007-0065-5               

Effects of drying method on preparation of nanometer α-Al₂O₃

XIAO Jin(肖 劲), WAN Ye(万 烨), DENG Hua(邓 华), LI Jie(李 劼), LIU Ye-xiang(刘业翔)

(School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China)

Abstract: Ammonium aluminum carbonate hydroxide (AACH) precursor was synthesized by the precipitation reaction of aluminum sulfate and ammonium carbonate. Then the precursor was dealt with five drying methods including ordinary drying, alcohol exchange, vacuum freeze-drying, glycol distillation, n-butanol azeotropic distillation respectively and calcined at 1 200 ℃ for 2 h to get α-Al₂O₃. The effects of drying methods on preparation of nanometer α-Al₂O₃ were discussed, and the optimal drying method was confirmed. The structural properties of powders were characterized by XRD, SEM and BET measurements. The results show that vacuum freeze-drying, glycol distillation and n-butanol azeotropic distillation can prevent the powders from aggregating, and among them the n-butanol azeotropic distillation is the best method. The nanometer α-Al₂O₃ powder with non-aggregation can be manufactured using n-butanol azeotropic distillation and the average particle size is about 40 nm.

key words: nanometer α-Al₂O_3; drying method; aggregation

1 Introduction

The nanometer α-Al₂O₃ powder, a kind of important structural and functional ceramic material, has excellent properties such as high surface activity, high light absorption, good wear resistance, high temperature stability and good corrosion resistance. It has been widely used in the fields of light, catalysis, electric, medical device and information^[1-5]. In order to obtain good forming and sintering property, the nanometer α-Al₂O₃ powder with high purity, narrow grain size distribution and spherical in size is needed.

Nano-sized α-Al₂O₃ powder is usually prepared by means of sol-gel method^[6], polyacrylamide gel method^[7], hydrolysis method^[8], micro-emulsion method^[9], liquid-phase precipitation method^[10]. DU et al^[11] prepared nano-size α-Al₂O₃ powder by a solid-state method using (NH₄)₂SO₄·Al₂(SO₄)₃·24H₂O as raw material and soluble starch as dispersant medium. Considering the fact that the liquid-phase precipitation methods have advantages such as easy fabrication and controllability, low-cost, and the nanometer alumina powders have excellent properties such as narrow grain-size distribution and high purity, the liquid-phase precipitation method becomes the most promising method^[12]. But the liquid-phase precipitation by method also has a lot of disadvantages including the complex condition and the agglomeration. Drying processes have significant influences on the properties of the powder. The agglomeration of nanometer alumina particles will take place if the drying methods are not properly chosen，which will have the adverse effect on its operational properties. How to control good dispersion of nanometer alumina powder during drying processes is one of the most important technology for preparing nanometer alumina powder^[13]. The drying methods of nanometer alumina powder such as alcohol exchange^[14], n-butanol heterogeneous azeotropic distillation^[12], vacuum freeze-drying^[15], supercritical drying have been systematically studied. YANG et al^[16] prepared non-aggregation nanometer alumina powder by using high-pressure drying method.

In this paper, a series of investigations were made to compare several drying methods including ordinary drying, alcohol exchange, vacuum freeze-drying, glycol distillation, n-butanol azeotropic distillation, and the best drying method for preparing nanometer α-Al₂O₃ powder was determined.

2 Experimental

2.1 Experimental process

0.1 mol Al₂(SO₄)₃(AR) was dissolved in deionized water to prepare solution with concentration of 0.3 mol/L by adding suitable amounts of PEG600 as dispersant.  0.4 mol (NH₄)₂CO₃(AR) was dissolved in deionized water to prepare 2.0 mol/L solution by adding suitable amounts of PEG1000 as dispersant. The precursor was prepared by rapidly adding Al₂(SO₄)₃ solution to a acutely stirred (1 400 r/min) (NH₄)₂CO₃ solution in the 55 ℃ thermostatic bath. When the slurry pH was adjusted to 8-10, the precipitate was aged in the container with constant stirring intensively for 1 h without removing the solution, the slurry was filtered and the filter cake was washed three times by deionized water, then carried on the operation as follows.

1) The filter cake was put in 80 ℃ drying oven for 12 h ordinary drying.

2) The filter cake was washed two times with absolute ethanol, and then dried for 12 h in the 80 ℃ vacuum oven.

3) The filter cake was cooled rapidly using liquid nitrogen, followed by 12 h vacuum freeze-drying.

4) The filter cake was washed two times by absolute ethanol, followed by glycol distillation, and then dried for 12 h in the 80 ℃ vacuum oven.

5) The filter cake was washed two times by absolute ethanol, followed by n-butanol azeotropic distillation, and then dried for 12 h in the 80 ℃ vacuum oven.

The above five samples were all calcined at       1 200 ℃ for 2 h to obtain α-Al₂O₃ powders.

