ABSTRACT

Alumina (Al₂O₃) has been synthesized through combustion synthesis (CS) technique. The calcined products were characterized using X-ray diffractional analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermo-gravimetric analysis (TGA). TG-DTA results reveal the various stages involved in transition from γ-phase to α-Al₂O₃ phase. The first phase γ-Al₂O₃ was presented in the temperature range from 600˚C - 875˚C as deduced from the XRD patterns with cubic crystal structure. The second stage occurs in the temperature range from 900˚C - 1000˚C. In the final step, above 1000˚C, the aluminium oxide appears completely as α-Al₂O₃, showing high crystallinity. The particle sizes are closely related to γ- to α-Al₂O₃ phase transition.

1. Introduction

Alumina (Al₂O₃) has been employed for a wide range of applications, like electronic packaging, catalysts, sensors, etc. [1,2]. There are 15 distinct crystallographic phases, on which γ and α forms are used for wide range of applications due to their distinct properties. The high surface area of γ-Al₂O₃ makes it useful for catalyst related applications [3,4], where as the polycrystalline α-Al₂O₃ is extensively used for ceramic applications [5,6].

Numerous synthesis techniques have been followed to obtain nanosized Al₂O₃ products. Treatment phase transformations in Al₂O₃ nanocrystallites from γ to α phase is a crucial factor to the monitored. The various phases of Al₂O₃ undergo a variety of transitions until the most stable α structure (α-Al₂O₃) is formed at a high temperature [1,2]. It is well known from reports in literature that for obtaining dense nanocrystalline Al₂O₃ products, either the phase transformation from γ to α phase has to be arrested or nanocrystalline α-Al₂O₃ powders have to be used [7-9]. Different types of high molecular weight organic fuels have been successfully employed to synthesize materials with high specific surface areas. However, with the use of these fuels, an additional calcinations step is needed to obtain crystalline materials, making the process very expensive [10]. The use of urea in direct synthesis procedure facilitated the formation of crystalline materials due to the generation of a temperature [11].

Generally, combustion synthesis is an excellent technique for preparing high temperature materials because of its low cost, high yield and the ability to achieve high purity and single or multi phase complex oxide powders in the as-synthesized state [12]. In this way the heat input needs only to reach an ignition temperature so that an exothermic, self-sustaining reaction takes place between precursors such as metal nitrates (oxidizers) and a carbonaceous fuel (reducer) [13]. In fact, this auto-ignition process along with the self-propagating high temperature synthesis (SHS) has emerged as safe, simple and most economic process, which yields high purity powders with excellent homogeneity and fine particle sizes in the case of composite materials. Furthermore, as this process does not require any further processes of washing, filtration, drying, calcinations, addition of any acid or base, it becomes very fast. Moreover, the combustion synthesis (CS) procedure has many advantages as compared to conventional synthesis procedures; CS process can be used to produce novel phases with unique properties in materials as pure compounds can be easily produced because of the high thermal gradient and rapid cooling rate. Because of the gas evolution, large particles or agglomerates can be disintegrated during the process and the products formed are of high purity. The resulting product is very fine particulates of friable agglomerates that can be easily ground to obtain a much finer particle size.

In this perspective, this paper aims to study the phase transformation (γ- to α-Al₂O₃) behaviour of Al₂O₃ samples prepared by combustion synthesis technique and to evaluate their characteristic properties. The different phases formed were characterized by various techniques.

2. Experimental Details

Analytical grade Aluminium nitrate Al(NO₃)₃∙6H₂O and urea [(NH₂)₂CO] without the addition of water in the molar ratio 1:2 were directly mixed. Aluminium nitrate being hygroscopic absorbs moisture from the air forming a transparent slurry mixture. After homogeneous mixing, the solutions were condensed on a hot plate and fired for 30 minutes in a muffle furnace at 500˚C. The resultant powders were subsequently annealed in the range of 600˚C - 1000˚C for 2 h to obtain nanocrystalline Al₂O₃. Thermal behaviours of the samples were studied by thermogravimetry analyzer (Nietzsche STA 409C) and differential thermal analyzer (WCR-1C) in N₂ at the heating rate of 20˚C/min up to 1000˚C. The powders were characterized to determine the crystalline size, structure and morphology. The structure of the samples was analyzed using Powder X-ray diffraction (XRD) technique (Rich Siefert, Model 3000) using CuKα1 (λ = 1.5406 Å) radiation. Then using XRDA software programme the lattice parameters were determined and the average crystallite size was calculated with DebyeScherrer formula. The Al₂O₃ particles were studied using transmission electron microscopy (TEM, JEM-1200EX, JEOL), the dilute suspension of the Al₂O₃ powders on the carbon-coated copper grids also dried in air. The surface features of the as prepared and annealed samples were observed by scanning electron microscope (SEM; Hitachi S-3000N).

3. Results and Discussion

3.1. Growth Process and Reaction Mechanism for Al₂O₃ Formation

The individual reactivity of Al(NO₃)₃ with respect to urea combustion reactions assumed to take place in the case of metal nitrate/fuel binary mixtures are represented by the Equation (1)

(1)

Al(NO₃)₃ the situation is quite opposite. The reaction of Al(NO₃)₃ with urea (Equation (1)) was an intense and rather fast flaming combustion process. The formed Al₂O₃ nanoparticles were used for the further characterization.

