Preparation and Sintering Behaviour of Alumina Powder by Ammonia Precipitation Method

In this paper, alumina precursor was prepared by the ammonia precipitation method which used Al (NO3) 3 9H2O as aluminum source and NH4OH as a precipitator, adding a small amount of PEG4000 as the surface active agent. Finally γ-Al2O3 was obtained at 900°C for 2h. The stable alumina crystal form of α-Al2O3 was got at 1100°C for 2h. The influence of precipitation agent on the precursor was studied by means of TG / DTA and Tem, XRD etc. The effects of the synthesis temperature and time on the phase composition and morphology of the alumina powder were also analysed.


Introduction
Alumina powder as one of the special functional materials, due to the high purity, fine particles and uniformly distributed, it has a broad prospect of application on the ceramic structure, catalyst carrier, fine ceramics for its small size , surface and quantum effect [1].
Preparation of nano alumina powder by precipitation method [2,3,4] has the advantages of low cost, high purity, large amount of production, simple equipment manufacturing and short process.At present, the main precipitation method for preparing alumina powder was used ammonia and urea as precipitation agent,by the reaction of aluminum ions and precipitation agent directly generated γ-AlO(OH),and then sintered this alumina precursor [5].
Based on the preliminary work [6,7], the process parameters such as thermal analysis, synthesis temperature and reaction time were studied by using ammonia precipitation method which took aluminum nitrate as raw material.This method had the advantages of simple process, low cost and short process flow, and it was suitable for the preparation of nanometer powder in the process of industrial production.

Experiment method
Using Al(NO 3 ) 3 •9H 2 O as aluminum source, NH 4 OH as precipitator, a small amount of PEG4000 was added in the experiment as the surface active agent to prevent the aggregation of the product particles.In order to reduce growth rate of the nucleation , the experimental temperature should be controlled below 10 , dropping speed was controlled at 3 ~ 5 ml/min, and the end point pH value was kept between 7 and 8.
After the end of the titration, the preparation of the precursor solution was prepared by aging and centrifugation, and then washed with anhydrous ethanol.
Finally, the resulting sample was placed in a rapid heating furnace, with a temperature of 10 /min rise to 900 for 2h, the sample was token with the furnace cooling, and finally ultra-fine alumina powder was got.

Effect of dispersant on particle size and morphology
Surface active agent as a good chemical dispersants can be adsorbed on the surface of the powder particles.It made the inter particles difficult to reunite and made the powder dispersed by electrostatic repulsion and VDW [8,9,10].Some research results shown that PEG4000 can form a uniform and thick protective film on the surface of the colloid, which can restrain the aggregation of colloidal particles very effectively [11].Therefore, PEG4000 was used as a dispersant to prepare ultrafine alumina powders.

TG-DSC comprehensive thermal analysis of alumina precursor
The TG-DSC curve of alumina precursor prepared by ammonia as precipitator was shown in Figure 1.With the increase of sintering temperature, weightlessness of Al(OH) 3 was a continuous process when the temperature was below 700 .The curve showed a continuous decline in this process.When the temperature was higher than 700 , the TG curve was almost a horizontal curve.This horizontal curve indicated that the decomposition of Al(OH) 3 was over.The TG curve was in a horizontal state for the Al(OH) 3 has begun to phase transformation,and the phase change process was a constant weight process.
An obvious endothermic peak was appeared in the DSC curve at 133.46 .The adsorption water was divided from the sample.Endothermic peaks was appeared nearly 207.80 for the volatilization of residual organic matter in the sample.A strong endothermic peak appeared at 245.98 for the alumina precursor was appeared the following decomposition reaction, see formula (1): (1) Water produced by the decomposition reaction was overflowed in the form of water vapor.With the increase of temperature, a strong exothermic peak was appeared at 299.77 for AlOOH was generated from the decomposition reaction, see formula (2).TG curves between 200 to 300 sharp declined for AlOOH was changed to Al 2 O 3 and Decomposition of water was evaporated from the sample.
(2) When the heating temperature rises to 1107 and 1163 respectively, two obvious endothermic peak were correspondingly appeared on the DSC curve.There was no obvious phenomenon of weightlessness in the TG curve.Stable alumina crystal can be obtained at 1000 .

Effect of calcination temperature on the phase composition of alumina
The XRD diagram of alumina precursor prepared by ammonia water as the precipitation agent at different temperatures was shown in Figure 2. Amorphous Al 2 O 3 was obtained when the sintering temperature reached 800 for 2h as shown from Fig. 2

Effect of sintering time on the weight loss rate of alumina powder
The sintering temperature range was determined by the comprehensive thermal analysis of alumina precursor.In order to determine the reasonable holding time, a certain amount of alumina precursor was taken under 900 and incubated for 1 hour, 1.5 hours, 2 and 2.5 hours.The relationship between the powder weight loss rate and the sintering time was as shown below in Fig. 3.The weight loss rate of the powder was basically unchanged at 900 for 2 hours compared with followed sintering time.Therefore, 2 hours of the sintering time was chosen due to the aggregation and growth of grains can be avoided.

Effect of calcination temperature on particle size and morphology of alumina powder
The relationship between different calcinationtemperature and average particle size of alumina powder prepared by using ammonia as precipitator was shown in Fig. 4. The different sintering temperature had a significant effect on the particle size of alumina.At the same sintering time, with the increase of the sintering temperature, the relationship between the sintering temperature and the average particle size firstly decreased, then increased followed by decreasing.And the minimum size of the particle size was obtained at 900 .The nano scale spherical alumina powder can be obtained from alumina precursor sintered by 900 for 2 hours, as shown in Fig. 5.

Conclusions
Nano alumina particles was got from the dried precursor by sintering 900 after 2 hours.Alumina precursor was prepared which used 0.3mol/l aluminum nitrate as raw material and 1:2 ammonia as precipitator, adding a certain amount of dispersant.In this precipitation process, temperature was controlled below 10 degrees.And pH was between 7 and 8.
After adding the dispersant PEG4000, fine particle size, appearance and good dispersion of the alumina powder can be obtained.In the sintering process of alumina precursor, the crystal components increased gradually, and the crystal composition increased with the increase of the sintering temperature.A stable crystal structure of the -Al 2 O 3 can be obtained with ammonia as the precipitation agent prepared by the alumina precursor sintered at 1100 for 2 h.

Figure 1 .
Figure 1.TG-DSC curve of Al 2 O 3 precursor prepared by ammonium hydroxide

Figure 3 .
Figure 3. Relation between weight loss and dwell time

Figure 4 .
Figure 4. Relationship between different sintering temperature and average particle size with ammonia as precipitator

Figure 5 .
Figure 5. SEM image of Al 2 O 3 powder prepared by ammonium