2.2 Sample characterization

The precursors and the calcined products were identified using a Rigaku X-ray powder diffractometer (XRD) with Cu K_α radiation, and the average grain size of α-Al₂O₃ (d_XRD) was calculated through FWHM. The particle shape and the dispersion of the powders were characterized by JSM-5600LV scanning electron microscope. The powders for SEM observation were scattered in acetone ultrasonically and then the suspended powders were mounted on a copper microgrit. The surface area of the alumina powders was characterized by ST-03 surface aperture cryoscopy, and the BET-equivalent primary particle diameter of α-Al₂O₃ (d_BET) was calculated through BET method.

3 Results and discussion

3.1 Material authentication

The X-ray diffraction patterns of five precursors and their calcined products are absolutely consentaneous. One of the X-ray diffraction patterns of precursors is shown in Fig.1, and one of the X-ray diffraction patterns of calcined products is shown in Fig.2.

Fig.1 shows that precursors have the structure of pure ammonium aluminum carbonate hydroxide (NH₄Al- (OH)₂CO₃). When the molar ratio of Al₂(SO₄)₃ to (NH₄)₂CO₃ is 1?4, we could predict the reaction in the reactor as follows:

Al₂(SO₄)₃+4(NH₄)₂CO₃+2H₂O=

2NH₄Al(OH)₂CO₃+3(NH₄)₂SO₄+2CO₂        (1)

Fig.1 XRD pattern of precursor

Fig.2 XRD pattern of calcined product(α-Al₂O₃)

   Fig.2 shows that the calcined products are pure α-Al₂O₃, which reveals that after calcining at 1 200 ℃ for 2 h, AACH is transformed into pure α-Al₂O₃.

3.2 Effect of drying method on dispersion of products

The SEM micrographs of the precursor treated by ordinary drying method and its calcined product are shown in Fig.3, showing that the precursor has a considerable aggregation phenomenon and the distribution of the particle size is inhomogeneous. The size of α-Al₂O₃ particles is large and the particles also have a visible aggregation with sintering neck.  

There might be two reasons for the phenomenon described above. Firstly, massive water molecules are stored in ammonium aluminum carbonate hydroxide particles. The liquid-gas interface has not produced at the beginning of evaporation, and there is no capillary force. When the ordinary drying method is adopted, with the continuously evaporating of the liquid, the void among the particles might result in great capillary pressure which presses the particles together, and produces the bond force, and the aggregation of the particles is formed.

Meanwhile, such bond force may be increased with the further drying process, and the aggregation becomes

Fig.3 SEM images of precursor (a) and α-Al₂O₃ (b) prepared by ordinary drying method

more serious. Secondly, the redundant free water molecules and the free hydroxyl on the surface of the powder particles interact each other by hydrogen bonding. When the particles contact closely, the hydrogen function between the redundant free water molecules and the hydroxyl on the surface of the particles is produced, and the bridging function appears. This bridging water molecules will be removed at the beginning of the drying, which causes the hydroxyl hydrogen bond function between the particles, and the covalent bonder will be introduced in the further dehydration process, then aggregation is produced. Fig.4 shows the possible mechanism of the formation of agglomerates in ordinary drying.

The SEM images of precursor treated by alcohol exchange method and its calcined product are shown in Fig.5, indicating that compared with the ordinary drying method, the dispersivity of the precursor and α-Al₂O₃ are improved by using alcohol exchange method. But the particle size of the precursor is still inhomogeneous, and the shape is also irregular. The particle size of α-Al₂O₃ powders which connect together is larger and considerable aggregation still exists. It can be concluded that when the precursor is washed by absolute ethanol, water among the particles is removed, and the capillary pressure is reduced, so the aggregation of the particles decreases. But water among the particles cannot be thoroughly removed by absolute ethanol, therefore, the hard aggregation of the particles can only be partially eliminated.

Fig.4 Possible mechanism of formation of agglomerates in ordinary drying

The SEM images of the precursors treated by vacuum freeze-drying, glycol distillation and n-butanol azeotropic distillation respectively and their calcined products are shown in Fig.6, showing that after the treatment of the three drying methods, the dispersion of precursors and their calcined products is improved, and the precursors are in non-aggregation with regular shape and even distribution of particle size. The obtained α-Al₂O₃ powders are also in non-aggregation with regular shape, and the average particle size is about   40 nm.

When the freeze drying method is used, water can be

Fig.5 SEM images of precursor (a) and α-Al₂O₃ (b) prepared by alcohol exchange method

Fig.6 SEM images of precursors ((a₁), (b₁), (c₁)) and α-Al₂O₃ ((a₂), (b₂), (c₂)) prepared by different drying methods

(a₁), (a₂)—Vacuum freeze-drying; (b₁), (b₂)—Glycol distillation; (c₁), (c₂)—n-butanol heterogeneous azeotropic distillation

removed by freezing drying method by the agency of sublimation under reduced pressure. In order to obtain a satisfactory drying rate, very low pressure is required. Most freeze drying is done at -50 ℃, and absolute pressures of about 2 666 Pa or less. In the freeze-drying process, the “liquid bridge” between the particles is frozen to “solid bridge”, the relative position of the two particles is fixed, and the surface tension of gas-liquid interface between the two particles is also eliminated. The hard aggregation can be effectively prevented by freezing drying with the decreased “solid bridge”. The freezen water within the microspheres can be easily removed, but the fraction of bound water is more slowly sublimed and still remained after freeze-drying for 12 h, so there is still some sintering neck in α-Al₂O₃ particles.