3.2. Crystal Structure Determination

Figure 1(a) shows XRD pattern shows formation of mixed phases of Al₂O₃ along with the presence of nonstiochiometric as-synthesized precursor. Because of air as carrier gas, the deficiency of oxygen atoms may have lead to the formation of Al₂O₃ phase in the powder samples. The nonstiochiometric phase can be removed by heat treatment. Table 1 gives the comparison of observed “d” values are in good agreement with standard “d” values and the diffraction peaks are indexed to the phase of Al₂O₃ [JCPDS No. 86-1410]. Figures 1(b)-(d) shows the XRD pattern of annealed γ-Al₂O₃ was revealed. It can be compared to transformation temperature the range from 600˚C to 875˚C, with low intensity which signified the transformation of mixed phase Al₂O₃ to γ-alumina is the only identified phase observed. Thermal treatment at 600˚C to 875˚C resulting with diffraction peaks at 2θ, 46.00˚ for (400) and 66.80˚ for (440) [JCPDS No. 29- 0063] shows the sign of γ-Al₂O₃ phase. Table 2 gives the comparison of observed “d” values are in good agreement with standard “d” values and the diffraction peaks are indexed to the phase of γ-Al₂O₃ [JCPDS No. 29-0063]. The degree of crystallinity was improved with increasing temperature. γ-Alumina was completely transformed to α-alumina at 900 to 1000˚C α-alumina is

the only present phase. The crystallite size of the α-Al₂O₃ it increases with increasing temperature. Figures 1(e)-(f). shows XRD pattern of Al₂O₃ annealed above 900 to 1000˚C, matches well with that of α-Al₂O₃ with the diffraction peaks at 2θ values of, 25.37˚ for (012), 34.89˚ for (104), 37.50˚ for (110), 43.04˚ for (113), 52.58˚ for (024), 57.26˚ for (116) and 66.28˚ for (214) [JCPDS No. 42-1468]. No more appearance of other phases could be found in that temperature. Table 3 gives the comparison of observed “d” values are in good agreement with standard “d” values and the diffraction peaks are indexed to the phase of α-Al₂O₃ [JCPDS No. 29-0063]. The crystallite sizes of the nanocrystalline Al₂O₃ sample have been calculated using Scherrer formula [14]. The change in grain size is observed with broadening of the diffraction peaks. The mean crystallite size is calculated from the Scherrer formula listed in Table 4.

The particle size of γ-Al₂O₃ phase was increase corresponding to the respective annealing temperature. It was shown that γ-Al₂O₃ phase crystallite almost disappeared without growing up and α-Al₂O₃ phase crystallite rapidly grew up as transformation was completed. The α-Al₂O₃ phase crystallites could be obtained either through interface migration from α-Al₂O₃ nuclei or by coalescence among the α-Al₂O₃ nuclei and the two modes of formation result in the complete transition of γ-/α-Al₂O₃ phase. A high transformation temperature always results in the coarsening of particles and formation of hard agglomerates in the powder. Thus, a reduction in the γ-Al₂O₃ → α-Al₂O₃ transformation temperature is crucial for the processing of highly reactive ultra-fine α alumina powder. Strong peaks at 2θ, 43.04 for (113) and 57.26 for (116),

are recognized, indicating α-Al₂O₃ phase according to the 42-1468 [JCPDS-PCPDFWIN, 2002].

3.3. Differential Thermalgravimetric Analysis

The thermal studies of these samples were carried out at the heating rate of 20˚C/min up to 1000˚C and the TG/DTA curves are given shown in Figure 2. From Figure 2, it is observed that the process of decomposition proceeds in two stages. The first step, ambient to endothermic peak at 195˚C is due to the dehydration of hydroxyl water of γ-AlOOH transformed to γ-Al₂O₃. In the temperature range from 150˚C to 340˚C, the 24.75% weight loss is much larger than the theoretical weight loss of the boehmite dehydration. It is known that the ultrafine boehmite powders have high special surface and easily adsorb water as hydration water which can exist stably to a certain temperature. It was reported [17] that pseudoboehmite typically contained excess water. The further weight loss at high temperature may result from the dehydration of the residual hydroxyl water in γ- Al₂O₃.The final step, from 340˚C to 630˚C, was due to the formation of Al₂O₃ with a weight loss of 11.556%. The net weight loss of the compound was found to be 36.30%. The DTA result of the as-prepared sample shows that the exothermic curves were associated with the corresponding weight loss in TGA curve of Al₂O₃ powder.

3.4. Differential Scanning Calorimetry

Figure 3 depicts the DSC trace of Al₂O₃ powders, in the temperature range of 100˚C to 550˚C at three different heating rates 10˚C, 15˚C and 20˚C/min. From the DSC trace, it is seen that at temperature below 260˚C, there are endothermic peaks due to the vaporization of physically bound absorbed water and the dehydration reaction of powders. It is important to note that these powders, irrespective of their urea-nitrate ratios, always show exothermal decomposition in their corresponding DSC