Water can be removed by glycol distillation method from the precipitation by distillation due to the high boiling point of the glycol. The boiling point of the glycol is 197.4 ℃ which is higher than that of the water. Water is evaporated when heating the mixture. Compared with the alcohol exchange method, the effect of removing water is greatly improved by glycol distillation. But water cannot be thoroughly removed, and there is still some sintering necks in α-Al₂O₃ particles.

When the precursor is treated by n-butanol azeotropic distillation method, at first, water in the distilled gels is replaced by n-butanol that has a low surface tension stress, leading to a decrease of aggregation. Secondly, vander Waals bonding instead of hydrogen bonding may be responsible for eliminating agglomeration by n-butanol azeotropic distillation treatment. In fact, the surface reaction between precursor and n-butanol may take place during distillation:

Al(OH)_n+HOC₄H₉→H­O+(OH)_n-1…Al—O—C₄H₉  (2)

The OH^- groups of the hydrates on the surface are substituted by —OC₄H₉, which corresponds to hydrophilicity and hydrophobicity, respectively. As a result, strong hydrogen bonding between the hydrate gels with H—O—H bridges becomes a van der Waals bond with —O—C₄H₉ bridges after distillation. So it is the best method to prevent the powder from forming aggregation.

3.3 Effect of drying method on properties of alumina particles        

The precursors treated by the above five drying methods are calcined at 1 200 ℃ for 2 h to obtain pure α-Al₂O₃ particles, the average grain size was calculated from the Scherrer formula:

d_XRD=Kλ/(βcos θ)            (3)

where  d_XRD is the average grain size in nm, K=0.89 is the Scherrer constant, λ=0.154 1 nm is the wave length of X-ray, β is the broadening of diffraction line measured half of its maximum intensity in radians, θ is the diffraction angle.

The average grain sizes of the samples are estimated from the line width of the (113) XRD peaks and are shown in Fig.7.

Assuming monodisperse, spherical primary particles, the BET-equivalent primary particle diameter (d_BET) of different samples was calculated by:

d_BET=6/(ρ_pS_m)              (4)

where  ρ_p is the density of α-Al₂O₃, S_m is the specific surface area. The BET-equivalent primary particle diameters of different samples are shown in Fig.8.

Fig.7 d_XRD of samples prepared by different drying methods

A—Ordinary drying; B—Alcohol exchange; C—Vacuum freeze-drying; D—Glycol distillation; E—n-butanol azeotropic distillation

Fig.8 d_BET of samples prepared by different drying methods

A—Ordinary drying; B—Alcohol exchange; C—Vacuum freeze-drying; D—Glycol distillation; E—n-butanol azeotropic distillation

The aggregation coefficient (f_a) of different samples can be calculated from the following relationship:

f_a= d_BET/d_XRD                       (5)

The aggregation coefficients of different samples are shown in Fig.9. If f_a is close to 1, the aggregation of the sample will be slightly, and if f_a deviates from 1, the aggregation of the sample will be serious. Fig.9 shows that the massive aggregation is formed by ordinary drying method (sample A), the slightly aggregation is formed by alcohol exchange method (sample B), and the products with good dispersion are formed by vacuum freeze-drying (sample C), glycol distillation (sample D) and n-butanol azeotropic distillation (sample E). It is demonstrated that vacuum freeze-drying, glycol distillation and n-butanol azeotropic distillation can effectively prevent the powder from forming aggregation during drying process, and obtain the α-Al₂O₃ powder with excellent properties, among them the n-butanol azeotropic distillation is the best method to prevent the powders from forming aggregation.

Fig.9 Aggregation coefficient of samples prepared by different drying methods

A—Ordinary drying; B—Alcohol exchange; C—Vacuum freeze-drying; D—Glycol distillation; E—n-butanol azeotropic distillation

4 Conclusions

1) Ammonium aluminum carbonate hydroxide (AACH) was synthesized by the precipitation and calcined at 1 200 ℃ for 2 h to obtain nanometer α-Al₂O₃ powder.

2) The gravely hard aggregation is formed by ordinary drying. The dispersity of powder is partially improved by alcohol exchange method, but the hard aggregation cannot be thoroughly solved.

3) Vacuum freeze-drying, glycol distillation and n-butanol azeotropic distillation can effectively prevent the powder from forming aggregation in drying process, among them the n-butanol azeotropic distillation is the best. Nanometer α-Al₂O₃ powder with non-aggregation can be manufactured though n-butanol azeotropic distillation and the average particle size is about 40 nm.

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(Edited by CHEN Wei-ping)

Foundation item: Project (5JJ30103) supported by the Natural Science Foundation of Hunan Province, China

Received date: 2006-08-26; Accepted date: 2006-10-18

Corresponding author: XIAO Jin, PhD; Tel: +86-731-8876454；E-mail: 13607445108@hnmcc.com